plate tectonics_subduction zones and island arcs

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PLATE TECTONICS: Lecture 5

SUBDUCTION ZONES and ISLAND ARCS

Subduction Zones are where cool lithospheric plates sink back into the mantle. It takes about50 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. Althoughthere 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 fastestmoving plate (ca. 10 cm/yr), and this is the plate that supplies most of the Earth's subductinglithosphere, and thus where the overall slab-pull force will be the larger. The normal argumentis that the cool ocean crust will more easily convert to dense eclogite which, as we have seenin 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, butthere 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 crustgrew 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 showssediment being scraped off the downgoing plate to form an accretionary wedge, and that aforearc basin is forming on top of the wedge as it is dragged down (and is presumably fed byvolcanic debris from the arc). However, the cartoon avoids the issue of how and where thevolcanic magmas come from. To what extent does the basaltic subducted slab contribute to arcmagmas? Is it just the fluids carried down in altered oceanic crust that migrate into the mantlewedge overlying the subduction zone and cause melting? Ot what extent do sediments carrieddown the subduction zone then contribute to arc magmas? Why are arc volcanoes nearlyalways situated about 110 km above the Benioff Zone? What happens to material taken downthe subduction zone?

MARGINAL BASINS & BACK ARC SPREADING

ATE TECTONICS: Lecture 2 http://www.le.ac.uk/geology/art/gl209/lecture5/lecture5.html

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Marginal basins are a common feature of the Western Pacific. Examples (north to south) arethe Sea of Japan, the West Philippine Basin, the Parace Vela & Shikoku Basins, the MarianaTrough, the Woodlark Basin, the Fiji and Lau Basins. By contrast marginal basins are rarer inthe 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, whichare separated from larger oceans by an island arc. Some marginal basins at continentalmargins 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), orrecently active (2). The third may represent basins formed by even older back-arc spreading, ornormal 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

(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 ofoff-scraped sediments, drilling has not substantiated this. It appears that - at least at

intraoceanic arcs - abyssal sediments on the downgoing plate are largely subducted.


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That the back-arc region is a zone of asthenospheric upwelling is supported by seismicevidence which suggests a low-Q (seismic attenuation) zone behind the arc, compatible with asmall amount of melt in the back-arc region:

Magnetic anomalies in back-arc basins are not so well developed, nor have such symmetricallinear patterns, as those in the normal ocean basins. There have been difficulties in identifyingthe anomalies. It has been suggested by Lawver & Hawkins (1978) that spreading may bemore 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.

Models for Back-arc Spreading (see Karig, 1974)

Active Diapirism: One of the earliest models, based on the Mariana Arc System, is that of anuprising diapir splitting the arc. The diapir is initiated either as a result of frictional heating atthe 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 separatedby seafloor spreading:

Passive Diapirism: This results from regional extensional stresses in the the lithosphereacross the arc system. In effect the downgoing slab, although acting like a conveyor belt, alsohas a vertical component that causes "roll-back". The arc and forearc then stays with thesubduction zone, as a result of a supposed trench suction force:

Stepwise Migration: Here it is assumed that the subducting slab is snapped off near thehinge, 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 asthenospherewells up behind it:


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Convection-driven: This model proposed by Toksoz & Bird (1978), and requires thatsubsidiary 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 ™ thelithosphere is thinner and thus more easily disrupted than under continents:

Uprising Harzburgite Diapir: This model (Oxburgh & Parmentier 1978) depends on the factthat refractory lithosphere (which has lost its basalt component at mid-ocean ridges) is lessdense and inherently more buoyant than normal fertile mantle. Thus it would rise if heated tosame temperature as surrounding mantle. Such diapirs could in theory be derived fromsubducting lithosphere, although it is doubtful that subducting l ithosphere could be heatedwithin 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 dipof 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 strongvertical component. Thus extensional conditions in back-arc region. In the E. Pacific, on theother hand, the lithosphere subducting beneath the Andes is young (Tertiary), warm and lessdense, and subducts at a shallow angle. Thus convergence is more compressive than


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extensional. Uyeda & Kanamori (1979) have characterised these two extreme types ofsubduction as Mariana and Chilean type respectively. See also Dewey (1981)

Other models: Various researchers have since commented on the possible causes of back-arcspreading, including assessments of dependence on absolute and relative plate motions.Consult some of references listed below. Experimental laboratory studies have been carried outby Kincaid & Olsen (1987), observing the effects of continued subduction where thesubducting slab 'hits' the 650 km discontinuity. The results show that steep subduction doesproduce a significant roll-back effect on the hinge, which will generate extensional conditions inthe back-arc region. Note that with subduction rates of about 7 cm/yr it would take about 10 mybefore newly subducted ocean lithosphere would 'hit' the 650 km discontinuity and begin toinitiate '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 studiedthrough marine geophysical studies, dredging and drilling (particularly Legs 58, 59 and 60 ofDSDP 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

(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-arcspreading. It appears to pre-date the Kyushu-Palau Ridge. Magnetic anomalies suggest activespreading in the early Tertiary (62-40 Ma) with the NW-SE trending Central Basin Fault as thespreading centre. The Oki-Daito Ridge in the northern West Philippine Sea is aligned parallelto this feature and has been regarded as an old remnant arc: however drilled samples from theOki-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 rateis 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 basinfloors. Consists of vesicular lava flows, dykes and sills, interbedded with volcaniclastic brecciaslying below Middle Oligocene oozes. Lavas all belong to Island Arc Tholeiite (IAT) Series,


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typical of the most primitive island arcs. Now an inactive Remnant Arc that was active betweenabout 42 and 32 my ago.

Parece-Vela and Shikoku Basins: Magnetic anomaly patterns indicate back-arc spreadingbetween 30 and 17 my in Parece-Vela and between 26 and 15 my in the Shikoku Basin innorth. 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. Drillingpenetrated about 1000 m of volcaniclastic material composed of basalts, basaltic andesites,rare andesites and plagioclase phenocrysts. Their character is calc-alkaline, with much highercontents of Ba and Sr than those of K-P Ridge. Arc was active 17-8 my ago. So now aRemnant 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 inMariana Trough are MORB-like, but have some arc characteristics. Vesicular. Spreading still inprogress. Further north, on Iwo-Jima Ridge, there is an incipient back-arc basin just beginningto form - the Bonin Trough.

Mariana Active Arc: This consists of numerous small islands and seamounts, on the easternedge of the extensive Fore-arc region. Lavas are mainly basalts, basaltic andesites andandesites.

Mariana Fore-arc: The forearc region shows a history of continual subsidence. The basement isEocene 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 Kyushu-Palau Ridge).These magmas can normally be easily distinguished from calc-alkaline basalts from moremature arc systems.

(2) Boninites , or high-magnesian andesites. These are unusual lavas, combining high Siwith high Mg, Ni and Cr. They are thought to have formed by wet-melting of ratherrefractory 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 isbeing subducted, and that the fore-arc itself is suffering tectonic erosion as a result of the


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rasping action of the downgoing slab.

TECTONIC EVOLUTION OF MARIANA ARC SYSTEM

Combining evidence from magnetic anomalies, drilling, dredging and geochronology, thegeologic history of the arc system can be pieced together. In the period immediately precedingthe development of the arc, the plate configuration in the eastern Indian Ocean and westernPacific was dominated by the rapid movement of India northward. There were some major N - Soriented transform faults at this time, so about 60 Ma ago the plate tectonic configurationprobably looked like this:

India was just about to collide into Asia to form the Himalayas, Australia had just begun toseparate from Antarctica, and note the very large ridge offsets on the N-S transforms. Thecritical point at this time was that slab-pull associated with the rapidly-moving Indian Plate willstop as soon as India collides. Similarly, the spreading ridge in the NE Pacific is going to pushitself under the Aleutians, when upon the slab-pull will also stop. This leaves the northerly pullforces 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 inHawaiian-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 transformfaults, with younger, warmer and thinner ocean ocean lithosphere to the west, and older, coolerand denser lithosphere to east. Drawn to scale, the position immediately before the change inplate 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 spreading ridge. After the change inplate motion direction, the map then looked like:

A new volcanic arc forms at the site of the easternmost transform, and many complicationsdevelop in SE Asia (Philippines, etc.) because of transforms turning into arcs, and varioussubduction-flips as thick (plateau-type) ocean crust refuses to subduct. A new subduction zonedevelops north of Australia.


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(2) Rapid build-up of Kyushu-Palau Arc in late Eocene – Oligocene through voluminouseruption of island arc tholeiites and high-Mg boninites. Activity continued for ca 10 my. So whathappened 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 verticalcomponent so that it is sinking into the mantle. At this point hot asthenosphere mantle rushesin to replace it. So in a rather unique rapidly extensional tectonic environment, wet alteredocean crust is juxtaposed next to very hot asthenospheric mantle. With an abundance of heatand water, it is not surprising that huge amounts of magma are generated. This tectonicsituation is actually even more extensional than at a mid-ocean ridge, so it may be expectedthat 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 & ShikokuBasins 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 ofcalc-alkaline basalts and basaltic andesites.

(5) Splitting of West Mariana Arc abut 6 my ago to form Mariana Trough by back-arcspreading, and leaving West Mariana Ridge as remnant arc.

(6) Formation of new Mariana Arc 5 my ago to present. Now erupting lavas with mixedcalc-alkaline - island arc tholeiite characteristics.

Presumably the Mariana Arc will continue migrating eastwards into the Pacific.

Magma Compositions

Arc Magmatism


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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 continentalcrust or sub-continental lithosphere involved).

The earliest lavas erupted (now seen on Kyushu-Palau Ridge and Mariana Fore-arc) are islandarc 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 havesimilarities 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 silicacontents more typical of andesites; they have high Cr and Ni contents, but have lower Ticontents and higher K, Rb, Ba and Sr contents than would normally be expected of high-Mgrocks.

Boninites are thought to result from wet melting of the rather refractory Mg-rich mantle wedgebeneath 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 extensivecrystal fractionation en route to the surface. Or they could represent melts of subducted oceanbasalt crust (only possible at the very start of subduction when the ocean lithosphere ispushed down into hot mantle).

After opening of the Parece Vela basin by back-arc spreading, arc volcanic activity wastransferred 17 my ago to the what is now the West Mariana Ridge, and continued building upthat arc for ca. 9 my. The lavas erupted however were mainly calc-alkaline basalts (CAB) andbasaltic andesites, with higher Al contents, much higher Sr and Ba contents and lightrare-earth enriched rather than depleted REE patterns. These lavas are more similar tocalc-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 animplication that the wedge may have been enriched in Ba, Sr, light REE, etc., perhaps as aresult of continued fluid transport of these elements into the wedge from the dehydratingsubducting slab.

Modern lavas erupted at the active Mariana Arc tend to be mainly andesites and basalticandesites having characteristics in between those of IAT and CAB. There is some evidence thata small component (ca. 0.5%) of subducted abyssal sediment is involved in their sourceregions.

Perhaps the most interesting aspect of the Mariana arc is that at least three distinct magmatypes appear to have been generated from the one subduction zone. Yet the whole arc systemevolved entirely within the oceanic environment.

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 uprisingmantle 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. Moreoverthey tend to have higher water contents and be vesicular - a consequence of fluids distilledfrom the subducting slab. These features are useful discriminants in trying to characteriseophiolites as being derived from either obducted ocean floor or marginal basin crust. SeeSaunders & Tarney (1984; 1991) for summary.


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Addition: Schematic cross-section across the Mariana Arc showing the components involved inmagma generation.

Fluids are released from the sub-ducting slab as "wet" amphibolite recrystallises at ca. 100kmdepth to dry dense eclogite. These fluids migrate upwards into the mantle wedge and inducemelting 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 eventsaffecting the deeper lithosphere.

More active mantle diapirism occurs in the back-arc region, and this results in much moremelting and active spreading. Hydrous fluids are still involved in these mamgas, but to a lesserextent 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 anythingwhich reduces the slab-pull force will encourage changes in the direction and speed of platemotion. 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 Pacificthe Kula Ridge began to subduct beneath the Aleutians - Kamchatka. A result was a markedreduction in the N™S slab-pull, because recently formed hot lithosphere is not very dense andnot keen to subduct. In combination with other plate re-configuring events worldwide, this mayhave been enough to cause switch in Pacific Plate motion from N – S to E – W. But seeRichards et al. (1996)

REFERENCES: Arcs and Marginal Basins

The references below lead to most aspects of interest to island arcs, even if you just look at theabstracts & diagrams!


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BLOOMER, S.H. 1987. Geochemical characteristics of boninite- and tholeiite-series volcanic rocks from theMariana 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 ageof 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 intra-oceanicarc-trench systems. Tectonophysics 102, 174-205.

HICKEY, R.L. & FREY, F.A. 1982. Geochemical characteristics of boninite series volcanics: implications fortheir 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 ofsubduction 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, 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.


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KINCAID, C. & OLSON, P. 1987. An experimental study of subduction and slab migration. Journal of

Geophysical Research 92, 13832-13840.

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 possibletectonic 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 theage 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: asynthesis 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, 183-204.

SAUNDERS, A.D. & TARNEY, J. 1984. Geochemical characteristics of basaltic volcanism within back-arcbasins. In KOKELAAR, B.P. & HOWELLS, M.F. (eds) Marginal Basin Geology . Geological Society of

London, Special Publication 16, 59-76.

SAUNDERS, A.D. & TARNEY, J. 1991. Back-arc basalts. In FLOYD, P.A. (ed) Oceanic Basalts . Blackie,Glasgow, pp. 219-263.

SHEMENDA, A.I. 1993. Subduction of the lithosphere and back arc dynamics: insights from physicalmodeling. Journal of Geophysical Research 98, 16167-16185.

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 arcsand 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. Geochemical aspectsof back-arc spreading in the Scotia Sea and Western Pacific. Philosophical 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. InTARNEY, J., PICKERING, K.T., KNIPE, R.J. & DEWEY, J.F. (eds ) Behaviour and Influence of Fluids in

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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.InTALWANI, M. & PITMAN, W.C. (eds) Island Arcs, Deep Sea Trenches and Back-arc Basins . American

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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 structurebeneath northeastern Japan. Journal of Geophysical Research 97, 19909-19928.

TECTONICS OF SUBDUCTION ZONES

Contrasts between West & East Pacific

Uyeda & Kanamori (1979) emphasised that there were two contrasting types of subductionzone: Mariana Type and Chilean Type - with of course many intermediate types. The MarianaType is characterised by a very steeply dipping slab; the Chilean Type by a shallow-dippingslab. 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.


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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 deeperportion

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.

Difference in seismic characteristics: The steep dip of the Benioff Zone in the Mariana typemeans that the contact interface between the subducting slab and the mantle wedgelithosphere is less than 100 km, hence not much frictional drag. In any case tectonic conditionsare extensional. In Chilean type however, the shallow slab dip and greater thickness ofcontinental lithosphere means that the contact interface can be as much as 400 km. Henceconsiderable resistance and friction and much greater seismic activity.

Tectonic Erosion and Accretion: In the Mariana Arc there is no accretion of abyssalsediments 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 undergoingtectonic erosion ("subcretion"). Most of the sediment is being subducted - only a smallproportion of it is re-cycled into arc volcanics. Along Chilean margin the sediment supplyvaries: 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 inNorthern Chile, but growing by sediment accretion in Southern Chile. Where sediment supplyis high, sediments may fill the trench and flood over on to the oceanic plate; thus depressing itso 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 "roll-back" of hinge and convergence rate. Ifroll-back is faster than convergence rate then back-arc extension results; if slower, thenback-arc compression.

Roll-back may be determined by age of subducting lithosphere (Molnar & Atwater 1978). Oldcold lithosphere is denser and subducts at steeper angle . . presumably takes less time toreach 650 km discontinuity. If it cannot penetrate discontinuity then splays back (seeexperiments of Kinkaid & Olsen (1987)) and induces roll-back of hinge at subduction zone,giving extensional tectonics. However, with shallower angle subduction of younger warmerlithosphere the slab will take longer to reach 650km discontinuity, and will warm up more andbecome less coherent and less able to induce roll-back effect. So no extension. An additionalfactor is that in the Eastern Pacific the American Plate is over-riding the Pacific (Nazca) Platedue 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 explainwhy intraoceanic island arcs are essentially a Phanerozoic phenomenon, and become rare orabsent in the middle to early Precambrian. Higher thermal gradients in Precambrian wouldmean greater ridge length and smaller plates (see Hargraves 1986), so subducting plateswould be younger and warmer, and less likely to subduct at steep angle. Hence much lesslikely to induce extensional conditions at convergent plate boundaries. Is it only when there isextension 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.


plate tectonics_subduction zones and island arcs

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