LATE CENOZOIC TECTONIC EVOLUTION OF THE PUNA PLATEAU AND ADJACENT FORELAND, NORTHWESTERN ARGENTINE ANDES

Kinematic analysis of approximately 1500 fault-slip measurements from the Puna plateau and adjacent foreland of northwestern Argentina suggests that two regional kinematic regimes characterize late Cenozoic deformation: a thrust phase with approximately WNW-ESE shortening and subvertical extension followed by a strike-slip phase with approximately ENE-WSW shortening and approximately NNW-SSE extension. Radiometric dating combined with field relationships demonstrate that thrust faulting started by 13 Ma and lasted, at least locally, until 1 Ma, and that strike-slip faulting started by 2 Ma and is still active. The shortening direction of the thrust phase, which accounts for most of the Andean shortening, differs from the coeval plate tectonic convergence direction and probably cannot be explained by later oroclinal bending. Paleostructural control of deformation kinematics and/or strike-slip faulting along the thermally weakened volcanic arc might explain the Mio-Pliocene shortening direction. The subhorizontal extension direction of the strike-slip phase is evident at all elevations studied, suggesting that local body forces do not drive it. A decrease in South America-Nazca plate convergence rate and/or complex three-dimensional effects, possibly including kinematic variation with depth in the crust, might provide a satisfactory explanation.