Collaborative Research: Shear Zone and Detachment Formation during Syn-convergent Extension in the Andes: Cordillera Blanca, Peru

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The dominant process in mountain building is shortening and thickening of the Earth?s crust across relatively narrow orogenic belts where tectonic plates converge. During the past two decades there has been growing appreciation of the importance of crustal extension occurring within mountain belts at the same time as shortening. Examples of such syn-convergent extension have been identified in active mountain belts including the Himalaya and Andes and many researchers have also found evidence for similar processes in rocks that formed in ancient mountain belts. The goal of this project is to document the mechanisms of ongoing syn-convergent extension within an active fault system that bounds the southwestern margin of the Cordillera Blanca in the Peruvian Andes. Data collected on this modern structure will help researchers interpret evidence for syn-convergent extension from ancient orogenic events as well as modern mountain belts. The Cordillera Blanca is the ideal structure to study these processes because extension is very recent ( 6 million years), the regional tectonic regime has remained fairly constant during the evolution of the fault and the rocks in the footwall are similar along the entire 200 km length of the detachment. These factors will facilitate comparison of data from different parts of the detachment and allow us extrapolate processes and rates of deformation into the past. Syn-convergent extension can be broadly divided into two general types according to the direction of extension relative to the mountain belt: orogen parallel extension involves stretching of the crust parallel to the trend of the mountain belt (perpendicular to the direction of shortening); transverse extension involves stretching across the trend of the mountain belt (parallel to the direction of shortening). Orogen parallel extension generally occurs on faults oriented perpendicular to the mountains while transverse extension occurs on structures parallel to the mountain belt. Transverse extension poses something of a conundrum: How can the crust shorten and extend at the same time and in the same direction? The answer seems to involve a partitioning of shortening and extension at different levels in the crust. By studying the evolution of the Cordillera Blanca through time as it is exhumed we will attempt to elucidate the conditions that allow extension while overall shortening continues across the Andes. This project will focus on shear-zone-scale processes including: (1) How have the thickness, strain rate, differential flow stress and temperature evolved during exhumation to shallower crustal levels, and (2) How is deformation on the master detachment related to slip on seismically active structures? By carefully integrating field mapping of structural geometry, kinematic studies, analysis of deformation mechanisms, temperature estimates and paleopiezometry, we hope to reconstruct a profile showing how the strength and mechanical behavior of the shear zone varied at different depths during deformation and exhumation of the Cordillera Blanca. Our research will provide results that will help researchers to better understand how extensional faults evolve during crustal extension in modern and ancient mountain belts. The research represents a collaborative effort between researchers at the University of Kentucky Knoxville and Montana State University. In addition to the scientific goals of the project, the project is supporting the training of graduate and undergraduate students in an STEM discipline and contributing to public outreach efforts based on the research outcomes.