Constraints on the Evolution of the Tharsis Region of Mars

Introduction: The western hemisphere of Mars is dominated by topography associated with the Tharsis volcanic province. A range of models for the formation and/or evolution of this region have been proposed and include (1) dynamic support of topography by a large mantle plume [1,2], (2) regional uplift due to underplating of crustal material derived from the northern hemisphere [3], (3) uplift due to solely mantle anomalies thermal and/or compositional [4], and including crustal thickening by intrusion [5], (4) flexural loading due to volcanic construction [6, 7]. Recent studies suggest that models invoking present-day dynamic support of the Tharsis rise are problematic due to the requirement of persistent plume structure for the 4 Ga history of Tharsis [7], and the indication that an upper mantle plume is responsible for less than 10% of the present-day geoid [8]. Support of the Tharsis rise purely by mantle thermal and compositional anomalies requires the maintenance of large lateral variations in density over billions of years. Clearly, none of the models alone can explain the complex history of the Tharsis region. Mars Global Surveyor (MGS) gravity, topography, magnetic, and imaging data provide new constraints on models for Tharsis. Here we summarize the primary aspects of the tectonic history of the region, the present-day gravity and topography, and the magnetic field. We suggest a mechanism for the formation and evolution of the Tharsis province that satisfies these observations qualitatively. We present preliminary results on constraints on the thermal history of the crust provided by the magnetic field observations.