Wrinkle ridges are among the most common tectonic structures on the terrestrial planets and provide important records of the history of planetary strain and geodynamics. The observed broad arches and superposed narrow wrinkles are thought to be the surface manifestation of blind thrust faults, which terminate in near-surface volcanic sequences and cause folding and layer-parallel shear. However, the subsurface tectonic architecture associated with the ridges remains a matter of debate. Here we present direct observations of a wrinkle ridge thrust fault where it has been exposed by erosion in the southern wall of Melas Chasma on Mars. The thrust fault has been made resistant to erosion, likely due to volcanic intrusion, such that later erosional widening of the trough exposed the fault plane as a 70km long ridge extending into the chasma. A plane fit to this ridge crest reveals a thrust fault with a dip of 13 degrees (+8 degrees, -7 degrees) between 1 and 3.5km depth below the plateau surface, with no evidence for listric character in this depth range. This dip is significantly lower than the commonly assumed value of 30 degrees, which, if representative of other wrinkle ridges, indicates that global contraction on Mars may have been previously underestimated.