OPTICALLY STIMULATED LUMINESCENCE (OSL) GEOCHRONOLOGY OF AN ACTIVE BLIND THRUST FAULT, WHEELER RIDGE, CALIFORNIA

Quantifying the magnitude, timing, and rate of fault slip along a laterally propagating blind thrust requires examining surface process response to differential uplift. Wheeler Ridge anticline, in the Southern San Joaquin Valley of California, is a well-known example of a young (<250 ka), and active (1952 Kern County Earthquake, Mw 7.3) eastward-propagating fold. From east to west, a water gap and wind gap separate three geomorphic domains: the east, central, and west. We present nine new preliminary Optically Stimulated Luminescence (OSL) numerical ages from tectonically isolated surfaces and geomorphic analysis of airborne light detection and ranging (lidar) data to illustrate the spatiotemporal evolution of Wheeler Ridge. Preliminary surface uplift rates use a surface idealized from adjacent undeformed fans to calculate the vertical distance (y) from OSL Sample locations along the fold crest.

The eastern section is minimally dissected and has well preserved relict alluvial fan morphology with landforms interpreted as fan swales and terraces. The youngest dated surface (y=13.5 m) has an OSL age of ~29 ka and surface uplift rate of ~0.46 m/kyr. About 1 km to the west, ages from a 30 m thick section of alluvial fan deposits range from ~63–109 ka. The inferred rate based on the stratigraphically highest sample is ~1.16 m/kyr (y=73 m). The central section has two distinct domains: broad sloped crest flanked by steep drainages on the forelimb and long, straight drainages on the backlimb. The central section has an age range of ~103 ka and uplift rate of ~1.22 m/kyr (y=126 m). The western section and oldest dated deposit is ~137 ka, yielding a rate of ~2.47 m/kyr (y=339 m). The western backlimb has long, linear drainages divided by rounded hillslope ridgelines. The backlimb of all geomorphic domains is completely in the hanging wall of the Wheeler Ridge thrust and thus backlimb topography is the best representation of the balance of surface processes responding to local rock uplift migrating towards the east over time.