IGPP Seminar Series

Development of the Tharsis Rise on Mars as a Result of Impact-induced Plate Subduction

by An Yin
Department of Earth and Space Sciences

Abstract

The Tharsis Rise is a broad northwest-trending plateau on Mars with an average elevation of ~11 km. It is ~6000 km long and ~ 3500 km wide, occupying ~25% of the surface area of the planet. The tectonic origin of the Tharsis Rise has been attributed to (1) long-lived hotspot activity, (2) impact-induced sustained partial melting in the mantle, and (3) edge-driven mantle convection along the highland-plain dichotomy boundary. A major problem with the above hypotheses is their inability to explain the temporal and spatial evolution of the Tharsis volcanism. Remote-sensing mapping indicates that the Tharsis volcanism was developed in three stages from southeast to northwest. The first stage was expressed by the emplacement of regionally extensive volcanic flows across much of the southeastern half of the Tharsis Rise in the Late Noachian to Early Hesperian. The eruption center is a northeast-trending zone located across the Thaumasia Planum and Thaumasia Highlands along the southeast margin of the Tharsis Rise. The second stage of volcanism was expressed by the initiation of a chain of shield volcanoes in the Late Hesperian along the northeast-trending Tharsis Montes located ~2500 km northwest of the older Thaumasia volcanic zone and was built on top of the Hesperian Thaumasia volcanic flows. The three shield volcanoes in the Tharsis Montes are spaced at ~720 km. The third stage of Tharsis volcanism was expressed by the development of the Olympus Mons and Alba Patera, which are aligned in the northeast direction, spaced at ~1800 km, and located about 1600 km to the northeast of the Tharsis Montes volcanic zone. The Olympus-Alba volcanic zone was initiated in the Early Amazonian. The Alba Patera was build on top of the Late Hesperian Tharsis Montes flows, creating a volcanic flank that superposes the younger volcanic flows over an older surface. The composite nature of the flanking surface around the Alba Patera has led to the false impression that the Alba Patera volcano was constructed in the Hesperian prior to the Olympus Mons. A clue to explain the above observations is the temporal and spatial relationships between the development of the Tharsis Rise and formation of the Argyre Planitia, the second largest impact basin (~1300 km in diameter) on the Highlands of Mars. The Argyre basin was created in the Early Noachian and lies ~450 km southeast of the Tharsis Rise. I propose that the Argyre impact event had caused strong northwest-southeast compression in the lithosphere and partial melting in the upper mantle across the highland-plain dichotomy boundary along the southeastern margin of the Tharsis Rise. Impact-induced compression triggered underthrusting of the thinner Lowland lithosphere (the Arcadia plate) below the thicker Highland lithosphere (the Tharsis plate), while partial melting produced a regionally extensive asthenosphere that allowed the Arcadia plate to sink downward into the mantle. Extent of impact-induced melting may be illustrated by volcanism in the Syrtis Major and Hesperian Planums around the Isidis impact basin (~ 1100 km in diameter), which are located between 1000 km and 2000 km away from the edge of the impact basin. Northwestward sinking of the Arcadia slab caused northwest-southeast extension in the overriding plate and eventually generated arc volcanism as the slab sank to the depth (~ 250 km) where dehydration reaction operates. The progressive sinking of the slab produced arcs and intra-arc/back-arc extension from southeast to northwest. The process created large volume of low density materials that allow the plateau to sustain its high elevation over a time span of 2-4 Ga. The subduction model explains the prominent negative free-air gravity along the northwestern edge of the Tharsis Rise and implies that the aureole zone on the northwest side of the Olympus Mons may be an accretionary prism accommodating recent plate convergence. The development of the northeast-trending Elysium volcanic chain could either be induced by the Utopia impact event to the west or by northwestward propagation of the subducting Arcadia plate from the Tharsis Rise.
Tuesday, 26 May 2009
3853 Slichter Hall
Refreshments at 3:45 PM
Lecture at 4:00 PM