Testing a two-stage tectonic model for the formation of closed basins in the northern Valles Marineris, Mars

The formation of closed basins with blunt or irregular edges in the northern Valles Marineris has been predominantly attributed to two end-member processes: (1) mass removal by subsurface dissolution or (2) rifting bounded by cross-faults under orthogonal extension. To test the above models, we conducted preliminary observations mainly from analysis of CTX images from Hebes and Candor Chasmata. Our work reveals the presence of folds and normal faults that are oriented obliquely to the trend of the troughs. We could not locate any features that supported the hypothesis of chasmata formation by subsurface removal of material. Our observed folds commonly trend west-northwest and normal faults strike northeast. Both folds and normal faults involve trough-fill and trough-wall rocks, suggesting a tectonic origin rather than creation by surface processes such as landslide emplacement. The geometric arrangement of the folds and normal faults is compatible with general left-slip faulting. However, trough development cannot be explained by the movements of a single straight fault, as it does not explain the classic room problem for the formation of Hebes Chasma. Here we propose that the troughs may have been created by two stages of left-slip tectonics with curved fault traces. Initially, the current site of the troughs was located at right-step restraining bends that had created oblique folds. Folding may have destroyed the original anisotropy characterized by horizontal bedding and also caused lateral expansion of the restraining bends. The latter made the fault geometry less favorable to accommodate fault motion. As a result, a branch of the master left-slip faults was deactivated and was replaced by a new fault strand in a left-step fashion that led to the development of releasing bends. From the irregular trough edges of Hebes Chasma, we postulate that the releasing bend consists of two faults bounding the north and south sides of the central trough high. We test this hypothesis by conducting systematic mapping of the two chasmata and sandbox analogue experiments.