Contemporary block tectonics: California and Nevada

Abstract

Well‐determined fault plane solution and the gross pattern of late‐Cenozoic faulting in California and Nevada show a systematic relation between the orientation of fault planes and slip directions. In general, normal faults have northerly strikes, reverse faults have easterly strikes, and dextral and sinstral strike slip faults have northwesterly and northeasterly strikes, respectively. Kinematically, this relation is consistent with the response of clusters of fault‐bounded crustal blocks to a regional stress field generated by the relative motion between the Pacific and North American plates. In this stress field, the greatest and least principal (compressive) stresses are restricted to northerly and easterly striking vertical planes, respectively. Clusters of crustal blocks bound by faults having the above attitudes form three basic regimes: (1) a spreading regime with a combination of normal and conjugate, strike slip faults, (2) a converging regime with a combination of reverse and conjugate, strike slip faults, and (3) a wrench regime with sets of subparallel, throughgoing, northwest striking (dextral) faults or northeast striking (sinstral) faults. These three regimes are typified by faulting patterns in the Basin and Range province (spreading), the Transverse Ranges (converging), and the San Andreas‐Garlock fault systems (wrench), respectively. The gross deformation of each regime resulting from relative displacements between individual blocks is characterized by north‐south shortening and east‐west extension with the ratio of extensional to shortening strains (and the areal dilitation) decreasing systematically from spreading to wrench to compressional regimes. The wrench regime involves a component of net rotational deformation (clockwise for dextral slip and counter‐clockwise for sinstral slip), while deformation of the spreading and converging regimes is irrotational. Local deviations from regional kinematic directions are concentrated along the boundaries between regimes reflecting the mismatch in gross deformation fields between regimes. Maximum principal and shear stress magnitudes will increase systematically from spreading to wrench to converging regimes provided that fault slip is controlled by frictional strength (Byerlee's law) along preexisting fractures and that pore pressure in the brittle crust is laterally uniform. A minimum strength difference between active, block‐bounding faults and block interiors is 15–30%. Simple arrangements of such block clusters mimic the gross kinematic pattern of Quaternary faulting in California and Nevada. Some implications for contemporary tectonics emphasized by this model involve the westward displacement of the Sierra Nevada block.with respect to the stable interior of the North American plates, oblique thrusting of the Salinian block over the Pacific plate, and a progressive increase in the offset of the San Andreas fault represented by the ‘big bend’ through the Transverse Ranges.