Transition from basement-involved to thin-skinned thrusting in the Cordillera Oriental of southern Bolivia

The eastern flank of the Central Andes in southern Bolivia and northern Argentina is underlain by a fold-and-thrust belt of Neogene age. It comprises an external thin-skinned segment with basal detachment in Silurian shales (Subandean Ranges) and an internal segment where units as deep as Precambrian metasedimentary and igneous rocks are involved in thrusting (eastern Cordillera Oriental). Between the Cordillera Oriental and the Subandean Ranges an intermediate segment (Interandean Zone) intervenes north of the Argentinian-Bolivian border (22 degrees S). The structure of the Interandean Zone is characterized by folded and thrust-faulted Cambrian to Triassic strata. Balanced cross sections across the western Subandean Ranges, the Interandean Zone, and the eastern Cordillera Oriental are based on previously available data and recent field work. They suggest that the geological structure can be interpreted in terms of two major basement-involved thrusts, here referred to as Interandean and Subandean thrusts. The more internal Interandean thrust caused the development of broad basement anticlines in the eastern Cordillera Oriental. However, at shallower levels, its displacement was accommodated by thin-skinned folding and thrusting in the Interandean Zone. In-sequence activation of the Subandean thrust uplifted the Interandean Zone on top of an imbricate basement sheet and caused the development of the thin-skinned Subandean Ranges farther east. Published radiometric data and subsidence history of the Subandean foreland basin suggest that the Interandean thrust was active between about 10 Ma and 5 Ma. The Subandean thrust is still active. Southward displacement transfer from the Subandean thrust onto the Interandean thrust and a southward dipping lateral ramp of the Interandean thrust result in the gradual transition from the Interandean Zone north of 22 degrees S to the Cordillera Oriental of northern Argentina. A similar but larger-scale transfer may be involved in the southern termination of the thin-skinned Subandean Ranges of Argentina at 23 degrees S. Shortening in the western part of the thrust belt is of the order of 80-90 km. Total shortening in the Eastern Andean thrust belt from the undeformed foreland to the eastern margin of the Cordillera Oriental is 140-150 km.