LITHOSPHERIC STRUCTURE, HEAT FLOW, AND TECTONIC STRAIN IN THE THAUMASIA HIGHLANDS AND THE COPRATES RISE, MARS

The Thaumasia Plateau, located southeast of the Tharsis volcanos, is one of the major geologic provinces in the western hemisphere of Mars. Its 3000 km long southern and eastern margins are defined by the Thaumasia Highlands and Coprates Rise mountain belts, which formed by compression and crustal shortening in the late Noachian and early Hesperian. Recent gravity modeling using data from the Mars Reconnaissance Orbiter (model MRO95A) provides important new constraints on the evolution of this region. (1) Assuming a typical density of 2900 kg per cubic meter, the lithospheric thickness is 25-30 km in the center of the mountain belts. This is considerably thicker than observed for other Noachian age units and implies that the heat flow is less than half of the value typical for Mars at this time. (2) The crust in the mountain belts is about 20% thicker than in the plateau to the north. Although there is some mapped volcanism, the bulk of this thickening appears to be tectonic in origin. (3) The gravity anomaly along the transition from the highlands to the lowland plains to the south and east can not be fit with the same model parameters as used for the mountain belts. One possible solution is that these regions have a lower density than in the mountain belts.

An explanation that is consistent with all of these observations is that this was a region of cold mantle downwelling during the Noachian, possibly serving to balance at least part of the hot upwelling in Tharsis. This would explain the thick lithosphere and low inferred heat flow, and the associated mantle flow pattern would explain the compressive deformation in the mountain belts and the overall tectonic thickening of the crust. The low density along the flanks of the plateau is consistent with these regions acting as accretionary wedges. The observed crustal thickening in the mountains implies about 100 km of crustal shortening across the width of the mountain belt. This implies that mantle convection on Mars was not in the stagnant lid regime when these mountains formed.