Formation of mountain belts on Venus: Evidence for large-scale convergence, underthrusting, and crustal imbrication in Freyja Montes, Ishtar Terra

Abstract

The Freyja Mottles, Venus, deformation zone consists of a distinctive linear orogenic belt (Freyja Montes), an adjacent plateau, a steep scarp separating the plateau from the North Polar Plains lying about 3 km below the plateau, and a volcanically flooded foredeep and rise at the base of the scarp. On the basis of mapping of radar images and interpretation of the geologic units and topography, the Freyja Montes deformation zone is interpreted to be the site of large-scale crustal convergence and imbrication involving the underthrusting of the North Polar Plains beneath Ishtar Terra. This deformation is interpreted to have begun with north-south regional compressional deformation causing flexure, buckling, and underthrusting of the crust and lithosphere of the North Polar Plains beneath the relatively higher proto-Ishtar/ Lakshmi. Further convergence and underthrusting caused crustal thickening and uplift and created compressive stresses in the upper part of the underthrusting layer. Failure of this layer led to a new site of underthrusting outboard of the rise and a new detachment surface. The detached layer and the wedge of volcanic flows in the intervening foredeep were further deformed and incorporated (sutured) onto the plateau and mountain range. This crustal underthrusting and imbrication caused crustal thickening and corresponding crustal uplift. The continuing process of convergence underthrusting, detachment, and imbrication produced large-scale crustal thickening, uplift, and the formation of the plateau behind the asymmetrical front of Freyja Montes. The present configuration of topography and units and the distinctive elevation of the scarp and Freyja Montes suggest that the process continues to operate at the present time.