VL Geodynamik & Tektonik, WS 080914.01.2009 Dynamik von Subduktionszonen Institut für...

VL Geodynamik & Tektonik, WS 080914.01.2009

Dynamik vonDynamik vonSubduktionszonenSubduktionszonen

Institut für Geowissenschaften Universität Potsdam

VL Geodynamik & Tektonik, WS 0809

Übersicht zur Vorlesung


Subduktions-zonen


simple scaling viewL

W

D

FR

FB

vplate T

density after expansion

t)1/2

cooling thickness time t

- bouyancy forceFR - resistance force


density size gravity

mass acceleration*

„bouyancy force“

stress

area„resistance force“

because of

Plate tectonics: scaling view (I)

FB = DW ) g

FR = v/LDW )

and = = vL


FB FR

~ Ra 2/3

Plate tectonics: scaling view (II)


T = 1400 K temperature difference

= 3 ·10-6 m2/s thermal expansion

= 1022 Pa s viscosity

= 10-6 m2/s thermal diffusivity

= 3 ·103 kg/m3 density

g = 10 m/s2 grav. acceleration

L = 3 ·106 m layer thickness

plate velocity ~ 14 cm/yr !


deformationtime scales

subductionzones


various kineticprocesses during

subduction

P. van Keken, 2004


What do we want to understand ..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


VolcanismVolcanism

Plate tectonics - potential hazards (I)Plate tectonics - potential hazards (I)


Magma GenesisMagma Genesis


Eruption of Mount St. Helens, Eruption of Mount St. Helens,

May 18, 1980May 18, 1980

http://en.wikipedia.org/wiki/1980_eruption_of_Mount_St._Helens


Mt. Saint Helens1980 eruption

USGS

Loma Prieta1989 earthquake


Eruption of Mount Eruption of Mount Pinatubo, Pinatubo,

June 15, 1991June 15, 1991


Complex plate Complex plate boundary zone boundary zone in South-East in South-East

AsiaAsia

Northward motion of India deforms all of the

region

Many small plates (microplates) and

blocks

Molnar & Tapponier, 1977

Sumatra Earthquake, Sumatra Earthquake, December 26, 2004December 26, 2004

Eruption Eruption Mt. Pinatubo, 2001Mt. Pinatubo, 2001


Tsunami wavesTsunami waves

Plate tectonics - potential hazards (II)Plate tectonics - potential hazards (II)


December 26, 2004

subductionthrust fault earthquake


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 yrLonger if some slip is aseismicFaults aren’t exactly periodic for reasons we don’t understand

Stein & Wysession, 2003

HOW OFTEN ?

Sumatra Earthquake, Sumatra Earthquake, December 26, 2004December 26, 2004


Banda Aceh, Sumatra, before tsunamihttp://geo-world.org/tsunami


Banda Aceh, Sumatra, after tsunamihttp://geo-world.org/tsunami


Large Large EarthquakesEarthquakes

Plate tectonics - potential hazards (III)Plate tectonics - potential hazards (III)


Largest earthquakes, 1900 - 2004Largest 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

VL Geodynamik & Tektonik, WS 0809USGS

Largest earthquakes, 1900 - 2004Largest earthquakes, 1900 - 2004


(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

3 components of earthquake hazard at SZ3 components of earthquake hazard at SZ

VL Geodynamik & Tektonik, WS 0809Earthquakes and subducted slabs beneath the Tonga-Fiji area(yellow marker - 2002 series, orange marker - 1986 series)

Triggeringmainshocks

Triggeredmainshocks

Deep EarthquakesDeep Earthquakes


SubductionSubduction

understanding of subduction process completedformation of theory of plate tectonics

provided mechanism for removing oceanic crustgenerated at mid-ocean ridges

one plate descends below another,oceanic crust is consumed


deepintermediateshallow

how was subduction “discovered”?how was subduction “discovered”?

“Wadati-Benioff” zones: zones of dipping earthquakes to 100’s kms depth (max: ~670 km)

seismicity


Wadati-Benioff zone

northern Japan

hypocentersepicenters

red dots are deepest earthquakes so they plot on map as farthest from trench

plate tectonics: convergent boundaries


variations in dips of Wadati-Benioff zones



“imaging” the subducting plate with seismic velocities- subducting plate is cooler than surrounding mantle -

fast: cooler

(denser material)

slow: hotter

(less dense material)

slowfast



oceanic lithosphere density > continental lithosphere

less buoyant plate dives below more buoyant plate

3 types of convergence

• ocean-continent convergence

• ocean-ocean convergence

• continent-continent convergence (collision)



• one oceanic plate subducts below another

• trench, accretionary wedge, forearc basin, volcanic arc

• earthquakes occur along interface between two plates

(1) ocean-ocean convergence


(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


(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

VL Geodynamik & Tektonik, WS 0809http://www.pmel.noaa.gov/vents/coax/coax.html

Example:well-developed

trenches in Indonesia/Phillippines

(1) ocean-ocean 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

(2) ocean-continent convergence


(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


arc-trench gapdistance between thetrench and volcanoes

because the depth at whichmagmas are generated

in subduction zonesis about 100-150 km,this distance depends

on the dip of the subducting plate

if the dip of the subducting plateis flat enough, no volcanoes form

subducted plate doesn’t go deep…

infer dip by looking at distancebetween volcanoes and trench


overriding plate

pushestrench

subducting plate

steepensand pulls

overriding plate

toward trench

trench can migrate through timeresponse to forcing either by overriding or subducting plate


neither plate wants to subduct(both are buoyant)

result is continental collision

• mountain belts

• thrust faults

• suture zone - plate boundary

• “detached” subducting plate

(3) continent-continent convergence


model for India and Asia collision



are part ofa long

mountain beltthat extends

to Alps

Himalayas

INDIAN PLATE

EURASIAN PLATE

AFRICAN PLATE



deformation from collision extends far into Tibet/Asia



ridge push: sea floor spreading and gravity

what causes plates to move ?

sliding of plate downhill from ridge to trenchwhile being pushed by sea floor spreading



slab pull: weight of subducting slab

subducting slab sinks into mantlefrom its own weight, pulling therest of the plate with it

as subducting slab descendsinto mantle, the higherpressures cause minerals totransform to denser forms(crystal structures compact)


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



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


Forsyth & Uyeda, 1975


mantle convection is the likely candidate,but is it the cause or an effectof ridge push and slab pull ?



How Mantle Slabs Drive Plate MotionsHow Mantle Slabs Drive Plate Motions

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 theaverage 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).


How Mantle Slabs Drive Plate MotionsHow Mantle Slabs Drive Plate Motions

bending forcesbending forces


Diagram showing the mantle flow associated with the "slab suction" plate-driving mechanism in which the sinking slab is detached from the subductingPlate 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).


Thermal-mechanical structureThermal-mechanical structureof subduction zonesof subduction zones

a more detailed quantitative understanding of subduction zones

VL Geodynamik & Tektonik, WS 0809Bodine et al., JGR 86 (1981) 3695-3707

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 surfacedividing the mechanically stronglithosphere into upper extensional and lower compressional zones.

Wadati & Benioff zonesWadati & Benioff zones


Simple thermal slab model (McKenzie, 1969)Simple thermal slab model (McKenzie, 1969)


Thermal modeling predicts maximum depth of isotherms in slab varies with thermal parameter

””

Simple thermal slab model (McKenzie, 1969)Simple thermal slab model (McKenzie, 1969)


Deepest earthquakes never exceed ~700 km

Maximum depth increases with

Earthquakes below 300 km occur only for slabs

with > 5000 km

Thermal modeling predicts maximum depth of isotherms Thermal modeling predicts maximum depth of isotherms in slab varies with thermal parameterin slab varies with thermal parameter

Kirby et al., 1996


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

- earthquakes are related to phase changes

Ringwood, 1979


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


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

Slabs are not thermally equilibrated with mantleSlabs are not thermally equilibrated with mantle

Stein & Stein, 1996


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

““SLAB PULL” plate driving forceSLAB PULL” plate driving force


““SLAB PULL” plate driving forceSLAB 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.


Forsyth and Uyeda, 1975

(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

Forces within subducting plates (I)Forces within subducting plates (I)


Forsyth and Uyeda, 1975, Wiens & Stein, 1984

Forces within subducting plates (II)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.

VL Geodynamik & Tektonik, WS 0809Stein & Wysession, Blackwell 2003

Different stresses result if weight of column of material supported in different ways

similar to what seismic focal mechanisms show !

Forces within subducting plates (III)Forces within subducting plates (III)


Clapeyron slope describes how mineral phase Clapeyron slope describes how mineral phase changes occur at different depths in cold slabschanges occur at different depths in cold slabs

use thermal model to find dT, phase

relations to find and thus dP

convert to convert to depth change depth change

dzdz


Opposite deflection of mineral phase boundariesOpposite 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.

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 H > 0. Because

this is a transformation to denser phases (V < 0), Clapeyron slope is negative, and the 660 km

discontinuity should be deeper in slabs than outside

Because spinel is denser than olivine, V < 0. This reaction is exothermic (gives off heat) so H <

0 is also negative, causing a positive Clapeyron slope. The slab is colder than the ambient mantle (T<0 ), so this phase change occurs at a lower

pressure (P<0), corresponding to shallower depth


Kirby et al., Rev. Geophys. 1996

Metastable delay of mineral phase transformationsMetastable 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.

Metastable delay of mineral phase transformationsMetastable delay of mineral phase transformations


Deep earthquakes from metastable olivine ?Deep earthquakes from metastable olivine ?


VL Geodynamik & Tektonik, WS 0809Vassiliou & Hager, Pageoph 128 (1988) 547-624

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.

Deep earthquakes due to large viscosity contrast Deep earthquakes due to large viscosity contrast between transition zone and lower mantle ?between transition zone and lower mantle ?


Intermediate depth earthquakes (I)Intermediate depth earthquakes (I)

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.

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.



Intermediate depth earthquakes (II)Intermediate depth earthquakes (II)

Support for this model comes from the fact that the intermediateearthquakes 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.

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.



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

Complex thermal structure, mineralogy & geometry Complex thermal structure, mineralogy & geometry of subducted slabs in the mantle transition zoneof subducted slabs in the mantle transition zone

VL Geodynamik & Tektonik, WS 080914.01.2009

ZusammenfassungZusammenfassung

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 auf

den Lebensraum des Menschen (Vulkanismus,Erdbeben, Tsunamis). Ihr quantitatives Verständnis erfordert das Zusammenwirken von mineralogisch-petrologischen Untersuchungen, geophysikalischer

Beobachtung und geodynamischer Modellierung.

VL Geodynamik & Tektonik, WS 080914.01.2009 Dynamik von Subduktionszonen Institut für...

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