We have identified flexural uplift at the flanks of the Coracis Fossae Rift valley. Modeling the topography the elastic thickness is constrained to 10.3--12.5 km, the corresponding mechanical thickness and thermal gradient are 15--19 km and 27--33 K km-1, respectively. The time of rifting is dated by evaluating the crater size-frequency distribution of key surface units related to the rifting. Thus, the heat flux between 3.5 and 3.9 Ga is calculated to be 54--66 mW m-2. Furthermore, the stresses supported by the rift bounding faults are estimated to be below 5 MPa. We speculate that this weakness may be connected to the presence of water below the surface.