EXPLORING KINEMATIC VORTICITY FROM METAMORPHIC CORE COMPLEXES IN THE NORTH AMERICAN CORDILLERA

Metamorphic core complexes (MCCs) stretch across the North American Cordillera and exhume relatively deep (mid crust), metamorphosed crustal rocks to be juxtaposed against relatively cold, brittle upper crust. Classic models for their formation differentiate between simple shear with low-angle detachment faults that exhume footwall rocks via isostatic rebound versus pure shear involving primarily thermally-induced buoyancy forces to advect hot (commonly migmatite-cored) footwall rocks. Most MCCs consist of an approximately ~0.75-km-thick mylonitic shear zone that separates metamorphosed lower plate and unmetamorphosed upper plate rocks. The integrated finite strain history of this shear zone can be used to better resolve the kinematics, dynamics, and rheology of the MCCs. Here we examined the relative contributions of pure-shear and simple-shear for mylonitic shear zones from several of the central and southern MCCs (e.g., the Snake Range, East Humboldt Range, and Whipple Mountains) using field, outcrop, and microstructural observations to explore their similarities and differences that may reflect variable strain rates, host-rock lithology, temperature, and partitioning. The central MCCs deform well-constrained stratigraphy, which allows for robust estimates of shear-zone attenuation (i.e., vertical shortening, horizontal stretching). They all experienced >80% attenuation. The southern MCCs deform orthogneiss and thus shear-zone attenuation is unconstrained. Determination of kinematic vorticity numbers (Wk) from mylonite samples across the shear zones suggest that most mylonites deformed by ~50-75% pure shear, which is consistent with macro-scale observations of severe host-rock attenuation. Numerical simulations show that advection of hot, buoyant viscous rocks drives significant pure shear strain along the margins of the rising diapir, which validates our observations that MCCs involve significant pure shear. Similar observations from other gneiss domes and the wallrocks of intruded plutons suggests that pure-shear attenuation may be a diagnostic feature around buoyantly driven advective processes in the crust. We further argue that many MCCs in North American involved buoyant doming, potentially facilitated by isostasy during removal of their hanging wall rocks.