CRUSTAL THICKNESS EVOLUTION OF SOUTHERN LAURENTIA'S PROTEROZOIC OROGENS

Dramatically different tectonic styles have been interpreted for the mid-Proterozoic (ca. 1.8 and 1.0 Ga). Some workers envision processes similar to the present (Condie, 1982; Whitmeyer and Karlstrom, 2007) while others have proposed a different tectonic paradigm associated with thinner crust, higher heat flow, and/or fewer mountain building events (e.g., Tang et al., 2021; Spencer et al., 2021). To test hypotheses about the nature of Proterozoic tectonics and crustal thickness, we examined the igneous and metamorphic records of the Proterozoic orogenic belts of the southwestern US, a region previously interpreted as an example of accretionary orogenesis and modern-style plate tectonics at 1.8-1.3 Ga. We constrained the spatial and temporal evolution of crustal thickness from metamorphic, xenolith, and phase equilibria datasets combined with and geochemical proxies (La/Yb; La/0.1Y). The earliest arc rocks (1.84-1.71 Ga) suggest a relatively thin, 30-40 km, crust. Granitoids dated to 1.71-1.65 Ga indicate thickening to ~55 km during the Yavapai orogeny. A slight decrease (~10 km) in crustal thickness ca. 1.70-1.68 Ga may reflect erosional and/or extensional processes, consistent with decompression recorded by clockwise P-T paths. Mesoproterozoic plutons (1.5-1.3 Ga) record crustal thicknesses of ≥50 km across the Southwest during the Picuris orogeny. Integration of metamorphic and igneous data suggest that thickening occurred by upper crustal loading in northern NM whereas lower crustal processes such as magmatic underplating were dominant in CO and AZ. Juvenile Mesoproterozoic crust south of the Nd line of Bickford et al. (2015) yield much thinner estimates of 20-30 km at ca. 1.5-1.45 Ga and 30-40 km at ca. 1.4-1.35 Ga. We interpret these results to support models in which accretion of a juvenile arc terrane (the Picuris province) drove inboard tectonism associated with the Picuris orogeny. The results do not support models suggesting thinner crust and/or orogenic quiescence in the Proterozoic. Taken together, our results support the interpretation of subduction-related, Andean-scale accretionary orogenesis across the southern margin of Laurentia from 1.8-1.3 Ga manifested by ~3-5 km-high mountains supported by ≥50 km thick crust, and the development of an orogenic plateau ca. 1.45-1.40 Ga.