BARBARA CARRAPA

The Central Andes represent the archetypical Cordilleran orogenic system, with a well-developed continental volcanic arc and some of the thickest crust on Earth. Yet the relative contributions of shortening and magmatic additions to crustal thickening remain difficult to quantify, which hinders understanding processes of crustal evolution in continental arcs. Cerro Aconcagua, the highest mountain in the Americas and a relict Miocene stratovolcano resting on 55 km-thick crust, is the ideal natural laboratory to address this issue in subduction-related magmatic arcs because it preserves a multi-million year record of magmatism and deformation within the Aconcagua fold-thrust belt. Estimates of paleo-crustal thickness in the Andes can be made using the geochemistry of subduction-related magmatic rocks, or minerals crystallized within them. This study applies a geochemical proxy approach for crustal thickness estimates to detrital syntectonic deposits of the Santa Maria Conglomerate derived from the Aconcagua stratovolcano to reconstruct paleo-crustal thickness of the Andes at this latitude. Detrital zircon trace-element data from ashes intercalated in the conglomerate, combined with previously published paleo-crustal thickness data, indicate crustal thicknesses of ∼35 km ca. 38 Ma and ∼44 km ca. 12 Ma, requiring ∼11 km of crustal thickening after ca. 12 Ma to achieve present-day crustal thickness of ∼55 km. In the absence of significant magmatism since ca. 10 Ma at this location, we show that more than half of the crustal thickening after 12 Ma, corresponding to 2 km of uplift, was achieved by Miocene shortening. Our study also reveals significant differences in crustal thicknesses between the Central Andes and the southern Central Andes which we speculate may be due to southward crustal flow during the last ∼20 My.

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.