A Geodynamic Model of Alba Patera; Hotspot Tectonics and Volcanism Under the Tharsis Stress Field

Abstract

Alba Patera, one of the largest volcanoes in the planetary system, is situated at the northern periphery of the Tharsis rise and is characterised by numerous graben-dike systems. A study of the fault or dike geometry combined with finite element modelling and analogue experiments allows to assess the influences of regional and local tectonics on the volcano. The graben configuration reflects a change of regional stress orientation and magnitude from the Tharsis centre to the periphery. To the south of Alba Patera, the branch of oldest grabens indicates a significant regional E-W extension. At higher latitudes, the direction of the regional extension turned towards NW-SE. Its influence on the structural pattern was important near the volcano centre and decreased toward the north. However, volcanism and tectonics at Alba Patera were largely uncoupled from the Tharsis activity. Broad uplift centred on Alba Patera better explains the radial pattern of dike swarms that occurred from the edifice centre to the northern pole along 1000 km distance. Coupled with the giant dike swarms, the widespread volcanism of Alba Patera's early phase is similar to the flood basalt provinces commonly associated to hotspot and continental rifting episodes on Earth. Local uplift was followed by subsidence of smaller wavelength responsible for the formation of concentric grabens on the upper and mid flanks of the volcano. The circular fracturing represents a long-term mechanism active during several ten or even hundreds of Myrs accounting for mantle dynamic processes. An increase of density of the mid and lower crust by intrusion and subsequent cooling below Alba Patera probably formed a local stress field that superposed on the regional tectonics. The study of Alba Patera allows to reconstruct the successive tectonic and magmatic events from the birth to the death of an hotspot.