Pre-Middle Jurassic Accretion-Related Metamorphism in the South-Central Mountains, Northern California

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The purpose of this study is to investigate the occurrence, nature and timing of pre-Middle Jurassic convergence-related metamorphism in accreted terranes of the south-central Klamath Mountains of northern California. A number of elongate, tectonometamorphic accreted terranes comprising the western Paleozoic and Triassic belt (WPzTr) contain abundant evidence of low-P/T metamorphism related to combined regional shortening and magmatic heating which occurred during Middle and Late Jurassic evolution of an really widespread continental-margin magmatic arc at the western edge of North America. One of the WpzTr terranes, the Stuart Fork, also contains evidence of an earlier pre-mid-Jurassic metamorphism related to subduction along a convergent margin, preserved as distinctive high-P/T blueschist metamorphic assemblages that are partly obscured within younger plutonic aureoles. Other terranes in the southern WPzTr contain poorly documented low-grade metamorphic mineral assemblages which are not easily attributed to particular metamorphic conditions, geothermal gradients or tectonic environments. The goal of this study is to conduct a detailed petrologic and thermochronlogic investigation of pre-Middle Jurassic metamorphism and deformation within these southern WPzTr terranes. Integrated field, petrologic and geochronologic data will help to constrain the origin, evolution and tectonic environment of these terranes prior to their assembly in mid- Jurassic time. These data will also be employed to model their accretion to the continental margin, and to address and unresolved problem of whether the early Mesozoic margin of the central Cordillera evolved as a single, diachronous subduction boundary, or as a system of parallel, perhaps, coeval, subduction zones. This integrated study will employ standard optical microscopy, detailed electron microprobe analysis (including back-scattered electron imagining), and XRD analysis. These methods are particularly well-suited to the study of low-grade metamorphic rocks. In addition, chronological and thermal- history constraints will be provided by 40Ar/39Ar age-dating in collaboration with Dr. Paul R. Renne of Princeton University. We plan to conduct this investigation over a period of two years, including two summer field seasons.