RECONSTRUCTION OF LATE QUATERNARY SURFACE OFFSET ON THE SALT LAKE FAULT: PROXY FOR EARTHQUAKES ON THE BLIND GREAT VALLEY THRUST FAULT BENEATH THE WESTERN MARGIN OF THE NORTHERN SACRAMENTO VALLEY, CALIFORNIA

Reconstruction of displacement per event on the Salt Lake fault provides indirect evidence for the late Quaternary activity, and likely size of earthquakes, on the underlying Funks segment of the Great Valley thrust fault, in the foothills region of the northwestern Sacramento Valley. The east-dipping Salt Lake thrust fault, and associated Sites anticline and Fruto syncline, have accommodated late Quaternary tectonic wedging above the blind, west-dipping Great Valley thrust fault. Results of a 12.4-km-long differential GPS survey of geomorphic surfaces and the stream channel thalweg along Hunters Creek, orthogonal to these structures, document the location, style, and relative amounts of late Quaternary deformation. Detailed geomorphic profiling documents that stream terraces along Hunters Creek diverge over the Sites anticline and converge toward the Fruto syncline axis. The amplitude of folding in older terraces is greater than that in younger terraces, which we interpret as evidence for progressive fold growth during the late Quaternary. Stream terrace deposits exposed within paleoseismic trenches have been offset by repeated displacement on the Salt Lake fault, producing an upstream-facing scarp in bedrock and overlying fluvial gravels, uplift and erosion of the hanging wall, and a series of colluvial wedge deposits in the foot wall block. Based on soil development within the uppermost offset colluvial wedge, the most recent fault offset occurred during the late Pleistocene to early Holocene (12 to 8 ka). Based on offset of late Pleistocene stratigraphic units, we document total vertical separation of 1.2 m along the Salt Lake fault within the last 30 to 70 ka. At least three discrete slip events, with average displacements of 0.4 m, are interpreted from reconstruction of multiple faulted scarp-derived colluvial wedges. Development of restorable strath surfaces on offset alluvial gravels and associated, well-developed buried soils strongly suggest periods of relative stability between faulting events. Elastic dislocation modeling suggests that the observed Quaternary displacements on the Salt Lake fault may have occurred as a result of triggered slip in response to large earthquakes (>1 m displacement) on the underlying blind Great Valley thrust fault.