Unveiling the origin of radial grabens on Alba Patera volcano by finite element modelling

The Tharsis region is an 8000-km-wide structural dome that incorporates a concentration of the main volcanic and tectonic activity on the Planet Mars. The area of structural doming is characterised by giant radial graben-dike systems. Nested on a set of these giant dikes to the northern side of Tharsis, is Alba Patera, one of the largest volcanoes in the planetary system. The regional dikes there are in arcuate arrangement and imply an E-W to NW-SE regional extension at Alba Patera. To assess the influence of regional and local tectonics, we studied the dike orientations on the volcano with Viking mosaic data and simulated plausible stress fields with finite element modelling. We found that the influence of a NW-SE regional extension was strong near the volcano centre but decreased rapidly in importance towards the northern pole, i.e., far from the Tharsis centre. By combining this regional stress with a broad uplift that is due to a buoyancy zone of about 1400 km in lateral extent and centred under Alba Patera, we reproduced the radial pattern of dike swarms that diverge from the Tharsis trend. Regional tectonics may have dominated the early stages of dike injection. During the evolution of Alba Patera, however, local updoming controlled the dike pattern, supporting the idea of a hotspot under Alba Patera. The well-expressed dike geometry and characteristics of Alba Patera provide an ideal example for comparative study with analogue hotspots on Earth where plate tectonics and active erosion may complicate the reconstruction of volcanic and tectonic history and the understanding of involved geodynamic processes.