Where did the Arizona-Plano Go? Protracted Thinning Via Upper- to Lower-Crustal Processes

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 similar to 60-55 km by the Late Cretaceous. Modern crustal thicknesses are similar to 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 similar to 40 Ma with limited evidence of denudation and similar to 10 km of crustal thinning. Major cooling occurred during detachment faulting and MCC formation at 26-19 Ma, corresponding to similar to 8 km of denudation and similar to 8 km of crustal thinning. Finally, we document a cooling phase at 17-11 Ma related to Basin and Range extension that corresponds with similar to 5 km of denudation and similar to 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 similar to 26 Ma recorded by thermochronology is insufficient to explain the magnitude of crustal thinning (similar to 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.

Automated summary

Subduction of the Farallon slab beneath North America during Mesozoic-Cenozoic period resulted in the formation of a regional orogenic plateau in the southwestern US. The study of low-temperature thermochronology and whole-rock geochemistry revealed crustal thinning in southern Arizona during the Late Cretaceous and Cenozoic periods.