Inherited weaknesses control deformation in the flat slab region of Central Argentina

Abstract:

The Sierras Pampeanas region of west-central Argentina has long been considered a geologic type-area for flat-slab induced thick-skinned deformation. Frictional coupling between the horizontal subducting plate and South American lithosphere from ~12 Ma to the present provides an obvious causal mechanism for the basement block uplifts that characterize this region. New low temperature thermochronometry data show basement rocks from the central Sierras Pampeanas (~ longitude 66 ̊ W) including Sierras Cadena de Paiman, Velasco and Mazan retain a cooling history of Paleozoic - Mesozoic tectonics events. Results from this study indicate that less than 2 km of basement has been exhumed since at least the Mesozoic. These trends recorded by both apatite fission track (AFT) and apatite helium (AHe) thermochronometry suggest that recent Mio-Pliocene thick-skinned deformation associated with flat-slab subduction follow inherited zones of weakness from Paleozoic terrane sutures and shear zones and Mesozoic rifting. If a Cenozoic foreland basin exisited in this region, its thickness was minimal and was controlled by paleotopography. Pre-Cenozoic cooling ages in these ranges that now reach as high as 4 km imply significant exhumation of basement rocks before the advent of flat slab subduction in the mid-late Miocene. It also suggests that thick-skinned deformation associated with flat slab subduction may at least be facilitated by inherited crustal-scale weaknesses. At the most, pre-existing zones of weakness may be required in regions of thick-skinned deformation. Although flat-slab subduction plays an important role in the exhumation of the Sierras Pampeanas, it is likely not the sole mechanism responsible for thick-skinned deformation in this region. This insight sheds light on the interpretation of modern and ancient regions of thick-skinned deformation in Cordilleran systems.