THERMAL EFFECTS OF NORMAL FAULTING DURING RIFTED BASIN FORMATION .1. A FINITE-DIFFERENCE MODEL

A finite difference model is presented, describing the thermal structu re of a region undergoing fault-controlled extension. The model is two -dimensional and time-dependent. We combine temperature changes result ing from conduction, advection due to fault movements, sediment blanke ting and heat production. Cooling curves can be derived at chosen poin ts in the area. This study mainly investigates the effect of the exten sion rate on the thermal field, which turns out to be very important. 20 km of extension with extension rates of 0.2, 2.0 and 20 mm/yr yield maximum deviations in temperatures from the steady-state situation of 2, 26 and 84%. The sediment blanketing effect can even be reversed by variations in the sedimentation rates. Fast sedimentation leads to cr ustal cooling, slow sedimentation to crustal heating. The turning-poin t value is lithology-dependent and corresponds to 2.0 mm/yr for shale, 2.2 mm/yr for clay, and 1.4 mm/yr for sand; a sandstone will cause an increased cooling al both high and low velocities. Our modelling show s that footwall cooling during or immediately after a rifting event ca nnot be explained by the downwards movement of the adjacent, relativel y cold hanging wall. Consequently, in areas where such a cooling is ob served, other cooling mechanisms must play an important role. These ca n be phenomena like footwall uplift or the fading out of an older ther mal anomaly.