Philippine Sea Plate
The IBM arc system is part of the Philippine Sea Plate, at least to the first approximation. Although the IBM arc deforms internally – and in fact in the south a small plate known as the Mariana Plate is separated from the Philippine Sea Plate by a spreading ridge in the Mariana Trough – it is still useful to discuss approximate rates and directions of the Philippine Sea Plate with its lithospheric neighbors, because these define, to a first order, how rapidly and along what streamlines material is fed into the Subduction Factory. The Philippine Sea Plate (PH) has four neighboring plates: Pacific (PA), Eurasian (EU), North American (NA), and Caroline (CR). There is minor relative motion between PH and CR; furthermore, CR does not feed the IBM Subduction Factory, so it is not discussed further. The North American plate includes northern Japan, but relative motion between it and Eurasia is sufficiently small that relative motion between PH and EU explains the motion of interest. The Euler pole for PH-PA as inferred from the NUVEL-1A model for current plate motions lies about 8°N 137.3°E, near the southern end of the Philippine Sea Plate. PA rotates around this pole CCW ~1°/Ma with respect to PH. This means that relative to the southernmost IBM, PA is moving NW and being subducted at about 20–30 mm/y, whereas relative to the northernmost IBM, PA is moving WNW and twice as fast. At the south end of IBM, there is almost no convergence between the Caroline Plate and the Philippine Sea Plate.