Paleoproterozoic rocks in the southwestern United States record tectonism at depths of 10–20 km during 1.75–1.65 Ga amalgamation of arcs to North America. These exposures of middle crust are characterized by great heterogeneity of composition, structure, and metamorphic grade. The crust is dominated by subvertical fabric and is laterally segmented into 10-km-scale blocks by subvertical shear zones. Metamorphic temperature gradients of 200 °C occur within <10 km length scales at various 10–20 km depths. Hottest areas are near granitoid plutons (10 km depths) or dike swarms (20 km depths) and are dominated by subhorizontal foliations. Lowest temperature blocks (depths <10 km) show brittle-ductile thrust belt geometries, and all levels show a range of brittle, semi-brittle, and ductile deformation features. Our synthesis suggests that the synorogenic middle crust exhibits a distinctive heterogeneity of tectonism influenced strongly by advective heating and resulting strength variations. Segmentation of the crust into tectonic blocks is interpreted to be related to the 10 km/10 m.y. space and time scales of plutonism and melt transfer in the middle crust. By analogy to experimental data we hypothesize that the middle crust was a spatially and temporally complex strength beam during deformation, with vertical fabric recording strain hardening during whole lithosphere failure. Heterogeneity and a dominance of vertical fabric are consistent with a general lack of middle crustal reflectivity and may have influenced differential uplift during subsequent tectonism.

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