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Two transverse dune fields occur among Late Amazonian volcanic and aeolian landforms in southwestern Tharsis, Mars. The first is located similar to 70 km northwest of Biblis Patera, around 5 degrees N, 125 degrees W. The second is located about 500 km northwest of Arsia Mons, at 2 degrees S, 130 degrees W. The latter is the largest dune field thus far documented to occur in the equatorial latitudes of Mars. Unlike other dunes on the planet, both dune fields in Tharsis have low thermal inertias (<2.7 x 10(-3) cal cm(-2) sec(-0.5) K-1) and high albedos (similar to 0.26) that are indistinct from their surrounding terrain. Both dune fields have superposed features, such as impact craters, lava flows, smooth-surfaced units, and bright wind streaks. The dune fields therefore appear to be inactive and mantled by fine-grained material (i.e., particles <60 mu m). To form, aeolian dunes require a supply of sand. On Earth, most dune sands are supplied by fluvial and littoral processes, but this is not the case in Tharsis on Mars. Because they are superposed on a Late Amazonian surface, the climate is assumed to have been hyper-arid throughout the time that the dunes have existed. Under these conditions, the only plausible source for quantities of sediment sufficient to form transverse dune fields is explosive volcanism. Therefore, the two dune fields in Tharsis are evidence that explosive volcanism has occurred in this region in the Late Amazonian Epoch. (C) 1997 Academic Press.