Izu-Bonin-Mariana Arc - Plate Motions

Plate Motions

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 (DeMets et al. 1994) 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 now convergence between the Caroline Plate and the Philippine Sea Plate. It should be noted that the IBM arc is not experiencing trench ‘roll-back’, that is, the migration of the oceanic trench towards the ocean. The trench is moving towards Eurasia, although a strongly extensional regime is maintained in the IBM arc system because of rapid PH-EU convergence. The nearly vertical orientation of the subducted plate beneath southern IBM exerts a strong “sea-anchor” force that strongly resists its lateral motion. Back-arc basin spreading is thought to be due to the combined effects of the sea-anchor force and rapid PH-EU convergence (Scholz & Campos 1995). The obliquity of convergence between PA and the IBM arc system change markedly along the IBM arc system. Plate convergence inferred from earthquake slip vectors is nearly strike-slip in the northernmost Marianas, adjacent to and south of the northern terminus of the Mariana Trough, where the arc has been ‘bowed-out’ by back-arc basin opening, resulting in a trench which strikes approximately parallel to the convergence vectors. Convergence is strongly oblique for most of the Mariana Arc system but is more nearly orthogonal for the southernmost Marianas and most of the Izu-Bonin segments. McCaffrey 1996 noted that the arc-parallel slip rate in the forearc reaches a maximum of 30 mm/yr in the northern Marianas. According to McCaffrey, this is fast enough to have produced geologically significant effects, such as unroofing of high-grade metamorphic rocks, and provides one explanation for why the forearc in southern IBM is tectonically more active than that in northern IBM.