The Mojave Extensional Belt of southern California

Abstract

It is proposed that the regional pattern of early Miocene low‐ and high‐angle normal faulting and dike swarm emplacement in the western and central Mojave Desert is the result of regional extension developed within a roughly east trending zone, named here the “Mojave Extensional Belt”. This zone of extension, although now disrupted by late Cenozoic right‐slip faulting, can be traced from the intersection of the San Andreas and Garlock faults to the eastern Mojave Desert near Bristol Lake and the Granite Mountains fault. Restoration of post‐Oligocene movement along the San Andreas fault system places the now buried western part of the Mojave Extensional Belt opposite a similar age‐extended area of west central California. Surface mapping and limited subsurface data suggest that the dominant tectonic process responsible for development of the Mojave Extensional Belt was low‐angle, normal sense, simple shear (detachment faulting). Strain within the Mojave Extensional Belt is partitioned between four domains (Edwards, Waterman, Daggett, and Bullion terranes) that each consist of one or more half‐grabens. Each half‐graben is composed of tilted, normal fault‐bounded blocks that lie above a rooted, low‐angle, brittle‐ductile normal sense shear zone. Tilting and extension of the upper plate resulted from the superposition of several generations of originally steeply dipping (∼80°) planar to slightly curviplanar normal faults. At least one generation of high‐angle faults postdates the formation of the detachments. Differential extension between the domains was accommodated by strike‐slip faults (transfer zones). Similarly, the lack of early Miocene extensional elements north of the western segment of the Garlock fault suggests that the Garlock may have been active in the early Miocene and served as the northern transfer boundary of the Mojave Extensional Belt. The Mojave Extensional Belt initially began to open about 22 m.y. ago, with the major phase of extension occurring between ∼22 and 20 Ma. Local, high‐angle, normal faults cut older detachments and were active between ∼20 and 17 Ma. Kinematic indicators suggest that the western part of the Mojave Extensional Belt opened in a NE–SW direction, whereas the eastern portion extended in an ENE–WSW direction (present‐day reference frame). Rotation of the region about vertical axes as revealed by paleomagnetic studies suggests that the direction of extension within the Mojave Extensional Belt was originally oriented ∼N–S and was rotated as much as 50° (clockwise) between ∼20 and 18 Ma (Early Miocene).