CAREER: Investigating Controls on Arc Flare-ups and the Growth of Lower Continental Crust

The proposed research program focuses on investigating the magmatic and thermal evolution of an exhumed, Cretaceous magmatic arc in Fiordland, New Zealand with the primary goal of determining the tempo and mechanisms of magmatic construction of lower arc crust. Geochronologic and geochemical data will be used to test three hypotheses: 1) magmatic fluxes into the lower and middle crust were episodic, spatially transient, and culminated in a brief, approximately 3 million year high flux event; 2) deep-crustal magmas were derived from both mantle wedge and pre-existing crustal sources and generated voluminous high-pressure eclogitic cumulates; and 3) the arc root experienced heterogeneous heating and cooling that is not easily explained by simple monotonic cooling models, but instead may be compatible with mafic underplating driven by asthenospheric upwelling beneath the lithospheric arc root. Addressing these hypotheses will be used to evaluate processes of arc root construction and modification, and to create a 4-dimensional petrologic model for the Cretaceous Fiordland arc. Magmatic arcs are the subduction factories on earth where large volumes of intermediate, or andesitic, continental crust are created. A fundamental problem in Earth sciences is that the dominant magmas generated from partial melting of the mantle in arc environments are silica poor basalts, rather than intermediate andesites. It has long been speculated that the evolution of mantle-derived magmas from basalt to andesite likely occurs in the deepest roots (25-60 kilometers depth) of arcs where fundamental processes such as fractional crystallization of dense minerals, magma mixing and assimilation of preexisting crust occur. Recently, high-precision isotopic dating of arc rocks has led to the idea that large volumes of magmas may be generated during flare-up events, characterized by intense periods of arc activity. These ideas suggest that continental crustal construction in magmatic arcs may be highly episodic and that much of the geochemical diversification of magmas that we see in modern arcs may occur during brief time periods within the roots of the magmatic arc. This project focuses on a unique, exhumed deep-crustal section of a continental arc in Fiordland, New Zealand where the deep crust is well exposed to 25-65 kilometers paleodepth. Preliminary data suggests that over 70% of the arc root was emplaced during a brief, high-flux event, and this project will allow the evaluation of the temporal and geochemical processes of continental crust construction in the deep roots of this magmatic arc.

In addition to the scientific goals of the project, it also emphasizes the integration of research with a dynamic teaching and mentoring program that aims to disseminate knowledge at all academic levels from K-12 to graduate education. California State University Northridge (CSUN) undergraduate students will participate in field and laboratory research in the United States and New Zealand. Latino minority and female students (both underrepresented groups in the geosciences and STEM) will be recruited and mentored through the 'ROCs' outreach program (Research Opportunities for CSUN students). The 'ROCs' program is aimed at early involvement and mentoring of underrepresented undergraduate CSUN students in research projects and providing guidance on graduate and geosciences career paths through workshops and weekly meetings. The PI and CSUN students will also work with the Deep Cove Outdoor Educational Trust in Fiordland National Park (a UNESCO World Heritage site) to create hands-on, geoscience activities for K-12 students from across the South Island of New Zealand. Activities and short videos highlighting unique aspects of Fiordland geology will be available on a CSUN-hosted website, YouTube and Vimeo for broad dissemination to the public.