Estimating Regional Crustal Thickness on Venus: A Morphological and Structural Analysis of Ridge Belts.

Abstract:

Despite Venus showing no evidence of a global system of tectonic deformation, a variety of tectonic landforms across Venus suggests a complex and active geologic history that is not fully understood. Widely distributed within Venus’ low-lying plains are linear to arcuate, positive-relief systems of shortening structures, termed “ridge belts” in the literature. Previous studies interpreted ridge belts as surface expressions of zones of concentrated crustal shortening accommodated by thrust faulting and folding. However, a robust morphological characterization of these landforms is yet to be fully described.

We acquired detailed morphometric data for a globally distributed set of ridge belts using stereophotogrammetry-derived topographic data. We then produced detailed structural maps, based on comparative morphology with shortening structures on Earth, of six ridge belts with ~100 meter-per-pixel synthetic aperture radar image data. Next, we used topographic profiles showing evidence of flexural signatures proximal to four ridge belts to acquire local elastic lithosphere thickness estimates, treating each ridge as a line load. Our results indicate a range in elastic lithospheric thickness of 13–25 km for these four study ridge belts.

Cross-sectional profiles across these belts display fore- and back-limb morphology consistent with thrust-related landforms. Morphometric analysis of the belts returns relatively low-relief values, with an average relief of 582 m and none in excess of 1 km. We interpret tectonic structures within the ridge belts as predominately thrust faults and related hanging wall anticlinal folds, the majority of which strike roughly parallel to the long axis of the host ridge belt. Our interpretation therefore leads to the view that ridge belts are complex systems of thrust fault duplexes, in contrast to shortening structures on other worlds that are often morphologically more simple. The uniformly low relief values for these ridges, together with our flexural results, agree with previous studies that concluded that Venus has a thin elastic lithosphere due to elevated surface temperatures. Forward modeling of the depth of penetration of ridge belt thrust faults offers a means to test this conclusion.