Subsidiary Fault Kinematics and Displacement Transfer at the Mill Creek-Mission Creek Fault Stepover, San Bernardino Mountains, California

Abstract

The geometry of principal slip zones and the structures of their damage zones can affect earthquake nucleation, propagation, and arrest. Geometric irregularities, such as bends and segmented faults (stepovers), can modify the far-field stress state locally, both on and off principal slip surfaces, and promote or inhibit rupture though these complexities. The broad, multi-fault system of complex kinematics that is the San Gorgonio Pass region in Southern California represents a perfect area to study earthquake rupture through geometrically complex regions. The objective of this study is to characterize the surface, brittle deformation within the damage zone of the Mill creek fault in order to better understand the role that this fault has played in accommodating the overall right-lateral shear along the San Andreas system within the San Bernardino Mountains.

This study identified three deformation events on the basis of fault fabrics and crosscutting relationships. The earliest deformation is associated with movement along the Mill Creek fault from straight segments into a restraining bend. A younger deformation event relates to the development of right-lateral oblique-normal transfer faults, which is recorded by the overprinting of epidote-mineralized faults by iron-oxide stained faults associated with the formation and movement along those faults. The youngest deformation event consists of reactivation of subsidiary faults due to shortening associated with the steep topography and mass wasting processes during rapid uplift and exhumation.

The subsidiary fault fabric of the damage zone of the Mill Creek fault indicates that the trend of the maximum principal compressive stress is oriented at a relatively low angle to the fault trace. This observation contrasts to that of other faults in the San Andreas system which document large angles to the main fault. This may indicate that the Mill Creek fault operated at a higher apparent friction than the other faults.

The restraining bend of the Mill Creek fault modifies the local stress state such that the maximum principal compressive stress steepens from nearly horizontal to approximately a 45⁰ plunge to the NW as the bend is approached. These observations imply a significant component of 3D deformation when compared to a similar-shaped, but relatively isolated, fault-bend along the North Branch San Gabriel fault.