Long-term kinematic and erosional record of the south-central Andes and implications for mechanisms of crustal thickening, Chile and Argentina (34–35°S)

Caden J Howlett, University of Montana Western
Chance B Ronemus, University of Idaho
Barbara Carrapa, University of Arizona
Peter G DeCelles, University of Arizona
,
,
,
,

Abstract

Mechanisms responsible for crustal thickening in cordilleran orogenic systems remain debated. Upper-crustal shortening and retroarc underthrusting, magmatic addition, and subduction-related underplating may all contribute, but resolving their relative roles requires integrated constraints on the timing and rates of deformation, exhumation, magmatism, and crustal thickening. Such records have proven reliable in the Central Andes but remain under studied in the south-central Andes.

New mapping, structural observations, and low-temperature thermochronology from an orogen-scale transect at 34.5°S provide insight into a transitional segment of the Andes, where shortening magnitude and crustal thickness experience a dramatic southward decrease. A balanced cross section from the Coastal Cordillera of Chile through the San Rafael Block of Argentina yields a minimum shortening estimate of 26.4 km, more than an order of magnitude lower than estimates from the Bolivian orocline. Mapping along the continental drainage divide reveals deeply exhumed structural levels above a major basement structure that transitions eastward into a zone of detachment folding controlled by Jurassic evaporites. New apatite fission-track and (U-Th-Sm)/He data reveal Late Cretaceous cooling (ca. 110–90 Ma) across the thrust belt and previously undated foreland uplifts, followed by focused Neogene exhumation in the High Andes. Young cooling ages, deformed Miocene and younger intrusions, and focused upper-crustal seismicity are consistent with uplift and erosion within a subcritically tapered orogenic wedge.

Thrust belt development at 34.5°S was/is influenced by a laterally irregular stratigraphy, reactivation of preexisting normal faults, and weak regional evaporite horizons. Together, these factors promoted a low-taper wedge and contributed to the along-strike decrease in shortening. The integration of thermochronology data with geochemical mohometry confirms a tight temporal overlap between major Neogene shortening and crustal thickening, and significant thickening was only possible following the establishment of a regional linked decollement capable of underthrusting retroarc lithosphere.

Session

Rates and dates