Collaborative Research: A Seismic Study of Oceanic-Arc Crustal Construction Processes at the Archetypal Andreanof Segment of the Aleutian Arc
This project aims to understand how crust is formed in volcanic arcs above subduction zones. Crustal formation within volcanic arcs is a profoundly important process that continually replenishes the mass of the continents, whose constant weathering and erosion provide the source of elemental nutrients that drive Earth's biogeochemical cycles. Remarkably, the creation of new continental material and the recycling of material back into the mantle have remained balanced on Earth over hundreds of millions of years. Understanding this balance is key to understanding the habitability of the planet. All of Earth's crust is created from partial melts of the deeper mantle that rise upward and then freeze to form the crust. The building blocks of continental crust form when water released from a subducting tectonic plate lowers the melting temperature of the mantle, creating magma that rises and accumulates in volcanic arcs. The process of arc-crust formation has been the focus of many studies, but our understanding of these process is far from a complete. Questions persist because arc processes are complex and episodic, they occur over a range of spatial scales and depend on variables such as the age of the subducting plate and the evolutionary stage of a given arc, which may never reach a steady state. The study location of this project is the Andreanof segment of the Aleutian arc. Coincidentally, this location coincides with the epicenter of the 1957 Great Aleutian earthquake, the third largest earthquake of the last century and with the largest aftershock zone of any earthquake ever recorded. Active-source seismic techniques will be used to image the seismic velocity structure of the entire crust along and across this arc segment to address many of the unanswered questions of arc-crust formation. These same methodologies can be used to address questions related to earthquake and volcanic hazards common to volcanic arcs. The education component of this project will train a cohort of the next generation of scientists in the acquisition and advance analyses of these types of data.
Through this grant an active-source seismic experiment will be conducted in the Andreanof segment of the Aleutian arc in order to study arc-crust formation processes. Many basic questions about arc-crust formation processes remain, including the source and variability of primary arc magmas, where and how these magmas fractionate to produce the characteristic arc geochemical signatures of crustal rocks, the role of water in all of these processes, the bulk composition of arc crust and its basic architecture and similarity to continental crust, the forcing functions for all of these processes and the temporal and spatial variability of those forcings, and the mass balance of the entire system. Substantial advance can be made by focused, multi-disciplinary study of an arc segment that has simple, well known magmatic and tectonic histories, well studied geochemistry, and sufficient along-strike variability to capture many of the processes that are believed to control arc-crust geochemical and structural evolution. The Andreanof segment is one of very few locations that satisfy these criteria. It consists of an intact, arc massif formed during three identifiable and dated magmatic episodes; it is only modestly deformed and exhibits along-strike variability in active volcanism from primarily basaltic in the east to andesitic/dacitic in the west. The seismic experiment supported by this grant will consist of one transect along the entire length of the segment and two arc-crossing transects. Multi-channel seismic (MCS) data, recorded on an 8-km-long streamer, and ocean-bottom seismic data, with instrument spacings of 10 to 15 km, will be acquired along each transect, and these data will be analyzed with advanced tomographic and MCS imaging methodologies to produce seismic velocity and reflectivity images that will reveal, for the first time, the full crustal architecture of an intact arc massif at a segment scale. These images will provide estimates of composition that will inform models of melt emplacement and fractionation, and enable a number of specific questions to be addressed, including: What is the bulk composition, thickness and basic architecture of oceanic-arc crust, How are variations in factors such as primary magma composition and crustal processing of magmas expressed in bulk arc crustal composition and structure, and What is the fate of mafic/ultra-mafic cumulates?