Assemblages of geologic/morphologic units in the northern ... · The geologic/morphologic map of the northern mid-to-high latitudes of Venus prepared by a Soviet science team on the

ASSEMBLAGES OF GEOLOGIC/MORPHOLOGIC UNITS IN THE

NORTHERN HEMISPHERE OF VENUS*

JAMES W. HEAD Department of Geological Sciences, Brown University, Providence, RI, U.S.A.

Abstract. The geologic/morphologic map of the northern mid-to-high latitudes of Venus prepared by a Soviet science team on the basis of Venera 15/16 mission radar image coverage is analyzed and used to define six discrete assemblages of geologic/morphologic units that have well-defined geographic distributions. These assemblages have distinctive and differing geological and tectonic expressions and include: Plains Assemblage - which is dominated by lowland smooth plains and lowland rolling plains interpreted to be of volcanic origin, and a high concentration of small volcanic domes; Plains-Corona Assemblage - which is dominated by lowland smooth plains and lowland rolling plains interpreted to be of volcanic origin, at least ten coronae structures concentrated in the northern half of the region, and at least five large volcanoes, generally concentrated in the southern and western half of the region; Plains-Ridge Belt Assemblage - which is dominated by lowland smooth plains and lesser amounts of lowland rolling plains, major occurrences of ridge belts in a distinctive fan-shaped pattern, and very minor and patchy occurrences of tessera; Plains-Corona-Tessera Assemblage - which is dominated by approximately equal amounts of lowland smooth plains and lowland rolling plains, at least five coronae concentrated in the northern part of the region, a small number of large volcanoes, also in the northern part of the region, and numerous small patches of tesserae scattered throughout, and the highest abundance of small volcanic domes observed in the northern hemisphere; Tessera-Ridge Belt Assem- blage - which is dominated by a few large areas (Fortuna, Laima, Tellus) and several smaller areas (Dekla, Meni) of tesserae, ridge belts generally arrayed in an angular and often orthogonal pattern different from the fan-shaped pattern of the Plains-Ridge Belt Assemblage, lowland rolling plains and lesser amounts of lowland smooth plains, and an upland rise (Bell Regio); Tessera-Mountain Belt Assemblage - which is centered on the two volcanoes Colette and Sacajawea in Lakshmi Planum, and characterized by the peripheral mountain belt/tessera pairs, with the tessera on the outboard side: Danu/Clotho (S), Akna/Atropos (W), Freyja/ltzpapalotl (N), and Maxwell/Fortuna (E).

The distribution and characteristics of assemblages demonstrate that vertical and horizontal tectonic forces are operating on the crust and lithosphere of Venus in different ways in specific localized areas. Alternative models are outlined for the origin of each assemblage and the relationship between assemblages, and important unresolved questions are identified. A key to the further understanding of these assemblages is the origin of ridge belts and tessera terrain.

Introduction

The Venera 15/16 missions provided altimetry and high resolution images of the northern 20% of the surface of Venus, complementing moderate resolution altimetry coverage from Pioneer-Venus (Pettengill et al., 1980; Masursky et al., 1980), and permitted the detailed geologic mapping and analysis of a portion of Venus previously poorly understood (Barsukov et al., 1986; Basilevsky et al., 1986; Ivanov et al., 1986; Barsukov and Basilevsky, 1986; Sukhanov et al., 1989). The resulting geologic/morphologic map (produced by a Soviet mapping team and shown in

* ‘Geology and Tectonics of Venus’, special issue edited by Alexander T. Basilevsky (USSR Acad. of Sci., Moscow), James W. Head (Brown University, Providence), Gordon H. Pettengill (MIT. Cambridge, Massachusetts) and R. S. Saunders (J.P.L., Pasadena).

Earth, Moon, and Planets 50151: 391-408. 1990. 0 1990 Kluwer Academic Publishers. Printed in the Netherlands.

392 JAMES W. HEAD

Barsukov and Basilevsky, 1986; Basilevsky and Head, 1988; Figure 1) outlines the distribution of geologic units (Barsukov et al., 1986), tectonic structure (Basilevsky et al., 1986), and the distribution of craters of impact origin (Basilevsky et al., 1987), permitting an estimate of the average age of the mapped area of less than about one billion years (Ivanov et al., 1986). Approximately 70% of the mapped

Fig. 1. Geologic/morphologic map of the northern latitudes of Venus compiled from radar images obtained by the Venera 15/16 spacecraft by a mapping team of associates from the Vernadsky Institute of Geochemistry and Analytical Chemistry and the Geological Institute of the USSR Academy of Sciences (see Barsukov and Basilevsky, 1986). The key to units is as follows: (1) lowland smooth plains; (2) lowland rolling plains; (3) highland plains; Lakshmi Planum; (4) mountain belts around Lakshmi Planum; (5) ridge belts; (6) tesserae; (7) large domal uplifts or domal rises; (8) volcanoes; (9) coronae or ovoids; (10) faults; (11) impact craters; (12) ‘arachnoids’. Detailed nomenclature designations, topography, radar image mosaic, and air-brush rendition of topography and morphology

can be found in ‘Maps of Part of the Northern Hemisphere of Venus’, (USGS, 1989).

ASSEMBLAGES OF GEOLOGIC/MORPHOLOGIC UNITS 393

area is dominated by plains of apparent volcanic origin, while units of tectonic origin make up less than 30%.

The distribution of geologic/morphologic units (Figure 1) is distinctly not ran- dom. The purpose of this paper is to define several basic assemblages of geologic/- morphologic units, to document their characteristics, spatial distribution, and correlation with topography and other surface properties, to analyze the relationships of these assemblages, and to discuss their possible origins.

Definition and Interpretation of Units

The major geologic/morphologic units and tectonic structures mapped on Venus (Figure 1 and caption) include, in approximate decreasing percentage of abundance, plains, tessera, ridge belts, coronae, volcanoes, and mountain belts. Plains units, with a variety of surface textures and broad topographic characteristics (smooth, rolling, etc.), cover over 70% of the surface mapped by Venera 15116, and have been interpreted to be solidified basaltic lava flows on the basis of the presence of radar dark and bright flow-like features, domes and volcanic centers, and Venera lander geochemical results (Barsukov and Basilevsky, 1986; Basilevsky and Head, 1988, and references therein). Numerous possible sources for flows and volcanic deposits can be seen, but the exact mode of formation of the plains deposits is often assumed, but not known (e.g., they may result from: (1) in situ formation from many distributed sources and vertical thickening with time (the volcanic plains, hot spot, or flood basalt analogy), (2) formation along zones of crustal spreading and lateral movement away to create large regions of crust of relatively similar thickness and features (the ocean floor analogy), or (3) some combination of the above two options (Head, 1990a)).

Tesserae are upland regions characterized by dense systems of ridges and grooves transecting each other in orthogonal, diagonal, chevron-like, or chaotic manner (Basilevsky et al., 1986; Bindschadler and Head, 1988a, 1988b, 1990). Tesserae occur in small isolated areas as well as in large regions, such as Fortuna and Tellus Regio. The morphology of tesserae seems to be related to deformation which has acted over broad areas (the area1 deformation style of Basilevsky et al., 1986), but the origin of the deformation is still controversial. A range of models for tessera origin include (Bindschadler and Head, 1988b, 1990): horizontal compression and crustal thickening; vertical uplift due to shallow mantle processes (Phillips, 1986; Bindschadler and Parmentier, 1987); gravity driven deformation manifested as gravity sliding (Sukhanov, 1986; Kozak and Schaber, 1986; Markov et al., 1989) or gravitational relaxation (Bindschadler and Parmentier, 1987); or analogy to the terrestrial sea floor where structural patterns are related to rise-crest and spreading processes (Head and Crumpler, 1987; Sotin et al., 1989; Head et al., 1990; Head, 199Oc).

Ridge belts are characterized by closely spaced, linear to sinuous, parallel to sub-parallel ridges 5 to 20 km wide arranged in sinuous bands which range from

394 JAMES W. HEAD

30 to 400 in width (the belt deformation style of Basilevsky et al., 1986). On the basis of their similarity to wrinkle ridges, these features were originally interpreted to be of compressional origin (Barsukov et al., 1986)) and several workers continue to interpret many of the structures in this manner (Kryuchkov, 1988; Frank and Head, 1990). Other workers (Sukhanov and Pronin, 1989) have proposed that ridge belts, particularly in the fan-shaped area east of Atalanta Planitia, are of extensional origin, and that crustal spreading is occurring along the ridge belt axes. In either case, horizontal stresses and deformation of surface layers charac- terize the regions where ridge belts occur.

Coronae are circular to elliptical structures 150-600 km across characterized by an annulus of concentric ridges and geologically complex interiors displaying volcanic flows, domes, and ridges of various orientations that are generally distinctive from the exterior deposits. Although coronae display a wide range of geologic features, the focused nature of the interpreted activity in individual corona, and the associated volcanism and deformation, has led most workers to conclude that their origin is related to vertical movement associated with diapirism and/or local thermal anomalies (hot spots), and subsequent deformation related to gravitational relaxation (Barsukov et al., 1986; Basilevsky et al., 1986; Stofan et al., 1988; Edmunds et al., 1988; Stofan and Head, 1990; Pronin and Stofan, 1988,199O). The smaller arachnoids (named for their spider-like appearance) are 50-200 km in diameter and consist of concentric and concentric/radial systems of narrow ridges, central domes, and arcuate scarps, and have been interpreted by Pronin and Borozdin (1988) to be the surface manifestation of diapirs caused by density inversions. The larger dome uplifts, or domal rises, such as Beta, Bell, and Ulfrun Regiones, have a plains-like surface morphology but their crests are modified by troughs, parallel grooves, scarps, and volcanoes and they are interpreted to represent tectonic doming and associated rifting and volcanism (Stofan et al., 1989).

Several tens of generally circular features 50-300 km in diameter and usually less than 1 km in height are interpreted as volcanoes because of their morphology, the frequent occurrence of central summit craters, and radial flow-like features (Slyuta, 1990; Slyuta and Kreslavsky, 1990). Mountain belts are characterized by narrow linear positive topography and associated features (ridges, troughs, scarps) that led Crumpler et al. (1986) to interpret them as erogenic belts, with associated anticlines, synclines and thrust faults. Virtually all workers interpret the mountain belts surrounding Lakshmi Planum in Western Ishtar Terra to be indications of large-scale horizontal compressional deformation (Campbell et al., 1983; Basilev- sky et al., 1986; Pronin, 1986; Vorder Bruegge et al., 1990; Head, et al., 1989, 1990; Head, 1990b), although the source of this deformation is debated.

Definition of Basic Assemblages and Their Interpretation

An assemblage of geologic/morphologic units is a region of the planet characterized by the dominance of single types of units (for example, different plains units), or


the association of several units (for example, plains and coronae), in a manner different from adjacent terrain. The purpose of defining these assemblages is to establish the relationships of various units, to obtain a better understanding of their distribution, and to establish how these assemblages and their relationships might provide insight into the origin of the units in general. On the basis of the area1 distribution of geologic/morphologic units in the northern high latitudes of Venus (Barsukov and Basilevsky, 1986), six basic assemblages can be defined (Figure 1).

Plains Assemblage: This assemblage is dominated by lowland smooth plains and lowland rolling plains interpreted to be of volcanic origin, and very minor and patchy occurrences of tesserae and a ridge belt. It is located in northern Guinevere Planitia and Sedna Planitia (about 290-O” and up to about 60”N), covers about 12.6 x lo6 km’, and is topographically lower than western Ishtar Terra to the north, and terrain to the west and east. The assemblage is distinctive in the predominance of volcanic plains, the large abundance of small volcanic domes (it has the second highest concentration of domes in the northern hemisphere; Aubele and Slyuta, 1990), and the general lack of other features and structures.

Two end-member models can be proposed for the origin of these plains (Figure 2a) on the basis of the observed characteristics of the assemblages and what is known about the nature of processes on Venus. The first of these is in situ formation from local sources (vertical crustal formation and evolution). In the area of this assemblage there are no large volcanic edifices mapped at this scale (Figure l), and domal rises are not sufficient in number to be the major source of plains deposits. However, the number of small domes (5-20 km diameter) in this area is very large, ranging from 50-400 domes per lo6 km2 (Slyuta et al., 1988; Aubele and Slyuta, 1990). Thus, these domes are candidates to produce this assemblage by local sources, and other hidden sources may also be spread over the region. Recent Arecibo data for the southern part of Guinevere and the region to the south also reveal numerous flows and local volcanic source vents (Campbell et al., 1989). The second end-member model is formation by crustul spreading (lateral crustal formation and evolution) (Figure 2a). The topography of this region is low and there is no apparent topographic or morphologic evidence for a spreading center in the assemblage. Spreading has been proposed in Western Aphrodite Terra in the equatorial region to the south (Head and Crumpler, 1987). This model for the formation of the plains assemblage would require similar processes operating outside this region (to the south?) and transport of newly formed crust veneered with volcanic plains into this region.

Plains-Corona Assemblage: This assemblage, extending from about 220” to 290”, and from the southern extent of the Venera coverage to the pole, covers about 20.2 x lo6 km2. It is dominated by lowland smooth plains and lowland rolling plains interpreted to be of volcanic origin, at least ten coronae structures concentrated in the northern half of the region, and at least five large volcanoes, generally concentrated in the southern and western half of the region. Two large dome rises

396 JAMES W. HEAD

MODEL 1

VERTK=AL FORMATlON

PLAINS ASSEMBLAGE

Fig. 2a.

MODEL 2

LATERAL FORMATKM

AND CRUSTAL SPREADBIG

PLAINS-RIDGE BEU PLAHS-CORONA PLAWS-CORONA-

MODEL1 PA!SSlVE

DEFORMATION AND HoTsPoTs

PLAINS-CORONA PLAINS-RIDGE BELT

> ASSEMBLAGES

PLAINS-CORONA-TESSERA

Fig. 2b.

MODEL 2

ACTIVE CMJSTAL SPREADWG


COMPRESSICNAL DEFOf7MATION AND

THlCKENlW

MODEL 2

LATERAL MCWEMENT AND CONVERGENCE OF BLOCKS OF THCKEMD

CRUST

TESSERA-RIDGE BELT ASSEMBLAGE

Fig. 2c.

MOOEL 1

MODEL 2

TESSERA-MOUNTAH BELT ASSEMBLAGE

Fig. 2d. Fig. 2. Models for the origin and re!ationships of different assemblages.

dominate the southeastern (Beta Regio) and southwestern (Ulfrun Regioj areas of this assemblage, and a third smaller domal rise (Metis Regio) is located in the northern part. Topography is generally characterized by intermediate elevations, with local high regions associated with the three domal rises and some of the volcanoes, and a low extending west from Guinevere to the region just north of Beta Regio.

398 JAMES W. HEAD

The presence of distinctive volcanic centers, domal rises, and coronae all point toward a range of thermal anomalies (hot spots) and diapirism at a variety of scales operating in this assemblage, and a focus on vertical movement associated with the formation and evolution of these features (Figure 2b, right side). The distribution and size of volcanoes and coronae suggest that there are a variety of deeper anomalies beneath this assemblage that cause surface deformation and volcanism over individual regions measured in the tens to several hundred kilometer range. The domal rises along the southern border may be even larger manifes- tations of the same process (Basilevsky and Head, 1988). If large volcanoes represent earlier stages of coronae formation (Stofan and Head, 1990), then the most recent activity in this assemblage is along the western margin.

Plains-Ridge Belt Assemblage: This assemblage, extending from about 140” to 220” at the southern edge of the coverage to a point near the north pole, covers about 26.8 x lo6 km2. It is dominated by lowland smooth plains and lesser amounts of lowland rolling plains, both interpreted to be of volcanic origin, major occurrences of ridge belts, and very minor and patchy occurrences of tesserae. The ridge belts are arrayed in a fan-like manner with central braided ridge belts to the north and more widely dispersed set of belts to the south. This assemblage also contains the second highest abundances of arachnoid structures (at least 20), which occur along the central braided ridge belt (Lauma Dorsa) and in a cluster in the southeastern part of the region. Topography is generally characterized by internediate elevations, with local linear high and low regions associated with the ridge belts and intervening plains respectively. The regional topography is lower in the western part (parts of Atalanta and Vellamo Planitia), forms an irregular linear high in the central region, and is relatively low in parts of the east (Ganiki Planitia). Sukhanov and Pronin (1988) present data on crater density which they interpret to indicate that this region is one of the youngest in the Venera 15/16 coverage.

The ridge belts and their fan-shaped distribution clearly dominate this assemblage and indicate that horizontal forces (either compressional deformation, extension and crustal spreading, or some combination of these) are fundamental to the formation and evolution of this region (Figure 2b, center). On the basis of the details of ridge belt structure, the general absence of highland terrain such as the tessera, the low crater density, and the wedge shape, Sukhanov and Pronin (1989) interpret the fan-shaped ridge belt region to be the site of crustal spreading, a process forming new crust and dividing older adjacent highlands. Other studies, focusing on individual ridge belts in this and other regions (Frank and Head, 1990), or on regional analysis of spacing of belts (Zuber, 1987), find evidence for compressional deformation and a compressional origin of many ridge belts. Nonetheless, it is clear that this assemblage is fundamentally different from the adjacent assemblages (Figures 1, 2b), and that its features represent horizontal forces in its formation and evolution.

Plains-Corona- Tessera Assemblage: This assemblage, extending from about 90”

ASSEMBLAGES OF CEOLOClC/MORPHOLOGIC UNITS 399

to 140” at the southern edge of the coverage to a point near the north pole, covers about 22.2 x lo6 km2. It is dominated by approximately equal amounts of lowland smooth plains and lowland rolling plains, both interpreted to be of volcanic origin, at least five coronae concentrated in the northern part of the region, a small number of volcanoes, also in the northern part of the region, and numerous small patches of tesserae scattered throughout the region. It also has the highest concentration of small volcanic domes (5-20 km in diameter) in the northern hemisphere (Slyuta et al., 1988; Aubele and Slyuta, 1990) with densities ranging from 50-550 domes per lo6 km2, and the borders of this high concentration coin- cide approximately with the assemblage borders. Topography is generally low in the northeast (Atalanta Planitia) and south (Niobe Planitia) and intermediate elevation elsewhere, being highest in the tessera regions of Tethus Regio in the northern part of the assemblage.

With the exception of localized occurrences of tessera and the abundance of small domes, this assemblage is very similar to the Plains-Corona Assemblage immediately to the east of the Plains-Ridge Belt Assemblage (Figures 1, 2b). Similarities include: (1) the occurrence of both coronae and volcanoes, (2) their concentration in the northern part of the assemblage, (3) flanking of this occurrence adjacent to the northern part of the fan-shaped ridge belt, (4) the presence of lowland smooth and rolling plains, and (5) the absence of ridge belts (Figures 1, 2b). Differences include: (1) the smaller number of coronae and volcanoes in this assemblage, (2) the greater abundance of tessera patches relative to the Plains- Corona Assemblage, and (3) the greater number of small volcanic domes relative to the Plains-Corona Assemblage. The general size and abundance of the coronae, volcanoes, and dome clusters point toward a range of thermal anomalies (hot spots) and diapirism at a variety of scales operating in this assemblage, and a focus on vertical movement associated with the formation and evolution of these features (Figure 2b). As in the Plains-Corona Assemblage to the east, the distribution and size of volcanoes, dome clusters, and coronae suggest that there are a variety of deeper anomalies beneath at least the northern part of this assemblage that cause surface deformation and volcanism over individual regions measured in the tens to several hundred kilometer range. The significance of the tessera in this region is presently not understood. The presence of the tessera patches suggests that this assemblage may have undergone more extensive tectonic deformation than its complementary assemblage to the east, or, alternatively, that the tessera may represent some variation on the mode of formation of plains units, such as locally thicker crust produced at spreading centers (Bindschadler and Head, 1988b, 1990; Sotin ef al., 1989; Head and Crumpler, 1990).

Tessera-Ridge Belt Asse’rnblage: This assemblage extends from about 0” to 90 at the southern edge of the coverage to a point near the north pole and covers about 29 x lo6 km2. It is dominated by a few large areas (Fortuna, Laima, Tellus) and several smaller patches (Dekla, Meni) of tesserae, ridge belts generally arrayed in an angular and often orthogonal pattern different from the fan-shaped pattern

400 JAMES W. HEAD

of the Plains-Ridge Belt Assemblage, lowland rolling plains and lesser amounts of lowland smooth plains. It also contains the highest abundance of arachnoids (at least forty) of any assemblage. These are located primarily in association with ridge belts. The assemblage has a small number of coronae and volcanoes, and an upland rise (Bell Regio). Topography is generally intermediate in elevation (0.5-2.0 km) above the mean planetary radius, particularly in the tessera and large domal rise areas, with local lows in the plains areas of Leda and Bereghinya Planitia.

This region is characterized by intense tectonic deformation in the form of: (1) highly deformed tessera (particularly in Fortuna Tessera; Sukhanov, 1986; Kozak and Schaber, 1986; Vorder Bruegge and Head, 1989, 1990), and (2) orthogonally arrayed ridge belts that appear to mark the boundaries between large structural blocks. This assemblage has the largest abundance of tesserae in the Venera 1506 coverage. In the north (Fortuna Tessera) much of the deformation appears to be related to convergence and crustal thickening (Vorder Bruegge and Head, 1989, 1990), while the orthogonal and angular ridge belts in the central and southern regions may be interpreted to mark zones of focused compressional deformation between the larger crustal blocks containing plains and tessera (Figure 2c) (Frank and Head, 1990). Western Aphrodite Terra, just to the southeast of this region, is the site of proposed crustal spreading (Head and Crumpler, 1987) and some of the deformation in this region may be linked to that activity. In any case, the observed tectonic structure that can be interpreted appears to be primarily linked to horizontal compression (Figure 2~).

Tessera-Mountain Belt Assemblage: This assemblage (Figure 1) is centered on western Ishtar Terra and covers the smallest area, approximately 5.2 x lo6 km2. The region is centered on the two volcanoes Colette and Sacajawea in Lakshmi Planum, and is characterized by surrounding terrain types of mountain belt/tessera pairs, with the tessera on the outboard side: Danu/Clotho to the south, Akna/ Atropos to the west, Freyja/Itzpapolotl to the north, and Maxwell/Fortuna to the east. The topography is high, with steep slopes in and adjacent to the mountain belts and at the edge of Ishtar Terra (Sharpton and Head, 1985; 1986) and the plains of Lakshmi are at abnormally high elevations (>2.5 km) (classified as highland plains in Barsukov et al., 1986).

The characteristics of the mountain belts suggest that they are erogenic belts (Crumpler et al., 1986) and represent the focal point of large-scale horizontal compressional deformation (Figure 2d). The tessera patches that are associated with each belt may represent more intense flanking deformation (e.g., as at Maxwell Montes; Vorder Bruegge et al., 1990), or the imbrication of crustal slices during the process of flexure and underthrusting (e.g., as at Freyja Montes; Head, 1990b). The mountain ranges arrayed around the edges of this assemblage suggest that horizontal compression is directed from the surrounding assemblages in toward central Lakshmi Planum, and that crustal thickening and melting may be responsible for the central volcanoes and plains (Magee and Head, 1988; Roberts

ASSEMBLAGES OF GEOLOGIC/MORPHOLOGLC UNITS 401

and Head, 1990a, b). An alternate view holds that the central volcanoes mark the location of a major hot spot and that the mountains represent deformation associated with the outward force of this plume (Pronin, 1986). In either case (Figure 2d), horizontal compressional deformation clearly dominates this unique region.

Ages

Counts of impact craters for the northern mid-to-high latitudes of Venus mapped by Venera 1506 permitted estimates for the average age of this region of 1.0 Ga (rtO.5 Ga) (Ivanov et al., 1986), and several hundred million years (Schaber et al., 1987). In either case, this region is one of the least cratered in the inner solar system (Barlow, 1988). Plaut and Arvidson (1987) have shown that the total number of impact craters is small enough so that a counting circle of about 5 X lo6 km2 is required for statistically significant counts. Therefore, statistically significant ages cannot be obtained for units on the scale individual geologic/- morphologic units mapped by Barsukov et al. (1986) (Figure 1). The assemblages of units mapped here each encompass a considerably larger surface area than single

CRATER DENSITY

0 0.5 I 1.5 2 2.5 3 35 3.75 NUMBER FER 10” KM*

Fig. 3. Map of impact crater density (from Burba. 1989 and Sukhanov and Pronin. 1988) with assemblage boundaries superimposed.

402 JAMES W. HEAD

occurrences of units, with the assemblages ranging in area from 5.2 x lo6 km2 to 29 x lo6 km2, with the average being about 19 x lo6 km2.

Burba (1989) and Sukhanov and Pronin (1989) present a crater density map for the number of impact craters per lo6 km2, and the assemblage boundaries have been superimposed on this map in Figure 3. The average number of craters in the area covered by Venera 15/16 is 1.1 craters per lo6 km2. In addition, new crater densities have been determined specifically for the assemblages mapped here by taking the tables of crater distribution of Basilevsky et al. (1987) and counting the number of craters in each assemblage (Table I). On the basis of these data, the Plains-Ridge Belt Assemblage appears to contain the fewest craters, while the Tessera-Mountain Belt Assemblage appears to contain the most craters, with the other assemblages being intermediate, but generally relatively lower, in total numbers of craters. For comparison, Campbell et al. (1989) found that the the density of craters of possible impact origin in the 32 X lo6 km2 of the equatorial region between Beta and Western Eistla Regio covered by new Arecibo data is 0.7 x lo6 km2. The density of craters of probable impact origin in this same area is 0.4 x lo6 km2 (Campbell et al., 1989).

Several assemblages, particularly the Plains-Corona Assemblage and the Plains- Ridge Belt Assemblage, show a distinctive clustering of craters in the north relative to their southern regions. This clustering, and the fact that the relatively heavily cratered Tessera-Mountain Belt Assemblage occurs at high latitudes, is also re- flected in the decreasing abundance of craters per unit area from the North Pole toward the equator documented by Head and Crumpler (1989). Acquisition of high-resolution images by Magellan should permit the more exact determination of crater diameters, determination of the stratigraphic relationship of craters and their surroundings, and the more confident distinction of impact and volcanic craters, so that crater size-frequency distribution data can confidently be plotted for each assemblage.

TABLE I

Number of impact craters per lo6 km* for each assemblage (data from Ivanov et al., 1986; Basilevsky ef al., 1987)

Assemblage Area ( 10h km*) Craters/lo6 km*

Plains Assemblage 12.6 1.1 Plains-Corona Assemblage 20.2 1.0 Plains-Ridge Belt Assemblage 26.8 1.1 Plains-Corona-Tessera Assemblage 22.2 1.0 Tessera-Ridge Belt Assemblage 29.0 1.2 Tessera-Mountain Belt Assemblage 5.2 2.7


Assemblage Relationships

Some assemblages appear to be more closely related than others. For example, the Plains-Corona Assemblage and the Plains-Corona-Tessera Assemblage are approximately equal in size and show a similar north-south distribution in major features such as coronae (Figure l), suggesting that the two assemblages may have common origins. In addition, the Plains-Ridge Belt Assemblage intervenes between these two assemblages and the ridge belts along the margins of the Plains- Ridge Belt Assemblage tend to parallel the boundaries with the two complementary flanking assemblages (Figures 1, 2b).

The sense of tectonic deformation, however, appears to be very different in the Plains-Ridge Belt Assemblage (horizontal) and the flanking assemblages (vertical). Two models can be proposed to explain the observed relationships and to serve as a guide for further studies (Figure 2b). In model 1, the assemblages operate independently, with vertical plumes causing the volcanoes and coronae in the slightly thicker crust of the flanking assemblages, while the thinner crust of the central assemblage is being deformed by either extension or compression, producing a relatively distinctive ridge spacing interval (Zuber, 1987). In model 2, the assemblages are linked in their origin, with the central assemblage being related to crustal spreading along a central rise (in the manner of Sukhanov and Pronin, 1988). Crustal underthrusting and remelting would occur at the edges and under- neath the flanking assemblages, producing diapiric rise of material to initiate volcanism and corona evolution. This scenario differs from that of Sukhanov and Pronin (1989) who interpret all ridge belts to be extensional in nature. In this model (2), some of the ridge belts along the edge of the central assemblage are interpreted to be of compressional origin (Frank and Head, 1990), and the boundaries may often mark the location of crustal underthrusting and loss (for example in the northern part of the Plains-Ridge Belt Assemblage where it intersects Freyja Montes; see Head, 1990b). More detailed mapping is required to distinguish which elements of these two models are applicable.

The nature of the Tessera-Ridge Belt Assemblage elements suggests lateral tectonic deformation, but the exact style is unknown. Two models are proposed to illustrate the observed relationships and to serve as a guide for further studies (Figure 2~). In model 1, tessera represent regions of crustal thickening caused by horizontal compressional deformation and ridge belts are predominantly compressional and are located along the flanks of blocks of tessera or between larger crustal blocks. In this model, the surface texture of the tessera is a combination of deformation associated with compression and crustal thickening, and with gravitational relaxation (Bindschadler and Parmentier, 1987) and possible gravity sliding (Sukhanov, 1986; Kozak and Schaber, 1989). Plains units would represent areas of thinner less-deformed crust. In model 2 (Figure 2c), the crust formed at a spreading center and was transported laterally, perhaps from the southeast (Head

404 JAMES W. HEAD

and Crumpler, 1987). Tessera represent areas of thicker than normal crust produced by thermal anomalies (hot spots) at the rise crest (Sotin et al., 1989; Head and Crumpler, 1990). Ridge belts represent the focus of deformation and interaction of blocks, and sometimes mark the location of underthrusting and crustal loss. In the northern part of this assemblage (Figure 2c; left-hand side of model 2), several tessera blocks have converged at the location of a zone of underthrusting, causing accretion of blocks with various structural orientations (Fortuna Tessera; Vorder Bruegge et al., 1990; Head et al., 1990). When tessera blocks merged, underthrusting of the thickened crust became difficult, and the zone of underthrusting terminated there and began in adjacent thinner crust. More detailed mapping is required to distinguish which elements of these two models (Figure 2c) are applicable.

The Tessera-Mountain Belt Assemblage is very distinctive in terms of its linear folded and faulted mountain belts and tessera segments surrounding an elevated volcanic plateau (Lakshmi Planum). There is a consensus about the horizontal deformation and shortening required to produce the mountain belts (e.g., Pronin, 1986; Crumpler et al., 1986), but there are two end-member models for the formation of the assemblage as a whole (Figure 2d). In model 1, a large plume or hot spot is responsible for much of the elevated topography of western Ishtar and Lakshmi Planum, and the two large volcanic shields are evidence of its presence (Pronin, 1986). The outward lateral movement of the plume flanks causes ‘asthenospheric drag’ which creates outwardly directed compression and produces the mountain belts surrounding Lakshmi Planum. In model 2, compression is directed laterally from the regions surrounding western Ishtar, resulting in crustal thickening and elevation of the topography, and formation of mountain ranges around the periphery. In places, underthrusting, crustal imbrication, and crustal loss are occurring (e.g., Freyja Montes; Head, 1990b), and the central volcanism is attributed to melting of the thickened crust (Roberts and Head, 1990a, b). Continuing horizontal convergence results in the docking of tessera fragments of thickened crust on the outboard margins of Maxwell Montes (Fortuna Tessera) and Freyja Montes (Atropos Tessera) (Head et al., 1990). In summary, model 1 emphasizes vertical movement and minor lateral movement, while model 2 emphasizes significant lateral movement and convergence. More detailed mapping is required to distinguish which elements of these two models are applicable.

The Plains Assemblage is the least complex of the assemblages in terms of the number and diversity of units and could be formed by vertical accumulation of lavas over a large number of sources throughout the region (model 1; Figure 2a), or by the creation of crust at a spreading center (to the south?) and lateral transport of the crust into this region (model 2). The distinctive northern boundary of this assemblage with the Tessera-Mountain Belt Assemblage (Danu Montes) suggests that there may have been some lateral movement of this assemblage from the southwest to the northeast toward western Ishtar Terra and possible underthrusting at Danu Montes (Head and Burt, 1990; Janle and Jannsen, 1984).

ASSEMBLAGES OF GEOLOGlC/MORPHOLOGIC UNITS 405

Additional data on the nature of the equatorial region to the south, and on the detailed structure of the plains region and its volcanic sources are required to distinguish between these two hypotheses.

Conclusions

1. Geological/morphological map units derived from Venera 1.5/16 data for the northern mid-to-high latitudes do not occur randomly and can be grouped into several basic assemblages.

2. These assemblages show that the dominant surface geological processes differ markedly from region to region on Venus and that the assemblages can be characterized in a tectonic sense as having been formed by either predominantly horizon- tally or predominantly vertically directed deformational forces. These characteristics and relationships indicate that the tectonic style of Venus is not dominated by one style of activity, movement, or deformation, but contains elements of several styles (Solomon and Head, 1982). These include: (1) thermal uplift and vertical motion possibly related to hot spots or mantle plumes at several different scales (Morgan and Phillips, 1983; Phillips and Malin, 1983, Pronin, 1986; Stofan et al., 1989; Stofan and Head, 1990) as seen in the domal rises, coronae, and volcanoes;’ (2) regional lateral motion including extension and compression at a range of scales (Crumpler et al., 1986; Sukhanov, 1986; Head, 1990b) as seen in the ridge belts, rifts, mountain belts, and in the more highly deformed tessera, and (3) gravitational relaxation (Stofan et al., 1988; Bindschadler and Parmentier, 1987) as seen in local areas around coronae and possibly in the tessera.

3. The Tessera-Mountain Belt Assemblage has the highest crater density, and the Plains-Ridge Belt Assemblage the lowest, although the statistical significance of these differences remains to be assessed in detail. Some assemblages may be related to each other in terms of tectonic style and deformation. For example, the formation and evolution of the fan-shaped Plains-Ridge Belt Assemblage may be linked to the adjacent Plains-Corona and Plains-Corona-Tessera assemblages.

4. The detailed origin and relationships of the assemblages is unclear primarily because there is at present no consensus on the origin of several of the major geologic units (e.g., ridge belts, tessera). Two end-member interpretations of the origin and relationships of these assemblages that emphasize vertical and lateral movements in the crust/lithosphere and mantle are presented as focal points for further discussion and analysis. Additional insight into the significance and lateral extent of these assemblages will come from linking analyses of the equatorial region (Senske, 1989) to the characteristics of the northern mid-to-high latitudes. The high resolution global coverage of the MAGELLAN Mission (Dallas and Nickle, 1987) will provide important new information on the nature of processes responsible for the units making up the assemblages, and the distribution of similar units and assemblages over the globe. This will permit further discrimination between the different models for the origin and relationship of assemblages.

406 JAMES W. HEAD

Acknowledgements

The financial support of National Aeronautics and Space Administration grants NGR-40-002-116 and NAGW-713, from the Planetary Geology and Geophysics Program of the Solar System Exploration Division, Office of Space Science and Applications, are gratefully acknowledged. Mary Ellen Murphy, Angel Hilliard, and Peter Neivert assisted greatly in the preparation of the manuscript. Particular thanks are extended to Jeff Burt, Ellen Stofan, and Marc Parmentier for helpful discussions relating to this paper, and to Ellen Stofan, Don Wilhelms, and Buck Sharpton for comments and reviews of an earlier version of this paper.

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Assemblages of geologic/morphologic units in the northern ... · The geologic/morphologic map of the northern mid-to-high latitudes of Venus prepared by a Soviet science team on the