Acheron Fossae, Mars: Tectonic rifting, volcanism, and implications for lithospheric thickness

Abstract

The Acheron Fossae region is an ancient crustal block exposed in the northwestern Tharsis province on Mars. It is named after the Acheron Fossae, a particularly well-developed system of extensional tectonic grabens. This study is the first to describe the tectonic inventory in detail, using topographic and image data from the Global Surveyor and Mars Express missions. We see the extensional tectonics along the east-west trending topographic high of Acheron Fossae as a surface expression of upwelling asthenospheric material that initiated regional uplift, crustal extension and breakup, and associated volcanic activity in Noachian time. The tectonic architecture and the dimensions of the extensional structures are comparable to those of terrestrial continental rifts. An area of elevated topography and strong erosion, which constitutes a regional landmark in the eastern Acheron Fossae region, is interpreted as a rift-related center of volcanism, caused by local magmatic uprise and involvement of the lithosphere. The extension across the Acheron Fossae reaches values between 1.2 km and 8.7 km, comparable to young continental rifts on Earth. Crater statistics indicate an absolute cratering model age between ~3.9 Ga and 3.7 Ga for the rifting. The uplift observed on the flanks of the Acheron Fossae indicates a fairly thin and thus hot lithosphere. Using flexural analysis, we have constrained the elastic lithosphere thickness at the time of rifting to 8.9-11.3 km, corresponding to thermal gradients between 28 and 41 K km^-1. These heat flows are substantially larger than Noachian heat flow values previously reported, but are consistent with the presence of rift-related volcanism and a magmatically very active environment.