COASTAL UPLIFT OF THE POINT REYES PENINSULA NORTH OF SAN FRANCISCO, CALIFORNIA

The Point Reyes Peninsula, located west of the San Andreas fault and north of San Francisco, is a large-scale syncline that has emerged from the sea during the Quaternary Period. Marine terraces that bevel the slopes record this deformation and have been used to quantify the rate and style of uplift. Luminescence dates from beach sediments on the lowest marine terrace suggest formation at sea level during the stage 5a highstand about 80 ka. Terrace elevations were measured with a Global Positioning System unit and used to calculate uplift rates of about 0.2 mm/yr near the fold hinge to over 1.0 mm/yr on the fold limbs. The limb rates are higher than other uplift rates along this part of the coast and may reflect the independent deformation of this crustal flake. The Point Reyes Peninsula is part of the Salinian terrane, which comprises the area west of the San Andreas fault in northern and central California. Higher uplift rates to the south are the result of restraining bends in the San Andreas fault, and to the north they are the result of contractional geometries near the Mendocino triple junction. At Point Reyes, folding and uplift seem to be accommodated by movement along reverse faults with northwest-southeast and west-east orientations. The crustal fragment may be caught in a vice created by these structures, some of which could be inherited from older episodes of deformation. The Point Reyes Peninsula is the primary component of the Point Reyes National Seashore. Most park visitors are aware of the strike-slip motions produced by the famous San Andreas fault, but it is the vertical motion along reverse faults that has created the distinctive topography of the region.