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Subduction of oceanic lithosphere • • • • • Evolution of the continental crust New crust is formed in marginal basins, island arcs, and oceanic plateaux New crust is accreted to the continents along the active continental margins. Crust is also added by magmatic underplating Continental crust is deformed in collision zones. It is eroded and recycled in the mantle in subduction zones. Some crust can be subducted in collision zones. What is the budget? Wednesday, April 14, 2010 • • • The age of continental crust appears younger at the margins than at the center of the continent. Suggestion by Wilson that continents grew from a central core is now superseded. However, new continental crust has been and is presently being added at the active margins, like the North American Cordillera. Wednesday, April 14, 2010 Subduction • • • Ocean-Ocean subduction (island arcs, marginal basins) Western pacific Ocean-Continent subduction. Eastern Pacific. The Cordilleras of N and S America. Continental collision. The Alps. Himalayas. Wednesday, April 14, 2010 • • • Formation of future continental crust Accretion of new crust by docking of terranes and magmatic underplating Recycling and Destruction of continental crust. Location of convergent plate margins on Earth. Active back arc basins are also shown. Wednesday, April 14, 2010 Subduction of the oceanic crust Wednesday, April 14, 2010 • • • The western Pacific contains most of Earth marginal basins and oceanic island arcs. This is where new continental crust is being formed It will eventually be docked on the active margins of the continents Wednesday, April 14, 2010 W Pacific. Topography Bathymetry Wednesday, April 14, 2010 W Pacific Free Air Gravity (from satellite data) Wednesday, April 14, 2010 • Tectonic setting of Southeast Asia. Thick lines with closed triangles are modern active trenches and those with open triangles are inactive. Thick arrows show the direction of plate movement in a fixed hotspot reference frame after Engebretson et al. (1985) and Royer and Sandwell (1989). Length of bar is motion for 10 Ma. Spreading centers are shown in double lines. Thin lines are active structural boundaries and thin broken lines are traces of offshore or buried structural boundaries. Abbreviation: T: Trench or Trough, JB: Japan Basin, NA: Nankai Trough, OT: Okinawa Trough, DRB: Daito ridges and basins, SB: Shikoku Basin, OD: Ogasawara Depression, MA: Mariana Trough, PB: Parece Vela Basin, WPB: West Philippine Basin, SCB: South China Basin, AB: Andaman Basin, PA: Palawan Trough, NE: Negros Trench, SU: Sulu Basin, CO: Cotobato Trench, CB: Celebes Basin, NS: North Sulawesi Trench, MB: Makassar Basin, BB: Banda Basin, MO: Molucca Collision, SO: Sorong Fault, CE: Ceram Trough, CAB: Caroline Basin, NG: New Guinea Trench, WE: West Melanesia Trench, NB: New Britain Trench and TR: Trobriand Trench, RF: Red River Fault, SF: Seribu Fault. Wednesday, April 14, 2010 Australia: Topography Bathymetry Wednesday, April 14, 2010 Australia: Free Air Gravity. Wednesday, April 14, 2010 Scotia Arc: Topography Bathymetry Wednesday, April 14, 2010 Scotia Arc: Free Air Gravity Wednesday, April 14, 2010 South America Topography and bathymetry Wednesday, April 14, 2010 Free Air gravity in the oceans around South America Wednesday, April 14, 2010 NE Pacific Topography Bathymetry Wednesday, April 14, 2010 NE Pacific Free Air Gravity Wednesday, April 14, 2010 Free Air gravity from GRACE (29/10/2004). Paired gravity anomalies Wednesday, April 14, 2010 Sea floor age Note subduction occurs at all ages. Wednesday, April 14, 2010 Distribution of ages of the sea floor The area of Sea floor of given age decreases with age. This indicates that sea floor is destroyed at constant rate regardless of age. Wednesday, April 14, 2010 Oceanic subduction zones • • Wednesday, April 14, 2010 (top) Schematic section through the upper 140 km of a subduction zone, showing the principal crustal and upper mantle components and their interactions. Note that the location of the “mantle wedge” (unlabeled) is that part of the mantle beneath the overriding plate and between the trench and the most distal part of the arc where subduction-related igneous or fluid activity is found. MF stands for magmatic front. (bottom) Schematic section through the center of the Earth, which shows better the scale of subduction zones. Subducted lithosphere is shown both penetrating the 660 km discontinuity (right) and stagnating above the discontinuity (left). A mantle plume is shown ascending from the site of an ancient subducted slab. Dashed box shows the approximate dimensions of the shallow subduction zone of Figure 1b. • Structure of the western Pacific subduction zones with a back-arc basin. Wednesday, April 14, 2010 Semilog scale plotting the depth dimension for subduction zones from 1 km below the surface to the core-mantle boundary. Mean depth of ocean (3880 m) is from Kennett [1982]. Typical crustal thickness (∼20 km) for juvenile arc crust is from Suyehiro et al. [1996]; crust associated with Andean-type convergent margins may be up to 70 km thick. Mean distance of arc volcanoes to trench (166 ± 60 km) and of depth beneath arc volcano to seismic zone (124 ± 38 km) are from Gill [1981]. Greatest depth of subducted material returned to surface (∼100 km). Wednesday, April 14, 2010 Seismic structure, focal mechanisms • • • • • Wadati Benioff zone Local seismic tomography Stresses in the subducted plate Stresses in the overriding plate Seismic tomography and the fate of the subducted slab. Wednesday, April 14, 2010 • Plate flexure causes a bulge (outer rise) up from the trench. Wednesday, April 14, 2010 • • The dipping seismogenic zone (Wadati-Benioff zone) delineates the subducting slab. Earthquakes show that the slab is cold and brittle. Wednesday, April 14, 2010 • • • • Quality factor Q measures how well energy is transmitted. Attenuation of seismic waves (loss of energy) is due to “friction”, i.e. viscous mantle. Comparing seismic signals that travel through the slab or below the volcanic arc show that waves that travel in the asthenosphere are more attenuated than waves that travel through the slab. The attenuation affects mostly the high frequencies that are attenuated Wednesday, April 14, 2010 • The Q factor (quality) measures the loss of energy of the seismic waves. Quality is high if there is no loss of energy. The attenuation of seismic waves usually is caused by partial melting. Wednesday, April 14, 2010 Velocity anomalies related to temperature • The subducting plate is seismic because it remains cold and brittle. The higher velocities are also due to the plate being cold. Wednesday, April 14, 2010 • Tomographic images show that the 650km discontinuity is a temporary barrier for some plates that can not penetrate into the lower mantle, but most eventually do. Wednesday, April 14, 2010 • • The dip of the slab is variable. Stresses seem mostly compressive in the shallow dipping slabs otherwise tensile. Wednesday, April 14, 2010 • Geophysical images of subduction zones. (a) P wave tomographic image of NE Japan (modified after Zhao et al. [1994]). There is no vertical exaggeration. (b) P wave tomographic image of Tonga subduction zone (modified after Zhao et al. [1997]). There is no vertical exaggeration. For both Figures 6a and 6b, red and blue colors denote regions where the P wave velocities are relatively slow and fast, respectively, compared to average mantle at the same depth. (c) P wave velocity structure across the Cascadia Subduction Zone (modified after Parsons et al. [1998]). Yellow dots show earthquakes during 1970–1996 between 45° and 47° latitude, >M1 at depths >25 km and >M4 at shallower depths. Note vertical exaggeration is 2x. Note that the subduction zones in Figure 6a and 6b subduct old, cold lithosphere, which is relatively easy to identify tomographically, whereas the subduction zone in Figure 6c subducts young lithosphere, which differs less in velocity relative to the surrounding mantle and is more difficult to image tomographically. See color version of Wednesday, April 14, 2010 Mantle structure beneath some subduction zones. Wednesday, April 14, 2010 Contrasting tectonic environments in subduction zones. Extension or compression in the overiding plate Tectonic variability found behind magmatic arcs, using the Bolivian Andes and the Lau Basin as examples. Dark shading denotes lithosphere; underlying white is asthenosphere. (a) Tectonic setting of Lau Basin (modified after Zhao et al. [1997]), emphasizing lithospheric structures. Dashed box shows region of detail shown in Figure 20b. (b) Tectonic setting of Andes between 20°S and 24°S (modified after Yuan et al. [2000]) emphasizing crustal structure. Area of detail shown in Figure 20d lies partly within the openended dashed rectangle. (c) Cross section across the Lau Basin, emphasizing lithospheric structure, at the latitude investigated by Ocean Drilling Progdram Leg (∼20°S) (modified after Hawkins [1994]). Extension rate is from GPS measurements of Bevis et al. [1995] for velocities of Tonga from Australia; note that the highest rate is at the northern end of Lau Basin (∼17°S), and the rate decreases to the south. (d) Cross section across Andean back arc region, emphasizing crustal structure, about 20°S (modified after Gubbels et al. [1993]). Shortening rate is from GPS measurements of Bevis et al. [2001]. Figures 20c and 20d have the same horizontal scale. There is no vertical exaggeration on any of the sections. Wednesday, April 14, 2010 • Compression in the eastern Pacific. • • • Wednesday, April 14, 2010 Focal mechanisms in the Peruvian Andes. Stress in the overriding plate is compressive in the Peruvian Andes where subduction is shallow. It suggests that there is crustal shortening in the overriding plate in the Andes. • • • Deformation in the Peruvian Andes. GPS measurement Geological rates of deformation Wednesday, April 14, 2010 • Extension in the western Pacific • • Focal mechanisms show tensile stress in the overriding plate in the Tonga Kermadec region. Stress consistent with back arc extension and marginal basin formation Wednesday, April 14, 2010 Stress regime determines tectonic style in the overriding plate • • End-member types of subduction zones, based on the buoyancy of lithosphere being subducted (modified after Uyeda and Kanamori [1979]). Usually young lithosphere is buoyant, but old lithosphere could be also be light if it includes oceanic plateaus or fossil ridges (e.g. Ontong-Java in the western Pacific, the northern Andes). Wednesday, April 14, 2010 Back arc spreading • Several mechanisms have been proposed • Convection entrained by subducting plate and enhanced by melting • Slab rollback (Scotia plate, and others) Wednesday, April 14, 2010 Example of back arc extension-east Scotia sea The back arc spreading forms normal sea floor with well developed marine magnetic anomalies. Interpreted magnetic anomaly map of the East Scotia Sea. Anomaly identifications are essentially the same as those of Barker [1995], but recently identified pseudofaults [Livermore et al., 1994, 1997], formed by ridge segment propagation [Hey, 1977], are incorporated in this interpretation. The central Bruhnes anomaly as well as anomalies 2A and 3 are shaded. Ridge crest segments E2–E9 are numbered. The 2500-m contour (dashed line) locates the South Sandwich island arc and the North and South Scotia Ridge. The ornamented line represents the trench. The islands identified are Zavodovski (Z), Candlemas (C), Montagu (M), and Southern Thule (ST). Wednesday, April 14, 2010 Wednesday, April 14, 2010 Back arc extension in the Philippine Basin at the Izu-Bonin trench Bathymetry Wednesday, April 14, 2010 Magnetics Gravity Wednesday, April 14, 2010 Wednesday, April 14, 2010 • Free-air gravity anomalies of the Ontong Java Plateau–Solomon Islands convergent zone derived from Geosat and ERS-1 altimetry data (Sandwell and Smith, 1997). Arrows indicate direction and rate of Pacific plate relative to adjacent plates from DeMets et al. (1994). Large yellow areas are known or inferred oceanic plateaus compiled by Coffin and Eldholm (1994); dashed yellow lines represent hotspot tracks or "tails". Key to abbreviations: NB=Nauru basin; ER=Eauripik Rise; LP=Louisiade Plateau; OJP=Ontong Java Plateau; VAT=Vanuatu trench; VT=Vitiaz trench; NFP=North Fiji Plateau; MP=Manihiki Plateau; SHS=Samoa hotspot; TT=Tonga trench; LR=Louisville ridge; HP=Hikurangi Plateau; THS=Tasminid hotspot; LHS=Lord Howe hotspot. Note the general difficulty in correlating individual hotspot tracks to oceanic plateaus because of intervening subduction zones. Wednesday, April 14, 2010 Evolution of the Tonga Fiji subduction? Tectonic reconstruction of the New Hebrides - Tonga region (modified and interpreted from Auzende et al. [1988], Pelletier et al. [1993], Hathway [1993] and Schellart et al.(2002a)) at (a) ~ 13 Ma, (b) ~ 9 Ma, (c) 5 Ma and (d) Present. The IndoAustralian plate is fixed. DER = d'Entrcasteaux Ridge, HFZ = Hunter Fracture Zone, NHT = New Hebrides Trench, TT = Tonga Trench, WTP = West Torres Plateau. Arrows indicate direction of arc migration. During opening of the North Fiji Basin, the New Hebrides block has rotated some 40-50° clockwise [Musgrave and Firth, 1999], while the Fiji Plateau has rotated some 70-115° anticlockwise [Malahoff et al., 1982]. During opening of the Lau Basin, the Tonga Ridge has rotated ~ 20° clockwise [Sager et al., 1994 Wednesday, April 14, 2010 Volcanic activity in the overriding plate • Magmatic arc complexities. Only an idealized section through an intraoceanic arc is shown; similar processes are expected beneath Andean-type arcs. Note that the asthenosphere is shown extending up to the base of the crust; delamination or negative diapirism is shown, with blocks of the lower crust sinking into and being abraded by convecting mantle. Regions where degassing of CO2 and H2O is expected are also shown. Wednesday, April 14, 2010 • What is the cause of the magmatic activity? • • Simple thermal models show that the down-going plate remains cold and could not melt. It is likely that melting occurs because fluids are expelled from the downgoing slab that lower the melting temperature in the mantle. Wednesday, April 14, 2010 Effect of water on the solidus Effect of water on melting of peridotite. (a) Pressuretemperature diagram for H2O undersaturated (0.2–0.5 wt % H2O) and anhydrous partial melting of mid-ocean ridge basalt (MORB) and pyrolite mantle composition (modified after Ulmer [2001]). Dashed lines indicate the stability limits of garnet peridotite (gar), spinel peridotite (sp), plagioclase peridotite (plg), and amphibole peridotite (amph). The dash-dot line corresponds to the average curent mantle adiabat (ACMA) corresponding to a potential temperature of 1280°C. (b) Plot of melt fraction versus temperatures of anhydrous batch melts of a depleted MORB mantle source at 1.5 GPa with the liquidus temperatures of hydrous batch melts from peridotite containing 0.15% H2O and 0.32% of simplified Mariana subduction component, which is an aqueous fluid with dissolved solutes [Stolper and Newman, 1994]. Upper axis shows forsterite content of equilibrium olivine. Figure 13b is from Gaetani and Grove [1998]. Wednesday, April 14, 2010 Melting occurs only in a very narrow range. It determines the location of volcanic arc. • • • This model accounts for the entrainment of the asthenosphere by the downgoing slab (which makes it colder at given depth. There is a narrow window where the geotherm intersects the solidus for wet mantle. (a) Material movement (arrows) and temperature structure (dashed lines) of a simplified convergent margin (modified after finite element models of Davies and Stevenson [1992]). Crust and mantle lithosphere of the subducting plate move at a constant velocity of 7 cm yr-1, dragging the asthenosphere with it beneath the overriding plate. Note also that the isotherms remain approximately parallel with the original seafloor, even deep into the subduction zone. The shaded area in the mantle wedge approximates the location of detail shown in Figure 12. (b) Temperature profile beneath the arc volcanoes (bold line) and melting curve for wet mantle (thin line). Note that the temperature reaches a maximum at ∼80 km depth and then decreases as the subducted lithosphere is approached, reaching a minimum at a depth of ∼120 km. The temperature is high enough for mantle melting only at a depth of ∼80 km, ∼40 km above the subducted plate. Wednesday, April 14, 2010 Volcanic Arc Model for dehydration of subducted materials (modified after Schmidt and Poli [1998]). Dehydration of subducted peridotite and oceanic crust occurs continuously to ∼150–200 km; thus water is continuously supplied to the overlying mantle. The shaded region in the mantle wedge labeled “partially molten region” is expected to melt to a significant degree. The volcanic front forms where the amount of melt can separate and rise to the surface. Open arrows indicate rise of fluid, solid arrows indicate rise of melts. Long arrows indicate flow of asthenosphere in the mantle wedge. Stippled area marks stability field of amphibole beneath the forearc. Dashed lines outline stability fields of hydrous phases in peridotite. The horizontally ruled region shows where talc is stable. For some thermal structures a portion of the peridotitic lithosphere will be <600°C at 62 kbar; serpentine will react to “phase A,” and thus H2O will be subducted to large depth. In the oceanic crust, temperatures can be low enough to preserve lawsonite and phengite to their maximum pressure stability; however, at somewhat warmer conditions (slower subduction, shallower angle, and younger crust), zoisite (zo) will be the last potassium-free phase to decompose, and the top of the oceanic crust (phengite-rich sedimentary layer) might melt. Single-phase transitions, which cause a potassium-rich fluid pulse, could result from the breakdown of phengite in oceanic crust Wednesday, April 14, 2010 • Detailed cartoon of the mantle wedge (region of detail depicted by shading in Figure 11), showing how fluids might move from the subducted slab into a region where melts could be generated (modified after Davies and Stevenson [1992] and Stern [1998]). Water is carried in the descending slab (A), and dense aqueous fluids are continuously released from subducted sediments, crust, and serpentinites (B). These fluids rise into the overlying mantle to form hydrous phases in mantle peridotite (C). Amphiboles are shown forming here, but it could also be another hydrous mineral. Metasomatized mantle descends with the subducted slab. At the maximum depth of stability for amphibole peridotite, ∼100 km, it breaks down to anhydrous peridotite and aqueous fluid (D). The fluid rises vertically, moving away from the subducted slab and toward hotter regions in the mantle. At some point the rising water will react with anhydrous peridotite to form amphibole again (E). This descends until the amphibole breaks down again (F). Amphibole peridotite forms (G), descends, and breaks down again (H). The dark shaded area is mantle that can melt if sufficient water is provided. Above point H the mantle is sufficiently hot that water added to it leads to melting (I). The mantle is still moving downward, so melt will not be able to rise until enough of it accumulates to form a sufficiently large diapir (K) as a result of RayleighTaylor instability. Partially molten diapirs are less dense than the surrounding mantle and can rise through it at a rate of perhaps 1 m yr-1 (L). At the base of the overlying mantle lithosphere (M), magma separates from the unmelted part of the diapir to feed an arc volcano. Wednesday, April 14, 2010 • • • Temporal evolution of island arc magmatism. Early stage expulsion of water (80-100km) causes partial melting of mantle with tholeitic magmas. At greater depths, the partially molten region ascents as a diapir and adiabatic decompression causes more melting and differentiated calc-alkaline magmas. Wednesday, April 14, 2010 • • • • • Wednesday, April 14, 2010 Andean style batholiths New continental crust is added directly to the continent through magmatic activity. Sr87/Sr86 indicate mantle origin with continental crust contamination. Be10 (1/2 life = 1.5 My) was found in some of the lavas. It could come only from sediments that were subducted, melted, and included in the magmas. Note large batholiths in continents used to detect ancient Andean type margin (e.g.dePas batholith in NQO). Proterozoic examples: Is the elongated de Pas batholith in the Core Zone anologuous to the batholith in the Andes? Suggests subduction of the Superior plate beneath the core zone during the NQO Wednesday, April 14, 2010 Wathaman batholith on margin of Hearne. No batholith in the Superior => No subduction beneath Superior during closure of Maniwekan ocean? Wednesday, April 14, 2010 • • • Wednesday, April 14, 2010 Andes elevation ~ 6km Crustal thickness is estimated 60-70km Crustal thickening by tectonic shortening and by magmatic underplating • Wednesday, April 14, 2010 Receiver function (RF) images and crustal models of the Central Andes along an east– west profile. In the images, red indicates positive, velocity increase downwards; blue indicates negative, velocity reduction downwards. a, Time domain RFs averaged over a 30-km-wide moving window, 2–10-s band pass filtered. b, Possible crustal S-wave velocity (vS) models resulting from the inversion of the RFs in a. c, Interpretative cartoon with depth-migrated RF image as background averaged over the Altiplano and Puna regions. Black plus signs show Moho depth data from wide-angle reflection studies4. Local crustal models and a global mantle reference model were used for migration. The TRAC1 and TRAC2 converters in a and c border the Andean lowvelocity zone (ALVZ) which dips westwards from below the Eastern Cordillera fault system (indicated in c by black lines at the surface) across the entire Altiplano/Puna to the Precordillera (red points at surface indicate Cenozoic volcanoes). Figure S1 in Supplementary Information shows this data set split in two at 23° S for judging north– south variations of the TRAC2 converter. Figure S2 in Supplementary Information shows the lateral extension of the ALVZ. The Nazca converter (thick black-white dashed line indicated in c) is interpreted as an image of the oceanic Moho. The upper boundary of the oceanic crust (slab shear zone) is set 10 km above the oceanic Moho in agreement with the results of waveform inversion (see text and Fig. 4). Oceanic crust is clearly visible from converted waves only down to Electrical conductivity of the mantle is measured by MT Contours: depth of Nazca plate Wednesday, April 14, 2010 High conductivity = hydrated mantle Wednesday, April 14, 2010 Metamorphism in subduction zones • • • Metamorphism in subduction zones High pressure in the downgoing slab Low pressure in the overiding plate Wednesday, April 14, 2010 Paired metamorphic belts in Japan Note paired metamorphic belts are found around the Pacific (with the right polarity) Wednesday, April 14, 2010 Thermal models of end-member (young and hot versus old and cold) subduction zones. (a) NE Japan arc, a good example of a cold subduction zone. (b) SW Japan arc, a good example of a hot subduction zone. Note the great difference in the temperature of the slab interface at 50 km depth (T50km) and beneath the volcanic front (Tvf) but the small difference in the maximum temperature of the mantle wedge beneath the volcanic front (Tmw). (c) Pressure-temperature diagram showing metamorphic facies and melting relations for basaltic oceanic crust, along with trajectories for crust subducted beneath NE and SW Japan Roman numerals identify fields for metamorphic facies: I, greenschist; II, epidote amphibolite; III, amphibolite; IV, granulite; V, epidote blueschist; VI, lawsonite blueschist (green field shows location of the eclogite field, with the dark dashed line separating this from blueschist); VII, chloritoid-amphibolite-zoisite eclogite; VIII, zoisitechloritoid eclogite; and IX, lawsonite-chloritoid eclogite. Note that oceanic crust subducted beneath SW Japan enters the eclogite field at ~40 km depth and, if hydrous, begins to melt at ~90 km, whereas oceanic crust subducted beneath NE Japan barely enters the eclogite field at 120 km depth. Figures 6a– 6c are modified after Peacock and Wang [1999] and Peacock [2002]. See color version of this figure at back of this issue. Wednesday, April 14, 2010 • • • • The Canadian Cordillera Accreted terranes in the Canadian Cordillera. Some terranes are of North American affinity Others are of oceanic affinity. Wednesday, April 14, 2010 • • • Main geological belts of the Canadian Cordillera and Bouguer gravity anomaly. Intermontane and Coast belts are accreted terranes. Omineca and Foreland belts are of north American affinity. Wednesday, April 14, 2010 Schematic cross section of the Canadian Cordillera Wednesday, April 14, 2010 Geodynamic model of the Canadian Cordillera after the LITHOPROBE studies • • • Intermontane belt transported on North America basement. Coast belt docked on the side. Sediments Wednesday, April 14, 2010 a, Unmigrated section showing the reflection, labelled JdF, from the top of the subducting Juan de Fuca plate west of Vancouver Island, the E reflection zone, and the F reflection which marks the top of the subducting Juan de Fuca plate further east. Reflections from the Leech River crustal fault, LRF, are truncated by the E reflections. b, Migrated section superimposed on a display of P wave velocities derived by 3D tomographic inversion of first arrivals from local earthquakes and wide-angle airgun shots around Vancouver Island9. The velocities were extracted from the 3D velocity model along the composite seismic profile shown by the red line in Fig. 1 and projected in the same way as the seismic reflection data. Earthquakes, which were relocated in the tomographic inversion, are shown by filled black circles. The nearlinear alignment of east-dipping inslab earthquakes just above the oceanic Moho at 45–55 km depth may indicate delaminatio of the oceanic crust Wednesday, April 14, 2010 • Wednesday, April 14, 2010 Comparison of scattered wave inversion results with thermal model. a, S-velocity perturbations below the array, recovered from the inversion of scattered waves in the P-wave coda of 31 earthquakes recorded at teleseismic distances. The image represents a bandpass-filtered version of the true perturbations to a one-dimensional, smoothly varying reference model. Discontinuities are present where steep changes in perturbation polarity occur. b, Thermal model of Cascadia subduction zone corresponding approximately to the profile in a. The cool subducting plate depresses isotherms in the forearc, rendering serpentine stable within that portion of the mantle encompassed by the dashed rectangle; solid lines indicate locations of subducting oceanic crust and continental Moho. Note temperature contour interval is 200 °C. c, Interpretation of structure in a. High degrees of mantle serpentinization where the subducting oceanic crust enters the forearc mantle results in an inverted continental Moho (high-velocity crust on low-velocity mantle), which gradually reverts eastward to normal polarity by -122.3° longitude. The signature of the subducting oceanic Moho diminishes with depth as a result of progressive eclogitization below 45 km. Inverted triangles in a and c show instrument locations. What happened to the Farallon plate? It is still down there! Wednesday, April 14, 2010 • Cross-section through the Cascadia subduction zone on the northwest coast of the United States. Water released from the subducting oceanic crust rises up and hydrates the forearc mantle; water released deeper down causes partial melting of rock to magma that erupts in the arc volcanoes. In the water-altered — or 'serpentinized' — forearc rocks, seismic waves travel more slowly and the usual gradient of wave speed is reversed, producing an effect known as an 'inverted Moho', which has been detected by Bostock et al.1. The slippery nature of the serpentinized forearc mantle limits the seismogenic depth of coupling between the two plates, and so might also limit the impact of earthquakes in the region. Wednesday, April 14, 2010 • Foreland basins are caused by the bending of the plate under the load of the accreted terranes Wednesday, April 14, 2010 The accretion of terranes over the western margin of Canada caused the formation of the Alberta basin Wednesday, April 14, 2010 Forearc basin-Accretionary prism End-member forearc types: (a) accretionary forearc (modified after Dickinson [1995]) and (b) nonaccretionary forearc (P. Fryer, personal communication, 2001). Note that the abundance of sediments associated with accretionary forearcs is manifested as an accretionary prism and as a thick forearc basin and that the relative lack of sediments leaves the nonaccretionary forearc exposed. Wednesday, April 14, 2010 • • Mechanical model of an accretionary wedge. Balance of forces implies a positive slope for the wedge Wednesday, April 14, 2010 Erosion and subduction of sediment in crust could exceed accretion of new crust. • Map showing the distribution of accreting versus eroding subduction zones considered within this study. Accretionary margins are shown with solid barbs on the plate boundary, while open barbs mark erosive margins. Wednesday, April 14, 2010 Compilation of profiles across accretionary plate margins. Profiles are redrawn and resized to a common scale in order to allow direct comparison of different margins. Sources for the original data are shown next to each profile. Wednesday, April 14, 2010 Compilation of profiles across nonaccretionary and erosive plate margins. Profiles are redrawn and resized to a common scale in order to allow direct comparison of different margins. Wednesday, April 14, 2010 Budget of all continental margins Diagram showing the integrated growth or erosion rate of each active plate margin in relation to the global average growth rate required to maintain the continental freeboard. Note that several erosive plate margins are actively growing crust despite rapid loss at the trench. Wednesday, April 14, 2010 • Pie chart showing the relative proportions of the major inputs and outputs from the global subduction systems with respect to the crust. Note the dominance of arc magmatism over subduction accretion as a source of new material. Wednesday, April 14, 2010 When did subduction processes begin on Earth ? • • Evidence for late Archean subduction? Seismic reflector in the mantle interpreted as a relict subducted slab. Wednesday, April 14, 2010 Mantle reflectors could be fossil Archean subduction zones. (example from the Slave Province, LITHOPROBE SNORCLE transect). Wednesday, April 14, 2010 Continental crust evolution? • • • • Today continental crust is being destroyed at about the same rate it is accreted. We do not know how the total volume of the continental crust has changed with time. Model 4 with a rapid growth of the continental crust at the end of Archean is plausible but so are the other models Episodic growth models have also been proposed. Wednesday, April 14, 2010 Summary • • • • • • • • • Subduction of sea floor independent of age But tectonic style might depend on age of the subducted slab (western vs eastern Pacific) Plate remains cold Expulsion of volatiles causes partial melting in the mantle Magmas in volcanic arc from mantle (but …) . Main source of continental growth High pressure low temperature metamorphism in subducted slab Accretion of terranes in North American cordillera Plate tectonic and subduction in late Archean at 2.7 Ga Budget of continental crust? Wednesday, April 14, 2010 Open questions • • • • • • What initiates subduction? Mechanism of accretion. When did present day style of subduction start? Meaning of Archean and Proterozoic subcrustal reflectors. What is crustal accretion destruction present budget? How did total volume of continental crust change with time? Wednesday, April 14, 2010 • Importance of convergence rate and age of subducted lithosphere. (a) Plot of length of seismic zone as a function of the product of convergence rate and age of subducted lithosphere. Approximate uncertainties are given by error bars. Dashed line corresponds to length of seismic zone equal to convergence rate multiplied by age divided by 10 (modified after Molnar et al. [1979]). (b) Plot of convergence rate versus lithosphere age, showing the strong influence this relationship has on seismicity (modified after Ruff and Kanamori [1980], using GPS-defined convergence rates for Pacific-Tonga [Bevis et al., 1995]. The number at each subduction zone is the associated maximum Mw (seismic moment magnitude), and the contours of constant Mw are defined by regression analysis. Shaded area outlines subduction zones associated with back arc spreading or interarc rifting. Note that most of the biggest earthquakes occur at Andean-type margins. Wednesday, April 14, 2010 • Diagnostic trace element differences between adakites and more common andesite-dacite-rhyolite (ADR) suites (modified after Defant and Drummond [1990]). Adakites form by melting at high pressure in equilibrium with garnet (garnet has a high partition coefficient for Y and low partition coefficients for Sr) and so have low-Y contents and high Sr/Y. ADR suites form at lower pressure in equilibrium with plagioclase (plagioclase has a high partition coefficient for Sr and low partition coefficients for Y) and so have high Y contents and low Sr/Y. Wednesday, April 14, 2010 • Incompatibility plot (spidergram) for important subduction zone inputs (global subducting sediment (GLOSS) and normal mid-ocean ridge basalt (NMORB)) and typical outputs (boninite, arc tholeiite, back arc basin basalt, dacite, and continental crust) from Table 2. Elements on the horizontal axis are listed in order of their incompatibility in the mantle relative to melt; elements on the left are strongly partitioned into the melt, whereas those on the right are strongly partitioned into peridotite minerals. Composition of NMORB and element order is after Hofmann [1988]. Note the characteristic enrichments of GLOSS and subduction zone outputs relative to MORB with respect to fluid-mobile large-ion lithophile elements: Rb, Ba, U, K, Pb, and Sr; note the relative depletion of these in high field strength elements: Nb, Ta, Zr, Ti, Y, and heavy rare earth elements. Note also the greater enrichment and overall similarity of the “continental suite” (GLOSS, continental crust, and Chilean dacite) on the one hand and the “oceanic suite” (boninite, arc tholeiite, back arc basin basalt, and MORB) on the other. Shaded field encompasses the continental suite. Wednesday, April 14, 2010 • Model of subducted slabs (modified after Stein and Rubie [1999]). (top) Predicted mineral phase boundaries and (bottom) resulting buoyancy forces in a downgoing slab with (left) equilibrium mineralogy and (right) for a nonequilibrium metastable olivine wedge. Assuming equilibrium mineralogy, the slab has significant negative thermal buoyancy (dark shading in bottom graphs), due to both its colder temperature and the elevated 410 km discontinuity, and significant positive compositional buoyancy (cross hatching) associated with the depressed 660 km discontinuity. If a metastable wedge is present, it adds positive buoyancy and hence decreases the net negative buoyancy force driving subduction. Units are in 103 N m-3. Wednesday, April 14, 2010 Map of Pacific and Caribbean LIP's compared to modern plate boundaries (yellow lines), active hotspots/mantle plumes (yellow stars), and rates of the Pacific and Caribbean plate relative to surrounding plates from global plate motion model of DeMets et al. (1994). Oceanic plateaus are subdivided into three tectonic settings as shown in the key to the upper right. Box denotes Solomon Islands– Ontong Java Plateau study area for papers in this volume. Key to abbreviations for larger LIP's and hotspot tracks: HE=Hawaii– Emperor seamount chain; SR=Shatsky Rise; SO=Sea of Okhotsk; OP=Ogasawara Plateau; MNR=Marcus Necker Ridge; OJP=Ontong Java Plateau; MP=Manihiki Plateau; HR=Hikurangi Plateau; NR=Nazca Ridge; CR=Carnegie Ridge; COR=Cocos Ridge; CP=Caribbean Plateau. Numbered LIP's 1–21 are keyed to Table 2A (oceanic plateau or hotspot tracks presently subducting at Pacific and Caribbean plate boundaries) and Table 2B (proposed oceanic plateaus accreted during Phanerozoic collision events involving island arcs or continental orogenic belts). (B) Schematic diagram summarizing four possible relationships between hotspot or mantle plume heads and hotspot tracks or "tails" in intraplate (1), subduction (2, 3) and in ancient orogenic belts (4). See text for discussion. Wednesday, April 14, 2010 • Potassium-silica diagram for representative arcs. Dashed line defines boundary between shoshonitic and calc-alkaline and tholeiitic suites (CATS). IzuBonin and Mariana arcs are exemplary of intraoceanic arcs (fields from Stern et al. [2002], note that volumetrically subordinate Mariana shoshonites [Sun and Stern, 2001] are omitted). Field for Andes, 16°S–26°S encompasses most of 606 Plio-Pleistocene and younger samples from the Central Volcanic Zone (CVZ) (G. Wörner, personal communication, 2002). Abbreviations are as follows: M, typical MORB from Table 2; B, back arc basin basalt from Table 2; I, mean composition of Izu-Bonin arc samples; MA, mean composition of Mariana arc samples; CC, bulk continental crust from Table 2; G, GLOSS from Table 2; A, mean composition of Andes CVZ lavas; and UC, composition of upper continental crust [from McLennan, 2001]. Dark shading encompasses mean and typical compositions of the “oceanic suite” (MORB, back arc basin basalt, and mean Mariana and Izu-Bonin lavas), and light shading encompasses mean and typical compositions of the “continental suite” (bulk continental crust, GLOSS, upper continental crust, and mean Andean dacite). Wednesday, April 14, 2010 Seismic tomography of two arcs • • • • Crustal structure of typical arcs, based on P wave velocities. Note vertical exaggeration is ∼5x. (a) Izu arc, 33°N (modified after Suyehiro et al. [1996]). (b) Eastern Aleutian arc (modified after Holbrook et al. [1999]). Note differences in thickness and velocity structure. We do not have comparable tomographic images of Andean-type arc crust. See color version of this figure at back of this issue. Wednesday, April 14, 2010 • Comparison of pattern of deformation observed in Ontong Java Plateau–Solomon arc convergent zone with examples of Phanerozoic and Precambrian patterns of deformation in collision zones. Key to lithospheric compositions: RED=upper crustal, mainly igneous rocks; YELLOW=upper crustal, mainly sedimentary rocks; gray and BLACK=upper mantle, mainly ultramafic rocks. (A) Ontong Java Plateau—initial subduction– accretion of uppermost plateau in western Malaita accretionary prism (modified from Rahardiawan et al., 2004). Open dots represent ISC hypocenters (M>4.0). (B) Ontong Java Plateau—more advanced subduction–accretion in eastern Malaita accretionary prism. (C) Cenozoic Alpine crustal-scale triangle zone or "flake" (modified from Oxburgh, 1972); L=base of lithosphere; M=subducted Moho. (D) Precambrian Canadian crustal-scale triangle zones or "flakes" (modified from Cook et al., 1998); M2=base of Slave Province mantle. (E) Precambrian African thrust-imbricated, subduction-related prism Wednesday, April 14, 2010