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Subduction-Related Accretionary Wedges (B-type Subduction) |
Department of Geology, San José State University, San José, CA 95192-0102, USA sedlock{at}geosun1.sjsu.edu
Structural data and field observations from Franciscan-type coherent blueschists in western Baja California, Mexico, are used to test proposed mechanisms for exhumation of these high-pressure, low-temperature (high P/T) metamorphic rocks. Analysis of specific observations that are predicted by, consistent with, or inconsistent with specific mechanisms supports extension and normal faulting as the dominant mechanism in Baja. Erosion and strike-slip faulting may have been active, but their contributions cannot be documented and are inferred to be minor. Buoyancy forces and ductile flow probably played a negligible role. Extension and exhumation probably occurred in Late Cretaceous and Paleogene time, while subduction continued at the convergent margin west of North America. Blueschists probably were exhumed to shallow crustal levels by 30 Ma, and possibly before a decrease in convergence rate at about 40 Ma. Coherent blueschists lie in the footwalls of major normal fault zones that contain serpentinite-matrix mélange with exotic blocks of greenschist, amphibolite, eclogite, and blueschist. Mélange probably formed before normal slip on the fault zones. Most mélange components are consistent with tectonic thinning of the mantle wedge and adjacent parts of the accretionary prism that once lay structurally higher than the lower-plate blueschists. Many of these findings may be applicable to similar rocks in California, including Franciscan blueschists.
