West Pacific subduction system
The present-day Philippine subduction zone, with the Philippine Sea plate subducting below the Eurasian plate, is thought to have started at about 9 Ma (e.g., Wu et al., 2016), after the collision of the Palawan continental block with the Philippine Mobile Belt (PMB) that occurred around ~20-11 Ma (Marchadier and Rangin, 1990; Yumul Jr. et al., 2003).
The Palawan block belongs to the Eurasian plate and drifted towards the southeast until it collided with the Philippine archipelago at the trench of the former, eastward subduction zone (Marchadier and Rangin, 1990). This collision likely induced a flip in subduction polarity, which initiated the westward Philippine subduction zone (Barrier et al., 1991) on the other side of the already existing volcanic arc.
Arc volcanism related to the east dipping subduction along the Philippine trench started at the latest around 6.6 Ma and is today recorded in rhyolitic volcanic rocks in the Bicol peninsula (i.e., in the northern part of today’s subduction trench). Arc magmatism shows a trend towards younger ages both north (1.5 Ma) and south of Bicol (3.5 Ma), potentially indicating a southward propagation of the subduction trench (Ozawa et al., 2004, Sajona et a., 1997).
In the model of Müller et al. (2016), the Philippine SZI event occurs at 14 Ma, in conjunction with the Halmahera SZI event (the subduction zones are contiguous but of opposite polarity). The initiation of the Philippine subduction zone immediately follows the cessation of a pre-existing subduction zone (the ‘East Philippine’ subduction zone) on the far (south) side of the Philippine arc; the Philippine subduction zone initiated on the north side of the arc, parallel to the orientation of the pre-existing subduction zone, but separated from it by ~350-500 km. Coincident with the SZI event (at 14 Ma), the motion of the Philippine Sea plate changes significantly, whereas the motion of the Australian plate remains unchanged.
The scenario with the collision-induced subduction polarity flip is also consistent with the tomographic image under the Bicol peninsula (Puspito et al., 1993). Although the Philippine slab is missing from the Atlas of the Underworld catalogue, the seismic tomography models used here for creating vote maps are consistent in predicting a slab until 700-800 km depth, where it overlaps with another deeper anomaly.
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