A BLISTER HYPOTHESIS FOR THE CENTRAL MOJAVE METAMORPHIC CORE COMPLEX NEAR BARSTOW, CALIFORNIA

The Waterman Hills detachment fault (WHDF) north of Barstow is reported as formed during passage of a rolling hinge, but kinematics are conflicting; N-directed upper-plate brittle strain in the Mud Hills and NE-directed lower-plate ductile strain in the Waterman Hills/Mitchell Range. In the Waterman Hills, the last slip events on the WHDF, determined from foliated gouge and the reorientation of directly subjacent L-tectonites, were S-directed, approximately antipolar to brittle strain in the Mud Hills to the north. We interpret the N-S shear strain on the WHDF to be associated with flexing of the detachment around E-W axes. The flexing and detachment shear are the youngest Miocene deformation and followed major fluid-flow across the fault, cataclasis, and vertical collapse. The flexing also post-dates main-phase extension (pre 18 Ma?) and bears no obvious spatial or genetic association with regional post-18-Ma strike-slip faulting. It records N-S shortening kinematiclly consistent with other major Miocene shortening strains including 1) the horizontal contractional component of widespread constrictional L tectonites formed in a lower-plate Miocene pluton, 2) strong folding around northeast axes of L and LS mylonites and ultramylonites in the pluton at scales ranging from sub-meter to hundreds of meters, 3) kilometer-scale steep-axis bending of previously steeply tilted early Miocene upper-plate volcanic and sedimentary rocks, and 4) development of the post-18 Ma east-west Barstow syncline 7 km to the north. Collectively, the assemblage of shortening strains, oriented either normal to the NE-trending axis of regional extension or approximately N-S, spans the full range of age and depth of known Miocene deformation in the central Mojave metamorphic core complex. These relations invite consideration of a tectonic model that is a) three dimensional, b) fundamentally constrictional in both the ductile and brittle realms and c) not driven by plane-strain extension as in the rolling hinge model. Also, the restricted extent (12x35 km), extension-normal long-axis shape, and 3-D aspect of high strain in the core complex are more consistent with a localized tectonic blister than a sub-horizontal crustal shear zone coupled to high-strain areas located 200 km to the east and west as favored in published reports.