The Timing and Style of Deformation in the Susques Basin of the Puna Plateau, Northwest Argentina

The Susques basin lies in the northeast quadrant of the Puna Plateau in northwest Argentina (approximately 23°S, 66° W). Fluvial excavation of this formerly closed basin has revealed a rich stratigraphy that constrains the timing and style of local deformation. Because Cenozoic thickening of the central Andean crust likely produced surface uplift of the Puna-Altiplano plateau, the timing of shortening may be used to estimate the time at which this landform was constructed. The Susques basin is bounded on the west by east-verging high-angle reverse faults that expose granites and granodiorites, while its eastern margin is defined by a west-verging fold-and-thrust belt that generally exposes shallower crustal levels at the latitude of the town of Susques. The reverse faults trend NE-SW, while the fold-and-thrust belt is sub-parallel to this, trending ENE-WSW. Stratigraphy within the basin includes Ordovician metasediments unconformably overlain by a thick section of fine- to medium-grained Tertiary continental clastic sediments. In some portions of the basin, the over-riding fold-and-thrust belt has sheared these strata into locally disharmonic folds. Syntectonic sedimentation is recorded both in piggy-back basins on the fold-and-thrust belt, and in a progressive unconformity along the western, fault-bounded range-front. The cessation of fold-and-thrust belt deformation is marked by an unconformable, undeformed clastic sequence. Importantly, the fold-and-thrust belt bounding the eastern basin margin apparently records significantly more deformation than do the high-angle reverse structures bounding the western basin margin. The Ordovician and Tertiary units are overlain by an ignimbrite that has been dated as 6.5 Ma. Deformation can therefore be broadly constrained as having occurred in the Tertiary, prior to 6.5 Ma. Fortunately, many of the Tertiary rocks in the basin contain intercalated volcanic ashes that further constrain the ages of these units. We are currently using 39/40Ar geochronology to analyze biotites from six samples from the Susques basin. These radiometric ages will further constrain the timing of deformation within the basin.