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

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.