Description and kinematics of the SE Loreto basin fault array, Baja California Sur, Mexico: A positive field test of oblique-rift models

Abstract

We present the results of mapping and kinematic analysis on an ∼ 18 km2 array of normal and oblique-slip faults in the SE Loreto basin, near Loreto, Baja California Sur, Mexico. The fault array developed on the western margin of the Gulf of California in late Pliocene time, during the early part of the modern stage of oblique rifting of the gulf. The SE Loreto fault array is an antithetic transfer zone between two large, E-dipping normal faults. The fault array is coherent and formed in one episode. The dominant N- to NNE-striking faults primarily dip W and display nearly pure normal slip. More NE-striking faults show normal-sinistral slip and NW-striking faults have dextral-normal to dextral slip. N- to NW-striking faults have steeper dips and a greater component of dextral slip with more northwesterly strike. Bulk extension, based on strain analysis of faults with striae, is oriented nearly E-W with little plunge and consistent throughout the fault array. The kinematic analysis of the SE Loreto fault array provides a well-constrained test of oblique-rift models. Our results support previous modeling of homogeneous extension in oblique-rifted margins when both instantaneous and finite strain are considered. Positive field tests of analytical and experimental models, such as this one, indicate that such models may be robust predictors of the obliquity of rifting in ancient mountain belts where brittle fault arrays are preserved, but plate motions are unknown.