Dr. Julie Fosdick has been awarded an NSF Earth Sciences Postdoctoral Fellowship to carry out a research and education plan at the University of Arizona, Tucson. This project aims to quantify temporal changes in crustal thickening and thrust belt exhumation along the central Andes of northwestern Argentina by employing bedrock and detrital geo-thermochronology of the Bermejo Basin and Precordillera fold-thrust belt. Results from this study will also characterize the source-sink thermal history of Tertiary foreland and intermontane basin strata. Findings will be evaluated in the larger regional context to elucidate the spatial pattern, timing, and rates of foreland exhumation and deformation in the central Precordillera of northwestern Argentina to investigate along-strike changes in structural style and basin evolution within the Central Andean orogen.
Convergent plate tectonic systems are represented by impressive mountain belts throughout the world, forming significant topographic features that provide sediment to the world's basins. However, existing models for continental deformation along subduction zones do not sufficiently explain the diversity and magnitude of tectonic structures, motivating the need to better understand the geologic processes that modulate the development of orogenic belts. These are also important processes to understand because they are linked to seismic and volcanic hazards and the distribution of natural resources. The project includes a detailed education and outreach program at the University of Arizona and the University of Buenos Aires, Argentina. Two upper-division and graduate level seminars will be taught by the fellow: Exhumation and Controlling Mechanisms in Cordilleran Orogenic Systems (Year 1, UA), and Thermochronology Applied to Tectonic Problems (Year 2, UBA). Lastly, the postdoctoral fellow will mentor student research projects to broaden her teaching and mentorship experience in preparation for an academic career in the geosciences.
Major mountain belts in convergent settings form due to the complex interaction between thickening of the Earth’s lithosphere due to deformation and erosional removal and redistribution of rocks into sedimentary basins. While these processes are generally well-understood in subduction zone settings such as the Andes Mountains of South America, existing models for continental deformation do not sufficiently explain the diversity and physiography of tectonic structures that occur along the mountain belt. The central Andean orogenic belt is composed of multiple tectonic domains with widely variable subducting plate geometries, lithospheric structure, deformational style, sedimentation patterns, physiography, and climate. The Argentine Precordillera fold-and-thrust belt in the Central Andes is an important player in Andean tectonics because it records the structural and exhumational response to a changing tectonic regime during the transition from steep to flat subduction of the Nazca plate beneath the South American Plate. Presently, the Argentine Precordillera and Sierras Pampeanas region (30-32 °S) is characterized by high seismicity and recent large magnitude earthquakes. The primary aim of this project was to characterize spatial patterns and rates of rock exhumation (movement of rocks toward the Earth’s surface) across the Argentine Precordillera in order to improve our knowledge of how different tectonic styles of deformation control rock exhumation and basin sedimentation patterns. The research and education activities associated with this project included two field campaigns, extensive geo-thermochronologic analysis, thermal modeling, and training and mentorship opportunities for NSF Earth Science Postdoctoral Fellow Julie Fosdick at the University of Arizona. This work was completed in order to evaluate the erosional response of the Argentine Precordillera to changes in plate interactions and test relationships between faulting and exhumation across different structural styles. A second goal of this project was to investigate the age and stratigraphic significance of the pre-foreland basin deposits in the genetically linked Bermejo foreland basin. The most significant findings of this research have been a regional assessment of the long-term exhumation history across the Argentine Precordillera. Our low-temperature thermochronology data constrain rock cooling through ~70-110 °C and show that the locus of exhumation correlates with timing of faulting during eastward growth of the mountain belt. These results also show that the highest magnitude of Pliocene (~ 4 to 2 million years ago) exhumation is concentrated along the most seismically active part of the region along the boundary between the Argentine Precordillera and the adjacent Sierras Pampeanas basement uplift province. The second primary contribution from this research is our characterization of early Cenozoic sedimentary strata beneath the classic foreland basin deposits of the Bermejo Basin. These strata were previously mapped as Permian units, indicating a substantial period of missing time in the sedimentary rock record prior to Oligocene-Miocene sedimentation in the foreland to the rising Andes. Our findings show that the continental redbed succession is in fact Eocene in age and should be interpreted within the context of an earlier foreland basin system in the Central Andes. Scientific findings from this project offer new insight into the exhumational behavior of fold-and-thrust belts characterized by multiple structural styles and are of broad interest to the community of researchers working in modern and ancient contractional mountain belts. This project provided new insight into how faulting and erosion progressed during growth of the Argentine Precordillera during the last ~16 million years. This work generated new datasets on the spatio-temporal relationship between exhumation and deformation to test thrust propagation models of erosion and orogenic wedge dynamics in the Precordillera thrust-belt. Apatite thermochronology data from thrust sheets suggest that exhumation tracks eastward with the Andean deformation front and that faulting in the hinterland portions of the thrust belt and synchronous exhumation are coeval with onset of Sierras Pampeanas basement uplifts. This project supported the professional development, research activities, and training of one postdoctoral fellow and mentoring of two graduate students in the geosciences. This work also fostered new collaborations and infrastructure between numerous American and Argentine geoscientists with interests in Andean geology. Our results and findings from this project have been presented at three national and international conferences and will be published in at least three peer-reviewed journals.
