Neogene sedimentation and provenance record of the Subandean zone and Chaco foreland basin, southern Bolivia

Evolution of the Subandean fold-thrust belt and adjacent Chaco foreland basin since their inception ~20 Myr ago has involved shortening, crustal thickening and flexural loading along the eastern margin of the central Andes in Bolivia. Few studies have assessed the linkages among surface processes and threshold responses to climate change, tectonics, and/or catchment-area modification in this thin-skinned fold-thrust belt and coupled foreland basin. Analyses of stratigraphic variations and sediment provenance within two of the proximal, westernmost sections of the Subandean zone (Bartolo and Emborozú localities) are consistent with an eastward advancing synorogenic fold-thrust system guiding deposition and facies migration within several foreland depozones. Results from Cenozoic detrital zircon U-Pb geochronology, conglomerate clast compositions, sandstone petrography and paleocurrent analyses provide information on the gradual eastward diachroneity of upward-coarsening successions and fluvial megafan deposits. New and published zircon U-Pb data for interbedded Miocene tuffs and an updated magnetostratigraphic analysis contribute to the interpretation of depositional ages and changes in sediment accumulation rates. Regionally, the Neogene sedimentation history shows a gradually subsiding basin until 8 Ma with progressively coarser facies up to 6 Ma. High accumulation rates for proximal alluvial-fan facies and proximal fluvial megafan deposits after 6 Ma are identified and correlated with their distal counterparts in the Chaco basin. Evidence for the introduction of progressively younger rocks in uplifted sediment source areas, as documented by conglomerate counts and detrital zircon age populations, lead us to interpret protracted unroofing and sediment dispersal from the Eastern Cordillera–Interandean Zone commencing at ~23 Ma, followed by initial propagation of the deformation front into the Subandean Zone at ~20-15 Ma (as estimated from new tuff ages and accumulation rates). A temporal shift from axial to transverse fluvial drainage recorded by paleocurrent data reinforces evidence for an eastward advancing depocenter and structural partitioning of the older segments of the foreland basin, particularly from 6 Ma to present. Continued exhumation and enhancement of the drainage areas to the west (i.e., Eastern Cordillera, Interandean Zone, and actively deforming Subandean zone) reflect changes in the accumulation of coarser lithofacies in proximal areas and delivery of significant sediment volumes to modern fluvial megafans of the Chaco plain.