Rifting

The features created by crustal movements may be mountain chains, like the Himalayas, where collision of continents causes extensive compression. Conversely, the depressions of the Red Sea and East African Rift Basin are formed by extensional plate movements. Both type of movements form large scale depressions into which sediments from the surrounding elevated areas ( highs ) are transported. These depressions are termed sedimentary basins (Fig. 2.3). The basin fill can attain a thickness of several kilometres. 

M. R. Rift and E. H. Covitz, Introduction to Organic Electrochemistry, Marcel Dekker, Inc., New York, 1974. 

Ritz, m.. Ritze, /. rift, cleft, fissure, crack, chink slit scratch. 

For many cases one needs to have values of thermodynamic variables for conditions very different from 298 K and 1.0 bar. These cases include reactions occurring above the tropo-pause, where pressures are several orders of magnitude less than 1.0 bar and temperatures are less than 200 K. The important reactions occurring in the high-temperature and high-pressure aqueous conditions of the mid-ocean rift zone, and the high-temperature and high-pressure conditions where important mineral transformations occur far below the Earth s surface are examples. 

Morgan, W. J. (1983). Hotspot tracks and the early rifting of the Atlantic. Tectonophysics 94,123-139. 

Several different hypotheses on the tectonic setting of the Kuroko mine area have been proposed. They include volcanic front of island arc (T. Sato, 1974 Horikoshi, 1975a), rifting of island arc (Cathles, 1983a), back-arc depression (Fujioka, 1983 Uyeda, 1983), and back-arc basin. 

Figure 1.34. Frequency histogram for MgO/FeO ratios (in wt%) of chlorite from the basalt studied (A) and MORE (B). Data sources are Shikazono and Kawahata (1987), Humphris and Thompson (1978) (M Mid-Atlantic Ridge) and Kawahata (1984) (C Costa Rica Rift, Galapagos Spreading Centre). The data on chlorite from MORE are taken from typical metabasalt and not from quartz-chlorite breccia and veins which formed in a hydrothermal upflow zone (Shikazono et al., 1987).

Figure 1.60. Variation of subsidence rate for syn-rift basins in the Uetsu district, northeast Honshu (Yamaji, 1990). The line of boxes shows the spatially averaged subsidence rate. The rate after 15 Ma is not clear because of uncertainty in paleobathymetry. However, the rate probably decreased to the order of 10-100 m/m.y. If the rate had been of the order of 1 km/m.y. after 15 Ma, the water depth of the inner arc region at 14 Ma would have been much deeper than modem, young, back-arc basins.

Figure 1.85. Relation between Fe20j content of epidote and that of original fresh rocks. S Seigoshi, Y Yugashima, N Noya, F Furotobe, O Ohtake, M Mid-Atlantic ridge, C Costa Rica rift. Mi Mitsuishi, Sh Shimokawa (Shikazono, 1984)...

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). 

Figure 1.164. Distribution of Cenozoic basalts and active rift systems in the northeast China region. Arrows indicate the horizontal convective current in the upper mantle associated with the upwelling of the asthenosphere beneath the region. A Baikal Rift B Shanxi Graben C Tancheng-Lujiang Fault D Okinawa Trough (Tatsumi et al., 1990).

INTRA-PLATE VOLCANISM BACKARC VOLCANIC CONTINENTAL RIFT SYSTEM BASIN BASIN... 

Cathles, L.M. (1983a) Kuroko-type massive sulfide deposits of Japan Products of an aborted Island-arc rift. Econ. GeoL Mon., 5, 96-114. 

Honnoretz, J., Laveme, C., Hubberten, H.W., Emmermann, R. and Muehlenbachs, K. (1983) Alteration processes in layer 2 basalts from deep sea drilling project Hole 504B, Costa Rica Rift. Initial Rep. Deep Sea Drill. Proj., 55, 509-542. 

Kawahata, H. (1984) Subseafloor hydrothermal alteration on Garapagos ridge, Costa Rica rift. Unpub. Doctoral Thesis, U. Tokyo. 

Kawahata, H. and Shikazono (1988) Sulfur isotope and total sulfur studies of basalts and greenstones from Hole 504B, Costa Rica Rift Implications for hydrothermal alteration. Can. Min. 26, 555-565. 

Kawahata, H., Kusakabe, M. and Kikuchi, Y. (1987) Strontium, oxygen, and hydrogen isotope geochemistry of hydrothermally altered and weatherted rocks in DSDP Hole 504B, Costa Rica Rift. Earth Planet. 

Yamaji, A. (1990) Rapid intra-arc rifting in Miocene mortheast Japan. Tectonics, 9, 365-378. 

Central Manus Basin Vienna Woods (3° ID S, 150°17 E) 2500 2-km-wide axial rift graben of the northeast spreading center. Mostly massive pillow lava floor. Sulfide chimneys up to 20 m high are venting clear, milky and black fluids. Sulfate smokers are also present. Sphalerite, wurtzite, pyrite, marcasite, chalcopyrite, galena, amorphous silica, barite. Sulfate chimney anhydrite, silica, barite. 

Tectonic setting Back-arc rift or continental rift Sediment-barren midoceanic Sediment-covered... 

Shinozuka et al. (1999) analyzed the host volcanic and intrusive rocks in the Minamidani mine district in the Maizuru tectonic Belt and found that these rocks formed in an island arc back-arc system near Laurasia during late Paleozoic. Probably the Yanahara deposits, one of the representative Hitachi subtype deposits, were formed in an island arc back-arc system as same as the Minamidani. Sato and Kase (1996) thought that the Hitachi-subtype deposits formed in back-arc rift or continental rift (Table 2.21). 

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