Shallow Seismic Reflection Imaging of the Salar de Uyuni, Bolivia: Quaternary Neotectonics and Stratigraphy

Results from a shallow seismic reflection experiment in 2000 on the Salar de Uyuni at the southern end of the Bolivian Altiplano will be presented. This salar is the largest salt flat in the world (~130 km x 80 km) and is a remnant of the Quaternary paleolake Minchin/Tauca. Paleoshorelines of this lake mapped by GPS show marked deformation (Bills et al., GRL, 1994). The salar also shows evidence for both Quaternary volcanic activity and active faulting from Andean tectonics. The paleolake stratigraphy of the salar, therefore, may record neotectonic deformation from three different processes: (1) isostatic rebound from the evaporation of the deep lakes that have occupied this region periodically during Quaternary climate fluctuations, (2) active faulting along local/regional fault systems, and (3) regional uplift and/or tilting of the Andes. The seismic experiment was designed, in part, to determine the extent to which the Quaternary lake stratigraphy documents neotectonic deformation and, if possible, to separate the effects of the different processes. The seismic profiles were collected using IRIS PASSCAL instruments and a hammer source. The stratigraphy is imaged to a depth of at least 200 m at surprisingly high frequencies and can be tied to the 220 m deep paleoclimate corehole drilled by Baker et al. (Nature, 2001). One 130-km transect was collected across the entire E-W width of the salar. A second 50-km N-S transect crosses at the corehole. These profiles are limited by very low spatial sampling but clearly show that the shallow Quaternary lake deposits now dip to the east and north and are offset by sets of faults, particularly on the eastern side of the salar. The tilting of the stratigraphy conforms with the thickening of the young surficial salt layer to the east, although the shallowmost seismic data quality doesn't allow one to determine the youngest age of layers offset by the imaged faults. Current efforts are to separate out the isostatic rebound signature from the deformation from faulting.