Formation mechanism of South Tharsis ridge belt, Mars, The

Abstract

The South Tharsis Ridge Belt (STRB) partially surrounds Tharsis in an arc around the southwest part of the rise. It consists of 29 large ridges separated by distances up to 110 km, with average relief of 1.5 km above the surrounding plains. Because the STRB is among the oldest tectonic features associated with Tharsis, it may provide key information on the early evolution of Tharsis and the ancient tectonic history of Mars. Earlier studies concluded that the ridges formed through compressional tectonism by a combination of a buckling instability and thrust faulting. However, when the shape, size, and separation of the ridges are considered, the STRB resembles the extensional Basin and Range province on Earth, both of which are characterized by series of parallel mountain ranges separated by broad valleys. In this thesis, we evaluate both extensional and compressional hypotheses for the origin of the ridges using evidence from topographic profiles, deformed craters, boundary element modeling, crustal thickness models, and thermal modeling. Though no single model explains all aspects of the ridges, the topography of the ridges and crustal thinning associated with the belt support an extensional origin. Thermal modeling results show that Basin and Range-style wide rifting in the STRB under expected thermal gradients of 15-20 K/km should occur for strain rates comparable to those in the Basin and Range. We suggest that the large amount of extensional strain inferred in the STRB may have been accommodated by the significant ductile deformation observed in the western Terra Sirenum craters. This has significant implications for the incipient development of Tharsis and possibly pre-Tharsis phases of Mars' evolution. However, with the available evidence we cannot unequivocally reject a compressional mechanism for forming the STRB or confirm an extensional mechanism.