A finite element computer program for one-dimensional heat flow was modified to apply to two-dimensional subduction where conductive heat transport parallel to the upper and lower plates was neglected. One-dimensional finite element columns were cut, displaced, and reassembled to model heat advection. Calculated temperature, depth-dependent pressure, stress levels associated with dislocation creep in olivine at constant strain rate, and a 1-GPa deviatoric stress maximum were used to calculate the form of the brittle-plastic failure envelope for profiles across subducted slabs. The failure envelopes were then used to calculate maximum sustainable compression parallel to the slab. Model result suggests the following: (1) In high-angle subduction without frictional heating and horizontal heat flow, temperatures along the interplate contact zone remain below about 120 ¿C, which is far below temperatures necessary for olivine plasticity. Without substantial frictional heating or lateral heat flow from the adjacent magmatic arc, the subducted slab will gain rather than lose strength before its upper surface contacts asthenosphere. In addition, viscous coupling due to olivine plasticity and associated ablation of the upper plate will be unimportant.

Slab strength reduction due to heating occurs rapidly after the top of the slab comes into contact with asthenosphere. (2) In low-angle subduction the top of the slab remains in contact with overlying lithosphere and does not contact asthenosphere. Slab strength reduction due to conductive heating from above occurs slowly because the upper plate is cooled by the lower plate and is the source of progressively less heat. Tempertures along the interplate contact zone may be sufficient for olivine plasticity at large distances from the trench, and viscous coupling between plates and associated ablation of the upper plate may be important in this tectonic setting. (3) Low-angle subduction for distances of over 1500 km during the Laramide orogeny in southwestern North America appears viable. At 50 Ma, slab strength beneath the Black Hills of South Dakota is predicted to have been comparable to, or greater than, strength at the trench. ¿ American Geophysical Union 1994