INFERENCES OF DEVIATORIC STRESS IN ACTIVELY DEFORMING BELTS FROM SIMPLE PHYSICAL MODELS

Observations of temperatures and heat flux near major thrust zones indicate that in their deep levels shear stresses may exceed 50-100 MPa. Within strike-slip zones shear stresses in the lower lithosphere may also approach 50-100 MPa, though shear stresses in the upper crust of those regions are probably much lower. The relationship of tectonic style to surface elevation in the Andes and Tibet yields an estimate of about 5 x 10(12) N m-1 for the force per unit length required to deform the lithosphere of these regions. This force per unit length is equivalent to an average shear stress of about 25 MPa through a lithosphere 100 km thick. The width-to-length ratios of active belts are consistent with deformation determined by the creep of the lower lithosphere rather than by friction on faults. The patterns of rotation of crustal blocks in western North America suggest that these blocks passively follow the deformation of a continuous substrate. The observations of deformation and the estimates of stress derived from them, both suggest that the upper continental crust is weak, relative to the lower parts of the lithosphere the deformation of which it follows passively. If this is so, determinations of stress in the upper crust may have only limited relevance to the deformation of the lithosphere as a whole.