Modeling of Neogene crustal block rotation: Case study of southeastern California

Abstract

Numerical modeling using a two‐dimensional distinct element method (DEM) was performed to give an additional understanding of the mechanism of crustal block rotation in a highly active portion of the Pacific‐North America transform plate boundary. Paleomagnetic and structural investigations have shown that large (30°–50°) horizontal rotations of crustal blocks occurred in the Eastern Transverse Zone and the Northeast Mojave Block (southeastern California) since the end of early Miocene. The main aspect of the modeling was to investigate whether an anisotropic horizontal stress state can be responsible for the rotation of crustal blocks with a length of 100 km or not. The simulations were performed using different stress boundary conditions and different parameters of the fault strength. Applying an assumed ratio of 2 for the horizontal stress components as boundary condition, block rotations in the order of 40° only occur if (1) the major horizontal stress SH is oriented N20°E with a maximal deviation of 15° to the east or west, and (2) the fault strength described by the friction coefficient is smaller than 0.12, or 14% only of the value 0.85 for intact rocks (Byerlee's law). For a weakening model of the fault strength an initial friction coefficient of 0.85 allows also rotations of about 40° if the friction coefficient decreases rapidly due to an increasing shear displacement to a basic friction coefficient of 0.12 or smaller. For the same amount of rotation (40°) a decreasing stress ratio requires a lowering of the friction coefficient used, whereas an increase of the ratio allows a higher friction coefficient. The simulation also show that the modeling with elastic blocks can adequately describe the observed main features of the crustal deformation, especially the cumulative slip values. Discrepancies in the observations and modeling results for the western parts of the Mojave Desert are the result of a neglect of the three‐dimensional deformation in our two‐dimensional modeling.