UNDERSTANDING THE ROLE LANDSLIDES PLAY IN DENUDATION OF EXTENSIONAL SYSTEM FOOTWALLS: PASSIVE-SOURCE SEISMIC ANALYSIS OF THE SLIDE MOUNTAIN LANDSLIDE COMPLEX, SIERRA NEVADA FRONTAL FAULT ZONE, WASHOE VALLEY, NEVADA, USA

Rangefront-bounding normal faults generate steep topography that over time is subdued by erosional processes including landsliding. However, our understanding of the overall importance of landslide events in footwall denudation remains limited, in part because these deposits become buried by valley fill through time. The presence of buried landslide deposits can be confirmed by expensive techniques like drilling boreholes. However, less expensive geophysical techniques like the refraction microtremor passive-source seismic method can be used rapidly over a wide area to identify buried landslide deposits.

Washoe Valley sits between the Carson and Virginia ranges in western Nevada. The Carson Range bounds the west side of the valley, with the Sierra Nevada Frontal fault system between. The eastern Carson Range is dominantly composed of Cretaceous granite. A wide swath of sheared and brecciated granite high in the range serves as the source area for landslides of the Slide Mountain landslide complex. This complex contains a stack of at least 10 Quaternary landslide deposits exposed in the northwestern part of Washoe Valley. No other landslide complex is apparent along the Carson Range in Washoe Valley.

Two passive-source curvilinear seismic arrays were collected on the western side of Washoe Valley. One array (Slide) was placed across the toe of the Slide Mountain landslide complex. The second array (Franktown) was ~2 km south where landslide deposition is not apparent. Each line consisted of 100 Fairfield 3‑component seismic nodes with ~22 m spacing for a total line lengths of 2.2 km. The nodes collected data for ~4 hours for each array and data were processed using ReMi 2dS^TM software from Terēan. The Slide array shows shear wave velocity heterogeneity in the upper ~50-100 m (including inversions) that are not present in the Franktown array. The heterogeneous velocity zone thickens toward the center of the landslide complex and includes ~65% of the area above the fault along the array. We interpret the Slide array heterogeneity to represent landslide deposits, with velocity heterogeneity resulting from the generally larger but variable clast size in the landslide deposits. The more homogeneous Franktown array is interpreted to not record any landslide deposits. This type of analysis could be used to estimate relative volume of landslide deposits in fault-bounded basins.