Crustal spreading on Venus: evidence from topography, morphology, symmetry, and map patterns

Abstract

Evidence for numerous features similar to terrestrial divergent plate boundaries and spreading centers are present in the topography and morphology of Western Aphrodite Terra, Venus (Head and Crumpler, 1987). Linear discontinuities cutting across the strike of Western Aphrodite Terra (Cross-Strike Discontinuities or CSDs) were examined in further detail. It was found that: 1. (1) They represent the most prominent regional structural trend in Western Aphrodite and are readily distinguishable from linear patterns caused by ground tracks in the Pioneer-Venus orbiter data; 2. (2) individual CSDs are either deep troughs or steep regional scarps and steps; 3. (3) the spacing between the more prominent CSDs ranges from about 500 to 1500 km (average spacing is 850 km); and 4. (4) there is a distinctive structural trend orthogonal to the strike of the CSDs. Additional smaller scarps and slopes with characteristics of known CSDs occur in the Arecibo altimetric profiles at spacings of as little as 50 km, in agreement with previous suggestions of Sotin et al. (1989) that other smaller CSDs with less offset may occur in domains between the more prominent ones previously mapped. The detailed characteristics of the CSDs (detailed trough and scarp-like form, association with ridge offsets, linear terminations, linear slope traces, parallelism, great length) are all very similar to the characteristics of transform faults and fracture zones on the Earth's seafloor, and this additional analysis supports the earlier interpretation that CSDs represent analogs to terrestrial oceanic fracture zones. Assessment of bilateral symmetry of topography and other elements across Western Aphrodite Terra indicate that: 1. (1) an accurate prediction of the large and small-scale symmetrical elements in both the altimetry data and radar images within each domain on one flank of the highlands may be made by substitution of a mirror image of the opposite flank, and 2. (2) smaller wavelength symmetrical elements are revealed in the altimetric profiles by subtracting the background regional symmetry and examining the residuals. Residual altimetry profiles obtained in this manner show that individual altimetry features several hundred kilometers wide and several hundred meters high are frequently located at the same distance from the axis of regional symmetry on the north and the south flanks of the highlands. Topography and geometric features such as ridge crests and transforms have specific predictable relationships in a crustal spreading environment. Tests were made in Western Aphrodite for such organized relationships predicted to occur in association with ridge crest offsets at transform faults and fracture zones. It was found that: 1. (a) topographic stepdowns occur across CSDs in accordance with the general consequences of juxtaposing lithospheres of different relative ages, 2. (b) the correct direction of stepdown (down toward older crust and lithosphere) occurs relative to that predicted from ridge crest offsets, and 3. (c) the observed amount of stepdown is similar in many cases to that predicted from the observed ridge crest offsets. These analyses of cross-strike discontinuities, topographic and image symmetry across Aphrodite, and predictions of topography based on ridge crest and fracture zone geometric relationships further support the earlier interpretation that the 7500 km extent of Western Aphrodite Terra represents the site of crustal spreading and displays many of the characteristics of terrestrial divergent plate boundaries.