Department of Geology Professor Nicholas Pinter  Geology Home Page   Nicholas Pinter Home Page

The Northern Channel Islands of California -- Anacapa, Santa Cruz, Santa Rosa, and San Miguel -- form the southern margin of the Santa Barbara Channel. The islands also form the southernmost range of the western Transverse Ranges, a region characterized by late Cenozoic folding, reverse faulting, and clockwise rotation of fault-bounded blocks. Several active faults cut the islands and the surrounding seafloor, including the left-lateral Santa Rosa Island fault, the Santa Cruz Island fault, and the Dume/Anacapa fault. These faults are predominately left-lateral, have smaller reverse components, and appear to be continuous with a system of similar faults at the northern margin of the Los Angeles Basin (the Malibu Coast, Santa Monica, Hollywood, and Raymond faults). Together, all of these faults appear to form a semi-continuous zone which has been called the Transverse Ranges Boundary fault zone, separating the western Transverse Ranges to the north from the northwest-southeast-trending structures to the south. The left-lateral surface faults of the Transverse Ranges Boundary system seem to coexist with low-angle thrust faults at depth. Geodetic measurements of the Channel Islands relative to the mainland suggest north-northeast shortening at rates of ~6±1 mm/yr. Total slip seems to be partitioned into: (1) thrust motion on the low-angle faults and (2) strike slip and oblique slip on the high-angle faults. Regional thrust structures have been inferred from balanced cross sections beneath the Santa Monica Mountains (the Santa Monica Mountains Thrust) and beneath the Northern Channel Islands (the Channel Islands Thrust). It has been suggested that uplift of the Northern Channel Islands chain is the result of south-vergent slip across a north-dipping ramp on the underlying thrust fault. In tectonically active coastal settings, uplifted coastal terraces are valuable tools for assessing the presence, pattern, and rates of late Quaternary deformation. Erosional coastal terraces consist of three principal elements: a wave-cut platform, a seacliff, and a shoreline angle. Coastal terraces form during the periods of time around the high stands of glacial-interglacial eustatic fluctuations, and the shoreline angle represents a marker of the highest position of sea level associated with that high stand. The nature, number, ages, and spacing of terraces on an uplifting coastline depend on the coastal processes active there, the rate of tectonic uplift, and the nature and strength of surface processes that act to erode or otherwise obscure the relict terraces over time. Each uplifted coastal terrace acts a geomorphic timeline for faults or other deformational structures that cross that surface, constraining the age of the most recent rupture and providing morphological and stratigraphic markers with which to measure average slip rate since terrace formation.

Precise measurements of terrace elevations and depths of subsided eustatic-lowstand delta clinforms have been used to show that subsidence around the Northern Channel Islands represents the flexural isostatic response to thrust 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. 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.