13-10 Cordilleran Orogenic Wedge Evolution in a Transitional Segment of the South-Central Andes (34.5°S), Chile and Argentina

Session: Toe to Toe: Cordilleran Systems from Trench to Retroarc Domains

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Abstract:

Subduction of oceanic lithosphere beneath the South American continent has driven the growth of an ~8,000 km-long fold and thrust belt. Despite continuous subduction since the Mesozoic, there is considerable along-strike variability in crustal shortening magnitude, orogenic width, and crustal thickness, reflecting fundamental differences in thrust belt evolution. New geologic 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 undergoes its most dramatic decrease. A balanced cross section from the Coastal Cordillera in Chile through the San Rafael Block in Argentina produces a minimum shortening estimate of 26.4 km, more than an order of magnitude less than estimates from the Bolivian orocline. Mapping along the continental drainage divide reveals deeply exhumed structural levels above a major basement structure that transitions into a zone of detachment folding controlled by Middle Jurassic evaporites. Thermochronology records ca. 100-90 Ma cooling in the frontal (Malargüe) thrust belt, although most new and existing ages from the high Andes are young (< 5 Ma). Young cooling ages alongside deformed Miocene and younger intrusions and focused upper crustal seismicity are consistent with recent uplift and erosion within a subcritically tapered wedge. Apatite fission-track and (U-Th-Sm)/He data from the San Rafael Block (SRB) intraforeland uplift ~200 kilometers east of the topographic front suggest limited Cenozoic foreland burial during the main phase of shortening. Thermal history modeling from the SRB also reveals ca. 110-90 Ma cooling, consistent with a regional, synchronous Late Cretaceous thermal event driven by either 1) an early phase of intraforeland contraction or 2) extensional processes during rifting.

Development of thrust belt at 34°S was influenced by a laterally irregular stratigraphy, the reactivation of preexisting normal faults, and weak regional evaporite horizons. The combination may have contributed to the along-strike decrease in shortening and crustal thickening by helping maintain a very low taper angle. However, shortening values are insufficient to explain the ~48 km regional crustal thickness, implying either high initial thickness or alternative thickening mechanisms such as magmatic addition or underplating of subducted material along the South American margin.

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

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