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For permission to copy, contact [email protected] 2005 Geological Society of America 147
Geosphere; December 2005; v. 1; no. 3; p. 147–172; doi: 10.1130/GES00016.1; 11 figures; 6 tables, 1 animation.
An animated tectonic reconstruction of southwestern North Americasince 36 Ma
Nadine McQuarrie*Brian P. WernickeDivision of Geological and Planetary Sciences, MS 100-23, California Institute of Technology, Pasadena,California 91125, USA
ABSTRACT
We present tectonic reconstructions andan accompanying animation of deformationacross the North America–Pacific plateboundary since 36 Ma. Intraplate defor-mation of southwestern North America wasobtained through synthesis of kinematicdata (amount, timing, and direction of dis-placement) along three main transectsthrough the northern (408N), central (368N–378N), and southern (348N) portions of theBasin and Range province. We combinedthese transects with first-order plate bound-ary constraints from the San Andreas faultand other areas west of the Basin andRange. Extension and strike-slip deforma-tion in all areas were sequentially restoredover 2 m.y. (0–18 Ma) to 6 m.y. (18–36 Ma)time intervals using a script written for theArcGIS program. Regions where the kine-matics are known constrain adjacent areaswhere the kinematics are not well defined.The process of sequential restoration high-lighted misalignments, overlaps, or largegaps in each incremental step, particularlyin the areas between data transects, whichremain problematic. Hence, the value of thereconstructions lies primarily in highlight-ing questions that might not otherwise berecognized, and thus they should be viewedmore as a tool for investigation than as afinal product.
The new sequential reconstructions showthat compatible slip along the entire north-south extent of the inland right-lateralshear zone from the Gulf of California tothe northern Walker Lane is supported byavailable data and that the east limit of ac-tive shear has migrated westward with re-spect to North America since ca. 10 Ma.The reconstructions also highlight newproblems regarding strain-compatible ex-
tension east and west of the Sierra Nevada–Great Valley block and strain-compatibledeformation between southern Arizona andthe Mexican Basin and Range. Our resultsshow ;235 km of extension oriented;N788W in both the northern (50% exten-sion) and central (200% extension) parts ofthe Basin and Range. Following the initia-tion of east-west to southwest-northeast ex-tension at 15–25 Ma (depending on longi-tude), a significant portion of right-lateralshear associated with the growing Pacific–North America transform jumped into thecontinent at 10–12 Ma, totaling ;100 kmoriented N258W, for an average of ;1 cm/yr since that time.
Keywords: Basin and Range, kinematic re-construction, extension, plate tectonics, ve-locity field.
INTRODUCTION
The large-scale horizontal velocity field atEarth’s surface is one of the main predictionsof physical models of lithospheric deforma-tion (e.g., England and McKenzie, 1982).Two-dimensional, cross-sectional models offinite deformation of mountain belts incorpo-rating strong heterogeneity in rheologic pa-rameters have been developed over the last de-cade (e.g., Lavier and Buck, 2002; Braun andPauselli, 2004). Owing to advances in com-putation, fully three-dimensional models ofplate boundary deformation zones, incorpo-rating both horizontal and vertical variationsin lithospheric rheology, will soon becomecommon. Thus, a key observational frontierwill be the determination of precise displace-
*Current address: Department of Geosciences,Guyot Hall, Princeton University, Princeton, NewJersey 08544, USA.
ment vector fields of continental deformationin order to test these models. The most dra-matic recent improvement in obtaining suchvelocity fields has been the advent of space-based tectonic geodesy (especially using con-tinuous global positioning systems [GPS]),which is yielding velocity fields that are un-precedented in terms of both the scale of ob-servation and the accuracy of the velocities.These data have already been used as tests forphysical models in southwestern North Amer-ica (e.g., Bennett et al., 1999, 2003; Flesch etal., 2000) and elsewhere (e.g., Holt et al.,2000). Substantial progress has also occurredover the last decade in determining longer-term velocity fields using the methods of platetectonics and regional structural geology.
These longer-term displacement historiesare essential for addressing the question ofhow the lithosphere responds to major varia-tions in plate geometry and kinematics (e.g.,Houseman and England, 1986; England andHouseman, 1986; Bird, 1998) because suchvariations occur on the million-year timescale. Plate tectonics is a precise method forconstraining the overall horizontal kinematicsof plate boundaries, using seafloor topograph-ic and magnetic data in concert with the geo-magnetic time scale. For the diffuse defor-mation that characterizes the continentallithosphere along plate boundaries, however,tectonic reconstruction at scales in the 100 kmto 1000 km range is not as straightforward. Itis based primarily on structural geology andpaleomagnetic studies and requires the iden-tification of large-scale strain markers andconsideration of plate tectonic constraints(e.g., Wernicke et al., 1988; Snow and Wer-nicke, 2000; McQuarrie et al., 2003). Regionalstrain markers within the continents may notexist in any given region, and even if they do,they may not be amenable to accurate recon-struction at large scales.
148 Geosphere, December 2005
N. MCQUARRIE and B.P. WERNICKE
Figure 1. Schematic diagram illustratingthe method of using regional structuralconstraints to limit possible displacementpaths in tectonic reconstructions. See textfor discussion and explanation of letters.
In southwestern North America, a zone ofplate boundary deformation on the order of1000 km wide is developed along the plateboundary. In mid-Tertiary time (36 Ma), thisboundary was strongly convergent, with theFarallon plate subducting eastward beneaththe North American plate. Beginning at ca. 30Ma, the Pacific-Farallon ridge came in contactwith the North American plate. Since then, thePacific–North America boundary has grownthrough the migration of triple junctions alongthe coast. Now, the entire margin from south-ern Baja California to Cape Mendocino is atransform Pacific–North America boundary,rather than a convergent Farallon–NorthAmerica boundary (Atwater, 1970). Thischange in the configuration of the plateboundary is both relatively simple and pro-found, making southwestern North Americaan ideal laboratory for investigating how con-tinental lithosphere responds to changes in rel-ative plate motion.
Refined plate tectonic reconstructions haveprovided an improved kinematic model of thechange from convergent to transform motionand have shown that there were significantvariations in the obliquity of the transform af-ter it developed. In particular, during the in-terval ca. 16 to ca. 8 Ma, Pacific–North Amer-ica motion was highly oblique and included amargin-normal extensional component of asmuch as 2 cm/yr, coeval with a rapid pulse ofMiocene extension that formed the Basin andRange province (Atwater and Stock, 1998;Wernicke and Snow, 1998). At ca. 8 Ma,Pacific–North America motion changed tomore purely coastwise motion, which appearsto have changed the intraplate tectonic regimefrom profound extension to a more complexmixture of extension, shortening, and trans-form motion, responsible for the opening ofthe Gulf of California, thrust faulting of thewestern Transverse Ranges, and developmentof the San Andreas fault–eastern Californiashear zone–Walker Lane, respectively.
Over the last several years, high-quality,large-scale kinematic constraints, many ofwhich resulted from decades of field work andattending debate, have become available,reaching the point where synthesis into alarge-scale velocity field is feasible. A rudi-mentary kinematic model using many of theconstraints along the plate boundary and inthe plate interior was incorporated into a pub-licly available animation of the post–38 Maevolution of the entire Pacific–Farallon–NorthAmerica system (Atwater and Stock, 1998;animation available at http://emvc.geol.ucsb.edu/download/nepac.php).
In this paper, we synthesize the current state
of information on the kinematics of the dif-fusely deforming North American plate since36 Ma, based on offsets of regional structuralmarkers, and construct a strain-compatible ki-nematic model of the horizontal motions at 2m.y. (0–18 Ma) and 6 m.y. (18–36 Ma) inter-vals, presented as a continuous animation. Themodel is by no means a final product, as newkinematic information and testing will requiresignificant modifications of the model. Rather,the model is an attempt to be quantitativelyrigorous in a way that will be useful for com-parison with large-scale, three-dimensionalphysical models and for the identification ofissues regarding the structural kinematics thatmight not otherwise be detected. Thus, in ad-dition to the animation, we have constructed‘‘instantaneous’’ velocity fields based on 2m.y. averages from 0 Ma to 18 Ma and 6 m.y.averages from 18 Ma to 36 Ma. These resultsare our best attempt at ‘‘paleogeodesy,’’ pre-senting the geology-based kinematic model ina format similar to modern GPS velocityfields, which in turn may be quantitativelycompared to physically based model velocityfields.
METHODS
By combining regional structural con-straints into a single model, the self-consistencyof the model (i.e., its strain compatibilitythrough time) provides powerful additionalconstraints on the kinematics in at least threeways. The first and most important is the factthat high-quality local kinematic informationimposes severe constraints on its surroundingswhere information may not be available. As ahypothetical example, consider a large regionof oblique extension between two undeformedblocks (Fig. 1). The strain and strain path neednot be known for each geological element inthe deforming region in order to constrain thelarge-scale kinematics. If the sum of fault dis-placements across just a single reconstructionpath (p) is known, restoring point A to a po-sition at point B, and it is known that theblocks have not rotated, then the single pathimposes a strong constraint on the overall ki-nematics of all of the other paths between theblocks (Fig. 1A).
The second additional constraint is on er-rors in reconstructions. In the example in Fig-ure 1, let us suppose that the minimum valueof all fault displacements along reconstructionpath p restores the block to point B, but thereis no constraint on the maximum value alongthe path itself. The side of the block contain-ing A would overlap the block on the otherside of the rift if the displacement along the
path were in excess of AC (Fig. 1B), violatingthe condition of strain compatibility. There-fore, the displacement is constrained to be be-tween AB and AC, rather than some valuegreater than AB.
A third and perhaps most useful additionalconstraint arises when local constraints con-tradict one another. For example, if recon-struction along path q (Fig. 1A) required thatpoint A restore to a position D, which is wellwithin the other block, then the violation ofstrain compatibility forces reevaluation of thegeological constraints. The geological recon-struction for displacement along q, the paleo-magnetic constraints on the blocks, and thepresumed rigidity of the blocks cannot all becorrect. Thus, the exercise of regional recon-struction focuses attention on information thatis most critical for improving the accuracy ofthe reconstruction. For southwestern NorthAmerica, there is now enough high-quality lo-cal kinematic information that large-scale self-consistency of the model imposes useful ad-ditional constraints in all of these ways.
In making the reconstruction, the methodsused in the local study of Wernicke et al.(1988) and Snow and Wernicke (2000) in theDeath Valley region of the central Basin andRange province were applied at large scale. InSnow and Wernicke (2000), each step in thereconstruction showed the paleoposition ofexisting mountain ranges. Although the recon-
Geosphere, December 2005 149
ANIMATED TECTONIC RECONSTRUCTION
Figure 2. Map of western North America showing the primary tectonic elements in thereconstruction. The gray shaded polygons represent the physiographic or geologic ex-pression of mountain ranges, which in the Basin and Range are fault bounded and sep-arated by alluvial valleys. Dashed boxes are the locations of Figures 3–5. The numbersrefer to specific mountain ranges identified in Tables 1–6.
struction allowed for the ranges to changeshape as extension is restored (i.e., the rangesmay decrease in area), in our reconstruction,the mountain ranges are shown as digitizedpolygons that approximate: (1) the modernbedrock-alluvium contact (e.g., a typical rangein the Basin and Range), (2) faults boundingindividual crustal blocks (e.g., the Santa YnezMountains block in the western TransverseRanges), or (3) the physiographic boundariesof large, intact crustal blocks (e.g., the Colo-rado Plateau). In some cases, especially wherelarge extensional strains are involved, the re-construction overlaps individual polygons toaccount for extension, essentially using themodern bedrock-alluvium contact as a geo-graphical reference marker. Because the strainis extensional, and in the case of metamorphiccore complexes, one range has literally movedoff of the top of another, these overlaps do notviolate strain compatibility.
The individual positions of polygons wererestored in each 2 m.y. time frame through anArcGIS script that reads and updates a tablelisting the kinematic data for each range. Thescript, created by Melissa Brenneman of theRedlands Institute at the University of Red-lands, is written in Visual Basic and is incor-porated as a tool in a custom ArcMap docu-ment. The script reads a dBASE 4 table thatcontains the movement parameters (direction,distance rotation, and time interval) for eachrange (Appendix 1).1 The movement parame-ters listed in the table include both the avail-able data (Figs. 2–5), as well as the motionrequired for strain compatibility. For the re-gions where kinematic data are not available,the kinematics could be defined by insertingdata from proximal areas, or individual rangescould be moved by hand with the motion up-dated and recorded in the dBASE table usingthe ArcGIS script. The ArcGIS format and ac-companying script allows for exact displace-ments to be incorporated into the model, aswell as the individual adjustment of ranges toensure strain compatibility. The GIS script re-cords the geographical position of the centroidof each range at each 2 m.y. or 6 m.y. epoch.This allows for the data to be displayed in avariety of ways, including palinspastic mapsfor each 2 m.y. or 6 m.y. epoch, instantaneousvelocity vectors at each 2 m.y. or 6 m.y. ep-och, ‘‘paths’’ that individual ranges take over
1GSA Data Repository item 2005200, Appendix1, Movement Table, paleogeographic maps, andArcGIS files (shape files for each reconstructed timestep), is available online at www.geosociety.org/pubs/ft2005.htm, or on request from [email protected] or Documents Secretary, GSA, P.O.Box 9140, Boulder, CO 80301-9140, USA
the 36 m.y. span of the reconstruction, or ananimation that shows the integrated motionover 36 m.y. Instantaneous geology velocityfields are obtained from connecting the cen-troids of specific ranges at one time with thecentroid of the same range in a later time.
DATA
The primary tectonic elements in the recon-struction are large crustal blocks comprisingflat-lying pre–36 Ma strata, or geologic ele-ments that are otherwise little deformed, andthe straining areas in between them. The largeunstrained blocks include the Great Plains–Rocky Mountains region (nominal NorthAmerica reference frame), the Sierra MadreOccidental, the Colorado Plateau, the SierraNevada–Great Valley block, and PeninsularRanges block (Fig. 2). The strained areasaround them include the Rio Grande rift andBasin and Range province, the Gulf of Cali-fornia, the Transverse Ranges, the CoastRanges, and the Continental Borderlandsprovince offshore of southern California andBaja California.
The constraints used in the reconstructionare organized into six major categories (Tables1–6). The first covers a range-by-range recon-struction path across the northern Basin andRange near latitude 408N (Fig. 3 and Table 1).The second includes a similar reconstructionpath across the central Basin and Range nearlatitude 378N (Fig. 4 and Table 2). These tworeconstructions collectively constrain the mo-tion of the Sierra Nevada–Great Valley block.The third includes constraints from the south-ern Basin and Range, mainly the mid-Tertiarymetamorphic core complexes of the ColoradoRiver corridor and southern Arizona, west ofthe Sierra Madre Occidental, and extensionacross the Rio Grande rift north and east ofthe Sierra Madre Occidental (Table 3). Thefourth includes the complex Oligocene to re-cent strike-slip and extensional displacementsof the Mojave region, which connect the Si-erran displacement to regions farther south(Fig. 5 and Table 4). The fifth includes paleo-magnetic and geologic constraints on verticalaxis rotations of large crustal blocks, includ-ing the Sierra Nevada and Colorado Plateau,as well as small, individual ranges within the
150 Geosphere, December 2005
N. MCQUARRIE and B.P. WERNICKE
Figure 3. Map of the northern Basin and Range showing the kinematic data incorporatedinto the model. Black numbers indicate horizontal displacement amount, red bold num-bers indicate age range of motion. Arrows indicate approximate magnitude and directionof individual relative displacements between polygons. Data compiled from Allmendingeret al., 1986; Armstrong et al., 2004; Bartley and Wernicke, 1984; Coogan and DeCelles,1996; DeCelles et al., 1995; Dilles and Gans, 1995; Faulds et al., 2003; Hardyman et al.,1984; Hintzi, 1973; Hudson and Oriel, 1979; Lee, 1999; Miller et al., 1999; Niemi, 2002;Niemi et al., 2004; Smith, 1992; Smith et al., 1990; Smith and Bruhn, 1984; Stockli, 2000,2001; Surpless, 1999.
central Basin and Range and Mojave regions(Table 5). Lastly, constraints on the large dis-placements along the San Andreas fault–Gulfof California shear system, and strains and ro-tations within the Continental Borderlands, in-cluding the large clockwise rotation of theSanta Ynez Mountains block, are included inFigure 6 and Table 6.
Northern Basin and Range
The extensional kinematics of the northernBasin and Range are dominated by two large-offset normal fault systems, the Snake Rangedetachment system (78 km of total offset) andthe Sevier Desert detachment (40 km of totaloffset). The Snake Range detachment systemaffects the Egan, Schell Creek, and SnakeRanges (Fig. 2, ranges 6–8). Although thecoupling of this system of faults to deep crust-al extension has been debated (e.g., Gans andMiller, 1983; Miller et al., 1983; Bartley andWernicke, 1984; Miller at al., 1999; Lewis etal., 1999), a magnitude of upper crustal exten-sion of 78 km 6 10 km, as determinedthrough mapped and restored stratigraphic
markers, is not controversial (Gans and Miller,1983; Bartley and Wernicke, 1984). Morecontroversial is the geometry of the exten-sional faults in the Sevier Desert basin (be-tween ranges 3 and 4, Fig. 2), including thevery existence of the Sevier Desert detach-ment, which is known only from interpreta-tions of seismic reflection profiles and welldata (Anders and Christie-Blick, 1994; Willset al., 2005). The 40-km offset along the Se-vier Desert detachment used in this paper isbased on restoring Sevier fold-thrust beltstructures that are offset by the detachment,and high-angle normal faults in the hangingwall imaged in the Consortium for ContinentalReflection Profiling (COCORP) and industryseismic reflection lines (Allmendinger et al.,1986; Allmendinger et al., 1995; Coogan andDeCelles, 1996). An opposing view to thelarge-offset kinematics of a shallow detach-ment suggests that the imaged reflection sur-face is a composite of aligned features thatincludes basin-bounding high-angle normalfaults, a subhorizontal thrust fault, and anevaporite horizon (Anders and Christie-Blick,1994). According to this interpretation, exten-
sion across ranges within and around the Se-vier Desert basin could be as little as 10 km(versus 40 km), which would subtract ;15%from our overall estimate of extension alongthe transect.
To the west of the Egan Range area, theremainder of the northern Basin and Rangedeformation is partitioned into extensional andright-lateral strike-slip offsets, both of whichaccommodate translation of the Sierra–GreatValley block away from the interior of NorthAmerica. The extension (94 km) is accom-modated by several systems of steeply tiltednormal fault blocks in the western Basin andRange, with individual fault systems accom-modating up to 16 km of extension (Fig. 2,ranges 13, 19, and 20) (Surpless, 1999; Dillesand Gans, 1995; Smith, 1992), and a numberof high-angle, presumably modest-offset nor-mal faults that define the Basin and Rangephysiography across the central part of the re-construction path, which we assume have 3–4 km of horizontal offset each.
Right-lateral shear is accommodated pre-dominantly through northwest-trending faultsconcentrated near the western edge of thenorthern Basin and Range in the northernWalker Lane Belt (Fig. 2, range 18). Right-lateral offset on a series of faults, which in-dividually have 5–15 km of offset, totals 20–56 km (Faulds et al., 2005; Hardyman et al.,1984). Because the faults strike more westerlythan the North American margin, their motionaccommodates a component of westward mo-tion of the plate boundary.
Timing of extension in the northern Basinand Range is constrained by a large body ofwork on the ages of faulted Cenozoic volcanicand sedimentary units and cooling ages of up-lifted footwall blocks. For example, the early‘‘core complex’’–related extension (ca. 35–25Ma) is seen in coeval faulting and volcanismat 35 Ma in the Egan Range (Gans and Miller,1983) and 40Ar/39Ar cooling ages indicative ofrapid cooling from 30 to 25 Ma in the westernportion of the northern Snake Range and from20 to 15 Ma in the eastern portion of the range(Lee, 1995). Apatite fission track (AFT) cool-ing ages from the northern Snake Range in-dicate 10–13 km of fault slip from 18 to 14Ma. Initiation of later ‘‘Basin and Range’’ ex-tension is seen predominantly in the fission-track and helium cooling ages of apatite andzircon. The cooling ages across the width ofthe extending zone cluster ca. 15 Ma (Stockli,1999), with 18 Ma ages in the footwall of theSnake Range detachment (Miller et al., 1999)(Fig. 2, range 6) and Sevier Desert detachment(Stockli et al., 2001) (Canyon Range, Figure2, range 3).
Geosphere, December 2005 151
ANIMATED TECTONIC RECONSTRUCTION
Figure 4. Map of the central Basin and Range showing the kinematic data incorporatedinto the model. Motion of the Sierra Nevada with respect to the Colorado Plateau in thisregion is predominantly constrained by two distinctive sedimentary deposits (indicated asstars and hexagons) offset along extensive normal and strike-slip fault systems. Arrowsindicate approximate magnitude and direction of individual relative displacements be-tween polygons. Black numbers indicate horizontal displacement amount, bold red num-bers indicate age range of displacement. Data from Axen et al., 1990; Brady et al., 2000;Burchfiel, 1968; Burchfiel et al., 1987; Cemen et al., 1985; Duebendorfer et al., 1998;Fitzgerald et al., 1991; Fowler and Calzia, 1999; Guth, 1989; Holm and Dokka, 1991,1993; Hoisch and Simpson, 1993; Niemi et al., 2001; Snow and Lux, 1999; Snow andWernicke, 1989, 2000; and Wernicke et al., 1988.
Central Basin and Range
The central Basin and Range province is inmany respects an ideal location for a province-wide restoration of Basin and Range extension(Snow and Wernicke, 2000, and referencestherein). A regionally conformable miogeo-cline, Mesozoic thrust structures and distinc-tive Tertiary sedimentary deposits tightly limitthe extensional history of both the Lake Mead(Fig. 2, ranges 22–25) and the Death Valley(Fig. 2, ranges 27–34) extensional systems(Wernicke et al., 1988; Wernicke, 1992). Mo-tion of the Sierra Nevada with respect to theColorado Plateau in this region is primarilyconstrained by displacements of two distinc-tive Miocene basins developed early in thehistory of the extension of each system (Fig.4).
In the Lake Mead system, restoring numer-ous proximal landslide breccias at FrenchmanMountain (Fig. 2, range 24) to their source
areas in the Gold Butte block (Fig. 2, range23) also restores piercing lines defined by thesouthward truncation of Triassic formationalboundaries by the basal Tertiary unconformityin both areas. The correlation of these featuresin the Frenchman Mountain and Gold Butteareas suggests 65 km 6 15 km of extensionbetween the two blocks (Fig. 4).
In the Death Valley system, Wernicke et al.(1988) initially proposed that the Panamintthrust at Tucki Mountain (Panamint Range,Figure 2, range 28) is correlative with the Chi-cago Pass thrust in the Nopah–RestingSprings Range (Fig. 2, range 27) and theWheeler Pass thrust in the Spring Mountains(Fig. 2, range 25), suggesting a total of 125km 6 7 km of post-Cretaceous, west-northwestern extension has separated them(Table 2). This offset is strengthened by cor-relations of additional contractile structuresexposed across the Death Valley extensionalsystem (Snow and Wernicke, 1989; Snow,
1992; Snow and Wernicke, 2000) and distinc-tive middle Miocene sedimentary depositsthat occur along the extensional path (Niemiet al., 2001). These include proximal con-glomeratic strata of the Eagle Mountain For-mation, which were derived from the north-eastern margin of the Hunter Mountainbatholith in the southern Cottonwood Moun-tains (Fig. 2, range 32). Recognition and cor-relation of this dismembered early extensionalbasin, in conjunction with stratigraphic con-straints from other Tertiary deposits in the re-gion, indicates that its fragmentation occurredmainly from 12 Ma to 2 Ma (Fig. 4). Thecorrelation of these deposits yields a displace-ment vector of 104 km 6 7 km orientedN678W between the Nopah–Resting SpringsRange (Fig. 2, range 27) and the CottonwoodMountains (Fig. 2, range 32).
To the ;170 km of displacement from theseconstraints, we add four additional estimatesto complete the reconstruction path. In theLake Mead system, 15 km of extension be-tween the Gold Butte area and the ColoradoPlateau (Fig. 2, range 23) (Brady et al., 2000)and a maximum of 8 km of extension betweenthe Spring Mountains (Fig. 2, range 25) andFrenchman Mountain (Fig. 2, range 24) (Wer-nicke et al., 1988) increases the total displace-ment of the Spring Mountains relative to theColorado Plateau to ;88 km. In the DeathValley system, an addition of 9 km of dis-placement in both the Panamint and OwensValleys increases the total displacement to;147 km between the Spring Mountains andthe Sierra Nevada.
The sum of all displacements along the pathis therefore 235 km 6 20 km (Table 2), whichrepresents a combination of areal dilation(crustal thinning) and plane strain (strike-slipfaulting). Approximately 80% of the elonga-tion is accommodated by vertical thinning and;20% by north-south contraction (Wernickeet al., 1988; Snow and Wernicke, 2000). Inaddition to this path, there are a number ofmore local offsets that were used to positionpolygons to the north and south, which areshown in Figure 4 and summarized in Table 2.
Southern Basin and Range–Rio GrandeRift
Extension in the southern Basin and Rangeis almost completely dominated by the for-mation of large-offset normal faults that formthe metamorphic core complexes (Coney,1980; Spencer and Reynolds, 1989; Dickin-son, 2002). The core complexes ring thesouthwestern margin of the Colorado Plateau(Fig. 2, ranges 37–44), and estimates of the
152 Geosphere, December 2005
N. MCQUARRIE and B.P. WERNICKE
Figure 5. Map of the Mojave and Southern Basin and Range region showing distributionof strike-slip faults (bold lines), vertical axis rotation data (curved arrows), and extensionaloffsets (straight arrows). For strike-slip faults, reported slip amounts (black numbers) arecontrasted with model slip (bold blue numbers). For extension and rotation constraints,black numbers indicate horizontal displacement or amount of rotation, bold red numbersindicate age range of deformation. Red shaded area is the area of the model that undergoes24–18 Ma core complex extension in the Mojave region. WRT indicates measured dis-placement is ‘‘with respect to’’ the Colorado Plateau. Data from Ballard, 1990; Bassettand Kupfer, 1964; Dokka, 1983, 1989; Foster et al., 1993; John and Foster, 1993; Ham-ilton, 1987; Howard and Miller, 1992; Miller, 1980; Miller and Morton, 1980; Powell,1981; Richard et al., 1992; Richard, 1993; Richard and Dokka, 1992; Spencer and Reyn-olds, 1989; Spencer et al., 1995; Schermer et al., 1996; Walker et al., 1995.
total extension they represent are remarkablysystematic in magnitude, direction, and rate(Table 3). The timing of extension varies inage from 28 Ma to 14 Ma as the extensionmigrates from southeast to northwest. The mi-gration of extension has been related to a sim-ilar migration in volcanism. Both extensionand volcanism have been proposed to be a re-sult of the northwestward foundering of theFarallon plate (e.g., Humphreys, 1995; Dick-inson, 2002).
In a similar time frame (ca. 26 Ma), vol-caniclastic sediments deposited east of theColorado Plateau in the Rio Grand Rift (Fig.2, location 45) have been interpreted as rep-resenting the onset of extension (Chapin andCather, 1994). Ingersoll (2001) counters thatthe early sediments are broad volcaniclasticaprons that show no evidence of syndeposi-tional faulting. He places the initiation of rift-ing slightly later (ca. 21 Ma). Based on initi-ation of half-graben sedimentation and strataltilting, rapid extension occurred between 17
and 10 Ma (Ingersoll, 2001; Chapin and Cath-er, 1994). The total magnitude of extension issmall and ranges from 6 km in the northernpart of the rift to 17 km in the south, consis-tent with the 1.58 clockwise rotation of theColorado Plateau (Chapin and Cather, 1994;Russell and Snelson, 1994).
Extension within the Rio Grande rift is con-tiguous with the broad extended region farthersouth, east of the Sierra Madre Occidental (inChihuahua), the magnitude of which is poorlyunderstood (Dickinson, 2002). Generally, ex-tension in the Mexican Basin and Range ispartitioned both in time and space. Early corecomplex extension is documented in north-western Mexico (in Sonora), just west of theSierra Madre Occidental (Nourse et al., 1994;Gans, 1997) (Fig. 2). Palinspastic reconstruc-tions over small regions in Sonora suggest cu-mulative extension of 90%, mostly between26 and 20 Ma, and more modest extension(10%–15%) between 20 and 17 Ma (Gans,1997). Limited crustal extension is also doc-
umented east of the Sierra Madre Occidentalduring the same time period (Dickinson,2002). Major extension occurred in both Chi-huahua (Henry and Aranda-Gomez, 2000;Dickinson, 2002) and Sonora Mexico (e.g.,Stock and Hodges, 1989; Henry, 1989; Lee etal., 1996) from ca. 12 Ma to 6 Ma as a preludeto the opening of the Gulf of California at 6Ma (Oskin et al., 2001). During the 12 Ma to6 Ma interval, very small magnitude east-west‘‘Basin and Range’’ extension affected Ari-zona (Spencer and Reynolds, 1989; Spenceret al., 1995).
Mojave Region
Cenozoic deformation of the Mojave regionoccurred in two main stages. Deformation be-gan in the late Oligocene–early Miocene withthe formation of large-offset normal faults andassociated core complexes (Glazner et al.,1989; Dokka, 1989; Walker et al., 1990). Ex-tension in the Mojave region (Fig. 2, location41, and Fig. 5) may be linked to core complexextension in the southern Basin and Rangecorridor (Fig. 2, ranges 38 and 44) through adiffuse transfer zone that involves both rota-tion and strike-slip faulting (Bartley and Glaz-ner, 1991; Martin et al., 1993). The magnitudeof extension is determined through alignmentof pre-extensional markers that include faciestrends in Paleozoic strata, a unique gabbro-granite complex, and late Jurassic dikes, in-dicating a total of 40–70 km of offset (Glazneret al., 1989; Walker et al., 1990; Martin et al.,1993). Extension began in synchronism withthe eruption and emplacement of 24–23 Maigneous rocks (Walker et al., 1995) and iscapped by the flat-lying, 18.5 Ma PeachSprings Tuff (Glazner et al., 2002). The frac-tion of the Mojave Desert region that was af-fected by mid-Tertiary extension is controver-sial (e.g., Dokka, 1989; Glazner et al., 2002).Glazner et al. (2002) propose that only a smallregion north of Barstow (Fig. 2, location 41)was affected by the early extension, with thesouthern boundary of this extensional domainlinked to core complex extension to the south-east through diffuse right-lateral shear. Thenorthern boundary of the extensional domainis more problematic; however, regional kine-matic compatibility requires a northern trans-fer zone that links Mojave extension to similarage extension to the north or west. Rotationof the Tehachapi Mountains and/or extensionin the southern San Joaquin Valley may rep-resent the northern portion of this system(McWilliams and Li, 1985; Plescia and Cald-erone, 1986; Tennyson, 1989; Goodman and
Geosphere, December 2005 153
ANIMATED TECTONIC RECONSTRUCTION
TA
BLE
1.D
AT
AU
SE
DF
OR
RE
ST
OR
ING
EX
TE
NS
ION
INN
OR
TH
ER
NB
AS
INA
ND
RA
NG
ELA
TIT
UD
E40
8N
Ran
ge/fa
ult
Hor
izon
tal
disp
lace
men
tof
rang
ebl
ock
(mod
elpo
lygo
n)
Hor
izon
tal
com
pone
ntof
faul
tsl
ip(d
ata)
Tim
ing
ofsl
ipR
ate
ofde
form
atio
n(h
oriz
onta
lcom
pone
ntof
faul
tsl
ip/d
urat
ion)
Dire
ctio
nof
mot
ion
Dat
aus
edS
ourc
eC
umul
ativ
epa
thdi
spla
cem
ent
(6cu
mul
ativ
eer
ror)
Long
.(x
)La
t.(y
)
San
Pitc
h/G
unni
son
(1)
[346
]4
km4
km6
2;
18–0
Ma
0.22
km/m
.y.
E-W
Str
atig
raph
icse
para
tion
Hin
tze,
1973
4km
62
km2
111.
7887
939
.502
97C
anyo
n/Le
van
and
Fay
ette
segm
ents
,W
asat
chfa
ult
(2)
[358
]
5km
5km
62
km;
10–0
Ma
0.5
km/m
.y.
E-W
Hol
ocen
eve
rtic
alof
fset
rate
san
dM
ioce
neex
hum
atio
nra
tes,
assu
me
;30
8di
p
Nei
mie
tal
.,20
04;
Mac
hette
etal
.,19
92;
Jack
son,
1991
;S
mith
and
Bru
hn,
1984
;
6km
63
km2
112.
1542
139
.560
62
Cric
ket
Ran
ge/S
evie
rD
eser
tD
etac
hmen
t(3
)[3
00]
34km
40km
62
km;
18–0
Ma
2.22
km/m
.y.
(tot
al)
E-W
Mat
chin
gC
Rcu
lmin
atio
nin
surf
ace
and
subs
urfa
ce.
Coo
gan
and
DeC
elle
s,19
96;
Coo
gan
etal
.,19
9546
km6
3.5
km2
112.
7558
239
.213
01
12km
12-1
0km
;19
–15
Ma
3km
/m.y
.A
FT
cool
ing
ages
assu
min
ga
34-4
0de
gree
faul
tS
tock
liet
al.,
2001
22km
16-3
0km
15–0
Ma
1km
/m.y
.S
tock
liet
al.,
2001
Hou
seR
ange
/‘‘R
eflec
tion
F’’
faul
t(4
)[3
50]
3.33
km4
km6
1km
;15
Ma
.267
km/m
.y.
E-W
Offs
etre
flect
oron
seis
mic
line
Allm
endi
nger
etal
.,19
8650
km6
3.6
km2
113.
3471
639
.234
02
Con
fusi
onR
ange
/Hou
seR
ange
faul
t(5
)[3
59]
4.66
km4
km(2
–3km
exhu
mat
ion)
;15
Ma
E-W
AF
Tco
olin
gag
eS
tock
li,19
992
113.
6144
638
.882
85
Sna
keR
ange
/nor
ther
nS
nake
Ran
gede
colle
men
t(6)
[366
]30
km(2
5–35
),22
km(0
–18)
30km
63
km(0
)22
63
km35
–25
Ma,
(pau
se25
–18M
a)18
–0M
a3
km/m
.y.
from
35–2
5M
a,2
km/m
.y.
from
18-0
Ma,
3km
/m.y
.fr
om18
-14
Ma,
1.7
km/m
.y.
from
14-0
Ma
E-W
Map
rela
tions
stra
tigra
phic
offs
et,
Ar/
Ar
cool
ing
ages
and
AF
Tda
ta(s
ugge
sts
10.5
-13
kmof
slip
betw
een
18-1
4M
a)
Gan
san
dM
iller
,19
83;
Bar
tley
and
Wer
nick
e,19
84;
Lee
1995
;M
iller
etal
.,19
99;
Lew
iset
al.,
1999
102
km6
5.5
km2
114.
1164
239
.435
17
Sch
ellC
reek
Ran
ge/S
prin
gV
alle
yfa
ult
(7)
[362
]11
km(0
–18)
11km
62
km18
–0M
a1.
6km
/m.y
.fr
om35
-25
Ma,
6km
/m.y
.fr
om18
-0
Ma
E-W
Map
rela
tions
,AF
T,
ZF
T,
Ar/
Ar
Bar
tley
and
Wer
nick
e,19
84;
Gan
set
al.,
1985
;Le
e,19
95;
Mill
eret
al.,1
999;
Lew
iset
al.,
1999
113
km6
6km
211
4.63
925
39.2
2082
But
teM
tns.
/Ega
nR
ange
low
-an
gle
faul
ts(8
)[8
0]16
km(2
5–35
)16
km6
2km
,5
kmex
hum
atio
n35
–25
Ma
16–1
7M
aE
-WB
artle
yan
dW
erni
cke,
1984
;G
ans
etal
.,19
85;
Lee,
1995
;S
tock
li,19
99
129
km6
6km
211
4.91
863
39.0
7945
Ega
nR
ange
toS
hosh
one
Mou
ntai
ns(9
–14)
45.6
km47
km6
10km
(26
excl
udin
gT
oiya
bera
nge)
16–0
Ma
26km
/16
m.y
./5ra
nges
E-W
Map
rela
tions
and
cros
sse
ctio
nre
stor
atio
n.S
mith
etal
.,19
9117
6km
611
.8km
Pin
eR
ange
/Gra
ntR
ange
and
Buc
kR
ange
faul
ts(9
)[3
48]
5km
6km
61.
5km
,;
5km
exhu
mat
ion
;15
km.3
25km
/m.y
.E
-W’’
Lund
etal
.,19
93;
Tay
lor
etal
.,19
892
115.
3773
240
.399
33
Dia
mon
dR
ange
/Pan
cake
Ran
ge/
Mah
ogan
yH
illsfa
ults
(10)
[360
]5
km5
kmof
26to
tal
unkn
own,
assu
med
;16
–0M
a.3
25km
/m.y
.E
-WM
apre
latio
nsan
dcr
oss
sect
ion
rest
orat
ion.
Sm
ithet
al.,
1991
211
5.82
060
39.8
3001
Sul
fur
Spr
ings
Ran
gefa
ults
(11)
5km
5km
of26
tota
las
sum
ed;
16–0
.325
km/m
.y.
E-W
’’S
mith
etal
.,19
91T
oqui
ma
Ran
ge/S
imps
onP
ark
Mou
ntai
nsfa
ult
(12)
[364
]5
km5
kmof
26to
tal
assu
med
;16
–0.3
25km
/m.y
.E
-W’’
Sm
ithet
al.,
1991
211
6.82
368
39.1
5694
Toi
yabe
Ran
gefa
ults
(13)
[368
]5
km(2
5–35
),16
km(0
–16)
21km
exte
nsio
n,;
5km
exhu
mat
ion
35–2
5M
a,16
–0M
a.5
km/m
.y.
(35-
25),
1km
/m.y
.(1
6-0)
E-W
’’S
mith
,19
92;
Sto
ckli,
1999
176
km6
11.8
km2
116.
8173
240
.337
06
Sho
shon
eM
ount
ain
faul
ts(1
4)[3
63]
4.6
km5
kmof
26to
tal
assu
med
;16
–0M
a.3
25km
/m.y
.N
64W
’’S
mith
etal
.,19
912
117.
3797
239
.471
25
Par
adis
eR
ange
faul
t(1
5)[3
51]
3.5
km3
km6
1as
sum
ed;
16–0
Ma
.188
km/m
.y.
N65
WP
arad
ixe,
C.
Alp
ine
and
Stil
lwat
erR
ange
sre
pres
ent
100
kmar
eath
atha
sge
ogra
phic
alex
tent
onm
ap
179
km6
11.8
km2
117.
5617
839
.321
98
C.
Alp
ine
Ran
gefa
ult
(16)
[349
]3
km3
km6
1as
sum
ed;
16–0
Ma
.188
km/m
.y.
N65
WE
xten
ded
anun
know
nam
ount
182
km6
11.8
km2
117.
7081
439
.751
61
Stil
lwat
erR
ange
faul
t(1
7)[3
54]
1.5
km3
km6
1as
sum
ed;
16–0
Ma
.188
km/m
.y.
N65
WU
nkno
wn/
best
estim
ate
185
km6
11.8
km2
117.
8555
740
.264
51
Gill
is/G
abbs
Val
ley
Ran
ge.
Gum
drop
hills
,In
dian
head
,B
ento
nsp
ring,
Pet
rified
Spr
ing
faul
ts.
(18)
[326
]
211
7.85
557
40.2
6451
youn
ger
than
25M
a6.
75km
/m.y
.(if
sinc
e8
Ma)
N14
WO
ffset
ofst
eep
beds
(Tria
ssic
age)
,gr
anite
intr
usio
nsan
dtu
ffs
Har
dym
anet
al.,
1984
203
km6
13.3
km2
118.
4691
538
.811
73
Nor
ther
nW
alke
rLa
nefa
ult
syst
em(1
8)29
(0–6
),13
(8–1
0)35
65
kmrig
htla
tera
lyo
unge
rth
an25
Ma
5-10
km/m
.y.
(ifsi
nce
5M
a)N
37W
Offs
etse
gmen
tsof
Eas
ttr
endi
ngO
ligoc
ene
pale
oval
ley
Fau
lds
etal
.,20
03
Was
sak
Ran
gefa
ult
syst
em(1
9)[3
54]
16km
11.7
66
1km
(13.
36
3km
)15
–0M
aE
-WC
ross
sect
ion
rest
orat
ion
plus
unro
ofing
ofgr
anite
Sur
ples
s,19
99;
Sto
ckli
etal
.,20
0221
5km
613
.6km
211
8.88
407
38.6
6236
10.5
km8.
71km
14–1
5M
a4.
3km
/m.y
.E
-Wfr
omva
lley
betw
een
Sin
gats
e’’
2km
2.1
km9–
7M
a1.
05km
/m.y
.E
-Wan
dW
assa
kra
nges
’’3.
4km
0.5
km(0
62)
,2.
56
27–
0M
a.3
km/m
.y.
E-W
’’
Sin
gats
eR
ange
faul
tsy
stem
(20)
[365
]15
.66
km13
62
15to
0M
aO
ffset
volc
anic
and
sedi
men
tary
rock
s,A
r/A
ris
otop
eag
es
Dill
esan
dG
ans,
1995
228
km6
13.8
km2
119.
2535
738
.959
20
7.66
kmM
ain
phas
e7.
2614
–12
Ma
3.63
km/m
.y.
E-W
’’4.
4km
1.7
km11
–8M
a.5
6km
/m.y
.E
-W’’
3.6
km4.
1km
7–0
Ma
.58
km/m
.y.
E-W
’’B
ucks
kin
Mtn
s./P
ine
Nut
Val
ley/
Pin
eN
utR
ange
Fau
lts(2
1)[3
52]
5km
76
2,2–
3km
ofex
hum
atio
n10
–0M
a.7
km/m
.y.
E-W
Offs
etfr
omm
appe
dun
its;
timin
g(1
0-5
Ma)
from
Sur
ples
s
Hud
son
and
Orie
l,19
79;
Sto
ckli,
1999
231
km6
14km
211
9.45
846
38.7
1853
Tot
alex
tens
ion
236
km23
56
14km
N78
8W
Not
e:Lo
ng.—
long
itude
;La
t.—la
titud
e.C
R—
Can
yon
Ran
ge;
AF
T—
Apa
tite
Fis
sion
Tra
ckco
olin
gag
e.N
umbe
rsin
pare
nthe
ses
refe
rto
spec
ific
rang
esid
entifi
edon
Fig
.2;
num
bers
inbr
acke
tsre
fer
toth
eR
AN
GE
pID
num
ber
for
indi
vidu
alra
nges
inth
eA
rcG
ISsh
ape
files
and
the
Mov
emen
tT
able
(Arc
GIS
files
,M
ovem
entT
able
[see
foot
note
1]).
The
cum
ulat
ive
erro
rre
port
edin
colu
mn
9eq
uals
the
squa
rero
otof
the
sum
ofth
esq
uare
sfo
rin
divi
dual
vect
ors.
154 Geosphere, December 2005
N. MCQUARRIE and B.P. WERNICKET
AB
LE2.
DA
TA
US
ED
FO
RR
ES
TO
RIN
GE
XT
EN
SIO
NIN
CE
NT
RA
LB
AS
INA
ND
RA
NG
ELA
TIT
UD
EO
F38
8N
Ran
ge/fa
ult
Hor
izon
tal
disp
lace
men
tof
rang
ebl
ock
(mod
elpo
lygo
n)
Hor
izon
tal
com
pone
ntof
faul
tsl
ip(d
ata)
Tim
ing
ofsl
ipR
ate
ofde
form
atio
n(h
oriz
onta
lcom
pone
ntof
faul
tsl
ip/d
urat
ion)
Dire
ctio
nof
mot
ion
Dat
aus
edR
efer
ence
sC
umul
ativ
edi
spla
cem
ent
offa
ult
slip
data
(6cu
mul
ativ
eer
ror)
Long
.(x
)La
t.(y
)
Mor
mon
Mou
ntai
nsar
ea/
Mor
mon
Pea
k,T
ule
Spr
ings
and
Cas
tleC
liff
deta
chm
ents
(22)
[73]
68km
546
1016
–12
Ma
13.5
km/m
.y.
S60
EC
ross
sect
ion
reco
nstr
uctio
nth
roug
hB
eave
rD
am/T
ule
Spr
ings
and
Mor
mon
Pea
kde
tach
men
tsys
tem
s
Axe
net
al.,
1990
54km
610
km2
114.
2455
237
.094
63
Gol
dB
utte
/Sou
thV
irgin
Mou
ntai
nsde
tach
men
t(23
)[5
7]
18km
156
5;
17–1
5M
a7.
5km
/m.y
.E
-WC
ross
sect
ion
reco
nstr
uctio
n,A
FT
cool
ing
ages
,ov
erla
ppin
g15
Ma
basa
lt
Wer
nick
eet
al.,
1988
;B
rady
etal
.,20
00;
Fitz
gera
ldet
al.,
1991
15km
65
km2
114.
2092
036
.387
48
Fre
nchm
anM
ount
ain/
Lake
Mea
dfa
ult
syst
em(2
4)[4
7]60
km60
min
90m
ax65
615
16–1
2M
a16
.25
km/m
.y.
N75
EM
egab
recc
iafr
omG
old
But
te,
pinc
hout
ofM
zfo
rmat
ions
,Virg
inM
ount
ains
deta
chm
ents
yste
m
Sno
wan
dW
erni
cke,
2000
;D
uebe
ndor
fer
and
Bla
ck,
1992
;D
uebe
ndor
fer
etal
.,19
98;
Wer
nick
eet
al.,
1988
80km
616
km2
114.
9699
936
.203
12
Spr
ing
Mou
ntai
ns/L
asV
egas
faul
tsy
stem
(25)
[53]
8km
8km
68
15–1
1M
aN
75E
Unk
now
nam
ount
ofex
tens
ion
betw
een
Spr
ing
Mou
ntai
nan
dF
renc
hman
Mtn
.
Wer
nick
eet
al.,
1988
88km
618
km2
115.
6091
236
.066
31
Las
Veg
assh
ear
zone
Und
eter
min
ed47
67
km15
–11
Ma
11.7
5km
/m.y
.A
lignm
ent
ofG
ass
Pea
kth
rust
(She
epR
ange
)w
ithW
heel
erP
ass
thru
st(S
prin
gM
t.).
Sno
wan
dW
erni
cke,
2000
;B
urch
fiele
tal
.,19
87;
Wer
nick
eet
al.,
1988
She
epR
ange
,P
intw
ater
Ran
ge,
Spo
tted
Ran
gede
tach
men
t(26
)[5
9,61
,64
]
23km
206
5km
13M
a–?
(9M
a)3.
67km
/m.y
.N
63W
Ext
ensi
onas
soci
ated
with
the
She
epR
ange
deta
chm
ent,
exte
nsio
nam
ount
base
don
cros
sse
ctio
nre
stor
atio
n
Gut
h,19
89;
Sno
wan
dW
erni
cke,
2000
;S
now
,19
9274
km6
11km
211
4.91
820
36.6
3221
Nop
ahR
ange
,R
estin
gS
prin
gR
ange
,S
prin
gM
tns.
deta
chm
ent(
27)
[50]
26km
;25
63
km14
–9M
a5
km/m
.y.
N63
WA
lignm
ent
ofth
etr
ace
ofth
eW
heel
erP
ass
thru
sts
with
resp
ect
toot
her
thru
sts
inth
esy
stem
Wer
mic
keet
al.,
1988
,S
now
and
Wer
nick
e,20
0011
3km
618
km2
116.
1889
236
.271
85
Cot
tonw
ood,
Pan
amin
t,B
lack
Mou
ntai
ns/A
mar
gosa
,cen
tral
Dea
thV
alle
y,E
mig
rant
deta
chm
ents
(28,
30,
32)
[54]
102
km10
4km
67
11–2
(?)
Ma
11.5
km/m
.y.
N67
WT
heor
igin
alex
tent
ofE
agle
Mou
ntai
nF
orm
atio
nm
ust
have
been
with
in20
kmof
sour
cear
eain
sout
hern
Cot
tonw
oods
.
Nei
mie
tal
.,20
0121
7km
619
km2
117.
1595
236
.203
16
Res
ting
Spr
ings
,N
opah
Ran
ges
and
Bla
ck,
Pan
amin
tM
ount
ains
/G
rape
vine
,A
mar
gosa
,ce
ntra
lDea
thV
alle
yar
eade
tach
men
ts(2
7,28
,30
)
105
km10
06
79–
5M
a22
.5km
/m.y
.N
78W
Will
owS
prin
gpl
uton
intr
uded
(10-
12km
)at
11.6
cool
edra
pidl
y(6
-7
Ma)
and
appe
ars
inbo
ulde
rsat
;5
Ma.
(Am
argo
sach
aos)
.A
lignm
ent
ofth
rust
-bel
tst
ruct
ures
Wer
nick
eet
al.,
1988
;S
now
and
Wer
nick
e,20
00;
Hol
met
al.,
1992
;H
olm
and
Dok
ka,
1993
;S
now
,19
92
Kin
gsto
nR
ange
/Kin
gsto
nR
ange
deta
chm
ent(
29)
[38]
6km
6km
13.1
–12
Ma
6km
/m.y
.E
-WD
epos
ition
inth
eS
hado
wV
alle
yB
asin
Fow
ler
and
Cal
zia,
1999
211
5.88
365
35.6
8015
Sou
ther
nB
lack
Mou
ntai
ns/
Kin
gsto
nR
ange
,A
mar
gosa
deta
chm
ents
(30)
[52]
25km
256
3km
12–8
Ma
6.25
km/m
.y.
E-W
Gra
nite
meg
abre
ccia
s(in
the
grea
ter
Am
argo
saba
sin)
poss
ibly
deriv
edfr
oma
disp
lace
dpo
rtio
nof
Kin
gsto
nR
ange
plut
on(w
est
ofba
sin)
.
Sno
wan
dW
erni
cke,
2000
211
6.59
671
36.0
3965
Fun
eral
,G
rape
vine
Mou
ntai
ns/
Poi
ntof
Roc
ksde
tach
men
t(3
1)[6
0]
Und
eter
min
ed;
10km
14–1
0M
a2.
5km
/m.y
.N
45E
Ope
ning
ofth
eex
tens
iona
lBul
lfrog
Bas
inC
emen
etal
.,19
85;
Sno
wan
dLu
x,19
992
116.
8703
036
.687
89
Gra
pevi
neM
ount
ains
,Bar
eM
ount
ain/
Bul
lfrog
deta
chm
ents
(31)
[72,
58]
60km
526
214
–7M
a7.
43km
/m.y
.S
68E
Cor
dille
rafo
ld-t
hrus
tbel
tre
cons
truc
tions
Sno
wan
dW
erni
cke,
2000
,19
892
116.
6792
436
.875
79
Gra
pevi
neM
ount
ains
,Fun
eral
Mou
ntai
ns/B
ound
ary
Can
yon
deta
chm
ent(
31)
[72]
32km
356
2km
9–5
Ma
8.75
km/m
.y.
S37
EC
ordi
llera
fold
-thr
ustb
elt
reco
nstr
uctio
ns,A
FT
,Z
FT
and
sphe
neF
Tco
olin
gag
es
Sno
wan
dW
erni
cke,
2000
,19
89;
Sno
w,
1992
,H
oisc
han
dS
imps
on,
1993
;H
olm
and
Dok
ka,
1991
211
7.21
776
37.1
2056
Nor
ther
nD
eath
Val
ley
206
105–
0M
a2-
6km
/m.y
.E
-WO
ffset
thru
stfa
ult
and
Qua
tern
ary
mar
kers
Reh
eis
1993
,R
ehei
san
dS
awye
r,19
97C
otto
nwoo
dM
ount
ains
/E
mig
rant
faul
tsy
stem
(32)
[70]
22km
226
3km
6–3.
2M
a5.
5km
/m.y
.S
45E
Cor
rela
tion
ofW
hite
Top
back
fold
inC
otto
nwoo
d,F
uner
alM
ount
ains
and
Spe
cter
Ran
ge
Wer
nick
eet
al.,
1988
;S
now
,19
92;
Sno
wan
dLu
x,19
99;
Sno
wan
dW
erni
cke,
2000
211
7.57
841
36.8
7160
Dar
win
Pla
teau
,In
yoM
ount
ains
/Hun
ter
faul
t(3
3)[5
1,66
]
9km
96
14.
8–0.
6M
a2.
25km
/m.y
.S
55E
Cor
rela
tion
ofS
alin
eR
ange
volc
anic
sB
urch
fiele
tal
.,19
8722
6km
619
km2
117.
6613
236
.174
92
Sie
rra
Nev
ada/
Ow
ens
Val
ley
faul
tsy
stem
(34)
[65]
;9
km9
66
0–4
Ma
2.25
km/m
.y.
S60
EE
stim
ate
of15
%6
10%
exte
nsio
nW
erni
cke
etal
.,19
8823
5km
620
km2
119.
8016
337
.885
03
Tot
alex
tens
ion
232
km23
56
20km
N78
8W
Not
e:Lo
ng.—
long
itude
;La
t.—la
titud
e;A
FT
—A
patit
eF
issi
onT
rack
cool
ing
age;
ZF
T—
Zirc
onfis
sion
trac
kag
e;F
T—
Fis
sion
Tra
ck.
Num
bers
inpa
rent
hese
sre
fer
tosp
ecifi
cra
nges
iden
tified
onF
ig.
2;nu
mbe
rsin
brac
kets
refe
rto
the
RA
NG
EpI
Dnu
mbe
rfo
rin
divi
dual
rang
esin
the
Arc
GIS
shap
efil
esan
dth
eM
ovem
entT
able
(Arc
GIS
files
,M
ovem
ent
Tab
le[
see
foot
note
1]).
The
cum
ulat
ive
erro
rre
port
edin
colu
mn
9eq
uals
the
squa
rero
otof
the
sum
ofth
esq
uare
sfo
rin
divi
dual
vect
ors.
Geosphere, December 2005 155
ANIMATED TECTONIC RECONSTRUCTIONT
AB
LE3.
DA
TA
US
ED
FO
RR
ES
TO
RIN
GE
XT
EN
SIO
NIN
SO
UT
HE
RN
BA
SIN
AN
DR
AN
GE
Ran
ge/fa
ult
Hor
izon
tal
disp
lace
men
tof
rang
ebl
ock
(mod
elpo
lygo
n)
Hor
izon
tal
com
pone
ntof
faul
tsl
ip(d
ata)
Tim
ing
ofsl
ipR
ate
ofde
form
atio
n(h
oriz
onta
lcom
pone
ntof
faul
tsl
ip/d
urat
ion)
Dire
ctio
nof
mot
ion
Dat
aus
edR
efer
ence
sLo
ng.
(x)
Lat.
(y)
McC
ullo
ugh
Ran
geto
Col
orad
oP
late
au(3
5)[3
7]80
km50
-80
km14
–16
Ma
(24)
8km
/m.y
.S
75W
Res
tora
tion
ofst
rike
slip
faul
tof
fset
s,til
ted
volc
anic
rock
s,po
rphy
ryco
pper
empl
acem
ent
dept
hs
Spe
ncer
and
Rey
nold
s,19
89;
John
and
Fos
ter,
1993
211
5.34
595
35.1
7415
Bla
ckM
ount
ains
(36)
[46]
36km
(tot
al)
.25
km15
–13.
4M
a2.
5km
/m.y
.N
70E
Tilt
edvo
lcan
icro
cks.
Fos
ter
etal
.,19
93;
Spe
ncer
etal
.,19
95;
Spe
ncer
and
Rey
nold
s,19
89
211
4.49
030
35.4
7688
Sac
ram
ento
,Che
meh
uevi
Eld
orad
oM
ount
ains
(37)
[44]
36km
.25
–30
km21
–15
Ma
5km
/m.y
.S
60W
Ar/
Ar,
AF
Tco
olin
gag
esJo
hnan
dF
oste
r,19
932
114.
7685
435
.311
17
Buc
kski
n,R
awhi
deM
ount
ains
(38)
[629
]66
km66
km6
8km
27–1
3M
a4.
7km
/m.y
.S
57W
Tim
ing
indi
cate
dby
tilte
dvo
lcan
icst
rata
,m
agni
tude
dete
rmin
edby
offs
etne
cess
ary
toex
pose
low
erpl
ate
Sco
ttet
al.,
1998
;F
oste
ret
al.,
1993
;S
penc
eret
al.,
1995
;S
penc
eran
dR
eyno
lds,
1991
211
3.74
331
34.0
9275
Cat
alin
a,R
inco
nM
ount
ains
(39)
[648
]28
km20
–30
km,
coul
dbe
upto
40km
27–2
0M
a4
km/m
.y.
S60
WC
orre
latio
nof
Pre
cam
bria
ngr
anite
and
pre-
mid
Ter
tiary
thru
stin
gD
avy
etal
.19
89;
Dic
kins
on,
1991
;F
ayon
etal
.,20
002
110.
7158
432
.350
01
Har
quah
ala,
Har
cuva
rM
ount
ains
(40)
[657
]66
km67
km6
17km
26–1
4M
a5.
6km
/m.y
.S
57W
Dis
plac
edbr
ecci
as(5
5km
)pl
usad
ditio
nale
xten
sion
invo
lcan
ican
dP
reca
mbr
ian
rock
s(1
26
7)
Ric
hard
etal
.,19
90;
Spe
ncer
and
Rey
nold
s,19
91.
211
3.51
995
33.8
6443
Pin
alen
oM
ount
ains
(41)
[647
]22
km20
–30?
km29
–19
Ma
2.5
km/m
.y.
S60
WA
r/A
rco
olin
gag
es,
appr
oxim
ate
amou
ntof
disp
lace
men
tne
cess
ary
toex
pose
low
erpl
ate
Long
etal
.,19
952
110.
0116
332
.772
88
Sou
thM
ount
ain
(43)
[653
,654
]50
km25
-19
Ma
S60
WR
eyno
lds,
1985
211
1.60
311
33.1
3175
Whi
pple
Mou
ntai
n(4
4)[6
65]
63km
71km
619
km22
–16
Ma
11.8
km/m
.y.
S57
WO
ffset
ofdi
kesw
arm
,til
ted
sedi
men
tary
stra
taD
avis
and
List
er,
1988
;S
penc
eran
dR
eyno
lds,
1991
211
4.10
627
34.3
1624
Rio
Gra
ndex
tens
ion
(45)
7km
6km
12–1
8M
a.8
75km
/m.y
.38
8Npa
linsp
astic
rest
orat
ion
ofse
ism
icco
ntro
lled
cros
sse
ctio
n,an
dgr
owth
stra
tain
basi
n
Cha
pin
and
Cat
her,
1994
;In
gers
oll,
2001
;R
usse
llan
dS
nels
on,
1994
13km
10km
12–1
8M
a1.
625
km/m
.y.
368N
Cha
pin
and
Cat
her,
1994
;In
gers
oll,
2001
;R
usse
llan
dS
nels
on,
1994
17km
17km
12–1
8M
a2
km/m
.y.
358N
Cha
pin
and
Cat
her,
1994
;In
gers
oll,
2001
;R
usse
llan
dS
nels
on,
1994
Not
e:Lo
ng.—
long
itude
;La
t.—la
titud
e;A
FT
—A
patit
eF
issi
onT
rack
cool
ing
age.
Num
bers
inpa
rent
hese
sre
fer
tosp
ecifi
cra
nges
iden
tified
onF
ig.
2;nu
mbe
rsin
brac
kets
refe
rto
the
RA
NG
EpI
Dnu
mbe
rfo
rin
divi
dual
rang
esin
the
Arc
GIS
shap
efil
esan
dth
eM
ovem
ent
Tab
le(A
rcG
ISfil
es,
Mov
emen
tT
able
[see
foot
note
1]).
Malin, 1992; Walker et al., 1995; Glazner etal., 2002).
Following this early phase of extensionaldeformation, a system of right- and left-lateralstrike-slip faults similar to those active todaywas established, with right-lateral shear alonga series of northwest-striking faults predomi-nant (Fig. 5). The total accumulated shearacross the Mojave, as documented by fieldstudies, is 53 km 6 6 km (Table 4). The tim-ing of right-lateral shear is not well con-strained. Motion on the faults is inferred to bepost–10 Ma based on strain compatibility withdeformation directly north and south (Tables2 and 5).
Vertical Axis Rotations East of the SanAndreas Fault
There are two zones of vertical-axis rotationeast of the San Andreas fault: the EasternTransverse Ranges located immediately southof the Mojave block and the northeastern Mo-jave rotational block (Carter et al., 1987;Schermer et al., 1996; Dickinson, 1996) (Fig.5 and Table 4).
The Eastern Transverse Ranges include aseries of structural panels separated by east-west–oriented, left-lateral faults (Dickinson,1996). Paleomagnetic studies show that 10 62 Ma rocks within this zone record the entire458 rotation (Carter et al., 1987), while 4.5 Mavolcanic rocks are unrotated (Richard, 1993).These constraints imply that all of the rotationand most of the right-lateral strike-slip motionin the Mojave region immediately to the northare ca. 10 Ma and younger.
The northeastern corner of the Mojave re-gion is another area of pronounced clockwiserotation. Schermer et al. (1996) proposed thatthe northeastern Mojave underwent 238 of ro-tation accompanied by 5 km of left-lateral slipon faults within the rotating region and 158 of‘‘rigid body’’ rotation. Total right-lateral shearpredicted by this model is 33 km.
San Andreas System and Areas to theWest
Deformation west of the San Andreas faultis defined by four first-order constraints (Fig.6 and Table 6). The first is motion on the SanAndreas fault itself, which is tightly con-strained in central California at 315 km 6 10km by restoring the Pinnacles volcanics westof the fault to the Neenach volcanics to theeast of it (Matthews, 1976; Graham et al.,1989; Dickinson, 1996). The offset volcanicswere extruded from 22 Ma to 24 Ma, but ten-tatively correlative late Miocene strata (7–8
156 Geosphere, December 2005
N. MCQUARRIE and B.P. WERNICKET
AB
LE4.
DA
TA
US
ED
FO
RR
ES
TO
RIN
GS
TR
IKE
-SLI
PD
ISP
LAC
EM
EN
TS
INT
HE
MO
JAV
ER
EG
ION
Fau
ltH
oriz
onta
ldis
plac
emen
tof
rang
ebl
ock
(mod
elpo
lygo
n)H
oriz
onta
lcom
pone
ntof
faul
tsl
ip(d
ata)
Tim
ing
ofsl
ipD
ata
used
(offs
etfe
atur
es)
Ref
eren
ces
Cum
ulat
ive
disp
lace
men
tof
right
-late
ralf
ault
slip
data
(6cu
mul
ativ
eer
ror)
Cen
tral
Moj
ave
(41)
63km
40–5
0km
24-1
8M
aO
ffset
feat
ures
that
incl
ude
Jura
ssic
dike
s,in
trus
ive
rock
s,an
dsh
elf
tooc
ean
faci
estr
ansi
tion
Gla
zner
etal
.,19
89;
Mar
tinet
al.,
1993
;F
letc
her
etal
.,19
95;
Inge
rsol
let
al.,
1996
Azt
ecM
ines
Was
hfa
ult
8km
8km
LLpo
stlo
wer
Mio
cene
Intr
usiv
eco
ntac
tbe
twee
nS
anG
abrie
lter
rane
and
Cre
tace
ous
plut
on
Pow
ell,
1981
Blu
eC
utfa
ult
4km
6-9–
9km
Ant
iform
ingn
eiss
icfo
liatio
nH
ope,
1966
Bul
lion/
Rod
man
/Pis
gah
faul
t13
km6.
4–14
.4km
RL
Kan
esp
rings
faul
tD
okka
,19
8310
.4km
64
kmC
hiria
sco
faul
t7–
16km
11km
LLD
acite
dike
,an
dst
eepl
ydi
ppin
gR
edC
loud
thru
stfa
ult
Pow
ell,
1981
Chu
ckw
alla
Val
ley
basi
n31
km(R
L)gr
avity
low
Gra
vity
data
indi
cate
com
plex
basi
nst
ruct
ure
Rot
stei
net
al.,
1976
;R
icha
rd,
1993
Cib
ola
faul
t7
km7
kmR
LW
est
dipp
ing
norm
alfa
ults
,an
dea
stdi
ppin
gco
ntac
tbe
twee
nla
vas
Ric
hard
etal
.,19
92
For
dLa
keN
orth
basi
n5
km3.
5km
RL
Bas
eof
McC
oyM
ount
ains
form
atio
nS
tone
and
Pel
ka,
1989
Indi
anW
ash
Bas
in7
km7
kmR
LS
trat
igra
phic
sepa
ratio
nR
icha
rd,
1993
Iron
Mou
ntai
nsfa
ult
5km
5.5
kmR
LR
ock
units
(uns
peci
fied)
How
ard
and
Mill
er,
1992
Lagu
nafa
ult
syst
em36
km2
kmR
LS
lipes
timat
edby
estim
atin
gex
tens
ion
nece
ssar
yfo
r20
8di
pin
cong
lom
erat
e
Ric
hard
,19
93
Mam
mot
hW
ash
faul
t;
12km
;9
kmLL
Inte
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ents
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ojav
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kmR
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aleo
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netic
rota
tions
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erm
er,
1996
;M
iller
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nt,
2002
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alsh
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inth
eM
ojav
e99
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e:R
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cate
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ectiv
ely.
Geosphere, December 2005 157
ANIMATED TECTONIC RECONSTRUCTIONT
AB
LE5.
DA
TA
US
ED
FO
RR
ES
TO
RIN
GV
ER
TIC
AL
AX
ISR
OT
AT
ION
INR
EC
ON
ST
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CT
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nsR
otat
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odel
Mea
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dro
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omda
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geR
ate
Dire
ctio
nR
efer
ence
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ng.
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orad
oP
late
au(4
5)[7
4]1.
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Ma
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cwP
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netis
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xten
sion
dete
rmin
edth
roug
hcr
oss
sect
ion
rest
orat
ion
Cha
pin
and
Cat
her,
1994
211
0.14
435
37.2
6894
Eas
tern
Tra
nsve
rse
Ran
ges
(46)
[90–
99]
458
41.4
86
7.7
afte
r10
.4M
acw
Pal
eom
agne
ticda
taC
arte
ret
al.,
1987
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thea
ster
nM
ojav
e(4
7)[m
ojav
e6]
368–
388
238
loca
lly63
8af
ter
10.4
Ma
cwP
aleo
mag
netic
data
Sch
erm
eret
al.,
1996
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ssic
dike
sin
NE
Moj
ave
(47)
40–5
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Ron
and
Nur
,19
96M
ojav
ero
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ns(4
7)30
821
–48
Ma
cwP
aleo
mag
netic
data
Dok
kaan
dT
ravi
s,19
90;
Dok
kaet
al.,
1998
Cot
tonw
ood
and
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eral
Mtn
s.(3
1,32
)[7
0,60
]40
8m
idM
ioce
neor
late
rcw
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eom
agne
ticda
taS
now
and
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1999
;S
now
and
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nick
e,20
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117.
5784
136
.871
60
Bla
ckM
ount
ains
(30)
[52]
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ter
mid
Mio
cene
plut
onP
aleo
mag
netic
data
Hol
met
al.,
1993
211
6.59
671
36.0
3965
Gra
pevi
neM
ount
ains
(31)
[72]
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8cc
wP
aleo
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data
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mi,
2002
;S
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nick
e,20
002
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ain
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[58]
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ater
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ippe
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ills
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een
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etal
.,19
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son
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.,19
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mi,
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211
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ther
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rLa
ne(1
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)[3
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netic
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onvo
lcan
icro
cks
Cas
hman
and
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tain
e,20
00
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rra
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ada
(34)
[65]
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cw)
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eom
agne
ticda
taF
reie
tal
.,19
84;
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i,19
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enan
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eick
ert,
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ilder
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ulty
,19
99
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tern
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nsve
rse
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ges
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1178
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1108
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cwP
aleo
mag
netic
data
Hor
nafiu
set
al.,
1986
;Lu
yend
yk,
1991
Not
e:T
able
5is
anin
com
plet
elis
ting
ofan
exte
nsiv
epa
leom
agne
ticda
tase
tfo
rw
este
rnN
orth
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eric
a.Lo
ng.—
long
itude
;La
t.—la
titud
e;cw
—cl
ockw
ise
rota
tions
;ccw
—co
unte
rclo
ckw
ise
rota
tions
.Num
bers
inpa
rent
hese
sre
fer
tosp
ecifi
cra
nges
iden
tified
onF
ig2;
num
bers
inbr
acke
tsre
fer
toth
eR
AN
GE
pID
num
ber
for
indi
vidu
alra
nges
inth
eA
rcG
ISsh
ape
files
and
the
Mov
emen
tTab
le(A
rcG
ISfil
es,M
ovem
entT
able
[see
foot
note
1]).
Ma) are apparently offset 255 km (Graham etal., 1989; Dickinson, 1996).
The second constraint is the ;1108 clock-wise rotation of major fault-bounded blocks inthe western Transverse Ranges (Hornafius etal., 1986; Luyendyk, 1991). Because of thelength and structural integrity of these blocks(in particular, the Santa Ynez Mountains), thisrotation requires a coast-parallel displacementof ;270 km (Hornafius et al., 1986).
The shear and rotation of these blocks areconfirmed by the third major constraint, re-construction of now-scattered outcrops of thedistinctive Eocene Poway Group. Exposuresalong the Channel Islands were rifted awayfrom counterparts in southernmost California,which are in turn offset from their source areain northern Sonora, Mexico, by the southernSan Andreas fault system (Abbott and Smith,1989). Rifting and rotation of the westernTransverse Ranges away from the PeninsularRanges formed the strongly attenuated crustof the Continental Borderlands on their trail-ing edge. The magnitude of this extension isproposed to be ;250 km based on seismicreflection data delineating the geometry of ex-tensional fault systems and correlation of‘‘mega key beds’’ or lithotectonic belts (fore-arc basin sediment, Franciscan subductioncomplex) (Crouch and Suppe, 1993; Bohan-non and Geist, 1998).
The final first-order constraint is the open-ing of the Gulf of California. Although offsetof the Poway Group suggests roughly 250 kmof displacement, recognition of correlative py-roclastic flows on Isla Tiburon and near Puer-tecitos on the Baja Peninsula dated at 12.6 Maand 6.3 Ma constrains the full transfer of BajaCalifornia to the Pacific plate to have occurredno earlier than ca. 6 Ma, with 255 km 6 10km of displacement along the plate boundarysince then (Oskin et al., 2001). Including ad-ditional deformation of the adjacent continen-tal margins increases the magnitude of dis-placement to as much as 276 km 6 10 km(Oskin and Stock, 2003).
WESTERN NORTH AMERICAANIMATION
The western North America animation (An-imation 1) combines 13 individual paleogeo-graphic maps (Figs. 7–9) (Appendix 1, paleo-geographic maps [see footnote 1])2 generatedby ArcGIS into a digital animation illustratinga model of how extension and right lateralshear evolved in the region. The color scheme
2If you are reading this offline, please visitwww.gsajournals.org to view Animation 1.
158 Geosphere, December 2005
N. MCQUARRIE and B.P. WERNICKE
Figure 6. A: First-order constraints for displacement along the San Andreas fault andassociated displacements to the west. Arrows indicate approximate magnitude and direc-tion of individual relative displacements between polygons. Black numbers indicate hor-izontal displacement amount, bold red numbers indicate age range of displacement. Stip-pled areas labeled EPG show distribution of Eocene Poway Group and equivalents.Stippled area labeled SEPG marks the location of the source area for the Eocene PowayGroup. Triangle pattern (Gulf of California), and stars (central California) show distri-bution of correlative volcanic units offset by the San Andreas–Gulf of California riftsystem. MTJ is modern location of the Mendocino triple junction. Data from Abbott andSmith, 1989; Bohannon and Geist, 1998; Crouch and Suppe, 1993; Dickinson and Wer-nicke, 1997; Dickinson, 1996; Graham et al., 1989; Hornafius et al., 1986; Luyendyk, 1991;Matthews, 1979; Oskin et al., 2001. B: Successive locations of the eastern edge of Pacificplate oceanic lithosphere relative to stable North America. The thick colored lines rep-resent minimum extent of oceanic lithosphere at the times shown (from Atwater and Stock,1998). These positions constrain the maximum westward extent of continental NorthAmerica through time.
for the animation includes yellow, orange, andred polygons on a white background. Thepolygon shape reflects the modern bedrock-alluvium contact, fault-bounded crustal blocksor the physiographic boundaries of large, in-tact crustal blocks. Yellow polygons indicateareas where there are no data for how the re-gion is deforming. Orange polygons (ranges)are ranges whose motion is directly con-strained by kinematic data. These polygonsturn red during the time period of motion (i.e.,Baja California turns red from 6 to 0 Ma as itseparates from North America and movesnorthward on the Pacific Plate). A notable ex-ception to this is the Colorado Plateau–Rio
Grand rift area. Neither the Colorado Plateau,nor the Rio Grande rift turn red during rota-tion and extension even though there are datathat describe this deformation (Table 5). Thespace created (additional white space betweenthe colored polygons) as the movie progressesin time indicate areas of extension. The re-moval of white space (as polygons move clos-er together) indicates areas of compression.The thick blue line on the left of the animationrepresents successive locations of the easternedge of Pacific plate oceanic lithosphere rel-ative to stable North America at the time pe-riod annotated on the upper left edge of theline (from Atwater and Stock, 1998). The po-
sition of this line constrains the maximumwestward extent of continental North Americaat the time indicated because it shows the min-imum east limit of extant oceanic crust. De-tails of the reconstruction can be seen by mov-ing the slider bar on the animation. To moveback and forth over a narrow window of time,just hold the mouse key down over the trian-gle on the slider bar and move it back andforth over the time window of interest.
DISCUSSION
The exercise of developing a self-consistent,strain-compatible model has raised a numberof issues that are difficult to resolve satisfac-torily in the reconstruction and require furtherinvestigation. The most apparent (amongmany!) are (1) the need for middle to lateMiocene right-lateral shear in the eastern Mo-jave region to make room for the northerlymotion of the Sierra Nevada determined fromthe central and northern Basin and Range re-construction paths; (2) the need for largeamounts of relatively young extension innorthern Mexico both east and west of the Si-erra Madre Occidental to reconcile core com-plex extension in Arizona and the late Miocene–Pliocene opening of the Gulf of California; (3)the apparent rotational history of the SierraNevada–Great Valley block; and (4) generallylarge amounts of Miocene-Pliocene shorteningand extension in the Transverse Ranges, CoastRanges, and Borderlands provinces, whicharise from the need to reconcile San Andreasoffset with the position of oceanic crust off-shore, differences in the age of extensionnorth and south of the Garlock fault, and largeclockwise rotation of the Santa Ynez Moun-tains block (Animation 1).
Eastern Mojave Region
The eastern California shear zone–WalkerLane belt is an ;120-km-wide zone of right-lateral, intraplate shear east of the Sierra Ne-vada and San Andreas fault. Geodetically thisshear zone accommodates up to 25% of thePacific–North America relative plate motion(Bennett et al., 2003; McClusky et al., 2001;Miller et al., 2001; Sauber et al., 1994). Geo-logic estimates of displacements vary alongthe north-south extent of the eastern Californiashear zone. Proposed net displacement alongthe eastern California shear zone (oriented;N208W) varies from 65 km in the Mojaveregion (Dokka and Travis, 1990) to 133 km inthe central Basin and Range (Snow and Wer-nicke, 2000; Wernicke et al., 1988). In thenorthern Walker Lane region, shear estimates
Geosphere, December 2005 159
ANIMATED TECTONIC RECONSTRUCTIONT
AB
LE6.
DA
TA
US
ED
FO
RR
ES
TO
RIN
GH
OR
IZO
NT
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gon)
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ate
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urat
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aus
edR
efer
ence
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reas
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ion
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–16
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r
N30
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ffset
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ene
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ogic
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atur
es,
early
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anic
and
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men
tary
rock
san
dM
ioce
nese
dim
enta
rybr
ecci
as
Atw
ater
and
Sto
ck,
1998
;D
icki
nson
and
Wer
nick
e,19
97;
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kens
on,
1996
;G
raha
met
al.,
1989
;S
ieh
and
Jahn
s,19
84;
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thew
s,19
79B
aja/
Isla
deT
ibro
n(4
9)(5
1)[8
5][1
09]
300
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.,20
01;
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,20
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lade
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ron/
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ora
(51)
[109
][1
70]
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tern
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nsve
rse
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ges
(50)
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e-12
Ma
N50
WC
orre
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ioce
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stra
taO
skin
and
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ck,
2003
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ene
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ayG
roup
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ces
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Mig
uel,
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taR
osa,
and
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taC
ruz
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inct
ive
volc
anic
clas
tsw
ithso
urce
area
inS
onor
aM
exic
o
Abb
ott
and
Sm
ith,
1989
Bor
derla
ndex
tens
ion
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kmat
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km;
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a14
km/m
.y.
;N
40W
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smic
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ctio
nda
ta,
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elat
ion
of‘‘m
ega
keyb
eds’
’
Cro
uch
and
Sup
pe,
1993
;B
ohan
non
and
Gei
st,
1998
Not
e:N
umbe
rsin
pare
nthe
ses
refe
rto
spec
ific
rang
esid
entifi
edon
Fig
.2,
num
bers
inbr
acke
tsre
fer
toth
eR
AN
GE
pID
num
ber
for
indi
vidu
alra
nges
inth
eA
rcG
ISsh
ape
files
and
the
Mov
emen
tTab
le(A
rcG
ISfil
es,
Mov
emen
tT
able
[see
foot
note
1]).
range from 20 km to 54 km (Faulds et al.,2005; Hardyman et al., 1984), plus an addi-tional component of northwest-directed exten-sion due to a change in extension direction inthe northern Basin and Range from east-westin the east to northwest-southeast in the west.
One of the goals of this study was to de-velop a kinematically consistent model of theeastern California shear zone that fits withinthe errors provided by both local and regionalstudies. We found 100 km 6 10 km right-lateral shear oriented N258W was compatiblewith data in both the northern and central Ba-sin and Range (Animation 1). In the Mojaveregion of the eastern California shear zone,however, available data suggest no more than53 km 6 6 km of right-lateral shear orientedN258W, about half of what is required to thenorth. Kinematic compatibility with the mag-nitude of deformation north of the Garlockfault requires ;100 km of right-lateral shearthough the Mojave region, with the majorityof additional shear located on the eastern edgeof the shear zone during its early (12–6 Ma)history (Figs. 5,7,8, Animation 1). The 27 kmand 45 km of right-lateral offset along theBristol Mountains–Granite Mountain andsouthern Death Valley fault zones is signifi-cantly greater than previous estimates (0–10km and 20 km, respectively), but solid pierc-ing points that limit the net offset are scarceand debatable (Howard and Miller, 1992;Dokka and Travis, 1990; Davis, 1977). The;30 km of displacement along the easternedge of the Mojave must be transferred south-ward along the Sheep Hole fault to the LagunaFault system of Richard (1993). The 36 km ofmodel offset is significantly greater than the 2km of right-lateral offset proposed by Richard(1993) (Table 4, Fig. 5). Additional faults withsignificantly greater offsets than that docu-mented by geology are the 11–13 km modeloffsets on the Camp Rock, Gravel Hills, andHarper Lake fault systems, where current es-timates suggest no more than 3 km of offseton any of these faults (Dibblee, 1964; Oskinand Iriondo, 2004; M. Strane, 2005, personalcommun.). The difference between the modeland data requires that the slip discrepancymust be taken up on other faults (most likelyto the east) in the Mojave shear system. Al-though the details concerning both timing anddistribution of shear within the eastern Cali-fornia shear zone will continue to evolve withtime, the strength of the central Basin andRange offsets combined with kinematic com-patibility constraints require reevaluation ofgeologic evidence for total magnitude of right-lateral shear through the Mojave. Therefore,we have modeled many of the faults in the
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Figure 7. Reconstructed paleogeographic maps (from 0–6 Ma) used in the reconstruction. The color scheme is the same as that for theanimation, yellow polygons on a white background. Yellow polygons indicate areas where there are no data for how the region isdeforming. Polygons (ranges) that have data associated with their motion are orange when associated faulting is inactive and red duringfault activity. Panels represent different time slices: (a) 0 Ma, (b) 2 Ma, (c) 4 Ma, and (d) 6 Ma.
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Figure 8. Reconstructed paleogeographic maps (from 8 to 14 Ma) used in the reconstruction. The color scheme is the same as Figure7. Panels represent different time slices: (a) 8 Ma, (b) 10 Ma, (c) 12 Ma, and (d) 14 Ma.
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Figure 9. Reconstructed paleogeographic maps (from 16 to 36 Ma) used in the reconstruction. The color scheme is the same as Figure7. Panels represent different time slices: (a) 16 Ma, (b) 18 Ma, (c) 24 Ma, (d) 30 Ma, and (e) 36 Ma.
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Figure 9. (continued).
Mojave with greater net offset than suggestedby offset markers. From the model slipamounts shown on Figure 5, we obtain 100km of right-lateral shear oriented N258Wsince 12 Ma, at a long-term rate of 8.3 mm/yr 6 1 mm/yr. We suggest that the discrep-ancy may be due to penetrative shear in thelargely granitic crust between the strike-slipfaults (e.g., Miller and Yount, 2002).
Increasing right-lateral shear in the easternMojave has implications on how shear is dis-tributed along the entire plate boundary. Sincemuch of the additional shear predates theopening of the Gulf of California (Oskin etal., 2001), it implies ;50 km of dextral shearbetween 6 and 12 Ma in the Sonora region.The total magnitude of separation of the BajaPeninsula from Sonora predicted from themodel is 350 km, 300 km of which is post 6Ma. This is slightly greater than the 296 6 20km, 276 6 10 km of which is post 6 Ma mea-sured by Oskin and Stock (2003), but still sig-nificantly less than the 450 km total continen-tal separation proposed by Fletcher (2003).
Dokka and Travis (1990) proposed that theeastern California shear zone accommodated9%–14% of total predicted relative motion be-tween plates if shear initiated at 10 Ma. Themodel of eastern California shear zone defor-mation that we propose here (Animation 1)suggests that eastern California shear zone de-
formation is ;28% of San Andreas motionaveraged since 12 Ma and 15% of total platemotion since 16 Ma.
Arizona–Mexican Basin and Range
The geographic region that has the fewestlocal kinematic constraints is Sonora–ChihuahuaMexico. However, the kinematics of Baja Cal-ifornia, based on plate tectonic reconstructions(Atwater and Stock, 1998), is an especiallypowerful constraint on intraplate deformationin this region. The constraint arises from thesimple fact that oceanic and continental lith-osphere cannot occupy the same surface areaat the same time (Atwater and Stock, 1998)(Fig. 6B). The plate tectonic constraint sug-gests ;330 km 6 50 km of extension between6 Ma and 24 Ma, because after restoring theoffset across the Gulf of California (Oskin etal., 2001; Oskin and Stock, 2003), this is thetotal overlap between continent and ocean. Inconcert with strong northeast-southwest exten-sion in Arizona, we suggest similar magni-tudes of extension (44 km and 86 km) oc-curred from 16 Ma to 24 Ma and was orientedN508E–N608E (making room for the brownand green curves in Fig. 6B). We show anoth-er pulse from 12 Ma to 8 Ma orientedN658W–N788W, reflecting the growing influ-ence of the Pacific plate’s northerly motion on
intraplate deformation, as appears to be thecase to the north (Animation 1).
Restoring 330 km of extension, however,particularly the northwesterly extension in thewindow of time from 16 Ma to 8 Ma, opensup a large northeast-trending gap in southernArizona and northern Sonora. This gap is aresult of differences in both magnitude andtiming of extension between southern Arizonaand northern Sonora and suggests (incorrect-ly) that there is ;60 km of NW-SE compres-sion between 16 and 8 Ma (Figs. 8–9). Largemagnitude core complex extension in southernArizona initiates at ca. 28 Ma and wanes from16 Ma to 14 Ma (Table 3). Significant exten-sion in Sonora occurs over a similar timerange (Nourse et al., 1994; Gans, 1997). Atca. 12 Ma, however, significant extension isrecorded in both the Gulf extensional provincewest of the Sierra Madre Occidental (e.g.,Stock and Hodges, 1989; Henry, 1989; Lee etal., 1996, Gans et al., 2003) and east of theSierra Madre Occidental (Henry and Aranda-Gomez, 2000), while only minimal magni-tudes of east-west extension are recorded insouthern Arizona. This problem is similar tothat arising from the difference in timing ofextension north of the Colorado River exten-sional corridor between the Mojave Desertand central Basin and Range. Here, the Gar-lock fault accommodates different amounts ofextension, not only from 10 Ma to the present(Davis and Burchfiel, 1973), but potentiallythroughout the history of extension in the re-gion (24–0 Ma). Although the difference intiming and magnitude of extension betweenthe Mojave region and the southern ArizonaBasin and Range versus the Mexican Basinand Range in Sonora and Chihuahua isgenerally recognized (e.g., Henry and Aranda-Gomez, 2000; Dickinson, 2002), the geometryand genetic relationship of the transfer systemthat must separate them is problematic.
In the model presented here, the amount ofextension in the Mexican Basin and Range ispartitioned between the extending regions east(;134 km) and west (;180 km) of the un-strained Sierra Madre Occidental block. Al-though both regions display numerous exten-sional structures, the exact magnitude ofextension is unknown. Because of the differ-ence in post–16 Ma extension in Chihuahuaand the Rio Grande rift (90 and 20 km, re-spectively) after ca. 16 Ma, the model includesa zone of right-lateral shear that extendsthrough southeastern Arizona between the twoprovinces (Animation 1). The existence of thisshear zone is unlikely, leaving two possiblesolutions. The first is that extension system-atically increases from the Rio Grande rift to
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Chihuahua Mexico due to clockwise rotationof the Sierra Madre Occidental (rotationwould need to be greater than the 1.58 rota-tional opening of the Rio Grande rift). Anoth-er solution would be partitioning a muchgreater magnitude of extension in the Gulf ex-tensional province (;270–300 km), but this isthus far not supported by mapping in the re-gion (Henry and Aranda-Gomez, 2000). Mostlikely some combination of these factors isnecessary to match the first-order geologicconstraints of the region.
Sierra Nevada–Great Valley BlockRotation
The reconstruction presented here showssignificant extension in the northern Basin andRange between 36 Ma and 24 Ma, with es-sentially no extension occurring over this timeperiod in the central Basin and Range. To ac-commodate this difference, the Sierra Nevada–Great Valley Block must rotate or deform in-ternally. We propose that the block behavesfairly rigidly and rotates counterclockwise(Animation 1). After initiation of extension inthe central Basin and Range at ;16 Ma, theSierra Nevada–Great Valley Block rotatesclockwise for a final net rotation of 28 (Table5, Appendix 1, Movement Table [see footnote1]). The animation shows the early 36–24 Marotation accommodated by ;35 km of dextralshear along the proto–Garlock fault and ac-companying compression in the southeasternSierra Nevada region (Animation 1). The ac-tual effects of this rigid body rotation on thedeformation of surrounding regions (particu-larly to the north and south) are highly depen-dent on the axis of rotation and how rigidlythe block behaved, both of which are un-known. The rotation of the Tehachapi Moun-tains may include this early counterclockwiserotation of the Sierras, as well as potentiallybeing linked to southern Basin and Rangecore-complex formation, which immediatelyfollowed (McWilliams and Li, 1985; Plesciaand Calderone, 1986; Walker et al., 1995;Glazner et al., 2002).
Areas West of the San Andreas Fault
Based on the timing and magnitude of dis-placement on a few fault systems (San An-dreas, northern Gulf of California, Mojave,central Basin and Range, and the Santa YnezMountains), continental basins must open(creation of white spaces in the movie [Ani-mation 1, Figs. 7–9] suggesting pulses of ex-tension) and close (closing of spaces or over-lap of polygons suggesting pulses of
contraction) from 24 Ma to 0 Ma. Even at thislarge and relatively simplified scale, extensionand contraction are spatially and temporallycomplex throughout the region west of theSan Andreas fault, and we expect even greatercomplexities in timing and magnitude at amore detailed level. The following discussionhighlights the magnitudes of displacement andsummarizes data that either support or conflictwith the model displacements.
Transverse RangesThe clockwise rotation of the Western
Transverse Ranges (Hornafius et al., 1986; Lu-yendyk, 1991) suggests regions of extensionand subsequent compression both north andsouth of the rotating Santa Ynez Mountainsblock (Fig. 2, range 50) (Animation 1). Themagnitude of predicted extension (Fig. 8) andcontraction (Fig. 7) (oriented ;north-south) isas great as 130 km to the north of the westernside of the block from ca. 12 Ma to the pres-ent. Motion of Baja California northward from6 Ma to the present suggests as much as 90km of shortening in the southern TransverseRanges (Santa Ynez and San Gabriel Moun-tains blocks) (Fig. 7, Animation 1). Transpres-sive motion involving the San Gabriel Moun-tains, San Bernardino Mountains, and Mojaveblocks implies ;40 km of north-south short-ening immediately north of the PeninsularRanges block. Balanced cross sectionsthrough the San Emigdio, Santa Ynez, andSan Gabriel Ranges indicate 53 km of short-ening since 3 Ma (Namson and Davis, 1988a).Although the shortening estimate is stronglydependent on the details of how the SantaYnez and Peninsular Ranges–Baja Californiablocks move, the reconstruction presentedhere suggests ;60 km of north-south short-ening at the longitude of the eastern SantaYnez Mountains block since 6 Ma. As sug-gested by Namson and Davis (1988a), short-ening of this magnitude in the upper mantlelithosphere is supported by a large volume ofhigh-velocity material imaged tomographical-ly beneath the region (e.g., Humphreys et al.,1984).
Coast RangesDifferences in the timing of extension with-
in the Mojave and Basin and Range north andsouth of the Garlock fault, in conjunction withplate tectonic constraints on the westernmostlimit of the North America continental edge(Atwater and Stock, 1998), indicate a periodof extension (20–16 Ma) and subsequent com-pression (14–0 Ma) to the west of the Sierra–Great Valley block (Figs. 7–9, Animation 1).Approximately 80 km of core-complex exten-
sion south of the Garlock fault occurred priorto significant extension in the central Basinand Range. In order to maintain a quasilinearocean-continent boundary, a zone of extensionroughly equal in magnitude to the core-complexextension is required north of the Garlockfault and west of the Sierra Nevada–Great Val-ley block. This becomes most visible in thereconstruction at 16 Ma (Fig. 9A). As exten-sion evolves in the central Basin and Range,this same zone undergoes contraction to main-tain the quasilinear plate boundary suggestedby the extant distribution of oceanic crustfrom 16 Ma to the present.
The Neogene tectonic and volcanic historyfrom the Great Valley to the edge of the con-tinent is broadly consistent with the model(data summarized in Tennyson, 1989). Al-though the model and geologic data are diffi-cult to compare quantitatively because thereare no obvious normal faults with measurableoffsets, the magnitude of extension (and sub-sequent compression) is significantly less thanthat predicted by the model. Development oflocal nonmarine basins and eroded highs, fol-lowed by significant subsidence at ;16–18Ma and the development of the relatively deepmarine Monterey basin strongly suggests anextensional event. Rotation of the TehachapiMountains and/or extension in the southernSan Joaquin Valley (McWilliams and Li,1985; Plescia and Calderone, 1986; Tennyson,1989; Goodman and Malin, 1992) may be in-dicative of this extension but may representfar less than the ;80 km predicted by themodel.
The subsequent compression in the CoastRanges is more quantifiable and appears to besignificantly less than that suggested by themodel. Estimates of compression in the CoastRanges east and west of the San Andreas faultrange from 20 km to 48 km (Page et al., 1998;Namson and Davis, 1990, 1988b), with all ofthe known shortening occurring post–10 Ma,and most of it post–4 Ma (Page et al., 1998;Namson and Davis, 1990). Therefore, themodel predicts an additional 32–50 km ofshortening prior to 10 Ma, for which there is(thus far) no evidence in the Coast Ranges.
Pausing Animation 1 at 15 Ma highlightsthe crux of the problem (Fig. 9a). To eliminatethe need of early 24–16 Ma extension in theCoast Ranges (and subsequent compression),the continental edge would need to bend east-ward north of the Mojave and then continuenorth along the western edge of the Great Val-ley (Animation 1). This bend in the continen-tal edge would create an ;80-km-wide,;300-km-long section of oceanic crust thatwould have to be subducted south of the
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northward migrating triple junction during theperiod of central Basin and Range extension.The solution to the space problem that themodel highlights may rest in a combination ofseveral possibilities which include allowingfor a warping of the North America coast line,finding greater magnitudes of deformation inthe region of the Coast Ranges, and less ex-tension in the central Basin and Range. How-ever, to truly evaluate the magnitude of eachof these options requires more detailed recon-struction of crustal blocks west of the San An-dreas fault.
Uncertainties in the Reconstruction
Statistically rigorous uncertainties are no-toriously difficult to quantify in geological re-constructions, largely because estimates ofgeologic offset do not have Gaussian or otherstandard probability distribution functions.The condition of strain compatibility or ‘‘nooverlap’’ sets a hard limit on the displacementestimate but does not distinguish higher orlower probability of any given position withinthose limits. Hence, the variance of any givenestimate cannot be rigorously quantified.
In map view, any given displacement esti-mate will have an irregularly shaped uncer-tainty region. Under the assumption of a uni-form probability distribution within theseuncertainty regions, Wernicke et al. (1988)used a Monte Carlo method to estimate thetotal uncertainty on the sum of displacementvectors for a path across the central Basin andRange. This method repeatedly summed ran-domly selected vectors from each uncertaintyregion to generate a probability distributionfor the net offset. The contour that excludedthe outermost 5% of the model runs was takenas an estimate of two standard deviations ofthe measurement. The estimate of total Sierranmotion thus derived was 247 km 6 56 km, S758 6 128E, and therefore a reasonable esti-mate of the standard deviation would be 28km. For this same estimate, the square root ofthe sum of the squares for individual vectors(in the direction of displacement, using valuesfrom Table 1 and Figure 10 in Wernicke et al.,1988) is only 15 km. This is perhaps not sur-prising because the Monte Carlo approachdoes not place greater weight on values nearthe center of the uncertainty polygon than onvalues at the edges.
Our revised displacement estimate for thecentral Basin and Range, 235 km 6 20 km(again the error is equal to the square root ofthe sum of the squares for individual vectors),is similar to that of Wernicke et al. (1988) ifone considers the 20 km figure as a crude es-
timate of the standard deviation (1-sigma).However, given the results from Wernicke etal. (1988), the real error may scale upward byas much as a factor of two, depending on thedegree to which our best estimate is moreprobable than values at the extremes. A simplesum of each uncertainty along a given pathfrom Table 1 and Table 2 gives an error esti-mate of 47 km and 45 km, respectively. Thus,as a rule of thumb, the uncertainty in positionof any given range or set of ranges at anygiven time is on the order of 20–40 km at onestandard deviation.
Because the reconstruction involves tem-poral information (which is also uncertain),the problem of rigorously estimating errorsbecomes even more difficult and is clearly be-yond the scope of this paper. Even thoughtemporal information adds to the uncertaintyof position at any given time, the self-consistency of the reconstruction mitigatesthese uncertainties to a substantial degree.
EVOLUTION OF THE REGIONALVELOCITY FIELD
Tracking the restored positions of the rang-es from the palinspastic maps, we have cre-ated ‘‘instantaneous’’ velocity fields based on2 m.y. averages from 0 Ma to 18 Ma and 6m.y. averages from 18 to 36 Ma. These pa-leogeodetic velocity fields depict how defor-mation has evolved in space and time acrossthe plate boundary deformation zone (Figs.10A–10G).
Figures 10F and 10G (30–18 Ma) illustratethe collapse of the Basin and Range awayfrom the stable Colorado Plateau through theformation of metamorphic core complexes ata time of active ignimbrite volcanism andPacific-Farallon convergence. Extension initi-ated first in the northern Basin and Range andthen in the southern Basin and Range. Thispulse of large-magnitude extension migratedsouth and north, respectively, until it con-verged in the central Basin and Range at ca.16 Ma. Figure 10E (14–16 Ma) emphasizesthe large extensional strains in the central Ba-sin and Range especially with respect to theconcurrent faulting to the north and south. The14–16 Ma time slice also shows the impact ofthe evolving plate boundary on the NorthAmerican continent as right-lateral shear is ac-commodated through the rotation of the West-ern Transverse Ranges and accompanyingshear and extension. The 10–12 Ma time slice(Fig. 10D) illustrates the uniform (systemati-cally increasing) strain in the northern Basinand Range and, in contrast, the westward-mi-grating extension in the central Basin and
Range. Significant extension is also necessaryin the Mexican Basin and Range due to plate-boundary constraints. It is during this time pe-riod that right-lateral shear migrates farther in-board into the continent through thedevelopment of the eastern California shearzone. South of the Garlock fault, the shear isoriented nearly parallel to the plate boundary(N258W). North of the Garlock fault, the shearplus extension creates a more oblique orien-tation of shear (;N678W). From 6 Ma to 8Ma, this same pattern of intracontinental right-lateral shear strengthens with shear partitioneddifferently south of the Garlock fault than inthe central Basin and Range and northern Ba-sin and Range portions of the eastern Califor-nia shear zone (Fig. 10C). In the Mexican Ba-sin and Range, deformation wanes andextension and right-lateral shear become con-centrated in the proto–Gulf of California.
The differences in the velocity fields fromthe 2–4 Ma average to the 0–2 Ma average ismost likely a function of limitations in thedata, rather than a significant slowing in therate of deformation over the last 2 m.y. (i.e.,the Mojave region) (Figs. 10A and 10B).Within the model, the lack of timing con-straints for right-lateral faults through the Mo-jave means that the rate of deformation therebecomes a function of the rate of deformationto the north and south. North of the Garlockfault, large magnitudes (104 km) of obliqueextension are focused predominately from 11Ma to 3 Ma (Niemi et al., 2001; Snow andWernicke, 2000; Snow and Lux, 1999). Southof the Mojave, the timing of deformation ispartially bracketed by the age of rotation ofthe Eastern Transverse Ranges (as mentionedearlier, ca. 10 Ma rocks record the entire 458of rotation whereas ca. 4 Ma rocks indicate norotation; Carter et al., 1987; Richard, 1993).These timing constraints suggest most of thedeformational shear in the Mojave occurredbetween 10 Ma and 2 Ma. However, the totaldisplacement across the eastern Californiashear zone (100 km 6 10 km) averaged overthe last 12 m.y. suggests a long-term rate of8.3 mm/yr 6 1 mm/yr. This rate is similar toor slightly less than the 8–12 mm/yr rate sug-gested by geodetic studies (McClusky et al.,2001; Miller et al., 2001; Sauber et al., 1994;Savage et al., 1990).
Another way to look at the evolution of thevelocity field and provide a direct comparisonbetween geologic data and geodynamicalmodel results is by mapping the paths that in-dividual ranges take over the deformationalinterval of interest (Fig. 11). Note that thebend in the path of the Pacific plate does notappear to be related to changes in the paths of
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Figure 10. ‘‘Instantaneous’’ velocity fields based on 2 m.y. averages from 0 to 18 Ma, and 6 m.y. averages from 18 to 36 Ma. Arrowsshow displacement with respect to stable North America and were determined by connecting the centroids of specific ranges at one timewith the centroid of the same range in a later time. Because the motion of individual ranges can be very slight at the eastern edge ofthe model, the line lengths representing each incremental offset were uniformly doubled. Map base is the palinspastic map from theyoungest time in the 2 or 6 m.y. interval.
the Sierra Nevada with respect to the ColoradoPlateau or changes in the paths of individualranges within the continent. The most signif-icant continental change in direction occurs at12 Ma. Because the plate constraints do notrequire the bend to occur at that time (it is
only a function of the times at which magneticanomalies constrain the position), it is possi-ble within the uncertainties of both the platereconstruction and geological reconstruction(Atwater and Stock, 1998; Wernicke andSnow, 1998) that these events more closely
correlate. As stated previously, the timing ofdevelopment of right-lateral shear depends onthe orientation and timing of early extensionin the Death Valley region, which if relativelyminor prior to 11 Ma would point toward alater time of onset of right-lateral shear in-
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Figure 10. (continued).
board of the Sierra Nevada. The distributionof north-northwest shear through the Mojaveis kinematically linked to the northwesterlymotion of the Sierra Nevada–Great Valleyblock, in turn requiring at least some amountof right-lateral shear within the Mojave regionbetween 12 Ma and 8 Ma.
CONCLUSIONS
Although orogen-scale reconstructions ofthe Basin and Range will continue to evolvewith time and adjust as more data is acquired,the exercise in kinematic compatibility wepresent here highlights what we understand
and more importantly what we still do not un-derstand regarding the evolution of the plateboundary.
Results that are robust and highlight whatwe do understand include: (1) 235 km 6 20km of extension oriented N788W in both thenorthern (50% extension) and central (200%
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Figure 11. Map illustrating paths of various ranges from 0 to 36 Ma. Solid black circlesindicate the positions of the ranges at 8 Ma, and open circles represent the positions ofthe ranges at 12 Ma. The westernmost point on each line represents the current locationof each range. The easternmost point on the path represents the location of the range at36 Ma. The blue arrows represent the motion of the Mendicino triple junction, with itsposition shown at 24, 15, 8, and 0 Ma (Atwater and Stock, 1998).
extension) parts of the province. An importantimplication of the model is that any significantchange in extension amount in a portion of theregion (i.e., a range on the path between theColorado Plateau and the Sierra Nevada) mustbe evaluated in light of how that change af-fects coevolving regions to the north andsouth. (2) A significant portion of boundary-parallel shear (in contrast to earlier extension)jumped into the continent at ca. 10–12 Ma,and once established, appears to have migrat-ed westward with time. (3) The magnitude ofslip on the eastern California shear zone ap-pears to be 100 km 6 10 km, although theexact structures that accommodate this shearin the Mojave, or how much of the relativemotion is accommodated by distributed shear,is not known.
Problems with the current reconstructions
are highlighted by large gaps in the model.These zones emphasize areas where morework is needed in refining our ideas abouthow intraplate deformation is accommodatedthrough time. Salient aspects of the model thatwe do not understand include: (1) compatibil-ity between timing of extension north andsouth of the Garlock fault and a smooth north-northwest–trending continental edge as im-plied by plate tectonic reconstructions. Tomaintain a relatively smooth continental edgewith different periods of extension across theGarlock fault, a triangular window of signifi-cant extension (.50 km; 24–16 Ma) followedby an equal amount of shortening (14–0 Ma)would have occurred in the Coast Range–Great Valley region. While known geologysupports extension and subsequent compres-sion in these time windows, the magnitude is
;25% of what is needed; and (2) differencesin magnitude and timing of extension betweensouthern Arizona and northern Sonora, Mex-ico, require a transfer zone or large lateral dis-placement gradient. The model displays thiszone as a gap that opens up (going backwardwith time) between the two provinces. Timingand magnitude of extension in the Sonora re-gion were constrained only by plate motionsto the west and broad assumptions as to sim-ilarities in timing and direction with areas tothe north. Data detailing the magnitude, tim-ing, and direction of extension through theMexican Basin and Range is necessary to re-solve this problem.
ACKNOWLEDGMENTS
This paper has benefited greatly from many con-versations with scientists familiar with westernNorth America geology. We specifically want to ac-knowledge Tanya Atwater, Bill Dickinson, AllenGlazner, Steve Graham, Jon Spencer, Joann Stock,Nathan Niemi, Mike Oskin, Jason Saleeby, andJohn Suppe. We are grateful to Melissa Brennemanat the University of Redlands for creating theArcMap document and script used for the recon-struction. The movie would not exist without thehelp of the University of California, Santa Barbara,Educational Multimedia Visualization Center, spe-cifically Carrie Glavich and Grace Giles. GaryAxen, Doug Walker, Craig Jones, and Randy Kellerall provided insightful feedback through the reviewprocesses that greatly improved the clarity of pre-sentation. This project was funded by the CaltechTectonics Observatory.
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