Late Cenozoic deformation is broadly distributed across the North American plate margin of the conterminous western United States and stretches from the San Andreas fault system eastward across the Mojave Desert and into the Basin and Range. The eastern California shear zone and Walker Lane of the western Great Basin form an active belt of structures accommodating about 25% of the relative motion between North America and the Pacific plate. From the Mojave Desert, the displacement is carried north, east of the southern Sierra Nevada, in a narrow zone of deformation bound on the west and east by the Owens Valley and Furnace Creek fault systems. North from the latitude of the central Sierra Nevada, the zone of deformation broadens to include the Walker Lane and central Nevada seismic belt in the northwestern Great Basin. The Sierra Nevada behaves as a coherent tectonic block with a northwest-directed motion of 10-14 mm/yr and forms the western boundary of the zone of distributed deformation in the Great Basin. A complex pattern of active structures underlies west-central Nevada where displacement is transferred from the eastern California shear zone to the Walker Lane and the central Nevada seismic belt. Integrated geologic, seismological, and GPS geodetic results indicate that the central Walker Lane serves as an displacement transfer system linking stepped northwest-trending transcurrent faults. Within the central Walker Lane displacement partitioning is active and may be accommodated by differential motion of tectonic blocks. A GPS velocity field exhibits a systematic increase in magnitude from east to west across the central Walker Lane and is consistent both with block translation and vertical-axis rotation. Zones of divergent, transcurrent, and convergent motion are observed across suspected block boundaries. The observed velocity field is not easily reconciled with the current understanding of fault displacements and points out the difficulty in comparing geodetic and geologic displacement fields.
The PI's propose an integrated geodetic and geologic investigation of the central Walker Lane to address two questions: (1) Is displacement transfer between transcurrent structures wholly accommodated by kinematically coordinated slip on throughgoing faults, and (2) are the kinematics of active displacement transfer derived from GPS geodesy consistent with the displacement field estimated by earthquake seismology and fault-slip inversion? The central Walker Lane is ideally suited for this study for three reasons: (1) the region is seismically active and well defined earthquake focal mechanisms exist for the displacement transfer fault system, (2) the faults are well exposed and have produced preliminary fault-slip estimates of deformation kinematics, and (3) a complex present-day displacement field with ~10 mm/yr of differential slip is recorded by a GPS geodetic network. These elements will allow characterization of the kinematics of displacement transfer and offer the opportunity to directly compare different means of measuring deformation kinematics. The PI's primary interest is to characterize the kinematics of deformation in the active displacement transfer stepover and to compare the geodetically determined displacement field with that derived from earthquake focal mechanisms and fault-slip inversion. To achieve their research objectives, several aspects of the extensional stepover system exposed in the central Walker Lane must be examined in greater detail. The primary tasks set out in this project are: (1) to establish the spatial distribution of active high-angle faults linking the bounding transcurrent faults, (2) to document the detailed geometry and slip history of fault systems that transfer displacement between transcurrent faults, and (3) to deploy a GPS network with sufficient density to differentiate between continuous versus discontinuous variations in the present-day velocity field. The PI's will address the tasks listed above with an integrated study utilizing detailed geologic mapping, structural analysis, and GPS geodesy. The investigators (Oldow and Satterfield) each have substantial experience working in the area and by building on previous studies, geological mapping and structural analysis will establish the areal limits of the transtensional fault system, document variable geometric relations between major and secondary fault systems, define the kinematic history of fault motion, and develop first-order estimates of recent slip magnitude by documenting offset landforms. Combined with other ongoing or proposed studies in the central Walker Lane, the results of this research should contribute to an unprecedented view of active transtensional displacement transfer and yield critical insight into the comparison of geologic and geodetic measures of deformation kinematics.
