Determining rates of Quaternary uplift across the Santa Rosa Island Fault, Channel Islands National Park, Southern California

Abstract:

The northern Channel Islands, southern California, form the southern margin of the western Transverse Ranges. They constitute a roughly east-west trending emergent ridge uplifted as a result of south-vergent shortening and left-lateral slip, mostly on west-striking, north-dipping faults. Late Quaternary uplift of these islands is recorded by two prominent wave-cut platforms, which occur at elevations of ~5-7 and ~6-24 meters above sea level, have been attributed by previous workers to Marine Isotope Stages (MIS) 5a (~80ka) and 5e (~120 ka), and indicate low and somewhat spatially variable, uplift rates of 0.1-0.2 mm/yr. We have documented evidence of older, higher marine features, such as shoreline angles, beveled surfaces buried by aeolian sand, and pholad borings, in the interior of Santa Rosa Island. Our simple steady uplift models indicate that the highest of these paleo shore line markers, which occur at ~275 m, could be anywhere from ~1.2 Ma to ~2.7 Ma. However, despite their importance for understanding longer term uplift rates associated with the Santa Rosa Island fault and/or other faults, their ages remain unknown.

As part of a cooperative effort with the National Park Service to complete a new Quaternary geologic map of the northern Channel Islands, we collected samples of quartz-bearing rock and sediment at seven locations on Santa Rosa Island, representative of 6 paleo shorelines spanning elevations between ~65 and ~275 meters above sea level, from both sides of the Santa Rosa Island fault. Rock samples were collected from moderately indurated fossiliferous marine sands that either contain pholad-bored clasts or overlie a beveled bedrock surface in order to determine ¹⁰Be exposure ages. Sediments were collected from where they directly overlie shoreline angles for determination of ¹⁰Be-²⁶Al burial ages. Ages of these paleo sea level markers will enable us to test previous assumptions of steady island uplift, as well as allow us to test hypotheses related to the timing of differential uplift across the Santa Rosa Island fault as documented by differential GPS surveys of the MIS platform edges. The timing and distribution of long-term uplift rates will allow us to distinguish between ramp-flat or listric models of deep, low-angle thrusts responsible for the long-term uplift of Santa Rosa Island.