ORIGIN OF THEATER-HEADED VALLEYS IN THE SAHARA: IMPLICATION FOR OUTFLOW CHANNELS ON MARS

The origin of the Martian outflow channels (OC) remains a subject of debate. Two popular models were proposed: groundwater sapping and megaflooding. Understanding the origin of the OC can provide constraints on the presence of life on Mars and on its timing. We adopted an integrated approach (remote sensing, geo-statistical, isotopic, geotechnical and field) to assess the origin of theater-headed valleys (THV) in the Sahara as a potential terrestrial analogue to the Martian landscape. Our findings include: (1) the Saharan THV are widely and densely distributed along fault-controlled escarpments, (2) although extensive erosion was associated with the Saharan THV (typical erosional volume: 2 x 10⁹ m³ to 3 x 10⁹ m³), watershed analysis revealed small upslope contributing areas (typically 8 to 12 km²), (3) escarpments strongly dissected by THV are characterized by strong-over-weak stratigraphy (capping limestone/sandstone: 27 to 52 N/mm²; underlying shale: 14-16 N/mm²) and show evidence of salt weathering (e.g. tafoni, granular disintegration, gypsum veins, polygonal halite crust) and dissolution features (4) spatial analysis reveal a strong spatial correlation between the distribution of THV, tufa and the majority of playas, (5) isotopic composition (δ¹⁸O) of tufa deposits (147 samples) are consistent with deposition from depleted fossil Nubian groundwater (δ¹⁸O: -12.8 to -8.0 ‰), and (7) geotechnical properties of the Moghra shale in the Qattara depression indicate that shale deposits have high swelling potential. We interpret these findings to indicate that pressurized groundwater accessed deep-seated faults, caused prolonged and extensive discharge, incised THV, facilitated salt weathering, caused basal support swelling, undermined escarpments, caused scarp retreat, deposited tufa on escarpment faces and playa deposits proximal to these cliffs, and modified the Saharan landscape. Because previous research and our analysis of MOLA, HRSC, MARSIS-SHARAD, and THEMIS images indicate that the Martian OC and the Saharan THV and their surroundings share similar morphological characteristics, extensive distribution, high structural control and absence of upslope streams, we suggest that groundwater sapping could have played a major role in the formation of the OC on Mars.