Description
Emergent marine terraces on San Clemente Island are examined in detail and used to assess the Pleistocene neotectonic activity of the island. This study completes the mapping of the terraces on the island and establishes a comprehensive and internally consistent terrace chronology which is used to calculate rates of vertical deformation and assess the geomorphic evolution of the island. San Clemente Island, a fault-bounded crustal island block on the southern California continental borderland, possesses the most complete record of Quaternary interglacial sea level high stands in North America. These are represented by a suite of up to 31 erosional marine terraces that reach an elevation of approximately 530 m. Across the island, the lowest terrace likely correlates with the oxygen isotope Substage Sa (80 ka) highstand, except where locally removed by erosion. The 2nd terrace level, which likely correlates to the oxygen isotope Substage 5e highstand, is moderately to well expressed, virtually continuous across the western side of the island, and moderately preserved along the steep eastern side. The elevations of both the 1st and 2nd terraces were found to be similar on both sides of the island. Previous studies have used a shoreline angle (SLA) elevation of 32 m for the 2nd terrace, generating a long-term tectonic uplift rate of ~0.2 m/ka, comparable to that for many coastal localities in southern California and the offshore islands. Surveyed terraces around the island, however, yield approximate SLA elevations of 5 to 6 m for the 1st terrace, and 20 to 22 mm for the 2nd. An elevation of 20 to 200 m for the SLA of the 2nd terrace yields a long-term uplift rate of 0.135 m/ka, about half that previously used. Evaluation of the uplift of the marine terraces using a technique outlined in earlier research shows that the uplift rates of the lower terraces average 0.135 m/ka for terraces up to approximately 86 m. The terraces from 97 m to approximately 169 m can be correlated with terraces from northern San Diego County, and suggest a compatible uplift rate that can be used to infer the amount of uplift across the island for the last approximately 1.25 Ma. The lower terraces located along the south end of the island near Pyramid Cove and Pyramid Head were found to have a higher uplift rate (0.20 m/ka) than other areas of the island, but the differential was most likely produced as a result of localized faulting. Evidence of active deformation other than uplift and minor faulting was not observed for the lowest two terraces around the island, suggesting that the island is not actively being tilted or folded. Thus, whatever mechanism was responsible for the observed doming of the island block, and tilting of the volcanic flows, is no longer active. Direct evidence of faulting of the 2nd terrace was found at Wilson Cove, although previous studies have noted that faults do not offset the lower terraces of the island. It is likely that other mapped faults on the island also have a lateral component that has been difficult to recognize or document.
