Deep seismic imaging of developing continental crust in oceanic island arcs

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Volcanic island arcs such as the present day Aleutian, Mariana and Solomon island chains in the Pacific Ocean form where an oceanic tectonic plate subducts into the mantle beneath another piece of oceanic lithosphere. Ancient island arcs are believed to have been the fundamental building blocks that were assembled by plate tectonics to create the continents over the last ~3 Ga. However, the process through which the relatively buoyant continental crust is derived from the denser rocks that form much island arc crust is unclear. When continental-like andesitic rocks are generated in the middle crust of an island arc, a larger volume of denser mafic-ultramafic rocks should also be produced in an underlying root. Seismic refraction surveys, which estimate a proxy for crustal composition, namely the seismic wave speed, indicate that arc crust is too thin (12-35 km) to contain this root. The root can be denser than the uppermost mantle, detach from the base of the evolving island arc, and sink into the underlying mantle, but the time-scale of this hypothesised foundering is unknown. Seismic reflection surveys have recently found evidence of two roots extending to 50 km depth beneath the Aleutian arc, and may be able to identify other "missing" dense arc roots if they are still present. The objective of this research proposal is to utilize previously recorded deep seismic reflection profiles over island arcs of different ages to determine the distribution of arc roots and to constrain the time-scale of their foundering. Crustal production rates are highest during initiation of an island arc, and these earliest arc rocks are now found in the outer forearc close to the deep bathymetric trench. Seismic reflection imaging across the forearc will help to unravel the early history of voluminous crustal production in island arcs and the subsequent transition to a more stable regime of crustal generation. Since reflections from the descending oceanic plate are also recorded, a valuable additional consequence of this work will be improved estimates of the width of the seismogenic zone of four subduction megathrust faults derived from changes in their characteristic reflection responses and/or intersections with the uppermost mantle where stable sliding occurs.