The role of enhanced erosion and clastic wedge progradation in the evolution of the Bolivian Subandean fold and thrust belt

The central Andean fold and thrust belt of Bolivian is a perfect archive for the tectonic, climatic, erosional, and uplift history of the middle central Andes. This fold and thrust belt reflects thin-skinned shortening and eastward propagation of the Andean deformation front. The interplay of tectonics, climate, and erosion in the deposition of up to 7.5 km of late Cenozoic strata remains unclear. To unravel this puzzle, we use industrial seismic reflection profiles, 8 new zircon U-Pb age data from Mio-Pliocene sedimentary strata and balanced cross-sections to evaluate the rates of thrust propagation, shortening, and deposition pinch-out migration of the late Cenozoic strata in the Bolivian Subandean fold and thrust belt. The results from our study indicate that thrusting arrived in the Subandean belt at ~12.4 ± 0.5 Ma and propagated rapidly westward up to ca. 6 Ma. This was followed by out-of-sequence deformation from ~ 4 to 2.1 Ma and by renewed propagation thereafter. Our results also show that the thrust front propagation- and deposition pinch-out migration rates mimic the sediment accumulation rate. The rates of deposition pinch-out migration and thrust propagation increased three-and two fold, respectively (8.25 mm/a; 3.3 mm/a) at 8-6 Ma. The three-fold increase in deposition pinch-out migration rate at this time is an indication of enhanced erosional efficiency in the hinterland, coupled with flexural rebound of the basin. Following the pulse of pinch-out migration at 6 Ma, the Subandean belt witnessed rapid ~70 km forelandward propagation of thrusting to the La Vertiente structure. As a result of no evidence for fast thrust front migration being linked to an increase in shortening rate, the enhanced frontal accretion suggests a shift to supercritical wedge taper. We propose that the supercritical state was due to a drop in basal strength, caused by sediment loading and pore fluid overpressure. This scenario implies that climate-controlled variation in erosional efficiency was the driver of the late Miocene mass redistribution which induced flexural rebound of the Subandean thrust belt, spreading of a large clastic wedge, and subsequent thrust front propagation.