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* Geology Department, Southern Illinois University, Carbondale, Illinois 62901-4324;
** Institute for Crustal Studies, University of California, Santa Barbara, California 93106
npinter{at}geo.siu.edu
In the Northern Channel Islands of California, preservation of paleo-shoreline features both as uplifted landforms onshore and subsided depositional features offshore has allowed precise measurement of late Quaternary fold growth relative to sea level and has helped to disentangle the relative contributions of tectonic forcing and isostatic response. We precisely measured several hundred points on uplifted Pleistocene coastal terraces of Santa Cruz and Anacapa islands, the elevations of which document anticlinal growth of the islands since at least 400 ka. We also interpreted a dense grid of seismic-reflection profiles that image a complex of eustatic lowstand deltas that record regional subsidence surrounding the uplifting structure. We combined onshore uplift with offshore subsidence measurements to produce a quantitative mass balance of structural growth since 400 ka.
The data outlined above were then used to test the hypothesis that subsidence around the Northern Channel Islands represents the flexural isostatic response to tectonic loading of the islands. Calculations of cumulative uplifted mass and cumulative subsided mass across the structure closely match the values predicted for density-driven isostatic compensation and local flexural support, although the wavelength of flexure predicted for the study area suggests that an additional component of flexural support may extend distally 10s of kilometers into the surrounding offshore basins.
In the Northern Channel Islands, isostatic subsidence is explicitly manifested because erosional mass removal is small, because the crust here is relatively thin and weak, and most importantly because sea level provides an absolute datum against which to measure vertical deformation. An explicit treatment of the interplay between crustal thickening, erosional denudation, surface uplift, and isostatic response helps to illustrate the conditions under which isostatic subsidence may or may not be a significant process. Where surface denudation rates approach the rate of rock uplift, for example, isostatic subsidence will approach zero. In contrast, failure to recognize isostatic subsidence in earthquake-hazard studies can lead to order-of-magnitude errors in slip-rate and hazard estimates.
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