LEE, CLAVIN F., and THOMAS L. HENYEY, University of Southern California, Los Angeles, CA

ABSTRACT: Tectonic Implications of Extensional Faults Along the Nicolas-Catalina Terrane Boundary, California Continental Borderland

The Nicolas-Catalina terrane boundary defines a distinct lithologic discontinuity between forearc sedimentary strata on the west and exhumed ophiolitic and blueschist rocks on the east. Analysis of multichannel seismic reflection records acquired by the U.S. Geological Survey suggests that this boundary developed as a result of (1) uplift and truncation of the Cretaceous Paleogene forearc strata along the eastern edge of the Nicolas terrane, and (2) west-northwest extension on north to northwest-trending normal faults along the eastern edge of the Nicolas terrane, accommodated by horizontal right-slip displacement along west-northwest-trending faults. The latter faults presently appear to be active reverse faults, but mismatching strata across many of these faults suggest that they ha a strike-slip history. The timing of the tectonic activity suggests that this abrupt lithologic boundary developed as a result of the early Neogene transition of the California continental borderland from a convergent to a transform margin.

A new geologic model is developed which proposes that at about 18 Ma, extension along one or more north-trending spreading centers or rift zones separated or pulled apart the

borderland between the California-Mexico border and the Vizcaino Peninsula. Uplift immediately prior to or accompanying extension, possibly because of the presence of hotter, more buoyant crust as the East Pacific-Farallon Ridge approached and collided with the continental margin (1) exposed the Cretaceous-Paleogene forearc to erosion, and (2) exhumed the underlying oceanic crust and subducted blueschists (Catalina Schist) within the inner borderland. Lateral displacements due to rifting and extension along the eastern and southern boundary of the Nicolas and Catalina terranes would be accommodated along west-northwest-trending transform or strike-slip faults oriented perpendicular to the zone of extension.

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