This project is providing new information on how thickened lower crust in mountain belts extends and thins during orogenic collapse. Field studies and numerical models demonstrate that the strength, thickness, and mechanical properties of the lower continental crust are key factors that influence the behavior of the entire lithosphere during orogenesis, including the localization of strain into shear zones. However, there still is uncertainty about how these properties change during orogenic cycles and how they influence patterns of deformation. Fiordland, New Zealand provides a natural laboratory for resolving these questions because it exposes over 3000 square kilometers of ancient Mesozoic lower crust. This project is determining the time scales, physical dimensions, and effects of mafic-intermediate magmatism, crustal melting, metamorphism, and deformation on the evolution of this lower crustal section. The work integrates field mapping with three-dimensional analyses of deformation, pressure-temperature-time-deformation paths for rocks, and uranium-lead and samarium-neodymium isotope geochronology. These tools are helping to resolve the thermal and mechanical structure of Fiordland's lower crust over 25 million years and at depths of 35-50 kilometers during a transition from shortening and crustal thickening to extension and crustal thinning. They also are revealing the changing nature of strain localization processes, including the formation of a symmetric style of extensional faults that border domes of high grade metamorphic rocks.
The central hypothesis of this research is that variations in temperature, composition, strength, and crustal thickness, resulting from magmatism and metamorphism, created a patchwork of large (1000 square kilometers) relatively undeformed regions separated by weak deforming regions. Strong regions resisting deformation develop shortly after the emplacement of the youngest plutons. Initially, extension was broadly distributed in diffuse shear zones that reflected the large size of weak areas. Later, rapid cooling of the igneous rocks and removal or crystallization of melt strengthened large sections, changing the location and size of weak areas. During cooling, the dry cores of plutons became stronger than the hydrous, quartz-rich host rocks, resulting in the migration of deformation out of pluton interiors and into softer host rock.
The research supported here is part of a collaborative project between scientists at the University of Vermont and the University of Alabama Tuscaloosa. In addition to the research objectives, the project is supporting graduate and undergraduate student training in a STEM discipline at both institutions. The students are involved in all aspects of the project, and are gaining experience from using state-of-the-art analytical facilities from a variety of sources, including those of their host institutions, the University of North Carolina Chapel Hill, and the U.S. Geological Survey. The project is enhancing collaborations among U.S. and New Zealand scientific institutions. A strong partnership with New Zealand scientists provides access to unpublished data and sample archives resulting from a recent program to map Fiordland at a 1:250,000 scale. Student participation in these collaborations is helping to advance discovery and strengthen these partnerships. Students are benefiting from cultural and scientific exchanges and are gaining important experience in some of the best research facilities. Research results are being integrated into classroom curricula, disseminated via presentations at professional science meetings and the peer-reviewed geologic literature, and resulting data is being archived in a variety of community databases.
