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.

Appendix

Table 4.1 lists the sections of the various wide-angle velocity models that were laterally averaged to create the 1D velocity-depth functions shown in Figs. 4.13 and 4.14. The x-coordinates correspond to those used originally in the publications cited. A maximum depth, below which the velocities are not considered well constrained, was chosen for each velocity function, and velocity functions are only displayed above these depths.

Fig. 4.15

Comparison of the average seismic velocity functions for island arcs with that of continental crust from Christensen and Mooney (1995). The different depth datum levels are indicated. Note that the range of data included in the island arc average decreases below 30 km depth, because there are few velocity models that extend this deep.

Table 4.1 Locations of derived 1D velocity-depth functions

Representative 1D velocity depth functions for island arc crust were calculated by averaging the sections of the 2D arc velocity models listed in Table 4.1 (but excluding the continental part of Aleutian line A3) using three difference reference datums: mean sea level, the seafloor (taken to be the 1.52 km s⁻¹ isovelocity contour), and the top of the igneous crust (Fig. 4.15). In many island arc settings, the top of the igneous crust is not well defined due to low velocity, high porosity volcanic rocks and intrusions into the overlying sedimentary section; so the 3.5 km s⁻¹ contour was used to approximate this boundary. To obtain a 1D velocity model for a particular reference datum, e.g. the seafloor, each velocity model was depth-shifted so that the velocity value immediately below the datum was aligned at zero across the 2D model. This results in higher velocities at zero depth than the selected datum value due to discretization of the velocity models. All the 2D models were then averaged. The contribution of a particular velocity model to the average velocity functions depends on the length of the model in Table 4.1. Thus the long strike lines along the Aleutian and Izu, Bonin and Mariana contribute more to the average than relatively short dip lines, such as that across South Sandwich arc.

Table 4.2 Reference 1D velocity-depth functions for island arcs (km s⁻¹)