EXTENSION OF STABLE CONTINENTAL LITHOSPHERE AND THE INITIATION OF LITHOSPHERIC SCALE FAULTS

We address the physical conditions which control the style of continen tal extension. Geological evidence suggests that once lithospheric sca le zones of localized deformation have been formed, they strongly affe ct continental deformation. It is the purpose of this paper to investi gate mechanisms which may cause lithospheric scale faults to initiate in stable continental lithosphere which is laterally fairly homogeneou s. Faults and shear zones cutting strong layers in the lithosphere wil l have a very significant influence on the evolution during extension. Based upon experimental flow laws, a strength maximum can be expected in the mantle directly beneath the Moho. Strain localization in the s hallow upper mantle is therefore expected to have a very pronounced ef fect on the evolution of the extending lithosphere. Low viscosities in the lower crust decouple the crust mechanically from the upper mantle . Therefore causes for strain localization in the sub-Moho mantle must be found in the mantle itself. Two potential causes satisfying this r equirement are boudinageing and strain weakening. We use thermal-mecha nical finite element models which incorporate the elastic, visco-plast ic, and viscous response of lithospheric rocks. The results of our mod el experiments suggest that boudins do not evolve during extension of continental lithosphere in most situations; only when the extension ra te is fast relative to thermal reequilibration may homogeneous boudina ge result. By its very nature, however, this type of boudinage cannot produce lithospheric scale faults. Our model results suggest that shea r zones may evolve after strain weakening. However, the style of exten sion on the scale of the lithosphere is pure shear like because the sh ear deformation in localized zones is balanced; in most cases the shea r zones occur in conjugate pairs. Initiation of lithospheric scale fau lts is concluded to be unlikely in stable and homogeneous lithosphere in interior parts of continental plates.