Funding Agency: National Science Foundation

Abstract:
The nature of the stratigraphic signature of orogeny continues to be a subject of debate, and in many cases stratigraphic estimates for the onset of orogeny lag significantly behind estimates based on structural, metamorphic, and geochronological data. As the type-example of a high-elevation retroarc thrust belt, the central Andes provide an opportunity to test conflicting models for synorogenic stratigraphic evolution. This project addresses a fundamental aspect of Andean orogeny - the timing of initial mountain building owing to crustal shortening and thickening. To this end, a team of researchers from the University of Arizona, the University of Texas, and the University of Wyoming in collaboration with Argentinean scientists are documenting the evolution of Paleogene basins in northwestern Argentina. Three models for basin evolution are being tested: (1) post-rift thermal subsidence, (2) foreland basin subsidence, and (3) intermontane basin development during tectonic disruption of the Andean foreland. Each model makes specific predictions about depositional environments, sediment distribution patterns, provenance, basin architecture, subsidence, and thermal history. The research team is collecting sedimentological data (lithofacies, thickness, paleocurrent directions, provenance) and chronological data (40Ar/39Ar tuff ages, palynology, U-Pb zircon and apatite ages, and apatite fission track ages) from Paleogene strata in a roughly 30,000 square kilometer region. Expected products of the research include a comprehensive dataset on the Paleogene stratigraphy of northwestern Argentina, and a concerted application of two new methods (detrital zircon and apatite U-Pb geochronology) to early Cenozoic tectonic problems in the central Andes. Moreover, the results will directly address the issue of how to read orogenic signatures in the stratigraphic record.

The Andes are widely regarded as the type area for mountain belts formed in response to subduction of oceanic lithosphere beneath continents and models based on the Andes are regularly used to understand the development of ancient mountain belts around the globe. Nevertheless, a fundamental aspect of Andean orogeny - the age of initial mountain building owing to crustal shortening and thickening -remains controversial. Through application of new analytical techniques and interpretation of the sedimentary record, this project would resolve this controversy for the Central Andes. The project would provide geologists with new methods to determine the timing of major mountain building events in the geological past.