UC Santa Barbara

The lower Colorado River extensional corridor (CREC) is an area of extreme crustal extension (>100%) which borders areas of moderate to minimal (<15%) extension. Regionally extensive detachment (low-angle normal) fault models commonly applied to the CREC define the margin of the extensional terrane as a discrete “breakaway” where the detachment fault originally intersected the surface. Results of field investigations along the CREC margin reveal locally complex deformation histories inconsistent with simple detachment models. In the Little Piute and northwest Piute Mountain areas, the dominant extension direction (NW-SE) is orthogonal to the commonly observed extension direction in the CREC (SW-NE), multiple generations of strike slip and oblique normal faults are present, and deformation occurred after roughly 17.7 Ma. These observations suggest a complex strain field evolved throughout the Miocene along the western margin of the CREC. Westward tilting of the Piute Mountain block indicates extensional deformation persisted beyond the previously hypothesized CREC breakaway. In the Piute Range, west-directed normal faults dip steeply (60°) and have cutoff angles near 90° showing no obvious relationship to east-dipping detachment faults documented to the south (Homer Mountain) and east (Newberry Mountains). In all study areas the observed deformation history is more complex than expected in simple detachment fault breakaway models, suggesting current models are inadequate in describing deformation patterns along the margin of highly extended terranes.