THE CASE FOR LATE CRETACEOUS EXTENSION IN THE FUNERAL MOUNTAINS METAMORPHIC CORE COMPLEX

The partial exhumation of mid-crustal rocks in the Late Cretaceous has been documented in metamorphic core complexes of the western US, from western Arizona to southern Idaho, and yet the presence or absence of Late Cretaceous extension, and the responsible geodynamic mechanism(s), remain controversial. In the Funeral Mountains, the relative components of Miocene versus Cretaceous top-NW mylonitic deformation recorded in the high-strain rocks between the Miocene Boundary Canyon detachment (BCD) and the migmatitic core have been a subject of debate as both events exhibit similar lineation and foliation orientations and shear directions. Here we review observations that support Late Cretaceous extensional exhumation. (1) Samples taken from a 13 km long swath along the slip direction for NW extension of middle-amphibolite facies Johnnie Formation in the footwall of the BCD, yielded muscovite 40Ar/39Ar cooling ages between 86 and 71 Ma. (2) Moderate temperature mylonitic quartzite (mixed <a> slip, regime 2 microstructures) at intermediate levels are associated with Eocene muscovite cooling ages, documenting pre-Eocene shear. (3) Early Miocene mafic dikes cut deeper high-temperature quartzite mylonite (prism <a> slip; regime 3 microstructures) and yet are highly deformed within calc-mylonites, suggesting Miocene strain localization in weak layers and Miocene reactivation of older shear fabrics. (4) Titanite within mylonitic rocks yielded Tera-Wasserburg lower intercept ages of 77 to 88 Ma whereas a low-strain, clinopyroxene-rich calcsilicate lens yielded a 167 Ma age, suggesting preferential Late Cretaceous growth/recrystallization/Pb loss during shearing; the Late Jurassic age is consistent with Lu-Hf garnet and U-Th-Pb monazite ages that document Middle Jurassic regional metamorphism. Late Cretaceous synconvergent extension may be a response to uplift related to removal of mantle lithosphere and/or edge effects of dynamic topography associated with low-angle subduction.