Assessing fault displacement and off-fault deformation in an extensional tectonic setting using 3-D ground-penetrating radar imaging

High-resolution geophysical imaging can aid in the study of active faults by providing subsurface information about the geologic structure and deformational patterns present. We collected ground-penetrating radar (GPR) data across the active Emigrant Peak Fault in order to image and quantify fault displacement and assess off-fault deformation in an alluvial fan setting. The Emigrant Peak Fault in Fish Lake Valley, Nevada, is part of the northwest-striking, right-lateral Walker Lane-Eastern California Shear Zone that accommodates a portion of the North America-Pacific plate boundary strain. The normal fault displacement along this northeast-striking fault plays an instrumental role in transferring right-lateral deformation through a regional releasing bend.The GPR surveys span the Emigrant Peak Fault and adjacent alluvial fan and consist of a high-resolution 50 MHz 3-D grid and a longer 25 MHz 2-D line. The 3-D grid covers an area of 115 m by 500 m straddling the fault, while the 2-D GPR data form a 1500 m long line perpendicular to the active trace of the Emigrant Peak Fault. Depth of imaging ranges between ~ 25 m for the 50 MHz data and ~ 40 m for the 25 MHz data. GPR data exhibit structural geometries dominated by NW-dipping normal faulting and corresponding antithetic faults. Near the main fault strand, alluvial fan strata dip consistently to the NW while the stratigraphic architecture becomes more complex in the down-dip direction, including colluvial wedges and small graben structures. At the main fault, GPR imaged reflectors showing offset of 1–2 m, which is in agreement with the minimum displacement observed at the youngest terraces along the fault scarp. Radar data reveal considerable off-the-main-fault deformation and complex structures not evident in surface observations, aerial photography, and differential GPS digital elevation models. Rapid rejuvenation of the active alluvial fan likely erases any surface expression of deformation away from the main fault. This study provides further evidence that 3-D subsurface imaging is necessary for accurate determination of fault displacement and off-fault deformation of active fault zones.