Active shortening and intermontane basin formation in the central Puna Plateau: Salar de Pocitos, NW Argentina (24°37’S, 67°03’W)

STRECKER, MR., BOOKHAGEN, B. AND ALONSO, RN.

SAN FRANCISCO

Congreso; AGU Fall Meeting; 2012

AMERICAN GEOPHYSICAL UNION

Similar to other Cenozoic orogenic plateaus, extensional tectonics associated with mafic volcanism typifies the Altiplano-Puna of the southern Central Andes, while the flanks of the plateau and adjacent forelands experience shortening. Extensional tectonism in the plateau region since the late Miocene has been explained with delamination of lithospheric mantle. In contrast, here we report on protracted shortening in the western interior of the Puna Plateau of Argentina from the Tertiary to the present-day. With average elevations of about 3.7 km the Altiplano-Puna is a first-order morphotectonic province of the southern central Andes and constitutes the world’s second largest orogenic plateau. With few exceptions the Andean plateau consists of internally drained, partly coalesced sedimentary basins that are mainly bordered by reverse-fault bounded ranges, 5 to 6 km high. The basins contain continental evaporites, volcanic and clastic deposits, typically between 3 and 5 km thick and record protracted sedimentation since the Eo-Oligocene. While there are many unifying basin characteristics in this region, including internal drainage, semi-arid to arid climate and associated deposition of evaporites, there are notable differences between the northern and southern parts of the plateau. The vast Altiplano basin of Bolivia characterizes the northern plateau region, whereas the Argentine Puna to the south comprises numerous smaller basins that reflect continued comparmentalization by the combined effects of tectonism and volcanic activity. The N-S oriented Salar de Pocitos basin is the vestige of a formerly contiguous sedimentary basin within the Puna interior. Unlike many other basins in this region it is bordered by the limb of an anticline developed in Tertiary sedimentary rocks on the west, while the eastern border is a reverse-faulted range front. To the north and south the basin is closed by transverse-oriented volcanic edifices. Evidence for sustained contractional tectonic activity exists along the western basin margin. Fanning of dipping strata and regraded, inclined gravel-covered pediment surfaces as well as wind gaps associated with gravel derived from a basement mountain range about 15 km farther west document late Tertiary to Pleistocene growth of the basin-bounding, approximately N-S oriented and N plunging anticline. Late Pleistocene and Holocene lake shorelines and lacustrine deposits are also tilted eastward along the same structure. In addition, INSAR measurements of deformed lake terraces in the Pocitos basin clearly document that the fold is growing and that the basin continues to be asymmetrically deformed. In light of ongoing basin-wide shortening in the Pocitos region the kinematic changeover from shortening to extension in the Puna must be highly disparate in space and time.