140-6 How to uplift a forearc: A case study in the Coastal Cordillera of the southern Atacama Desert (26-29°S).

Session: Building the South American Cordillera, Paleozoic to Recent: Insights from Geo-Thermochronology

Presenting Author:

Authors:

Abstract:

The topography of most forearcs in subduction zones is characterized by a coastal high separated from the arc by a depression. Along the Chilean subduction zone the coastal high corresponds to the Coastal Cordillera (CC). Tectonic underplating is an active process along the entire margin, even in the turbidite-starved northern Chilean margin, where the underplated material is continental crust removed by tectonic erosion at shallower levels. A notable feature of the Chilean CC is the segmentation of uplift along-strike, which spatially correlates with deep structures of the South American basement and the subduction of bathymetric anomalies of the Nazca plate. However, it is unclear whether upper and lower plate structures control the amount of material that turns into tectonic slices at the plate interface.

The CC in the southern Atacama Desert serves as natural laboratory for studying the role of upper and lower plate structures in plate interface deformation. This region is of particular interest due to the confluence of a subducted bathymetric anomaly, the Copiapó Ridge (CR), which contributes fluids to the plate interface; and a major subvertical fault, the Atacama Fault System (AFS), which is thought to be rooted at the subduction interface. Where the AFS is closer to the trench, a steep coastal cliff is observed, and highly incised pediment surfaces are preserved at the summits of the western CC. This suggests that the AFS may interact with the plates interface to enhance uplift in these areas. Conversely, where the CR is subducted, no coastal cliff is observed. Instead, a wide coastal platform and broad alluvial valleys dominate the landscape of the western CC. This suggests that excess fluids diminish friction at the plates interface, lowering uplift.

The scientific ideas presented herein will be corroborated by using Miocene sedimentary rocks and pediment surfaces as deformational markers to constrain uplift timing and patterns. The age of these markers will be determined using ⁴⁰Ar/³⁹Ar dating on interbedded tuffs and ¹⁰Be/²⁶Al exposure ages. Exhumation patterns will be determined through multi-sample modeling of apatite fission tracks (AFT) and U-Th/He (AHe). Finally, we will compare uplift and exhumation patterns with the lithospheric structure as interpreted from 3D tomography images, considering that some tectonic factors, such as subduction angle and convergence velocity, may have changed.

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

doi: 10.1130/abs/2025AM-6945

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