Intellectual Merit: Constraining the water budget at subduction zones is of great importance for understanding many Earth processes, including island arc volcanism, intermediate and deep earthquakes, and the transport of water to the deeper mantle. Perhaps the largest uncertainty is the hydration state of the upper mantle as the oceanic plate enters the trench. Although the amount of water contained in the uppermost mantle of the down-going plate is potentially much larger than in subducting sediments and hydrated crust, the degree to which initially anhydrous oceanic mantle is hydrated during its passage from ridge to trench is unknown. The most popular and influential model for mantle hydration involves serpentinization by fluid flow along faults produced during bending at the outer rise of subduction zones. If old oceanic plates are hydrated to depths of ~45 km as suggested by the width of intermediate-depth seismic zones and the depths of outer rise earthquakes it will have great implications for the earth's water budget.
In this project ocean bottom seismograph (OBS) data collected by the LABATTS experiment will be used to constrain changes in mantle seismic velocity resulting from serpentinization. Since seismic velocity is a linear function of the degree of serpentinization, mapping the seismic velocity change near the trench will constrain the amount of water entering the Tonga subduction zone in the form of serpentinized mantle. The LABATTS deployment is the only previously collected dataset that offers the opportunity to image to depths of ~ 45 km the possible effects of progressive serpentinization of incoming oceanic mantle from old ocean lithosphere through the trench axis. Data from five OBSs and one land station (Niue Island) extending from the trench to 300 km onto the Pacific plate in the Tonga outer rise will be analyzed with various techniques to image velocity changes in the uppermost oceanic mantle. Seismic wave delay times will be determined using cross-correlation techniques to assess changes in total integrated delay time due to changes in upper mantle velocity. The ratio of seismic compressional and shear wave anomalies will be diagnostic of serpentinization as compared to other sources of travel time anomalies. This study will also clarify the lateral and depth extent of earthquake faulting, as well as the depth of the neutral surface between extensional and compressional seismicity that may pose a barrier to fluid penetration. The results of the earthquake study will be used to constrain the role of faulting in the progressive serpentinization of the incoming plate.
Broader Impacts: This project supports a young researcher (Conder), as well as a graduate student. The project will also support an undergraduate researcher for 2 summers, providing research experiences for promising undergraduates. The PI (Douglas Wiens) has a strong record of mentoring undergraduate students, and several past undergrad researchers from his lab have gone on to PhDs and faculty positions. This project will also ensure that the LABATTS experiment ocean bottom seismic data are archived properly at the IRIS Data Management Center, to be made available openly to researchers in future.
