The old plates may snuggle willingly

The Izu-Bonin-Mariana arc system is suspected to be initiated spontaneously or forcibly. A new drill core in the Philippine Sea Plate holds the hand of the spontaneous subduction initiation model.

Sinking of old and cooled oceanic lithosphere (or plate) into the mantle beneath the overriding lithosphere, called subduction, plays a key role in the Earth system: raising new continental crust materials as well as the deep cycles of numerous elements. In spite of the presence of various theories on subduction system, the initiation of subduction is poorly understood. In September 2015 issue of Nature Geoscience, Arculus et al.1 present a new drilling core data from the Izu-Bonin-Mariana (IBM) arc, a young subduction system considered to be initiated about 52 million years ago (Ma) (ref. 2) The data provide strong evidence for one of the previously proposed models: spontaneous subduction initiation.

The fundamental problem of the researches on subduction initiation is that once the subduction becomes ‘self-sustaining’, the traces of the earliest stage are mostly obliterated so that it is impractical to discern the mode of initiation (ref. 3). Thus many researches have focused on geophysical constraints; geologists scoured for rocks which may preserve the early signals of subduction system (e.g. ref. 3, 4). These endeavors have been established broadly two models of subduction initiation: induced and spontaneous nucleation of subduction zones (ref. 5).

The model of induced nucleation requires additional tectonic compressional force to overcome tension beneath the oceanic plate so that the plate can penetrate into the mantle (ref. 3). Meanwhile, the model of spontaneous nucleation is based on more intuitive physical mechanisms: gravitational instability due to the density inversion. As the oceanic lithosphere become cooled and heavier, at a certain moment the lithosphere will ‘sink’ into the mantle. If the subduction has begun spontaneously, overlying plate should be subject to the extensional tectonic regime, whereas induced subduction model accompanies the compressional structures in the overlying plate (ref. 3,6).

Figure 1. Comparative geochemical plots of compiled (ref. 10) and averages (ref. 13) of mid-ocean ridge compositions (grey and brown) and subduction-related basalts from fore-arc (ref. 2,4,11,12) and WPB1 regions. The high Y/Zr (a) and V/Ti (b) ratios are indicative of derivation of basalts from highly depleted upper mantle sources (ref. 2,14)

The extensional force at the overlying plate in the subduction system generates fore-arc spreading followed by subsequent magma activity. A combination of water supply from the slab and decompression causes extensive melting of lherzolite and harzburgite whose products will be boninites and severely depleted arc tholeiites (ref. 5).

The IBM arc system is one of the youngest subduction system in the Earth, whose upper plate is the Philippine Sea Plate overlying the Pacific Sea Plate. Previous researches on the IBM fore-arc crust and exposed rocks on islands found occurrence of tholeiites and boninites of Eocene age, and proposed the stretching as a consequence of spontaneous subduction initiation (ref. 2,4,7). Nonetheless, structure and physical models on the West Philippine Basin (WPB), which lies on the western side of the back-arc ridge, have suggested that the weak zones in the WPB is not the cause of subduction initiation, and even that the initiation of the IBM arc subduction was induced by pre-existing compressional regime of the WPB (e.g. ref. 8).

Arculus et al. (ref. 1) analyzed the new drilling-core sample in the Amami-Sankaku Basin (ASB), which is located on the eastern margin of the WPB. The authors’ results revealed for the first time the chemistries and ages of the WPB basement rocks and its overlying sediments. The biostratigraphy and paleomagnetic-based age-depth plot suggests that the basement rock of tholeiite basalt is Eocene age (~ 55 Ma). The bulk composition of the tholeiite basalt is striking, because the compositions of those rocks are much more depleted in magma-phile trace elements than normal mid-oceanic ridge basalts (MORBs).

So how the presence of the highly depleted tholeiite basalt in the WPB support the idea of spontaneous initiation of the IBM arc system? The induced subduction initiation model excludes the possibility of widespread depleted tholeiitic basalt in the WPB which cannot be produced by over-thrusting of the older plate. The new drilling cores in the WPB do not contain any normal MORB signals, supporting that the old Pacific Plate had ‘snuggled’ willingly into the warm mantle beneath the WPB.

In order to establish a well-defined tectonic model in a region, a high-precision age determination such as 40Ar/39Ar radiometric dating is necessary. The Eocene epoch of western Pacific Ocean is critical moment for tectonic evolution of eastern Asia, Australia and Zealandia, considering the rearrangement of their plate boundaries and paleo-motions (e.g. ref. 2, 9). Although the refinement of previous models of the Philippine Sea Plate must be followed, the Arculus and colleagues’ demonstration of the presence of extensional regime and its depleted products remains as a strong evidence for the spontaneous nucleation of the IBM arc system.

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