GEOLOGIC STRAIN PARTITIONING IN THE EASTERN CALIFORNIA SHEAR ZONE: RIDING CRUSTAL WAVES IN THE MOJAVE DESERT

The 1990s earthquake ruptures in the Mojave Desert, and numerous other northwest-striking dextral faults of the eastern California shear zone, lie within a neotectonic fold-and-thrust belt that is manifested by large-scale physiographic waves. The belt trends WNW parallel to the San Andreas fault. It is 200-250 km long, 100 km wide, and contains two pairs of topographic crests and troughs. The southern crest comprises the central and eastern Transverse Ranges. The northern crest comprises low mountains in the southern Mojave Desert between Silver Mountain and the Coxcomb Mountains, which we collectively term the Bullion Mountains highlands. The southern trough consists of lowlands directly north of the Transverse Ranges between Victorville and Dale Lake, and the northern trough comprises a chain of valleys between Barstow and Danby Lake. The wavelength is about 30-50 km, and the amplitude decreases northward from about 2 km to 1 km.

The crustal waves are perpendicular to the contemporary NNE-SSW regional compression direction and they apparently are contractional in origin. Based on dated alluvial sediments deposited in the troughs, the waves began forming about 4-5 Ma. Their development controlled the origin and downstream expansion of the Mojave River, which drains northward from the central Transverse Ranges. A stratigraphic and structural record of recurrent Pleistocene warping is present north of Victorville where the river crosses the westernmost Bullion Mountains highlands. There, the base of the river deposits is an early Pleistocene (1.2-1.8 Ma) angular unconformity and their top is a deformed, late Pleistocene (60-70 ka) geomorphic surface. Recent arching of the southern Mojave Desert probably also induced incision of the modern Mojave River canyon between Victorville and Barstow. The upper crust of the Mojave Desert may be buckling above a decollement at the base of the seismogenic zone at 7-10 km depth. However, given that extensive strike-slip faulting is occurring in the same region, Quaternary strain apparently is partitioned into discrete components of contraction and dextral shearing.