Lunar mare deposits_associated_with_orientale_basin

Lunar mare deposits associated with the Orientale impact basin:New insights into mineralogy, history, mode of emplacement,and relation to Orientale Basin evolution from Moon MineralogyMapper (M3) data from Chandrayaan‐1

Jennifer Whitten,1 James W. Head,1 Matthew Staid,2 Carle M. Pieters,1 John Mustard,1

Roger Clark,3 Jeff Nettles,1 Rachel L. Klima,1,4 and Larry Taylor5

Received 31 August 2010; revised 23 November 2010; accepted 30 December 2010; published 22 April 2011.

[1] Moon Mineralogy Mapper (M3) image and spectral reflectance data are combined toanalyze mare basalt units in and adjacent to the Orientale multiring impact basin. Modelsare assessed for the relationships between basin formation and mare basalt emplacement.Mare basalt emplacement on the western nearside limb began prior to the Orientale event asevidenced by the presence of cryptomaria. The earliest post‐Orientale‐event mare basaltemplacement occurred in the center of the basin (Mare Orientale) and postdated theformation of the Orientale Basin by about 60–100Ma. Over the next several hundred millionyears, basalt patches were emplaced first along the base of the Outer Rook ring (Lacus Veris)and then along the base of the Cordillera ring (Lacus Autumni), with some overlap inages. The latest basalt patches are as young as some of the youngest basalt deposits onthe lunar nearside. M3 data show several previously undetected mare patches on thesouthwestern margins of the basin interior. Regardless, the previously documented increasein mare abundance from the southwest toward the northeast is still prominent. We attributethis to crustal and lithospheric trends moving from the farside to the nearside, withcorrespondingly shallower density and thermal barriers to basaltic magma ascent anderuption toward the nearside. The wide range of model ages for Orientale mare deposits(3.70–1.66 Ga) mirrors the range of nearside mare ages, indicating that the small amount ofmare fill in Orientale is not due to early cessation of mare emplacement but rather to limitedvolumes of extrusion for each phase during the entire period of nearside mare basaltvolcanism. This suggests that nearside and farside source regions may be similar but thatother factors, such as thermal and crustal thickness barriers to magma ascent and eruption,may be determining the abundance of surface deposits on the limbs and farside. Thesequence, timing, and elevation of mare basalt deposits suggest that regional basin‐relatedstresses exerted control on their distribution. Our analysis clearly shows that Orientale servesas an excellent example of the early stages of the filling of impact basins with mare basalt.

Citation: Whitten, J., J. W. Head, M. Staid, C. M. Pieters, J. Mustard, R. Clark, J. Nettles, R. L. Klima, and L. Taylor (2011),Lunar mare deposits associated with the Orientale impact basin: New insights into mineralogy, history, mode of emplacement, andrelation to Orientale Basin evolution from Moon Mineralogy Mapper (M3) data from Chandrayaan‐1, J. Geophys. Res., 116,E00G09, doi:10.1029/2010JE003736.

1. Introduction

[2] About 20% of the lunar surface is covered with maredeposits, including mare patches on the farside and limbs

[Head, 1976;Wilhelms, 1987; Yingst and Head, 1997, 1999]and larger more continuous deposits lying within and adja-cent to the large impact basins on the nearside of the Moon[Head, 1976; Wilhelms, 1987; Head and Wilson, 1992] [seeYingst and Head, 1997, Figure 1;Hiesinger and Head, 2006,Figure 1.21]. The generation, ascent, and eruption of marebasalt magmas have been subjects of intense study and debatefor many years. Petrologists have proposed and assessedmodels for the generation and petrogenetic evolution ofbasaltic rocks returned from the Moon [e.g., Hess andParmentier, 1995, 2001; Shearer et al., 2006]. Geologistshave described and classified the nature of the vents anddeposits and from this information inferred eruption condi-tions and durations [e.g., Head, 1976; Wilson and Head,

1Department of Geological Sciences, Brown University, Providence,Rhode Island, USA.

2Planetary Science Institute, Tucson, Arizona, USA.3U.S. Geological Survey, Denver, Colorado, USA.4Now at Johns Hopkins University Applied Physics Laboratory, Laurel,

Maryland, USA.5Planetary Geosciences Institute, University of Tennessee, Knoxville,

Tennessee, USA.

Copyright 2011 by the American Geophysical Union.0148‐0227/11/2010JE003736

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http://dx.doi.org/10.1029/2010JE003736

1980; Head and Wilson, 1992; Hiesinger and Head, 2006].Spectroscopists have shown the mineralogical diversity ofmare basalts and linked surface units to returned samples[e.g., Pieters, 1978; Pieters et al., 1993; Staid et al., 1996;

Lucey et al., 2006]. Geochronologists have provided achronology of mare basalt emplacement and flux throughdating of returned samples [e.g., Nyquist and Shih, 1992] andimpact crater size–frequency distribution analyses of mareunits [e.g., Stöffler et al., 2006; Hiesinger et al., 2000, 2003,2011]. Volcanologists have modeled the processes of magmageneration, ascent, and eruption and made predictions aboutthe nature, volumes, and styles of eruptions [e.g.,Wilson andHead, 1981; Head and Wilson, 1992]. Geophysicists havemodeled the thermal history and structure of the lunar interiorand provided a range of candidate scenarios for mare basaltgeneration [e.g., Wieczorek et al., 2006], as well as the rela-tionship of mare basalt emplacement and lithospheric thick-ness [e.g., Solomon and Head, 1979, 1980].[3] Despite significant advances in all of these areas,

several of the most fundamental questions in mare basaltpetrogenesis and emplacement remain unresolved [e.g.,Jolliff et al., 2006; Jolliff, 2008]. Mare basalts are known tooccur preferentially on the lunar nearside, where both thecrust [e.g., Ishihara et al., 2009] and lithosphere [Solomonand Head, 1980] are thinnest, and to be concentrated withinand adjacent to impact basins, such as Imbrium, Serenitatis,Crisium and Humorum. The following are among the out-standing questions.[4] 1. What is the relationship of impact basin formation

andmare basalt genesis and emplacement? For example, doesthe formation of an impact basin generate or assist in theformation of mare basalts [e.g., Ivanov and Melosh, 2003;Elkins‐Tanton et al., 2004; Elkins‐Tanton and Hager, 2005]?Does the presence of impact basins influence where marebasalts are likely to be extruded [e.g., Head and Wilson,1992]? When mare basalts are emplaced in lunar basins,does the filling and loading influence further stages, styles,and locations of eruption and emplacement [e.g., Solomonand Head, 1979, 1980]?[5] 2. What is the influence of crustal thickness and litho-

spheric thickness on the emplacement of mare basalts? Aremare basalts inhibited from ascending to the surface bybuoyancy traps at the base of the crust or lithosphere [e.g.,Head and Wilson, 1992]? Or are there other reasons for thedifferences in the nearside/farside distribution of mare basaltsunrelated to these factors, such as fundamental differencesin deep thermal structure and thus mare basalt genesis [e.g.,Zhong et al., 2000]?[6] The Orientale multiring basin offers the opportunity to

assess many of these questions. Located on the western limbof the lunar nearside (Figure 1), Orientale is the youngestmultiring basin on the Moon [e.g., Wilhelms, 1987] and islocated in the transitional region between thin nearside crustand thick farside crust [e.g., Ishihara et al., 2009]. It issparsely filled with mare basalts and thus the relationshipsbetween mare basalts and a well‐preserved basin structure

Figure 1. (a) Location map of Orientale Basin with fourmain rings and deposits labeled. (b) M3 thermal image(2936 nm) mosaic of Orientale Basin. This wavelengthband is effective at displaying topography. (c) Standard M3

color composite, highlighting mafic mineral absorptions(R, 1.0 mm integrated band depth (IBD); G, the 2.0 mmIBD; and B, reflectance at 1489 nm). Yellow regions indicateareas of strong 1.0 and 2.0 mm IBDs.

WHITTEN ET AL.: VOLCANISM IN THE LUNAR ORIENTALE BASIN E00G09E00G09

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can be readily assessed. Furthermore, the remote sensingcharacteristics of the mare basalts have been previouslystudied in a variety of analyses [e.g., Head, 1974; Scott et al.,1977; Greeley et al., 1993; Kadel et al., 1993; Yingst andHead, 1997; Bussey and Spudis, 1997, 2000; Staid, 2000].[7] Recent acquisition of high spatial and spectral resolu-

tion spectrometer data for the Orientale region of the Moonby the Moon Mineralogy Mapper (M3) instrument on theChandrayaan‐1 mission permits further characterization ofmare basalt deposits in this region, and enables an analysisof their mineralogy, ages, affinities, and their relationshipsto Orientale impact basin deposits, topography and evo-lution. In this analysis, we utilize a range of M3 and relateddata (e.g., Lunar Orbiting Laser Altimeter, LOLA) to addressthese outstanding questions concerning (1) the relation ofmare basalt origin and emplacement to impact basins and(2) crustal/lithospheric thickness differences.[8] The purpose of this paper is to investigate the char-

acteristics of volcanic features and deposits in the OrientaleBasin using the latest available data, including (1) the ages ofthe various volcanic deposits, the melt sheet, and the basinevent itself; (2) the areal extent and volume of the identifiedmare deposits; (3) the spectral characteristic and composi-tional implications of Mare Orientale and other volcanicdeposits; (4) their altimetry, including both elevation andlocal or regional tilts; and (5) the volcanic features associatedwith the basin, including sinuous rilles, domes, and vents.Combining all of this new information can assist in furtherdefining the relationship between large impact basins andvolcanism on the lunar surface in a nearside/farside transi-tion region.[9] We begin with a description of the new M3 data and

other data and techniques in order to provide background onhow we have utilized these data to bring new insight into theproblems outlined above.

2. The Moon Mineralogy Mapper (M3)Experiment, Related Data, and Analysis

2.1. Background: The M3 Instrument

[10] Within the last several years, petabytes of new datahave been returned to Earth from instruments on severallunar orbiters (e.g., Kaguya, Chang’ E, Chandrayaan‐1,Lunar Reconnaissance Orbiter). Among these instrumentsis the Moon Mineralogy Mapper (M3), a NASA imagingspectrometer onboard the Indian Chandrayaan‐1 spacecraft.M3 was designed to operate in two modes, a global mode anda targeted mode. The data used in this paper were obtained inglobal mode, which collected reflectance spectra in 85 bands,ranging from 400 to 3000 nm with a spatial resolution of140 m/pixel from a 100 km orbit [e.g., Green et al., 2007;Pieters et al., 2009]. Data from the M3 instrument were usedin crater counting, measurement of the area and volume of oldand newly identified mare ponds, determination of the lengthand width of sinuous rilles, and the definition of the spectralcomposition of Mare Orientale and related deposits.

2.2. Methods

2.2.1. Altimetry[11] The Lunar Orbiting Laser Altimeter (LOLA), one of

the instruments carried by the NASA Lunar Reconnaissance

Orbiter (LRO), provides a precise global lunar topographicmodel and geodetic grid by pulsing a laser instrument duringorbit [Zuber et al., 2010; Smith et al., 2010]. A 64 pixel perdegree (∼473 m/pixel) global gridded data set was used forbasin profiles and mare pond elevation analyses in this study.In addition, LOLA profile data were used to determine thewidths and depths of the identified sinuous rilles.2.2.2. Impact Crater Size‐Frequency Distributions[12] Crater counting was conducted using a M3 mosaic of

Orientale Basin at ∼140 m/pixel [Boardman et al., 2011], aswell as the CraterStats program [Michael and Neukum, 2010].The chronology and production functions of Neukum et al.[2001] were used to calculate the ages from analysis of thenew M3 data; crater ages were obtained by plotting unbinneddata. Specific count regions of similarly sized area weredefined for both the Maunder Formation (melt sheet deposit)and the Hevelius Formation (basin ejecta deposit) to obtainmodel ages for the basin‐forming event. Each count area forthe Maunder Formation was between 1,000 and 3,000 km2

and count areas for the Hevelius Formation ranged from190,000 to 250,000 km2. For dating the basin‐forming eventonly primary craters larger than 5 km were counted, whereasfor the mare ponds primary craters greater than ∼0.75 km indiameter were counted. Only if the crater population above0.75 km was too small to give statistically significant ageswere craters from 0.5 km in diameter considered. We foundno obvious flow fronts and no distinct differences in marecomposition in the individual mare ponds in Lacus Veris andLacus Autumni; thus, we assume that individual mare pondsare the result of single eruptive events [Yingst and Head,1997, 1999]. There is some indication of subtle differ-ences within individual mare pond composition, which couldeither be due to lateral and vertical mixing of feldspathicmaterial with mare soils or actual mineralogical differences[Kadel, 1993; Staid, 2000]. Further investigation is neededto assess these trends in mixing and apparent mineralogicaldifferences.2.2.3. Spectral Properties and Background on LunarNear‐Infrared Reflectance Spectroscopy[13] The mineralogic composition of the lunar surface has

been investigated for decades using the techniques of visible,near‐infrared (VNIR) spectroscopy, owing to the abundanceof diagnostic mineral absorptions in this region of the elec-tromagnetic spectrum [e.g., McCord et al., 1981; Pieterset al., 1996; Ohtake et al., 2009; Isaacson et al., 2011].These diagnostic absorptions in the VNIR are the result ofmolecular electronic transitions that are specific to crystal-lographic sites in mineral crystals. The characteristics of thesecrystallographic sites vary between different minerals andmineral compositions [Burns, 1993]. Thus, the shape andlocation of mineral absorptions are indicative of composition.The two most common mafic minerals on planetary surfacesare olivine and pyroxene, both having diagnostic absorptionsthat vary with composition. Pyroxenes have two diagnosticabsorptions near 1 and 2 mm [e.g., Adams, 1974; Cloutis andGaffey, 1991], and olivines have a combination absorptionfeature near 1 mm, composed of three separate overlappingmineral absorptions [e.g., Burns, 1970, 1974; Sunshine andPieters, 1998].[14] Oftentimes the continuum slope of spectra are

described as either “blue” or “red,” referring to the slope of


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the visible part of the electromagnetic spectrum. Blue light isthe shortest wavelength and red light is manifested at longerwavelengths. Thus, when a spectrum is described as “blue”that means the shorter wavelength region of the electromag-netic spectrum is dominating and the spectrum has a flatslope; “red” refers to positive continuum slopes controlled bythe longer wavelength regions of the electromagnetic spec-trum. This continuum slope is controlled by composition of ageologic material, whether it is mixed with another spectrallydistinct geologic material, and the maturity or degree of spaceweathering experienced by the surface and the composition ofthe material [e.g., Staid, 2000; Staid and Pieters, 2000].[15] Approximately 120 M3 spectra from Mare Orientale

were collected to characterize basalt compositions. Opticalperiod 1B (OP1B) [Boardman et al., 2011] data strips fromtwo regions of Mare Orientale, previously identified asuncontaminated with feldspathic basinmaterial [Staid, 2000],were utilized for data collection. To avoid complications re-sulting from highland contamination, spectra were collectedfrom craters < ∼1 km in diameter that showed an immaturesignature in a 0.989 mm/0.750 mm ratio parameter map.Craters have an excavation depth that is approximately 1/10thof the diameter [Melosh, 1989]; thus, collecting spectra fromcraters < ∼1 km in diameter ensures that only the surfacedeposit is being sampled. The 0.989 mm band, located atapproximately the center of one of the main absorptions formafic minerals, and the 0.750 mm band, located outside of theabsorption on the continuum part of a spectrum, provide agood indication of the strength of the 1 mm absorption inmafic minerals. The closer the value of the ratio is to 1, theweaker the mineral absorption.[16] It was necessary to take spectral measurements of the

more freshly exposed areas to avoid any effects of maturityor space weathering, such as decreased depth of mineralabsorption bands, increased red slope of the spectra, and adecrease the overall albedo of the surface [e.g., Pieters et al.,2000; Anand et al., 2004; Noble et al., 2007]. Calibration ofthe data is explained in detail by R. O. Green et al. (TheMoonMineralogy Mapper (M3) imaging spectrometer for lunarscience: Instrument, calibration, and on‐orbit measurementperformance, submitted to Journal of Geophysical Research,2011). Separate from general corrections, a mild spectralcorrection was applied to the data in order to suppress residualartifacts in M3 data. A detailed explanation of this correctionis provided by Isaacson et al. [2011].[17] In this analysis, only the 1.0 mm absorption band was

investigated due to factors, such as thermal effects, becomingincreasingly important to the 2.0 mm absorption, which isin the process of being resolved in the ongoing calibration ofthe new M3 data. Features of the 1 mm absorption that werestudied include the band center and the band strength. It hasbeen shown in numerous studies that absorptions centeredaround 1.0 mm and 2.0 mm shift to longer wavelengths asFe2+ and Ca2+ substitute for Mg2+ in pyroxenes [e.g., Adams,1974; Cloutis and Gaffey, 1991; Pieters et al., 1996; Klimaet al., 2007]. Measurements of the position of the 1 mmabsorption band center were made using the methods ofCloutis and Gaffey [1991]. In these calculations, it wasassumed that the absorptions were the result of a singleelectronic transition absorption, where spectroscopic min-eral absorptions are actually composed of several over-lapping electronic transition absorption bands. This more

basic approach was used in place of the modified Gaussianmodel (MGM), which can derive much more specific com-positional information by deconvolving individual absorp-tion bands [e.g., Sunshine et al., 1990; Pieters et al., 1996;Klima et al., 2007], in order to reach initial conclusions aboutthe composition of basalts in Mare Orientale.

3. Review of Models of Mare Basalt GenesisRelated to Impact Basin Formation/Evolutionand Global Crustal Thickness Variations

[18] Early in the modern study of the Moon, basic modelsfor the formation of mare basalts were postulated thatinvolved internal radiogenic heat sources and sequentialpartial melting of a layered mantle [e.g., Runcorn, 1974;Solomon, 1975]. Subsequent petrological evidence suggesteda wide range of source depths for mare basalts. The docu-mentation of increasing complexities in the ages and petro-genesis of mare basalts led to the proposal of several majornew ideas for their generation (see summary by Shearer et al.[2006]). Models for the generation, ascent and eruption ofmare basalts (as observed in the distribution of mare deposits)can be classified into five types, as follows.

3.1. Model 1: Nearside/Farside Mare BasaltAsymmetry Due to Crustal Thickness Differences

[19] One of the more fundamental characteristics of theMoon is the nearside‐farside asymmetry in the distribution ofmare basalts [e.g., Head, 1976]. Early models accounted forthis by calling on differences in nearside‐farside crustalthickness [e.g., Solomon, 1975; Head and Wilson, 1992]: forglobally heterogeneous mare basalt source regions at depth,dikes rising to a constant level were muchmore likely to eruptonto the surface in the thinner nearside crust than dikes risingin a thicker farside crust. The general paucity of farside marebasalts and the concentration of most farside basalts in therelatively deep South Pole–Aitken basin seemed consistentwith this hypothesis.[20] Global altimetry obtained by the Clementine mission

[e.g., Zuber et al., 1994; Neumann et al., 1996] revealed,however, that the depth of the South Pole–Aitken basin wascomparable to mare basalt elevations in basins on the near-side, and yet the SPA basin was not as extensively flooded bymare basalts as the nearside basins [e.g., Head et al., 1993;Greeley et al., 1993; Yingst and Head, 1997, 1999; Pieterset al., 2001]. These observations cast doubt on the assump-tion of global mare basalt source symmetry and the role ofcrustal thickness in explaining nearside‐farside mare basaltasymmetry, and pointed out the necessity of adding addi-tional factors to models of magma ascent and eruption [e.g.,Wieczorek et al., 2001]. This led to the development of sev-eral alternative models for the emplacement of basalts in theSouth Pole–Aitken basin and for theMoon as a whole, raisingthe questions (1) were there fundamental differences in thenearside‐farside source regions established early in lunarhistory [e.g., Zhong et al., 2000; Parmentier et al., 2002];(2) did the Procellarum KREEP Terrain (PKT) play a fun-damental role in mare basalt generation [e.g., Wieczorek andPhillips, 2000]; (3) was there a basic difference in thenearside‐farside thermal gradient that influenced both basinrelaxation and mare basalt generation, ascent and eruption[e.g., Solomon and Head, 1980]; and (4) could the formation


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of a basin the size of SPA have induced sufficient convectionto have stripped away a subsurface KREEP layer, and thusto have inhibited the formation of mare basalts below thebasin [e.g., Arkani‐Hamed and Pentecost, 2001]?[21] Model 1a (correlation with crustal/lithospheric thick-

ness) thus predicts a close correlation between these factors,and model 1b (no correlation) predicts not only no correla-tion, but differences in the nature and distribution of marebasalts in space and time due to nearside‐farside mantlesource region differences (Table 1).

3.2. Model 2: Impact Basin Pressure‐Release Meltingand Associated Secondary Convection

[22] Pressure release melting is known to be an importantmechanism for basalt generation, but the lunar pressuregradient, combined with the composition of the crust and theapparent depth of origin of mare basalts, has led to this mech-anism generally not being favored for mare basalt formation.Recently, Elkins‐Tanton et al. [2004] have reassessed themagmatic effects of large basin formation, and introduced atwo‐stage model for melt creation beneath lunar basins trig-gered by basin formation itself. In the initial stage, craterexcavation depressurizes and uplifts underlying mantlematerial so that it melts in situ instantaneously, forming largequantities of melt below the basin (in addition to impactmelt in the cavity). This model thus predicts huge quantitiesof in situ pressure release melt (98–100% of the melt created)produced instantaneously and available to be extruded intothe impact basin as lunar mare basalts.[23] In the second stage, the cratered region rises isostati-

cally, warping isotherms upward and inducing convection, atwhich time adiabatic melting can occur. This second stage isminiscule in terms of melt produced (1–2% of the total) butcan last for a longer period of time, up to ∼350 Ma. In theElkins‐Tanton et al. [2004] model, mafic mantle melts can begenerated from depths of 150–560 km, depending on mantlepotential temperature. Assuming that 10% of the melt gen-erated erupts, Elkins‐Tanton et al. [2004, Figure 5] find thatthe volumes of magma reported for basins are similar to theirpredictions. Elkins‐Tanton et al. [2004] also model the originand emplacement of high‐alumina, high‐TiO2, KREEP‐rich,

and picritic magmas and predict an order of eruption, with themost primitive, lowest‐titanium magmas last.[24] In a more recent treatment of impact‐induced con-

vection, Ghods and Arkani‐Hamed [2007] used a suite ofnumerical models to show that this mechanism might be ableto account for the formation of mare basalts, the range ofdepths of their source regions, the observed delay betweenimpact basin formation and initiation of basaltic volcanism,and the long duration of emplacement of mare basalts.These models treat basins of different sizes (e.g., rangingfrom Orientale, through Imbrium, to South Pole–Aitken),make different predictions about the record of mare basaltemplacement for each, and can be readily tested against thegeological record of mare basalt volcanism.[25] Models for pressure release melting predict initially

large volumes of mare basalts in the basin center, followed bysmaller amounts emplaced over several hundred millionyears, and the lowest‐Ti content lavas last (Table 1). Key teststhus involve the timing, duration, volumes, styles and thecomparison of the record in different sized basins.

3.3. Model 3: Enhanced Sub‐Procellarum KREEPLayer

[26] Although clearly outside the Orientale Basin region,more than 60% of the mare basalts by area occur within theboundaries of the Procellarum KREEP Terrain (PKT), whichmakes up only ∼16% of the surface of the Moon. Thisobservation was one of the major factors that led Wieczorekand Phillips [2000] to propose that there was a cause andeffect in terms of the occurrence of KREEP and the genera-tion of mare basalts. They postulated that the enhancement ofheat‐producing elements implied by the elevated KREEPlevels significantly influenced the thermal evolution of theregion, causing the underlying mantle to partially melt overmuch of lunar history to generate the observed basaltic vol-canic sequence. The thermal model ofWieczorek and Phillips[2000] predicts that melting occurs only directly below thePKT. Partial melting of the mantle begins immediately afterthe model is started at 4.5 Ga and continues to a lesser degreeto the present. Melting initiates immediately beneath theKREEP basalt layer and becomes deeper with time, with the

Table 1. Summary of the Various Models of Mare Basalt Genesis

Model Summary Predictions

1 Nearside/farside asymmetry due to crustal thickness differences 1a) A close correlation exists between crustal thickness anddistribution of mare basalts.

1b) No correlation exists between these two variables, insteaddifferences in the nature and distribution are due to nearside/farside mantle source region differences.

2 Pressure release melting and associated secondary convection Large quantities in situ pressure release melting are producedinstantaneously and located in the basin center. Smallerquantities of basalts are produced as a result of adiabatic meltinginduced by convection and persist for extended periods of timeafter impact (i.e., 350 Ma). The lowest‐titanium content marebasalts erupt last.

3 Enhanced KREEP layer induces volcanism in the Westernnearside Procellarum KREEP Terrain (PKT)

The presence of KREEP and possible derivatives (i.e., "red spot"volcanism such as the Gruithuisen Domes) might control thedistribution of mare basalts.

4 Large‐scale overturn causes eruption of mare basalts High‐titanium basalts erupt earliest, followed by a suite of othertypes of basalts. Model is independent of basin formation.

5 Mare basalt emplacement is related to global thermal evolutionand basin evolution

Early mare volcanism is located in the center of the basin, and theyoungest deposits can be found on the outside rim of the basin.This model is not necessarily related to basin‐forming impacts.


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maximum depth of melting being ∼600 km, and the KREEPlayer is kept above its solidus for most of lunar history.[27] Does this hypothesis account for the origin of mare

basalts in terms of the timing, duration, areal distribution,volumes, and changes in depth with time? If not, is it relatedto any other volcanism within the PKT region, such as the“red spot” volcanic domes and related deposits? Hess andParmentier [2001] pointed out several difficulties with thismodel in terms of (1) petrogenetic evolution, (2) geophysicalevidence against the long‐term duration of a near‐liquidKREEP layer, and (3) the fact that such a layer might form animpenetrable barrier to the eruption of mare basalts (e.g., howcan denser mare basalt liquids penetrate through a >30 kmthick layer of less dense, KREEP‐rich liquid?). This modelpredicts (Table 1) that the presence of KREEP and possiblederivatives, such as “red spots,”might control the distributionof mare basalts, and thus we examine the new data for suchevidence.

3.4. Model 4: Large‐Scale Overturn of Initial UnstableStratification

[28] In model 4, crystallization of the lunar magma ocean(LMO) forms a chemically stratified lunar interior with ananorthositic crust separated from the primitive lunar interiorby magma ocean cumulates. Dense, ilmenite‐rich cumulateswith high concentrations of incompatible radioactive ele-ments are the last magma ocean cumulates to form, andunderlying olivine‐orthopyroxene cumulates are also strati-fied with later crystallized, denser, more Fe‐rich composi-tions at the top. These layers are gravitationally unstable.Rayleigh‐Taylor instabilities cause the dense cumulates tosink toward the center of the Moon and to form a dense core[Hess and Parmentier, 1995]. Subsequently, the ilmenite‐rich cumulate core undergoes radioactive heating and thisheats the overlying mantle, causing melting. The sourceregion for high‐TiO2 basalts is thus envisioned to be a mixedzone above the core‐mantle boundary containing variableamounts of ilmenite and KREEP and involves deep, high‐pressure melting, delayed for a period of time subsequent toLMO formation and overturn. Thermal plumes rise intochemically stratified surroundings of the mantle (chemicallyless dense but colder) above the core and cause mixing andhomogenization. The resulting lower thermal boundary layermay be partially to wholly molten depending on mineralogyand the range of input parameters.[29] Melting at the top of the mixed layer to produce mare

basalt magmas must occur at low enough pressure for meltbuoyancy and at high enough pressure to satisfy the depthindicated by phase equilibria. The onset time of mare volca-nism is constrained by bulk core radioactivity, and TiO2‐richmare basalt liquids must be positively buoyant enough toform dikes rather than sink. This model predicts (Table 1)early high‐Ti mare basalts followed by a suite of other typesand is completely independent of impact basin formation.

3.5. Model 5: Nature and Location of Source Regionsand Emplacement Styles Related to Global ThermalEvolution and Basin Evolution but Independent of BasinFormation

[30] Inmodel 5, mare basalt emplacement is not necessarilyrelated to the formation of the impact basin in which thebasalts reside. Rather, the shape of the basin, the sequence

and geometry of fill and the resulting distribution of loading‐induced stresses (influenced by global thermal evolution andthe net state of stress in the lithosphere) controls the locationof eruptive vents and the style of emplacement [e.g., Solomonand Head, 1979, 1980; McGovern and Litherland, 2010].Using linear tectonic rilles and wrinkle ridges, Solomon andHead [1980] established that there are two stress systems,one local and another global, that affect the volcanism thatoccurred in the large lunar basins. The local stresses are theresult of lithospheric loading by the basalt fill of the basinsand global stresses originate from the thermal evolution of theMoon. The onset of mare volcanism in each basin is favoredby the presence of the local extensional stresses in the litho-sphere. Loading by central mare fill induces flexure andfavors migration of vents to the margins of the basin.[31] This hypothesis leads to the prediction (Table 1) that

early mare volcanism will be focused in the basin center andthat the youngest mare volcanism will have occurred on theedges of the basin. Once the global compressive stress, fromglobal cooling of the Moon, overrides the local extensionalstresses, ascent and eruption of mare basalts becomes moredifficult, and ultimately terminates.

3.6. Tests of the Models

[32] Although these five models treat different aspects ofthe origin, evolution and emplacement of mare basalts, eachmakes different predictions (Table 1) and can be tested withthe further characterization of mare basalts in the OrientaleBasin. Our analysis of mare basalts in Orientale thus has thefollowing elements: (1) When and where were Orientaledeposits first emplaced? (2) When and where did the terminalstages of volcanism in Orientale basin occur (what is theirage, what is their mode of emplacement)? (3) What was theflux of mare basalt volcanism? What is the volume of marebasalt deposits as a function of time, when did it peak, werethere multiple peaks, and what was the shape of the declinetoward the present? (4) How do the Orientale mare depositscompare to the global distribution of mare basalts as a func-tion of time? (5) How does the mineralogy of the Orientalemare deposits compare to the global distribution of marebasalt types? (6) How do eruption styles compare to theglobal distribution of mare basalt vent types and impliederuption conditions (e.g., J. W. Head and L. Wilson, Lunarvolcanic vent types, landforms and deposits: A synthesis andassessment of modes of emplacement, manuscript in prepa-ration, 2011)? (7) How do the individual and total volumes ofOrientale mare basalts help constrain the total volume ofmantle melting that has occurred (using estimates of extrusionto intrusion ratios, and depths of origin, to provide an order ofmagnitude assessment of the total melting that occurred,updating earlier estimates)? (8) How does the distribution ofpyroclastics in Orientale compare to the nature and globaldistribution of mare pyroclastic deposits [e.g., Gaddis et al.,2003], vent characteristics, eruption conditions, and associ-ated mare basalt types? (9) How do the model ages and modelage ranges of Orientale mare basalt emplacement relate to thegeneral thermal evolution of the Moon and to the loading andsubsidence observed in other basins [e.g., Solomon andHead,1980]? (10) Is there any evidence in Orientale for the type of“red spot” extrusive domes and related deposits [e.g.,Chevrelet al., 1999; Wagner et al., 2002, 2010; Wilson and Head,2003] that may relate to suspected KREEP basalt extrusions


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[e.g., Spudis et al., 1988] or to upland and Cayley plains thatmight represent aluminous basalts or cryptomaria [e.g.,Antonenko et al., 1995; Antonenko, 1999]?

4. The Orientale Basin: Background and Setting

[33] The Orientale multiring impact basin, located on thewestern limb of the Moon (19°S and 93°W), is the youngestlunar basin, dated to the Upper Imbrium period of lunarhistory [e.g., Wilhelms, 1987; Kadel, 1993] (Figure 1).Orientale is particularly interesting because, unlike mostlunar basins, its interior has not been completely filled withmare deposits, permitting investigations into the volcanicprocesses involved in the evolution and emplacement of marebasalts [e.g., Head, 1974; Greeley, 1976; Spudis et al., 1984;Head and Wilson, 1992; Bussey and Spudis, 1997, 2000;Yingst and Head, 1997]. The preservation of the Orientalering topography distinctly separates its various mare depositsand allows for a closer examination of their ages, compo-sitions and modes of emplacement. The nature, ages, andlocation of volcanic vents and deposits within multiringedbasins provide evidence for the role the basins play in thegeneration of volcanism and provide important informationon the thermal evolution of the lunar interior.[34] There are four main rings associated with Orientale

Basin (Figure 1a) [Head, 1974; Moore et al., 1974; Scottet al., 1977; Wilhelms, 1987; Spudis, 1993; Head et al.,1993; Head, 2010]. The ∼930 km Cordillera Mountain ringis an inward facing mountain scarp that defines the basin. Thesecond largest ring, the Outer Rook ring, is an interconnectednetwork of massifs spanning ∼620 km in diameter. The nextinnermost ring is the Inner Rook ring, ∼480 km in diameter. Itis composed of many isolated massifs resembling centralpeaks in complex craters and together, these resemble a ringof peaks as seen in peak ring basins. Themost interior ring is acentral depression ∼320 km in diameter that has been inter-preted to have formed by ∼3 km of thermal subsidence [Brattet al., 1985a]. The preservation of this ring topographyseparates the various mare deposits and allows for a closerexamination of their ages, compositions, duration of volca-nism and modes of emplacement.[35] The three large previously defined mare deposits in

Orientale Basin include (Figure 1) Mare Orientale, LacusVeris, and Lacus Autumni. Mare Orientale is located in thecenter of the basin, almost entirely encompassed by the rim ofthe central depression, and is the largest of the mare deposits.Included in the defined volcanic units of Mare Orientale is thepolygonal mare deposit directly to the southwest of the center.Next in areal extent is Lacus Veris, located between the InnerRook and Outer Rook rings. This mare deposit is composedof five large ponds along with several smaller scattered pondsall oriented in an arcuate belt, extending from ∼NNW to E.Similar in broad location within the basin interior is LacusAutumni, positioned between the Outer Rook and Cordilleramountains between ∼ENE to E. Only three relatively shallowponds comprise Lacus Autumni. The deposits within each setof rings become progressively smaller in area with distancefrom the center of the basin. A large mafic ring occurs in theSSW part of the Orientale interior, centered on the OuterRook ring (Figure 1c).[36] In addition to the volcanic deposits located between

the basin rings, there are four main deposits related to the

underlying Orientale Basin: the Hevelius Formation, theMontes Rook Formation, the Maunder Formation and relatedplains material [Scott et al., 1977]. The Hevelius Formationconsists of the radial basin ejecta blanket located outside theCordillera mountain ring. Interior to that deposit is theMontes Rook Formation, between the Cordillera and OuterRook rings. It consists of hummocky knobs situated in arough textured matrix. The outer facies of the Maunder For-mation has a highly fractured, undulating and corrugatedsurface and is interpreted to be composed of melt sheetmaterial. Last, the inner facies of the Maunder Formationconsists of higher‐albedo plains material that has a smoothsurface texture, embays topography and is broken by distinctlinear fracture patterns; this facies is interpreted to be morepure impact melt lacking the abundance of admixed claststhat result in draping and cracking of the outer part of theMaunder Formation [Head, 1974;Moore et al., 1974; Spudiset al., 1984].

5. Distribution, Areas, and Volumes of MareBasalts and Related Volcanic Features

[37] Using M3 data, we first examined the Orientale regionfor any evidence of previously undetected mare patches. Theincreased spatial and spectral resolution of M3 has allowedfor the identification of several additional mare deposits to thewest and south of the basin interior (Figure 2), although themajor distribution of mare from the southwest to the north-east of the basin remains unchanged. Orientale Basin itself is∼930 km in diameter and covers an area of ∼700,000 km2

[Head, 1974]. Within the basin, the largest of the maredeposits is Mare Orientale, which covers an area of52,700 km2 (Table 2). Determination of the volume ofMare Orientale requires information on the thickness of thedeposit. Previous estimates of the thickness ofMare Orientalewere less than ∼1–2 km [Head, 1974; Solomon and Head,1980], and perhaps up to ∼1 km thick [Greeley, 1976; Scottet al., 1977].[38] We used M3 spectral reflectance data to assess post-

mare impact craters that penetrate into and possibly throughthe mare deposit, and stratigraphic relationships, to assess therange of mare thickness and to derive an average thicknessestimate (Figure 3a). Craters that impacted into the mare andshowed mare signatures in their interior and ejecta providedminimum thickness estimates, while impacts into mare thatexcavated nonmare highlands material provided maximumestimates of the thickness of fill. These craters were chosenbased on their location in the mare deposits, although, theappearance of the crater and ejecta spectra were important aswell. It was necessary for the geologic materials to still havean identifiable spectrum in order to differentiate between thebasaltic mare and more feldspathic highland signatures. Marespectroscopic signatures contain mineral absorptions at both1 and 2 mm, owing to the presence of olivine and pyroxene inthe mare basalt, and have lower albedos (lower reflectancevalues), as compared to the highland material. Spectral sig-natures of highland material lack absorption features, buthave higher albedo values. This is due to the fact that thehighlands are thought to be composed largely of anorthosite,which is usually spectrally neutral on the Moon. Anorthositehas an absorption around 1.2 mm if Fe2+ is included in themineral structure [e.g., Bell and Mao, 1973; Adams and


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Goullaud, 1978] and if the rock has not been subjected tosufficiently high shock pressures to erase evidence of thissignature [e.g., Adams, 1979; Johnson and Hörz, 2003].[39] Crater impacts throughout lunar history have promp-

ted lateral and vertical mixing of geologic materials over theentire surface of the Moon. Mixing of feldspathic highlandmaterial and mafic mare material can influence estimatesof the thickness of mare deposits as well as the compositionof mare basalt. Highland material could contaminate marematerial and change the shape of the spectra, including thestrength of the absorptions, and the location of absorptionband centers [e.g., Staid, 2000]. Several researchers haveinvestigated the mixing relationships along mare‐highlandboundaries, investigating the importance of lateral and ver-tical transport of materials [e.g., Fischer and Pieters, 1995;Mustard and Head, 1996; Mustard et al., 1998; Li andMustard, 2000]. Mustard and Head [1996] investigated theeffects of three contact geometries and found lateral mixingto be the dominant transport mechanism when consideringboundaries with highland massifs bordering mare deposits.[40] The ∼13 km diameter crater Il’in, in northwest Mare

Orientale, lacks a highland feldspathic signature in its impactejecta and interior [see also Staid, 2000]. Based on the depth/

diameter relationships defined by Pike [1977], Il’in excavatesbetween ∼0.8 km [Stöffler et al., 1975] and ∼1.3 km [Melosh,1989] into mare material (Figures 3a and 3b). The crater Il’in,however, is superposed on a mare wrinkle ridge (Figure 3b),and on the basis of the interpreted three‐dimensional struc-ture of these contractional features, it is possible that themare basalts in which the arch and ridges formed have beenthickened by the folding and thrusting thought to accompanytheir formation [e.g., Lucchitta, 1976; Sharpton and Head,1988].[41] The crater Hohmann, ∼100 km east of Il’in, has a

nonmare feldspathic signature on its rim and is partially filledwith mare material. Hohmann is ∼17 km in diameter, andbased on the relationships defined by Stöffler et al. [1975] andMelosh [1989] is estimated to have sampled between ∼1.1 kmand ∼1.7 km into the crust (Figures 3a and 3c). Hohmanncrater rim is composed entirely of highland material, makingthis crater the maximum limit for the depth ofMare Orientale.[42] Interspersed throughout Mare Orientale are several

smaller craters with diameters on the order of 4–5 km exca-vating feldspathic basin material. Diameters of this sizeindicate sampling depths of < ∼300 m (Figure 3a). In addi-tion, mare deposits along the margins of Mare Orientale show

Figure 2. Areal distribution of mare ponds, sinuous rilles, and domes in Orientale Basin. Ponds are repre-sented by gray regions with black numbers, sinuous rilles are represented by red curvy lines with red letterlabels, and domes are outlined in yellow.


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evidence for lateral mixing, where highlands ejecta has beenthrown in from the adjacent basin deposits, and verticalmixing, caused by material being excavated from below thinmare deposits and incorporated into the regolith. For instance,a good example of predominantly lateral mixing is observedin the rectangular mare deposit to the southwest of MareOrientale. It is mantled with feldspathic soils while most ofthe craters show a mafic signature. However, the larger of thecraters (∼2 km in diameter) have a faint feldspathic signatureon their floor, similar to the soil signature in the polygon. Incomparison, small craters on the order of several hundredmeters in diameter have only a mafic signature. This suggeststhat locally this mare unit is very thin, less than 40 m thick(Figure 3d).[43] Furthermore, there are numerous kipukas (islands of

basin deposits surrounded by, and protruding through, themaria) whose spectral properties are consistent with theMaunder Formation protruding through Mare Orientale(Figures 1c and 3e). The abundance of these preexistingtopographic features in the mare, combined with the shallowslopes from mare margins suggested by the decrease inkipuka density (Figure 3e), support the interpretation thatMare Orientale is shallow in these specific areas. For exam-ple, there is a line of preexisting basin topography through thecenter of the basin protruding through the surface of the maredeposit (Figures 3a and 3e). Thus, it appears that the centerof the basin, in a north‐south direction, is much shallowerthan adjacent mare to its east and west. The juxtaposition ofall these features demonstrates that the basin topographyunderlying Mare Orientale varies significantly, especiallyfrom east to west. On the basis of these observations and data,our best estimates for the thickness and volumes of MareOrientale are as follows: small crater depth of sampling data

suggest a typical thickness of ∼200 m (Figure 3a), and kipukadistribution data (Figure 3e) are consistent with this. Apply-ing this as an average thickness to the area of Mare Orientaleprovides an estimated volume of ∼10,440 km3 (Table 2).[44] Volumes and areas of terrestrial continental flood

basalts have been calculated by many researchers [e.g.,Richards et al., 1989; Camp et al., 2003; Reidel, 2005; Jayand Widdowson, 2008]. For instance, estimated volumesfor the Deccan Traps are between 0.5 and 2 × 106 km3 overan area of 1 × 106 km2. In comparison to the Deccan Traps,all of the flows that make up Columbia River Basalts haveboth a smaller calculated volume (∼2 × 105 km3) [Jay andWiddowson, 2008] and areal extent (>2 × 105 km2) [Reidel,2005]. Mare Orientale has an area of ∼52,700 km2 and avolume of ∼10,400 km3, much less in extent and volume thanthe Columbia River basalts.[45] Lacus Veris (Figure 1) is the next largest mare deposit,

with each of its five ponds varying considerably in area. Thelargest of the Lacus Veris ponds is ∼8,890 km2 and thesmallest pond is ∼145 km2. Using the largest craters to con-strain the thickness of these deposits [Yingst and Head, 1997]gives minimal volume estimates ranging from ∼10 km3 to∼1,385 km3 (Table 2). The largest pond in Lacus Verisappears to vary in thickness throughout its extent in a mannersimilar to Mare Orientale. In the northern part of the pond a∼7 km diameter crater excavates into mafic material, butapproximately 30 km southeast a ∼5 km diameter cratersituated between kipukas excavates feldspathic material(Figure 3f).[46] Lacus Autumni is composed of several large ponds

as well. The largest of its three ponds covers an area of∼2,060 km2 and the smallest is ∼815 km2. These mare pondsare some of the shallowest in the basin, as evidenced by the

Table 2. Areas, Volumes, and Elevations of Mare Ponds in Orientale Basin

Identifiera Area (km2) Y‐H Areab (km2) Minimum Volume (km3) Y‐H Minimum Volumeb (km3) Elevation (km)

Mare Orientale 52200 ‐ 10440 ‐ −2.741 25 ‐ 1 ‐ 2.372 230 ‐ 15 ‐ −0.1553 935 ‐ 65 ‐ −0.4274 80 ‐ 10 ‐ −0.1705 160 ‐ 10 ‐ 0.4096 160 ‐ 15 ‐ −0.3757 1620 ‐ 235 ‐ −2.378 75 ‐ 2 ‐ −0.6539 1580 1740 300 870 −1.0410 145 165 10 20 −1.0611 1505 1640 135 820 −1.0812 2200 2560 240 1280 −0.70713 720 ‐ 50 ‐ −2.4014 8890 11510 1385 5755 −0.52615 55 ‐ 3 ‐ −0.42516 140 ‐ 5 ‐ −0.77317 515 845c 25 150 −0.98118 165 ‐ 25 ‐ −1.1319 100 145 3 20 −0.6520 105 ‐ 5 ‐ −0.1921 815 905 65 160 −0.36922 2060d 2240 115 390 −0.23323 985 945 110 120 −0.14924 510d 730 25 130 −1.58

aIdentifier refers to the numbering system introduced in Figure 2.bAreas and volumes calculated by Yingst and Head [1997] (Y‐H).cPond is defined differently; Yingst and Head [1997] pond is a combination of our ponds 16 and 17.dSections of these ponds were missing in our mosaic. Therefore, other data sets (i.e., Clementine albedo images) were used to fill in the gaps.


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abundance of kipukas. Based on current crater investigations,ponds in Lacus Autumni have volumes between ∼65 km3 and∼115 km3 (Table 2).[47] Yingst and Head [1997] previously investigated the

areas and volumes of Lacus Veris and Lacus Autumni tounderstand better the mode and rates of emplacement forthese deposits. Both volume and area values were recal-culated in the current analysis because of the availabilityof higher‐resolution image, compositional and topographicdata, allowing for more precise identification of ponds and

small craters. The new total calculated area and volume forponds 1–24 from this study (19,350 km2 and 2,400 km3) issmaller than the previous estimates of Yingst and Head[1997] (23,400 km2 and 9,700 km3). The total (minimum)volume of mare in the Orientale Basin is ∼46,000 km3,throughout an area of ∼700,000 km2. The smaller areas andvolumes are very likely due to higher‐resolution data used indefining the margins of the deposits, the spectral data thathelped define the mare nature of smooth mare areas and the

Figure 3. M3 thermal images (1489 nm) showing the variable thickness in Mare Orientale and LacusVeris. (a) Map of Mare Orientale with thickness labeled, as inferred from crater excavation depths[Stöffler et al., 1975]. Craters highlighted in yellow represent those excavating mare material and thosein white represent craters ejecting feldspathic basin material. (b) Il’in crater (∼13 km in diameter) excavatesonly mafic material. (c) Hohmann crater (∼17 km in diameter) flooded with mare, has a feldspathic rim.(d) Southwest polygon. (e) Kipukas protruding from mare surface in the center of the basin. (f) Close‐up of the north central region in Lacus Veris with distinct example of thickness variation. Shares thesame color scheme as Figure 3a.


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high spatial and spectral resolution data that permitted muchbetter estimates of the thickness of the mare basalts.[48] Yingst and Head [1997] found that South Pole–

Aitken contains ∼153,000 km3 of mare over its areal extent(∼4,000,000 km2). Comparing the basin areas and mare arealextent of Orientale and South Pole–Aitken shows that, pro-portionally, Orientale Basin has more mare covering itsinterior than South Pole–Aitken, when taking Mare Orientaleinto account. In contrast to these two incompletely filledbasins, most other lunar nearside basins have volumesbetween 200,000 km3 (e.g., Tranquillitatis and Fecunditatis)to 1,000,000 km3 (e.g., Crisium, Serenitatis) [Bratt et al.,1985b], 4 to 20 times more mare than Orientale.[49] We identified several ponds (1, 4, 5, 6, 8) using M3

data (Figure 1). Pond 4 (see identification scheme in Figure 2)is ∼80 km2 and lies just inside massifs on the western side ofthe Inner Rook ring. Ponds 2 and 3, ∼230 km2 and ∼935 km2,have been noted previously in Zond and Galileo data [Scottet al., 1977; Kadel, 1993; Head et al., 1993] and both arelocated adjacent to the Outer Rook ring, in a similar positionto the ponds of Lacus Veris. Ponds 1, 5, and 6, having areas of∼25 km2, ∼160 km2, and ∼160 km2, lie along the scarp of theCordillera mountain ring and mirror the occurrence of theponds of Lacus Autumni. Placing volumetric constraints onthese identified ponds is more difficult because they havevery few impacts owing to their small size. Table 2 showsminimum estimates of these pond volumes derived fromcraters with mafic signatures.[50] Other small deposits have been identified around and

between previously known mare deposits. Pond 20, covering∼105 km2, has been identified to the east of the Outer Rookring. Unlike the other deposits identified by M3, this marepond appears to mantle the preexisting topography instead ofponding in a topographic low. Many other small depositshave been identified between the ponds in Lacus Autumniand spread around the eastern base of the Outer Rook ring

(Figure 2). Despite their small size, these deposits identifiedwith M3 data show that volcanism in Orientale was activethroughout the entire basin area and not simply confined tothe eastern part of Orientale.[51] A large, 154 km diameter dark annular ring was first

discovered and documented in Soviet Zond 8 images [Lipsky,1975], and is located in the south‐southwestern part ofOrientale Basin centered on the Outer Rook ring (Figure 4).Earlier interpretations concluded that the dark mantle ringdeposit (DMRD) consisted of a large number of individualvents producing an annulus of dark mantle pyroclasticdeposits; the mantle was interpreted to have erupted fromvents marking the location of a 175 km diameter pre‐Orientale Basin crater, thus explaining its circular nature[Schultz and Spudis, 1978]. This interpretation had a signif-icant influence on attempts to locate the Orientale Basintransient cavity rim crest, since the presence, and thus pres-ervation, of such a large crater would tend to place the tran-sient cavity rim crest inside its location, at or inside theInner Rook ring [Schultz and Spudis, 1978].[52] Clementine data revealed the presence of a 7.5 km

wide by 16 km long elongate depression located at the centerof the DMRD [Weitz et al., 1998; Head et al., 2002].This elongate depression, similar to features seen in associ-ation with other dark mantle deposits (e.g., Sulpicius Gallus[Lucchitta and Schmidt, 1974]), had no obvious adjacentdeposits of darkmantle or mare, but on the basis of its locationin the approximate center of the DMRD, Weitz et al. [1998]and Head et al. [2002] investigated the possibility that itcould be a source crater for an eruption producing the DMRD.They proposed that the dark ring is the manifestation of apyroclastic eruption originating at a fissure vent (the elongatedepression) and forming an Ionian‐like eruption plume.Headet al. [2002] outlined a scenario in which the event producingthe eruption began with a dike rapidly emplaced from sub-crustal depths to within ∼3–4 km of the surface. The dike

Figure 4. Pyroclastic ring in the southwest of Orientale Basin. Elongate vent is located within the OuterRook ring. (a) Thermal M3 image (2936 nm). (b) StandardM3 color composite (R, 1 mm IBD; G, 2 mm IBD;B, 1489 nm reflectance).


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stabilized and degassed over ∼1.7 years to form an upperfoam layer, which then penetrated to the surface to cause aneruption, lasting ∼1–2 weeks. This eruption produced analmost 40 km high symmetrical spray of pyroclasts into thelunar vacuum at velocities of ∼350 to ∼420 m/s; as a result ofthis eruption, the pyroclastic material accumulated in asymmetrical ring around the vent to produce the DMRD.Head et al. [2002] attributed the paucity of pyroclastic ringsof this type on the Moon to the low probability of a dikestalling at just the right depth (∼3–4 km) to create theseeruption conditions.[53] In general, lunar volcanic glasses can be identified in

spectra by broad absorptions at 1.0 and 2.0 mm. The amountof titanium in the glasses can be determined by an iron‐titanium charge transfer absorption feature in the visibleregion of the electromagnetic spectrum. The higher the Ticontent in the glasses, the stronger the absorption edge in thevisible [Bell et al., 1976]. Black beads that represent thecrystallized equivalents of the orange glasses have a strong,broad absorption centered at ∼0.6 mm that is due to thepresence of ilmenite.[54] Galileo images provided the first multispectral

images of the DMRD region and the dark halo deposit itself;Pieters et al. [1993] showed that the ring deposit is some-what brighter than nearside mantling deposits, has a veryweak 1 mm absorption band, and has an ultraviolet‐visible(UV/VIS) ratio that is relatively high but lower than that seenin nearside deposits of black spheres. They concluded thatthese characteristics could be consistent with contaminationof the deposit by highland material (as suggested by Greeleyet al. [1993]). They also proposed an alternative interpre-tation, that the actual spectral properties of the pyroclasticsthemselves dominate the measured spectral properties. In thefirst case the deposits could be interpreted as similar toilmenite‐rich dark mantling material such as the black beadssampled at Apollo 17. In the second case, however, the weak1 mm band could indicate that the deposits are not homoge-neous glass but are in a crystallized form. The low UV/VISratio, relative to black beads seen elsewhere, could be due tothe lower abundance of ilmenite, the opaque component thatcauses darkening. In the latter case, the Orientale ring depositmight have affinities with the local medium‐Ti depositsidentified in the Orientale region [Greeley et al., 1993].[55] The Clementine UV/VIS spectral data for the

Orientale DMRD show a slight absorption at 0.9–1.0 mm,similar to that seen in Aristarchus DMD spectra [Weitz et al.,1998]. On the basis of these characteristics, the beads thatcompose the Orientale DMRD were interpreted to be domi-nated by glasses, rather than the crystallized beads typical ofthe Taurus‐Littrow DMD deposit. The implications are thatthe DMRD beads cooled rapidly after eruption at the surface,preventing crystals from forming. Low optical density in theeruption plume, due to either a high gas content or a lowmasseruption rate, is required to enhance rapid cooling and explainthe dominance of glasses in the deposit. This implies rela-tively rapid cooling times for the eruption products, consis-tent with theHead et al. [2002] scenario of a dike stalling andbuildup of volatile foams prior to eruption. Although analysisis not yet complete, M3 data suggest that the pyroclasticdeposits have been weathered and are glass‐rich only atsmall fresh craters. Although the age of these deposits is notknown due to the difficulty of dating friable mantle deposits,

there is no reason to believe that they lie outside the range ofmodel ages for the mare deposits.[56] The total duration of volcanic activity in Orientale

spans 0.85–1.50 Ga (see section 7), but despite this lengthyduration, the ponds have relatively low volumes [Yingst andHead, 1997]. In comparison to other large impact basins onthe Moon, Orientale contains a very small total volume ofmare material, ∼46,000 km3. Bratt et al. [1985b] calculatedtotal volumes for basins on the nearside of the Moon, whichinclude the volume of the topographic depression and marefill. For comparison, these volumes are taken as themaximumvolume of mare. Serenitatis and Nectaris, two basins similarin size to Orientale, have total basin volumes of 1 × 106 km3

and 7 × 105 km3, respectively. According to Bratt et al.[1985b], Orientale has a total basin volume of 7 × 105 km3,meaning that there is ∼6.55 × 105 km3 that was not filledwith mare. Consequently the mare load in the center ofOrientale Basin is not as massive as the initial central loadsthat are likely to have characterized the deeper basin interiorin other lunar basins. For example, Solomon and Head [1980]estimated the thickness of Mare Orientale to be 1–2 km inmodeling the mare load and its influence on flexure. Themantle plug that was uplifted into the excavated cavity whenit collapsed [Wieczorek and Phillips, 2000], and its super-isostatic state may thus be of more significance in terms ofa cause of the local extensional stress along the edge of thebasin.

6. Craters Kopff and Maunder

[57] Two large craters in the interior of Orientale, Maunderand Kopff (Figure 5), have attracted attention for years duetheir similar sizes but contrasting morphology. A first‐orderexamination of the 55 km diameter Maunder crater indicatesthat it is a classic example of an impact crater, having a centralpeak, a flat floor, terraced walls, a raised rim and continuousejecta deposit, and a system of radial rays [El‐Baz, 1974;Pike, 1980]. In contrast, the 42 km diameter Kopff craterhas no central peak, no wall terraces, an unusual rim shape,and unusual smooth crater ejecta deposits and secondaries.Several authors hypothesized that Kopff crater was volcanicin origin based on its subdued morphology [McCauley, 1968;Guest and Greeley, 1977], with one hypothesis favoring anexplosion caldera [Pai et al., 1978]; other researchers favoredan unusual impact event, for example, into partially moltenmaterial [Wilhelms and McCauley, 1971;Guest and Greeley,1977; Spudis et al., 1984]. Still others hypothesized thatKopff was a volcanically altered impact crater [Schultz, 1976;Wilhelms, 1987].[58] The rim crest of Kopff is very irregular, with multiple

undulations along its circumference. The largest crater rim‐to‐floor relief is ∼1.7 km. This is less than half of the craterrim crest‐to‐floor measurement ofMaunder, ∼4 km deep. Thedepth of Kopff is too shallow to be a typical fresh impactcrater and its floor diameter of ∼36 km is too large [Pike,1980] (Figures 6a and 6b). Both these characteristics, inaddition to the observed floor flooding, indicate that the craterhas undergone at least some volcanic modification. Mea-surements of morphologic features that would not be affectedby volcanic modification, such as rim height and width,indicate that Kopff has characteristics consistent with animpact crater (Figures 6c and 6d). Its rim height value is on


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the lower end of collectedmeasurements, but this could be theresult of impact into partially molten material. In comparison,the rim height ofMaunder is slightly above the trend, likely tobe the result of its location on preexisting topography alongthe edge of the inner depression.[59] An impact crater ∼3 km in diameter located on the

eastern edge of the mare‐covered floor of Kopff has exca-vated anorthositic material (<2 wt % FeO) [Bussey andSpudis, 2000], confirmed here with M3 data (Figure 5b). Inthe standard M3 color composite (Figure 5b) the rim of Kopffhas a distinctly feldspathic signature, suggesting that it iscomposed largely of premare basin material. The depth ofexcavation of the ∼3 km superposed crater is ∼0.2 km, con-straining the mare deposit volume to be <145 km3, usingtechniques described by Pike [1977], Stöffler et al. [1975],and Yingst and Head [1997]. If Kopff was a typical impactcrater, its depth should be on the order of ∼3.5 km (Figure 6a).

[60] In order to evaluate processes of modification forKopff, Maunder crater was “flooded” to an elevation ofapproximately −2.6 km from its rim crest (comparable to thedepth of the floor of Kopff below the rim crest) to evaluate itsmorphology (Figure 7). Comparing the still uncovered por-tion of Maunder with the morphology of Kopff could help todetermine if the flooding scenario is possible. Assuming aninitial depth of 3.5 km for the floor of Kopff, this floodedelevation is similar to the amount of flooding Kopff isassumed to have experienced. At this flood level (∼2.2 km),Maunder is filled with a volume of ∼1900 km3. Using thevarious morphometric impact crater measurements for Kopff[Pike, 1980] then yields a volume of ∼1700 km3. This cal-culated volume is significantly different from that determinedfrom compositional information. The ∼3 km diameter crateron the east of Kopff floor that excavated nonmare materialmay have impacted into an uplifted crater floor, or possibly aburied slump terrace.[61] The floor of Kopff is very asymmetric due to a small

depression in its center and a slight bulge on its eastern side,superposed by a floor fracture. The uneven nature of the marefloor indicates that after flooding, some type of volcanicactivity continued to occur and eventually warped the surface.The bulge and fracturing of the floor could result from a smallmare intrusion forming beneath the crater floor, creating afloor‐fractured crater [Schultz, 1976].[62] In addition to the unusual morphology of Kopff, crater

count statistics provide additional evidence concerning itsorigin. The Maunder Formation has been dated at ∼3.64 Gaon average, and Kopff formed at ∼3.63 Ga, based on modelages derived from its smooth ejecta deposit (Table 3). TheKopff impact occurred soon after the emplacement of themeltsheet. If the central uplifted area of the Orientale Basin andthe overlying melt sheet had not thermally equilibrated after∼10 Ma, perhaps the thermal state of the substrate influencedthe formation and early modification of the morphology ofKopff [e.g., Spudis et al., 1984]. Dates on the volcanicflooding of the interior of Kopf are ∼3.36 Ga, significantlylater than the formation of the crater itself. This favors vol-canic modification of an unusual crater type.

7. Ages of Mare Basalts

[63] Crater production and chronology functions [e.g.,Neukum, 1983; Ivanov et al., 1999, 2001; Hartmann et al.,2000; Neukum et al., 2001] have been developed that per-mit the assignment of ages from crater size‐frequency dis-tributions for the surfaces of the terrestrial planets. Cratercounting is very useful for dating lunar surface units due tothe high degree of preservation of impact craters. This datingmethod can be employed to determine the timing of largebasin‐forming impacts, ages of different volcanic units orflows, and the duration of volcanism. Important considera-tions when dating volcanic surfaces include the effects ofresurfacing events, secondary craters and endogenic craters[e.g., Hiesinger et al., 2000; Greeley and Gault, 1971;Oberbeck and Morrison, 1974], all of which can produceages that differ from the ages of the actual surface units.[64] Crater count analyses have been conducted on Mare

Orientale, Lacus Veris and Lacus Autumni by previousresearchers [e.g., Greeley et al., 1993; Kadel, 1993; Morotaet al., 2010]. Initially, Hartmann and Yale [1968] interpreted

Figure 5. (a) M3 thermal image (2936 nm) of Maunder andKopff craters. (b) Standard M3 color composite (R, 1 mmIBD; G, 2 mm IBD; B, 1489 nm reflectance).


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early crater counts to mean that Lacus Veris and Autumnioccurred soon after the Orientale event, followed later byinfilling of Mare Orientale. Using Lunar Orbiter photo-graphs, Greeley et al. [1993] determined that the oldest marewas emplaced in south central Mare Orientale at ∼3.70 Ga,followed by the emplacement of maria in western andsoutheastern Mare Orientale at ∼3.45 Ga. Moving furtherfrom the center of the basin, Greeley et al. [1993] showedthat Lacus Veris had an average model age of ∼3.50 Ga, andLacus Autumni is the youngest deposit with a model age of∼2.85 Ga. Even younger model ages have been reported forcentral Lacus Veris, at ∼2.59 and ∼2.29 Ga [Kadel, 1993].Based on these findings, mare volcanism in Orientale wasestimated to have lasted between 0.85 and 1.50 Ga [Greeleyet al., 1993], beginning ∼100 Ma after basin formation.[65] Using high‐resolution M3 data at 140 m/pixel, as

compared to the Lunar Orbiter ∼3.5–7.6 km/pixel resolutionused by Greeley et al. [1993], crater counts were conductedon the ejecta deposit of Orientale Basin, parts of MareOrientale, the Maunder Formation [see Head, 1974; Bussey

and Spudis, 2000] and the identified mare ponds in LacusVeris and Lacus Autumni of sufficient size to produce reliablemodel ages (Table 3). Previous estimates of the model ageof the Orientale Basin event are ∼3.8 Ga [Baldwin, 1974;Nunes et al., 1974; Schaeffer and Husain, 1974; Neukum,1983; Baldwin, 1987; Wilhelms, 1987; Kadel, 1993].Updated crater age equations produce a model age of∼3.68 Ga for the Orientale impact event (Figure 8). The olderNeukum [1983] production and chronology functions yield amodel age of ∼3.8 Ga, using the counted regions from thisstudy. This difference in calculated ages arises from an updateof Neukum production and chronology equations. Newcounts on impact craters in and around Orientale Basin per-mitted a reestimation of the size‐frequency distribution curvefor craters between 1 km and 20 km [Neukum et al., 2001].Earlier basin model age estimates made using the Hartmannproduction function could differ as a result of the inherentdifferences between the two approaches for deriving a produc-tion function. Neukum uses an 11th‐order polynomial, whereasHartmann uses a log incremental equation to approximate

Figure 6. Graphs modified from Pike [1980] to include Maunder and Kopff crater characteristics. Allvalues are measured in kilometers. (a) Crater depth versus rim crest diameter. (b) Floor diameter versus rimcrest diameter. (c) Rim height versus rim crest diameter. (d) Rim width versus rim crest diameter.


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the size‐frequency distribution curve. These two functionsdiffer most at diameter bins between 2 and 20 km [Neukumet al., 2001, Figure 8].[66] Our data suggest that the melt sheet material of the

Maunder Formation solidified shortly after the basin formingevent, ∼3.64 Ga. In theory, the melt sheet and basin ejectaages should be the same, since the melt is produced instan-taneously with the impact, and comes to rest during the short‐term modification stage, the terminal stage of the crateringevent. The discrepancy between ages is likely to be due todifferences in the degree of crater preservation, or possiblydue to differences in material properties [e.g., van der Bogertet al., 2010]. Melt sheet material tends to drape topography,creating smooth surfaces that allow for the preservation ofsmaller craters (∼<10 km). Ejecta blankets tend to haverougher textures that may lead to poorer preservation ofsmaller craters. Thus, the melt sheet age is typically the morereliable of the two for the age of the impact event. However,we counted craters > 5 km on the Hevelius Formation and

craters > 0.75 km on the Maunder Formation in an attempt tocount the most preserved craters on each deposit. Our cal-culated crater retention ages are almost the same, ∼3.68 Gafor the ejecta and ∼3.64 for the melt sheet; this suggestsour efforts were successful. Mare Orientale was emplacedshortly after the Maunder Formation ∼3.58 Ga (Figure 8 andTable 3). This delay of ∼60–100Ma between basin formationand volcanism in Mare Orientale, and later extended vol-canism in Lacus Veris and Lacus Autumni argues againstOrientale impact pressure release melting (Table 1) being asignificance factor in mare basalt production [e.g., Elkins‐Tanton et al., 2004], at least for the last volcanic depositsthat reset the age of these surfaces.[67] Impact craters were counted, size‐frequency distribu-

tions were compiled and ages were calculated for the fivelargest ponds in Lacus Veris; this yielded an age range of∼3.20 to ∼3.69 Ga (Figures 9a–9f and Table 3). These modelages are relatively consistent with those calculated fromprevious studies, but these are differences in grouping and

Figure 7. (a) M3 thermal image (2936 nm) of Maunder crater in its current state. (b) M3 thermal image ofKopff crater. (c) M3 thermal image of Maunder “flooded” to the same proportion that Kopff is suspected tobe flooded based on crater morphology relationships defined by Pike [1980]. (d) Graph of the volume ofmare versus the elevation of the mare surface at 200 m steps of flooding in Maunder.


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separation of the various ponds [e.g., Greeley et al., 1993;Kadel, 1993]. Greeley et al. [1993] grouped several of theponds in Lacus Veris together, while Kadel [1993] separatedindividual ponds into separate units.[68] The largest discrepancy in model ages between our

study and previous work occurs in the ponds of LacusAutumni. Our crater counts give an age range between ∼3.47and ∼1.66 Ga for the ponds (Figures 9j–9l; note the modelage uncertainties). The young model age of ∼1.66 Ga fallswell within the calculated time range of mare volcanismoccurring on the lunar nearside [Hiesinger et al., 2000](Figure 10). The new model ages calculated for ponds 22 and23 in Lacus Autumni do not wholly agree with ages reportedby previous researchers; Kadel [1993] calculated a model ageof 3.00 Ga for pond 23 and 2.85 Ga for pond 22, differencesof 0.57 Ga and 1.19 Ga, respectively. This discrepancy in themodel ages could be the result of sizable differences in thedefinition of the count area in these two ponds, or from countstatistics. However, the fact that most of the other ponds

in Lacus Veris and Autumni have similar model ages inboth studies suggests that generally, the two crater countingmethodologies do not appear to differ significantly.[69] The time of impact for bothMaunder andKopff craters

was calculated by dating their continuous ejecta deposits.These two large craters are of particular interest becausethough similar in location and size, they are very different inmorphology. The mare floor of Kopff is dated at ∼3.36 Gaand its ejecta at ∼3.63 Ga, supporting the idea that Kopffmight have been produced by impact into material withdifferent thermal or mechanical properties [e.g., Wilhelmsand McCauley, 1971; Guest and Greeley, 1977]. Maunderejecta is dated ∼2.88 Ga. This younger model age forMaunder crater is supported by stratigraphic relationships,such as the superposition of Maunder ejecta on Kopff’s marefloor.[70] The range of newly calculated model ages for the mare

deposits coincides with the climax of nearside lunar volca-nism. The majority of our model ages plot around ∼3.5 Ga

Table 3. Model Ages of Orientale and Datable Mare Ponds Thereina

Neukum et al. [2001] Neukum [1983]

Age (Ga) Positive Error (Ga) Negative Error (Ga) Age (Ga) Positive Error (Ga) Negative Error (Ga)

Orientale Event1 3.75 0.03 0.05 3.87 0.03 0.042 3.64 0.05 0.08 3.76 0.04 0.063 3.71 0.04 0.06 3.82 0.04 0.054 3.64 0.05 0.08 3.77 0.04 0.06Average 3.68 0.02 0.03 3.80 0.02 0.02

Melt Sheet1 3.71 0.04 0.07 3.71 0.04 0.072 3.67 0.04 0.05 3.67 0.04 0.053 3.68 0.04 0.06 3.68 0.04 0.064 3.71 0.04 0.05 3.72 0.04 0.055 3.53 0.06 0.11 3.53 0.06 0.116 3.58 0.05 0.07 3.58 0.05 0.077 3.70 0.04 0.05 3.71 0.04 0.05Average 3.64 0.02 0.02 3.64 0.02 0.02

Mare Orientaleb

1 3.64 0.04 0.06 3.64 0.04 0.062 3.52 0.08 0.20 3.51 0.08 0.213 3.63 0.05 0.07 3.63 0.05 0.074 3.33 0.15 0.81 3.33 0.15 0.815 3.70 0.04 0.07 3.70 0.04 0.07Polygon (7) 3.57 0.06 0.11 3.57 0.06 0.11Average 3.58 0.03 0.03 3.58 0.03 0.03

Lacus Verisb

(9) 3.44 0.09 0.28 3.44 0.09 0.28(10) 3.69 0.09 0.48 3.69 0.09 0.49(11) 3.22 0.21 1.10 3.21 0.22 1.10(12) 3.36 0.12 0.47 3.36 0.12 0.47(14) 3.20 0.13 0.30 3.20 0.13 0.30

Lacus Autumnib

(21) 3.47 0.11 0.67 3.47 0.11 0.68(22) 1.66 0.83 0.96 1.65 0.83 0.96(23) 2.38 0.88 11.50 2.38 0.88 1.50

Kopff b

Interior (13) 3.36 0.17 1.60 3.37 0.17 1.60Ejecta 3.63 0.03 0.05 3.63 0.03 0.05

MaunderEjecta 2.88 0.30 0.50 2.86 0.31 0.50

New Pondsb

(3) 3.20 0.19 0.70 3.19 0.19 0.70

aModel ages calculated using the CraterStats program [Michael andNeukum, 2010]; data are unbinned and both the 1983 and 2001Neukum chronology andproduction functions are used. Error estimates are only a function of count statistics and do not incorporate any systematic uncertainties in the absolute agecalibration.

bNumbers in parentheses correspond to pond identification numbers from Figure 2.


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(Figure 10a), similar to nearside flow units dated byHiesingeret al. [2011]. Thus, Orientale mare deposits were emplacedin the same era of significant volcanism that occurred else-where on the Moon.

8. Mineralogy of Mare Basalts

[71] The mineralogy of Orientale Basin mare basalts is ofparticular interest, especially the question of how they com-pare with the compositional range seen in nearside lunarbasalts. Morphological analysis of Orientale basalts has been

interpreted to mean that mare ponds were emplaced in singleeruptive events [Yingst and Head, 1997], which has impli-cations for the size of the source region beneath this part ofthe Moon. Several researchers [Greeley et al., 1993; Kadel,1993; Staid, 2000] have investigated the composition of themare basalts in Orientale Basin, but none have been able toestablish the actual mineralogy due to limitations in instru-ment spatial and spectral resolution. Previous studies havefound that Mare Orientale and other mare ponds within thebasin are highly contaminated by the local highland material[Spudis et al., 1984; Staid, 2000]; despite this, Spudis et al.

Figure 8. (a) Production function fit plot showing the average calculated ages for the Orientale ejecta, themelt sheet and Mare Orientale with model isochrons. Cumulative frequency plots of (b) the Orientale ejectablanket, (c) the melt sheet (Maunder Formation) and (d) Mare Orientale, with age isochrons and errors foreach data point included.


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Figure 9. Cumulative frequency plots of each of the dated ponds. (a) Pond 9. (b) Pond 10. (c) Pond 11.(d) Pond 12. (e) Pond 14. (f) Maunder ejecta. (g) Kopff ejecta. (h) Pond 13. (i) Pond 3. (j) Pond 21.(k) Pond 22. (l) Pond 23.


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[1984] concluded that the basalts in Mare Orientale, LacusVeris and Autumni are probably of similar composition tonearside basalts.[72] Many researchers have focused on the TiO2 contents

of the mare ponds in Orientale [Greeley et al., 1993; Kadel,

1993]. Using Galileo Sold Sate Imaging (SSI) 0.41/0.56mm spectral reflectance ratio images, Greeley et al. [1993]found that units in Mare Orientale are medium‐high‐Tibasalt soils (3–7 wt % TiO2), with lower‐Ti signatures in thenortheast and west central Mare Orientale. Lacus Veris con-tains medium‐ to high‐Ti basalt soils and Lacus Autumni hasmedium‐ to high‐Ti basalt soils in the north and medium‐Tibasalt soils (<4 wt % TiO2) in the south. The authors pointedout that contamination from Orientale feldspathic materialscould be influencing the observed compositions. UsingGalileo EM‐1multispectral images,Kadel [1993] determinedthat Mare Orientale had a titanium content of 3–7 wt %TiO2.With Clementine data, Staid [2000] calculated titaniumvalues of ∼4% TiO2 using the methods from Charette et al.[1974]. Currently M3 data is being used to investigate near-side lunar basalts with varying TiO2 contents [e.g., Staidet al., 2011; Dhingra et al., 2010]. Determining the TiO2

content of mare basalts across the lunar surface is an ongoingproject. However, generally M3 data does not appear tonegate previous TiO2 estimates.[73] From crater ages, Greeley et al. [1993] concluded that

volcanism in Orientale spanned ∼0.85 Ga and that for sucha long time period the narrow range of mare compositionswas unusual compared to the nearside mare. They concludedthat this could indicate multiple eruptions from a relativelyhomogeneous source or multiple sources with subtle com-positional differences [Greeley et al., 1993]. It remainsunclear as to whether multiple flows of different mineralogyexist in Mare Orientale, or whether the mineralogy is gener-ally similar, but appears different due to mixing with adjacentand underlying Orientale Basin material. The two mostuncontaminated regions in Mare Orientale (as defined byStaid [2000]) were investigated in depth using the M3 1.0 mmband centers and band strength values to determine whethermultiple flows exist.[74] Evidence for vertical mixing through thin basalts can

be seen in the central portions of the interior deposits as wellas in the ejecta of craters near mare boundaries. Elevatedkipukas of basin materials, including the rim of Hohmanncrater (17 km), are embayed by subsequent mare flows,indicating that large areas of the central basalt deposits arerelatively thin. Lateral mixing has also brightened and alteredthe spectral properties of the Orientale basalts over large areasof the northern portion of the interior deposits and near mareboundaries [Staid, 2000]. In the northern portion of the basin,dark halo craters are observed to penetrate through ejectafrom Maunder crater (55 km), exposing spectrally bluer anddarker or less contaminated basalts from beneath brighter andredder ejecta‐contaminated mare surfaces.[75] In the northwest region, no distinct pattern emerges in

the band center data (Figure 11a). The average band center in

Figure 10. (a) Histogram of the temporal distribution ofmodel ages for basalts on the near side and far side (modifiedfrom Hiesinger et al. [2011]). N = 330 represents nearsidebasalt ages from Hiesinger et al. [2011]. N = 16 representsthe counts performed for Orientale Basin in this study.(b) Orientale Basin mare deposits color coded according toscheme from Hiesinger et al. [2011]. (c) Spatial distributionof model ages for defined units on both the lunar nearside andfarside [from Hiesinger et al., 2011].


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this region is 0.997mm,with values ranging from 0.981mm to1.019 mm. The strength of the 1 mm absorption is anotherparameter that could reveal additional existing compositionaldifferences. In theory, collecting spectra from a number ofsmall fresh craters should eliminate any significant differ-ences in maturity in our sample population. Thus, anyobserved differences in the absorption strength are inherent inthe minerals themselves. In the northwest study region(Figure 11c), the majority of the strong 1 mm absorptionsoccur to the south, while the weaker absorptions are con-centrated in the north. However, as was the case with the bandcenter data, an obvious correlation does not exist. One pos-sible explanation for this might be the impact of Il’in crater(13 km), whose ejecta material blanketed a significant area inthis region of Mare Orientale. M3 spectral data (Figure 1c)indicates that ejecta from Il’in crater is dominated by marebasalt signatures, is less weathered than surrounding soils and

lacks a detectable component of the underlying feldspathicbasin materials, indicating that basalts are thicker in thisportion of the basin.[76] Additionally, redder spectral properties that may result

from the presence of lower‐Ti basalts are observed in someregions of Mare Orientale. TiO2 rich mare basalts have aflatter, or bluer, continuum slope than lower‐Ti basalts [Staidand Pieters, 2000]. These red regions appear to have lowerUV/VIS ratios and higher albedos than the bluest regions ofMare Orientale, while retaining stronger mafic signaturesthan areas which have obvious nonmare mixing (e.g., regionssouth of Maunder crater). Such areas include deposits insouthwestern Mare Orientale and small regions north andeast of Il’in crater. However, due to the large amountsof nonmare contamination throughout the Orientale depo-sits, confirmation of such potential variations in emplacedbasalt compositions will require more detailed analysis of

Figure 11. Close‐ups of the compositional study regions in Mare Orientale, located in the northwest andsoutheast of the deposit. (a) Map of the areal distribution of band center positions for samples collected fromthe northwest. (b) Map of the band centers of the southeast spectra. (c) Northwest map of the spectral bandstrengths. (d) Areal distribution of band strengths for the collected spectra in the southeast.


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associated mare craters in another study using lower illumi-nation angle M3 data acquired in later optical periods ofthe mission. Future research will need to be conducted toquantify how much mixing contributes to spectral signaturesin Orientale Basin mare deposits.[77] Comparing the 1 mm characteristics of the northwest

study region with the southeast region clearly shows that thenorthwest lacks the diversity in band center position found inthe southeast (Figures 11a and 11b). The southeast region hasspectra band centers at both longer and shorter wavelengths;absorption band centers range from 0.978 mm to 1.053 mm,with an average of 1.003 mm. Proximity to the edge of themelt sheet does not appear to have an effect on band center,as the range in values is quite large for craters located alongthis boundary. Surface albedo differences and distance fromkipukas do not appear to affect the band center values either.In addition, the 1 mm band strength data was investigatedto determine if any mineralogical differences exist in themare composition (Figure 11d).[78] As more Fe2+ and Ca2+ substitutes for Mg2+ in the

pyroxene structure the absorption centers shift to longerwavelengths, which is reflected in the spectra collected fromthe northwest and southeast regions in Mare Orientale.Typically, orthopyroxenes have their first electronic transi-tion absorption centered around 0.9 mm; another absorptionof roughly equal strength is centered about 1.9 mm [e.g.,Adams, 1974; Cloutis and Gaffey, 1991; Klima et al., 2007].High‐Ca clinopyroxenes exhibit absorption bands at longerwavelengths, centered near 1 mm and 2.3 mm. The collectedspectra have 1 mm band centers that are shifted to longerwavelengths, from 0.978 mm to 1.053 mm, indicating that thepyroxene mineralogy is dominated by a high‐Ca clinopyr-oxene. From this study alone it is difficult to state exactlywhich elements comprise the minerals and in what propor-tions they are present. While this study is simply a first look atthe composition of basalts in Mare Orientale, further spectralanalysis, such as MGM or radiative transfer modeling, isneeded to better quantify the compositions of pyroxenesthroughout Orientale Basin.[79] In Lacus Veris and Autumni the mare basalts appear to

have similar spectral properties as those observed within thedeposits of Mare Orientale (Figure 1c). Mare soils in theseregions are relatively blue, but contain significant and vari-able nonmare highland material contamination, leading toreddening and brightening of surfaces. The surfaces of theolder Lacus Veris basalts north of Orientale appear redder,on average, than younger Lacus Veris deposits to the east.Nonmare contamination appears to be at least partiallyresponsible for these properties as spectrally bluer dark halocraters are observed within several areas of these northerndeposits. Lower‐Ti basalts [Kadel, 1993; Staid, 2000] in theLacus Veris deposits north of Orientale, if present, may alsocontribute to the redder spectral properties of surface soils.However, distinguishing lower‐titanium deposits from con-taminated higher‐Ti soils within northern Lacus Veris willrequire a more detailed examination of nonmare mixing inthis region.[80] The deposits in Lacus Autumni also have UV/VIS

spectral properties that are similar to basalts in eastern LacusVeris and most of Mare Orientale, consistent with relativelyhigh‐Ti contents (3–7 wt % TiO2). Although these depositsappear to be homogeneous in terms of titanium content, small

variations in 1 and 2 mm band strength are observed in ther-mally corrected M3 integrated band depth images. Thesevariations appear to result both from different degrees ofnonmare mixing as well as actual differences in emplacedbasalt mineralogy. One region that contains relatively strongvariations in 1 and 2 mm mafic band strength occurs as aspectral boundary between western and eastern deposits inLacus Autumni. These differences are not associated withobvious differences in UV/VIS slope or brightness and,therefore, are likely to result from differences in mineralogyrather than feldspathic contamination. Similar spectral bound-aries occur in the M3 standard color composite in other areasof Lacus Veris, but are more subtle and, in some cases, mayresult from different levels of nonmare mixing. Many of thespectral variations observed for mare soils within thesecomposite images are nonunique and more detailed analysisof mare crater spectra associated with these spectral bound-aries will be necessary to reveal potential variations in maficmineralogy within these deposits.[81] It is unlikely that all the diversity is due to feldspathic

contamination. There are indications of some variations inUV/VIS slope, 1.0 versus 2.0 mm band strength and bandshape that are associated with the basalts themselves. How-ever, the differences appear to be relatively small in mostareas, such that we are unable to map obvious spectral unitsgiven the large amount of mixing and the high phase anglesof the data investigated thus far. Overall, these are relativelyhomogeneous basalts considering the spatial and temporalextent of their emplacement and the full range of spectralproperties observed in the M3 global mosaics.[82] Our analyses are consistent with a basaltic com-

position for Mare Orientale that contains several pyroxeneminerals with various amounts of Fe2+ and Ca2+. Most vari-ation in the deposit is likely due to lateral and vertical mixingwith feldspathic basin materials. Lacus Veris and Autumniappear similar in composition to Mare Orientale, but certainregions display spectral characteristics indicative of eitherfeldspathic mixing or slight differences in basalt composition.Further investigation is needed to determine the actual causein apparent spectral differences. All mare basalts in OrientaleBasin appear to have approximately the same titanium con-tent, a medium‐high value between 3 and 7 wt % TiO2.Calculations of titanium content using the Charette et al.[1974] method indicate that compared to other lunar basins(excepting Tranquillitatis and Western Procellarum), Orien-tale has a relatively high‐Ti content [Staid, 2000]. Using theLucey et al. [1998] method, Orientale has a titanium valuethat is generally comparable to the majority of nearside lunarmare basin deposits [Staid, 2000].

9. Volcanological Features Associated With MareBasalt Deposits

[83] Lunar mare deposits are characterized by a widevariety of volcanological features [Guest, 1971;Head, 1976],including flow fronts, lava channels, collapsed lava tubes,sinuous rilles, domes and cones, vents and collapse pits. Alsoobserved are multiple flow units distinguished on the basisof spectral contrast and impact crater size‐frequency distri-butions [e.g., Hiesinger et al., 2000, 2008, 2011]. Sourceregions observed in lunar nearside mare deposits includelinear fissures formed by dikes propagating to the surface and


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erupting along the fissure to form a fissure vent. These fea-tures are often characterized by a linear array of cones andsmall domes, and they commonly have a larger edifice or pitthat represents the widest part of the dike, where effusion wasconcentrated following the initial eruptive phase [Wilson andHead, 1988]. Lava distributary features observed in nearsidemare regions include lava channels within broader lava flows(centralized channels defined by marginal levees). These aregenerally considered to be constructional features, formed bythe dynamics of the flow process within the flow itself [e.g.,Greeley, 1971; Hulme, 1974]. Channels can also be roofedover, forming an insulating carapace and tube‐fed flows;rows of pits and channel segments provide evidence for thepresence of tube‐fed flows. Fissure vents or point sourcevents form sinuous rilles, features that are thought to beerosional in nature. Sinuous rilles may form by thermal [e.g.,Hulme, 1973; Williams et al., 2000] and mechanical erosion[e.g.,Gregg andGreeley, 1993] of the substrate, in contrast tothe constructional style of channel‐fed flows. The char-acteristics and distribution of these various features provideimportant information on the styles of eruption and processesof magma ascent and eruption. Several of these describedfeatures are observed in the Orientale Basin.[84] Greeley [1976] conducted an analysis of the volcanism

in Orientale to determine the modes of emplacement of themare deposits. Initially investigating basaltic terrains onEarth, Greeley [1976] defined three terrain types he consid-ered most important for interpreting basaltic regions on otherterrestrial planets: flood basalts, shield volcanoes, andbasaltic plains. Using this classification scheme, Greeley[1976] interpreted the modes of emplacement for the vari-ous mare deposits in the Orientale Basin. Based on the lackof preserved flow features, areal extent and large impliedvolume, Mare Orientale was classified by Greeley [1976] asflood basalt style volcanism. Lacus Veris is much different incharacter, having an abundance of sinuous rilles (Figure 2),thought to be lava channels or collapsed lava tubes, andseveral small coalescing shield‐like structures [Greeley,1976]. Lacus Autumni has a large number of sinuous rillesas well. As a result of implied fissure vents and rille pointsources, Lacus Veris and Lacus Autumni were classified byGreeley [1976] as basaltic plains style volcanism.[85] The high spatial and spectral resolution M3 data add

new insight into themodes ofmare emplacement in Orientale.The absence of (1) multiple, distinctively different flowsubunits distinguished by spectral data; (2) morphologicallyprominent flow fronts; or (3) areal differences in impact cratersize‐frequency distributions (as is the case in Mare Imbrium)[Bugiolacchi and Guest, 2008], all suggest that MareOrientale may have been emplaced in a single eruptive phase,consistent with a flood basalt style [Greeley, 1976; Yingst andHead, 1997]. On the other hand, several prominent maredome‐like structures exist in the central part of the basin, andthese could potentially have been point sources for some ofthe plains within Mare Orientale (Figure 12).[86] One such dome feature (∼20 km in diameter) is located

west of Hohmann crater among the chain of kipukas in thecenter of the basin (Figures 12a and 12b). There is a crater onthe summit region measuring ∼1.6 km in diameter. Based oncrater diameter/basal dome diameter relationships establishedby Head and Gifford [1980] through the evaluation of over200 lunar mare domes, crater diameter is 19–26% of the basal

dome diameter. The summit crater of this Orientale domedoes not fit this relationship, suggesting that this mare domeis not a lunar volcano. In addition, the visible outline of thedome does not appear distinct in LOLA data (Figure 12c)nor does it have a distinct spectral signature in M3 data(Figure 12b). Instead, this dome may be categorized moreappropriately as a class 5 dome [Head andGifford, 1980]. It isirregular in shape, fits within the delineated diameter range,does not have a real typical summit crater and is closelyassociated with highland units. This dome is likely to haveoriginated when the center of Orientale Basin was floodedand mare draped over preexisting topography.[87] Another group of mare domes is located in pond 14 of

Lacus Veris (Figures 12d, 12e, and 12f). They are ∼6–8 kmin diameter and look like small terrestrial shield volcanoesrising ∼170–230 m high. However, despite their volcano‐likeappearance, all of these mare domes lack a distinct summitcrater. The peaks of several of these domes have a higheralbedo than their slopes that display a feldspathic signature intheM3 data. Thus, this group of domes is likely to be the resultof highland material being draped with mare material (a class5 dome according to Head and Gifford [1980]). Their prox-imity to the edge of the mare pond, coupled with M3 spectraldata, supports this interpretation.[88] In east central Mare Orientale, a dome‐like complex

with a superposed fracture (∼7.5 km long) is interpreted toindicate continued volcanic activity after solidification of amare deposit (Figures 12g–12i). It stands ∼150–280 m abovethe mare surface and is ∼29 km in diameter. The domecomplex lacks distinct morphologic and spectral signaturesto distinguish it from Mare Orientale basaltic material. Thiscomplex may have formed during late stage volcanism,perhaps draping of mare basalt over the substrate duringcooling and contraction or upwarping by a subsurfacemagma chamber.[89] Another volcanic feature associated with Mare

Orientale is a sinuous rille that is cut into the southern rim ofthe basin interior. Rille A (Figure 13a), just to the south of thesouthern margin ofMare Orientale, was previously thought tobe a channel draining impact melt sheet material [Greeley,1976]. The new M3 data, however, clearly indicate that thechannel was a conduit for mare material. Mafic signatures areseen both at the head of the rille and halfway down the rille, ina location where mare material ponded before continuing toflow into the basin floor (Figures 1c and 13b). This feature isthe longest sinuous rille in Orientale (Table 4), originating inthe Maunder Formation and extending into Mare Orientalefor a distance of 80 km. Its flow path has been significantlyinfluenced by fractures in the Maunder Formation, makingthe rille less sinuous than others in the basin. Narrowing of thechannel along its length and other morphologic character-istics are consistent with thermal erosion of the coherentMaunder Formation, interpreted as impact melt; erosion ispredicted to be greatest near the vent (where the lava is at thehighest temperature) and decrease down rille, at least in theabsence of other changes in setting (background slope oreroding substrate) [Hulme, 1973].[90] A combination of high‐resolution M3 and LOLA data

has permitted the identification of several new sinuous rilles(e.g., rilles T, P, and K; Figures 2 and 13). Other previouslyidentified sinuous rilles [Greeley, 1976] have not been con-firmed in the new high‐resolution data (Figure 2; compare


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with Figure 2 of Greeley [1976]). Areas reported to havesinuous rilles on the basis of previous data were examinedfirst with M3 data to confirm the existence of sinuous struc-tures. If a definitive determination could not be made, LOLAprofiles were analyzed to assess whether or not a troughexisted. Several sinuous rilles identified by Greeley [1976]did not appear in LOLA profiles, suggesting that sinuousrilles do not exist in these locations. Table 4 enumeratesthe different rille characteristics observed (length, width

and depth). Most of the rilles in Orientale Basin haveshorter lengths than those on the lunar nearside; however,the average calculated length of 30 km is consistent withprevious measurements (Figure 14). Schubert et al. [1970]measured lengths of sinuous rilles on the lunar nearside inregions covered by the Orbiter IV high‐resolution photo-graphs, finding that the average length was 34 km long,though a few rilles were greater than 300 km in length(Figure 14b).

Figure 12. Thermal image (2936 nm) and standard M3 color composite (R, 1 mm IBD; G, 2 mm IBD;B, reflectance at 1489 nm) of the mare domes in Orientale Basin. (a) Thermal image of dome located∼40 kmwest of Hohmann crater. (b) Standard M3 color composite of Figure 12a. (c) West‐east profile fromLOLA data. (d) Dome complex in pond 14 of Lacus Veris. (e) Standard M3 color composite of Figure 12c.(f) West‐east profile of dome. (g) Dome complex located in eastern Mare Orientale ∼100 km southwest ofKopff crater. (h) Standard M3 color composite of Figure 12e. (i) West‐east profile of dome complex.


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Figure 13. Close‐ups of selected rilles and topographic profiles in Orientale Basin. Images are takenfrom M3 mosaics, the thermal band (2936 nm) and the standard M3 color composite (R, 1 mm IBD;G, 2 mm IBD; B, reflectance at 1489 nm). Locations of profile images are indicated by the white arrows.(a–c) Rille A. (d–f) Rille S. (g–i) Rille P. (j–l) Rille T.


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[91] Generally in Orientale, sinuous rilles or parts of sinu-ous rilles that cut into feldspathic material have greater widthscompared to rilles in the volcanic mare deposits. This increasein width is potentially the result of mechanical erosion by lavaflowing over more poorly consolidated material. Those thatcut into both mare and feldspathic material vary in width from∼0.3 to ∼0.5 km, becoming narrower after transitioning ontomare material. With the current resolution data it is difficultto state whether or not changes in slope play a role in thisobserved change in width.

[92] Differences in channel width and type of materialbeing eroded could have significant implications for thedominant type of erosion, either thermal or mechanical [e.g.,Carr, 1974; Hurwitz et al., 2010a]. Rilles that have largedepressed heads are proposed to be indicative of thermalerosion: as the volcanic material is being erupted, the hottestmaterial lands closest to the vent and causes the most erosion[e.g.,Wilson and Head, 1980; Head and Wilson, 1980; J. W.Head and L.Wilson, manuscript in preparation, 2011]. Alongwith other characteristics, we believe a depressed head sug-gests that thermal erosion is likely the dominant process inthese particular rilles.[93] In comparison to rille A (Figure 13a), rille S

(Figure 13d) is one of the more sinuous of the lava channelsobserved in Orientale, likely related to the small elevationdifference in pond 19. The walls of rille S have a strong maficsignature (Figure 13e), and it by far the widest sinuous rille(Figure 13f) extending entirely through mare material. Sincethe width remains fairly consistent along the entire rille itis difficult to say which end is the source. Based on thesecharacteristics we suggest that thermal erosion was thedominant process because it seems likely that excess energyis required to carve a channel that large and sinuous throughgradually sloped volcanic material.[94] Rille P originates in feldspathic basin material and,

although it does not end in a mare deposit, its tail trendstoward a small mare deposit east of pond 22 (Figures 13g and13h). There is little evidence for a residual mafic signature onany observable walls of the rille, likely due to its locationon feldspathic material (Figure 13h). The tail end of rille Pappears to either disappear into a dark depression, potentiallythe opening to a roofed part of the lava channel, or endabruptly at the base of the slope (Figure 13g); it is difficult tostate definitively without higher‐resolution data. Mechanicalerosion may have played a larger role in the formation ofthis sinuous rille, as feldspathic material on the slopes ofOrientale massifs and mountain rings appears unconsolidatedand prone to mass wasting.

Table 4. Characteristics of Identified Sinuous Rilles in OrientaleBasin

Identifiera MaterialbLength(km)

Average Width(m)

Average Depth(m)

A Feldspathic 80 1090 170Bc Mare 15 515 20C Feld./Mare 25 750 20D Feldspathic 35 1050 255E Feldspathic 20 870 40F Mare 40 315 10Gd Mare 10 385 30Hd Mare 25 440 10I Mare 30 330 5J Feld./Mare 60 480 35Kd Mare 15 435 ‐L Mare 40 655 15M Feldspathic 20 755 120N Mare 45 460 25O Mare 5 470 20P Feldspathic 30 260 285Q Feld./Mare 10 1040 50R Mare 20 535 10S Mare 30 865 205T Feld./Mare 30 840 95

aIdentifier refers to the labels in Figure 2.bMaterial denotes what type of material the rille cuts through.cPossibly an older degraded rille, difficult to tell with current data

available.dRilles that appear to follow circumferential graben.

Figure 14. Histogram of rille lengths. (a) Measured rilles in Orientale. (b) Measured rilles on the lunarnearside [from Schubert et al., 1970].


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[95] Another new rille, T, is associated with one of thenewly discovered mare deposits (pond 6). This sinuous rilletransitions from feldspathic to mafic material, correspondingto a change in its width from ∼1.1 km to ∼0.60 km(Figures 13j–13l). This change in width is gradual, making itdifficult to say if it is a product of a change in material orwhether other variables, such as a change in slope, may be acontrolling factor. The bottom half of the rille still has a strongmafic signature in NIR data (Figure 13k). Compared to otherrilles and ponds this system is unique. It is very isolated fromother Orientale mare deposits, suggesting that the entire marepond has been deposited by the rille. Further analysis of rille Twill facilitate the placement of constraints on the flux ofmaterial through the rille. The formation process for rille T ismore difficult to predict. Narrowing and shallowing of therille as it enters the mare pond could be caused by thermalerosion, where dissipation of heat results in an inabilityto efficiently melt and remove material, or the observationcould be explained by mechanical erosion, with the geologicmaterial at the end of the rille being more resistant to thisprocess. Future analyses, which focus on high‐resolutiontopography and morphology [e.g., Hurwitz et al., 2010b]and spectral data, will help resolve many of the remainingambiguities associated with sinuous rille formation.[96] Our current study broadly agrees with the classifica-

tion scheme of Greeley [1976], but the new high‐resolution

M3 data indicate that Mare Orientale contains several localsources, including a sinuous rille in the southern part of thebasin as well as additional complexities observed in domesand related structures. Sinuous rilles and lava channels at leastpartially contributed to the formation of Lacus Veris andLacus Autumni. More detailed analysis is necessary to dis-tinguish between constructional lava channel and sinuousrille formation processes, and between the relative roles ofthermal and mechanical erosion.

10. Altitude Distribution of and Deformationof Mare Basalts

[97] The LOLA instrument onboard LRO is providing thehighest‐resolution altimetric data yet obtained for the Moon[Smith et al., 2010]. Coupling this topographic data with ageand compositional data can aid in deciphering the relationshipbetween multiring basin formation and volcanic activity. Theaverage elevations of the mare ponds (Table 2) accentuatethe stair‐like shape of the basin from Mare Orientale out tothe Cordillera Mountain ring ∼8 km above (Figures 1a, 15,and 16). Mare Orientale is the lowest lying basaltic deposit,followed by the ponds of Lacus Veris and, lastly, those ofLacus Autumni. A plot of the average pond elevations versustheir ages (Figure 17) shows the order of emplacement as itrelates to the basinmorphology. A very broad trend shows theyoungest mare deposits being higher in elevation, whichcorresponds with greater distances from the center of thebasin. Mare Orientale was the first and lowest basalt deposit,followed generally by the basalts of Lacus Veris, by and largebeing emplaced from the west to the east and eventuallysucceeded by the ponds of Lacus Autumni. Notably, LacusVeris has ponds that span a large portion of the duration ofvolcanism, while Mare Orientale only erupted at the begin-ning of the basin history and Lacus Autumni erupted near theend of volcanism in the basin.[98] The surfaces of each of the ponds were investigated for

any suggestion of a regional tilt by measuring topographicprofiles from a LOLA DEM (Figure 15). Current data showno indication of a significant radial tilt inward toward thecenter of the basin or an east‐west or north‐south regional tilt.The only consistent trend was found in the ponds in LacusAutumni, which were found to all tilt toward the center of thebasin, but the scale of the tilt was so small, on the order ofthousandths of a degree, that it appears insignificant. Exam-ination of suspected small‐scale radial tilts in the mare pondsshowed that many were the result of local topography, forexample, from mass wasting and impact cratering. Thus, to afirst order, the interior of the Orientale Basin did not appear toundergo regional subsidence and tilting subsequent to theemplacement of Lacus Veris and Autumni.[99] An interesting feature observed in the LOLA data is

the profile of the mare polygon to the southwest of MareOrientale. Its northern end dips in the direction of MareOrientale (Figure 15). The mare appears to have beendeposited within the fractured polygon, and later tilted towardthe center of the basin. This tilt is likely to be the result ofthe thermal subsidence that created the inner depression inOrientale Basin. The formation of compressive features (e.g.,wrinkle ridges) and uneven cooling underneath the center ofthe basin could result in some areas subsiding more thanothers.

Figure 15. LOLA DEM superposed over a M3 albedomosaic showing the topography of Orientale Basin (data fromMarch 2010 PDS release).


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[100] Mare deposits within Mare Orientale, however, havebeen significantly deformed by a series of wrinkle ridges(Figure 15) extending across the central part of the basin.There is a series of 2–3 ridges in the south central part of thebasin that parallel preexisting basin topography for ∼130 km.Near the northern end of this group of ridges another ∼190 kmlong wrinkle ridge begins in the western part of MareOrientale. These features are superposed on topographyassociated with the modification of the surface of theMaunder Formation. Scarps and fractures indicate that cool-ing of the impact melt sheet, thermal equilibration of theraised isotherms and impact‐related heat, together producedup to ∼3 km of subsidence in the period following the basin‐forming event (Figure 15). The superposition of the wrinkleridges on Mare Orientale indicates they were produced afterthat mare deposit was emplaced. Lack of prominent wrinkleridges in Lacus Veris and Autumni suggest that this com-pression in the center of the basin occurred before theiremplacement. Based on our crater retention ages, this datesthe compressional event to between 3.58 and 3.47 Ga.[101] Outside of Mare Orientale, there is one wrinkle ridge

(∼10 km long) in the southern portion of pond 22. Similar tothose in Mare Orientale, this wrinkle ridge had to occur afterthe mare in pond 22 was emplaced, which in this case was∼1.66 Ga. This little ridge might be the manifestation ofcompressional stresses on the outside edge of the basin,created just after the last phases of volcanism in the basin.[102] A ∼340 km long circumferential graben occurs just

inside of Lacus Veris. This extensional feature can be seen

where it cuts into the Maunder Formation and is embayed bythe mare deposits of Lacus Veris. These stratigraphic relationsindicate that the graben formed before the emplacement ofpond 14 in Lacus Veris, but after the Maunder Formationcooled. Remnants of another graben can be seen in pond 12.Here a similar situation exists, where the graben is cutting intothe feldspathic basin material while being covered in certainareas by mare material. Thus, based on stratigraphic relation-ships, these graben likely formedbetween 3.64Ga and 3.36Ga.

11. Evaluation of Models for Formationand Evolution of Mare Basalts

[103] Analyzing the various characteristics (e.g., ages, area,volumes, elevations, spectral compositions and associatedvolcanic features) of Orientale Basin has assisted in furtherunderstanding the relationship between the formation of lunarimpact basins and the onset of mare volcanism. In comparisonto the other lunar basins, Orientale offers great insight into theinitial stages of mare emplacement due to its incompleteflooding.With the latest surge of lunar data, from instrumentssuch as M3 and LOLA, new information has been obtained tohelp clarify this relationship between large lunar basins andvolcanism. We now use this synthesis data to assess modelsof mare basalt genesis, ascent and eruption.

11.1. Model 1

[104] Model 1 of the origin and evolution of mare basaltsassumes that the nearside‐farside asymmetry in the distribu-

Figure 16. Cross section through Orientale Basin, highlighting the basin rings and the location of mareponds between the various rings. Graph produced using a LOLA DEM from Figure 15 (data from March2010 PDS release).


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tion of mare basalts is due to differences in crustal thickness(Table 1). Orientale Basin provides a view into the impor-tance of the crustal thickness differences since it is situatedon the boundary between the thick lunar farside and thethinner nearside crust. Despite the discovery of several newdeposits to the south and southwest of Orientale, the generaltrend in the abundance of mare deposits, increasing from thesouthwest to the northeast is still prevalent. Our data showthat the genesis of mare basalt was symmetric over the lunarsurface. Differences in the distribution of mare lies in thedifficulty of getting melt products to the surface due tothermal and buoyancy barriers.

11.2. Model 2

[105] In model 2 [Elkins‐Tanton et al., 2004], basin‐forming impacts induce pressure release melting and asso-ciated secondary convection to explain the distribution oflunar mare basalts (Table 1). The first stage predicts largevolumes of mare basalts in the center of the basin (98–100%of the total melt), followed by smaller amounts of basalts (1–2% of the total melt) emplaced over a much longer period oftime. Calculated volumes of mare basalts in Orientale Basinsuggest Mare Orientale contains ∼94% of the total mareby volume and the rest (∼6%) resides in Lacus Veris andAutumni. Despite this general agreement in the locations andtheir relative volumes of mare, dating the basaltic surfaces inOrientale has shown that this process of pressure releasemelting is not consistent with the predicted timing. The basinand melt sheet are dated between 3.68 and 3.64 Ga, whilecentral Mare Orientale was not emplaced until 3.58 Ga, a time

gap of ∼60–100 Ma (Table 3). This time gap shows that theextrusion of Mare Orientale was not instantaneous andtherefore, was not the product of impact induced pressurerelease melting.

11.3. Model 3

[106] Another model (model 3) [Wieczorek and Phillips,2000] suggests that the presence of an enhanced sub‐Pro-cellarum KREEP layer explains the existence of mare basaltsand viscous domes in the western nearside PKT (Table 1).Signifiers of this influence on the thermal evolution of theMoon are manifested as “red spot” volcanism, which hasbeen associated with KREEPy rock compositions [Malin,1974; Chevrel et al., 1999]. “Red spot” volcanism on thelunar nearside has been identified as a postbasin, premareevent. This nonmare, spectrally distinct expression of vol-canism appears to be absent from both the edges of OrientaleBasin and the mare pond deposits, and no radioactive ele-mental anomalies have been detected. All volcanic featuresand deposits appear basaltic in composition.

11.4. Model 4

[107] Instabilities in the lunar mantle, caused by large‐scale overturn are another mechanism suggested to controlthe emplacement and evolution of mare basalts [Hess andParmentier, 1995] (Table 1). Unlike previous models,model 4 is completely independent of basin formation pro-cesses. The presence of early high‐Ti basalts, superposed onbasalts with lower titanium values would support this model.However, in Orientale the composition of the mare basalts

Figure 17. Graph of age versus elevation for all the datable mare ponds in Orientale. Early episodes oferuption occur more frequently than later eruptions. Mare in the center of the basin erupts early and thenstops. Continued later eruptions occur within the basin rings.


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appears to be fairly homogeneous despite the eruption ofbasalts over a period of ∼1.9 Ga. The basalts in Mare Or-ientale have medium‐ to high‐Ti contents, and Lacus Verisand Autumni have relatively high titanium values. In the caseof Orientale Basin it does not appear that any low‐Ti depositswere emplaced after the medium‐to high‐Ti (3–7 wt % TiO2)deposits that are observed.

11.5. Model 5

[108] In model 5, the emplacement of mare basalts is relatedto both the global thermal evolution and basin evolution(Table 1). Rather than relying on the basin‐forming event,this model depends on local and global lunar stress fields aswell as the geometry of the lunar basins. Based on the modelpredictions, local stresses control the location of deposits inthe lunar basins. Early mare volcanism will be focused in thecenter of the basin and the youngest deposits should occuralong the edges. As the lunar lithosphere cools, its thicknessincreases with time. The increasing density of the cooling,thickening lithosphere increases the load on the center of thebasin, consequently creating the local extensional stresses onthe exterior of the basin. Unloading‐related extensionalstresses are greatest immediately after the basin‐formingimpact and decrease with time as the lunar lithospherethickens and evens out underneath the entire basin. Eventu-ally global compressive stresses, from cooling of the Moon,dominate local extensional stresses and lead to the termina-tion of the ascent and eruption of mare basalts.[109] Model age dates from this study confirm that the

earliest volcanism did occur in the center of the basin(Table 3) and was independent of the basin‐forming impact,evidenced by the time lag between basin formation andemplacement of Mare Orientale. The younger deposits inLacus Veris and Lacus Autumni show a weak positive cor-relation between age and increasing distance from the centerof the basin; deposits young as they are emplaced further fromthe basin center. In addition, the areal extent of the depositsdecreases with distance from the center of the basin, possiblybecause there is more crust to transect (Figures 15 and 16).

12. Conclusions

[110] Analysis of the character and distribution of maredeposits in the Orientale Basin has provided key informationon the relationship between the formation of large lunarimpact basins and mare basalt volcanism. Results from ourtests of the five models indicate the following.[111] 1. The limited distribution of mare basalts in Orientale

provides clues to the initial stages of mare basalt filling. Marefilling began in the center of the basin ∼3.58 Ga, approxi-mately a hundred million years after basin formation (∼3.68Ga), assuming this fill is the result of a single volcanic event.The central deposit erupted as a flood basalt and was followedclosely by later pulses of volcanism that produced the mareponds in Lacus Veris and lastly those in Lacus Autumni(Table 3). Estimated flow unit volumes range from 30 to7,700 km3, averaging between 590 and 940 km3 [Hiesingeret al., 2002]. This time sequence correlates well with thetheory that early flooding is dominated by larger depositthicknesses over a smaller area, and later flooding is oppositein character, with shallower deposits that cover a larger area[Head, 1982]. The maximum difference of 50–130 Ma

between the basin forming impact event and the average ageofMare Orientale represents a significant time gap and arguesagainst near instantaneous depressurization melting related tothe impact event.[112] 2.Model age dating has shown that the youngest mare

deposits occur on the edges of the basin (i.e., LacusAutumni),varying in age from 3.47 to 1.66 Ga. A few ponds in LacusVeris have crater retention ages within this range as well.Generally, lava flow emplacement occurred first in LacusVeris and finally in Lacus Autumni, with some overlap. Theirabundance of sinuous rilles indicates they represent a basalticplains style volcanism dominated by both point sources (e.g.,vents and rilles) and fissures. The trend between the modelages and elevations of mare ponds (Figure 17) shows how theyoungest ponds are both the furthest from the center of thebasin and at the highest elevations. This observation supportsthe idea that extensional stresses within the basin propagateradially outward over time. The correlation with the postbasinstate of stress in the lithosphere favors dike emplacement inthe ring regions where annulus‐normal basin stresses arelikely, as the significant remaining basin topography adjuststhermally and mechanically [Bratt et al., 1985a].[113] 3. The model age distribution of Orientale mare ba-

salts shows that they span nearly the entire range of nearsidemodel ages but that the volumes are considerably smaller thanfor the mare filling typical nearside impact basins.[114] 4. Model age data indicate that Orientale mare basalts

were erupting onto the surface over a significant time period(∼1.9 Ga; Table 3). The majority of them were depositedduring the highest frequency of eruptions on the lunar near-side, around 3.5 Ga (Figure 10) [Hiesinger et al., 2011].[115] 5. Mare Orientale was determined to be the result of a

single eruptive phase, lacking any compelling compositionalevidence for the presence of multiple distinct flow units.Lacus Veris and Autumni appear similar in composition toMare Orientale, but display spectral characteristics that couldbe indicative of both feldspathic mixing and slight differencesin basalt composition. Most of the mare deposits in Orientaleappear to have a medium‐ to high‐Ti content (3–7 wt % TiO2

from Kadel [1993]).[116] 6. Mare eruption locations are focused along the

margins of the different basin rings, mostly in the form ofvents and sinuous rilles. Most of the obvious vents and all ofthe rilles are located on the eastern side of the Orientale Basin.No rilles exist on the southwestern side of Orientale possiblybecause the crustal thickness there was too great to allow forthe propagation of dikes to the surface, hindering the openingof vents and extrusion of significant quantities of mare basaltover a sustained time period. Thus, the minor deposits presentto the west and southwest of Orientale are likely the result ofsmall fissures or vents that have since been covered up by thedeposits themselves.[117] 7. There is no evidence in Orientale Basin for “red

spot” extrusive domes and related deposits. Thus, there doesnot appear to be any KREEPy basalt extrusions or aluminousbasalts.[118] 8. Kopff crater, previously interpreted as a volcanic

caldera [e.g., Pai et al., 1978], is shown to be a volcanicallymodified impact crater. However, the unusual character of itsmorphology suggests an impact into central basin material ofunusual thermal properties.


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[119] Acknowledgments. M3 is supported as a NASA DiscoveryProgrammission of opportunity. Both the science results and the science val-idation are supported through NASA contract NNM05AB26C. Thanks areextended to members of the M3 team for assistance in the preparation of thismanuscript. Additionally, theM3 team is grateful to ISRO for the opportunityto fly as a guest instrument on Chandrayaan‐1 and is grateful to all on theISRO team that enabled M3 data to be returned. We gratefully acknowledgethe valuable input to this project by the public release of LROLaser Altimeterdata.

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R. Clark, U.S. Geological Survey, MS964, Box 25046, Federal Center,Denver, CO 80225, USA.

J. W. Head, J. Mustard, J. Nettles, C. M. Pieters, and J. Whitten,Department of Geological Sciences, Brown University, Box 1846,Providence, RI 02912, USA. ([email protected])R. Klima, Johns Hopkins University Applied Physics Laboratory, 11100

Johns Hopkins Rd., Laurel, MD 20732‐0000, USA.M. Staid, Planetary Science Institute, 1700 E. Fort Lowell, Ste. 106,

Tucson, AZ 85719, USA.L. Taylor, Planetary Geosciences Institute, Department of Earth and

Planetary Sciences, University of Tennessee, 1412 Circle Dr., Knoxville,TN 37996‐1410, USA.


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Technology

Lunar mare deposits_associated_with_orientale_basin