A PLUME TECTONICS MODEL FOR THE THARSIS PROVINCE, MARS

Morphological and structural data from the whole Tharsis province sugg est that a number of shallow grabens radially oriented about the Thars is bulge on Mars are underlain by dykes, which define giant radiating swarms similar to, e.g. the Mackenzie dyke swarm of the Canadian shiel d. Mechanisms for graben formation are proposed, and the depth, width, and height of the associated dykes are estimated. Structural mapping leads to define successive stages of dyke emplacement, and provide str ess-trajectory maps that indicate a steady source of the regional stre ss during the whole history of the Tharsis province. A new tectonic mo del of Tharsis is presented, based on an analogy with dyke swarms on t he Earth that form inside hot spots. This model successfully matches t he following features: (1) the geometry of the South Tharsis Ridge Bel t, which may have been a consequence of the compressional stress field at the boundary between the uplifted and non-uplifted areas in the up per part of the lithosphere at the onset of hot spot activity; (2) ext ensive lava flooding, interpreted as a consequence of the high thermal anomaly at the onset of plume (hot spot) activity; (3) wrinkle ridge geometry in the Tharsis hemisphere, the formation of which is interpre ted as a consequence of buoyant subsidence of the brittle crust in res ponse to the lava load; (4) Valles Marineris limited stretching by pre liminary passive rifting, and uplift, viewed as a necessary consequenc e of adiabatic mantle decompression induced by stretching. The geometr ical analysis of dyke swarms suggests the existence of a large, Tharsi s-independent extensional state of stress during all the period of tec tonic activity, in which the minimum compressive stress is roughly N-S oriented. Although magmatism must have loaded the lithosphere signifi cantly after the plume activity ceased and be responsible for addition al surface deformations, there is no requirement for further loading s tress to explain surficial features. Comparison with succession of mag matic and tectonic events related to hotspots on the Earth suggests th at the total time required to produce all the surface deformation obse rved in the Tharsis province over the last 3.8 Ga does probably not ex ceed 10 or 15 Ma. Copyright (C) 1996 Elsevier Science Ltd.