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Applicant -- Evolution of Venus, a global perspective — from exogenic to endogenic over time

Wednesday, March 16, 2016
Vicki
Hansen

NASA’s Magellan mission of two decades ago revealed that Venus lacks plate tectonics, yet Venus’ surface evolution and operating geodynamic system has remained elusive.  A variety of geologic mapping and surface modeling studies lead to the emergence of holistic understanding of Venus’ global evolution that recognizes both exogenic and endogenic processThe picture that emerges is one of early exogenic-instigated events and global thin lithosphere, followed by an evolution to thicker lithosphere and largely endogenic-driven processes that became regionally more focused with time. This comprehensive perspective is informed by: a) Monte Carlo crater modeling; b) impact crater characteristics, evolution and relative ages; c) ancient ribbon-tessera terrain character, distribution, and history; d) radial dike swarm characteristics, global distribution, and temporal constraints; e) global and regional wrinkle-ridge patterns, and the relative ages of wrinkle-ridge suites; f) the age of volcanic plains units relative to adjacent features; g) the nature and distribution of shield terrain; h) the surface expression and evolution of Artemisa vast circular feature marked by two concentric troughs of ~2400 and >5000 km diameter, a 12,000 km-diameter radial dike swarm, a 13,000 km-diameter suite of concentric wrinkle ridges, and an interior region of unique volcano-tectonic fabricsi) the characteristics and relative ages of crustal plateaus and volcanic rises; and j) constraints derived from geologic mapping of numerous VMap regions (~7.5x106 km2 each) by independent workers.

The emergent global evolution is relatively simple at a long-wavelength time-scale, although rich histories develop locally. Crustal plateaus and ribbon-tessera terrain formed early marking an era of globally thin lithosphere and bolide-impact drive processes; deformation belts likely formed late during this era, representing endogenic-driven processes. Time-transgressive evolution of the Artemis superplume marks the Artemis era that affected ~30% of the surface, and also resulted in the rise of Ishtar Terra above a complimentary downwelling. This relatively singular mantle flow pattern subsequently evolved to more localized mantle plumes–marked by coronae-dominated volcanic rises (e.g., Bell, Eistla), and downwellingsforming broad lowland basins. Further gradational thickening of the lithosphere lead to an increase in localized regional endogenic-driven tectonomagmaticactivity marked by corona-chasma chains, fracture zones, and large rift-dominated volcanic rises. During thin lithosphere time Venus experienced dominantly steady-state resurfacing driven by large bolide impact and crustal plateau formation. With time, large bolides waned, the lithosphere thickened, and craters accumulated across the surface, with the most dramatically modified craters preserved in the young tectonomagmatic Bata-Alta-Themisregion.

 

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