TECTONIC SUBSIDENCE AND CRUSTAL FLEXURE IN THE NEOGENE CHACO BASIN OFBOLIVIA

We examined the evolution of the geometry of the Chaco foreland basin of Bolivia (Rio Grande-Parapeti area, latitude 18-degrees-50'-20-degre es-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 u sed the Bouguer gravity anomaly and the power spectrum of the gravity anomalies to estimate the mean depth of the compensating interfaces. I n 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 migra tion of the forebulge during the Neogene of about 90 km. Analysis of m ultiple geophysical data in the Chaco foreland basin of Bolivia permit s us to sketch out its evolution and to tentatively describe its succe ssive geometries and topographies since 10 Ma.