167-11 Crustal Thickness Evolution of the Southern Central Andes (~35 °S): Insights from Igneous Paleomohometry and Zircon Petrochronology

Session: Integrating Geochronology and Geochemistry to Decipher the Tectonic Evolution of Orogenic Belts

Presenting Author:

Chance Ronemus

Authors:

Ronemus, Chance¹, Howlett, Caden², Carrapa, Barbara³, Echaurren, Andrés⁴, Barrionuevo, Matías⁵, Mosolf, Jesse⁶, Foley, Michelle⁷

Abstract:

Long-lived Cordilleran orogenesis has dramatically thickened the Andean crust, although the precise mechanics of this process are incompletely understood. The development of chemical mohometery—leveraging igneous geochemical parameters sensitive to Moho depth—has enabled quantitative reconstruction of crustal thickness histories in magmatic arc settings. Application of this technique in the Central Andes has revealed protracted Cretaceous–Cenozoic thickening to > 60 km, consistent with long-lived upper-plate shortening in the region. However, the thickness evolution remains poorly constrained farther south, where the modern crust is thinner (< 50 km at ≥ 35 °S), Paleogene deformation is debated, and recent studies suggest that the magnitude of Cenozoic thickening exceeded than explainable by shortening.

 

To address this knowledge gap, we examined a > 3.5 km-thick Cretaceous–Quaternary volcanic succession in the Tinguiririca valley of central Chile (~35 °S). We used 136 whole-rock geochemical analyses (11 new, 125 compiled) to reconstruct Moho depths through time via the Geochemical Arc Moho Estimator (GAME) program. We also analyzed 294 zircon grains from 21 samples for U-Pb geochronology and trace-element geochemistry, evaluating several trace-element ratios as crustal thickness proxies.

 

Our results suggest modest Late Cretaceous crustal thinning, consistent with intra-arc extension recorded by normal fault-related growth strata. However, we find no evidence of major Paleogene crustal thickness changes, contrasting both with thickening reported farther north and with models predicting thinning from extensional opening of the Abanico basin. The crust thickened by at least 8.4 ± 4.7 km between ca. 20 Ma and 12 Ma, coinciding with documented retroarc shortening but exceeding magnitudes explainable by shortening alone. Zircon Dy/Yb, Gd/Yb, and Sm/Yb track whole-rock mohometer trends, whereas zircon Eu anomalies (Eu/Eu*) yield a dissimilar evolution and unreasonable modern thickness estimates (~60 km).

 

We conclude that: (1) the crustal thickness evolution of the Central and southern Central Andes diverged in the Late Cretaceous–Paleogene, with thickening of northern regions contrasting with stable or decreasing thicknesses farther south; (2) Andean crustal thickening at ~35 °S occurred mainly during the Early–Middle Miocene and likely involved processes additional to retroarc shortening; and (3) light-to-medium over heavy rare earth element ratios in zircon track crustal thickness evolution with greater fidelity than Eu/Eu* and offer promising petrochronologic proxies amenable to future calibration as quantitative mohometers.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

© Copyright 2025 The Geological Society of America (GSA), all rights reserved.