The Ups and Downs of the Inner California Borderlands Constrained by Submarine Terraces

Recently acquired seismic reflection data in the Inner California Borderlands (ICB) have imaged flights of submarine terraces, that constrain the regional vertical displacement from Middle Miocene ( 15 Ma) to present. The morphology of the broad shelf around San Clemente Island (SCI) has recorded multiple marine transgressions and consequent wave-base erosion. The outboard terrace located at 90 to 115mbsl appears to correlate with local relative sea level depth of the Last Glacial Maximum (LGM), consistent with the reported Quaternary uplift rate for SCI (0.13m/ky). Submarine terraces are of similar depths around the shelf of SCI, suggesting uniform Quaternary uplift, an observation consistent with onshore mapping of SCI's terraces. Terraces deeper than the local LGM levels ( 115 mbsl) are observed offshore both SCI and Santa Catalina Island (CI) at similar depths. Previous researchers have interpreted these deeper terraces as Quaternary in age; such an interpretation requires recent subsidence to explain the depths of the terraces as they are deeper than LGM sea levels. The purported subsidence to explain the depth of these terraces is not observed in multichannel seismic data in the Catalina Basin. Knudsen sub-bottom profiler (3.5kHz) data recently collected off CI image a prograding low-stand wedge that downlaps onto this deeper terraces (-255m) and suggest they predate the late Quaternary. Based on our nested seismic reflection, bathymetric, geochemical and macrofossil data, we provide a new explanation for the abrasion platforms observed at depths greater than local sea level depths during the LGM ( 115 mbsl). In our model, the deeper abrasion platforms observed offshore Catalina and San Clemente Islands are middle to late Miocene in age ( 12- 9 Ma) when the region was shallow and subject to wave-base erosion. Subsequent subsidence of the ICB by conductive cooling continued into the Pliocene ( 4 Ma). The subsidence was followed by Quaternary uplift of the ICB as recorded by the flights of terraces observed along the southern California Coast and offshore islands. Age dating yields the following Quaternary uplift rates for the region: Southern California coast - 0.13m/ky; San Clemente Island - 0.135m/ky; Northern Channel Islands - 0.12m/ky-0.15m/ky.