SORLIEN, CHRISTOPHER C., Institute for Crustal Studies, University of California, Santa Barbara, CA, 93106; LEONARDO SEEBER, Lamont-Doherty Earth Observ. Columbia University, Palisades, NY 10964; and ANDREW T. SCOTT, Geology Dept., Southern Illinois University, Carbondale, IL 62901
Abstract: Rapid Subsidence and South Propagation of a Thrust Wedge into the Southern California Borderland
We used dense grids of seismic reflection data to interpret sequence stratigraphy of the shelf and slope north of Santa Cruz and Anacapa Islands. Lowstand paleo-shelf breaks of a NE-prograding delta lobe have subsided by 150-400 m. One sequence was traced into the deep basin and an age of 600-900 ka was interpolated between dated 160 ka and 1 Ma horizons. Additional age control comes from a correlation of the 3 youngest stacked prograding sequences to the last 3 eustatic lowstands. The tilt of the ~750 ka sequence is 1.8°, and, 25 km west, the tilt of the inferred ~270 ka sequence is 0.6°, using a line of zero vertical motion on the inner shelf. A progressive down-to the north tilt is proved because older sequences have subsided more and are more tilted than younger ones, the outer shelf and slope have subsided more than the inner shelf, and the islands have uplifted. A N-to-S progression from uplift and erosion to subsidence and aggradation is also seen. In the saddle between the islands and the Santa Monica Mountains, even the crest of the anticline is now subsiding, with a planar erosion surface now at 45° m. This surface probably represented a low-stand wavewashed bank, speculatively until ~1 Ma, before it subsided. NWSE right-lateral faults intersect the SMMCI anticline from the south, basins formed east of such intersections, and the thrust front collapsed 10 km south into Santa Monica basin. Only then could submarine channels cross the subsided anticline crest to source the Hueneme fan. The regional S-migrating subsidence may be a response to S-propagation of a thrust sheet. The offshore tilting can be explained by an active, listric Channel Islands thrust, and activity on the linked Santa Monica Mountains blind thrust must be evaluated in light of regional subsidence.
AAPG Search and Discovery Article #90920©1999 AAPG Pacific Section Meeting, Monterey, California