LAUSR

Abstract In this study we address the question of whether the Tharsis rise that occupies ~25 % of the surface area of Mars was once covered by an ice cap, and the sudden ice-cap melting was responsible for the catastrophic development of the Late Hesperian (3.5-3.6 Ga) circum-Tharsis giant (>1000s km in length) outflow channels. To achieve this goal, we conducted geomorphologic mapping across the Oudemans crater basin (~125 km in diameter) at elevations of ~2.7 km to ~5.0 km and its bounding plateau plains at elevations of ~5.3 km to ~6.7 km in the central Tharsis rise. Our work shows that the Oudemans-impact-induced landforms were superposed by a suite of younger landform assemblages, which consist of horn-like peaks, arete-like ridges, cirque-like depressions, hanging-valley-like features, trim-line-like escarpments, lobate ridges, striated, pitted and hummocky terrains, and features resembling drumlins, crag-and-tails and roches moutonnees. A plateau-plain striated terrain on a hummocky surface exhibits streamlined linear ridges that are 400-2000 m wide and up to ~17 km long. The average length-width ratio (~9), shape, surface texture, and associated washboard landform pattern of the linear morphological features in the striated terrain on Mars are comparable to mega-scale glacial lineations (MSGLs) on Earth. Using well-understood Earth analogues as a guide, we interpret the post-Oudemans-impact landform assemblages to have formed during regional ice-sheet-style glaciation. We further suggest the striated terrain on the plateau plains to have been generated by a fast-moving ice stream in a regional ice sheet that filled 4-km deep Oudemans crater and covered the crater-bounding plateau plains in the central Tharsis rise. The volume of the glacier ice stored in the Oudemans crater basin alone is estimated on the order of ~200,000 km3. The shape of the inferred glacier-induced landforms and their cross-cutting relationships require early northward-flowing northward-advancing glaciation, followed by southward-flowing northward-retreating glaciation. The size-frequency distribution of the craters superposed on top of the interpreted glaciated landforms in the mapped area yields a glaciation age of ca. 3.5 Ga, coeval with the estimated age of the interpreted glacial landforms along the 2000-km long Valles Marineris trough zone east of the study area. The inferred glaciation age is also coeval with the development of the circum-Tharsis outflow channels. Although our work supports the Tharsis ice-cap hypothesis, the full extent of the ice cap within and possibly beyond the Tharsis rise remains unconstrained.