The Ups And Downs Of The Santa Rosa Island Fault, Northern Channel Islands, California: More Than Simple Strike Slip

It has long been recognized that Santa Rosa Island, part of an east-west chain of four islands bordering the Santa Barbara Channel, is transected by a large, broadly arcuate, east-west-striking fault known as the Santa Rosa Island fault (SRIF). Earlier interpretations call for mainly sinistral strike slip along the fault during Pliocene and Quaternary time. The terrain south of the SRIF is generally more rugged, elevated, and densely and deeply dissected compared to the tablelands north of the fault, which are cut by widely spaced and commonly flat-floored canyons. A possible cause for this topographic contrast is that SRIF movement includes a component of south-side-up dip slip. However, this sense of vertical displacement is at odds with other observations: (1) the SRIF forms a south-facing (i.e., north-side-up) erosionally modified scarp that gradually increases in height from the east and west sides of the island to a maximum of ~100 m a few km east of the island's center (at Black Mountain); and (2) prominent, deep stream canyons dissect the tablelands north of the fault. These geomorphic features are consistent with an episode of uplift north of the fault and consequent downcutting of northward-flowing streams on the northern part of the island. To help evaluate and discriminate between these various SRIF geomorphic-tectonic scenarios, we made detailed fault-slip observations at 5 sites along the fault trace, two in Pleistocene marine-terrace deposits exposed in the western and eastern sea cliffs and three in Tertiary bedrock in the island's interior. The principal SRIF fault strands dip northward (54o-83o) or are subvertical at all of the sites. Slip surfaces generally preserve sinistral and sinistral-normal strike-slip striae accompanied by patches of normal and reverse oblique- to dip-slip striae. Where oblique-reverse and (or) reverse striae are identified, they mostly overprint and thus postdate strike-slip striae. Only reverse-slip striae are present where the fault cuts late Pleistocene (120- or 80-ka?) marine-terrace deposits exposed in the eastern sea cliffs, but normal, strike-slip, and reverse striae are all common on older subparallel possible SRIF fault strands exposed nearby in Miocene bedrock. Collectively, these fault kinematic results support a SRIF tectonic model whereby up-to-south normal or sinistral-normal slip characterizes the fault in the Pliocene and (or) early Pleistocene. We hypothesize that sinistral strike slip becomes dominant through much of the Pleistocene, but it is modified by progressively greater components of up-to-north reverse slip and coeval growth of the south-facing fault scarp in the late Quaternary.