REGIONAL GEOLOGIC MAP ACROSS THE ANDEAN RETROARC FOLD-THRUST BELT OF SOUTHERN BOLIVIA: NEW INSIGHTS ON THE SUBANDEAN ZONE, INTERANDEAN ZONE, AND EASTERN CORDILLERA AT 21°S

In order to understand the structural geometry of the Andean retroarc fold-thrust belt, regional geologic mapping was performed between 21°10’ S and 22°10’ S along a ~300 km east-west transect from the Chaco Plain to the eastern Altiplano. Mapping was undertaken at scales of 1:50,000 and 1:24,000, and is displayed at 1:200,000. Mapping was conducted on field sheets using topographic base maps from the Instituto Geografico Militar of Bolivia, as well as Bing Aerial Imagery overlain with a 20 m topographic contour interval generated in ArcGIS using 90 m SRTM data. Map data were collected principally on the roads connecting the towns of Villamontes, Tarija, Tupiza, and San Vincente, with ancillary transects focused near Tarija and Tupiza. In the Chaco foreland, published seismic reflection data were extrapolated to the surface. In addition, the new mapping was incorporated with published geologic maps in specific areas that were not accessed in this study. ArcGIS was used for compilation of published maps and digitization of field sheets, which provided data for a regional balanced cross section. The cross section was constructed using field data projected from east-west mapping traverses, which are oriented perpendicular to the regional strike of thrust faults and folds. Because field data could not be collected in an uninterrupted transect, map data were projected from six across-strike traverse segments. Each segment contains a high density of field measurements, and the segment breaks were placed at stratigraphic or structural contacts that are continuous along strike. The deformed and corresponding restored cross sections are displayed at 1:200,000 scale. The deformed and restored sections were hand-drafted simultaneously, by measuring bed line lengths in individual thrust sheets on the deformed section, and drafting the corresponding restored thrust sheet by matching bed lengths and maintaining unit thicknesses. Viability of the cross section is established by conserving slip across the thrust belt and balancing basement shortening with upper crustal shortening. The total shortening estimate for the retroarc is 337±40 km (36 ±3%).