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 high-strain zones. New field, 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 <12 km (below pressures equivalent to the aluminosilicate triple point) to ∼25 km depths (∼0.7 GPa) between ca. 1.70 and 1.69 Ga, (3) decompression back to ∼12 km depths (0.3–0.4 GPa) by ca. 1.68 Ga, and (4) a protracted period of near-isobaric cooling (ca. 200–250 Ma). The general geometry of this looping P-T path is similar for rocks across the entire traverse; however, significant differences in peak temperatures are recorded (∼500 to >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 confirms 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 field 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 flow 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 flow 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 reflect erosion synchronous with penetrative development of steeply dipping NE-striking foliations and steeply plunging stretching lineations, consistent with an orogen-scale strain field involving NW-SE subhorizontal shortening and subvertical extension during crustal thickening.

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