Abstract

We used submerged paleoshorelines as strain markers to investigate Holocene and late Pleistocene vertical tectonic movement at the intersection of the offshore Santa Cruz–Catalina Ridge with the southern boundary of the Western Transverse Ranges, within the California Continental Borderland. Past submerged shoreline positions were identified using high-resolution multibeam bathymetry, side-scan sonar, submersible observations, and the presence of intertidal and subtidal invertebrate fossils. Numerous accelerator mass spectrometry (AMS) 14C ages of shells from these paleoshorelines were found to be between ~27,000 yr radiocarbon (RC) and 11,500 yr before present, indicative of shoreline colonization during and following the Last Glacial Maximum (LGM), establishing these paleoshorelines as a usable datum for measuring vertical change since this time. Removal of the nontectonic component of vertical change using an ice-volume-equivalent eustatic sea-level compilation indicates between 20 and 45 m of uplift of the eastern part of the Northern Channel Islands block since the LGM lowstand, resulting in an uplift rate of 1.50 ± 0.59 mm/yr over the last 23 k.y. This rate closely matches uplift predicted by published slip rates for the Channel Islands thrust, which underlies the Northern Channel Islands platform. Results from post-LGM shoreline features on Pilgrim Banks are somewhat more ambiguous. Submarine paleoshoreline uplift, together with the extensive upper-crustal fold-thrust style of deformation, illustrates the transpressional interaction of the Borderland and the Western Transverse Ranges blocks where the Santa Cruz–Catalina Ridge and northern Channel Islands intersect.

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