GEOMETRIC AND EXPERIMENTAL-STUDY OF A PIGGYBACK BASIN - EXAMPLE OF THE ALTO-BENI SYNCLINORIUM (CENTRAL ANDES)

Regional study of the northern Subandean Zone of Bolivia provided evid ence of a Neogene piggyback basin - the Alto Beni basin and allowed to define its structural geometry and its sedimentary environment. This basin corresponds to a synclinorium Filled with 7 000 m of continental synorogenic sediments. The construction of a balanced cross section s hows that the basin was carried piggyback on a major thrust sheet, and in front of a duplex. A synsedimentary analog sandbox experiment, sui ted to the regional context of the study, permitted us to explain the genesis and the structure of this type of piggyback basin. The model w as performed in a normal gravity field, and was gradually shortened in the deformation box Structurator, designed to use the X-ray tomograph y technique. Acquisition of tomographic images of serial cross section s and ''time'' cross sections made possible a geometric and kinematic analysis of the deformation. In the experiment, the piggyback basin de velops in the core of a syncline which is structuring on the roof thru st of the duplex. The development of the syncline borders synchronizes with the duplex shortening which constitutes the ''motor'' of the bas in formation. The modelling shows how the erosion and sedimentation pr ocess control the geometry of some faults, and the sequence of thrusts . The Alto Beni basin constitutes a natural prototype of piggyback bas in whose deep structure and development can be directly compared to an analog modelling. Its mechanism of development should be applied to o ther natural examples.