Missing shortening in the thick-skinned retroarc thrust belt of the central Andes, northwestern Argentina, ~25°S

A very large discrepancy exists among estimated retroarc shortening magnitudes in the thin-skinned thrust belt of Bolivia and the thick-skinned thrust belt of northwestern Argentina. Fieldwork and structural analysis from this study at ~25°S latitude in northwestern Argentina confirm the presence of a mainly west verging, thick-skinned style of shortening in the region which, taken together with thermochronological data ((U-Th)/He in zircon and apatite and published apatite fission track results), imply up to 10 km of rapid, Miocene exhumation. Although these results suggest that significant exhumation occurred in the region, displacements on mapped, discrete faults are insufficient in magnitude (by ~15-20%) to generate the observed ~58 km thick crust (Yuan et al., 2002). We suggest that additional, unrecognized shortening or crustal addition is required in the region to explain the thick crust and occurred by 1) crustal flow from neighboring regions of thickened crust; 2) passive roof thrusting whereby major, likely mid-crustal shortening was fed to higher structural levels; 3) tectonic underplating of trench or forearc rocks; and/or 4) significant penetrative strain not accommodated by through going faults. We do not discard other mechanisms of crustal thickening and/or addition in the region, but magnitudes of upper crustal shortening required by excess area calculations are best explained by penetrative deformation within rheologically weak metaturbidites of the Neoproterozoic to Cambrian Puncoviscana Formation. Although penetrative strain was generally not considered in cross section restorations in the retroarc of Bolivia, a relative lack of these rheologically weak rocks there probably precludes penetrative deformation as a significant mode of deformation and may partially reconcile the discrepancy in along-strike shortening magnitudes. Significant shortening has been accommodated by penetrative strain in other orogens worldwide, including western North America (e.g., Mitra, 1994; Duebendorfer and Meyer, 2002) and southwestern China (e.g., Burchfiel et al., 2007), and may reflect an often overlooked yet fundamental style of crustal deformation.