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Plate Tectonics: GL209
Prof. John Tarney
Lecture 6: Thermal Aspects of Subduction Zones
How can subduction zones give rise to the following range of
magmas? Surely this must imply a range of P-T conditions that
involve both slab and wedge melting?
PLATE TECTONICS: Lecture 6
THERMAL ASPECTS OF SUBDUCTION ZONES
Boninites (High-Mg andesites): usually formed at early stage of
island arcs
Island Arc Tholeiites (IAT): normally restricted to primitive island
arcs
Calc-alkaline basalts & andesites: found in mature island arcs and
continental margins
Bajaites (Adakites): High-Mg andesites (but different from
boninites) where ridge subduction occurs or mafic rocks have been
underplated.
Shoshonites: Often late-subduction or post-subduction: high-Ba, Sr
magmas
Archaean TTG suite: Distinctive, and thought to be derived from
subducted ocean crust (they resemble adakites).
For the last 2 decades, geologists, geophysicists and geochemists
have argued about the physical and chemical conditions which allow
melting to occur in subduction zones. Whereas it is easy to explain
magmatism at ocean ridges where hot mantle is rising, it is not at
first easy to explain why magmas appear in abundance when a cold
slab is pushed into the mantle at subduction zones. It used to be
thought that friction between the overriding and under-riding plates
was responsible, but calculations have showed that friction is most
unlikely: there is probably too much hydrous fluid and soft oozy
subducted sediment that act as a lubricant. It is important to try to
understand the thermal structure of subduction zones.
In a classic review paper, Ringwood (1974) suggested that the most
primitive island arc lavas (IAT), which are basaltic, could be related
to dehydration of the hydrated ocean crust (amphibolite) as it
transforms to dense eclogite at depths of ca. 100km. The hydrous
fluids rise up into the peridotite mantle wedge, promoting melting
(magmas form at much lower temperatures in the presence of water).
These magmas then rise slowly up to the arc volcanoes above, and
crystallise Mg-rich olivines and pyroxenes as they ascend, so the
magmas become more iron-rich. The eruption of basalt (tholeiite) is
non-violent. This is shown in cartoon form:
The critical points of issue are:
(a) under what conditions does the slab melt?
(b) what is the difference between subducting old ocean crust and
young ocean crust?
(c) do magmas originate instead in the mantle wedge?
(d) what is the mineralogy of the wedge. Are minerals like
hornblende, phlogopite & K-richterite stable in subduction
zones?
(e) how can the difference between primitive island arcs (e.g.
Marianas) that tend to erupt basalts, and mature arcs (e.g.
Andean margin) that tend to erupt andesite, be explained?
Anderson et al. (1978; 1980) were the first to consider the thermal
structure of subduction zones seriously. Wyllie & co-workers, in a
series of papers (e.g. Wyllie, 1988), used experimental petrology to
try to constrain what will melt under hydrous conditions, and what
the magma compositions would be. He produced some useful
cartoon models, one of which is shown below:
For the calc-alkaline, more silicic andesitic and dacitic magmas or
more mature arcs, Ringwood suggested a slightly different
mechanism based on his experimental work on eclogite. Hydrous
melting of eclogite (if Si-poor garnet stays in the residue produces
silica-rich dacitic magmas. These then react with the mantle wedge
and rise up as diapirs and erupt as much more violent hydrous
magmas, of which Mt. St. Helens is a good example.
However, there are a number of problems with these simple models,
and it is now accepted that they only acount for a minor number of
features of subduction zone magmas. For instance, the primitive
Mariana arc tholeiites are really a result of fore-arc diapirism
connected with the initiation of a new subduction zone, following a
change in plate motion, as outlined in the last lecture.
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Plate Tectonics: GL209
Prof. John Tarney
Lecture 6: Thermal Aspects of Subduction Zones
The important points to note are that the ocean crust reaching a
subduction zone will be relatively "cold" and "wet". Just how cold it
will be will depend on just how many hundreds or thousands of km it
has travelled from the spreading ridge. It will be wet as a result of
hydrothermal alteration near the ridge axis. As the plate subducts the
basaltic crust will undergo a progressive increase in metamorphic
grade – Greenshist > Amphibolite > Eclogite facies – which is also a
series of dehydration reactions to about 100km depth.
More recently, Peacock (1991) and Bickle & Davies (1991) have
produced much better thermal models. For instance, Peacock (1991)
has produced useful thermal numerical models. He explores the
thermal effect of:
(a) Age of the oceanic crust being subducted. Clearly young warm
ocean crust will be more likely to melt if subducted than old cold
lithosphere. The diagram below shows the increase in temperature at
1 my intervals (dots) as ocean crust ranging in age from 5 my [A] to
200 my [D] is subducted to 200 km. The surprising result is that only
when quite young crust is being subducted is there a possibility of
melting (i.e. temperatures reach >900°C beneath the arc). So as
subduction continues and older and older crust begins to be
subducted, it is less easily melted:
The diagram also shows how the top and base of oceanic crust heats
up. The base is initially hotter, but the top eventually gets hotter
because of heat conducted from the mantle wedge.
(c) Magmas from the mantle wedge? Curves E and F show the
temperatures of the mantle wedge (straddling the depths at which
magmas are generated below arc volcanoes) at 10 m.y. and 20 m.y.
after the start of subduction, but without allowing any convection in
the mantle wedge. The cooling effect of the slab is very important,
quickly taking the wedge below temperatures at which magmas
would be generated.
The blocks labelled eclogite and blueschist show the P-T conditions
found in exhumed subduction complexes (e.g. the Franciscan of
California) which are consistent with an average age of 50 Ma old
for subucted ocean crust.
(b) Amount of previously subducted lithosphere. Clearly the more
you stuff cool oceanic lithosphere into the upper mantle, the more it
will cool it (the iced drink analogy!). With subduction rates of 10
cm/yr it is possible to subduct 100 km of ocean crust per m.y. [the
diagrams are drawn for a much slower subduction rate of 3 cm/yr]
The implication from the diagram below is that the cooling effect of
continued subduction is quite severe, So after less than 600 km (= 6
m.y.) of ocean crust subduction, temperatures are below those at
which the slab melts. But what about the mantle wedge?
However, Curve G shows the effect of allowing induced convection
in the mantle wedge (a similar curve linked with E would be even
higher temperature . . . ). In this case the temperatures stay above
950 °C as the wedge material is dragged down, and so hydrous
melting would be possible. An important implication from this
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Plate Tectonics: GL209
Prof. John Tarney
diagram is that it is much more likely that arc magmas are derived
from the mantle wedge: conditions for slab melting are very
restricted.
Lecture 6: Thermal Aspects of Subduction Zones
curve turns over at ~70-80 km to become pressure-sensitive –
hornblende in mantle breaks down at ~100+ km. This means that a
lot of fluid will be released from hornblende at these depths, which
could promote melting. Is this why most arc volcanoes lie ~100 km
above the Benioff Zone?
(d) Effect of induced convection on slab. Briefly, the modelling
shows that induced convection can enhance the meltability of older
slab, but the effect on young ocean crust is not important:
(f) Upward and Downward flow in mantle wedge
There is increasng interest in subduction-induced flow in the mantle
wedge. At shallow levels (25-50km) the massive amounts of water
entering the subduction zone may hydrate the mantle wedge to give
serpentinite: this rock contains >12% water and is significantly less
dense than normal mantle, and so can rise diapirically and "intrude"
(solid state flow) the fore-arc regions, whether formed of arc
volcanics or accreted sediment.
Further down, cooling of the wedge by the subduction zone itself
may make it negatively buoyant (i.e. denser) and help drag the
wedge down, promoting hornblende breakdown and fluid release.
This will enhance induced convection effects. The amount of
coupling between slab and mantle wedge would however be reduced
by soft sediment at the interfact between the two.
Upward flow further back in the mantle wedge would compensate
these effects, particularly if enhanced by low-density fluid and
magmas. (to be continued . . . )
REFERENCES
ANDERSON, R.N., DELONG, S.E. & SCHWARTZ, W.M. 1978.
Thermal model for subduction with dehydration in the
downgoing slab. Journal of Geology 86, 731-739.
ANDERSON, R.N., DELONG, S.E. & SCHWARTZ, W.M. 1980.
Dehydration, asthenospheric convection and seismicity in
subduction zones. Journal of Geology 88, 445-451.
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.
DAVIES, J.H. & BICKLE, M.J. 1991. A physical model for the
volume and composition of melt produced by hydrous fluxing
above subduction zones. Philosophical Transactions of the Royal
Society, London A335, 355-364.
DAVIES, J.H. & STEVENSON, D.J. 1992. Physical model of source
region of subduction zone magmatism. Journal of Geophysical
Research 97, 2037-2070.
DEFANT, M.J. & DRUMMOND, M.S. 1990. Derivation of some
modern arc magmas by melting of young subducted lithosphere.
Nature 347, 662-665.
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.
GARFUNKEL, Z., ANDERSON, C.A. & SCHUBERT, G. 1986.
Mantle circulation and the lateral migration of subducted slabs.
Journal of Geophysical Research 91, 7205-7223.
HARGRAVES, R.B. 1986. Faster spreading or greater ridge length
in the Archean? Geology 14, 750-752.
KINCAID, C. & OLSON, P. 1987. An experimental study of
subduction and slab migration. Journal of Geophysical Research
92, 13832-13840.
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.
(e) Temperature or pressure control on magma generation?
The diagram above shows effect of water on the melting behaviour
of basaltic oceanic crust. Under dry conditions melting increases
with pressure (red dashed line). However, under water-saturated
conditions (red full line) melting temperatures plummet by almost
400°C at depths of 50 km.
Importantly, the blue curve shows how hornblende becomes an
important mineral under hydrous conditions; however, note that the
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Plate Tectonics: GL209
Prof. John Tarney
Lecture 6: Thermal Aspects of Subduction Zones
THE FATE OF SEDIMENTS AT SUBDUCTION ZONES
PEACOCK, S.M. 1987. Thermal effects of metamorphic fluids in
subduction zones. Geology 15, 1057-1060.
PEACOCK, S.M. 1991. Numerical simulations of subduction zone
pressure-temperature-time paths: constraints on fluid production
and arc magmatism. Philosophical Transactions of the Royal
Society, London A335, 341-353.
RINGWOOD, A.E. 1974. The petrological evolution of island arc
systems. Journal of the Geological Society, London 130, 183204.
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.
SUDO, A. & TATSUMI, Y. 1990. Phlogopite and K-amphibole in
the upper mantle: implications for magma genesis in subduction
zones. Geophysics Research Letters 17, 29-32.
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.
The floors of the world’s oceans are covered by sediment up to 1 km
thick (age dependent) as a result of slow accumulation of calcareous
and siliceous biogenic oozes capped by fine clays that have been
carried in suspension to the middle of oceans. Additionally, nearer
continents there may be much thicker accumulations of clastic
sediments brought in by deltas and turbidity currents, and further redistributed by strong bottom water currents. Sooner or later this
sedimenty must finish up at a subduction zone. What happens to it?
Does it get scraped-off, or does it get dragged down the subduction
zone? If the latter, does it just disappear into the deep mantle, or does
it get recycled into island arc magmas? The balance is shown as
follows:
Effectively, subduction at active margins can be likened to a
conveyor belt carrying a lot of loose rubbish moving against a
buttress: some material is going to get scraped-off:
There are many variables in the whole process. So it is important to
look at a number at different tectonic situations.
(1) Primitive Island Arcs: no sediment accretion
At intraoceanic island arcs, such as the Marianas, there is
no sediment supply from the continent (this is trapped by the backarc basin), and the arc itself produces only a minor amount of
volcanic ash (the eruptions are basaltic and not violent). Most of the
sediment arriving at the subduction zone is abyssal ooze and clay
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Plate Tectonics: GL209
Prof. John Tarney
carried on the subducting plate (on old ocean crust, at least 0.5km
thick). It used to be thought that this abyssal sediment was scraped
off to form an accretionary wedge in the fore-arc. However,
dredging and drilling in the Mariana forearc and trench has shown
that there is little on no sediment in the Mariana trench. Yet during
the 40 my since the arc system has been in existence, up to 40 km³ of
sedimen / km length of arc should have been scraped off the
subducting plate (which is subducting at 10 cm/year).
The sediment must be subducted - but how? The answer
seems to be that, as the subducting plate bends over to become
vertical, the flexure causes horsts and graben to develop. Sediments
are scraped off from the horsts into the graben and thus encased as
the ocean lithosphere deforms (for this reason it was thought this
would be a good place to dispose of nuclear waste!) In fact the
ocean crust acts as a gigantic rasp on the arc too - the forearc is
gradually, but slowly, eroded:
Lecture 6: Thermal Aspects of Subduction Zones
the coast, close to the trench? (Although difficult to prove it was
there when it has gone!).
Where sediment supply is a little higher, trench gets partly filled with
sediment. Some of this sediment may get scraped off. But drilling in
the Middle America Trench suggests that the abyssal ocean floor
sediments are still subducted (soft oozes act as a lubricant)
(3) S. Chile and Alaska: high sediment input
Here the climate is temperate and wet. Abundant rivers, some
deriving from glaciers. Floods common. High rate of sediment
supply to the ocean. Sediment supply was even higher during the
Pleistocene (and there has not been time yet to subduct them).
Result is that large amount of sediment is carried into the
trench. Trench quickly gets filled, and sediment then carried out onto
subducting plate. As this continues the weight of sediment actually
depresses the plate as it approaches the trench so that angle of dip is
smaller (dip increases under the continental margin proper).
With a shallower dip, no horsts & graben form, and
sediment is scraped off. This can readily be seen from reflection
profiles. Layering of sediments disappears as continent is
approached. Low angle thrusts appear. Younger sediments are
progressively underplated. If sedimentation rates are high (as they
are in high northern/southern latitudes) this can give rise to lateral
growth of continents. The process is called subduction-accretion and
the structures are called Accretionary Prisms. The general features
are shown below:
However geochemical studies have shown that very little of the
sediment is actually incorporated into the arc volcanics, so most of it
must be cycled into the deeper mantle. Presumably, as the slab at the
Marianas is avalanching into the lower mantle, the sediments may be
taken down also.
(4) Characteristics of Accretionary Wedges/Prisms
(2) Northern Chile: no sediment to subduct
Lateral continental growth by subduction-accretion is dependent on
(a) the supply of material from the ocean, and (b) the sediment
supply from the continent. These two might vary over a large range.
Here the sediment supply is also very limited because of the arid
climate. Many of the rivers from the high Andes never quite make it
to the ocean, and in any case there few floods (which produce the
turbidity currents that carry the sediment out into the ocean proper).
Also, major faults parallel to the coast tend to obstruct the rivers,
forming saline lakes (were common in N. Chile).
(a) Material accreted from oceans
The ocean floor is not smooth. Study of the Pacific map
shows that the pre-Tertiary ocean floor is considerably rougher than
that generated in the Tertiary. There are more oceanic plateaus,
aseismic ridges, ocean island chains and arcs – in large part this
results from the spate of mantle plumes which punched through the
Pacific ocean plate in the late Cretaceous (120 - 80 Ma).. Many of
these upstanding structures are capped by carbonate banks, because
they stayed above the carbonate compensation depth (CCD) much
longer than normal ocean floor.
Ocean floor that is rough and upstanding is more likely to
be scraped off when it reaches subduction zones at active margins.
So this sceaped-off material will be a mixture of mafic rocks
(metamorphosed to amphibolite) associated with thick limestone
(marble) sequences, as well as sileceous and carbonate oozes (=
"cypoline schists") and lithified cherts. Large oceanic structures such
as plateaus and arcs may "choke" the subduction zone, causing backstepping of the subduction zone, the arcs being left as an ophiolite
So the situation is similar to that in the Marianas, although the dip of
the subducting slab is not so great. Some geologists have suggested
that the rasping action of the subducting slab has actually eroded
back the continental margin of N. Chile and Peru. Is this why the
locus of volcanic activity continually moves eastwards with time in
the N. Andes? And why Palaeozoic batholiths are exposed right at
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Plate Tectonics: GL209
Prof. John Tarney
(e.g. the classic Troodos complex on Cyprus). However, normal
ocean floor, which is smooth and cold, may not be scraped off at all
(it is th is that converts to eclogite to provide the slab-pull force), so
the soft carbonate-siliceous oozes and cloay may not be scraped off
quite so readily.
Lecture 6: Thermal Aspects of Subduction Zones
continental crust or oceanic crust (e.g. ocean plateaus) many
thousands of km in just a few tens of m.y., and as plates can change
their direction of motion (c.f. kinik in Hawaiian chain), this can lead
to the juxtaposition of segments of crust that have a completely
different geological histories. So it is not just collision of major
continents (e.g. India and Asia to form Himalayas) but also on a
much smaller scale. In particular, major transform faults can
transport differnt crustal segments laterally for many 1000s of km
(e.g. San Andreas Fault). Of course terranes are usually fault- or
thrust-bounded.
(b) Material supplied from the continents
This is largely material supplied by river systems feeding
active continental margins. Of course at the present day there are not
many rivers feeding active continental margins -- they are mostly
still feeding the passive margins of the Atlantic, the Indian ocean and
around Antarctica/Australia.
It is important to note that in the Upper Palaeozoic and early
Mesozoic, the southern continents formed part of Gondwanaland - a
very large continental landmass. Moreover much of Gondwanaland
was rimmed by active margins. The margin had low relief (the
present high Andes is not typical, and results from Miocene
deformation and uplift). So it is probable that very large rivers were
dumping sediment onto the subducting plate, and the sediment was
then accreted back on to the continental margin . . . now exhumed
and exposed, particularly in southern Chile, where they are of late
Palaeozoic age (before the Andean magmatic cycle), and South
Island, New Zealand. But they can also be seen in Alaska, and of
course occur in older mountain belts (commonly termed Flysch).
Compared with the partly-lithified material scraped off from the
oceanic plate, the material coming from the continent is unlithified
clastic sediment. The two get tectonically intermixed and intensely
deformed (the subduction interface allows thousands of km of
relative movement in just a few tens of Ma ™ far more than with
continental collision), so most rocks from this environment have
strong penetrative foliations and linear fabrics (see New Harbour
Group on Anglesey) and finish up as teconic melanges -- lenses of
oceanic rocks in deformed soft sediment.
As soft wet sediment (greywacke-shale) is continually underplated
beneath the accretionary wedge, it heats up slowly. Water is
progressively driven off. Hot water dissolves silica from sandy beds,
and deposits it at higher levels as abundant cross-cutting quartz
veins. However, because underplating is continuous process,
sediments and quartz veins become progressively and very strongly
deformed. Can be almost mylonite-like fabric. No bedding remains.
Cross-cutting quartz veins are stretched out to become sub-parallel to
foliation. Very characteristic rock type. Many tens or even hundreds
of km of 'new' crust can accrete laterally onto continental margins in
this way.
Erosion of upper part of accretionary wedge may occur,
and younger sediments deposited on top in fore-arc basins. These
may also become deformed, but less so (could the South Stack Series
on Anglesey may represent such fore-arc basin rocks?).
References
DAVIES, J.H. & von BLANCKENBURG, F. 1995. Slab breakoff: A
model of lithosphere detachment and its test in the magmatism
and deformation of collisional orogens. Earth and Planetary
Science Letters 129, 85-102.
von HUENE, R. & SCHOLL, D.W. 1993. The return of sialic
material to the mantle indicated by terrigenous material
subducted at convergent margins. Tectonophysics 219, 163-175.
Terrane Terminology (Jargon)
"A fault-bounded package of strata that has a geological history
distinct from the adjoining geologic units"
Howell (1989) divided terranes as follows:
Stratigraphic
fragments of continents
(1) representing
(2) fragments
ofcontinental margin
(3) fragments of
volcanic arc
(4) fragments of
ocean basins
Disruptive
Metamorphic
However, a genetic terminology is also prevalent:
Exotic, Suspect, Displaced or Accreted terranes: this
implies that the terrane has
been transported some distance to its current
position.
Pericratonic: Contains cratonal detritus and formed on
attenuated continental crust.
Terranes are sometimes described in terms of tectonic assemblages,
which are rock-stratigraphic units formed in actualistic tectonic
settings, such as island arcs or ocean floors. A terrane may consist of
one or more tectonic assemblages
Domain: A volume of rock, bounded by compositionalor structural
discontinuities, within which there is structural homogeneity; these
may contain minor stratigraphic distinctions as well andcan be
viewed as subterranes.
Superterranes: A composite terrane, consisting of two or more
compound terranes, that were amalgamated prior to subsequent
orogenesis.
REFERENCES (General)
CONEY, P., JONES, D.L. & MONGER, J.W. 1980. Cordilleran
suspect terranes. Nature 288, 329-333.
BEN-AVRAHAM, Z., NUR, A., JONES, D. & COX, A. 1981.
Continental accretion: from oceanic plateaus to allochthonous
terranes. Science 213, 47-54.
HOWELL, D.G. 1989. Tectonics of Suspect Terranes. Chapman &
Hall, NewYork, 232pp.
BEBOUT, G.E. & BARTON, M.D. 1989. Fluid flow and
metasomatism in a subduction zone hydrothermal system:
Catalina Schist terrane, California. Geology 17, 976-980.
TERRANES
"Terrane" concepts are now quite widely used in interpreting
geological relationships in many parts of the world, and in rocks of
many ages. Basically plate tectonics can move segments of
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Plate Tectonics: GL209
Prof. John Tarney
Lecture 6: Thermal Aspects of Subduction Zones
References On Andean Terranes
ASPDEN, J.A. & McCOURT, W.J. 1986. Mesozoic oceanic terrane
in the central Andes of Colombia. Geology 14, 415-418.
References On Alaskan Terranes
VROLIJK, P., MYERS, G. & MOORE, J.C. 1987. Warm fluid
migration along tectonic melanges in the Kodiak accretionary
complex, Alaska. Journal of Geophysical Research 93, 1031310324.
BARKER, F., JONES, D.L., BUDAHN, J.R. & CONEY, P.J. 1988.
Oceanic plateau-seamount origin of basaltic rocks, Angayuchan
Terrane, Central Alaska. Journal of Geology 96, 368-374.
Baltic Shield Proterozoic Terranes
PARK, A.F. 1991. Continental growth by accretion: a
tectonostratigraphic terrane analysis of the evolution of the
western and central Baltic Shield, 2.50 to 1.75 Ga. Bulletin of the
Geological Society of America 103, 522-537.
References On Caledonian Terranes
DEWEY, J.F. & SHACKLETON, R.M. 1984. A model for the
evolution of the Grampian tract in the Caledonides and
Appalachians. Nature 312, 115-121
MURPHY, F.C. & HUTTON, D.H.W. 1986. Is the Southern
Uplands of Scotland really an accretionary prism? Geology 14,
54-57.
HUTTON, D.H.W. 1987, Strike-slip terranes and a model for the
evolution of the British and Irish Caledonides. Geological
Magazine 124, 405-425.
BENTLEY, M.R., MALTMAN, A.J. & FITCHES, W.R. 1988.
Colonsay and Islay: a suspect terrane within the Scottish
Caledonides. Geology 16, 26-28.
HAUGHTON, P.D.W. 1988. A cryptic Caledonian flysch terrane in
Scotland. Journal of the Geological Society, London 145, 685703.
MARCANTONIO, F., DICKIN, A.P., McNUTT, R.H., &
HEAMAN, L.M. 1988. A 1800 million year old Proterozoic
gneiss terrane in Islay with implications for the crustal structure
evolution of Britain. Nature 335, 62-64.
SOPER, N.J., GIBBONS, W. & McKERROW, W.S. 1989.
Displaced terranes in Britain and Ireland. Journal of the
Geological Society, London 146, 365-367.
THIRLWALL, M.F. 1989. Movement on proposed terrane
boundaries in northern Britain: constraints from OrdovicianDevonian igneous rocks. Journal of the Geological Society,
London 146, 373-376.
BLUCK, B.J. & DEMPSTER, T.J. 1991. Exotic metamorphic
terranes in the Caledonides: Tectonic history of the Dalradian
block, Scotland. Geology 19, 1133-1136.
RYAN, P.D. & DEWEY, J.F. 1991. A geological and tectonic crosssection of the Caledonides of western Ireland. Journal of the
Geological Society, London 148, 173-180.
MURPHY, F.C., ANDERSON, T.B., DALY, J.S. & 16 others, 1991
An appraisal of Caledonian suspect terrains in Ireland. Irish
Journal of Earth Sciences 11, 11-41.
SOPER, N.J., ENGLAND, R.W., SNYDER, D.B. & RYAN, P.D.
1992. The Iapetus suture zone in England, Scotland and eastern
Ireland: a reconciliation of geological and deep seismic data.
Journal of the Geological Society, London 149, 697-700.
BROWN, C. & WHELAN, J.P. 1995. Terrane boundaries in Ireland
inferred from the Irish Magnetotelluric Profile and other
geophysical data. Journal of the Geological Society, London 152,
523-534.
References On Archaean Terranes
(to be continued)
References On Appalachian Terranes
WILLIAMS, H. & HATCHER, R.D. 1982. Suspect terranes and
accretionary history of the Appalachian region. Geology 10, 530536.
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