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Dynamik von Subduktionszonen

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1 Dynamik von Subduktionszonen
Institut für Geowissenschaften Universität Potsdam

2 Übersicht zur Vorlesung

3 Subduktions- zonen

4 simple scaling view L D W d d FR vplate DT d ~ (kt)1/2 r0 , a time t h
cooling thickness r0 , a time t D h r0 a DT density after expansion k FR - resistance force FB - bouyancy force

5 FB = ra DT/2 (d DW ) g FR = ( v/L) (DW )
Plate tectonics: scaling view (I) FB = ra DT/2 (d DW ) g „bouyancy force“ density size gravity mass acceleration * FR = ( v/L) (DW ) „resistance force“ stress s area because of and s =   e = v / L

6 Plate tectonics: scaling view (II)
FB FR ~ Ra 2/3

7 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 plate velocity ~ 14 cm/yr !

8 subduction zones deformation time scales

9 various kinetic processes during subduction P. van Keken, 2004

10 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)

11 Plate tectonics - potential hazards (I)

12 Magma Genesis

13 Eruption of Mount St. Helens, May 18, 1980

14 Mt. Saint Helens 1980 eruption USGS Loma Prieta 1989 earthquake

15 Eruption of Mount Pinatubo, June 15, 1991

16 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

17 Plate tectonics - potential hazards (II)
Tsunami waves

18 subductionthrust fault earthquake
December 26, 2004 subductionthrust fault earthquake

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 = mm ~ takes mm / 50 mm/yr = 200 yr Longer if some slip is aseismic Faults aren’t exactly periodic for reasons we don’t understand Sumatra Earthquake, December 26, 2004 HOW OFTEN ?

20 Banda Aceh, Sumatra, before tsunami

21 Banda Aceh, Sumatra, after tsunami

22 Plate tectonics - potential hazards (III)
Large Earthquakes

23 Largest earthquakes, 1900 - 2004 USGS 1. Chile 1960 05 22 9.5 38.24 S
73.05 W 5. Off the West Coast of Northern Sumatra 9.3 3.30 N 95.78 E 2. Prince William Sound, Alaska 9.2 61.02 N W 3. Andreanof Islands, Alaska 9.1 51.56 N W 4. Kamchatka 9.0 52.76 N E 6. Off the Coast of Ecuador 8.8 1.0 N 81.5 W 7. Rat Islands, Alaska 8.7 51.21 N E 8. Assam - Tibet 8.6 28.5 N 96.5 E 9. 8.5 54.0 N 161.0 E 10. Banda Sea, Indonesia 5.05 S E 11. Kuril Islands 44.9 N 149.6 E USGS

24 Largest earthquakes, USGS

25 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

26 Deep Earthquakes Triggered mainshocks Triggering mainshocks
Earthquakes and subducted slabs beneath the Tonga-Fiji area (yellow marker series, orange marker series)

27 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

28 how was subduction “discovered”?
“Wadati-Benioff” zones: zones of dipping earthquakes to 100’s kms depth (max: ~670 km) deep shallow intermediate seismicity

29 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

30 variations in dips of Wadati-Benioff zones
plate tectonics: convergent boundaries variations in dips of Wadati-Benioff zones

31 plate tectonics: convergent boundaries
“imaging” the subducting plate with seismic velocities - subducting plate is cooler than surrounding mantle - slow fast fast: cooler (denser material) slow: hotter (less dense material)

32 3 types of convergence 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)

33 (1) ocean-ocean convergence
• one oceanic plate subducts below another • earthquakes occur along interface between two plates • trench, accretionary wedge, forearc basin, volcanic arc

34 (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

35 (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 km, creating magma that rises to the surface to form island volcanoes

36 (1) ocean-ocean convergence
Example: well-developed trenches in Indonesia/ Phillippines

37 (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

38 (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

39 arc-trench gap distance between the trench and volcanoes
because the depth at which magmas are generated in subduction zones is about 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

40 trench can migrate through time
response to forcing either by overriding or subducting plate subducting plate steepens and pulls overriding toward trench overriding plate pushes trench

41 (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

42 (3) continent-continent convergence
model for India and Asia collision

43 (3) continent-continent convergence
EURASIAN PLATE Himalayas are part of a long mountain belt that extends to Alps INDIAN PLATE AFRICAN PLATE

44 (3) continent-continent convergence
deformation from collision extends far into Tibet/Asia

45 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

46 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)

47 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

48 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

49 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 ?

50 How Mantle Slabs Drive Plate Motions
C.P. Conrad and C. Lithgow-Bertelloni "How mantle slabs drive plate tectonics" Science, 298, , 2002

51 Observed plate motions
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).

52 How Mantle Slabs Drive Plate Motions
bending forces

53 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.

54 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).

55 The "slab pull" plate-driving mechanism
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.

56 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.

57 Preferred model for how mantle slabs drive plate motions
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.

58 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).

59 Thermal-mechanical structure
a more detailed quantitative understanding of subduction zones Thermal-mechanical structure of subduction zones

60 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 km. These constrain the neutral surface dividing the mechanically strong lithosphere into upper extensional and lower compressional zones. Bodine et al., JGR 86 (1981)

61 Simple thermal slab model (McKenzie, 1969)

62 Simple thermal slab model (McKenzie, 1969)
Thermal modeling predicts maximum depth of isotherms in slab varies with thermal parameter ” f ”

63 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  > 5000 km Kirby et al., 1996

64 deep earthquakes stop at 660 km, perhaps because:
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

65 Seismicity decreases to minimum ~300 km, and then increases again
Deep earthquakes below ~ 300 km treated as distinct from intermediate earthquakes with depths km Deep earthquakes peak at about 600 km, and then decline to an apparent limit at ~ km

66 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

67 “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

68 “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.

69 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

70 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

71 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

72 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

73 Opposite deflection of mineral phase boundaries
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 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 Upward deflection of the 410 km and downward deflection of the 660 km discontinuities have been observed in travel time studies.

74 Metastable delay of mineral phase transformations
Kirby et al., Rev. Geophys. 1996

75 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.

76 Deep earthquakes from metastable olivine ?
Kirby et al., Rev. Geophys. 1996

77 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 km and increase from km. Vassiliou & Hager, Pageoph 128 (1988)

78 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

79 Support for this model comes from the fact that the intermediate
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

80 phases are no longer thermodynamically stable.
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

81 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 mineralogisch- petrologischen Untersuchungen, geophysikalischer Beobachtung und geodynamischer Modellierung.

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