Go to 'figures/overhead' section.
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
Abstract.
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:
Global distribution of arc systems.
Map of the Sierra Nevada Arc / Franciscan Melange
Section through the Sierra Nevada Arc / Franciscan Melange of California
