This is an ambitious project that has the potential to fill in important gaps in the overall picture of orogenesis in the central Andes, and of convergent-margin tectonism in general. The project is constructed around a well defined basic-science question, did the Andes rise in a rapid pulse, or did they rise gradually? Producing elevations and crustal thicknesses of the magnitude found in this study area remains a key problem in continental tectonics.
This question provides a foundation from which the PIs develop a variety of linked projects, including: 3-D structural analysis of fold-thrust belt shortening in the Andes, testing of new methods of paleo-elevation analysis, use of seismic studies to characterize the roots of the range (both in the deep crust and in the underlying mantle), creative use of petrologic and isotopic data to constrain thickened crust at times in the past. The project has the potential to address 3-D mass balance issues during orogeny, as well as the impact of a rising mountain belt on continent-scale weather systems. Of note, to put the analysis of orographic weather studies in context, the PIs will also undertake a broader paleo-climate study. All of the questions to be studied are current and important, and are of interest across traditional disciplinary boundaries and, the research strategy as outlined has a high potential to answer the questions that it poses.
Intellectual Merit. This project addressed current debates over the timing of uplift in the Central Andes of South America by focusing on the sedimentary and tectonic history of southern Peru and Bolivia. This is a critical region where geologic records have suggested a long-term, apparently continuous history of crustal shortening (roughly 40-0 Myr (million years) ago) that strongly conflicts with recent evidence for rapid surface uplift in the late Neogene (largely focused at 10-5 Myr ago). The project integrated sedimentology, stratigraphy, basin analysis, magnetostratigraphy, structural geology, geochronology, and thermochronology to assess the evolution of hinterland and foreland regions of southern Peru and Bolivia, including the Western Cordillera, Altiplano, Eastern Cordillera, and Subandean Zone. Emerging techniques include a new paleoaltimeter employing geochemical signatures preserved in volcanic glasses, which are abundant in the Central Andes. The stable isotopic results for deuterium have proven instrumental in assessing past elevation change within regions that are currently situated above 3800-4000 m above sea level. In this case, results from southern Peru suggest rapid surface uplift in early Miocene time, such that the Western Cordillera were attained by roughly 19-16 Myr ago. In addition, our results have demonstrated the utility of detrital zircon U-Pb geochronology in defining the precise timing of faulting in elevated hinterland regions dominated by proximal basin fill. Constraining the depositional (stratigraphic) ages of such coarse-grained, proximal deposits is notoriously difficult, largely due to the lack of interbedded volcanic rocks and well-preserved fossils. However, our results show the power of U-Pb dating of proximal sandstones with a significant component of volcanic detritus. In this case, U-Pb ages of detrital zircon grains show the timing of displacement along several thrust faults. When integrated with paleomagnetic data (magnetic polarity stratigraphy), this age control shows several phases of extremely rapid sediment accumulation, which appear to correlate with phases of fault displacement along two thrust faults. This correlation suggests that upper crustal processes are sufficient to explain rapid basin growth, without the need for deeper crustal/lithospheric processes, such as rapid removal of lower lithosphere. Broader Impacts. Methods. The project employed established sedimentologic, stratigraphic, structural, magnetostratigraphic, and geochemical techniques and developed additional laboratory methods for collection of stable isotopic and geochronological data. Several novel aspects include the utilization of (a) hydrogen stable isotopic (deuterium) data to evaluate changes in paleoelevation and (b) U-Pb geochronology of detrital materials to assess precise ages of deposition for growth strata and associated upper crustal structures. Personnel and Training. This project provided training for one postdoctoral associate, two graduate research assistants, and over ten undergraduate researchers who received hands-on laboratory and field training in sedimentary geology, paleomagnetism, U-Pb geochronology, and U-Th/He thermochronology laboratories. At the University of Texas at Austin, both postdocs (Joel Saylor, N. Ryan McKenzie) and PhD students (Nicholas Perez, Amanda Calle) gained valuable research experience as well as an introduction to mentoring with their interactions with the undergraduate students who worked on this project. During the course of this proposal, postdoc Saylor started a tenure-track faculty position at the University of Houston. Graduate assistant Perez also successfully obtained a three-year NSF Graduate Research Fellowship. The undergraduate students from UT-Austin as well as field assistants from Peruvian and Bolivian universities obtained instruction in field-work fundamentals for sedimentary and structural geology. Partnerships. The project strengthened collaborative ties with international partner Ecopetrol-ICP (Instituto Colombiano del Petróleo), a research laboratory within the national oil company of Colombia. Ecopetrol and UT Austin personnel have been involved in a scientific exchange program designed to provide mutual access to research facilities and local experts in Colombia and the USA. Ecopetrol personnel have helped provide access to restricted sites, helped in obtaining maps and aerial photographs, and offered discussions of local and regional geology, as well as logistical insights regarding safe travel in different parts of the country. Outreach. PI Horton has been involved in workshops targeted at prospective and incoming graduate students as part of the Academic English Program administered by the UT-Austin International Office. Graduate research assistant Perez and postdoc Saylor have been involved in science outreach events at several local elementary and middle school and has led activities introducing students to geology, sedimentology, and tectonics.
