Effekt der
T-abhängigen
Viskosität von
Mantelgestein
stagnant lid
(Lithosphäre)
Plattentektonik
Ozeane
Kontinente
3 Typen von
Plattengrenzen
Vergleich zw. Kontinent - Ozean
Fowler 1990
Vergleich zw. Kontinent - Ozean
Auswirkungen auf die
Plattengrenzen
•  Konvergenz von ozeanischer Lithosphäre
führt zu Subduktion oder Obduktion
•  Konvergenz von kontinentaler Lithosphäre
führt zu Kollision
Begriff der „Tektosphäre“
Plate tectonics: Scaling view
L
W
δ
δ ∼ (κt)1/2
D
vplate
cooling thickness
ρ α ΔΤ
0
density after
expansion
FR - resistance force
δ
time t
FR
ΔT
ρ , α
η
κ
0
FB
- bouyancy force
Plate tectonics: scaling view (I)
„bouyancy force“
density
size
mass
„resistance force“
because of
stress
σ
and
area
*
gravity
acceleration
Plate tectonics: scaling view (II)
FB
FR
~ Ra 2/3
ρ = 3*103 kg/m3
α = 3*10-6 m2/s
0
ΔT = 1400 K η = 1022 Pa s g = 10 m/s2 L = 3*106 m κ = 10-6 m2/s density
thermal expansion
temperature difference
viscosity
grav. acceleration
layer thickness
thermal diffusivity
plate velocity ~ 14 cm/yr !
Triggered
mainshocks
Triggering
mainshocks
Earthquakes and subducted slabs beneath the Tonga-Fiji area
(yellow marker - 2002 series, orange marker - 1986 series)
deformation
time scales
subduction
zones
How was subduction “discovered”?
Wadati-Benioff zones: zones of dipping earthquakes to
"
"
"
100’s kms depth (max: ~670 km)
deep
intermediate
shallow
continent-continent collision
model for India and Asia collision
from: http://pubs.usgs.gov/publications/text
deformation from collision extends far into Tibet/Asia
plate tectonics: what is driving mechanism ?
must explain: sea floor spreading and subduction;
"
" heat flow, warm and elevated ridges;
"
" cold and deep trenches
mantle convection is likely candidate
but is mantle the cause or an effect
"of ridge push and slab pull?
plate tectonics: what causes plates to move ?
one idea…
ridge push: sea floor spreading and gravity
sliding of plate downhill from ridge to trench
while being pushed by sea floor spreading
plate tectonics: what causes plates to move ?
another idea…
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)
plate tectonics: what causes plates to move ?
slab sinking causes roll back and trench suction
slab pull is more important than ridge push
how do we know this?
plates that have the greatest length of subduction boundary
have the fastest velocities
C.P. Conrad and C. Lithgow-Bertelloni,
"How mantle slabs drive plate tectonics,"
Science, 298, 207-209, 2002
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).
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.
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).
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.
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.
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.
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).
Langsame und
schnelle Subduktion
Compare models for younger
and slower subducting slabs
(φ ~ 2500 km), approximating
Aleutian arc, and older faster
subducting slabs (φ ~ 17000
km), approximating Tonga arc:
Slabs with higher thermal
parameter warms up more slowly
and are thus colder.
Prediction consistent with
observation that Tonga has deep
earthquakes whereas Aleutians
do not.
Test thermal models using
earthquake locations (should be
in cold interior) & seismic
velocities from tomography.
Stein & Stein, 1996
Dämpfung
seismischer Wellen
Cold slabs transmit seismic
energy with less attenuation
than its surroundings
Seismograms from deep
earthquake at station NIU,
to which waves travel
through downgoing slab,
have more short period
energy than at VUN, to
which waves arrive through
surrounding mantle.
Short period energy more
absorbed on the path to
VUN than on the more rigid
slab path to NIU
Oliver and Isacks, 1967"
Berechnung der ”slab pull” Kraft
Thermal model gives force driving
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.
Depends on thermal density contrast
so force increases for faster
subducting velocity and thicker &
hence older resp. colder plate.
Expression similar to that for
”ridge push” since both are thermal
buoyancy forces.
Eine Erläuterung zur ”slab pull” Kraft:
The net effect of a subduction zone on the remainder of
the plate is not a ”pull”,
so term ”slab pull” is somewhat misleading.
Instead, as implied by slab stress models, ”slab pull” force
is balanced by local resistive forces, 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.
Stein & Stein, 1996
Beide Kräfte, ’’ridge push'’ und ’’slab pull”, haben die gleiche Ursache thermischer Auf- bzw. ’’Ab-’’trieb infolge der Temperaturdifferenz
zwischen Lithosphere und umgebenden Mantel
Ridge push is due to oceanic lithosphere cooling after it forms; slab pull is due
to the cooled lithosphere heating up again as it subducts.
Although it is useful to think of the forces separately, both are parts of the
net buoyancy force due to mantle convection.
Stein & Stein, 1996
Hinweise auf die ”slab pull” Kraft
(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"
Forsyth and "
Uyeda, 1975
Wiens & Stein, 1984"