MECHANICS OF EXTENSIONAL WEDGES AND GEOMETRY OF NORMAL FAULTS

Based on the assumption that rocks are fractured simultaneously where the principle stresses reach the critical values of the modified Griff ith criterion presented in this paper, a possible formation mechanism for upper crustal extensional wedges is discussed. Regarding the stres s in the crust as the superposition of a lithostatic state, in which g ravity and fluid pressure are included, and a complementary stress, wh ich possibly implies the superposed tectonic stress, an elastic slab m odel of the upper crust above the brittle-ductile transition is consid ered. It is proposed that the strain discontinuity between the failed and unfailed regions in the slab may lead to large-scale curved normal faults, which cannot be explained by traditional fault theory. Three types of extensional wedges are produced, as the magnitude of superpos ed tensile stress decreases, remains unchanged and increases with dept h. Slab length. tensile strength, friction cofficient and fluid pressu re, significantly affect the geometry of extensional wedges and their boundary faults.