SMITH, DOUGLAS P., Geology Department, Vanderbilt University, Nashville, TN, and CATHY J. BUSBY, Department of Geological Sciences, University of California, Santa Barbara, CA

ABSTRACT: Cretaceous Extension and Tertiary Translation along the Southwest Edge of North America? Answers from the Valle Group: Vizcaino Terrane

The late Mesozoic Valle Group of western-central Baja California was deposited atop the leading edge of North America, making it a sensitive recorder of tectonic activity deforming the edge of the plate. Recent stratigraphic, sedimentologic, and paleomagnetic work on the Valle Group provides the following insights into the evolution of the plate margin.

The edge of North America experienced brittle extension in the middle Cretaceous. That deformation is coincident with the timing of peak blueschist metamorphism in the lower plate of the convergent margin and may be related to initial uplift of the high-pressure rocks that now lie faulted against unmetamorphosed Valle Group strata.

Data of various kinds taken from the Valle Group provide ambiguous or directly contradictory answers to the question of Vizcaino terrane motion. Paleomagnetic data robustly show that Valle Group strata and associated basement rocks traveled significantly farther than that required to open the Gulf of California. Faunal data from the Valle Group weakly suggest that the paleomagnetic data are in error; in particular, the presence of Collignonicerous woollgari in the Valle Group may link the Valle to a more Boreal, rather than Tethyan, province. U-Pb data from granitoid clasts in the Valle conglomerates suggest that the granitoids were not likely derived from any nearby provenances, supporting post Albian-Aptian terrane travel. Conversely, isotopic data from quartzite clasts in Valle con lomerates seem to link the basin with nearby sources located in the ancient passive margin of North America.

AAPG Search and Discovery Article #90992©1993 AAPG Pacific Section Meeting, Long Beach, California, May 5-7, 1993.