High-resolution seismic imaging, Mono Lake fault zone, eastern Sierra region, Walker Lane, California

Multiple strands of the Mono Lake fault zone (MLfz), a segment of the Sierra Nevada frontal fault zone, have been imaged on several high-resolution seismic reflection profiles collected during 2009 and 2011 at Mono Lake. The profiles show coherent reflectors to about 30-40 ms depth below the lake bottom (~30 m thick section) in nearshore areas north of the Lee Vining delta. The MLfz is well imaged on 8 lines including 4 lines ~normal to the trend of the fault zone. The fault zone is ~ 0.75 km wide. Deep reflection horizons appear gently tilted and rotated into the fault zone with a prominent clastic wedge overlying the west-tilted horizons. Shallow reflectors above the clastic wedge are generally east-sloping, but noticeably less inclined above the fault zone. At least two ruptures offset Holocene deposits, with ~0.5-1.8 m dip-slip offset around 2.5 ka and ~3.6-6.13 m dip-slip offset around 4.7 to 6.25 ka. The ages of reflection horizons are estimated using published Holocene and late Pleistocene sedimentation rates, as well as correlation with a published nearby shallow core. The short term fault slip rate based on the timing of the most recent event and multiple events in the profile lines suggests fault slip rates of about 0.26 to 0.55 m/ka using ages based only on sedimentation rate and of about 0.31 to 0.34 m/ka using correlation ages from nearby shallow core. This offshore dip-slip rate is significantly lower than previous published 1.0-2.0 m/ka dip-slip rates estimated using cosmogenic dating of Tioga glacial moraines in Lundy Canyon and offset older moraines. The offset on the large scarp in Lundy Canyon (~20 m) decreases both north and south of the canyon and flanking lateral moraines where the scarp is on the order of only ~4-7 m high. A possible explanation for the apparent difference in MLfz slip rates onshore and offshore in this part of Mono Basin may be soft sediment deformation of saturated glacial-deltaic sediment within Lundy Canyon that causes local lateral spreading and an unusually large scarp, or to occurrence of multiple fault strands. Profile ML-112 showing Mono Lake fault zone,