Auswirkungen auf die Plattengrenzen

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Auswirkungen auf die Plattengrenzen

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

W

L

D

δ

δ ∼ (κt) 1/2

cooling thickness

ρ 0 α ΔΤ

density after

expansion

v plate

δ

time t

F R

ΔT

ρ 0 , α

η

κ

F R - resistance force

F B

- bouyancy force


Plate tectonics: scaling view (I)

„bouyancy force“

density size gravity

mass

*

acceleration

„resistance force“

because of

and

stress

σ

area


Plate tectonics: scaling view (II)

F B

F R

~ Ra 2/3


ρ 0 = 310 3 kg/m 3 density

α = 310 6 m 2 /s thermal expansion

ΔT = 1400 K

temperature difference

η = 10 22 Pa s

viscosity

g = 10 m/s 2 grav. acceleration

L = 3*10 6 m

layer thickness

κ = 10 -6 m 2 /s

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"

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