Abstract This two-day trip explores the northern San Andreas fault in the Gualala area between Fort Ross and Point Arena (Fig. 1 ). The first stop overlooks the Golden Gate Bridge and includes a discussion of its in-progress seismic retrofit. Several subsequent stops are at paleoseismic sites on the San Andreas fault. The stop at Annapolis Road includes a short hike along the fault through the redwood forest. This section of the fault is locked and has not moved since the 1906 earthquake. Additional stops visit Quaternary marine terraces and include discussion of associated tectonic deformation.

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 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.

Abstract Detailed studies of emergent marine terraces in south-central coastal California provide new data for assessing the nature and rates of late Quaternary uplift along the Pacific-North American transpressional plate margin west of the San Andreas fault. Between Morro Bay and the Santa Maria Valley, remnants of at least 12 marine platforms are present. The ages of the lowest two platforms (∼80 ka and —120 ka) are well constrained by uranium-series ages, amino-acid racemization analyses, and paleoclimatic analyses of invertebrate faunal assemblages. Uplift rates based on the present altitude of the 120-ka terrace in this region range from 0.06 to 0.23 m/ka. The rates of uplift change abruptly across several fault zones that intersect the coast and thus provide a measure of the vertical component of slip on these faults. Based on the assumption of constant uplift, we estimate that paleosea level during the ∼80-ka high sea stand in this region was —4±1 m (relative to present sea level). Near San Simeon, flights of four and five marine terraces are mapped to the northeast and southwest, respectively, of the northwest-trending San Simeon fault zone, a predominantly right-lateral strike-slip fault with a slip rate of 1 to 3 mm/yr. We estimate the ages of these terraces to be 60 or 80 ka, 80 or 105 ka, 120 ka, 210 ka, and 330 ka. Outside of the fault zone, the spatial distribution and present altitudes of the terraces indicate relatively uniform coastal-uplift rates of approximately 0.17 m/ka southwest and 0.16 m/ka northeast of the fault. The uplift rate for the uplifted and deformed areas within the fault zone is approximately 0.24 m/ka.