Late Cretaceous channel flow in Western Arizona: Implications for the ancestral origin of metamorphic core complexes

Thomas Lamont, Shane Scoggin, Johannes Haemmerli, Gabe Epstein, Michael Wells, Alex Scandore, Meaghan Boyle

The origin of ancestral ’extensional’ shear fabrics in metamorphic core complexes (MCCs) in the Colorado River Extensional Corridor remains a tectonic mystery. Here we show that Late-Cretaceous metamorphic rocks and migmatites in the Harcuvar-Harquahala and Granite Wash Mountains MCCs reached peak metamorphic conditions of ca. 0.75 GPa and 780°C and were extruded towards the SW onto lower grade metamorphic rocks via ’channel flow’ between ca. 76–70 Ma, ∼40 Myrs before regional extension. The channel is bounded by a top-NE ’extensional’ roof shear zone and a top-SW thrust-sense base, associated with an inverted metamorphic gradient. U-(Th)-Pb and Rb-Sr geochronology confirms that shearing at the roof and base occurred concurrently. To explain these features, we propose that low-angle subduction of the Farallon plate released volatiles directly into the base of the North American crust, which induced water fluxed crustal melting and rheological weakening, facilitating flow of deep crustal rock to shallower depths. Consequently, ancestral ’extensional’ fabrics in MCCs may represent the roof of an extruding channel formed in an overall contractional
setting.