Crustal segmentation, composite looping pressure-temperature paths, and magma-enhanced metamorphic field gradients: Upper Granite Gorge, Grand Canyon, USA

Abstract

The Paleoproterozoic orogen of the southwestern United States is characterized by a segmented, block-type architecture consisting of tens of kilometer-scale blocks of relatively homogeneous deformation and metamorphism bounded by subvertical highstrain zones. New fi eld, microstructural, and petrologic observations combined with previously published structural and geochronological data are most consistent with a tectonometamorphic history characterized by a clockwise, looping pressure-temperature (P-T) path involving: (1) initial deposition of volcanogenic and turbiditic supracrustal rocks at ca. 1.75‐1.74 Ga, (2) passage from 750 °C). Notable variations along the transect are also primarily thermal in nature and include differences in the temperature of the prograde history (i.e., early andalusite versus kyanite), equilibrium pressures recorded at peak temperatures, and intensity of late-stage thermal spikes due to local dike emplacement. High-precision ΔPT “relative” thermobarometry confi rms lateral temperature variations on the order of 100‐250 °C with little to no variation in pressure. The Upper Granite Gorge thus represents a subhorizontal section of lowermost middle continental crust (~0.7 GPa). Results imply that the entire ~70-km-long transect decompressed from ~0.7 to ~0.3‐ 0.4 GPa levels as one large coherent block in the Paleoproterozoic. The transect represents a 100% exposed fi eld laboratory for understanding the heterogeneity and rheologic behavior of lowermost middle continental crust during orogenesis. Hot blocks achieved partial melting conditions during penetrative subvertical fabric development. Although these blocks were weak, large-scale horizontal channel fl ow was apparently inhibited by colder, stronger blocks that reinforced and helped preserve the block-type architecture. Development of dramatic lateral thermal gradients and discontinuities without breaks in crustal level is attributed to: (1) spatially heterogeneous advective heat fl ow delivered by dense granitic pegmatite dike complexes and (2) local transcurrent displacements along block-bounding high-strain zones over an ~15‐20 m.y. time interval. Exhumation of the transect from 25 to 12 km depths is interpreted to refl ect erosion synchronous with penetrative development of steeply dipping NE-striking foliations and steeply plunging stretching lineations, consistent with an orogen-scale strain fi eld involving NW-SE subhorizontal shortening and subvertical extension during crustal thickening.