SUBMERGED PALEO-SHORELINES OF THE CALIFORNIA CONTINENTAL BORDERLAND: TESTING THE HYPOTHESIS OF HYDRO-ISOSTATIC ADJUSTMENT

Numerical modeling predicts isostatic subsidence and tilting of the continental shelves during periods of sea-level rise, caused by flexure of the crust due to increasing meltwater loads. To assess this phenomenon using empirical data, we mapped the depths of regional paleo-shoreline features of Oxygen Isotope Stage 2 (OIS2) age and younger, using seismic-reflection profiles and bathymetric data from the Southern California Continental Borderland (SCCB). Vertical displacement rates were calculated over a variety of time intervals, including from emergent marine terraces and from permanent GPS stations on the offshore islands and on the mainland coast.

We identify up to five submerged paleo-shoreline or shoreline-related features in the SCCB, at depths ranging from -30 m to -140 m. The OIS2 and other recent paleo-shorelines are assumed to be "bathtub rings"—found at equal depths except where deformed by local tectonics—however that is not the case. The depth of each of these latest Pleistocene paleo-shorelines becomes gradually deeper with increasing distance offshore from the mainland. Also, the depth differences are greatest for the oldest paleo-shoreline (~40 m difference), and progressively diminish for the younger features.

Despite the presence of emergent marine terraces on nearly all the SCCB islands—attesting to long term uplift of the islands—vertical GPS rates show slow (-0.29 to -1.06 mm/yr) rates of present-day subsidence of the islands and many areas of the mainland coast. The offshore deepening of the submerged paleo-shorelines and the mismatch between vertical GPS rates and terrace-derived uplift rates in the SCCB are consistent with hydro-isostatic adjustment to sea-level rise after the Last Glacial Maximum. Our results suggest that glacio- and hydro-isostatic adjustment has, and continues, to exert significant control on sea-level position across the Pacific Coast of North America. Moreover, our results advise caution in the use of submerged sea-level markers to track vertical deformation in the marine environment without consideration of ongoing regional isostatic adjustment.