WRINKLE RIDGES ON SOLIS DORSA, MARS: ARE THEY THRUST-RELATED FOLDS OR MORAINE RIDGES?

Wrinkle ridges are commonly considered to be thrust-related folds induced by crustal shortening. This mechanism requires warping of pre-existing strata, testable in areas where geologic context is well understood. This may be the case in the Tharsis rise where pre-folding strata are possibly layered volcanic flows similar to those at nearby Valles Marineris. Using HiRISE images (25-50 cm/pixel), we conducted preliminary photogeologic mapping at Solis Dorsa in the SE Tharsis. Our work shows that the ridges are composed of matrix-supported boulders with various albedos, which implies a poly-lithologic clast composition and a sedimentary origin. The ridges are ubiquitously associated with variously shaped (e.g., circular, rectangular, curvilinear strip, and oval) and sized (100s m to > 5 km in the longest dimension) depressions, which are steeply walled locally with raised rims. Such landforms are not present on the nearby ridge-bounding plains. The close association of boulders, depressions, and the ridges suggests a genetic link among boulder deposition, depression generation, and wrinkle-ridge formation. Esker-like features and locally disharmonic folds discordant to the overall trend and shape of the wrinkle ridges are also present within the ridges (ESP_040119_1525). Smaller lobate boulder-bearing ridges, resembling paired hill-hole moraine ridges on Earth, emanate from the edges of the wrinkle ridges, which are unaccountable by the tectonic-origin model (ESP_037402_1545). The ridge surfaces are locally striated (ESP_012968_1535), or display interconnected curvilinear ridges linked with foreland esker-like features. The esker-like ridges in turn terminate at depressions filled by finer-grained materials on the ridge-bounding plains (PSP_006797_1545). Our preliminary observations led us to conclude that the wrinkle ridges in the SE Tharsis region have a glacial origin. The lack of extensive melt-water channels requires cold-based glaciation during which wrinkle ridges formed by supraglacial deposition and steep-walled depressions by dead-ice melting/sublimation after glaciation.