We investigate the morphology and topography of one of the largest fault-bounded tectonic structures on Mars, a complex, approximately N-S trending system of troughs and scarps at the western border of the Thaumasia plateau in the Claritas region (hereinafter referred to as Thaumasia graben, or TG). It is located between 15¿S and 38¿S latitude and at ~255¿E longitude. No detailed investigation of its morphotectonic setting has been performed yet. The region is a complexly fractured area with a number of different fault sets, including simple and complex graben. The TG extends over more than 1000 km along its trend, averaging 100 km in width and 1.6 km in depth. Crustal extension is accommodated by the formation of a system of asymmetric graben, or halfgraben. On the basis of fault orientation and trough depth, the TG can be subdivided in a north-south direction into three segments. Except for the northernmost segment, the predominant master fault system is located along the eastern flank of the TG, highlighting the overall asymmetric architecture. Fault length segments vary from 50 to 90 km with observable displacements of 1.3--2.2 km. Crustal extension, inferred from gridded MOLA topography across scarp offsets, varies along trend between 0.5 km and ~4 km, assuming a fault dip of 60¿. This is relatively moderate extension if compared to terrestrial continental rifts, but consistent with extension measured across the Tempe Rift on Mars. We find that the Thaumasia graben displays some characteristics which are common to terrestrial continental rifts, whereas other properties are distinctively not rift-like.