Where did the Arizona‐Plano Go? Protracted Thinning Via Upper‐ to Lower‐Crustal Processes. Issue 4 (3rd April 2022)

• Record Type: Journal Article
• Authors: Jepson, Gilby; Carrapa, Barbara; George, Sarah W. M.; Reeher, Lauren J.; Kapp, Paul A.; Davis, George H.; Thomson, Stuart N.; Amadori, Chiara; Clinkscales, Christopher; Jones, Sean; Gleadow, Andrew J. W.; Kohn, Barry P.
• Is Part Of: Journal of geophysical research. Volume 127:Issue 4(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-04-03
• Subjects: Arizona‐plano -- crustal thickness estimates -- thermochronology -- lower‐crustal flow -- metamorphic core complex; Geomagnetism -- Periodicals; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 551.1
• DOI: 10.1029/2021JB023850
• Languages: English
• ISSNs: 2169-9313

Abstract

Abstract: Mesozoic‐Cenozoic subduction of the Farallon slab beneath North America generated a regionally extensive orogenic plateau in the southwestern US during the latest Cretaceous, similar to the modern Central Andean Plateau. In Nevada and southern Arizona, estimates from whole‐rock geochemistry suggest crustal thicknesses reached ∼60–55 km by the Late Cretaceous. Modern crustal thicknesses are ∼28 km, requiring significant Cenozoic crustal thinning. Here, we compare detailed low‐temperature thermochronology from the Catalina metamorphic core complex (MCC) to whole rock Sr/Y crustal thickness estimates across southern Arizona. We identify three periods of cooling. A minor cooling phase occurred prior to ∼40 Ma with limited evidence of denudation and ∼10 km of crustal thinning. Major cooling occurred during detachment faulting and MCC formation at 26–19 Ma, corresponding to ∼8 km of denudation and ∼8 km of crustal thinning. Finally, we document a cooling phase at 17–11 Ma related to Basin and Range extension that corresponds with ∼5 km of denudation and ∼9 km of crustal thinning. During the MCC and Basin and Range extension events, the amount of denudation recorded by low‐temperature thermochronology can be explained by corresponding decreases in the crustal thickness. However, the relatively limited exhumation prior to detachment faulting at ∼26 Ma recorded by thermochronology is insufficient to explain the magnitude of crustal thinning (∼10 km) observed in the whole rock crustal thickness record. Therefore, we suggest that crustal thinning of the Arizona‐plano was facilitated via ductile mid‐ to lower‐crustal flow, and limited upper‐crustal extension at 50–30 Ma prior to detachment faulting and Basin and Range extension. Plain Language Summary: In this study, we integrate low‐temperature thermochronology with regional paleo‐crustal thickness estimates from whole rock geochemistry and geophysical proxies applied to the southwestern USA, which is a natural laboratory for crustal thickening and thinning processes. Understanding crustal evolution is paramount for our understanding of the processes involved in mountain building and surface uplift with implications on various fields of Geosciences including tectonics, petrology and surface processes. Through this approach, we are able to compare the total amount of crustal thinning to the amount of denudation experienced by the upper crust. Thus, we provide a quantitative, mass‐balance calculation of crustal thinning. We apply this approach to the relatively well understood Basin and Range province in Arizona, southwestern USA. At 70 Ma the region was hypothesized to have existed as a thick, orogenic plateau (∼55 km thick crust), however, today the crust is relatively thin as observed from teleseismic receiver functions (∼28 km). Thus, we pose the question: when and how was this thick crust removed? This study has major implications for considering the relative contributions of the upper and lower crust to the demise of orogenically‐thickened crust and offers a new workflow for linking upper‐ and mid‐ to lower‐crustal processes. Key Points: Crustal thickness estimates, geophysical proxies, and thermochronology provide independent constraints on the thinning history western USA Where crustal thickness estimates and thermochronology match, we can assess that upper‐crustal processes control crustal thinning Where these systems diverge, we assess that lower‐crustal processes are controlling crustal thinning