Abstract This three-day field trip will examine the nature of contemporary tectonic processes in the western Basin and Range province by focusing on the historical faulting and paleoseismology of the central Nevada seismic belt (CNSB) (Fig. 1 ). The trip begins in Reno and includes travel through Fallon, Nevada to major faulting sites in the 1954 Rainbow Mountain, Fairview Peak, and Dixie Valley rupture area ~125 km east of Reno (Fig. 2 ). The principal topics addressed by this trip will include: Map showing surface ruptures (bold lines) and focal mechanisms reported for major historical earthquakes of the central Nevada-eastern California seismic belt. Other Quaternary faults of the Basin and Range are shown as thinner lines. Compressional quadrants of focal mechanism are black. Focal mechanisms are from Doser (1986) and the National Earthquake Information Center. Area of Figure 2 is shown by the gray box. Within the central Nevada seismic belt, right-normal-oblique surface ruptures of 1954 Rainbow Mountain-Fairview Peak sequence mark a transition between dominantly right-lateral events to the south within the northwest-trending Walker Lane belt ( Stewart, 1988 ), and the dominantly dip-slip Dixie Valley and Pleasant Valley earthquakes to the north, where the Basin and Range is characterized by a consistent north-to northeast-trending structural grain. The structural pattern, distribution, and characteristics of surface faulting and secondary effects associated with large-magnitude historical earthquakes as they bear on the nature of modern tectonic processes in the CNSB. The paleoseismicity of the historical fault zones within

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.

Emergent Quaternary marine terraces are present along most of the south-central California coastline from San Simeon on the north to the Santa Maria Valley on the south. Detailed mapping of these terraces provides new data for assessing the locations, style, and rates of Quaternary deformation in the region. The distribution, correlation, and ages of the terraces have been studied near San Simeon and on the flanks of the San Luis Range between Morro Bay and the Santa Maria Valley. In the San Simeon study area, sequences of four and five marine terraces have been mapped to the northeast and southwest, respectively, of the southern onshore reach of the San Simeon fault zone. From youngest to oldest, they are the Point (Q p ), San Simeon (Q s ), Tripod (Q t ), Oso (Q o ), and La Cruz (Q lc ) terraces. They are interpreted to correlate with marine oxygen isotope stages 3 or 5a (60 or 80 ka), 5a or 5c (80 or 105 ka), 5e (120 ka), 7 (210 ka), and 9 (330 ka). A uranium-series age of 46 ± 2 ka and a weighted mean average thermoluminescence age of 95 ± 13 ka have been obtained for samples collected from the lowest two emergent terraces, respectively, on the southwestern side of the fault zone. Estimated ages and correlation of terraces across the San Simeon fault zone are based on lateral correlation of the Tripod (Q t ) terrace to the well-dated ∼120-ka Cayucos terrace, comparison of relative soil profile development, and comparison of geomorphic expression and terrace altitudinal spacing. Comparison of the relative altitudinal spacing of terraces with paleosea-level curves developed from worldwide data indicates uplift rates of approximately 0.17 ± 0.02 m/kyr southwest, and 0.16 ± 0.01 m/kyr northeast of the fault, and approximately 0.24 m/kyr for the uplifted and warped areas within the fault. Terrace altitudinal spacing for the lowest three terraces on San Simeon Point, however, indicates that uplift during the past 120,000 yr in this area has not been uniform adjacent to the active traces of the San Simeon fault zone. In the San Luis Range study area, a flight of at least 12 elevated marine terraces is present between Morro Bay and the northwestern margin of the Santa Maria Valley. The lower two terraces (Q 1 and Q 2 ) in this sequence are interpreted to correlate to marine oxygen isotope substages 5a (80 ka), and 5e (120 ka), respectively. These correlations are well constrained by 12 uranium-series ages of coral and vertebrate bone samples, 12 amino acid racemization analyses, and 14 paleoclimatic analyses of invertebrate faunal assemblages. The ages of the lower two terraces provide local calibration of the terrace sequence for correlation with paleosea-level curves developed from worldwide data. For terraces equal to or younger than about 330 ka, we have estimated terrace ages and uplift rates by correlating shoreline angle altitude and terrace altitudinal spacing to these curves. Uplift rates based on the present altitude of the 120-ka terrace in this region range from approximately 0.06 to 0.23 m/kyr. Late Pleistocene uplift rates throughout the entire coastal region between San Simeon to the Santa Maria Valley are comparable to rates observed elsewhere in California, which are approximately 0.1 to 0.3 m/kyr in tectonic regimes characterized by predominantly strike-slip faulting. The rates are considerably less than maximum rates of 3 to 5 m/kyr for the region directly south of the Mendocino Triple Junction, 5 to 7 m/kyr for areas characterized by significant crustal shortening, such as the Ventura anticline in the Transverse Ranges, and 0.8 m/kyr for the Santa Cruz Mountains region adjacent to the restraining bend in the San Andreas fault. Estimates of the position of sea level (with respect to the present) during the ∼80-ka sea-level highstand range from about −19 m to near the present level. Estimated paleosea level during the ∼80-ka high stand in the San Luis Range study area, assuming a +6 m paleosea-level estimate for the ∼120-ka terrace and uniform uplift since formation of the ∼120-ka terrace, is −4 ± 1 m (relative to present sea level). This value is in general agreement with other recent estimates from coastal California, Mexico, and Japan, but is significantly higher than previous estimates from New Guinea and Barbados.

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 Wilmar Avenue fault is a late Quaternary reverse fault in southwestern San Luis Obispo County, California, that one of us (S.P.N.) identified in 1986. Mapping shows that the fault is at the southeastern base of the San Luis Range and contributed to the uplift of the range. The fault is divided into two structural sections: a western section that is a discrete fault zone that places lower Miocene rocks above overturned upper Pliocene strata, and an eastern segment that is partly a blind fault expressed at the surface in a monoclinal fault propagation fold. In both sections an upper Pliocene marine sand unit is offset vertically by 250 to 300 m, and offsets late Pleistocene marine terraces. Late Quaternary slip rates for both segments derived from the offset terraces are estimated at between 0.04 and 0.07 m/Ka. Although Holocene slip-rate data are lacking, the Wilmar Avenue fault zone is considered to be part of the seismogenic southwestern boundary zone of the San Luis/Pismo block (Slemmons and Clark, 1991) and could pose a seismic hazard to nearby communities.

The San Luis Range, a prominent west-northwest-trending topographic and structural high along the coast of south-central California, is one of a series of elongated structural blocks in the Los Osos/Santa Maria (LOSM) domain. The range is uplifting as a relatively rigid crustal block along bordering northwest-striking reverse faults. Altitudes and ages of marine terraces show that the range is uplifting at rates of between 0.12 and 0.23 m/kyr, with little or no internal deformation. Major geologic structures within the range, including the Pismo syncline and the San Miguelito, Edna, and Pismo faults, do not deform Quaternary deposits or landforms and are not active structures in the contemporary tectonic setting. The northeastern margin of the range is bordered by the Los Osos fault zone, a southwest-dipping reverse fault that separates the uplifting San Luis Range from the subsiding or southwest-tilting Cambria block to the northeast. The fault zone has had recurrent late Pleistocene and Holocene displacement at a long-term slip rate of 0.2 to 0.7 mm/yr. Uplift of the range is accommodated, entirely or in part, by displacement along this fault zone. The southwest margin of the San Luis Range is bordered by a complex system of late Quaternary reverse faults that separates the range from the subsiding Santa Maria Basin to the southwest. The fault system includes the Wilmar Avenue, San Luis Bay, Olson, Pecho, and Oceano faults, all of which dip moderately to steeply to the northeast. The cumulative net dip-slip rate of displacement for this system of faults ranges from about 0.16 to about 0.30 mm/yr. Slip rates on individual faults generally range from 0.04 to 0.11 mm/yr. We infer that the style and rates of deformation occurring within and bordering the San Luis Range are representative of the style and rates of deformation occurring elsewhere in the LOSM domain. Crustal shortening in the domain is accommodated primarily by reverse faulting along the margins of structural blocks and by uplift, subsidence, or tilting of the blocks. In the southern and southeastern parts of the domain, crustal shortening also may be accommodated by active folding and thrust faulting. The west-northwest structural grain and tectonic style within the LOSM domain is unique in the south-central coastal California region, and is transitional between the west-trending structural grain of the western Transverse Ranges and the north-northwest-trending grain of the Santa Lucia and San Rafael Ranges. We interpret that Quaternary deformation within the domain is related to transpression along the North America/Pacific plate margin, renewed late Cenozoic clockwise rotation of the western Transverse Ranges, and convergence of the domain against the relatively stable Salinian crust that underlies much of the Santa Lucia and San Rafael Ranges to the northeast.

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.

Detailed geologic investigations that included mapping, geomorphic analysis, drilling, and logging of natural and trench wall exposures were performed to characterize the Holocene behavior of the southeastern onshore reach of the San Simeon fault near San Simeon, California. Our field investigations revealed that the San Simeon fault consists of two and possibly four or more major strands that define a southeast-tapering zone that is about 400 m wide at Oak Knoll Creek, narrowing to about 120 m at San Simeon Cove, 2.6 km to the southeast. Geologic and soils data from four sites within this fault reach show that the primary San Simeon fault traces are northwest-trending, vertical to near-vertical, right-slip faults that have subhorizontal striae and slickensides. The ratio of strike slip to dip slip on primary traces is > 10:1 at the Borrow Pit site and about 8:1 to 10:1 at Airport Creek. These faults have undergone multiple slip events during the Holocene. We estimate a slip rate for the fault zone of 0.9 to 3.4 mm/yr, with a best constrained value of 1.0 to 1.4 mm/yr. Although our studies are confined to one major strand within the fault zone, analysis of deformed marine terrace strandlines suggests this estimate may closely approximate the value for the fault zone as a whole. Two fault strands, one at Oak Knoll Creek and the other at Airport Creek, yield net slip estimates of 1 to 2 m per event. Based on estimates of both slip rate and net slip per event, recurrence frequencies for the San Simeon fault are estimated to fall within the range of 265 to 2,000 yr, with best constrained values between approximately 600 and 1,800 yr. Evidence at Airport Creek suggests that slip events have not occurred at uniform intervals.

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.