STRESS MODELS FOR THARSIS FORMATION, MARS

A critical survey is presented of most stress models proposed for the formation of the tectonic structures in the Tharsis volcano-tectonic p rovince on Mars and provides new constraints for further models. First papers, in the 1970s, attempted to relate the Tharsis formation to as thenospheric movements and lithosphere loading by magma bodies. These processes were then quantified in terms of stress trajectory and magni tude models in elastic lithosphere (e.g. Banerdt et al., J. Geophys. R es. 87(B12), 9723-9733, 1982). Stresses generated by dynamic lithosphe re uplift were rapidly dismissed because of the poor agreement between the stress trajectories provided by the elastic models and the struct ural observations. The preferred stress models involved lithosphere lo ading, inducing isostatic compensation, and then lithosphere flexure. Some inconsistency with structural interpretation of Viking imagery ha s been found. In the early 1990s, an attempt to solve this problem res ulted in a model involving the existence of a Tharsis-centred brittle crustal cap, detached from the strong mantle by a weak crustal layer ( Tanaka et al., J. Geophys. Res. 96(E1), 15617-15633, 1991). Such a con figuration should produce loading stresses akin to those predicted by some combination of the two I loading modes. This model has not been q uantified yet, however it is expected to reconcile stress trajectories and most structural patterns. Nevertheless, some inconsistencies with observed structures are also expected to remain. Parallel to this app roach focused on loading mechanisms, the idea that volcanism and tecto nic structures could be related to mantle circulation began to be cons idered again through numerical convection experiments, whose results h ave however not been clearly correlated with surface observations. Str uctural clues to early Tharsis dynamic uplift are reported. These stru ctures have little to do with those predicted by elastic stress modell ing of dynamic lithosphere uplift. They denote the existence of unstea dy I stress trajectories responsible for surface deformations that can not be readily predicted by elastic models. These structures illustrat e that improving current stress models for Tharsis formation shall com e from deeper consideration of rock failure criterion and load growth in the lithosphere(e.g. Schultz and Zuber; J. Geophys. Res. 99(E7), 14 691-14702, 1994), Improvements should also arise from better understan ding rheological layering in the lithosphere and its evolution with ti me, and from consideration of stress associated to magma emplacement i n the crust, which mag. have produced many tectonic structures before loading stress resulting from magma freezing became significant (Mege and Masson, Planet. Space Sci. 44, 1499-1546 1996a). Copyright (C) 199 6 Elsevier Science Ltd.