Geochronology, especially U-Pb zircon geochronology, has made important contributions to our understanding of the Jurassic Coast Range ophiolite of California. However, much of the older work is primitive by modern standards, and even some recent U-Pb work is limited in its precision and accuracy by a range of factors. We apply a new zircon analysis method, chemical abrasion–thermal ionization mass spectrometry (CA-TIMS), to generate high-precision, high-accuracy multistep 206 Pb*/ 238 U plateau ages for zircons from plagiogranites from the Point Sal (Coast Range ophiolite) and San Simeon (Coast Range ophiolite) ophiolite remnants. These remnants have been postulated to have been part of a single, contiguous remnant prior to offset along the San Gregorio–San Simeon–Hosgri fault system. Two fractions of zircon from a Point Sal Coast Range ophiolite plagiogranite, and one fraction of zircon from a San Simeon Coast Range ophiolite plagiogranite yield 206 Pb*/ 238 U plateau ages that are indistinguishable from one another—a mean age for the three determinations is 165.580 ± 0.038 Ma (95% confidence, mean square of weighted deviates [MSWD] = 0.47). The error quoted is an internal precision, which is appropriate for comparison of the ages to one another. The fact that the San Simeon and Point Sal ages are indistinguishable, even with such very small internal precision errors, is a remarkably robust confirmation of the correlation between the San Simeon and Point Sal ophiolite remnants.

Transpressive plate motion in the coastal region between Monterey Bay and Los Angeles is distributed over a complex system of active strike-slip faults, subparallel reverse and reverse-oblique faults, and related folds. Seismotectonic responses to interplate stresses vary markedly along this portion of the plate margin. Coastal central California is divided into structurally and physiographically distinct seismotectonic domains separated by major, predominantly Quaternary, boundary faults. Internally, seismotectonic domains are marked by distinctive styles and orientations of Quaternary faulting and folding, historical seismicity patterns, geomorphic expression, and basement rock characteristics. Five principal seismotectonic domains are recognized in this study: Transverse Ranges domain, Santa Maria Basin-San Luis Range domain, coastal Franciscan domain, Salinian domain, and western San Joaquin Valley domain. Major domain boundaries include the San Andreas, Nacimiento-Rinconada, San Gregorio-Hosgri, Big Pine, and Santa Monica-Raymond-Sierra Madre-Cucamonga faults. The Transverse Ranges domain is characterized by pronounced north-northeast-oriented maximum horizontal compressive stress and associated Quaternary crustal shortening, west-trending reverse and left-lateral reverse-oblique faults and earthquake focal mechanisms, and a frequent occurrence of damaging earthquakes. The Santa Maria Basin-San Luis Range domain has low to moderate rates of Quaternary tectonism, active west- to northwest-striking reverse faults, and low to moderate seismicity with mainly reverse and left-lateral reverse-oblique focal mechanisms. The coastal Franciscan domain includes numerous northwest-striking, mainly northeast-dipping, faults with uncertain earthquake potentials. Moderate seismicity and reverse and right-lateral reverse-oblique earthquake focal mechanisms indicate significant northeast-directed convergence and broad internal deformation of weak Franciscan Complex basement. The Salinian domain includes a moderate- to high-relief western region marked by abundant northwest-striking faults with uncertain Quaternary histories, and an eastern region with generally low relief and few recognized surface faults. Seismicity within the domain is sparse, typically with right-lateral strike-slip focal mechanisms. The western San Joaquin Valley domain is marked by young folds associated with active thrust and reverse faults in its central and southern portions and both shear and contractional deformation in the north. Seismicity occurs at a low to moderate rate, with mainly reverse and thrust fault focal mechanisms.

Mesoscale fault slip data were gathered in rocks ranging in age between Cretaceous and Quaternary to evaluate the evolution of regional stress tensor orientations in the San Luis Obispo-Santa Maria area of coastal central California. We applied the numerical inversion method of Carey and Brunier (1974) for fault slip data to obtain a mean direction of the maximum principal stress (σ 1 ) trending 202°, 5° for the late Pliocene-Quaternary. Similar orientations were determined from data of older units. In many late Pliocene-Quaternary sediments, conjugate sets of reverse faults are present, and based on the Anderson (1951) model of faulting, yield the same northeast-southwest-oriented axis of maximum compression. The predominant joint sets strike 030° and 110°, almost parallel and othogonal to large-scale fold axes of the area. The northeast σ 1 agrees with the present regional stress tensor determined from focal mechanisms, and indicates that since the late Pliocene the deformation of this area was by northeast-oriented crustal shortening. The slip indicators related to this tensor appear to overprint and often mask most of the indicators related to earlier stress regimes.

The Los Osos fault zone is a west-northwest-trending reverse fault in the Pacific coastal region of San Luis Obispo County, California. The fault zone extends as a discontinuous en echelon zone from the Hosgri fault zone in Estero Bay southeast to an intersection with the West Huasna fault zone near Twitchell Reservoir, a distance of up to 57 km. The fault zone is divided into four segments based on distinct changes in recency of activity and slip rate along the fault: abrupt changes in elevation of the bordering San Luis Range, en echelon separation of fault traces, intersection with known or inferred branching or crossing structures (e.g., faults, subsiding basins), and changes in geomorphic expression from a range-front fault to an intrarange fault. From northwest to southeast, we propose naming the segments the Estero Bay, Irish Hills, Lopez Reservoir, and Newsom Ridge segments. The Estero Bay segment, 11 to 15 km long, lies primarily offshore in Estero Bay. Its recency of activity is unknown. The segment is poorly imaged on seismic reflection data and is weakly expressed in sea-floor bathymetry, suggesting a low rate of late Quaternary activity. The Irish Hills segment, which is 17 to 21 km long, exhibits the strongest expression of Holocene activity and is a well-defined range-front fault. Detailed mapping of marine terraces and trenching of fluvial deposits show that this segment has had recurrent late Pleistocene and Holocene movement at a long-term slip rate of 0.2 to 0.4 mm/yr. The adjacent Lopez Reservoir segment is a 15- to 19-km-long, poorly defined range-front fault that displaces older Quaternary alluvium. Detailed mapping and trenching indicate no Holocene activity. The Newsom Ridge segment is an 8-km-long, intrarange fault that has poor geomorphic expression and appears not to displace late Pleistocene deposits.

The San Simeon fault zone disrupts a flight of emergent marine terraces and offsets a series of drainages near San Simeon Point along the coast of south-central California. Detailed studies of the offset marine terraces and drainages have provided data that we have used to estimate the late Pleistocene slip rate for this fault zone. In this study, we mapped four and five marine terraces to the northeast and southwest, respectively, of the southern onshore reach of the San Simeon fault zone. These terraces correlate with sea-level highstands at ∼60 or 80, ∼80 or 105, ∼120, ∼210, and ∼330 ka. The marine terrace strandlines are displaced by the San Simeon fault zone along two or possibly three primary fault traces within a zone of shearing and warping up to 500 m wide. Ratios of horizontal to vertical slip are 8:1 to greater than 50:1, demonstrating that the fault is predominantly a right-lateral strike-slip fault. Estimated slip rates based on the present locations of strandlines for the San Simeon (80 or 105 ka), Tripod (120 ka), and Oso (210 ka) terraces, and paleogeographic reconstructions of the shoreline configurations during their development, range from about 0.4 to 11 mm/yr, with the best constrained values ranging from 1 to 3 mm/yr. Slip rates based on deflections and apparent offset of drainages across the primary active traces of the San Simeon fault zone are in agreement with the 1-t o 3-mm/yr values estimated from the marine terrace study. The San Simeon fault zone, therefore, accommodates a significant amount of transpressional strain along the North America-Pacific plate margin. The fault zone is part of the larger San Gregorio-San Simeon-Hosgri system of near-coastal faults. The geologically determined slip rate of 1 to 3 mm/yr is comparable to geodetically modeled estimates of fault-parallel shear west of the San Andreas fault.

Analysis of the properties of soils developed in marine terrace deposits that are displaced by the San Simeon fault zone in central California allows for their correlation across the fault. Based on a suite of 7 soil and stratigraphic parameters determined from 17 soil profiles, the second, third, and fourth marine terraces west of the fault correlate best with the first, second, and third terraces east of it. Limited radiometric age control suggests that the first terrace west of the fault probably correlates to oxygen isotope Stage 3 at about 60 ka, whereas the second through forth terraces correlate to late and early Stage 5 and Stage 7 at about 80 to 105,120, and 200 to 230 ka, respectively.

The Hosgri fault zone (HFZ) is the name given to the southern section of the major coastal fault in central California. The Hosgri separates Transverse Range structure from offshore Santa Maria Basin structure and is a key element for any tectonic model that includes this economically significant region. Previous published maps have not adequately defined the southern termination of the HFZ, the style of faulting on the HFZ, and the relation of the HFZ to surrounding structures. Using more than 1,500 mi of processed seismic reflection data, we have mapped upper Miocene and Pliocene structure in the region of the HFZ offshore from Point Sal in the north, to Point Conception in the south where the HFZ ends against east-west structures in the westernmost Santa Barbara Channel. In the same area, east-west-trending structures in the western Transverse Ranges north of the channel abut against the HFZ. The HFZ is an oblique right-slip fault along most of its length, but significant changes in the style of faulting are associated with variations in fault trend. North of Point Arguello, the HFZ appears to dip at a high angle in the upper 2,000 m of section and is distinguishable from thrust and reverse faults developed to its west. Between Point Arguello and Point Conception it may be a northeast-dipping thrust. Along its mapped length, east-side-up vertical separation is typical and may be more than 400 m on a Pliocene unconformity. Older horizons show more separation; the lower Miocene is up on the east by almost 1 km off Purisima Point. However, individual en echelon segments of the fault show west-side-up vertical separation where expected in an oblique right-slip fault system. No piercing points were found to define strike separation. Pliocene drag folds indicate dextral slip in Pliocene and later time.

This work includes new interpretations of shallow offshore geologic structure between Point Arguello and the Santa Maria River within California’s 3-mi coastal limit. These interpretations are based on multi-sensor high-resolution seismic reflection data collected during January and February 1986. Water depths within the survey area range from 16 m (50 ft) nearshore to 70 m (230 ft) 3 mi west of Point Arguello. The sea floor slopes between 0.3 and 0.5° south-westward. The thickness of unconsolidated Quaternary sediment in the survey area ranges from 0 (bedrock outcrop) to almost 50 m (165 ft) off Point Arguello. The survey area crosses the boundary between the northwest-trending Coast Ranges and the east-trending Transverse Ranges. The onshore faults and folds can be traced offshore in the seismic sections. From north to south, these faults include: (1) Pezzoni-Casmalia-Orcutt frontal fault, (2) Lions Head fault, (3) Santa Ynez River fault system, (4) Lompoc-Solvang fault, (5) Cañada-Honda fault, and (6) several unnamed faults offshore Point Arguello. These faults are tentatively classified as potentially active because they do not offset a Pleistocene erosion surface and the Holocene unconsolidated sediments overlying that surface do not show offset in the seismic records. Although the faults are tentatively classified as potentially active, they may be seismically active as suggested by the limited earthquake data in the area. The seismic data show that the north-northwest-striking Hosgri fault zone decreases in both vertical and right-slip displacement toward the south. In the northern and central parts of the survey area, the fault zone consists of two subparallel branches. In the south, near Purisima Point and near the boundary between the Transverse and Coast Ranges, the north-northwest strike of the fault zone changes toward the east and the fault zone shows splays. Our interpretation of the data is that this area of splays may be the terminus of the Hosgri. If that is correct, then the amount of surface rupture due to earthquakes along this segment of the Hosgri is likely to be small, if surface ruptures occur at all.

The seismotectonic pattern determined along a 65-km-wide corridor across the central California Coast Ranges (Dehlinger and Bolt, 1987), from the San Andreas fault to seaward of the Hosgri fault, is used to identify associated structures in the upper crust. The seismogenic zone is approximately 12 km thick in the corridor, and forms a 90-km-wide border zone of the upper Pacific lithospheric plate. This border includes three provinces (from northeast to southwest): the San Andreas fault zone, an adjacent 40- to 50-km-wide seismically quiescent province, and a 40- to 50-km-wide compressive province along the coastline. These provinces are characterized by distinct focal parameters and distinct rock types, and transition boundaries between these provinces are relatively narrow. The upper crust in the quiescent province consists of high-strength granites of the Salinian block; in both the San Andreas and the contractional provinces, this part of the crust consists of low-strength Franciscan rocks. We conclude that differential strengths of the upper crustal rocks in the corridor have modified the broader, more regional stress fields acting across the North American-Pacific plates to produce the observed seismicity. The extent to which the earthquake data in the corridor corroborate the existence of a proposed deep detachment surface is examined. The detachment model has been suggested to account for the crustal shortening observed across the Coast Ranges (Crouch et al., 1984; Eaton, 1985), where lower crustal materials are being recycled into the mantle. The set of earthquake data analyzed here does not imply the presence of a detachment within the seismogenic zone; neither, although less directly, does it imply such a detachment at greater depth. If a deep detachment due to horizontal shortening does exist, it would be restricted to the southwest half of the corridor, as strike-slip, not horizontal shortening, predominates in the northeast half of the corridor. An alternate model, in which deformation beneath the seismogenic zone occurs by creep and flow over an extensive depth range, can be made to conform with upper crustal shortening in the southwest part of the corridor and with horizontal slip in the quiescent and the San Andreas provinces. Such types of deeper deformation are more consistent with the earthquake focal parameters in the corridor than is a detachment at or below the base of the seismogenic zone.