Deformation History of the Susques Basin (~23°S, 66°W), Puna Plateau, NW Argentina: New Constraints by Apatite (U-Th)/He Thermochronology and 40Ar-39Ar Geochronology

New geochronologic and thermochronologic data from the Susques basin in the central Puna plateau of northwest Argentina provide information about the timing of deformation and exhumation in this region. Seventeen samples for 39Ar/40Ar geochronology were collected in the Susques basin and the adjacent Sierra del Tanque. These include ashes in deformed and undeformed sections, as well as reworked units and pristine, undisturbed units. Single grain analyses of locations previously dated by multi-grain analyses or whole-rock K-Ar methods reveal the presence of several vastly different age populations within reworked samples, forcing a re- assessment of existing geochronology in this area of the Puna. The oldest undeformed ash yields an age of 6.59 ± 0.027 Ma, indicating that deformation ceased prior to this time. Samples for Apatite (U-Th)/He dating were collected from 2 transects in the basin-bounding Sierra del Tanque and Sierra de Cobres. Helium ages range from ~47-29 Ma in the Sierra del Tanque, and from ~32-21 Ma in the Sierra de Cobres. Geochronology from the ashes intercalated within deformed and undeformed basin sediments and (U-Th)/He thermochronology from the two transects suggest that: 1. Deformation in the Susques basin, which occurred primarily by thrust and reverse faulting, ceased by ~6.5 Ma, and 2. Less than 2 km of exhumation has occurred over the past ~20 Ma, and only ~500 m of exhumation occurred between 20-50 Ma. The cessation of tectonic activity was rapidly followed by the deposition of large-volume, undeformed ignimbrites between ~6.5 Ma and 900 ka. No balanced structural sections exist in this area. However, the limited exhumation documented by the (U-Th)/He system suggests that regional deformation was also likely limited. Since significant amounts of shortening are not recorded in the unroofing history of the basin-bounding ranges, a mechanism other than recent distributed crustal shortening must be invoked to explain the high elevation and thick crust of this area of the Puna plateau. Our observations, combined with the presence of voluminous, late Cenozoic ignimbrites, suggest that a mechanism that involves addition or removal of material from the lower crustal and/or mantle lithosphere may play a significant role in creating the high topography of this area.