HOLOCENE(?) AND LATEST PLEISTOCENE SURFACE RUPTURES ON THE SOUTHERN INYO MOUNTAIN FAULT, SOUTHERN OWENS VALLEY, EASTERN CALIFORNIA SHEAR ZONE

The Inyo Mountain fault (IMF) is discontinuous and locally cuts alluvial deposits at the base of the Inyo’s between Mazourka Canyon and Hwy 190. The southern section of the IMF trends ~N20-40W for ~12 km at the base of, and within the range-front east of Keeler. Shutter ridges with interfluves of late Mio(?)-Pleistocene alluvium indicate long-term dextral motion along prominent NE facing scarps. Newly identified paleoseismic sites in ‘slate’ and Cerro Gordo Canyons show evidence for at least 2 ruptures that are likely <20 ka, with the youngest event probably a M~6.5 in the last 10 ka. An ~0.5 m NE facing scarp formed by the youngest event cuts an abandoned alluvial gravel bar (Q3b/a) within a wide active wash informally known as ‘slate canyon’ for an abandoned slate quarry. Several right-stepping en echelon ~0.5 m deep depressions pond fine sediment parallel to this scarp. An ~4 m high channel cut on the north side of the bar exposes an ~45 m wide zone of steeply NE dipping faults and fractures, as well as a disturbed zone with vertically rotated clasts underlying the surface scarp. Strong evidence of lateral motion is indicated by an ~20-40 cm wide vertical shear zone with vertically aligned clasts separating dissimilar horizontally imbricated gravels on either side. Secondary faults dip ~80° NE with apparent normal displacements of ~3 cm and common vertically rotated clasts. Faults and fractures terminate ~60 cm below the surface into a silt-rich bioturbated zone.

The scarp and faults exposed in the gravel bar (Q3b/a) are younger than a fluvial terrace flanking the wash that we infer is the maximum aggradation surface from the last glacial maximum (~20 ka). The ~10 ka or younger age of the ~0.5 m scarp is supported by a ~1.5 m high exposure of Holocene alluvium on the north side of the active wash that shows a near-vertical fault/fracture that is overlain by unfaulted gravels ~30 cm depth. This fault also has vertically aligned clasts and fissure fills.

In Cerro Gordo Canyon, an ~3.0 m NE facing compound fault scarp with a younger single-event scarp ~0.5-0.75 m high at the base cuts a preserved alluvial fan surface inferred to be <20 ka. The geomorphic relations are confirmed by an exposure in the bottom of an adjacent active channel that has several N35W striking vertical shears and fractures filled with CaCO₃ in sandy sediment within an 8 m wide zone.