Download PLATE TECTONICS: Lecture 5

PLATE TECTONICS: Lecture 5
SUBDUCTION ZONES and ISLAND ARCS
Subduction Zones are where cool lithospheric plates sink back into the
mantle. It takes about 50 my for the ocean lithosphere that formed in the hot
(>1000°C) environment at mid-ocean ridges to cool to an equilibrium state
and sink to its maximum depth below sea-level. Although there is no universal
agreement on the balance of forces that drives plate tectonics, the "slab-pull"
force is thought to be an important one. For instance the Pacific Plate is the
fastest moving plate (ca. 10 cm/yr), and this is the plate that supplies most of
the Earth's subducting lithosphere, and thus where the overall slab-pull force
will be the larger. The normal argument is that the cool ocean crust will more
easily convert to dense eclogite which, as we have seen in Lecture 1, is much
more dense than pyrolite.
What is most surprising is the great variation in geological features associated
with subduction. There is a huge difference between the East Pacific and the
West Pacific. Not only that, but there are differences along the Andean
margin, and also quite major differences as we go back in time. But it is
important to understand subduction because this is where the continental
crust grew progressively with time.
Subduction is where tectonics, structural geology, sedimentation, igneous
petrology, metamorphism, geochemistry, geophysics and applied geology all
interact. Typical "textbook" features of a mature continental margin subduction
zone are shown below. The cartoon shows sediment being scraped off the
downgoing plate to form an accretionary wedge, and that a forearc basin is
forming on top of the wedge as it is dragged down (and is presumably fed by
volcanic debris from the arc). However, the cartoon avoids the issue of how
and where the volcanic magmas come from.
To what extent does the basaltic subducted slab contribute to arc magmas?
Is it just the fluids carried down in altered oceanic crust that migrate into the
mantle wedge overlying the subduction zone and cause melting?
Ot what extent do sediments carried down the subduction zone then
contribute to arc magmas?
Why are arc volcanoes nearly always situated about 110 km above the
Benioff Zone? What happens to material taken down the subduction zone?
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MARGINAL BASINS & BACK ARC SPREADING
Marginal basins are a common feature of the Western Pacific. Examples
(north to south) are the Sea of Japan, the West Philippine Basin, the Parace
Vela & Shikoku Basins, the Mariana Trough, the Woodlark Basin, the Fiji and
Lau Basins. By contrast marginal basins are rarer in the Eastern Pacific. The
two examples in the Atlantic are the Caribbean and the Scotia Sea.
Marginal basins are small oceanic basins, usually adjacent or "marginal" to a
continent, which are separated from larger oceans by an island arc. Some
marginal basins at continental margins may be imperfectly developed and
represented by thinned crust, often associated with basic volcanism. Karig
(1971, 1974) divided marginal basins into:
(1) Active marginal basins with high heat flow.
(2) Inactive marginal basins with high heat flow.
(3) Inactive marginal basins with normal heat flow.
The first two are thought to have formed by back-arc spreading, either still
active (1), or recently active (2). The third may represent basins formed by
even older back-arc spreading, or normal ocean crust that has been "trapped"
behind a recently developed oceanic island arc.
FRAMEWORK OF AN ISLAND ARC SYSTEM
The commonly held model of an arc - back-arc system has the following
components:
(1) Subduction Zone
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(2) Fore-arc region with accretionary sedimentary prism
(3) Frontal Arc
(4) Active Arc
(5) Marginal Basin with spreading centre
(6) Remnant Arc
(7) Inactive Marginal Basin
Although the extensive fore-arc region of many island arcs was thought to be
composed of off-scraped sediments, drilling has not substantiated this. It
appears that - at least at intraoceanic arcs - abyssal sediments on the
downgoing plate are largely subducted.
That the back-arc region is a zone of asthenospheric upwelling is supported
by seismic evidence which suggests a low-Q (seismic attenuation) zone
behind the arc, compatible with a small amount of melt in the back-arc region:
Magnetic anomalies in back-arc basins are not so well developed, nor have
such symmetrical linear patterns, as those in the normal ocean basins. There
have been difficulties in identifying the anomalies. It has been suggested by
Lawver & Hawkins (1978) that spreading may be more diffuse and not
constrained to one central well-defined spreading centre. Good dateable
magnetic anomaly patterns were first described from the Scotia Sea back-arc
basin (IA Hill). Spreading in some basins may be asymmetric, with accretion
favoured on the active arc side.
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Models for Back-arc Spreading (see Karig, 1974)
Active Diapirism: One of the earliest models, based on the Mariana Arc
System, is that of an uprising diapir splitting the arc. The diapir is initiated
either as a result of frictional heating at the subduction zone, or more likely
through fluids released from the dehydrating subducting slab. The rising diapir
then splits the arc in two and the two halves are progressively separated by
seafloor spreading:
Passive Diapirism: This results from regional extensional stresses in the the
lithosphere across the arc system. In effect the downgoing slab, although
acting like a conveyor belt, also has a vertical component that causes "rollback". The arc and forearc then stays with the subduction zone, as a result of
a supposed trench suction force:
Stepwise Migration: Here it is assumed that the subducting slab is snapped
off near the hinge, presumably because something on the downgoing slab is
too light to go down, and so a new subduction is initiated oceanwards. The
arc stays near the hinge and the asthenosphere wells up behind it:
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Convection-driven: This model proposed by Toksoz & Bird (1978), and
requires that subsidiary convection cells are driven by the downward drag of
the downgoing slab. Calculations suggest that spreading would occur about
10 my after the start of subduction. This might explain why back-arc spreading
is more common in oceanic regions ™ the lithosphere is thinner and thus
more easily disrupted than under continents:
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Uprising Harzburgite Diapir: This model (Oxburgh & Parmentier 1978)
depends on the fact that refractory lithosphere (which has lost its basalt
component at mid-ocean ridges) is less dense and inherently more buoyant
than normal fertile mantle. Thus it would rise if heated to same temperature as
surrounding mantle. Such diapirs could in theory be derived from subducting
lithosphere, although it is doubtful that subducting lithosphere could be heated
within 10 my; more likely it takes 1000 - 2000 my according to megalith
concepts of Ringwood (1982):
Old and Young Lithosphere: Molnar & Atwater (1978) have argued that it
depends on the dip of the subducting slab whether extension occurs in the
back arc region. In the W. Pacific it is old (Jurassic), cold and dense
lithosphere that is subducting - with very steep dip and strong vertical
component. Thus extensional conditions in back-arc region. In the E. Pacific,
on the other hand, the lithosphere subducting beneath the Andes is young
(Tertiary), warm and less dense, and subducts at a shallow angle. Thus
convergence is more compressive than extensional. Uyeda & Kanamori
(1979) have characterised these two extreme types of subduction as Mariana
and Chilean type respectively. See also Dewey (1981)
Other models: Various researchers have since commented on the possible
causes of back-arc spreading, including assessments of dependence on
absolute and relative plate motions. Consult some of references listed below.
Experimental laboratory studies have been carried out by Kincaid & Olsen
(1987), observing the effects of continued subduction where the subducting
slab 'hits' the 650 km discontinuity. The results show that steep subduction
does produce a significant roll-back effect on the hinge, which will generate
extensional conditions in the back-arc region. Note that with subduction rates
of about 7 cm/yr it would take about 10 my before newly subducted ocean
lithosphere would 'hit' the 650 km discontinuity and begin to initiate 'roll-back'
of the hinge, and thus extensional conditions.
EVOLUTION OF MARIANA ARC SYSTEM
The Mariana Arc is perhaps the type intra-oceanic arc system, and the most
extensively studied through marine geophysical studies, dredging and drilling
(particularly Legs 58, 59 and 60 of DSDP in late 1970's). From west to east it
consists of the following features:
(1) West Philippine Basin
(2) Kyushu-Palau Ridge (a remnant arc)
(3) Shikoku & Parece-Vela Basins
(4) West Mariana Ridge (a remnant arc)
(5) Mariana Trough
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(6) Active Mariana Arc
(7) Mariana Fore-arc (made of old arc)
(8) Mariana Trench (up to 11 km deep)
(9) The subducting Pacific Plate (Jurassic age)
West Philippine Basin: This may be 'trapped' in origin and not strictly formed
by back-arc spreading. It appears to pre-date the Kyushu-Palau Ridge.
Magnetic anomalies suggest active spreading in the early Tertiary (62-40 Ma)
with the NW-SE trending Central Basin Fault as the spreading centre. The
Oki-Daito Ridge in the northern West Philippine Sea is aligned parallel to this
feature and has been regarded as an old remnant arc: however drilled
samples from the Oki-Daito Ridge are alkaline basalts, not island arc basalts.
Drilled samples from the W. Philippine Basin are fairly typical MORB.
The Philippine Basin is slowly subducting to the west beneath Taiwan, etc.
The subduction rate is much less than that of the Pacific Plate beneath the
Marianas.
Kyushu-Palau Ridge: This is over 2000 km long and rises 2 km above the
adjacent basin floors. Consists of vesicular lava flows, dykes and sills,
interbedded with volcaniclastic breccias lying below Middle Oligocene oozes.
Lavas all belong to Island Arc Tholeiite (IAT) Series, typical of the most
primitive island arcs. Now an inactive Remnant Arc that was active between
about 42 and 32 my ago.
Parece-Vela and Shikoku Basins: Magnetic anomaly patterns indicate
back-arc spreading between 30 and 17 my in Parece-Vela and between 26
and 15 my in the Shikoku Basin in north. Basaltic sills common in sediments
near basement, indicating high rates of sedimentation near near ridge axis.
Basalts are vesicular. Similar to MORB.
West Mariana Ridge: Shallower and younger than the Kyushu-Palau Ridge.
Drilling penetrated about 1000 m of volcaniclastic material composed of
basalts, basaltic andesites, rare andesites and plagioclase phenocrysts. Their
character is calc-alkaline, with much higher contents of Ba and Sr than those
of K-P Ridge. Arc was active 17-8 my ago. So now a Remnant Arc. Arc built
up when spreading in P-V / Shikoku Basins ceased.
Mariana Trough: This is 1500 km long, 250 km wide. Rough topography,
high heat flow. Magnetic lineations poorly developed, but suggest back arc
spreading from about 6 my ago - i.e. when activity on West Mariana Ridge
ceased. Near the West Mariana Ridge metabasalts, gabbros and anorthositic
cumulates were drilled - deeper part of a rifted-apart arc? Basalts in Mariana
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Trough are MORB-like, but have some arc characteristics. Vesicular.
Spreading still in progress. Further north, on Iwo-Jima Ridge, there is an
incipient back-arc basin just beginning to form - the Bonin Trough.
Mariana Active Arc: This consists of numerous small islands and seamounts,
on the eastern edge of the extensive Fore-arc region. Lavas are mainly
basalts, basaltic andesites and andesites.
Mariana Fore-arc: The forearc region shows a history of continual
subsidence. The basement is Eocene in age (similar to Kyushu-Palau Ridge)
and consists of two distinct lava types:
(1) Island Arc Tholeiites (very similar in character to those of KyushuPalau Ridge). These magmas can normally be easily distinguished
from calc-alkaline basalts from more mature arc systems.
(2) Boninites, or high-magnesian andesites. These are unusual lavas,
combining high Si with high Mg, Ni and Cr. They are thought to have
formed by wet-melting of rather refractory lithosphere.
(3) Dacites also occur on Guam.
Drilling and dredging in the trench area of the fore-arc has recovered mainly
volcanic materials. No scraped-off sediments from the oceanic plate - with the
implication that all sediment is being subducted, and that the fore-arc itself is
suffering tectonic erosion as a result of the rasping action of the downgoing
slab.
TECTONIC EVOLUTION OF MARIANA ARC SYSTEM
Combining evidence from magnetic anomalies, drilling, dredging and
geochronology, the geologic history of the arc system can be pieced together.
In the period immediately preceding the development of the arc, the plate
configuration in the eastern Indian Ocean and western Pacific was dominated
by the rapid movement of India northward. There were some major N - S
oriented transform faults at this time, so about 60 Ma ago the plate tectonic
configuration probably looked like this:
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India was just about to collide into Asia to form the Himalayas, Australia had
just begun to separate from Antarctica, and note the very large ridge offsets
on the N-S transforms. The critical point at this time was that slab-pull
associated with the rapidly-moving Indian Plate will stop as soon as India
collides. Similarly, the spreading ridge in the NE Pacific is going to push itself
under the Aleutians, when upon the slab-pull will also stop. This leaves the
northerly pull forces on the Pacific plate very weak, and very vulnerable to
change in plate motion direction. So about 40 my ago the Pacific Plate
changed motion from northwards to westward (c.f. kink in Hawaiian-Emperor
seamount chain). The sequence of events can be tracked as follows:
(1) The Kyushu-Palau Ridge is thought to mark the position of one of these
major transform faults, with younger, warmer and thinner ocean ocean
lithosphere to the west, and older, cooler and denser lithosphere to east.
Drawn to scale, the position immediately before the change in plate motion
probably looked like this:
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It can easily be envisaged how the eastern side would easily subduct under
the new young warm lithosphere to the west that had recently formed at a
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spreading ridge. After the change in plate motion direction, the map then
looked like:
A new volcanic arc forms at the site of the easternmost transform, and many
complications develop in SE Asia (Philippines, etc.) because of transforms
turning into arcs, and various subduction-flips as thick (plateau-type) ocean
crust refuses to subduct. A new subduction zone develops north of Australia.
(2) Rapid build-up of Kyushu-Palau Arc in late Eocene – Oligocene through
voluminous eruption of island arc tholeiites and high-Mg boninites. Activity
continued for ca 10 my. So what happened to bring about such a rapid rate of
magma production. It is possible that the earliest stages of subduction looked
as follows:
Note that the downgoing plate not only has "conveyor-belt" motion, but also a
strong vertical component so that it is sinking into the mantle. At this point hot
asthenosphere mantle rushes in to replace it. So in a rather unique rapidly
extensional tectonic environment, wet altered ocean crust is juxtaposed next
to very hot asthenospheric mantle. With an abundance of heat and water, it is
not surprising that huge amounts of magma are generated. This tectonic
situation is actually even more extensional than at a mid-ocean ridge, so it
may be expected that all the features of a "type" mid-ocean ridge are
reproduced: pillow lavas, sheeted dykes, gabbros, etc. This is shown below:
(to come)
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(3) Splitting of K-P Arc in half about 30 my ago with formation of
Parece-Vela & Shikoku Basins by back-arc spreading. Spreading
stopped about 16 my ago.
(4) Formation of West Mariana Arc between about 17 and 8 my ago
through eruption of calc-alkaline basalts and basaltic andesites.
(5) Splitting of West Mariana Arc abut 6 my ago to form Mariana
Trough by back-arc spreading, and leaving West Mariana Ridge as
remnant arc.
(6) Formation of new Mariana Arc 5 my ago to present. Now erupting
lavas with mixed calc-alkaline - island arc tholeiite characteristics.
Presumably the Mariana Arc will continue migrating eastwards into the
Pacific.
Magma Compositions
Arc Magmatism
The magmas erupted at the Mariana Arc show a gradual evolution in
composition with time. Note that the whole arc system has evolved entirely
within the oceanic regime (no continental crust or sub-continental lithosphere
involved).
The earliest lavas erupted (now seen on Kyushu-Palau Ridge and Mariana
Fore-arc) are island arc tholeiites (IAT) and boninites. These are characteristic
of very primitive oceanic island arcs, and are not usually erupted on
continents or in the later stages of arc development. IAT have similarities with
mid-ocean ridge basalts (MORB), in having depleted rare-earth element
(REE) patterns, but are usually more Fe-rich and with low Cr and Ni contents,
very low Nb and Ta, higher K contents and high K/Rb ratios. Boninites are
high-Mg lavas, but have high silica contents more typical of andesites; they
have high Cr and Ni contents, but have lower Ti contents and higher K, Rb,
Ba and Sr contents than would normally be expected of high-Mg rocks.
Boninites are thought to result from wet melting of the rather refractory Mgrich mantle wedge beneath the developing arc - with the wedge being
contaminated with elements such as K, Rb, Ba, Sr transported from the
subduction zone during dehydration of the hydrous ocean crust.
IAT could be melts of the more fertile asthenosphere, the magmas then
undergoing extensive crystal fractionation en route to the surface. Or they
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could represent melts of subducted ocean basalt crust (only possible at the
very start of subduction when the ocean lithosphere is pushed down into hot
mantle).
After opening of the Parece Vela basin by back-arc spreading, arc volcanic
activity was transferred 17 my ago to the what is now the West Mariana
Ridge, and continued building up that arc for ca. 9 my. The lavas erupted
however were mainly calc-alkaline basalts (CAB) and basaltic andesites, with
higher Al contents, much higher Sr and Ba contents and light rare-earth
enriched rather than depleted REE patterns. These lavas are more similar to
calc-alkaline lavas erupted at continental margins (though the latter are
usually dominated by andesite rather than basaltic andesites).
These CAB magmas may have been derived from the mantle wedge. But if so
there is an implication that the wedge may have been enriched in Ba, Sr, light
REE, etc., perhaps as a result of continued fluid transport of these elements
into the wedge from the dehydrating subducting slab.
Modern lavas erupted at the active Mariana Arc tend to be mainly andesites
and basaltic andesites having characteristics in between those of IAT and
CAB. There is some evidence that a small component (ca. 0.5%) of
subducted abyssal sediment is involved in their source regions.
Perhaps the most interesting aspect of the Mariana arc is that at least three
distinct magma types appear to have been generated from the one
subduction zone. Yet the whole arc system evolved entirely within the oceanic
environment.
Back-arc Basalts
In many respects marginal basin basalts (MBB) are similar to normal midocean ridge basalts (N-type MORB). However during the early stages of backarc spreading, when the uprising mantle diapir splits the volcanic arc, the
basalt magmas are derived from the sub-arc mantle. These basalts tend to
have an arc-like geochemical signature. Thus their REE patterns may be
slightly light REE enriched, they have higher Ba, Sr, K and Rb, but low Nb and
Ta. Moreover they tend to have higher water contents and be vesicular - a
consequence of fluids distilled from the subducting slab. These features are
useful discriminants in trying to characterise ophiolites as being derived from
either obducted ocean floor or marginal basin crust. See Saunders & Tarney
(1984; 1991) for summary.
Addition: Schematic cross-section across the Mariana Arc showing the
components involved in magma generation.
Fluids are released from the sub-ducting slab as "wet" amphibolite
recrystallises at ca. 100km depth to dry dense eclogite. These fluids migrate
upwards into the mantle wedge and induce melting of the sub-arc lithosphere.
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(The more water, the more melting, and higher the magma production?).
However, this mantle varies in it's fertility because of previous metasomatic
events affecting the deeper lithosphere.
More active mantle diapirism occurs in the back-arc region, and this results in
much more melting and active spreading. Hydrous fluids are still involved in
these mamgas, but to a lesser extent than in the arc rocks.
WHAT CAUSED THE CHANGE IN PACIFIC PLATE MOTION THAT
PRODUCED THE MARIANA ARC?
If we bear in mind that plate motions are dominantly controlled by 'slab pull',
then anything which reduces the slab-pull force will encourage changes in the
direction and speed of plate motion. It is notable that in the southeastern
Pacific the Aluk Ridge (spreading centre) began to progressively subduct
along the Antarctic Peninsula; at the same time, the northwestern Pacific the
Kula Ridge began to subduct beneath the Aleutians - Kamchatka. A result
was a marked reduction in the N™S slab-pull, because recently formed hot
lithosphere is not very dense and not keen to subduct. In combination with
other plate re-configuring events worldwide, this may have been enough to
cause switch in Pacific Plate motion from N – S to E – W. But see Richards et
al. (1996)
REFERENCES: Arcs and Marginal Basins
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The references below lead to most aspects of interest to island arcs, even if
you just look at the abstracts & diagrams!
BLOOMER, S.H. 1987. Geochemical characteristics of boninite- and tholeiite-series volcanic
rocks from the Mariana forearc and the role of an incompatible element-enriched fluid in
arc petrogenesis. Geological Society of America, Special Paper 215, 151-164.
CARLSON, R.L., HILDE, T.W.C. & UYEDA, S. 1983. The driving mechanism of plate
tectonics: relation to age of the lithosphere at trenches. Geophysics Research Letters
10, 297-300.
CHASE, C.G. 1978. Extension behind island arcs and motions relative to hot spots. Journal of
Geophysical Research 83, 5385-5387.
CHASE. C.G. 1979. Asthenospheric counterflow: a kinematic model. Geophysical Journal of
the Royal Astronomical Society 56, 1-18.
CRAWFORD, A.J., BECCALUVA, L. & SERRI, G. 1981. Tectono-magmatic evolution of the
West Philippine-Mariana region and the origin of boninites. Earth and Planetary
Science Letters 54, 346-356.
DAVIES, J.H. & STEVENSON, D.J. 1992. Physical model of source region of subduction
zone magmatism. Journal of Geophysical Research 97, 2037-2070.
GARFUNKEL, Z., ANDERSON, C.A. & SCHUBERT, G. 1986. Mantle circulation and the
lateral migration of subducted slabs. Journal of Geophysical Research 91, 7205-7223.
HAMILTON, W.B. 1988. Plate tectonics and island arcs. Geological Society of America
Bulletin 100, 1503-1527.
HASTON, R. & FULLER, M. 1991. Palaeomagnetic data from the Philippine Sea plate and
their significance. Journal of Geophysical Research 96, 6073-6098.
HAWKINS, J.W., BLOOMER, S.H., EVANS, C.A. & MELCHIOR, J.T. 1984. Evolution of intraoceanic arc-trench systems. Tectonophysics 102, 174-205.
HICKEY, R.L. & FREY, F.A. 1982. Geochemical characteristics of boninite series volcanics:
implications for their source. Geochimica et Cosmochimica Acta 46, 2099-2115.
HILDE, T.W., UYEDA, S. & KROENKE, L. 1977. Evolution of the western Pacific and its
margin. Tectonophysics 38, 145-167.
HOLE, M. J., SAUNDERS, A. D., MARRINER, G. F. & TARNEY, J. 1984. Subduction of
pelagic sediment: implications for the origin of Ce-anomalous basalts from the Mariana
Islands. Journal of the Geological Society, London 141, 453-472.
HSUI, A.T., MARSH, B.D. & TOKSOZ, M.N. 1983. On melting of the subducted ocean crust:
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IDA, Y. 1983. Convection in the mantle wedge above the slab and tectonic processes in
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JURDY, D.M. 1979. Relative plate motions and the formation of marginal basins. Journal of
Geophysical Research 84, 6796-6802.
JURDY, D.M. & STEFANICK, M. 1983. Flow models for back-arc spreading. Tectonophysics
99, 191-200.
KARIG, D.E. 1974. Evolution of arc systems in the Western Pacific. Annual Reviews of Earth
and Planetary Sciences 2, 51-78.
KARIG, D.E. 1971. Structural history of the Mariana island arc system. Geological Society of
America Bulletin 82, 323-344.
KARIG, D.E. 1971. Origin and development of marginal basins in the Western Pacific. Journal
of Geophysical Research 76, 2542-2561.
KARIG, D.E. 1982. Initiation of subduction zones - Implications for arc evolution and ophiolite
development. Geological Society of London, Special Publication 10, 563-576.
KINCAID, C. & OLSON, P. 1987. An experimental study of subduction and slab migration.
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KUSHIRO, I. 1990. Partial melting of mantle wedge and evolution of island arc crust. Journal
of Geophysical Research 95, 15929-15939.
LAWVER, L.A. & HAWKINS, J.W. 1978. Diffuse magnetic anomalies in marginal basins: their
possible tectonic and petrologic significance. Tectonophysics 45, 323-339.
MARSH, B.D. 1979. Island arc development: some observations, experiments and
speculations. Journal of Geology 87, 687-713.
MOLNAR, P. & ATWATER, T. 1978. Interarc spreading and cordilleran tectonics as alternates
related to the age of subducted ocean lithosphere. Earth and Planetary Science Letters
41, 330-340.
MUELLER, S. & PHILLIPS, R.J. 1991. On the initiation of subduction. Journal of Geophysical
Research 96, 651-665.
NATLAND, J.H. & TARNEY, J. 1982. Petrological evolution of the Mariana Arc and Back-arc
Basin System: a synthesis of drilling results in the South Philippine Sea. Initial Reports
of the Deep Sea Drilling Project 60, 877-908 (Washington: U.S. Government Printing
Office).
PEACOCK, S. M. 1990. Fluid processes in subduction zones. Science 248, 329-337.
RICHARDS, M.A. & LITHGOW-BERTELLONI, C. 1996. Plate motion changes, the
Hawaiian™Emperor bend, and the apparent success and failure of geodynamic
models. Earth and Planetary Science Letters 137, 19-27.
RINGWOOD, A.E. 1974. The petrological evolution of island arc systems. Journal of the
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SAUNDERS, A.D. & TARNEY, J. 1984. Geochemical characteristics of basaltic volcanism
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SAUNDERS, A.D. & TARNEY, J. 1991. Back-arc basalts. In FLOYD, P.A. (ed) Oceanic
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SHEMENDA, A.I. 1993. Subduction of the lithosphere and back arc dynamics: insights from
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SPENCE, W. 1987. Slab pull and the seismotectonics of subducting lithosphere. Reviews of
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STERN, R.J. & BLOOMER, S.H. 1992. Subduction-zone infancy - Examples from the Eocene
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STERN, R.J., BLOOMER, S.H., LIN, P.-N. & SMOOT, N.C. 1989. Submarine arc volcanism in
the southern Mariana arc as an ophiolite analogue. Tectonophysics 168, 151-170.
TARNEY, J., SAUNDERS, A.D. & WEAVER, S.D. 1977. Geochemistry of volcanic rocks from
the island arcs and marginal basins of the Scotia Arc region. In: TALWANI, M. &
PITMAN, W.C. (eds) Island Arcs, Deep Sea Trenches and Back-arc Basins. American
Geophysical Union, Maurice Ewing Series 1, 367-378.
TARNEY, J., SAUNDERS, A. D., MATTEY, D. P., WOOD, D. A. & MARSH, N. G. 1981.
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TATSUMI, Y., MURASAKI, M. & NOHDA, S. 1992. Across-arc variation of lava chemistry in
the Izu-Bonin Arc: identification of subduction components. Journal of Volcanology and
Geothermal Research 49, 179-190.
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TOKSOZ, N. & BIRD, P. 1977. Formation and evolution of marginal basins and continental
plateaus.In TALWANI, M. & PITMAN, W.C. (eds) Island Arcs, Deep Sea Trenches and
Back-arc Basins. American Geophysical Union, Maurice Ewing Series 1, 379-393.
UYEDA, S. & KANAMORI, H. 1979. Back-arc opening and mode of subduction. Journal of
Geophysical Research 84, 1049-1061.
WYLLIE, P.J. 1988. Magma genesis, plate tectonics and chemical differentiation of the earth.
Reviews of Geophysics 26, 370-404.
ZHAO, D., HASEGAWA, A. & HORIUCHI, S. 1992. Tomographic imaging of P and S wave
velocity structure beneath northeastern Japan. Journal of Geophysical Research 97,
19909-19928.
TECTONICS OF SUBDUCTION ZONES
Contrasts between West & East Pacific
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Uyeda & Kanamori (1979) emphasised that there were two contrasting types
of subduction zone: Mariana Type and Chilean Type - with of course many
intermediate types. The Mariana Type is characterised by a very steeply
dipping slab; the Chilean Type by a shallow-dipping slab. These differences
were further amplified by Dewey (1981).
Mariana Type has:
1. Deep open trench (up to 11 km deep) that subducts old cold Jurassic crust.
2. A very steep Benioff Zone
3. Extensive faulting, subsidence and tectonic erosion of the outer trench wall.
4. Widespread intra-arc extension and back-arc spreading.
5. More earthquakes in the under-riding than in the over-riding plate.
6. A rather thin mafic-intermediate composition volcanic-plutonic crust.
7. Extensive volcanism; mainly basaltic with only minor andesites.
8. Little or no sedimentary accretion at the trench.
9. Subdued morphological expression.
10 Lavas have quiet eruptive style.
11 Volcanoes are mainly submerged cones with fringing reefs.
12 Poorly developed volcaniclastic dispersal fans.
Chilean Type has:
1. Shallower trench (up to 6 km) that subducts younger, warmer, Eocene age
oceanic crust.
2. Thrust faulting common on outer trench wall.
3. Major thrust faulting in the under-riding Nazca Plate up to 200 km west of
the trench.
4. A Benioff Zone with a very shallow dip down to about 200 km, and then a
steeper deeper portion
below a seismic gap.
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5. Widespread intra-arc compression and back-arc thrusting over a foreland
trough.
6. More, and higher energy, earthquakes in the over-riding than in the underriding plate.
7. Plutonism is dominant over volcanism.
8. Volcanism is dominantly of andesite-dacite-rhyolite type; basalts being
much rarer.
9. Thick (ca 70km) continental crust gradually tapering trenchward to less
than 10 km.
10 Because of dominant compression, continental arc has high uplift rates.
11 Violent eruptive style. High viscosity lavas. Extensive volcaniclastic
dispersal fans.
12 Spectacular geomorphological expression.
Difference in seismic characteristics: The steep dip of the Benioff Zone in the
Mariana type means that the contact interface between the subducting slab
and the mantle wedge lithosphere is less than 100 km, hence not much
frictional drag. In any case tectonic conditions are extensional. In Chilean type
however, the shallow slab dip and greater thickness of continental lithosphere
means that the contact interface can be as much as 400 km. Hence
considerable resistance and friction and much greater seismic activity.
Tectonic Erosion and Accretion: In the Mariana Arc there is no accretion of
abyssal sediments at the trench. Yet considerable volumes of sediment are
entering the trench: sediments are 0.5km thick on Pacific Plate entering the
trench, subduction rate 10 cm/yr for ca. 40 m.y. (work out how many cubic km
per unit length of arc!). Instead forearc is undergoing tectonic erosion
("subcretion"). Most of the sediment is being subducted - only a small
proportion of it is re-cycled into arc volcanics. Along Chilean margin the
sediment supply varies: very little in north where desert conditions, but much
more in south where rainfall is high. It has been suggested that the continental
basement may be eroding by subcretion in Northern Chile, but growing by
sediment accretion in Southern Chile. Where sediment supply is high,
sediments may fill the trench and flood over on to the oceanic plate; thus
depressing it so that it approaches subduction zone at a shallow angle.
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Explanation for differences between East and West Pacific Margins
Contrast cannot be explained simply by differences in convergence rate, since
Chilean, Mariana, Japanese and Tonga arcs all have head-on convergence
rate of about 10 cm/yr. Contrast must be related to balance between "rollback" of hinge and convergence rate. If roll-back is faster than convergence
rate then back-arc extension results; if slower, then back-arc compression.
Roll-back may be determined by age of subducting lithosphere (Molnar &
Atwater 1978). Old cold lithosphere is denser and subducts at steeper angle .
. presumably takes less time to reach 650 km discontinuity. If it cannot
penetrate discontinuity then splays back (see experiments of Kinkaid & Olsen
(1987)) and induces roll-back of hinge at subduction zone, giving extensional
tectonics. However, with shallower angle subduction of younger warmer
lithosphere the slab will take longer to reach 650km discontinuity, and will
warm up more and become less coherent and less able to induce roll-back
effect. So no extension. An additional factor is that in the Eastern Pacific the
American Plate is over-riding the Pacific (Nazca) Plate due to the opening of
the Atlantic . . although the rate is quite small.
Wider implications: If the balance between compression and extension at
convergent plate margins is related to dip of slab (and hence age of
lithosphere subducting), then it may explain why intraoceanic island arcs are
essentially a Phanerozoic phenomenon, and become rare or absent in the
middle to early Precambrian. Higher thermal gradients in Precambrian would
mean greater ridge length and smaller plates (see Hargraves 1986), so
subducting plates would be younger and warmer, and less likely to subduct at
steep angle. Hence much less likely to induce extensional conditions at
convergent plate boundaries. Is it only when there is extension that island
arcs are produced?
References
DEWEY, J.F. 1981. Episodicity, sequence and style at convergent plate boundaries. In: The
Continental Crust and its Mineral Deposits. Geological Association of Canada, Special
Paper 20, 553-572.
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