HOW TO BUILD THE MID-CRUSTAL STRENGTH BEAM: THE ROLE OF MELT EMPLACEMENT IN THE UPPER GRANITE GORGE, GRAND CANYON

The strength of the continental crust exerts a first order control on the evolution and expression of orogens. Based on our current rheological paradigm, the mid-crust is one of the strongest portions of the lithospheric column, commonly called the “mid crustal strength beam”. However, this simplified model does not take into account many geologic complexities known to weaken or strengthen the crust during orogenesis and crustal evolution. For example, the transient presence of melt, generated in-situ or emplaced from below, is a key crustal weakening mechanism. However, after emplaced melt crystallizes, the crust can be strengthened relative to its syn- or pre-melting condition. Here, we present field and microstructural observations from the Upper Granite Gorge (UGG) of the Grand Canyon, AZ that constrain evolving crustal rheology during Proterozoic collisional orogenesis. The UGG exposes polydeformed mid-crustal metamorphic rocks with an alternating hot block-cold block architecture (5-7 kbar; cold block < 550°C, hot blocks > 650°C), variably intruded by pegmatite swarms. Steeply dipping, high-strain NE-SW striking fabrics of the second deformation phase (D₂) dominate the UGG, with earlier, shallowly dipping D₁ fabrics preserved in microlithons and inclusion trails in local low-D2-strain domains. Pegmatites, dominated by coarse grained feldspars, intruded as both parallel and crosscutting bodies to S₁/S₂ fabric and some are deformed by F₂ folds, indicating that crystallization of these dike swarms was broadly syn-D₂. There is a strong spatial correlation between pegmatite swarms and map-scale D₂ folds, in and around which D₁ fabrics are best preserved. We suggest that the rapid crystallization of feldspar-rich pegmatites (relative to deformation timescales) created a 3-D grid of strong rocks (akin to steel rebar in concrete), especially in D₁ dominated cold blocks, and prevented the development of D₂ high-strain structures. In some hot blocks where pegmatites intruded, they are dynamically recrystallized and deformed, consistent with crystal plasticity expected in feldspar at elevated temperatures. This study highlights the importance of melt emplacement not only as a weakening agent, but also as a way to enhance the strength the crust and build the mid-crustal strength beam.