Crustal structure of the Mariana Island arc system and old Pacific plate from seismic refraction data

Abstract

Twenty-six new explosion refraction profiles from the Mariana island arc system and old Pacific plate along a 750 km east-west line at about 18°N latitude are analyzed to determine crustal structure across the Mariana basin (old Pacific plate ), arc-trench gap, Mariana ridge and Mariana trough. The Pacific plate east of the Mariana trench has a crustal thickness of 6.8 km, similar to that of average Pacific ocean crust, but has thicker than average layers 2A and 2B. This is most likely due to lower velocity material introduced by local seamount volcanism. Large scale vertical faulting in the arc-trench gap is supported by the data in this study, while plate accretion is not. Thick sequences of low-velocity material are also found in the arc-trench gap, and no mantle velocities are found. Crustal structure on the Mariana ridge is roughly similar to that of the arc-trench gap. There is evidence here of arrivals from a relatively high velocity body under the ridge, possibly a volcanic plug or sill. In the Mariana trough, an average crustal thickness of S km is found, with evidence of crustal thinning toward the center of the trough. These data, together with depth-age and depth-heat flow relationships, indicate crustal formation in the Mariana marginal basin differs considerably from that at mid-ocean ridges. Crust which forms in the marginal basin is thinner, cools faster and sinks more quickly than oceanic crust.