There are two major collisions between active volcanic arcs carried by the PSP and the adjacent margins. One is in the west with the Luzon volcanic arc. It has resulted in the Taiwan orogeny since the Pliocene e. The other is in the north with the IBM arc. Lallemand described the deformation modes of the subducting PSP associated with these two arc-continent collisions.
The PSP itself appears as a mosaic of oceanic basins, aseismic ridges, plateaus, fracture zones, volcanic arcs and fore-arcs, fossil, and active spreading centers. Despite some controversy on the age of the small westernmost Huatung Basin see Fig. Mrozowski et al. Its unusual depth, which reaches a maximum of m, has been explained by post-spreading amagmatic extension Fujioka et al. Several ridge jumps, plateau emplacements and overlapping spreading centers were interpreted by Deschamps et al.
Another striking difference from the initial model proposed by Hilde and Lee concerns the northernmost and southernmost parts of the WPB. According to their interpretation, rifting may have started as soon as 55 Ma ago behind the proto-Philippine Arc. Close-up view of the bathymetry of the northern half of the WPB. Recently, Sasaki et al. Their magnetic model, even poorly constrained, provides an age of 35—40 Ma, which is quite young compared to its counterpart in the north.
Whatever the age of this basin, it is now clear that it is made of normal oceanic crust and spreading occurred E-W in its present position south of the Mindanao FZ. Other authors Taylor and Goodliffe ; Sasaki et al.
Unlike most previous authors, Sasaki et al. The age of this small basin is highly controversial. Estimates vary from to Ma Deschamps et al. Further details and an interesting discussion about this controversy are available in Eakin et al. Both basins rifted right after the cessation of spreading in the WPB, i. Sdrolias et al. The back-arc region ruptured simultaneously in the north on the Shikoku rift and in the south on the Parece Vela rift about km apart. The dramatic change in spreading orientation and rate after 20 Ma is interpreted by Sdrolias et al.
Many authors have studied the morphological, geophysical, and petrological characteristics of this basin Karig b ; Baker et al.
This is partly responsible for the curvature of the arc and its stretching, especially south of Guam where a rift crossing the arc and the fore-arc accommodates the spreading Ribeiro et al. The PSP oceanic basins are separated by aseismic ridges and plateaus, except along the eastern edge where active volcanic arcs bound the Mariana Trough and Sumisu Rift see Fig.
Tokuyama , , based on petrology, concluded that the Amami Plateau, composed of three km-long E-W-trending ridges, was an active island arc before the Eocene Nishizawa et al. It is presently subducting beneath the Ryukyu Arc. It also subducts on its western side and connects with the KPR on its eastern side. Basalts collected there also indicate an island arc setting Tokuyama et al. Further south, the Oki-Daito Ridge s. Most of the Daito ridges probably originated from island arcs, based on their similar velocity structures Nishizawa et al.
The Oki-Daito Rise, southwest of the main ridge, is an exception in terms of velocity structure, as it has a thinner crustal section. Nishizawa et al. The three small basins sandwiched between the Daito ridges, i.
Their crustal thickness is typical of oceanic crust Nishizawa et al. Finally, the small oceanic basins in the ADO region are roughly contemporaneous with the first stages of WPB spreading and could be considered to be the result of short-lived multiple spreading centers originating from the same regional magmatic event Deschamps and Lallemand Velocity and gravity models across the ridge suggest an episode of westward underthrusting of the WPB oceanic crust beneath the Huatung Basin Deschamps et al.
The Gagua Ridge thus represents failed subduction along a former fracture zone or transform boundary between two oceanic basins. That episode of short-lived subduction occurred in the Oligocene according to Deschamps and Lallemand , whereas Eakin et al. It extends from the mid-point between Luzon Island and Benham Rise to the Ryukyu Trench south of Miyako Island and then parallels the trench until at least the fore-arc area off Okinawa Island see Figs. It is difficult to estimate the total offset along the LOFZ because the fossil spreading center west of the fracture zone is entirely subducted beneath the Ryukyu fore-arc and because its location between the northern termination of the CBF rift and the LOFZ is not clear.
Based on magnetic identifications, the age of the oceanic crust west of the LOFZ ranges from 54 Ma in the south to younger than Shinjo and Ishizuka noticed that all samples collected east of the LOFZ have geochemical plume signatures, contrary to those reported west of the fracture zone. This is a strong argument for a large offset of perhaps several hundred kilometers along that fault zone. It experienced a succession of magmatic episodes accompanying, and recording, subduction initiation as well as several periods of seafloor rifting and spreading.
The first spreading phase, at the origin of the West Philippine Basin, was contemporaneous with its initiation and building, probably in interaction with the Oki-Daito mantle plume Ishizuka et al. The stratigraphic section of the fore-arc crust consists, from bottom to top, of peridotites, gabbroic rocks, a sheeted dike complex, basaltic lava flows, lavas and dikes of boninite and their differentiates, transitional high-Mg andesites, and tholeiitic and calc-alkaline arc lavas Ishizuka et al.
Reagan et al. This discovery indicates that the area of initial seafloor spreading contemporaneous with subduction initiation extended from the present-day fore-arc to the region west of the present-day KPR Ishizuka et al. Fewer data are available for the Yap and Palau arc systems, but it has been established that the Yap arc is no longer active volcanically since the Late Oligocene or Miocene, right after its collision with the Caroline Ridge Hawkins and Batiza ; McCabe and Uyeda Fujioka et al.
Metamorphic and gabbroic rocks from the Yap Arc appear quite similar to those exposed in the Parece Vela Basin Ohara et al.
The islands and submerged arc of Palau are thought to be the southern extension of the KPR Kobayashi They consist of 20—38 Ma arc-type volcanic rocks including basalt, andesite, and dacite fringed by coral limestones Meijer et al. A significant part of the PSP has been subducted beneath the Ryukyu Arc as attested by Wadati-Benioff zone seismicity and regional scale tomographic studies e.
This is consistent with the edge of the seismicity around km according to Wei et al. Lallemand et al. Wei et al. That deep slab could be connected with the detached slab east of the Gagua Ridge see Fig. Zhao et al. PSP imagery beneath southwest Japan is complicated by the fact that the subducting Shikoku Basin is very young 30—15 Ma and lies above the old subducting Pacific slab. Cao et al. It is apparently interrupted by a low-velocity anomaly north of Kyushu, extending northwestward from 80 km to greater depths.
Huang et al. In their model, both the buoyancy of the KPR and the directional change in the motion of the PSP play a role in the tearing process.
The corner flow produced by the subducting Pacific slab, together with its dehydration, probably heat the PSP slab from below, thus producing low-velocity anomalies Zhao et al. We will later assume that about km of PSP slab has been subducted beneath the Ryukyu and southwest Japan arcs.
Oceanic basins with their subducted parts, present and paleo-volcanic arcs and trenches of the PSP. Oceanic basins are outlined with their respective ages and the main directions of spreading grey arrows. The PSP slabs dotted areas have been unfolded and projected at the surface. The location of the Eo-Oligo-Lower Miocene trench in orange has been estimated based on the amount of IBM arc consumption by tectonic erosion.
Mesozoic terrane occurrences in the PSP are reported with green circles , in which the numbers refer to Table 1 where more details are presented.
High intraslab velocity parallel to the trench is consistent with the main spreading direction of the WPB see grey arrows in Fig. Park et al. One is east of Kyushu. It may have been caused by the non-offset ridge segment. The other is south of Shikoku. It might have been caused by the dextrally offset segment. In this case, the width of the subducted Shikoku Basin would be narrower Fig.
Based on the Wadati-Benioff zone, the slab is no longer than km at the latitude of Mindanao Island and is shorter both north and south of there Lallemand et al. The subduction can be traced from offshore Mortal Island in the south to Luzon Island in the north Cardwell et al. The short length of the subducted slab as well arc volcanism ages support a Pliocene age for the start of PSP subduction offshore Mindano. The subduction then propagated both northward and southward Lallemand et al. In detail, that region underwent several episodes of subduction, collision, and terrane accretion as depicted by many authors McCaffrey et al.
Shortening and even subduction of the WPB probably occurred north of the Philippine Trench along the East Luzon Trough as attested by a negative gravity anomaly and compressional features.
Lewis and Hayes proposed that subduction was active there during the Oligocene. Such short-lived subduction of the WPB along its western margin was also proposed by Deschamps et al. Transpression along the Gagua Ridge has been also proposed by Eakin et al. Accretion of crust or sediment may occur, contributing to a net growth of the upper plate, but sedimentary or crustal erosion also called tectonic erosion is also possible, resulting in a net loss of upper plate material.
Von Huene and Scholl estimated that about half of the convergent margins were accreting and half were erosive. There are two main indicators for a net global loss of crust at a convergent margin: a significant fore-arc subsidence increasing trenchward together with a landward retreat of the active volcanic front von Huene and Lallemand ; Lallemand Most of the time, the two indicators are observed to be simultaneous Fig.
Since the PSP is subducting along its western boundaries, we will focus on its eastern limit, which includes the IBM, Yap and Palau margins, to determine whether the plate has grown or shrunk.
Sketch showing the shrinking of an active margin undergoing tectonic erosion. The Pacific plate carries numerous seamounts Fig. As a consequence, the trench inner slope is steep and no sediment accretion is observed at the toe of the margin. Instead, basement rocks have been drilled close to the trench and even serpentinite mud volcanoes have been observed all along the Mariana fore-arc Natland and Tarney ; Bloomer ; Fryer ; Lagabrielle et al.
The diversity of metabasic rocks found in one of these seamounts can be explained by recycling of fore-arc materials through tectonic erosion and subduction of the fore-arc Fryer et al. The IBM margin has been classified as one of the most erosive margins in the world e.
In parallel research, Mitchell et al. Johnson et al. These rocks were created from boninite parental magmas that were modified into tonalites and are now part of a midcrustal layer. Their exposure near the trench is another evidence of tectonic erosion processes. Volcanic arc retreat sometimes results from a shallowing of slab dip, but in this case, and especially in the Mariana Arc, the present-day slab dip is almost vertical.
By extension, the whole margin, from southern Mariana to northern Japan has been severely affected by tectonic erosion processes during the Neogene and maybe earlier Lallemand , This retreat occurred simultaneously with a subsidence of the margin by more than 6 km von Huene and Lallemand As a consequence, lower crustal and even upper mantle sections of the PSP are exposed on the inner slope of the Yap Trench Fujiwara et al. Significant arc consumption occurred there at least during the Neogene.
This observation is extremely important for correctly interpreting the petrological and chronological data collected along that arc. We distinguish between two types of Mesozoic rock occurrences in the PSP depending on their location within, or along, the edges of the PSP.
Some were observed in a central position like the ADO region. They necessarily belong to the proto-PSP. Others were observed near the plate boundaries, i. They might have been accreted after the PSP formed. Starting with the ADO region, Matsuda et al. Based on the geochemical characteristics of the basalts, she concluded that the Early Cretaceous subduction zone that formed the Amami Plateau might have been the site of slab melting compatible with the subduction of a young, hot plate at that time.
The Amami tonalites were probably formed by fractional crystallization from the basaltic magma or partial melting of basaltic arc crust. Mizuno et al. More recently, andesites dredged on the Daito Ridge 8 and 9 in Table 1 and Fig.
Tani et al. Jurassic to Cretaceous zircon U-Pb ages have been obtained from the plutonic rocks 7 in Table 1 and Fig. More recently, volcanic rocks have been also sampled in the northern part of the IBM fore-arc. East of the Bonin Islands, pillow-lavas were dated at As discussed in section 1. The two dredge sites are located on basement highs west of the Gagua Ridge 11 and 12 in Table 1 and Fig.
These cherts likely belonged to the former oceanic plate Huatung Basin on which the Miocene volcanic arc was built Deschamps et al. Hickey-Vargas et al. For comparison, sources for igneous rocks from the Amami Plateau, thought to be an Early Cretaceous island arc, have Indian Hf and Nd presubduction mantle and Pacific Pb subduction component isotope characteristics Hickey-Vargas et al.
Except in its northernmost part north Luzon and the Batan islands , the Philippine archipelago does not belong to the present-day PSP, but before the inception of the Philippine Trench in the Late Miocene, it can be considered to have been part of the PSP. The Philippine region has a composite basement containing numerous ophiolite complexes, most of them being of supra subduction origin Pubellier et al.
Based on gravity data, these authors suggested that the present Miocene volcanic arc in Luzon lies directly on the southern extension of the Huatung Basin. Yumul provided a compilation of ophiolites all along the mobile belt.
He proposed a zonation from Late Cretaceous amphibolites with quartz-albite metamorphic soles distributed mainly in the eastern belt, then Early to Late Cretaceous dismembered ophiolites with mostly tectonic melanges in the central belt and finally Late Cretaceous to Oligocene ophiolitic complexes in the western belt. Such a zonation is discussed by Tani et al. They showed these to be Eocene in age and thus probably genetically associated with the WPB opening.
The geodynamic context of the proto-PSP isolation has long been a matter of debate. A variant of this model was later proposed by Jolivet et al. Lewis et al. Based on considerable geological and paleomagnetic constraints, especially those imposed from ocean drilling data and the Philippine islands, Hall et al. Hall later refined the plate tectonic evolution of the whole western Pacific region.
In the earlier stages of WPB opening, they have accounted for the presence of a plume Macpherson and Hall in the generation of the Benham Rise, Urdaneta Plateau, numerous overlapping spreading centers and ridge jumps.
However, this point needs to be clarified because, if most authors agree that the inception of IBM subduction marked the onset of the PSP, there is still a debate about the best marker for subduction initiation. Ishizuka et al. From a petrological point of view, most authors e. By contrast, the first occurrence of boninites, which required a source of volatiles, certainly marks the subduction of a hydrated lithosphere.
Considering that the transition from strike-slip movement along a transform boundary to convergent movement would unlikely produce high convergence rates normal to the boundary in the earlier stages, subduction inception would reasonably have started a few m.
Thus we finally converge with the estimate by Ishizuka et al. This oldest age for the WPB crust has been confirmed by Doo et al. Since there was no IBM subduction when the WPB rifted and spreading began, we should consider that it started to open behind the proto-Philippine Arc under the influence of the Oki-Daito plume. Consequently, the timing of this major event is much younger than the initiation of subduction along the proto-IBM trench prior to 50 Ma. Furthermore, Faccenna et al.
Faccenna et al. Plume-induced subduction initiation has been invoked by Gerya et al. First of all, reasonable numerical models indicate that compression across the boundary fault is required e. Secondly, no present or recent subduction onsets, the Philippine, New Hebrides, Flores or Wetar trench, for example, have involved such gravitational scenarios. All these recent incipient subductions required compressional tectonic forces to initiate them Hall et al.
Thirdly, transpression along a former transform boundary between two oceanic plates may allow the younger plate to subduct beneath the older as presently observed along the Hjort Trench Meckel et al. Rapid subduction of the still-spreading IP ridge, over a vast distance, likely triggered a chain reaction of tectonic plate reorganization Fig. Ridge-push and slab pull forces acting on the Pacific plate changed drastically, resulting in a switch of the absolute motion from northwest to west-directed around 55 Ma.
Since the Australian plate absolute motion also changed during the same period Whittaker et al. Zones of active seafloor spreading can also occur behind the island arc, known as back-arc basins.
These are often associated with submarine volcanoes. Where oceanic crust meets continental crust The denser oceanic plate is subducted, often forming a mountain range on the continent. The Andes is an example of this type of collision. Where continental crust meets continental crust Both continental crusts are too light to subduct so a continent-continent collision occurs, creating especially large mountain ranges. The most spectacular example of this is the Himalayas. The space created can also fill with new crustal material sourced from molten magma that forms below.
Divergent boundaries can form within continents but will eventually open up and become ocean basins. On land Divergent boundaries within continents initially produce rifts, which produce rift valleys.
Under the sea The most active divergent plate boundaries are between oceanic plates and are often called mid-oceanic ridges. The relative motion of the plates is horizontal. They can occur underwater or on land, and crust is neither destroyed nor created.
At a destructive plate margin , the plates move towards one another. This usually involves an oceanic plate and a continental plate. The collision process is known as subduction. The oceanic plate is denser heavier than the continental plate. As the plates move towards each other, the oceanic plate is forced beneath the continental plate. When the plate sinks into the mantle it melts to form magma. The magma escapes under pressure through cracks in the rock and rises up through a volcano.
The volcanic eruptions can be violent, with lots of steam, gas and ash.
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