Lithospheric Contraction on Mars: A 3D Model of the Amenthes Thrust Fault System

Amenthes Rupes is the topographic expression of a main fault belonging to a thrust fault system located parallel to the martian dichotomy boundary. A 3D forward model has been applied to the Amenthes thrust fault system, constraining fault geometries at depth, variations of slip along strike, and structural parameters controlling the formation of fault propagation folds. Our results provide a complex 3D view of the tectonic framework of the area, with implications for tectonic evolution, regional shortening distribution, and the main mechanical discontinuities in the lithosphere. The modeled fault surfaces show planar morphologies combined with listric geometries at depth. The obtained depths of faulting for the major faults of this fault system suggest a depth of the brittle-ductile transition (at the time of formation) of 20-24 km, somewhat shallower than previous estimates for this area. A possible mechanical discontinuity located at 10.5-13 km deep can be deduced from the faulting depths of the secondary faults. The listric geometries at depth imply that slip is transmitted from the decollement, which, together with the inclusion in the model of secondary and subsidiary faults, allow us to estimate the horizontal shortening recorded in this area ranging from 2-3 km up to similar to 5.5 km in the southeastern part of the fault system. This range increases the previous shortening estimates in this area between similar to 60% and similar to 200%. Consequently, global shortening estimates based on global fault maps are biased by the detail of mapping, and shortening would substantially increase if secondary faults were included. Plain Language Summary Amenthes Rupes is a large topographic relief known as a lobate scarp, formed by the displacement of a contractional fault belonging to a fault system located in the Amenthes Region, Mars. The study of the faults forming these reliefs provides insights on the faulting and folding processes, which are related to the state of the lithosphere at the time of formation. A 3D modeling has been applied to Amenthes fault system, modeling fault geometries at depth, interaction between faults, associated slip values, depth of faulting, and other parameters that control the formation of the relief. These results provide a complex 3D view of how the area contracted. The obtained fault surfaces show curved geometries at depth. The major faults in the fault system root at a depth where ductile deformation in the lithosphere begins, estimated to be at 20-24 km. The estimated regional horizontal shortening accommodated by the fault system range between 2 and similar to 5.5 km, which increases the shortening in this area between 60% and 200% compared with previous estimates. This recognizes that the inclusion of secondary faults in global contraction models would increase substantially the global shortening estimates.