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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 (Qp), San Simeon (Qs), Tripod (Qt), Oso (Qo), and La Cruz (Qlc) 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 (Qt) 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 (Q1 and Q2) 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.

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