The formation of the South Tharsis Ridge Belt: Basin and Range‐style extension on early Mars?

Abstract

The South Tharsis Ridge Belt (STRB) is located along the northeastern edge of Terra Sirenum and partially surrounds the southwestern part of Tharsis in an arc. It consists of 29 large ridges separated by distances 130 to 260 km, with average relief of 1.5 km above the surrounding plains. Because the STRB is among the oldest tectonic features associated both spatially and developmentally with Tharsis, it may provide key information on the early evolution of Tharsis and possibly pre-Tharsis processes. Earlier studies concluded that the ridges formed through compressional tectonism. However, the shape, size, and separation of the ridges support the interpretation that the STRB resembles the extensional Basin and Range Province on Earth. Both regions are characterized by series of parallel mountain ranges separated by broad valleys. In this study, we evaluate both extensional and compressional hypotheses for the origin of the ridges using evidence from topography, deformed craters, crustal thickness models, and strain modeling. Though neither interpretation explains all aspects of the ridges, the topography of the ridges and crustal thinning associated with the western part of the ridge belt support an extensional origin. Strain models predict that Basin and Range-style wide rifting would be expected for early Martian conditions. This extension may have been initiated by plume-induced uplift in the early stages of Tharsis formation, but the large amount of extensional strain inferred in the western STRB must have been accommodated by compression elsewhere, possibly in the heavily deformed craters of western Terra Sirenum.One of these ancient tectonic features is the South Tharsis Ridge Belt (STRB) (Figure 1), mapped and interpreted by Schultz and Tanaka [1994] to consist of a system of 29 ridges forming an arcuate shape around southern Tharsis. The ridges are separated by distances of~150-500 km and have reliefs of~1.5 km above the surrounding plains. The STRB is among the oldest tectonic features associated with Tharsis, but the KARASOZEN ET AL.FORMATION OF THE SOUTH THARSIS RIDGE BELT 916 PUBLICATIONS