ABSTRACT Geophysical images and structural cross sections of accretionary wedges are usually aligned orthogonal to the subduction trench axis. These sections often reveal underplated duplexes of subducted oceanic sediment and igneous crust that record trench-normal shortening and wedge thickening facilitated by down-stepping of the décollement. However, this approach may underrecognize trench-parallel strain and the effects of faulting associated with flexure of the downgoing plate. New mapping of a recently exposed transect across a portion of the Marin Headlands terrane, California, United States, documents evidence for structural complexity over short spatio-temporal scales within an underplated system. We documented the geometry, kinematics, vergence, and internal architecture of faults and folds along ~2.5 km of section, and we identified six previously unmapped intraformational imbricate thrusts and 13 high-angle faults that accommodate shortening and flattening of the underthrust section. Thrust faults occur within nearly every lithology without clear preference for any stratigraphic horizon, and fold vergence varies between imbricate sheets by ~10°–40°. In our map area, imbricate bounding thrusts have relatively narrow damage zones (≤5–10 m) and sharp, discrete fault cores and lack veining, in contrast to the wide, highly veined fault zones previously documented in the Marin Headlands terrane. The spacing of imbricate thrusts, combined with paleoconvergence rates, indicates relatively rapid generation of new fault surfaces on ~10–100 k.y. time scales, a process that may contribute to strain hardening and locking within the seismogenic zone. The structural and kinematic complexity documented in the Marin Headlands is an example of the short spatial and temporal scales of heterogeneity that may characterize regions of active underplating. Such features are smaller than the typical spatial resolution of geophysical data from active subduction thrusts and may not be readily resolved, thus highlighting the need for cross-comparison of geophysical data with field analogues when evaluating the kinematic and mechanical processes of underplating.

Abstract Small dams are often situated on low-order tributaries that drain grazed hill-slopes in dry regions of the western United States. In this paper, we use remote-sensing techniques in a case study to explore the effects of multiple small stock-pond dams on tributaries to Chileno Creek, a coastal watershed in central California. Dam density, or number of dams per drainage area, is 0.76 dams per km 2 , with most of the tributaries containing one or more dams. The total eroding channel length downstream of dams is ∼11% greater than total eroding length upstream. The relatively high density of the small stock-pond dams leads to cumulative effects that elevate the magnitude of (1) increased downstream erosion and (2) fragmentation of longitudinal connectivity between tributary headwaters and the main channel. The total headwater area producing sediment blocked by small dams equals 30% of the Chileno watershed. From these data, we infer that reduced sediment load due to the presence of the dams slows downstream riparian recovery. Results of the Chileno Creek case study emphasize that basin-scale management approaches and restoration strategies to restore connectivity are imperative in watersheds with high dam density. Uncertainty related to the biophysical effects of small dams and their removal may be investigated through analysis of baseline and long-term monitoring data, and adaptive assessment and management.

Abstract The main destination of this field trip is the San Andreas fault in Marin County, where the ground rupture of the 1906 earthquake is well preserved within the boundaries and easements of Point Reyes National Seashore. In addition to three stops along the fault, the field guide also describes stops to view the Golden Gate Bridge and White's Hill slide on Sir Francis Drake Boulevard near the town of Fairfax, and it discusses the geology along the way. Figure 1 shows the location of the stops for this field trip. Excellent online fieldtrip guides to the geology of Point Reyes peninsula, the Marin Headlands, and the San Andreas fault are available on the Internet (Stoffer, 2005 ; Elder, 2005 ). The great San Francisco earthquake of 18 April 1906 was generated by rupture of at least 435 km of the northern San Andreas fault (Lawson, 1908 ). The earthquake produced maximum horizontal offsets of 16–20 ft (5–6 m) along the San Andreas fault north of San Francisco and smaller offsets south of the city. In Marin County, there has been very little urbanization along the fault. Prior to the establishment of the National Seashore in 1962, most of the region was used for dairy farming and cattle ranching. Because the region remains largely as it was in the late nineteenth century, conditions are ideal for investigating how the morphology of the rupture has changed in the 100 years since the earthquake. Furthermore, this section of the San Andreas fault continues to yield important data about dates of prehistoric earthquakes and the slip rate of the fault. Two fundamentally different types of bedrock underlie Marin County (Fig. 2 ). Right-lateral shear along the San Andreas transform plate boundary during the late Cenozoic has juxtaposed Franciscan subduction zone rocks on the east against the Salinian terrane of Point Reyes peninsula to the west. The Franciscan Assemblage (Complex) is a highly deformed, lithologically heterogeneous sequence of metamorphosed volcanic and sedimentary rocks accreted to western North American during subduction of the Farallon plate in the Mesozoic. The Salinian terrane is a displaced fragment of continental crust that consists of Cretaceous plutonic and older metamorphic rock overlain by lower Eocene to Pliocene marine sedimentary rocks (Clark and Brabb, 1997 ). In between the Franciscan and Salinian terranes lies a valley created by the San Andreas fault zone that is characterized by Quaternary deposition and low ridges and depressions elongated parallel or subparallel to the fault. Along the route of this field trip on our way to the San Andreas fault, road cuts expose the world-famous, Franciscan Accretionary Complex rocks including oceanic pillow basalts (greenstone) overlain by radiolarian chert, graywacke sandstone, and “mélange” (from the French word for “mixture”), with inclusions of greenstone, chert, serpentinite, and graywacke. Isolated outcrops or knobs of erosion-resistant rocks within a surrounding matrix of highly sheared shale of the mélange typify the topography of grass-covered slopes of eastern Marin County. During the trip we will also travel through a forest of redwood trees near Samuel Taylor State Park en route to the Douglas-fir–covered Point Reyes peninsula.