The Thaumasia “rift”, Mars – Is It a Rift?

We describe the morphology of a large and complex graben structure in the western Thauma-sia region (the " Thaumasia graben " [1] or TG). We consider possible fault geometries, determine extension , and discuss shortly possible models for its origin. Background: Situated at the western border of the Thaumasia plateau, the TG is one of the most prominent fault-bounded structures on Mars. It is ~100 km wide, >1000 km long, and strikes N15°-20°W (Fig. 1a). Several authors [e.g., 1,2,3,4] ascribe it to rifting, while others [5,6,7] favour right-lateral strike-slip faulting accomodating a relative motion of Thaumasia toward SE. Roof collapse after late-stage magma withdrawal from Syria Planum has also been hypothesized [8]. The TG formed during the last stage of Thaumasia tectonics, probably in Late Hesperian [9] or Early Amazonian [2]. It is superimposed on the Early Noa-chian tectonic center of Claritas (27°S, 106°W [10]). Architecture: Principal morphotectonic features of the TG and adjacent areas are (Fig.1a): (i) smooth lava plains of Syria Planum, bordered to the W by (ii) an escarpment made up by an en echelon series of steeply W-dipping faults marking the eastern border of the TG, (iii) the graben floors of segments A and B (see Fig. 1b for location), and (iv) the curvature of steeply E-dipping faults at 18-21°S, defining the master faults of segment A and separating it from a topog-raphical high towards NW. Between 22°S and 33°S, the western border fault system is more diffuse. Extension across segment A has been accomodated by an asymmetric (half)graben about 150 km in length and 100 km in width. Steeply E-to SE-dipping normal faults with fault length segments of 50-80 km and displacements of 2.0 km form the master fault system (profile 2, Fig. 1c). The graben is characterized by step-fault platforms with displacements of up to 150 m on antithetic faults. Internal block faulting is often controlled by reactivated trends of older fractures. At 21°S, the master fault changes over to the E-flank of the TG, and the elevation of the graben floor decreases to elevations 3500-4000 m. Segment B is about 250 km long and up to 100 km wide. As shown by profile 3 (Fig. 1c), the graben floor is tilted towards the steeply W-dipping normal (planar) border fault system. Master fault lengths range from 50-80 km and observable throws from 1.5-2.2 km. MOLA data suggest limited block rotation on synthetic normal faults. …