Introduction
[93] The surface of Venus has been modified by a wide range of tectonic processes. This deformation has produced a complex surface characterized by fractured plains, ridge belts, rifts, fracture belts, complex ridge terrain (tesserae), and mountain belts [Masursky et al., 1980; Barsukov et al., 1986; Basilevsky et al., 1986; Solomon et al., 1991 and 1992]. Reliable identification and mapping of these tectonic features are critical to formulating and testing models for the evolution of Venus [Solomon and Head, 1982 and 1991]. Recognition of tectonic features in radar images is often difficult and strongly dependent on viewing geometry and incidence angle [MacDonald, 1980; Ford, 1980]. In this chapter, the techniques for identifying and interpreting tectonic features in radar images are discussed.
Features of Extensional Origin
Troughs and Graben
Troughs and valleys interpreted to be primarily extensional in origin are clearly identifiable in many regions of Venus. These features range from individual troughs within the plains to radial or circumferential graben associated with calderas and coronae, to major rift valleys (e.g., Devana Chasma) (Figures 8- 1 (a) through (c)). A discussion of trough morphology in general is useful for identifying the wide range of linear depressions seen in the Magellan data. The interpretation that troughs are extensional in origin must be used with caution. For example, numerous sinuous rifles and lava channels that are not formed by extension are abundant on Venus. The morphology of the troughs, the regional structure, the types of associated features, and an understanding of the geologic setting must all be taken into account to determine if troughs have resulted from crustal or lithospheric stretching and are therefore graben.
Troughs are identified in both Magellan synthetic aperture radar (SAR) and altimetry data. As with illumination by natural sunlight, illumination by radar reveals the presence of positive or negative topography. Troughs or depressions are characterized by a dark band closer to the direction of illumination than the following bright band, as shown in Figure 8-2. The ability to identify troughs is constrained by the spatial resolution of the Magellan SAR. The 210-m average resolution of the image data makes it difficult to distinguish troughs with widths less than a kilometer. These narrow depressions are usually resolved as radar-bright lineaments that are often indistinguishable from small-scale ridges. In addition, troughs with small depth-to-width ratios may not have slopes sufficiently steep to cause noticeable brightness differences. In contrast, troughs in the range of 20 km to several hundred km in width can be easily identified in the Magellan altimetry data or radar stereo images. The 4- to 20-km altimeter footprint is useful in identifying the morphology of major rift zones whose widths are greater than 10 km.
The direction of radar illumination relative to a trough may either enhance or subdue the structure of a depression. Features aligned parallel to the radar look direction are often difficult to identify; a slight change in the look azimuth, in a direction away from an alignment parallel to a feature, can reveal the structure of a feature. Because of this dependence on look direction, multiple data sets (Magellan, Arecibo, and Venera 15/16) with various viewing geometries should be used for the positive identification of troughs and when attempting to compile rose diagrams to quantify the orientation of surface structures [Stofan et al., 1989].
![(c) [below] rift valley Devana Chasma, 1050 km long and 80 to 240 km wide, in Beta Regio (28°N, 283°E). Illumination is from the left in each image.](p95s.jpg)
.Radar image
and interpretative cross section of the rift valley, Guor Linea
(18.8°N,0.3°E). In areas such as this where the slope of
the scarp is less than the incidence angle (45 deg), the radar can
image the surfaces of backslopes.
[97] Scarp and Normal Fault Morphology
A number of large-scale troughs on Venus (e.g., Devana Chasma and Ganis Chasma) are characterized by bright lineaments interpreted to be normal faults aligned parallel to the strike of the trough and that in some places cover a region broader than the trough itself (Figure 8-2(b)). These features are interpreted to be rifts produced by broad-scale lithospheric extension, similar to that which occurs at the East African rift on Earth [McGill et al., 1981; Stofan et al., 1989; Senske et al., 1991].
In contrast to the large rift valleys, some regions within the plains contain bright lineaments that have a low degree of sinuosity and are interpreted to be individual normal faults, fractures, or small graben produced by extension. In some places, intersecting sets of linear features (e.g., the ""ridded plains" [Saunders et al., 1991]) are interpreted to be fractures produced by lithospheric extension [Banerdt and Sammis, 1992] (Figure 8-3).
There is an abundance of scarps within the plains and on the highlands that are interpreted to be the result of normal faulting. If scarps are not high (i.e., less than 200 m of vertical offset) or do not have a rough surface, they can be difficult to distinguish from other lineaments on radar images. Compared with the surrounding plains, they appear typically as bright, relatively straight, linear features if they face the radar illumination or dark lineaments if they face away. They also appear compressed when facing the illumination direction and extended when facing away. If scarps lie parallel to the direction of illumination, they may be visible only at points where they are faceted or deviate from the parallel.
Features of Compressional Origin
Mountain Belts
Three major mountain belts on Venus-Akna (Figure 8-4), Freyja, and Maxwell Montes, located within Ishtar Terra-are interpreted to have been formed by compression and crustal shortening, which produced folding and thrust faulting [Solomon and Head, 1984; Crumpler et al., 1986; Vorder Bruegge and Head, 1990; Solomon et al., 1992; Kaula et al., 1992]. Features interpreted to be folds are characterized by bright/dark lineament pairs, and have a high degree of sinuosity along strike (Figures 8-S(a) and (b)). Many features interpreted to be folds occur in chevron patterns (Figure 8-6). On Earth, most folded mountain belts are heavily eroded. In comparison, inferred low rates of erosion suggest that folds on Venus are likely to be primary features. Some asymmetric lineaments are interpreted to be thrust faults within the mountains [Barsukov et al., 1986]. In ....
....addition, evidence for strike-slip faulting can be identified by fractures or folds offset in a consistent sense along a linear zone that cuts across the strike of most mountain belts. Typically, cross-cutting relationships can be used to determine the sequence of events, with most mountain belts characterized by early folding and imbrication followed by late-stage extension (cross-cutting graben) indicative of gravitational relaxation [Smrekar and Phillips, 1988; Solomon et al., 1991 and 1992; Kaula, et al., 1992; Smrekar and Solomon, 1992]. Mountain belts on Venus are located at high....
...latitudes where the incidence angle of the Magellan radar ranges from 20 to 25 deg. At these low incidence angles, roughness effects are less significant and layover is quite common (Figure 8-7); therefore asymmetric features in images of high latitude areas should be interpreted with caution.
Ridges
Features identified as ridges are characterized by elevated topography and a high degree of sinuosity; they fall into two groups: (1) widely spaced sinuous ridges and (2) collections of ridges that form belts. The widely spaced sinuous ridges tend to be only 2 to 5 km across and are found in almost all plains regions on Venus (Figure 8-8). Since they are found at most latitudes, care must be exercised when comparing features of similar appearance that have been imaged at different incidence angles. Plains ridges bear many similarities to wrinkle ridges found on other terrestrial planets [Watters, 1988 and 1992]. On the basis of their high degree of sinuosity and frequent asymmetry, these ridges are generally interpreted to be thrust faults [Solomon et al., 1992]. In some places, the radar signature is indicative of a more flat-topped, steep-sided feature consistent with a fault-bend fold or a horst [Suppe and Connors, 1992].
Ridge Belts
Ridge belts are found along the borders of major regions of complex ridge terrain (tesserae) and in two major groups located in Lavinia Planitia and in the Atalanta/Vinmara Planitae region (Figure 8-9). The belts are characterized by relatively elevated arch-shaped topography and sinuous ridges; occasionally, troughs lie along the summit and flanks of the elevated topography. Ridges and troughs within the belts tend to be spaced approximately 5 km apart. In some places, faults and folds that exhibit a consistent sense of offset suggest shear. On the basis of a high degree of sinuosity and the presence of arches indicative of folding and thrusting, ridge belts are interpreted to be compressional in origin [Frank and Head, 1990; Solomon et al., 1991 and 1992; Squyres et al., 1992a], although an extensional origin has also been suggested [Sukhanov and Pronin, 1988]. Belts containing extensional graben (Figure 8-9(b)) are also interpreted to be of compressional origin, with summit graben produced by bending stresses as the crust is warped by compressional forces [Squyres et al., 1992a].
= 30 deg, symmetric folds appear asymmetric in the
image. Foreslopes are compressed and backslopes are expanded. (b)
Radar image and interpretive cross section of a series of folds in
Lavinia Planitia located at 36.4°S, 337.9°E. Illumination
is from the left at an incidence angle of 29 deg.
Features of Complex or Indeterminate Origin
Complex Ridge Terrain (Tesserae)
All complexly deformed regions on Venus are categorized as complex ridge terrain (CRT), but they vary in style and sequence of deformation and may differ in origin. CRT, also called tesserae, was first identified in Venera 15/16 images of Venus [Barsukov et al., 1986]. It consists of raised, plateau-shaped regions characterized by at least three sets of intersecting lineaments (Figure 8- 10). In a number of places, relatively small outliers of embayed highly deformed terrain are found within the plains and at locations distal to larger areas of CRT. These relationships may indicate that laterally extensive regions of low-lying CRT have been flooded by later plains volcanism [Ivanov et al., 1992]. Theories for the origin of CRT include (1) deformation of a highly silicic basement [Nikolayeva et al., 1988]; (2) late-stage plume plateaus [Herrick and Phillips, 1990]; and (3) crustal shortening followed by gravitational relaxation over a mantle downwelling [Bindschadler and Parmentier, 1990; Bindschadler et al., 1992a]. Structural features within CRT include folds, thrust faults, graben, and strike-slip faults [Solomon et al., 1991 and 1992; Bindschadler et al., 1992a]. The sequence of events within some major regions of CRT indicate crustal shortening followed by extension [Bindschadler et al., 1992b]. However, detailed kinematic studies of most of these regions are presently lacking.
Coronae
Coronae are circular to elongate features that range from 75- to over 2000-km across and are primarily characterized by an annulus of troughs and/or ridges (Figure 8-11) [Barsukov et al., 1986; Pronin and Stofan, 1990]. The annul) around coronae are 10- to 150-km across and generally correspond to elevated topography. Topographically, coronae are domes, plateaus, plateaus with interior depressions, and rimmed depressions. The majority of the features are surrounded by a peripheral trough. Coronae are also characterized by an association of volcanic features, such as small volcanic domes, lava flows, and moderate-size edifices (see Chapter 9). A group of features with a morphology similar to coronae but lacking a distinct annulus is classified as corona-like features [Stofan et al., 1992].
Coronae are concentrated in a few clusters and along chains, such as those at Parga and Hecate Chasmata. Coronae are interpreted to be surface manifestations of mantle upwelling [Stofan and Head, 1990; Stofan et al., 1991 and 1992; Squyres et al., 1992b]. Within the complexly deformed rims of some large coronae, such as Quetzalpetlatl and Nightingale, layover and foreshortening are observed in images. Stereo imaging and/or analyses that combine radar with altimeter data are critical to interpreting the tectonic structures at coronae and the geologic sequence of events.
Arachnoids
Arachnoids are circular to elongate features that may be generically related to coronae. Arachnoids were first identified in Venera 15/16 data as "spider and cobweb structures" due to their complex appearance. Arachnoids are characterized by a central structure, either a dome or a central circular, depression, surrounded by circumferential and radial linear features (Figure 8-12). The inner dome or depression tends to....
....be 10 to 30 km across, with radiating fractures extending from SO to over I SO km from the central feature. The fractures may have been produced when magma intruded near the surface, forming a small central edifice and producing a system of radiating dikes and fractures [McKenzie et al., 1992]. The emplacement of dikes has been used to understand regional stress patterns at the time of their emplacement [e.g., Muller and Pollard, 1977].
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