Circum-Tharsis wrinkle ridges at Lunae Planum: Morphometry, formation, and crustal implications

Abstract

Wrinkle ridges are one of the most widespread tectonic landforms on Mars developed under a compressional stress regime within the lithosphere. In this study, we investigate the morphology and structure of wrinkle ridges at northern Lunae Planum, which belongs to the circum-Tharsis wrinkle ridge system. We use methods to construct balanced cross-sections of fold-and-thrust belts that are based on conservation of area and volume to assess the nature of deformation and to quantify the amount of shortening and the depth of detachment faulting. Our morphometric analyses indicate that height and width of wrinkle ridges at Lunae Planum systematically decrease with increasing distance from the center of the Tharsis rise from west to east. The results indicate that horizontal shortening decreases from 4.75% to 1.25%, or ~116 m to ~56 m from west to east, and this shortening is accommodated by the formation of complex asymmetric ridges. Sections through wrinkle ridges provided by an escarpment at Kasei Valles and steep walls of impact craters expose thrust faults beneath the wrinkle ridges that dip at 38° +/− 5°. We model wrinkle ridges as fault-propagation folds and show that the observed thrust faults form along the frontal fold axial planes of the anticlines. At depth these faults merge into more gentle thrust ramps of 20°+/− 5° dip. Kinematic modeling further implies that the thrust ramps transition into a sub-horizontal detachment, which rises from −4 km in the west to −2.6 km in the east, measured from the surface. The very gentle eastward dipping topographic slope of Lunae Planum and the eastward rising detachment define a narrow wedge that is interpreted as an incipient critical taper with a tectonic push induced by the Tharsis rise. We propose here that the mechanically weak detachment is localized in a water-bearing crustal layer underneath a crust whose pore space is sealed by ice. Fluid overpressure conditions may enable yielding and slip on the detachment. Hydrous mineral reactions on the slip planes and flood basalt layers that gently dip to the Tharsis center due to flexural loading may additionally weaken the basaltic rocks and contribute to detachment faulting.