tessera terrain, venus: characterization and models for ... · tessera terrain, venus'...

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 96, NO. B4, PAGES 5889-5907, APRIL 10, 1991

Tessera Terrain, Venus' Characterization and Models for Origin and Evolution

DUANE L. BINDSCHADLER 1 AND JAMES W. HEAD

Department of Geological Sciences, Brown University, Providence, Rhode Island

Tessera terrain is the dominant tectonic landform in the northem high latitudes of Venus mapped by the Venera 15 and 16 orbiters and is concentrated in the region between the mountain ranges of western Ishtar Terra and Aphrodite Terra. Tesserae are characterized by regionally high topography, a high degree of small scale surface roughness, and sets of intersecting tectonic features. Available Pioneer Venus line of sight gravity data suggest that tessera terrain is compensated at shallow depths relative to many topographic highs on Venus and may be supported by cmstal thickness variations. Three types of tessera terrain can be defined on the basis of structural patterns: subparallel ridged terrain (Tsr), trough and ridge terrain (Ttr), and disrupted terrain (Tds). Observed characteristics of tessera terrain are compared to predictions of models in order to begin to address the question of its origin and evolution. Formational models, in which high topography is created along with surface deformation, include (1) horizontal convergence, (2) mantle upwelling, (3) crustal underplating, and (4) a seafloor spreading analogy. Modificational models, in which deformation occurs as a response to the presence of elevated regions, consist of (1) gravity sliding and (2) gravitational relaxation. We find that horizontal convergence and late stage gravitational relaxation are the most consistent with basic observations for subparallel ridged terrain and disrupted terrain. Understanding of the basic structural characteristics of trough and ridge terrain is more tentative, and models involving a spreading process or convergence and relaxation merit further study.

INTRODUC•ON [Campbell et al., 1983]). In addition, Sukhanov [1986] interprets structures within some plains regions as tessera

Some of the large topographic uplands in northern Venus, terrain partially buried beneath plains units, thus increasing such as eastern Ishtar Terra and Tellus Regio, were noted to the area of the tessera terrain to ~15% of northern Venus (or possess distinctive radar properties in data gathered by the ~17x106 km2). Tesserae are found in association with a number Pioneer Venus (PV)orbiter [Head et al., 1985]. Subsequent of units and are a major part of several geologic unit investigations by the Venera 15 and 16 orbiters [Kotelnikov et assemblages in the northern hemisphere of Venus: the plains- al., 1985; Rzhiga, 1984] revealed that these areas and corona-tessera, tessera-ridge belt, and tessera-mountain belt numerous other regions are characterized by a complexly assemblages [Head, 1990a]. Tesserae are strongly concentrated deformed surface called tessera terrain (also known informally between longitudes 0øE and 150øE (Figure 1), between a as parquet terrain). The mapped distribution of tesserae, adapted proposed center of crustal extension and divergence in from Barsukov and Basilevsky [1986], is shown in Figure 1. Tessera terrain was subsequently defined as regions of ridges and troughs that intersect at a variety of angles ranging from orthogonal to oblique [Basilevsky et al., 1986]. Tesserae lie at higher elevations than most other surface units [Bindschadler and Head, 1989] and appear to have undergone extensive horizontal deformation, resulting in a complex pattern of ridges and troughs. The complexity of tessera terrain is typified by a portion of central Tellus Regio (Figure 2). Individual ridges and troughs tend to be short and to vary greatly in

Aphrodite Terra [Schaber, 1982; Head and Crumpier, 1987] and a proposed region of compressional deformation and crustal convergence in western Ishtar Terra [Campbell et al., 1983; Pronin, 1986; Crumpler et al., 1986]. The broad distribution, location, and apparently intense deformation recorded by the tessera terrain all suggest that it is an integral part of Venus tectonics.

The process or processes by which tessera terrain is formed are not well characterized or understood. Several modes of

origin have been suggested, including deformation driven by appearance along strike and are typically spaced of the order of horizontal flow within the asthenosphere of Venus 10 km apart. Basilevsky et al. [1986] referred to the deformation in the tesserae as areal deformation so as to

distinguish it from more linear or arcuate features such as ridge belts and the mountain belts of western Ishtar.

Mapped tessera terrain covers approximately 10% of the region imaged by the two Venera orbiters [Sukhanov, 1986; Bindschadl•r and Head, 1989], more area than any of the other tectonic units observed on the surface (coronae, ridge belts [Barsukov et al., 1986], and mountain belts or banded terrain

1Now at Department of Earth and Space Sciences, University of Califomia, Los Angeles.

Copyright 1991 by the American Geophysical Union.

Paper number 90JB02742. 0148-0227/91/90JB-02742505.00

[Basilevsky, 1986] and gravity sliding over "gentle upwellings" in the mantle [Sukhanov, 1986]. These qualitative models are described principally to suggest their ability to create complex patterns of ridges and troughs similar to those observed in the tessera terrain. However, tesserae are also

characterized by properties measured by the PV spacecraft, including relatively high topography, rough surfaces at meter and sub-meter scales, and relatively small line of sight (LOS) gravity anomalies (where such data are available). In addition, we observe three distinct morphologic types of tessera terrain: subparallel ridged terrain (Tsr), trough and ridge terrain (Ttr), and disrupted terrain (Tds), each of which represents a particular sequence and style of deformation. Successful models for the origin and evolution of tessera terrain must be consistent with these basic characteristics of tessera terrain.

Tessera terrain is characterized by a relatively broad range of elevations [Bindschadler and Head, 1989] but in virtually all

5889

5890 BINDSCHADLER AND HEAD: MODELS FOR VENUS TESSERA TERRAIN

%

150 ø

Fig. 1. Distribution of tessera terrain, adapted from the work of Barsukov et al. [1986]. The outlined region is the full extent of Venera 15/16 data. From north to south in the 0øE-90øE quadrant, the three large regions of tessera terrain are Fortuna Tessera, Laima Tessera, and Tellus Regio. For reference, Maxwell Montes is centered on 65øN, 5øE, and western Aphrodite Terra lies between --60øE and 150øE longitude.

formation of an elevated, deformed region and consider horizontal convergence, mantle upwelling, crustal underplating, and a process analogous to seafloor spreading as possible models. In the second set we consider the structural and tectonic consequences of gravity on a region of high topography. The first of these (gravity sliding) considers thin- skinned deformation of the upper crust caused by topographic gradients. The second (gravitational relaxation) considers the deformation of the entire crust that can result from relief along both the surface and the crust-mantle boundary.

Using modificational models, we consider the possibility that tessera terrain evolves over time; that structures formed by formational processes may be altered or crosscut by structures formed during modification. A simple example is a two-stage model, for example, convergence and crustal thickening followed in time by gravitational relaxation and extension of the resulting region of high topography. In more complex examples the two processes may act at the same time in different parts of a region of tessera terrain.

Formational and modificational models are evaluated by comparing their predictions to observed characteristics of tesserae. Because of the diverse structure and morphology of tessera terrain, we address its dominant characteristics and thus the processes which dominate its formation and evolution. Thus models which are not favored as dominant processes for tessera formation/modification still might have operated in a small region or regions of tessera terrain. Of the six models, we find that three are unlikely to dominate formation/modification of tessera terrain: mantle upwelling, crustal underplating, and

cases appears to lie at a higher elevation than surrounding gravity sliding. Of the remaining three, horizontal plains. Given this pervasive topographic relationship, we convergence best explains the subparallel ridged terrain (Tsr), describe and evaluate two sets of models: formational models and continued convergence, in some cases followed by and modificational models. In the first set we treat the gravitational relaxation, best explains the disrupted terrain

Fig. 2. Venera data from central Tellus Regio, representing the typical complexity of tessera terrain structures. Image is centered on 34.5 ø N, 80 ø E and is 550 km E-W by 350 km N-S. The radar look azimuth in this and all subsequent images is approximately westward. Thus bright slopes are eastward facing (radar facing) and dark slopes are westward facing (away facing). The diffuse vertical stripes are an artifact of computer mosaicking and can be seen to varying degrees in most Venera images.

BINDSCIqADLER AND HEAD: MODELS FOR VENUS TESSERA TERRAIN 5891

(Tds). A seafloor spreading analogy can explain a number of the characteristics of the trough and ridge terrain, but there also appear to be differences between the structure of Ttr and that of the terrestrial seafloor. More detailed examination of Venera

and Magellan data will be required to establish the process(es) that have formed trough and ridge terrain.

CHARACTERISTICS OF TESSERA TERRAIN

Tessera terrain is found in at least three large regions, each at least 1000 km in its smallest dimension, and numerous smaller regions, typically < 500 km in their largest dimension [Barsukov et al., 1986; Barsukov and Basilevsky, 1986]. The three large regions are Fortuna Tessera, Laima Tessera, and Tellus Regio (Figure 1). Numerous smaller tesserae are located near these three regions. Most tessera terrain is found between 0øE and 140øE longitude, although a number of small regions are located to the west, bordering Akna, Freyja, and Danu montes, which themselves surround Lakshmi Planum. Surface properties inferred from PV data were used to predict the locations of tessera terrain south of Venera coverage [Kreslavsky et al., 1988; Bindschadler et al., 1990a]. Evaluation of that prediction using a variety of radar data, including recent Arecibo images [Campbell et al., 1989] suggests that tessera terrain is widely distributed throughout the equatorial and southern regions of Venus [Bindschadler et al., 1990a].

Regions of tessera terrain appear similar in Pioneer Venus (PV) data but are characterized by diverse appearances in Venera 15/16 images. We find that this diversity can be characterized in terms of three major terrain types, each representing a particular style and sequence of deformation. These and other morphologic characteristics of tessera terrain can be used to evaluate the various formational and modificational model for

the tessera terrain.

Topography and Radar Properties

Regions of tessera are among the most distinctive of the Venera units in terms of elevation and PV surface radar

properties [Bindschadler and Head, 1988a, 1989]. Elevations within tesserae typically range from 1 to 3 km above the mean planetary radius (6051.9 km), with an average near 2 km [Bindschadler and Head, 1989]. Almost all tesserae lie at higher elevations than surrounding units (typically plains), even relatively small regions such as Ananke Tessera (~50øN, 135øE). Tesserae commonly have relatively steep boundaries and somewhat lower relief in the interior, resulting in a characteristic plateau shape. In other cases, the boundary between plains and tessera terrain is less topographically distinct [Sukhanov, 1986]. Measurements of surface properties by the PV orbiter [Pettengill et al., 1980, 1982, 1988] indicate that the surface is extremely rough at scales ranging from 5 cm up to 10 m [Bindschadler and Head, 1988a, 1989]. Such surface roughness is strongly associated with tectonic units mapped from Venera 15/16 data and is thought to be related to tectonic deformation [Bindschadler and Head, 1988a, 1989].

LOS Gravity

Another important source of data is line of sight (LOS) gravity data obtained from Doppler radio tracking of the PV spacecraft [Sjogren et al., 1983]. Because of limitations related to spacecraft altitude, maps of LOS anomalies are restricted to

the region between approximately 20øS and 50øN latitude. Three large tesserae lie at least partly within this equatorial band: Tellus Regio, Laima Tessera, and Alpha Regio. All three regions are characterized by anomalies of less than 5 mGal [Sjogren et al., 1983], despite the fact that elevations within these regions are principally greater than 1 km above surrounding terrain. This is in distinct contrast to the larger gravity anomalies observed over topographic highs such as Beta, Aria, Eistla, and Bell regiones, and also contrasts with the often-cited positive correlation of gravity and topography for Venus [e.g., Phillips and Malin, 1983]. In particular, Tellus Regio exhibits a very low correlation between gravity and topography in comparison to regions such as Beta and Aria [Sjogren et al., 1983]. These characteristics suggest the possibility of fundamental differences in mechanisms of compensation between tessera terrain and regions such as Bell, Beta, and Atla regiones. A recent comparison of geoid- topography ratios (GTRs) for topographic features showed that proposed hotspots (e.g., Beta, Bell) and known and predicted tessera [Bindschadler et al., 1990a] formed largely distinct groups in terms of GTR and characteristic wavelength or size [Smrekar and Phillips, 1990]. Tesserae were characterized by smaller values of GTR than proposed hotspots. Thus the gravity and topography of tesserae are most consistent with compensation due to crustal thickness variations or to shallow mantle/lithospheric processes.

Types of Tessera

Tessera terrain was first defined from its appearance in Venera 15/16 radar images to consist of orthogonal to obliquely oriented intersecting sets of ridges and troughs [Basilevsky et al., 1986] (Figure 2). However, regions that fit this broad definition are diverse [Sukhanov, 1986]; in some cases, different morphologies can be seen within a single region of tessera. Examining Venera images of the three large regions of tessera (Figure 1), we define three types of tessera on the basis of a characteristic morphology, consisting of ridges, troughs, grooves, and lineations, their continuity, and angular and crosscutting relationships. Interpreting these morphologic elements as tectonic structures, we suggest that each type of tessera reflects a particular style and sequence of deformation. These types of tessera thus represent important constraints on models for formation and evolution of the terrain.

Subparallel ridged terrain. This terrain type is characterized by the presence of numerous subparallel ridges and troughs and cross-strike lineations. The type area for subparallel ridged terrain (Tsr) is located to the east of Maxwell Montes, in Fortuna Tessera (Figure 3a). Subparallel ridged terrain is also found in western Tellus Regio and in Atropos Tessera, adjacent to the western edges of the banded terrain in Akna Montes. Although similar in morphology to the banded terrain, Tsr is distinguished by the presence of abundant lineations which disrupt ridge trends.

The most prominent structures in the Tsr are subparallel ridges, which strike approximately N-S to NE-SW in the type area (Figure 3b). Ridge spacings are typically 10-15 kin, and most ridges are continuous for over 50 km along strike. Major ridges are typically continuous for up to 150 km. In addition to ridge structures, Tsr contains numerous lineations. These are defined by discontinuities in the subparallel ridges and by a few distinct trough or ridge structures (Figure 3b) oriented approximately N60øE and N60øW and are typically continuous

5892 BINDSCHADLER AND HEAD: MODELS FOR VENUS TESSERA TERILA, IN

Fig. 3a. Venera image of type area for subparallel ridged terrain (Tsr), located to the east of Maxwell Montes, in Fortuna Tessera. Image is centered at approximately 67øN, 16øE. The three major ridges to the left and below the center of the image appear to be crosscut by lineations in some cases, crosscut lineations in others, and typify the complex relationships between ridges and lineations (see Figure 3b).

L

It ,j

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I J / ridge 100 200 km '- -- - - _ lineation

Fig. 3b. Sketch map of type area for subparallel ridged terrain.

over distances greater than 100 kin. Lineations in the type area were noted previously by Ronca and Basilevsky [1986] and can alsoi be observed in Arecibo images of the region. synchronous with) conjugate strike-slip faulting. If the ridges

Morphologic characteristics of ridges in the Tsr suggest were extensional, conjugate strike-slip faults would be that they formed by compression. Ridges and troughs form expected to strike approximately N30øE and N30øW, repetitive sets of subparallel structures, are continuous over inconsistent with the observed trends. The partly synchronous relatively long distances (--100 kin), and appear to be nature of ridges and lineations indicates that the two sets of relatively symmetric, characteristics most commonly features did not originate independently. Thus we interpret Tsr associated with compressional structures. Ridges in the Tsr are structures to originate by compressional and strike-slip or thus similar to ridges observed in Maxwell, Akna, and Freyja shear deformation. Lineations similar to those in the Tsr are montes, which have been interpreted as compressional in observed in Akna, Freyja, [Crumpier et al., 1986], and Maxwell origin [Crumpier et al., 1986; Pronin, 1986; Basilevsky et al., montes [Vorder Bruegge et al., 1990] and have also been 1986; Vorder Bruegge et al., 1990]. Ridges in the Tsr also lie interpreted as strike-slip faults or indications of shearing. parallel to ridges in Maxwell Montes. It seems likely that both Trough and ridge terrain. Structures in the trough and ridge sets of structures were formed by the same process. terrain (Ttr) are expressed as troughs in one direction and as

Characteristics of ridge-lineation intersections suggest that ridges and valleys oriented orthogonal to the troughs. The type lineations are due to strike-slip or shear deformation. At the area for Ttr is located in eastern Laima Tessera (Figure 4a). intersections of ridges and lineations, subparallel ridges Trough and ridge terrain is also found in Meshkenet Tessera, in undergo a number of changes. Most typically, ridge trends stop Tethus Regio. at such intersections. In other cases ridges, change strike in the Troughs appear as both broad (--50 km) and narrow (--10-20 near vicinity of lineations or change character, becoming kin) structures, with spacing between troughs typically greater significantly less prominent and exhibiting less relief. In than 30 kin. Broad troughs commonly consist of two distinct some cases there is little or no interruption of ridge continuity scarps separated by a flat floor composed of smooth plains as it passes through a lineation, indicating that formation of deposits and are continuous over distances of up to 1000 km. ridges and lineations overlapped somewhat in time. Angular Given the low erosion rates on Venus [lvanov et al., 1986], the and crosscutting relationships between ridges and lineations apparent lack of extensive soil deposits [McGill et al., 1983; are consistent with compression followed by (and partly Bindschadler and Head, 1988a, 1989], and the pervasive nature

BINDSCHADLER AND HEAD: MODELS FOR VENUS TESSERA TERRAIN 5893

Fig. 4a. Venera image of type area for trough and ridge terrain (Ttr), located in central and eastern Laima Tessera. Image center is 53øN, 53øE. Exceptions to the generally parallel nature of troughs are found in the southern portions of the region, particularly in the southeast corner.

of plains volcanism, smooth plains within the large troughs are most likely to be of volcanic origin. Narrow troughs may be similar, but their floors are not clearly resolved in the Venera data. Trough structures could originate in a number of ways. Their similarity in shape to graben suggests that they may be extensional features. Head [1990b] has suggested that they may be analogous to oceanic fracture zones on Earth, citing their continuity and parallelism. Troughs could also be the result of strike-slip faulting or shearing.

In contrast to trough structures, ridge and valley structures are small, closely spaced, and continuous over shorter distances, resulting in a corrugated appearance. Widths of the corrugations vary from -20 km down to features at the 1-3 km limit of Venera resolution. In some cases, ridges appear flat- crested and steep-sided, similar to horst structures. Similarly, valleys commonly resemble graben structures. In the southern portions of the type area, corrugations are typically expressed as distinct grooves (Figures 4a and 4b), which are distinguished from troughs and valleys by the presence of raised rims. The shapes of corrugations and groove structures are most consistent with an extensional origin, although a compressional origin cannot be ruled out from presently available data.

Ridge and valley structures typically appear to terminate within domains between troughs, although examples can be seen in which they appear to crosscut trough structures (e.g., arrow in Figure 4b). Higher-resolution data will be required to definitively establish whether a consistent crosscutting relationship is present or not, particularly in the northeastern portion of the type area, where structures are relatively small and closely spaced (Figure 4a).

/

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I I /• ridge .c-r-r- scarp o lOO 200 km • groove '"% trough Fig. 4b. Sketch map of type area for trough and ridge terrain (Ttr). The heavy line in the eastern portion of the sketch delimits the boundary of tessera terrain. The arrow in the southern portion of the figure points to a region where ridge and valley structures appear to crosscut a large trough.

Disrupted terrain. Central Tellus Regio is the type area for disrupted terrain (Tds, Figure 5a). Disrupted terrain is the most common type of tessera, is found in all three large regions of tessera, and predominates among smaller regions. Ridges, troughs, grooves, and lineations are all found within the Tds. Disrupted terrain is characterized by a more chaotic appearance than the other two types of tessera, primarily due to a deficit of continuous ridges longer than-•50 km. However, lineations defined by discontinuities in ridges and short, discontinuous troughs and ridges tend to maintain consistent orientations over a region. Grooves are relatively common within the Tds as compared to the other two terrain types and tend to be the most continuous, throughgoing structures in the Tds.

Ridges in the Tds tend to be symmetric and in some cases form subparallel sets (Figure 5b). In regions where Tds is bounded by Tsr, the strike of ridges as well as their appearance change only very gradually from one type of tessera to the other. This suggests that Tds ridges originate in the same manner as Tsr ridges. We therefore interpret them as compressional features. Lineations within the Tds disrupt ridges in the same manner as was observed within the Tsr and possess a similar morphology. This suggests that Tds lineaments also originate by strike-slip or shear deformation. Troughs and grooves in the Tds could be (1) extensional features, (2) fracture zone analogs, as suggested for similar features in the Ttr [Head, 1990b], or (3) the surface expressions

5894 BINDSCHADLER AND •: MODELS FOR VENUS TESSERA TERRAIN

Fig. 5a. Venera image of type area for disrupted terrain (Tds), located in central Tellus Regio. Image is centered on 38øN, 79øE. The volcano near the center of the image is the largest volcano found within a region of tessera and appears to sit in a distinct trough. Structures within the disrupted terrain tend to be much less continuous and more I variable in orientation than those in the other two types of tessera. 0

• I • ridge ,z( < scarp 100 200 km • groove ...-lineation

trough of strike-slip faults. Troughs usually lie along or parallel to lineations, suggesting that these structures share a similar origin. The raised rims of grooves are most consistent with an extensional origin. In addition, grooves are typically associated with intratessera plains deposits found in local topographic lows (see discussion of intratessera plains, below), also consistent with an extensional origin.

Many areas of the disrupted terrain appear to record compressional, extensional, and shear deformation. Ridges appear to be predominantly compressional in origin, while troughs and lineaments are most likely to originate by strike- slip faulting or shearing. Grooves appear to be extensional features and probably represent the most recent deformation within the Tds. Grooves tend to be more continuous than ridges or troughs and in most cases crosscut other Tds structures whenever intersections occur. In areas where ridges, grooves, and lineations occur, disrupted terrain appears to have undergone compression, followed by and partly synchronous with strike-slip deformation, followed by extension.

Tessera-Plains Boundaries

Boundaries of tesserae and plains regions are characterized by two morphologies [Sukhanov, 1986; Bindschadler and Head, 1988b]. The first (type I) is the most common, is highly irregular at the 100-km scale, and is represented by southeastern Tellus Regio (Figure 6a). Contacts between plains and regions of tessera terrain occur as onlap of plains onto

Fig. 5b. Sketch map of type area for disrupted terrain (Tds). The arrow points toward a set of subparallel ridges and troughs. Such sets are characteristic of subparallel ridged terrain (Tsr).

tessera or as distinct scarps (Figures 6a and 6b). In the former case, structures appear subdued, consistent with embayment. Scarp orientations are typically unrelated to dominant structural trends within the tessera. For example, in Figure 6a, most scarps strike approximately N-S, while the dominant trends within Tellus are approximately NE-SW and NW-SE. Type I boundaries appear to form principally by embayment. The topographic scarps observed between plains and tesserae are either primary features which formed before embayment or relatively late-stage tectonic features which may serve to downdrop blocks of tessera terrain.

Type II boundaries are relatively linear at the 100 km scale (Figure 7a) and are commonly characterized by steep regional slopes [Sharpton and Head, 1985]. Several ridge belts have been mapped near such boundaries, such as the region between the plains-tessera boundary and the dashed line in Figure 7b. However, such mapped "ridge belts" are morphologically distinct from the ridge belt structures located in the region between 150øE and 240øE longitude [Frank and Head, 1990]. They lack the anastomosing nature of structures in the ridge belts, are found in areas with over 1 km of relief, and possess transitional boundaries with disrupted terrain (Tds). These


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Fig. 6a. Venera image of type I boundary between tessera terrain and surrounding plains, located in southeastern Tellus Regio. Image is approximately 600 km E-W by 500 km N-S and is centered on 32.5øN, 85øE. Note the subdued appearance of tessera structures near the boundary, particularly in the upper central part of the image.

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Fig. 6b. Sketch map of the region in Figure 6a. Structural symbols are the same as those used in Figure 5b. The stippled pauem represents plains regions.


Fig. 7a. Venera image of type II boundary between tessera terrain and surrounding plains, located in western Tellus Regio. Image is centered on 33'N, 73'E.

I i I 0 1 O0 200 km

Fig. 7b. Sketch map of the region in Figure 7a. Lines indicate ridges or grooves; barbed lines are scarps (e.g., Figure 5b). Stippled pattern

characteristics match those described above for subparallel represents intratessera plains (Pit), which lie along a regional ridged terrain (Tsr), rather than those of ridge belts. topographic high. The dashed lines indicate regions previously

Numerous small, mare-like ridges are present within plains adjacent to type H boundaries, trending parallel to the boundary (e.g., Figure 7a). The overall linearity of the boundary and the structural continuity suggested by parallel ridges in the tessera across the features (Figure 7) reveal that the plains lie in local terrain and within the plains suggest that deformation topographic troughs. Examination of PV and Venera postdates most plains-forming events. The presence of Tsr is topography shows that the local troughs of the Pit in Tellus interpreted to indicate compressional deformation. Type II Regio lie along broader-scale regional topographic highs. boundaries thus appear to represent a tectonic relationship The formation of Pit appears to postdate most of the between plains and tessera, while type I boundaries represent observed deformation within tessera terrain. Plains surfaces are embayment of tessera terrain by plains units. clearly undeformed and commonly superpose otherwise

continuous structural features or trends in the surrounding

Intratessera Plains tessera terrain. Most boundary-parallel structures (usually grooves) are aligned with regional structural trends but are

The three large regions of tessera all contain oval to relatively sharp in appearance, continuous, and throughgoing polygonal regions of intratessera smooth plains, in comparison to surrounding tessera structures. As noted approximately 50-200 km in width. Typically, intratessera above, grooves usually crosscut other structures within the Tds plains (Pit)are surrounded by groove structures that parallel or and are thus relatively young. The fact that they bound Pit define the plains boundary. A group of Pit occur in western suggests that the origin of both features is related and that both Tellus Regio (Figure 7). Examination of PV reflectivity, RMS are young compared to most other structures within the Tellus slope, PV synthetic aperture radar (SAR), and Venera SAR data Regio. reveals that the Pit are much smoother than surrounding Intra-tessera plains are also present in Laima and Fortuna tesserae at scales of 5 cm to 10 m and are similar to typical Tesserae. In Laima they tend to be oval in shape, and boundary- Venus plains regions in terms of radar properties [Bindschadler parallel structures are less distinct or occasionally absent. and Head, 1988a, 1989]. The plains deposits are therefore Although they retain the trough-like shape of Pit in Tellus interpreted as volcanic [Barsukov et al., 1986; Bindschadler et Regio, these plains do not lie along regional topographic al., 1986]. In Venera SAR data, variations in radar backscatter highs. Intra-tessera plains also occur in central Fortuna


Tessera, in a region referred to as chevron tessera [Vorder Bruegge and Head, 1989]. These plains regions are distinctly polygonal to linear or arcuate, display relatively indistinct boundary-parallel structures, and lie within a broad topographic low in Fortuna Tessera. Some of the larger Pit near the southern boundary of Fortuna appear to be deformed, displaying a pattern of small (.--3 km wide) ridges on the plains. Deformation is minor compared to the surrounding tessera terrain. Despite some differences, Pit in Laima and Fortuna are similar to those in Tellus in terms of postdating most deformation, and thus they appear to be relatively young.

MODELS

Previous models for the tessera terrain include deformation

driven by asthenospheric flow [Basilevsky, 1986], gravity sliding above asthenospheric upwellings [Sukhanov, 1986; Smrekar and Phillips, 1988], and gravity sliding driven solely by topographic gradients [Kozak and Schaber, 1986]. Unresolved questions as to the nature of Venus tectonics [e.g., Kaula and Phillips, 1981; Arvidson and Davies, 1981; Solomon and Head, 1982; Phillips and Malin, 1983; Morgan and Phillips, 1983; Phillips, 1986; Grimm and Solomon, 1987; Head and Crumpier, 1987; Turcotte, 1989] and the diverse morphologies of tesserae have led us to formulate a number of models for the formation and evolution of this terrain by considering these and other tectonic processes. Models are then evaluated by comparing their predictions to the basic characteristics of tessera terrain.

Horizontal Compression

Mantle Upwelling

Crustal Underplating / Intrusion

Fig. 8. Sketches representing formational models for tessera terrain. Question marks indicate uncertain nature of forces driving horizontal convergence.

Two types of models are considered: formational and modificational models. Formational models lead to the creation of a deformed region of high topography and include horizontal convergence, mantle upwelling, crustal underplating, and a process analogous to seafloor spreading (Figure 8). Modificational models consider the effects of body forces on a region of high topography, opposed by the strength of the lithosphere and by any formational processes that continue to operate. On Earth, gravitational collapse may be an important process in the history of mountain belts [e.g., Froidevaux and Ricard, 1987; Dewey, 1988], but the action of gravity is overshadowed by water-related erosional processes which weather material in elevated regions, transport it, and ultimately deposit it at or below sea level. On Venus, there is no free water on the surface, and erosion rates are much lower than on Earth [lvanov et al., 1986; Sharpton and Head, 1985; Bindschadler and Head, 1988a]. Tectonic processes may be much more important for reducing surface relief than on Earth. We consider two modificational models: gravity sliding and gravitational relaxation of compensated topography (Figure 9). These models imply that the morphology of tessera terrain evolves over time and that this evolution can involve more

than a single process.

Gravitational Relaxation extension

Gravity Sliding breakaway fault

extension

compression I /' ./•:?• ii ii?i

Fig. 9. Sketches representing modificational models for tessera terrain. Inset box for gravity sliding indicates possible relative scale of gravitational relaxation and gravity sliding.

Horizontal Convergence and Crustal Thickening

In this model, the high topography of the tesserae is due to compressional thickening of crustal material. Such deformation may be driven by the convergence of large lithospheric plates, as it is on Earth, or in response to downwelling flow within the mantle [Bindschadler and Parmentier, 1990], as represented by the arrows in Figure 8. Structures due to convergence should be dominated by horizontal compressional features. Unless the crust attains

5898 BINDSCHADLER AND HFAd): MODELS FOR VENUS TESSERA TERRAIN

large thicknesses (~100 km), gravity anomalies are expected to be relatively small. Features on Venus thought to be due to convergence include Akna, Freyja, and Maxwell montes. These mountain belts are dominated by compressional features and also contain features interpreted as strike-slip faults or shear zones [Crumpier et al., 1986; Pronin, 1986; Basilevsky et al., 1986; Head, 1990c; Vorder Bruegge et al., 1990].

Based on terrestrial examples, convergence can cause sufficient crustal thickening to explain the observed elevation of regions of tessera. On Earth, the Tibetan Plateau appears to be a region where the crust is approximately double the thickness of typical continental crust [Chen and Molnar, 1981]. Even if the density contrast between Venus crust and manfie is relatively small (e.g., Pc = 3.0 g cm '3 and Pm = 3.3 g cm'3), and we assume a typical crustal thickness of 25 km [e.g., Zuber, 1987; Grimm and Solomon, 1988], doubling such a crust yields ~2.3 km of isostatically supported relief. Approximately 90% of tesserae lies within 2.3 km of the mean planetary radius [Bindschadler and Head, 1989]. If the density contrast between crustal and mantle materials is greater, lesser amounts of crustal thickening are required to produce high elevations. Observed LOS gravity anomalies over Alpha and Tellus regiones are relatively small, consistent with crustal thickening and Airy compensation.

Considered in terms of the examples of terrestrial orogenic belts and Venusian mountain belts, horizontal convergence and compressional deformation predict many of the features of subparallel ridged terrain (Tsr) and disrupted terrain (Tds). There are no compressional structures in the trough and ridge terrain (Ttr) (unless the interpretation of ridge and valley slructures as extensional features is incorrect), and thus the Ttr does not appear to be the result of convergence. Ridges and lineations within the Tsr and Tds (Figures 3 and 5) are consistent with compressional and strike-slip deformation, respectively. Moreover, lineations are found in the Tsr in the orientation expected for conjugate strike-slip faults formed during compression. The transitional nature of boundaries between Tds and Tsr may be explained if disrupted terrain is formed by progressive strike-slip faulting, and possibly block rotation, of subparallel ridged terrain. Tapponnier and Molnar [1976] suggest that compressional deformation involving significant thickening of the crust will lead to reorientation of principal stresses. Following Anderson's theory of faulting, compression initially results in thrust faulting, since the least compressive principal stress (133) is vertical. As topography grows and crustal thickness increases, vertical normal stresses become increasingly compressional until 133 lies within the horizontal plane and strike-slip deformation occurs. On Earth, strike-slip features have been associated with orogenesis. In addition, extensional features are found within high plateaus formed at convergent plate boundaries [Molnar and Tapponnier, 1978; Dalmayrac and Molnar, 1981] and are thought by a number of workers to result from continued growth of topography and increasing vertical normal stresses. Extensional features are found within the disrupted terrain and are discussed further in the section on gravitational relaxation.

The relationship between the mountain ranges of western Ishtar Terra and tessera terrain is also consistent with a

horizontal convergence model. The mountain ranges that border Lakshmi Planurn (Akna, Freyja, Maxwell, and Danu montes) are themselves bordered by tessera terrain. These regions of tessera form a plateau outboard of Lakshmi Planurn and appear to be a significant part of the architecture of

mountain belts on Venus [Head et al., 1990]. Moreover, boundaries between the mountain belts and the tesserae are

transitional in nature and are characterized by a great deal of structural continuity. Type II boundaries (Figure 7) between plains units and tesserae are also consistent with convergence and crustal thickening and are most commonly associated with T sr. These relationships suggest that the origin of the Lakshmi-surrounding regions of tessera are related to horizontal convergence [Head, 1990c], as are other regions of Tsr and Tds associated with type 11 boundaries.

Mantle Upwelling

In the second model, a mantle hotspot or plume causes thermal and/or dynamic uplift of the surface (Figure 8). The primary candidates for such upwellings are regions such as Beta, Atla, and Bell regiones. These swell-like regions have been suggested to be related to hotspot activity within the mantle on the basis of pervasive volcanism, apparent depths of compensation of the order of 150-300 km, and correspondingly large gravity anomalies [Esposito et al., 1982; Janle et al., 1987; Senske and Head, 1989]. It would then be expected that tessera terrain is related to these domal uplifts [Barsukov et al., 1986], with tessera terrain perhaps representing a later stage in the evolution of domal uplifts. Deformation due to uplift would include circumferential extensional features due to flexure [e.g., Banerdt, 1986] and might also include features due to shear tractions applied at the base of the lithosphere by mantle flow [Phillips, 1986, 1990] . If upwelling flow persists for sufficiently long times, significant crustal thinning and surface subsidence may result [Bindschadler and Parmentier, 1990]. Deformational features due to mantle upwelling are expected to be dominated by extensional structures.

Topography and LOS gravity strongly constrain the applicability of the upwelling hypothesis to the formation of tessera terrain. Where both are available, these data suggest that regions of tessera terrain are compensated at relatively shallow depths (less than 100 km) and are characterized by small ratios of geoid to topography compared to proposed hotspot features [Smrekar and Phillips, 1989, 1990]. This suggests that regions of tessera are not presently supported by deep-seated variations in mantle temperature.

Pronin [1986] suggests that Lakshmi Planurn is a locus of mantle upwelling and that the mountain ranges there were created by shear along the base of the lithosphere due to flow of mantle material away from the upwelling. According to this model, regions of tessera outboard of Lakshmi could form as a result of upwelling. However, a quantitative model for mantle flow tectonics [Bindschadler and Parmentier, 1990] indicates that such a scenario is unlikely. If the mountain ranges are the result of crustal thickening due to manfie flow, then this study indicates that crustal thinning of equal or greater magnitude is expected to occur in Lakshmi Planurn. Extensional features are largely absent on the Planurn surface, aside from a few small (~10 km wide) graben or fractures [Roberts and Head, 1990]. In fact, the deformation and topography associated with western Ishtar Terra and Lakshmi Planurn are better explained in terms of mantle downwelling [Bindschadler and Parmentier, 1990; Bindschadler et al., 1990b].

Several morphologic characteristics of tessera terrain are also inconsistent with a mantle upwelling origin. Swells such as Beta and Bell regiones are characterized by dome-shaped

BINDSCHADLER AND •: MODELS FOR VENUS TESSERA TERRAIN 5899

topography. In contrast, tessera terrain is commonly characterized by a steep-sided plateau shape. Domal uplifts are commonly characterized by large rift zones (e.g., Beta and Atla regiones), and all domal uplifts contain at least two large (-200 km diameter) shield volcanoes [McGill et al., 1981; Janle et al., 1987; Stofan et al., 1989; Senske and Head, 1989]. The only shield-type volcanic structure observed in the tessera terrain is located in Tellus Regio (Figure 5a) and is no more than 50 km in diameter. Moreover, deposits related to the volcano clearly embay tessera structures, indicating that the feature postdates deformation.

Some of the differences between domal uplifts and tesserae might be explained if tesserae represented older, more evolved mantle upwellings, in which the mantle source has died out or moved away. However, no structures are apparent in the tessera terrain that represent good candidates for altered or degraded versions of rifts or large shield volcanoes. Regions of tessera are present on the flanks of both Bell [Barsukov et al., 1986; Janle et al., 1987] and Beta regiones [Bindschadler et al., 1990a; Campbell et al., 1989]. In both cases, the tessera terrain appear to be relatively old, embayed by plains volcanism. While it is possible that the formation of these few small regions of tessera is related to mantle upwelling, available data strongly suggest that most regions of tessera did not form in this way.

Crustal Underp lating

Emplacement of a large volume of low-density (crustal) intrusive material at or near the crust-mantle boundary should result in surface uplift and deformation (Figure 8). On the basis of the distribution of elevations for the tessera terrain, Nikolaeva et al. [1988] suggest that tessera terrain is composed of highly feldspathic material, such as anorthosite. Rapid emplacement of low-density material such as anorthosite near the crust-mantle boundary can be modeled as a gravitational relaxation process, in which surface topography grows in order to balance the excess low-density root at depth. Intruded/underplated material could be of either basaltic or anorthositic composition; the only requirement is that it be less dense than underlying mantle materials. Underplating is predicted to result in uplift and extensional deformation of the surface [Bindschadler, 1990]. Unless underplating proceeds very rapidly and has occurred relatively recently, gravity anomalies should only reflect the thickened crust. There are several reasons why such a model is unlikely to explain the formation of any of the types of tessera terrain.

Surface deformation due to underplating is exclusively extensional in nature. Observations suggest a compressional origin for subparallel ridged terrain (Tsr), as well as much of the disrupted terrain (Tds), which shares transitional boundaries with the Tsr. Trough and ridge terrain (Ttr) structures are consistent with extensional deformation but require that the underplating event create a highly organized orthogonal pattern of structures.

The volume, surface area, and heat input corresponding to such an intrusion are extremely large. For Tellus Regio alone, the intrusion would correspond to a surface area of -1.5x106 km 2. Assuming a relatively large density contrast between intrusive material and mantle material (20%), a minimum of 7.5x106 km 3 of underplated material is required to support the topography of Tellus Regio through Airy isostasy. To differentiate this volume of material from the mantle of Venus

would require a minimum of 1025 J, assuming a latent heat of fusion of 400 kJ kg '1. This corresponds to the entire global heat loss for Venus over a period of ~105 years, based on a nominal rate of 3.4x1013 W [Solomon and Head, 1982]. Such a differentiation event might occur over a much longer period of time. However, relaxation of stresses due to underplating should occur quickly. The characteristic time for such

relaxation is x = 4nlX/pg3,, where I.t is mantle viscosity, p is density, and )• is the characteristic wavelength of a region of noncompensated topography [Turcotte and Schubert, 1982]. For )• =1000 km (e.g., Laima Tessera), I.t = 1021 Pa s, and p = 3000 kg m '3, we find x = 1.6x10 4 years. Thus a single differentiation event of the suggested scale would require more energy than is available from the planet. The organized structural pattern of the Ttr requires that the geometry of the intrusion in plan view and thus the orientation of principal stresses remain approximately constant. It is highly unlikely that repetitive intrusion events, at least 106 years apart to satisfy heat flow constraints, could also repeat the same geometry time after time. Although underplating may occur on Venus, it does not appear to be feasible as a mechanism for the production of the observed high topography and deformation of tesserae.

Seafloor Spreading Analogy

This model is based upon the hypothesis that a process analogous to seafloor spreading occurs in western Aphrodite Terra [Head and Crumpier, 1987] and has been suggested to apply specifically to Laima Tessera [Head, 1990b]. As modeled by Sotin et al. [1989], spreading on Venus is expected to result in crustal thicknesses somewhat greater than on Earth (-15 km), due to higher average mantle temperature. To produce the relatively high elevations and thick crust of tessera terrain, a region of anomalously high mantle temperature (e.g., a hotspot) is required, similar to the situation postulated by Sotin et al. [1989] for Ovda Regio. On Earth, enhanced crustal thicknesses (up to -9 km greater than average) are found along the Mid-Atlantic Ridge at its intersection with the Iceland hotspot [Pallmason and Saemundsson, 1974]. Thus a spreading- like process appears to be capable of producing a topographically high region characterized by relatively shallow compensation.

Structures created by the spreading process on Earth'include fracture zones and abyssal hills in approximately orthogonal orientations. The exact mechanism(s) by which abyssal hills form on Earth is controversial, but both spreading-related extension and volcanism are thought to contribute. Tectonic processes associated with near-spreading center extension, thrusting, and block rotation are thought to dominate slow spreading ridges (-1-3 cm yr -1 full spreading rate) [e.g., Harrison and Stieltjes, 1977; Macdonald, 1986], while episodic volcanism occurring at the ridge crest becomes increasingly important along medium (-3-9 cm yr '1) and fast spreading ridges (>9 cm yr -1) [Kappel and Ryan, 1986; Barone and Ryan, 1988; Pockalny et al., 1988]. Suggested rates of spreading on Venus [Kaula and Phillips, 1981; Crumpler and Head, 1988; Sotin et al., 1989] are in the range of slow spreading ridges. On the basis of our understanding of the terrestrial spreading process, abyssal hills on Venus should be predominantly tectonic in origin, consistent with the tectonic origin of ridge and trough structures in the Ttr. However, the transition between tectonic-dominated and volcanic-dominated


abyssal hills may be a function of lithospheric strength in the vicinity of the spreading center, rather than spreading rate. If so, the higher surface temperature of Venus may extend the volcanic-dominated regime to slower spreading rates. In addition, the presence of a mantle hotspot might also be expected to increase the amount of volcanic topography and/or resurfacing because of increased availability of melt.

The roughly orthogonal pattern of terrestrial seafloor is most like the structural pattern of trough and ridge terrain (Ttr), with troughs corresponding to fracture zones and ridge and valley structures corresponding to abyssal hills [Bindschadler and Head, 1988c; Head, 1990b]. If disrupted or subparallel ridged terrain (Tds, Tsr) originally possessed an orthogonal pattern, then that pattern has been so strongly overprinted by subsequent deformation as to be unrecognizable. Morphologic observations suggest that only the Ttr structural pattern could have originated as purely the result of a spreading process. We also note that the western Aphrodite region displays high values of RMS slope and diffuse scattering [Bindschadler and Head, 1988a], properties characteristic of much of the tessera terrain [Bindschadler and Head, 1989], including Laima Tessera (the type area for Ttr). A spreading process appears to be consistent with some of the basic properties of tessera terrain.

Specific comparisons can be made between a number of features in the trough and ridge terrain and the terrestrial seafloor that may help to resolve the applicability of the spreading hypothesis. Unlike many terrestrial fracture zones, troughs in the Ttr are not necessarily parallel and have not been demonstrated to define the distinct changes in regional elevations that result from the juxtaposition of lithosphere of different ages. Troughs also appear to be loci of plains volcanism, which is not a common characteristic of terrestrial fracture zones. Although important, such differences do not allow us to rule out a spreading analogy. On Earth, changes in poles of motion of plates and propagation of spreading centers can result in complex, nonparallel geometries for fracture zones. A lack of distinct elevation changes across fracture zones is commonly observed in terrestrial lithosphere older than ~80 m.y. and is thought to be due to thermal equilibration. The presence of volcanic deposits in the floor of troughs may simply reflect higher surface temperatures, thinner lithosphere, or more pervasive volcanism on Venus.

One characteristic of terrestrial seafloor is the presence of numerous small volcanoes or seamounts, many of which are thought to originate near a spreading center. Their distribution is not well characterized over the entire ocean floor, but statistical studies suggest number densities of the order of 200 seamounts with over 1 km of relief per 106 km 2 [Smith and Jordan, 1988]. Numerous small volcanic domes are observed on Venus [Barsukov et al., 1986] and have been suggested to be analogous to terrestrial seamounts [Aubele and Slyuta, 1990]. According to Smith and Jordan's [1988] values, there should be of the order of 300 seamounts within Laima Tessera (the location of most of the Ttr) with elevations >1 km and corresponding basal diameters >10 km. Differences between large volcanoes on Earth and Venus suggest that Venusian seamounts might be significantly lower in elevation [Head and Wilson, 1986] but would therefore have correspondingly larger diameters. Such factors, as well as unknown differences in the

spreading process between Earth and Venus might lead to both lesser production of seamounts and difficulties in identifying those that are present. It is equally clear, however, that there

are no easily identifiable volcanic domes larger than 10 km in diameter within the type area for Ttr (Figure 4).

A major identifiable characteristic of slow spreading terrestrial ridges is a rift valley, defined by fault-bounded escarpments. Deformation of newly formed crust is accommodated by numerous normal faults within the rift valley. However, even major faults (with up to 200 m of throw) persist for only 1-2 km along strike, while major features such as the rift valley walls persist for up to -•40-60 km along strike [Macdonald, 1986]. This lack of continuity contrasts strongly with the length and continuous nature of transform faults and their aseismic extensions and suggests that regions between transform faults deform independently of one another. If formed by a Venusjan spreading process, ridge and valley structures in the Ttr should appear continuous across troughs only by coincidence. Most ridge and valley structures in the Ttr appear to be discontinuous across trough structures, but some do appear to crosscut troughs (Figure 4). A critical test would be to see whether most ridge and valley structures appear to disrupt the floors of troughs not partly filled by smooth plains. This test could be made using Magellan data.

A seafloor spreading process must be considered a candidate for the origin of the trough and ridge terrain. On the basis of analogies with terrestrial spreading, this process is consistent with the basic constraints (topography, gravity, orthogonal structural pattern) offered by observations. However, morphologic observations clearly show that if such a process is responsible for the Ttr, it produces a surface that is lacking in seamounts and commonly exhibits nonparallel transforms which are also loci for plains volcanism. The differences between terrestrial seafloor and Ttr suggest that further investigation will be needed to resolve the the origin of Ttr, possibly including analysis of high-resolution Magellan data. Important tests of the spreading hypothesis include understanding the nature of structural and age relationships between the troughs and ridge and valley structures and the nature of deformation that formed ridge and valley structure.

Gravity-Driven Modification

A number of terms have been used to denote gravity-driven tectonic processes, including gravity sliding, gravity spreading, and gravitational relaxation. In terms of Venus tectonics, two modes have been discussed. The first involves the formation of a detachment surface, creating either a brittle or ductile d6collement over which a relatively thin slice or wedge of crust slides [e.g., Sukhanov, 1986; Smrekar and Phillips, 1988]. This process will be referred to as "gravity sliding." Over the large areas and at the relatively small regional slopes that characterize tessera terrain, gravity sliding is expected to occur at geologic strain rates (<10 -14 s-l), rather than as a catastrophic event, such as a landslide. The second mode involves whole crustal deformation driven by gradients in vertical normal stresses associated with relief at the surface

and/or along the crust-mantle boundary [e.g., Ramberg, 1968; Artyushkov, 1973] and will be referred to as "gravitational relaxation."

Gravity sliding. Previous studies suggesting a gravity- sliding origin for tessera terrain include Sukhanov [1986], who suggested that gravity sliding occurred above gentle asthenospheric upwellings, and Kozak and Schaber [1986], who conclude that Laima Tessera originated by gravity sliding. Smrekar and Phillips [1988] formulated a one-dimensional


Fig. 10a. Venera image of possible gravity slide feature southwest of Lakshmi Planum. Image is centered on 59.5øN, 310øE.

model for gravity sliding and suggested that the process should of deformation. Based on this evaluation, we suggest that result in significant surface deformation on Venus. None of gravity sliding is not a significant process in trough and ridge these models specifically consider the deformational pattern terrain (Ttr) but might have occurred in regions where that would be expected to result from gravity sliding. To subparallel ridged (Tsr) lies downslope from disrupted terrain evaluate this model, we first consider the pattern of (Tds). deformation expected to result from gravity sliding, then Downslope movement of a wedge of material would be describe a potential example of gravity sliding, and finally expected to produce extensional features in the upslope region compare observed tessera structures with the predicted pattern and compressional features near the toe of the wedge (Figure 9,

-0.5

O. 0

0.0 •,• 0.0 0.5

2.0

•' ridge

',,•._t rough

scarp

lineation

-0.5

0 100 200 km

I I Fig. 10b. Sketch map showing structures and topographic contours of possible gravity slide. Topography taken from Venera data; contour interval is 500 m. Stippled region represent the distinctive lobate region which is similar to the structures surrounding some coronae.


see also Figure 1 of Smrekar and Phillips [1988]). This basic deformational pattern is observed in proposed examples of gravity sliding such as the Olympus Mons aureole on Mars [Francis and Wadge, 1983]. Subsidiary tectonic features might include strike-slip faults that parallel the direction of motion of the wedge and accommodate lateral variations in the rate of downslope movement, as suggested for Laima Tessera [Kozak and Schaber, 1986] or the Olympus Mons aureole [Francis and Wadge, 1983; Carras, 1987].

Some of the steepest topographic slopes on Venus are found surrounding Lakshmi Planurn [Sharpton and Head, 1985], and therefore gravity sliding is more likely to occur in this region than in most areas. We have examined a feature that lies to the

southwest of Lakshmi (Figure 10) and which may be an example of gravity sliding on Venus. Although mapped as tessera terrain [Barsukov et al., 1986] and named Moira Tessera, the feature does not correspond to any of the three types of tessera described here, nor is more than one distinct set of structures present within most of the feature. Venera images show a series of slope-normal fractures and a structurally complex lobate region that lies at the distal portions of the feature, away from Lakshmi (Figure 10a). A map of topography and structures (Figure 10b) shows that the lobate region forms a southwest facing slope, while the fractures lie at relatively higher elevations, on a gentle northeast facing slope. Ridges and troughs within the lobe tend to be relatively short and are often quite small, near the 1-2 km resolution of the Venera radar. The complex lobate region is similar to the outer surround of some coronae, particularly Nightingale Corona [Barsukov et al., 1986]. These lobate regions and associated ridges are thought to be compressional features [Stofan and Head, 1990], but it is not posiible to rule out an extensional origin for the features in Moira. We suggest a possible interpretion of this feature as an example of gravity sliding of material away from the steep southern slope of Vesta Rupes. A topographic profile and interpretational cross section are shown in Figure 11. In the context of gravity sliding, the lobe is due to compressional deformation at the toe of the wedge, while fractures are the result of extension in the upslope region. This deformation occurred as material moved down and away from the surface of Lakshmi Planurn.

If Moira "Tessera" (Figure 10) represents a typical example of gravity sliding on Venus, then it is clear that most tesserae did not form due to this process. However, if gravity sliding were to modify an already deformed terrain, a more complex appearance might be expected (e.g., additional tectonic trends and structures not associated with gravity sliding). We therefore consider potential example of gravity-sliding modification to be regions exhibiting extensional deformation

at high elevations, with parallel compressional features at lower elevations.

Trough and ridge terrain (Ttr) could represent the upper portion of a gravity slide, if lower elevation compressional features were found to strike parallel to either the long trough structures or the discontinuous ridge and trough structures (Figure 4). The only potential example of such a pattern is found along the eastern boundary of Laima Tessera, where ridges in Kamari Dorsa lie at lower elevations that the Ttr in Laima and might be considered to strike roughly parallel to the troughs there. However, topographic slopes in the tessera trend approximately SSE-NNW, or nearly perpendicular to the direction required for gravity sliding to form ridges in Kamari Dorsa. Moreover, ridges in Kamari Dorsa clearly crosscut both sets of tessera structures, suggesting that troughs in Laima and ridges in Kamari Dorsa were formed at different times by distinct processes.

Like trough and ridge terrain, most regions of Tsr and Tds do not exhibit the structural pattern expected for gravity sliding, even taking into account that such a pattern might be superposed onto or convolved with preexisiting structures. Most regions Tsr are found surrounding the mountain ranges in western Ishtar Terra [Bindschadler, 1990] and lie downslope of compressional mountain ranges such as Akna, Freyja, and Maxwell montes [Pronin, 1986; Crumpier et al., 1986], not extensional features. Most regions of Tds are not consistent with the structural pattern predicted for gravity sliding, with the exception of the western and eastern boundaries of Tellus Regio. However, examination of topography and structures in eastern Tellus (described below) leads us to favor relaxation as the dominant mode of gravity-driven modification of tessera. Thus, although gravity sliding may occur on Venus, it does not appear to dominate the formation or modification of tessera terrain.

Gravitational relaxation. Relaxation is potentially significant in the high-temperature Venus environment [Weertrnan, 1979], particularly for regions of relatively thick crust, consistent with gravity and topography of tesserae. A region of thick crust is subject to horizontal gradients in vertical normal stresses, which can lead to flow of crustal material away from the region, and surface deformation [Ramberg, 1968; Artyushkov, 1973; Banerdt, 1986]. Quantitative relaxation models show that significant horizontal strain (deformation) of surface materials can occur subsequent to creation of topography. In particular, for compensated topography, relaxation results in extension within the interior of a high region and possible compression at lower elevations around the periphery of the region, subject to regional stress fields [Bindschadler, 1990; D.L.

2.1

Fig. 11. Topographic profile and structural interpretation of possible gravity slide. Profile is taken from Pioneer Venus topography data. Horizontal scale is the same as Figure 10. Stippled pattern represents the region beneath the hypothesized decollement surface.

BINDSCHADLER AND •: MODELS FOR VENUS TESSERA TERRAIN 5903

B indschadler et al., manuscript in preparation, 1991]. Extensional features formed by relaxation should be younger than those created during formation of the topographic high and are therefore expected to superpose or crosscut structures created during a formational stage. Large regions of tessera are commonly characterized by relatively steep boundaries and lower relief interiors, yielding a plateau-like shape. Model results [Bindschadler, 1990] show that if the upper crust is stronger than the lower crust [Zuber, 1987; Banerdt and Golombek, 1988], then these steep sides are expected to decay more quickly than the overall relief of a region of tessera. This would allow high-relief boundaries between tesserae and plains regions (type II boundaries) to evolve into lower-relief type I boundaries.

Relaxation does not appear to be consistent with most regions of subparallel ridged terrain (Tsr). Most areas of Tsr are not associated with any extensional features and are unlikely to have resulted from gravity-driven processes. Troughs and ridge- and-trough structures within the Ttr are consistent with an extensional origin. However, simple models of the relaxation process do not explain why extension in the Ttr would be manifested differently in one orientation (troughs) than in another (ridges/valleys).

It is more likely that some regions of disrupted terrain have been modified by relaxation. Grooves are relatively common within the Tds (Figure 5) and appear to indicate relatively late stage extensional deformation. Intratessera plains in Tellus Regio, within the disrupted terrain, tend to occur at relatively high elevations (Figure 7). All other factors being equal (such as the availability of magma at depth),. potential energy considerations suggest that eruptions should occur preferentially at lower elevations. However, high elevations may favor eruptions for two reasons, both consistent with relaxation. In the first, if the thermal gradient is sufficiently steep, crustal materials (presumably basaltic or diabasic) will cross the solidus before they cross the basalt-eclogite phase transition. If so, melting can occur, and magma is available to be erupted. Temperatures near the solidus also tend to enhance thermally activated creep and favor relaxation of Airy- compensated topography. The second reason is that relaxation models predict an extensional stress regime throughout the crust, which tends to enhance crack propagation and allow melt easier access to the surface.

One region within Tellus Regio exhibits features which are best explained by gravitational relaxation (Figure 12). In this region, three linear troughs crosscut the disrupted terrain and are therefore relatively young. This crosscutting relationship is most clearly developed in the westernmost trough. The southern part of the trough is floored by smooth plains material, but slightly to the north, remnants of the ESE trending slxuctures which dominate the terrain to the west of the trough are crosscut by the N-S trending structures of the trough itself. The troughs are interpreted to be extensional in nature on the basis of their topographic shape and the presence of numerous discontinuous subparallel structures similar to those observed in Devana Chasma and interpreted as fault scarps [Stofan et al., 1989]. Between troughs there are numerous grooves and small, faint, linear structures that parallel the strike of the troughs. These features also appear to crosscut ENE trending structures and are interpreted to indicated pervasive extensional deformation throughout the region. These extensional features lie at high elevations (Figure 12b) and are approximately parallel to a broad trend defined by the highest elevations in Tellus Regio. The plains-tesserae

boundary in this region is similar to type II boundaries (Figure 7) by virtue of its relative linearity at the 100-krn scale and the presence of ridges in the adjacent plains, but the boundary lacks a region of subparallel ridged terrain. Plains ridges lie at elevations below --1.5 km and appear to be asymmetric. However, this appearance is likely to be due to layover and/or foreshortening in the radar image, caused by the steep topographic slope (Figure 12b). Similar effects are observed within Alma Montes. We therefore interpret the ridges as compressional features.

The topographic relationship of extensional and compressional structures in eastern Tellus, their close proximity, and the relatively late stage nature of extensional structures in this region are all consistent with gravitational relaxation. While gravity sliding might explain some aspects of the structural framework of the region, the large area involved and the large width of features such as the troughs suggest that deformation is not thin-skinned. In addition, the westernmost of the three troughs lies along a relatively genfie westward facing slope, yet no parallel compressional features are observed in the tessera terrain to the west of this trough. Therefore we favor a relaxation model over one involving thin- skinned deformation. None of the four formational models

predicts the structural and topographic relationships described in this part of eastern Tellus. On the basis of this example and the consistency of observations in the Tds with the model predictions, we suggest that relaxation is a significant part of the evolution of tessera terrain.

Relaxation, or gravitational collapse, is also suggested by numerous workers to be a significant process in many terrestrial mountain belts, including the Basin and Range Province [Froidevaux and Ricard, 1987], the Hercynian belt in southern Europe [Mdnard and Molnar, 1988], and the Appalachians [Dewey, 1988]. Extension has also occurred from the Quaternary to the present within the Altiplano [Dalmayrac and Molnar, 1981; Sdbrier et al., 1985] and the Tibetan Plateau [Chen and Molnar, 1981; Arrnijo et al., 1986]. This process may be an integral part of the evolution of terrestrial mountain belts [Froidevaux and Ricard, 1987; Dewey, 1988], which are the principal loci of crustal thickening on the Earth. All other factors being equal, the high surface temperature of Venus should enhance such a process [Weertman, 1979].

SUMMARY AND CONCLUSIONS

We have tested a set of basic tectonic models for the

formation and modification of tessera terrain against observations derived from PV and Venera data. Mantle

upwelling, crustal underplating, and gravity sliding do not appear to satisfy basic observational constraints for the majority of tesserae. Mantle upwelling fails to predict shallow apparent depths of compensation, and tessera terrain contains neither large shield volcanoes nor large rift systems that would be consistent with more evolved or fossil upwellings. As presently understood, crustal underplating produces no compressional features and therefore is not responsible for the formation of Tsr or Tds. In order to produce a region of Ttr such as Laima Tessera for underplating requires unrealistically large heat input within a geologically short period of time. Gravity sliding may be important on a local scale (e.g., Moira "Tessera", Figure 10) but is not favored for larger regions of tessera because the predicted structural framework and its


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smooth plains

Fig. 12b. Sketch map of eastern Tellus Regio showing major troughs in the tesserae, interpreted as extensional features, and plains ridges, interpreted as compressional features. Dashed lines are topographic contours taken from Venera 15/16 data. Contour interval is 500 m. Asymmetric appearance of plains ridges may be due to layover and/or foreshortening, indicating topographic slopes of near to or greater than 10 ø.

correlation with topography are not generally observed in the formation of tessera terrain. three major types of tessera terrain. These three processes may Horizontal convergence and crustal thickening, as operate on Venus and may even have formed some small areas exemplified by the formation of terrestrial orogenic belts and defined as tessera terrain but they do not appear to dominate the venusian mountain belts, are consistent with the basic

BINDSCHADLER AND HF. AD: MODELS FOR VENUS TESSERA TERRAIN 5905

characteristics of subparallel ridged (Tsr) and disrupted terrain (Tds). In particular, shallow apparent depths of compensation, the presence of compressional and strike-slip features in these regions, the association of these terrains with Venusian mountain belts, and the presence of late stage extensional features in the Tds are all consistent with this model. Further examinations of the tessera terrain should include consideration of how the driving forces for convergence might be identified.

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The diverse appearance and characteristics of tessera terrain 1988b. suggest that either diverse processes are responsible for the Bindschadler, D.L., and J.W. Head, Models for the origin of tessera terrain on Venus, Lunar Planet. Sci., XIX, 78-79, 1988c. formation of tesserae or that different regions and types of Bindschadler, D.L., and J.W. Head, Characterization of Venera 15/16 tesserae represent different points along an evolutionary geologic units from Pioneer Venus reflectivity and roughness data, continuum. This work suggests that both are possible. Sub- Icarus, 77, 3-20, 1989. parallel ridged and disrupted terrain may result from a sequence Bindschadler, D.L., and E.M. Parmentier, Mantle flow tectonics' The

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Acknowledgments. Discussions with Tim Byrne and Ellen Stofan helped to sharpen many of our ideas about tessera terrain. Thanks to Tim Byrne, Annette deCharon, Richard Vorder Bmegge, and Roger Phillips for their comments. We gratefully acknowledge careful reviews by George McGill, Bruce Banerdt, and Steve Squyres, and the support of NASA grant NAGW-713 and the William Madar Foundation.

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D.L. Bindschadler, Department of Earth and Space Sciences, UCLA, Los Angeles, CA 90024.

J.W. Head, Department of Geological Sciences, Box 1846, Brown University, Providence, RI 02912.

(Received September 27, 1989; revised December 10, 1990;

accepted December 18, 1990.)

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