Download Plate Tectonics: GL209 Prof. John Tarney Lecture 5: Subduction

Plate Tectonics: GL209
Prof. John Tarney
Lecture 5: Subduction Zones
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.
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?
FRAMEWORK OF AN ISLAND ARC SYSTEM
The commonly held model of an arc - back-arc system has the
following components:
(1) Subduction Zone
(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:
MARGINAL BASINS & BACK ARC SPREADING
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
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
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Plate Tectonics: GL209
Prof. John Tarney
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.
Lecture 5: Subduction Zones
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:
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:
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):
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 "roll-back". 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:
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 backarc 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)
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Prof. John Tarney
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.
Lecture 5: Subduction Zones
West Mariana Ridge: Shallower and younger than the KyushuPalau 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 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.
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:
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) 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
(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)
(1) Island Arc Tholeiites (very similar in character to those of
Kyushu-Palau 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.
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.
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.
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.
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Prof. John Tarney
Lecture 5: Subduction Zones
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:
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 spreading ridge. After the change in plate motion
direction, the map then looked like:
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:
A new volcanic arc forms at the site of the easternmost transform,
and many complications develop in SE Asia (Philippines, etc.)
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Lecture 5: Subduction Zones
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.
Magma Compositions
(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:
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 subcontinental 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 Mg-rich 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 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.
Arc Magmatism
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)
(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.
Back-arc Basalts
In many respects marginal basin basalts (MBB) are similar
to normal mid-ocean ridge basalts (N-type MORB). However during
the early stages of back-arc 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.
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Plate Tectonics: GL209
Prof. John Tarney
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.
Lecture 5: Subduction Zones
REFERENCES: Arcs and Marginal Basins
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, 53855387.
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. Tectonomagmatic 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 intra-oceanic 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: effects of subduction induced mantle
flow. Tectonophysics 99, 207-220.
IDA, Y. 1983. Convection in the mantle wedge above the slab and
tectonic processes in subduction zones. Journal of Geophysical
Research 88, 7449-7456.
JURDY, D.M. 1979. Relative plate motions and the formation of
marginal basins. Journal of Geophysical Research 84, 67966802.
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.
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. (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)
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Prof. John Tarney
KARIG, D.E. 1971. Origin and development of marginal basins in
the Western Pacific. Journal of Geophysical Research 76, 25422561.
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. Journal of Geophysical Research
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KUSHIRO, I. 1990. Partial melting of mantle wedge and evolution
of island arc crust. Journal of Geophysical Research 95, 1592915939.
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, 330340.
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 Geological Society, London 130, 183204.
SAUNDERS, A.D. & TARNEY, J. 1984. Geochemical
characteristics of basaltic volcanism within back-arc basins. In
KOKELAAR, B.P. & HOWELLS, M.F. (eds) Marginal Basin
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SAUNDERS, A.D. & TARNEY, J. 1991. Back-arc basalts. In
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SHEMENDA, A.I. 1993. Subduction of the lithosphere and back arc
dynamics: insights from physical modeling. Journal of
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SPENCE, W. 1987. Slab pull and the seismotectonics of subducting
lithosphere. Reviews of Geophysics 25, 55-69.
STERN, R.J. & BLOOMER, S.H. 1992. Subduction-zone infancy Examples from the Eocene Izu-Bonin-Mariana and Jurassic
California arcs. Geological Society of America Bulletin 104,
1621-1636.
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-
Lecture 5: Subduction Zones
arc Basins. American Geophysical Union, Maurice Ewing Series
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TARNEY, J., SAUNDERS, A. D., MATTEY, D. P., WOOD, D. A.
& MARSH, N. G. 1981. Geochemical aspects of back-arc
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Transactions of the Royal Society of London A300, 263-285.
TARNEY, J., PICKERING, K.T., KNIPE, R.J. & DEWEY, J.F.
1991. Fluids and subduction zone processes. In TARNEY, J.,
PICKERING, K.T., KNIPE, R.J. & DEWEY, J.F. (eds)
Behaviour and Influence of Fluids in Subduction Zones. The
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TATSUMI, Y., MURASAKI, M. & NOHDA, S. 1992. Across-arc
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of subduction. Journal of Geophysical Research 84, 1049-1061.
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differentiation of the earth. Reviews of Geophysics 26, 370-404.
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7
Plate Tectonics: GL209
Prof. John Tarney
TECTONICS OF SUBDUCTION ZONES
Lecture 5: Subduction Zones
interface can be as much as 400 km. Hence considerable resistance
and friction and much greater seismic activity.
Contrasts between West & East Pacific
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.
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.
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 under-riding 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.
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 headon convergence rate of about 10 cm/yr. Contrast must be related to
balance between "roll-back" of hinge and convergence rate. If rollback 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 rollback 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.
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
Wider implications: If the balance between compression and
extension at convergent plate margins is related to dip of slab (and
8
Plate Tectonics: GL209
Prof. John Tarney
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.
9
Lecture 5: Subduction Zones