T250 The Andes from Top to Bottom (Posters)

Wednesday, 18 October 2023
8:00 AM - 5:30 PM
Hall B

ASSESSING THE STRUCTURAL GEOMETRY OF THE ANDES ABOVE THE PERUVIAN FLAT SLAB (10-12° S) USING KINEMATIC MODELLING AND COOLING AGES

Paper number: T250-3
Program: Technical Sessions
Time: 8:00 AM – 5:30 PM

Authors

Abstract

We present a new geologic map and cross-section from field-based structural observations, orientation measurements, lithologic data, geomorphic indicators, and preliminary cooling ages. The stratigraphy exhibits significant stratigraphic changes due to a pre-compression extensional environment. The thickness of the section increases from ~4 kilometers (km) in the east to ~12 km in the west. Preliminary cooling ages are from three mineral systems: Apatite (U-Th)/He (AHe), Apatite Fission Track (AFT), and Zircon (U-Th)/He (ZHe). AHe ages range from 2 to 65 million years (Ma) with the largest cluster of ages between 5 to 9 Ma and are located between 180 to 210 km from the deformation front. Reset ZHe ages range from 8 to 30 Ma with 11 samples indicating partial reset ages of 43 to 181 Ma. The youngest ages for AHe and ZHe are located 180-200 km and 189 km from the deformation front (respectively) and show a sharp increase in ages (older) to the southwest. These young ages are indicative of active uplift and exhumation. The increase in ages to the southwest strongly suggests exhumation over a basement ramp. Geomorphic indicators of active uplift include high topography and high KSN. The topographic mean reaches elevations of 4 km along this section at 205 km and the maximum KSN value is located at 180 km from the deformation front. We propose that the high topographic mean is a function of the lower edge of an active basement footwall ramp while the maximum KSN is a function of the upper edge. These edges set the boundaries for the region with the youngest AHe ages and the youngest ZHe age. The associated hanging wall ramp is located ~60 km to the east where KSN drops to background values. Our balanced cross-section exhibits multiple basement structures. Exposed basement west of 205 km requires a western basement thrust to promote structural elevation between 205-290, an active basement thrust between 205-120 km, and a broken foreland basin with basement exposures in the Subandean Zone <10 km from the deformation front. We propose an active basement ramp in the region of high topography, high KSN, and young cooling ages between 180 and 205 km from the deformation front.

Geological Society of America Abstracts with Programs. Vol. 55, No. 6

doi: 10.1130/abs/2023AM-394875

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