The earth’s continental crust is generally thought to be created in subduction zone settings where one of the two colliding plates subducts and molten rock (or magma) is produced from melting of the earth’s interior. Subduction zones play an important dualistic role in human society. On the one hand they are zones of destruction typified by volcanic eruptions and related geologic hazards such as earthquakes, landslides, lahars, ashfalls and pyroclastic flows. On the other hand, subduction zones are beneficial to society because they are regions where hydrothermal fluids provide geothermal energy, precious ore minerals are mined, and fertile volcanic soils are used to grow crops. One of the outstanding questions in geology centers on how magmas form in subduction zones and how the tempo, or rate, of magma production varies through time and space. The answers to these questions ultimately lead to a better understanding of how continental crust forms in subduction zones and this is the central theme of this project.

Understanding how continental crust is created in subduction zones requires examining the ‘deep plumbing system’ of a magmatic arc because this is where magmas are thought to form and become chemically modified. We investigated a well-exposed section of >3000 km² of Mesozoic middle and lower crust in Fiordland National Park, New Zealand. In this area, faulting related to movement along the Alpine fault exposed a unique deep-crustal cross-section of a Phanerozoic magmatic arc. In the course of our study, we produced new radiometric ages (U-Pb zircon, titanite and rutile), whole-rock geochemical and isotope analyses, and zircon O- and Lu-Hf isotope analyses with the goals of understanding the tempo of magmatism and metamorphism, and sources involved in the production of magmas. These data are now published in 9 peer-reviewed articles, >30 abstracts presented at national/international meetings, and 7 California State University MS student theses.

In our investigation, we found that the lower crust of Zealandia (earth’s newest continent) formed during a surge of mafic to intermediate magmas that were emplaced during a 10 m.y. period from 124 to 114 Ma. This surge of magma signified a major transition in magma chemistry and was characterized by zircon oxygen isotope values that signified production from the underlying mantle (rather than remelting of existing crust). Metamorphic titanite dates from the lower crust also indicate that magma emplacement was associated with high-temperature conditions (900-750ºC) and the lower crust remained hot (>650ºC) for more than 15 Myr after the cessation of magmatism. Despite these high-temperature conditions, our data indicate that existing crustal rocks did not contribute significantly to the chemistry of the magmas. The surge of magmatism in Zealandia was also found to be linked to (1) transpression and regional thrust faulting from ca. 130 to 105 Ma, (2) crustal thickening from 128 to 116 Ma, and (3) continent-ward migration of the arc towards Gondwana and away from the trench. We interpreted these features as indicating a major transition in subduction zone dynamics likely resulting from flattening of the slab and/or changes in subduction zone geometry like a slab tear. Our observations also strongly support production of magmas from the underlying mantle with only limited contributions from pre-existing crust. This interpretation is novel because workers in other arcs (e.g., southern and central Sierra Nevada, North America) have previously proposed that magmatic surges such as the one in Zealandia were triggered by intra-crustal melting of hydrous mineral assemblages in the lower crust. Instead, our data document another end member scenario whereby large volumes of continental crust can be generated by ‘mantle-driven’ surges with little input from older continental crust.

In addition to findings related to continental crust production, this project also supported undergraduate students at California State University Northridge (CSUN) through the newly developed ‘ROCs’ program (Research Opportunities for CSUN students). CSUN is certified by the U.S. Department of Education as both a Hispanic Serving Institution and an Asian American Native American-Pacific Islander Serving Institution. In the College of Science and Math, 42% of undergraduate students are ethnic minorities, and 33% are ethnic minorities in the Department of Geological Sciences. Goals of the ROCs program are to 1) encourage aspiring CSUN geology undergraduate students, especially minorities and females, to pursue research projects with faculty mentors, 2) broaden students’ understanding of career and graduate school opportunities in the geosciences, and 3) create a sense of community among underserved students with the goal of increasing retention in geoscience. Since the beginning of the program in 2014, the ROCs program has mentored >50 students, and all ROCs participants have successfully completed B.S. Geology degrees. In addition, over 90% of former ROCs participants have careers in geoscience or are enrolled in M.S. or Ph.D programs.

Last Modified: 08/18/2020
Modified by: Joshua Schwartz