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Volcanic Arcs & Marginal Basins

        Volcanic complexes associated with subduction may be located on  either oceanic or continental crust. A typical example of the former would be the Mariana arc in the western Pacific, whereas a typical example of the latter would  be the Andes of the South American Cordillera.   In this respect the Pacific ‘ring of  fire’ is asymmetrical.
        Arcs of the western Pacific are invariably associated with back-arc oceanic domains (Marianas, Tonga/Fiji) which form by splitting of the arc as the result of the generation of a secondary mantle convection system above the subducting plate. The active arc, migrating oceanward in association with trench roll-back and the formation of an 'accretionary prism'/trench-forearc system, becomes progressively separated from the remnant arc by the growing marginal basin. At some point however arc growth may be interrupted by consumption of the marginal basin(s), resulting in amalgamation (stage d) of the remanant  arcs with the active arc to form a composite arc.
         The Japanese arc is unusual in that although it is a migrating arc, separated from mainland Asia by the Japanese Sea, it is located on continental crust. Andean continental arcs, on the other hand,  tend to maintain their integrity and grow both vertically and horizontally with time. (The arc system of the Western United States is a special case complicated by the subduction  of the East Pacific oceanic ridge.)
        Between the locus of formation of oceanic crust at mid-ocean ridges and  the locus of ocean consumption at a subduction zone, oceanic crust is hydrated  and oxidized - olivine is converted to serpentine; pyroxenes are changed to  amphibole, plagioclase is made into epidote and chlorite, and magnetite is  oxidized to hematite.  As the oceanic crust is transferred back into the mantle  along the subduction zone, the hydrated mineral phases are converted back into  olivine, pyroxene, etc.  The water released, which contains various but probably large amounts of  dissolved solid material (e.g. Si, Fe, K, U, Zr) in solution, passes upwards into  the overlying mantle, which consequently melts and rises to the Earth's surface  to form arc volcanic complexes with a distinctive chemical composition.
       The crystallization sequence in arc volcanics is usually:
       Olivine -> clinopyroxene+magnetite/ -> orthopyroxene -> plagioclase ->  amphibole, rather than Olivine -> plagioclase -> clinopyroxene -> magnetite in  the case of mid-ocean ridge basalt. (Why would magnetite crystallize early in arc  volcanic rocks?) Consequently, arc rocks tend to have low Ti contents. (Why?)
        Primitive arcs and arcs close to the oceanic interface are rich in copper  and gold whereas more evolved arcs and those parts of arcs located away from  the oceanic interface tend to be more enriched in zinc, lead, and silver. Some  arcs, such as those of Malaysia, may also show an enrichment in tin. A similar vector variation is seen in the mineralogical and chemical variation of arc volcanic rocks - those closer to the trench tend to be quartz rich and poor in alkalies (K, Na, Ba), whereas those rocks further from the trench may contain amphibole and display enrichment in alkali elements. The oldest volcanic rocks formed closest to the trench during the initial stages of arc formation are relatively enriched in Mg and Si and are called boninites.
        It is usually assumed that oceanic and arc-derived sediments accumulating in the region between the arc and the trench (the 'accretionary prism')  are subducted back into  the mantle (the 'Andean graben bucket' mechanism). However, within the Sierra Nevadan arc (click here to see cross section) of California, the Franciscan complex  is thought to represent an example of  subducted sediments that have been 'regurgitated' back to the Earth's surface.   Such rocks are known as 'melanges' and are characterised by the presence of  ‘knockers’ of high pressure mafic rocks such as eclogite (rocks composed of  jadeite-bearing clinopyroxene and garnet) and blueschist (rocks containing the  blue amphibole glaucophane).

Blueschist/eclogite 'knocker' in the Franciscan of the San Francisco Bay area.
Dr Young examining a large Californian 'knocker'.
Eclogite: note the smooth, rounded, and grooved surface of this eclogite block.
Typical outcrop of Franciscan blueschist melange.
Franciscan melange: mixture of shale and broken-up greywacke material.
Franciscan melange: greywacke and shale.

   Since there appears to a deficit of arc rocks in most orogenic belts, it seems likely that arcs themselves are capable of being subducted and are thereby  recycled back into the mantle (e.g. the Tessier-Taylor terrane of the Yukon  region). In contrast, the Late Proterozoic Arabian-Nubian system of north east  Africa and Arabia contains an excessive concentration of arc material - it is  known as the ‘arc graveyard’ of East Africa.

    Overhead sequence:
Dietz’s miogeoclinal model. (11dietz.gif)
Global distribution of oceanic and continental margin arcs. (11arcs.gif)
Arcs and marginal basins of the western Pacific - trench-slope break; fore-arc  basin; accretionary prism;
The arc-trench model. (11karig.gif)
Crustal structure of Japan and the Japan Sea marginal basin (11japan.gif)
The Andean system. (11andes.gif)
Chain bucket model of sediment subduction. Arc accretion (11karig.gif)

    The following was obtained from: http://www.gps.caltech.edu/~gurnis/papers.html

        Dynamic interaction between tectonic plates, subducting slabs, and the mantle
       Shijie Zhong and Michael Gurnis Seismological Laboratory California Institute of Technology

    Mantle convection models have been formulated to investigate the  relation between plate kinematics and mantle dynamics. The cylindrical  geometry models incorporate mobile, faulted plate margins, a phase change at  670 km depth, non-Newtonian rheology, and tectonic plates. Models with a  variety of parameters indicate that a relatively stationary trench is more likely to be associated with a subducted slab which penetrates into the lower mantle with a steep dip angle. However, a subducted slab which is deflected above the 670 km phase change with a shallow dip is more likely to be associated with a margin which has undergone rapid retrograde trench migration. This relation between slab morphology and plate kinematics is consistent with seismic tomography and plate reconstruction of Western Pacific subduction zones. The efficiency of slab penetration through the 670 km phase change is controlled by both the buoyancy of the subducting plate and the mobility of the overriding plate. While older subducting plates have a greater propensity for slab  penetration, trench mobility reduces the propensity for slab penetration. Smaller overriding plates have a greater mobility. When subducted slabs approach the bottom thermal boundary layer, hot fluid is pushed aside, and plumes form on the periphery of slab accumulations. There are sharp temperature contrasts between subducted slab and the thermal boundary layer at the core mantle boundary (CMB). Old subducted slabs and a thermal boundary layer lead to large-scale lateral structure near the CMB.  The entire Paper is available as PDF file
Table 1  Figure 1  Figure 2  Figure 3  Figure 4  Figure 5
Figure 1, Model setup for cases within a full cyinder.
Figure 2, Time history of plate velocities (A) and trench migration (B) for Case 1.
Symbols O and C in A indicate the velocities of oceanic and continental plates, respectively. The green dashed line in A indicates zero velocity. Figure 3, Time history of plate velocities (A) and migration of plate margins (B) for Case 2. Symbols O-U, C-U, O-L, and C-L in A indicate the velocities of oceanic and continental plates in the upper and lower hemispheres, respectively.
In B, symbols F1 and F2 dictate the migration of faulted converging margins in the upper and lower hemispheres, respectively, while symbols R1 and R2 stand for the spreading centers bounded by two oceanic plates and two continental plates, respectively. The green dashed line in A indicates zero velocity.
Figure 4, Time history of plate velocities (A) and migration of plate margins (B) for Case 3. See Figure 3 for symbol convention.
Figure 5, Time history of plate velocities (A) and migration of plate margins (B) for Case 4. See Figure 3 for symbol convention.

Animations - this abstract and the associated animations (may take several minutes to download, and needs the Quicktime plugin)can be seen at:

Subduction Models


Dietz's miogeoclinal model.

Global distribution of arc systems.

A model oceanic arc.

The Japanese arc.

The Andean arc.

Arc accretion.

Map of the Sierra Nevada Arc / Franciscan Melange

Section through the Sierra Nevada Arc / Franciscan Melange of California

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