Download Subduction of oceanic lithosphere

Subduction of oceanic lithosphere
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Evolution of the continental crust
New crust is formed in marginal basins, island arcs, and oceanic plateaux
New crust is accreted to the continents along the active continental margins.
Crust is also added by magmatic underplating
Continental crust is deformed in collision zones. It is eroded and recycled in
the mantle in subduction zones. Some crust can be subducted in collision
zones.
What is the budget?
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The age of continental crust
appears younger at the
margins than at the center
of the continent.
Suggestion by Wilson that
continents grew from a
central core is now
superseded.
However, new continental
crust has been and is
presently being added at
the active margins, like the
North American Cordillera.
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Subduction
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Ocean-Ocean subduction (island
arcs, marginal basins) Western
pacific
Ocean-Continent subduction.
Eastern Pacific. The Cordilleras of
N and S America.
Continental collision. The Alps.
Himalayas.
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Formation of future continental
crust
Accretion of new crust by docking
of terranes and magmatic
underplating
Recycling and Destruction of
continental crust.
Location of convergent plate margins on Earth. Active back arc
basins are also shown.
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Subduction of the oceanic crust
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The western Pacific contains
most of Earth marginal
basins and oceanic island
arcs.
This is where new
continental crust is being
formed
It will eventually be docked
on the active margins of the
continents
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W Pacific. Topography Bathymetry
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W Pacific Free Air Gravity (from satellite data)
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Tectonic setting of Southeast Asia. Thick lines
with closed triangles are modern active trenches
and those with open triangles are inactive. Thick
arrows show the direction of plate movement in a
fixed hotspot reference frame after Engebretson et
al. (1985) and Royer and Sandwell (1989). Length
of bar is motion for 10 Ma. Spreading centers are
shown in double lines. Thin lines are active
structural boundaries and thin broken lines are
traces of offshore or buried structural boundaries.
Abbreviation: T: Trench or Trough, JB: Japan
Basin, NA: Nankai Trough, OT: Okinawa Trough,
DRB: Daito ridges and basins, SB: Shikoku Basin,
OD: Ogasawara Depression, MA: Mariana
Trough, PB: Parece Vela Basin, WPB: West
Philippine Basin, SCB: South China Basin, AB:
Andaman Basin, PA: Palawan Trough, NE:
Negros Trench, SU: Sulu Basin, CO: Cotobato
Trench, CB: Celebes Basin, NS: North Sulawesi
Trench, MB: Makassar Basin, BB: Banda Basin,
MO: Molucca Collision, SO: Sorong Fault, CE:
Ceram Trough, CAB: Caroline Basin, NG: New
Guinea Trench, WE: West Melanesia Trench, NB:
New Britain Trench and TR: Trobriand Trench,
RF: Red River Fault, SF: Seribu Fault.
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Australia: Topography Bathymetry
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Australia: Free Air Gravity.
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Scotia Arc: Topography Bathymetry
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Scotia Arc: Free Air Gravity
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South America Topography and bathymetry
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Free Air gravity in the oceans around South America
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NE Pacific Topography Bathymetry
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NE Pacific Free Air Gravity
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Free Air gravity from GRACE (29/10/2004).
Paired gravity anomalies
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Sea floor age
Note subduction occurs at all ages.
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Distribution of ages of the sea floor
The area of Sea floor of given
age decreases with age. This
indicates that sea floor is
destroyed at constant rate
regardless of age.
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Oceanic subduction zones
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(top) Schematic section through the upper
140 km of a subduction zone, showing the
principal crustal and upper mantle
components and their interactions. Note that
the location of the “mantle
wedge” (unlabeled) is that part of the
mantle beneath the overriding plate and
between the trench and the most distal part
of the arc where subduction-related igneous
or fluid activity is found. MF stands for
magmatic front.
(bottom) Schematic section through the
center of the Earth, which shows better the
scale of subduction zones. Subducted
lithosphere is shown both penetrating the
660 km discontinuity (right) and stagnating
above the discontinuity (left). A mantle
plume is shown ascending from the site of
an ancient subducted slab. Dashed box
shows the approximate dimensions of the
shallow subduction zone of Figure 1b.
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Structure of the western Pacific subduction zones with a back-arc basin.
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Semilog scale plotting the depth dimension for subduction zones from 1 km below the surface to the core-mantle
boundary. Mean depth of ocean (3880 m) is from Kennett [1982]. Typical crustal thickness (∼20 km) for juvenile
arc crust is from Suyehiro et al. [1996]; crust associated with Andean-type convergent margins may be up to 70
km thick. Mean distance of arc volcanoes to trench (166 ± 60 km) and of depth beneath arc volcano to seismic
zone (124 ± 38 km) are from Gill [1981]. Greatest depth of subducted material returned to surface (∼100 km).
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Seismic structure, focal mechanisms
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Wadati Benioff zone
Local seismic tomography
Stresses in the subducted plate
Stresses in the overriding plate
Seismic tomography and the fate of the subducted slab.
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Plate flexure causes a bulge (outer rise) up from the trench.
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The dipping seismogenic zone (Wadati-Benioff zone) delineates the
subducting slab.
Earthquakes show that the slab is cold and brittle.
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Quality factor Q measures
how well energy is
transmitted.
Attenuation of seismic waves
(loss of energy) is due to
“friction”, i.e. viscous mantle.
Comparing seismic signals
that travel through the slab or
below the volcanic arc show
that waves that travel in the
asthenosphere are more
attenuated than waves that
travel through the slab.
The attenuation affects mostly
the high frequencies that are
attenuated
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The Q factor (quality) measures the loss of energy of the seismic waves. Quality is
high if there is no loss of energy. The attenuation of seismic waves usually is
caused by partial melting.
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Velocity anomalies related to temperature
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The subducting plate is seismic because it remains cold and brittle.
The higher velocities are also due to the plate being cold.
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Tomographic images show that the 650km
discontinuity is a temporary barrier for some plates
that can not penetrate into the lower mantle, but most
eventually do.
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The dip of the slab is variable.
Stresses seem mostly compressive in the shallow dipping slabs otherwise
tensile.
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Geophysical images of subduction
zones. (a) P wave tomographic image
of NE Japan (modified after Zhao et al.
[1994]). There is no vertical
exaggeration. (b) P wave tomographic
image of Tonga subduction zone
(modified after Zhao et al. [1997]).
There is no vertical exaggeration. For
both Figures 6a and 6b, red and blue
colors denote regions where the P
wave velocities are relatively slow and
fast, respectively, compared to average
mantle at the same depth. (c) P wave
velocity structure across the Cascadia
Subduction Zone (modified after
Parsons et al. [1998]). Yellow dots
show earthquakes during 1970–1996
between 45° and 47° latitude, >M1 at
depths >25 km and >M4 at shallower
depths. Note vertical exaggeration is
2x. Note that the subduction zones in
Figure 6a and 6b subduct old, cold
lithosphere, which is relatively easy to
identify tomographically, whereas the
subduction zone in Figure 6c subducts
young lithosphere, which differs less in
velocity relative to the surrounding
mantle and is more difficult to image
tomographically. See color version of
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Mantle structure beneath some subduction zones.
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Contrasting tectonic environments in subduction zones.
Extension or compression in the overiding plate
Tectonic variability found behind magmatic
arcs, using the Bolivian Andes and the Lau
Basin as examples. Dark shading denotes
lithosphere; underlying white is asthenosphere.
(a) Tectonic setting of Lau Basin (modified
after Zhao et al. [1997]), emphasizing
lithospheric structures. Dashed box shows
region of detail shown in Figure 20b. (b)
Tectonic setting of Andes between 20°S and
24°S (modified after Yuan et al. [2000])
emphasizing crustal structure. Area of detail
shown in Figure 20d lies partly within the openended dashed rectangle. (c) Cross section across
the Lau Basin, emphasizing lithospheric
structure, at the latitude investigated by Ocean
Drilling Progdram Leg (∼20°S) (modified after
Hawkins [1994]). Extension rate is from GPS
measurements of Bevis et al. [1995] for
velocities of Tonga from Australia; note that the
highest rate is at the northern end of Lau Basin
(∼17°S), and the rate decreases to the south. (d)
Cross section across Andean back arc region,
emphasizing crustal structure, about 20°S
(modified after Gubbels et al. [1993]).
Shortening rate is from GPS measurements of
Bevis et al. [2001]. Figures 20c and 20d have
the same horizontal scale. There is no vertical
exaggeration on any of the sections.
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• Compression in the eastern
Pacific.
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Focal mechanisms in the Peruvian
Andes.
Stress in the overriding plate is
compressive in the Peruvian Andes
where subduction is shallow.
It suggests that there is crustal
shortening in the overriding plate
in the Andes.
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Deformation in the Peruvian Andes.
GPS measurement
Geological rates of deformation
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• Extension in the western
Pacific
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Focal mechanisms show tensile stress
in the overriding plate in the Tonga
Kermadec region.
Stress consistent with back arc
extension and marginal basin
formation
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Stress regime determines tectonic style in the overriding plate
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End-member types of subduction
zones, based on the buoyancy of
lithosphere being subducted (modified
after Uyeda and Kanamori [1979]).
Usually young lithosphere is buoyant,
but old lithosphere could be also be
light if it includes oceanic plateaus or
fossil ridges (e.g. Ontong-Java in the
western Pacific, the northern Andes).
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Back arc spreading
• Several
mechanisms have
been proposed
• Convection
entrained by
subducting plate
and enhanced by
melting
• Slab rollback
(Scotia plate, and
others)
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Example of back arc extension-east Scotia sea
The back arc spreading forms normal
sea floor with well developed marine
magnetic anomalies.
Interpreted magnetic anomaly map of
the East Scotia Sea. Anomaly
identifications are essentially the same
as those of Barker [1995], but recently
identified pseudofaults [Livermore et
al., 1994, 1997], formed by ridge
segment propagation [Hey, 1977], are
incorporated in this interpretation. The
central Bruhnes anomaly as well as
anomalies 2A and 3 are shaded. Ridge
crest segments E2–E9 are numbered.
The 2500-m contour (dashed line)
locates the South Sandwich island arc
and the North and South Scotia Ridge.
The ornamented line represents the
trench. The islands identified are
Zavodovski (Z), Candlemas (C),
Montagu (M), and Southern Thule
(ST).
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Back arc extension in the Philippine Basin at the Izu-Bonin trench
Bathymetry
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Magnetics
Gravity
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Free-air gravity anomalies of the Ontong Java Plateau–Solomon Islands convergent zone derived from Geosat and ERS-1
altimetry data (Sandwell and Smith, 1997). Arrows indicate direction and rate of Pacific plate relative to adjacent plates
from DeMets et al. (1994). Large yellow areas are known or inferred oceanic plateaus compiled by Coffin and Eldholm
(1994); dashed yellow lines represent hotspot tracks or "tails". Key to abbreviations: NB=Nauru basin; ER=Eauripik Rise;
LP=Louisiade Plateau; OJP=Ontong Java Plateau; VAT=Vanuatu trench; VT=Vitiaz trench; NFP=North Fiji Plateau;
MP=Manihiki Plateau; SHS=Samoa hotspot; TT=Tonga trench; LR=Louisville ridge; HP=Hikurangi Plateau;
THS=Tasminid hotspot; LHS=Lord Howe hotspot. Note the general difficulty in correlating individual hotspot tracks to
oceanic plateaus because of intervening subduction zones.
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Evolution of the Tonga Fiji subduction?
Tectonic reconstruction of the
New Hebrides - Tonga region
(modified and interpreted from
Auzende et al. [1988], Pelletier
et al. [1993], Hathway [1993]
and Schellart et al.(2002a)) at (a)
~ 13 Ma, (b) ~ 9 Ma, (c) 5 Ma
and (d) Present. The IndoAustralian plate is fixed. DER =
d'Entrcasteaux Ridge, HFZ =
Hunter Fracture Zone, NHT =
New Hebrides Trench, TT =
Tonga Trench, WTP = West
Torres Plateau. Arrows indicate
direction of arc migration.
During opening of the North Fiji
Basin, the New Hebrides block
has rotated some 40-50°
clockwise [Musgrave and Firth,
1999], while the Fiji Plateau has
rotated some 70-115°
anticlockwise [Malahoff et al.,
1982]. During opening of the
Lau Basin, the Tonga Ridge has
rotated ~ 20° clockwise [Sager et
al., 1994
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Volcanic activity in the overriding plate
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Magmatic arc complexities. Only an
idealized section through an
intraoceanic arc is shown; similar
processes are expected beneath
Andean-type arcs. Note that the
asthenosphere is shown extending up
to the base of the crust; delamination
or negative diapirism is shown, with
blocks of the lower crust sinking into
and being abraded by convecting
mantle. Regions where degassing of
CO2 and H2O is expected are also
shown.
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• What is the cause of the
magmatic activity?
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Simple thermal models show that the
down-going plate remains cold and could
not melt.
It is likely that melting occurs because
fluids are expelled from the downgoing
slab that lower the melting temperature
in the mantle.
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Effect of water on the solidus
Effect of water on melting of
peridotite. (a) Pressuretemperature diagram for H2O
undersaturated (0.2–0.5 wt %
H2O) and anhydrous partial
melting of mid-ocean ridge basalt
(MORB) and pyrolite mantle
composition (modified after
Ulmer [2001]). Dashed lines
indicate the stability limits of
garnet peridotite (gar), spinel
peridotite (sp), plagioclase
peridotite (plg), and amphibole
peridotite (amph). The dash-dot
line corresponds to the average
curent mantle adiabat (ACMA)
corresponding to a potential
temperature of 1280°C. (b) Plot
of melt fraction versus
temperatures of anhydrous batch
melts of a depleted MORB
mantle source at 1.5 GPa with the
liquidus temperatures of hydrous
batch melts from peridotite
containing 0.15% H2O and
0.32% of simplified Mariana
subduction component, which is
an aqueous fluid with dissolved
solutes [Stolper and Newman,
1994]. Upper axis shows
forsterite content of equilibrium
olivine. Figure 13b is from
Gaetani and Grove [1998].
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Melting occurs only in a very narrow range. It
determines the location of volcanic arc.
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This model accounts for the entrainment of the
asthenosphere by the downgoing slab (which
makes it colder at given depth. There is a narrow
window where the geotherm intersects the solidus
for wet mantle.
(a) Material movement (arrows) and temperature
structure (dashed lines) of a simplified convergent
margin (modified after finite element models of
Davies and Stevenson [1992]). Crust and mantle
lithosphere of the subducting plate move at a
constant velocity of 7 cm yr-1, dragging the
asthenosphere with it beneath the overriding plate.
Note also that the isotherms remain approximately
parallel with the original seafloor, even deep into
the subduction zone. The shaded area in the
mantle wedge approximates the location of detail
shown in Figure 12.
(b) Temperature profile beneath the arc volcanoes
(bold line) and melting curve for wet mantle (thin
line). Note that the temperature reaches a
maximum at ∼80 km depth and then decreases as
the subducted lithosphere is approached, reaching
a minimum at a depth of ∼120 km. The
temperature is high enough for mantle melting
only at a depth of ∼80 km, ∼40 km above the
subducted plate.
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Volcanic Arc
Model for dehydration of subducted materials
(modified after Schmidt and Poli [1998]).
Dehydration of subducted peridotite and oceanic
crust occurs continuously to ∼150–200 km; thus
water is continuously supplied to the overlying
mantle. The shaded region in the mantle wedge
labeled “partially molten region” is expected to
melt to a significant degree. The volcanic front
forms where the amount of melt can separate and
rise to the surface. Open arrows indicate rise of
fluid, solid arrows indicate rise of melts. Long
arrows indicate flow of asthenosphere in the mantle
wedge. Stippled area marks stability field of
amphibole beneath the forearc. Dashed lines
outline stability fields of hydrous phases in
peridotite. The horizontally ruled region shows
where talc is stable. For some thermal structures a
portion of the peridotitic lithosphere will be
<600°C at 62 kbar; serpentine will react to “phase
A,” and thus H2O will be subducted to large depth.
In the oceanic crust, temperatures can be low
enough to preserve lawsonite and phengite to their
maximum pressure stability; however, at somewhat
warmer conditions (slower subduction, shallower
angle, and younger crust), zoisite (zo) will be the
last potassium-free phase to decompose, and the
top of the oceanic crust (phengite-rich sedimentary
layer) might melt. Single-phase transitions, which
cause a potassium-rich fluid pulse, could result
from the breakdown of phengite in oceanic crust
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Detailed cartoon of the mantle wedge (region of detail
depicted by shading in Figure 11), showing how fluids
might move from the subducted slab into a region where
melts could be generated (modified after Davies and
Stevenson [1992] and Stern [1998]). Water is carried in
the descending slab (A), and dense aqueous fluids are
continuously released from subducted sediments, crust,
and serpentinites (B). These fluids rise into the overlying
mantle to form hydrous phases in mantle peridotite (C).
Amphiboles are shown forming here, but it could also be
another hydrous mineral. Metasomatized mantle
descends with the subducted slab. At the maximum
depth of stability for amphibole peridotite, ∼100 km, it
breaks down to anhydrous peridotite and aqueous fluid
(D). The fluid rises vertically, moving away from the
subducted slab and toward hotter regions in the mantle.
At some point the rising water will react with anhydrous
peridotite to form amphibole again (E). This descends
until the amphibole breaks down again (F). Amphibole
peridotite forms (G), descends, and breaks down again
(H). The dark shaded area is mantle that can melt if
sufficient water is provided. Above point H the mantle is
sufficiently hot that water added to it leads to melting (I).
The mantle is still moving downward, so melt will not be
able to rise until enough of it accumulates to form a
sufficiently large diapir (K) as a result of RayleighTaylor instability. Partially molten diapirs are less dense
than the surrounding mantle and can rise through it at a
rate of perhaps 1 m yr-1 (L). At the base of the overlying
mantle lithosphere (M), magma separates from the
unmelted part of the diapir to feed an arc volcano.
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Temporal evolution of island arc
magmatism.
Early stage expulsion of water (80-100km)
causes partial melting of mantle with
tholeitic magmas.
At greater depths, the partially molten
region ascents as a diapir and adiabatic
decompression causes more melting and
differentiated calc-alkaline magmas.
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Andean style batholiths
New continental crust is added
directly to the continent through
magmatic activity.
Sr87/Sr86 indicate mantle origin
with continental crust
contamination.
Be10 (1/2 life = 1.5 My) was
found in some of the lavas. It
could come only from sediments
that were subducted, melted, and
included in the magmas.
Note large batholiths in
continents used to detect ancient
Andean type margin (e.g.dePas
batholith in NQO).
Proterozoic examples:
Is the elongated de Pas
batholith in the Core Zone
anologuous to the batholith in
the Andes?
Suggests subduction of the
Superior plate beneath the core
zone during the NQO
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Wathaman batholith on margin of Hearne. No batholith in the
Superior => No subduction beneath Superior during closure of
Maniwekan ocean?
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Andes elevation ~ 6km
Crustal thickness is
estimated 60-70km
Crustal thickening by
tectonic shortening and by
magmatic underplating
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Receiver function (RF) images and crustal
models of the Central Andes along an east–
west profile. In the images, red indicates
positive, velocity increase downwards; blue
indicates negative, velocity reduction
downwards. a, Time domain RFs averaged
over a 30-km-wide moving window, 2–10-s
band pass filtered. b, Possible crustal S-wave
velocity (vS) models resulting from the
inversion of the RFs in a. c, Interpretative
cartoon with depth-migrated RF image as
background averaged over the Altiplano and
Puna regions. Black plus signs show Moho
depth data from wide-angle reflection
studies4. Local crustal models and a global
mantle reference model were used for
migration. The TRAC1 and TRAC2
converters in a and c border the Andean lowvelocity zone (ALVZ) which dips westwards
from below the Eastern Cordillera fault
system (indicated in c by black lines at the
surface) across the entire Altiplano/Puna to the
Precordillera (red points at surface indicate
Cenozoic volcanoes). Figure S1 in
Supplementary Information shows this data
set split in two at 23° S for judging north–
south variations of the TRAC2 converter.
Figure S2 in Supplementary Information
shows the lateral extension of the ALVZ. The
Nazca converter (thick black-white dashed
line indicated in c) is interpreted as an image
of the oceanic Moho. The upper boundary of
the oceanic crust (slab shear zone) is set
10 km above the oceanic Moho in agreement
with the results of waveform inversion (see
text and Fig. 4). Oceanic crust is clearly
visible from converted waves only down to
Electrical conductivity of the mantle
is measured by MT
Contours: depth of Nazca plate
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High conductivity = hydrated mantle
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Metamorphism in subduction zones
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Metamorphism in subduction zones
High pressure in the downgoing slab
Low pressure in the overiding plate
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Paired metamorphic belts in Japan
Note paired metamorphic belts are found around the Pacific (with the right polarity)
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Thermal models of end-member (young and hot
versus old and cold) subduction zones. (a) NE Japan
arc, a good example of a cold subduction zone. (b)
SW Japan arc, a good example of a hot subduction
zone. Note the great difference in the temperature of
the slab interface at 50 km depth (T50km) and
beneath the volcanic front (Tvf) but the small
difference in the maximum temperature of the mantle
wedge beneath the volcanic front (Tmw). (c)
Pressure-temperature diagram showing metamorphic
facies and melting relations for basaltic oceanic crust,
along with trajectories for crust subducted beneath
NE and SW Japan Roman numerals identify fields for
metamorphic facies: I, greenschist; II, epidote
amphibolite; III, amphibolite; IV, granulite; V, epidote
blueschist; VI, lawsonite blueschist (green field
shows location of the eclogite field, with the dark
dashed line separating this from blueschist); VII,
chloritoid-amphibolite-zoisite eclogite; VIII, zoisitechloritoid eclogite; and IX, lawsonite-chloritoid
eclogite. Note that oceanic crust subducted beneath
SW Japan enters the eclogite field at ~40 km depth
and, if hydrous, begins to melt at ~90 km, whereas
oceanic crust subducted beneath NE Japan barely
enters the eclogite field at 120 km depth. Figures 6a–
6c are modified after Peacock and Wang [1999] and
Peacock [2002]. See color version of this figure at
back of this issue.
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The Canadian Cordillera
Accreted terranes in the Canadian
Cordillera.
Some terranes are of North
American affinity
Others are of oceanic affinity.
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Main geological belts of the
Canadian Cordillera and Bouguer
gravity anomaly.
Intermontane and Coast belts are
accreted terranes.
Omineca and Foreland belts are of
north American affinity.
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Schematic cross section of the Canadian Cordillera
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Geodynamic model of the Canadian Cordillera after the LITHOPROBE studies
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Intermontane belt transported on North America basement.
Coast belt docked on the side.
Sediments
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a, Unmigrated section showing the reflection, labelled JdF, from the top of the subducting Juan de Fuca plate west of
Vancouver Island, the E reflection zone, and the F reflection which marks the top of the subducting Juan de Fuca plate further
east. Reflections from the Leech River crustal fault, LRF, are truncated by the E reflections.
b, Migrated section superimposed on a display of P wave velocities derived by 3D tomographic inversion of first arrivals from
local earthquakes and wide-angle airgun shots around Vancouver Island9. The velocities were extracted from the 3D velocity
model along the composite seismic profile shown by the red line in Fig. 1 and projected in the same way as the seismic
reflection data. Earthquakes, which were relocated in the tomographic inversion, are shown by filled black circles. The nearlinear alignment of east-dipping inslab earthquakes just above the oceanic Moho at 45–55 km depth may indicate delaminatio
of the oceanic crust
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Comparison of scattered wave inversion results
with thermal model. a, S-velocity perturbations
below the array, recovered from the inversion of
scattered waves in the P-wave coda of 31
earthquakes recorded at teleseismic distances. The
image represents a bandpass-filtered version of the
true perturbations to a one-dimensional, smoothly
varying reference model. Discontinuities are
present where steep changes in perturbation
polarity occur. b, Thermal model of Cascadia
subduction zone corresponding approximately to
the profile in a. The cool subducting plate
depresses isotherms in the forearc, rendering
serpentine stable within that portion of the mantle
encompassed by the dashed rectangle; solid lines
indicate locations of subducting oceanic crust and
continental Moho. Note temperature contour
interval is 200 °C. c, Interpretation of structure in
a. High degrees of mantle serpentinization where
the subducting oceanic crust enters the forearc
mantle results in an inverted continental Moho
(high-velocity crust on low-velocity mantle),
which gradually reverts eastward to normal
polarity by -122.3° longitude. The signature of the
subducting oceanic Moho diminishes with depth
as a result of progressive eclogitization below
45 km. Inverted triangles in a and c show
instrument locations.
What happened to the Farallon plate?
It is still down there!
Wednesday, April 14, 2010
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Cross-section through the Cascadia subduction zone on the northwest coast of
the United States. Water released from the subducting oceanic crust rises up and
hydrates the forearc mantle; water released deeper down causes partial melting
of rock to magma that erupts in the arc volcanoes. In the water-altered — or
'serpentinized' — forearc rocks, seismic waves travel more slowly and the usual
gradient of wave speed is reversed, producing an effect known as an 'inverted
Moho', which has been detected by Bostock et al.1. The slippery nature of the
serpentinized forearc mantle limits the seismogenic depth of coupling between
the two plates, and so might also limit the impact of earthquakes in the region.
Wednesday, April 14, 2010
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Foreland basins are caused by the bending of the plate under
the load of the accreted terranes
Wednesday, April 14, 2010
The accretion of terranes over the western margin of Canada caused the
formation of the Alberta basin
Wednesday, April 14, 2010
Forearc basin-Accretionary prism
End-member forearc types: (a) accretionary
forearc (modified after Dickinson [1995])
and (b) nonaccretionary forearc (P. Fryer,
personal communication, 2001). Note that
the abundance of sediments associated with
accretionary forearcs is manifested as an
accretionary prism and as a thick forearc
basin and that the relative lack of sediments
leaves the nonaccretionary forearc exposed.
Wednesday, April 14, 2010
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Mechanical model of an accretionary wedge.
Balance of forces implies a positive slope for the wedge
Wednesday, April 14, 2010
Erosion and subduction of sediment in crust could exceed accretion of new
crust.
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Map showing the distribution of accreting versus eroding subduction zones
considered within this study. Accretionary margins are shown with solid
barbs on the plate boundary, while open barbs mark erosive margins.
Wednesday, April 14, 2010
Compilation of profiles across
accretionary plate margins. Profiles are
redrawn and resized to a common scale
in order to allow direct comparison of
different margins. Sources for the
original data are shown next to each
profile.
Wednesday, April 14, 2010
Compilation of profiles across nonaccretionary and erosive plate margins.
Profiles are redrawn and resized to a common scale in order to allow direct
comparison of different margins.
Wednesday, April 14, 2010
Budget of all continental margins
Diagram showing the integrated growth
or erosion rate of each active plate
margin in relation to the global average
growth rate required to maintain the
continental freeboard. Note that several
erosive plate margins are actively
growing crust despite rapid loss at the
trench.
Wednesday, April 14, 2010
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Pie chart showing the relative proportions
of the major inputs and outputs from the
global subduction systems with respect to
the crust. Note the dominance of arc
magmatism over subduction accretion as a
source of new material.
Wednesday, April 14, 2010
When did subduction processes begin on Earth ?
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Evidence for late Archean subduction?
Seismic reflector in the mantle interpreted as a relict subducted slab.
Wednesday, April 14, 2010
Mantle reflectors could be fossil
Archean subduction zones.
(example from the Slave Province, LITHOPROBE
SNORCLE transect).
Wednesday, April 14, 2010
Continental crust evolution?
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Today continental crust is being destroyed at about the same rate it is accreted.
We do not know how the total volume of the continental crust has changed with
time.
Model 4 with a rapid growth of the continental crust at the end of Archean is
plausible but so are the other models
Episodic growth models have also been proposed.
Wednesday, April 14, 2010
Summary
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Subduction of sea floor independent of age
But tectonic style might depend on age of the subducted slab (western vs
eastern Pacific)
Plate remains cold
Expulsion of volatiles causes partial melting in the mantle
Magmas in volcanic arc from mantle (but …) . Main source of continental
growth
High pressure low temperature metamorphism in subducted slab
Accretion of terranes in North American cordillera
Plate tectonic and subduction in late Archean at 2.7 Ga
Budget of continental crust?
Wednesday, April 14, 2010
Open questions
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What initiates subduction?
Mechanism of accretion.
When did present day style of subduction start?
Meaning of Archean and Proterozoic subcrustal reflectors.
What is crustal accretion destruction present budget?
How did total volume of continental crust change with time?
Wednesday, April 14, 2010
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Importance of convergence rate and age
of subducted lithosphere. (a) Plot of
length of seismic zone as a function of
the product of convergence rate and age
of subducted lithosphere. Approximate
uncertainties are given by error bars.
Dashed line corresponds to length of
seismic zone equal to convergence rate
multiplied by age divided by 10
(modified after Molnar et al. [1979]).
(b) Plot of convergence rate versus
lithosphere age, showing the strong
influence this relationship has on
seismicity (modified after Ruff and
Kanamori [1980], using GPS-defined
convergence rates for Pacific-Tonga
[Bevis et al., 1995]. The number at each
subduction zone is the associated
maximum Mw (seismic moment
magnitude), and the contours of
constant Mw are defined by regression
analysis. Shaded area outlines
subduction zones associated with back
arc spreading or interarc rifting. Note
that most of the biggest earthquakes
occur at Andean-type margins.
Wednesday, April 14, 2010
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Diagnostic trace element differences
between adakites and more common
andesite-dacite-rhyolite (ADR) suites
(modified after Defant and Drummond
[1990]). Adakites form by melting at
high pressure in equilibrium with
garnet (garnet has a high partition
coefficient for Y and low partition
coefficients for Sr) and so have low-Y
contents and high Sr/Y. ADR suites
form at lower pressure in equilibrium
with plagioclase (plagioclase has a
high partition coefficient for Sr and
low partition coefficients for Y) and so
have high Y contents and low Sr/Y.
Wednesday, April 14, 2010
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Incompatibility plot (spidergram) for important subduction zone inputs
(global subducting sediment (GLOSS) and normal mid-ocean ridge basalt
(NMORB)) and typical outputs (boninite, arc tholeiite, back arc basin
basalt, dacite, and continental crust) from Table 2. Elements on the
horizontal axis are listed in order of their incompatibility in the mantle
relative to melt; elements on the left are strongly partitioned into the melt,
whereas those on the right are strongly partitioned into peridotite minerals.
Composition of NMORB and element order is after Hofmann [1988]. Note
the characteristic enrichments of GLOSS and subduction zone outputs
relative to MORB with respect to fluid-mobile large-ion lithophile elements:
Rb, Ba, U, K, Pb, and Sr; note the relative depletion of these in high field
strength elements: Nb, Ta, Zr, Ti, Y, and heavy rare earth elements. Note
also the greater enrichment and overall similarity of the “continental
suite” (GLOSS, continental crust, and Chilean dacite) on the one hand and
the “oceanic suite” (boninite, arc tholeiite, back arc basin basalt, and
MORB) on the other. Shaded field encompasses the continental suite.
Wednesday, April 14, 2010
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Model of subducted slabs (modified after Stein and Rubie [1999]). (top) Predicted mineral phase boundaries
and (bottom) resulting buoyancy forces in a downgoing slab with (left) equilibrium mineralogy and (right)
for a nonequilibrium metastable olivine wedge. Assuming equilibrium mineralogy, the slab has significant
negative thermal buoyancy (dark shading in bottom graphs), due to both its colder temperature and the
elevated 410 km discontinuity, and significant positive compositional buoyancy (cross hatching) associated
with the depressed 660 km discontinuity. If a metastable wedge is present, it adds positive buoyancy and
hence decreases the net negative buoyancy force driving subduction. Units are in 103 N m-3.
Wednesday, April 14, 2010
Map of Pacific and Caribbean LIP's compared to
modern plate boundaries (yellow lines),
active hotspots/mantle plumes (yellow stars),
and rates of the Pacific and Caribbean plate
relative to surrounding plates from global
plate motion model of DeMets et al. (1994).
Oceanic plateaus are subdivided into three
tectonic settings as shown in the key to the
upper right. Box denotes Solomon Islands–
Ontong Java Plateau study area for papers in
this volume. Key to abbreviations for larger
LIP's and hotspot tracks: HE=Hawaii–
Emperor seamount chain; SR=Shatsky Rise;
SO=Sea of Okhotsk; OP=Ogasawara
Plateau; MNR=Marcus Necker Ridge;
OJP=Ontong Java Plateau; MP=Manihiki
Plateau; HR=Hikurangi Plateau; NR=Nazca
Ridge; CR=Carnegie Ridge; COR=Cocos
Ridge; CP=Caribbean Plateau. Numbered
LIP's 1–21 are keyed to Table 2A (oceanic
plateau or hotspot tracks presently
subducting at Pacific and Caribbean plate
boundaries) and Table 2B (proposed oceanic
plateaus accreted during Phanerozoic
collision events involving island arcs or
continental orogenic belts). (B) Schematic
diagram summarizing four possible
relationships between hotspot or mantle
plume heads and hotspot tracks or "tails" in
intraplate (1), subduction (2, 3) and in
ancient orogenic belts (4). See text for
discussion.
Wednesday, April 14, 2010
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Potassium-silica diagram for representative arcs.
Dashed line defines boundary between shoshonitic
and calc-alkaline and tholeiitic suites (CATS). IzuBonin and Mariana arcs are exemplary of
intraoceanic arcs (fields from Stern et al. [2002],
note that volumetrically subordinate Mariana
shoshonites [Sun and Stern, 2001] are omitted).
Field for Andes, 16°S–26°S encompasses most of
606 Plio-Pleistocene and younger samples from
the Central Volcanic Zone (CVZ) (G. Wörner,
personal communication, 2002). Abbreviations are
as follows: M, typical MORB from Table 2; B,
back arc basin basalt from Table 2; I, mean
composition of Izu-Bonin arc samples; MA, mean
composition of Mariana arc samples; CC, bulk
continental crust from Table 2; G, GLOSS from
Table 2; A, mean composition of Andes CVZ
lavas; and UC, composition of upper continental
crust [from McLennan, 2001]. Dark shading
encompasses mean and typical compositions of
the “oceanic suite” (MORB, back arc basin basalt,
and mean Mariana and Izu-Bonin lavas), and light
shading encompasses mean and typical
compositions of the “continental suite” (bulk
continental crust, GLOSS, upper continental crust,
and mean Andean dacite).
Wednesday, April 14, 2010
Seismic tomography of two arcs
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Crustal structure of typical arcs, based on P
wave velocities. Note vertical exaggeration
is ∼5x.
(a) Izu arc, 33°N (modified after Suyehiro
et al. [1996]).
(b) Eastern Aleutian arc (modified after
Holbrook et al. [1999]).
Note differences in thickness and velocity
structure. We do not have comparable
tomographic images of Andean-type arc
crust. See color version of this figure at
back of this issue.
Wednesday, April 14, 2010
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Comparison of pattern of deformation
observed in Ontong Java Plateau–Solomon
arc convergent zone with examples of
Phanerozoic and Precambrian patterns of
deformation in collision zones. Key to
lithospheric compositions: RED=upper
crustal, mainly igneous rocks;
YELLOW=upper crustal, mainly
sedimentary rocks; gray and BLACK=upper
mantle, mainly ultramafic rocks. (A)
Ontong Java Plateau—initial subduction–
accretion of uppermost plateau in western
Malaita accretionary prism (modified from
Rahardiawan et al., 2004). Open dots
represent ISC hypocenters (M>4.0). (B)
Ontong Java Plateau—more advanced
subduction–accretion in eastern Malaita
accretionary prism. (C) Cenozoic Alpine
crustal-scale triangle zone or
"flake" (modified from Oxburgh, 1972);
L=base of lithosphere; M=subducted Moho.
(D) Precambrian Canadian crustal-scale
triangle zones or "flakes" (modified from
Cook et al., 1998); M2=base of Slave
Province mantle. (E) Precambrian African
thrust-imbricated, subduction-related prism
Wednesday, April 14, 2010