Paper No. 12
Presentation Time: 4:15 PM
IMPROVED AGE CONTROL AND INITIAL DETRITAL ZIRCON PROVENANCE FOR THE CENTRAL ANDEAN FORELAND BASIN SYSTEM OF SOUTHERN BOLIVIA
HORTON, Brian K. and CALLE, Amanda Z., Institute for Geophysics and Department of Geological Sciences, Jackson School of the Geosciences, University of Texas at Austin, Austin, TX 78712, horton@jsg.utexas.edu
Zircon U-Pb geochronology of clastic basin fill and interbedded volcanic tuffs provide insights into detrital provenance and chronostratigraphy of the Cenozoic foreland basin system exposed over a ~200 km proximal-to-distal transect in southern Bolivia at 20-21°S. New U-Pb provenance results, the most extensive reported for the central Andean fold-thrust belt and foreland basin, are presented for Cenozoic synorogenic sandstones (11 samples) and pre-Cenozoic sources (6 samples) in the Eastern Cordillera, Interandean Zone, Subandean Zone, and Chaco Plain. U-Pb age spectra for Paleozoic-Mesozoic stratigraphic units (Cambrian-Ordovician, Silurian, Devonian, Carboniferous, Triassic, Cretaceous) reveal (1) the dominance of Neoproterozoic (680-540 Ma) age populations with progressive upsection increases in (2) Sunsás-Grenville (1200-1000 Ma) signatures and (3) subordinate Famatinian-Ocloyic (490-440 Ma) ages. U-Pb results for Oligocene to Quaternary deposits (Petaca, Tariquia, Guandacay, Emborozú formations, and modern fluvial megafans) show (1) Neoproterozoic and (2) Sunsás-Grenville ages consistent with uplifted hinterland sources of widely distributed Ordovician rocks dominating the Eastern Cordillera, with upsection appearances of (3) Famatinian-Ocloyic ages attributable to middle to late Miocene initiation of shortening involving Silurian–Cretaceous rocks of the Interandean and Subandean zones and (4) Cenozoic (<25 Ma) ages derived from the Andean magmatic arc.
New magnetostratigraphic results and U-Pb age control for interbedded tuffs (20.8 ± 0.4, 8.4 ± 0.3, 3.8 ± 0.1 Ma) within studied Subandean sections (Bartolo, Camiri, Emborozú, Villamontes) provide improved constraints on depositional patterns, accumulation rates, and major provenance shifts. Further efforts will focus on pinpointing the eastward advance of fold-thrust deformation from the Eastern Cordillera (where shortening was largely focused at 40-25 Ma) to the Subandean Zone (where 10-0 Ma shortening is generally assumed). Key issues include a potentially long lived (>10 Myr) hiatus in shortening followed by an abrupt forelandward jump in deformation that may have been triggered by hinterland plateau uplift, climate change, and/or cyclical interactions among arc magmatism and upper-crustal deformation.
