NEAR-SURFACE SEISMIC REFLECTION STUDY OF QUATERNARY NEOTECTONICS AND STRATIGRAPHY ON THE BOLIVIAN ALTIPLANO

We conducted a high-resolution seismic reflection experiment on the Salar de Uyuni at the southern end of the Bolivian Altiplano in the summer/fall of 2000. 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 and others, GRL, 1994). The salar also lies between the mostly volcanic Western Cordillera and the mostly tectonic Eastern Cordillera of the Andes and shows local evidence for both Quaternary volcanic activity and active faulting. The stratigraphy of this region, 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 the Quaternary, (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 document the extent to which the Quaternary lake stratigraphy is affected by neotectonic deformation and, if possible, to separate the effects of the different processes.

The seismic profiles were acquired using high-resolution seismic equipment from IRIS PASSCAL and a jackhammer source. Data were recorded to 500 ms at 0.25 ms sampling. Depth penetration exceeded 300 m with numerous reflections and surprisingly high frequency content (over 500 Hz in the shallow section). Two different types of profiles were collected to satisfy the nearly exclusive need for both high-resolution data with dense spatial sampling and very long profiles (up to 130 km). Seven standard 60-channel profiles were collected along an east-west transect of the salar at 2 m shot/receiver spacing to provide detailed stratigraphy in segments of 1 km or less. (Two of these are tied to the 220 m deep corehole drilled by Baker and others (Nature, 2001) to look at paleoclimate history.) In addition, two transects were collected E-W and N-S across the salar with 6-channel records at 1 km spacing. Despite the low spatial sampling, these long transects clearly show faulting and tilting across the basin.