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Investigation of the neotectonics of the Sacramento Delta area along the eastern margin of the Coast Ranges has located several active near-surface compression-related structures along a 40 km long east-west transect: The Pittsburg/Kirby Hills fault (renamed from Kirby Hills or Montezuma fault), a portion of the Midland fault, and a splay of the Los Medanos thrust running beneath Ryer Island. Interpretation of crustal-scale geology is based on integration of multichannel marine seismic profile data, high-resolution shallow seismic profiles, velocity modeling from wide-angle refraction, (data generated from BASDC, Bay Area Seismic Imaging experiment); seismicity data from the Northern California Seismic Network, subsurface geologic data, and surface geology. Tectonic wedges beneath active reverse faults are moving relatively east over a crystalline basement on a mid-crustal decollement, at depths averaging about 15 km beneath the study area and shallowing to the east. Depth and location of earthquake hypocenters indicates additional deformation at 16 to 24 km depth, within the crystalline basement. INTRODUCTION Recognition that thrust-related earthquakes are an integral component of deformation within the predominantly strike-slip San Andreas fault system has sparked a search for potentially destructive thrust faults throughout the California Coast Ranges. The 1983 ML 6.7 Coalinga, 1989 ML 7.1 Loma Prieta, and 1994 ML 6.7 Northridge earthquakes occurred on previously unknown compressive structures related in some way to displacement along the San Andreas fault system. One aim of the Bay Area Seismic Imaging experiment (B ASDC) program was to locate and characterize active and potentially active buried thrust faults, thereby aiding assessment of seismic hazards as well as building an understanding of transpressional fault dynamics. BASDC was a project conducted by a consortium of research groups involving acquisition, processing and interpretation of seismic reflection and refraction data across the central California Coast Ranges using marine geophysical techniques along the course of the Sacramento River through San Francisco Bay and offshore. This paper presents an interpretation of crustal structure in the eastern Coast Ranges in the Sacramento River delta area (Fig. 1 & 2) as synthesized from BASDC and complementary geologic data (expanded from Karageorgi et al., 1992 and Band et al., 1993). An unforeseen outgrowth of the investigation was recognition of very deep seismicity occurring beneath surface-mapped faults (25 km compared to an average 10 km in the Coast Ranges), in the basement. Realization that such deep deformation is occurring within the crystalline basement led geologists to reconsider the tectonic models applied to the Coast Ranges (Furlong et al., 1989; Jones et al., 1994). Geologic Setting The California Coast Ranges are an amalgam of accretionary prism material (Franciscan Assemblage) added to the western margin of North America during late Jurassic through early Tertiary subduction of the Farallon Plate. Adjacent to the Coast Ranges (Fig. 1) lies the Central Valley, an asymmetrical syncline comprised of Upper Jurassic and Cretaceous strata (known as the Great Valley Sequence) and Tertiary elastics originally deposited on a crystalline basement. Originally flat-lying Great Valley Sequence is now exposed in an east-dipping homocline along the eastern margin of the Coast Ranges as well as being folded and faulted into the Coast Ranges themselves. The nature of the structure responsible for the homocline is thought to be either an ancient subduction zone (Dickinson, 1981) or an east-moving tectonic wedge of Franciscan rocks (Wentworth et al., 1984; Jones et al., 1994). Since the conversion of this margin to largely transform motion, beginning about 8 Ma (Atwater, 1970), strike-slip faults (Fig. 1) sliced through the Coast Ranges, cutting older faults and folds (Irwin, 1990). While right-lateral strike-slip is the dominant style of geodetically determined deformation, geologic evidence for young (<3.5 Ma) deformation along the eastern margin of the Coast Ranges is compressive. Examples of young compressive deformation are the active folds at Coalinga Anticline (Wentworth et al., 1983; Wentworth and Zoback, 1989), Kettleman Hills (Bloch et al., 1993) and Rumsey Hills (Unruh and Moores, 1992). Additionally, tilted and offset Pleistocene and younger units are observed in outcrop (Harwood and Helley, 1987) and on seismic reflection profiles all along this margin. Occurrence of large earthquakes with no apparent surface rupture suggests seismogenic blind thrusting is taking place beneath the folds (Wong et al., 1988; Wong, 1992). Similar fault associations in New Zealand (Cashman et al., 1992) are interpreted as examples of strain partitioning of far-field stresses where weak strike-slip faults accommodate the shear component, and folds and thrusts accommodate the normal component of deformation in an area of oblique tectonic convergence. However, unlike New Zealand, the latest plate motion models based on geodetics show no active convergence between the plates across the boundary marked by the San Andreas fault system, (DeMets et al., 1990). Midland well =T~ .2 km to bsmt Cross-section of Fig. 4 Figure 1: Regional map and study area showing BASIX cruise track and study area centered on the Sacramento Delta. Bold lines mark major faults: CGVF = Concord/Green Valley fault, VF = Vaca fault, SAP = San Andreas fault, HF = Hayward fault, CF = Calaveras fault, MCF = Marsh Creek fault, GF = Greenville fault, SG-HF = San Gregorio-Hosgri fault, PKHF = Pittsburg/Kirby Hills fault, MF = Midland fault. Note how the line marking the eastern topographic margin of the Coast Ranges "bays" in at the delta; the topography in this case in structurally controlled. The BASIX Experiment The BASK program was prompted by the 1989 Loma Prieta earthquake to search for similar thrust-type structures associated with East Bay faults (Hayward, Calaveras & Greenville faults). The BASIX profiling cruise took place in 1991; details are described by McCarthy and Hart (1993). The specific objective was to image the entire crust to define the geometry of crustal-scale faults and their apparent relationship. Three complementary methods were used: Multichannel seismic profiling (McCarthy and Hart, 1993), wideangle reflection/refraction profiling (Brocher and Moses, 1993; Brocher and Pope, 1994), and high-resolution seismic profiling (Anima and Williams, 1991; Anima et al., 1992). However, data quality of the multichannel seismic profile was compromised by noise and other problems associated with acquisition in a high-current channel setting (described by McCarthy and Hart, 1993). The multichannel profile, designed to reveal structure to 6-10 sec two-way travel time, recovered useful data to only about 4 sec twtt, and that only east of Suisun Bay (Figure 2). The multichannel data were processed at the U.S. Geological Survey (McCarthy and Hart, 1993) (Fig. 3). The same source used for multichannel profile was recorded along a wide-spaced refraction array reaching from San Francisco to the Sierra Foothills on portable 3and 5component seismometers (described in Brocher and Moses, 1993). These data provide reliable velocity information to depths of about 5 km, deeper data are less well-defined. High-resolution reflection data (Anima and Williams, 1991) image strata to depths shallower than 200 m.

Active tectonic deformation at the eastern margin of the California Coast Ranges; results of the BASIX and CALCRUST programs