TECTONIC SUBSIDENCE AND CRUSTAL FLEXURE IN THE NEOGENE CHACO BASIN OF BOLIVIA

We examined the evolution of the geometry of the Chaco foreland basin of Bolivia (Rio Grande-Parapeti area, latitude 18-degrees-50'-20-degrees-S) using seismic reflection, gravity and well-log data. Subsidence curves enabled us to identify three tectonic stages with a subsidence rate which ranged from 0.1 to 0.4 km/Myr during the late Miocene. We used the Bouguer gravity anomaly and the power spectrum of the gravity anomalies to estimate the mean depth of the compensating interfaces. In order to understand the evolution of the crustal flexure, we used an elastic model with a flexural rigidity of almost-equal-to 10(23) N.m and crustal thickness of almost-equal-to 30-31 km. We observed a migration of the forebulge during the Neogene of about 90 km. Analysis of multiple geophysical data in the Chaco foreland basin of Bolivia permits us to sketch out its evolution and to tentatively describe its successive geometries and topographies since 10 Ma.