Active shortening along the northern edge of the western Transverse Ranges recorded by deformed fluvial terraces along Santa Cruz Creek in the eastern Santa Ynez Valley, California

Vertical-axis rotation of the western Transverse Ranges in southern California was accompanied by folding and reverse faulting at the northern edge of the rotated domain during late Miocene to Pliocene time. Deformed Pleistocene strata in the Santa Ynez Valley suggest that this rotational folding and faulting is still occurring. Fluvial terraces are present along several major drainages that cross the faults and folds and provide an opportunity to evaluate the timing and magnitude of Late Quaternary deformation at the northern edge of the rotated western Transverse Ranges. Three flights of well-preserved fluvial strath terraces are exposed along a 9.3 km stretch of the Santa Cruz Creek drainage in the eastern Santa Ynez Valley. Fluvial deposits that make up the terrace treads have been lifted 18.5 m (T-1) to 110m (T-3) above the active channel and are unpaired across the drainage. The terrace surfaces are underlain by a thin 0.6m-6m veneer of fluvial deposits resting on an erosional surface cut into the underlying Pleistocene Paso Robles Formation. GPS profiles (accurate to .1m) of the terrace surfaces and the Santa Cruz Creek stream profile show evidence of Late Quaternary deformation. The T1 surface (lowest terrace surface) is offset vertically 3.5m across the Los Alamos/Baseline fault, the T2 surface is offset 7m vertically across this feature, and the T2 and T3 terraces terminate abruptly at the Little Pine Fault indicating Quaternary displacement across these structures. Optically Stimulated Luminescence (OSL) dating is being used to date the terraces in order to calculate rates of uplift and deformation.