Petrologic constraints on the unroofing history of the Funeral Mountain Metamorphic Core Complex, California

Abstract

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The northern Funeral Mountains, southeastern California, comprise a metamorphic core complex that lies within a region of extreme Neogene extension but did not experience a significant late Cenozoic thermal event. This circumstance, unusual among core complexes of the North American Cordillera, permits direct examination of the preextensional thermal evolution of a portion of the Cordilleran hinterland. For pelitic schists from the highest-grade portions of the Funeral metamorphic core, thermobarometry and thermodynamic modeling of garnet zonation define a P-T trajectory showing: (1) attainment of “peak” metamorphic conditions at 800–850 K and 800–1000 MPa (30–37 km depths); followed by (2) 400 to 600 MPa of decompression (15–22 km of exhumation) with no substantial change in temperature. Available geochronologic data indicate that peak metamorphism occurred in Early Cretaceous time and that the decompression path developed over the Early to Late Cretaceous interval. The maximum pressures indicated by the petrologic data require substantial Late Jurassic(?)-Early Cretaceous tectonic burial even though most recognized thrust faults at this latitude that have large stratigraphie throws are assumed or demonstrated to have early Mesozoic ages. We postulate that post-Early Cretaceous extensional faults may have excised the necessary middle Mesozoic thrust structures. While the majority of extensional structures responsible for this excisement is likely to be of Neogene age, associated with Basin and Range extension, the Cretaceous decompression path described in this paper is similar to the theoretical P-T paths derived from numerical modeling of extensional unroofing. Given recent evidence for the development of extensional structures in compressional regimes like the Himalayan and Alpine orogens, it seems prudent to search for evidence of Mesozoic extensional structures in future studies of the hinterland of the North American Cordillera.