Late Crustal Growth on Mars: Evidence From Lithospheric Extension

Abstract

Rift valleys on Mars have attracted considerable attention in the last couple of years and recently been observed with the High Resolution Stereo Camera (HRSC) on board the Mars Express mission. The images provided by the HRSC cover very large areas (up to 10⁵km²) with high spatial resolution and are an ideal data base for age determinations based on crater statistics. It was thus possible to constrain the time of rifting at the Coracis Fossae in the Thaumasia region to 3.5-3.9 Gyr, the spatial resolution of the images being ~20 m per pixel. Continental extension on Earth proceeds in either narrow rifting, wide rifting or core complex mode, depending on the ambient initial conditions. The type of lithospheric extension at the Coracis Fossae is narrow rifting, the region subject to intense normal faulting narrower than 150 km. We have applied the pure shear model of lithospheric extension to determine the range of initial conditions such as thermal gradient and crustal thickness that allow for narrow rifts to form. Using the constraints posed on the thermal gradient by the flexure of the rift flank, we have determined the crustal thickness at the Coracis Fossae at 3.5-3.9 Gyr. We find that for the given thermal gradient of 27-33 K km^-1 the crustal thickness cannot have exceeded 40 km. This is considerably lower than present day estimates of the crustal thickness in that region and interpreted as an indication for continued crustal production at that time.