Building the Andean Crust at ∼35°S: Arc Geochemistry Reveals Cretaceous Thinning and Rapid Neogene Thickening

Quantification of the timing and mechanisms of crustal thickening in Cordilleran orogenic hinterlands remains challenging. We address this in the Andes using whole‐rock geochemistry and zircon petrochronology from a >70 Myr volcanic record in the Tinguiririca valley of Chile (∼35S), which captures a pivotal transition from extensional to contractional deformation. Zircon (Sm, Gd, Dy)/Yb ratios generally track trends in whole‐rock geochemical crustal thickness (paleomohometry) models across this record, with Dy/Yb showing the most consistent correspondence. Our results indicate modest crustal thinning coeval with Late Cretaceous normal faulting and oblique plate convergence. Eocene and Early Miocene samples yield similar crustal thicknesses (∼35 km), suggesting limited local net attenuation during proposed intra‐arc rifting (Abanico basin). The crust then thickened by ∼10 km to near‐modern values (∼46 km) during the Neogene; Quaternary estimates match modern geophysical constraints, validating the paleomohometry approach. Thickening was most rapid between ca. 17 and 13 Ma, coinciding with accelerated retroarc shortening and plate convergence. Additional zircon geochronologic and geochemical proxies (Th/U, phosphorus content, and oxygen fugacity) indicate enhanced Miocene crustal melting and magmatic productivity. Along‐strike comparisons reveal north‐to‐south younging and decreasing magnitude of thickening, mirroring trends in retroarc shortening. These findings indicate that underthrusting of foreland lithosphere during retroarc shortening drove crustal thickening, magmatic flare‐up, and generation of orogenic topography in the southern Central Andes.