Relative timing of crustal plateau magmatism and tectonism at Tellus Regio, Venus

Crustal plateaus, subcontinent-sized regions of thickened crust that stand 1-4 km above surrounding low-lying plains, define a major physiographic province of Venus. Thus their mode of emplacement plays a fundamental part in the evolution of the entire planet. Local topographic lows up to several hundred kilometers across filled with radar-dark volcanic flows, termed intratessera floodlava basins (ITBs), are common within crustal plateaus. ITBs record abundant crustal plateau volcanism and have implications for crustal plateau emplacement and evolution. We ask the fundamental question: Did ITBs develop before, during, or after crustal plateau development? To address this question, we mapped in detail the geology a particular ITB in Tellus Regio to determine temporal relationships between ITB volcanic flows and local crustal plateau tectonism. Mechanical arguments and crosscutting relationships indicate that ITB volcanism occurred during crustal plateau formation. Extending the timing relationships resolved at this basin, we suggest that ITB magmatism played a major role in crustal plateau genesis. The geologic history discerned is consistent with the upwelling model for crustal plateau evolution in which early surface extension and volcanism would be expected in a hotspot environment. Conversely, early widespread extension and volcanism would not be expected if crustal plateaus formed above a mantle downwelling because (1) predicted horizontal and radial convergence would obstruct early surface extension and (2) lower than normal geothermal gradients would impede, rather than enhance, volcanism.