Asthenosphere

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). 

Injection of the asthenosphere was associated with the eastward horizontal convective flow caused by the upwelling of asthenosphere beneath the northeastern China region (Fig. 1.165). 

Otofuji (1996) proposed a double door opening mode with a fast spreading rate of 21 cm/year for the evolution of the Japan Sea, caused by the injection of asthenosphere with a low viscosity beneath the Japan Sea area. 

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).

Figure 1.165. Schematic diagram showing the effect of upwelling of asthenosphere beneath a continent which is adjacent to an arc-trench. system (Tatsumi et al., 1990).

Tatsumi, Y., Maruyama, S. and Nohda, S. (1990) Mechanism of back arc opening in the Japan sea role of asthenospheric injection. Tectonophysics, 181, 299-306. 

Olafsson M, Eggler DH (1983) Phase relations of amphibole, amphibole-carbonate, and phlogopite-carbonate peridohte petrologic constraints on the asthenosphere. Earth Planet Sci Lett 64 305-315 Olson P, Schubert G, Anderson C, Goldman P (1988) Plume formahon and lithosphere erosion a comparison of laboratory and numerical experiments. J Geophys Res 93 15065-15084 Pearson DG, Shirey SB, Carlson RW, Boyd FR, Nixon PH (1995) Stabilisahon of Archean lithospheric manhe A Re-Os isotope isotope study of peridohte xenoliths. Earth Planet Sci Lett 134 341-357... 

Herrstrom EA, Reagan MK, Morris JD (1995) Variations in lava composition associated with flow of asthenosphere beneath southern Central America. Geology 23 617-620 Hildreth W, Moorbath S (1988) Crastal contributions to arc magmatism in the Andes of Central Chile. Contrib Mineral Petrol 98 455-489... 

Many properties and characteristics of the Earth are determined by plate tectonics, according to the theory of which the lithosphere is not a closed shell instead it consists of about a dozen large, rigid plates. These are constantly in motion—on a geological timescale. Each of the plates moves as an independent unit and swims on the softer, but more dense, asthenosphere (Press and Siever, 1995). 

Fig. 2.5 The outer shell of the Earth, the lithosphere, is a solid, rigid layer. It consists of the crust and the outer parts of the mantle. The lithosphere swims on the flexible, partially melted part of the mantle (the asthenosphere). Figure simplified after Press and Siever (1994). With permission of W. H. Freeman and Company, New York...

Figure 5.5 Least-square mixing hyperbola for the isotopic data on Heard Island of Barling and Goldstein (1989). Data from Table 5.10. The 87Sr/86Sr value of the MORB source ( 0.7025) lies below the horizontal asymptote. Asthenosphere and oceanic lithosphere are unlikely source components of Heard Island basalts.

Isostacy The condition of gravitational equilibrium that is attained by lithospheric units as a result of adjustments in their positions as they float in the asthenosphere. 

Figure 3.1 A cross section of the crusts, rigid upper mantle, and plastic mantle asthenosphere of the Earth. The Mohorovicic Discontinuity is the boundary between the crust and upper mantle. The lithosphere includes the crusts and the upper mantle.

Asthenosphere A plastic and partially molten layer in the mantle under the lithosphere. The asthenosphere usually extends from about 70 to 200km below the Earth s surface. 

Spreading zone A divergent boundary between two tectonic plates where magma from the asthenosphere rises and creates new crust on the adjacent plates. The Mid-Atlantic Ridge is an example of a spreading zone. 

Tectonic plate A section of the Earth s lithosphere (the crusts and underlying rigid upper mantle) that slowly moves on the underlying plastic asthenosphere. The Earth s lithosphere is divided into about 15 tectonic plates. Some plates consist entirely of oceanic crust, while others have substantial continental crust. The movement of tectonic plates over time is described by the theory of plate tectonics. 

Fig. 8.6. Sr-Nd (A) and Pb (B) isotope diagrams for the Sicily Province. Compositions of HIMU, DMM, MORB and European Asthenospheric Reservoir (EAR) are also shown. See text for explanation.

In contrast, Esperanca and Crisci (1995) and Trua et al. (1998) propose that magmatism in the Sicily Province is derived from young lithosphere that was enriched by addition of asthenosphere-derived melts. As recalled earlier, this is a relatively young process, which occurred during Permian-Triassic times. The young age of metasomatism is supported by Sr-Nd isotopic studies on the Iblei xenoliths, which give a pseudo-isochron of about 200 Ma (Tonarini et al. 1996). 

The variable isotopic compositions (especially for Pb) reveal geochemi-cally heterogeneous sources. The origin of this heterogeneity is debated and may derive either from shallow mantle processes, such as variable me-tasomatic modifications of the lithosphere by asthenospheric melts, or from mixing between a deep-mantle plume and asthenosphere-lithosphere material. Etna and Ustica show some trace element and isotopic characteristics (e.g. Rb/Nb, Ce/Pb, and boron isotopes), indicative of a contribution from subduction-derived components. 

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