Warrego Valles Revisited: Valley Network Formation, Modification, and Climatic and Structural Controls

Introduction: This investigation focused on Warrego Valles, a region of well-developed valley systems located in Thaumasia Planum of the southern highlands at ~42°S, 267°E. Some previous workers interpreted the valley systems in this area as being of fluvial origin [1-3], while others focused on sapping [4-6] or hydrothermal activity [7-10] as formative mechanisms. It is clear that no single model can adequately elucidate the disparate valley system morphologies observed at Warrego Valles and many other sites on Mars, but rather that different processes predominated during multiple epochs and in various geologic settings with unique implications for Martian paleoclimate. Review of Proposed Valley Formation Processes: The nomenclature has become a bit muddled, but in general " valley " only implies a topographic feature: an elongate low-lying area surrounded by higher terrain, while a " channel " typically refers to the bed of a stream; therefore, valleys may contain channels and channels may contain streams [2, 11]. Fluvial Systems. Runoff fed, high-density (km/km 2), dendritic networks epitomize fluvial systems on Earth. These systems originate with rills that feed into ephemeral gullies and then into perennial streams. Fluvial systems, which are characterized by numerous branching tributaries, yield higher Strahler orders than comparable sapped drainage systems. These systems cut distinctive V-shaped valley planforms that widen progressively downstream. The primary implication of fluvial systems of this type on Mars is abundant precipitation in the past—requiring a vastly different climate system than is observed today [3]. Some workers argue that a precipitative climate is not reconcilable with the asymmetric distribution of fluvial valley systems that is observed [8]. Groundwater Sapping. Sapping is an erosive, channel-forming process in which groundwater outflow (seepage) weakens valley head-and side-walls leading to collapse and retreat (primarily by removing basal material at the site of outflow). Sapping produces U-shaped valleys characterized by relatively constant widths and theater-style heads. On Earth, the influence of sapping in valley system formation is usually swamped by the predominating influence of runoff [12]. Some well-known examples