EAR-PF Continuous vs. mixed-mode deformation during construction of the southern central Andes: Investigating the spatiotemporal scales of Cordilleran tectonic regimes

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Dr. Chelsea Mackaman-Lofland has been awarded an NSF EAR Postdoctoral Fellowship to conduct research, professional development, and educational activities at the University of Connecticut (UConn) under the mentorship of Professors Julie Fosdick and Michael Hren. This project investigates the spatial and temporal scales of crustal deformation during construction of the southern central Andes (~30°S) in South America. Project results will improve scientific understanding of the forces and geodynamic mechanisms driving subduction-related mountain building. The Andes define key examples of mountain building during the subduction of oceanic crust beneath a continental plate, and are integral to Earth systems including ocean/atmospheric circulation patterns, biological diversity, and natural hazard and resource distribution. At ~30°S, contrasting models of Andean mountain building invoke continuous crustal shortening vs. episodic shortening, extensional, and possibly neutral conditions in the overriding plate. This project integrates structural mapping, basin analysis, geo/thermochronology, and paleoaltimetry of bedrock and hinterland basin fill at ~30°S to (1) resolve conflicting shortening, extensional, and/or neutral deformation mode interpretations and (2) quantify the timing, magnitude, and rates of deformation, crustal evolution, surface uplift, and establishment of the Andean Cordillera. Research and professional development will involve geological field training for Argentine undergraduate students and training for an undergraduate thesis project at the host institution. The UConn undergraduate will be recruited through the newly funded GEOPAths internship and career-mentoring program, which emphasizes the recruitment and retention of underrepresented student groups. Project results will be incorporated into a field trip for Argentine and Chilean students and activities for the “Mountains, Rivers, and Carbon” and “Paleogeography, Tectonics, and Global Climate” educational outreach programs at UConn. Subduction-related mountain building has long been attributed to continuous shortening and thickening of the overriding plate following margin-scale changes in plate convergence. However, an increasing body of structural, stratigraphic, and magmatic evidence points to episodic deformation, with potentially regional shortening, extension, and/or neutral regimes during the construction of Cordilleran orogens. Along the transition from central to southern Andes (~25−35°S), upper-plate deformation involved an apparent shift from continuous shortening to mixed-mode deformation. Characterizing the nature of this transition is fundamental to understanding the timing, tempo, and spatial continuity of convergent margin processes, but existing reconstructions invoke conflicting tectonic conditions most apparent along a single orogenic transect at ~30°S. This research seeks to resolve conflicting deformation mode interpretations and provide geographic and timing parameters with which to interrogate potential regional or plate-scale mechanisms controlling Cordilleran deformation. Combined zircon Lu-Hf (Lutetium-Hafnium) and U-Pb (Uranium-Lead) geochronology, low-temperature thermochronology, and stable isotope paleoaltimetry analyses will establish quantitative relationships between deformation, exhumation, crustal thickening, and surface uplift. Reconstruction of precursor deformation and sedimentation history will provide a framework with which to investigate the influence of inherited tectonic regimes and basin architectures on Neogene growth of the modern Andes.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.