Slow Long-Term Exhumation of the West Central Andean Plate Boundary, Chile.

Abstract

We present a regional analysis of new low‐temperature thermochronometer ages from the Central Andean fore arc to provide insights into the exhumation history of the western Andean margin. To derive exhumation rates over 10 million‐year timescales, 38 new apatite and zircon (U‐Th)/He ages were analyzed along six ~500‐km long near‐equal‐elevation, coast parallel, transects in the Coastal Cordillera (CC) and higher‐elevation Precordillera (PC) of the northern Chilean Andes between latitudes 18.5°S and 22.5°S. These transects were augmented with age‐elevation profiles where possible. Results are synthesized with previously published thermochronometric data, corroborating a previously observed trenchward increase in cooling ages in Peru and northern Chile. One‐dimensional thermal‐kinematic modeling of all available multichronometer equal‐elevation samples reveals mean exhumation rates of <0.2 km/Myr since ~50 Ma in the PC and ~100 Ma in the CC. Regression of pseudovertical age‐elevation transects in the CC yields comparable rates of ~0.05 to ~0.12 km/Myr between ~40 and 80 Ma. Differences between the long‐term mean 1‐D rates and shorter‐term age‐elevation‐derived rates indicate low variability in the exhumation history. Modeling results suggest similar background exhumation rates in the CC and PC; younger ages in the PC are largely a function of increased heat flow and consequently an elevated geothermal gradient near the arc. Slow exhumation rates are suggestive of semiarid conditions across the region since at least the Eocene and deformation and development of the Andean fore arc around this time.