Kinematics and Timing of Superposed Deformation in the Funeral Mountains Metamorphic Core Complex, Death Valley, CA
Creators: name: Sauer, Katrina Marie
Publisher: name: University of Nevada, Las Vegas
Year: 2014

description: The Funeral Mountains metamorphic core complex (FMMCC) in Death Valley, California exposes middle to lower crustal rocks of the Sevier-Laramide orogen in the footwall of the Miocene Boundary Canyon detachment. The structurally deepest rocks in the FMMCC are exposed in Monarch Canyon, where the Meso- to Neoproterozoic metasedimentary rocks record upper amphibolite facies metamorphism with migmatites developed at the deepest levels. Distributed ductile deformation and stratigraphically-localized high-strain zones, termed intracore shear zones, are responsible for attenuation and local stratigraphic omission during top-northwest non-coaxial deformation. The structurally deepest Monarch Spring shear zone (MSSZ) juxtaposes the migmatitic paragneisses below against greenschist to amphibolite facies marbles, pelitic and calcsilicate schists above. Below the MSSZ, the migmatitic paragneisses lack the top-northwest fabrics and instead exhibit a northeast-trending mineral lineation and local, strong fabric asymmetry indicative of top-southwest shear. We hypothesize that the MSSZ represents a deformed anatectic front, and an apparent zone of structural decoupling between orthogonally-directed shear fabrics at deep crustal levels during the Sevier-Laramide orogen. U-Pb zircon geochronology on leucogranite sills that are folded with the top-SW fabric yield crystallization ages of 68 Ma, while a dike cutting the top-SW fabric and another cutting isoclinal folds yield ages of 61 Ma and 57 Ma, respectively. This geochronology along with EBSD and structural observations indicate that the orthogonally-directed flow above and below the MSSZ may have developed coevally and synchronous with continued regional compression during the Sevier-Laramide orogeny, supporting the hypothesis of synorogenic extension.
The timing of extension within Cordilleran metamorphic core complexes remains controversial, with the opposing views of Tertiary extension during a single tectonic event, or a polyphase extensional history beginning in the Late Cretaceous and continuing through the Miocene. The relative contributions of Late Cretaceous-early Tertiary and Miocene extensional strains in the FMMCC, which manifest in the top-northwest fabric, are addressed here using thermochronologic, microstructural, and EBSD studies. The EBSD and microstructural data show predominantly mixed <a> to prism </a><a> slip and subgrain rotation to grain boundary migration, suggesting deformation temperatures ca. 400-550° C. One quartzite lies adjacent to a marble that yields an 40Ar/39Ar muscovite cooling age of 44 Ma, suggesting pre-44 Ma deformation. The timing of deformation along the intracore shear zones remains elusive, although an 8 m.y. discrepancy (78 Ma versus 86 Ma) across the Monarch Canyon shear zone may indicate extension along the MCSZ occurring in the Late Cretaceous. Groupings of 40Ar/39Ar muscovite ages in the proximity of Monarch Canyon indicate cooling during the Oligocene and Eocene, hinting at the potential for an early Tertiary phase of extension that has not been previously recognized. Regardless, the thermochronologic and microstructural data support a polyphase extensional history for the Funeral Mountains metamorphic core complex, beginning in the Late Cretaceous and extending to the Miocene.</a>