Slip rates and slip gradients in the eastern California shear zone—the geology

Walker, J. Douglas, Department of Geology, University of Kansas, Lawrence, KS, 66045; jdwalker@ku.edu

This talk presents data and summarizes interpretations for deformation in the eastern California shear zone based on observations of the geology. The shear zone consists of roughly NNW to NW trending fault zones that interact with the Garlock fault, a fundamental cross-strike structure. Two important approaches to understanding deformation in the eastern California shear zone are: 1) establishing slip rates on strike-slip and oblique-slip fault systems using geologic data; and 2) determining gradients in slip in the geologic deformation field. These facets are interesting on their own, and in combination give data for interpreting both the geodynamic development and the geologic history of the region. We gain further insights by comparing these data to the geodetically established deformation field, the subject of the companion presentation.

Geologic slip rates are established for most of the fault zones in the eastern California shear zone. Because slip rates have been determined primarily for the roughly NNW to NW trending faults of the shear zone, the strike-slip component of deformation is best resolved; normal or reverse components are typically inferred from minor structures. Slip rates across the Mojave Desert account for about 7 mm/yr of dextral shear with a component of transpression. This estimate uses data for 5 to 6 fault zones and an interpretation for rates of block rotation in the northeastern part of the Mojave block. Transpression results from the main strike-slip faults being oriented more westerly than the overall shear zone. Dextral shear in the southwestern Basin and Range occurs at a similar rate of about 6.5 mm/yr in a system that is transtension in character. This estimate uses data from 5 fault zones that are well developed about 30 km north of the Garlock fault. Although both estimates have large uncertainty (probably around 2 mm/yr), they agree surprisingly well and are distinctly less than the approximately 13 mm/yr of dextral shear derived from geodetic measurements. Several different explanations, such as time-varying strain and non-elastic crustal behavior, may resolve this apparently robust disagreement between geodetic and geologic rates.

Although many workers have studied the strain partitioning and strain transfer aspects of fault slip in the eastern California shear zone, few have focused on strain gradients and changes in deformation style as fault zones approach the Garlock fault. As noted by previous studies, faults in the Mojave Desert appear to lose slip and end before intersecting the Garlock fault. A similar situation exists for faults in the Basin and Range province. For example, the Panamint Valley fault zone consists of several discrete and well marked branches with clear Holocene motion about 30 km and farther north of the Garlock fault, but narrows to a single fault southward. No young scarps are discernable within 10 km of the Garlock. Because of the changes in slip on faults and the fact that the trace of the Garlock fault is continuous, strain associated with faults systems of the eastern California shear zone must change markedly near the Garlock fault. These faults apparently feed into complex deformation (in addition to sinistral strike-slip faulting) within the Garlock fault zone. In the eastern part of the Garlock, there is clear evidence for contractional deformation along numerous folds and thrust faults oriented at a small northeast oblique angle to the main fault zone. To the west of the Blackwater/Little Lake faults, deformation at the Garlock contains a significant component of fault-perpendicular extension. Numerous workers interpret that shear associated with the eastern California shear zone causes oroclinal bending of the Garlock fault; however, the connection between shear zone deformation, changes in strain, and deformation along the Garlock fault remain uncertain. Crust to mantle scale models for deformation in the eastern California shear zone must honor not only the slip rates determined from geologic data, but also the smaller scale deformation field.