Kinematic analysis of rotated fractures and blocks in simple shear

Abstract

Deformation in part of the surface rupture of the Imperial Valley, California, earthquake of 15 October 1979 provides a geometric and mechanical analog for the model of tectonic evolution of southern California in Miocene time as proposed by Luyendyk et al. (1980). The N35W-striking fault broke as an irregular series of left-stepping en-echelon R shears across a field that prior to the earthquake had been scored by N-S oriented furrows. Between overlapping segments of the R shears, elongate blocks of soil separated along the furrows and rotated up to 70° clockwise. The furrows functioned as planar discontinuities which guided and accommodated shear strain in those parts of the mole track between the R shears. The relatively rigid blocks detached on an irregular horizontal decollement from the less rigid subsoil. Triangular gaps opened between blocks where rotation was variable; blocks buckled or were thrust out of the shear zone where they were constrained by the walls of the shear zone. The geometry and kinematics of structures in the mole track virtually duplicate the larger scale structures and requisite movements in the regional tectonic model, and show that the regional model is plausible.