T388 Challenges in Tectonics: Dynamic Interactions among Earth-Surface Processes, Landscape Evolution, and Tectonics (Posters)

Wednesday, 25 October 2017
9:00 AM - 6:30 PM
Halls 4EF

OROGENIC WEDGE DYNAMICS OF THE CENTRAL ANDEAN RETROARC IN SOUTHERN BOLIVIA (~21°S): INSIGHTS INTO FEEDBACK EFFECTS FROM APATITE (U-TH)/HE, APATITE FISSION TRACK, AND ZIRCON (U-TH)/HE AGES

Paper number: T388-8
Program: Technical Sessions
Time: 9:00 AM – 6:30 PM

Authors

Abstract

The Andean retroarc is an ideal setting to test predicted linkages between first-order processes operating within Cordilleran systems (e.g., cyclicity, critical taper). Paleoaltimetry data suggest the hinterland plateau (~21°S) underwent rapid surface uplift at ~13 Ma, which is predicted to induce rapid thrust belt propagation as a means to reduce wedge taper. We refine the deformation history of the Bolivian retroarc (21°S) in order to test this prediction and provide insight into wedge dynamics. We present 20 zircon (U-Th)/He (ZHe) ages, 19 apatite fission track (AFT) ages, and 59 apatite (U-Th)/He (AHe) ages across the Eastern Cordillera (EC), Interandean Zone (IAZ), and Subandean Zone (SAZ). ZHe ages reveal exhumation initiated in the eastern EC at 39-35 Ma, followed by westward propagation and development of a taper-building backthrust belt across the western EC from ~36-27 Ma. Eastward propagation resumed by ~29 Ma, as illustrated by rapid cooling through ZHe, AFT, and AHe closure in the eastern EC, but may have stagnated by ~22 Ma. Out of sequence deformation persisted in the western EC from ~26-10 Ma. Isostatic effects from a dense lithospheric root may be linked to this second taper building phase given that most out of sequence shortening (~21-12 Ma) precedes ~13 Ma rapid uplift of the hinterland. Eastward migration of deformation resumed by ~19 Ma, and AFT ages record two episodes of rapid thrust front advance that occurred in the western IAZ at ~19-15 Ma, and in the eastern IAZ-western SAZ at ~12-8 Ma. The earlier episode occurred as the thrust front encountered mechanically weak Silurian shales at the EC-IAZ boundary, resulting in a supercritical wedge state. The latter episode may be a second supercritical wedge phase induced by changes in surface elevation, as it closely followed rapid hinterland uplift at ~13 Ma. In the eastern SAZ, AFT and AHe ages show deformation rapidly progressed to the modern thrust front from 6.3±1.5 Ma to the present. Development of an orographic barrier and a shift to wetter climatic conditions on the eastern flank of the mountain belt occurred at ~8.5 Ma as the hinterland reached critical elevations (>3 km). Low denudation rates and mass accumulation in the orogen interior drove outward expansion of the wedge due to increasing lithostatic stresses, despite focused erosion in the SAZ.

Geological Society of America Abstracts with Programs. Vol. 49, No. 6

doi: 10.1130/abs/2017AM-304165

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