The Seismic Structure of Island Arc Crust

Abstract

Intraoceanic island arcs are considered to be fundamental building blocks of continental crust that are accreted during arc–continent collision. P wave velocity models derived from wide-angle seismic surveys can constrain the thickness and composition of arc crust. The variations of P wave velocity with depth of the Aleutian, Izu-Bonin-Mariana, Lesser Antilles, Solomon, South Sandwich, and Tonga island arcs are compared, and the unextended Aleutian arc is contrasted in detail with the Izu-Bonin-Mariana arc-back-arc system, which has been variably subject to extension and arc rifting. The Aleutian arc is interpreted to be 35 km thick along much of its eastern section, while close to the volcanic line the Izu arc is 26–35 km thick, the Bonin arc 10–22 km thick, and the Mariana arc 16–24 km thick, with these variations in thickness primarily related to the amount of extension that has affected the different segments of the arc. Both wide-angle refraction and normal incidence reflection surveys indicate that the crust–mantle transition can extend 4–10 km beneath the top-Moho reflector used to determine most crustal thicknesses. At depths greater than 8–10 km, i.e., a confining pressure of ~0.2 GPa, all surveyed island arcs exhibit higher seismic velocities than continental crust, and are thus on average more mafic. However, at depths less than 8–10 km, P wave velocities in island arcs generally fall within the broad range of values corresponding to continental crust. These upper crustal velocities are consistent with the presence of tonalitic rocks, but at shallow depths felsic rocks cannot be readily discriminated from more mafic rocks with elevated porosity. Nevertheless lateral variations in seismic velocity along the Izu-Bonin arc on the scale of ~50 km can be correlated with the chemistry of arc volcanoes, suggesting a link between seismic velocity, crustal composition, and the magmatic evolution of the arc. Prior to arc–continent collision, sedimentary rocks derived from the approaching continent can accumulate across the forearc and in the back-arc basin, and may reach thicknesses as great as 12 km.