Cenozoic geological and plate tectonic evolution of SE Asia and the SW Pacific: computer-based reconstructions, model and animations

http://www2.ess.ucla.edu/~yin/05-Publications/papers/124-Yin-2010-Tectonophysics.pdf

Cenozoic tectonic evolution of Asia: A preliminary synthesis

The discovery of metal-depositing hot springs on the sea floor, and especially their link to chemosynthetic life, was among the most compelling and significant scientific advances of the twentieth century. More than 300 sites of hydrothermal activity and sea-floor mineralization are known on the ocean floor. About 100 of these are sites of high-temperature venting and polymetallic sulfide deposits. They occur at mid-ocean ridges (65%), in back-arc basins (22%), and on submarine volcanic arcs (12%). Although high-temperature, 350°C, black smoker vents are the most recognizable features of sea-floor hydrothermal activity, a wide range of different styles of mineralization has been found. Different volcanic substrates, including mid-ocean ridge basalt, ultramafic intrusive rocks, and more evolved volcanic suites in both oceanic and continental crust, as well as temperature-dependent solubility controls, account for the main geochemical associations found in the deposits. Although end-member hydrothermal fluids mainly originate in the deep volcanic basement, the presence of sediments and other substrates can have a large effect on the compositions of the vent fluids. In arc and backarc settings, vent fluid compositions are broadly similar to those at mid-ocean ridges, but the arc magmas also supply a number of components to the hydrothermal fluids.
The majority of known black smoker vents occur on fast-spreading mid-ocean ridges, but the largest massive sulfide deposits are located at intermediate- and slow-spreading centers, at ridge-axis volcanoes, in deep backarc basins, and in sedimented rifts adjacent to continental margins. The range of deposit sizes in these settings is similar to that of ancient volcanic-associated massive sulfide (VMS) deposits. Detailed mapping, and in some cases drilling, indicates that a number of deposits contain 1 to 5 million tons (Mt) of massive sulfide (e.g., TAG hydrothermal field on the Mid-Atlantic Ridge, deposits of the Galapagos Rift, and at 13°N on the East Pacific Rise). Two sediment-hosted deposits, at Middle Valley on the Juan de Fuca Ridge and in the Atlantis II Deep of the Red Sea, are much larger (up to 15 and 90 Mt, respectively).
In the western Pacific, high-temperature hydrothermal systems occur mainly at intraoceanic back-arc spreading centers (e.g., Lau basin, North Fiji basin, Mariana trough) and in arc-related rifts at continental margins (e.g., Okinawa trough). In contrast to the mid-ocean ridges, convergent margin settings are characterized by a range of different crustal thicknesses and compositions, variable heat flow regimes, and diverse magma types. These variations result in major differences in the compositions and isotopic systematics of the hydrothermal fluids and the mineralogy and bulk compositions of the associated mineral deposits. Intraoceanic back-arc basin spreading centers host black smoker vents that, for the most part, are very similar to those on the mid-ocean ridges. However, isotopic data from both the volcanic rocks and the sulfide deposits highlight the importance of subduction recycling in the origin of the magmas and hydrothermal fluids. Back-arc rifts in continental margin settings are typically sediment-filled basins, which derive their sediment load from the adjacent continental shelf. This has an insulating effect that enhances the high heat flow associated with rifting of the continental crust and also helps to preserve the contained sulfide deposits. Large hydrothermal systems have developed where initial rifting of continental crust or locally thickened arc crust has formed large calderalike sea-floor depressions, similar to those that contained major VMS-forming systems in the geologic record. Hydrothermal vents also occur in the summit calderas of submarine volcanoes at the volcanic fronts of arcs. However, this contrasts with the interpreted settings of most ancient VMS deposits, which are considered to have formed mainly during arc rifting. Hydrothermal vents associated with arc volcanoes show clear evidence of the direct input of magmatic volatiles, similar to magmatic-hydrothermal systems in subaerial volcanic arcs. Several compelling examples of submarine epithermal-style mineralization, including gold-base metal veins, have been found on submarine arc volcanoes,and this type of mineralization may be more common than is presently recognized. 
Mapping and sampling of the sea floor has dramatically improved geodynamic models of different submarine volcanic and tectonic settings and has helped to establish a framework for the characterization of many similar ancient terranes. Deposits forming at convergent margins are considered to be the closest analogs of ancient VMS. However, black smokers on the mid-ocean ridges continue to provide critically important information about metal transport and deposition in sea-floor hydrothermal systems of all types. Ongoing sea-floor exploration in other settings is providing clues to the diversity of mineral deposit types that occur in different environments and the conditions that are favorable for their formation.

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Sea-Floor Tectonics and Submarine Hydrothermal Systems

▪ Abstract Growth of the Japanese arc system, which has mainly taken place along the continental margin of Asia since the Permian, is the result of subduction of the ancient Pacific ocean floor. Backarc basin formation in the Tertiary shaped the present-day arc configuration. The neotectonic regime, which is characterized by strong east-west compression, has been triggered by the eastward motion of the Amur plate in the Quaternary. The tectonic evolution of the Japanese arc system includes formation of rock assemblages common in most orogenic belts. Because the origin and present-day tectonics of these assemblages are better defined in the case of the Japanese arc system, study of the system provides useful insight into orogenesis and continental crust evolution.

/pdf/tectonic-evolution-of-the-japanese-island-arc-system-1gm8prse3z.pdf

Tectonic Evolution of the Japanese Island Arc System

The Okinawa Trough is an incipient intracontinental back arc basin that has developed behind the Ryukyu arc-trench system. To explore its magmatic evolution and the nature of the mantle source, we present major and trace element and Sr-Nd isotopic data for mafic volcanic rocks from the Middle and Southern Okinawa Trough and the Central Ryukyu Arc. Two episodes of activity formed the latter: older (∼21–13 Ma) and younger (∼6–4 Ma). Although Quaternary basalts from the Middle Okinawa Trough (MOT) have major element compositions comparable to basalts from intra-oceanic back arc basins, they are characterized by relative enrichments of large ion lithophile elements and Pb and by depletions of high field strength elements analogous to those observed in Ryukyu Arc volcanics. Two components have been involved in the generation of MOT basalt, which we identify as an E-MORB type (or oceanic island basalt type) upwelling asthenospheric mantle and a “subduction component.” Quaternary basalts from the Southern Okinawa Trough (SOT) have uniform Nd but heterogeneous Sr isotopic ratios and incompatible trace element compositions. This may be ascribed to more complicated tectonic and magmatic processes in the SOT compared with the MOT, such as oblique subduction of the Philippine Sea Plate and interaction with postcollisional extension in the northern Taiwan orogenic belt. Integrating geological information available from nearby regions, we emphasize that the SOT is an “atypical” back arc basin because its development essentially occurred synchronous with or even prior to development of the arc-trench system.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/1999JB900040

Geochemical and Sr‐Nd isotopic characteristics of volcanic rocks from the Okinawa Trough and Ryukyu Arc: Implications for the evolution of a young, intracontinental back arc basin

The Okinawa Trough, lying between Japan and Taiwan, is a backarc basin formed by extension within the continental lithosphere behind the Ryukyu trench-arc system Stress directions associated with the two last extensional phases in the southwestern Okinawa Trough have been deduced from a comparison with analog modeling: the direction of extension is N 150 o for the Pleistocene phase of extension (2-01 Ma) and N 170 o for the late Pleistocene-Holocene phase of extension (01-0 Ma) The present-day Ryukyu volcanic arc, a narrow continuous feature extending from Japan to Taiwan, is located on the eastern side of the Okinawa Trough, 80-100 km above the Wadati-Benioff zone, the minimum depth for emplacement of arc magmatism Scarce present-day backarc volcanism appears in the middle and southern Okinawa Trough within linear en echelon bathymetric depressions A N045 o oriented seamount volcanic chain cuts across obliquely the southwestern Okinawa Trough and lies in the direct line of extension of the Gagua ridge, a N-S linear volcanic feature of the Philippine Sea plate Associated with this extension of the Gagua ridge, a large reentrant located at the base of the Ryukyu prism, the uplift of part of the Nanao forearc basin and the deformation of the sedimentary arc suggest that the voluminous cross-backarc volcanism could be tied to the subduction of the Gagua ridge located there at a depth of 80-100 km beneath the backarc basin A second area of anomalous volcanism has been identified in the middle Okinawa Trough in the ENE extension of the Daito ridge, a WNW-ESE 400-km-long volcanic feature of the Philippine Sea plate We suggest that the Gagua and Daito ridges initially induced stress at the base of the arc which is still brittle and cracks propagated through the overlying brittle lithosphere, allowing magmas with arc affinities to erupt at the seafloor This excessive magmatism reaches the seafloor through conduits which preferentially follow in their shallowest portion the crustal normal faults of the backarc rifts The Okinawa Trough is consequently still in an early stage of evolving from arc type to backarc activity

/pdf/okinawa-trough-backarc-basin-early-tectonic-and-magmatic-4l4l901r65.pdf

Okinawa trough backarc basin: Early tectonic and magmatic evolution

Marine observations offshore of Taiwan indicate intense deformation of the Luzon arc-forearc complex, with episodic eastward migration of the active deformation front across the complex. The Philippine Sea Plate (PHS) began colliding with the Eurasian continental margin in Pliocene time. Because of the obliquity of plate convergence, the collision has propagated through time from north of Taiwan to the south with the more advanced stages being presently observed to the north, whereas the subduction of the oceanic lithosphere of the South China Sea beneath the PHS occurs to the south. Offshore, the collision zone is characterized by deformation of the arc including the forearc region to the south. This active tectonic domain absorbs a significant amount of shortening between the Eurasia margin and the PHS, which is moving towards N 310° E at about 8 cm/yr relative to Eurasia. Swath bathymetry and backscattering data, together with seismic reflection and geopotential data obtained during the ACT cruise onboard the R/V L'Atalante, showed major north to south changes in the tectonic style in both the indenting arc and the host margin. In the southern domain, left-lateral transpression is recorded by deformed and folded series of the forearc domain that are unconformably overlain by collision-derived sediments of the Southern Longitudinal Trough (SLT). Today, the loci of deformation has jumped to the east and it is characterized by the growth of a sedimentary ridge (the Huatung ridge, rear portion of the former Manila oceanic accretionary wedge including forearc and intra-arc sequences), which overthrusts the basement of the island arc. In the northern domain, north of 22°30'N, active westward thrusting of the Coastal Range (remnants of the island arc and forearc basins) over the Lichi melange develops onland along the Longitudinal Valley. Offshore, at the base of the eastern slope, prominent fault scarps suggest an active eastward thrusting of parts of the arc (volcanic edifices and intra-arc or forearc sediments) onto the oceanic crust of the Philippine Sea Plate. It accounts today for part of the convergence. The

/pdf/arc-continent-collision-in-taiwan-new-marine-observations-1f7dlebxg1.pdf

Arc-continent collision in Taiwan: New marine observations and tectonic evolution

Resume Les deux dernieres phases d'extension identifiees dans la partie centrale du bassin arriere-arc d'Okinawa sont datees du Pleistocene et du Pleistocene terminal-Holocene. Les deux systemes de failles correspondants ont ete caracterises a partir des donnees de reflexion sismique, de 3,5 kHz et de bathymetrie multifaisceaux, acquises au cours de la campagne Donghai. Les directions des contraintes ont alors ete etablies en utilisant les resultats de modelisations analogiques a deux couches fragile et ductile.

Nicolas Thareau

Papers

Okinawa trough backarc basin: Early tectonic and magmatic evolution

Arc-continent collision in Taiwan: New marine observations and tectonic evolution

Mécanismes d'extension dans la partie centrale du bassin d'Okinawa (Pacifique occidental)