| Popis: |
The origin of the major Andean orocline in Bolivia, where the north trending southern Andes abruptly change direction by 45° and continue northwestward into Peru, has been investigated by photogeologic techniques utilizing Landsat imagery and Shuttle Imaging Radar (SIR-A) in conjunction with published literature and maps. The evidence suggests that this orocline might have been formed, beginning in the Miocene, by the Pacific coastal region being bent around the resistant Brazilian Shield as a result of the relative motions between the eastward-moving coastal region and the westward-moving South American continent. The eastward motion of the coast was further increased about 10 m.y. B.P. when the opening of the East Pacific Rise increased the rate of eastward motion of the subducting Nazca Plate. The Brazilian Shield, with its deep roots, provided greater resistance to this motion, resulting in the decoupling of the Andes along a wide, left-lateral, strike-slip fault zone extending east-west from the Altiplano, through Santa Cruz and into the interior. Indicated left-lateral motion across this fault decreases from 180 km on the east side of the Andes to 50 or 60 km in the Altiplano, the difference being taken up in compressing the Eastern Cordillera and sub-Andean Foredeep from an average width of 350 km in southern Bolivia and Argentina to an average width of 220 km in northern Bolivia and Peru. The resistance of the Shield may also have resulted in counter-clockwise rotation of, and lateral angular shear in, the Andes north of the break zone. |
