ALBERT

Description: [1]  The Grizzly Valley fault system (GVFS) strikes northwestward across Sierra Valley, a low-relief basin situated within a network of active dextral-slip faults in the northern Walker Lane, California. Quaternary motion along the Grizzly Valley fault system has not been previously documented. We used high-resolution (0.25 m) airborne LiDAR data in combination with high-resolution, P-wave, seismic-reflection imaging to evaluate Quaternary deformation associated with the GVFS. We identified suspected tectonic lineaments using the LiDAR data and collected seismic-reflection data along six profiles across the lineaments. The seismic-reflection images reveal a deformed basal marker that we interpret to be the top of Tertiary volcanic rocks overlain by a 120- to 450-m-thick suite of subhorizontal reflectors that we interpret to be Plio-Pleistocene lacustrine deposits. Three profiles image features that we interpret to be the principle active trace of the GVFS, which is a steeply dipping fault zone that vertically offsets the volcanic rocks and the lacustrine basin fill. These data suggest that the GVFS may have been active in latest Quaternary time because: 1) the LiDAR data show subtle surficial geomorphic features that are typical of youthful faulting, including a topographic lineament marked by a ~1-m-high ridge composed of discontinuous, left-stepping lobes; and 2) the seismic profiles demonstrate shallow faulting of the lacustrine strata that coincides with the left-stepping ridge. This investigation illustrates the potential for unidentified, low rate, strike-slip faults in transtensional basins and emphasizes the value of high-resolution topographic data and subsurface imaging as a means of identifying these structures.

Description: The Yakima folds of central Washington, USA are prominent anticlines that are the primary tectonic features of the backarc of the northern Cascadia subduction zone. What accounts for their topographic expression and how much strain do they accommodate and over what time period? We investigate Manastash anticline, a north-vergent fault propagation fold typical of structures in the fold province. From retrodeformation of line- and area-balanced cross sections, the crust has horizontally shortened by 11% (0.8-0.9 km). The fold, and by inference all other folds in the fold province, formed no earlier than 15.6 Ma as they developed on a landscape that was reset to negligible relief following voluminous outpouring of Grande Ronde Basalt. Deformation is accommodated on two fault sets including west-northwest-striking frontal thrust faults and shorter north- to northeast-striking faults. The frontal thrust fault system is active with late Quaternary scarps at the base of the range front. The fault-cored Manastash anticline terminates to the east at the Naneum anticline and fault; activity on the north-trending Naneum structures predates emplacement of the Grande Ronde Basalt. The west-trending Yakima folds and west-striking thrust faults, the shorter north to northeast striking faults, and the Naneum fault together constitute the tectonic structures that accommodate deformation in the low-strain-rate environment in the backarc of the Cascadia Subduction Zone.