About 10% of the Tharsis bulge surface is intensely faulted, with a fault density up to 1 fault/km. In these intensely faulted areas, 54 impact craters which exhibit faulted walls and floor are now elliptical in shape. These ellipticities indicate an average strain value of 10% inside these provinces. This strain value is 4 times greater than the strain deduced from normal fault morphometric analysis. This high value indicates that the strain is not only due to 60¿ dip normal faults, and that some additional tectonic processes exist such as boudinage. This important strain value corresponds to an elongation of about 50 km across Thaumasia and Tempe-Mareotis provinces, and 20 km across Alba Patera Regio. The amount of elongation for the entire Tharsis Region is 2 orders of magnitude higher than the strains and motions related to standard models of bulge formation. These intensely faulted areas are restricted areas where faults are both radial to the bulge and perpendicular to the topographic slope. The stretching does not influence the regional topography and there are no topographic shoulders or important level differences between faulted and nonfaulted areas, which would indicate that the crust is not thinned under the stretched provinces. Outside these intensely faulted areas the graben width is remarkably constant all over the Tharsis bulge, indicating the existence of a brittle ductile transition or an incompetent level at about 1--2 km below the surface. A new origin for the Tharsis tectonic features is proposed as a preliminary hypothesis: the intensely faulted areas correspond to the uphill parts and to denudation zones of gravity slidings which move on a 2km-deep incompetent level.

The gliding of the slab on the very low dipping flanks of Tharsis needs three conditions: (1) the viscosity of the incompetent layer must be equal to the viscosity of the ice, (2) the brittle layer must have been previously faulted by the bulging itself, (3) these preexisting radial fault directions must be perpendicular to topographic slope. The ridged plain units in Lunae Palus and Coprates provinces thus correspond to a zone of blocking inside the gliding slab, due to an interruption of the decollement level. ¿ American Geophysical Union 1993