SEDIMENT ACCUMULATION AND SURFACE UPLIFT IN THE ALTIPLANO BASIN

Sediment accumulation rates and depositional environments in the Altiplano basin viewed in light of surface uplift history can help constrain the geodynamic processes that formed the Andean plateau. The relative roles of the removal of mantle lithosphere and lower crustal flow in the surface uplift of regions that show relatively little upper crustal shortening have been debated in both the Andean and Tibetan plateaus. Each of these processes should produce distinctive spatial and temporal patterns of sedimentation and subsidence that can be distinguished from well-dated stratigraphic successions. Here we present new ⁴⁰Ar/³⁹Ar and magnetostratigraphic age constraints from the northern Altiplano. By combining these data with existing age constraints for middle to late Miocene strata (Roperch et al., 1999), we compile a history of sediment accumulation, depositional environment, and surface uplift between ~14.2 Ma and 5.2 Ma. This record shows that the time period between ~13 Ma and 8.6 Ma, leading up to the rapid rise of the Altiplano was punctuated by an increase in the rate of sedimentation in the Altiplano and the development of widespread lacustrine deposition. By 8.6 Ma, sediment accumulation rates decrease dramatically from an average rate of 0.88 mm/yr to 0.12 mm/yr, and fluvial sedimentation resumes at this time. We suggest that rapid subsidence and underfilled (i.e., lacustrine) basin conditions prior to surface uplift was associated with the growth of a perturbation in dense lower lithosphere. The downward growth, thinning, and ultimate detachment of eclogitic lower crust and mantle lithosphere is consistent with paleoaltimetry estimates that indicate that the Altiplano rose by ~2.5 to 3.5 ± 0.5 km between 10 Ma and 6.8 Ma (Garzione et al., 2006; Ghosh et al., 2006). Flow of lower crustal material into the Altiplano region would be expected to cause a decrease in sediment accumulation rates. Although we cannot rule out lower crustal flow as a significant process for crustal thickening; sediment accumulation rates, surface uplift rates, and depositional environments suggest that this process could not have contributed significantly to surface uplift of the Altiplano until after ~8.7 Ma.