A COMPARISON OF THE SEISMIC STRUCTURE OF MODERN ISLAND ARC CRUST AND LITHOPROBE RESULTS FROM ARCHEAN CONTINENTAL CRUST

The western Pacific, which contains several island arcs and oceanic plateau, is frequently cited as a modern day analogue for the tectonic regime that existed when the Archean cratons were assembled. Seismic velocity models derived from refraction surveys can determine the crustal thickness and constrain the composition of Earth’s crust. I present a global review of seismic velocity models from recent surveys of island arcs: crustal thicknesses vary from as little as 10 km in the Bonin arc to 35 km in the Aleutian and northern Izu arcs with the primary control being the degree of extension to which the arc has been subject. Although globally island arcs appear to have a more mafic composition than both typical continental crust and Archean cratons such as the Superior Province surveyed by Lithoprobe, intermediate composition crust is inferred in parts of the Izu arc. Seismic velocity models of island arcs are commonly poorly constrained at depths of 20-50 km beneath the arc due to the difficulty in recording first arrivals at the long offsets required to directly infer the velocity; however, the absence of a sharp velocity contrast at the Moho appears to be a first order characteristic of island arc crust, and indicates the existence of a broad crust-mantle transition zone. Multichannel seismic reflection surveys complement refraction surveys by revealing structures associated with variations of density and seismic velocity at the scale of a few hundred meters or less to depths of 60 km or more. Parts of the Eocene Mariana arc and of the eastern Aleutian arc are characterized by reflectors that extend continuously from the lower arc crust to below the inferred crust-mantle transition, >50 km depth in some areas. Since relative to the overlying crust the upper mantle is commonly homogeneous and hence non-reflective, we interpret these reflectors to be gabbro, garnet gabbro, and pyroxenite intrusions within crustal roots that represent a heterogeneous transition from mafic lower crustal rocks to ultramafic mantle rocks. We suggest that the reflectivity is linked to repeated differentiation and intrusion of mantle-derived melts into the sub-arc lithosphere, and that the depth of these roots shows that fractionation of arc crust can extend well below the seismically determined Moho.