Abstract

The sub-Andean foreland basin of southern Bolivia chronicles erosional unroofing of the central Andean fold-thrust belt during Cenozoic shortening. Analyses of five stratigraphic sections document regional paleosol development in forebulge to distal foredeep depozones, followed by proximal accumulation of a greater than 4 km (2.5 mi) thick upward coarsening and thickening succession of mixed fluvial to megafan deposits. New timing constraints from zircon U-Pb ages for sandstones and interbedded volcanic horizons indicate Oligocene to late Miocene facies migration and eastward progradation from growing structures and point sources of sediment. Detrital zircon U-Pb results, sandstone/conglomerate compositions, and paleocurrent data reveal subarkosic and sublitharenitic sand derived from Andean sources to the west, with (1) contributions from upper Paleozoic strata (420–570 Ma zircon age components) of the inter-Andean Zone; (2) variable input from the Eastern Cordillera, including lower Paleozoic strata (650–800 Ma zircon ages), Mesozoic strata (66–360 Ma zircon ages), and recycled Paleogene basin fill; and (3) later arrival of a cosmopolitan age assemblage from upper Paleozoic to Cenozoic strata of the incipient sub-Andean Zone. Eastward advance of the fold-thrust belt corresponds to an increase in sediment accumulation from 3 m (10 ft)/m.y. to 90–2000 m (295–6562 ft)/m.y. The integrated results suggest a progressive early–middle Miocene transition from foredeep to wedge-top deposition within the eastern inter-Andean Zone to western sub-Andean Zone, with a generally post–12 Ma age for most sub-Andean structures. We propose that either (1) pre-Cenozoic stratigraphic and structural heterogeneities promoted nonsystematic activation and shortening advance, with an unsteady eastward migration of flexural foreland subsidence, or (2) the fold-thrust belt and foreland basin advanced systematically eastward, with a marked southward reduction in the magnitude of shortening along the inter-Andean-sub-Andean Zone. Surface uplift associated with advancing deformation guided the creation of topographic barriers in the fold-thrust belt and proximal foreland, inducing variable unroofing patterns, drainage network evolution, and deposition of fluvial megafans.