A new high-resolution velocity model of the Mariana arc-backarc system obtained from active-source seismic profiling demonstrates velocity variations within the arc middle and lower crusts of intermediate to felsic and mafic compositions. The characteristics of the oceanic-island-arc crust are a middle crust with velocity of ∼6 km/s, laterally heterogeneous lower crust with velocities of ∼7 km/s, and unusually low mantle velocities. Petrologic modeling suggests that the volume of the lower crust, composed of restites and olivine cumulates after the extraction of the middle crust, should be significantly larger than is observed, suggesting that a part of the lower crust, especially the cumulates, is seismically a part of the mantle.

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