COMPARISON OF EVOLUTIONARY HISTORIES FOR THARSIS, MARS, AND METIS REGIO, VENUS

Detailed geological mapping, geomorphological assessment of landscapes, and analyses of the topographic and geophysical data show histories that reflect similar-trending evolutionary stages in magma-driven development during proposed stagnant-lid convection regimes for the Tharsis magmatic complex on Mars and the Metis Regio quadrangle on Venus. While Tharsis may be the result of a superplume on Mars, the Metis region exemplifies styles of geologic activity that appear to be the result of mantle processes, including the manifestation of hundreds of lesser magma plumes distributed across Venus. Both regions include well-documented, temporal and spatial associations among magmatic and tectonic features (e.g., volcanic edifices and putative intrusive bodies plus fracture, graben, ridge, and/or rift systems). We also observe a transition from early, much more intensive, magma-driven tectonic activity (Stage 1) to moderate tectonic deformation of widespread volcanic plains (Stage 2). Later, a substantial waning in magmatic-driven tectonism is observed and regional volcanic lava plains are deformed by wrinkle ridges (Stage 3), and finally the formation of large volcanic constructs, domes, and extensive lobate flow fields (Stages 4 and 5). Concentrated magma-driven tectonic activity, including possible reactivation of basement structures, is also observed for both Mars and Venus at the Tharsis and Metis Regio regions, later in the recorded history. There are four major differences between the two areas. (1) The exposed record length is more than 3 billion years for Mars and less than 500 million years for Venus. (2) Venus displays greater populations of tectonic structures and centers of magma-driven activity. (3) Venus has a more diverse suite of magmatic-tectonic feature types than does Mars. These include well-defined tesserae; coronae; rift systems; ridge, fracture and mountain belts; and diverse volcanic edifices and flow fields whose structural and topographic relations indicate complex evolutionary histories. (4) Venus lacks the equivalent of the Tharsis and Elysium superplumes, which dominate martian geological history. The parallel evolutional histories will be further assessed through geophysical and geochemical modeling.