Joint Meeting Pacific Section, AAPG & Cordilleran Section GSA April 29–May 1, 2005, San José, California

Tectonic Geomorphologic Evidence for Spatially and Temporally Varying Uplift Rates Adjacent to the San Andreas Fault on the Point Reyes Peninsula

Karen Grove¹, Anne Marie Scherer², and John Caskey^1
1 Department of Geosciences, San Francisco State Univ, 1600 Holloway Ave, San Francisco, CA 94132, [email protected]
² CH2M HILL, Seattle, WA 98195

The Point Reyes Peninsula is part of the eastern edge of the Bodega Basin. Like other nearby offshore basins, the Bodega Basin is part of a large, northwest-trending fold and thrust belt associated with a varied sea floor of alternating bathymetric highs and lows. Unlike the other offshore basins, the Bodega Basin is underlain by granitoid basement rocks and has been partially uplifted above sea level, forming the Point Reyes Peninsula. The peninsula is structurally bounded on the southwest by the offshore Point Reyes thrust, which bends eastward around the south side of the Point Reyes headland to join other east-west-trending structures in Drakes Bay, and on the east by the western boundary fault (WBF), which is the westernmost strand of the San Andreas fault zone from near Bolinas to Tomales Bay.

Late Pleistocene coastal terraces on the Point Reyes Peninsula record uplift of Inverness Ridge along the WBF, and the Point Reyes headland along the Point Reyes thrust. These ridges are limbs of a broad, large-scale syncline, with the low-lying region of Drakes Estero marking the fold hinge in between. We measured terrace elevations, used GIS tools to analyze geomorphic features, and sampled sediments for luminescence dating. Age data for the youngest marine terrace suggest that it correlates to oxygen isotopic stage 5a terraces (~80 ka). On the east limb of the fold, this terrace increases dramatically in elevation from the hinge area near Drakes Estero (~0.2 mm/yr) southeastward to Bolinas (~1.0 mm/yr), indicating that the fold is a late Pleistocene and probably actively forming structure. Fault facets along the WBF are also higher and steeper toward the south, suggesting that the north-to-south increase in uplift is associated with increased uplift rates along the WBF toward the south. The only noteworthy earthquake in the region since 1906 was the 1999 M5 reverse-slip earthquake at Bolinas, where we measure our highest uplift rates. Point Reyes headland, on the west limb of the syncline, appears to have immerged above sea level since ~100 ka, suggesting that the Point Reyes thrust is also a late Pleistocene and potentially active structure. Our analysis of older terraces on Inverness Ridge suggests that the uplift rate has increased with time, accelerating since ~500 ka.

Posted with permission of The Geological Society of America; abstract also online (http://gsa.confex.com/gsa/2005CD/finalprogram/abstract_85715.htm). © Copyright 2005 The Geological Society of America (GSA).