Style and history of Andean deformation, Puna plateau, northwestern Argentina

Topographically, the Puna plateau of northwestern Argentina is the southern continuation of the Bolivian Altiplano. Its thickening and consecutive upl ift result from the Andean orogeny. To better constrain the structural styl e and its progressive development, we have studied field data, topographic and satellite imagery, balanced cross sections, seismic reflection data, ki nematic analysis of fault slip data, anisotropy of magnetic susceptibility (AMS), paleomagnetic data, and apatite fission track (AFT) data. Across the Puna plateau, Precambrian and Paleozoic basement ranges, bounded by high-a ngle reverse faults (dips greater than or equal to 60 degrees), alternate w ith Cenozoic intermontane: basins. Major thrusts trend NNE-SSW and do not s how a preferred vergence. Intermontane basins have various degrees of symme try, depending on the geometries and attitudes of associated thrusts as wel l as on the magnitudes of their offsets. There is a close correlation betwe en the surface expression of a basin and the amount of internal deformation . A line-balanced cross section of the Puna at 25 degreesS has yielded a Ce nozoic shortening of 10-15%, in a direction subperpendicular to the orogen. By kinematic analysis of Cenozoic fault slip data we have obtained princip al directions of strain rate across the Puna. Shortening axes are subhorizo ntal and trend on average WNW-ESE (similar to N110 degrees), stretching axe s are subvertical, and intermediate axes are subhorizontal and trend on ave rage NNE-SSW. Strain ellipsoids are dominantly of plane strain type. and th ey represent dip-slip thrusting. From paleomagnetic and AMS data, shortenin g axes form a radial pattern around the eastern edge of the central Andes. The pattern is attributed to an inhomogeneous stress field. reflecting the eastward convex shape of the central Andean thrust front. From the history of burial and uplift, Andean shortening reached the northeastern part of th e Puna in the late Eocene and the adjacent Eastern Cordillera in the late E ocene or early Oligocene. This shortening was presumably due to the Incaic phase of the Andean orogeny. In the eastern part of the orogen the onset of shortening was probably guided by preexisting Paleozoic and Mesozoic struc tures, so that Andean deformation propagated unevenly eastward.