T267 Plate Tectonics, Arc-Trench Systems, Cordilleran Tectonics, Sedimentary Basins, Sandstone Provenance, and Geoarchaeology (Posters): A Celebration of William R. Dickinson's Career

Tuesday, 27 September 2016
9:00 AM - 6:30 PM
Exhibit Hall E/F

RETROARC CRUSTAL SHORTENING AND STRUCTURAL ARCHITECTURE OF THE ANDEAN FOLD-THRUST BELT OF SOUTHERN BOLIVIA (21°S): IMPLICATIONS FOR KINEMATIC DEVELOPMENT AND CRUSTAL THICKENING OF THE CENTRAL ANDES

Paper number: T267-28
Program: Technical Sessions
Time: 9:00 AM – 6:30 PM

Authors

Abstract

Reliable estimates of shortening magnitude, deformation kinematics, and thrust belt geometry provide a critical framework for testing the mechanisms influencing the growth of convergent orogenic systems. We calculate total shortening, present a kinematic model for thrust belt development, and assess the crustal thickening budget of the central Andean retroarc region of southern Bolivia (21°S) by incorporating new geologic mapping and existing geophysical data to construct a balanced cross section that encompasses the Subandean Zone (SAZ), Interandean Zone (IAZ), and Eastern Cordillera (EC). The SAZ, defined by 10-20 km wavelength, 4-6 km amplitude fault-bend folds above a 10-12 km-deep regional décollement in Silurian strata, accommodated 82 km (36%) of east-west shortening. The IAZ, a bivergent zone of 2-4 km-thick thrust sheets of mainly Silurian-Devonian rocks that are structurally elevated ~10 km relative to adjacent SAZ levels, accommodated 70 km (70%) of shortening. The EC is a bivergent zone of 2-10 km-thick thrust sheets of Cambrian-Ordovician rocks structurally elevated ~6 km relative to the IAZ and Altiplano. However, we now account for a previously unbalanced segment of the central EC, increasing the previous shortening estimate (95 km) to 120 km (37%). When combined with a published estimate of Altiplano shortening (65 km), total retroarc shortening is estimated at 337±40 km (36 ±3%). We interpret Cenozoic development of the retroarc thrust belt by eastward propagation and stacking of two ~12 km-thick basement megathrust sheets that fed slip from a mid-crustal décollement up regional-scale footwall ramps into an upper décollement at the basement-cover interface. This kinematic model is consistent with the pattern of published cooling ages across the retroarc region and the long-term eastward migration of the central Andean foreland basin system, and also accounts for the large-scale structural steps observed across individual morphotectonic zones. Calculating a crustal thickness budget using the new shortening estimate accounts for 93-101% of the geophysically observed crustal area (assuming 35 km thick initial crust), and produces an excess of ~6-15% crustal area (assuming 40 km thick initial crust), raising the possibility of lower crustal flow or loss due to tectonic thickening.

Geological Society of America Abstracts with Programs. Vol. 48, No. 7

doi: 10.1130/abs/2016AM-284079

© Copyright 2016 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.