MESOZOIC ANCESTRY OF THE FUNERAL MOUNTAINS METAMORPHIC CORE COMPLEX: INFLUENCE OF THRUST BURIAL AND INCLINED RHEOLOGICAL LAYERING ON POLYPHASE MESOZOIC AND CENOZOIC EXTENSIONAL REACTIVATION

Many metamorphic core complexes in the western US show evidence for the following relationships demonstrating tectonic inheritance in the geometry, kinematics and localization of Cenozoic extension: (1) localization in sites of significant Mesozoic crustal thickening; (2) development and Cenozoic reactivation of Late Cretaceous to Paleocene synconvergent extensional structures; and (3) a geometric and kinematic influence of initially inclined lithologic layering and rheological boundaries on Mesozoic and Cenozoic extensional structures. We review results of studies in the Funeral Mountains metamorphic core complex (FMCC) to demonstrate these relationships as well as to argue for the reactivation of a Jurassic thrust by the Miocene Boundary Canyon detachment (BCD). The FMCC shows clear evidence for significant localized crustal thickening in development of a metamorphic field gradient over a 40 km distance in Proterozoic metasedimentary rocks, from lower greenschist facies rocks to the SE to upper amphibolite facies/migmatites to the NW. Lu-Hf garnet dating constrains the age of initial metamorphism to the Late Jurassic (Oxfordian), and associated PT paths show garnet growth during compression. Muscovite ⁴⁰Ar/³⁹Ar ages beneath the Miocene BCD across 35 kilometers parallel to the transport direction provide evidence for a Late Cretaceous, extensional top-to-northwest structure antecedent to the BCD; ages show a northwest-decreasing age pattern from pre-burial Proterozoic detrital ages in the southeast, through 154 to 91 Ma cooling ages in the Sterling Quartzite, to 75 to 70 Ma ages in the subjacent Johnnie Formation in the northwest. Zircon (U-Th)/He analyses from quartzite samples collected along the same transect reveal a pattern of ages decreasing towards the northwest, and capture the pre-Miocene-extension partial retention zone and the onset of BCD slip at ~ 10-11 Ma with rapid slip until 7.4 Ma. These data compel the following history: (1) Jurassic crustal thickening and underthrusting of supracrustal rocks leading to development of NW dipping lithologic panel; (2) reactivation of the thrust zone during Late Cretaceous extension; and (3) subsequent reactivation of the composite deformation zone during Late Miocene extension.