Transcript Die Erde in Bewegung

Dynamik von
Subduktionszonen
Institut für Geowissenschaften
Universität Potsdam
14.01.2009
VL Geodynamik & Tektonik, WS 0809
Übersicht zur Vorlesung
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Subduktionszonen
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simple scaling view
L
W
d
d ~ (kt)
D
1/2
vplate
cooling thickness
r a DT
0
density after
expansion
FR - resistance force
d
time t
FR
DT
r ,a
h
k
0
FB - bouyancy force
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Plate tectonics: scaling view (I)
FB = ra DT/2 (d DW) g
„bouyancy force“
density
size
mass
*
gravity
acceleration
FR = (h v/L) (DW )
„resistance force“
s
because of
s = h  e
stress
and
e = v / L
area
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Plate tectonics: scaling view (II)
FB
FR
~ Ra
2/3
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r = 3 ·103 kg/m3
a = 3 ·10-6 m2/s
0
DT = 1400 K
h = 1022 Pa s
g = 10 m/s2
L = 3 ·106 m
k = 10-6 m2/s
density
thermal expansion
temperature difference
viscosity
grav. acceleration
layer thickness
thermal diffusivity
plate velocity ~ 14 cm/yr !
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deformation
time scales
subduction
zones
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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 -> eclogite transformation occur at equilibrium condition?
Do dehydration reactions cause earthquakes?
– could dehydration reactions at high-P ( V<0) 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
http://en.wikipedia.org/wiki/1980_eruption_of_Mount_St._Helens
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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
Eruption
Mt. Pinatubo, 2001
Sumatra Earthquake,
December 26, 2004
Molnar & Tapponier, 1977
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Plate tectonics - potential hazards (II)
Tsunami waves
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December
26, 2004
subduction
thrust fault
earthquake
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INTERSEISMIC:
India subducts beneath
Burma microplate
at 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
Stein & Wysession, 2003
Fault slipped ~ 10 m = 10000 mm
~ takes 10000 mm / 50 mm/yr = 200 yr
Longer if some slip is aseismic
Faults aren’t exactly periodic
for reasons we don’t understand
HOW OFTEN ?
Sumatra Earthquake,
December 26, 2004
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Banda Aceh, Sumatra, before tsunami
http://geo-world.org/tsunami
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Banda Aceh, Sumatra, after tsunami
http://geo-world.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
Triggered
mainshocks
Triggering
mainshocks
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
100’s kms depth (max: ~670 km)
deep
intermediate
shallow
seismicity
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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)
fast
slow
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plate tectonics: convergent boundaries
less buoyant plate dives below more buoyant plate
oceanic lithosphere density > 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 doesn’t 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
overriding
plate
pushes
trench
subducting
plate
steepens
and pulls
overriding
plate
toward
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
• “detached” subducting plate
• suture zone - plate boundary
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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
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subducting plates toward subduction zones.
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
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(compare to observed plate motions, shown in Fig. 1).
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 f
Earthquakes below 300
km occur only for slabs
with f > 5000 km
Kirby et al., 1996
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Transition zone between
upper & lower mantles
bounded by 410 km and
660 km discontinuities
corresponding to
mineral phase changes
deep earthquakes stop at
660 km, perhaps because:
- slabs equilibrate
thermally
- slabs cannot penetrate
660 km
Ringwood,
1979
- earthquakes are related
to phase changes
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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
< 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
Stein & Stein, 1996
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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 !
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Stein & Wysession, Blackwell 2003
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
Because spinel is denser than olivine, DV < 0.
This reaction is exothermic (gives off heat) so DH <
0 is also negative, causing a positive Clapeyron
slope. The slab is colder than the ambient mantle
(DT<0 ), so this phase change occurs at a lower
pressure (DP<0), corresponding to shallower depth
In contrast, the ringwoodite ( spinel phase) to
perovoskite plus magnesiowustite transition,
thought to give rise to the 660 km discontinuity, is
endothermic (absorbs heat) so DH > 0. Because
this is a transformation to denser phases (DV < 0),
Clapeyron slope is negative, and the 660 km
discontinuity should be deeper in slabs than outside
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 300410 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 eclogiteforming 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 gekennzeichnet
durch die komplexe Wechselwirkung tektonischer,
mineralogisch-petrologischer und geophysikalischer
Prozesse auf verschiedensten Raum- und Zeitskalen.
Diese hochgradig nichtlinear miteinander verbundenen
Prozesse haben einen entscheidenden Einfluss auf
den Lebensraum des Menschen (Vulkanismus,
Erdbeben, Tsunamis). Ihr quantitatives Verständnis
erfordert das Zusammenwirken von mineralogischpetrologischen Untersuchungen, geophysikalischer
Beobachtung und geodynamischer Modellierung.
14.01.2009
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