Subduction Model

During Stage I (Fig. 4.27) two parallel westwarddipping subduction zone existed along the paleo-Pacific 

T T I Admiralty Intrusives metarmorphism LP, MP, HP = low/medium/high pressure, g = greenschist facies 

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At present there are a number of competing lines of evidence in support of the TTG being formed by remelting at the base of thickened crust, and in the subducted slab. Support for the subduction model is reviewed by Martin (1995), whereas evidence for the role of... 

Another objection to the subduction model cited by Weaver et al. (1984) is that the boundary zone of the Lanterman fault does not contain crustal fragments of the subduction-zone complex. In addition, they noted that plutons of the S-type Granite Harbor Intrusives, that... 

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. 

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)-...

Pickett et al. 1997) in MORE samples played an important role, not only to address the problem of magma transfer times, but also in the development of dynamic melting models able to sustain such disequilibria over long periods. In subduction zone volcanoes, the combined study of and Ra- °Th disequilibria place unique constraints on the... 

The preceding sections have established that the U-series information from arc lavas is a complex end-product of several different processes and this is especially true for U-Th isotopes. The two observations that seem to be most robust are (1) the need to transfer the Ra-excess signal from the subducting plate into the melting zone and to the surface in less than a few 1000 years, and (2) the need for a partial melting process which allows for Pa in-growth. There are a number of different models which can take account of these two requirements but which have different implications for the U-Th isotope arrays. 

Ayers JC, Dittmer SK, Layne GD (1997) Partitioning of elements between peridotite and H2O at 2.0-3.0 GPa and 900-1000°C, and application to models of subduction zone processes. Earth Planet Sci Lett 150 381-398... 

Davies JH, Biekle MJ (1991) A physical model for the volume and composition of melt produeed by hydrous fluxing above subduction zones. Phil Trans R Soc Lond 335 355-364 Davies JH, Stevenson DJ (1992) Physical model of source region of subduction zone voleanies. J Geophys Res 97 2037-2070... 

Ellam RM, Hawkesworth CJ (1988) Elemental and isotopic variations in subduction related basalts evidenee for a three component model. Contrib Mineral Petrol 98 72-80 Elkins Tanton LT, Grove TL, Doimelly-Nolan J (2001) Hot, shallow mantle melting under the Caseades voleanoe are. Geology 29 631-634... 

Brenan JM, Ryerson FJ, Shaw HF (1998) The role of aqueous fluids in the slab-to-mantle transfer of boron, beryllium, and lithium during subduction Experiments and models. Geochim Cosmochim Acta 62 3337-3347... 

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