FLATTENING STRAINS AND COMPLEX KINEMATICS OF THE 96-MILE ULTRAMYLONITE, GRAND CANYON BASEMENT

The nature of high strain zones within orogenic settings, including the timing of their initiation, P-T-conditions, strain geometry, and kinematic history are key to understanding both orogenic history and the rheologic nature of orogenic crust. The Grand Canyon, Arizona, USA, exposes >200 miles of continuous exposure of a deeply exhumed Proterozoic orogen (“Grand Canyon Basement”) that records multiple episodes of deformation, metamorphism and plutonism. The resulting crustal architecture is one of variably deformed crustal blocks separated by high strain zones. Here, we present optical petrography, electron backscatter diffraction (EBSD) data, and strain analysis from the ultramylonitic 96-mile shear zone in the basement of the Upper Gorge of the Grand Canyon. The 96-Mile shear zone is a subvertical zone of ultramylonitic to mylonitic metavolcanic rocks that separates the low-T Topaz Canyon block from the high-T Trinity Creek block. Large quartz rich zones were sampled from two locations within the shear zone. These quartz-rich zones preserve a textbook quartz ribbon core-mantle microstructure with fine serrated edges and recrystallized grains, indicating a grain boundary bulging mechanism. Large feldspar grains act as porphyroclasts within quartz-dominated domains, displaying both bookshelf fracturing as well as recrystallization textures. Quartz ribbons have similar axial ratios in both the standard kinematic plane (XZ) and lineation normal plane (YZ), suggesting a flattening-type strain geometry. Quartz ribbons also exhibit asymmetry in both principal planes and record east-side-up as well as dextral components of shearing. Quartz (one point per grain) CPO pole figures calculated from EBSD maps consistently record a flattening strain geometry pattern, with minor asymmetry consistent with an east-side-up sense of shear. These data are discussed within the context of new thermochronology and geochronology data that suggest that the shear zone juxtaposes blocks that cooled at significantly different times, with the uplifted eastern side cooling through ~350°C at 1.45-1.40 Ga. We conclude that the 96-Mile shear zone is a significant transpressional structure that uplifted the Trinity Creek block with respect to the adjacent Topaz Canyon block at this time.