VL Geodynamik & Tektonik, WS 0809 simple scaling view L W D FRFR FBFB v plate TT density after expansion t) 1/2 cooling thickness time t - bouyancy force F R - resistance force
VL Geodynamik & Tektonik, WS 0809 densitysize gravity massacceleration * „bouyancy force“ stress area „resistance force“ because of Plate tectonics: scaling view (I) F B = DW ) g F R = v/L DW ) and = = v L
VL Geodynamik & Tektonik, WS 0809 FBFB FRFR ~ Ra 2/3 Plate tectonics: scaling view (II)
VL Geodynamik & Tektonik, WS 0809 T = 1400 K temperature difference = 3 ·10 -6 m 2 /sthermal expansion = Pa sviscosity = m 2 /sthermal diffusivity = 3 ·10 3 kg/m 3 density g = 10 m/s 2 grav. acceleration L = 3 ·10 6 mlayer thickness plate velocity ~ 14 cm/yr !
VL Geodynamik & Tektonik, WS 0809 deformation time scales subduction zones
VL Geodynamik & Tektonik, WS 0809 various kinetic processes during subduction P. van Keken, 2004
VL Geodynamik & Tektonik, WS 0809 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)
VL Geodynamik & Tektonik, WS 0809 Magma Genesis
VL Geodynamik & Tektonik, WS 0809 Eruption of Mount St. Helens, May 18, 1980
VL Geodynamik & Tektonik, WS 0809 Mt. Saint Helens 1980 eruption USGS Loma Prieta 1989 earthquake
VL Geodynamik & Tektonik, WS 0809 Eruption of Mount Pinatubo, June 15, 1991
VL Geodynamik & Tektonik, WS 0809 Complex plate boundary zone in South-East Asia Northward motion of India deforms all of the region Many small plates (microplates) and blocks Molnar & Tapponier, 1977 Sumatra Earthquake, December 26, 2004 Eruption Mt. Pinatubo, 2001
VL Geodynamik & Tektonik, WS 0809 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 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 Stein & Wysession, 2003 HOW OFTEN ? Sumatra Earthquake, December 26, 2004
VL Geodynamik & Tektonik, WS 0809 Banda Aceh, Sumatra, before tsunami
VL Geodynamik & Tektonik, WS 0809 Banda Aceh, Sumatra, after tsunami
VL Geodynamik & Tektonik, WS 0809 Largest earthquakes, Chile S73.05 W 5.Off the West Coast of Northern Sumatra N95.78 E 2.Prince William Sound, Alaska N W 3.Andreanof Islands, Alaska N W 4.Kamchatka N E 6.Off the Coast of Ecuador N81.5 W 7.Rat Islands, Alaska N E 8.Assam - Tibet N96.5 E 9.Kamchatka N161.0 E 10.Banda Sea, Indonesia S E 11.Kuril Islands N149.6 E USGS
VL Geodynamik & Tektonik, WS 0809 USGS Largest earthquakes,
VL Geodynamik & Tektonik, WS 0809 (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 SZ
VL Geodynamik & Tektonik, WS 0809 Earthquakes and subducted slabs beneath the Tonga-Fiji area (yellow marker series, orange marker series) Triggering mainshocks Triggered mainshocks Deep Earthquakes
VL Geodynamik & Tektonik, WS 0809 Subduction understanding of subduction process completed formation of theory of plate tectonics provided mechanism for removing oceanic crust generated at mid-ocean ridges one plate descends below another, oceanic crust is consumed
VL Geodynamik & Tektonik, WS 0809 deep intermediate shallow how was subduction “discovered”? “Wadati-Benioff” zones: zones of dipping earthquakes to 100’s kms depth (max: ~670 km) seismicity
VL Geodynamik & Tektonik, WS 0809 Wadati-Benioff zone northern Japan hypocenters epicenters red dots are deepest earthquakes so they plot on map as farthest from trench plate tectonics: convergent boundaries
VL Geodynamik & Tektonik, WS 0809 variations in dips of Wadati-Benioff zones plate tectonics: convergent boundaries
VL Geodynamik & Tektonik, WS 0809 “imaging” the subducting plate with seismic velocities - subducting plate is cooler than surrounding mantle - fast: cooler (denser material) slow: hotter (less dense material) slow fast plate tectonics: convergent boundaries
VL Geodynamik & Tektonik, WS 0809 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) plate tectonics: convergent boundaries
VL Geodynamik & Tektonik, WS 0809 one oceanic plate subducts below another trench, accretionary wedge, forearc basin, volcanic arc earthquakes occur along interface between two plates (1) ocean-ocean convergence
VL Geodynamik & Tektonik, WS 0809 (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
VL Geodynamik & Tektonik, WS 0809 (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
VL Geodynamik & Tektonik, WS 0809 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
VL Geodynamik & Tektonik, WS 0809 (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
VL Geodynamik & Tektonik, WS 0809 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
VL Geodynamik & Tektonik, WS 0809 overriding plate pushes trench subducting plate steepens and pulls overriding plate toward trench trench can migrate through time response to forcing either by overriding or subducting plate
VL Geodynamik & Tektonik, WS 0809 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
VL Geodynamik & Tektonik, WS 0809 model for India and Asia collision (3) continent-continent convergence
VL Geodynamik & Tektonik, WS 0809 are part of a long mountain belt that extends to Alps Himalayas INDIAN PLATE EURASIAN PLATE AFRICAN PLATE (3) continent-continent convergence
VL Geodynamik & Tektonik, WS 0809 deformation from collision extends far into Tibet/Asia (3) continent-continent convergence
VL Geodynamik & Tektonik, WS 0809 ridge push: sea floor spreading and gravity what causes plates to move ? sliding of plate downhill from ridge to trench while being pushed by sea floor spreading
VL Geodynamik & Tektonik, WS 0809 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)
VL Geodynamik & Tektonik, WS 0809 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 what causes plates to move ?
VL Geodynamik & Tektonik, WS 0809 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 what causes plates to move ? Forsyth & Uyeda, 1975
VL Geodynamik & Tektonik, WS 0809 mantle convection is the likely candidate, but is it the cause or an effect of ridge push and slab pull ? what causes plates to move ?
VL Geodynamik & Tektonik, WS 0809 How Mantle Slabs Drive Plate Motions C.P. Conrad and C. Lithgow-Bertelloni "How mantle slabs drive plate tectonics" Science, 298, , 2002
VL Geodynamik & Tektonik, WS 0809 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).
VL Geodynamik & Tektonik, WS 0809 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.
VL Geodynamik & Tektonik, WS 0809 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).
VL Geodynamik & Tektonik, WS 0809 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.
VL Geodynamik & Tektonik, WS 0809 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.
VL Geodynamik & Tektonik, WS 0809 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.
VL Geodynamik & Tektonik, WS 0809 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).
VL Geodynamik & Tektonik, WS 0809 Thermal-mechanical structure of subduction zones a more detailed quantitative understanding of subduction zones
VL Geodynamik & Tektonik, WS 0809 Bodine et al., JGR 86 (1981) 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. Wadati & Benioff zones
VL Geodynamik & Tektonik, WS 0809 Thermal modeling predicts maximum depth of isotherms in slab varies with thermal parameter ” ” Simple thermal slab model (McKenzie, 1969)
VL Geodynamik & Tektonik, WS 0809 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 in slab varies with thermal parameter Kirby et al., 1996
VL Geodynamik & Tektonik, WS 0809 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
VL Geodynamik & Tektonik, WS 0809 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
VL Geodynamik & Tektonik, WS 0809 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 mantle Stein & Stein, 1996
VL Geodynamik & Tektonik, WS 0809 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 force
VL Geodynamik & Tektonik, WS 0809 “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.
VL Geodynamik & Tektonik, WS 0809 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)
VL Geodynamik & Tektonik, WS 0809 Forsyth and Uyeda, 1975, Wiens & Stein, 1984 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 0809 Stein & 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)
VL Geodynamik & Tektonik, WS 0809 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
VL Geodynamik & Tektonik, WS 0809 Opposite 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
VL Geodynamik & Tektonik, WS 0809 Kirby et al., Rev. Geophys Metastable delay of mineral phase transformations
VL Geodynamik & Tektonik, WS 0809 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 transformations
VL Geodynamik & Tektonik, WS 0809 Deep earthquakes from metastable olivine ? Kirby et al., Rev. Geophys. 1996
VL Geodynamik & Tektonik, WS 0809 Vassiliou & Hager, Pageoph 128 (1988) 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. Deep earthquakes due to large viscosity contrast between transition zone and lower mantle ?
VL Geodynamik & Tektonik, WS 0809 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. Kirby et al., Rev. Geophys. 1996
VL Geodynamik & Tektonik, WS 0809 Intermediate depth earthquakes (II) 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. 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. Kirby et al., Rev. Geophys. 1996
VL Geodynamik & Tektonik, WS 0809 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 of subducted slabs in the mantle transition zone
VL Geodynamik & Tektonik, WS 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.