Subduction

Figure 1.1. Outline index map of the Japanese subduction zones. Thick lines with teeth are converging plate boundaries. Arrows indicate relative plate motions. Abbreviations su, Suruga trough sa, Sagami trough sf, South Fossa Magna triple junction och, Off Central Honshu triple junction ISTL, Itolgawa-Shizuoka Tectonic Line KSM, Kashima VLBl station (Uyeda, 1991).

These different sites of hydrothermal and ore-forming activity may have resulted from the mode of subduction of the Pacific Plate. Mariana-type subduction (characterized by a steep angle of subduction and back-arc basin formation Uyeda and Kanamori, 1979) during middle Miocene caused WNW-ESE extension, submarine hydrothermal activity, thick accumulation of bimodal (basaltic and dacitic) volcanic activity (Green tuff) and Kuroko-type formation (Shikazono and Shimizu, 1993). Plio-Pleistocene Chilean-type subduction (shallow-dipping subduction zone, E-W compression Uyeda and Kanamori, 1979) and oblique subduction of the Pacific Plate beneath the North American Plate led to uplift and expansion of land area, subaerial hydrothermal activity accompanied by meteoric water circulation, subaerial andesitic volcanic activity and formation of vein-type deposits. 

Lead isotopes. Sato and Sasaki (1973) concluded on the basis of a remarkable narrow range in lead isotopic composition of Kuroko ores that lead of Kuroko ore came from deep-seated source which originated from subducting pelagic sediments. 

Watanabe (1986, 1989, 1990a,b, 1991) studied the vein pattern, the age of vein-type deposits and the volcanic rocks in southwest Hokkaido and showed that the major veins such as those at the Toyoha and Chitose have been formed at dextral strike-slip movement of an E-W trend, and those veins are situated at the west-southwest extension of the maximum displaced zone within the dextral shear belt along the Kuril arc. Watanabe (1990b) also showed that the veins in the Sapporo-Iwanai district strike E-W and are oblique to the NW-SE volcanic chains which are sub-parallel to the maximum principal stress estimated in southwest Hokkaido during Late Miocene to Holocene and oblique subduction of Pacific Plate was active during the Plio-Pleistocene age. 

Uyeda and Kanamori (1979) divided mode of subduction into two types Mariana-type characterized steep subduction and Chilean-type characterized by gentle subduction estimated from the dip of Benioff-Wadati zones (Fig. 1.161). The geological phenomena associated with these subductions are shown in Fig. 1.162. It is inferred that the change in mode of subduction from Mariana-type to Chilean-type occurred at ca. 5 Ma in Northeast Honshu. Kuroko deposits are associated with Mariana-type, whereas epithermal vein-type deposits (particularly Au-Ag deposits) with Chilean-type. 

Figure 1.162. Two kinds of subduction boundaries (Uyeda and Kanamori, 1979).

Injection of the asthenosphere into the mantle wedge and the dam-up effect of the subducted slab explains the rifting process in the Japan Sea (Fig. 1.165). 

Model 4 is also a plate kinematic model. The retreat of a fore arc plate forms a back-arc basin. This model seems attractive. Jackson et al. (1975) found the periodicities of rotational motions of the Pacific plate. When the direction of the Pacific plate changed and obliquely subducted, the compressional force of oceanic plate to continental plate decreases. That means that the retreat of fore arc plate occurs. 

In any model, back-arc basins form under the extensional stress regime and are associated by Mariana-type subduction by Uyeda and Kanamori (1979) rather than Chilean-type subduction. 

Figure 1.197, Subduction mode of the Pacific plate beneath the Northeast Japan arc, and style of the epithermal systems in the Kameda peninsula, Hokkaido. Normal, oblique and highly oblique subduction are tentatively defined by the angle (a) between the assumed subduction direction of the Pacific plate and the trend of the Northeast Japan arc. Normal subduction (60° < a = 90°), oblique subduction (30° < a = 60°) and highly oblique subduction (0° < a = 30°). Highly oblique subduction has not occurred along the Northeast Japan arc since 11 Ma. (Watanabe et al., 1996).

Niitsuma, N. and Akiba, F. (1984) Subduction of plate and tectonics of the Japanese Islands of Neogene age. Kaiyokagaku (Ocean Science), 163, 4-9 (in Japanese). 

Uyeda, S. (1983) Hydrothermal circulation in the Mariana trough, Kuroko and the mode of subduction. Mining Geology Special Issue, 11, 37-53 (in Japanese with English abst.). 

Uyeda, S. and Nishiwaki, C. (1980) Stress field, metallogenesis, and mode of subduction. Geol. Soc. Canada, Special Paper, 20, 323-339. 

Honma et al. (1991) have shown that the Okinawa Trough basalts have significantly high K, Rb and Sr contents and D/H, 0/ 0 and Sr/ Sr ratios than N-Morb have and these are due to generation of magma from normal-type mantle peridotite modified by component from the subducted slab and crustal contamination. 

Several hydrothermal sites have been discovered on the seafloor of Izu-Bonin arc that is located at the eastern margin of the Philippine Sea plate (Fig. 2.32). This arc has been formed, related to the westward subduction of the Pacific plate (Fig. 2.32). Hydrothermal mineralization occurs both in back-arc depression and volcanic chain (Shichito-Iwojima Ridge). Hydrothermal venting and mineralizations are found... 

As mentioned already in Chapter 2, submarine volcanism occurs not only at midoceanic ridges but also at subduction-related tectonic settings such as the Shikoku and Daito Basins, Farce Vela Basins, and Mariana Trough, Okinawa Trough and Izu Bonin Arc (e.g.. Wood et al., 1980 Dick, 1982 Delaney and Boyle, 1986). 

At the convergent plate boundaries, CO2 degasses not only from back-arc basins by hydrothermal solutions but also from terrestrial subduction zones by volcanic gases and hydrothermal solutions. However, the studies on CO2 degassing from terrestrial subduction zones are not many. Seward and Kerrich (1996) have shown that hydrothermal CO2 flux from terrestrial geothermal system (such as Taupo volcanic zone in New Zealand) exceeds lO mol/year which is comparable to that of midoceanic ridges (Table 3.4). 

Sano and Williams (1996) calculated present-day volcanic carbon flux from subduction zones to be 3.1 x 10 mol/year based on He and C isotopes and C02/ He ratios of volcanic gases and fumaroles in circum-Pacific volcanic regions. Williams et al. (1992) and Brantley and Koepenich (1995) reported that the global CO2 flux by subaerial volcanoes is (0.5-2.0) x lO mol/m.y. and (2-3) x 10 mol/m.y. (maximum value), respectively. Le Guern (1982) has compiled several measurements from terrestrial individual volcanoes to derive a CO2 flux of ca. 2 x 10 mol/m.y. Le Cloarec and Marty (1991) and Marty and Jambon (1987) estimated a volcanic gas carbon flux of 3.3 X 10 mol/m.y. based on C/S ratio of volcanic gas and sulfur flux. Gerlach (1991) estimated about 1.8 x 10 mol/m.y. based on an extrapolation of measured flux. Thus, from previous estimates it is considered that the volcanic gas carbon flux from subduction zones is similar to or lower than that of hydrothermal solution from back-arc basins. 

Sulfur in the sediments and oceanic crust which is derived from seawater subducts to deeper parts. This subduction flux is estimated to be ca. 4 x lO mol/m.y. (Shikazono, 1997). Therefore, degassing S flux from back-arc and island arc ((2.3-8.2) x lO mol/m.y.) seems to be not different from the subduction flux, although uncertainty of estimated degassing and subduction flux is large. 

We speculate from the above argument that primordial sulfur degasses from midoceanic ridges even at present time as well as He, because subduction flux to mantle seems to be small. However, we need more detailed study on long-term S cycle including hydrothermal S flux to evaluate this speculation. 

Geochemical balance of arsenic in ocean and subduction flux (g/year) (Shikazono, 1993)... 

The geochemical balance of As in ocean and subduction flux of As are summarized in Table 3.6. 

Sano, Y. and Williams, S.N. (1996) Fluxes of mantle and subducted carbon along convergent plate boundaries. Geophy.s. Res. Udt., 23, 2749-2752. 

During the middle Miocene, Kuroko deposits, polymetallic vein-type deposits, gold-quartz vein-type deposits and Sb and Hg vein-type deposits were formed (see sections 1.3 and 1.6). Many vein-type deposits were formed not only in and nearby the Japanese Islands, but also at middle Miocene in northwest USA (Basin and Range Lipman, 1982), and elsewhere in the circum-Pacific regions (e.g., Peru). It is probable that large amounts of CO2 effused into the atmosphere from hydrothermal solution associated with this widespread mineralization and volcanic gas from subduction zones, causing an increase in temperature. 

Fig. 4.11. Atmospheric CO2 variation estimated by modified GEOCARB II model including volcanic eruption rate of circum-Pacific region by Kennett et al. (1977) (Kashiwagi et al., 2000). y represents the contribution of the flux from back arc basin to that from subduction zones at present. Rco = PcOi/PcOi 02 Pfesent-day PC02)-...

It is suggested that the mode of subduction of the Pacific Plate since the middle Miocene age related to Jackson s episode, hence oscillation of direction of lateral movement of Pacific plate. Synchronized igneous and hydrothermal activities and Jackson s episode indicate that the formations and characteristics of hydrothermal ore deposits (Kuroko and epithermal vein-type deposits) are largely influenced by plate tectonics (mode of subduction, direction of plate movement, etc.). For example, sulfur isotopic composition of sulfides is not controlled by /o and pH, but by of... 

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