Dynamik von Subduktionszonen

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Dynamik vonSubduktionszonen

• Institut für Geowissenschaften Universität Potsdam

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Übersicht zur Vorlesung
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Subduktions- zonen
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simple scaling view
L
W
d
d
FR
vplate
DT
d (kt)1/2 cooling thickness
r0 , a
time t
h
D
r0 a DT density after expansion
k
- bouyancy force
FB
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Plate tectonics scaling view (I)
FB ra DT/2 (d DW ) g
bouyancy force
gravity
density
size
mass
acceleration

FR (? v/L) (DW )
resistance force
stress s
area
because of
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Plate tectonics scaling view (II)
Ra 2/3
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r0 3 103 kg/m3 density
a 3 10-6 m2/s thermal expansion
DT 1400 K temperature difference
h 1022 Pa s viscosity
g 10 m/s2 grav. acceleration
L 3 106 m layer thickness
k 10-6 m2/s thermal diffusivity
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deformation time scales
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various kinetic processes during subduction
P. van Keken, 2004
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What do we want to understand ..

• What is the flow pattern in the wedge mantle?
• Temperature distribution (how hot is the corner?)
• 2-D laminar flow versus 3-D flow involving
  trench parallel component?
• Do subducting slabs contain a large amount of
  water (serpentine)?
• What is the distribution of water in the wedge
  mantle?
• Is the wedge mantle wet throughout, or is it
  wet only in limited regions? (Comparison to the
  continental tectosphere.)
• Does basalt -gt eclogite transformation occur at
  equilibrium condition?
• Do dehydration reactions cause earthquakes?
• could dehydration reactions at high-P (?Vlt0)
  cause instability?

open todo list, MARGINS workshop, Ann Arbor
(2002) http//www.nsf-margins.org/MTEI.html
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Plate tectonics - potential hazards (I)
Volcanism
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Magma Genesis
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Eruption of Mount St. Helens, May 18, 1980
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Mt. Saint Helens 1980 eruption
USGS
Loma Prieta 1989 earthquake
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Eruption of Mount Pinatubo, June 15, 1991
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Complex plate boundary zone in South-East
Asia Northward motion of India deforms all of
the region Many small plates (microplates) and
blocks
Molnar Tapponier, 1977
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Plate tectonics - potential hazards (II)
Tsunami waves
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December 26, 2004 subductionthrust fault
earthquake
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INTERSEISMIC India subducts beneath Burma
microplateat about 50 mm/yr (precise rate hard
to infer given complex geometry) Fault interface
is locked EARTHQUAKE (COSEISMIC) Fault
interface slips, overriding plate rebounds,
releasing accumulated motion
Fault slipped 10 m 10000 mm takes 10000 mm
/ 50 mm/yr 200 yr Longer if some slip is
aseismic Faults arent exactly periodic for
reasons we dont understand
Sumatra Earthquake, December 26, 2004
HOW OFTEN ?
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Banda Aceh, Sumatra, before tsunami http//geo-wor
ld.org/tsunami
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Banda Aceh, Sumatra, after tsunami http//geo-worl
d.org/tsunami
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Plate tectonics - potential hazards (III)
Large Earthquakes
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Largest earthquakes, 1900 - 2004
1. Chile 1960 05 22 9.5 38.24 S 73.05 W
5. Off the West Coast of Northern Sumatra 2004 12 26 9.3 3.30 N 95.78 E
2. Prince William Sound, Alaska 1964 03 28 9.2 61.02 N 147.65 W
3. Andreanof Islands, Alaska 1957 03 09 9.1 51.56 N 175.39 W
4. Kamchatka 1952 11 04 9.0 52.76 N 160.06 E
6. Off the Coast of Ecuador 1906 01 31 8.8 1.0 N 81.5 W
7. Rat Islands, Alaska 1965 02 04 8.7 51.21 N 178.50 E
8. Assam - Tibet 1950 08 15 8.6 28.5 N 96.5 E
9. Kamchatka 1923 02 03 8.5 54.0 N 161.0 E
10. Banda Sea, Indonesia 1938 02 01 8.5 5.05 S 131.62 E
11. Kuril Islands 1963 10 13 8.5 44.9 N 149.6 E
USGS
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Largest earthquakes, 1900 - 2004
USGS
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3 components of earthquake hazard at SZ
(1) Large interplate thrust (rare, but
paleoseismology tsunami history from Japan find
big one in 1700) largest earthquakes but further
away (2) Intraslab (Juan de Fuca) earthquakes
smaller but closer to population (3) Overriding
(North American) plate smaller but closer to
population
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Deep Earthquakes
Earthquakes and subducted slabs beneath the
Tonga-Fiji area(yellow marker - 2002 series,
orange marker - 1986 series)
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Subduction
one plate descends below another, oceanic crust
is consumed
understanding of subduction process
completed formation of theory of plate tectonics
provided mechanism for removing oceanic
crust generated at mid-ocean ridges
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how was subduction discovered?
Wadati-Benioff zones zones of dipping
earthquakes to 100s kms depth (max
670 km)
deep
shallow
seismicity
intermediate
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plate tectonics convergent boundaries
Wadati-Benioff zone northern Japan
epicenters
hypocenters
red dots are deepest earthquakes so they plot on
map as farthest from trench
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plate tectonics convergent boundaries
variations in dips of Wadati-Benioff zones
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plate tectonics convergent boundaries
imaging the subducting plate with seismic
velocities - subducting plate is cooler than
surrounding mantle -
fast cooler (denser material) slow hotter
(less dense material)
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plate tectonics convergent boundaries
less buoyant plate dives below more buoyant plate
oceanic lithosphere density gt continental
lithosphere
3 types of convergence
ocean-ocean convergence
ocean-continent convergence
continent-continent convergence (collision)
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(1) ocean-ocean convergence
one oceanic plate subducts below another
earthquakes occur along interface between two
plates
trench, accretionary wedge, forearc basin,
volcanic arc
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(1) ocean-ocean convergence
trench deep, narrow valley where oceanic plate
subducts
accretionary wedge sediments that accumulated
on subducting plate as it traveled from ridge
are scraped off and accreted (added) to
overriding plate
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(1) ocean-ocean convergence
forearc basin between accretionary wedge and
volcanic arc
volcanic arc mantle is perturbed by subduction
process and melts at depths of 100-150 km,
creating magma that rises to the surface to
form island volcanoes
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(1) ocean-ocean convergence
Example well-developed trenches in
Indonesia/ Phillippines
http//www.pmel.noaa.gov/vents/coax/coax.html
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(2) ocean-continent convergence
oceanic plate subducts below less dense
continental crust
features same as with ocean-ocean convergence
except that volcanoes are built on continental
crust and in some cases a backarc thrust belt
may form
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(2) ocean-continent convergence
volcanoes (magmatic arc) more silicic from
addition of continental material batholiths
form at depth
backarc thrust belt thrust faults form behind
arc in response to convergence stickiness
between plates
Andes Cascades
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arc-trench gap
distance between the trench and volcanoes
because the depth at which magmas are
generated in subduction zones is about 100-150
km, this distance depends on the dip of the
subducting plate
if the dip of the subducting plate is flat
enough, no volcanoes form subducted plate doesnt
go deep
infer dip by looking at distance between
volcanoes and trench
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trench can migrate through time response to
forcing either by overriding or subducting plate
subducting plate steepens and pulls overriding
plate toward trench
overriding plate pushes trench
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(3) continent-continent convergence
neither plate wants to subduct (both are buoyant)
result is continental collision
mountain belts
thrust faults
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(3) continent-continent convergence
model for India and Asia collision
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(3) continent-continent convergence
EURASIAN PLATE
Himalayas
are part of a long mountain belt that extends to
Alps
INDIAN PLATE
AFRICAN PLATE
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(3) continent-continent convergence
deformation from collision extends far into
Tibet/Asia
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what causes plates to move ?
ridge push sea floor spreading and gravity
sliding of plate downhill from ridge to
trench while being pushed by sea floor spreading
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what causes plates to move ?
slab pull weight of subducting slab
subducting slab sinks into mantle from its own
weight, pulling the rest of the plate with it
as subducting slab descends into mantle, the
higher pressures cause minerals to transform to
denser forms (crystal structures compact)
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what causes plates to move ?
slab pull is more important than ridge push
How do we know ? - Plates that have the greatest
length of subduction boundary have the fastest
velocities
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what causes plates to move ?
slab pull is more important than ridge push
Forsyth Uyeda, 1975
How do we know ? - Plates that have the greatest
length of subduction boundary have the fastest
velocities
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what causes plates to move ?
mantle convection is the likely candidate,
but is it the cause or an effect of ridge push
and slab pull ?
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How Mantle Slabs Drive Plate Motions
C.P. Conrad and C. Lithgow-Bertelloni "How
mantle slabs drive plate tectonics" Science,
298, 207-209, 2002
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Observed plate motions. Arrow lengths and colors
show velocity relative to the average velocity.
Note that subducting plates (Pacific, Nazca,
Cocos, Philippine, Indian-Australian plates in
the center of this Pacific-centered view) move
about 4 times faster than non-subducting plates
(North and South American, Eurasian, African,
Antarctic plates around the periphery).
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How Mantle Slabs Drive Plate Motions
bending forces
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Diagram showing the mantle flow associated with
the "slab suction" plate-driving mechanism in
which the sinking slab is detached from the
subducting Plate and sinks under its own weight.
This induces mantle flow that drives both the
overriding and subducting plates toward each
other at approximately equal rate.
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Predicted plate velocities for the "slab suction"
plate-driving model. Note that subducting and
non-subducting plates travel at approximately
the same speed, which is not what is observed
(compare to Fig. 1).
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The "slab pull" plate-driving mechanism. Here the
slab pulls directly on the subducting plate,
drawing it rapidly toward the subduction zone.
The mantle flow induced by this motion tends to
drive the overriding plate away from the
subduction zone. This results in an asymmetrical
pattern of plate motions.
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Plate motions driven by the slab pull
plate-driving mechanism. In this case, plates
move with about the right relative speeds, but
overriding plates move away from trenches,
instead of toward them as is observed.
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Preferred model for how mantle slabs drive plate
motions. Slabs in the upper mantle pull directly
on surface plates driving their rapid motion
toward subduction zones. Slab descending in the
lower mantle induce mantle flow patterns that
excite the slab suction mechanism. This flow
tends to push both overriding and subducting
plates toward subduction zones.
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Predicted plate motions from our combined model
of slab suction from lower mantle slabs and slab
pull from upper mantle slabs (Fig. 6). This
model predicts both the relative speeds of
subducting and overriding plates, as well as the
approximate direction of plate motions (compare
to observed plate motions, shown in Fig. 1).
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a more detailed quantitative understanding of
subduction zones
Thermal-mechanical structure of subduction zones
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Wadati Benioff zones
Some earthquakes appear to result from flexural
bending of the downgoing plate as it enters
the trench. Focal depth studies show a pattern
of normal faulting in the upper part of the plate
to a depth of 25 km, and thrusting in its lower
part, between 40-50 km. These constrain the
neutral surface dividing the mechanically
strong lithosphere into upper extensional and
lower compressional zones.
Bodine et al., JGR 86 (1981) 3695-3707
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Simple thermal slab model (McKenzie, 1969)
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Simple thermal slab model (McKenzie, 1969)
Thermal modeling predicts maximum depth of
isotherms in slab varies with thermal parameter
f
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Thermal modeling predicts maximum depth of
isotherms in slab varies with thermal parameter
Deepest earthquakes never exceed 700 km Maximum
depth increases with ? Earthquakes below 300 km
occur only for slabs with ? gt 5000 km
Kirby et al., 1996
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Transition zone between upper lower mantles
bounded by 410 km and 660 km discontinuities corre
sponding to mineral phase changes deep
earthquakes stop at 660 km, perhaps because -
slabs equilibrate thermally - slabs cannot
penetrate 660 km - earthquakes are related to
phase changes
Ringwood, 1979
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Seismicity decreases to minimum 300 km, and then
increases again Deep earthquakes below 300
km treated as distinct from intermediate
earthquakes with depths 70-300 km Deep
earthquakes peak at about 600 km, and then
decline to an apparent limit at 600-700 km
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Slabs are not thermally equilibrated with mantle
Coldest portion reaches only half mantle
temperature in about 10 Myr, about the time
required for the slab to reach 660 km. Thus
restriction of seismicity to depths lt 660 km does
not indicate that the slab is no longer a
discrete thermal and mechanical entity. From
thermal standpoint, there is no reason for slabs
not to penetrate into lower mantle. When a slab
descends through lower mantle at the same rate
(it probably slows due to the more viscous lower
mantle), it retains a significant thermal anomaly
at the core-mantle boundary, consistent with
models of that region
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SLAB PULL plate driving force
Thermal modeling gives a driving force for
subduction due to the integrated negative
buoyancy (sinking) of cold dense slab from
density contrast between it and the warmer and
less dense material at same depth outside.
Negative buoyancy is associated with the cold
downgoing limb of mantle convection
pattern. Since the driving force depends on
thermal density contrast, it increases for (i)
Higher v, faster subducting hence colder
plate (ii) Higher L, thicker and older hence
colder plate Expression is similar to that for
ridge push since both forces are thermal
buoyancy forces
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SLAB PULL plate driving force
Significance for stresses in slabs and for
driving plate motions depends on their magnitude
relative to resisting forces at the subduction
zone As slabs sink into the viscous mantle,
displacement of mantle material causes force
depending on the viscosity of mantle and slab
subduction rate. Slabs are also subject to drag
forces on their sides and resistance at the
interface between overriding and downgoing
plates, which are frequently manifested as
earthquakes.
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Forces within subducting plates (I)
(1) Average absolute velocity of plates increases
with the fraction of their area attached to
downgoing slabs, suggesting that slabs are a
major determinant of plate velocities (2)
Earthquakes in old oceanic lithosphere have
thrust mechanisms showing deviatoric
compression
Forsyth and Uyeda, 1975
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Forces within subducting plates (II)
The slab pull'' force is balanced by local
resistive forces, a combination of the effects of
viscous mantle and the interface between plates.
This situation is like an object dropped in a
viscous fluid, which is accelerated by its
negative buoyancy until it reaches a terminal
velocity determined by its density and shape, and
the viscosity and density of the fluid.
Forsyth and Uyeda, 1975, Wiens Stein, 1984
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Forces within subducting plates (III)
Different stresses result if weight of column of
material supported in different ways
similar to what seismic focal mechanisms show !
Stein Wysession, Blackwell 2003
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Clapeyron slope describes how mineral phase
changes occur at different depths in cold slabs
use thermal model to find dT, phase relations to
find ? and thus dP
convert to depth change dz
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Opposite deflection of mineral phase boundaries
Upward deflection of the 410 km and downward
deflection of the 660 km discontinuities have
been observed in travel time studies.
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Metastable delay of mineral phase transformations
Kirby et al., Rev. Geophys. 1996
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Metastable delay of mineral phase transformations
Predicted mineral phase boundaries and resulting
buoyancy forces in slab with and without
metastable olivine wedge For equilibrium
mineralogy cold slab has negative thermal
buoyancy, negative compositional buoyancy from
elevated 410 km discontinuity, and positive
compositional buoyancy from depressed 660 km
discontinuity Metastable wedge gives positive
compositional buoyancy and decreases force
driving subduction
Stein Rubie, Science 1999
negative buoyancy favours subduction, whereas
positive buoyancy opposes it.
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Deep earthquakes from metastable olivine ?
Kirby et al., Rev. Geophys. 1996
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Deep earthquakes due to large viscosity contrast
between transition zone and lower mantle ?
Predicted stress orientations are similar to
those implied by focal mechanisms. Moreover,
magnitude of the stress varies with depth in a
fashion similar to the depth distribution of
seismicity - minimum at 300-410 km and increase
from 500-700 km.
Vassiliou Hager, Pageoph 128 (1988) 547-624
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Intermediate depth earthquakes (I)
Oceanic crust should undergo two important
mineralogic transitions as it subducts. Hydrous
(water-bearing) minerals formed at fractures and
faults warm up and dehydrate. Gabbro transforms
to eclogite, rock of same composition composed
of denser minerals.
Under equilibrium conditions, eclogite should
form by the time slab reaches 70 km depth.
However, travel time studies in some slabs find
low-velocity waveguide interpreted as subducting
crust extending to deeper depths. Hence
eclogite-forming reaction may be slowed in cold
downgoing slabs, allowing gabbro to persist
metastably.
Kirby et al., Rev. Geophys. 1996
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Intermediate depth earthquakes (II)
In this model intermediate earthquakes occur by
slip on faults, but phase changes favor faulting.
The extensional focal mechanisms may also reflect
the phase change, which would produce extension
in the subducting crust.
Support for this model comes from the fact that
the intermediate earthquakes occur below the
island arc volcanoes, which are thought to result
when water released from the subducting slab
causes partial melting in the overlying
asthenosphere.
Kirby et al., Rev. Geophys. 1996
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Complex thermal structure, mineralogy geometry
of subducted slabs in the mantle transition zone
Deep subduction process is a chemical reactor
that brings cold shallow minerals into
temperature and pressure conditions of mantle
transition zone where these phases are no longer
thermodynamically stable. Because there is no
direct way of studying what is happening and what
comes out, one seeks to understand the system by
studying earthquakes that somehow reflect what is
happening.
Kirby et al., 1996
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Zusammenfassung
Die Dynamik von Subduktionszonen ist
gekennzeichnetdurch die komplexe Wechselwirkung
tektonischer, mineralogisch-petrologischer und
geophysikalischer Prozesse auf verschiedensten
Raum- und Zeitskalen. Diese hochgradig
nichtlinear miteinander verbundenenProzesse
haben einen entscheidenden Einfluss aufden
Lebensraum des Menschen (Vulkanismus, Erdbeben,
Tsunamis). Ihr quantitatives Verständnis
erfordert das Zusammenwirken von
mineralogisch-petrologischen Untersuchungen,
geophysikalischer Beobachtung und geodynamischer
Modellierung.