TESTING A SLAB ROLL BACK MODEL FOR THE DEVELOPMENT OF THE THARSIS RISE

The Tharsis rise occupying 25% of Mars surface is characterized by zonal volcanism migrating from southeast to northwest and a change in style of volcanism from effusive eruption to central-volcano construction. Within each volcanic zone, central volcanoes change along strike from thin and tall geometry in the southwest (i.e., Tharsis Montes and Olympus Mons) to wide and flat geometry in the northeast (e.g., Alba Patera) across a NE-trending boundary. This implies an abrupt decrease in lava viscosity across the boundary. The Tharsis rise is also dominated by NE-trending grabens that extend >1500 km away from the margins of the rise. Their wide spatial distribution has been a major mechanical obstacle to all the existing Tharsis models advocating vertical loading for its origin, either by a plume from below or by a large volcanic construct from above. To explain Tharsis evolution, I propose that its volcanism and graben development were induced by slab rollback of a southeast-dipping plate. Subduction was initiated near Syria Planum by early Hesperian volcanic flooding, possibly triggered by the Argyre impact. The positive feedback between volcanic loading and downward flexural warping may have eventually led to lithospheric foundering and subsequent subduction near the dichotomy boundary. In this model, the whole lithosphere was subducted below the Tharsis rise while only the mantle lithosphere was delaminated from the overlying crust and subducted below the Uranius Patera and Alba Patera to the northeast and the Sirenum Terra to the southwest. This model not only explains the widely distributed grabens but also places diverse Tharsis features under a unified tectonic framework: (1) the Olympus Mons aureole zone as a subduction zone, (2) the Ere Montes as a fore-bulge, (3) the NW-trending normal faults at Gordii Dorsum and Acheron Fossae as accommodation structures to have facilitated slab rollback, (4) the lack of crustal magnetic anomalies in the Tharsis as a result of juvenile-crust genesis by arc and back-arc magmatism, (5) the Thaumasia-Coprates Rise orogen as a retra-arc thrust belt during initial Tharsis subduction, and (6) the left-slip Valles Marineris fault zone as a transform structure linking NE-trending normal faults in a back-arc setting during later back-arc extension.