Remnants of Eoarchean continental crust derived from a subducted proto-arc.

PubMed

Ge, Rongfeng; Zhu, Wenbin; Wilde, Simon A; Wu, Hailin

2018-02-01

Eoarchean [3.6 to 4.0 billion years ago (Ga)] tonalite-trondhjemite-granodiorite (TTG) is the major component of Earth's oldest remnant continental crust, thereby holding the key to understanding how continental crust originated and when plate tectonics started in the early Earth. TTGs are mostly generated by partial melting of hydrated mafic rocks at different depths, but whether this requires subduction remains enigmatic. Recent studies show that most Archean TTGs formed at relatively low pressures (≤1.5 GPa) and do not require subduction. We report a suite of newly discovered Eoarchean tonalitic gneisses dated at ~3.7 Ga from the Tarim Craton, northwestern China. These rocks are probably the oldest high-pressure TTGs so far documented worldwide. Thermodynamic and trace element modeling demonstrates that the parent magma may have been generated by water-fluxed partial melting of moderately enriched arc-like basalts at 1.8 to 1.9 GPa and 800° to 830°C, indicating an apparent geothermal gradient (400° to 450°C GPa -1 ) typical for hot subduction zones. They also locally record geochemical evidence for magma interaction with a mantle wedge. Accordingly, we propose that these high-pressure TTGs were generated by partial melting of a subducted proto-arc during arc accretion. Our model implies that modern-style plate tectonics was operative, at least locally, at ~3.7 Ga and was responsible for generating some of the oldest continental nuclei.

Remnants of Eoarchean continental crust derived from a subducted proto-arc

PubMed Central

Ge, Rongfeng; Zhu, Wenbin; Wilde, Simon A.; Wu, Hailin

2018-01-01

Eoarchean [3.6 to 4.0 billion years ago (Ga)] tonalite-trondhjemite-granodiorite (TTG) is the major component of Earth’s oldest remnant continental crust, thereby holding the key to understanding how continental crust originated and when plate tectonics started in the early Earth. TTGs are mostly generated by partial melting of hydrated mafic rocks at different depths, but whether this requires subduction remains enigmatic. Recent studies show that most Archean TTGs formed at relatively low pressures (≤1.5 GPa) and do not require subduction. We report a suite of newly discovered Eoarchean tonalitic gneisses dated at ~3.7 Ga from the Tarim Craton, northwestern China. These rocks are probably the oldest high-pressure TTGs so far documented worldwide. Thermodynamic and trace element modeling demonstrates that the parent magma may have been generated by water-fluxed partial melting of moderately enriched arc-like basalts at 1.8 to 1.9 GPa and 800° to 830°C, indicating an apparent geothermal gradient (400° to 450°C GPa−1) typical for hot subduction zones. They also locally record geochemical evidence for magma interaction with a mantle wedge. Accordingly, we propose that these high-pressure TTGs were generated by partial melting of a subducted proto-arc during arc accretion. Our model implies that modern-style plate tectonics was operative, at least locally, at ~3.7 Ga and was responsible for generating some of the oldest continental nuclei. PMID:29487901

Detachments of the subducted Indian continental lithosphere based on 3D finite-frequency tomographic images

NASA Astrophysics Data System (ADS)

Liang, X.; Tian, X.; Wang, M.

2017-12-01

Indian plate collided with Eurasian plate at 60 Ma and there are about 3000 km crustal shortening since the continental-continental collision. At least one third of the total amount of crustal shortening between Indian and Eurasian plates could not be accounted by thickened Tibetan crust and surface erosion. It will need a combination of possible transfer of lower crust to the mantle by eclogitization and lateral extrusion. Based on the lithosphere-asthenosphere boundary images beneath the Tibetan plateau, there is also at least the same amount deficit for lithospheric mantle subducted into upper/lower mantle or lateral extrusion with the crust. We have to recover a detailed Indian continental lithosphere image beneath the plateau in order to explain this deficit of mass budget. Combining the new teleseismic body waves recorded by SANDWICH passive seismic array with waveforms from several previous temporary seismic arrays, we carried out finite-frequency tomographic inversions to image three-dimensional velocity structures beneath southern and central Tibetan plateau to examine the possible image of subducted Indian lithosphere in the Tibetan upper mantle. We have recovered a continuous high velocity body in upper mantle and piece-wised high velocity anomalies in the mantle transition zone. Based on their geometry and relative locations, we interpreted these high velocity anomalies as the subducted and detached Indian lithosphere at different episodes of the plateau evolution. Detachments of the subducted Indian lithosphere should have a crucial impact on the volcanism activities and uplift history of the plateau.

New Geodetic Results from the Hauraki Rift: Slow Continental Rifting Oblique to Subduction, North Island, New Zealand

NASA Astrophysics Data System (ADS)

Pickle, R. C.; Eccles, J. D.; Hreinsdottir, S.; Palmer, N.; Rowland, J. V.

2016-12-01

The Hauraki Rift, an active but slow-deforming narrow intra-continental rift in northern New Zealand paradoxically strikes nearly normal to the Pacific-Australian oblique subduction boundary 300+ km to the southeast. Both the driving mechanism and quantitative details of the rift's current activity are unknown. Past GPS/GNSS geodetic surveying in the area has been coarse and erratic (e.g. single 8-hour surveys in 1995). In 2015 and again in 2016 a 37 station network of existing benchmarks around the rift was measured with the aim of gaining better insight into deformation in the region. We find that it is primarily extensional ( 0.9 mm/yr) with a small portion of right-lateral shearing ( 0.1 mm/yr) relative to a fixed Australian plate in ITRF2008. Closer to the plate boundary, the oblique westward subduction of the Pacific plate generates a strong clockwise angular strain signature in the over-riding plate; this same angular stress field is the simplest explanation for the Hauraki Rift's axis-perpendicular strain and in consistent with previous geophysical observations. Additionally, several short wavelength dislocations between our velocity solutions hint at the existence of undocumented active faults which will have implications to the seismic hazard to Auckland, New Zealand's largest city, located just 50km west of the rift.

Basins in ARC-continental collisions

USGS Publications Warehouse

Draut, Amy E.; Clift, Peter D.; Busby, Cathy; Azor, Antonio

2012-01-01

Arc-continent collisions occur commonly in the plate-tectonic cycle and result in rapidly formed and rapidly collapsing orogens, often spanning just 5-15 My. Growth of continental masses through arc-continent collision is widely thought to be a major process governing the structural and geochemical evolution of the continental crust over geologic time. Collisions of intra-oceanic arcs with passive continental margins (a situation in which the arc, on the upper plate, faces the continent) involve a substantially different geometry than collisions of intra-oceanic arcs with active continental margins (a situation requiring more than one convergence zone and in which the arc, on the lower plate, backs into the continent), with variable preservation potential for basins in each case. Substantial differences also occur between trench and forearc evolution in tectonically erosive versus tectonically accreting margins, both before and after collision. We examine the evolution of trenches, trench-slope basins, forearc basins, intra-arc basins, and backarc basins during arc-continent collision. The preservation potential of trench-slope basins is low; in collision they are rapidly uplifted and eroded, and at erosive margins they are progressively destroyed by subduction erosion. Post-collisional preservation of trench sediment and trench-slope basins is biased toward margins that were tectonically accreting for a substantial length of time before collision. Forearc basins in erosive margins are usually floored by strong lithosphere and may survive collision with a passive margin, sometimes continuing sedimentation throughout collision and orogeny. The low flexural rigidity of intra-arc basins makes them deep and, if preserved, potentially long records of arc and collisional tectonism. Backarc basins, in contrast, are typically subducted and their sediment either lost or preserved only as fragments in melange sequences. A substantial proportion of the sediment derived from

Subduction zone decoupling/retreat modeling explains south Tibet (Xigaze) and other supra-subduction zone ophiolites and their UHP mineral phases

NASA Astrophysics Data System (ADS)

Butler, Jared P.; Beaumont, Christopher

2017-04-01

The plate tectonic setting in which proto-ophiolite 'oceanic' lithosphere is created remains controversial with a number of environments suggested. Recent opinions tend to coalesce around supra-subduction zone (SSZ) forearc extension, with a popular conceptual model in which the proto-ophiolite forms during foundering of oceanic lithosphere at the time of spontaneous or induced onset of subduction. This mechanism is favored in intra-oceanic settings where the subducting lithosphere is old and the upper plate is young and thin. We investigate an alternative mechanism; namely, decoupling of the subducting oceanic lithosphere in the forearc of an active continental margin, followed by subduction zone (trench) retreat and creation of a forearc oceanic rift basin, containing proto-ophiolite lithosphere, between the continental margin and the retreating subduction zone. A template of 2D numerical model experiments examines the trade-off between strength of viscous coupling in the lithospheric subduction channel and net slab pull of the subducting lithosphere. Three tectonic styles are observed: 1) C, continuous subduction without forearc decoupling; 2) R, forearc decoupling followed by rapid subduction zone retreat; 3) B, breakoff of subducting lithosphere followed by re-initiation of subduction and in some cases, forearc decoupling (B-R). In one case (BA-B-R; where BA denotes backarc) subduction zone retreat follows backarc rifting. Subduction zone decoupling is analyzed using frictional-plastic yield theory and the Stefan solution for the separation of plates containing a viscous fluid. The numerical model results are used to explain the formation of Xigaze group ophiolites, southern Tibet, which formed in the Lhasa terrane forearc, likely following earlier subduction and not necessarily during subduction initiation. Either there was normal coupled subduction before subduction zone decoupling, or precursor slab breakoff, subduction re-initiation and then decoupling

Retrodeforming the Arabia-Eurasia collision zone : Age of collision and magnitude of continental subduction

NASA Astrophysics Data System (ADS)

McQuarrie, N.; van Hinsbergen, D. J. J.

2012-04-01

When did continents collide, and how is convergence partitioned after collision are first order questions that seem to defy consensus along the Alpine-Himalyan orogen. Estimates on the age of collision for Arabia and Eurasia range from late Cretaceous to Pliocene, based on a wide variety of presumed geologic responses. Both lower Miocene synorgenic strata with growth structures adjacent to the main Zagros fault and upper Oligocene to lower Miocene overlap strata over post-collisional thrusts are derived from Eurasia and require that collision was underway at least by ~25-24 Ma. However, upper plate deformation, exhumation and sedimentation are used to argue for an older, 35 Ma collision age. Africa-North America-Eurasia plate circuit rotations, combined with Red Sea rotations provides precise estimates of the relative positions between the northern Arabian margin and the southern Eurasia margin. Plate circuits indicate, from NW to SE along the collision zone 490-650 km of post-25 Ma Arabia-Eurasia convergence and 810-1070 km since 35 Ma. To assess the consequences of these collision ages for the amount of Arabian continental subduction, we compile all documented shortening within the orogen. The Zagros fold-thrust belt consists of thrusted upper crust that was offscraped from subducted Arabian continental lithosphere. Balanced cross-sections give 105-180 km of Zagros shortening (including estimates from the Zagros proper, 45-90 km, and the Zagros "crush" zone, 60-90 km). Shortening within Eurasia is estimated to be 53-75 km through the Kopet Dagh and Alborz Mountains, plus 38 km across Central Iran. These estimates suggest that the orogen has shortened 200 to 300 km since the early Miocene. Both a 25 and a 35 Ma collision estimate thus requires that a considerable portion of the Arabian plate subducted without recognized accretion of its upper crust. To balance plate circuits and documented shortening requires whole-sale subduction of ~500-800 km of continental

Continental Subduction: Mass Fluxes and Interactions with the Wider Earth System

NASA Astrophysics Data System (ADS)

Cuthbert, S. J.

2011-12-01

Substantial parts of ultra-high pressure (UHP) terrains probably represent subducted passive continental margins (PCM). This contribution reviews and synthesises research on processes operating in such systems and their implication for the wider Earth system. PCM sediments are large repositories of volatiles including hydrates, nitrogen species, carbonates and hydrocarbons. Sediments and upper/ mid-crustal basement are rich in incompatible elements and are fertile for melting. Lower crust may be more mafic and refractory. Juvenile rift-related mafic rocks also have the potential to generate substantial volumes of granitoid melts, especially if they have been hydrated. Exposed UHP terrains demonstrate the return of continental crust from mantle depths, show evidence for substantial fluxes of aqueous fluid, anatexis and, in entrained orogenic peridotites, metasomatism of mantle rocks by crust- derived C-O-H fluids. However, substantial bodies of continental material may never return to the surface as coherent masses of rock, but remain sequestered in the mantle where they melt or become entrained in the deeper mantle circulation. Hence during subduction, PCM's become partitioned by a range of mechanisms. Mechanical partitioning strips away weaker sediment and middle/upper crust, which circulate back up the subduction channel, while denser, stronger transitional pro-crust and lower crust may "stall" near the base of the lithosphere or be irreversibly subducted to join the global mantle circulation. Under certain conditions sediment and upper crustal basement may reach depths for UHPM. Further partitioning takes place by anatexis, which either aids stripping and exhumation of the more melt-prone rock-masses through mechanical softening, or separates melt from residuum so that melt escapes and is accreted to the upper plate leading to "undercrusting", late-orogenic magmatism and further refinement of the crust. Melt that traverses sections of mantle will interact with

Pervasive Eclogitization Due to Brittle Deformation and Rehydration of Subducted Basement: Effects on Continental Recycling?

NASA Astrophysics Data System (ADS)

Engi, Martin; Giuntoli, Francesco; Lanari, Pierre; Burn, Marco; Kunz, Barbara; Bouvier, Anne-Sophie

2018-03-01

The buoyancy of continental crust opposes its subduction to mantle depths, except where mineral reactions substantially increase rock density. Sluggish kinetics limit such densification, especially in dry rocks, unless deformation and hydrous fluids intervene. Here we document how hydrous fluids in the subduction channel invaded lower crustal granulites at 50-60 km depth through a dense network of probably seismically induced fractures. We combine analyses of textures and mineral composition with thermodynamic modeling to reconstruct repeated stages of interaction, with pulses of high-pressure (HP) fluid at 650-670°C, rehydrating the initially dry rocks to micaschists. SIMS oxygen isotopic data of quartz indicate fluids of crustal composition. HP growth rims in allanite and zircon show uniform U-Th-Pb ages of ˜65 Ma and indicate that hydration occurred during subduction, at eclogite facies conditions. Based on this case study in the Sesia Zone (Western Italian Alps), we conclude that continental crust, and in particular deep basement fragments, during subduction can behave as substantial fluid sinks, not sources. Density modeling indicates a bifurcation in continental recycling: Chiefly mafic crust, once it is eclogitized to >60%, are prone to end up in a subduction graveyard, such as is tomographically evident beneath the Alps at ˜550 km depth. By contrast, dominantly felsic HP fragments and mafic granulites remain positively buoyant and tend be incorporated into an orogen and be exhumed with it. Felsic and intermediate lithotypes remain positively buoyant even where deformation and fluid percolation allowed them to equilibrate at HP.

Relationship Between Subduction Erosion, Seamount Subduction, Fluid Venting and Mound Formation on the Slope of the Costa Rican Continental Margin

NASA Astrophysics Data System (ADS)

Petersen, C.; Klaucke, I.; Weinrebe, W.

2006-12-01

The oceanic crust off central Costa Rica northwest of the Cocos Ridge is dominated by chains of seamounts rising 1-2 km above the seafloor with diameters of up to 20 km. The subduction of these seamounts leads to strong indentations, scars and slides on the continental margin. A smoother segment of about 80 km width is located offshore Nicoya peninsula. The segment ends at a fracture zone which marks the transition of oceanic crust created at the Cocos-Nazca spreading center (CNS) and at the East Pacific Rise (EPR). Offshore Nicaragua the incoming EPR crust is dominated by bending related faults. To investigate the relationship between subduction erosion, fluid venting and mound formation, multibeam bathymetry and high-resolution deep-tow sidescan sonar and sediment echosounder data were acquired during R/V Sonne cruises SO163 and SO173 (2002/2003). The deep-tow system consisted of a dual-frequency 75/410 kHz sidescan sonar and a 2-12 kHz chirp sub-bottom profiler. The connection of the observed seafloor features to deeper subduction related processes is obtained by analysis of multi-channel streamer (MCS) data acquired during cruises SO81 (1992) and BGR99 (1999). Data examples and interpretations for different settings along the margin are presented. Near the Fisher seamount the large Nicoya slump failed over the flank of a huge subducted seamount. The sidescan and echosounder data permit a detailed characterization of fault patterns and fluid escape structures around the headwall of the slump. Where the fracture zone separating CNS and EPR crust subducts, the Hongo mound field was mapped in detail. Several mounds of up to 100 m height are located in line with a scar possibly created by a subducting ridge of the fracture zone. MCS data image a topographic high on the subducting oceanic crust beneath the mound field which lead to uplift and possibly enabled ascent of fluids from the subducting plate. The combined analysis of geoacoustic and seismic MCS data

Experimental constraints on the fate of subducted upper continental crust beyond the "depth of no return"

NASA Astrophysics Data System (ADS)

Zhang, Y.; Wu, Y.; WANG, C.; Jin, Z.

2015-12-01

Large-scale oceanic/continental subduction introduces a range of crustal materials into the Earth's mantle. These subducted material will be gravitationally trapped in the deep mantle when they have been transported to a depth of greater than ~250-300 km ("depth of no return"). However, little is known about the fate of these trapped continental material. Here, we conduct experimental study on a natural continental rock which compositionally similar to the average upper continental crust (UCC) over a pressure and temperature range of 9-16 GPa and 1300-1800 oC to constraint the fate of these trapped continental materials. The experimental results demonstrate that subducted UCC produces ~20-30 wt% K-rich melt (>55 wt% SiO2) in the upper mantle (9-13 GPa). The melting residue is mainly composed of coesite/stishovite + clinopyroxene + kyanite. In contrast, partial melting of subducted UCC in the MTZ produces ~10 wt% K-rich melt (<50 wt% SiO2), together with stishovite, clinopyroxene, K-Hollandite, garnet and CAS-phase as the residue phases. The melting residue phases achieve densities greater than the surrounding mantle, which provides a driving force for descending across the 410 km seismic discontinuity into the MTZ. However, this density relationship is reversed at the base of MTZ, leaving the descended residues being accumulated above the 660 km seismic discontinuity and may contribute to the stagnated "second continent". On the other hand, the melt is ~0.3-0.7 g/cm3 less dense than the surrounding mantle and provides a buoyancy force for the ascending of melt to shallow depth. The ascending melt preserves a significant portion of the bulk-rock REEs and LILEs. Thus, chemical reaction between the melt and the surrounding mantle would leads to a variably metasomatised mantle. Re-melting of the metasomatised mantle may contribute to the origin of the "enriched mantle sources" (EM-sources). Therefore, through subduction, stagnation, partial melting and melt segregation

Continental basalts record the crust-mantle interaction in oceanic subduction channel: A geochemical case study from eastern China

NASA Astrophysics Data System (ADS)

Xu, Zheng; Zheng, Yong-Fei

2017-09-01

Continental basalts, erupted in either flood or rift mode, usually show oceanic island basalts (OIB)-like geochemical compositions. Although their depletion in Sr-Nd isotope compositions is normally ascribed to contributions from the asthenospheric mantle, their enrichment in large ion lithophile elements (LILE) and light rare earth elements (LREE) is generally associated with variable enrichments in the Sr-Nd isotope compositions. This indicates significant contributions from crustal components such as igneous oceanic crust, lower continental crust and seafloor sediment. Nevertheless, these crustal components were not incorporated into the mantle sources of continental basalts in the form of solidus rocks. Instead they were processed into metasomatic agents through low-degree partial melting in order to have the geochemical fractionation of the largest extent to achieve the enrichment of LILE and LREE in the metasomatic agents. Therefore, the mantle sources of continental basalts were generated by metasomatic reaction of the depleted mid-ocean ridge basalts (MORB) mantle with hydrous felsic melts. Nevertheless, mass balance considerations indicate differential contributions from the mantle and crustal components to the basalts. While the depleted MORB mantle predominates the budget of major elements, the crustal components predominate the budget of melt-mobile incompatible trace elements and their pertinent radiogenic isotopes. These considerations are verified by model calculations that are composed of four steps in an ancient oceanic subduction channel: (1) dehydration of the subducting crustal rocks at subarc depths, (2) anataxis of the dehydrated rocks at postarc depths, (3) metasomatic reaction of the depleted MORB mantle peridotite with the felsic melts to generate ultramafic metasomatites in the lower part of the mantle wedge, and (4) partial melting of the metasomatites for basaltic magmatism. The composition of metasomatites is quantitatively dictated by

Crustal growth in subduction zones

NASA Astrophysics Data System (ADS)

Vogt, Katharina; Castro, Antonio; Gerya, Taras

2015-04-01

There is a broad interest in understanding the physical principles leading to arc magmatisim at active continental margins and different mechanisms have been proposed to account for the composition and evolution of the continental crust. It is widely accepted that water released from the subducting plate lowers the melting temperature of the overlying mantle allowing for "flux melting" of the hydrated mantle. However, relamination of subducted crustal material to the base of the continental crust has been recently suggested to account for the growth and composition of the continental crust. We use petrological-thermo-mechanical models of active subduction zones to demonstrate that subduction of crustal material to sublithospheric depth may result in the formation of a tectonic rock mélange composed of basalt, sediment and hydrated /serpentinized mantle. This rock mélange may evolve into a partially molten diapir at asthenospheric depth and rise through the mantle because of its intrinsic buoyancy prior to emplacement at crustal levels (relamination). This process can be episodic and long-lived, forming successive diapirs that represent multiple magma pulses. Recent laboratory experiments of Castro et al. (2013) have demonstrated that reactions between these crustal components (i.e. basalt and sediment) produce andesitic melt typical for rocks of the continental crust. However, melt derived from a composite diapir will inherit the geochemical characteristics of its source and show distinct temporal variations of radiogenic isotopes based on the proportions of basalt and sediment in the source (Vogt et al., 2013). Hence, partial melting of a composite diapir is expected to produce melt with a constant major element composition, but substantial changes in terms of radiogenic isotopes. However, crustal growth at active continental margins may also involve accretionary processes by which new material is added to the continental crust. Oceanic plateaus and other

Experimental constraints on the fate of subducted upper continental crust beyond the "depth of no return"

NASA Astrophysics Data System (ADS)

Zhang, Yanfei; Wu, Yao; Wang, Chao; Zhu, Lüyun; Jin, Zhenmin

2016-08-01

The subducted continental crust material will be gravitationally trapped in the deep mantle after having been transported to depths of greater than ∼250-300 km (the "depth of no return"). However, little is known about the status of this trapped continental material as well as its contribution to the mantle heterogeneity after achieving thermal equilibrium with the surrounding mantle. Here, we conduct an experimental study over pressure and temperature ranges of 9-16 GPa and 1300-1800 °C to constrain the fate of these trapped upper continental crust (UCC). The experimental results show that partial melting will occur in the subducted UCC along normal mantle geotherm to produce K-rich melt. The residual phases composed of coesite/stishovite + clinopyroxene + kyanite in the upper mantle, and stishovite + clinopyroxene + K-hollandite + garnet + CAS-phase in the mantle transition zone (MTZ), respectively. The residual phases achieve densities greater than the surrounding mantle, which provides a driving force for descent across the 410-km seismic discontinuity into the MTZ. However, this density relationship is reversed at the base of the MTZ, leaving the descended residues to be accumulated above the 660-km seismic discontinuity and may contribute to the "second continent". The melt is ∼0.6-0.7 g/cm3 less dense than the surrounding mantle, which provides a buoyancy force for ascent of melt to shallow depths. The ascending melt, which preserves a significant portion of the bulk-rock rare earth elements (REEs), large ion lithophile elements (LILEs), and high-filed strength elements (HFSEs), may react with the surrounding mantle. Re-melting of the metasomatized mantle may contribute to the origin of the "enriched mantle sources" (EM-sources). Therefore, the deep subducted continental crust may create geochemical/geophysical heterogeneity in Earth's interior through subduction, stagnation, partial melting and melt segregation.

The initial superposition of oceanic and continental units in the southern Western Alps: constraints on geometrical restoration and kinematics of the continental subduction wedge

NASA Astrophysics Data System (ADS)

Dumont, Thierry; Schwartz, Stéphane; Matthews, Steve; Malusa, Marco; Jouvent, Marine

2017-04-01

older in the oceanic rocks (Malusà et al. 2015). Finally, further SE, the Voltri massif shows a huge volume of serpentinized mantle which locally overlies continental basement (strongly metamorphosed), and is interpreted as an exhumed remnant of the subduction channel (Federico et al., 2007). In all these localities the transport directions during initial pulses of stacking were consistently oriented generally towards the NW to N, taking into account the subsequent Oligocene and younger collision-related deformation (complex folds, thrusts, backfolds and backthrusts, and block-rotations). It is thus possible to attempt reconstructing an early stage continental subduction wedge involving these different elements from the subduction channel to the most frontal part of the accretionary complex. However, this early Alpine orogen which was active throughout the Eocene is interpreted to have propagated generally towards the NW to N, prior to subsequent pulses of more westerly directed deformation from the Oligocene onwards within the southern part of the Western Alps arc. It is therefore essential to continually improve high-resolution 3D geophysical imaging to facilitate a better understanding of the complex western termination of the Alpine orogen. References: Dumont T., Schwartz S., Guillot S., Simon-Labric S., Tricart P. & Jourdan S. (2012), Structural and sedimentary record of the Oligocene revolution in the Western Alpine arc. Jour. Geodynamics, doi:10.1016/j.jog.2011.11.006 Federico L., Crispini L., Scambelluri M. & Capponi G. (2007), Ophiolite mélange zone records exhumation in a fossil subduction channel. Geology, 35, p. 499-502 Malusà M.G., Faccenna C., Baldwin S.L., Fitzgerald P.G., Rossetti F., Balestrieri M.L., Danišík M., Ellero A., Ottria G. & Piromallo C. (2015), Contrasting styles of (U)HP rock exhumation along the Cenozoic Adria-Europe plate boundary (Western Alps, Calabria, Corsica). Geochem. Geophys. Geosyst. ,16, p. 1786-1824 Tricart P. & Schwartz S

Modeling Archean Subduction Initiation from Continental Spreading with a Free-Surface

NASA Astrophysics Data System (ADS)

Adams, A.; Thielmann, M.; Golabek, G.

2017-12-01

Earth is the only planet known to have plate tectonics, however the onset of plate tectonics and Earth's early tectonic environment are highly uncertain. Modern plate tectonics are characterized by the sinking of dense lithosphere at subduction zones; however this process may not have been feasible if Earth's interior was hotter in the Archean, resulting in thicker and more buoyant oceanic lithosphere than observed at present [van Hunen and van den Berg, 2008]. Previous studies have proposed gravitational spreading of early continents at passive margins as a mechanism to trigger early episodes of plate subduction using numerical simulations with a free-slip upper boundary condition [Rey et al., 2014]. This study utilizes 2D thermo-mechanical numerical experiments using the finite element code MVEP2 [Kaus, 2010; Thielmann et al., 2014] to investigate the viability of this mechanism for subduction initiation in an Archean mantle for both free-slip and free-surface models. Radiogenic heating, strain weakening, and eclogitization were systematically implemented to determine critical factors for modeling subduction initiation. In free-slip models, results show episodes of continent spreading and subduction initiation of oceanic lithosphere for low limiting yield stresses (100-150 MPa) and increasing continent width with no dependency on radiogenic heating, strain weakening, or eclogitization. For models with a free-surface, subduction initiation was observed at low limiting yield stresses (100-225 MPa) with increasing continent width and only in models with eclogitization. Initial lithospheric stress states were studied as a function of density and viscosity ratios between continent and oceanic lithosphere, and results indicate the magnitude of lithospheric stresses increases with increasing continental buoyancy. This work suggests continent spreading may trigger episodes of subduction in models with a free-surface with critical factors being low limiting yield stresses

The influence of tectonic inheritance on crustal extension style following failed subduction of continental crust: applications to metamorphic core complexes in Papua New Guinea

NASA Astrophysics Data System (ADS)

Biemiller, J.; Ellis, S. M.; Little, T.; Mizera, M.; Wallace, L. M.; Lavier, L.

2017-12-01

The structural, mechanical and geometric evolution of rifted continental crust depends on the lithospheric conditions in the region prior to the onset of extension. In areas where tectonic activity preceded rift initiation, structural and physical properties of the previous tectonic regime may be inherited by the rift and influence its development. Many continental rifts form and exhume metamorphic core complexes (MCCs), coherent exposures of deep crustal rocks which typically surface as arched or domed structures. MCCs are exhumed in regions where the faulted upper crust is displaced laterally from upwelling ductile material along a weak detachment fault. Some MCCs form during extensional inversion of a subduction thrust following failed subduction of continental crust, but the degree to which lithospheric conditions inherited from the preceding subduction phase control the extensional style in these systems remains unclear. For example, the Dayman Dome in Southeastern Papua New Guinea exposes prehnite-pumpellyite to greenschist facies rocks in a smooth 3 km-high dome exhumed with at least 24 km of slip along one main detachment normal fault, the Mai'iu Fault, which dips 21° at the surface. The extension driving this exhumation is associated with the cessation of northward subduction of Australian continental crust beneath the oceanic lithosphere of the Woodlark Plate. We use geodynamic models to explore the effect of pre-existing crustal structures inherited from the preceding subduction phase on the style of rifting. We show that different geometries and strengths of inherited subduction shear zones predict three distinct modes of subsequent rift development: 1) symmetric rifting by newly formed high-angle normal faults; 2) asymmetric rifting along a weak low-angle detachment fault extending from the surface to the brittle-ductile transition; and 3) extension along a rolling-hinge structure which exhumes deep crustal rocks in coherent rounded exposures. We

Pliocene granodioritic knoll with continental crust affinities discovered in the intra-oceanic Izu-Bonin-Mariana Arc: Syntectonic granitic crust formation during back-arc rifting

NASA Astrophysics Data System (ADS)

Tani, Kenichiro; Dunkley, Daniel J.; Chang, Qing; Nichols, Alexander R. L.; Shukuno, Hiroshi; Hirahara, Yuka; Ishizuka, Osamu; Arima, Makoto; Tatsumi, Yoshiyuki

2015-08-01

A widely held hypothesis is that modern continental crust of an intermediate (i.e. andesitic) bulk composition forms at intra-oceanic arcs through subduction zone magmatism. However, there is a critical paradox in this hypothesis: to date, the dominant granitic rocks discovered in these arcs are tonalite, rocks that are significantly depleted in incompatible (i.e. magma-preferred) elements and do not geochemically and petrographically represent those of the continents. Here we describe the discovery of a submarine knoll, the Daisan-West Sumisu Knoll, situated in the rear-arc region of the intra-oceanic Izu-Bonin-Mariana Arc. Remotely-operated vehicle surveys reveal that this knoll is made up entirely of a 2.6 million year old porphyritic to equigranular granodiorite intrusion with a geochemical signature typical of continental crust. We present a model of granodiorite magma formation that involves partial remelting of enriched mafic rear-arc crust during the initial phase of back-arc rifting, which is supported by the preservation of relic cores inherited from initial rear-arc source rocks within magmatic zircon crystals. The strong extensional tectonic regime at the time of intrusion may have allowed the granodioritic magma to be emplaced at an extremely shallow level, with later erosion of sediment and volcanic covers exposing the internal plutonic body. These findings suggest that rear-arc regions could be the potential sites of continental crust formation in intra-oceanic convergent margins.

Formation and metasomatism of continental lithospheric mantle in intra-plate and subduction-related tectonic settings

NASA Astrophysics Data System (ADS)

Ionov, Dmitri

2010-05-01

, major and trace element and isotope compositions of fertile lherzolites and thus cannot provide viable alternatives to the concept of melt extraction from pristine mantle as the major mechanism of CLM formation. Published data on xenoliths from andesitic volcanoes and on supra-subduction oceanic peridotites [4] show that the most common rocks in mantle wedge lithosphere are highly refractory harzburgites characterized by a combination of variable but generally high modal opx (18-30%) with very low modal cpx (1.5-3%). At a given olivine (or MgO) content, they have higher opx and silica, and lower cpx, Al and Ca contents than normal refractory peridotite xenoliths in continental basalts; the Mg-Si and Al-Si trends in those rocks resemble those in cratonic peridotites. These features may indicate either fluid fluxing during melting in the mantle wedge or selective post-melting metasomatic enrichments in silica to transform some olivine to opx. High oxygen fugacities and radiogenic Os-isotope compositions in those rocks may be related to enrichments by slab-derived fluids, but these features are not always coupled with trace element enrichments or patterns commonly attributed to "subduction zone metasomatism" deduced from studies of arc volcanic rocks and experiments. The valuable insights provided by experimental work and xenolith case studies are difficult to apply to many natural peridotite series because late-stage processes commonly overlap the evidence for initial melting. References: [1] Herzberg C., J. Petrol. 45: 2507 (2004). [2] Ionov D. & Sobolev A., GCA 72 (S1): A410 (2008). [3] Ionov D., Contrib. Miner. Petrol. (2007) [4] Ionov D., J. Petrol. doi: 10.1093/petrology/egp090 (2010)

The interplay between subduction and lateral extrusion: A case study for the European Eastern Alps based on analogue models

NASA Astrophysics Data System (ADS)

van Gelder, I. E.; Willingshofer, E.; Sokoutis, D.; Cloetingh, S. A. P. L.

2017-08-01

A series of analogue experiments simulating intra-continental subduction contemporaneous with lateral extrusion of the upper plate are performed to study the interference between these two processes at crustal levels and in the lithospheric mantle. The models demonstrate that intra-continental subduction and coeval lateral extrusion of the upper plate are compatible processes leading to similar deformation structures within the extruding region as compared to the classical setup, lithosphere-scale indentation. Strong coupling across the subduction boundary allows for the transfer of stresses to the upper plate, where strain regimes are characterized by crustal thickening near a confined margin and dominated by lateral displacement of material near a weak lateral confinement. The strain regimes propagate laterally during ongoing convergence creating an area of overlap characterized by transpression. When subduction is oblique to the convergence direction, the upper plate is less deformed and as a consequence the amount of lateral extrusion decreases. In addition, strain is partitioned along the oblique plate boundary resulting in less subduction in expense of right lateral displacement close to the weak lateral confinement. Both oblique and orthogonal subduction models have a strong resemblance to lateral extrusion tectonics of the Eastern Alps (Europe), where subduction of the adjacent Adriatic plate beneath the Eastern Alps is debated. Our results imply that subduction of Adria is a valid mechanisms to induce extrusion-type deformation within the Eastern Alps lithosphere. Furthermore, our findings suggest that the Oligocene to Late Miocene structural evolution of the Eastern Alps reflects a phase of oblique subduction followed by a later stage of orthogonal subduction conform a Miocene shift in the plate motion of Adria. Oblique subduction also provides a viable mechanism to explain the rapid decrease in slab length of the Adriatic plate beneath the Eastern Alps

The Two Subduction Zones of the Southern Caribbean: Lithosphere Tearing and Continental Margin Recycling in the East, Flat Slab Subduction and Laramide-Style Uplifts in the West

NASA Astrophysics Data System (ADS)

Levander, A.; Bezada, M. J.; Niu, F.; Schmitz, M.

2015-12-01

The southern Caribbean plate boundary is a complex strike-slip fault system bounded by oppositely vergent subduction zones, the Antilles subduction zone in the east, and a currently locked Caribbean-South American subduction zone in the west (Bilham and Mencin, 2013). Finite-frequency teleseismic P-wave tomography images both the Atlanic (ATL) and the Caribbean (CAR) plates subducting steeply in opposite directions to transition zone depths under northern South America. Ps receiver functions show a depressed 660 discontinuity and thickened transition zone associated with each subducting plate. In the east the oceanic (ATL) part of the South American (SA) plate subducts westward beneath the CAR, initiating the El Pilar-San Sebastian strike slip system, a subduction-transform edge propagator (STEP) fault (Govers and Wortel, 2005). The point at which the ATL tears away from SA as it descends into the mantle is evidenced by the Paria cluster seismicity at depths of 60-110 km (Russo et al, 1993). Body wave tomography and lithosphere-asthenosphere boundary (LAB) thickness determined from Sp and Ps receiver functions and Rayleigh waves suggest that the descending ATL also viscously removes the bottom third to half of the SA continental margin lithospheric mantle as it descends. This has left thinned continental lithosphere under northern SA in the wake of the eastward migrating Antilles subduction zone. The thinned lithosphere occupies ~70% of the length of the El Pilar-San Sebastian fault system, from ~64oW to ~69oW, and extends inland several hundred kilometers. In northwestern SA the CAR subducts east-southeast at low angle under northern Colombia and western Venezuela. The subducting CAR is at least 200 km wide, extending from northernmost Colombia as far south as the Bucaramanga nest seismicity. The CAR descends steeply under Lake Maracaibo and the Merida Andes. This flat slab is associated with three Neogene basement cored, Laramide-style uplifts: the Santa Marta

Detrital fingerprints of fossil continental-subduction zones (Axial Belt Provenance, European Alps)

NASA Astrophysics Data System (ADS)

Resentini, Alberto; Garzanti, Eduardo; Vezzoli, Giovanni; Andò, Sergio; Malusà, Marco G.; Padoan, Marta; Paparella, Paolo

2010-05-01

Alpine-type collision orogens are generated by attempted subduction of thinned continental margins. Because of complex tectonic structure, orogenic detritus is characterized by a range of detrital signatures, making its recognition an arduous task (Dickinson and Suczek, 1979). Among the various orogenic sub-provenances, Axial Belt Provenance, derived from the erosion of the neometamorphic axial pile, can be regarded as the most typifying signature of collision orogens (Garzanti et al., 2007). In the Austroalpine Cretaceous and Penninic Eocene axial belts of the Alps, we ideally distinguish three structural levels, each characterized by diagnostic detrital fingerprints. The shallow level chiefly consists of offscraped remnant-ocean turbidites and unmetamorphosed continental-margin sediments, and mostly produces lithic to lithoquartzose sedimentaclastic sands yielding very-poor heavy-mineral suites including ultrastable minerals. The intermediate level includes low-grade metasediments and polymetamorphic basements, and sheds lithoquartzose to quartzolithofeldspathic metamorphiclastic sands yielding moderately-rich epidote- amphibole suites with chloritoid or garnet. The deep level contains eclogitic remnants of continent- ocean transitions, and supplies quartzofeldspathic to quartzolithic high-rank metamorphiclastic to lithic ultramaficlastic sands yielding rich to extremely-rich suites dominated by garnet, hornblende, or epidote depending on protoliths (continental vs. oceanic) and pressure/temperature paths followed during exhumation. Although widely overprinted under greenschist-facies or amphibolite-facies conditions, occurrence of ultradense eclogite in source areas is readily revealed by the Heavy Mineral Concentration (HMC) index, which mirrors the average density of source rocks in the absence of hydraulic-sorting effects (Garzanti and Andò 2007). The Metamorphic Index (MI, Garzanti and Vezzoli, 2003) and Hornblende Colour Index (HCI) reflect peak

Why Does Some Subducted Continental Crust Escape Deformation and Transformation?

NASA Astrophysics Data System (ADS)

Garber, J. M.; Kylander-Clark, A. R.; Stearns, M.; Seward, G.; Hacker, B. R.

2016-12-01

Titanite geochemistry and geochronology from the Western Gneiss Region (WGR) of Norway shows that large portions of continental crust were deeply subducted and exhumed without significantly deforming or transforming, indicating heterogeneous mechanical behavior of crust at mantle depths. Titanite is stable in granitic rocks over a significant P-T range, and contains numerous major, minor, and trace elements that record 1) P-T conditions, 2) interactions with other major and accessory phases, and 3) information about the composition of coeval melts and fluids. A large titanite LA-ICP-MS dataset from WGR granitic gneisses and leucosomes yields a record that depends strongly on textural setting: some titanites formed after the decomposition of other phases at mantle depths, but other titanites preserve inherited isotopic dates and chemistry with variable recrystallization at grain rims. Differences in rock textures, as well as changes in volatile abundances (F and OH) among different titanite populations, suggest that the persistence of crustal minerals at mantle depths is coupled with limited prograde fluid infiltration, which restricted the efficacy of metamorphic reactions, likely increased rock strength, and caused heterogeneities in the density of the subducting slab. The observed expanses of untransformed, undeformed granitic crust may have been critical to coupling the continental crust to underlying mantle lithosphere, which prevented significant internal deformation of the slab (e.g,. Young et al., 2007; Butler et al., 2015). Finally, our statistical approach shows the utility of dimensional reduction in geochemical studies: rather than comparing individual element or isotopic abundances or ratios, principal components or discriminant analyses can condense variables and help efficiently distinguish between distinct geologic agents or geochemical reservoirs.

Melting of subducted continental crust: Geochemical evidence from Mesozoic granitoids in the Dabie-Sulu orogenic belt, east-central China

NASA Astrophysics Data System (ADS)

Zhao, Zi-Fu; Liu, Zhi-Bin; Chen, Qi

2017-09-01

Syn-collisional and postcollisional granitoids are common in collisional orogens, and they were primarily produced by partial melting of subducted continental crust. This is exemplified by Mesozoic granitoids from the Dabie-Sulu orogenic belt in east-central China. These granitoids were emplaced in small volumes in the Late Triassic (200-206 Ma) and the Late Jurassic (146-167 Ma) but massively in the Early Cretaceous (111-143 Ma). Nevertheless, all of them exhibit arc-like trace element distribution patterns and are enriched in Sr-Nd-Hf isotope compositions, indicating their origination from the ancient continental crust. They commonly contain relict zircons with Neoproterozoic and Triassic U-Pb ages, respectively, consistent with the protolith and metamorphic ages for ultrahigh-pressure (UHP) metaigneous rocks in the Dabie-Sulu orogenic belt. Some granitoids show low zircon δ18O values, and SIMS in-situ O isotope analysis reveals that the relict zircons with Neoproterozoic and Triassic U-Pb ages also commonly exhibit low δ18O values. Neoproterozoic U-Pb ages and low δ18O values are the two diagnostic features that distinguish the subducted South China Block from the obducted North China Block. Thus, the magma source of these Mesozoic granitoids has a genetic link to the subducted continental crust of the South China Block. On the other hand, these granitoids contain relict zircons with Paleoproterozoic and Archean U-Pb ages, which are present in both the South and North China Blocks. Taken together, the Mesozoic granitoids in the Dabie-Sulu orogenic belt and its hanging wall have their magma sources that are predominated by the continental crust of the South China Block with minor contributions from the continental crust of the North China Block. The Triassic continental collision between the South and North China Blocks brought the continental crust into the thickened orogen, where they underwent the three episodes of partial melting in the Late Triassic, Late

Circum-Pacific accretion of oceanic terranes to continental blocks: accretion of the Early Permian Dun Mountain ophiolite to the E Gondwana continental margin, South Island, New Zealand

NASA Astrophysics Data System (ADS)

Robertson, Alastair

2016-04-01

Accretionary orogens, in part, grow as a result of the accretion of oceanic terranes to pre-existing continental blocks, as in the circum-Pacific and central Asian regions. However, the accretionary processes involved remain poorly understood. Here, we consider settings in which oceanic crust formed in a supra-subduction zone setting and later accreted to continental terranes (some, themselves of accretionary origin). Good examples include some Late Cretaceous ophiolites in SE Turkey, the Jurassic Coast Range ophiolite, W USA and the Early Permian Dun Mountain ophiolite of South Island, New Zealand. In the last two cases, the ophiolites are depositionally overlain by coarse clastic sedimentary rocks (e.g. Permian Upukerora Formation of South Island, NZ) that then pass upwards into very thick continental margin fore-arc basin sequences (Great Valley sequence, California; Matai sequence, South Island, NZ). Field observations, together with petrographical and geochemical studies in South Island, NZ, summarised here, provide evidence of terrane accretion processes. In a proposed tectonic model, the Early Permian Dun Mountain ophiolite was created by supra-subduction zone spreading above a W-dipping subduction zone (comparable to the present-day Izu-Bonin arc and fore arc, W Pacific). The SSZ oceanic crust in the New Zealand example is inferred to have included an intra-oceanic magmatic arc, which is no longer exposed (other than within a melange unit in Southland), but which is documented by petrographic and geochemical evidence. An additional subduction zone is likely to have dipped westwards beneath the E Gondwana margin during the Permian. As a result, relatively buoyant Early Permian supra-subduction zone oceanic crust was able to dock with the E Gondwana continental margin, terminating intra-oceanic subduction (although the exact timing is debatable). The amalgamation ('soft collision') was accompanied by crustal extension of the newly accreted oceanic slab, and

Update on GPS-Acoustics Measurements on the Continental Slope of the Cascadia Subduction Zone

NASA Astrophysics Data System (ADS)

Chadwell, C. D.

2017-12-01

Land-based GPS measurements suggest the megathrust is locked offshore along the Cascadia Subduction Zone. However, land-based data alone lack geometric resolution to constrain the how the slip is distributed. GPS-Acoustic measurements can provide these constraints, but using traditional GPS-Acoustic approaches employing a ship is costly. Wave Gliders, a wave- and solar-powered, remotely-piloted sea surface platform, provide a low cost method for collecting GPS-A data. We have adapted GPS-Acoustic technology to the Wave Glider and in 2016 began annual measurements at three sites in the Cascadia Subduction Zone (CSZ). Here, we review positioning results collected during summer 2017 at two sites on the continental slope of the Cascadia Subduction Zone: One site is approximately 45 NM offshore central Oregon and the other approximately 50 NM offshore central Washington State. A third site is approximately 90 NM offshore central Oregon on the incoming Juan de Fuca plate. We will report on initial results of the GPS-A data collection and operational experiences of the missions in 2016 and 2017. Wave Glider based GPS-A measurement have the potential to significantly increase the number and frequency of measurements of strain accumulation in Cascadia Subduction Zone and elsewhere.

Geological constraints on continental arc activity since 720 Ma: implications for the link between long-term climate variability and episodicity of continental arcs

NASA Astrophysics Data System (ADS)

Cao, W.; Lee, C. T.

2016-12-01

Continental arc volcanoes have been suggested to release more CO2 than island arc volcanoes due to decarbonation of wallrock carbonates in the continental upper plate through which the magmas traverse (Lee et al., 2013). Continental arcs may thus play an important role in long-term climate. To test this hypothesis, we compiled geological maps to reconstruct the surface distribution of granitoid plutons and the lengths of ancient continental arcs. These results were then compiled into a GIS framework and incorporated into GPlates plate reconstructions. Our results show an episodic nature of global continental arc activity since 720 Ma. The lengths of continental arcs were at minimums during most of the Cryogenian ( 720-670 Ma), the middle Paleozoic ( 460-300 Ma) and the Cenozoic ( 50-0 Ma). Arc lengths were highest during the Ediacaran ( 640-570 Ma), the early Paleozoic ( 550-430 Ma) and the entire Mesozoic with peaks in the Early Triassic ( 250-240 Ma), Late Jurassic-Early Cretaceous ( 160-130 Ma), and Late Cretaceous ( 90-65 Ma). The extensive continental arcs in the Ediacaran and early Paleozoic reflect the Pan-African events and circum-Gondwana subduction during the assembly of the Gondwana supercontinent. The Early Triassic peak is coincident with the final closure of the paleo-Asian oceans and the onset of circum-Pacific subduction associated with the assembly of the Pangea supercontinent. The Jurassic-Cretaceous peaks reflect the extensive continental arcs established in the western Pacific, North and South American Cordillera, coincident with the initial dispersal of the Pangea. Continental arcs are favored during the final assembly and the early-stage dispersal of a supercontinent. Our compilation shows a temporal match between continental arc activity and long-term climate at least since 720 Ma. For example, continental arc activity was reduced during the Cryogenian icehouse event, and enhanced during the Early Paleozoic and Jurassic-Cretaceous greenhouse

On the initiation of subduction

NASA Technical Reports Server (NTRS)

Mueller, Steve; Phillips, Roger J.

1991-01-01

Estimates of shear resistance associated with lithospheric thrusting and convergence represent lower bounds on the force necessary to promote trench formation. Three environments proposed as preferential sites of incipient subduction are investigated: passive continental margins, transform faults/fracture zones, and extinct ridges. None of these are predicted to convert into subduction zones simply by the accumulation of local gravitational stresses. Subduction cannot initiate through the foundering of dense oceanic lithosphere immediately adjacent to passive continental margins. The attempted subduction of buoyant material at a mature trench can result in large compressional forces in both subducting and overriding plates. This is the only tectonic force sufficient to trigger the nucleation of a new subduction zone. The ubiquitous distribution of transform faults and fracture zones, combined with the common proximity of these features to mature subduction complexes, suggests that they may represent the most likely sites of trench formation if they are even marginally weaker than normal oceanic lithosphere.

Bulldozing of Basal Continental Mantle Lithosphere During Flat-Slab Subduction

NASA Astrophysics Data System (ADS)

Axen, G. J.; van Wijk, J.; Currie, C. A.

2017-12-01

Flat-slab subduction occurs along 10% of subduction margins, forming magmatic gaps and causing inland migration of upper-plate deformation. We suggest that basal continental mantle lithosphere (CML) can be bulldozed ahead of the flat portion of horizontally-subducted oceanic lithosphere, forming a growing and advancing keel of thickened CML. This process fills the asthenospheric mantle wedge with CML, precluding melting. The bulldozed CML keel may transmit tectonic stresses ahead of the flat slab itself, causing upper-plate deformation ahead of the slab hinge. We designed 2-D numerical models after the North American Laramide orogeny, with subduction of a thick, buoyant oceanic plateau (conjugate Shatsky Rise) and with the continent advancing trenchward over the initial slab hinge. This results in slab-flattening, and removal of CML material. In our models, the thickness of the CML layer removed by this process depends on overriding plate rheology and is up to 25 km. The removed material is bulldozed ahead of the hinge and may fill up the asthenospheric wedge. Low-density (depleted) CML favors formation of bulldozed keels, which increase in width as CML strength decreases. Regular-density and/or stronger CML forms smaller bulldozed keels that are more likely to sink with the slab as eclogitization and densification proceed. When the flat slab rolls back, it leaves a step in the CML at the farthest extent of the slab. Relics of this step may remain below North America or may have dripped off. We interpret an upper-mantle fast-velocity anomaly below SE New Mexico and W Texas as a drip/keel, and the step in lithosphere thickness in southwestern Colorado as a fossil step, caused by the removal of the CML layer. Our model predicts that the Laramide bulldozed CML keel may have aided in stress transmission that caused basement uplifts as far as NE Wyoming and subsurface folds even farther N and E. Modern examples may exist in South American flat slab segments.

Late Cretaceous-Early Eocene Climate Change Linked to Tectonic Eevolution of Neo-Tethyan Subduction Systems

NASA Astrophysics Data System (ADS)

Jagoutz, O. E.; Royden, L.; Macdonald, F. A.

2015-12-01

In this presentation we demonstrate that the two tectonic events in the late Cretaceous-Early Tertiary triggered the two distinct cooling events that followed the Cretaceous Thermal Maximum (CTM). During much of the Cretaceous time, the northern Neo Tethyan ocean was dominated by two east-west striking subduction system. Subduction underneath Eurasia formed a continental arc on the southern margin of Eurasia and intra oceanic subduction in the equatorial region of the Neo Tethys formed and intra oceanic arc. Beginning at ~85-90 Ma the western part of the TTSS collided southward with the Afro-Arabian continental margin, terminating subduction. This resulted in southward obduction of the peri-Arabian ophiolite belt, which extends for ~4000 km along strike and includes the Cypus, Semail and Zagros ophiolites. At the same time also the eastern part of the TTS collided northwards wit Eurasia. After this collisional event, only the central part of the subduction system remained active until it collided with the northern margin of the Indian continent at ~50-55 Ma. The collision of the arc with the Indian margin, over a length of ~3000 km, also resulted in the obduction of arc material and ophiolitic rocks. Remnants of these rocks are preserved today as the Kohistan-Ladakh arc and ophiolites of the Indus-Tsangpo suture zone of the Himalayas. Both of these collision events occurred in the equatorial region, near or within the ITCZ, where chemical weathering rates are high and are contemporaneous with the onset of the global cooling events that mark the end of the CTM and the EECO. The tectonic collision events resulted in a shut down of subduction zone magmatism, a major CO2 source and emplacement of highly weatherable basaltic rocks within the ITCZ (CO2 sink). In order to explore the effect of the events in the TTSS on atmospheric CO2, we model the potential contribution of subduction zone volcanism (source) and ophiolite obduction (sink) to the global atmospheric CO2

Reconstruction of Northeast Asian Deformation Integrated with Western Pacific Plate Subduction since 200 Ma

NASA Astrophysics Data System (ADS)

Liu, S.; Gurnis, M.; Ma, P.; Zhang, B.

2017-12-01

The configuration and kinematics of continental deformation and its marginal plate tectonics on the Earth's surface are intrinsic manifestations of plate-mantle coupling. The complex interactions of plate boundary forces result in plate motions that are dominated by slab pull and ridge push forces and the effects of mantle drag; these interactions also result in continental deformation with a complex basin-mountain architecture and evolution. The kinematics and evolution of the western Pacific subduction and northeast Asian continental-margin deformation are a first-order tectonic process whose nature and chronology remains controversial. This paper implements a "deep-time" reconstruction of the western Pacific subduction, continental accretion or collision and basin-mountain deformation in northeast Asia since 200 Ma based on a newly revised global plate model. The results demonstrate a NW-SE-oriented shortening from 200-137 Ma, a NWW-SEE-oriented extension from 136-101 Ma, a nearly N-S-oriented extension and uplift with a short-term NWW-SEE-oriented compressional inversion in northeast China from 100-67 Ma, and a NW-SE- and nearly N-S-oriented extension from 66 Ma to the present day. The western Pacific oceanic plate subducted forward under East Asia along Mudanjiang-Honshu Island during the Jurassic, and the trenches retreated to the Sikhote-Alin, North Shimanto, and South Shimanto zones from ca. 137-128 Ma, ca. 130-90 Ma, and in ca. 60 Ma, respectively. Our time-dependent analysis of plate motion and continental deformation coupling suggests that the multi-plate convergent motion and ocean-continent convergent orogeny were induced by advance subduction during the Jurassic and earliest Cretaceous. Our analysis also indicates that the intra-continent rifting and back-arc extension were triggered by trench retreat during the Cretaceous and that the subduction of oceanic ridge and arc were triggered by trench retreat during the Cenozoic. Therefore, reconstructing

High Frequency Cut-off Characteristics of Strong Ground Motion Records at Hard Sites, Subduction and Intra-Slab Earthquakes

NASA Astrophysics Data System (ADS)

Kagawa, T.; Tsurugi, M.; Irikura, K.

2006-12-01

A study on high frequency cut-off characteristics of strong ground motion is presented for subduction and intra- slab earthquakes in Japan. In the latest decade, observed records at hard sites are published by NIED, National Research Institute for Earth Science and Disaster Prevention, and JCOLD, Japan Commission on Large Dams. Especially, KiK-net and K-NET maintained by NIED have been providing high quality data to study high-frequency characteristics. Kagawa et al.(2003) studied the characteristics for crustal earthquakes. We apply the same methodology to the recently observed Japanese records due to subduction and intra-slab earthquakes. We assume a Butterworth type high-cut filter with limit frequency (fmax) and its power factor. These two parameters were derived from Fourier spectrum of observed records fitting the theoretical filter shape. After analyzing the result from view points of site, path, or source effects, an averaged filter model is proposed with its standard deviation. Kagawa et al.(2003) derived average as 8.3 Hz with power factor of 1.92. It is used for strong ground motion simulation. We will propose parameters for the high-cut filters of subduction and intra-slab earthquakes and compare them with the results by Kagawa et al.(2003). REFERENCES: Kagawa et al. (2003), 27JEES (in Japanese with English Abstract).

Dynamics of subduction, accretion, exhumation and slab roll-back: Mediterranean scenarios

NASA Astrophysics Data System (ADS)

Tirel, C.; Brun, J.; Burov, E. B.; Wortel, M. J.; Lebedev, S.

2010-12-01

A dynamic orogen reveals various tectonic processes brought about by subduction: accretion of oceanic and continental crust, exhumation of UHP-HP rocks, and often, back-arc extension. In the Mediterranean, orogeny is strongly affected by slab retreat, as in the Aegean and Tyrrhenian Seas. In order to examine the different dynamic processes in a self-consistent manner, we perform a parametric study using the fully coupled thermo-mechanical numerical code PARAFLAM. The experiments reproduce a subduction zone in a slab pull mode, with accretion of one (the Tyrrhenian case) and two continental blocks (the Aegean case) that undergo, in sequence, thrusting, burial and exhumation. The modeling shows that despite differences in structure between the two cases, the deformation mechanisms are fundamentally similar and can be described as follows. The accretion of a continental block at the trench beneath the suture zone begins with its burial to UHP-HP conditions and thrusting. Then the continental block is delaminated from its subducting lithosphere. During the subduction-accretion process, the angle of the subducting slab increases due to the buoyancy of the continental block. When the oceanic subduction resumes, the angle of the slab decreases to reach a steady-state position. The Aegean and Tyrrhenian scenarios diverge at this stage, due naturally to the differences of their accretion history. When continental accretion is followed by oceanic subduction only, the continental block that has been accreted and detached stays at close to the trench and does not undergo further deformation, despite the continuing rollback. The extensional deformation is located further within the overriding plate, resulting in continental breakup and the development of an oceanic basin, as in the Tyrrhenian domain. When the continental accretion is followed first by oceanic subduction and then by accretion of another continental block, however, the evolution of the subduction zone is

Introduction to the structures and processes of subduction zones

NASA Astrophysics Data System (ADS)

Zheng, Yong-Fei; Zhao, Zi-Fu

2017-09-01

Subduction zones have been the focus of many studies since the advent of plate tectonics in 1960s. Workings within subduction zones beneath volcanic arcs have been of particular interest because they prime the source of arc magmas. The results from magmatic products have been used to decipher the structures and processes of subduction zones. In doing so, many progresses have been made on modern oceanic subduction zones, but less progresses on ancient oceanic subduction zones. On the other hand, continental subduction zones have been studied since findings of coesite in metamorphic rocks of supracrustal origin in 1980s. It turns out that high-pressure to ultrahigh-pressure metamorphic rocks in collisional orogens provide a direct target to investigate the tectonism of subduction zones, whereas oceanic and continental arc volcanic rocks in accretionary orogens provide an indirect target to investigate the geochemistry of subduction zones. Nevertheless, metamorphic dehydration and partial melting at high-pressure to ultrahigh-pressure conditions are tectonically applicable to subduction zone processes at forearc to subarc depths, and crustal metasomatism is the physicochemical mechanism for geochemical transfer from the slab to the mantle in subduction channels. Taken together, these provide us with an excellent opportunity to find how the metamorphic, metasomatic and magmatic products are a function of the structures and processes in both oceanic and continental subduction zones. Because of the change in the thermal structures of subduction zones, different styles of metamorphism, metasomatism and magmatism are produced at convergent plate margins. In addition, juvenile and ancient crustal rocks have often suffered reworking in episodes independent of either accretionary or collisional orogeny, leading to continental rifting metamorphism and thus rifting orogeny for mountain building in intracontinental settings. This brings complexity to distinguish the syn-subduction

Introduction to the structures and processes of subduction zones

NASA Astrophysics Data System (ADS)

Zheng, Yong-Fei; Zhao, Zi-Fu

2017-09-01

Subduction zones have been the focus of many studies since the advent of plate tectonics in 1960s. Workings within subduction zones beneath volcanic arcs have been of particular interest because they prime the source of arc magmas. The results from magmatic products have been used to decipher the structures and processes of subduction zones. In doing so, many progresses have been made on modern oceanic subduction zones, but less progresses on ancient oceanic subduction zones. On the other hand, continental subduction zones have been studied since findings of coesite in metamorphic rocks of supracrustal origin in 1980s. It turns out that high-pressure to ultrahigh-pressure metamorphic rocks in collisional orogens provide a direct target to investigate the tectonism of subduction zones, whereas oceanic and continental arc volcanic rocks in accretionary orogens provide an indirect target to investigate the geochemistry of subduction zones. Nevertheless, metamorphic dehydration and partial melting at high-pressure to ultrahigh-pressure conditions are tectonically applicable to subduction zone processes at forearc to subarc depths, and crustal metasomatism is the physicochemical mechanism for geochemical transfer from the slab to the mantle in subduction channels. Taken together, these provide us with an excellent opportunity to find how the metamorphic, metasomatic and magmatic products are a function of the structures and processes in both oceanic and continental subduction zones. Because of the change in the thermal structures of subduction zones, different styles of metamorphism, metasomatism and magmatism are produced at convergent plate margins. In addition, juvenile and ancient crustal rocks have often suffered reworking in episodes independent of either accretionary or collisional orogeny, leading to continental rifting metamorphism and thus rifting orogeny for mountain building in intracontinental settings. This brings complexity to distinguish the syn-subduction

Processes in continental collision zones: Preface

NASA Astrophysics Data System (ADS)

Zheng, Yong-Fei; Zhang, Lifei; McClelland, William C.; Cuthbert, Simon

2012-04-01

Formation and exhumation of high-pressure (HP) to ultrahigh-pressure (UHP) metamorphic rocks in continental subduction zones are the two fundamental geodynamic aspects of collisional orogensis. This volume is based on the Session 08c titled "Geochemical processes in continental collision zones" at Goldschmidt 2010 in Knoxville, USA. It focuses on micro- to macro-scale processes that are temporally and spatially linked to different depths of crustal subduction/exhumation and associated mineralogical changes. They are a key to understanding a wide spectrum of phenomena, involving HP/UHP metamorphism and syn-/post-collisional magmatism. Papers in this volume report progresses in petrological, geochronological and geochemical studies of UHP metamorphic rocks and their derivatives in China, with tectonic settings varying from arc-continent collision to continent-continent collision. Microbeam in-situ analyses of metamorphic and magmatic minerals are successfully utilized to solve various problems in the study of continental deep subduction and UHP metamorphism. In addition to their geochronological applications to dating of HP to UHP metamorphic events during continental collision, microbeam techniques have also served as an efficient means to recognize different generations of mineral growth during continental subduction-zone metamorphism. Furthermore, metamorphic dehydration and partial melting of UHP metamorphic rocks during subduction and exhumation are highlighted with respect to their effects on fluid action and element mobilization. These have provided new insights into chemical geodynamics in continental subduction zones.

Initiation of Subduction Zones: A Consequence of Lateral Compositional Buoyancy Contrast Within the Lithosphere

NASA Astrophysics Data System (ADS)

Niu, Y.; O'Hara, M. J.; Pearce, J. A.

2001-12-01

Subduction of oceanic lithosphere into deep mantle is one of the key aspects of plate tectonics. Pull by the subducting-slab due to its negative buoyancy is widely accepted as the major driving force for plate motion and plate tectonics. Hence, there would be no plate tectonics if there were no subduction zones. Yet how a subduction zone initiates remains poorly known. Here we show that lateral compositional (vs. thermal) buoyancy contrast within the lithosphere creates the favored and necessary condition for the initiation of a subduction zone by (1) comparing the compositional and density differences between normal oceanic lithosphere (NOL) represented by abyssal peridotites (AP) and subarc lithosphere (SAL) represented by forearc peridotites (FP), and (2) simple physical analysis. As the gravitational attraction is the principal driving force of the subducting slab, it would be optimal if one part of the lithosphere experiences a greater gravitational attraction than its adjacent neighbor prior to or during the initiation of a subduction. This requires the pre-existence of a density contrast within the lithosphere. If the lithosphere is thermally uniform as is often the case, then the density contrast must result from a compositional contrast. This hypothesis can be tested by examining the lithospheric materials on both sides of a subduction zone. Subduction of a dense NOL beneath a buoyant continental lithosphere is straightforward, but intra-oceanic subduction such as in the western Pacific requires a scrutiny. Our data show that FP of Mariana and Tonga - two of the most important intra-oceanic subduction zones on Earth - are compositionally more depleted than AP: Cr#-sp (mean+/- 1σ ) = 0.584+/-0.084(FP) vs. 0.307+/-0.134(AP); Mg#-ol = 0.915+/-0.006(FP) vs. 0.898+/-0.082(AP); Mg#-opx = 0.917+/-0.006(FP) vs. 0.908+/-0.006(AP); Mg#-cpx = 0.929+/-0.021(FP) vs. 0.917+/-0.011(AP). As a result, SAL is > 0.7% less dense than NOL. This density contrast due to

The Generation of Continents through Subduction Zone Processing of Large Igneous Provinces: A Case Study from the Central American Subduction Zone

NASA Astrophysics Data System (ADS)

Harmon, N.; Rychert, C.

2013-12-01

Billions of years ago primary mantle magmas evolved to form the continental crust, although no simple magmatic differentiation process explains the progression to average andesitic crustal compositions observed today. A multiple stage process is often invoked, involving subduction and or oceanic plumes, to explain the strong depletion observed in Archean xenoliths and as well as pervasive tonalite-trondhjemite-granodiorite and komatiite protoliths in the greenstone belts in the crust in the cratons. Studying modern day analogues of oceanic plateaus that are currently interacting with subductions zones can provide insights into continental crust formation. Here we use surface waves to image crustal isotropic and radially anisotropic shear velocity structure above the central American subduction system in Nicaragua and Costa Rica, which juxtaposes thickened ocean island plateau crust in Costa Rica with continental/normal oceanic crust in Nicaragua. We find low velocities beneath the active arc regions (3-6% slower than the surrounding region) and up to 6% radially anisotropic structures within the oceanic crust of the Caribbean Large Igneous Province beneath Costa Rica. The low velocities and radial anisotropy suggest the anomalies are due to pervasive deep crustal magma sills. The inferred sill structures correlate spatially with increased silicic outputs in northern Costa Rica, indicating that deep differentiation of primary magmas is more efficient beneath Costa Rica relative to Nicaragua. Subduction zone alteration of large igneous provinces promotes efficient, deep processing of primary basalts to continental crust. This scenario can explain the formation of continental lithosphere and crust, by both providing strongly depleted mantle lithosphere and a means for rapidly generating a silicic crustal composition.

Subduction or obduction of continental crust in the northern Norwegian Caledonides? An example from the Nordmannvik Nappe

NASA Astrophysics Data System (ADS)

Faber, Carly; Stünitz, Holger; Jeřábek, Petr; Gasser, Deta; Konopásek, Jiří; Kraus, Katrin

2017-04-01

The debate about how and why continental crust is subducted is ongoing (Ingalls et al., 2016). This work uses the tectonmetamorphic history of a the Nordmannvik nappe in the northern Scandinavian Caledonides to discuss mid- to lower-crustal processes involved in the subduction of continental crust during the Caledonian Orogeny. The Nordmannvik Nappe, together with the underlying Kåfjord and Vaddas nappes, constitutes the Reisa Nappe Complex (RNC). The RNC overlies continental rocks of the Kalak Nappe Complex (KNC), and a clear oceanic suture between Baltican basement, the KNC and the RNC is missing. The RNC consists mainly of paragneisses of mostly unknown depositional age. Rare fossils in the Vaddas Nappe indicate that it at least partly consists of Ordovician-Silurian (>460 Ma) metasediments (Binns and Gayer, 1980). Both the Nordmannvik and Vaddas Nappes were intruded by gabbroic melt around 439 Ma at 9 kbar (c. 30 km) (Getsinger et al., 2013). Therefore, the host and intrusive rocks were already buried to positions far deeper than oceanic crust prior to nappe stacking. Nordmannvik nappe rocks show at least two distinct metamorphic fabrics; 1) an early high-grade kyanite-present migmatitic fabric and 2) a pervasive mylonitic fabric. Based on microstructural observations and pseudosection modeling these two fabrics are estimated to have formed at 770-800 °C and 9.4-11 kbar and 580-630 °C and 8-9.8 kbar, respectively. The presence of sillimanite in garnet cores (confirmed by Raman spectra) and garnet core compositions also suggest that an earlier, less well constrained, history exists with metamorphism around 815 °C and 8.7 kbar, similar to that recognized in the KNC, where it is dated to be pre-Caledonian. The lack of ocean floor rocks between the Nordmannvik Nappe and the Baltica basement suggests that the Nordmannvik Nappe and nappe units below were fairly proximal to Baltica prior to the Caledonian Orogeny. Their position below the Lyngen Nappe (Iapetus

Evolution of a Subduction Zone

NASA Astrophysics Data System (ADS)

Noack, Lena; Van Hoolst, Tim; Dehant, Veronique

2014-05-01

The purpose of this study is to understand how Earth's surface might have evolved with time and to examine in a more general way the initiation and continuance of subduction zones and the possible formation of continents on an Earth-like planet. Plate tectonics and continents seem to influence the likelihood of a planet to harbour life, and both are strongly influenced by the planetary interior (e.g. mantle temperature and rheology) and surface conditions (e.g. stabilizing effect of continents, atmospheric temperature), but may also depend on the biosphere. Employing the Fortran convection code CHIC (developed at the Royal Observatory of Belgium), we simulate a subduction zone with a pre-defined weak zone (between oceanic and continental crust) and a fixed plate velocity for the subducting oceanic plate (Quinquis et al. in preparation). In our study we first investigate the main factors that influence the subduction process. We simulate the subduction of an oceanic plate beneath a continental plate (Noack et al., 2013). The crust is separated into an upper crust and a lower crust. We apply mixed Newtonian/non-Newtonian rheology and vary the parameters that are most likely to influence the subduction of the ocanic plate, as for example density of the crust/mantle, surface temperature, plate velocity and subduction angle. The second part of our study concentrates on the long-term evolution of a subduction zone. Even though we model only the upper mantle (until a depth of 670km), the subducted crust is allowed to flow into the lower mantle, where it is no longer subject to our investigation. This way we can model the subduction zone over long time spans, for which we assume a continuous inflow of the oceanic plate into the investigated domain. We include variations in mantle temperatures (via secular cooling and decay of radioactive heat sources) and dehydration of silicates (leading to stiffening of the material). We investigate how the mantle environment influences

Models of Active Glacial Isostasy Roofing Warm Subduction: Case of the South Patagonian Ice Field

NASA Technical Reports Server (NTRS)

Klemann, Volker; Ivins, Erik R.; Martinec, Zdenek; Wolf, Detlef

2007-01-01

Modern geodetic techniques such as precise Global Positioning System (GPS) and high-resolution space gravity mapping (Gravity Recovery and Climate Experiment, GRACE) make it possible to measure the present-day rate of viscoelastic gravitational Earth response to present and past glacier mass changes. The Andes of Patagonia contain glacial environments of dramatic mass change. These mass load changes occur near a tectonically active boundary between the Antarctic and South American plates. The mechanical strength of the continental side of this boundary is influenced by Neogene ridge subduction and by the subduction of a youthful oceanic slab. A ridge of young volcanos parallels the Pacific coastline. Release of volatiles (such as water) at depth along this ridge creates a unique rheological environment. To assess the influence of this rheological ridge structure on the observational land uplift rate, we apply a two dimensional viscoelastic Earth model. A numerical study is presented which examines the sensitivity of the glacial loading-unloading response to the complex structure at depth related to the subducting slab, the viscous wedge between slab and continental lithosphere, and the increase of elastic thickness from oceanic to continental lithosphere. A key feature revealed by our numerical experiments is a continuum flow wherein the slab subdues the material transport toward oceanic mantle and crust. The restricted flow is sensitive to the details of slab mechanical strength and penetration into the upper mantle. The reduced viscosity within the mantle wedge, however, enhances the load-induced material transport everywhere within the asthenosphere.

Switching deformation mode and mechanisms during subduction of continental crust: a case study from Alpine Corsica

NASA Astrophysics Data System (ADS)

Molli, Giancarlo; Menegon, Luca; Malasoma, Alessandro

2017-04-01

The switching in deformation mode (from distributed to localized) and mechanism (viscous versus frictional) represent a relevant issue in the frame of processes of crustal deformation in turn connected with the concept of the brittle-"ductile" transition and seismogenesis. On the other hand the role of brittle precursors in nucleating crystal-plastic shear zones has received more and more consideration being now recognized as having a fundamental role in the localization of deformation and shear zone development, thus representing a case in which switching deformation mode and mechanisms interact and relate to each other. This contribution analyses an example of a crystal plastic shear zone localized by brittle precursor formed within a host granitic-mylonite during deformation in subduction-related environment. The studied sample come from the external Corsican continental crust units involved in alpine age subduction and characterized by a low grade blueschist facies peak assemblages. The blueschist facies host rock is cut by a thin (< 1 cm thick) brittle-viscous shear zone that preserves domains with a cataclastic microstructure overprinted by mylonitic deformation. Blue amphibole is stable in the shear zone foliation, which therefore formed under HP/LT metamorphic conditions in a subduction environment. Quartz microstructure in the damage zone flanking the brittle-viscous shear zone shows evidence of both microcracking and dislocation glide, with limited recrystallization localized in intracrystalline bands. In the mylonite portion of the shear zone, quartz forms polycrystalline ribbons of dynamically recrystallized grains with a crossed-girdle c-axis CPO. Extrapolation of laboratory-derived flow laws indicates strain rate of ca. 3.5 * 10-12 s-1 during viscous flow in the shear zone. The studied structures, possibly formed by transient instability related to episodic stress/strain rate variations, may be considered as a small scale example of fault behaviour

Mineralogical Evidence for the Bulk Transformation of Continental Crust to Ultrahigh-Pressure Conditions in Subduction Zones

NASA Astrophysics Data System (ADS)

Peterman, E. M.; Hacker, B. R.; Kylander-Clark, A. R.

2005-12-01

Evidence for (ultra)high-pressure --(U)HP-- metamorphism in modern orogenic belts and the preservation of exhumed (U)HP terranes around the world suggest that subduction and exhumation of continental crust plays an important role in Phanerozoic plate tectonics. The Western Gneiss region (WGR) of Norway, a major (U)HP province extending over 60,000 km2, provides an excellent opportunity to study how subduction to depths >100 km affects continental crust. By studying a ~60 km wide transect bounded to the north by Vartdalsfjorden and Rovdefjorden and the south by the Möre og Romsdal county boundary, we are able to examine mineralogical changes that occurred during subduction and exhumation within a rock composed predominantly of orthogneiss and variably transformed mafic bodies, which indicate the depths to which these rocks were subducted. Previous studies (e.g. Hacker et al., 2005) have suggested that Caledonian deformation in WGR host gneisses is primarily limited to brittle-ductile fabrics characterized by greenschist to lower-amphibolite facies metamorphism; the majority of the deformation in the rocks, including the pervasive foliation and foliation-parallel isoclinal folds, occurred between 1200 and 900 Ma. On the northern half of our study area, however, locally occurring neoblastic garnet crosscuts the foliation in the gneiss. The boundary of this garnet zone coincides with the local HP-UHP boundary, as determined by the presence of coesite in eclogite. Because garnet can retain information about changes in pressure and temperature, as well as the availability of water within the crust to catalyze chemical reactions, our findings suggest that 1) portions of the orthogneiss did transform at high pressures, 2) the presence of garnet within the orthogneiss may indicate conditions that approximate UHP and can therefore be useful in defining the boundaries between UHP and HP conditions, and 3) the growth of garnet during (U)HP metamorphism may be controlled by

Compression-extension transition of continental crust in a subduction zone: A parametric numerical modeling study with implications on Mesozoic-Cenozoic tectonic evolution of the Cathaysia Block

PubMed Central

Chan, Lung Sang; Gao, Jian-Feng

2017-01-01

The Cathaysia Block is located in southeastern part of South China, which situates in the west Pacific subduction zone. It is thought to have undergone a compression-extension transition of the continental crust during Mesozoic-Cenozoic during the subduction of Pacific Plate beneath Eurasia-Pacific Plate, resulting in extensive magmatism, extensional basins and reactivation of fault systems. Although some mechanisms such as the trench roll-back have been generally proposed for the compression-extension transition, the timing and progress of the transition under a convergence setting remain ambiguous due to lack of suitable geological records and overprinting by later tectonic events. In this study, a numerical thermo-dynamical program was employed to evaluate how variable slab angles, thermal gradients of the lithospheres and convergence velocities would give rise to the change of crustal stress in a convergent subduction zone. Model results show that higher slab dip angle, lower convergence velocity and higher lithospheric thermal gradient facilitate the subduction process. The modeling results reveal the continental crust stress is dominated by horizontal compression during the early stage of the subduction, which could revert to a horizontal extension in the back-arc region, combing with the roll-back of the subducting slab and development of mantle upwelling. The parameters facilitating the subduction process also favor the compression-extension transition in the upper plate of the subduction zone. Such results corroborate the geology of the Cathaysia Block: the initiation of the extensional regime in the Cathaysia Block occurring was probably triggered by roll-back of the slowly subducting slab. PMID:28182640

Three-Dimensional Numerical Modeling of Crustal Growth at Active Continental Margins

NASA Astrophysics Data System (ADS)

Zhu, G.; Gerya, T.; Tackley, P. J.

2011-12-01

Active margins are important sites of new continental crust formation by magmatic processes related to the subduction of oceanic plates. We investigate these phenomena using a three-dimensional coupled petrological-geochemical-thermomechanical numerical model, which combines a finite-difference flow solver with a non-diffusive marker-in-cell technique for advection (I3ELVIS code, Gerya and Yuen, PEPI,2007). The model includes mantle flow associated with the subducting plate, water release from the slab, fluid propagation that triggers partial melting at the slab surface, melt extraction and the resulting volcanic crustal growth at the surface. The model also accounts for variations in physical properties (mainly density and viscosity) of both fluids and rocks as a function of local conditions in temperature, pressure, deformation, nature of the rocks, and chemical exchanges. Our results show different patterns of crustal growth and surface topography, which are comparable to nature, during subduction at active continental margins. Often, two trench-parallel lines of magmatic activity, which reflect two maxima of melt production atop the slab, are formed on the surface. The melt extraction rate controls the patterns of new crust at different ages. Moving free water reflects the path of fluids, and the velocity of free water shows the trend of two parallel lines of magmatic activity. The formation of new crust in particular time intervals is distributed in finger-like shapes, corresponding to finger-like and ridge-like cold plumes developed atop the subducting slabs (Zhu et al., G-cubed,2009; PEPI,2011). Most of the new crust is basaltic, formed from peridotitic mantle. Granitic crust extracted from melted sediment and upper crust forms in a line closer to the trench, and its distribution reflects the finger-like cold plumes. Dacitic crust extracted from the melted lower crust forms in a line farther away from the trench, and its distribution is anticorrelated with

Kinematics of Late Cretaceous subduction initiation in the Neo-Tethys Ocean reconstructed from ophiolites of Turkey, Cyprus, and Syria

NASA Astrophysics Data System (ADS)

Maffione, Marco; van Hinsbergen, Douwe; de Gelder, Giovanni; van der Goes, Freek; Morris, Antony

2017-04-01

Formation of new subduction zones represents one of the cornerstones of plate tectonics, yet both the kinematics and geodynamics governing this process remain enigmatic. A major subduction initiation event occurred in the Late Cretaceous, within the Neo-Tethys Ocean between Gondwana and Eurasia. Supra-subduction zone (SSZ) ophiolites (i.e., emerged fragments of ancient oceanic lithosphere accreted at supra-subduction spreading centers) were generated during this subduction event, and are today distributed in the eastern Mediterranean region along three E-W trending ophiolitic belts. Current models associate these ophiolite belts to simultaneous initiation of multiple, E-W trending subduction zones at 95 Ma. Here we report paleospreading direction data obtained from paleomagnetic analysis of sheeted dyke sections from seven Neo-Tethyan ophiolites of Turkey, Cyprus, and Syria, demonstrating that these ophiolites formed at NNE-SSW striking ridges parallel to the newly formed subduction zones. This subduction system was step-shaped and composed of NNE-SSW and ESE-WNW segments. The eastern subduction segment invaded the SW Mediterranean, leading to a radial obduction pattern similar to the Banda arc. Emplacement age constraints indicate that this subduction system formed close to the Triassic passive and paleo-transform margins of the Anatolide-Tauride continental block. Because the original Triassic-Jurassic Neo-Tethyan spreading ridge must have already subducted below the Pontides before the Late Cretaceous, we infer that the Late Cretaceous Neo-Tethyan subduction system started within ancient lithosphere, along NNE-SSW oriented fracture zones and faults parallel to the E-W trending passive margins. This challenges current concepts suggesting that subduction initiation occurs along active intra-oceanic plate boundaries.

Vertical slab sinking and westward subduction offshore of Mesozoic North America

NASA Astrophysics Data System (ADS)

Sigloch, Karin; Mihalynuk, Mitchell G.

2013-04-01

hotspot reference frame, with elongate slab walls predicts where and when the intra-oceanic trenches would have been overridden by the westward-moving continent. Land geology plays the role of a validating data set: trench override is predicted to coincide with accretion of buoyant arc terranes, deformation of the continental margin and slab window volcanism. We find excellent agreement between predicted and observed accretion episodes, validating both vertical sinking (within observational uncertainties of a few hundred kilometers laterally), and westward subduction beneath an archipelago of island arcs west of Jura-Cretaceous North America. Amalgamation of the arcs with North America occurred as the intervening ocean crust was consumed. Implied slab sinking rates are of 10±2 mm/a, uniformly for three different slab walls. We conclude that the hypothesis of essentially vertical slab sinking produces a self-consistent model that explains first-order observations of 200 Ma - 50 Ma Cordilleran geology. By contrast, the standard scenario of a continental Farallon trench requires massive amounts of slab to be laterally displaced by 1000+ km after subduction, and offers no explanation for a long series of Cretaceous terrane accretions.

Geodynamic models of terrane accretion: Testing the fate of island arcs, oceanic plateaus, and continental fragments in subduction zones

NASA Astrophysics Data System (ADS)

Tetreault, J. L.; Buiter, S. J. H.

2012-08-01

Crustal growth at convergent margins can occur by the accretion of future allochthonous terranes (FATs), such as island arcs, oceanic plateaus, submarine ridges, and continental fragments. Using geodynamic numerical experiments, we demonstrate how crustal properties of FATs impact the amount of FAT crust that is accreted or subducted, the type of accretionary process, and the style of deformation on the overriding plate. Our results show that (1) accretion of crustal units occurs when there is a weak detachment layer within the FAT, (2) the depth of detachment controls the amount of crust accreted onto the overriding plate, and (3) lithospheric buoyancy does not prevent FAT subduction during constant convergence. Island arcs, oceanic plateaus, and continental fragments will completely subduct, despite having buoyant lithospheric densities, if they have rheologically strong crusts. Weak basal layers, representing pre-existing weaknesses or detachment layers, will either lead to underplating of faulted blocks of FAT crust to the overriding plate or collision and suturing of an unbroken FAT crust. Our experiments show that the weak, ultramafic layer found at the base of island arcs and oceanic plateaus plays a significant role in terrane accretion. The different types of accretionary processes also affect deformation and uplift patterns in the overriding plate, trench migration and jumping, and the dip of the plate interface. The resulting accreted terranes produced from our numerical experiments resemble observed accreted terranes, such as the Wrangellia Terrane and Klamath Mountain terranes in the North American Cordilleran Belt.

Origin of ophiolite complexes related to intra-oceanic subduction initiation: implications of IODP Expedition 352 (Izu-Bonin fore arc)

NASA Astrophysics Data System (ADS)

Robertson, Alastair; Avery, Aaron; Carvallo, Claire; Christeson, Gail; Ferré, Eric; Kurz, Walter; Kutterolf, Steffen; Morgan, Sally; Pearce, Julian; Reagan, Mark; Sager, William; Shervais, John; Whattam, Scott; International Ocean Discovery Program Expedition 352 (Izu-Bonin-Mariana Fore Arc), the Scientific Party of

2015-04-01

Ophiolites, representing oceanic crust exposed on land (by whatever means), are central to the interpretation of many orogenic belts (e.g. E Mediterranean). Based mostly on geochemical evidence, ophiolites are widely interpreted, in many but by no means all cases, as having formed within intra-oceanic settings above subduction zones (e.g. Troodos ophiolite, Cyprus). Following land geological, dredging and submersible studies, fore arcs of the SW Pacific region became recognised as likely settings of supra-subduction zone ophiolite genesis. This hypothesis was tested by recent drilling of the Izu-Bonin fore arc. Four sites were drilled, two on the outer fore arc and two on the upper trench slope. Site survey seismic data, combined with borehole data, indicate that three of the sites are located in fault-controlled sediment ponds that formed in response to dominantly down-to the-west extensional faulting (with hints of preceding top-to-the-east compressional thrusting). The sediments overlying the igneous basement, of maximum Late Eocene to Recent age, document ash and aeolian input, together with mass wasting of the fault-bounded sediment ponds. At the two more trenchward sites (U1440 and U1441), mostly tholeiitic basalts were drilled, including massive and pillowed lavas and hyaloclastite. Geochemically, these extrusives are of near mid-oceanic ridge basalt composition (fore arc basalts). Subtle chemical deviation from normal MORB can be explained by weakly fluid-influenced melting during decompression melting in the earliest stages of supra-subduction zone spreading (not as 'trapped' older MORB). The remaining two sites, c. 6 km to the west (U1439 and U1442), penetrated dominantly high-magnesian andesites, known as boninites, largely as fragmental material. Their formation implies the extraction of highly depleted magmas from previously depleted, refractory upper mantle in a supra-subduction zone setting. Following supra-subduction zone spreading, the active

Continental underplating after slab break-off

NASA Astrophysics Data System (ADS)

Magni, V.; Allen, M. B.; van Hunen, J.; Bouilhol, P.

2017-09-01

We present three-dimensional numerical models to investigate the dynamics of continental collision, and in particular what happens to the subducted continental lithosphere after oceanic slab break-off. We find that in some scenarios the subducting continental lithosphere underthrusts the overriding plate not immediately after it enters the trench, but after oceanic slab break-off. In this case, the continental plate first subducts with a steep angle and then, after the slab breaks off at depth, it rises back towards the surface and flattens below the overriding plate, forming a thick horizontal layer of continental crust that extends for about 200 km beyond the suture. This type of behaviour depends on the width of the oceanic plate marginal to the collision zone: wide oceanic margins promote continental underplating and marginal back-arc basins; narrow margins do not show such underplating unless a far field force is applied. Our models show that, as the subducted continental lithosphere rises, the mantle wedge progressively migrates away from the suture and the continental crust heats up, reaching temperatures >900 °C. This heating might lead to crustal melting, and resultant magmatism. We observe a sharp peak in the overriding plate rock uplift right after the occurrence of slab break-off. Afterwards, during underplating, the maximum rock uplift is smaller, but the affected area is much wider (up to 350 km). These results can be used to explain the dynamics that led to the present-day crustal configuration of the India-Eurasia collision zone and its consequences for the regional tectonic and magmatic evolution.

Subduction dynamics: From the trench to the core-mantle boundary

NASA Astrophysics Data System (ADS)

Kincaid, Chris

1995-07-01

Subduction occurs along convergent plate boundaries where one of the colliding lithospheric plates descends into the mantle. Subduction zones are recognized where plates converge at ˜2-15 cm/yr, although well developed trenches and volcanic arcs (e.g. the line of active volcanoes lying parallel to most ocean trenches, such as the Aleutian Islands in the North Pacific) occur when convergence rates are higher, 4-10 cm/yr. This report is meant to provide a brief review on the general topic of subduction dynamics. A recent spin on subduction studies is the growing realization that the need to understand this global Earth process may be argued not only on purely scientific grounds, but also in terms of societal relevance. While subducting slabs of oceanic lithosphere clearly provide the dominant driving force for mantle dynamics and plate tectonics, over half of the Earth's present 40,000 km of subduction zones are associated with continental margins where a large and rapidly increasing percentage of the Earth's population resides. Subductioninduced hazards along active continental margins include those associated with volcanic hazards (Blong, 1984; Tilling, 1989) such as lava flows, pyroclastic flows and ash fallout and tectonic processes, such as faulting, tsunamis and earthquakes. With regards to earthquake hazards, all of the great (magnitude >9) earthquakes in recorded history have occurred at subduction zones, with 50% of all energy released since 1900 being in four events (1964-Alaska; 1960-Chile; 1957- Aleutians; 1952-Kamchatka). Subduction zone hazards have significant impact on long time scales, such as contributions to global climate change (Robock, 1991; Simarski, 1992; Johnson, 1993; Bluth et al., 1993) and short time scales such as airline safety (Casadevall, 1992). Moreover, accretionary wedges are important in terms of resource potential and trenches have occasionally been suggested as nuclear waste disposal sites.

Trans-Alaska Crustal Transect and continental evolution involving subduction underplating and synchronous foreland thrusting

USGS Publications Warehouse

Fuis, G.S.; Moore, Thomas E.; Plafker, G.; Brocher, T.M.; Fisher, M.A.; Mooney, W.D.; Nokleberg, W.J.; Page, R.A.; Beaudoin, B.C.; Christensen, N.I.; Levander, A.R.; Lutter, W.J.; Saltus, R.W.; Ruppert, N.A.

2008-01-01

We investigate the crustal structure and tectonic evolution of the North American continent in Alaska, where the continent has grown through magmatism, accretion, and tectonic underplating. In the 1980s and early 1990s, we conducted a geological and geophysical investigation, known as the Trans-Alaska Crustal Transect (TACT), along a 1350-km-long corridor from the Aleutian Trench to the Arctic coast. The most distinctive crustal structures and the deepest Moho along the transect are located near the Pacific and Arctic margins. Near the Pacific margin, we infer a stack of tectonically underplated oceanic layers interpreted as remnants of the extinct Kula (or Resurrection) plate. Continental Moho just north of this underplated stack is more than 55 km deep. Near the Arctic margin, the Brooks Range is underlain by large-scale duplex structures that overlie a tectonic wedge of North Slope crust and mantle. There, the Moho has been depressed to nearly 50 km depth. In contrast, the Moho of central Alaska is on average 32 km deep. In the Paleogene, tectonic underplating of Kula (or Resurrection) plate fragments overlapped in time with duplexing in the Brooks Range. Possible tectonic models linking these two regions include flat-slab subduction and an orogenic-float model. In the Neogene, the tectonics of the accreting Yakutat terrane have differed across a newly interpreted tear in the subducting Pacific oceanic lithosphere. East of the tear, Pacific oceanic lithosphere subducts steeply and alone beneath the Wrangell volcanoes, because the overlying Yakutat terrane has been left behind as underplated rocks beneath the rising St. Elias Range, in the coastal region. West of the tear, the Yakutat terrane and Pacific oceanic lithosphere subduct together at a gentle angle, and this thickened package inhibits volcanism. ?? 2008 The Geological Society of America.

Origin of back-arc basins and effects of western Pacific subduction systems on eastern China geology

NASA Astrophysics Data System (ADS)

Niu, Y.

2013-12-01

Assuming that subduction initiation is a consequence of lateral compositional buoyancy contrast within the lithosphere [1], and recognizing that subduction initiation within normal oceanic lithosphere is unlikely [1], we can assert that passive continental margins that are locations of the largest compositional buoyancy contrast within the lithosphere are the loci of future subduction zones [1]. We hypothesize that western Pacific back-arc basins were developed as and evolved from rifting at passive continental margins in response to initiation and continuation of subduction zones. This hypothesis can be tested by demonstrating that intra-oceanic island arcs must have basement of continental origin. The geology of the Islands of Japan supports this. The highly depleted forearc peridotites (sub-continental lithosphere material) from Tonga and Mariana offer independent lines of evidence for the hypothesis [1]. The origin and evolution of the Okinawa Trough (back-arc basin) and Ryukyu Arc/Trench systems represents the modern example of subduction initiation and back-arc basin formation along a (Chinese) continental margin. The observation why back-arc basins exit behind some subduction zones (e.g., western Pacific) but not others (e.g., in South America) depends on how the overlying plate responds to subduction, slab-rollback and trench retreat. In the western Pacific, trench retreat towards east results in the development of extension in the upper Eurasian plate and formation of back-arc basins. In the case of South America, where no back-arc basins form because trench retreat related extension is focused at the 'weakest' South Mid-Atlantic Ridge. It is thus conceptually correct that the South Atlantic is equivalent to a huge 'back-arc basin' although its origin may be different. Given the negative Clayperon slope of the Perovskite-ringwoodite phase transition at the 660 km mantle seismic discontinuity (660-D), slab penetration across the 660-D is difficult and

Seamount subduction and related deformation and seismicity of the continental slope off Manzanillo, Mexico, as evidenced by multibeam data

NASA Astrophysics Data System (ADS)

Bandy, W. L.; Castillo Maldonado, M.; Mortera-Gutierrez, C. A.

2014-12-01

The west coast of Mexico presents a complex pattern of deformation related to the convergence and subduction of the Rivera plate beneath the Jalisco Block/North American plate. Previous single beam bathymetric data have evidenced a large bathymetric high at 104.6218oW, 18.7123oN, in the continental slope region off Manzanillo, Mexico. One school of thought held that this high was the offshore extension of the onshore Manzanillo horst, although the two features are offset in a right-lateral sense. Alternatively, given the presence of a large positive magnetic anomaly near the bathymetric high, the high could also be caused by the collision and subsequent subduction of a large seamount. Given that the offset between the two structures was the main evidence for proposing the existence of a forearc sliver in the offshore area of the Jalisco margin, resolving the nature of this bathymetric high is quite important in our attempts to understand the plate kinematics and tectonics of this region. Thus, to better define the deformation pattern associated with the bathymetric high, multibeam bathymetric data (obtained using the Kongsberg EM300 multibeam system), subbottom profiles (obtained using the Kongsberg TOPAS18 system), and total field magnetic data (obtained using the Geometrics G877 marine proton precession magnetometer) were collected in the continental slope region between Manzanillo, Colima, and Chamela, Jalisco, during several cruises of UNAM´s research vessel the B.O. EL PUMA. The morphology and structural deformation patterns obtained in this study indicate very clearly that a large seamount is in the process of subducting beneath the continental slope off Manzanillo. The results also indicate that not only has the seamount uplifted the slope but has resulted in slumping of the area of the slope landward of the seamount. Given these results the proposal of the existence of an independent forearc sliver in the offshore area of the southern Jalisco block needs

A thermo-mechanical model of horizontal subduction below an overriding plate

NASA Astrophysics Data System (ADS)

van Hunen, Jeroen; van den Berg, Arie P.; Vlaar, Nico J.

2000-10-01

Subduction of young oceanic lithosphere cannot be explained by the gravitational driving mechanisms of slab pull and ridge push. This deficiency of driving forces can be overcome by obduction of an actively overriding plate, which forces the young plate either to subduct or to collide. This mechanism leads to shallow flattening of the slab as observed today under parts of the west coast of North and South America. Here this process is examined by means of numerical modeling. The convergence velocity between oceanic and continental lithospheric plates is computed from the modeling results, and the ratio of the subduction velocity over the overriding velocity is used as a diagnostic of the efficiency of the ongoing subduction process. We have investigated several factors influencing the mechanical resistance working against the subduction process. In particular, we have studied the effect of a preexisting lithospheric fault with a depth dependent shear resistance, partly decoupling the oceanic lithosphere from the overriding continent. We also investigated the lubricating effect of a 7 km thick basaltic crustal layer on the efficiency of the subduction process and found a log-linear relation between convergence rate and viscosity prefactor characterizing the strength of the oceanic crust, for a range of parameter values including values for basaltic rocks, derived from empirical data. A strong mantle fixes the subducting slab while being overridden and prevents the slab from further subduction in a Benioff style. Viscous heating lowers the coupling strength of the crustal interface between the converging plates with about half an order of magnitude and therefore contributes significantly to the subduction process. Finally, when varying the overriding velocity from 2.5 to 10 cm yr -1, we found a non-linear increase of the subduction velocity due to the presence of non-linear mantle rheology. These results indicate that active obduction of oceanic lithosphere by an

Deep seismic reflection evidence for ancient subduction and collision zones within the continental lithosphere of northwestern Europe

NASA Astrophysics Data System (ADS)

Balling, N.

2000-12-01

Deep seismic profiling experiments in the region of NW Europe (including BABEL in the Gulf of Bothnia and the Baltic Sea, Mobil Search in the Skagerrak and MONA LISA in the North Sea) have demonstrated the existence of seismic reflectors in the mantle lithosphere beneath the Baltic Shield, the Tornquist Zone and the North Sea basins. Different sets of reflectors are observed, notably dipping and sub-horizontal. Dipping, distinct reflectivity, which may be followed from Moho/Moho offsets into the deeper parts of the continental lithosphere, is of special interest because of its tectonic and geodynamic significance. Such reflectivity, observed in several places, dipping 15-35° and covering a depth range of 30-90 km, constrained by surface geological information and radiometric age data, is interpreted to represent fossil, ancient subduction and collison zones. Subduction slabs with remnant oceanic basaltic crust transformed into eclogite is assumed, in particular, to generate deep seismic reflectivity. Deep seismic evidence is presented for subduction, crustal accretion and collision processes with inferred ages from 1.9 to 1.1 Ga from the main structural provinces within the Baltic Shield including Svecofennian, Transscandinavian Igneous Belt, Gothian and Sveconorwegian. Along the southwestern border of Baltica (in the southeastern North Sea) south-dipping crustal and sub-crustal reflectivity is observed down to a depth of about 90 km, close to the lithosphere-asthenosphere boundary. These structures are interpreted to reveal a lithosphere-scale Caledonian (ca. 440 Ma) suture zone resulting from the closure of the Tornquist Sea/Thor Ocean and the amalgamation of Baltica and Eastern Avalonia. These results demonstrate that deep structures within the continental lithosphere, originating from early crust-forming plate tectonic processes, may survive for a very long time and form seismic marker reflectivity of great value in geotectonic interpretation and

Seismicity, Deformation, and Metamorphism in the Western Hellenic Subduction Zone: New Constraints From Tomography

NASA Astrophysics Data System (ADS)

Halpaap, Felix; Rondenay, Stéphane; Ottemöller, Lars

2018-04-01

The Western Hellenic Subduction Zone is characterized by a transition from oceanic to continental subduction. In the southern oceanic portion of the system, abundant seismicity reaches depths of 100 km to 190 km, while the northern continental portion rarely exhibits deep earthquakes. Our study investigates how this oceanic-continental transition affects fluid release and related seismicity along strike. We present results from local earthquake tomography and double-difference relocation in conjunction with published images based on scattered teleseismic waves. Our tomographic images recover both subducting oceanic and continental crusts as low-velocity layers on top of high-velocity mantle. Although the northern and southern trenches are offset along the Kephalonia Transform Fault, continental and oceanic subducting crusts appear to align at depth. This suggests a smooth transition between slab retreat in the south and slab convergence in the north. Relocated hypocenters outline a single-planed Wadati-Benioff Zone with significant along-strike variability in the south. Seismicity terminates abruptly north of the Kephalonia Transform Fault, likely reflecting the transition from oceanic to continental subducted crust. Near 90 km depth, the low-velocity signature of the subducting crust fades out and the Wadati-Benioff Zone thins and steepens, marking the outline of the basalt-eclogite transition. Subarc melting of the mantle is only observed in the southernmost sector of the oceanic subduction, below the volcanic part of the arc. Beneath the nonvolcanic part, the overriding crust appears to have undergone large-scale silica enrichment. This enrichment is observed as an anomalously low Vp/Vs ratio and requires massive transport of dehydration-derived fluids updip through the subducting crust.

A numerical investigation of continental collision styles

NASA Astrophysics Data System (ADS)

Ghazian, Reza Khabbaz; Buiter, Susanne J. H.

2013-06-01

Continental collision after closure of an ocean can lead to different deformation styles: subduction of continental crust and lithosphere, lithospheric thickening, folding of the unsubducted continents, Rayleigh-Taylor (RT) instabilities and/or slab break-off. We use 2-D thermomechanical models of oceanic subduction followed by continental collision to investigate the sensitivity of these collision styles to driving velocity, crustal and lithospheric temperature, continental rheology and the initial density difference between the oceanic lithosphere and the asthenosphere. We find that these parameters influence the collision system, but that driving velocity, rheology and lithospheric (rather than Moho and mantle) temperature can be classified as important controls, whereas reasonable variations in the initial density contrast between oceanic lithosphere and asthenosphere are not necessarily important. Stable continental subduction occurs over a relatively large range of values of driving velocity and lithospheric temperature. Fast and cold systems are more likely to show folding, whereas slow and warm systems can experience RT-type dripping. Our results show that a continent with a strong upper crust can experience subduction of the entire crust and is more likely to fold. Accretion of the upper crust at the trench is feasible when the upper crust has a moderate to weak strength, whereas the entire crust can be scraped-off in the case of a weak lower crust. We also illustrate that weakening of the lithospheric mantle promotes RT-type of dripping in a collision system. We use a dynamic collision model, in which collision is driven by slab pull only, to illustrate that adjacent plates can play an important role in continental collision systems. In dynamic collision models, exhumation of subducted continental material and sediments is triggered by slab retreat and opening of a subduction channel, which allows upward flow of buoyant materials. Exhumation continues

Subduction initiation and Obduction: insights from analog models

NASA Astrophysics Data System (ADS)

Agard, P.; Zuo, X.; Funiciello, F.; Bellahsen, N.; Faccenna, C.; Savva, D.

2013-12-01

Subduction initiation and obduction are two poorly constrained geodynamic processes which are interrelated in a number of natural settings. Subduction initiation can be viewed as the result of a regional-scale change in plate convergence partitioning between the set of existing subduction (and collision or obduction) zones worldwide. Intraoceanic subduction initiation may also ultimately lead to obduction of dense oceanic "ophiolites" atop light continental plates. A classic example is the short-lived Peri-Arabic obduction, which took place along thousands of km almost synchronously (within ~5-10 myr), from Turkey to Oman, while the subduction zone beneath Eurasia became temporarily jammed. We herein present analog models designed to study both processes and more specifically (1) subduction initiation through the partitioning of deformation between two convergent zones (a preexisting and a potential one) and, as a consequence, (2) the possible development of obduction, which has so far never been modeled. These models explore the mechanisms of subduction initiation and obduction and test various triggering hypotheses (i.e., plate acceleration, slab crossing the 660 km discontinuity, ridge subduction; Agard et al., 2007). The experimental setup comprises an upper mantle modelled as a low-viscosity transparent Newtonian glucose syrup filling a rigid Plexiglas tank and high-viscosity silicone plates. Convergence is simulated by pushing on a piston at one end of the model with plate tectonics like velocities (1-10 cm/yr) onto (i) a continental margin, (ii) a weakness zone with variable resistance and dip (W), (iii) an oceanic plate - with or without a spreading ridge, (iv) a subduction zone (S) dipping away from the piston and (v) an upper active continental margin, below which the oceanic plate is being subducted at the start of the experiment (as for the Oman case). Several configurations were tested over thirty-five parametric experiments. Special emphasis was

Absolute Plate Motion Control Since the Triassic from the Cocos Slab and its Associated Subduction Record in Mexico

NASA Astrophysics Data System (ADS)

Boschman, L.; Van Hinsbergen, D. J. J.; Langereis, C. G.; Molina-Garza, R. S.; Kimbrough, D. L.; Spakman, W.

2017-12-01

A positive wave speed anomaly interpreted as the Cocos slab stretches from the uppermost mantle at the Middle America trench in the west, to the lowermost mantle below the Atlantic in the east. The length and continuity of this slab indicates long-lived, uninterrupted eastward subduction of the attached Cocos Plate and its predecessor, the Farallon Plate. The geological record of Mexico contains Triassic to present day evidence of subduction, of which the post-Late Cretaceous phase is of continental margin-style. Interpretations of the pre-Upper Cretaceous subduction-related rock assemblages are under debate, and vary from far-travelled exotic intra-oceanic island arc character to in-situ extended continental margin origin. We present new paleomagnetic data that show that Triassic, Jurassic and Cretaceous subduction-related rocks from the Vizcaíno Peninsula and the Guerrero terrane have a paleolatitudinal plate motion history that is equal to that of the North American continent. This suggests that these rock assemblages were part of the overriding plate and were perhaps only separated from the North American continent by temporal fore- or back-arc spreading. The entire Triassic-present day subduction record, and hence, reconstructed trench location, can therefore be linked to the Cocos slab, which provides control on longitudinal plate motion of North America since the time of Pangea. Compared to the latest state of the art mantle frames, in which longitudes are essentially unconstrained for pre-Cretaceous times, our reconstructed absolute position of North America requires a significant westward longitudinal shift for Mesozoic times.

Diapir versus along-channel ascent of crustal material during plate convergence: constrained by the thermal structure of subduction zones

NASA Astrophysics Data System (ADS)

Liu, M. Q.; Li, Z. H.

2017-12-01

Crustal rocks can be subducted to mantle depths, interact with the mantle wedge, and then exhume to the crustal depth again, which is generally considered as the mechanism for the formation of ultrahigh-pressure metamorphic rocks in nature. The crustal rocks undergo dehydration and melting at subarc depths, giving rise to fluids that metasomatize and weaken the overlying mantle wedge. There are generally two ways for the material ascent from subarc depths: one is along subduction channel; the other is through the mantle wedge by diapir. In order to study the conditions and dynamics of these contrasting material ascent modes, systematic petrological-thermo-mechanical numerical models are constructed with variable thicknesses of the overriding and subducting continental plates, ages of the subducting oceanic plate, as well as the plate convergence rates. The model results suggest that the thermal structures of subduction zones control the thermal condition and fluid/melt activity at the slab-mantle interface in subcontinental subduction channels, which further strongly affect the material transportation and ascent mode. Thick overriding continental plate and low-angle subduction style induced by young subducting oceanic plate both contribute to the formation of relatively cold subduction channels with strong overriding mantle wedge, where the along-channel exhumation occurs exclusively to result in the exhumation of HP-UHP metamorphic rocks. In contrast, thin overriding lithosphere and steep subduction style induced by old subducting oceanic plate are the favorable conditions for hot subduction channels, which lead to significant hydration and metasomatism, melting and weakening of the overriding mantle wedge and thus cause the ascent of mantle wedge-derived melts by diapir through the mantle wedge. This may corresponds to the origination of continental arc volcanism from mafic to ultramafic metasomatites in the bottom of the mantle wedge. In addition, the plate

On the role of subducting oceanic plateaus in the development of shallow flat subduction

NASA Astrophysics Data System (ADS)

van Hunen, Jeroen; van den Berg, Arie P.; Vlaar, Nico J.

2002-08-01

Oceanic plateaus, aseismic ridges or seamount chains all have a thickened crust and their subduction has been proposed as a possible mechanism to explain the occurrence of flat subduction and related absence of arc magmatism below Peru, Central Chile and at the Nankai Trough (Japan). Their extra compositional buoyancy could prohibit the slab from sinking into the mantle. With a numerical thermochemical convection model, we simulated the subduction of an oceanic lithosphere that contains an oceanic crustal plateau of 18-km thickness. With a systematic variation, we examined the required physical parameters to obtain shallow flat subduction. Metastability of the basaltic crust in the eclogite stability field is of crucial importance for the slab to remain buoyant throughout the subduction process. In a 44-Ma-old subducting plate, basalt must be able to survive a temperature of 600-700 °C to keep the plate buoyant sufficiently long to cause a flat-slab segment. We found that the maximum yield stress in the slab must be limited to about 600 MPa to allow for the necessary bending to the horizontal. Young slabs show flat subduction for larger parameter ranges than old slabs, since they are less gravitationally unstable and show less resistance against bending. Hydrous weakening of the mantle wedge area and lowermost continent are required to allow for the necessary deformation of a change in subduction style from steep to flat. The maximum flat slab extent is about 300 km, which is sufficient to explain the observed shallow flat subduction near the Nankai Trough (Japan). However, additional mechanisms, such as active overthrusting by an overriding continental plate, need to be invoked to explain the flat-slab segments up to 500 km long below Peru and Central Chile.

The Taitao Granites: I-type granites formed by subduction of the Chile Ridge and its implication in growth of continental crusts

NASA Astrophysics Data System (ADS)

Anma, Ryo

2016-04-01

Late Miocene to Early Pliocene granite plutons are exposed at the tip of the Taitao peninsula, the westernmost promontory of the Chilean coast, together with a contemporaneous ophiolite with a Penrose-type stratigraphy. Namely, the Taitao granites and the Taitao ohiolite, respectively, are located at ~30 km southeast of the Chile triple junction, where a spreading center of the Chile ridge system is subducting underneath the South America plate. This unique tectonic setting provides an excellent opportunity to study the generation processes of granitic magmas at a ridge subduction environment, and the complex magmatic interactions between the subducting ridge, overlying crust and sediments, and mantle. This paper reviews previous studies on the Taitao ophiolite/granite complex and use geochemical data and U-Pb age distributions of zircons separated from igneous and sedimentary rocks from the area to discuss the mechanism that formed juvenile magma of calc-alkaline I-type granites during ridge subduction. Our model implies that the magmas of the Taitao granites formed mainly due to partial melting of hot oceanic crust adjacent to the subducting mid-oceanic ridge that has been under influence of deep crustal contamination and/or metasomatized sub-arc mantle through slab window. The partial melting took place under garnet-free-amphibolite conditions. The juvenile magmas then incorporated a different amount of subducted sediments to form the I-type granites with various compositions. The Taitao granites provide an ideal case study field that shows the processes to develop continental crusts out of oceanic crusts through ridge subduction.

Recycling Revisited: Where did all the Subducted Sediments go?

NASA Astrophysics Data System (ADS)

Hofmann, A. W.; Chauvel, C.; Lewin, E.; Kelemen, P. B.; Hacker, B. R.

2016-12-01

Several lines of reasoning have revived the idea [1] that subduction has recycled continent-derived sediments into the mantle on a massive scale. For example, well-known peaks in zircon ages have been reinterpreted as reflecting variable rates of crust destruction via erosion and sediment subduction [2]. In addition, assessment of the trace element budgets of subducted sediments and arc volcanics, as well as geological and geophysical studies of accretionary wedges have led to estimates that about one mass of present-day continental crust has been returned to the mantle [3]. If these ideas are correct, then recycled sedimentary components should be present in MORB and OIB sources. As previously established, Nb/U and 87Sr/86Sr are negatively correlated in all EM2-type OIBs, clearly indicating continental/sedimentary input. However, the MORB source reservoir, being depleted in incompatible elements, is particularly susceptible to "pollution" by subducted sediments. Chauvel et al. [4] modeled the Hf-Nd isotopic array of MORBs+OIBs and concluded that it requires the addition of up to 6 % subducted sediment. We revisit this issue and show that global MORBs show no decrease in Nb/U with increasing 87Sr/86Sr, ruling out extensive addition of recycled sediment into global MORB sources. Instead, the Hf-Nd array can be obtained by recycled alkali basalts derived from subducted seamounts and ocean islands, rather than sediments. Moreover, mantle plumes with clearly identifiable sediment input contribute less than 20% of the total plume flux. We conclude that most of the subducted sediment flux is not returned to the convecting mantle. Instead, its most plausible fate is to be underplated beneath existing continental crust via "relamination" [5]. These results imply that continental recycling is subordinate and the growth of the continental crust has been largely irreversible. [1] Armstrong, 1968, Rev. Geophys. 6, 175. [2] Hawkesworth et al., 2009, Science 323, 49. [3] Porter

Growth of early continental crust by water-present eclogite melting in subduction zones

NASA Astrophysics Data System (ADS)

Laurie, A.; Stevens, G.

2011-12-01

The geochemistry of well preserved Paleo- to Meso-Archaean Tonalite-Trondhjemite-Granodiorite (TTG) suite rocks, such as the ca 3.45 Ga trondhjemites from the Barberton greenstone belt in South Africa, provides insight into the origins of Earth's early felsic continental crust. This is particularly well demonstrated by the high-Al2O3 variety of these magmas, such as the Barberton rocks, where the geochemistry requires that they are formed by high pressure (HP) melting of a garnet-rich metamafic source. This has been interpreted as evidence for the formation of these magmas by anatexis of the upper portions of slabs within Archaean subduction zones. Most of the experimental data relevant to Archaean TTG genesis has been generated by studies of fluid-absent melting of metabasaltic sources. However, water drives arc magmatism within Phanerozoic subduction zones and thus, understanding the behaviour of water in Archaean subduction zones, may have considerable value for understanding the genesis of these TTG magmas. Consequently, this study investigates the role of HP water-present melting of an eclogite-facies starting material, in the production of high-Al2O3 type TTG melts. Water-saturated partial melting experiments were conducted between 1.9 and 3.0GPa; and, 870°C and 900°C. The melting reaction is characterized by the breakdown of sodic Cpx, together with Qtz and H2O, to form melt in conjunction with a less sodic Cpx: Qtz + Cpx1 + Grt1 + H2O = Melt + Cpx2 + Grt2. In many of the experimental run products, melt segregated efficiently from residual crystals, allowing for the measurement of a full range of trace elements via Laser Ablation Inductively Coupled Plasma Mass Spectroscopy. The experimental glasses produced by this study have the compositions of peraluminous trondhjemites; and they are light rare earth element, Zr and Sr enriched; and heavy rare earth element, Y and Nb depleted. The compositions of the experimental glasses are similar to high-Al2O3 type

Dynamic topography in subduction zones: insights from laboratory models

NASA Astrophysics Data System (ADS)

Bajolet, Flora; Faccenna, Claudio; Funiciello, Francesca

2014-05-01

The topography in subduction zones can exhibit very complex patterns due to the variety of forces operating this setting. If we can deduce the theoretical isostatic value from density structure of the lithosphere, the effect of flexural bending and the dynamic component of topography are difficult to quantify. In this work, we attempt to measure and analyze the topography of the overriding plate during subduction compared to a pure shortening setting. We use analog models where the lithospheres are modeled by thin-sheet layers of silicone putty lying on low-viscosity syrup (asthenosphere). The model is shorten by a piston pushing an oceanic plate while a continental plate including a weak zone to localize the deformation is fixed. In one type of experiments, the oceanic plate bends and subducts underneath the continental one; in a second type the two plates are in contact without any trench, and thus simply shorten. The topography evolution is monitored with a laser-scanner. In the shortening model, the elevation increases progressively, especially in the weak zone, and is consistent with expected isostatic values. In the subduction model, the topography is characterized, from the piston to the back-wall, by a low elevation of the dense oceanic plate, a flexural bulge, the trench forming a deep depression, the highly elevated weak zone, and the continental upper plate of intermediate elevation. The topography of the upper plate is consistent with isostatic values for very early stages, but exhibits lower elevations than expected for later stages. For a same amount of shortening of the continental plate, the thickening is the same and the plate should have the same elevation in both types of models. However, comparing the topography at 20, 29 and 39% of shortening, we found that the weak zone is 0.4 to 0.6 mm lower when there is an active subduction. Theses values correspond to 2.6 to 4 km in nature. Although theses values are high, there are of the same order as

Joint geophysical and petrological models for the lithosphere structure of the Antarctic Peninsula continental margin

NASA Astrophysics Data System (ADS)

Yegorova, Tamara; Bakhmutov, Vladimir; Janik, Tomasz; Grad, Marek

2011-01-01

The Antarctic Peninsula (AP) is a composite magmatic arc terrane formed at the Pacific margin of Gondwana. Through the late Mesozoic and Cenozoic subduction has stopped progressively from southwest to northeast as a result of a series of ridge trench collisions. Subduction may be active today in the northern part of the AP adjacent to the South Shetland Islands. The subduction system is confined by the Shackleton and Hero fracture zones. The magmatic arc of the AP continental margin is marked by high-amplitude gravity and magnetic anomaly belts reaching highest amplitudes in the region of the South Shetland Islands and trench. The sources for these anomalies are highly magnetic and dense batholiths of mafic bulk composition, which were intruded in the Cretaceous, due to partial melting of upper-mantle and lower-crustal rocks. 2-D gravity and magnetic models provide new insights into crustal and upper-mantle structure of the active and passive margin segments of the northern AP. Our models incorporate seismic refraction constraints and physical property data. This enables us to better constrain both Moho geometry and petrological interpretations in the crust and upper mantle. Model along the DSS-12 profile crosses the AP margin near the Anvers Island and shows typical features of a passive continental margin. The second model along the DSS-17 profile extends from the Drake Passage through the South Shetland Trench/Islands system and Bransfield Strait to the AP and indicates an active continental margin linked to slow subduction and on-going continental rifting in the backarc region. Continental rifting beneath the Bransfield Strait is associated with an upward of hot upper mantle rocks and with extensive magmatic underplating.

Initiation of Extension in South China Continental Margin during the Active-Passive Margin Transition: Thermochronological and Kinematic Constraints

NASA Astrophysics Data System (ADS)

Zuo, X.; Chan, L. S.

2015-12-01

The South China continental margin is characterized by a widespread magmatic belt, prominent NE-striking faults and numerous rifted basins filled by Cretaceous-Eocene sediments. The geology denotes a transition from active to passive margin, which led to rapid modifications of crustal stress configuration and reactivation of older faults in this area. Our zircon fission-track data in this region show two episodes of exhumation: The first episode, occurring during 170-120Ma, affected local parts of the Nanling Range. The second episode, a more regional exhumation event, occurred during 115-70Ma, including the Yunkai Terrane and the Nanling Range. Numerical geodynamic modeling was conducted to simulate the subduction between the paleo-Pacific plate and the South China Block. The modeling results could explain the fact that exhumation of the granite-dominant Nanling Range occurred earlier than that of the gneiss-dominant Yunkai Terrane. In addition to the difference in rock types, the heat from Jurassic-Early Cretaceous magmatism in Nanling may have softened the upper crust, causing the area to exhume more readily than Yunkai. Numerical modeling results also indicate that (1) high lithospheric geothermal gradient, high slab dip angle and low convergence velocity favor the reversal of crustal stress state from compression to extension in the upper continental plate; (2) late Mesozoic magmatism in South China was probably caused by a slab roll-back; and (3) crustal extension could have occurred prior to the cessation of plate subduction. The inversion of stress regime in the continental crust from compression to crustal extension imply that the Late Cretaceous-early Paleogene red-bed basins in South China could have formed during the late stage of the subduction, accounting for the occurrence of volcanic events in some sedimentary basins. We propose that the rifting started as early as Late Cretaceous, probably before the cessation of subduction process.

Terrane-Scale Metastability in Subducted Himalayan Continental Crust as Revealed by Integrated Petrological and Geodynamic Modeling

NASA Astrophysics Data System (ADS)

Palin, R. M.; Reuber, G. S.; White, R. W.; Kaus, B. J. P.; Weller, O. M.

2017-12-01

The Tso Morari massif, northwest India, is one of only two regions in the Himalayan Range that exposes subduction-related ultrahigh-pressure (UHP) metamorphic rocks. The tectonic evolution of the massif is strongly debated, however, as reported pressure estimates for peak metamorphism range between 2.4 GPa and 4.8 GPa. Such ambiguity hinders effective lithospheric-scale modeling of the early stages of the orogen's evolution. We present the results of integrated petrological and geodynamic modeling (Palin et al., 2017, EPSL) that provide new quantitative constraints on the prograde-to-peak pressure-temperature-time (P-T-t) path, and predict the parageneses that felsic and mafic components of the massif crust should have formed under equilibrium conditions. Our model shows that peak P-T conditions of 2.6-2.8 GPa and 600-620 °C, representative of subduction to 90-100 km depth (assuming lithostatic pressure), were reached just 3 Myr after the onset of collision. These P-T-t constraints correlate well with those reported for similar UHP eclogite in the along-strike Kaghan Valley, Pakistan, suggesting that the northwest Himalaya contains dismembered remnants of a 400-km long UHP terrane comparable in size to the Western Gneiss Region, Norway, and the Dabie-Sulu belt, China. The extremely high pressures (up to 4.8 GPa) for peak metamorphism reported by some workers are likely to be unreliable due to thermobarometry having been performed on minerals that did not represent equilibrium assemblages. Furthermore, key high-P minerals predicted to form in subducted Tso Morari continental crust (e.g. jadeite, Mg-rich garnet) are absent from natural samples in the region, reflecting the widespread metastable preservation of lower-pressure protolith assemblages during subduction and exhumation. This result questions the reliability of geodynamic simulations of orogenesis that are commonly predicated on equilibrium metamorphism operating continuously throughout tectonic cycles.

Diapir versus along-channel ascent of crustal material during plate convergence: Constrained by the thermal structure of subduction zones

NASA Astrophysics Data System (ADS)

Liu, Ming-Qi; Li, Zhong-Hai; Yang, Shao-Hua

2017-09-01

Subduction channel processes are crucial for understanding the material and energy exchange between the Earth's crust and mantle. Crustal rocks can be subducted to mantle depths, interact with the mantle wedge, and then exhume to the crustal depth again, which is generally considered as the mechanism for the formation of ultrahigh-pressure metamorphic rocks in nature. In addition, the crustal rocks generally undergo dehydration and melting at subarc depths, giving rise to fluids that metasomatize and weaken the overlying mantle wedge. There are generally two ways for the material ascent from subarc depths: one is along subduction channels; the other is through the mantle wedge by diapir. In order to study the conditions and dynamics of these contrasting material ascent modes, systematic petrological-thermo-mechanical numerical models are constructed with variable thicknesses of the overriding and subducting continental plates, ages of the subducting oceanic plate, as well as the plate convergence rates. The model results suggest that the thermal structures of subduction zones control the thermal condition and fluid/melt activity at the slab-mantle interface in subcontinental subduction channels, which further strongly affect the material transportation and ascent mode. The thick overriding continental plate and the low-angle subduction style induced by young subducting oceanic plate both contribute to the formation of relatively cold subduction channels with strong overriding mantle wedge, where the along-channel exhumation occurs exclusively to result in the exhumation of HP-UHP metamorphic rocks. In contrast, the thin overriding lithosphere and the steep subduction style induced by old subducting oceanic plate are the favorable conditions for hot subduction channels, which lead to significant hydration and metasomatism, melting and weakening of the overriding mantle wedge and thus cause the ascent of mantle wedge-derived melts by diapir through the mantle wedge

Magmatic tectonic effects of high thermal regime at the site of active ridge subduction: the Chile Triple Junction model

NASA Astrophysics Data System (ADS)

Lagabrielle, Yves; Guivel, Christèle; Maury, René C.; Bourgois, Jacques; Fourcade, Serge; Martin, Hervé

2000-11-01

High thermal gradients are expected to be found at sites of subduction of very young oceanic lithosphere and more particularly at ridge-trench-trench (RTT) triple junctions, where active oceanic spreading ridges enter a subduction zone. Active tectonics, associated with the emplacement of two main types of volcanic products, (1) MORB-type magmas, and (2) calc-alkaline acidic magmas in the forearc, also characterize these plate junction domains. In this context, MORB-type magmas are generally thought to derive from the buried active spreading center subducted at shallow depths, whereas the origin of calc-alkaline acidic magmas is more problematic. One of the best constrained examples of ridge-trench interaction is the Chile Triple Junction (CTJ) located southwest of the South American plate at 46°12'S, where the active Chile spreading center enters the subduction zone. In this area, there is a clear correlation between the emplacement of magmatic products and the migration of the triple junction along the active margin. The CTJ lava population is bimodal, with mafic to intermediate lavas (48-56% SiO 2) and acidic lavas ranging from dacites to rhyolites (66-73% SiO 2). Previous models have shown that partial melting of oceanic crust plus 10-20% of sediments, leaving an amphibole- and plagioclase-rich residue, is the only process that may account for the genesis of acidic magmas. Due to special plate geometry in the CTJ area, a given section of the margin may be successively affected by the passage of several ridge segments. We emphasize that repeated passages will lead to the development of very high thermal gradients allowing melting of rocks of oceanic origin at temperatures of 800-900°C and low pressures, corresponding to depths of 10-20 km depth only. In addition, the structure of the CTJ forearc domain is dominated by horizontal displacements and tilting of crustal blocks along a network of strike-slip faults. The occurrence of such a deformed domain implies

Evolution of passive continental margins and initiation of subduction zones

NASA Astrophysics Data System (ADS)

Cloetingh, S. A. P. L.; Wortel, M. J. R.; Vlaar, N. J.

1982-05-01

Although the initiation of subduction is a key element in plate tectonic schemes for evolution of lithospheric plates, the underlying mechanisms are not well understood. Plate rupture is an important aspect of the process of creating a new subduction zone, as stresses of the order of kilobars are required to fracture oceanic lithosphere1. Therefore initiation of subduction could take place preferentially at pre-existing weakness zones or in regions where the lithosphere is prestressed. As such, transform faults2,3 and passive margins4,5 where the lithosphere is downflexed under the influence of sediment loading have been suggested. From a model study of passive margin evolution we found that ageing of passive margins alone does not make them more suitable sites for initiation of subduction. However, extensive sediment loading on young lithosphere might be an effective mechanism for closure of small ocean basins.

Impacts of continental arcs on global carbon cycling and climate

NASA Astrophysics Data System (ADS)

Lee, C. T.; Jiang, H.; Carter, L.; Dasgupta, R.; Cao, W.; Lackey, J. S.; Lenardic, A.; Barnes, J.; McKenzie, R.

2017-12-01

On myr timescales, climatic variability is tied to variations in atmospheric CO2, which in turn is driven by geologic sources of CO2 and modulated by the efficiency of chemical weathering and carbonate precipitation (sinks). Long-term variability in CO2 has largely been attributed to changes in mid-ocean ridge inputs or the efficiency of global weathering. For example, the Cretaceous greenhouse is thought to be related to enhanced oceanic crust production, while the late Cenozoic icehouse is attributed to enhanced chemical weathering associated with the Himalayan orogeny. Here, we show that continental arcs may play a more important role in controlling climate, both in terms of sources and sinks. Continental arcs differ from island arcs and mid-ocean ridges in that the continental plate through which arc magmas pass may contain large amounts of sedimentary carbonate, accumulated over the history of the continent. Interaction of arc magmas with crustal carbonates via assimilation, reaction or heating can significantly add to the mantle-sourced CO2 flux. Detrital zircons and global mapping of basement rocks shows that the length of continental arcs in the Cretaceous was more than twice that in the mid-Cenozoic; maps also show many of these arcs intersected crustal carbonates. The increased length of continental arc magmatism coincided with increased oceanic spreading rates, placing convergent margins into compression, which favors continental arcs. Around 50 Ma, however, nearly all the continental arcs in Eurasia and North America terminated as India collided with Eurasia and the western Pacific rolled back, initiating the Marianas-Tonga-Kermadec intra-oceanic subduction complex and possibly leading to a decrease in global CO2 production. Meanwhile, extinct continental arcs continued to erode, resulting in regionally enhanced chemical weathering unsupported by magmatic fluxes of CO2. Continental arcs, during their magmatic lifetimes, are thus a source of CO2, driving

Modeling the influence of plate motions on subduction

NASA Astrophysics Data System (ADS)

Hillebrand, Bram; Thieulot, Cedric; van den Berg, Arie; Spakman, Wim

2014-05-01

Subduction zones are widely studied complex geodynamical systems. Their evolution is influenced by a broad range of parameters such as the age of the plates (both subducting and overriding) as well as their rheology, their nature (oceanic or continental), the presence of a crust and the involved plate motions to name a few. To investigate the importance of these different parameters on the evolution of subduction we have created a series of 2D numerical thermomechanical subduction models. These subduction models are multi-material flow models containing continental and oceanic crusts, a lithosphere and a mantle. We use the sticky air approach to allow for topography build up in the model. In order to model multi-material flow in our Eulerian finite element code of SEPRAN (Segal and Praagman, 2000) we use the well benchmarked level set method (Osher and Sethian, 1988) to track the different materials and their mode of deformation through the model domain. To our knowledge the presented results are the first subduction model results with the level set method. We will present preliminary results of our parametric study focusing mainly on the influence of plate motions on the evolution of subduction. S. Osher and J.A. Sethian. Fronts propagating with curvature-dependent speed: Algorithms based on hamilton-jacobi formulations. JCP 1988 A. Segal and N.P. Praagman. The SEPRAN package. Technical report, 2000 This research is funded by The Netherlands Research Centre for Integrated Solid Earth Science (ISES)

Deep Structure of Northern Apennines Subduction Orogen (Italy) as Revealed by a Joint Interpretation of Passive and Active Seismic Data

NASA Astrophysics Data System (ADS)

Piana Agostinetti, Nicola; Faccenna, Claudio

2018-05-01

The Apennines is a well-studied orogeny formed by the accretion of continental slivers during the subduction of the Adriatic plate, but its deep structure is still a topic of controversy. Here we illuminated the deep structure of the Northern Apennines belt by combining results from the analysis of active seismic (CROP03) and receiver function data. The result from combining these two approaches provides a new robust view of the structure of the deep crust/upper mantle, from the back-arc region to the Adriatic subduction zone. Our analysis confirms the shallow Moho depth beneath the back-arc region and defines the top of the downgoing plate, showing that the two plates separate at depth about 40 km closer to the trench than reported in previous reconstructions. This spatial relationship has profound implications for the geometry of the shallow subduction zone and of the mantle wedge, by the amount of crustal material consumed at trench.

Evidence for ˜80-75 Ma subduction jump during Anatolide-Tauride-Armenian block accretion and ˜48 Ma Arabia-Eurasia collision in Lesser Caucasus-East Anatolia

NASA Astrophysics Data System (ADS)

Rolland, Yann; Perincek, Dogan; Kaymakci, Nuretdin; Sosson, Marc; Barrier, Eric; Avagyan, Ara

2012-05-01

Orogens formed by a combination of subduction and accretion are featured by a short-lived collisional history. They preserve crustal geometries acquired prior to the collisional event. These geometries comprise obducted oceanic crust sequences that may propagate somewhat far away from the suture zone, preserved accretionary prism and subduction channel at the interplate boundary. The cessation of deformation is ascribed to rapid jump of the subduction zone at the passive margin rim of the opposite side of the accreted block. Geological investigation and 40Ar/39Ar dating on the main tectonic boundaries of the Anatolide-Tauride-Armenian (ATA) block in Eastern Turkey, Armenia and Georgia provide temporal constraints of subduction and accretion on both sides of this small continental block, and final collisional history of Eurasian and Arabian plates. On the northern side, 40Ar/39Ar ages give insights for the subduction and collage from the Middle to Upper Cretaceous (95-80 Ma). To the south, younger magmatic and metamorphic ages exhibit subduction of Neotethys and accretion of the Bitlis-Pütürge block during the Upper Cretaceous (74-71 Ma). These data are interpreted as a subduction jump from the northern to the southern boundary of the ATA continental block at 80-75 Ma. Similar back-arc type geochemistry of obducted ophiolites in the two subduction-accretion domains point to a similar intra-oceanic evolution prior to accretion, featured by slab steepening and roll-back as for the current Mediterranean domain. Final closure of Neotethys and initiation of collision with Arabian Plate occurred in the Middle-Upper Eocene as featured by the development of a Himalayan-type thrust sheet exhuming amphibolite facies rocks in its hanging-wall at c. 48 Ma.

Continental Evolution Involving Subduction Underplating and Synchronous Foreland Thrusting: Evidence from the Trans-Alaska Crustal Transect

NASA Astrophysics Data System (ADS)

Fuis, G. S.; Moore, T. E.; Plafker, G.; Brocher, T. M.; Fisher, M. A.; Mooney, W. D.; Nokleberg, W. J.; Page, R. A.; Beaudoin, B. C.; Christensen, N. I.; Levander, A.; Lutter, W. J.; Saltus, R. W.; Ruppert, N. A.

2010-12-01

We investigated the crustal structure and tectonic evolution of the North American continent in Alaska, where the continent has grown through magmatism, accretion, and tectonic underplating. In the 1980’s and early 1990’s, we conducted a geological and geophysical investigation, known as the Trans-Alaska Crustal Transect (TACT), along a 1350-km-long corridor from the Aleutian Trench to the Arctic coast. The most distinctive crustal structures and the deepest Moho along the transect are located near the Pacific and Arctic margins. Near the Pacific margin, we infer a stack of tectonically underplated oceanic layers interpreted to be remnants of the extinct Kula (or Resurrection) Plate. Continental Moho just north of this underplated stack is more than 55 km deep. Near the Arctic margin, the Brooks Range is underlain by north-vergent, crustal-scale duplexes that overlie a ramp on autochthonous North Slope crust. There, Moho has been depressed to nearly 50-km depth. In contrast, the Moho of central Alaska is on average 32 km deep. In the Paleogene, tectonic underplating of Kula- (or Resurrection-) Plate fragments overlapped in time with duplexing in the Brooks Range. Possible tectonic models linking these two widely separated regions include “flat-slab” subduction and an “orogenic-float” model. In the Neogene, the collision of the Yakutat terrane (YAK), in southern Alaska, correlates with renewed compression in northeast Alaska and northwest Canada, in a fashion somewhat similar to the tectonics in the Paleogene. The Yakutat terrane, riding atop the subducting Pacific oceanic lithosphere (POL), spans a newly interpreted tear in the POL. East of the tear, POL is interpreted to subduct steeply and alone beneath the Wrangell arc volcanoes because the overlying YAK has been left behind as tectonically underplated rocks beneath the rising St. Elias Range in the coastal region. West of the tear, the YAK and POL are interpreted to subduct together at a gentle

Early Carboniferous adakite-like and I-type granites in central Qiangtang, northern Tibet: Implications for intra-oceanic subduction and back-arc basin formation within the Paleo-Tethys Ocean

NASA Astrophysics Data System (ADS)

Liu, Jin-Heng; Xie, Chao-Ming; Li, Cai; Wang, Ming; Wu, Hao; Li, Xing-Kui; Liu, Yi-Ming; Zhang, Tian-Yu

2018-01-01

Recent studies have proposed that the Late Devonian ophiolites in the central Qiangtang region of northern Tibet were formed in an oceanic back-arc basin setting, which has led to controversy over the subduction setting of the Longmucuo-Shuanghu-Lancangjiang Suture Zone (LSLSZ) during the Late Devonian to Early Carboniferous. In this paper we present new data about a suite of granite plutons that intrude into ophiolite in central Qiangtang. Our aim was to identify the type of subduction and to clarify the existence of an intra-oceanic back-arc basin in the LSLSZ during the Late Devonian to Early Carboniferous. The suite of granites consists of monzogranites, syenogranites, and granodiorites. Our laser ablation-inductively coupled plasma-mass spectrometry zircon U-Pb data yielded Early Carboniferous crystallization ages of 357.2 Ma, 357.4 Ma and 351.1 Ma. We subsequently investigated the petrogenesis and tectonic setting of these granites based on their geochemical and Hf isotopic characteristics. First, we divided the granites into high Sr/Y (HSG) and low Sr/Y granites (LSG). The HSG group contains monzogranites and granodiorites that have similar geochemical characteristics to adakites (i.e., high Sr/Y and La/Yb ratios, low MgO, Y, and Yb contents, and no pronounced negative Eu anomaly), although they have slightly lower Sr and Al2O3 contents, caused by crystal fractionation during late magmatic evolution. Therefore, we define the HSG group as adakite-like granites. The study of the HSG shows that they are adakite-like granites formed by partial melting of oceanic crust and experience fractional crystallization process during late evolution. However, some differences between the monzogranites and granodiorites indicate that there are varying degree contributions of subducted sediments during diagenesis. The LSG group contains syenogranites that have distinct negative correlations between their P2O5 and SiO2 contents, and Y and Th contents have significant positive

3D Numerical Examination of Continental Mantle Lithosphere Response to Lower Crust Eclogitization and Nearby Slab Subduction

NASA Astrophysics Data System (ADS)

Janbakhsh, P.; Pysklywec, R.

2017-12-01

2D numerical modeling techniques have made great contribution to understanding geodynamic processes involved in crustal and lithospheric scale deformations for the past 20 years. The aim of this presentation is to expand the scope covered by previous researchers to 3 dimensions to address out-of-plane intrusion and extrusion of mantle material in and out of model space, and toroidal mantle wedge flows. In addition, 3D velocity boundary conditions can create more realistic models to replicate real case scenarios. 3D numerical experiments that will be presented are designed to investigate the density and viscosity effects of lower crustal eclogitization on the decoupling process of continental mantle lithosphere from the crust and its delamination. In addition, these models examine near-field effects of a subducting ocean lithosphere and a lithospheric scale fault zone on the evolution of the processes. The model solutions and predictions will also be compared against the Anatolian geology where subduction of Aegean and Arabian slabs, and the northern boundary with the North Anatolian Fault Zone are considered as two main contributing factors to anomalous crustal uplift, missing mantle lithosphere, and anomalous surface heat flux.

Fluid-induced Crystallization of Majoritic Garnet During Deep Continental Subduction (Western Gneiss Region, Norway)

NASA Astrophysics Data System (ADS)

Scambelluri, M.; Pettke, T.; van Roermund, H. L.

2008-12-01

In ultrahigh pressure (UHP) rocks, garnet containing pyroxene exsolutions derives from breakdown of majorite crystallized at depths > 200 km. Presence of microdiamonds in some of these rocks [1], including those of the Western Gneiss Region (WGR) of Norway [2], may suggest a fluid-bearing environment for the genesis of majorite. The WGR UHP gneisses host garnet peridotite and websterite recording uplift from extraordinary depths prior to uptake in a subducting slab [2]. These ultramafic rocks (islands of Otrøy and Bardane) derive from depleted Archean transition-zone mantle (350 km depth) upwelled and accreted to a cratonic lithosphere (M2 stage). Evidence for this are decimetric garnets (grt) preserved in Otrøy, hosting up to 20 volume% orthopyroxene (opx) and clinopyroxene (cpx) exsolved from precursor majoritic garnet (M1 stage). The pyroxene lamellae (20-30 ¥ìm thick, hundreds of ¥ìm long) [3] were exsolved under high-T, as shown by the garnet/cpx REE distribution [4]. This Archean-mid Proterozoic record is overprinted by the 425- 390 Ma Scandian continental subduction (M3 stage), forming new grt + cpx + opx + phlogopite (phl) + spinel (sp) that contain diamond-bearing micro-inclusions witnessing deep COH subduction fluids [2]. Here we document formation of new majoritic garnet in the M3 assemblage and in veins at Bardane [5]. Textural characteristics, together with the LREE and LILE enrichments of the M3 minerals, indicate that the new majorite is linked to infiltration of subduction fluids during renewed burial towards sub-lithospheric depths. It represents the deepest occurrence of fluid-related microstructures in mantle rocks. The new majoritic garnet crystallized at grain boundaries and in micro-veins at 7 Gpa and 900-1000 °C. It hosts thin pyroxene needles (5 mm thick, 100 mm long) exsolved under comparatively low-T, as indicated by the garnet/cpx REE distribution. The trace element signature of the majorite-bearing subduction assemblage is LREE

Geochemical and petrological evidence of the subduction of delaminated Adriatic continental lithosphere in the genesis of the Neogene-Quaternary magmatism of central Italy

NASA Astrophysics Data System (ADS)

Serri, G.; Innocenti, F.; Manetti, P.

1993-07-01

Serri, G., Innocenti, F. and Manetti, P., 1993. Geochemical and petrological evidence of the subduction of delaminated Adriatic continental lithosphere in the genesis of the Neogene-Quaternary magmatism of central Italy. In: M.J.R. Wortel, U. Hansen and R. Sabadini (Editors), Relationships between Mantle Processes and Geological Processes at or near The Earth's Surface. Tectonophysics, 223: 117-147. The Neogene-Quaternary magmatism of the northern Apenninic arc took place in four phases separated in space and time which become progressively younger from west to east: Phase I, 14 Ma; Phase II, 7.3-6.0 Ma; Phase III, 5.1-2.2 Ma; Phase IV, 1.3-0.1 Ma. This magmatism is the result of the activation of three physically separate sources: (1) the Adriatic continental crust, extracted from the mantle in the late Proterozoic; (2) a strongly refractory, recently K-enriched harzburgitic mantle located in the mechanical boundary layer (MBL) of the lithosphere; and (3) a recently metasomatized, cpx-rich mantle, compositionally variable from Iherzolite to wehrlite-clinopyroxenite, interpreted as an ephemerally K-enriched asthenosphere. The Adriatic continental crust is the dominant source of the acid plutonic and volcanic rocks of the Tuscan region. The acid magmatism is mostly found inside an ellipsoidal area (about 150 × 300 km) centred on Giglio Island, here defined as the Tuscan Crustal Dome. Within this area, mantle-derived magmas unaffected by important crustal contamination processes and mixing with crustal anatectic melts have so far not been found. Pure crustal magmas are rare but are represented, for example by some of the San Vincenzo and Roccastrada rhyolites. Virtually all the Tuscan acid centres show evidence of mixing with potassic mantle-derived magmas. Major and trace elements, as well as {87Sr }/{86Sr } and {143Nd }/{144Nd } data, on primitive rocks (Mg# > 65) reveal two groups of mantle-derived magmas. These define two distinct mantle enrichment trends, both

Structure of the Cascadia Subduction Zone Imaged Using Surface Wave Tomography

NASA Astrophysics Data System (ADS)

Schaeffer, A. J.; Audet, P.

2017-12-01

Studies of the complete structure of the Cascadia subduction zone from the ridge to the arc have historically been limited by the lack of offshore ocean bottom seismograph (OBS) infrastructure. On land, numerous dense seismic deployments have illuminated detailed structures and dynamics associated with the interaction between the subducting oceanic plate and the overriding continental plate, including cycling of fluids, serpentinization of the overlying forearc mantle wedge, and the location of the upper surface of the Juan de Fuca plate as it subducts beneath the Pacific Northwest. In the last half-decade, the Cascadia Initiative (CI), along with Neptune (ONC) and several other OBS initiatives, have instrumented both the continental shelf and abyssal plains off shore of the Cascadia subduction zone, facilitating the construction of a complete picture of the subduction zone from ridge to trench and volcanic arc. In this study, we present a preliminary azimuthally anisotropic surface-wave phase-velocity based model of the complete system, capturing both the young, unaltered Juan de Fuca plate from the ridge, to its alteration as it enters the subduction zone, in addition to the overlying continent. This model is constructed from a combination of ambient noise cross-correlations and teleseismic two station interferometry, and combines together concurrently running offshore OBS and onshore stations. We furthermore perform a number of representative 1D depth inversions for shear velocity to categorize the pristine oceanic, subducted oceanic, and continental crust and lithospheric structure. In the future the dispersion dataset will be jointly inverted with receiver functions to constrain a 3D shear-velocity model of the complete region.

3D dynamics of crustal deformation driven by oblique subduction: Northern and Central Andes

NASA Astrophysics Data System (ADS)

Schütt, Jorina M.; Whipp, David M., Jr.

2017-04-01

The geometry and relative motion of colliding plates will affect how and where they deform. In oblique subduction systems, factors such as the dip angle of the subducting plate and the convergence obliquity, as well as the presence of weak zones in the overriding plate, all influence how oblique convergence is partitioned onto various fault systems in the overriding plate. The partitioning of strain into margin-normal slip on the plate-bounding fault and horizontal shearing on a strike-slip system parallel to the margin is mainly controlled by the margin-parallel shear forces acting on the plate interface and the strength of the continental crust. While these plate interface forces are influenced by the dip angle of the subducting plate (i.e., the length of plate interface in the frictional domain) and the obliquity angle between the normal to the plate margin and the plate convergence vector, the strength of the continental crust in the upper plate is strongly affected by the presence or absence of weak zones such as regions of arc volcanism, pre-existing fault systems, or boundaries of stronger crustal blocks. In order to investigate which of these factors are most important in controlling how the overriding continental plate deforms, we compare results of lithospheric-scale 3D numerical geodynamic experiments from two regions in the north-central Andes: the Northern Volcanic Zone (NVZ; 5°N - 3°S) and adjacent Peruvian Flat Slab Segment (PFSS; 3°S -14°S). The NVZ is characterized by a 35° subduction dip angle with an obliquity angle of about 40°, extensive volcanism and significant strain partitioning in the continental crust. In contrast, the PFSS is characterized by flat subduction (the slab flattens beneath the continent at around 100 km depth for several hundred kilometers), an obliquity angle of about 20°, no volcanism and minimal strain partitioning. The plate geometry and convergence obliquity for these regions are incorporated in 3D (1600 x 1600 x

Subduction processes related to the Sea of Okhotsk

NASA Astrophysics Data System (ADS)

Zabarinskaya, Ludmila P.; Sergeyeva, Nataliya

2017-04-01

It is obviously important to study a role of subduction processes in tectonic activity within the continental margins. They are marked by earthquakes, volcanic eruptions, tsunami and other natural disasters hazardous to the people,plants and animals that inhabit such regions. The northwest part of the Sea of Okhotsk including the northern part of Sakhalin Island and the Deryugin Basin is the area of the recent intensive tectonic movements. The geological and geophysical data have made it possible to construct the geodynamic model of a deep structure of a lithosphere for this region. This geodynamic model has confirmed the existence of the ophiolite complex in the region under consideration. It located between the North Sakhalin sedimentary basin and the Deryugin basin. The Deryugin basin was formed on the side of an ancient deep trench after subducting the Okhotsk Sea Plate under Sakhalin in the Late Cretaceous-Paleogene. The North Sakhalin Basin with oil and gas resources was formed on the side of back-arc basin at that time. Approximately in the Miocene period the subduction process, apparently, has stopped. The remains of the subduction zone in the form of ophiolite complex have been identified according to geological and geophysical data. On a surface the subduction zone is shown as deep faults stretched along Sakhalin.

Gravity modelling of the Hellenic subduction zone — a regional study

NASA Astrophysics Data System (ADS)

Casten, U.; Snopek, K.

2006-05-01

The Hellenic subduction zone is clearly expressed in the arc-shaped distribution of earthquake epicenters and gravity anomalies, which connect the Peloponnesos with Crete and Anatolia. In this region, oceanic crust of the African plate collides northward with continental crust of the Aegean microplate, which itself is pushed apart to the south-west by the Anatolian plate and, at the same time, is characterised by crustal extension. The result is an overall collision rate of up to 4 cm/year and a retreating subduction process. Recent passive and active seismic studies on and around Crete gave first, but not in all details consistent, structural results useful for supporting gravity modelling. This was undertaken with the aim of presenting the first 3D density structure of the entire subduction zone. Gravity interpretation was based on a Bouguer map, newly compiled using data from land, marine and satellite sources. The anomalies range from + 170 mGal (Cretan Sea) to - 10 mGal (Mediterranean Ridge). 3D gravity modelling was done applying the modelling software IGMAS. The computed Bouguer map fits the low frequency part of the observed one, which is controlled by variations in Moho depth (less than 20 km below the Cretan Sea and extending 30 km below Crete) and the extremely thick sedimentary cover (partly up to 18 km) of the Mediterranean Ridge. The southernmost edge of the Eurasian plate, with its more triangular-shaped backstop area, was traced south off Crete. Only 50 to 100 km further to the south, the edge of the African continent was traced as well. In between these boundaries there is African oceanic crust, which has a clear arc-shaped detachment line situated at the Eurasian continental edge. The subduction arc is open towards the north, its slab separates hotter mantle material (lower density) below the updoming Moho of the Cretan Sea from colder one (higher density) in the south. Subjacent to the upper continental crust of Crete is a thickened layer of

Tomography and Dynamics of Western-Pacific Subduction Zones

NASA Astrophysics Data System (ADS)

Zhao, D.

2012-01-01

the Japan Sea and the East Asia margin may be related to a metastable olivine wedge in the subducting Pacific slab. The Pacific slab becomes stagnant in the mantle transition zone under East Asia, and a big mantle wedge (BMW) has formed above the stagnant slab. Convective circulations and fluid and magmatic processes in the BMW may have caused intraplate volcanism (e.g., Changbai and Wudalianchi), reactivation of the North China craton, large earthquakes, and other active tectonics in East Asia. Deep subduction and dehydration of continental plates (such as the Eurasian plate, Indian plate and Burma microplate) are also found, which have caused intraplate magmatism (e.g., Tengchong) and geothermal anomalies above the subducted continental plates. Under Kamchatka, the subducting Pacific slab shortens toward the north and terminates near the Aleutian-Kamchatka junction. The slab loss was induced by friction with the surrounding asthenosphere, as the Pacific plate rotated clockwise 30 Ma ago, and then it was enlarged by the slab-edge pinch-off by the asthenospheric flow. The stagnant slab finally collapses down to the bottom of the mantle, which may trigger upwelling of hot mantle materials from the lower mantle to the shallow mantle. Suggestions are also made for future directions of the seismological research of subduction zones.

Subduction of Proterozoic to Late Triassic continental basement in the Guatemala suture zone: A petrological and geochronological study of high-pressure metagranitoids from the Chuacús complex

NASA Astrophysics Data System (ADS)

Maldonado, Roberto; Ortega-Gutiérrez, Fernando; Ortíz-Joya, Guillermo A.

2018-05-01

Many continental subduction complexes contain abundant granitic rocks coexisting with minor volumes of eclogite-facies rocks. Characterization of granitic protoliths is crucial to decipher the origin of subducted continental crust, whereas knowledge of its metamorphic evolution is required to constrain the mechanisms of burial and exhumation. In this work we present geochronological and petrological evidence that demonstrate the occurrence of a subducted Proterozoic to Late Triassic granitic basement in the Chuacús complex of central Guatemala. Metagranitoids exposed in this area are interlayered with eclogite and other high-pressure rocks, and their structure is considerably variable due to strain partitioning during deformation. Laser ablation-inductively coupled plasma-mass spectrometry U-Pb zircon data from two ferroan metagranites yield protolith crystallization ages of ca. 1.1 Ga and their trace-element abundances suggest an origin related to intraplate magmatism, while a high-silica, peraluminous metagranite is dated at 1.0 Ga and was probably originated by partial melting of a high-grade continental crust. On the other hand, two megacrystic to augen metagranitoids yield protolith crystallization ages of ca. 224 Ma, which are identical within errors to the protolith age of hosted eclogitic metabasites. Their high incompatible trace element abundances together with the observed spatial-temporal relationships with mafic protoliths suggest that Late Triassic bimodal magmatism in the Chuacús complex was probably originated in a within-plate setting. Regardless of their age or structure, the studied metagranites preserve evidences for high-pressure metamorphic equilibration, such as the occurrence of Ca-rich garnet (XCa up to 0.52) in association with phengite (Si contents of up to 3.4 pfu) and rutile. The integration of Zr-in-rutile thermometry and phengite barometry allows the peak metamorphic conditions to be constrained at 640-680 °C and 13 kbar. This

Continental crust melting induced by subduction initiation of the South Tianshan Ocean: Insight from the Latest Devonian granitic magmatism in the southern Yili Block, NW China

NASA Astrophysics Data System (ADS)

Bao, Zihe; Cai, Keda; Sun, Min; Xiao, Wenjiao; Wan, Bo; Wang, Yannan; Wang, Xiangsong; Xia, Xiaoping

2018-03-01

The Tianshan belt of the southwestern Central Asian Orogenic Belt was generated by Paleozoic multi-stage subduction and final closure of several extinct oceans, including the South Tianshan Ocean between the Kazakhstan-Yili and Tarim blocks. However, the subduction initiation and polarity of the South Tianshan Ocean remain issues of highly debated. This study presents new zircon U-Pb ages, geochemical compositions and Sr-Nd isotopes, as well as zircon Hf isotopic data of the Latest Devonian to Early Carboniferous granitic rocks in the Wusun Mountain of the Yili Paleozoic convergent margin, which, together with the spatial-temporal distributions of regional magmatic rocks, are applied to elucidate their petrogenesis and tectonic linkage to the northward subduction initiation of the South Tianshan Ocean. Our zircon U-Pb dating results reveal that these granites were emplaced at the time interval of 362.0 ± 1.2-360.3 ± 1.9 Ma, suggesting a marked partial melting event of the continental crust in the Latest Devonian to Early Carboniferous. These granites, based on their mineral compositions and textures, can be categorized as monzogranites and K-feldspar granites. Geochemically, both monzogranites and K-feldspar granites have characters of I-type granites with high K2O contents (4.64-4.83 wt.%), and the K-feldspar granites are highly fractionated I-type granites, while the monzogranites have features of unfractionated I-type granites. Whole-rock Sr-Nd isotopic modeling results suggest that ca. 20-40% mantle-derived magmas may be involved in magma mixing with continental crust partial melts to generate the parental magmas of the granites. The mantle-derived basaltic magmas was inferred not only to be a major component of magma mixture but also as an important heat source to fuse the continental crust in an extensional setting, which is evidenced by the high zircon saturation temperatures (713-727 °C and 760-782 °C) of the studied granites. The Latest Devonian to

Magmatism evolution on the last Neoproterozoic development stage of the western Siberian active continental margin

NASA Astrophysics Data System (ADS)

Vernikovskaya, Antonina E.; Vernikovsky, Valery A.; Matushkin, Nikolay Yu.; Kadilnikov, Pavel I.; Romanova, Irina V.

2017-04-01

Rocks from active continental margin complexes are characterized by a wide variety of chemical compositions from depleted in alkali to alkali differentiates. When addressing issues of geodynamic settings in which such rocks form, it is important to understand the evolution of the host tectonic structure, as well as the chemical affiliation of the various rocks composing it. The Yenisey Ridge orogen located in the south-western framing of Siberia is one of the more studied regions with a long history of Neoproterozoic magmatic events. This orogen was formed during the collision of the Central Angara terrane with Siberia, which took place 761-718 Ma. Subsequent subduction-related events in the orogen have been recorded in the coeval magmatism (711-629 Ma) of two complexes: one is the active continental margin complex (Nb enriched igneous rocks - gabbroids, trachybasalts, A-type granites and carbonatites, including contact metasomatites zones with Nb mineralization), and the other one is an island arc complex (differentiated series volcanics, gabbroids and plagiogranites). The rocks of these complexes are respectively located in two suture zones: the Tatarka-Ishimba zone that formed due to the collision mentioned above, and the Yenisei suture marking the subduction zone [Vernikovsky et al., 2003; 2008]. The final Neoproterozoic stage in the evolution of the active margin of Siberia is manifested as adakite-gabbro-anorthosite magmatism in the 576-546 Ma interval. Our results indicate a genetic relationship between the adakites and their host NEB-type metabasites of the Zimovey massif. These Neoproterozoic adakites could have formed in a setting of transform-strike-slip drift of lithospheric plates after the subduction stopped, both from a crustal and mantle-crustal source, similarly to the Cenozoic magmatic complexes of the transform margin in the eastern framing of Eurasia [Khanchuk et al., 2016]. Vernikovsky V.A., Vernikovskaya A.E., Kotov A.B., Sal'nikova E

Density Sorting During the Evolution of Continental Crust

NASA Astrophysics Data System (ADS)

Kelemen, P. B.; Behn, M. D.; Hacker, B. R.

2015-12-01

We consider two settings - in addition to "delamination" of arc lower crust - in which dense, mafic eclogites founder into the convecting mantle while buoyant, felsic lithologies accumulate at the base of evolving continental crust. Arc processes play a central role in generating continental crust, but it remains uncertain how basaltic arc crust is transformed to andesitic continental crust. Dense, SiO2-poor products of fractionation may founder from the base of arc crust by "delamination", but lower arc crust after delamination has significantly different trace elements compared to lower continental crust (LCC). In an alternative model, buoyant magmatic rocks generated at arcs are first subducted, mainly via subduction erosion. Upon heating, these buoyant lithologies ascend through the mantle wedge or along a subduction channel, and are "relaminated" at
the base of overlying crust (e.g., Hacker et al EPSL 11, AREPS 15). Average buoyant lavas and plutons
for the Aleutians, Izu-Bonin-Marianas, Kohistan and Talkeetna arcs fall within the range of estimated LCC major and trace elements. Relamination is more efficient in generating continental crust than delamination. Himalayan cross-sections show Indian crust thrust beneath Tibetan crust, with no intervening mantle. There is a horizontal Moho at ca 80 km depth, extending from thickened Indian crust, across the region where Tibetan crust overlies Indian crust, into thickened Tibetan crust. About half the subducted Indian crust is present, whereas the other half is missing. Data (Vp/Vs; Miocene lavas formed by interaction of continental crust with mantle; xenolith thermometry) indicate 1000°C or more from ca 50 km depth to the Moho since the Miocene. We build on earlier studies (LePichon et al Tectonics 92, T'phys 97; Schulte-Pelkum et al Nature 05; Monsalve et al JGR 08) to advance the hypothesis that rapid growth of garnet occurs at 70-80 km and 1000°C within subducting Indian crust. Dense eclogites founder

The dynamic history of the Trans-Mexican Volcanic Belt and the Mexico subduction zone

NASA Astrophysics Data System (ADS)

Ferrari, Luca; Orozco-Esquivel, Teresa; Manea, Vlad; Manea, Marina

2012-02-01

The Trans-Mexican Volcanic Belt (TMVB) is a 1000 km long Neogene continental arc showing a large variation in composition and volcanic style, and an intra-arc extensional tectonics. It overlies the Rivera and Cocos slabs, which display marked changes in geometry. Geophysical studies indicate that lithospheric mantle is very thin or absent beneath the forearc and arc, the fluids from the slab are released in a 40 to 100 km wide belt beneath the frontal part of the arc, and the lower crust beneath the arc is partially molten. East of 101°W the TMVB is built on a Precambrian to Paleozoic crust with thickness of 50-55 km. West of 101°W the TMVB is underlain by Jurassic to Cenozoic marine and continental arcs with a 35-40 km thick crust. The evolution of the TMVB occurred in four stages: 1) from ~ 20 to 10 Ma the initial andesitic arc moved inland showing progressively drier melting and, eventually, slab melting, suggesting flattening of the subducted slab; 2) since ~ 11 Ma a pulse of mafic volcanism migrated from west to east reaching the Gulf of Mexico by 7 Ma. This mafic lavas marks the lateral propagation of a slab tear, triggered by cessation of subduction beneath Baja California; 3) thereafter, the volcanic front started moving trenchward, with a marked phase of silicic volcanism between 7.5 and 3 Ma, local emplacement of small volume intraplate-like basalts since 5 Ma, and development of extensional faulting. These features are related to slab rollback, enhancing asthenophere flux into the mantle wedge and promoting partial melting of the crust; 4) the modern arc consists of a frontal belt dominated by flux and slab melting, and a rear belt characterized by more differentiated rocks or by mafic lavas with little or no evidence of subduction fluids but higher asthenosphere fingerprint.

Are diamond-bearing Cretaceous kimberlites related to shallow-angle subduction beneath western North America?

NASA Astrophysics Data System (ADS)

Currie, C. A.; Beaumont, C.

2009-05-01

The origin of deep-seated magmatism (in particular, kimberlites and lamproites) within continental plate interiors remains enigmatic in the context of plate tectonic theory. One hypothesis proposes a relationship between kimberlite occurrence and lithospheric subduction, such that a subducting plate releases fluids below a continental craton, triggering melting of the deep lithosphere and magmatism (Sharp, 1974; McCandless, 1999). This study provides a quantitative evaluation of this hypothesis, focusing on the Late Cretaceous- Eocene (105-50 Ma) kimberlites and lamproites of western North America. These magmas were emplaced along a corridor of Archean and Proterozoic lithosphere, 1000-1500 km inboard of the plate margin separating the subducting Farallon Plate and continental North America Plate. Kimberlite-lamproite magmatism coincides with tectonic events, including the Laramide orogeny, shut-down of the Sierra Nevada arc, and eastward migration of volcanism, that are commonly attributed to a change in Farallon Plate geometry to a shallow-angle trajectory (<25° dip). Thermal-mechanical numerical models demonstrate that rapid Cretaceous plate convergence rates and enhanced westward velocity of North America result in shallow-angle subduction that places the Farallon Plate beneath the western edge of the cratonic interior of North America. This geometry is consistent with the observed continental dynamic subsidence that lead to the development of the Western Interior Seaway. The models also show that the subducting plate has a cool thermal structure, and subducted hydrous minerals (serpentine, phengite and phlogopite) remain stable to more than 1200 km from the trench, where they may break down and release fluids that infiltrate the overlying craton lithosphere. This is supported by geochemical studies that indicate metasomatism of the Colorado Plateau and Wyoming craton mantle lithosphere by an aqueous fluid and/or silicate melt with a subduction signature

Tracing subducted crustal materials in the mantle by using magnesium isotopes

NASA Astrophysics Data System (ADS)

Teng, F. Z.

2016-12-01

Recent studies show that some continental basalt, mantle-metasomatised peridotite and cratonic eclogite have heterogeneous Mg isotopic compositions. These isotopically distinct Mg isotopic compositions have been explained by the incorporation of subducted materials in their mantle sources though the detailed mechanisms are still not well understood. In particular, how Mg-poor crustal materials can modify Mg isotopic systematics of Mg-rich mantle is unknown. Subduction zones are the most efficient sites for crust and mantle interactions, hence should be where the most prominent Mg isotopic variation occurs. However, to date, little is known on Mg isotope systematics in the subduction factory. Here I first review and report new Mg isotopic data for arc lava, subarc peridotite and the subducted slab (marine sediment, altered basalt and abyssal peridotite), then use them to constrain the origins of mantle Mg isotopic heterogeneity and lay the foundation for using Mg isotopes as new tools for tracing crust-mantle interactions. The main conclusions are 1) fluid-rock interactions can modify Mg isotopic systematics of abyssal peridotites; 2) island arc lavas have non-MORB Mg isotopic compositions, reflecting distinct surbarc mantle Mg isotopic signature; 3) continental arcs have non-MORB Mg isotopic compositions, likely resulting from crustal contamination and 4) the isotopically heterogeneous continental basalts are mainly produced by mixing of isotopically distinct magmas instead of being partial melting products of metasomatised mantle peridotites.

Why Archaean TTG cannot be generated by MORB melting in subduction zones

NASA Astrophysics Data System (ADS)

Martin, Hervé; Moyen, Jean-François; Guitreau, Martin; Blichert-Toft, Janne; Le Pennec, Jean-Luc

2014-06-01

produced. Consequently, internal recycling of oceanic plateaus does not appear to be a suitable process for the genesis of Archaean continental crust. A possible alternative to this scenario is the subduction of oceanic plateaus. This hypothesis is supported by a present-day analog. In Ecuador, the Carnegie ridge, which is an oceanic plateau resulting from the Galapagos hot spot activity, is being subducted beneath the South American plate. Not only are the resulting magmas adakitic (TTG-like) in composition, but the volcanic productivity is several times greater than in other parts of the Andean volcanic arc. Above the location where the plateau is subducted, the arc is wide and the quaternary volcanoes numerous (about 80 active edifices). The volcanic productivity of each individual volcano also is more intense than away from the subduction focal point with an average output rate of about 0.4-0.5 km3·ka- 1 compared with only about 0.05-0.2 km3·ka- 1 for production rates at volcanoes erupting in the rest of the arc. Consequently, we infer that occasional subduction of oceanic plateaus throughout Earth's history can account for the episodic nature of crustal growth. Additionally, the generation by this mechanism of huge volumes of TTG-like magmas would readily dominate the crustal growth record.

Petrogenesis of the Majiari ophiolite (western Tibet, China): Implications for intra-oceanic subduction in the Bangong-Nujiang Tethys

NASA Astrophysics Data System (ADS)

Huang, Qiang-tai; Liu, Wei-liang; Xia, Bin; Cai, Zhou-rong; Chen, Wei-yan; Li, Jian-feng; Yin, Zheng-xin

2017-09-01

The Majiari ophiolite lies in the western Bangong-Nujiang Suture Zone, which separates the Qiangtang and Lhasa blocks in central Tibet. The ophiolite consists of peridotite, gabbro/diabase and basalt. Zircon U-Pb dating yielded an age of 170.5 ± 1.7 Ma for the gabbro, whereas 40Ar/39Ar dating of plagioclase from the same gabbro yielded ages of 108.4 ± 2.6 Ma (plateau age) and 112 ± 2 Ma (isochron age), indicating that the ophiolite was formed during the Middle Jurassic and was probably emplaced during the Early Cretaceous. Zircons from the gabbro have εHf(t) values ranging from +6.9 to +10.6 and f(Lu/Hf) values ranging from -0.92 to -0.98. Mafic lavas plot in the tholeiitic basalt field but are depleted in Nb, Ta and Ti and enriched in Rb, Ba and Th in the N-MORB-normalized trace element spider diagram. These lavas have whole-rock εNd(t) values of +5.9 to +6.6, suggesting that they were derived from a depleted mantle source, which was probably modified by subducted materials. The Majiari ophiolite probably formed in a typical back-arc basin above a supra-subduction zone (SSZ) mantle wedge. Intra-oceanic subduction occurred during the Middle Jurassic and collision of the Lhasa and South Qiangtang terranes likely occurred in the Early Cretaceous. Thus, closure of the Bangong-Nujiang Tethys Ocean likely occurred before the Early Cretaceous.

The Lord Howe Rise continental ribbon: a fragment of eastern Gondwana that reveals the drivers of continental rifting and plate tectonics

NASA Astrophysics Data System (ADS)

Saito, S.; Hackney, R. I.; Bryan, S. E.; Kimura, J. I.; Müller, D.; Arculus, R. J.; Mortimer, N. N.; Collot, J.; Tamura, Y.; Yamada, Y.

2016-12-01

Plate tectonics and resulting changes in crustal architecture profoundly influence global climate, oceanic circulation, and the origin, distribution and sustainability of life. Ribbons of continental crust rifted from continental margins are one product of plate tectonics that can influence the Earth system. Yet we have been unable to fully resolve the tectonic setting and evolution of huge, thinned, submerged, and relatively inaccessible continental ribbons like the Lord Howe Rise (LHR), which formed during Cretaceous fragmentation of eastern Gondwana. Thinned continental ribbons like the LHR are not easily explained or predicted by plate-tectonic theory. However, because Cretaceous rift basins on the LHR preserve the stratigraphy of an un-accreted and intact continental ribbon, they can help to determine whether plate motion is self-organised—passively driven by the pull of negatively-buoyant subducting slabs—or actively driven by convective flow in the mantle. In a self-organising scenario, the LHR formed in response to ocean-ward retreat of the long-lived eastern Gondwana subduction zone and linked upper-plate extension. In the mantle-driven scenario, the LHR resulted from rifting near the eastern edge of Gondwana that was triggered by processes linked to emplacement of a silicic Large Igneous Province. These scenarios can be distinguished using the ribbon's extensional history and the composition and tectonic affinity of igneous rocks within rift basins. However, current knowledge of LHR rift basins is based on widely-distributed marine and satellite geophysical data, limited dredge samples, and sparse shallow drilling (<600 m below-seafloor). This limits our ability to understand the evolution of extended continental ribbons, but a recent deep crustal seismic survey across the LHR and a proposed IODP deep stratigraphic well through a LHR rift basin provide new opportunities to explore the drivers behind rifting, continental ribboning and plate tectonics.

Amphibious Shear Velocity Structure of the Cascadia Subduction Zone

NASA Astrophysics Data System (ADS)

Janiszewski, H. A.; Gaherty, J. B.; Abers, G. A.; Gao, H.

2017-12-01

The amphibious Cascadia Initiative crosses the coastline of the Cascadia subduction zone (CSZ) deploying seismometers from the Juan de Fuca ridge offshore to beyond the volcanic arc onshore. This allows unprecedented seismic imaging of the CSZ, enabling examination of both the evolution of the Juan de Fuca plate prior to and during subduction as well as the along strike variability of the subduction system. Here we present new results from an amphibious shear velocity model for the crust and upper mantle across the Cascadia subduction zone. The primary data used in this inversion are surface-wave phase velocities derived from ambient-noise Rayleigh-wave data in the 10 - 20 s period band, and teleseismic earthquake Rayleigh wave phase velocities in the 20 - 160 s period band. Phase velocity maps from these data reflect major tectonic structures including the transition from oceanic to continental lithosphere, Juan de Fuca lithosphere that is faster than observations in the Pacific for oceanic crust of its age, slow velocities associated with the accretionary prism, the front of the fast subducting slab, and the Cascades volcanic arc which is associated with slower velocities in the south than in the north. Crustal structures are constrained by receiver functions in the offshore forearc and onshore regions, and by active source constraints on the Juan de Fuca plate prior to subduction. The shear-wave velocities are interpreted in their relationships to temperature, presence of melt or hydrous alteration, and compositional variation of the CSZ.

Continental crustal formation and recycling: Evidence from oceanic basalts

NASA Technical Reports Server (NTRS)

Saunders, A. D.; Tarney, J.; Norry, M. J.

1988-01-01

Despite the wealth of geochemical data for subduction-related magma types, and the clear importance of such magmas in the creation of continental crust, there is still no concensus about the relative magnitudes of crustal creation versus crustal destruction (i.e., recycling of crust into the mantle). The role of subducted sediment in the formation of the arc magmas is now well documented; but what proportion of sediment is taken into the deeper mantle? Integrated isotopic and trace element studies of magmas erupted far from presently active subduction zones, in particular basaltic rocks erupted in the ocean basins, are providing important information about the role of crustal recycling. By identifying potential chemical tracers, it is impossible to monitor the effects of crustal recycling, and produce models predicting the mass of material recycled into the mantle throughout long periods of geological time.

Future accreted terranes: a compilation of island arcs, oceanic plateaus, submarine ridges, seamounts, and continental fragments

NASA Astrophysics Data System (ADS)

Tetreault, J. L.; Buiter, S. J. H.

2014-12-01

Allochthonous accreted terranes are exotic geologic units that originated from anomalous crustal regions on a subducting oceanic plate and were transferred to the overriding plate by accretionary processes during subduction. The geographical regions that eventually become accreted allochthonous terranes include island arcs, oceanic plateaus, submarine ridges, seamounts, continental fragments, and microcontinents. These future allochthonous terranes (FATs) contribute to continental crustal growth, subduction dynamics, and crustal recycling in the mantle. We present a review of modern FATs and their accreted counterparts based on available geological, seismic, and gravity studies and discuss their crustal structure, geological origin, and bulk crustal density. Island arcs have an average crustal thickness of 26 km, average bulk crustal density of 2.79 g cm-3, and three distinct crustal units overlying a crust-mantle transition zone. Oceanic plateaus and submarine ridges have an average crustal thickness of 21 km and average bulk crustal density of 2.84 g cm-3. Continental fragments presently on the ocean floor have an average crustal thickness of 25 km and bulk crustal density of 2.81 g cm-3. Accreted allochthonous terranes can be compared to these crustal compilations to better understand which units of crust are accreted or subducted. In general, most accreted terranes are thin crustal units sheared off of FATs and added onto the accretionary prism, with thicknesses on the order of hundreds of meters to a few kilometers. However, many island arcs, oceanic plateaus, and submarine ridges were sheared off in the subduction interface and underplated onto the overlying continent. Other times we find evidence of terrane-continent collision leaving behind accreted terranes 25-40 km thick. We posit that rheologically weak crustal layers or shear zones that were formed when the FATs were produced can be activated as detachments during subduction, allowing parts of the FAT

Velocities of Subducted Sediments and Continents

NASA Astrophysics Data System (ADS)

Hacker, B. R.; van Keken, P. E.; Abers, G. A.; Seward, G.

2009-12-01

The growing capability to measure seismic velocities in subduction zones has led to unusual observations. For example, although most minerals have VP/ VS ratios around 1.77, ratios <1.7 and >1.8 have been observed. Here we explore the velocities of subducted sediments and continental crust from trench to sub-arc depths using two methods. (1) Mineralogy was calculated as a function of P & T for a range of subducted sediment compositions using Perple_X, and rock velocities were calculated using the methodology of Hacker & Abers [2004]. Calculated slab-top temperatures have 3 distinct depth intervals with different dP/dT gradients that are determined by how coupling between the slab and mantle wedge is modeled. These three depth intervals show concomitant changes in VP and VS: velocities initially increase with depth, then decrease beyond the modeled decoupling depth where induced flow in the wedge causes rapid heating, and increase again at depth. Subducted limestones, composed chiefly of aragonite, show monotonic increases in VP/ VS from 1.63 to 1.72. Cherts show large jumps in VP/ VS from 1.55-1.65 to 1.75 associated with the quartz-coesite transition. Terrigenous sediments dominated by quartz and mica show similar, but more-subdued, transitions from ~1.67 to 1.78. Pelagic sediments dominated by mica and clinopyroxene show near-monotonic increases in VP/ VS from 1.74 to 1.80. Subducted continental crust that is too dry to transform to high-pressure minerals has a VP/ VS ratio of 1.68-1.70. (2) Velocity anisotropy calculations were made for the same P-T dependent mineralogies using the Christoffel equation and crystal preferred orientations measured via electron-backscatter diffraction for typical constituent phases. The calculated velocity anisotropies range from 5-30%. For quartz-rich rocks, the calculated velocities show a distinct depth dependence because crystal slip systems and CPOs change with temperature. In such rocks, the fast VP direction varies from slab

Eclogite-facies metamorphism in impure marble from north Qaidam orogenic belt: Geodynamic implications for early Paleozoic continental-arc collision

NASA Astrophysics Data System (ADS)

Chen, Xin; Xu, Rongke; Schertl, Hans-Peter; Zheng, Youye

2018-06-01

In the North Qaidam ultrahigh-pressure (UHP) metamorphic belt, impure marble and interbedded eclogite represent a particular sedimentary provenance and tectonic setting, which have important implications for a controversial problem - the dynamic evolution of early Paleozoic subduction-collision complexes. In this contribution, detailed field work, mineral chemistry, and whole-rock geochemistry are presented for impure marble to provide the first direct evidence for the recycling of carbonate sediments under ultrahigh-pressures during subduction and collision in the Yuka terrane, in the North Qaidam UHP metamorphic belt. According to conventional geothermobarometry, pre-peak subduction to 0.8-1.3 GPa/485-569 °C was followed by peak UHP metamorphism at 2.5-3.3 GPa/567-754 °C and cooling to amphibolite facies conditions at 0.6-0.7 GPa/571-589 °C. U-Pb dating of zircons from impure marble reveals a large group with ages ranging from 441 to 458 Ma (peak at 450 Ma), a smaller group ranging from 770 to 1000 Ma (peak at 780 Ma), and minor >1.8 Ga zircon aged ca. 430 Ma UHP metamorphism. The youngest detrital zircons suggest a maximum depositional age of ca. 442 Ma and a burial rate of ca. 1.0-1.1 cm/yr when combined with P-T conditions and UHP metamorphic age. The REE and trace element patterns of impure marble with positive Sr and U anomalies, negative high field strength elements (Nb, Ta, Zr, Hf, and Ti), and Ce anomalies imply that the marble had a marine limestone precursor. Impure marble intercalated with micaschist and eclogite was similar to limestone and siltstone protoliths deposited in continental fore-arc or arc setting with basic volcanic activity. Therefore, the Yuka terrane most likely evolved in a continental island arc setting during the Paleozoic. These data suggest that metasediments were derived from a mixture of Proterozoic continental crust and juvenile early Paleozoic oceanic and/or island arc crust. In addition, their protoliths were likely

Implications of estimated magmatic additions and recycling losses at the subduction zones of accretionary (non-collisional) and collisional (suturing) orogens

USGS Publications Warehouse

Scholl, D. W.; von Huene, Roland E.

2009-01-01

Arc magmatism at subduction zones (SZs) most voluminously supplies juvenile igneous material to build rafts of continental and intra-oceanic or island arc (CIA) crust. Return or recycling of accumulated CIA material to the mantle is also most vigorous at SZs. Recycling is effected by the processes of sediment subduction, subduction erosion, and detachment and sinking of deeply underthrust sectors of CIA crust. Long-term (>10-20 Ma) rates of additions and losses can be estimated from observational data gathered where oceanic crust underruns modern, long-running (Cenozoic to mid-Mesozoic) ocean-margin subduction zones (OMSZs, e.g. Aleutian and South America SZs). Long-term rates can also be observationally assessed at Mesozoic and older crust-suturing subduction zone (CSSZs) where thick bodies of CIA crust collided in tectonic contact (e.g. Wopmay and Appalachian orogens, India and SE Asia). At modern OMSZs arc magmatic additions at intra-oceanic arcs and at continental margins are globally estimated at c. 1.5 AU and c. 1.0 AU, respectively (1 AU, or Armstrong Unit,= 1 km3 a-1 of solid material). During collisional suturing at fossil CSSZs, global arc magmatic addition is estimated at 0.2 AU. This assessment presumes that in the past the global length of crustal collision zones averaged c. 6000 km, which is one-half that under way since the early Tertiary. The average long-term rate of arc magmatic additions extracted from modern OMSZs and older CSSZs is thus evaluated at 2.7 AU. Crustal recycling at Mesozoic and younger OMSZs is assessed at c. 60 km3 Ma-1 km-1 (c. 60% by subduction erosion). The corresponding global recycling rate is c. 2.5 AU. At CSSZs of Mesozoic, Palaeozoic and Proterozoic age, the combined upper and lower plate losses of CIA crust via subduction erosion, sediment subduction, and lower plate crustal detachment and sinking are assessed far less securely at c. 115 km3 Ma-1 km-1. At a global length of 6000 km, recycling at CSSZs is accordingly c. 0

Submarine slope failures along the convergent continental margin of the Middle America Trench

NASA Astrophysics Data System (ADS)

Harders, Rieka; Ranero, CéSar R.; Weinrebe, Wilhelm; Behrmann, Jan H.

2011-06-01

We present the first comprehensive study of mass wasting processes in the continental slope of a convergent margin of a subduction zone where tectonic processes are dominated by subduction erosion. We have used multibeam bathymetry along ˜1300 km of the Middle America Trench of the Central America Subduction Zone and deep-towed side-scan sonar data. We found abundant evidence of large-scale slope failures that were mostly previously unmapped. The features are classified into a variety of slope failure types, creating an inventory of 147 slope failure structures. Their type distribution and abundance define a segmentation of the continental slope in six sectors. The segmentation in slope stability processes does not appear to be related to slope preconditioning due to changes in physical properties of sediment, presence/absence of gas hydrates, or apparent changes in the hydrogeological system. The segmentation appears to be better explained by changes in slope preconditioning due to variations in tectonic processes. The region is an optimal setting to study how tectonic processes related to variations in intensity of subduction erosion and changes in relief of the underthrusting plate affect mass wasting processes of the continental slope. The largest slope failures occur offshore Costa Rica. There, subducting ridges and seamounts produce failures with up to hundreds of meters high headwalls, with detachment planes that penetrate deep into the continental margin, in some cases reaching the plate boundary. Offshore northern Costa Rica a smooth oceanic seafloor underthrusts the least disturbed continental slope. Offshore Nicaragua, the ocean plate is ornamented with smaller seamounts and horst and graben topography of variable intensity. Here mass wasting structures are numerous and comparatively smaller, but when combined, they affect a large part of the margin segment. Farther north, offshore El Salvador and Guatemala the downgoing plate has no large seamounts but

Interaction of the subduction process and forearc tectonics: An example from the active N - Chilean margin

NASA Astrophysics Data System (ADS)

Victor, P.; Sobiesiak, M.

2005-12-01

Convergent plate boundaries at continental margins belong to the tectonically most active areas on earth and are endangered by devastating earthquakes and tsunamis. The north Chilean margin is a high strain continental margin driven by fast plate convergence rate. The greatest amount of strain is accommodated along the subduction interface. Nevertheless there is extensive crustal deformation obvious by surface ruptures along reactivated segments of large fault systems and vertical surface motions reflecting the interaction between subducting and overriding plates. The historical seismicity record indicates that great earthquakes affect the Chilean Forearc with recurrence intervals of about 112+/- 21 y . The last great event in northern Chile occurred in 1995 near Antofagasta. The Mw= 8.0 event ruptured the subduction interface 180 km along strike with an average slip of about 5m in the depth interval between 10-50 km. From careful evaluation of the aftershock sequence by examining the different catagories of aftershock focal mechanisms we can define three segments of the seismogenic zone affected by the Antofagasta main shock. The non-ruptured northern segment beneath Mejillones Peninsula is seperated by a broad transition zone from the central segment which hosts the earthquakes' rupture plane. The southern fault plane boundary is identified by linear alignment of all apparent aftershock mechanisms. Along this southern boundary the strike slip mechanisms are exclusively left lateral whereas the strike slip mechanisms along the northern transition zone are right lateral. The orientations of summed moment tensors calculated from aftershock fault plane solutions on the northern segment and in the northern transition zone differ from the orientations exhibited by moment tensors on the central segment. This might indicate a rotational component in the coseismic movement of the ruptured segment relative to the non-ruptured segment. The observed segmentation of the

History and evolution of Subduction in the Precambrium

NASA Astrophysics Data System (ADS)

Fischer, R.; Gerya, T.

2013-12-01

Plate tectonics is a global self-organising process driven by negative buoyancy at thermal boundary layers. Phanerozoic plate tectonics with its typical subduction and orogeny is relatively well understood and can be traced back in the geological records of the continents. Interpretations of geological, petrological and geochemical observations from Proterozoic and Archean orogenic belts however (e.g. Brown, 2006), suggest a different tectonic regime in the Precambrian. Due to higher radioactive heat production the Precambrian lithosphere shows lower internal strength and is strongly weakened by percolating melts. The fundamental difference between Precambrian and Phanerozoic subduction is therefore the upper-mantle temperature, which determines the strength of the upper mantle (Brun, 2002) and the further subduction history. 3D petrological-thermomechanical numerical modelling experiments of oceanic subduction at an active plate at different upper-mantle temperatures show these different subduction regimes. For upper-mantle temperatures < 175 K above the present day value a subduction style appears which is close to present day subduction but with more frequent slab break-off. At upper-mantle temperatures 175 - 250 K above present day values steep subduction changes to shallow underplating and buckling. For upper-mantle temperatures > 250 K above the present day value no subduction occurs any more. The whole lithosphere starts to delaminate and drip-off. But the subduction style is not only a function of upper-mantle temperature but also strongly depends on the thickness of the subducting plate. If thinner present day oceanic plates are used in the Precambrian models, no shallow underplating is observed but steep subduction can be found up to an upper-mantle temperature of 200 K above present day values. Increasing oceanic plate thickness introduces a transition from steep to flat subduction at lower temperatures of around 150 K. Thicker oceanic plates in the

Subduction Related Crustal and Mantle Deformations and Their Implications for Plate Dynamics

NASA Astrophysics Data System (ADS)

Okeler, Ahmet

Ocean-continent convergence and subsequent continental collision are responsible for continental growth, mountain building, and severe tectonic events including volcanic eruptions and earthquake activity. They are also key driving forces behind the extensive thermal and compositional heterogeneities at crustal and mantle depths. Active subduction along the Calabrian Arc in southern Italy and the Hellenic Arc are examples of such collisional tectonics. The first part of this thesis examines the subduction related deformations within the crust beneath the southern Apennines. By modeling regional surface wave recordings of the largest temporary deployment in the southern Apennines, a lower-crustal/upper-mantle low-velocity volume extending down to 50 km beneath the mountain chain is identified. The magnitude (˜ 0.4 km/s slower) and anisotropic nature (˜ 10%) of the anomaly suggest the presence of hot and partially molten emplacement that may extend into the upper-crust towards Mt. Vulture, a once active volcano. Since the Apulian basement units are deformed during the compressional and consequent extensional events, our observations favor the "thick-skin" tectonic growth model for the region. In the deeper mantle, active processes are thermodynamically imprinted on the depth and strength of the phase transitions. This thesis examines more than 15000 SS precursors and provides the present-day reflectivity structure and topography associated with these phase transitions. Through case studies I present ample evidence for both slab penetration into the lower mantle (beneath the Hellenic Arc, Kurile Island and South America) and slab stagnation at the bottom of the Mantle Transition Zone (beneath the Tyrrhenian Sea and eastern China). Key findings include (1) thermal anomalies (˜ 200 K) at the base of the MTZ, which represent the deep source for Cenozoic European Rift Zone, Mount Etna and Mount Cameroon volcanism, (2) significant depressions (by 20-40 km) at the bottom

Subduction-modified oceanic crust in the sources of continental picrite dikes from the Karoo LIP?

NASA Astrophysics Data System (ADS)

Heinonen, J. S.; Carlson, R. W.; Riley, T. R.; Luttinen, A. V.; Horan, M. F.

2013-12-01

The Ahlmannryggen mountain range in East Antarctica hosts unusual LILE-depleted, but Fe- and Ti-enriched ultramafic dikes (Group 3) that belong to the Jurassic (~180 Ma) Karoo continental flood basalt (CFB) province. Their high initial ɛNd (+5 to +9) indicates their origin within the sublithospheric mantle beneath the Gondwana supercontinent. Using the new Pb and Os isotopic data and previously published geochemical and mineral chemical data, we try to constrain their mantle sources. The dikes that lack evidence of crustal contamination exhibit very radiogenic ɛNd (+8.6 to +9.0), relatively radiogenic 206Pb/204Pb (18.2-18.4) and 87Sr/86Sr (0.7035-0.7037), and unradiogenic 187Os/188Os (0.124-0.125) at 180 Ma. These isotopic compositions are unlike those typical of MORBs, excluding depleted mantle as the sole source contributor. The Pb isotopic composition of the dikes plots close to the 4.43 Ga geochron and hence is compatible with derivation from an early-depleted reservoir (EDR), recently suggested to be a major source component in CFBs. However, the high ɛNd of the dikes exceeds the ɛNd estimated for EDR (+4.9 to +8.5 at 180 Ma) and the relative Nb, Fe, and Ti enrichment (pyroxenite fingerprint) of the dikes is not readily ascribed to EDR source. Based on our isotopic and trace element modeling, we regard that the mantle source of the picrite dikes contained seawater-altered and subduction-modified MORB with a recycling age of 0.8 Ga. Such a source component would explain the unusual combination of elevated initial 87Sr/86Sr, ɛNd, and 206Pb/204Pb, relative depletion in fluid-mobile LILE, U, Th, Pb, and LREE, and relative enrichment in Nb, Fe, Ti, and other HFSE. Behavior of Re and Os in subduction environments is not well constrained, but loss of Re from recycled MORB, as observed in some subduction-associated eclogites and blueschists, and predominant contribution of Os from depleted peridotite matrix could have produced the observed low 187Os/188Os

Future accreted terranes: a compilation of island arcs, oceanic plateaus, submarine ridges, seamounts, and continental fragments

NASA Astrophysics Data System (ADS)

Tetreault, J. L.; Buiter, S. J. H.

2014-07-01

Allochthonous accreted terranes are exotic geologic units that originated from anomalous crustal regions on a subducting oceanic plate and were transferred to the overriding plate during subduction by accretionary processes. The geographical regions that eventually become accreted allochthonous terranes include island arcs, oceanic plateaus, submarine ridges, seamounts, continental fragments, and microcontinents. These future allochthonous terranes (FATs) contribute to continental crustal growth, subduction dynamics, and crustal recycling in the mantle. We present a review of modern FATs and their accreted counterparts based on available geological, seismic, and gravity studies and discuss their crustal structure, geological origin, and bulk crustal density. Island arcs have an average crustal thickness of 26 km, average bulk crustal density of 2.79 g cm-3, and have 3 distinct crustal units overlying a crust-mantle transition zone. Oceanic plateaus and submarine ridges have an average crustal thickness of 21 km and average bulk crustal density of 2.84 g cm-3. Continental fragments presently on the ocean floor have an average crustal thickness of 25 km and bulk crustal density of 2.81 g cm-3. Accreted allochthonous terranes can be compared to these crustal compilations to better understand which units of crust are accreted or subducted. In general, most accreted terranes are thin crustal units sheared off of FATs and added onto the accretionary prism, with thicknesses on the order of hundreds of meters to a few kilometers. In addition many island arcs, oceanic plateaus, and submarine ridges were sheared off in the subduction interface and underplated onto the overlying continent. And other times we find evidence of collision leaving behind accreted terranes 25 to 40 km thick. We posit that rheologically weak crustal layers or shear zones that were formed when the FATs were produced can be activated as detachments during subduction, allowing parts of the FAT crust to

Hanging canyons of Haida Gwaii, British Columbia, Canada: Fault-control on submarine canyon geomorphology along active continental margins

NASA Astrophysics Data System (ADS)

Harris, Peter T.; Barrie, J. Vaughn; Conway, Kim W.; Greene, H. Gary

2014-06-01

Faulting commonly influences the geomorphology of submarine canyons that occur on active continental margins. Here, we examine the geomorphology of canyons located on the continental margin off Haida Gwaii, British Columbia, that are truncated on the mid-slope (1200-1400 m water depth) by the Queen Charlotte Fault Zone (QCFZ). The QCFZ is an oblique strike-slip fault zone that has rates of lateral motion of around 50-60 mm/yr and a small convergent component equal to about 3 mm/yr. Slow subduction along the Cascadia Subduction Zone has accreted a prism of marine sediment against the lower slope (1500-3500 m water depth), forming the Queen Charlotte Terrace, which blocks the mouths of submarine canyons formed on the upper slope (200-1400 m water depth). Consequently, canyons along this margin are short (4-8 km in length), closely spaced (around 800 m), and terminate uniformly along the 1400 m isobath, coinciding with the primary fault trend of the QCFZ. Vertical displacement along the fault has resulted in hanging canyons occurring locally. The Haida Gwaii canyons are compared and contrasted with the Sur Canyon system, located to the south of Monterey Bay, California, on a transform margin, which is not blocked by any accretionary prism, and where canyons thus extend to 4000 m depth, across the full breadth of the slope.

From rifting to subduction: the role of inheritance in the Wilson Cycle

NASA Astrophysics Data System (ADS)

Beaussier, Stéphane; Gerya, Taras; Burg, Jean-Pierre

2017-04-01

The Wilson Cycle entails that oceans close and reopen. This cycle is a fundamental principle in plate tectonics, inferring continuity from divergence to convergence and that continental rifting takes place along former suture zones. This view questions the role of inherited structures at each stage of the Wilson Cycle. Using the 3D thermo-mechanical code, I3ELVIS (Gerya and Yuen 2007) we present a high-resolution continuous model of the Wilson cycle from continental rifting, breakup and oceanic spreading to convergence and spontaneous subduction initiation. Therefore, all lateral and longitudinal structures of the lithospheres are generated self-consistently and are consequences of the initial continental structure, tectono-magmatic inheritance and material rheology. In the models, subduction systematically initiates off-ridge and is controlled by the convergence-induced swelling of the ridge. Geometry and dynamics of the developing off-ridge subduction is controlled by four main factors: (1) the obliquity of the ridge with respect to the convergence direction; (2) fluid-induced weakening of the oceanic crust; (3) irregularity of ridge and margins inherited from rifting and spreading; (4) strain localization at transform faults formed during ocean floor spreading. Further convergence can lead to obduction of the oceanic crust and segments of ridge after the oceanic lithosphere is entrained into subduction. We show that the main parameters controlling the occurrence and geometry of obducted ophiolite are the convergence rate and the inherited structure of the passive margins and ridge. Our numerical experiments results show the essential role played by inheritance during the Wilson Cycle and are consistent with nature observations such as the tectonic history of the Oman subduction-obduction system. REFERENCES Gerya, T. V., and D. A. Yuen. 2007: "Robust Characteristics Method for Modelling Multiphase Visco-Elasto-Plastic Thermo-Mechanical Problems, Physics of the

The effects of subduction termination on the continental lithosphere: Linking volcanism, deformation, surface uplift, and slab tearing in central Anatolia

NASA Astrophysics Data System (ADS)

Delph, Jonathan R.; Abgarmi, Bijan; Ward, Kevin M.; Beck, Susan L.; Arda Ozacar, A.; Zandt, George; Sandvol, Eric; Turkelli, Niyazi; Kalafat, Dogan

2017-04-01

The lithospheric evolution of Anatolia is largely defined by processes associated with the terminal stages of subduction along its southern margin. Central Anatolia represents the transition from the subduction of oceanic lithosphere at the Aegean trench in the west to the Arabian - Eurasian continental collision in the east. In the overriding plate, this complicated transition is contemporaneous with uplift along the southern margin of central Anatolia (2 km in 6 Myr), voluminous felsic-intermediate ignimbrite eruptions (>1000 km3), extension, and tectonic deformation reflected by abundant low-magnitude seismic activity. The addition of 72 seismic stations as part of the Continental Dynamics - Central Anatolian Tectonics project, along with development of a new approach to the joint inversion of receiver functions and dispersion data, enables us obtain a high-resolution 3D shear wave velocity model of central Anatolia down to 150 km. This new velocity model has important implications for the complex interactions between the downgoing, segmenting African lithosphere and the overriding Anatolian Plate. These results reveal that the lithosphere of central Anatolia and the northern Arabian Plate is thin (<50 to 80 km). The Central Taurus Mountains, which have experienced 2 km of uplift in the past 6 Ma, are underlain by the fastest shear velocities in the region (>4.5 km/s), indicating the presence of the Cyprean slab beneath central Anatolia. Thus, uplift of the Central Taurus Mountains may be due to slab rebound after the detachment of the oceanic portion of the Cyprean slab beneath Anatolia rather than the presence of shallow asthenospheric material. These fast velocities extend to the northern margin of the Central Taurus Mountains, giving way to a NE-SW trend of very slow upper mantle shear wave velocities (<4.2 km/s) beneath the Central Anatolian Volcanic Province. These slow velocities are interpreted to be shallow, warm asthenosphere in which melt is present

Crustal Gravitational Potential Energy Change and Subduction Earthquakes

NASA Astrophysics Data System (ADS)

Zhu, P. P.

2017-05-01

Crustal gravitational potential energy (GPE) change induced by earthquakes is an important subject in geophysics and seismology. For the past forty years the research on this subject stayed in the stage of qualitative estimate. In recent few years the 3D dynamic faulting theory provided a quantitative solution of this subject. The theory deduced a quantitative calculating formula for the crustal GPE change using the mathematic method of tensor analysis under the principal stresses system. This formula contains only the vertical principal stress, rupture area, slip, dip, and rake; it does not include the horizontal principal stresses. It is just involved in simple mathematical operations and does not hold complicated surface or volume integrals. Moreover, the hanging wall vertical moving (up or down) height has a very simple expression containing only slip, dip, and rake. The above results are significant to investigate crustal GPE change. Commonly, the vertical principal stress is related to the gravitational field, substituting the relationship between the vertical principal stress and gravitational force into the above formula yields an alternative formula of crustal GPE change. The alternative formula indicates that even with lack of in situ borehole measured stress data, scientists can still quantitatively calculate crustal GPE change. The 3D dynamic faulting theory can be used for research on continental fault earthquakes; it also can be applied to investigate subduction earthquakes between oceanic and continental plates. Subduction earthquakes hold three types: (a) crust only on the vertical up side of the rupture area; (b) crust and seawater both on the vertical up side of the rupture area; (c) crust only on the vertical up side of the partial rupture area, and crust and seawater both on the vertical up side of the remaining rupture area. For each type we provide its quantitative formula of the crustal GPE change. We also establish a simplified model (called

Evolution of supercritical fluid in deeply subducted continental crust: a case study of composite granite-quartz veins in the Sulu belt, China

NASA Astrophysics Data System (ADS)

Wang, S.; Wang, L.; Brown, M.

2016-12-01

Although fluid plays a key role in element transport and rock strength during subduction to and exhumation from ultrahigh pressure (UHP) metamorphic conditions, the source of supercritical fluid at P above the second critical endpoints (SCE) and the subsequent evolution are not well constrained. To provide insight into the evolution of supercritical fluid in continental subduction zones, we undertook an integrated study of composite granite-quartz veins in retrogressed and migmatitic UHP eclogite at General's Hill, N of Qingdao, in the central Sulu belt. The composite veins are irregularly distributed in the eclogite, which occurs as blocks within gneiss. The granite component is enriched in large ion lithophile elements and light rare earth elements but depleted in high field strength elements and heavy rare earth elements, indicating crystallization from a melt phase of crustal origin. Additionally, the granite contains high modal phengite (22-30 vol%) and clinozoisite/epidote (3-10 vol%), implying precipitation from a H2O-rich silicate melt. By contrast, the quartz component is dominated by SiO2 (99.10 wt%), and contains low total rare earth elements (ΣREE = 0.46 ppm), indicating precipitation from an aqueous fluid. The crystallization age of the composite veins is 221 ± 2 Ma, which is younger than the UHP metamorphism in the Sulu belt at ca 230 Ma, consistent with formation during exhumation. Initial 176Hf/177Hf ratios and δ18O values of metamorphic zircons from the composite veins, and Sr-Nd isotope compositions of the granites all lie between values for eclogite and gneiss, indicating a mixed source. Accordingly, we propose that a supercritical fluid generated from the gneiss and the included blocks of eclogite at P-T conditions above the SCE for both compositions became trapped in the eclogite during exhumation. At P below the SCE for the hydrous granite system, the mixed supercritical fluid separated into immiscible aqueous melt and aqueous fluid and

Transition from strike-slip faulting to oblique subduction: active tectonics at the Puysegur Margin, South New Zealand

NASA Astrophysics Data System (ADS)

Lamarche, Geoffroy; Lebrun, Jean-Frédéric

2000-01-01

South of New Zealand the Pacific-Australia (PAC-AUS) plate boundary runs along the intracontinental Alpine Fault, the Puysegur subduction front and the intraoceanic Puysegur Fault. The Puysegur Fault is located along Puysegur Ridge, which terminates at ca. 47°S against the continental Puysegur Bank in a complex zone of deformation called the Snares Zone. At Puysegur Trench, the Australian Plate subducts beneath Puysegur Bank and the Fiordland Massif. East of Fiordland and Puysegur Bank, the Moonlight Fault System (MFS) represents the Eocene strike-slip plate boundary. Interpretation of seafloor morphology and seismic reflection profiles acquired over Puysegur Bank and the Snares Zone allows study of the transition from intraoceanic strike-slip faulting along the Puysegur Ridge to oblique subduction at the Puysegur Trench and to better understand the genetic link between the Puysegur Fault and the MFS. Seafloor morphology is interpreted from a bathymetric dataset compiled from swath bathymetry data acquired during the 1993 Geodynz survey, and single beam echo soundings acquired by the NZ Royal Navy. The Snares Zone is the key transition zone from strike-slip faulting to subduction. It divides into three sectors, namely East, NW and SW sectors. A conspicuous 3600 m-deep trough (the Snares Trough) separates the NW and East sectors. The East sector is characterised by the NE termination of Puysegur Ridge into right-stepping en echelon ridges that accommodate a change of strike from the Puysegur Fault to the MFS. Between 48°S and 47°S, in the NW sector and the Snares Trough, a series of transpressional faults splay northwards from the Puysegur Fault. Between 49°50'S and 48°S, thrusts develop progressively at Puysegur Trench into a decollement. North of 48°S the Snares Trough develops between two splays of the Puysegur Fault, indicating superficial extension associated with the subsidence of Puysegur Ridge. Seismic reflection profiles and bathymetric maps show a

Neogene subduction beneath Java, Indonesia: Slab tearing and changes in magmatism

NASA Astrophysics Data System (ADS)

Cottam, Michael; Hall, Robert; Cross, Lanu; Clements, Benjamin; Spakman, Wim

2010-05-01

Java is a Neogene calc-alkaline volcanic island arc formed by the northwards subduction of the Indo-Australian Plate beneath Sundaland, the continental core of SE Asia. The island has a complex history of volcanism and displays unusual subduction characteristics. These characteristics are consistent with the subduction of a hole in the down going slab that was formed by the arrival of a buoyant oceanic plateau at the trench. Subduction beneath Java began in the Eocene. However, the position and character of the calc-alkaline arc has changed over time. An older Paleogene arc ceased activity in the Early Miocene. Volcanic activity resumed in the Late Miocene producing a younger arc to the north of the older arc, and continues to the present day. An episode of Late Miocene thrusting at about 7 Ma is observed throughout Java and appears to be linked to northward movement of the arc. Arc rocks display typical calc-alkaline characteristics and reflect melting of the mantle wedge and subducted sediments associated with high fluid fluxes. Between West Java and Bali the present arc-trench gap is unusually wide at about 300 km. Seismicity identifies subducted Indian Ocean lithosphere that dips north at about 20° between the trench and the arc and then dips more steeply at about 60-70° from 100 to 600 km depth. In East Java there is gap in seismicity between about 250 and 500 km. Seismic tomography shows that this gap is not an aseismic section of the subduction zone but a hole in the slab. East Java is also unusual in the presence of K-rich volcanoes, now inactive, to the north of the calc-alkaline volcanoes of the active arc. In contrast to the calc-alkaline volcanism of the main arc, these K-rich melts imply lower fluid fluxes and a different mantle source. We suggest that all these observations can be explained by the tearing of the subducting slab when a buoyant oceanic plateau arrived at the trench south of East Java at about 8 Ma. With the slab unable to subduct

Proterozoic Eastern Sayan ophiolites (Central Asian Orogenic Belt) record subduction initiation in vicinity of continental block

NASA Astrophysics Data System (ADS)

Belyaev, Vasilii; Gornova, Marina; Medvedev, Alexander; Dril, Sergey; Karimov, Anas

2017-04-01

volcanics and dikes are low-Ca and intermediate-Ca boninites, andesite-basalts, andesites, dacites of calc-alkaline (CA) affinity with rare evolved island-arc tholeiitic (IAT) andesite-basalts. They resemble appropriate rocks of intraoceanic island arcs, forearcs, and ophiolites. Boninites and CA-andesites are LREE-enriched (La/SmPM 1.2-3.8) at low HREE (0.5-1.6 ppm Yb) contents while evolved IAT show flat REE (La/SmPM = 1.1) and higher abundances (2.4-2.8 ppm Yb), and both have negative Nb anomalies. Nd-isotopic data expressed as epsilon Nd(1020Ma) values are -2.3 to +4.1 in cumulates, -2.8 to +0.4 in boninites and andesites, and +2.3 to +2.7 in IAT (compared to epsilon Nd(1020Ma) +7.8 in depleted mantle). The ophiolites obducted on the Gargan continental block, which contains Archean gneisses with epsilon Nd(1020) = -20 to -281. Subduction and recycling of sediments derived from these gneisses could explain enriched Nd isotopic characteristics of the studied ophiolitic rocks. The boninite-andesite-IAT association is usually found in subduction initiation settings recorded by modern forearc regions and forearc ophiolites. The difference of the Eastern Sayan ophiolites is their supposed formation close to ancient continental block which supplied recycled material into newly formed subduction zone. 1. Sklyarov et al (2016) Russ Geol Geophys 57, 127-140 2. Sobolev et al (1996) Petrology 3, 326-336.

A possible model for evaluating the topographic and mechanical effects of subducted oceanic plate with irregular surface

NASA Astrophysics Data System (ADS)

Kodama, K.

1987-02-01

A simple model is proposed to evaluate the possible mechanical and topographic effects that might be caused when an oceanic plate with irregular surface is subducted beneath a continental plate. The model focuses on the effects of such features as oceanic ridges, seamounts, plateaus and other bathymetric highs. The model approximates a continental plate as a two-dimensional incompressible Newtonian viscous fluid with uniform thickness, and a subducting oceanic plate as a rigid basement slipping beneath the viscous fluid with a constant velocity. In this model, bathymetric highs on the oceanic plate are approximated by topographic irregularities of the rigid basement. Based on the fundamental solutions given by Budd (1970) which were applied originally to the analysis of glacial movements, the shear stress near the basement and surface profile of the overriding medium are calculated, for instance, for a model whose basement profile is represented simply as exp( -ax 2) where a is the geometrical constant representing the degree of regional slope, and x the horizontal distance from the axis of the ridge. Assuming reasonable values for viscosity (10 23 poise), density (3 g/cm 3), thickness of the viscous medium (30 km), elevation of the top of the ridge from the surroundings (1 km), geometrical constant of the ridge shape (10 -2 km -2), and slip velocity (10 cm/a), the resultant surface profile was inferred to be asymmetrical shape with its highest elevation of 1360 m and the lowest of -620 m, while the magnitude of the shear stress near the base showed a symmetrical distribution with the maximum of 12.7 kbar. The results from these calculations do not only allow us to make quantitative estimates of the geological consequences of the subduction of bathymetric highs beneath continental plates but also give possible explanation for some of the uneven seismic activities found around present subduction zones.

Sculpting the Philippine archipelago since the Cretaceous through rifting, oceanic spreading, subduction, obduction, collision and strike-slip faulting: Contribution to IGMA5000

NASA Astrophysics Data System (ADS)

Aurelio, Mario A.; Peña, Rolando E.; Taguibao, Kristine Joy L.

2013-08-01

The Philippine archipelago resulted from a complex series of geologic events that involved continental rifting, oceanic spreading, subduction, ophiolite obduction, arc-continent collision, intra-arc basin formation and strike-slip faulting. It can be divided into two tectono-stratigraphic blocks, namely; the Palawan-Mindoro Continental Block (PCB) and the Philippine Mobile Belt (PMB). The PCB was originally a part of the Asian mainland that was rifted away during the Mesozoic and drifted in the course of the opening of the South China Sea (SCS) during Late Paleogene. On the other hand, the PMB developed mainly from island arcs and ophiolite terranes that started to form during the Cretaceous. At present, the PMB collides with the PCB in the Visayas in the central-western Philippines. This paper discusses recent updates on Philippine geology and tectonics as contribution to the establishment of the International Geologic Map of Asia at 1:5 M scale (IGMA5000).

Considering bioactivity in modelling continental growth and the Earth's evolution

NASA Astrophysics Data System (ADS)

Höning, D.; Spohn, T.

2013-09-01

The complexity of planetary evolution increases with the number of interacting reservoirs. On Earth, even the biosphere is speculated to interact with the interior. It has been argued (e.g., Rosing et al. 2006; Sleep et al, 2012) that the formation of continents could be a consequence of bioactivity harvesting solar energy through photosynthesis to help build the continents and that the mantle should carry a chemical biosignature. Through plate tectonics, the surface biosphere can impact deep subduction zone processes and the interior of the Earth. Subducted sediments are particularly important, because they influence the Earth's interior in several ways, and in turn are strongly influenced by the Earth's biosphere. In our model, we use the assumption that a thick sedimentary layer of low permeability on top of the subducting oceanic crust, caused by a biologically enhanced weathering rate, can suppress shallow dewatering. This in turn leads to greater vailability of water in the source region of andesitic partial melt, resulting in an enhanced rate of continental production and regassing rate into the mantle. Our model includes (i) mantle convection, (ii) continental erosion and production, and (iii) mantle water degassing at mid-ocean ridges and regassing at subduction zones. The mantle viscosity of our model depends on (i) the mantle water concentration and (ii) the mantle temperature, whose time dependency is given by radioactive decay of isotopes in the Earth's mantle. Boundary layer theory yields the speed of convection and the water outgassing rate of the Earth's mantle. Our results indicate that present day values of continental surface area and water content of the Earth's mantle represent an attractor in a phase plane spanned by both parameters. We show that the biologic enhancement of the continental erosion rate is important for the system to reach this fixed point. An abiotic Earth tends to reach an alternative stable fixed point with a smaller

Structures in the transition zone of the northeast South China Sea: serpentinite dome vs mantle exhumation, or evidence of Mesozoic active subduction transferring to Cenozoic passive extension?

NASA Astrophysics Data System (ADS)

Sun, Z.; Zhou, D.

2013-12-01

Complete sedimentary sequences and weak erosion make the transition zone of the South China Sea the optimal place to study the entire evolution history of marginal sea basins, as well as the transition mechanism from active subduction to passive extension. 2D long cable seismic profiles revealed that both Baiyun and Liwan sag in the northeast South China Sea margin were lack of large controlling faults, especially in Liwan sag, syn-rift sequences waved above the basement. Dome-like uplifts(serpetinite uplifts?) or diapirs(?) came from below the basement, caused the syn-rift sequences pushed up around 36Ma(T80). Gravity inversion based on seismic reflection indicated that the dome has a lower density and a lower layer velocity than normal crust. Also around the Continent-Ocean Boundary (COB), a small segment similar to the lower crust was exposed. Between this exposed segment and the Cenozoic oceanic crust, mantle seems to be exhumed along the breakup point. Between the COB and roughly the shelf break, high velocity lower crust was discriminated in the northeast continental margin. Structures in northeast South China Sea seems having many similarities with Newfoundland-Iberia margin, by serpentinite(?) dome and exhumed mantle, although spreading rate here is intermediate. In fact, regional background suggests that there might be another interpretation: transition from Mesozoic subduction to Cenozoic extension occurred through paleo oceanic crust breakup in the northeast, which in turn retained Mesozoic subduction system beneath the northeast continental margin. Confined with magnetic anomaly, Bouguer gravity gradient anomaly, and well drilling lithological evidences, Cenozoic Baiyun sag developed upon Mesozoic fore-arc, while Cenozoic Liwan sag developed upon Mesozoic accretionary prism. The high velocity lower crust was caused by both remnant subducted slab and by Oceanic-Continent interaction due to subduction. There might also be serpentinite dome and exhumed

Seismic Structure of Mantle Transition Zone beneath Northwest Pacific Subduction Zone and its Dynamic Implication

NASA Astrophysics Data System (ADS)

Li, J.; Guo, G.; WANG, X.; Chen, Q.

2017-12-01

The northwest Pacific subduction region is an ideal location to study the interaction between the subducting slab and upper mantle discontinuities. Various and complex geometry of the Pacific subducting slab can be well traced downward from the Kuril, Japan and Izu-Bonin trench using seismicity and tomography images (Fukao and Obayashi, 2013). Due to the sparse distribution of seismic stations in the sea, investigation of the deep mantle structure beneath the broad sea regions is very limited. In this study, we applied the well- developed multiple-ScS reverberations method (Wang et al., 2017) to analyze waveforms recorded by the Chinese Regional Seismic Network, the densely distributed temporary seismic array stations installed in east Asia. A map of the topography of the upper mantle discontinuities beneath the broad oceanic regions in northwest Pacific subduction zone is imaged. We also applied the receiver function analysis to waveforms recorded by stations in northeast China and obtain the detailed topography map beneath east Asia continental regions. We then combine the two kinds of topography of upper mantle discontinuities beneath oceanic and continental regions respectively, which are obtained from totally different methods. A careful image matching and spatial correlation is made in the overlapping study regions to calibrate results with different resolution. This is the first time to show systematically a complete view of the topography of the 410-km and 660-km discontinuities beneath the east Asia "Big mantle wedge" (Zhao and Ohtani, 2009) covering the broad oceanic and continental regions in the Northwestern Pacific Subduction zone. Topography pattern of the 660 and 410 is obtained and discussed. Especially we discovered a broad depression of the 410-km discontinuity covering more than 1000 km in lateral, which seems abnormal in the cold subducting tectonic environment. Based on plate tectonic reconstruction studies and HTHP mineral experiments, we

40K-(40)Ar constraints on recycling continental crust into the mantle

PubMed

Coltice; Albarede; Gillet

2000-05-05

Extraction of potassium into magmas and outgassing of argon during melting constrain the relative amounts of potassium in the crust with respect to those of argon in the atmosphere. No more than 30% of the modern mass of the continents was subducted back into the mantle during Earth's history. It is estimated that 50 to 70% of the subducted sediments are reincorporated into the deep continental crust. A consequence of the limited exchange between the continental crust and the upper mantle is that the chemistry of the upper mantle is driven by exchange of material with the deep mantle.

Evolution of the long-wavelength, subduction-driven topography of South America since 150 Ma

NASA Astrophysics Data System (ADS)

Flament, N. E.; Gurnis, M.; Williams, S.; Bower, D. J.; Seton, M.; Müller, D.

2014-12-01

Subduction to the west of South America spans 6000 km along strike and has been active for over 250 Myr. The influence of the history of subduction on the geodynamics of South America has been profound, driving mountain building and arc volcanism in the Andean Cordillera. Here, we investigate the long-wavelength changes in the topography of South America associated with subduction and plate motion and their interplay with the lithospheric deformation associated with the opening of the South Atlantic. We pay particular attention to the topographic expression of flat-lying subduction zones. We develop time-dependent geodynamic models of mantle flow and lithosphere deformation to investigate the evolution of South American dynamic and total topography since the late Jurassic (150 Ma). Our models are semi-empirical because the computational cost of fully dynamic, evolutionary models is still prohibitive. We impose the kinematics of global plate reconstructions with deforming continents in forward global mantle convection models with compositionally distinct crust and continental lithosphere embedded within the thermal lithosphere. The shallow thermal structure of subducting slabs is imposed, allowing us to investigate the evolution of dynamic topography around flat slab segments in time-dependent models. Multiple cases are used to investigate how the evolution of South American dynamic topography is influenced by mantle viscosity, the kinematics of the opening of the South Atlantic and alternative scenarios for recent and past flat-slab subduction. We predict that the migration of South America over sinking oceanic lithosphere resulted in continental tilt to the west until ~ 45 Ma, inverting to an eastward tilt thereafter. This first-order result is consistent with the reversal of the drainage of the Amazon River system. We investigate which scenarios of flat-slab subduction since the Eocene are compatible with geological constraints on the evolution of the Solimoes

Thermal impact of magmatism in subduction zones

NASA Astrophysics Data System (ADS)

Rees Jones, David W.; Katz, Richard F.; Tian, Meng; Rudge, John F.

2018-01-01

Magmatism in subduction zones builds continental crust and causes most of Earth's subaerial volcanism. The production rate and composition of magmas are controlled by the thermal structure of subduction zones. A range of geochemical and heat flow evidence has recently converged to indicate that subduction zones are hotter at lithospheric depths beneath the arc than predicted by canonical thermomechanical models, which neglect magmatism. We show that this discrepancy can be resolved by consideration of the heat transported by magma. In our one- and two-dimensional numerical models and scaling analysis, magmatic transport of sensible and latent heat locally alters the thermal structure of canonical models by ∼300 K, increasing predicted surface heat flow and mid-lithospheric temperatures to observed values. We find the advection of sensible heat to be larger than the deposition of latent heat. Based on these results we conclude that thermal transport by magma migration affects the chemistry and the location of arc volcanoes.

Slab seismicity in the Western Hellenic Subduction Zone: Constraints from tomography and double-difference relocation

NASA Astrophysics Data System (ADS)

Halpaap, Felix; Rondenay, Stéphane; Ottemöller, Lars

2016-04-01

The Western Hellenic subduction zone is characterized by a transition from oceanic to continental subduction. In the southern oceanic portion of the system, abundant seismicity reaches intermediate depths of 100-120 km, while the northern continental portion rarely exhibits deep earthquakes. Our study aims to investigate how this oceanic-continental transition affects fluid release and related seismicity along strike, by focusing on the distribution of intermediate depth earthquakes. To obtain a detailed image of the seismicity, we carry out a tomographic inversion for P- and S-velocities and double-difference earthquake relocation using a dataset of unprecedented spatial coverage in this area. Here we present results of these analyses in conjunction with high-resolution profiles from migrated receiver function images obtained from the MEDUSA experiment. We generate tomographic models by inverting data from 237 manually picked, well locatable events recorded at up to 130 stations. Stations from the permanent Greek network and the EGELADOS experiment supplement the 3-D coverage of the modeled domain, which covers a large part of mainland Greece and surrounding offshore areas. Corrections for the sphericity of the Earth and our update to the SIMULR16 package, which now allows S-inversion, help improve our previous models. Flexible gridding focusses the inversion on the domains of highest gradient around the slab, and we evaluate the resolution with checker board tests. We use the resulting velocity model to relocate earthquakes via the Double-Difference method, using a large dataset of differential traveltimes obtained by crosscorrelation of seismograms. Tens of earthquakes align along two planes forming a double seismic zone in the southern, oceanic portion of the subduction zone. With increasing subduction depth, the earthquakes appear closer to the center of the slab, outlining probable deserpentinization of the slab and concomitant eclogitization of dry crustal

Subduction-zone magnetic anomalies and implications for hydrated forearc mantle

USGS Publications Warehouse

Blakely, R.J.; Brocher, T.M.; Wells, R.E.

2005-01-01

Continental mantle in subduction zones is hydrated by release of water from the underlying oceanic plate. Magnetite is a significant byproduct of mantle hydration, and forearc mantle, cooled by subduction, should contribute to long-wavelength magnetic anomalies above subduction zones. We test this hypothesis with a quantitative model of the Cascadia convergent margin, based on gravity and aeromagnetic anomalies and constrained by seismic velocities, and find that hydrated mantle explains an important disparity in potential-field anomalies of Cascadia. A comparison with aeromagnetic data, thermal models, and earthquakes of Cascadia, Japan, and southern Alaska suggests that magnetic mantle may be common in forearc settings and thus magnetic anomalies may be useful in mapping hydrated mantle in convergent margins worldwide. ?? 2005 Geological Society of America.

New seismic observation on the lithosphere and slab subduction beneath the Indo-Myanmar block: Implications for continent oblique subduction and transition to oceanic slab subduction

NASA Astrophysics Data System (ADS)

Jiang, M.; He, Y.; Zheng, T.; Mon, C. T.; Thant, M.; Hou, G.; Ai, Y.; Chen, Q. F.; Sein, K.

2017-12-01

The Indo-Myanmar block locates to the southern and southeastern of the Eastern Himalayan Syntax (EHS) and marks a torsional boundary of the collision between the Indian and Eurasian plates. There are two fundamental questions concerned on the tectonics of Indo-Myanmar block since the Cenozoic time. One is whether and how the oblique subduction is active in the deep; the other is where and how the transition from oceanic subduction and continental subduction operates. However, the two problems are still under heated debate mainly because the image of deep structure beneath this region is still blurring. Since June, 2016, we have executed the China-Myanmar Geophysical Survey in the Myanmar Orogen (CMGSMO) and deployed the first portable seismic array in Myanmar in cooperation with Myanmar Geosciences Society (MGS). This array contains 70 stations with a dense-deployed main profile across the Indo-Myanmar Range, Central Basin and Shan State Plateau along latitude of 22° and a 2-D network covering the Indo-Myanmar Range and the western part of the Central Basin. Based on the seismic data collected by the new array, we conducted the studies on the lithospheric structure using the routine surface wave tomography and receiver function CCP stacking. The preliminary results of surface wave tomography displayed a remarkable high seismic velocity fabric in the uppermost of mantle beneath the Indo-Myanmar Range and Central Basin, which was interpreted as the subducted slab eastward. Particularly, we found a low velocity bulk within the high-velocity slab, which was likely to be a slab window due to the slab tearing. The preliminary results of receiver function CCP stacking showed the obvious variations of the lithospheric structures from the Indo-Myanmar Range to the Central Basin and Shan State Plateau. The lithospheric structure beneath the Indo-Myanmar Range is more complex than that beneath the Central Basin and Shan State Plateau. Our resultant high-resolution images

Subduction Orogeny and the Late Cenozoic Evolution of the Mediterranean Arcs

NASA Astrophysics Data System (ADS)

Royden, Leigh; Faccenna, Claudio

2018-05-01

The Late Cenozoic tectonic evolution of the Mediterranean region, which is sandwiched between the converging African and European continents, is dominated by the process of subduction orogeny. Subduction orogeny occurs where localized subduction, driven by negative slab buoyancy, is more rapid than the convergence rate of the bounding plates; it is commonly developed in zones of early or incomplete continental collision. Subduction orogens can be distinguished from collisional orogens on the basis of driving mechanism, tectonic setting, and geologic expression. Three distinct Late Cenozoic subduction orogens can be identified in the Mediterranean region, making up the Western Mediterranean (Apennine, external Betic, Maghebride, Rif), Central Mediterranean (Carpathian), and Eastern Mediterranean (southern Dinaride, external Hellenide, external Tauride) Arcs. The Late Cenozoic evolution of these orogens, described in this article, is best understood in light of the processes that govern subduction orogeny and depends strongly on the buoyancy of the locally subducting lithosphere; it is thus strongly related to paleogeography. Because the slow (4–10 mm/yr) convergence rate between Africa and Eurasia has preserved the early collisional environment, and associated tectonism, for tens of millions of years, the Mediterranean region provides an excellent opportunity to elucidate the dynamic and kinematic processes of subduction orogeny and to better understand how these processes operate in other orogenic systems.

Continental Growth and Mantle Hydration as Earth System Feedback Cycles and possible Effects of the Biosphere

NASA Astrophysics Data System (ADS)

Höning, D.; Spohn, T.

2016-12-01

The evolution of Earth is charcterized by intertwined feedback cycles. We focus on two feedback cycles that include the mantle water budget and the continental crust and study possible effects of the Earth's biosphere. The first feedback loop includes cycling of water into the mantle at subduction zones and outgassing at volcanic chains and mid-ocean ridges. Water will reduce the viscosity of mantle rock, and therefore the speed of mantle convection and plate subduction will increase with the mantle water concentration, eventually enhancing the rates of mantle water regassing and outgassing. A second feedback loop includes the production and erosion of continental crust. Continents grow by volcanism above subduction zones, whose total length is determined by the total area of the continents. Furthermore, the erosion rate of the continents is proportional to the total surface area of continental crust. The rate of sediment subduction affects the rate of transport of water to the mantle and the production rate of new continental crust. We present a model that includes both cycles and show how the system develops stable and unstable fixed points in a plane defined by mantle water concentration and surface are of continents. The stable points represent either an Earth mostly covered by continents and a wet mantle or an Earth mostly covered by oceans with a dry mantle. The presently observed Earth is inbetween these extreme states but the state is intrinsically unstable. We couple the feedback model to a parameterized thermal evolution model. We show how Earth evolved towards its present unstable state. We argue that other feedback cycles such as the carbonate silicate cycle may act to stabilize the present state, however. By enhancing continental weathering and erosion, and eventually the sediment transport into subduction zones, the biosphere impacts both feedback cycles and might play a crucial role in regulating Earth's system and keep continental crust coverage and

Geochronology, Geochemistry and Tectonics of Subduction-Related Late Triassic Rift Basins in Northern Chile (24º-26ºS).

NASA Astrophysics Data System (ADS)

Espinoza, M. E.; Oliveros, V.; Celis, C.

2016-12-01

As plate-tectonic processes ultimately control the location, initiation, and evolution of sedimentary basins, the study of these is crucial to understand the geodynamic framework of a specific period. In northern Chile, Late Triassic depocenters crop out along the Coastal Cordillera and Precordillera. These basins have been typically associated to a continental rifting unrelated to subduction prior to the Andean orogeny. In this work, we characterize these basins and present field and analytical data suggesting the development of these basins during an active subduction system. U-Pb geochronology show the opening of these basins probably during the Anisian-Carnian (>233 Ma) with the deposition of highly mature sediments in fluvial systems, followed by the initiation of the volcanism and associated fluvial-alluvial redeposition. Furthermore, a continental (fluvial and lacustrine) deposition and its transition to shallow marine facies are recorded during the Norian to Raethian (212-200 Ma), contemporaneous with the development of acidic volcanic centers. The sedimentary provenance evidence a main detrital supply of Early Permian age ( 297-283 Ma) corresponding to volcanic and plutonic basement rocks and a minor supply close to 478 Ma related to the exhumed Famatinian arc to the east. Geochemical results from volcanic products present in the basins show a typical subduction signal (calc-alkaline trend, low HFS/LILE ratio and Nb-Ta negative anomalies), while petrography indicate a wide compositional variation more than a bimodal distribution. These basins present half-graben geometries with the recognition of structural highs separating local depocenters. Kinematic analyses carried in synrift extensional faults show a bimodal distribution of the maximum strain axes from a NE-SW to a subordinate NW-SE direction of elongation. This bimodality could be related to the co-existence of two competing strain directions associated to the breakup of Pangea and the presence of a

Paleoseismicity and neotectonics of the Aleutian Subduction Zone—An overview

NASA Astrophysics Data System (ADS)

Carver, Gary; Plafker, George

The Aleutian subduction zone is one of the most seismically active plate boundaries and the source of several of the world's largest historic earthquakes. The structural architecture of the subduction zone varies considerably along its length. At the eastern end is a tectonically complex collision zone where the allochthonous Yakutat terrane is moving northwest into mainland Alaska. West of the collision zone a shallow-dipping subducted plate beneath a wide forearc, nearly orthogonal convergence, and a continental-type subduction regime characterizes the eastern part of the subduction zone. In the central part of the subduction zone, convergence becomes increasingly right oblique and the forearc is divided into a series of large clockwise-rotated fault-bounded blocks. Highly oblique convergence and island arc tectonics characterize the western part of the subduction zone. At the extreme western end of the arc, the relative plate motion is nearly pure strike-slip. A series of great subduction earthquakes ruptured most of the 4000-km length of the subduction zone during a period of several decades in the mid 1900s. The majority of these earthquakes broke multiple segments as defined by the large-scale structure of the overriding plate margin and patterns of historic seismicity. Several of these earthquakes generated Pacific-wide tsunamis and significant damage in the southwestern and south-central regions of Alaska. Characterization of previous subduction earthquakes is important in assessing future seismic and tsunami hazards. However, at present such information is available only for the eastern part of the subduction zone. The 1964 Alaska earthquake (M 9.2) ruptured about ˜950 km of the plate boundary that encompassed the Kodiak and Prince William Sound (PWS) segments. Within this region, nine paleosubduction earthquakes in the past ˜5000 years are recognized on the basis of geologic evidence of sudden land level change and, at some sites, coeval tsunami

Constraining Sources of Subducted and Recycled Carbon Along the Sunda Arc

NASA Astrophysics Data System (ADS)

House, B. M.; Bebout, G. E.; Hilton, D. R.; Rodriguez, B.; Plank, T. A.

2014-12-01

From sediment subduction rates and C contents at ODP/DSDP sites 765 and 211, we estimate the rate of C subduction along ~2000 km of the East Sunda Arc to be ~0.4 Tg C yr-1, representing a significant source of subducted volatiles [1]. However volatile recycling efficiency and the provenance of recycled volatiles in this region remain poorly understood. With new δ13C measurements of both carbonate and organic carbon from sites 211 and 765, we present the most detailed study yet of the spatial variability of subducted C and recycled CO2 provenance along the strike of the arc. Furthermore we demonstrate the importance of oceanic crustal carbonate as a C source in a subduction zone that is otherwise carbonate starved. Carbonate content throughout the sediment column decreases dramatically between site 765, approximately 250 km from the Australian continental margin, and site 211, approximately 300 km southwest of the trench and outboard of the Sunda Strait between Sumatra and Java. Continental and shelf carbonate input from the Australian margin dominates shallow deposits at site 765, but underlying pelagic sediments are thought to contribute the majority of inorganic C to the arc. The paucity of carbonate in sediments at site 211 suggests that along this segment essentially all carbonate subducted is derived from altered ocean crust, presenting an opportunity to study the effects of crustal carbonate input. While previous C provenance studies relied on globally-averaged δ13C values for organic and inorganic C in subducted sediments, we present new estimates based on measured δ13CVPDB of carbonate (average of ~2‰ in subducted sediments) and organic carbon (-22.5 to -23‰ average) along with previously published efflux data [2]. These estimates suggest that the arc-averaged ratio of carbonate to organic C subducted along the East Sunda Arc is nearly identical to the inorganic to organic C ratio represented in volcanic and hydrothermal CO2 output, suggesting that

Overview of Recent Coastal Tectonic Deformation in the Mexican Subduction Zone

NASA Astrophysics Data System (ADS)

Ramírez-Herrera, M. Teresa; Kostoglodov, Vladimir; Urrutia-Fucugauchi, Jaime

2011-08-01

Holocene and Pleistocene tectonic deformation of the coast in the Mexico subudction margin is recorded by geomorphic and stratigraphic markers. We document the spatial and temporal variability of active deformation on the coastal Mexican subduction margin. Pleistocene uplift rates are estimated using wave-cut platforms at ca. 0.7-0.9 m/ka on the Jalisco block coast, Rivera-North America tectonic plate boundary. We examine reported measurements from marine notches and shoreline angle elevations in conjunction with their radiocarbon ages that indicate surface uplift rates increasing during the Holocene up to ca. 3 ± 0.5 m/ka. In contrast, steady rates of uplift (ca. 0.5-1.0 m/ka) in the Pleistocene and Holocene characterize the Michoacan coastal sector, south of El Gordo graben and north of the Orozco Fracture Zone (OFZ), incorporated within the Cocos-North America plate boundary. Significantly higher rates of surface uplift (ca. 7 m/ka) across the OFZ subduction may reflect the roughness of subducting plate. Absence of preserved marine terraces on the coastal sector across El Gordo graben likely reflects slow uplift or coastal subsidence. Stratigraphic markers and their radiocarbon ages show late Holocene (ca. last 6 ka bp) coastal subsidence on the Guerrero gap sector in agreement with a landscape barren of marine terraces and with archeological evidence of coastal subsidence. Temporal and spatial variability in recent deformation rates on the Mexican Pacific coast may be due to differences in tectonic regimes and to localized processes related to subduction, such as crustal faults, subduction erosion and underplating of subducted materials under the southern Mexico continental margin.

The effect of atmospheric variability at intra-seasonal time scale on the SST of the Southwestern Atlantic Continental Shelf

NASA Astrophysics Data System (ADS)

Simionato, Claudia; Clara, Moira Luz; Jaureguizar, Andrés

2017-04-01

The Southwestern Atlantic Continental Shelf is characterized by large SST variability which origin remains unknown. In this work, we use blended SST data provided by NOAA CoastWatch Program, which combine the information coming from infrared and microwave sensors to provide daily images of an intermediate spatial resolution (11 km) with a noise floor of less than 0.2 °C. The data base starts at the middle of 2002, when an increase in signal variance is observed due to the fact that the Advanced Microwave Scanning Radiometer became available and as a consequence to its near all-weather coverage. Several years of observations are thus available, and even though the temporal and spatial resolution of these data is intermediate, they are reasonable for observing and characterizing the most significant patterns of SST variability in the (atmospheric) synoptic to intra-seasonal time scales, so as to help on understanding the physical processes which occur in the area and their forcing mechanisms. As we hypothesize that most of the variability in those time scales is wind forced, the study is complemented with the use of atmospheric observations -coming from remote sensing and reanalysis-. To perform the analysis, the long-term trend, inter-annual and seasonal variability are subtracted to the SST data to obtain the signal on intra-seasonal time scales. Then, Principal Components (EOF) analysis is applied to the data and composites of SST and several meteorological variables (wind, sea level pressure, air temperature, OLR, etc.) are computed for the days when the leading modes are active. It is found that the first three modes account for more than 70% of the variance. Modes 1 and 2 seem to be related to atmospheric waves generated in the tropical Pacific. Those waves, through atmospheric teleconnections, affect the SST on the southwestern South Atlantic Continental Shelf very rapidly. The oceanic anomalies exceed 0.7°C and are quite persistent. Mode 2 seems to be

Teleseismic P-wave tomography of the Sunda-Banda Arc subduction zone

NASA Astrophysics Data System (ADS)

Harris, C. W.; Miller, M. S.; Widiyantoro, S.; Supendi, P.; O'Driscoll, L.; Roosmawati, N.; Porritt, R.

2017-12-01

The Sunda-Banda Arc is the site of multiple ongoing tectonic deformation processes and is perhaps the best example of the transition from subduction of oceanic lithosphere to an active arc-continent collision. Investigating the mantle structure that has resulted from the collision of continental Australia, as well as the concurrent phenomena of continental subduction, slab-rollback, lithospheric tearing, and subduction polarity reversal is possible through seismic tomography. While both regional scale and global tomographic models have previously been constructed to study the tectonics this region, here we use 250 seismic stations that span the length of this convergent margin to invert for P-wave velocity perturbations in the upper mantle. We combine data from a temporary deployment of 30 broadband instruments as part of the NSF-funded Banda Arc Project, along with data from permanent broadband stations maintained by the Meteorological, Climatological, and Geophysical Agency of Indonesia (BMKG) to image mantle structure, in particular the subducted Indo-Australian plate. The BMKG dataset spans 2009-2017 and includes >200 broadband seismometers. The Banda Arc array (network YS) adds coverage and resolution to southeastern Indonesia and Timor-Leste, where few permanent seismometers are located but the Australian continent-Banda Arc collision is most advanced. The preliminary model was computed using 50,000 teleseismic P-wave travel-time residuals and 3D finite frequency sensitivity kernels. Results from the inversion of the combined dataset are presented as well as resolution tests to assess the quality of the model. The velocity model shows an arcuate Sunda-Banda slab with morphological changes along strike that correlate with the tectonic collision. The model also features the double-sided Molucca Sea slab and regions of high velocity below the bottom of the transition zone. The resolution added by the targeted USC deployment is clear when comparing models that

Nurture Versus Nature: Accounting for the Differences Between the Taiwan and Timor active arc-continent collisions

NASA Astrophysics Data System (ADS)

Harris, R. A.

2011-12-01

The active Banda arc/continent collision of the Timor region provides many important contrasts to what is observed in Taiwan, which is mostly a function of differences in the nature of the subducting plate. One of the most important differences is the thermal state of the respective continental margins: 30 Ma China passive margin versus 160 Ma NW Australian continental margin. The subduction of the cold and strong NW Australian passive margin beneath the Banda trench provides many new constraints for resolving longstanding issues about the formative stages of collision and accretion of continental crust. Some of these issues include evidence for slab rollback and subduction erosion, deep continental subduction, emplacement or demise of forearc basement, relative amounts of uplift from crustal vs. lithospheric processes, influence of inherited structure, partitioning of strain away from the thrust front, extent of mélange development, metamorphic conditions and exhumation mechanisms, continental contamination and accretion of volcanic arcs, does the slab tear, and does subduction polarity reverse? Most of these issues link to the profound control of lower plate crustal heterogeneity, thermal state and inherited structure. The thermomechanical characteristics of subducting an old continental margin allow for extensive underthrusting of lower plate cover units beneath the forearc and emplacement and uplift of extensive nappes of forearc basement. It also promotes subduction of continental crust to deep enough levels to experience high pressure metamorphism (not found in Taiwan) and extensive contamination of the volcanic arc. Seismic tomography confirms subduction of continental lithosphere beneath the Banda Arc to at least 400 km with no evidence for slab tear. Slab rollback during this process results in massive subduction erosion and extension of the upper plate. Other differences in the nature of the subducting plates in Taiwan in Timor are differences in the

The Fairway-Aotea Basin and the New Caledonia Trough, witnesses of the Pacific-Australian plate boundary evolution : from mid-Cretaceous cessation of subduction to Eocene subduction renewal

NASA Astrophysics Data System (ADS)

Collot, J.; Geli, L. B.; Lafoy, Y.; Sutherland, R.; Herzer, R. H.; Roest, W. R.

2009-12-01

The geodynamical history of the SW Pacific is controlled since the Mesozoic by the evolution of peri-Pacific subduction zones, in a trench retreat by slab roll-back process, which successively occurred along the Eastern Gondwana margin. In this context, most basins which formed after 45 Ma reached a stage of seafloor spreading, have recorded the inversions of the earth's magnetic field and present typical oceanic crust morphologies. By contrast, the New Caledonia and Fairway basins, which are narrower and present thick sedimentary covers have a less known and more controversial origin. Based on a regional geological synthesis and on interpretation of multichannel seismic reflection and refraction data, combined with drill hole data off New Zealand and a compilation of regional potential data, we distinguish 2 phases of the evolution of the Fairway-Aotea Basin (FAB) and the New Caledonia Trough (NCT), which reflect the evolution of the Gondwana-Pacific plate boundary: Phase 1: Mid Cretaceous formation of the FAB in a continental intra- or back- arc position of the Pacific-Gondwana subduction system. The formation of this shallow basin reflects the onset of continental breakup of the Eastern Gondwana margin during Cenomanian which was most probably caused by a dynamic change of the subduction zone through a « verticalization » of the slab. This event may be the result of the 99 Ma kinematic plate reorganization which probably led to subduction cessation along the Gondwana-Pacific plate boundary. A tectonic escape mechanism, in relation with the locking of the subduction zone by the Hikurangi Plateau, could also be responsible of the trench retreat leading to backarc extension. Phase 2: Regional Eocene-Oligocene uplift followed by rapid subsidence (3-4 km) of the system « Lord Howe Rise - FAB - Norfolk Ridge ». The structural style of this deformation leads us to suggest that detachment of the lower crust is the cause of subsidence. We therefore propose a model in

Petrology and tectonics of Phanerozoic continent formation: From island arcs to accretion and continental arc magmatism

USGS Publications Warehouse

Lee, C.-T.A.; Morton, D.M.; Kistler, R.W.; Baird, A.K.

2007-01-01

Mesozoic continental arcs in the North American Cordillera were examined here to establish a baseline model for Phanerozoic continent formation. We combine new trace-element data on lower crustal xenoliths from the Mesozoic Sierra Nevada Batholith with an extensive grid-based geochemical map of the Peninsular Ranges Batholith, the southern equivalent of the Sierras. Collectively, these observations give a three-dimensional view of the crust, which permits the petrogenesis and tectonics of Phanerozoic crust formation to be linked in space and time. Subduction of the Farallon plate beneath North America during the Triassic to early Cretaceous was characterized by trench retreat and slab rollback because old and cold oceanic lithosphere was being subducted. This generated an extensional subduction zone, which created fringing island arcs just off the Paleozoic continental margin. However, as the age of the Farallon plate at the time of subduction decreased, the extensional environment waned, allowing the fringing island arc to accrete onto the continental margin. With continued subduction, a continental arc was born and a progressively more compressional environment developed as the age of subducting slab continued to young. Refinement into a felsic crust occurred after accretion, that is, during the continental arc stage, wherein a thickened crustal and lithospheric column permitted a longer differentiation column. New basaltic arc magmas underplate and intrude the accreted terrane, suture, and former continental margin. Interaction of these basaltic magmas with pre-existing crust and lithospheric mantle created garnet pyroxenitic mafic cumulates by fractional crystallization at depth as well as gabbroic and garnet pyroxenitic restites at shallower levels by melting of pre-existing lower crust. The complementary felsic plutons formed by these deep-seated differentiation processes rose into the upper crust, stitching together the accreted terrane, suture and former

Paleomagnetism of Cretaceous Oceanic Red Beds (CORBs) from Gyangze, northern Tethys Himalaya: Evidence for Intra-oceanic Subduction System and Southern Paleolatitute Limit for the Lhasa Block

NASA Astrophysics Data System (ADS)

Tan, Xiaodong

2016-04-01

indicate a paleolatitude of 10±2 degree north, ~2000 km distance from the southern Tethys Himalaya. Therefore, the formation is not deposited near the greater Indian continental margins. Based on recent plate tectonic reconstruction, the CORBs are very likely formed within a back-arc basin between the equatorial intra-oceanic subduction system and the Asian continental margin. Due to coeval development of abundant red beds in the Lhasa block, the characteristic pigments of hematite born in the CORBs are likely of terrestrial origin. In addition, the new data indicate that the Lhasa block is unlikely to be at low paleolatitude in the Late Cretaceous and Tertiary as some of the paleomagnetic results show.

Archean greenstone belt magmatism and the continental growth-mantle evolution connection: constraints from Th-U-Nb-LREE systematics of the 2.7 Ga Wawa subprovince, Superior Province, Canada

NASA Astrophysics Data System (ADS)

Polat, Ali; Kerrich, Robert

2000-01-01

An extensive database, including Th-;U-Nb-REE systematics, for diverse magmatic and sedimentary lithologies of 2.7 Ga Wawa greenstone belts provide new constraints on the mechanism of crustal growth in the southern Superior Province, and controls on its composition. The greenstone belts are characterized by collages of oceanic plateaus, oceanic island arcs, and trench turbidites; these lithotectonic fragments were tectonically assembled in a large subduction-accretion complex. Following juxtaposition, these diverse lithologies were collectively intruded by syn-kinematic TTG (tonalite-trondhjemite-granodiorite) plutons and ultramafic to felsic dykes and sills, with subduction zone geochemical signatures. Intra-oceanic basalts are characterized by near-flat REE patterns, and Nb/U and Nb/Th ratios generally greater than primitive mantle values, consistent with positive ɛNd values. They are associated with komatiites, the association being interpreted as an ocean plateau sequence erupted from a mantle plume. Bimodal arc volcanic sequences, trench turbidites, and contemporaneous TTG suites are characterized by fractionated REE, with Nb/U and Nb/Th ratios less than primitive mantle values. Mixing hyperbolae between oceanic plateau and magmatic arc sequences pass through the estimated composition of bulk continental crust, suggesting that crustal growth in the late Archean was by tectonic, sedimentary, and chemical mixing of oceanic plateau and arc sequences at convergent plate boundaries. Mixing calculations suggest that oceanic plateau and subduction zone components in the Wawa continental crust are represented by 6-12% and 88-94%, respectively. High Nb/U and Nb/Th ratios of plateau tholeiitic basalts are interpreted as a complementary reservoir to arc magmatism (low Nb/U and Nb/Th), hundreds of millions of years prior to recycling of oceanic lithosphere through a subduction zone (high Nb/U, Nb/Th), and its incorporation into a mantle plume from which 2.7 Ga plateau

Plume-induced subduction initiation at the Cretaceous India-Arabia transform plate boundary: paleomagnetic constraints from the Semail ophiolite, Oman

NASA Astrophysics Data System (ADS)

Van Hinsbergen, D. J. J.; Maffione, M.; Koornneef, L.; Guilmette, C.

2016-12-01

The Neotethyan realm hosts a prominent belt of Cretaceous supra-subduction zone ophiolites from Turkey and Cyprus in the west, to Oman in the east. Associated crustal and metamorphic sole ages tightly cluster at 95-90 Ma, interpreted to shortly post-date subduction initiation in an intra-oceanic setting along transform faults or ridge segments (or ridge-parallel oceanic detachments). This subduction episode ended when the Arabian-African continental lithosphere arrived in the trench in the late Cretaceous and the leading edge of the overriding oceanic lithosphere obducted as ophiolites, including the famous Semail ophiolite of Oman. This catastrophic subduction initiation phase is assumed to be as response to some far-field trigger. Here, we analyzed whether the Semail ophiolite was generated at an E-W trending Neotethyan ridge or at a N-S trending transform. Therefore we paleomagnetically analyzed 10 localities in sheeted dyke sections of the Semail ophiolite that trend parallel to the obduction front of the ophiolite taken to reflect the paleo-trench. We demonstrate that the sheeted dyke sections, and thus also the trench, had an initial N-S strike, indicating that subduction below the Semail ophiolite probably initiated along a N-S striking transform plate boundary between the Indian and Arabian plate rather than at a Neotethyan mid-ocean ridge. Sometime before 83 Ma, India broke away from Madagascar, and underwent a counterclockwise rotation relative to Africa/Arabia around an Euler pole just north of Madagascar, likely triggered by the arrival of the Morondova mantle plume, the associated large igneous province formed since at least 91 Ma. Numerical models have shown that plume push was a likely driver for the inception of India-Madagascar spreading and associated Indian rotation. North of the associated Euler pole, E-W convergence India-Arabia must have occurred during India-Madagascar break-up. This has already been related to 96-90 Ma subduction initiation

The life cycle of continental rifts: Numerical models of plate tectonics and mantle convection.

NASA Astrophysics Data System (ADS)

Ulvrova, Martina; Brune, Sascha; Williams, Simon

2017-04-01

Plate tectonic processes and mantle convection form a self-organized system whose surface expression is characterized by repeated Wilson cycles. Conventional numerical models often capture only specific aspects of plate-mantle interaction, due to imposed lateral boundary conditions or simplified rheologies. Here we study continental rift evolution using a 2D spherical annulus geometry that does not require lateral boundary conditions. Instead, continental extension is driven self-consistently by slab pull, basal drag and trench suction forces. We use the numerical code StagYY to solve equations of conservation of mass, momentum and energy and transport of material properties. This code is capable of computing mantle convection with self-consistently generated Earth-like plate tectonics using a pseudo-plastic rheology. Our models involve an incompressible mantle under the Boussinesq approximation with internal heat sources and basal heating. Due to the 2D setup, our models allow for a comparably high resolution of 10 km at the mantle surface and 15 km at the core mantle boundary. Viscosity variations range over 7 orders of magnitude. We find that the causes for rift initiation are often related to subduction dynamics. Some rifts initiate due to increasing slab pull, others because of developing trench suction force, for instance by closure of an intra-oceanic back-arc basin. In agreement with natural settings, our models reproduce rifts forming in both young and old collision zones. Our experiments show that rift dynamics follow a characteristic evolution, which is independent of the specific setting: (1) continental rifts initiate during tens of million of years at low extension rates (few millimetres per year) (2) the extension velocity increases during less than 10 million years up to several tens of millimetres per year. This speed-up takes place before lithospheric break-up and affects the structural architecture of rifted margins. (3) high divergence rates

Plume-stagnant slab-lithosphere interactions: Origin of the late Cenozoic intra-plate basalts on the East Eurasia margin

NASA Astrophysics Data System (ADS)

Kimura, Jun-Ichi; Sakuyama, Tetsuya; Miyazaki, Takashi; Vaglarov, Bogdan S.; Fukao, Yoshio; Stern, Robert J.

2018-02-01

Intra-plate basalts of 35-0 Ma in East Eurasia formed in a broad backarc region above the stagnant Pacific Plate slab in the mantle transition zone. These basalts show regional-scale variations in Nd-Hf isotopes. The basalts with the most radiogenic Nd-Hf center on the Shandong Peninsula with intermediate Nd-Hf at Hainan and Datong. The least radiogenic basalts occur in the perimeters underlain by the thick continental lithosphere. Shandong basalts possess isotopic signatures of the young igneous oceanic crust of the subducted Pacific Plate. Hainan and Datong basalts have isotopic signatures of recycled subduction materials with billions of years of storage in the mantle. The perimeter basalts have isotopic signatures similar to pyroxenite xenoliths from the subcontinental lithospheric mantle beneath East Eurasia. Hainan basalts exhibit the highest mantle potential temperature (Tp), while the Shandong basalts have the lowest Tp. We infer that a deep high-Tp plume interacted with the subducted Pacific Plate slab in the mantle transition zone to form a local low-Tp plume by entraining colder igneous oceanic lithosphere. We infer that the subducted Izanagi Plate slab, once a part of the Pacific Plate mosaic, broke off from the Pacific Plate slab at 35 Ma to sink into the lower mantle. The sinking Izanagi slab triggered the plume that interacted with the stagnant Pacific slab and caused subcontinental lithospheric melting. This coincided with formation of the western Pacific backarc marginal basins due to Pacific Plate slab rollback and stagnation.

Detection of earthquake swarms at subduction zones globally: Insights into tectonic controls on swarm activity

NASA Astrophysics Data System (ADS)

Nishikawa, T.; Ide, S.

2017-07-01

Earthquake swarms are characterized by an increase in seismicity rate that lacks a distinguished main shock and does not obey Omori's law. At subduction zones, they are thought to be related to slow-slip events (SSEs) on the plate interface. Earthquake swarms in subduction zones can therefore be used as potential indicators of slow-slip events. However, the global distribution of earthquake swarms at subduction zones remains unclear. Here we present a method for detecting such earthquake sequences using the space-time epidemic-type aftershock-sequence model. We applied this method to seismicity (M ≥ 4.5) recorded in the Advanced National Seismic System catalog at subduction zones during the period of 1995-2009. We detected 453 swarms, which is about 6.7 times the number observed in a previous catalog. Foreshocks of some large earthquakes are also detected as earthquake swarms. In some subduction zones, such as at Ibaraki-Oki, Japan, swarm-like foreshocks and ordinary swarms repeatedly occur at the same location. Given that both foreshocks and swarms are related to SSEs on the plate interface, these regions may have experienced recurring SSEs. We then compare the swarm activity and tectonic properties of subduction zones, finding that swarm activity is positively correlated with curvature of the incoming plate before subduction. This result implies that swarm activity is controlled either by hydration of the incoming plate or by heterogeneity on the plate interface due to fracturing related to slab bending.

Tearing, segmentation, and backstepping of subduction in the Aegean: New insights from seismicity

NASA Astrophysics Data System (ADS)

Bocchini, G. M.; Brüstle, A.; Becker, D.; Meier, T.; van Keken, P. E.; Ruscic, M.; Papadopoulos, G. A.; Rische, M.; Friederich, W.

2018-06-01

This study revisits subduction processes at the Hellenic Subduction Zone (HSZ) including tearing, segmentation, and backstepping, by refining the geometry of the Nubian slab down to 150-180 km depth using well-located hypocentres from global and local seismicity catalogues. At the western termination of the HSZ, the Kefalonia Transform Fault marks the transition between oceanic and continental lithosphere subducting to the south and to the north of it, respectively. A discontinuity is suggested to exist between the two slabs at shallow depths. The Kefalonia Transform Fault is interpreted as an active Subduction-Transform-Edge-Propagator-fault formed as consequence of faster trench retreat induced by the subduction of oceanic lithosphere to the south of it. A model reconstructing the evolution of the subduction system in the area of Peloponnese since 34 Ma, involving the backstepping of the subduction to the back-side of Adria, provides seismological evidence that supports the single-slab model for the HSZ and suggests the correlation between the downdip limit of the seismicity to the amount of subducted oceanic lithosphere. In the area of Rhodes, earthquake hypocentres indicate the presence of a NW dipping subducting slab that rules out the presence of a NE-SW striking Subduction-Transform-Edge-Propagator-fault in the Pliny-Strabo trenches region. Earthquake hypocentres also allow refining the slab tear beneath southwestern Anatolia down to 150-180 km depth. Furthermore, the distribution of microseismicity shows a first-order slab segmentation in the region between Crete and Karpathos, with a less steep and laterally wider slab segment to the west and a steeper and narrower slab segment to the east. Thermal models indicate the presence of a colder slab beneath the southeastern Aegean that leads to deepening of the intermediate-depth seismicity. Slab segmentation affects the upper plate deformation that is stronger above the eastern slab segment and the seismicity

The Continental Margin of East Asia: a collage of multiple plates formed by convergence and extension from multiple directions

NASA Astrophysics Data System (ADS)

Mao, J.; Wang, T.; Ludington, S.; Qiu, Z.; Li, Z.

2017-12-01

East Asia is one of the most complex regions in the world. Its margin was divided into 4 parts: Northeast Asia, North China, South China and Southeast Asia. During the Phanerozoic, continental plates of East Asia have interacted successively with a) the Paleo Tethyan Ocean, b) the Tethyan and Paleo Pacific Oceans and c) the Pacific and Indian. In the Early Mesozoic, the Indosinian orogeny is characterized by the convergence and extension within multiple continental plates, whereas the Late Mesozoic Yanshanian orogeny is characterized by both convergence and compression due to oceanic subduction and by widespread extension. We propose this combination as "East Asia Continental Margin type." Except in Northeast Asia, where Jurassic and Cretaeous accretionary complexes are common, most magmatic rocks are the result of reworking of ancient margins of small continental plates; and oceanic island arc basalts and continental margin arc andesites are largely absent. Because South China is adjacent to the western margin of the Pacific Plate, some effects of its westward subduction must be unavoidable, but juvenile arc-related crust has not been identified. The East Asian Continental Margin is characterized by magmatic rocks that are the result of post-convergent tectonics, which differs markedly from the active continental margins of both South and North America. In summary, the chief characteristics of the East Asian Continental Margin are: 1) In Mesozoic, the periphery of multiple blocks experienced magmatism caused by lithospheric delamination and thinning in response to extension punctuated by shorter periods of convergence. 2) The main mechanism of magma generation was the partial melting of crustal rocks, due to underplating by upwelling mafic magma associated with the collapse of orogenic belts and both extension and compression between small continental blocks. 3) During orogeny, mostly high Sr/Y arc-related granitoids formed, whereas during post-orogenic times, A

Magnetism at Depth: A View from an Ancient Continental Subduction and Collision Zone

NASA Astrophysics Data System (ADS)

McEnroe, Suzanne A.; Robinson, Peter; Church, Nathan; Purucker, Michael

2018-04-01

Recent sophisticated global data compilations and magnetic surveys have been used to investigate the nature of magnetization in the lower crust and upper mantle. Two approaches to constraining magnetizations are developed, providing minimum (0.01 SI) and maximum (0.04 SI) susceptibility estimates, given some assumed thickness (15+ km here). These values are higher than are found in many continental rocks. Are there rocks deeper in the crust or upper mantle that are more magnetic than expected, or are the model assumptions incomplete? What is the magnetic behavior of deep-crustal and upper mantle rocks, when slightly cooler than the Curie or Néel temperatures of their magnetic minerals, after being exhumed from locations of high-grade metamorphism at greater depth? Different sets of equilibrium metamorphic minerals can be considered that would form under different conditions. Results on 1,501 samples from the Western Gneiss Region (WGR) Norway, mainly from mafic and ultramafic bodies subducted to depths of 60-200 km and temperatures of 750 up to 950°C at the very highest pressures, show that rocks did not fully equilibrate to the dominant metamorphic-facies conditions. There is a large variation in petrophysical properties, oxide minerals, and mineral assemblages in WGR samples, though they cannot explain the broad high-amplitude (deep-seated) anomalies measured in this region. The presence of magnetite, and exsolved titanohematite and hemoilmenite in samples, shows those magnetic phases are preserved even at eclogite-facies conditions, in part because complete eclogite-facies equilibrium was rarely achieved.

Cainozoic Orogenic Magmatism In The Western-central Mediterranean Area: Implications For The Subduction-modified Mantle Sources

NASA Astrophysics Data System (ADS)

Beccaluva, L.; Bianchini, G.; Coltorti, M.; Siena, F.; Verde, M.

In this contribution new REE and Sr-Nd isotopic data carried out on Cainozoic subduction-related volcanic rocks from the western-central Mediterranean are dis- cussed within a general review of the Cainozoic orogenic magmatism of the area. These volcanic events are related to subduction processes which occurred along the Paleo-European margin at least since Eocene and migrated (trough passive sinking and slab roll-back) southeastward up to the present in the peri-Tyrrhenian margin of Italy. Orogenic rocks from Provence (34-20 Ma) are characterised by 87Sr/86Sr be- tween 0.70453 and 0.70579, and 143Nd/144Nd between 0.51292 and 0.51265, which are consistent with mantle sources modified by subduction fluids released by altered oceanic crust. Sr-Nd isotopic composition of orogenic rocks from Sardinia (32-13 Ma), show a more complex picture: some compositions with relatively low 87Sr/86Sr (<0.706) and high 143Nd/144Nd (>0.5125), are compatible with the subduction of pure oceanic crust, while compositions with very high 87Sr/86Sr (up to 0.7113) and low 143Nd/144Nd (down to 0.51219) require additional components of continental crust affinity in the mantle wedge (partial fusion of subducted terrigenous sediments?). As concerns the Aeolian volcanics (< 1.3 Ma), compositions are compatible with man- tle sources solely enriched by fluid components from subducted oceanic crust. How- ever, it is interesting to note that shoshonites from the younger series of Stromboli display distinctly higher 87Sr/86Sr (up to 0.7075) and lower 143Nd/144Nd composi- tion (down to 0.51242), thus requiring once again recycle of continental crust materials in their mantle sources. The influence of such continental crust-derived components appear to be even more important in the mantle sources of the Campania volcanics, where extreme Sr-Nd isotopic compositions are recorded (87Sr/86Sr up to 0.7097; 143Nd/144Nd down to 0.5122).

Results from CAT/SCAN, the Calabria-Apennine-Tyrrhenian/Subduction-Accretion-Collision Network

NASA Astrophysics Data System (ADS)

Steckler, M. S.; Amato, A.; Guerra, I.; Armbruster, J.; Baccheschi, P.; Diluccio, F.; Gervasi, A.; Harabaglia, P.; Kim, W.; Lerner-Lam, A.; Margheriti, L.; Seeber, L.; Tolstoy, M.; Wilson, C. K.

2005-12-01

The Calabrian Arc region is the final remnant of a Western Mediterranean microplate driven by rollback. Calabria itself is an exotic block that rifted off Sardinia and opened the Tyrrhenian Sea back-arc basin in its wake. The Calabrian Arc rapidly advanced to the southeast, with subduction ahead and extension behind, following subduction rollback of the Mesozoic seafloor. The subduction zone meanwhile collided progressively with the Apulia to form the Apennines in peninsular Italy and with the Africa to form the Maghrebides in Sicily. The Calabrian Arc is where the transition from subduction to continental collision is occurring. The collisions on either side of Calabria have restricted oceanic subduction to a narrow 200-km salient with well-defined edges and seismicity that extends to over 500 km depth. The collisions have also slowed, or possibly even halted, the rapid advance of the arc. Whether rollback of the oceanic lower plate of the Ionian Sea continues and whether the upper plate of Calabria continues to move as an independent plate are both uncertain. The Calabrian-Apennine-Tyrrhenian/Subduction-Collision-Accretion Network (CAT/SCAN) is a passive experiment to study of the Calabrian Arc and the transition to the southern Apennines. The land deployment consisted of three phases. The initial phase included an array of 39 broadband seismometers onshore, deployed in the winter of 2003/4. In September 2004, the array was reduced to 28 broadband and 8 short-period instruments. In April 2005, the array was reduced once again to 20 broadband and 2 short-period instruments. The field deployment was completed in October 2005. Offshore, 12 broadband Ocean Bottom Seismometers (OBSs) were deployed in the beginning of October 2004. Data from 4 OBSs have been recovered so far with deployment durations from a few weeks to almost one year. Fishing activity has been strongly implicated in the early recoveries, (with one instrument returned by fishermen), and is suspected

The influence of a subduction component on magmatism in the Okinawa Trough: Evidence from thorium and related trace element ratios

NASA Astrophysics Data System (ADS)

Guo, Kun; Zeng, Zhi-Gang; Chen, Shuai; Zhang, Yu-Xiang; Qi, Hai-Yan; Ma, Yao

2017-09-01

The Okinawa Trough (OT) is a back-arc, initial continental marginal sea basin located behind the Ryukyu Arc-Trench System. Formation and evolution of the OT have been intimately related to subduction of the Philippine Sea Plate (PSP) since the late Miocene; thus, the magma source of the trough has been affected by subduction components, as in the case of other active back-arc basins, including the Lau Basin (LB) and Mariana Trough (MT). We review all the available geochemical data relating to basaltic lavas from the OT and the middle Ryukyu Arc (RA) in this paper in order to determine the influence of the subduction components on the formation of arc and back-arc magmas within this subduction system. The results of this study reveal that the abundances of Th in OT basalts (OTBs) are higher than that in LB (LBBs) and MT basalts (MTBs) due to the mixing of subducted sediments and EMI-like enriched materials. The geochemical characteristics of Th and other trace element ratios indicate that the OTB originated from a more enriched mantle source (compared to N-mid-ocean ridge basalt, N-MORB) and was augmented by subducted sediments. Data show that the magma sources of the south OT (SOT) and middle Ryukyu Arc (MRA) basalts were principally influenced by subducted aqueous fluids and bulk sediments, which were potentially added into magma sources by accretion and underplating. At the same time, the magma sources of the middle OT (MOT) and Kobi-syo and Sekibi-Syo (KBS+SBS) basalts were impacted by subducted aqueous fluids from both altered oceanic crust (AOC) and sediment. The variable geochemical characteristics of these basalts are due to different Wadati-Benioff depths and tectonic environments of formation, while the addition of subducted bulk sediment to SOT and MRA basalts may be due to accretion and underplating, and subsequent to form mélange formation, which would occur partial melting after aqueous fluids are added. The addition of AOC and sediment aqueous fluid

Geophysical signature of hydration-dehydration processes in active subduction zones

NASA Astrophysics Data System (ADS)

Reynard, Bruno

2013-04-01

inclusions in arc lavas. High electrical conductivities up to 1 S/m in the hydrated wedge of the hot subductions (Ryukyu, Kyushu, Cascadia) reflect high fluid concentration, while low to moderate (<0.01 S/m) conductivities in the cold subductions (N-E Japan, Bolivia) reflect low fluid flow. This is consistent with the seismic observations of extensive shallow serpentinization in hot subduction zones, while serpentinization is sluggish in cold subduction zones. Bezacier, L., et al. 2010. Elasticity of antigorite, seismic detection of serpentinites, and anisotropy in subduction zones. Earth and Planetary Science Letters, 289, 198-208. Reynard, B., 2012. Serpentine in active subduction zones. Lithos, http://dx.doi.org/10.1016/j.lithos.2012.10.012. Reynard, B., Mibe, K. & Van de Moortele, B., 2011. Electrical conductivity of the serpentinised mantle and fluid flow in subduction zones. Earth and Planetary Science Letters, 307, 387-394. Reynard, B., Nakajima, J. & Kawakatsu, H., 2010. Earthquakes and plastic deformation of anhydrous slab mantle in double Wadati-Benioff zones. Geophysical Research Letters, 37, L24309.

Modelling the Impact of Life on Continental Growth - Mechanisms and Results

NASA Astrophysics Data System (ADS)

Höning, D.; Spohn, T.

2013-12-01

The complexity of planetary evolution increases with the number of interacting reservoirs. On Earth, even the biosphere is speculated to interact with the interior. It has been argued (e.g., Rosing et al. 2006; Sleep et al, 2012) that the formation of continents could be a consequence of bioactivity harvesting solar energy through photosynthesis to help build the continents and that the mantle should carry a chemical biosignature. Through plate tectonics, the surface biosphere can impact deep subduction zone processes and the interior of the Earth. Subducted sediments are particularly important, because they influence the Earth's interior in several ways, and in turn are strongly influenced by the Earth's biosphere. In our model, we use the assumption that a thick sedimentary layer of low permeability on top of the subducting oceanic crust, caused by a biologically enhanced weathering rate, can suppress shallow dewatering. This in turn leads to greater availability of water in the source region of andesitic partial melt, resulting in an enhanced rate of continental production and regassing rate into the mantle. Our model includes (i) mantle convection, (ii) continental erosion and production, and (iii) mantle water degassing at mid-ocean ridges and regassing at subduction zones. The mantle viscosity of our model depends on (i) the mantle water concentration and (ii) the mantle temperature, whose time dependency is given by radioactive decay of isotopes in the Earth's mantle. Boundary layer theory yields the speed of convection and the water outgassing rate of the Earth's mantle. Our results indicate that present day values of continental surface area and water content of the Earth's mantle represent an attractor in a phase plane spanned by both parameters. We show that the biologic enhancement of the continental erosion rate is important for the system to reach this fixed point. An abiotic Earth tends to reach an alternative stable fixed point with a smaller

Subduction and Plate Edge Tectonics in the Southern Caribbean

NASA Astrophysics Data System (ADS)

Levander, A.; Schmitz, M.; Niu, F.; Bezada, M. J.; Miller, M. S.; Masy, J.; Ave Lallemant, H. G.; Pindell, J. L.

2012-12-01

The southern Caribbean plate boundary consists of a subduction zone at at either end connected by a strike-slip fault system: In the east at the Lesser Antilles subduction zone, the Atlantic part of the South American plate subducts beneath the Caribbean. In the north and west in the Colombia basin, the Caribbean subducts under South America. In a manner of speaking, the two plates subduct beneath each other. Finite-frequency teleseismic P-wave tomography confirms this, imaging the Atlantic and the Caribbean subducting steeply in opposite directions to transition zone depths under northern South America (Bezada et al, 2010). The two subduction zones are connected by the El Pilar-San Sebastian strike-slip fault system, a San Andreas scale system that has been cut off at the Bocono fault, the southeastern boundary of the Maracaibo block. A variety of seismic probes identify where the two plates tear as they begin to subduct (Niu et al, 2007; Clark et al., 2008; Miller et al. 2009; Growdon et al., 2009; Huang et al., 2010; Masy et al., 2011). The El Pilar system forms at the southeastern corner of the Antilles subduction zone with the Atlantic plate tearing from South America. The deforming plate edges control mountain building and basin formation at the eastern end of the strike-slip system. In northwestern South America the Caribbean plate very likely also tears, as its southernmost element subducts at shallow angles under northernmost Colombia and the northern, nonsubducting part underthrusts the continental edge. The subducting segment rapidly descends to transition zone depths under Lake Maracaibo (Bezada et al., 2010). We believe that the flat slab produces the Merida Andes, the Perija, and the Santa Marta ranges. The nonsubducting part of the Caribbean plate underthrusts northern Venezuela to about the width of the coastal mountains (Miller et al., 2009), where the plate edge supports the coastal mountains, and controls continuing deformation.

An image of P- to S-wave velocity ratios in the forearc of the Central Andean subduction zone

NASA Astrophysics Data System (ADS)

Wasja Bloch, Nikolai; Kummerow, Jörn; Wigger, Peter; Shapiro, Serge

2014-05-01

The ratio of seismic P- to S-wave velocities (the Vp/Vs ratio) of a given rock volume is a sensitive proxy for the detection of fluids and melts. In subduction regimes it has often been inferred from seismic tomography and been used, e.g., to detect pathways of ascending melt above the seismogenic zone, where tomographic methods have their highest resolution. We present Vp/Vs ratios that were computed using only seismic arrival time observations following the approach of Lin and Shearer (2007). This approach has its highest sensitivity in the source volume of a set of nearby seismic events and is hence particularly well suited to directly probe the plate interface. We present data from a temporary local network of short period seismometers that was in operation in the forearc of the Central Andean subduction zone at 21° S between 2005 and 2012. From this database we were able to localize 3253 seismic events (Ml ~0.5--4) with high precision, yielding a detailed image of the seismicity distribution in this region. Seismicity is pervasive within the entire crust of the South American continental plate and exhibits three distinct bands in the subducting slab, the lowermost one being located in the lithospheric mantle of the subducting plate. The highest concentration of seismic events is found in the contact zone between the continental and the oceanic lithosphere at depths between 30 and 50 km. We group seismic events into approximately 100 subsets of nearby events that origin from the same geological structure. For about half of these subsets we are able to extract a reliable local Vp/Vs ratio. In the middle continental crust, Vp/Vs ratios show slightly enhanced values (~1.75). In the lower continental crust towards the plate interface they tend to increase from this value updip and decrease downdip. At the plate interface itself, we observe higher Vp/Vs ratios (>1.8) at shallower depths (between 20 and 40 km). Downdip (40--60 km depth) Vp/Vs ratios decrease to

Modeling Slab-Slab Interactions: Dynamics of Outward Dipping Double-Sided Subduction Systems

NASA Astrophysics Data System (ADS)

Király, Ágnes; Holt, Adam F.; Funiciello, Francesca; Faccenna, Claudio; Capitanio, Fabio A.

2018-03-01

Slab-slab interaction is a characteristic feature of tectonically complex areas. Outward dipping double-sided subduction is one of these complex cases, which has several examples on Earth, most notably the Molucca Sea and Adriatic Sea. This study focuses on developing a framework for linking plate kinematics and slab interactions in an outward dipping subduction geometry. We used analog and numerical models to better understand the underlying subduction dynamics. Compared to a single subduction model, double-sided subduction exhibits more time-dependent and vigorous toroidal flow cells that are elongated (i.e., not circular). Because both the Molucca and Adriatic Sea exhibit an asymmetric subduction configuration, we also examine the role that asymmetry plays in the dynamics of outward dipping double-sided subduction. We introduce asymmetry in two ways; with variable initial depths for the two slabs ("geometric" asymmetry), and with variable buoyancy within the subducting plate ("mechanical" asymmetry). Relative to the symmetric case, we probe how asymmetry affects the overall slab kinematics, whether asymmetric behavior intensifies or equilibrates as subduction proceeds. While initial geometric asymmetry disappears once the slabs are anchored to the 660 km discontinuity, the mechanical asymmetry can cause more permanent differences between the two subduction zones. In the most extreme case, the partly continental slab stops subducting due to the unequal slab pull force. The results show that the slab-slab interaction is most effective when the two trenches are closer than 10-8 cm in the laboratory, which is 600-480 km when scaled to the Earth.

A Mesozoic orogenic cycle from post-collision to subduction in the southwestern Korean Peninsula: New structural, geochemical, and chronological evidence

NASA Astrophysics Data System (ADS)

Park, Seung-Ik; Kwon, Sanghoon; Kim, Sung Won; Hong, Paul S.; Santosh, M.

2018-05-01

The Early to Middle Mesozoic basins, distributed sporadically over the Korean Peninsula, preserve important records of the tectonic history of some of the major orogenic belts in East Asia. Here we present a comprehensive study of the structural, geochemical, geochronological, and paleontological features of a volcano-sedimentary package, belonging to the Oseosan Volcanic Complex of the Early to Middle Mesozoic Chungnam Basin, within the Mesozoic subduction-collision orogen in the southwestern Korean Peninsula. The zircon U-Pb data from rhyolitic volcanic rocks of the complex suggest Early to Middle Jurassic emplacement age of ca. 178-172 Ma, harmonious with plant fossil taxa found from the overlying tuffaceous sedimentary rock. The geochemical data for the rhyolitic volcanic rocks are indicative of volcanic arc setting, implying that the Chungnam Basin has experienced an intra-arc subsidence during the basin-expanding stage by subduction of the Paleo-Pacific (Izanagi) Plate. The Jurassic arc-related Oseosan Volcanic Complex was structurally stacked by the older Late Triassic to Early Jurassic post-collisional basin-fill of the Nampo Group by the Jangsan fault during basin inversion. The Late Jurassic to Early Cretaceous K-feldspar and illite K-Ar ages marked the timing of inversion tectonics, contemporaneous with the magmatic quiescence in the southern Korean Peninsula, likely due to flat-lying or low-angle subduction. The basin evolution history preserved in the Mesozoic Chungnam Basin reflects a Mesozoic orogenic cycle from post-collision to subduction in the southwestern Korean Peninsula. This, in turn, provides a better understanding of the spatial and temporal changes in Mesozoic tectonic environments along the East Asian continental margin.

Partially Melted UHP Eclogite in the Sulu Orogenic Belt, China and its rheological significance to deep continental subduction: Micro- to Macro-scale Evidence

NASA Astrophysics Data System (ADS)

Wang, Lu; Kusky, Timothy; Polat, Ali; Wang, Songjie; Jiang, Xingfu; Zong, Keqing; Wang, Junpeng; Deng, Hao; Fu, Jianmin

2015-04-01

Partially Melted UHP Eclogite in the Sulu Orogenic Belt, China and its rheological significance to deep continental subduction: Micro- to Macro-scale Evidence Numerous studies have described partial melting processes in low-high pressure meta-sedimentary rocks, some of which may generate melts that coalesce to form plutons. However, migmatized ultrahigh pressure (UHP) eclogite has never been clearly described from the microscale to macroscale, though experimental studies prove dehydration partial melting of eclogite at high pressure condition1 and low degrees of partially melted eclogite have been reported from the Qaidam UHP orogenic belt in NW China2,3 or inferred from multiphase solid (MS) inclusions within eclogite4 in the Sulu UHP belt. We present field-based documentation of decompression partial melting of UHP eclogite from Yangkou and General's Hill, Sulu Orogen. Migmatized eclogite shows successive stages of anatexis, initially starting from intragranular and grain boundary melt droplets, which grow into a 3D interconnected intergranular network, then segregate and accumulate in pressure shadow areas, and finally merge to form melt channels and dikes that transport melts to upper lithospheric levels. In-situ phengite breakdown-induced partial melting is directly identified by MS inclusions of Kfs+ barium-bearing Kfs + Pl in garnet, connected by 4-10 μm wide veinlets consisting of Bt + Kfs + Pl next to the phengite. Intergranular veinlets of plagioclase + K-feldspar first form isolated beads of melt along grain boundaries and triple junctions of quartz, and with higher degrees of melting, eventually form interconnected 3D networks along grain boundaries in the leucosome, allowing melt to escape from the intergranular realm and collect in low-stress areas. U-Pb (zircon) dating and petrological analyses on residue and leucocratic rocks shows that partial melting occurred at 228-219 Ma, shortly after peak UHP metamorphism (~230 Ma), and at depths of 30-90 km

Geochemistry of subduction zone serpentinites: A review

NASA Astrophysics Data System (ADS)

Deschamps, Fabien; Godard, Marguerite; Guillot, Stéphane; Hattori, Kéiko

2013-09-01

Over the last decades, numerous studies have emphasized the role of serpentinites in the subduction zone geodynamics. Their presence and role in subduction environments are recognized through geophysical, geochemical and field observations of modern and ancient subduction zones and large amounts of geochemical database of serpentinites have been created. Here, we present a review of the geochemistry of serpentinites, based on the compilation of ~ 900 geochemical data of abyssal, mantle wedge and exhumed serpentinites after subduction. The aim was to better understand the geochemical evolution of these rocks during their subduction as well as their impact in the global geochemical cycle. When studying serpentinites, it is essential to determine their protoliths and their geological history before serpentinization. The geochemical data of serpentinites shows little mobility of compatible and rare earth elements (REE) at the scale of hand-specimen during their serpentinization. Thus, REE abundance can be used to identify the protolith for serpentinites, as well as magmatic processes such as melt/rock interactions before serpentinization. In the case of subducted serpentinites, the interpretation of trace element data is difficult due to the enrichments of light REE, independent of the nature of the protolith. We propose that enrichments are probably not related to serpentinization itself, but mostly due to (sedimentary-derived) fluid/rock interactions within the subduction channel after the serpentinization. It is also possible that the enrichment reflects the geochemical signature of the mantle protolith itself which could derive from the less refractory continental lithosphere exhumed at the ocean-continent transition. Additionally, during the last ten years, numerous analyses have been carried out, notably using in situ approaches, to better constrain the behavior of fluid-mobile elements (FME; e.g. B, Li, Cl, As, Sb, U, Th, Sr) incorporated in serpentine phases

Topographic and sedimentary features in the Yap subduction zone and their implications for the Caroline Ridge subduction

NASA Astrophysics Data System (ADS)

Dong, Dongdong; Zhang, Zhengyi; Bai, Yongliang; Fan, Jianke; Zhang, Guangxu

2018-01-01

The Yap subduction zone in the western Pacific presents some unique features compared to normal intra-oceanic subduction zones such as the subduction of an oceanic plateau. However, due to the relative paucity of geophysical data, the detailed structure remains unknown in this area. In this study, we present the latest high-quality swath bathymetry and multi-channel seismic data acquired synchronously in 2015 across the Yap subduction zone. The topographic and sedimentary features are intensively investigated and a modified evolutionary model of the Yap subduction zone is proposed. The two-stage evolution of the Parece Vela Basin (PVB) produced fabrics that are N-S trending and NW-SE trending. Our seismic data clearly reveal landslide deposits at the upper slope break of the forearc, to the north of the Yap Island, which was identified as the fault notch denoting a lithological boundary in previous work. The swath bathymetry and seismic profile reveal detailed horst and graben structures, including a crescent-shaped fault zone near the contact between the Yap Trench and the Caroline Ridge. A simple geometric model is proposed to explain the structure formation, indicating that the higher topography of the Caroline Ridge resulted in enhanced bending-related extension. A seismic angular unconformity (named R1) is identified in the Sorol Trough, marking the onset of rifting in the trough. Based on the sequence thickness and deposition rate by Deep Sea Drilling Project (DSDP), it is deduced that the Sorol Trough formed at 10 Ma or even earlier. A modified model for the Yap subduction zone evolution is proposed, incorporating three major tectonic events: the proto-Yap Arc rupture in the Oligocene, the collision of the Caroline Ridge and the Yap Trench in the late Oligocene or middle Miocene, and the onset of the Sorol Trough rifting in the late Miocene.

Fore- and Back-Arc Structures Along the Hikurangi-Kermadec Subduction Zone

NASA Astrophysics Data System (ADS)

Scherwath, M.; Kopp, H.; Flueh, E. R.; Henrys, S. A.; Sutherland, R.

2009-04-01

The Hikurangi-Kermadec subduction zone northeast of New Zealand represents an ideal target to study lateral variations of subduction zone processes. The incoming Pacific plate changes from being a large igneous province, called the Hikurangi Plateau, in the south to normal oceanic plate north of the Rapuhia Scarp. The overriding Australian plate is continental in the south, forming the North Island of New Zealand, and changes to an island arc in the north. Further lateral variability exists in changes in volcanic and hydro-thermal activity, transitions from accretion to subduction erosion, backarc spreading and rifting, and is accompanied by northward increasing seismicity. As part of the MANGO project (Marine Geoscientific Investigations on the Input and Output of the Kermadec Subduction Zone), four marine geophysical transects of largely seismic reflection and refraction data provide constraints on the upper lithospheric structures across the Hikurangi-Kermadec Trench between 29-38 degrees South. On MANGO profile 1 in the south, the initially shallow subduction of the incoming plateau coincides with crustal underplating beneath the East Cape ridge. To the west lies the 100 km wide and over 10 km deep Raukumara Basin. Seismic velocities of the upper arc mantle are around 8 km/s and are considered normal. In contrast, on MANGO profile 4, about 1000 km to the north around the volcanically active Raoul Island, the incoming oceanic crust appears to bend considerably steeper and thus causes a 50 km narrower forearc with a smaller forearc basin. Furthermore, the upper mantle velocities in both plates are relatively low (7.4-7.7 km/s), likely indicating strong bending related deformation of the incoming plate and thermal activity within the arc possibly due to spreading. Here, arc volcanism is relatively active, with many large volcanoes directly on the ridge. The central two transects MANGO 2 and 3, though without data coverage of the structure of the incoming plate

Switches in subduction polarity, slab tearing and the opening of slab gaps along the Alpine chain - a view from the bottom up

NASA Astrophysics Data System (ADS)

Handy, M. R.; Ustaszewski, K. M.; Kissling, E. H.

2013-12-01

Kinematic reconstructions of the Alpine orogen from Late Cretaceous to present time reveal that slab tearing and switches of subduction polarity are related to two slab gaps presently imaged as low-velocity anomalies at the transition of the Eastern and Central Alps, and beneath the northern Dinarides. A lithosphere-scale transfer fault at the Alps-Dinarides join (ADT) linked S-directed subduction of the oceanic part of the European plate in the Alps with N-directed subduction of the continental part of the Adriatic plate in the Dinarides in Late Cretaceous to Paleogene time. Transfer faulting in the Dinarides was initially situated along a suture zone, then jumped westward no later than 40 Ma as thrusting and subduction affected more external units of the Alps and Dinarides. Late Eocene Alpine collision led to a slowing of Adria-Europe convergence and initial rupturing of the European and Adriatic slabs in Eocene-Oligocene time, when most of the oceanic lithosphere broke off. This thermally preconditioned the lithosphere for a radical reorganization of slabs and mantle flow in the Alpine domain beginning in early Miocene time. This included the onset of Carpathian rollback subduction, as well as counterclockwise rotation and N-ward subduction of Adriatic continental lithosphere into the space beneath the Eastern Alps that was vacated by foundering and renewed tearing of the European slab in Oligocene-early Miocene time. Our plate reconstructions indicate that this tear nucleated at the tip of a subducted sliver of European continental lithosphere coinciding with the present location of the narrow slab gap between the Eastern and Central Alps. This tear then propagated horizontally to the NE along the subducted boundary of the European margin and the Carpathian embayment of the Alpine Tethyan ocean. The surface response to slab tearing included peneplainization and uplift of part of the Eastern Alps. Transfer faulting along the ADT gave way to back-arc extension

Structural control of the upper plate on the down-dip segmentation of subduction dynamics

NASA Astrophysics Data System (ADS)

Shi, Q.; Barbot, S.; Karato, S. I.; Shibazaki, B.; Matsuzawa, T.; Tapponnier, P.

2017-12-01

The geodetic and seismic discoveries of slow earthquakes in subduction zones have provided the observational evidence for the existence of the transition between megathrust earthquakes and the creeping behaviors. However, the mechanics behind slow earthquakes, and the period differential motion between the subducting slab and the overlying plate below the seismogenic zone, remain controversial. In Nankai subduction zone, the very-low-frequency earthquakes (VLFE), megathrust earthquakes, long-term slow earthquakes (duration of months or years) and the episodic tremor and slip zone (ETS) are located within the accretionary prism, the continental upper crust, the continental lower crust and the upmost mantle of the overriding plate, respectively. We use the rate-and-state friction law to simulate the periodic occurrence of VLFEs, megathrust earthquakes and the tremors in the ETS zone because of relatively high rock strength within these depth ranges. However, it is not feasible to use frictional instabilities to explain the long-term slow earthquakes in the lower crust where the ductile rock physics plays a significant role in the large-scale deformation. Here, our numerical simulations show that slow earthquakes at the depth of the lower crust may be the results of plastic instabilities in a finite volume of ductile material accompanying by the grain-size evolution. As the thickness of the fault zone increases with depth, deformation becomes distributed and the dynamic equilibrium of grain size, as a competition between thermally activated grain growth and damage-related grain size reduction, results in cycles of strain acceleration and strain deficit. In addition, we took into account the elevated pore pressure in the accretinary prism which is associated with small stress drop and low-frequency content of VLFEs and may contribute to the occurrence of tsunamigenic earthquakes. Hence, in our numerical simulations for the plate boundary system in Nankai, the down

Cyclic stressing and seismicity at strongly coupled subduction zones

USGS Publications Warehouse

Taylor, M.A.J.; Zheng, G.; Rice, J.R.; Stuart, W.D.; Dmowska, R.

1996-01-01

We use the finite element method to analyze stress variations in and near a strongly coupled subduction zone during an earthquake cycle. Deformation is assumed to be uniform along strike (plane strain on a cross section normal to the trench axis), and periodic earthquake slip is imposed consistent with the long-term rate of plate convergence and degree of coupling. Simulations of stress and displacement rate fields represent periodic fluctuations in time superimposed on an average field. The oceanic plate, descending slab, and continental lithosphere are assumed here to respond elastically to these fluctuations, and the remaining mantle under and between plates is assumed to respond as Maxwell viscoelastic. In the first part of the analysis we find that computed stress fluctuations in space and time are generally consistent with observed earthquake mechanism variations with time since a great thrust event. In particular, trench-normal extensional earthquakes tend to occur early in the earthquake cycle toward the outer rise but occur more abundantly late in the cycle in the subducting slab downdip of the main thrust zone. Compressional earthquakes, when they occur at all, have the opposite pattern. Our results suggest also that the actual timing of extensional outer rise events is controlled by the rheology of the shallow aseismic portion of the thrust interface. The second part of the analysis shows the effects of mantle relaxation on the rate of ground surface deformation during the earthquake cycle. Models without relaxation predict a strong overall compressional strain rate in the continental plate above the main thrust zone, with the strain rate constant between mainshocks. However with significant relaxation present, a localized region of unusually low compressional, or even slightly extensional, strain rate develops along the surface of the continental plate above and somewhat inland from the downdip edge of the locked main thrust zone. The low strain rate

Vertical tectonics at an active continental margin

NASA Astrophysics Data System (ADS)

Houlié, N.; Stern, T. A.

2017-01-01

Direct observations of vertical movements of the earth's surface are now possible with space-based GPS networks, and have applications to resources, hazards and tectonics. Here we present data on vertical movements of the Earth's surface in New Zealand, computed from the processing of GPS data collected between 2000 and 2015 by 189 permanent GPS stations. We map the geographical variation in vertical rates and show how these variations are explicable within a tectonic framework of subduction, volcanic activity and slow slip earthquakes. Subsidence of >3 mm/yr is observed along southeastern North Island and is interpreted to be due to the locked segment of the Hikurangi subduction zone. Uplift of 1-3 mm/yr further north along the margin of the eastern North Island is interpreted as being due to the plate interface being unlocked and underplating of sediment on the subduction thrust. The Volcanic Plateau of the central North Island is being uplifted at about 1 mm/yr, which can be explained by basaltic melts being injected in the active mantle-wedge at a rate of ∼6 mm/yr. Within the Central Volcanic Region there is a 250 km2 area that subsided between 2005 and 2012 at a rate of up to 14 mm/yr. Time series from the stations located within and near the zone of subsidence show a strong link between subsidence, adjacent uplift and local earthquake swarms.

Transformation from Paleo-Asian Ocean closure to Paleo-Pacific subduction: New constraints from granitoids in the eastern Jilin-Heilongjiang Belt, NE China

NASA Astrophysics Data System (ADS)

Ma, Xing-Hua; Zhu, Wen-Ping; Zhou, Zhen-Hua; Qiao, Shi-Lei

2017-08-01

The eastern Jilin-Heilongjiang Belt (EJHB) of NE China is a unique orogen that underwent two stages of evolution within the tectonic regimes of the Paleo-Asian and Paleo-Pacific oceans. 158 available zircon U-Pb ages, including 26 ages obtained during the present study and 132 ages from the literature, were compiled and analyzed for the Mesozoic and Cenozoic granitoids from the EJHB and the adjacent Russian Sikhote-Alin Orogenic Belt (SAOB), to examine the temporal-spatial distribution of the granitoids and to constrain the tectonic evolution of the East Asian continental margin. Five stages of granitic magmatism can be identified: Early Triassic (251-240 Ma), Late Triassic (228-215 Ma), latest Triassic to Middle Jurassic (213-158 Ma), Early Cretaceous (131-105 Ma), and Late Cretaceous to Paleocene (95-56 Ma). The Early Triassic granitoids are restricted to the Yanbian region along the Changchun-Yanji Suture, and show geochemical characteristics of magmas from a thickened lower crust source, probably due to the final collision of the combined NE China blocks with the North China Craton. The Late Triassic granitoids, with features of A-type granites, represent post-collisional magmatic activities that were related to post-orogenic extension, marking the end of the tectonic evolution of the Paleo-Asian Ocean. The latest Triassic to Paleocene granitoids with calc-alkaline characteristics were NE-trending emplaced along the EJHB and SAOB and young towards the coastal region, and represent continental marginal arc magmas that were associated with the northwestwards subduction of the Paleo-Pacific Plate. Two periods of magmatic quiescence (158-131 and 105-95 Ma) correspond to changes in the subduction direction of the Paleo-Pacific Plate from oblique relative to the continental margin to subparallel. Taking all this into account, we conclude that: (1) the final closure of the Paleo-Asian Ocean occurred along the Changchun-Yanji Suture during the Early Triassic; (2) the

Coesite Assemblages in Deep Continental Lithosphere: Additional Evidence for a Protolith from Subduction of Oceanic Crust

NASA Astrophysics Data System (ADS)

Sobolev, N.

2005-12-01

Inclusions in diamonds (DIs) represent an important source of information about the composition of continental lithospheric mantle. The isolated coesite inclusions in two diamonds (Harris, 1968) and a full set of eclogitic minerals (coesite (Cs), garnet (Ga), omphacite Cpx)) in two Yakutian diamonds (Sobolev et al., 1976), followed by finds of Cs-eclogite xenoliths (Smyth and Hatton, 1977; Ponomarenko et al., 1977) testify to the importance of coesite as a constituent of eclogitic rocks in deep lithospheric environment. Since these earlier times, coesite has been documented in more than 250 natural diamonds from 25 localities worldwide. Some 40 xenoliths of Cs-eclogites were found both in South African and Yakutian kimberlites. However, >50% of DIs of coesite are related to only four (4) diamond localities, including Guaniamo, Venezuela (Sobolev et al., 1998, 2003), Argyle (Jaques et al., 1989; Sobolev et al., 1989), New South Wales, all Australia (Sobolev et al., 1984; Meyer et al., 1997), and North Yakutian alluvials (Sobolev et al., 1999). All described DIs with coesite are from a wide range of assemblages: websterites to kyanite eclogites; grospydites and calcsilicate assemblages, with a large range in Gt [3.7-28.7 wt.% CaO] and Cpx [ 0.9-8.8 wt.% Na2O] compositions. In spite of these occurrences in diamonds, to the present, no coesite has been detected within the assemblage of minerals making up some 400 diamondiferous-eclogite xenoliths; similarly, no diamonds have been found in any Cs-eclogite xenoliths. This apparent paradox may be caused by coesite alteration in the diamondiferous eclogites, whereas coesite eclogites may have formed only outside of the diamond stability field. Indeed, coesite eclogites (without diamonds) may occupy a shallower position within continental lithosphere compared with the normal E-type diamond source. This indicates a broadly basaltic chemistry of the deep eclogitic environment, additional evidence for a protolith from the

In search of transient subduction interfaces in the Dent Blanche-Sesia Tectonic System (W. Alps)

NASA Astrophysics Data System (ADS)

Angiboust, Samuel; Glodny, Johannes; Oncken, Onno; Chopin, Christian

2014-09-01

In this paper we study the Alpine metamorphic history of a major tectonic zone which formed during Alpine orogeny, the Dent Blanche Thrust (DBT). This contact, located in the Northern Western Alps, juxtaposes some ophiolitic metasediment-rich remnants of the Liguro-Piemontese ocean (Tsaté Complex) with a composite continental, km-sized complex (Dent Blanche Tectonic System, DBTS) of Adriatic affinity thrusted over the ophiolite. In order to better understand the geodynamic meaning of the DBT region and adjacent units, we have reconstructed the pressure-temperature-time-deformation (P-T-t-d) history of these two units using modern thermobarometric tools, Rb/Sr geochronology, and field relationships. We show that the Tsaté Complex is formed by a stack of km-thick calcschists-bearing tectonic slices having experienced variable maximum burial temperatures between 360 °C and 490 °C at depths of ca. 25-40 km. Associated deformation ages span a range between 37 Ma and 41 Ma. The Arolla gneissic mylonites at the base of the DBTS experienced high-pressure (12-14 kbar), top-to-NW deformation at ca. 450 °C between 43 and 48 Ma. A first age of ca. 58 Ma has been obtained for high-pressure ductile deformation in the Valpelline shear zone, atop Arolla gneisses. Some of the primary, peak metamorphic fabrics have been reworked and later backfolded during exhumation and collisional overprint (ca. 20 km depth, 37-40 Ma) leading to the regional greenschist-facies retrogression which is particularly prominent within Tsaté metasediments. We interpret the Dent Blanche Thrust, at the base of the Arolla unit, as a fossilized subduction interface active between 43 and 48 Ma. Our geochronological results on the shear zone lining the top of the Arolla unit, together with previous P-T-t estimates on equivalent blueschist-facies shear zones cutting the Sesia unit, indicate an older tectonic activity between 58 and 65 Ma. We demonstrate here that observed younger ages towards lowermost

Formation of an active thrust triangle zone associated with structural inversion in a subduction setting, eastern New Zealand

NASA Astrophysics Data System (ADS)

Barnes, Philip M.; Nicol, Andrew

2004-02-01

We analyze a thrust triangle zone, which underlies the continental shelf of Hawke Bay, eastern New Zealand, within the Hikurangi subduction margin. This triangle zone differs from many other examples in that it is active, 90 km from the leading edge of the overriding plate, and formed due to polyphase deformation involving opposed dipping thrust duplex and backthrust, with the later structure forming in response to inversion of an extensional graben. The component structures of the zone mainly developed sequentially rather than synchronously. High-quality marine seismic reflection lines, tied to well and seabed samples, reveal the three-dimensional structure of the zone, together with its 25 Myr evolution and late Quaternary activity. The triangle zone occurs in the lateral overlap between a stack of NW dipping blind thrusts, and a principal backthrust, the Kidnappers fault. The NW dipping thrusts initiated in the early-middle Miocene during the early stages of subduction, with subsequent thrust duplex formation producing major uplift and erosion in the late Miocene-early Pliocene. The active backthrust formed during the late Miocene to early Pliocene as a thin-skinned listric extensional fault confined to the cover sequence. Structural inversion of the extensional fault commenced in the early-middle Pliocene, produced the backthrust and marks the formation of the thrust triangle zone. The thrust duplex and backthrust accrued strain following inversion; however, the later structure accommodated most of the surface deformation in the Quaternary. Section balancing of the triangle zone together with a detailed analysis of reverse displacements along the backthrust reveal spatial and temporal variations of strain accumulation on the two principal components of the zone. Although the formation of the triangle zone is strongly influenced by regional tectonics of the subduction system, these variations may also, in part, reflect local fault interaction. For example, high

Strain measurements and the potential for a great subduction earthquake off the coast of washington.

PubMed

Savage, J C; Lisowski, M

1991-04-05

Geodetic measurements of deformation in northwestern Washington indicate that strain is accumulating at a rate close to that predicted by a model of the Cascadia subduction zone in which the plate interface underlying the continental slope and outer continental shelf is currently locked but the remainder of the interface slips continuously. Presumably this locked segment will eventually rupture in a great thrust earthquake with a down-dip extent greater than 100 kilometers.

Sediment underthrusting within a continental magmatic arc: Coast Mountains batholith, British Columbia

NASA Astrophysics Data System (ADS)

Pearson, David M.; MacLeod, Douglas R.; Ducea, Mihai N.; Gehrels, George E.; Jonathan Patchett, P.

2017-10-01

Though continental magmatic arcs are factories for new continental crust, a significant proportion of continental arc magmas are recycled from supracrustal material. To evaluate the relative contributions of retroarc underthrusting and trench side partial sediment subduction for introducing supracrustal rocks to the middle and lower crust of continental magmatic arcs, we present results from the deeply exposed country rocks of the Coast Mountains batholith of western British Columbia. Prior work demonstrates that these rocks underwent widespread partial melting that contributed to the Coast Mountains batholith. We utilize U-Pb zircon geochronology, Sm-Nd thermochronology, and field-based studies to document the protoliths and early burial history of amphibolite and granulite-facies metasedimentary rocks in the Central Gneiss Complex. U-Pb detrital zircon data from the structurally highest sample localities yielded 190 Ma unimodal age peaks and suggest that retroarc rocks of the Stikine terrane constitute a substantial portion of the Central Gneiss Complex. These supracrustal rocks underwent thrust-related burial and metamorphism at >25 km depths prior to 80 Ma. These rocks may also be underlain at the deepest exposed structural levels by Upper Cretaceous metasedimentary rocks, which may have been emplaced as a result of trench side underplating or intraarc burial. These results further our understanding of the mechanisms of material transport within the continental lithosphere along Cordilleran subduction margins.

Earth's first stable continents did not form by subduction

NASA Astrophysics Data System (ADS)

Johnson, Tim; Brown, Michael; Gardiner, Nicholas; Kirkland, Christopher; Smithies, Hugh

2017-04-01

The geodynamic setting in which Earth's first stable cratonic nuclei formed remains controversial. Most exposed Archaean continental crust comprises rocks of the tonalite-trondhjemite-granodiorite (TTGs) series that were produced from partial melting of low magnesium basaltic source rocks and have 'arc-like' trace element signatures that resemble continental crust produced in modern supra-subduction zone settings. The East Pilbara Terrane, Western Australia, is amongst the oldest fragments of preserved continental crust of Earth. Low magnesium basalts of the Paleoarchaean Coucal Formation, at the base of the Pilbara Supergroup, have trace element compositions consistent with the putative source rocks for TTGs. These basalts may be remnants of the ≥35 km-thick pre-3.5 Ga plateau-like basaltic crust that is predicted to have formed if mantle temperatures were much hotter than today. Using phase equilibria modelling of an average uncontaminated Coucal basalt, we confirm their suitability as TTG source rocks. The results suggest that TTGs formed by 20-30% melting along high geothermal gradients (≥700 °C/GPa), which accord with apparent geotherms recorded by >95% of Archaean rocks worldwide. Moreover, the trace element composition of the Coucal basalts demonstrates that they were derived from an earlier generation of mafic/ultramafic rocks, and that the arc-like signature in Archaean TTGs was inherited through an ancestral source lineage. The protracted multistage process required for production and stabilisation of Earth's first continents, coupled with the high geothermal gradients, are incompatible with modern-style subduction and favour a stagnant lid regime in the early Archaean.

Seismic evidence for overpressured subducted oceanic crust and megathrust fault sealing.

PubMed

Audet, Pascal; Bostock, Michael G; Christensen, Nikolas I; Peacock, Simon M

2009-01-01

Water and hydrous minerals play a key part in geodynamic processes at subduction zones by weakening the plate boundary, aiding slip and permitting subduction-and indeed plate tectonics-to occur. The seismological signature of water within the forearc mantle wedge is evident in anomalies with low seismic shear velocity marking serpentinization. However, seismological observations bearing on the presence of water within the subducting plate itself are less well documented. Here we use converted teleseismic waves to obtain observations of anomalously high Poisson's ratios within the subducted oceanic crust from the Cascadia continental margin to its intersection with forearc mantle. On the basis of pressure, temperature and compositional considerations, the elevated Poisson's ratios indicate that water is pervasively present in fluid form at pore pressures near lithostatic values. Combined with observations of a strong negative velocity contrast at the top of the oceanic crust, our results imply that the megathrust is a low-permeability boundary. The transition from a low- to high-permeability plate interface downdip into the mantle wedge is explained by hydrofracturing of the seal by volume changes across the interface caused by the onset of crustal eclogitization and mantle serpentinization. These results may have important implications for our understanding of seismogenesis, subduction zone structure and the mechanism of episodic tremor and slip.

Initiation of extension in South China continental margin during the active-passive margin transition: kinematic and thermochronological constraints

NASA Astrophysics Data System (ADS)

ZUO, Xuran; CHAN, Lung

2015-04-01

The southern South China Block is characterized by a widespread magmatic belt, prominent NE-striking fault zones and numerous rifted basins filled by Cretaceous-Eocene sediments. The geology denotes a transition from an active to a passive margin, which led to rapid modifications of crustal stress configuration and reactivation of older faults in this area. In this study, we used zircon fission-track dating (ZFT) and numerical modeling to examine the timing and kinematics of the active-passive margin transition. Our ZFT results on granitic plutons in the SW Cathaysia Block show two episodes of exhumation of the granitic plutons. The first episode, occurring during 170 Ma - 120 Ma, affected local parts of the Nanling Range. The second episode, a more regional exhumation event, occurred during 115 Ma - 70 Ma. Numerical geodynamic modeling was conducted to simulate the subduction between the paleo-Pacific plate and the South China Block. The modeling results could explain the observation based on ZFT data that exhumation of the granite-dominant Nanling Range occurred at an earlier time than the gneiss-dominant Yunkai Terrane. In addition to the difference in geology between Yunkai and Nanling, the heating from Jurassic-Early Cretaceous magmatism in the Nanling Range may have softened the upper crust, causing the area to exhume more readily. Numerical modeling results also indicate that (1) high slab dip angle, high geothermal gradient of lithosphere and low convergence velocity favor the subduction process and the reversal of crustal stress state from compression to extension in the upper plate; (2) the late Mesozoic magmatism in South China was probably caused by a slab roll-back; and (3) crustal extension could have occurred prior to the cessation of plate subduction. The inversion of stress regime in the continental crust from compression to crustal extension has shed light on the geological condition producing the red bed basins during Late Cretaceous

Effects of geodynamic setting on the redox state of fluids released by subducted mantle lithosphere

NASA Astrophysics Data System (ADS)

Evans, K. A.; Reddy, S. M.; Tomkins, A. G.; Crossley, R. J.; Frost, B. R.

2017-05-01

high hydrogen activity (aH2). Ultramafic rocks are associated with two distinctly different fluid compositions during pre-subduction and subduction serpentinisation. Initially, while olivine is in equilibrium with infiltrating fluid, mineral assemblages that include awaruite in the rocks buffer fluids to H2-bearing, low aO2 compositions. Deserpentinisation of incompletely serpentinised rocks in which awaruite is present also produces H2-bearing fluids. Once awaruite is exhausted, H2-poor, high aO2 fluids co-exist with awaruite-absent assemblages, and deserpentinisation of such rocks would produce H2O-rich fluids. Thus, deserpentinisation of ultramafic rocks could produce either hydrogen-bearing fluids that could infiltrate and reduce the sub-arc mantle, or more oxidised fluids, which could transfer redox budget to other geochemical reservoirs such as the sub-arc mantle. Therefore, the redox contribution of subducted ultramafic rocks to the deep Earth and sub-arc mantle depends on the extent of protolith serpentinisation. Pre-subduction settings that promote extensive serpentinisation by oxidised fluids at high fluid:rock ratios in open systems, such as slow and ultraslow spreading ridges, transform faults, oceanic core complexes, and exhumed mantle at rifted continental margins, may produce more oxidised fluids than those associated with less pervasive serpentinisation and fluids that may be rock-buffered to a reduced state.

Multiple subduction imprints in the mantle below Italy detected in a single lava flow

NASA Astrophysics Data System (ADS)

Nikogosian, Igor; Ersoy, Özlem; Whitehouse, Martin; Mason, Paul R. D.; de Hoog, Jan C. M.; Wortel, Rinus; van Bergen, Manfred J.

2016-09-01

Post-collisional magmatism reflects the regional subduction history prior to collision but the link between the two is complex and often poorly understood. The collision of continents along a convergent plate boundary commonly marks the onset of a variety of transitional geodynamic processes. Typical responses include delamination of subducting lithosphere, crustal thickening in the overriding plate, slab detachment and asthenospheric upwelling, or the complete termination of convergence. A prominent example is the Western-Central Mediterranean, where the ongoing slow convergence of Africa and Europe (Eurasia) has been accommodated by a variety of spreading and subduction systems that dispersed remnants of subducted lithosphere into the mantle, creating a compositionally wide spectrum of magmatism. Using lead isotope compositions of a set of melt inclusions in magmatic olivine crystals we detect exceptional heterogeneity in the mantle domain below Central Italy, which we attribute to the presence of continental material, introduced initially by Alpine and subsequently by Apennine subduction. We show that superimposed subduction imprints of a mantle source can be tapped during a melting episode millions of years later, and are recorded in a single lava flow.

Structure and seismic activity of the Lesser Antilles subduction zone

NASA Astrophysics Data System (ADS)

Evain, M.; Galve, A.; Charvis, P.; Laigle, M.; Ruiz Fernandez, M.; Kopp, H.; Hirn, A.; Flueh, E. R.; Thales Scientific Party

2011-12-01

Several active and passive seismic experiments conducted in 2007 in the framework of the European program "Thales Was Right" and of the French ANR program "Subsismanti" provided a unique set of geophysical data highlighting the deep structure of the central part of the Lesser Antilles subduction zone, offshore Dominica and Martinique, and its seismic activity during a period of 8 months. The region is characterized by a relatively low rate of seismicity that is often attributed to the slow (2 cm/yr) subduction of the old, 90 My, Atlantic lithosphere beneath the Caribbean Plate. Based on tomographic inversion of wide-angle seismic data, the forearc can clearly be divided into an inner forearc, characterised by a high vertical velocity gradient in the igneous crust, and an outer forearc with lower crustal velocity gradient. The thick, high velocity, inner forearc is possibly the extension at depth of the Mesozoic Caribbean crust outcropping in La Désirade Island. The outer forearc, up to 70 km wide in the northern part of the study area, is getting narrower to the south and disappears offshore Martinique. Based on its seismic velocity structure with velocities higher than 6 km/s the backstop consists, at least partly, of magmatic rocks. The outer forearc is also highly deformed and faulted within the subducting trend of the Tiburon Ridge. With respect to the inner forearc velocity structure the outer forearc basement could either correspond to an accreted oceanic terrane or made of highly fractured rocks. The inner forearc is a dense, poorly deformable crustal block, tilted southward as a whole. It acts as a rigid buttress increasing the strain within both the overriding and subducting plates. This appears clearly in the current local seismicity affecting the subducting and the overriding plates that is located beneath the inner forearc. We detected earthquakes beneath the Caribbean forearc and in the Atlantic oceanic plate as well. The main seismic activity is

Numerical modeling of Farallon Plate flat-slab subduction: Influence of lithosphere structure and rheology on slab dynamics

NASA Astrophysics Data System (ADS)

Liu, X.; Currie, C. A.

2017-12-01

The subducted Farallon plate is believed to have evolved to a flat geometry underneath North America plate during Late Cretaceous, triggering Laramide deformation within the continental interior. However, the mechanism that caused the oceanic slab to flatten and the factors that control the flat-slab depth remain uncertain. In this work, we use 2D thermal-mechanical models using the SOPALE code to study the subduction dynamics from 90 Ma to 50 Ma. During this period, an oceanic plateau (Shatsky Conjugate) is inferred to have subducted beneath western North America and interacted with the continental lithosphere, including areas of thicker lithosphere such as the Colorado Plateau and Wyoming Craton. Based on seismic tomography and plate reconstruction data sets, we built a set of models to examine the influence of the structure and rheology of the oceanic and continental plates on slab dynamics. Models include a 600 km wide oceanic plateau consisting of 18 km thick crust and a 36 km thick underlying harzburgite layer, and we ran a series of model experiments to test different continental thicknesses (80 km, 120 km, & 180 km) and continental mantle lithosphere strengths (approximating conditions from wet olivine to dry olivine). Consistent with earlier studies, we find that creation of a long flat slab requires a buoyant oceanic plateau (i.e., non-eclogitized crust) and trenchward motion of the continent. In addition, our models demonstrate the upper plate has an important control on slab dynamics. A flat slab requires either a thin continent or, if the continent is thick, its mantle lithosphere must be relatively weak so that it can be displaced by the flattening slab. The depth of the flat slab is mainly controlled by two factors: (1) the continental thickness and (2) the strength of the continental mantle lithosphere. For the same initial lithosphere thickness (120 km), a shallower flat slab ( 90 km depth) occurs for the weakest mantle lithosphere ( wet olivine

Neoarchean Subduction Recorded in the Northern Margin of the Yangtze Craton, South China

NASA Astrophysics Data System (ADS)

Zhang, S. B.; Zheng, Y. F.

2016-12-01

formed in an active continental margin and the A-type granites formed as a response to the back-arc extension. The identification of 2.65-2.7 Ga subduction provides a possible link of the Yangtze Craton with the other cratons worldwide. This is also a new clue to reconstruct the Neoarchean supercontinent Kenorland.

Influence of sediment recycling on the trace element composition of primitive arc lavas

NASA Astrophysics Data System (ADS)

Collinet, M.; Jagoutz, O. E.

2017-12-01

Primitive calc-alkaline lavas from continental arcs are, on average, enriched in incompatible elements compared to those from intra-oceanic arcs. This relative enrichment is observed in different groups of trace elements: LILE (e.g. K, Rb), LREE to MREE (La-Dy) and HFSE (e.g.Zr, Nb) and is thought to result from (1) a transfer of material from the subducting slab to the mantle wedge at higher temperature than in intra-oceanic margins and/or (2) lower average degrees of melting in the mantle wedge, as a consequence of thicker overlying crusts and higher average pressures of melting. In addition to thicker overlying crusts and generally higher slab temperatures, continental margins are characterized by larger volumes of rock exposed above sea level and enhanced erosion rates compared to intra-oceanic arcs. As several geochemical signatures of arc lavas attest to the importance of sediment recycling in subduction zones, we explore the possibility that the high concentrations of incompatible elements in primitive lavas from continental arcs directly reflect a larger input of sediment to the subduction system. Previous efforts to quantify the sediment flux to oceanic trenches focused on the thickness of pelagic and hemipelagic sediments on top of the plate entering the subduction zone (Plank and Langmuir, 1993, Nature). These estimates primarily relied on the sediment layer drilled outboard from the subduction system and likely underestimate the volume of sediment derived from the arc itself. Accordingly, we find that such estimates of sediment flux do not correlate with the concentration of incompatible elements in primitive arc lavas. To account for regional contributions of coarser detrital sediments, usually delivered to oceanic trenches by turbidity currents, we apply to arc segments a model that quantifies the sediment load of rivers based on the average relief, area, temperature and runoff of their respective drainage areas (Syvitski et al., 2003, Sediment. Geol

New Insight Into the Crustal Structure of the Continental Margin offshore NW Sabah/Borneo

NASA Astrophysics Data System (ADS)

Barckhausen, U.; Franke, D.; Behain, D.; Meyer, H.

2002-12-01

The continental margin offshore NW Sabah/Borneo (Malaysia) has been investigated with reflection and refraction seismics, magnetics, and gravity during the recent cruise BGR01-POPSCOMS. A total of 4000 km of geophysical profiles has been acquired, thereof 2900 km with reflection seismics. The focus of investigations was on the deep water areas. The margin looks like a typical accretionary margin and was presumably formed during the subduction of a proto South China Sea. Presently, no horizontal movements between the two plates are being observed. Like in major parts of the South China Sea, the area seaward of the Sabah Trough consists of extended continental lithosphere which is characterised by a pattern of rotated fault blocks and half grabens and a carbonate platform of Early Oligocene to Early Miocene age. We found evidence that the continental crust also underlies the Sabah Trough and the adjacent continental slope, a fact that raises many questions about the tectonic history and development of this margin. The tectonic pattern of the Dangerous Grounds' extended continental crust can be traced a long way landward of the Sabah Trough beneath the sedimentary succession of the upper plate. The magnetic anomalies which are dominated by the magnetic signatures of relatively young volcanic features also continue under the continental slope. The sedimentary rocks of the upper plate, in contrast, seem to generate hardly any magnetic anomalies. Based on the new data we propose the following scenario for the development of the NW Sabah continental margin: Seafloor spreading in the present South China Sea started at about 30 Ma in the Late Oligocene. The spreading process separated the Dangerous Grounds area from the SE Asian continent and ceased in late Early Miocene when the oceanic crust of the proto South China Sea was fully subducted in eastward direction along the Borneo-Palawan Trough. During Lower and/or Middle Miocene, Borneo rotated counterclockwise and was

Seismic anisotropy and mantle flow in the Hellenic subduction zone: The possible effects of trench retreat and slab tear at both ends.

NASA Astrophysics Data System (ADS)

Evangelidis, Christos

2017-04-01

The upper mantle anisotropy pattern in the entire area of the Hellenic subduction zone have been analyzed for fast polarization directions and delay times to investigate the complex 3D pattern of mantle flow around the subducting slab. All previous studies do incorporate a significant number of measurements in the backarc area of the Aegean and in two cross-sections along the Hellenic subduction system. However, the transitional area from oceanic to continental subduction in the Western Hellenic trench has not been adequately sampled so far. Moreover, the eastern termination of the Hellenic subduction and the possible origin of a trench parallel anisotropy remains unclear. Here, I focus on the two possible ends of the high curvature Hellenic arc. I have now measured SKS splitting parameters from all broadband stations of the Hellenic Unified Seismic Network (HUSN), that they have not been measured before, specially concentrated in the transitional area from oceanic to continental subduction system. Complementary, using the Source-Side splitting technique to teleseismic S-wave records from intermediate depth earthquake in the Hellenic trench, the anisotropy measurements are increased in regions where no stations are installed. In western Greece, the Hellenic subduction system is separated by the Cephalonia Transform Fault (CTF), a dextral offset of 100 km, into the northern and southern segments, which are characterized by different convergence rates and slab composition. Recent seismic data show that north of CTF there is a subducted continental lithosphere in contrast to the region south of CTF where the on-going subduction is oceanic. The new measurements, combined with previously published observations, provide the most complete up-to-date spatial coverage for the area. Generally, the pronounced zonation of seismic anisotropy across the subduction zone, as inferred from other studies, is also observed here. Fast SKS splitting directions are trench-normal in the

Early Carboniferous magmatism in Lhasa generated in passive continental margin: constrained by new SIMS dating from Carboniferous arc in Qiantang terrane, Tibet

NASA Astrophysics Data System (ADS)

Zhang, X. Z.; Dan, W.; Wang, Q.; Hao, L. L.; Qi, Y.

2016-12-01

In today's oceans, they are rarely undergone subduction on one side and extension on the opposite side. In contrast, there are a few magmatisms in the passive continental margins in the Tethys Ocean. However, because of their long and complex evolution of the northern continental margin of the Gondwana, the geodynamics of the magmatism occurred in this area is speculative or highly depute. One of these examples is the geodynamics of the 360-350 Ma magmatism in southern Lhasa, Tibet. Many authors speculated that it was generated in back-arc setting. Our recent new high-resolution SIMS zircon U-Pb dating reveals that there is a subduction arc with ages of 370-350 Ma in the Qiangtang terrane. The arc rocks compose of andesites, plagiogranites, A-type granites and cumulated gabbros, indicating an initial subduction. This initial subduction arc is located on the north margin of the eastern Paleo-Tethys Ocean, and it was formed slightly earlier than the 360-350 Ma magmatism in southern Lhasa, located on the south margin of the eastern Paleo-Tethys Ocean. Combined with similar aged magmatism generating the back-arc basin in the Sanjiang area, the 360-350 Ma magmatism in southern Lhasa was proposed to be generated in a passive continental margin, and induced by the regional extensional setting related to the subduction in the north margin of the eastern Paleo-Tethys Ocean.

Lithospheric Subduction on Earth and Venus?

NASA Astrophysics Data System (ADS)

Sandwell, D. T.; Garcia, E.; Stegman, D. R.; Schubert, G.

2016-12-01

There are three mechanisms by which terrestrial planets can shed excess heat: conduction across a surface thermal boundary layer; advection of heat through volcanic pipes; and mobile plates/subduction. On the Earth about 30% is released by conduction and 70% by subduction. The dominant mode of heat transport on Venus is largely unknown. Plate flexure models rule out significant heat loss by conduction and the resurfacing from active volcanism is in discordance with a surface age of 600 Ma. There are 9000 km of trenches on Venus that may have been subduction sites but they do not appear active today and are only 25% of the length of the subduction zones on the Earth. Turcotte and others have proposed an episodic recycling model that has short bursts ( 150 Ma) of plate tectonic activity followed by long periods ( 450 Ma) of stagnant lid convection. This talk will review the arguments for and against subduction zones on Venus and discuss possible new satellite observations that could help resolve the subduction issue. Figure Caption. (a) Global mosaic of Magellan SAR imagery. (b) Zoom of area along the Artemis trench, which has similar topography and fracture patterns as the Aleutian subduction zone on Earth. Trench and outer rise lines were digitized from the matching topography image (not shown). The Magellan SAR imagery and topography, displayed on Google Earth, can be downloaded at http://topex.ucsd.edu/venus/index.html

Cyclic growth in Atlantic region continental crust

NASA Technical Reports Server (NTRS)

Goodwin, A. M.

1986-01-01

Atlantic region continental crust evolved in successive stages under the influence of regular, approximately 400 Ma-long tectonic cycles. Data point to a variety of operative tectonic processes ranging from widespread ocean floor consumption (Wilson cycle) to entirely ensialic (Ampferer-style subduction or simple crustal attenuation-compression). Different processes may have operated concurrently in some or different belts. Resolving this remains the major challenge.

Slab detachment during continental collision: Influence of crustal rheology and interaction with lithospheric delamination

NASA Astrophysics Data System (ADS)

Duretz, T.; Gerya, T. V.

2013-08-01

Collision between continents can lead to the subduction of continental material. If the crust remains coupled to the downgoing slab, a large buoyancy force is generated. This force slows down convergence and promotes slab detachment. If the crust resists to subduction, it may decouple from the downgoing slab and be subjected to buoyant extrusion. We employ two-dimensional thermo-mechanical modelling to study the importance of crustal rheology on the evolution of subduction-collision systems. We propose simple quantifications of the mechanical decoupling between lithospheric levels (σ*) and the potential for buoyant extrusion of the crust (ξ*). The modelling results indicate that a variable crustal rheological structure results in slab detachment, delamination, or the combination of both mechanisms. A strong crust provides coupling at the Moho (low σ*) and remains coherent during subduction (low ξ). It promotes deep subduction of the crust (180 km) and slab detachment. Exhumation occurs in coherent manners via eduction and thrusting. Slab detachment triggers the development of topography (> 4.5 km) close to the suture. A contrasting style of collision occurs using a weak crustal rheology. Mechanical decoupling at the Moho (high σ*) promotes the extrusion of the crust (high ξ), disabling slab detachment. Ongoing shortening leads to buckling of the crust and development of topography on the lower plate. Collisions involving rheologically layered crust allow decoupling at mid-crustal depths. This structure favours both the extrusion of upper crust and the subduction of the lower crust. Such collisions are successively affected by delamination and slab detachment. Topography develops together with the buoyant extrusion of crust onto the foreland and is further amplified by slab detachment. Our results suggest that the occurrence of both delamination (Apennines) and slab detachment (Himalayas) in orogens may indicate differences in the initial crustal structure of

Tracing Geophysical Indicators of Fluid-Induced Serpentinization in the Pampean Flat Slab Subduction Region of Chile

NASA Astrophysics Data System (ADS)

Bourke, J. R.; Nikulin, A.; Park, J. J.

2016-12-01

An activity gap in the Andean volcanic arc in the Pampean section of the subduction zone in Chile ( 28°-33°S) marks a section of flat-slab subduction. Past studies connected this change in geometry to the collision and subduction of the Juan Fernandez Ridge and the resulting migration of both the thrust front and magmatism eastward to the Sierras Pampeanas. The fate of fluids released from the subducting Nazca slab remains uncertain and the degree of their interaction with the basal layer of the continental lithosphere is poorly understood. We present initial results of a receiver-function investigation and forward-modeling effort at station GO03 operated by the Chilean National Seismic Network. Receiver function analysis of 75 well-recorded teleseismic earthquake events recorded at GO03 allow us to constrain the position of the subducting Nazca slab and to address the physical properties of the interplate contact zone. Critically, our analysis indicates presence of a highly-anisotropic zone of low velocities directly above the subucting Nazca slab. We point out a remarkable similarity in geophysical characteristics between the observed seismic anomaly at GO03 and a volume of proposed serpentinization in an area of sub-horizontal subduction above the Juan de Fuca slab in Cascadia. This interpretation is further supported by forward-modeling receiver functions at GO03 relying on a velocity model that incorporates a serpentinized interplate region. The newly-identified low-velocity highly-anisotropic layer may extend beyond the GO03 area and act as a mineral reservoir that captures and, possibly, transports fluids derived from the dehydrating Nazca Plate as it subducts below South America. It is likely that there is a relationship between this feature and the lack of volcanic activity in the Pampean flat slab region. Figure Caption: A) Backazimuth sweep of receiver functions recorded at station GO03 with predicted phase arrivals plotted for 55 km, 65 km, 75 km and

Constraints on continental crustal mass loss via chemical weathering using lithium and its isotopes

PubMed Central

Liu, Xiao-Ming; Rudnick, Roberta L.

2011-01-01

Chemical weathering, as well as physical erosion, changes the composition and shapes the surface of the continental crust. However, the amount of continental material that has been lost over Earth’s history due to chemical weathering is poorly constrained. Using a mass balance model for lithium inputs and outputs from the continental crust, we find that the mass of continental crust that has been lost due to chemical weathering is at least 15% of the original mass of the juvenile continental crust, and may be as high as 60%, with a best estimate of approximately 45%. Our results suggest that chemical weathering and subsequent subduction of soluble elements have major impacts on both the mass and the compositional evolution of the continental crust. PMID:22184221

Imaging the transition from Aleutian subduction to Yakutat collision in central Alaska, with local earthquakes and active source data

USGS Publications Warehouse

Eberhart-Phillips, D.; Christensen, D.H.; Brocher, T.M.; Hansen, R.; Ruppert, N.A.; Haeussler, Peter J.; Abers, G.A.

2006-01-01

In southern and central Alaska the subduction and active volcanism of the Aleutian subduction zone give way to a broad plate boundary zone with mountain building and strike-slip faulting, where the Yakutat terrane joins the subducting Pacific plate. The interplay of these tectonic elements can be best understood by considering the entire region in three dimensions. We image three-dimensional seismic velocity using abundant local earthquakes, supplemented by active source data. Crustal low-velocity correlates with basins. The Denali fault zone is a dominant feature with a change in crustal thickness across the fault. A relatively high-velocity subducted slab and a low-velocity mantle wedge are observed, and high Vp/Vs beneath the active volcanic systems, which indicates focusing of partial melt. North of Cook Inlet, the subducted Yakutat slab is characterized by a thick low-velocity, high-Vp/Vs, crust. High-velocity material above the Yakutat slab may represent a residual older slab, which inhibits vertical flow of Yakutat subduction fluids. Alternate lateral flow allows Yakutat subduction fluids to contribute to Cook Inlet volcanism and the Wrangell volcanic field. The apparent northeast edge of the subducted Yakutat slab is southwest of the Wrangell volcanics, which have adakitic composition consistent with melting of this Yakutat slab edge. In the mantle, the Yakutat slab is subducting with the Pacific plate, while at shallower depths the Yakutat slab overthrusts the shallow Pacific plate along the Transition fault. This region of crustal doubling within the shallow slab is associated with extremely strong plate coupling and the primary asperity of the Mw 9.2 great 1964 earthquake. Copyright 2006 by the American Geophysical Union.

Metamorphic records for subduction erosion and subsequent underplating processes revealed by garnet-staurolite-muscovite schists in central Qiangtang, Tibet

NASA Astrophysics Data System (ADS)

Zhang, Xiu-Zheng; Dong, Yong-Sheng; Wang, Qiang; Dan, Wei; Zhang, Chunfu; Xu, Wang; Huang, Ming-Liang

2017-01-01

Subduction erosion is confirmed as a crucial geodynamic process of crustal recycling based on geological, geochemical, and geophysical observations at modern convergent plate margins. So far, not a single metamorphic record has been used for constraining a general tectonic evolution for subduction erosion. Here we first revealed metamorphic records for a subduction erosion process based on our study of the Late Paleozoic garnet-staurolite-muscovite schists in the central Qiangtang block, Tibet. Provenance analyses suggest that the protoliths of garnet-staurolite-muscovite schists have the Northern Qiangtang-affinity and were deposited in an active continental margin setting. Mineral inclusion data show that the early metamorphic stage (M1) recorded blueschist facies pressure-temperature (P-T) conditions of 0.8-1.1 GPa and 402-441°C, indicating that a part of the material from the overriding plate had been abraded into the subduction channel and undergone high-pressure/low-temperature metamorphism. The peak metamorphic stage (M2) recorded amphibolite facies P-T conditions of 0.3-0.5 GPa and 470-520°C. The 40Ar/39Ar cooling ages (263-259 Ma) yielded from muscovite suggest the amphibolite facies metamorphism (>263 Ma) occurred at oceanic subduction stage. The distinctly staged metamorphism defines a clockwise and warming decompression P-T-t path which reveals an underplating process following the early subduction erosion. During the tectonic process, the eroded low-density material escaped from the cold subduction channel and rise upward into the warm middle-lower crust of the upper plate, undergoing amphibolite facies metamorphism. Our new results revealed a complete evolutional process from the early subduction erosion to the subsequent underplating during the northward subduction of the Paleo-Tethys Ocean.

Three-dimensional thermal structure and seismogenesis in the Tohoku and Hokkaido subduction system

NASA Astrophysics Data System (ADS)

van Keken, P. E.; Kita, S.; Nakajima, J.; Bengtson, A. K.; Hacker, B. R.; Abers, G. A.

2010-12-01

The Northern Japan arc is characterized by fast subduction of old oceanic lithosphere. The high density instrumentation and high seismicity make this an ideal natural laboratory to study the interplay between subduction zone dynamics, dehydration, migration of fluids, and seismogenesis. In this study we use high resolution finite element models to predict the thermal structure of the subduction slab below Tohoku (Northern Honshu) and Hokkaido. These models allow us to predict the pressure, temperature and mineralogy of the subducted crust and mantle. We use these models to predict the (p,T) conditions of earthquakes that are relocated with a precision of around 1 km by double difference techniques. Below Northern Hokkaido and Tohoku we find that the earthquake activity is strong in crust and the uppermost mantle for temperatures < 450 C. Above this temperature earthquakes occur more sporadically and have significantly reduced integrated seismic moment. The strongest 3D variations in this arc occur below southern Hokkaido. This 200 km wide region is characterized by a change in trench geometry, anomalously low heatflow and an anomalous velocity structure in the mantle wedge. Tomographic imaging suggest that continental crust is subducted to significant depth, thereby insulating the subducting slab from the hot mantle wedge at least at intermediate depths. The thermal insulation is also suggested by the deepening of the earthquakes in the slab (Kita et al., EPSL, 2010). This region may be characterized by active crustal erosion which would lead to a further blanketing of the crust by a sedimentary layer. Further modifications in thermal structure are possible due to the 3D wedge flow that is generated by the along-arc variations in trench geometry. We quantitatively verify the relative importance of these processes using 2D and 3D dynamical models. Without the seismically imaged crustal structure the earthquake temperatures are significantly elevated compared to the

Along-strike complex geometry of subduction zones - an experimental approach

NASA Astrophysics Data System (ADS)

Midtkandal, I.; Gabrielsen, R. H.; Brun, J.-P.; Huismans, R.

2012-04-01

Recent knowledge of the great geometric and dynamic complexity insubduction zones, combined with new capacity for analogue mechanical and numerical modeling has sparked a number of studies on subduction processes. Not unexpectedly, such models reveal a complex relation between physical conditions during subduction initiation, strength profile of the subducting plate, the thermo-dynamic conditions and the subduction zones geometries. One rare geometrical complexity of subduction that remains particularly controversial, is the potential for polarity shift in subduction systems. The present experiments were therefore performed to explore the influence of the architecture, strength and strain velocity on complexities in subduction zones, focusing on along-strike variation of the collision zone. Of particular concern were the consequences for the geometry and kinematics of the transition zones between segments of contrasting subduction direction. Although the model design to some extent was inspired by the configuration along the Iberian - Eurasian suture zone, the results are also of significance for other orogens with complex along-strike geometries. The experiments were set up to explore the initial state of subduction only, and were accordingly terminated before slab subduction occurred. The model wasbuilt from layers of silicone putty and sand, tailored to simulate the assumed lithospheric geometries and strength-viscosity profiles along the plate boundary zone prior to contraction, and comprises two 'continental' plates separated by a thinner 'oceanic' plate that represents the narrow seaway. The experiment floats on a substrate of sodiumpolytungstate, representing mantle. 24 experimental runs were performed, varying the thickness (and thus strength) of the upper mantle lithosphere, as well as the strain rate. Keeping all other parameters identical for each experiment, the models were shortened by a computer-controlled jackscrew while time-lapse images were

Geochemistry of serpentinites in subduction zones: A review

NASA Astrophysics Data System (ADS)

Deschamps, Fabien; Godard, Marguerite; Guillot, Stéphane; Hattori, Kéiko

2013-04-01

Over the last decades, numerous studies have emphasized the role of serpentinites in the subduction zones geodynamics. Their presence and effective role in this environment is acknowledged notably by geophysical, geochemical and field observations of (paleo-) subduction zones. In this context, with the increasing amount of studies concerning serpentinites in subduction environments, a huge geochemical database was created. Here, we present a review of the geochemistry of serpentinites, based on the compilation of ~ 900 geochemical analyses of abyssal, mantle wedge and subducted serpentinites. The aim was to better understand the geochemical evolution of these rocks during their subduction history as well as their impact in the global geochemical cycle. When studying serpentinites, it is often a challenge to determine the nature of the protolith and their geological history before serpentinisation. The present-day (increasing) geochemical database for serpentinites indicates little to no mobility of incompatible elements at the scale of the hand-sample in most serpentinized peridotites. Thus, Rare Earth Elements (REE) distribution can be used to identify the initial protolith for abyssal and mantle wedge serpentinites, as well as magmatic processes such as melt/rock interactions taking place before serpentinisation. In the case of subducted serpentinites, the interpretation of trace element data is more difficult due to secondary enrichments independent of the nature of the protolith, notably in (L)REE. We propose that these enrichments reflect complex interactions probably not related to serpentinisation itself, but mostly to fluid/rock or sediment/rock interactions within the subduction channel, as well as intrinsic feature of the mantle protolith which could derive from the continental lithosphere exhumed at the ocean-continent transition. Additionally, during the last ten years, numerous studies have been carried out, notably using in situ approaches, to better

Construction of Source Model of Huge Subduction Earthquakes for Strong Ground Motion Prediction

NASA Astrophysics Data System (ADS)

Iwata, T.; Asano, K.; Kubo, H.

2013-12-01

It is a quite important issue for strong ground motion prediction to construct the source model of huge subduction earthquakes. Iwata and Asano (2012, AGU) summarized the scaling relationships of large slip area of heterogeneous slip model and total SMGA sizes on seismic moment for subduction earthquakes and found the systematic change between the ratio of SMGA to the large slip area and the seismic moment. They concluded this tendency would be caused by the difference of period range of source modeling analysis. In this paper, we try to construct the methodology of construction of the source model for strong ground motion prediction for huge subduction earthquakes. Following to the concept of the characterized source model for inland crustal earthquakes (Irikura and Miyake, 2001; 2011) and intra-slab earthquakes (Iwata and Asano, 2011), we introduce the proto-type of the source model for huge subduction earthquakes and validate the source model by strong ground motion modeling.

Sandbox Simulations of the Evolution of a Subduction Wedge following Subduction Initiation

NASA Astrophysics Data System (ADS)

Brandon, M. T.; Ma, K. F.; DeWolf, W.

2012-12-01

Subduction wedges at accreting subduction zones are bounded by a landward dipping pro-shear zone (= subduction thrust) and a seaward-dipping retro-shear zone in the overriding plate. For the Cascadia subduction zone, the surface trace of the retro-shear zone corresponds to the east side of the Coast Ranges of Oregon and Washington and the Insular Mountains of Vancouver Island. This coastal high or forearc high shows clear evidence of long-term uplift and erosion along its entire length, indicating that it is an active part of the Cascadia subduction wedge. The question addressed here is what controls the location of the retro-shear zone? In the popular double-sided wedge model of Willet et al (Geology 1993), the retro-shear zone remains pinned to the S point, which is interpreted to represent where the upper-plate Moho intersects the subduction zone. For this interpretation, the relatively strong mantle is considered to operate as a flat backstop. That model, however. is somewhat artificial in that the two plates collide in a symmetric fashion with equal crustal thicknesses on both sides. Using sandbox experiments, we explore a more realistic configuration where the upper and lower plate are separated by a gentle dipping (10 degree) pro-shear zone, to simulate the initial asymmetric geometry of the subduction thrust immediately after initiation of subduction. The entire lithosphere must fail along some plane for subduction to begin and this failure plane must dip in the direction of subduction. Thus, the initial geometry of the overriding plate is better approximated as a tapered wedge than as a layer of uniform thickness, as represented in the Willett et al models. We demonstrate this model using time-lapse movies of a sand wedge above a mylar subducting plate. We use particle image velocimetry (PIV) to show the evolution of strain and structure within the overriding plate. Material accreted to the tapered end of the overriding plate drives deformation and causes

Effect of rheological approximations on slab detachment in 3D numerical simulations of continental collision

NASA Astrophysics Data System (ADS)

Pusok, Adina E.; Kaus, Boris; Popov, Anton

2017-04-01

It is commonly accepted that slab detachment results from the development of extensional stresses within the subducting slab. Subduction slowdown due to arrival of buoyant continental material at the trench is considered to cause such stress build up in the slab. Following slab detachment, slab pull partially or completely loses its strength and hot asthenosphere may flow through the slab window, which can have major consequences for continental collision. The dynamics of slab detachment has been extensively studied in 2D (i.e. analytical and numerical), but 3D models of slab detachment during continental collision remain largely unexplored. Some of the previous 3D models have investigated the role of an asymmetric margin on the propagation of slab detachment (van Hunen and Allen, 2011), the impact of slab detachment on the curvature of orogenic belts (Capitanio and Replumaz, 2013), the role of the collision rate on slab detachment depth (Li et al., 2013) or the effect of along-trench variations on slab detachment (Duretz et al., 2014). However, rheology of mantle and lithosphere is known to have a major influence on the dynamics of subduction. Here, we explore a range of different rheological approximations to understand their sensitivity on the possible scenarios. We employ the code LaMEM (Kaus et al., 2016) to perform 3D simulations of subduction/continental collision in an integrated lithospheric and upper-mantle scale model. The models exhibit a wide range of behaviours depending on the rheological law employed: from linear, to temperature-dependent visco-elasto-plastic rheology that takes into account both diffusion and dislocation creep. For example, we find that slab dynamics varies drastically between end member models: in viscous approximations, slab detachment is slow, dominated by viscous thinning, while for a non-linear visco-elasto-plastic rheology, slab detachment is relatively fast, dominated by plastic breaking and inducing strong mantle flow in

Geothermal He and CO2 variations at Changbaishan intra-plate volcano (NE China) and the nature of the sub-continental lithospheric mantle

NASA Astrophysics Data System (ADS)

Hahm, D.; Hilton, D. R.; Cho, M.; Wei, H.; Kim, K.-R.

2008-11-01

We report new He and CO2 data for geothermal fluids from Changbaishan Volcano located on the border of China and North Korea. Helium isotope ratios reach a maximum of 5.4 RA (where RA = 3He/4He in air) whereas carbon isotope values (δ 13C) fall between -2 and -7 ‰ (vs. PDB). The ratio of CO2/3He varies between 0.5-7.1 (×109) for gas samples but is considerably higher (~1012) in waters. The combined He-CO2 systematics reveal the clear imprint of a dominant subducted slab contribution to the total CO2 inventory. We conclude that these geothermal volatiles originate from the sub-continental lithospheric mantle (SCLM) in NE China and represent ancient fluids captured by prior metasomatic events.

Arc-continent collision and the formation of continental crust: A new geochemical and isotopic record from the Ordovician Tyrone Igneous Complex, Ireland

USGS Publications Warehouse

Draut, Amy E.; Clift, Peter D.; Amato, Jeffrey M.; Blusztajn, Jerzy; Schouten, Hans

2009-01-01

Collisions between oceanic island-arc terranes and passive continental margins are thought to have been important in the formation of continental crust throughout much of Earth's history. Magmatic evolution during this stage of the plate-tectonic cycle is evident in several areas of the Ordovician Grampian-Taconic orogen, as we demonstrate in the first detailed geochemical study of the Tyrone Igneous Complex, Ireland. New U-Pb zircon dating yields ages of 493 2 Ma from a primitive mafic intrusion, indicating intra-oceanic subduction in Tremadoc time, and 475 10 Ma from a light rare earth element (LREE)-enriched tonalite intrusion that incorporated Laurentian continental material by early Arenig time (Early Ordovician, Stage 2) during arc-continent collision. Notably, LREE enrichment in volcanism and silicic intrusions of the Tyrone Igneous Complex exceeds that of average Dalradian (Laurentian) continental material that would have been thrust under the colliding forearc and potentially recycled into arc magmatism. This implies that crystal fractionation, in addition to magmatic mixing and assimilation, was important to the formation of new crust in the Grampian-Taconic orogeny. Because similar super-enrichment of orogenic melts occurred elsewhere in the Caledonides in the British Isles and Newfoundland, the addition of new, highly enriched melt to this accreted arc terrane was apparently widespread spatially and temporally. Such super-enrichment of magmatism, especially if accompanied by loss of corresponding lower crustal residues, supports the theory that arc-continent collision plays an important role in altering bulk crustal composition toward typical values for ancient continental crust. ?? 2009 Geological Society of London.

Subduction Drive of Plate Tectonics

NASA Astrophysics Data System (ADS)

Hamilton, W. B.

2003-12-01

shrinking oceans, forcing rapid Pacific spreading. Slabs suck forward overriding arcs and continental lithosphere, plus most subjacent mantle above the transition zone. Changes in sizes of oceans result primarily from transfer of oceanic lithosphere, so backarcs and expanding oceans spread only slowly. Lithosphere parked in, or displaced from, the transition zone, or mixed into mid-upper mantle, is ultimately recycled, and regional variations in age of that submerged lithosphere may account for some regional contrasts in MORB. Plate motions make no kinematic sense in either the "hotspot" reference frame (HS; the notion of fixed plumes is easily disproved) or the no-net-rotation frame (NNR) In both, for example, many hinges roll forward, impossible with gravity drive. Subduction-drive predictions are fulfilled, and paleomagnetic data are satisfied (as they are not in HS and NNR), in the alternative framework of propulsionless Antarctica fixed relative to sluggish lower mantle. Passive ridges migrate away from Antarctica on all sides, and migration of these and other ridges permits tapping fresh asthenosphere. (HS and NNR tend to fix ridges). Ridge migration and spreading rates accord with subduction drive. All trenches roll back when allowance is made for back-arc spreading and intracontinental deformation. Africa rotates slowly toward subduction systems in the NE, instead of moving rapidly E as in HS and NNR. Stable NW Eurasia is nearly stationary, instead of also moving rapidly, and S and E Eurasian deformation relates to subduction and rollback. The Americas move Pacificward at almost the full spreading rates of passive ridges behind them. Lithosphere has a slow net westward drift. Reference: W.B. Hamilton, An alternative Earth, GSA Today, in press.

Seismological constraints on the crustal structures generated by continental rejuvenation in northeastern China

PubMed Central

Zheng, Tian-Yu; He, Yu-Mei; Yang, Jin-Hui; Zhao, Liang

2015-01-01

Crustal rejuvenation is a key process that has shaped the characteristics of current continental structures and components in tectonic active continental regions. Geological and geochemical observations have provided insights into crustal rejuvenation, although the crustal structural fabrics have not been well constrained. Here, we present a seismic image across the North China Craton (NCC) and Central Asian Orogenic Belt (CAOB) using a velocity structure imaging technique for receiver functions from a dense array. The crustal evolution of the eastern NCC was delineated during the Mesozoic by a dominant low seismic wave velocity with velocity inversion, a relatively shallow Moho discontinuity, and a Moho offset beneath the Tanlu Fault Zone. The imaged structures and geochemical evidence, including changes in the components and ages of continental crusts and significant continental crustal growth during the Mesozoic, provide insight into the rejuvenation processes of the evolving crust in the eastern NCC caused by structural, magmatic and metamorphic processes in an extensional setting. The fossil structural fabric of the convergent boundary in the eastern CAOB indicates that the back-arc action of the Paleo-Pacific Plate subduction did not reach the hinterland of Asia. PMID:26443323

3D Numerical modelling of topography development associated with curved subduction zones

NASA Astrophysics Data System (ADS)

Munch, Jessica; Ueda, Kosuke; Burg, Jean-Pierre; May, Dave; Gerya, Taras

2017-04-01

Curved subduction zones, also called oroclines, are geological features found in various places on Earth. They occur in diverse geodynamic settings: 1) single slab subduction in oceanic domain (e.g. Sandwich trench in the Southern Atlantic); 2) single slab subduction in continental domain, (e.g. Gibraltar-Alboran orocline in the Western Mediterranean) 3); multi-slab subduction (e.g. Caribbean orocline in the South-East of the Gulf of Mexico). These systems present various curvatures, lengths (few hundreds to thousands of km) and ages (less than 35 Ma for Gibraltar Alboran orocline, up to 100 Ma for the Caribbean). Recent studies suggested that the formation of curved subduction systems depends on slab properties (age, length, etc) and may be linked with processes such as retreating subduction and delamination. Plume induced subduction initiation has been proposed for the Caribbean. All of these processes involve deep mechanisms such as mantle and slab dynamics. However, subduction zones always generate topography (trenches, uplifts, etc), which is likely to be influenced by surface processes. Hence, surface processes may also influence the evolution of subduction zones. We focus on different kinds of subduction systems initiated by plume-lithosphere interactions (single slab subduction/multi-slab subduction) and scrutinize their surface expression. We use numerical modeling to examine large-scale subduction initiation and three-dimensional slab retreat. We perform two kinds of simulations: 1) large scale subduction initiation with the 3D-thermomechanical code I3ELVIS (Gerya and Yuen, 2007) in an oceanic domain and 2) large scale subduction initiation in oceanic domain using I3ELVIS coupled with a robust new surface processes model (SPM). One to several retreating slabs form in the absence of surface processes, when the conditions for subduction initiation are reached (c.f. Gerya et al., 2015), and ridges occur in the middle of the extensional domain opened by slab

Zircon evidence for incorporation of terrigenous sediments into the magma source of continental basalts.

PubMed

Xu, Zheng; Zheng, Yong-Fei; Zhao, Zi-Fu

2018-01-09

Crustal components may be incorporated into continental basalts by either shallow contamination or deep mixing. While the former proceeds at crustal depths with common preservation of refractory minerals, the latter occurs at mantle depths with rare survival of relict minerals. Discrimination between the two mechanisms has great bearing to subcontinental mantle geochemistry. Here we report the occurrence of relict zircons in Cenozoic continental basalts from eastern China. A combined study of zircon U-Pb ages and geochemistry indicates that detrital zircons were carried by terrigenous sediments into a subcontinental subduction zone, where the zircon were transferred by fluids into the magma sources of continental basalts. The basalts were sampled from three petrotectonic units with distinct differences in their magmatic and metamorphic ages, making the crustal contamination discernible. The terrigenous sediments were carried by the subducting oceanic crust into the asthenospheric mantle, producing both soluble and insoluble materials at the slab-mantle interface. These materials were served as metasomatic agents to react with the overlying mantle wedge peridotite, generating a kind of ultramafic metasomatites that contain the relict zircons. Therefore, the occurrence of relict zircons in continental basalts indicates that this refractory mineral can survive extreme temperature-pressure conditions in the asthenospheric mantle.

Plate tectonic constraints on the cessation of subduction beneath the Baja California peninsula, Mexico

NASA Astrophysics Data System (ADS)

Stock, J. M.

2007-05-01

I review published models, existing global plate tectonic data and published marine geophysical observations west of Baja California to assess the timing and conditions under which subduction ceased along the W margin of Baja California. The relative motion of the Farallon microplate fragments can be reconstructed using Pacific- North America global plate motions (from the Pacific-Antarctica-Nubia-North America plate circuit) added to the local velocities of the microplates with respect to the Pacific plate. Because the Pacific plate was moving obliquely away from North America, the time at which subduction stopped has often been taken to be the time at which the microplates joined the Pacific plate (the ages of dead spreading centers preserved west of North America on the Pacific plate). The timing of cessation of subduction west of what is now northern Baja California is not recorded by a dead ridge offshore but is inferred to be coincident with extension and rotation in the continental borderland (early-middle Miocene). The Arguello microplate stopped spreading relative to the Pacific plate at about 13 Ma, providing a younger age limit on the cessation of subduction in the sector N of the Shirley transform fault. The time of cessation of spreading of the Magdalena-Pacific (M-P) ridge has been proposed by Michaud et al. (2006 Geology) to be as young as 8 Ma. However, the clockwise rotation of the M-P ridge before it ceased, and its inferred slow spreading rate away from the Pacific plate implies transcurrent motion with virtually no convergence between the Magdalena microplate and the North America plate during the last stages of activity of the M-P ridge. Subduction can occur by motion of forearc fragments without any convergence of the major bounding plates (e.g., the modern South Shetland Trench), but this may be ruled out for Baja California due to the small spatial scale of the microplates compared to the scale of the stable Baja California peninsula block

Subduction of hydrated basalt of the oceanic crust: Implications for recycling of water into the upper mantle and continental growth

NASA Technical Reports Server (NTRS)

Rapp, R. P.

1994-01-01

Subduction zones are presently the dominant sites on Earth for recycling and mass transfer between the crust and mantle; they feed hydrated basaltic oceanic crust into the upper mantle, where dehydration reactions release aqueous fluids and/or hydrous melts. The loci for fluid and/or melt generation will be determined by the intersection of dehydration reaction boundaries of primary hydrous minerals within the subducted lithosphere with slab geotherms. For metabasalt of the oceanic crust, amphibole is the dominant hydrous mineral. The dehydration melting solidus, vapor-absent melting phase relationships; and amphibole-out phase boundary for a number of natural metabasalts have been determined experimentally, and the pressure-temperature conditions of each of these appear to be dependent on bulk composition. Whether or not the dehydration of amphibole is a fluid-generating or partial melting reaction depends on a number of factors specific to a given subduction zone, such as age and thickness of the subducting oceanic lithosphere, the rate of convergence, and the maturity of the subduction zone. In general, subduction of young, hot oceanic lithosphere will result in partial melting of metabasalt of the oceanic crust within the garnet stability field; these melts are characteristically high-Al2O3 trondhjemites, tonalites and dacites. The presence of residual garnet during partial melting imparts a distinctive trace element signature (e.g., high La/Yb, high Sr/Y and Cr/Y combined with low Cr and Y contents relative to demonstrably mantle-derived arc magmas). Water in eclogitized, subducted basalt of the oceanic crust is therefore strongly partitioned into melts generated below about 3.5 GPa in 'hot' subduction zones. Although phase equilibria experiments relevant to 'cold' subduction of hydrated natural basalts are underway in a number of high-pressure laboratories, little is known with respect to the stability of more exotic hydrous minerals (e.g., ellenbergite) and

Subducting plate geology in three great earthquake ruptures of the western Alaska margin, Kodiak to Unimak

USGS Publications Warehouse

von Huene, Roland E.; Miller, John J.; Weinrebe, Wilhelm

2012-01-01

Three destructive earthquakes along the Alaska subduction zone sourced transoceanic tsunamis during the past 70 years. Since it is reasoned that past rupture areas might again source tsunamis in the future, we studied potential asperities and barriers in the subduction zone by examining Quaternary Gulf of Alaska plate history, geophysical data, and morphology. We relate the aftershock areas to subducting lower plate relief and dissimilar materials in the seismogenic zone in the 1964 Kodiak and adjacent 1938 Semidi Islands earthquake segments. In the 1946 Unimak earthquake segment, the exposed lower plate seafloor lacks major relief that might organize great earthquake rupture. However, the upper plate contains a deep transverse-trending basin and basement ridges associated with the Eocene continental Alaska convergent margin transition to the Aleutian island arc. These upper plate features are sufficiently large to have affected rupture propagation. In addition, massive slope failure in the Unimak area may explain the local 42-m-high 1946 tsunami runup. Although Quaternary geologic and tectonic processes included accretion to form a frontal prism, the study of seismic images, samples, and continental slope physiography shows a previous history of tectonic erosion. Implied asperities and barriers in the seismogenic zone could organize future great earthquake rupture.

Seismic structure and activity of the north-central Lesser Antilles subduction zone from an integrated approach: Similarities with the Tohoku forearc

NASA Astrophysics Data System (ADS)

Laigle, M.; Hirn, A.; Sapin, M.; Bécel, A.; Charvis, P.; Flueh, E.; Diaz, J.; Lebrun, J.-F.; Gesret, A.; Raffaele, R.; Galvé, A.; Evain, M.; Ruiz, M.; Kopp, H.; Bayrakci, G.; Weinzierl, W.; Hello, Y.; Lépine, J.-C.; Viodé, J.-P.; Sachpazi, M.; Gallart, J.; Kissling, E.; Nicolich, R.

2013-09-01

The 300-km-long north-central segment of the Lesser Antilles subduction zone, including Martinique and Guadeloupe islands has been the target of a specific approach to the seismic structure and activity by a cluster of active and passive offshore-onshore seismic experiments. The top of the subducting plate can be followed under the wide accretionary wedge by multichannel reflection seismics. This reveals the hidden updip limit of the contact of the upper plate crustal backstop onto the slab. Two OBS refraction seismic profiles from the volcanic arc throughout the forearc domain constrain a 26-km-large crustal thickness all along. In the common assumption that the upper plate Moho contact on the slab is a proxy of its downdip limit these new observations imply a three times larger width of the potential interplate seismogenic zone under the marine domain of the Caribbean plate with respect to a regular intra-oceanic subduction zone. Towards larger depth under the mantle corner, the top of the slab imaged from the conversions of teleseismic body-waves and the locations of earthquakes appears with kinks which increase the dip to 10-20° under the forearc domain, and then to 60° from 70 km depth. At 145 km depth under the volcanic arc just north of Martinique, the 2007 M 7.4 earthquake, largest for half a century in the region, allows to document a deep slab deformation consistent with segmentation into slab panels. In relation with this occurrence, an increased seismic activity over the whole depth range provides a new focussed image thanks to the OBS and land deployments. A double-planed dipping slab seismicity is thus now resolved, as originally discovered in Tohoku (NE Japan) and since in other subduction zones. Two other types of seismic activity uniquely observed in Tohoku, are now resolved here: "supraslab" earthquakes with normal-faulting focal mechanisms reliably located in the mantle corner and "deep flat-thrust" earthquakes at 45 km depth on the interplate

Structure and Deformation of the Hikurangi-Kermadec Subduction Zone - Transitions Revealed by Seismic Wide-angle Data

NASA Astrophysics Data System (ADS)

Scherwath, M.; Kopp, H.; Flueh, E. R.; Henrys, S. A.; Sutherland, R.

2008-12-01

The Hikurangi-Kermadec subduction zone northeast of New Zealand represents an ideal target to study lateral variations of subduction zone processes. The incoming Pacific plate changes from being a large igneous province, called the Hikurangi Plateau, in the south to normal oceanic plate north of the Rapuhia Scarp. The overriding Australian plate of continental character in the south, forming the North Island of New Zealand, and changes to an island arc in the north. Further lateral variability exists in changes in volcanic and hydro-thermal activity, transitions from accretion to subduction erosion, backarc spreading and rifting, and is accompanied by northward increasing seismicity. As part of the MANGO project (Marine Geoscientific Investigations on the Input and Output of the Kermadec Subduction Zone), four marine geophysical transects of largely seismic reflection and refraction data provide constraints on the upper lithospheric structures across the Hikurangi-Kermadec Trench between 29-38 deg S. On MANGO profile 1 in the south, the initially shallow subduction of the incoming plateau coincides with crustal underplating beneath the East Cape ridge. To the west lies the 100 km wide and over 10 km deep Raukumara Basin. Seismic velocities of the upper mantle of both plates are around 8 km/s and are considered normal. In contrast, on MANGO profile 4, about 1000 km to the north around the volcanically active Raoul Island, the incoming oceanic crust appears to bend considerably steeper and thus causes a 50 km narrower forearc with a smaller forearc basin. Furthermore, the upper mantle velocities in both plates are relatively low (7.4-7.7 km/s), likely indicating strong bending related deformation of the incoming plate and thermal activity within the arc possibly due to spreading. The central two transects MANGO 2 and 3, though without data coverage of the structure of the incoming plate, are more similar to MANGO 4. The arc regions appear to be strongly affected by

Common Pb isotope mapping of UHP metamorphic zones in Dabie orogen, Central China: Implication for Pb isotopic structure of subducted continental crust

NASA Astrophysics Data System (ADS)

Shen, Ji; Wang, Ying; Li, Shu-Guang

2014-10-01

We report Pb isotopic compositions for feldspars separated from 57 orthogneisses and 2 paragneisses from three exhumed UHPM slices representing the North Dabie zone, the Central Dabie zone and the South Dabie zone of the Dabie orogen, central-east China. The feldspars from the gneisses were recrystallized during Triassic continental subduction and UHP metamorphism. Precursors of the orthogneisses are products of Neoproterozoic bimodal magmatic events, those in north Dabie zone emplaced into the lower crust and those in central and south Dabie zones into middle or upper crust, respectively. On a 207Pb/204Pb vs. 206Pb/204Pb diagram, almost all orthogneisses data lie to the left of the 0.23 Ga paleogeochron and plot along the model mantle evolution curve with the major portion of the data plotting below it. On a 208Pb/204Pb vs. 206Pb/204Pb diagram the most of data of north Dabie zone extend in elongate arrays along the lower crustal curve and others extend between the lower crustal curve to near the mantle evolution curve for the plumbotectonics model. This pattern demonstrates that the Pb isotopic evolution of the feldspars essentially ended at 0.23 Ga and the orthogneiss protoliths were principally dominated by reworking of ancient lower crust with some addition of juvenile mantle in the Neoproterozoic rifting tectonic zone. According to geological evolution history of the locally Dabie orogen, a four-stage Pb isotope evolution model including a long time evolution between 2.0 and 0.8 Ga with a lower crust type U/Pb ratio (μ = 5-6) suggests that magmatic emplacement levels of the protoliths of the orthogneisses in the Dabie orogen at 0.8 Ga also play an important role in the Pb evolution of the exhumed UHPM slices, corresponding to their respective Pb characters at ca. 0.8-0.23 Ga. For example, north Dabie zone requires low μ values (3.4-9.6), while central and south Dabie zones require high μ values (10.9-17.2). On the other hand, Pb isotopic mixing between

Processes of lithosphere evolution: New evidence on the structure of the continental crust and uppermost mantle

USGS Publications Warehouse

Artemieva, I.M.; Mooney, W.D.; Perchuc, E.; Thybo, H.

2002-01-01

We discuss the structure of the continental lithosphere, its physical properties, and the mechanisms that formed and modified it since the early Archean. The structure of the upper mantle and the crust is derived primarily from global and regional seismic tomography studies of Eurasia and from global and regional data on seismic anisotropy. These data as documented in the papers of this special issue of Tectonophysics are used to illustrate the role of different tectonic processes in the lithospheric evolution since Archean to present. These include, but are not limited to, cratonization, terrane accretion and collision, continental rifting (both passive and active), subduction, and lithospheric basal erosion due to a relative motion of cratonic keels and the convective mantle. ?? 2002 Elsevier Science B.V. All rights reserved.

Seismic imaging of lithospheric discontinuities and continental evolution

NASA Astrophysics Data System (ADS)

Bostock, M. G.

1999-09-01

Discontinuities in physical properties within the continental lithosphere reflect a range of processes that have contributed to craton stabilization and evolution. A survey of recent seismological studies concerning lithospheric discontinuities is made in an attempt to document their essential characteristics. Results from long-period seismology are inconsistent with the presence of continuous, laterally invariant, isotropic boundaries within the upper mantle at the global scale. At regional scales, two well-defined interfaces termed H (˜60 km depth) and L (˜200 km depth) of continental affinity are identified, with the latter boundary generally exhibiting an anisotropic character. Long-range refraction profiles are frequently characterized by subcontinental mantle that exhibits a complex stratification within the top 200 km. The shallow layering of this package can behave as an imperfect waveguide giving rise to the so-called teleseismic Pn phase, while the L-discontinuity may define its lower base as the culmination of a low velocity zone. High-resolution, seismic reflection profiling provides sufficient detail in a number of cases to document the merging of mantle interfaces into lower continental crust below former collisional sutures and magmatic arcs, thus unambiguously identifying some lithospheric discontinuities with thrust faults and subducted oceanic lithosphere. Collectively, these and other seismic observations point to a continental lithosphere whose internal structure is dominated by a laterally variable, subhorizontal layering. This stratigraphy appears to be more pronounced at shallower lithospheric levels, includes dense, anisotropic layers of order 10 km in thickness, and exhibits horizontal correlation lengths comparable to the lateral dimensions of overlying crustal blocks. A model of craton evolution which relies on shallow subduction as a principal agent of craton stabilization is shown to be broadly compatible with these characteristics.

A revised subduction inception model to explain the Late Cretaceous, doubly vergent orogen in the pre-collisional western Tethys: evidences from the Northern Apennine

NASA Astrophysics Data System (ADS)

Meneghini, Francesca; Marroni, Michele; Pandolfi, Luca

2017-04-01

Orogenic processes are widely demonstrated to be strongly controlled by inherited structures. The paleogeography of the converging margins, and the tectonic processes responsible for their configuration, will influence the location of subduction initiation, the distribution of deformation between upper and lower plate, the shape of the accretionary prism and of the subsequent orogeny, through controlling the development of single or doubly-vergent orogens, and, as a corollary, the modality of exhumation of metamorphosed units. The "alpine age" collisional belts of the Mediterranean area are characterized by tangled architectures derived from the overlapping of several deformation events related to a multiphase, long history that comprises not only the collision of continental margins, but that can be regarded as an heritage of both the rifting-related configuration of the continental margins, and the subduction-related structures. The Northern Apennines is a segment of these collisional belts that originated by the Late Cretaceous-Middle Eocene closure of the northern branch of the western Tethys, and the subsequent Late Eocene-Early Oligocene continental collision between the Europe and Adria plates. Due to a different configuration of the paired Adria and Europe continental margins, inherited from a rifting phase dominated by asymmetric, simple-shear kinematics, the Northern Apennines expose a complex groups of units, referred to as Ligurian Units, that record the incorporation into the subduction factory of either fragments of the Ligure-Piemontese oceanic domain (i.e. Internal Ligurian Units), and various portions of the thinned Adria margin (i.e. External Ligurian Units), describable as an Ocean-Continent Transition Zone (OCTZ). The structural relationships between these groups of Units are crucial for the definition of the pre-collisional evolution of the belt and have been the subject of big debates in the literature, together with the location and

A Triassic to Cretaceous Sundaland-Pacific subduction margin in West Sarawak, Borneo

NASA Astrophysics Data System (ADS)

Breitfeld, H. Tim; Hall, Robert; Galin, Thomson; Forster, Margaret A.; BouDagher-Fadel, Marcelle K.

2017-01-01

Metamorphic rocks in West Sarawak are poorly exposed and studied. They were previously assumed to be pre-Carboniferous basement but had never been dated. New 40Ar/39Ar ages from white mica in quartz-mica schists reveal metamorphism between c. 216 to 220 Ma. The metamorphic rocks are associated with Triassic acid and basic igneous rocks, which indicate widespread magmatism. New U-Pb dating of zircons from the Jagoi Granodiorite indicates Triassic magmatism at c. 208 Ma and c. 240 Ma. U-Pb dating of zircons from volcaniclastic sediments of the Sadong and Kuching Formations confirms contemporaneous volcanism. The magmatic activity is interpreted to represent a Triassic subduction margin in westernmost West Sarawak with sediments deposited in a forearc basin derived from the magmatic arc at the Sundaland-Pacific margin. West Sarawak and NW Kalimantan are underlain by continental crust that was already part of Sundaland or accreted to Sundaland in the Triassic. One metabasite sample, also previously assumed to be pre-Carboniferous basement, yielded Early Cretaceous 40Ar/39Ar ages. They are interpreted to indicate resumption of subduction which led to deposition of volcaniclastic sediments and widespread magmatism. U-Pb ages from detrital zircons in the Cretaceous Pedawan Formation are similar to those from the Schwaner granites of NW Kalimantan, and the Pedawan Formation is interpreted as part of a Cretaceous forearc basin containing material eroded from a magmatic arc that extended from Vietnam to west Borneo. The youngest U-Pb ages from zircons in a tuff layer from the uppermost part of the Pedawan Formation indicate that volcanic activity continued until c. 86 to 88 Ma when subduction terminated.

Continental growth by successive accretion of oceanic lithosphere: Evidence from tilted seismic anisotropy

NASA Astrophysics Data System (ADS)

Babuska, V.; Plomerova, J.; Karato, S. I.

2012-04-01

Although many studies indicate that subduction-related accretion, subduction-driven magmatism and tectonic stacking are major crustal-growth mechanisms, how the mantle lithosphere forms remains enigmatic. Cook (AGU Geod. Series 1986) published a model of continental 'shingling' based on seismic reflection data indicating dipping structures in the deep crust of accreted terranes. Helmstaedt and Gurney (J. Geoch. Explor. 1995) and Hart et al. (Geology 1997) suggest that the Archean continental lithosphere consists of alternating layers of basalt and peridotite derived from subducted and obducted Archean oceanic lithosphere. Peridotite xenoliths from the Mojavian mantle lithosphere (Luffi et al., JGR 2009), as well as xenoliths of eclogites underlying the Sierra Nevada batholith in California (Horodynskij et al., EPSL 2007), are representative for oceanic slab fragments successively attached to the continent. Recent seismological findings also seem to support a model of continental lithosphere built from systems of paleosubductions of plates of ancient oceanic lithosphere (Babuska and Plomerova, AGU Geoph. Monograph 1989), or by stacking of the plates (Helmstaedt and Schulze, Geol. Soc. Aust. Spec. Publ. 1989). Seismic anisotropy in the oceanic mantle lithosphere, explained mainly by the olivine A- (or D-) type fabric (Karato et al., Annu. Rev. Earth Planet. Sci. 2008), was discovered almost a half century ago (Hess, Nature 1964). Though it is difficult to determine seismic anisotropy within an active subducting slab (e.g., Healy et al., EPSL 2009; Eberhart-Phillips and Reyners, JGR 2009), field observations and laboratory experiments indicate the oceanic olivine fabric might be preserved there to a depth of at least 200-300 km. Dipping anisotropic fabrics in domains of the European mantle lithosphere were interpreted as systems of 'frozen' paleosubductions (Babuska and Plomerova, PEPI 2006), and the lithosphere base as a boundary between a fossil anisotropy in the

Crustal volumes of the continents and of oceanic and continental submarine plateaus

NASA Technical Reports Server (NTRS)

Schubert, G.; Sandwell, D.

1989-01-01

Using global topographic data and the assumption of Airy isostasy, it is estimated that the crustal volume of the continents is 7182 X 10 to the 6th cu km. The crustal volumes of the oceanic and continental submarine plateaus are calculated at 369 X 10 to the 6th cu km and 242 X 10 to the 6th cu km, respectively. The total continental crustal volume is found to be 7581 X 10 to the 6th cu km, 3.2 percent of which is comprised of continental submarine plateaus on the seafloor. An upper bound on the contintental crust addition rate by the accretion of oceanic plateaus is set at 3.7 cu km/yr. Subduction of continental submarine plateaus with the oceanic lithosphere on a 100 Myr time scale yields an upper bound to the continental crustal subtraction rate of 2.4 cu km/yr.

USArray Imaging of North American Continental Crust

NASA Astrophysics Data System (ADS)

Ma, Xiaofei

The layered structure and bulk composition of continental crust contains important clues about its history of mountain-building, about its magmatic evolution, and about dynamical processes that continue to happen now. Geophysical and geological features such as gravity anomalies, surface topography, lithospheric strength and the deformation that drives the earthquake cycle are all directly related to deep crustal chemistry and the movement of materials through the crust that alter that chemistry. The North American continental crust records billions of years of history of tectonic and dynamical changes. The western U.S. is currently experiencing a diverse array of dynamical processes including modification by the Yellowstone hotspot, shortening and extension related to Pacific coast subduction and transform boundary shear, and plate interior seismicity driven by flow of the lower crust and upper mantle. The midcontinent and eastern U.S. is mostly stable but records a history of ancient continental collision and rifting. EarthScope's USArray seismic deployment has collected massive amounts of data across the entire United States that illuminates the deep continental crust, lithosphere and deeper mantle. This study uses EarthScope data to investigate the thickness and composition of the continental crust, including properties of its upper and lower layers. One-layer and two-layer models of crustal properties exhibit interesting relationships to the history of North American continental formation and recent tectonic activities that promise to significantly improve our understanding of the deep processes that shape the Earth's surface. Model results show that seismic velocity ratios are unusually low in the lower crust under the western U.S. Cordillera. Further modeling of how chemistry affects the seismic velocity ratio at temperatures and pressures found in the lower crust suggests that low seismic velocity ratios occur when water is mixed into the mineral matrix

From continental to oceanic rifting in the Gulf of California

NASA Astrophysics Data System (ADS)

Ferrari, Luca; Bonini, Marco; Martín, Arturo

2017-11-01

The continental margin of northwestern Mexico is the youngest example of the transition from a convergent plate boundary to an oblique divergent margin that formed the Gulf of California rift. Subduction of the Farallon oceanic plate during the Cenozoic progressively brought the East Pacific Rise (EPR) toward the North America trench. In this process increasingly younger and buoyant oceanic lithosphere entered the subduction zone until subduction ended just before most of the EPR could collide with the North America continental lithosphere. The EPR segments bounding the unsubducted parts of the Farallón plate remnants (Guadalupe and Magdalena microplates) also ceased spreading (Lonsdale, 1991) and a belt of the North American plate (California and Baja California Peninsula) became coupled with the Pacific Plate and started moving northwestward forming the modern Gulf of California oblique rift (Nicholson et al., 1994; Bohannon and Parsons, 1995). The timing of the change from plate convergence to oblique divergence off western Mexico has been constrained at the middle Miocene (15-12.5 Ma) by ocean floor morphology and magnetic anomalies as well as plate tectonic reconstructions (Atwater and Severinghaus, 1989; Stock and Hodges, 1989; Lonsdale, 1991), although the onset of transtensional deformation and the amount of right lateral displacement within the Gulf region are still being studied (Oskin et al., 2001; Fletcher et al., 2007; Bennett and Oskin, 2014). Other aspects of the formation of the Gulf of California remain not well understood. At present the Gulf of California straddles the transition from continental transtension in the north to oceanic spreading in the south. Seismic reflection-refraction data indicate asymmetric continent-ocean transition across conjugate margins of rift segments (González-Fernández et al., 2005; Lizarralde et al., 2007; Miller and Lizarralde, 2013; Martín-Barajas et al., 2013). The asymmetry may be related to crustal

From an active continental plate margin to continental collision: New constraints from the petrological, structural and geochronological record of the (ultra) high-P metamorphic Rhodope domain (N-Greece)

NASA Astrophysics Data System (ADS)

Mposkos, E.; Krohe, A.; Wawrzenitz, N.; Romer, R. L.

2012-04-01

The Rhodope domain occupies a key area along the suture between the European and the Apulian/Adriatic plate (Schmid et al., 2008), which collided in the early Tertiary (closure of the Vardar/Axios ocean, cf. Mposkos & Krohe, 2006). An integrated study of the geochronological, tectonic and petrological data of the Rhodope domain provides the unique opportunity resolving a 160 my lasting metamorphic evolution (Jurassic to Miocene) of an active plate margin to a high degree. The Greek Rhodope consists of several composite metamorphic complexes bounded by the Nestos thrust and several normal detachment systems. The PT- and structural records of the complexes constrain metamorphic, magmatic and tectonic processes, associated with subduction along a convergent plate margin including UHP metamorphism, MP to HP metamorphism associated with continental collision, and core complex formation linked to Aegean back arc extension. We focus on the Sidironero Complex that shows a polymetamorphic history. This is documented by SHRIMP and LA-ICP-MS U-Pb zircon ages of ca. 150 Ma from garnet-kyanite gneisses that are interpreted to record the HP/UHP metamorphism (Liati, 2005; Krenn et al., 2010). SHRIMP zircon ages of ca. 51 Ma from an amphibolitized eclogite is interpreted by Liati (2005) to record a second Eocene HP metamorphic event. We present new data from an integrated petrological, geochronological and tectonic study. Granulite facies and upper amphibolite facies metamorphic conditions are recorded by the mineral assemblage Grt-Ky-Bt-Pl-Kfs-Qtz-Rt and Grt-Ky-Bt-Ms-Pl-Qtz-Rt, respectively, in deformed migmatitic metapelites. Deformation occurred under granulite facies conditions. Monazites from the matrix, that formed during the granulite facies deformation, lack core/rim structures and are only locally patchy zoned. Monazite chemical compositions are related to varying reaction partners. Single grains and fractions of few grains yield ID-TIMS U-Pb ages that plot along the

Buoyant subduction on Venus: Implications for subduction around coronae

NASA Astrophysics Data System (ADS)

Burt, J. D.; Head, J. W.

1993-03-01

Potentially low lithospheric densities, caused by high Venus surface and perhaps mantle temperatures, could inhibit the development of negative buoyancy-driven subduction and a global system of plate tectonics/crustal recycling on that planet. No evidence for a global plate tectonic system was found so far, however, specific features strongly resembling terrestrial subduction zones in planform and topographic cross-section were described, including trenches around large coronae and chasmata in eastern Aphrodite Terra. The cause for the absence, or an altered expression, of plate tectonics on Venus remains to be found. Slab buoyancy may play a role in this difference, with higher lithospheric temperatures and a tendency toward positive buoyancy acting to oppose the descent of slabs and favoring under thrusting instead. The effect of slab buoyancy on subduction was explored and the conditions which would lead to under thrusting versus those allowing the formation of trenches and self-perpetuating subduction were defined. Applying a finite element code to assess the effects of buoyant forces on slabs subducting into a viscous mantle, it was found that mantle flow induced by horizontal motion of the convergent lithosphere greatly influences subduction angle, while buoyancy forces produce a lesser effect. Induced mantle flow tends to decrease subduction angle to near an under thrusting position when the subducting lithosphere converges on a stationary overriding lithosphere. When the overriding lithosphere is in motion, as in the case of an expanding corona, subduction angles are expected to increase. An initial stage involved estimating the changes in slab buoyancy due to slab healing and pressurization over the course of subduction. Modeling a slab, descending at a fixed angle and heated by conduction, radioactivity, and the heat released in phase changes, slab material density changes due to changing temperature, phase, and pressure were derived.

Structural deformation and detailed architecture of accretionary wedge in the northern Manila subduction zone

NASA Astrophysics Data System (ADS)

Gao, J.; Wu, S.; Yao, Y.; Chen, C.

2017-12-01

The South China Sea (SCS) which located at the southeast edge of the Eurasian plate, is heavily influenced by the Philippine Sea plate and the Indo-Australian plate. As eastern boundary of the SCS, Manila subduction zone was created by the northwestern movement of the Philippine Sea plate, recorded the key information on formation and evolution of the SCS and often triggered off earthquakes and tsunami in the East and South Asia. Using high resolution multi-channel seismic data across the northern Manila subduction zone, this study analyzed sedimentary characteristics of oceanic basin and trench, and fine described features of structural deformation and architecture of accretionary wedge and magmatism to discuss the time of subduction inception, thrust motion and influence of seamount subduction on the geometry of the Manila trench. Results show that lower slope of accretionary wedge mainly consist of imbricated thrusts with blind thrust as the frontal fault and structural wedge whereas upper slope was obscure for intensely structural deformation and magmatism. All the thrust faults merged into a detachment fault/surface which may root in Lower Miocene or even older strata, cut off the Miocene near buried seamount and extended the Pliocene upward, suggesting that this detachment fault was obviously influenced by buried seamount and basement high below the accretionary wedge. Magmatism began to be active from late Miocene and continued to be intense during Pliocene and Quaternary in the oceanic basin, trench and accretionary wedge. Based on characteristics of sedimentary and structural deformation, this study proposed that accretionary wedge of the northern Manila subduction zone formed before 16.5 Ma and propagated to the SCS through piggyback propagation thrusting when seafloor spreading of the SCS was still ongoing before 15 Ma. Subduction of extended continental crust in the northeastern SCS created a significantly concaving eastward to geometric shape of the

Early Cretaceous adakitic magmatism in central eastern China controlled by ridge subduction

NASA Astrophysics Data System (ADS)

Ling, M.; Luo, Z.; Sun, W.

2017-12-01

Early Cretaceous adakites are widely distributed in central eastern China, e.g., Lower Yangtze River Belt (LYRB), Dabie orogen and south Tan-Lu Fault (STLF) area. Adakite from the LYRB is closely associated with mineralization, while adakites from Dabie orogen and STLF are ore barren. Their origins, however, remain controversial. Detailed geochemical comparison between these adakites indicates that the LYRB adakite are formed by partial melting of oceanic crust, i.e., slab melting, whereas those from Dabie orogen and STLF (e.g., Guandian pluton) have origin of lower continental crust (LCC) 1,2. Base on the distribution of igneous rocks, e.g., adakite, A-type granite and Nb-enriched basalts, as well as other lines of evidence, ridge subduction of the Pacific and Izanagi plates was proposed to explain the genesis of Cretaceous magmatism and associated mineralization in the LYRB 1. Ridge subduction is a special plate tectonic process that can provide both physical erosion and thermal erosion 3. Flat subduction of a spreading ridge will result in strong physical subduction-related erosion, and trigger destruction (e.g., in the Dabie orogen) or delamination (e.g., in the STLF) of the thickened LCC. Subsequently, ridge subduction, accompanied by opening of a slab window, will facilitate partial melting of the LCC by thermal erosion. References: 1. Ling, M. X. et al. Cretaceous ridge subduction along the Lower Yangtze river belt, eastern China. Econ. Geol. 104, 303-321, doi:10.2113/gsecongeo.104.2.303 (2009). 2. Ling, M. X., Wang, F. Y., Ding, X., Zhou, J. B. & Sun, W. D. Different origins of adakites from the Dabie Mountains and the Lower Yangtze River Belt, eastern China: Geochemical constraints. International Geology Review 53, 727-740 (2011). 3. Ling, M. X. et al. Destruction of the North China Craton Induced by Ridge Subductions. Journal of Geology 121, 197-213 (2013).

Long distance transport of eclogite and blueschist during early Pacific Ocean subduction rollback

NASA Astrophysics Data System (ADS)

Tamblyn, Renee; Hand, Martin; Kelsey, David; Phillips, Glen; Anczkiewicz, Robert

2017-04-01

The Tasmanides in eastern Australia represent a period of continental crustal growth on the western margin of the Pacific Ocean associated with slab rollback from the Cambrian until the Triassic. During rollback numerical models predict that subduction products can become trapped in the forearc (Geyra et al., 2002), and can migrate with the trench as it retreats. In a long-lived subduction controlled regime such as the Tasmanides, this should result in an accumulation of subduction products with protracted geochronological and metamorphic histories. U-Pb, Lu-Hf, Sm-Nd and Ar-Ar geochronology and phase equilibria modelling of lawsonite-eclogite and garnet blueschist in the Southern New England Fold Belt in Australia demonstrate that high-P low-T rocks remained within a subduction setting for c. 40 Ma, from c. 500 to 460 Ma. High-P metamorphic rocks initially formed close to the Australian cratonic margin during the late Cambrian, and were subsequently transported over 1500 Ma oceanward, during which time subducted material continued to accumulate, resulting in the development of complex mélange which records eclogite and blueschist metamorphism and partial exhumation over 40 Ma. The duration of refrigerated metamorphism approximates the extensional evolution of the upper plate which culminated in the development of the Lachlan Fold Belt. The protracted record of eclogite and blueschist metamorphism indicates that rapid exhumation is not necessarily required for preservation of high-pressure metamorphic rocks from subduction systems. Reference: Gerya, T. V., Stockhert, B., & Perchuk, A. L. (2002). Exhumation of high-pressure metamorphic rocks in a subduction channel: A numerical simulation. Tectonics, 21(6), 6-1-6-19. doi:10.1029/2002tc001406

Oxygen isotopes in garnet and accessory minerals to constrain fluids in subducted crust

NASA Astrophysics Data System (ADS)

Rubatto, Daniela; Gauthiez-Putallaz, Laure; Regis, Daniele; Rosa Scicchitano, Maria; Vho, Alice; Williams, Morgan

2017-04-01

Fluids are considered a fundamental agent for chemical exchanges between different rock types in the subduction system. Constraints on the sources and pathways of subduction fluids thus provide crucial information to reconstruct subduction processes. Garnet and U-Pb accessory minerals constitute some of the most robust and ubiquitous minerals in subducted crust and can preserve multiple growth zones that track the metamorphic evolution of the sample they are hosted in. Microbeam investigation of the chemical (major and trace elements) and isotopic composition (oxygen and U-Pb) of garnet and accessory minerals is used to track significant fluid-rock interaction at different stages of the subduction system. This approach requires consideration of the diffusivity of oxygen isotopes particularly in garnet, which has been investigated experimentally. The nature of the protolith and ocean floor alteration is preserved in relict accessory phases within eclogites that have been fully modified at HP conditions (e.g. Monviso and Dora Maira units in the Western Alps). Minerals in the lawsonite-blueschists of the Tavsanli zone in Turkey record pervasive fluid exchange between mafic and sedimentary blocks at the early stage of subduction. High pressure shear zones and lithological boundaries show evidence of intense fluid metasomatism at depth along discontinuities in Monviso and Corsica. In the UHP oceanic crust of the Zermatt-Saas Zone, garnet oxygen isotopes and tourmaline boron isotopes indicate multistage fluid infiltration during prograde metamorphism. Localized exchanges of aqueous fluids are also observed in the subducted continental crust of the Sesia-Lanzo Zone. In most cases analyses of distinct mineral zones enable identification of multiple pulses of fluids during the rock evolution.

Seismic constraints of thinning and fragmenting continental lithosphere beneath the Korean Peninsula

NASA Astrophysics Data System (ADS)

Kim, S.; Tauzin, B.; Tkalcic, H.; Rhie, J.

2017-12-01

Modification of the continental lithosphere is still an enigmatic process. The Korean Peninsula (KP) is one of ideal place to depict the process by interactions with subducting oceanic slabs. We detect a significant thickness change (>50 km) of the continental lithosphere beneath the KP that is confirmed by two independent approaches: (1) 3D imaging using ambient noise analysis and (2) receiver function CCP stacking. A series of transdimensional and hierarchical Bayesian joint inversions is performed to obtain a high-resolution 3D model from different types of surface wave dispersion data. For the stacking of receiver function waveforms, the coda waveforms of crustal multi-modes (PpPs and PpSs) are combined together to better image the lithosphere-asthenosphere boundary. We estimate the relatively deeper rooted lithosphere (>100 km) in the southwestern part of the KP compared to shallower surrounding regions. The lithospheric structure is underlain by lower velocity anomalies (Vs<4.1 km/s), which extends from back-arc regions near subducting slabs horizontally and connects to low velocity anomalies in the deeper upper mantle vertically. The imaged features clearly show that the effect of the oceanic slab subduction is a key factor controlling the modification process. We further examine the implication for the occurrence of intraplate volcanoes and the relationship to the mantle transition zone heterogeneities due to stagnant slabs in the northeast Asia.

Tomographic images of subducted oceans matched to the accretionary records of orogens - Case study of North America and relevance to Central Asia

NASA Astrophysics Data System (ADS)

Sigloch, Karin; Mihalynuk, Mitchell G.; Hosseini, Kasra

2016-04-01

Accretionary orogens are the surface record of subduction on the 100-million-year timescale; they aggregate buoyant crustal welts that resisted subduction. The other record of subduction is found in the deep subsurface: oceanic lithosphere preserved in the mantle that records ocean basin closure between successive generations of arcs. Seismic tomography maps out these crumpled paleo-oceans down to the core-mantle boundary, where slab accumulates. One such accumulation of enormous scale is under Eastern Asia, recording the assembly of the Central Asian Orogenic Belt (CAOB). Deep CAOB slab has hardly been explored because tomographic image resolution in the lowermost mantle is limited, but this is rapidly improving. We present new images of the CAOB slabs from our P-wave tomography that includes core-diffracted waves as a technical novelty. The previous slab blur sharpens into the type of elongated geometries expected to trace paleo-trench lines. Since the North American Cordillera is younger than the CAOB (mostly <200 m.y. versus ~650-250 m.y.), its slabs have descended only to mid-mantle depths (<2000 km), where tomographic resolution is much better. Hence we can make a detailed, spatiotemporal match between 3-D slab geometries and the accretion history of the Cordillera - a blueprint for continental-scale investigations in other accretionary orogens, including what may become possible for the CAOB. Lower-mantle slabs beneath North America reveal evolving configurations of arc-trench positions back to the breakup of Pangea. These can be combined with quantitative plate reconstructions to show where and when the westward-drifting continent overrode pre-existing, intra-oceanic subduction zones, and accreted their associated arcs and basement terranes in Jurassic and Cretaceous times. Tectonic predictions from this "tomographic time machine" can be checked against the geological record. To demonstrate, we propose a resolution to the longstanding debate of how and when

On the Role of Subduction Dynamics on Emplacement of Metamorphic Core Complexes and Geothermal Systems

NASA Astrophysics Data System (ADS)

Roche, V. M.; Sternai, P.; Guillou-Frottier, L.; Menant, A.; Jolivet, L.; Bouchot, V.; Gerya, T.

2017-12-01

Subduction-induced extensional tectonics in back-arc domains results in the development of metamorphic core complexes (MCCs) and low-angle normal faults (detachments) that also control magma ascent and fluid circulation. However, possible links with the genesis of high-enthalpy geothermal resources (HEGRs) remain barely explored, and no unifying mechanism responsible for both the generation of MCCs and emplacement of HEGRs has yet been recognized. Although discussions on the possible role of magmatic intrusions beneath these systems are still active, another source of heat is required when one considers the scale of a geothermal Province. An additional source of heat, for instance, could arise from the deep dynamics implied by large-scale tectonic processes such as subduction. Firstly, we investigate subduction dynamics through 3D numerical geodynamic models involving slab rollback and tearing constrained primarily by, geothermal anomaly measurements from western Turkey. Our results show that subduction-induced extensional tectonics controls the genesis and distribution of crustal-scale thermal domes, analogous to crustal and lithospheric boudinage. The thermal domes weaken the crust, localize deformation and enhance development of crustal-scale detachments. Thus, these thermo-mechanical instabilities primarily trigger and control the distribution of MCCs. In addition, subduction-related asthenospheric return flow and shear heating in the mantle increase the temperature of the Moho by up to 250°C. Such forcing is observed in natural settings such as the Menderes (western Anatolia) and the Basin and Range (Western United States). Secondly, the numerically-obtained subduction-induced thermal signature at the base of the continental crust is then imposed as basal thermal condition for 2D high-resolution crustal models dedicated to the understanding of fluid flow around detachments. Our results show that permeable detachments control the bulk of the heat transport and

Reconstructing the Alps-Carpathians-Dinarides as a key to understanding switches in subduction polarity, slab gaps and surface motion

NASA Astrophysics Data System (ADS)

Handy, Mark R.; Ustaszewski, Kamil; Kissling, Eduard

2015-01-01

Palinspastic map reconstructions and plate motion studies reveal that switches in subduction polarity and the opening of slab gaps beneath the Alps and Dinarides were triggered by slab tearing and involved widespread intracrustal and crust-mantle decoupling during Adria-Europe collision. In particular, the switch from south-directed European subduction to north-directed "wrong-way" Adriatic subduction beneath the Eastern Alps was preconditioned by two slab-tearing events that were continuous in Cenozoic time: (1) late Eocene to early Oligocene rupturing of the oppositely dipping European and Adriatic slabs; these ruptures nucleated along a trench-trench transfer fault connecting the Alps and Dinarides; (2) Oligocene to Miocene steepening and tearing of the remaining European slab under the Eastern Alps and western Carpathians, while subduction of European lithosphere continued beneath the Western and Central Alps. Following the first event, post-late Eocene NW motion of the Adriatic Plate with respect to Europe opened a gap along the Alps-Dinarides transfer fault which was filled with upwelling asthenosphere. The resulting thermal erosion of the lithosphere led to the present slab gap beneath the northern Dinarides. This upwelling also weakened the upper plate of the easternmost part of the Alpine orogen and induced widespread crust-mantle decoupling, thus facilitating Pannonian extension and roll-back subduction of the Carpathian oceanic embayment. The second slab-tearing event triggered uplift and peneplainization in the Eastern Alps while opening a second slab gap, still present between the Eastern and Central Alps, that was partly filled by northward counterclockwise subduction of previously unsubducted Adriatic continental lithosphere. In Miocene time, Adriatic subduction thus jumped westward from the Dinarides into the heart of the Alpine orogen, where northward indentation and wedging of Adriatic crust led to rapid exhumation and orogen-parallel escape of

15 Years Of Ecuadorian-French Research Along The Ecuadorian Subduction Zone

NASA Astrophysics Data System (ADS)

Charvis, P.

2015-12-01

The Ecuadorian segment of the Nazca/South America subduction zone is an outstanding laboratory to study the seismic cycle. Central Ecuador where the Carnegie ridge enters the subduction marks a transition between a highly coupled segment that hosted one of the largest seismic sequence during the 20thcentury and a ~1200-km long weakly coupled segment encompassing southern Ecuador and northern Peru. A shallow dipping subduction interface and a short trench-coast line distance ranging from 45 to 80 km, together with La Plata Island located only 33 km from the trench axis, allow to document subduction processes in the near field with an exceptional resolution. Since 2000, a close cooperation between the Institute of Geophysics (Quito), INOCAR (Oceanographic Institute of the Ecuadorian Navy) with French groups allowed us to conduct up to 6 marine geophysics cruises to survey the convergent margin and jointly develop dense GPS and seismological networks. This fruitful collaboration now takes place in the framework of an International Joint Laboratory "Earthquakes and Volcanoes in the Northern Andes" (LMI SVAN), which eases coordinating research projects and exchanges of Ecuadorian and French scientists and students. This long-term investigation has already provided a unique view on the structure of the margin, which exhibits a highly variable subduction channel along strike. It allowed us to evidence the contrast between creeping and coupled segments of subduction at various scale, and the existence of large continental slivers whose motion accommodates the obliquity of the Nazca/South America convergence. Finally, we could evidence the first Slow Slip Events (SSE) that oppositely to most SSE documented so far, are accompanied with intense micro-seismicity. The recent support of the French National Research Agency and the Ecuadorian Agency for Sciences and Technology (Senescyt) will enable us to integrate the already obtained results, in an attempt to develop an

Forearc Basin Structure in the Andaman-Nicobar Segment of the Sumatra-Andaman Subduction Zone: Insight from High-Resolution Seismic Reflection Data

NASA Astrophysics Data System (ADS)

Moeremans, R. E.; Singh, S. C.

2014-12-01

The Andaman-Nicobar subduction is the northernmost segment of the Sumatra-Andaman subduction zone and marks the western boundary of the Andaman Sea, which is a complex backarc extensional basin. We present the interpretation of a new set of deep seismic reflection data acquired across the Andaman-Nicobar forearc basin, from 8°N to 11°N, to understand the structure and evolution of the forearc basin, focusing on how the obliquity of convergence affects deformation in the forearc, as well as on the Diligent (DF) and Eastern Margin Faults (EMF). Constraining the evolution of this basin, which is strongly related to the collision of India and Eurasia, can help shed light onto present-day deformation processes along this segment of the subduction zone, where convergence is highly oblique and little data is available. We find that he DF is a backthrust and corresponds to the Mentawai (MFZ) and West Andaman Fault (WAF) systems further south, offshore Sumatra. The DF is expressed as a series of mostly landward verging folds and faults, deforming the early to late Miocene sediments. The DF seems to root from the boundary between the accretionary complex and the continental backstop, where it meets the EMF. The EMF marks the western boundary of the forearc basin; it is associated with subsidence and is expressed as a deep piggyback basin, associated with recent Pliocene to Pleistocene subsidence at the western edge of the forearc basin. The eastern edge of the forearc basin is marked by the Invisible Bank (IB), which is thought to be tilted and uplifted continental crustal block. Subsidence along the EMF and uplift and tilting of the IB seem to be related to different opening phases in the Andaman Sea. The sliver Andaman-Nicobar Fault (ANF), which is the active northward extension of the Great Sumatra sliver Fault (GSF), lies to the east of the IB, and marks the boundary between continental crust underlying the forearc basin and crust accreted at the Andaman Sea Spreading

Carboniferous Proto-type Basin Evolution of Junggar Basin in Northwest China: Implications for the Growth Models of Central Asia Orogenic Belt

NASA Astrophysics Data System (ADS)

He, D.

2016-12-01

The Junggar Basin locates in the central part of Paleo-Asian Ocean tectonic domain, and records the dynamic processes of the Central Asian Orogenic Belt from subduction-accretion-collision to later intracontinental deformations. Carboniferous is the key period from subduction to closure in the tectonic evolution of Paleo-Asian Ocean. Based on the borehole, outcrop, seismic and gravity and magnetic anomaly data, the paper made analysis of the Carboniferous basin evolution.Geo-chronological results for the borehole volcanic rocks suggest that the Junggar Basin and adjacent area had five periods of volcanic activities, including two periods in the Early Carboniferous (359-347Ma 347-331Ma and 331-324Ma) and three periods in the Late Carboniferous (323-307Ma and 307-300Ma). Regional unconformities divided the Carboniferous into two tectono-stratigraphic sequences: Lower Carboniferous and Upper Carboniferous. The former is characterized by compressional structures and involves massive calc-alkaline basalts, andesites, dacites and rhyolites, whereas the later is mainly controlled by extensional faults and dominated by intermediate-mafic volcanic rocks, with bimodal volcanic rocks in parts. The paper determined four Carboniferous arc-basin belts in the Junggar Basin and adjacent area from north to south: the Saur-Fuhai-Dulate, Heshituoluogai-Wulungu-Yemaquan, Darbut-Luliang-Karamaili, and Zhongguai-Mosuowan-Baijiahai-Qitai, and identified multi-type basins, such as fore-arc basin, retro-arc basin, intra-arc rift basin, foreland basin and passive continental margin basin,etc.. The Carboniferous proto-type basin evolution of the Junggar Basin can be divided into three phases such as, the early to middle Early Carboniferous subduction-related compressional phase, the late Early Carboniferous to middle Late Carboniferous subduction-related extensional phase and the late Late Carboniferous intra-continental fault-sag phase. The study discloses that the Junggar Basin is likely

Late Cretaceous-Cenozoic subduction-collision history of the Southern Neotethys: new evidence from the Çağlayancerit area, SE Turkey

NASA Astrophysics Data System (ADS)

Akıncı, Ahmet Can; Robertson, Alastair H. F.; Ünlügenç, Ulvi Can

2016-01-01

Evidence of the subduction-collision history of the S Neotethys is well exposed in the frontal part of the SE Anatolian thrust belt and the adjacent Arabian continental margin. The foreland succession in the study area begins with Eocene shelf carbonates, ranging from shallow marine to deeper marine, without sedimentary input from the Tauride continent to the north. After a regional hiatus (Oligocene), sedimentation resumed during the Early Miocene with terrigenous gravity-flow deposition in the north (Lice Formation) and shallow-marine carbonates further south. Clastic detritus was derived from the Tauride continent and oceanic accretionary material. The base of the overriding Tauride allochthon comprises ophiolite-derived debris flows, ophiolite-related mélange and dismembered ophiolitic rocks. Above this, the regional-scale Bulgurkaya sedimentary mélange (an olistostrome) includes blocks and dismembered thrust sheets of metamorphic rocks, limestone and sandstone, which include Late Cretaceous and Eocene foraminifera. The matrix is mainly strongly deformed Eocene-Oligocene mudrocks, hemipelagic marl and sandstone turbidites. The thrust stack is topped by a regionally extensive thrust sheet (Malatya metamorphic unit), which includes greenschist facies marble, calcschist, schist and phyllite, representing Tauride continental crust. Beginning during the Late Mesozoic, the S Neotethys subducted northwards beneath a backstop represented by the Tauride microcontinent (Malatya metamorphic unit). Ophiolites formed within the S Neotethys and accreted to the Tauride active margin. Large-scale sedimentary mélange developed along the Tauride active margin during Eocene-Oligocene. On the Arabian margin, a sedimentary hiatus and tilting (Oligocene) is interpreted to record initial continental collision. The Early Miocene terrigenous gravity flows represent a collision-related flexural foreland basin. Southward overthrusting of the Tauride allochthon took place during Early

3D absolute hypocentral determination - 13 years of seismicity in Ecuadorian subduction zone

NASA Astrophysics Data System (ADS)

Font, Yvonne; Segovia, Monica; Theunissen, Thomas

2010-05-01

In Ecuador, the Nazca plate is subducting beneath the North Andean Block. This subduction triggered, during the last century, 4 major earthquakes of magnitude greater than 7.7. Between 1994 and 2007, the Geophysical Institute (Escuela National Politecnica, Quito) recorded about 40 000 events in whole Ecuador ranging from Mb 1.5 to 6.9. Unfortunately, the local network shows great density discrepancy between the Coastal and Andean regions where numerous stations were installed to survey volcanic activity. Consequently, seismicity in and around the interplate seismogenic zone - producer of the most destructive earthquakes and tsunamis - is not well constrained. This study aims to improve the location of 13 years seismicity occurred during an interseismic period in order to better localize the seismic deformation and gaps. The first step consists in the construction of a 3D "georealistic" velocity model. Because local tomography cannot provide satisfactory model, we combined all local crustal/lithospheric information on the geometry and velocity properties of different geological units. Those information cover the oceanic Nazca plate and sedimentary coverture the subducting plate dip angle; the North Andean Block margin composed of accreted oceanic plateaus (the Moho depth is approximated using gravity modeling); the metamorphic volcanic chain (oceanic nature for the occidental cordillera and inter-andean valley, continental one for the oriental cordillera); The continental Guyana shield and sedimentary basins. The resulting 3D velocity model extends from 2°N to 6.5°S and 277°E to 283°E and reaches a depth of 300 km. It is discretized in constant velocity blocks of 12 x 12 x 3 km in x, y and z, respectively. The second step consists in selecting an adequate sub-set of seismic stations in order to correct the effect of station density disequilibrium between coastal and volcanic regions. Consequently, we only keep the most representative volcanic stations in terms

a New Animation of Subduction Processes for Undergraduates

NASA Astrophysics Data System (ADS)

Stern, R. J.; Lieu, W. K.; Mantey, A.; Ward, A.; Todd, F.; Farrar, E.; Sean, M.; Windler, J.

2015-12-01

The subduction of oceanic lithosphere beneath convergent plate margins is a fundamental plate tectonic concept and an important Earth process. It is responsible for some of Earth's most dangerous natural hazards including earthquakes and volcanic eruptions but also produced the continental crust and important mineral deposits. A range of geoscientific efforts including NSF MARGINS and GeoPRISMS initiatives have advanced our understanding of subduction zone processes. In spite the importance of subduction zones and our advancing understanding of how these function, there are few animations that clearly explain the subduction process to non-expert audiences. This deficiency reflects the disparate expertises between geoscientists who know the science but have weak animation skills and digital artists and animators who have strong skills in showing objects in motion but are not experts in natural processes like plate tectonics. This transdisciplinary gap can and should be bridged. With a small grant from NSF (DUE-1444954) we set about to generate a realistic subduction zone animation aimed at the university undergraduate audience by first working within our university to rough out a draft animation and then contract a professional to use this to construct the final version. UTD Geosciences faculty (Stern) and graduate student (Lieu) teamed up with faculty from UTD School of Arts, Technology, and Emerging Communication (ATEC)(Farrar, Fechter, and McComber) to identify and recruit talented ATEC undergraduate students (Mantey, Ward) to work on the project. Geoscientists assembled a storyboard and met weekly with ATEC undergraduates to generate a first draft of the animation, which guided development of an accompanying narrative. The draft animation with voice-over was then handed off to professional animator Windler (Archistration CG) to generate the final animation. We plan to show both the student-generated draft version and the final animation during our presentation

Geophysical evidence for a transform margin in Northwestern Algeria: possible vestige of a Subduction-Transform Edge Propagator

NASA Astrophysics Data System (ADS)

Badji, R.; Charvis, P.; Bracene, R.; Galve, A.; Badsi, M.; Ribodetti, A.; Benaissa, Z.; Klingelhoefer, F.; Medaouri, M.; Beslier, M.

2013-12-01

This work is part of the Algerian-French SPIRAL program (Sismique Profonde et Investigation Régionale du Nord de l'Algérie) which provides unprecedented images of the deep structure of the western Algerian Margin based on several wide-angle and multichannel seismic data shot across the Algerian Margin. One of the different hypotheses for the opening of the western Mediterranean Sea, we are testing is that the western part of the Algerian margin was possibly part of the southern edge of the Alboran continental block during its westward migration related to the rollback of the Betic-Rif-Alboran subduction zone. A tomographic inversion of the first arrival traveltimes along a 100-km long wide-angle seismic profile shot over 40 Ocean Bottom Seismometers, across the Margin offshore Mostaganem (Northwestern Algerian Margin) was conducted. The final model reveals striking feature in the deep structure of the margin from north to south: 1- the oceanic crust is as thin as 4-km, with velocities ranging from 5.0 to 7.1 km/s, covered by a 3.3 km thick sedimentary pile (seismic velocities from 1.5 to 5.0 km/s) characterized by an intense diapiric activity of the Messinian salt layer. 2- a sharp transition zone, less than 10 km wide, with seismic velocities intermediate between oceanic seismic velocities (observed northward) and continental seismic velocities (observed southward). This zone coincides with narrow and elongated pull apart basins imaged by multichannel seismic data. No evidence of volcanism nor of exhumed serpentinized upper mantle as described along many extensional continental margins are observed along this segment of the margin. 3- a thinned continental crust coincident with a rapid variation of the Moho depth imaged from 12 to ~20 km with a dip up to 50%. The seafloor bathymetry is showing a steep continental slope (>20%). Either normal or inverse faults are observed along MCS lines shot in the dip direction but they do not present large vertical

Continental origin of the Gubaoquan eclogite and implications for evolution of the Beishan Orogen, Central Asian Orogenic Belt, NW China

NASA Astrophysics Data System (ADS)

Saktura, Wanchese M.; Buckman, Solomon; Nutman, Allen P.; Belousova, Elena A.; Yan, Zhen; Aitchison, Jonathan C.

2017-12-01

The Gubaoquan eclogite occurs in the Paleozoic Beishan Orogen of NW China. Previously it has been interpreted as a fragment of subducted oceanic crust that was emplaced as a mélange within continental rocks. Contrary to this, we demonstrate that the Gubaoquan eclogite protolith was a Neoproterozoic basic dyke/sill which intruded into Proterozoic continental rocks. The SHRIMP Usbnd Pb zircon dating of the metamorphic rims of the Gubaoquan eclogite yields an age 466 ± 27 Ma. Subdued heavy rare earth element abundances and lack of negative Eu anomalies of the metamorphic zircon domains confirm that this age represents eclogite facies metamorphism. The host augen orthogneiss has a Usbnd Pb zircon age of 920 ± 14 Ma, representing the timing of crystallization of the granitic protolith. A leucogranitic vein which intrudes the eclogite has a Usbnd Pb zircon age of 424 ± 8.6 Ma. This granitic vein marks the end of high-grade metamorphism in this area. The overcomplication of tectonic history of the Beishan Orogen is partially caused by inconsistent classifications and nomenclature of the same rock units and arbitrary subdivisions of Precambrian blocks as individual microcontinents. In an attempt to resolve this, we propose a simpler model that involves the partial subduction of the northern passive margin of the Dunhuang Block beneath the active continental margin developing on the Mazongshan-Hanshan Block to the north. Ocean closure and continental collision during the Late Ordovician resulted in continental thickening and eclogite facies metamorphism recorded by the mafic dykes/sills (now the Gubaoquan eclogite). In the light of the new data, the tectonothermal evolution of the Beishan Orogen is reviewed and integrated with the evolution of the Central Asian Orogenic Belt.

Buoyancy-Driven, Rapid Exhumation of Ultrahigh-Pressure Metamorphosed Continental Crust

NASA Astrophysics Data System (ADS)

Ernst, W. G.; Maruyama, S.; Wallis, S.

1997-09-01

Preservation of ultrahigh-pressure (UHP) minerals formed at depths of 90-125 km require unusual conditions. Our subduction model involves underflow of a salient (250 ± 150 km wide, 90-125 km long) of continental crust embedded in cold, largely oceanic crust-capped lithosphere; loss of leading portions of the high-density oceanic lithosphere by slab break-off, as increasing volumes of microcontinental material enter the subduction zone; buoyancy-driven return toward midcrustal levels of a thin (2-15 km thick), low-density slice; finally, uplift, backfolding, normal faulting, and exposure of the UHP terrane. Sustained over ≈ 20 million years, rapid (≈ 5 mm/year) exhumation of the thin-aspect ratio UHP sialic sheet caught between cooler hanging-wall plate and refrigerating, downgoing lithosphere allows withdrawal of heat along both its upper and lower surfaces. The intracratonal position of most UHP complexes reflects consumption of an intervening ocean basin and introduction of a sialic promontory into the subduction zone. UHP metamorphic terranes consist chiefly of transformed, yet relatively low-density continental crust compared with displaced mantle material--otherwise such complexes could not return to shallow depths. Relatively rare metabasaltic, metagabbroic, and metacherty lithologies retain traces of phases characteristic of UHP conditions because they are massive, virtually impervious to fluids, and nearly anhydrous. In contrast, H2O-rich quartzofeldspathic, gneissose/schistose, more permeable metasedimentary and metagranitic units have backreacted thoroughly, so coesite and other UHP silicates are exceedingly rare. Because of the initial presence of biogenic carbon, and its especially sluggish transformation rate, UHP paragneisses contain the most abundantly preserved crustal diamonds.

Buoyancy-driven, rapid exhumation of ultrahigh-pressure metamorphosed continental crust.

PubMed

Ernst, W G; Maruyama, S; Wallis, S

1997-09-02

Preservation of ultrahigh-pressure (UHP) minerals formed at depths of 90-125 km require unusual conditions. Our subduction model involves underflow of a salient (250 +/- 150 km wide, 90-125 km long) of continental crust embedded in cold, largely oceanic crust-capped lithosphere; loss of leading portions of the high-density oceanic lithosphere by slab break-off, as increasing volumes of microcontinental material enter the subduction zone; buoyancy-driven return toward midcrustal levels of a thin (2-15 km thick), low-density slice; finally, uplift, backfolding, normal faulting, and exposure of the UHP terrane. Sustained over approximately 20 million years, rapid ( approximately 5 mm/year) exhumation of the thin-aspect ratio UHP sialic sheet caught between cooler hanging-wall plate and refrigerating, downgoing lithosphere allows withdrawal of heat along both its upper and lower surfaces. The intracratonal position of most UHP complexes reflects consumption of an intervening ocean basin and introduction of a sialic promontory into the subduction zone. UHP metamorphic terranes consist chiefly of transformed, yet relatively low-density continental crust compared with displaced mantle material-otherwise such complexes could not return to shallow depths. Relatively rare metabasaltic, metagabbroic, and metacherty lithologies retain traces of phases characteristic of UHP conditions because they are massive, virtually impervious to fluids, and nearly anhydrous. In contrast, H2O-rich quartzofeldspathic, gneissose/schistose, more permeable metasedimentary and metagranitic units have backreacted thoroughly, so coesite and other UHP silicates are exceedingly rare. Because of the initial presence of biogenic carbon, and its especially sluggish transformation rate, UHP paragneisses contain the most abundantly preserved crustal diamonds.

Dynamics of Mid-Palaeocene North Atlantic rifting linked with European intra-plate deformations.

PubMed

Nielsen, Søren B; Stephenson, Randell; Thomsen, Erik

2007-12-13

The process of continental break-up provides a large-scale experiment that can be used to test causal relations between plate tectonics and the dynamics of the Earth's deep mantle. Detailed diagnostic information on the timing and dynamics of such events, which are not resolved by plate kinematic reconstructions, can be obtained from the response of the interior of adjacent continental plates to stress changes generated by plate boundary processes. Here we demonstrate a causal relationship between North Atlantic continental rifting at approximately 62 Myr ago and an abrupt change of the intra-plate deformation style in the adjacent European continent. The rifting involved a left-lateral displacement between the North American-Greenland plate and Eurasia, which initiated the observed pause in the relative convergence of Europe and Africa. The associated stress change in the European continent was significant and explains the sudden termination of a approximately 20-Myr-long contractional intra-plate deformation within Europe, during the late Cretaceous period to the earliest Palaeocene epoch, which was replaced by low-amplitude intra-plate stress-relaxation features. The pre-rupture tectonic stress was large enough to have been responsible for precipitating continental break-up, so there is no need to invoke a thermal mantle plume as a driving mechanism. The model explains the simultaneous timing of several diverse geological events, and shows how the intra-continental stratigraphic record can reveal the timing and dynamics of stress changes, which cannot be resolved by reconstructions based only on plate kinematics.

Seismic reflection imaging of two megathrust shear zones in the northern Cascadia subduction zone.

PubMed

Calvert, Andrew J

2004-03-11

At convergent continental margins, the relative motion between the subducting oceanic plate and the overriding continent is usually accommodated by movement along a single, thin interface known as a megathrust. Great thrust earthquakes occur on the shallow part of this interface where the two plates are locked together. Earthquakes of lower magnitude occur within the underlying oceanic plate, and have been linked to geochemical dehydration reactions caused by the plate's descent. Here I present deep seismic reflection data from the northern Cascadia subduction zone that show that the inter-plate boundary is up to 16 km thick and comprises two megathrust shear zones that bound a >5-km-thick, approximately 110-km-wide region of imbricated crustal rocks. Earthquakes within the subducting plate occur predominantly in two geographic bands where the dip of the plate is inferred to increase as it is forced around the edges of the imbricated inter-plate boundary zone. This implies that seismicity in the subducting slab is controlled primarily by deformation in the upper part of the plate. Slip on the shallower megathrust shear zone, which may occur by aseismic slow slip, will transport crustal rocks into the upper mantle above the subducting oceanic plate and may, in part, provide an explanation for the unusually low seismic wave speeds that are observed there.

3-D Numerical Modelling of Oblique Continental Collisions with ASPECT

NASA Astrophysics Data System (ADS)

Karatun, L.; Pysklywec, R.

2017-12-01

Among the fundamental types of tectonic plate boundaries, continent-continent collision is least well understood. Deformation of the upper and middle crustal layers can be inferred from surface structures and geophysical imaging, but the fate of lower crustal rocks and mantle lithosphere is not well resolved. Previous research suggests that shortening of mantle lithosphere generally may be occurring by either: 1) a distributed thickening with a formation of a Raleigh-Tailor (RT) type instability (possibly accompanied with lithospheric folding); or 2) plate-like subduction, which can be one- or two-sided, with or without delamination and slab break-off; a combination of both could be taking place too. 3-D features of the orogens such as along-trench material transfer, bounding subduction zones can influence the evolution of the collision zone significantly. The current study was inspired by South Island of New Zealand - a young collision system where a block of continental crust is being shortened by the relative Australian-Pacific plate motion. The collision segment of the plate boundary is relatively small ( 800 km), and is bounded by oppositely verging subduction zones to the North and South. Here, we present results of 3-D forward numerical modelling of continental collision to investigate some of these processes. To conduct the simulations, we used ASPECT - a highly parallel community-developed code based on the Finite Element method. Model setup for three different sets of models featured 2-D vertical across strike, 3-D with periodic front and back walls, and 3-D with open front and back walls, with velocities prescribed on the left and right faces. We explored the importance of values of convergent velocity, strike-slip velocity and their ratio, which defines the resulting velocity direction relative to the plate boundary (obliquity). We found that higher strike-slip motion promotes strain localization, weakens the lithosphere close to the plate boundary and

Multi-stage barites in partially melted UHP eclogite: implications for fluid/melt activities during deep continental subduction in the Sulu orogenic belt

NASA Astrophysics Data System (ADS)

Wang, Songjie; Wang, Lu

2015-04-01

Barite (BaSO4) is well-known from deep-sea sedimentary environments but has received less attention to its presence in high-grade metamorphic rocks. Recently, barite in ultrahigh pressure (UHP) eclogite has drawn increasing attention from geologists, especially in the Dabie-Sulu orogen, since it is an important indicator for high-salinity fluid events, thus aiding in further understanding HP-UHP fluid / melt evolution. However, its formation time and mechanism in UHP eclogite are still controversial, with three representative viewpoints: (1) Liu et al. (2000) found barite-anhydrite-coesite inclusions in zircon and interpreted them to have formed by UHP metamorphic fluids; (2) Zeng et al. (2007) recognized isolated barite within K-feldspar (Kfs) and Quartz (Qz) surrounded by radial cracks in omphacite, and interpreted Kfs+Qz to be reaction products of potassium-rich fluid/melt and coesite, with the barite formed by prograde metamorphic fluids; (3) Gao et al. (2012) and Chen et al. (2014) found barite-bearing Multiphase Solid (MS) inclusions within garnet and omphacite and assumed that the barite formed by phengite breakdown possibly caused by eclogite partial melting during exhumation, though no direct evidence were proposed. The controversy above is mainly due to the lack of direct formation evidence and absence of a clear link with the metamorphic evolution of UHP eclogite along the subduction-exhumation path. We report detailed petrological and micro-structural analyses revealing four types of barites clearly linked with (1) the prograde, (2) earlier stage of partial melting and (3) later stage of crystallization differentiation, as well as (4) high-grade amphibolite-facies retrogression of a deeply subducted and partially melted intergranular coesite-bearing eclogite from Yangkou Bay, Sulu Orogen. Round barite inclusions (type-I) within UHP-stage garnet and omphacite are formed by internally buffered fluids from mineral dehydration during prograde metamorphism

Predicting and testing continental vertical motion histories since the Paleozoic

NASA Astrophysics Data System (ADS)

Zhang, N.; Zhong, S.; Flowers, R. M.

2011-12-01

Dynamic topography at the Earth's surface caused by mantle convection can affect a range of geophysical and geological observations including bathymetry, sea-level change, continental flooding, sedimentation and erosion. These observations provide important constraints on and test of mantle dynamic models. Based on global mantle convection models coupled with the surface plate motion history, we compute dynamic topography and its history for the last 400 Ma associated with Pangea assembly and breakup, with particular focus on continental cratonic regions. We propose that burial-unroofing histories of continental cratons inferred from thermochronology data can be used as a new diagnostic to test dynamic topography and mantle dynamic models. Our models show that there are currently two broad dynamic topography highs in the Pacific and Africa for the present-day Earth that are associated with the broad, warm structures (i.e., superplumes) in the deep mantle, consistent with previous proposals of dynamical support for the Pacific and African superswells. Our models reveal that Pangea assembly and breakup, by affecting subduction and mantle upwelling processes, have significant effects on continental vertical motions. Our models predict that the Slave craton in North America subsides before Pangea assembly at 330 Ma but uplifts significantly from 330 Ma to 240 Ma in response to pre-Pangea subduction and post-assembly mantle warming. The Kaapvaal craton of Africa is predicted to undergo uplift from ~180 Ma to 90 Ma after Pangea breakup, but its dynamic topography remains stable for the last 90 Ma. The predicted histories of elevation change for the Slave and Kaapvaal cratons compare well with the burial-unroofing histories inferred from thermochronology studies, thus supporting our dynamic models including the development of the African superplume mantle structure. The vertical motion histories for other cratons can provide further tests and constraints on our mantle

Petrography and geochemistry of clastic rocks within the Inthanon zone, northern Thailand: Implications for Paleo-Tethys subduction and convergence

NASA Astrophysics Data System (ADS)

Hara, Hidetoshi; Kunii, Miyuki; Hisada, Ken-ichiro; Ueno, Katsumi; Kamata, Yoshihito; Srichan, Weerapan; Charusiri, Punya; Charoentitirat, Thasinee; Watarai, Megumi; Adachi, Yoshiko; Kurihara, Toshiyuki

2012-11-01

The provenance, source rock compositions, and sediment supply system for a convergence zone of the Paleo-Tethys were reconstructed based on the petrography and geochemistry of clastic rocks of the Inthanon Zone, northern Thailand. The clastic rocks are classified into two types based on field and microscopic observations, the modal composition of sandstone, and mineral compositions: (1) lithic sandstone and shale within mélange in a Permo-Triassic accretionary complex; and (2) Carboniferous quartzose sandstone and mudstone within the Sibumasu Block. Geochemical data indicate that the clastic rocks of the mélange were derived from continental island arc and continental margin settings, which correspond to felsic volcanic rocks within the Sukhothai Zone and quartz-rich fragments within the Indochina Block, respectively. The results of a mixing model indicate the source rocks were approximately 35% volcanic rocks of the Sukhothai Zone and 65% craton sandstone and upper continental crust of the Indochina Block. In contrast, Carboniferous quartzose sedimentary rocks within the Sibumasu Block originated from a continental margin, without a contribution from volcanic rocks. In terms of Paleo-Tethys subduction, a continental island arc in the Sukhothai Zone evolved in tandem with Late Permian-Triassic forearc basins and volcanic activity during the Middle-early Late Triassic. The accretionary complex formed contemporaneously with the evolution of continental island arc during the Permo-Triassic, supplied with sediment from the Sukhothai Zone and the Indochina Block.

Mechanical decoupling along a subduction boundary fault: the case of the Tindari-Alfeo Fault System, Calabrian Arc (central Mediterranean Sea)

NASA Astrophysics Data System (ADS)

Maesano, F. E.; Tiberti, M. M.; Basili, R.

2017-12-01

In recent years an increasing number of studies have been focused in understanding the lateral terminations of subduction zones. In the Mediterranean region, this topic is of particular interest for the presence of a "land-locked" system of subduction zones interrupted by continental collision and back-arc opening. We present a 3D reconstruction of the area surrounding the Tindari-Alfeo Fault System (TAFS) based on a dense set of deep seismic reflection profiles. This fault system represents a major NNW-SSE trending subduction-transform edge propagator (STEP) that controls the deformation zone bounding the Calabrian subduction zone (central Mediterranean Sea) to the southwest. This 3D model allowed us to characterize the mechanical and kinematic evolution of the TAFS during the Plio-Quaternary. Our study highlights the presence of a mechanical decoupling between the deformation observed in the lower plate, constituted by the Ionian oceanic crust entering the subduction zone, and the upper plate, where a thick accretionary wedge has formed. The lower plate hosts the master faults of the TAFS, whereas the upper plate is affected by secondary deformation (bending-moment faulting, localized subsidence, stepovers, and restraining/releasing bends). The analysis of the syn-tectonic sedimentary basins related to the activity of the TAFS at depth allow us to constrain the propagation rate of the deformation and of the vertical component of the slip-rate. Our findings provide a comprehensive framework of the structural setting that can be expected along a STEP boundary where contractional and transtensional features coexist at close distance from one another.

Subduction of Young Oceanic Lithosphere and Extensional Orogeny in Southwestern North America during Mid-Tertiary Time

NASA Astrophysics Data System (ADS)

Elston, Wolfgang E.

1984-04-01

An "extensional orogeny" deformed the Basin and Range province, probably beginning in the late Eocene (about 40 ± 3 Ma). Its characteristics include partial melting of the continental lithosphere during the "ignimbrite flareup," massive ductile extension (including detachment faulting), and rise of metamorphic core complexes. The affected zone became about 1200 km wide, possibly double its original width. It rose an average of 1-2 km, despite crustal thinning. Locally, some of the highest mountains of North America, up to 4.3 km high, rose through resurgence of ignimbrite cauldrons and isostatic uplift of underlying plutons. The climax of extension occurred prior to the development of the present basin and range topography. Modeling of major and trace elements and Sr and Pb isotopes strongly suggests that mid-Tertiary volcanic magmas equilibrated, and probably originated, in the continental lithosphere. Components attributable to subducted oceanic lithosphere have not yet been identified. The rocks seem to belong to two provinces, separated by the quartz diorite boundary line of Moore (1959), which also marks the western limit of North America at the end of the late Paleozoic Sonoman orogeny. To the west, low-K rocks rest on a basement of predominantly oceanic accreted terranes; to the east, high-K rocks rest on an autochthonous sialic basement. Within the high-K province, potassium variations can be correlated with crustal thickness; there is no need to invoke a K-h relationship. Conventional models of plate convergence and back arc extension which involve subduction of old, rigid, cool, and dense oceanic lithosphere may not apply to the mid-Tertiary Basin and Range province. The overridden Farallon plate is more likely to have been young, hot, ductile, buoyant, and no denser than continental asthenosphere, having been generated in a spreading center close to North America. Under these conditions, motion of the subducting plate slows and slab-pull is likely to

Drainage Asperities on Subduction Megathrusts

NASA Astrophysics Data System (ADS)

Sibson, R. H.

2012-12-01

Geophysical observations coupled with force-balance analyses suggest that the seismogenic shear zone interface of subduction megathrusts is generally fluid-overpressured to near-lithostatic values (λv = Pf/σv > 0.9) below the forearc hanging-wall, strongly modulating the profile of frictional shear resistance. Fluid sources include the accretionary prism at shallow levels and, with increasing depth, metamorphic dehydration of material entrained within the subduction shear zone together with progressive metamorphism of oceanic crust in the downgoing slab. Solution transfer in fine-grained material contained within the deeper subduction shear zone (150 < T < 350°C) likely contributes to hydrothermal sealing of fractures. A dramatic difference may therefore exist between low prefailure permeability surrounding the megathrust and high postfailure fracture permeability along the rupture zone and adjacent areas of aftershock activity. Observed postseismic changes in the velocity structure of the fore-arc hanging-wall led Husen and Kissling (2001) to propose massive fluid loss across the subduction interface following the 1995 Antofagasta, Chile, Mw8.0 megathrust rupture. Such trans-megathrust discharges represent a variant of 'fault-valve' action in which the subduction interface itself acts as a seal trapping overpressured fluids derived from metamorphic dehydration beneath. In low-permeability assemblages the maximum sustainable overpressure is limited by the activation or reactivation of brittle faults and fractures under the prevailing stress state. Highest overpressures tend to occur at low differential stress in compressional stress regimes. Loci for fluid discharge are likely determined by stress heterogeneities along the megathrust (e.g. the hangingwall of the rupture at its downdip termination). Discharge sites may be defined by swarm aftershocks defining activated fault-fracture meshes. However, fluid loss across a subduction interface will be enhanced when

Chemical and oxygen isotope zonings in garnet from subducted continental crust record mineral replacement and metasomatism

NASA Astrophysics Data System (ADS)

Vho, Alice; Rubatto, Daniela; Regis, Daniele; Baumgartner, Lukas; Bouvier, Anne-Sophie

2017-04-01

Garnet is a key mineral in metamorphic petrology for constraining pressure, temperature and time paths. Garnet can preserve multiple growth stages due to its wide P-T stability field and the relatively slow diffusivity for major and trace elements at sub-solidus temperatures. Pressure-temperature-time-fluid paths of the host rock may be reconstructed by combining metamorphic petrology with microscale trace element and oxygen isotope measurements in garnet. Subduction zones represent relevant geological settings for geochemical investigation of element exchanges during aqueous fluid-rock interactions. The Sesia Zone consists of a complex continental sequence containing a variety of mono-metamorphic and poly-metamorphic lithologies such as metagranitoids, sediments and mafic boudins. The precursor Varisican-Permian amphibolite-facies basement (6-9 kbar 650-850°C; Lardeaux and Spalla, 1991; Robyr et al., 2013) experienced high pressure metamorphism (15-22 kbar 500-550°C; Regis, et al. 2014; Robyr et al., 2013) during Alpine subduction. In different lithologies of the Internal Complex (Eclogitic Micaschist Complex), including metabasites from the Ivozio Complex, Ti-rich metasediments from Val Malone and pre-Alpine Mn-quartzites associated to metagabbros from Cima Bonze, garnet is abundant and shows a variety of complex textures that cannot be reconciled with typical growth zoning, but indicate resorption and replacement processes and possible metasomatism. In-situ, microscale oxygen isotopes analysis of garnet zones was performed by ion microprobe with the SwissSIMS Cameca IMS 1280-HR at University of Lausanne and SHRIMP-SI at the Australian National University. Each sample has a distinct δ18O composition, and the δ18O values show different degrees of variation between domains. Homogeneously low values of < 5‰ are measured in the garnets from the Ivozio Complex metagabbro. Intragrain variations of up to 3.5‰ in the porphyroblasts from Val Malone metasediments

Seismicity and structure of Nazca Plate subduction zone in southern Peru

NASA Astrophysics Data System (ADS)

Lim, H.; Kim, Y.; Clayton, R. W.; Thurber, C. H.

2016-12-01

We define subducting plate geometries in the Nazca subduction zone by (re)locating intra-slab earthquakes in southern Peru (2-18°S) and taking previously published converted phase analysis results, to clarify the slab geometry and inferred relationships to the seismicity. We also provide both P- and S-wave velocities of the subducting Nazca Plate and mantle wedge portions close to the slab using double-difference tomography (Zhang and Thurber, 2003) to understand upper plate volcanism and subduction process. A total of 492 regional earthquakes from August 2008 to February 2013 recorded from the dense seismic array (PeruSE, 2013) are selected for the relocation and tomography. The relocated seismicity shows a smooth contortion in the slab-dip transition zone for 400 km between the shallow (25°)-to-flat dipping interface in the north and 40°-dipping interface in the south. We find a significant slab-dip difference (up to 10°) between our results and previously published slab models along the profile region sampling the normal-dip slab at depth (>100 km). Robust features in both P- and S-wave tomography inversions are dipping low-velocity slabs down to 100 km transitioning to higher-velocities at 100-140 km in both flat slab and dipping slab regions. Differences in the velocities of the mantle wedge between the two regions may indicate different hydration states in the wedge.

Subduction disfigured mantle plumes: Plumes that are not plumes?

NASA Astrophysics Data System (ADS)

Druken, K. A.; Stegman, D. R.; Kincaid, C. R.; Griffiths, R. W.

2012-12-01

"Hotspot" volcanism is generally attributed to upwelling of anomalously warm mantle plumes, the intra-plate Hawaiian island chain and its simple age progression serving as an archetypal example. However, interactions of such plumes with plate margins, and in particular with subduction zones, is likely to have been a common occurrence and leads to more complicated geological records. Here we present results from a series of complementary, three-dimensional numerical and laboratory experiments that examine the dynamic interaction between negatively buoyant subducting slabs and positively buoyant mantle plumes. Slab-driven flow is shown to significantly influence the evolution and morphology of nearby plumes, which leads to a range of deformation regimes of the plume head and conduit. The success or failure of an ascending plume head to reach the lithosphere depends on the combination of plume buoyancy and position within the subduction system, where the mantle flow owing to downdip and rollback components of slab motion entrain plume material both vertically and laterally. Plumes rising within the sub-slab region tend to be suppressed by the surrounding flow field, while wedge-side plumes experience a slight enhancement before ultimately being entrained by subduction. Hotspot motion is more complex than that expected at intraplate settings and is primarily controlled by position alone. Regimes include severely deflected conduits as well as retrograde (corkscrew) motion from rollback-driven flow, often with weak and variable age-progression. The interaction styles and surface manifestations of plumes can be predicted from these models, and the results have important implications for potential hotspot evolution near convergent margins.

Evolution of the Grenada and Tobago basins and the onset of the Lesser Antilles subduction zone

NASA Astrophysics Data System (ADS)

Zitter, T. A. C.; Rangin, C.

2012-04-01

The Lesser Antilles active island arc marks the eastern boundary of the Caribbean plate, where the Atlantic oceanic crust is subducted. Geodynamic history of the Grenada and Tobago basins, accepted as both the back arc and fore arc basins respectively for this convergent zone, is the key for a better understanding of the Antilles arc subduction onset. Still, recent studies propose that these two basins formed as a single paleogene depocenter. Analysis of industrial and academical seismic profiling supports this hypothesis, and shows these basins are two half-graben filled by 15 kilometers of cenozoic sediments. The seismic profiles across these basins, and particularly the Geodinos Bolivar seismic profiles, indicate that the Antilles magmatic arc develops in the midst of the previously-extended Grenada-Tobago basin from Miocene time to present. The pre-cenozoic basement of the Grenada-Tobago basin can be traced from the Aves ridge to the Tobago Island where cretaceous meta-volcanic rocks are cropping out. Therefore, this large basin extension has been initiated in early Paleocene time during stretching or subsidence of the great cretaceous Caribbean arc and long time before the onset of the lesser Antilles volcanic arc. The question arises for the mechanism responsible of this intra-plate extension. The Tobago Ridge consists of the backstop of the Barbados prism. The innermost wedge is particularly well imaged on seismic data along the Darien Ridge, where the isopach paleogene sediments are jointly deformed in latest Oligocene. This deformation is starved with the early miocene piggy-back basin. Hence, we conclude the innermost wedge in contact with the butresss is late Oligocene in age and can be considered as the onset of the subduction along the Antilles arc. This 30 Ma subduction onset is also supported by the 750 km long Atlantic slab, imaged in tomography, indicating this subduction was active with constant velocity of 2.5 km/yr. Consequently, another

Intrusive rocks of the Wadi Hamad Area, North Eastern Desert, Egypt: Change of magma composition with maturity of Neoproterozoic continental island arc and the role of collisional plutonism in the differentiation of arc crust

NASA Astrophysics Data System (ADS)

Basta, Fawzy F.; Maurice, Ayman E.; Bakhit, Bottros R.; Azer, Mokhles K.; El-Sobky, Atef F.

2017-09-01

The igneous rocks of the Wadi Hamad area are exposed in the northernmost segment of the Arabian-Nubian Shield (ANS). These rocks represent part of crustal section of Neoproterozoic continental island arc which is intruded by late to post-collisional alkali feldspar granites. The subduction-related intrusives comprise earlier gabbro-diorites and later granodiorites-granites. Subduction setting of these intrusives is indicated by medium- to high-K calc-alkaline affinity, Ta-Nb troughs on the spider diagrams and pyroxene and biotite compositions similar to those crystallized from arc magmas. The collisional alkali feldspar granites have high-K highly fractionated calc-alkaline nature and their spider diagrams almost devoid of Ta-Nb troughs. The earlier subduction gabbro-diorites have lower alkalis, LREE, Nb, Zr and Hf values compared with the later subduction granodiorites-granites, which display more LILE-enriched spider diagrams with shallower Ta-Nb troughs, reflecting variation of magma composition with arc evolution. The later subduction granitoids were generated by lower degree of partial melting of mantle wedge and contain higher arc crustal component compared with the earlier subduction gabbro-diorites. The highly silicic alkali feldspar granites represent extensively evolved melts derived from partial melting of intermediate arc crustal sources during the collisional stage. Re-melting of arc crustal sources during the collisional stage results in geochemical differentiation of the continental arc crust and the silicic collisional plutonism drives the composition of its upper part towards that of mature continental crust.

Temperature Models for the Mexican Subduction Zone

NASA Astrophysics Data System (ADS)

Manea, V. C.; Kostoglodov, V.; Currie, C.; Manea, M.; Wang, K.

2002-12-01

It is well known that the temperature is one of the major factors which controls the seismogenic zone. The Mexican subduction zone is characterized by a very shallow flat subducting interplate in its central part (Acapulco, Oaxaca), and deeper subduction slabs northern (Jalisco) and southern (Chiapas). It has been proposed that the seismogenic zone is controlled, among other factors, by a temperature. Therefore, we have developed four two-dimensional steady state thermal models for Jalisco, Guerrero, Oaxaca and Chiapas. The updip limit of the seismogenic zone is taken between 100 §C and 150 §C, while the downdip limit is thought to be at 350 §C because of the transition from stick-slip to stable-sliding. The shape of the subducting plate is inferred from gravity and seismicity. The convergence velocity between oceanic and continental lithospheric plates is taken as the following: 5 cm/yr for Jalisco profile, 5.5 for Guerrero profile, 5.8 for Oaxaca profile, and 7.8 for Chiapas profile. The age of the subducting plates, since they are young, and provides the primary control on the forearc thermal structure, are as the following: 11 My for Jalisco profile, 14.5 My for Guerrero profile, 15 My for Oaxaca profile, and 28 My for Chiapas profile. We also introduced in the models a small quantity of frictional heating (pore pressure ratio 0.98). The value of 0.98 for pore pressure ratio was obtained for the Guerrero profile, in order to fit the intersection between the 350 §C isotherm and the subducting plate at 200 Km from trench. The value of 200 km coupling zone from trench is inferred from GPS data for the steady interseismic period and also for the last slow aseismic slip that occurred in Guerrero in 2002. We have used this value of pore pressure ratio (0.98) for all the other profiles. For the others three profiles we obtained the following coupling extents: Jalisco - 100 km, Oaxaca - 170 km and Chiapas - 125 km (from the trench). Independent constrains of the

Incorporating Cutting Edge Scientific Results from the Margins-Geoprisms Program into the Undergraduate Curriculum: The Subduction Factory

NASA Astrophysics Data System (ADS)

Penniston-Dorland, S.; Stern, R. J.; Edwards, B. R.; Kincaid, C. R.

2014-12-01

The NSF-MARGINS Program funded a decade of research on continental margin processes. The NSF-GeoPRISMS Mini-lesson Project, funded by NSF-TUES, is designed to integrate fundamental results from the MARGINS program into open-source college-level curriculum. Three Subduction Factory (SubFac) mini-lessons were developed as part of this project. These include hands-on examinations of data sets representing 3 key components of the subduction zone system: 1) Heat transfer in the subducted slab; 2) Metamorphic processes happening at the plate interface; and 3) Typical magmatic products of arc systems above subduction zones. Module 1: "Slab Temperatures Control Melting in Subduction Zones, What Controls Slab Temperature?" allows students to work in groups using beads rolling down slopes as an analog for the mathematics of heat flow. Using this hands-on, exploration-based approach, students develop an intuition for the mathematics of heatflow and learn about heat conduction and advection in the subduction zone environment. Module 2: "Subduction zone metamorphism" introduces students to the metamorphic rocks that form as the subducted slab descends and the mineral reactions that characterize subduction-related metamorphism. This module includes a suite of metamorphic rocks available for instructors to use in a lab, and exercises in which students compare pressure-temperature estimates obtained from metamorphic rocks to predictions from thermal models. Module 3: "Central American Arc Volcanoes, Petrology and Geochemistry" introduces students to basic concepts in igneous petrology using the Central American volcanic arc, a MARGINS Subduction Factory focus site, as an example. The module relates data from two different volcanoes - basaltic Cerro Negro (Nicaragua) and andesitic Ilopango (El Salvador) including hand sample observations and major element geochemistry - to explore processes of mantle and crustal melting and differentiation in arc volcanism.

Subduction starts by stripping slabs

NASA Astrophysics Data System (ADS)

Soret, Mathieu; Agard, Philippe; Dubacq, Benoît; Prigent, Cécile; Plunder, Alexis; Yamato, Philippe; Guillot, Stéphane

2017-04-01

Metamorphic soles correspond to tectonic slices welded beneath most large-scale ophiolites. These slivers of oceanic crust metamorphosed up to granulite facies conditions are interpreted as having formed during the first My of intra-oceanic subduction from heat transfer from the incipient mantle wedge towards the top of the subducting plate. Our study reappraises the formation of metamorphic sole through detailed field and petrological work on three classical key sections across the Semail ophiolite (Oman and United Arab Emirates). Geothermobarometry and thermodynamic modelling show that metamorphic soles do not record a continuous temperature gradient, as expected from simple heating by the upper plate or by shear heating and proposed by previous studies. The upper, high-temperature metamorphic sole is subdivided in at least two units, testifying to the stepwise formation, detachment and accretion of successive slices from the downgoing slab to the mylonitic base of the ophiolite. Estimated peak pressure-temperature conditions through the metamorphic sole are, from top to bottom, 850˚C - 1GPa, 725°C - 0.8 GPa and 530°C - 0.5 GPa. These estimates appear constant within each unit but separated by a gap of 100 to 200˚C and 0.2 GPa. Despite being separated by hundreds of kilometres below the Semail ophiolite and having contrasting locations with respect to the ophiolite ridge axis, metamorphic soles show no evidence for significant petrological variations along strike. These constraints allow to refine the tectonic-petrological model for the genesis of metamorphic soles, formed through the stepwise stacking of several homogeneous slivers of oceanic crust and its sedimentary cover. Metamorphic soles do not so much result from downward heat transfer (ironing effect) but rather from progressive metamorphism during strain localization and cooling of the plate interface. The successive thrusts are the result of rheological contrasts between the sole (initially at the

Multidisciplinary approach for the characterization of a new Late Cretaceous continental arc in the Central Pontides (Northern Turkey)

NASA Astrophysics Data System (ADS)

Ellero, Alessandro; Ottria, Giuseppe; Sayit, Kaan; Catanzariti, Rita; Frassi, Chiara; Cemal Göncüoǧlu, M.; Marroni, Michele; Pandolfi, Luca

2016-04-01

In the Central Pontides (Northern Turkey), south of Tosya, a tectonic unit consisting of not-metamorphic volcanic rocks and overlying sedimentary succession is exposed inside a fault-bounded elongated block. It is restrained within a wide shear zone, where the Intra-Pontide suture zone, the Sakarya terrane and the Izmir-Ankara-Erzincan suture zone are juxtaposed as result of strike-slip activity of the North Anatolian shear zone. The volcanic rocks are mainly basalts and basaltic andesites (with their pyroclastic equivalents) associated with a volcaniclastic formation made up of breccias and sandstones that are stratigraphically overlain by a Marly-calcareous turbidite formation. The calcareous nannofossil biostratigraphy points to a late Santonian-middle Campanian age (CC17-CC21 Zones) for the sedimentary succession. The geochemistry of the volcanic rocks reveals an active continental margin setting as evidenced by the enrichment in Th and LREE over HFSE, and the Nb-enriched nature of these lavas relative to N-MORB. As highlighted by the performed arenite petrography, the occurrence of continent-derived clastics in the sedimentary succession supports the hypothesis of a continental arc-derived volcanic succession. Alternative geodynamic reconstructions are proposed, where this tectonic unit could represent a slice derived from the northern continental margin of the Intra- Pontide or Izmir-Ankara-Erzincan oceanic basins.

Subduction metamorphism in the Himalayan ultrahigh-pressure Tso Morari massif: An integrated geodynamic and petrological modelling approach

NASA Astrophysics Data System (ADS)

Palin, Richard M.; Reuber, Georg S.; White, Richard W.; Kaus, Boris J. P.; Weller, Owen M.

2017-06-01

The Tso Morari massif is one of only two regions where ultrahigh-pressure (UHP) metamorphism of subducted crust has been documented in the Himalayan Range. The tectonic evolution of the massif is enigmatic, as reported pressure estimates for peak metamorphism vary from ∼2.4 GPa to ∼4.8 GPa. This uncertainty is problematic for constructing large-scale numerical models of the early stages of India-Asia collision. To address this, we provide new constraints on the tectonothermal evolution of the massif via a combined geodynamic and petrological forward-modelling approach. A prograde-to-peak pressure-temperature-time (P-T-t) path has been derived from thermomechanical simulations tailored for Eocene subduction in the northwestern Himalaya. Phase equilibrium modelling performed along this P-T path has described the petrological evolution of felsic and mafic components of the massif crust, and shows that differences in their fluid contents would have controlled the degree of metamorphic phase transformation in each during subduction. Our model predicts that peak P-T conditions of ∼2.6-2.8 GPa and ∼600-620 ∘C, representative of 90-100 km depth (assuming lithostatic pressure), could have been reached just ∼3 Myr after the onset of subduction of continental crust. This P-T path and subduction duration correlate well with constraints reported for similar UHP eclogite in the Kaghan Valley, Pakistan Himalaya, suggesting that the northwest Himalaya contains dismembered remnants of what may have been a ∼400-km-long UHP terrane comparable in size to the Western Gneiss Region, Norway, and the Dabie-Sulu belt, China. A maximum overpressure of ∼0.5 GPa was calculated in our simulations for a homogeneous crust, although small-scale mechanical heterogeneities may produce overpressures that are larger in magnitude. Nonetheless, the extremely high pressures for peak metamorphism reported by some workers (up to 4.8 GPa) are unreliable owing to conventional thermobarometry

The relationship between continental collision process and metamorphic pattern in the Himalayan collision belts

NASA Astrophysics Data System (ADS)

Oh, Chang-Whan

2015-04-01

Both UHP and HP eclogites are reported from the Kaghan Valley and Tso Morari Massif in the western part of the Himalayan collision belt (Ghazanfar and Chaudhry, 1987; Thakur, 1983). UHP eclogites in the Kaghan record peak metamorphic conditions of 770 °C and 30 kbar (O'Brien et al., 2001) and was retrograded into the epidote-amphibolite or blueschist (580-610 °C, 10-13 kbar; Lombardo and Rolfo, 2000). Sensitive high-resolution ion microprobe dating of zircon reveals that the UHP eclogite formed at ca. 46 Ma (Kaneko et al., 2003; Parrish et al., 2006). The Tso Morari UHP eclogite had formed at 750 °C, > 39 kbar (Mukheerjee et al., 2003; Bundy, 1980) and underwent amphibolite facies retro-grade metamorphism (580 °C, 11 kbar) during uplift (Guillot et al., 2008). Peak metamorphism of the Tso Morari Massif was dated at ca. 53-55 Ma (Leech et al., 2005). Only HP eclogites have been reported from the mid-eastern part of the Himalayan collision belt (Lombardo and Rolfo, 2000; Corrie et al., 2010). The HP eclogite in the mid-eastern part may have formed at ca. > 780 °C and 20 kbar and was overprinted by high-pressure granulite facies metamorphism (780-750°C, 12-10 kbar) at ca. 30 Ma (Groppo et al. 2007; Corrie et al., 2010). HP granulite (890 °C, 17-18 kbar) is reported from the NBS, at the eastern terminus of the Himalayan collision belt; the granulite was subjected to retrograde metamorphism to produce lower-pressure granulite (875-850°C, 10-5 kbar), representing near-isothermal decompression (Liu and Zhong, 1997). The HP granulite metamorphism may have occurred at ca. 22-25 Ma. Along the Himalayan collision belt, peak metamorphism changes eastward from UHP eclogite facies through HP eclogite facies to high-pressure granulite facies, indicating a progressive eastwards decrease in the depth of subduction of continental crust and an eastwards increase in the geothermal gradient. The peak metamorphic ages also decrease from 53-46 Ma in the west to 22-25 Ma in the

Propagation of back-arc extension in the arc of the southern New Hebrides Subduction Zone (South West Pacific) and possible relation to subduction initiation.

NASA Astrophysics Data System (ADS)

Fabre, M.; Patriat, M.; Collot, J.; Danyushevsky, L. V.; Meffre, S.; Falloon, T.; Rouillard, P.; Pelletier, B.; Roach, M. J.; Fournier, M.

2015-12-01

Geophysical data acquired during three expeditions of the R/V Southern Surveyor allows us to characterize the deformation of the upper plate at the southern termination of the New Hebrides subduction zone where it bends 90° eastward along the Hunter Ridge. As shown by GPS measurements and earthquake slip vectors systematically orthogonal to the trench, this 90° bend does not mark a transition from subduction to strike slip as usually observed at subduction termination. Here the convergence direction remains continuously orthogonal to the trench notwithstanding its bend. Multibeam bathymetric data acquired in the North Fiji Basin reveals active deformation and fragmentation of the upper plate. It shows the southward propagation of a N-S back-arc spreading ridge into the pre-existing volcanic arc, and the connection of the southern end of the spreading axis with an oblique active rift in the active arc. Ultimately the active arc lithosphere is sheared as spreading progressively supersedes rifting. Consequently to such incursion of back-arc basin extension into the arc, peeled off and drifted pieces of arc crust are progressively isolated into the back-arc basin. Another consequence is that the New Hebrides arc is split in two distinct microplates, which move independently relative to the lower plate, and thereby define two different subduction systems. We suggest arc fragmentation could be a consequence of the incipient collision of the Loyalty Ridge with the New Hebrides Arc. We further speculate that this kinematic change could have resulted, less than two million year ago, in the initiation of a new subduction orthogonal to the New Hebrides Subduction possibly along the paleo STEP fault. In this geodynamic setting, with an oceanic lithosphere subducting beneath a sheared volcanic arc, a particularly wide range of primitive subduction-related magmas have been produced including adakites, island arc tholeiites, back-arc basin basalts, and medium-K subduction

Subduction obliquity as a prime indicator for geotherm in subduction zone

NASA Astrophysics Data System (ADS)

Plunder, Alexis; Thieulot, Cédric; van Hinsbergen, Douwe

2016-04-01

The geotherm of a subduction zone is thought to vary as a function of subduction rate and the age of the subducting lithosphere. Along a single subduction zone the rate of subduction can strongly vary due to changes in the angle between the trench and the plate convergence vector, namely the subduction obliquity. This phenomenon is observed all around the Pacific (i.e., Marianna, South America, Aleutian…). However due to observed differences in subducting lithosphere age or lateral convergence rate in nature, the quantification of temperature variation due to obliquity is not obvious. In order to investigate this effect, 3D generic numerical models were carried out using the finite element code ELEFANT. We designed a simplified setup to avoid interaction with other parameters. An ocean/ocean subduction setting was chosen and the domain is represented by a 800 × 300 × 200 km Cartesian box. The trench geometry is prescribed by means of a simple arc-tangent function. Velocity of the subducting lithosphere is prescribed using the analytical solution for corner flow and only the energy conservation equation is solved in the domain. Results are analysed after steady state is reached. First results show that the effect of the trench curvature on the geotherm with respect to the convergence direction is not negligible. A small obliquity yields isotherms which are very slightly deflected upwards where the obliquity is maximum. With an angle of ˜30°, the isotherms are deflected upwards of about 10 kilometres. Strong obliquity (i.e., angles from 60° to almost 90°) reveal extreme effects of the position of the isotherms. Further model will include other parameter as the dip of the slab and convergence rate to highlight their relative influence on the geotherm of subduction zone.

Possible emplacement of crustal rocks into the forearc mantle of the Cascadia Subduction Zone

USGS Publications Warehouse

Calvert, A.J.; Fisher, M.A.; Ramachandran, K.; Trehu, A.M.

2003-01-01

Seismic reflection profiles shot across the Cascadia forearc show that a 5-15 km thick band of reflections, previously interpreted as a lower crustal shear zone above the subducting Juan de Fuca plate, extends into the upper mantle of the North American plate, reaching depths of at least 50 km. In the extreme western corner of the mantle wedge, these reflectors occur in rocks with P wave velocities of 6750-7000 ms-1. Elsewhere, the forearc mantle, which is probably partially serpentinized, exhibits velocities of approximately 7500 ms-1. The rocks with velocities of 6750-7000 ms-1 are anomalous with respect to the surrounding mantle, and may represent either: (1) locally high mantle serpentinization, (2) oceanic crust trapped by backstepping of the subduction zone, or (3) rocks from the lower continental crust that have been transported into the uppermost mantle by subduction erosion. The association of subparallel seismic reflectors with these anomalously low velocities favours the tectonic emplacement of crustal rocks. Copyright 2003 by the American Geophysical Union.

Kinematics of Late Cretaceous subduction initiation in the Neo-Tethys Ocean reconstructed from ophiolites of Turkey, Cyprus, and Syria

NASA Astrophysics Data System (ADS)

Maffione, Marco; van Hinsbergen, Douwe J. J.; de Gelder, Giovanni I. N. O.; van der Goes, Freek C.; Morris, Antony

2017-05-01

Formation of new subduction zones represents one of the cornerstones of plate tectonics, yet both the kinematics and geodynamics governing this process remain enigmatic. A major subduction initiation event occurred in the Late Cretaceous, within the Neo-Tethys Ocean between Gondwana and Eurasia. Suprasubduction zone ophiolites (i.e., emerged fragments of ancient oceanic lithosphere formed at suprasubduction spreading centers) were generated during this subduction event and are today distributed in the eastern Mediterranean region along three E-W trending ophiolitic belts. Several models have been proposed to explain the formation of these ophiolites and the evolution of the associated intra-Neo-Tethyan subduction zone. Here we present new paleospreading directions from six Upper Cretaceous ophiolites of Turkey, Cyprus, and Syria, calculated by using new and published paleomagnetic data from sheeted dyke complexes. Our results show that NNE-SSW subduction zones were formed within the Neo-Tethys during the Late Cretaceous, which we propose were part of a major step-shaped subduction system composed of NNE-SSW and WNW-ESE segments. We infer that this subduction system developed within old (Triassic?) lithosphere, along fracture zones and perpendicular weakness zones, since the Neo-Tethyan spreading ridge formed during Gondwana fragmentation would have already been subducted at the Pontides subduction zone by the Late Cretaceous. Our new results provide an alternative kinematic model of Cretaceous Neo-Tethyan subduction initiation and call for future research on the mechanisms of subduction inception within old (and cold) lithosphere and the formation of metamorphic soles below suprasubduction zone ophiolites in the absence of nearby spreading ridges.

A global census of continental rift activity since 250 Ma reveals a missing element of the deep carbon cycle

NASA Astrophysics Data System (ADS)

Brune, Sascha; Williams, Simon; Müller, Dietmar

2017-04-01

The deep carbon cycle connects CO2 concentrations within the atmosphere to the vast carbon reservoir in Earth's mantle: subducted lithosphere carries carbon into the mantle, while extensional plate boundaries and arc volcanoes release it back to Earth's surface. The length of plate boundaries thereby exerts first-order control on global CO2 fluxes on geological time scales. Here we provide a global census of rift length from the Triassic to present day, combining a new plate reconstruction analysis technique with data from the geological rift record. We find that the most extensive rift phase during the fragmentation of Pangea occurred in the Jurassic/Early Cretaceous with extension along the South Atlantic (9700 km) and North Atlantic rifts (9100 km), within East Gondwana (8500 km), the failed African rift systems (4900 km), and between Australia and Antarctica (3700 km). The combined extent of these and other rift systems amounts to more than 50.000 km of simultaneously active continental rifts. During the Late Cretaceous, in the aftermath of this massive rift episode, the global rift length dropped by 60% to 20.000 km. We further show that a second pronounced rift episode starts in the Eocene with global rift lengths of up to 30.000 km. It is well-accepted that volcanoes at plate boundaries release large amounts of CO2 from the Earth's interior. Recent work, however, highlights the importance of deep-cutting faults and diffuse degassing on CO2 emissions in the East African Rift, which appear to be comparable to CO2 release rates at mid-ocean ridges worldwide. Upscaling measured CO2 fluxes from East Africa to all concurrently active global rift zones with due caution, we compute the first-order history of cumulative rift-related CO2 degassing rates for the last 250 Myr. We demonstrate that rift-related CO2 release in the Early Cretaceous may have reached 400% of present-day rates. In first-order agreement with paleo-atmospheric CO2 concentrations from proxy

A model of diffuse degassing at three subduction-related volcanoes

NASA Astrophysics Data System (ADS)

Williams-Jones, Glyn; Stix, John; Heiligmann, Martin; Charland, Anne; Sherwood Lollar, Barbara; Arner, N.; Garzón, Gustavo V.; Barquero, Jorge; Fernandez, Erik

Radon, CO2 and δ13C in soil gas were measured at three active subduction-related stratovolcanoes (Arenal and Poás, Costa Rica; Galeras, Colombia). In general, Rn, CO2 and δ13C values are higher on the lower flanks of the volcanoes, except near fumaroles in the active craters. The upper flanks of these volcanoes have low Rn concentrations and light δ13C values. These observations suggest that diffuse degassing of magmatic gas on the upper flanks of these volcanoes is negligible and that more magmatic degassing occurs on the lower flanks where major faults and greater fracturing in the older lavas can channel magmatic gases to the surface. These results are in contrast to findings for Mount Etna where a broad halo of magmatic CO2 has been postulated to exist over much of the edifice. Differences in radon levels among the three volcanoes studied here may result from differences in age, the degree of fracturing and faulting, regional structures or the level of hydrothermal activity. Volcanoes, such as those studied here, act as plugs in the continental crust, focusing magmatic degassing towards crater fumaroles, faults and the fractured lower flanks.

Multi-stage mixing in subduction zone: Application to Merapi volcano, Indonesia

NASA Astrophysics Data System (ADS)

Debaille, V.; Doucelance, R.; Weis, D.; Schiano, P.

2003-04-01

Basalts sampling subduction zone volcanism (IAB) often show binary mixing relationship in classical Sr-Nd, Pb-Pb, Sr-Pb isotopic diagrams, generally interpreted as reflecting the involvement of two components in their source. However, several authors have highlighted the presence of minimum three components in such a geodynamical context: mantle wedge, subducted and altered oceanic crust and subducted sediments. The overlying continental crust can also contribute by contamination and assimilation in magma chambers and/or during magma ascent. Here we present a multi-stage model to obtain a two end-member mixing from three components (mantle wedge, altered oceanic crust and sediments). The first stage of the model considers the metasomatism of the mantle wedge by fluids and/or melts released by subducted materials (altered oceanic crust and associated sediments), considering mobility and partition coefficient of trace elements in hydrated fluids and silicate melts. This results in the generation of two distinct end-members, reducing the number of components (mantle wedge, oceanic crust, sediments) from three to two. The second stage of the model concerns the binary mixing of the two end-members thus defined: mantle wedge metasomatized by slab-derived fluids and mantle wedge metasomatized by sediment-derived fluids. This model has been applied on a new isotopic data set (Sr, Nd and Pb, analyzed by TIMS and MC-ICP-MS) of Merapi volcano (Java island, Indonesia). Previous studies have suggested three distinct components in the source of indonesian lavas: mantle wedge, subducted sediments and altered oceanic crust. Moreover, it has been shown that crustal contamination does not significantly affect isotopic ratios of lavas. The multi-stage model proposed here is able to reproduce the binary mixing observed in lavas of Merapi, and a set of numerical values of bulk partition coefficient is given that accounts for the genesis of lavas.

Flat-slab subduction, whole crustal faulting, and geohazards in Alaska: Targets for Earthscope

NASA Astrophysics Data System (ADS)

Gulick, S. P.; Pavlis, T. L.; Bruhn, R. L.; Christeson, G. L.; Freymueller, J. T.; Hansen, R. A.; Koons, P. O.; Pavlis, G. L.; Roeske, S.; Reece, R.; van Avendonk, H. J.; Worthington, L. L.

2010-12-01

Crustal structure and evolution illuminated by the Continental Dynamics ST. Elias Erosion and tectonics Project (STEEP) highlights some fundamental questions about active tectonics processes in Alaska including: 1) what are the controls on far field deformation and lithospheric stabilization, 2) do strike slip faults extend through the entire crust and upper mantle and how does this influence mantle flow, and 3) how does the transition from “normal” subduction of the Pacific along the Aleutians to flat slab subduction of the Yakutat Terrane beneath southeast and central Alaska to translation of the Yakutat Terrane past North American in eastern Alaska affect geohazard assessment for the north Pacific? Active and passive seismic studies and geologic fieldwork focusing on the Yakutat Terrane show that the Terrane ranges from 15-35 km thick and is underthrusting the North American plate from the St. Elias Mountains to the Alaska Range (~500 km). Deformation of the upper plate occurs within the offshore Pamplona Zone fold and thrust belt, and onshore throughout the Robinson Mountains. Deformation patterns, structural evolution, and the sedimentary products of orogenesis are fundamentally influenced by feedbacks with glacial erosion. The Yakutat megathrust extends beneath Prince William Sound such that the 1964 Mw 9.2 great earthquake epicenter was on this plate boundary and jumped to the adjacent Aleutian megathrust coseismically; this event illuminates the potential for transitional tectonic systems to enhance geohazards. The northern, southern, and eastern limits of the Yakutat microplate are strike-slip faults that, where imaged, appear to cut the entire crustal section and may allow for crustal extrusion towards the Bering Sea. Yakutat Terrane effects on mantle flow, however, have been suggested to cross these crustal features to allow for far-field deformation in the Yukon, Brooks Range, and Amerasia Basin. From the STEEP results it is clear that the Yakutat

Seafloor Structural Geomorphic Evolution in Response to Seamount Subduction, Poverty Bay Indentation, New Zealand

NASA Astrophysics Data System (ADS)

Bodger, K. L.; Pettinga, J. R.; Barnes, P. M.

2006-12-01

More than 4000 km2 of high quality bathymetric and backscatter imaging of the Poverty Bay Indentation across the northern part of the Hikurangi subduction zone provide new insights into the relationship between seafloor morphology and active structures. The swath bathymetry extends from the edge of the continental shelf to the abyssal plain, at depths of between 100 to 3500 metres. The origin of the slope re-entrant is inferred to be related to multiple seamount impacts, and these collisions have initiated numerous large-scale gravitational collapse structures, multiple debris flow and avalanche deposits, which range in down-slope length from a few hundred metres to more than 40 km. The Poverty Bay Indentation has been simultaneously eroded by canyon systems that exhibit many of the features of incised river systems onshore. The swath images are complemented by the availability of excellent high-quality processed multi-channel seismic reflection data, single channel high-resolution 3.5 kHz seismic reflection data, as well as a limited number of core samples. Seismic reflection profiles and seafloor morphology are used to provide three morpho-structural sections. The comparison of these sections highlights the different effects of seamount subduction on the evolution of the margin and the re-entrant. The northern two sections are located to the north side of the re-entrant and reveal the role of seamount impact on the interrelationship between the structural evolution with respect to seafloor morphology. Here the development of an over-steepened margin with fault reactivation, inversion and over- printing leads to very complex structural styles of deformation and geometry in both seismic reflection profiles and seafloor morphology. There is evidence of an older, inactive thrust front buried beneath the upper and mid- slope basins. Beneath the mid-slope a subducted seamount is revealed by the presence of relief on the subduction interface and associated structural

Self-Consistent Generation of Continental Crust in Global Mantle Convection Models

NASA Astrophysics Data System (ADS)

Jain, C.; Rozel, A. B.; Tackley, P.

2016-12-01

Numerical modelling commonly shows that mantle convection and continents have strong feedbacks on each other (Philips and Coltice, JGR 2010; Heron and Lowman, JGR 2014), but the continents are always inserted a priori while basaltic (oceanic) crust is generated self-consistently in such models (Rolf et al., EPSL 2012). We aim to implement self-consistent generation of continental crust in global models of mantle convection using StagYY (Tackley, PEPI 2008). The silica-rich continental crust appears to have been formed by fractional melting and crystallization in episodes of relatively rapid growth from late Archaean to late Proterozoic eras (3-1 Ga) (Hawkesworth & Kemp, Nature 2006). It takes several stages of differentiation to generate continental crust. First, the basaltic magma is extracted from the pyrolitic mantle. Second, it goes through eclogitic transformation and then partially melts to form Na-rich Tonalite-Trondhjemite-Granodiorite (TTG) which rise to form proto-continents (Rudnick, Nature 1995; Herzberg & Rudnick, Lithos 2012). TTGs dominate the grey gneiss complexes which make up most of the continental crust. Based on the melting conditions proposed by Moyen (Lithos, 2011), we parameterize TTG formation and henceforth, the continental crust. Continental crust can also be destroyed by subduction or delamination. We will investigate continental growth and destruction history in the models spanning the age of the Earth.

Self-consistent generation of continental crust in global mantle convection models

NASA Astrophysics Data System (ADS)

Jain, Charitra; Rozel, Antoine; Tackley, Paul

2017-04-01

Numerical modeling commonly shows that mantle convection and continents have strong feedbacks on each other (Philips and Coltice, JGR 2010; Heron and Lowman, JGR 2014), but the continents are always inserted a priori while basaltic (oceanic) crust is generated self-consistently in such models (Rolf et al., EPSL 2012). We aim to implement self-consistent generation of continental crust in global models of mantle convection using StagYY (Tackley, PEPI 2008). The silica-rich continental crust appears to have been formed by fractional melting and crystallization in episodes of relatively rapid growth from late Archean to late Proterozoic eras (3-1 Ga) (Hawkesworth & Kemp, Nature 2006). It takes several stages of differentiation to generate continental crust. First, the basaltic magma is extracted from the pyrolitic mantle. Second, it goes through eclogitic transformation and then partially melts to form Na-rich Tonalite-Trondhjemite-Granodiorite (TTG) which rise to form proto-continents (Rudnick, Nature 1995; Herzberg & Rudnick, Lithos 2012). TTGs dominate the grey gneiss complexes which make up most of the continental crust. Based on the melting conditions proposed by Moyen (Lithos, 2011), we parameterize TTG formation and henceforth, the continental crust. Continental crust can also be destroyed by subduction or delamination. We will investigate continental growth and destruction history in the models spanning the age of the Earth.

Paleo movement of continents since 300 Ma, mantle dynamics and large wander of the rotational pole

NASA Astrophysics Data System (ADS)

Greff-Lefftz, Marianne; Besse, Jean

2012-09-01

Apparent polar wander (APW) is known to be mainly linked to internal mass distribution changes and in particular to changes in subduction and large-scale upwellings in the mantle. We investigate plate motions during the last 410 million years in a reference frame where Africa is fixed. Indeed, Africa has remained a central plate from which most continents diverged since the break-up of Pangea. The exact amount of subduction is unknown prior to 120 Ma. We propose an approach, based on one hand on the study of the past subduction volcanism to locate ancient subduction activity, and on the other hand microplate motion history in the Tethyan area derived from geology and paleomagnetism. The peri-Pacific subductions seem to be a quasi-permanent feature of the Earth's history at least since the Paleozoic, with however localized interruptions. The “Tethyan” subductions have a complex history with successive collisions of continental blocs (Hercynian, Indo-Sinian, Alpine and Himalayan) and episodical rebirth of E-W subduction trending zones. Assuming that subducted slabs sink vertically into the mantle and taking into account large-scale upwellings derived from present-day tomography and intra-plate volcanism in the past, we compute the time variation of mantle density heterogeneities since 280 Ma. Due to conservation of the angular momentum of the Earth, the temporal evolution of the rotational axis is computed in a mantle reference frame where the Africa plate is fixed, and compared to the apparent polar wander (APW) observed by paleomagnetism since 280 Ma. We find that a major trend of both paleomagnetic and computed APW are successive oscillatory clockwise or counter-clockwise motions, with tracks separated by abrupt cusps (around 230 Ma, 190 Ma and 140-110 Ma). We find that cusps result from earlier major geodynamic events: the 230 Ma cusp is related to the end of active subduction due to the closure of the Rheic Ocean basin after the Hercynian continental

New zircon U-Pb LA-ICP-MS ages and Hf isotope data from the Central Pontides (Turkey): Geological and geodynamic constraints

NASA Astrophysics Data System (ADS)

Çimen, Okay; Göncüoğlu, M. Cemal; Simonetti, Antonio; Sayit, Kaan

2018-05-01

The Central Pontides in northern Anatolia is located on the accretionary complex formed by the closure of Neotethyan Intra-Pontide Ocean between the southern Eurasian margin (Istanbul-Zonguldak Terrane) and the Cimmerian Sakarya Composite Terrane. Among other components of the oceanic lithosphere, it comprises not yet well-dated felsic igneous rocks formed in arc-basin as well as continent margin settings. In-situ U-Pb age results for zircons from the arc-basin system (öangaldağ Metamorphic Complex) and the continental arc (Devrekani Metadiorite and Granitoid) yield ages of 176 ± 6 Ma, 163 ± 9 Ma and 165 ± 3 Ma, respectively. Corresponding in-situ average (initial) 176Hf/177Hf initial ratios are 0.28261 ± 0.00003, 0.28267 ± 0.00002 and 0.28290 ± 0.00004 for these units and indicative of a subduction-modified mantle source. The new U-Pb ages and Hf isotope data from these oceanic and continental arc units together with regional geological constraints support the presence of a multiple subduction system within the Intra-Pontide Ocean during the Middle Jurassic.

Multichannel Seismic Imaging of the Rivera Plate Subduction at the Seismogenic Jalisco Block Area (Western Mexican Margin)

NASA Astrophysics Data System (ADS)

Bartolome, R.; Gorriz, E.; Danobeitia, J.; Barba, D. C., Sr.; Martí, D.; L Cameselle, A.; Nuñez-Cornu, F. J.; Bandy, W. L.; Mortera, C.; Nunez, D.; Alonso, J. L.; Castellon, A.; Prada, M.

2016-12-01

During the TSUJAL marine geophysical survey, conducted in February and March 2014 Spanish, Mexican and British scientists and technicians explored the western margin of Mexico, considered one of the most active seismic zones in America. This work aims to characterize the internal structure of the subduction zone of the Rivera plate beneath the North American plate in the offshore part of the Jalisco Block, to link the geodynamic and the recent tectonic deformation occurring there with the possible generation of tsunamis and earthquakes. For this purpose, it has been carried out acquisition, processing and geological interpretation of a multichannel seismic reflection profile running perpendicular to the margin. Crustal images show an oceanic domain, dominated by subduction-accretion along the lower slope of the margin with a subparallel sediment thickness of up to 1.6 s two way travel time (approx. 2 km) in the Middle American Trench. Further, from these data the region appears to be prone to giant earthquake production. The top of the oceanic crust (intraplate reflector) is very well imaged. It is almost continuous along the profile with a gentle dip (<10°); however, it is disrupted by normal faulting resulting from the bending of the plate during subduction. The continental crust presents a well-developed accretionary prism consisting of highly deformed sediments with prominent slumping towards the trench that may be the result of past tsunamis. Also, a Bottom Simulating Reflector (BSR) is identified in the first half a second (twtt) of the section. High amplitude reflections at around 7-8 s twtt clearly image a discontinuous Moho, defining a very gentle dipping subduction plane.

Multichannel Seismic Imaging of the Rivera Plate Subduction at the Seismogenic Jalisco Block Area (Western Mexican Margin)

NASA Astrophysics Data System (ADS)

Bartolome, Rafael; Górriz, Estefanía; Dañobeitia, Juanjo; Cordoba, Diego; Martí, David; Cameselle, Alejandra L.; Núñez-Cornú, Francisco; Bandy, William L.; Mortera-Gutiérrez, Carlos A.; Nuñez, Diana; Castellón, Arturo; Alonso, Jose Luis

2016-10-01

During the TSUJAL marine geophysical survey, conducted in February and March 2014, Spanish, Mexican and British scientists and technicians explored the western margin of Mexico, considered one of the most active seismic zones in America. This work aims to characterize the internal structure of the subduction zone of the Rivera plate beneath the North American plate in the offshore part of the Jalisco Block, to link the geodynamic and the recent tectonic deformation occurring there with the possible generation of tsunamis and earthquakes. For this purpose, it has been carried out acquisition, processing and geological interpretation of a multichannel seismic reflection profile running perpendicular to the margin. Crustal images show an oceanic domain, dominated by subduction-accretion along the lower slope of the margin with a subparallel sediment thickness of up to 1.6 s two-way travel time (approx. 2 km) in the Middle American Trench. Further, from these data the region appears to be prone to giant earthquake production. The top of the oceanic crust (intraplate reflector) is very well imaged. It is almost continuous along the profile with a gentle dip (<10°); however, it is disrupted by normal faulting resulting from the bending of the plate during subduction. The continental crust presents a well-developed accretionary prism consisting of highly deformed sediments with prominent slumping towards the trench that may be the result of past tsunamis. Also, a bottom simulating reflector (BSR) is identified in the first half a second (twtt) of the section. High amplitude reflections at around 7-8 s twtt clearly image a discontinuous Moho, defining a very gentle dipping subduction plane.

Plate Speed-up and Deceleration during Continental Rifting: Insights from Global 2D Mantle Convection Models.

NASA Astrophysics Data System (ADS)

Brune, S.; Ulvrova, M.; Williams, S.

2017-12-01

The surface of the Earth is divided into a jigsaw of tectonic plates, some carrrying continents that disperse and aggregate through time, forming transient supercontinents like Pangea and Rodinia. Here, we study continental rifting using large-scale numerical simulations with self-consistent evolution of plate boundaries, where continental break-up emerges spontaneously due to slab pull, basal drag and trench suction forces.We use the StagYY convection code employing a visco-plastic rheology in a spherical annulus geometry. We consider an incompressible mantle under the Boussinesq approximation that is basally and internally heated.We show that continental separation follows a characteristic evolution with three distinctive phases: (1) A pre-rift phase that typically lasts for several hundreds of millions of years with tectonic quiescence in the suture and extensional stresses that are slowly building up. (2) A rift phase that further divides into a slow rift period of several tens of millions of years where stresses continuously increase followed by a rift acceleration period featuring an abrupt stress drop within several millions of years. The speed-up takes place before lithospheric break-up and therefore affects the structural architecture of the rifted margins. (3) The drifting phase with initially high divergence rates persists over tens of millions of years until the system adjust to new conditions and the spreading typically slows down.By illustrating the geodynamic connection between subduction dynamics and rift evolution, our results allow new interpretations of plate tectonic reconstructions. Rift acceleration within the second phase of rifting is compensated by enhanced convergence rates at subduction zones. This model outcome predicts enhanced subduction velocities, e.g. between North America and the Farallon plate during Central Atlantic rifting 200 My ago, or closure of potential back-arc basins such as in the proto-Andean ranges of South America

What governs the enrichment of Pb in the continental crust? An answer from the Mexican Volcanic Belt

NASA Astrophysics Data System (ADS)

Goldstein, S. L.; Lagatta, A.; Langmuir, C. H.; Straub, S. M.; Martin-Del-Pozzo, A.

2009-12-01

One of Al Hofmann’s many important contributions to our understanding of geochemical cycling in the Earth is the observation that Pb behaves like the light rare earth elements Ce and Nd during melting to form oceanic basalts, but is enriched in the continental crust compared to the LREE by nearly an order of magnitude (Hofmann et al. 1986). This is unusual behavior, and has been called one of the Pb paradoxes, since in most cases, the ratios of elements are effectively the same in the continental crust and oceanic basalts if they show similar mantle melting behavior. One of several mechanisms suggested to mediate this special enrichment is hydrothermal circulation at ocean ridges, which preferentially transports Pb compared to the REE from the interior of the ocean crust to the surface. We confirm the importance of hydrothermal processes at the East Pacific to mediate Pb enrichment at the Trans-Mexican Volcanic Belt (TMVB, through comparison of Pb isotope and Ce/Pb ratios of TMVB lavas with sediments from DSDP Site 487 near the Middle America trench. The lavas of the Trans-Mexican Volcanic Belt include “high Nb” alkali basalts (HNAB), whose trace element patterns lack subduction signatures. The HNAB basalts and hydrothermally affected sediments from DSDP 487, form end-members that bound calcalkaline lavas from volcanoes Colima, Toluca, Popocatépetl, and Malinche in Ce/Pb versus Pb isotope space. The HNAB represent the high Ce/Pb and high Pb-isotope end-member. The hydrothermal sediments have Pb isotopes like Pacific MORB but Ce/Pb ratios typical of the arcs and the continental crust, and an order of magnitude lower than MORB. No analyzed calcalkaline lavas are have compositions outside of the bounds formed by the HNAB and the hydrothermal sediments. The Ce/Pb and Pb isotope ratios show that the calcalkaline lava compositions are inconsistent with contributions from HNAB and EPR MORB, rather the contributions are from HNAB upper mantle and subducted

Storage of fluids and melts at subduction zones detectable by seismic tomography

NASA Astrophysics Data System (ADS)

Luehr, B. G.; Koulakov, I.; Rabbel, W.; Brotopuspito, K. S.; Surono, S.

2015-12-01

During the last decades investigations at active continental margins discovered the link between the subduction of fluid saturated oceanic plates and the process of ascent of these fluids and partial melts forming a magmatic system that leads to volcanism at the earth surface. For this purpose the geophysical structure of the mantle and crustal range above the down going slap has been imaged. Information is required about the slap, the ascent paths, as well as the reservoires of fluids and partial melts in the mantle and the crust up to the volcanoes at the surface. Statistically the distance between the volcanoes of volcanic arcs down to their Wadati Benioff zone results of approximately 100 kilometers in mean value. Surprisingly, this depth range shows pronounced seismicity at most of all subduction zones. Additionally, mineralogical laboratory investigations have shown that dehydration of the diving plate has a maximum at temperature and pressure conditions we find at around 100 km depth. The ascent of the fluids and the appearance of partial melts as well as the distribution of these materials in the crust can be resolved by seismic tomographic methods using records of local natural seismicity. With these methods these areas are corresponding to lowered seismic velocities, high Vp/Vs ratios, as well as increased attenuation of seismic shear waves. The anomalies and their time dependence are controlled by the fluids. The seismic velocity anomalies detected so far are within a range of a few per cent to more than 30% reduction. But, to explore plate boundaries large and complex amphibious experiments are required, in which active and passive seismic investigations should be combined to achieve best results. The seismic station distribution should cover an area from before the trench up to far behind the volcanic chain, to provide under favorable conditions information down to 150 km depth. Findings of different subduction zones will be compared and discussed.

Geochemical and Sr-Nd-Pb-Li isotopic characteristics of volcanic rocks from the Okinawa Trough: Implications for the influence of subduction components and the contamination of crustal materials

NASA Astrophysics Data System (ADS)

Guo, Kun; Zhai, Shikui; Yu, Zenghui; Wang, Shujie; Zhang, Xia; Wang, Xiaoyuan

2018-04-01

The Okinawa Trough is an infant back-arc basin developed along the Ryukyu arc. This paper provides new major and trace element and Sr-Nd-Pb-Li isotope data of volcanic rocks in the Okinawa Trough and combines the published geochemical data to discuss the composition of magma source, the influence of subduction component, and the contamination of crustal materials, and calculate the contribution between subduction sediment and altered oceanic crust in the subduction component. The results showed that there are 97% DM and 3% EMI component in the mantle source in middle trough (MS), which have been influenced by subduction sediment. The Li-Nd isotopes indicate that the contribution of subduction sediment and altered oceanic crust in subduction component are 4 and 96%, respectively. The intermediate-acidic rocks suffer from contamination of continental crust material in shallow magma chamber during fractional crystallization. The acidic rocks in south trough have experienced more contamination of crustal material than those from the middle and north trough segments.

Partial melting of deeply subducted eclogite from the Sulu orogen in China

PubMed Central

Wang, Lu; Kusky, Timothy M.; Polat, Ali; Wang, Songjie; Jiang, Xingfu; Zong, Keqing; Wang, Junpeng; Deng, Hao; Fu, Jianmin

2014-01-01

We report partial melting of an ultrahigh pressure eclogite in the Mesozoic Sulu orogen, China. Eclogitic migmatite shows successive stages of initial intragranular and grain boundary melt droplets, which grow into a three-dimensional interconnected intergranular network, then segregate and accumulate in pressure shadow areas and then merge to form melt channels and dikes that transport magma to higher in the lithosphere. Here we show, using zircon U–Pb dating and petrological analyses, that partial melting occurred at 228–219 Myr ago, shortly after peak metamorphism at 230 Myr ago. The melts and residues are complimentarily enriched and depleted in light rare earth element (LREE) compared with the original rock. Partial melting of deeply subducted eclogite is an important process in determining the rheological structure and mechanical behaviour of subducted lithosphere and its rapid exhumation, controlling the flow of deep lithospheric material, and for generation of melts from the upper mantle, potentially contributing to arc magmatism and growth of continental crust. PMID:25517619

A Model of Continental Growth and Mantle Degassing Comparing Biotic and Abiotic Worlds

NASA Astrophysics Data System (ADS)

Höning, D.; Hansen-Goos, H.; Spohn, T.

2012-12-01

While examples for interaction of the biosphere with the atmosphere can be easily cited (e.g., production and consumption of O2), interaction between the biosphere and the solid planet and its interior is much less established. It has been argued (e.g., Rosing et al. 2006; Sleep et al, 2012) that the formation of continents could be a consequence of bioactivity harvesting solar energy through photosynthesis to help build the continents and that the mantle should carry a chemical biosignature. We present an interaction model that includes mantle convection, mantle water vapor degassing at mid-oceanic ridges and regassing through subduction zones, continental crust formation and erosion and water storage and transport in a porous oceanic crust that includes hydrous mineral phases. The mantle viscosity in this model depends on the water concentration in the mantle. We use boundary layer theory of mantle convection to parameterize the mantle convection flow rate and assume that the plate speed equals the mantle flow rate. The biosphere enters the calculation through the assumption that the continental erosion rate is enhanced by a factor of several through bioactivity and through an assumed reduction of the kinetic barrier to diagenetic and metamorphic reactions (e.g., Kim et al. 2004) in the sedimentary basins in subduction zones that would lead to increased water storage capacities. We further include a stochastic model of continent-to-continent interactions that limits the effective total length of subduction zones. We use present day parameters of the Earth and explore a phase plane spanned by the percentage of surface coverage of the Earth by continents and the total water content of the mantle. We vary the ratio of the erosion rate in a postulated abiotic Earth to the present Earth, as well as the activation barrier to diagenetic and metamorphic reactions that affect the water storage capacity of the subducting crust. We find stable and unstable fixed points in

2D/3D Numerical Models of the Taiwan Orogen: Oblique Arc-Continent Collision overlying Orthogonal Subduction Systems

NASA Astrophysics Data System (ADS)

Kanda, R. V.; Suppe, J.; Wu, J. E.

2013-12-01

Recent plate-tectonic reconstructions based on mapping of subducted slabs imaged by state-of-the-art tomographic models, and constrained by paleomagnetic data demonstrate that the Philippine Sea Plate (PSP) was originally part of the Sunda Plate (SP). These reconstructions show that the PSP has moved northward with Australia across 25° of latitude since the early Eocene (~ 43 Ma). Most of this motion of the PSP was accommodated on the north and east by overriding a southward subducting East Asian Sea (EAS) ocean basin that was contiguous with the present-day Eurasian Plate (EP). On the western margin of the PSP, this northward advance was accommodated by a N-S transform system. Ages of the Luzon volcanic arc suggest that by early Miocene (~ 15-20 Ma), the EP seafloor west of this transform started subducting eastwards, and highly obliquely, underneath a NNW moving PSP that was detached from the SP. Further, by late Miocene (~10 Ma), northward subduction of the PSP along the present Ryukyu Trench began as a result of arc-continent collision of the PSP along the Eurasian continental margin and flipping of subduction polarity due to slab break-off of the south-subducting EAS. A significant rotation of the PSP-EP convergence to the present more northwesterly direction occurred only over the last ~2 Ma. This present-day juxtaposition of orthogonal subduction polarities beneath Taiwan can be understood in terms of a margin-parallel lithospheric STEP fault, that accomplishes the progressive SW extension of the Ryukyu Trench (RT), and also marks the northern limit of the EP subduction. The torn edge of the Eurasian lithosphere is imaged tomographically. Further support for this tearing comes from our newly developed multi-resolution stress maps based on focal-mechanism inversions and the seismicity distribution. Our inferred stress orientations indicate orthogonal contact between the subducting PSP and the Eurasian lithospheres, resulting in present-day E-W strike

A change in the geodynamics of continental growth 3 billion years ago.

PubMed

Dhuime, Bruno; Hawkesworth, Chris J; Cawood, Peter A; Storey, Craig D

2012-03-16

Models for the growth of continental crust rely on knowing the balance between the generation of new crust and the reworking of old crust throughout Earth's history. The oxygen isotopic composition of zircons, for which uranium-lead and hafnium isotopic data provide age constraints, is a key archive of crustal reworking. We identified systematic variations in hafnium and oxygen isotopes in zircons of different ages that reveal the relative proportions of reworked crust and of new crust through time. Growth of continental crust appears to have been a continuous process, albeit at variable rates. A marked decrease in the rate of crustal growth at ~3 billion years ago may be linked to the onset of subduction-driven plate tectonics.

Seismic imaging of deep crustal melt sills beneath Costa Rica suggests a method for the formation of the Archean continental crust

NASA Astrophysics Data System (ADS)

Harmon, Nicholas; Rychert, Catherine A.

2015-11-01

Continental crust formed billions of years ago but cannot be explained by a simple evolution of primary mantle magmas. A multi-step process is required that likely includes re-melting of wet metamorphosed basalt at high pressures. Such a process could occur at depth in oceanic crust that has been thickened by a large magmatic event. In Central America, variations in geologically inferred, pre-existing oceanic crustal thickness beneath the arc provides an excellent opportunity to study its effect on magma storage, re-melting of meta-basalts, and the potential for creating continental crust. We use surface waves derived from ambient noise tomography to image 6% radially anisotropic structures in the thickened oceanic plateau crust of Costa Rica that likely represent deep crustal melt sills. In Nicaragua, where the arc is forming on thinner oceanic crust, we do not image these deep crustal melt sills. The presence of these deep sills correlates with more felsic arc outputs from the Costa Rican Arc suggesting pre-existing thickened crust accelerates processing of primary basalts to continental compositions. In the Archean, reprocessing thickened oceanic crust by subsequent hydrated hotspot volcanism or subduction zone volcanism may have similarly enhanced formation of early continental crust. This mechanism may have been particularly important if subduction did not initiate until 3 Ga.

Eastern Mediterranean geothermal resources and subduction dynamics

NASA Astrophysics Data System (ADS)

Roche, Vincent; Sternai, Pietro; Guillou-Frottier, Laurent; Jolivet, Laurent; Gerya, Taras

2017-04-01

The Aegean-Anatolian retreating subduction and collision zones have been investigated through 3D numerical geodynamic models involving slab rollback/tearing/breakoff constrained by, for instance, seismic tomography or anisotropy and geochemical proxies. Here, we integrate these investigations by using the well documented geothermal anomalies geothermal anomalies. First, we use 3D high-resolution thermo-mechanical numerical models to quantify the potential contribution of the past Aegean-Anatolian subduction dynamics to such present-day measured thermal anomalies. Results suggest an efficient control of subduction-related asthenospheric return flow on the regional distribution of thermal anomalies. Our quantification shows that the slab-induced shear heating at the base of the crust could partly explain the high heat flow values above the slab tear (i.e. in the Menderes Massif, Western Turkey). Second, the associated thermal signature at the base of the continental crust is used as basal thermal boundary condition for 2D crustal-scale models dedicated to the understanding of heat transfer from the abnormally hot mantle to the shallow geothermal reservoir. These models couple heat transfer and fluid flow equations with appropriate fluid and rock physical properties. Results suggest that permeable low-angle normal faults (detachments) in the back-arc region can control the bulk of the heat transport and fluid circulation patterns. We suggest that detachments can drain crustal and/or mantellic fluids up to several kilometers depths. At the basin-scale, we show that the permeability of detachments may control the reservoirs location. Temperatures at the base of detachments may be subject to protracted increase (due to anomalously high basal heat flow) through time, thereby generating dome-shaped thermal structures. These structures, usually with 20km characteristic wavelength, may reach the Moho involving lateral rheological contrasts and possibly crustal

Fast rates of subduction erosion along the Costa Rica Pacific margin: Implications for nonsteady rates of crustal recycling at subduction zones

USGS Publications Warehouse

Vannucchi, P.; Ranero, C.R.; Galeotti, S.; Straub, S.M.; Scholl, D. W.; McDougall-Ried, K.

2003-01-01

At least since the middle Miocene (???16 Ma), subduction erosion has been the dominant process controlling the tectonic evolution of the Pacific margin of Costa Rica. Ocean Drilling Program Site 1042 recovered 16.5 Ma nearshore sediment at ???3.9 km depth, ???7 km landward of the trench axis. The overlying Miocene to Quaternary sediment contains benthic foraminifera documenting margin subsidence from upper bathyal (???200 m) to abyssal (???2000 m) depth. The rate of subsidence was low during the early to middle Miocene but increased sharply in the late Miocene-early Pliocene (5-6.5 Ma) and at the Pliocene-Pleistocene boundary (2.4 Ma). Foraminifera data, bedding dip, and the geometry of slope sediment indicate that tilting of the forearc occurred coincident with the onset of rapid late Miocene subsidence. Seismic images show that normal faulting is widespread across the continental slope; however, extension by faulting only accounts for a minor amount of the post-6.5 Ma subsidence. Basal tectonic erosion is invoked to explain the subsidence. The short-term rate of removal of rock from the forearc is about 107-123 km3 Myr-1 km-1. Mass removal is a nonsteady state process affecting the chemical balance of the arc: the ocean sediment input, with the short-term erosion rate, is a factor of 10 smaller than the eroded mass input. The low 10Be concentration in the volcanic arc of Costa Rica could be explained by dilution with eroded material. The late Miocene onset of rapid subsidence is coeval with the arrival of the Cocos Ridge at the subduction zone. The underthrusting of thick and thermally younger ocean crust decreased the subduction angle of the slab along a large segment of the margin and changed the dynamic equilibrium of the margin taper. This process may have induced the increase in the rate of subduction erosion and thus the recycling of crustal material to the mantle. Copyright 2003 by the American Geophysical Union.

Earth's first stable continents did not form by subduction.

PubMed

Johnson, Tim E; Brown, Michael; Gardiner, Nicholas J; Kirkland, Christopher L; Smithies, R Hugh

2017-03-09

The geodynamic environment in which Earth's first continents formed and were stabilized remains controversial. Most exposed continental crust that can be dated back to the Archaean eon (4 billion to 2.5 billion years ago) comprises tonalite-trondhjemite-granodiorite rocks (TTGs) that were formed through partial melting of hydrated low-magnesium basaltic rocks; notably, these TTGs have 'arc-like' signatures of trace elements and thus resemble the continental crust produced in modern subduction settings. In the East Pilbara Terrane, Western Australia, low-magnesium basalts of the Coucal Formation at the base of the Pilbara Supergroup have trace-element compositions that are consistent with these being source rocks for TTGs. These basalts may be the remnants of a thick (more than 35 kilometres thick), ancient (more than 3.5 billion years old) basaltic crust that is predicted to have existed if Archaean mantle temperatures were much hotter than today's. Here, using phase equilibria modelling of the Coucal basalts, we confirm their suitability as TTG 'parents', and suggest that TTGs were produced by around 20 per cent to 30 per cent melting of the Coucal basalts along high geothermal gradients (of more than 700 degrees Celsius per gigapascal). We also analyse the trace-element composition of the Coucal basalts, and propose that these rocks were themselves derived from an earlier generation of high-magnesium basaltic rocks, suggesting that the arc-like signature in Archaean TTGs was inherited from an ancestral source lineage. This protracted, multistage process for the production and stabilization of the first continents-coupled with the high geothermal gradients-is incompatible with modern-style plate tectonics, and favours instead the formation of TTGs near the base of thick, plateau-like basaltic crust. Thus subduction was not required to produce TTGs in the early Archaean eon.

Extending Alaska's plate boundary: tectonic tremor generated by Yakutat subduction

USGS Publications Warehouse

Wech, Aaron G.

2016-01-01

The tectonics of the eastern end of the Alaska-Aleutian subduction zone are complicated by the inclusion of the Yakutat microplate, which is colliding into and subducting beneath continental North America at near-Pacific-plate rates. The interaction among these plates at depth is not well understood, and further east, even less is known about the plate boundary or the source of Wrangell volcanism. The drop-off in Wadati-Benioff zone (WBZ) seismicity could signal the end of the plate boundary, the start of aseismic subduction, or a tear in the downgoing plate. Further compounding the issue is the possible presence of the Wrangell slab, which is faintly outlined by an anemic, eastward-dipping WBZ beneath the Wrangell volcanoes. In this study, I performed a search for tectonic tremor to map slow, plate-boundary slip in south-central Alaska. I identified ∼11,000 tremor epicenters, which continue 85 km east of the inferred Pacific plate edge marked by WBZ seismicity. The tremor zone coincides with the edges of the downgoing Yakutat terrane, and tremors transition from periodic to continuous behavior as they near the aseismic Wrangell slab. I interpret tremor to mark slow, semicontinuous slip occurring at the interface between the Yakutat and North America plates. The slow slip region lengthens the megathrust interface beyond the WBZ and may provide evidence for a connection between the Yakutat slab and the aseismic Wrangell slab.

Defining Incipient Subduction by Detecting Serpentenised Mantle in the Regional Magnetic Field

NASA Astrophysics Data System (ADS)

Pires, Rui; Clark, Stuart; Reis, Rui

2017-04-01

Keywords: Subduction initiation, Incipient Subduction, Active Margins, Southeast Asia, Mantle wedge The mechanisms of subduction initiation are poorly understood. One idea is to look for incipient subduction zones in the present day and see what features are common in these zones. However, incipient subduction zones are very difficult to detect and debate surrounds particular cases as to whether they qualify as incipient or not. In the analysis conducted in this work, we use the signal of the presence of a mantle wedge in the magnetic anomaly field as an indicator of incipient subduction. Each subduction zone exhibits variations in the particular responses of the system, such as slab-dip angle, maximum earthquake depths and volcanism to various parameters. So far, attempts to reduce the system to a dominate controlling parameter have failed, probably as a result of the limited number of cases and the large variety of controlling parameters. Parameters such as down-going and overriding plate morphology and velocity, mantle flow, the presence of plumes or not, sediment transport into the trench are a few of the parameters that have been studied in the literature. However, one of the characteristics associated with a subduction zones is the presence of a mantelic wedge as a result of the partial melt of the subducting plate and the development of a mantle wedge between the subducting plate and the overriding plate. The wedge is characterised by the presence of water (coming from sediments in the down-going plate) as well as lower temperatures (because the wedge is between two relatively cold lithospheres). As a results a serpentinized mantle wedge is formed that contains hydrous minerals, of which magnetite is an example, that alter the composition and properties of this region. According to Blakely et.al. (2005), this region exhibits both higher magnetic susceptibility and lower densities than the surrounding medium. We analysed five active margin boundaries located

On the initiation of subduction zones

NASA Astrophysics Data System (ADS)

Cloetingh, Sierd; Wortel, Rinus; Vlaar, N. J.

1989-03-01

Analysis of the relation between intraplate stress fields and lithospheric rheology leads to greater insight into the role that initiation of subduction plays in the tectonic evolution of the lithosphere. Numerical model studies show that if after a short evolution of a passive margin (time span a few tens of million years) subduction has not yet started, continued aging of the passive margin alone does not result in conditions more favorable for transformation into an active margin. Although much geological evidence is available in supporting the key role small ocean basins play in orogeny and ophiolite emplacement, evolutionary frameworks of the Wilson cycle usually are cast in terms of opening and closing of wide ocean basins. We propose a more limited role for large oceans in the Wilson cycle concept. In general, initiation of subduction at passive margins requires the action of external plate-tectonic forces, which will be most effective for young passive margins prestressed by thick sedimentary loads. It is not clear how major subduction zones (such as those presently ringing the Pacific Basin) form but it is unlikely they form merely by aging of oceanic lithosphere. Conditions likely to exist in very young oceanic regions are quite favorable for the development of subduction zones, which might explain the lack of preservation of back-arc basins and marginal seas. Plate reorganizations probably occur predominantly by the formation of new spreading ridges, because stress relaxation in the lithosphere takes place much more efficiently through this process than through the formation of new subduction zones.

Continental crustal growth and the supercontinental cycle: evidence from the Central Asian Orogenic Belt

NASA Astrophysics Data System (ADS)

Hong, Dawei; Zhang, Jisheng; Wang, Tao; Wang, Shiguang; Xie, Xilin

2004-09-01

Studies of supercontinental cycle are mainly concentrated on the assembly, breakup and dispersal of supercontinents, and studies of continental crustal growth largely on the growth and loss (recycling) of the crust. These two problems have long been studied separately from each other. The Paleozoic-Mesozoic granites in the Central Asian Orogenic Belt have commonly positive ɛNd values, implying large-scale continental crustal growth in the Phanerozoic. They coincided temporally and spatially with the Phanerozoic Pangea supercontinental cycle, and overlapped in space with the P-wave high- V anomalies and calculated positions of subducted slabs for the last 180 Ma, all this suggests that the Phanerozoic Laurasia supercontinental assembly was accompanied by large-scale continental crustal growth in central Asia. Based on these observations, this paper proposes that there may be close and original correlations between a supercontinental cycle, continental crustal growth and catastrophic slab avalanches in the mantle. In this model we suggest that rapid continental crustal growth occurred during supercontinent assembly, whereas during supercontinental breakup and dispersal new additions of the crust were balanced by losses, resulting in a steady state system. Supercontinental cycle and continental crustal growth are both governed by changing patterns of mantle convection.

Tectonics and geology of spreading ridge subduction at the Chile Triple Junction: a synthesis of results from Leg 141 of the Ocean Drilling Program

USGS Publications Warehouse

Behrmann, J.H.; Lewis, S.D.; Cande, S.C.

1994-01-01

An active oceanic spreading ridge is being subducted beneath the South American continent at the Chile Triple Junction. This process has played a major part in the evolution of most of the continental margins that border the Pacific Ocean basin. A combination of high resolution swath bathymetric maps, seismic reflection profiles and drillhole and core data from five sites drilled during Ocean Drilling Program (ODP) Leg 141 provide important data that define the tectonic, structural and stratigraphic effects of this modern example of spreading ridge subduction. A change from subduction accretion to subduction erosion occurs along-strike of the South American forearc. This change is prominently expressed by normal faulting, forearc subsidence, oversteepening of topographic slopes and intensive sedimentary mass wasting, overprinted on older signatures of sediment accretion, overthrusting and uplift processes in the forearc. Data from drill sites north of the triple junction (Sites 859-861) show that after an important phase of forearc building in the early to late Pliocene, subduction accretion had ceased in the late Pliocene. Since that time sediment on the downgoing oceanic Nazca plate has been subducted. Site 863 was drilled into the forearc in the immediate vicinity of the triple junction above the subducted spreading ridge axis. Here, thick and intensely folded and faulted trench slope sediments of Pleistocene age are currently involved in the frontal deformation of the forearc. Early faults with thrust and reverse kinematics are overprinted by later normal faults. The Chile Triple Junction is also the site of apparent ophiolite emplacement into the South American forearc. Drilling at Site 862 on the Taitao Ridge revealed an offshore volcanic sequence of Plio-Pleistocene age associated with the Taitao Fracture Zone, adjacent to exposures of the Pliocene-aged Taitao ophiolite onshore. Despite the large-scale loss of material from the forearc at the triple junction

A large mantle water source for the northern San Andreas Fault System: A ghost of subduction past

USGS Publications Warehouse

Kirby, Stephen H.; Wang, Kelin; Brocher, Thomas M.

2014-01-01

Recent research indicates that the shallow mantle of the Cascadia subduction margin under near-coastal Pacific Northwest U.S. is cold and partially serpentinized, storing large quantities of water in this wedge-shaped region. Such a wedge probably formed to the south in California during an earlier period of subduction. We show by numerical modeling that after subduction ceased with the creation of the San Andreas Fault System (SAFS), the mantle wedge warmed, slowly releasing its water over a period of more than 25 Ma by serpentine dehydration into the crust above. This deep, long-term water source could facilitate fault slip in San Andreas System at low shear stresses by raising pore pressures in a broad region above the wedge. Moreover, the location and breadth of the water release from this model gives insights into the position and breadth of the SAFS. Such a mantle source of water also likely plays a role in the occurrence of Non-Volcanic Tremor (NVT) that has been reported along the SAFS in central California. This process of water release from mantle depths could also mobilize mantle serpentinite from the wedge above the dehydration front, permitting upward emplacement of serpentinite bodies by faulting or by diapiric ascent. Specimens of serpentinite collected from tectonically emplaced serpentinite blocks along the SAFS show mineralogical and structural evidence of high fluid pressures during ascent from depth. Serpentinite dehydration may also lead to tectonic mobility along other plate boundaries that succeed subduction, such as other continental transforms, collision zones, or along present-day subduction zones where spreading centers are subducting.

The recycling of chromitites in ophiolites from southwestern North America

NASA Astrophysics Data System (ADS)

González-Jiménez, José M.; Camprubí, Antoni; Colás, Vanessa; Griffin, William L.; Proenza, Joaquín A.; O'Reilly, Suzanne Y.; Centeno-García, Elena; García-Casco, Antonio; Belousova, Elena; Talavera, Cristina; Farré-de-Pablo, Júlia; Satsukawa, Takako

2017-12-01

Podiform chromitites occur in mantle peridotites of the Late Triassic Puerto Nuevo Ophiolite, Baja California Sur State, Mexico. These are high-Cr chromitites [Cr# (Cr/Cr + Al atomic ratio = 0.61-0.69)] that contain a range of minor- and trace-elements and show whole-rock enrichment in IPGE (Os, Ir, Ru). That are similar to those of high-Cr ophiolitic chromitites crystallised from melts similar to high-Mg island-arc tholeiites (IAT) and boninites in supra-subduction-zone mantle wedges. Crystallisation of these chromitites from S-undersaturated melts is consistent with the presence of abundant inclusions of platinum-group minerals (PGM) such as laurite (RuS2)-erlichmanite (OsS2), osmium and irarsite (IrAsS) in chromite, that yield TMA ≈ TRD model ages peaking at 325 Ma. Thirty-three xenocrystic zircons recovered from mineral concentrates of these chromitites yield ages (2263 ± 44 Ma to 278 ± 4 Ma) and Hf-O compositions [ɛHf(t) = - 18.7 to + 9.1 and 18O values < 12.4‰] that broadly match those of zircons reported in nearby exposed crustal blocks of southwestern North America. We interpret these chromitite zircons as remnants of partly digested continental crust or continent-derived sediments on oceanic crust delivered into the mantle via subduction. They were captured by the parental melts of the chromitites when the latter formed in a supra-subduction zone mantle wedge polluted with crustal material. In addition, the Puerto Nuevo chromites have clinopyroxene lamellae with preferred crystallographic orientation, which we interpret as evidence that chromitites have experienced high-temperature and ultra high-pressure conditions (< 12 GPa and 1600 °C). We propose a tectonic scenario that involves the formation of chromitite in the supra-subduction zone mantle wedge underlying the Vizcaino intra-oceanic arc ca. 250 Ma ago, deep-mantle recycling, and subsequent diapiric exhumation in the intra-oceanic basin (the San Hipólito marginal sea) generated during an

Tectonic Reorganization of the Western Pacific in Eocene Time: Missing Pieces in the Subduction Initiation Puzzle

NASA Astrophysics Data System (ADS)

Bloomer, S. H.; Stern, R. J.

2002-12-01

The initiation of subduction is probably the geologic process most responsible for large-scale changes in the motions and interactions of plates. To the extent that subduction drives mantle convection, the initiation of subduction also drives major changes in the convection of the mantle. The mechanisms of subduction initiation remain, however, obscure, but it is becoming increasingly clear that Eocene sequences in the western Pacific provide an outstanding opportunity to study this phenomenon. The major subduction zones of the western Pacific (Tonga, Mariana, Izu, Bonin) all first produced volcanic products in early Eocene time (55-48 Ma). The similarity of timing and of the characteristics of these margins suggests that there may be a common process involved. There is no evidence in the forearc crust of any of these convergent margins for proximity to a continental margin at the time of initiation. Current models of plate motion (particularly given recent reinterpretations of the Hawaiian hotspot bend) show no major plate reorganization that might have provided excess compressional stress across the western Pacific margins. The only mechanically viable mechanism for subduction initiation in the region appears to be spontaneous failure due to gravitational instability of cold, old oceanic lithosphere. There are a number of geologic and geophysical unknowns in assessing the viability of such spontaneous nucleation. The lithosphere becomes stronger as it ages as well as becoming denser. Failure of such crust to form a nascent subduction zone requires a crustal weakness such as a fault and a mechanism to decrease the bending strength of the plate. Paleomagnetic data and plate reconstructions for both the IBM and the Tonga-Kermedec region provide no clear answer to these issues and in fact conflict with interpretations placing large transform faults at the site of subduction nucleation. The large-scale rotations inferred from those data for the IBM conflict, or at

Mega-thrust and Intra-slab Earthquakes Beneath Tokyo Metropolitan Area

NASA Astrophysics Data System (ADS)

Hirata, N.; Sato, H.; Koketsu, K.; Hagiwara, H.; Wu, F.; Okaya, D.; Iwasaki, T.; Kasahara, K.

2006-12-01

In central Japan the Philippine Sea plate (PSP) subducts beneath the Tokyo Metropolitan area, the Kanto region, where it causes mega-thrust earthquakes, such as the 1703 Genroku earthquake (M8.0) and the 1923 Kanto earthquake (M7.9) which had 105,000 fatalities. The vertical proximity of this down going lithospheric plate is of concern because the greater Tokyo urban region has a population of 42 million and is the center of approximately 40% of the nation's economic activities. A M7+ earthquake in this region at present has high potential to produce devastating loss of life and property with even greater global economic repercussions. The M7+ earthquake is evaluated to occur with a probability of 70% in 30 years by the Earthquake Research Committee of Japan. In 2002, a consortium of universities and government agencies in Japan started the Special Project for Earthquake Disaster Mitigation in Urban Areas, a project to improve information needed for seismic hazards analyses of the largest urban centers. Assessment in Kanto of the seismic hazard produced by the Philippine Sea Plate (PSP) mega-thrust earthquakes requires identification of all significant faults and possible earthquake scenarios and rupture behavior, regional characterizations of PSP geometry and the overlying Honshu arc physical properties (e.g., seismic wave velocities, densities, attenuation), and local near-surface seism ic site effects. Our study addresses (1) improved regional characterization of the PSP geometry based on new deep seismic reflection profiles (Sato etal.,2005), reprocessed off-shore profiles (Kimura et al.,2005), and a dense seismic array in the Boso peninsular (Hagiwara et al., 2006) and (2) identification of asperities of the mega-thrust at the top of the PSP. We qualitatively examine the relationship between seismic reflections and asperities inferred by reflection physical properties. We also discuss the relation between deformation of PSP and intra-slab M7+ earthquakes: the

Brittle deformation during Alpine basal accretion and the origin of seismicity nests above the subduction interface

NASA Astrophysics Data System (ADS)

Menant, Armel; Angiboust, Samuel; Monié, Patrick; Oncken, Onno; Guigner, Jean-Michel

2018-04-01

Geophysical observations on active subduction zones have evidenced high seismicity clusters at 20-40 km depth in the fore-arc region whose origin remains controversial. We report here field observations of pervasive pseudotachylyte networks (interpreted as evidence for paleo-seismicity) in the now-exhumed Valpelline continental unit (Dent Blanche complex, NW. Alps, Italy), a tectonic sliver accreted to the upper plate at c. 30 km depth during the Paleocene Alpine subduction. Pre-alpine granulite-facies paragneiss from the core of the Valpelline unit are crosscut by widespread, mm to cm-thick pseudotachylyte veins. Co-seismic heating and subsequent cooling led to the formation of Ti-rich garnet rims, ilmenite needles, Ca-rich plagioclase, biotite microliths and hercynite micro-crystals. 39Ar-40Ar dating yields a 51-54 Ma age range for these veins, thus suggesting that frictional melting events occurred near peak burial conditions while the Valpelline unit was already inserted inside the duplex structure. In contrast, the base of the Valpelline unit underwent synchronous ductile and brittle, seismic deformation under water-bearing conditions followed by a re-equilibration at c. 40 Ma (39Ar-40Ar on retrograded pseudotachylyte veins) during exhumation-related deformation. Calculated rheological profiles suggest that pseudotachylyte veins from the dry core of the granulite unit record upper plate micro-seismicity (Mw 2-3) formed under very high differential stresses (>500 MPa) while the sheared base of the unit underwent repeated brittle-ductile deformation at much lower differential stresses (<40 MPa) in a fluid-saturated environment. These results demonstrate that some of the seismicity clusters nested along and above the plate interface may reflect the presence of stiff tectonic slivers rheologically analogous to the Valpelline unit acting as repeatedly breaking asperities in the basal accretion region of active subduction zones.

Measurement of inter- and intra-annual variability of landscape fire activity at a continental scale: the Australian case

NASA Astrophysics Data System (ADS)

Williamson, Grant J.; Prior, Lynda D.; Jolly, W. Matt; Cochrane, Mark A.; Murphy, Brett P.; Bowman, David M. J. S.

2016-03-01

Climate dynamics at diurnal, seasonal and inter-annual scales shape global fire activity, although difficulties of assembling reliable fire and meteorological data with sufficient spatio-temporal resolution have frustrated quantification of this variability. Using Australia as a case study, we combine data from 4760 meteorological stations with 12 years of satellite-derived active fire detections to determine day and night time fire activity, fire season start and end dates, and inter-annual variability, across 61 objectively defined climate regions in three climate zones (monsoon tropics, arid and temperate). We show that geographic patterns of landscape burning (onset and duration) are related to fire weather, resulting in a latitudinal gradient from the monsoon tropics in winter, through the arid zone in all seasons except winter, and then to the temperate zone in summer and autumn. Peak fire activity precedes maximum lightning activity by several months in all regions, signalling the importance of human ignitions in shaping fire seasons. We determined median daily McArthur forest fire danger index (FFDI50) for days and nights when fires were detected: FFDI50 varied substantially between climate zones, reflecting effects of fire management in the temperate zone, fuel limitation in the arid zone and abundance of flammable grasses in the monsoon tropical zone. We found correlations between the proportion of days when FFDI exceeds FFDI50 and the Southern Oscillation index across the arid zone during spring and summer, and Indian Ocean dipole mode index across south-eastern Australia during summer. Our study demonstrates that Australia has a long fire weather season with high inter-annual variability relative to all other continents, making it difficult to detect long term trends. It also provides a way of establishing robust baselines to track changes to fire seasons, and supports a previous conceptual model highlighting multi-temporal scale effects of climate in

The Paleozoic - Mesozoic Mekele Sedimentary Basin in Ethiopia: An example of an exhumed IntraCONtinental Sag (ICONS) basin

NASA Astrophysics Data System (ADS)

Alemu, Tadesse; Abdelsalam, Mohamed G.; Dawit, Enkurie L.; Atnafu, Balemwal; Mickus, Kevin L.

2018-07-01

We investigated the evolution of the Mekele Sedimentary Basin (MSB) in northern Ethiopia using geologic field and gravity data. The depth to Moho and lithospheric structure beneath the basin was imaged using two-dimensional (2D) radially-averaged power spectral analysis, Lithoflex three-dimensional (3D) forward and inverse modeling, and 2D forward modeling of the Bouguer gravity anomalies. Previous studies proposed that the basin was formed as part of a multi-branched rift system related to the breakup of Gondwana. Our results show that the MSB: (1) is circular to elliptical in map view and saucer shaped in cross sectional view, (2) is filled with terrestrial and shallow marine sedimentary rocks, (3) does not significantly structurally control the sedimentation and the major faults are post-depositional, (4) is characterized by a concentric gravity minima, (5) is underlain by an unstretched crust (∼40 km thick) and thicker lithosphere (∼120 km thick). These features compare positively with a group of basins known as IntraCONtinental Sags (ICONS), especially those ICONS formed over accretionary orogenic terranes. Since the MSB is located above the Neoproterozoic accretionary orogenic terranes of the Arabian-Nubian Shield (ANS), we propose that the formation of the MSB to be related to cooling and thickening of a juvenile sub-continental lithospheric mantle beneath the ANS, which most probably provided negative buoyancy, and hence subsidence in the MSB, leading to its formation as an ICONS. The MSB could be used as an outcrop analog for information about the internal facies architecture of ICONS because it is completely exhumed due to tectonic uplift on the western flank of the Afar Depression.

Interaction Between Magmatism and Continental Extension, Insight From an Extensional Terrain in the Iranian Plateau

NASA Astrophysics Data System (ADS)

Malekpour Alamdari, A.; Axen, G. J.; Hassanzadeh, J.

2014-12-01

Our knowledge about the spatial and temporal relationship between continental extension and its related magmatism is mainly from the western US where removal of a flat subducting slab from under the continent controlled thermal weakening and some extensional collapse. The Iranian plateau, where flat-slab subduction and its subsequent rollback is suggested for the Tertiary magmatic evolution, is an ideal place to see if a similar interaction exists. Between the Late Cretaceous and, at least, the Early Eocene, large-scale continental extension affected the NE Iranian plateau. An ~100 km-long, SE tilted upper to mid-crustal section was exhumed by slip along a low-angle, NW-dipping detachment fault. From SE to NW (young to old) this section includes late Cretaceous pelagic limestones of the Kashmar ophiolites, Late and Early Cretaceous sedimentary rocks, and the Late Triassic and older crystalline rocks of the Biarjmand-Shotor Kuh metamorphic core complex. Little pre-extensional magmatic activity exists in the tilted sequence and in surrounding regions, as Late Jurassic and Early Cretaceous dikes. Similarly, syn-extensional magmatism is absent. In contrast, the tilted sequence is unconformably overlain by >4000 m of volcanic rocks with age ranging from the Middle Eocene (explosive, calc-alkaline?) to the Late Eocene (effusive, alkaline). The absence of considerable pre-extensional magmatism in the NE Iranian plateau does not support magma underplating, subsequent thermal weakening and collapse as a mechanism for the extension in this region. It also indicates that the models that consider waning of volcanism as a controlling mechanism for triggering of extensional faulting (Sonder & Jones, 1999) is not applicable for this region. The amagmatic extension may reflect magma crystallization at depth due to reduced confining pressure resulted from active normal faulting and fracturing (Gans & Bohrson, 1998). The extension and related asthenospheric rise may be developed in

Transition from Subduction to Strike-Slip in the Southeast Caribbean: Effects on Lithospheric Structures and Overlying Basin Evolution

NASA Astrophysics Data System (ADS)

Alvarez, T.; Mann, P.; Wood, L. J.; Vargas, C. A.; Latchman, J. L.

2013-12-01

lithospheric weakness during subduction. We propose that this fracture zone is a key transition point between the subduction of South American/Atlantic oceanic lithosphere; which descends into the mantle, to the northeast, and the under-thrusting of transitional to continental South American lithosphere which resists subduction to the southwest. Maps of South American basement and its overlying Cretaceous passive margin illustrates a northwesterly basement dip with a distinct change in angle of the northwest dip across the paleo-fracture zone consistent with our tomographic model. We propose that flexure of the subducting South American plate at this location exerts a critical control on the formation and evolution of the basins and the lateral distribution of Cretaceous through Pleistocene stratigraphic fill. East of the fracture zone, the overlying strata is deformed by active subduction and accretionary prism processes with a wider zone of shortening with lower overall topography, while to the west of the fracture zone there is active oblique collision with a narrower zone of shortening and greater uplift.

Intra-Extra Vehicular Activity (IEVA) Russian and Gemini Spacesuits

NASA Technical Reports Server (NTRS)

Thomas, Kenneth S.

2016-01-01

Kenneth Thomas will discuss the Intra-Extra Vehicular Activity Russian and Gemini spacesuits. While the United States and Russia adapted to existing launch- and reentry-type suits to allow the first human ventures into the vacuum of space, there were differences in execution and capabilities. Mr. Thomas will discuss the advantages and disadvantages of this approach compared to exclusively intra-vehicular or extra-vehicular suit systems.

Foreland sedimentary record of Andean mountain building during advancing and retreating subduction

NASA Astrophysics Data System (ADS)

Horton, Brian K.

2016-04-01

-skinned basement inversion with geometrically and kinematically linked thin-skinned thrust structures at shallower levels in the eastern foreland, including well-dated late Miocene growth strata. The mid-Cenozoic hiatus potentially signifies nondeposition during passage of a flexural forebulge or nondeposition during neutral to extensional conditions possibly driven by a transient retreating-slab configuration along the western margin of South America. Similar long-lived stratigraphic gaps are commonly observed in other foreland records of continental convergent margins. It is proposed that Andean orogenesis along the South American convergent margin has long been sensitive to variations in subduction dynamics throughout Mesozoic-Cenozoic time, such that shifts in relative convergence and degree of mechanical coupling along the subduction interface (i.e., transitions between advancing versus retreating modes of subduction) have governed fluctuating contractional, extensional, and neutral conditions. Unclear is whether these various modes affected the entire convergent margin simultaneously due to continental-scale changes (e.g., temporal shifts in plate convergence, absolute motion of upper plate, or mantle wedge circulation) or whether parts of the margin behaved independently due to smaller-scale fluctuations (e.g., spatial variations in the age of the subducted plate, buoyant asperities in the downgoing slab, or asthenospheric anomalies).

Cretaceous subduction in the Pyrenees: Iberian plate-kinematics in a mantle reference frame

NASA Astrophysics Data System (ADS)

Vissers, Reinoud; van Hinsbergen, Douwe; van der Meer, Douwe; Spakman, Wim

2016-04-01

During the Cretaceous, Iberia was a microplate separated from Laurasia and Gondwana by ridges and transforms, and by a convergent margin to its northeast along which the Pyrenean fold-thrust belt developed. As a microplate, Iberia underwent a well-defined but ill-understood Albian-Aptian ~ 35° counterclockwise rotation relative to Eurasia. Three competing kinematic scenarios for Iberian motion in the late Mesozoic are all compatible with the Pyrenean geological record and comprise (1) transtensional eastward motion of Iberia versus Eurasia, (2) strike-slip motion followed by orthogonal extension and (3) scissor-style opening of the Bay of Biscay coupled with subduction in the Pyrenean realm. The last scenario is the only one consistent with paleomagnetic and ocean floor anomaly constraints showing Iberia's rotation, but is criticized because the upper mantle below the Pyrenees contains no evidence for a subducted slab. Here we show that when taking absolute plate motions into account, Aptian oceanic subduction in the Pyrenees followed by Albian slab break-off should leave a slab remnant in the present-day mid-mantle below NW Africa instead of below the Pyrenees. Mantle tomography shows a positive seismic velocity anomaly that matches the predicted position and dimension of such a slab remnant between 1900 and 1500 km depth below Reggane in Southern Algeria. Seismic tomographic imaging of the mantle structure therefore does not falsify the Pyrenean subduction hypothesis, and provides no basis to discard marine magnetic and paleomagnetic constraints on Iberia's kinematic history. Slab break-off explains the well-dated Albian-Cenomanian high-temperature metamorphism in the Pyrenees that hitherto has been interpreted as an expression of continental break-up and hyperextension. We suspect that subduction in the Pyrenees may have played a key role in driving the rapid Aptian rotation of the Iberian microplate.

The Cascadia Subduction Zone: two contrasting models of lithospheric structure

USGS Publications Warehouse

Romanyuk, T.V.; Blakely, R.; Mooney, W.D.

1998-01-01

The Pacific margin of North America is one of the most complicated regions in the world in terms of its structure and present day geodynamic regime. The aim of this work is to develop a better understanding of lithospheric structure of the Pacific Northwest, in particular the Cascadia subduction zone of Southwest Canada and Northwest USA. The goal is to compare and contrast the lithospheric density structure along two profiles across the subduction zone and to interpet the differences in terms of active processes. The subduction of the Juan de Fuca plate beneath North America changes markedly along the length of the subduction zone, notably in the angle of subduction, distribution of earthquakes and volcanism, goelogic and seismic structure of the upper plate, and regional horizontal stress. To investigate these characteristics, we conducted detailed density modeling of the crust and mantle along two transects across the Cascadia subduction zone. One crosses Vancouver Island and the Canadian margin, the other crosses the margin of central Oregon.

Continental lithospheric subduction and intermediate-depth seismicity: Constraints from S-wave velocity structures in the Pamir and Hindu Kush

NASA Astrophysics Data System (ADS)

Li, Wei; Chen, Yun; Yuan, Xiaohui; Schurr, Bernd; Mechie, James; Oimahmadov, Ilhomjon; Fu, Bihong

2018-01-01

The Pamir has experienced more intense deformation and shortening than Tibet, although it has a similar history of terrane accretion. Subduction as a primary way to accommodate lithospheric shortening beneath the Pamir has induced the intermediate-depth seismicity, which is rare in Tibet. Here we construct a 3D S-wave velocity model of the lithosphere beneath the Pamir by surface wave tomography using data of the TIPAGE (Tien Shan-Pamir Geodynamic program) and other seismic networks in the area. We imaged a large-scale low velocity anomaly in the crust at 20-50 km depth in the Pamir overlain by a high velocity anomaly at a depth shallower than 15 km. The high velocity anomalies colocate with exposed gneiss domes, which may imply a similar history of crustal deformation, partial melting and exhumation in the hinterland, as has occurred in the Himalaya/Tibet system. At mantle depths, where the intermediate-depth earthquakes are located, a low velocity zone is clearly observed extending to about 180 km and 150 km depth in the Hindu Kush and eastern Pamir, respectively. Moreover, the geometry of the low-velocity anomaly suggests that lower crustal material has been pulled down into the mantle by the subducting Asian and Indian lithospheric mantle beneath the Pamir and Hindu Kush, respectively. Metamorphic processes in the subducting lower crust may cause the intermediate-depth seismicity down to 150-180 km depth beneath the Pamir and Hindu Kush. We inverted focal mechanisms in the seismic zone for the stress field. Differences in the stress field between the upper and lower parts of the Indian slab imply that subduction and detachment of the Indian lithosphere might cause intense seismicity associated with the thermal shear instability in the deep Hindu Kush.

Jurassic subduction initiation in the western and central Neo-Tethys and the origin of the Balkan ophiolites

NASA Astrophysics Data System (ADS)

Van Hinsbergen, D. J. J.; Maffione, M.

2017-12-01

Jurassic subduction initiation in the Neo-Tethys Ocean was the first, critical step of a long tectonic process that eventually led to the collision of the Adria-Africa and Eurasia plates and the formation of a 6000 km long Alpine orogenic belt spanning from the Balkan Peninsula to Iran. Investigating the process of subduction initiation in the Neo-Tethys during the Jurassic is crucial to (i) reconstruct the complex geological evolution of this orogen from its initial stages, and (ii) shed new lights over the enigmatic kinematics and driving mechanisms of subduction initiation. Records of the initial closure of the Neo-Tethys are today preserved in a fragmented belt of Middle Jurassic ophiolites (170-160 Ma) distributed above the Alpine orogen. In particular, the well-preserved and extensively studied ophiolites of the Balkan Peninsula offer a unique chance to study the mechanisms leading to the closure of the western domain of the Neo-Tethys. Here we provide the first quantitative constraints on the geometry of the Jurassic Neo-Tethyan subduction system using a net tectonic rotation analysis based on paleomagnetic and structural geological data from the sheeted dyke complexes of various ophiolites of Serbia (Maljen, Ibar) and Greece (Othris, Pindos, Vourinos, Guevgueli). Our results show that closure of the western Neo-Tethys was accommodated by two subduction zones, one intra-oceanic, formed at the N-S trending Neo-Tethyan ridge, the other initiated at the European passive margin and curving southward from a N-S to a NW-SE direction following the shape of the passive margin. We propose that these two subduction zones formed upon propagation of subduction(s) initiated in the central Neo-Tethys (modern Turkey) in the late Early Jurassic ( 185-180 Ma).

Geologic constraints on the setting and dynamics of subduction initiation

NASA Astrophysics Data System (ADS)

Encarnacion, John; Keenan, Timothy

2017-04-01

Understanding where and how subduction zones have and can initiate is important because, besides being a critical step in the plate tectonic system, it can provide insight into the complex interactions of crust and mantle rheology, forces acting on the plates, strain, metamorphic reactions, and erosional and depositional processes at the surface. Insight into the possibilities of where and how subduction zones start has been provided by numerical and analog modeling. All sites for subduction initiation are potential weak zones in the lithosphere and include the continent-ocean boundary, oceanic arc-oceanic crust boundary, oceanic transform faults and fracture zones, oceanic detachment faults, and active or recently extinct oceanic ridges/spreading centers. Within the constraints of modeling, it has also been shown that the forces involved in the initiation of subduction can be largely horizontal (induced by a collision, say, or through 'ridge push') or vertical (driven by density contrasts). The latter scenario is often referred to as "spontaneous" subduction initiation, whereas the former situation may be called "forced"or "induced" subduction initiation. It is prudent, however, not to assume that "what can happen, did happen." So, the challenge for geologists is to infer from the rock record, through structural mapping, thermochronology, thermobarometry, geochemistry, paleomagnetics, and sedimentological studies, how any given subduction zone began. Even with a complete data set, it is not always possible to fully constrain the specific geologic setting or dynamics involved in the initiation of a given subduction zone. One can, however, often rule out certain scenarios, increasing the probability of others. Part of the geologic record of subduction initiation preserved at some subduction zones are so-called "metamorphic soles," which include high-temperature (T) and high-pressure (P) metamorphosed oceanic crust that was underthrust to asthenospheric mantle depths

Geophysical evidence for a transform margin offshore Western Algeria: a witness of a subduction-transform edge propagator?

NASA Astrophysics Data System (ADS)

Badji, Rabia; Charvis, Philippe; Bracene, Rabah; Galve, Audrey; Badsi, Madjid; Ribodetti, Alessandra; Benaissa, Zahia; Klingelhoefer, Frauke; Medaouri, Mourad; Beslier, Marie-Odile

2015-02-01

For the first time, a deep seismic data set acquired in the frame of the Algerian-French SPIRAL program provides new insights regarding the origin of the westernmost Algerian margin and basin. We performed a tomographic inversion of traveltimes along a 100-km-long wide-angle seismic profile shot over 40 ocean bottom seismometers offshore Mostaganem (Northwestern Algeria). The resulting velocity model and multichannel seismic reflection profiles show a thin (3-4 km thick) oceanic crust. The narrow ocean-continent transition (less than 10 km wide) is bounded by vertical faults and surmounted by a narrow almost continuous basin filled with Miocene to Quaternary sediments. This fault system, as well as the faults organized in a negative-flower structure on the continent side, marks a major strike-slip fault system. The extremely sharp variation of the Moho depth (up to 45 ± 3°) beneath the continental border underscores the absence of continental extension in this area. All these features support the hypothesis that this part of the margin from Oran to Tenes, trending N65-N70°E, is a fossil subduction-transform edge propagator fault, vestige of the propagation of the edge of the Gibraltar subduction zone during the westward migration of the Alborán domain.

Seismic anisotropy in the Hellenic subduction zone: Effects of slab segmentation and subslab mantle flow

NASA Astrophysics Data System (ADS)

Evangelidis, C. P.

2017-12-01

The segmentation and differentiation of subducting slabs have considerable effects on mantle convection and tectonics. The Hellenic subduction zone is a complex convergent margin with strong curvature and fast slab rollback. The upper mantle seismic anisotropy in the region is studied focusing at its western and eastern edges in order to explore the effects of possible slab segmentation on mantle flow and fabrics. Complementary to new SKS shear-wave splitting measurements in regions not adequately sampled so far, the source-side splitting technique is applied to constrain the depth of anisotropy and to densify measurements. In the western Hellenic arc, a trench-normal subslab anisotropy is observed near the trench. In the forearc domain, source-side and SKS measurements reveal a trench-parallel pattern. This indicates subslab trench-parallel mantle flow, associated with return flow due to the fast slab rollback. The passage from continental to oceanic subduction in the western Hellenic zone is illustrated by a forearc transitional anisotropy pattern. This indicates subslab mantle flow parallel to a NE-SW smooth ramp that possibly connects the two subducted slabs. A young tear fault initiated at the Kefalonia Transform Fault is likely not entirely developed, as this trench-parallel anisotropy pattern is observed along the entire western Hellenic subduction system, even following this horizontal offset between the two slabs. At the eastern side of the Hellenic subduction zone, subslab source-side anisotropy measurements show a general trench-normal pattern. These are associated with mantle flow through a possible ongoing tearing of the oceanic lithosphere in the area. Although the exact geometry of this slab tear is relatively unknown, SKS trench-parallel measurements imply that the tear has not reached the surface yet. Further exploration of the Hellenic subduction system is necessary; denser seismic networks should be deployed at both its edges in order to achieve

Earth’s first stable continents did not form by subduction

NASA Astrophysics Data System (ADS)

Johnson, Tim E.; Brown, Michael; Gardiner, Nicholas J.; Kirkland, Christopher L.; Smithies, R. Hugh

2017-02-01

The geodynamic environment in which Earth’s first continents formed and were stabilized remains controversial. Most exposed continental crust that can be dated back to the Archaean eon (4 billion to 2.5 billion years ago) comprises tonalite-trondhjemite-granodiorite rocks (TTGs) that were formed through partial melting of hydrated low-magnesium basaltic rocks; notably, these TTGs have ‘arc-like’ signatures of trace elements and thus resemble the continental crust produced in modern subduction settings. In the East Pilbara Terrane, Western Australia, low-magnesium basalts of the Coucal Formation at the base of the Pilbara Supergroup have trace-element compositions that are consistent with these being source rocks for TTGs. These basalts may be the remnants of a thick (more than 35 kilometres thick), ancient (more than 3.5 billion years old) basaltic crust that is predicted to have existed if Archaean mantle temperatures were much hotter than today’s. Here, using phase equilibria modelling of the Coucal basalts, we confirm their suitability as TTG ‘parents’, and suggest that TTGs were produced by around 20 per cent to 30 per cent melting of the Coucal basalts along high geothermal gradients (of more than 700 degrees Celsius per gigapascal). We also analyse the trace-element composition of the Coucal basalts, and propose that these rocks were themselves derived from an earlier generation of high-magnesium basaltic rocks, suggesting that the arc-like signature in Archaean TTGs was inherited from an ancestral source lineage. This protracted, multistage process for the production and stabilization of the first continents—coupled with the high geothermal gradients—is incompatible with modern-style plate tectonics, and favours instead the formation of TTGs near the base of thick, plateau-like basaltic crust. Thus subduction was not required to produce TTGs in the early Archaean eon.

Study of crustal structure and stretch mechanism of central continental shelf of northern South China Sea

NASA Astrophysics Data System (ADS)

Cao, J.; Xia, S.; Sun, J.; Wan, K.; Xu, H.

2017-12-01

Known as a significant region to study tectonic relationship between South China block and South China Sea (SCS) block and the evolution of rifted basin in continental margin, the continental shelf of northern SCS documents the evolution from continental splitting to seafloor spreading of SCS. To investigate crustal structure of central continental shelf in northern SCS, two wide-angle onshore-offshore seismic experiments and coincident multi-channel seismic (MCS) profiles were carried out across the onshore-offshore transitional zone in northern SCS, 2010 and 2012. A total of 34 stations consisted of ocean bottom seismometers, portable and permanent land stations were deployed during the survey. The two-dimensional precise crustal structure models of central continental shelf in northern SCS was constructed from onshore to offshore, and the stretching factors along the P-wave velocity models were calculated. The models reveal that South China block is a typical continental crust with a 30-32 km Moho depth, and a localized high-velocity anomaly in middle-lower crust under land area near Hong Kong was imaged, which may reflect magma underplating caused by subduction of paleo-Pacific plate in late Mesozoic. The littoral fault zone is composed of several parallel, high-angle, normal faults that mainly trend northeast to northeast-to-east and dip to the southeast with a large displacement, and the fault is divided into several segments separated by the northwest-trending faults. The shelf zone south of LFZ was consisted of a differential thinning upper and lower continental crust, which indicate stretch thinning of passive continent margin during the Cenozoic spreading of the SCS. The results appear to further confirm that the northern margin of SCS experienced a transition from active margin to passive one during late Mesozoic and Cenozoic.

Double-Sided Wedge Model For Retreating Subduction Zones: Applications to the Apenninic and Hellenic Subduction Zones (Invited)

NASA Astrophysics Data System (ADS)

Brandon, M. T.; Willett, S.; Rahl, J. M.; Cowan, D. S.

2009-12-01

We propose a new model for the evolution of accreting wedges at retreating subduction zones. Advance and retreat refer to the polarity of the velocity of the overriding plate with respect to subduction zone. Advance indicates a velocity toward the subduction zone (e.g., Andes) and retreat, away from the subduction zone (e.g. Apennines, Crete). The tectonic mode of a subduction zone, whether advancing or retreating, is a result of both the rollback of the subducting plate and the absolute motion of the overriding plate. The Hellenic and Apenninic wedges are both associated with retreating subduction zones. The Hellenic wedge has been active for about 100 Ma, whereas the Apenninic wedge has been active for about 30 Ma. Comparison of maximum metamorphic pressures for exhumed rocks in these wedges (25 and 30 km, respectively) with the maximum thickness of the wedges at present (30 and 35 km, respectively) indicates that each wedge has maintained a relatively steady size during its evolution. This conclusion is based on the constraint that both frictional and viscous wedges are subject to the constraint of a steady wedge taper, so that thickness and width are strongly correlated. Both wedges show clear evidence of steady accretion during their full evolution, with accretionary fluxes of about 60 and 200 km2 Ma-1. These wedges also both show steady drift of material from the front to the rear of the wedge, with horizontal shortening dominating in the front of the wedge, and horizontal extension within the back of the wedge. We propose that these wedges represent two back-to-back wedges, with a convergent wedge on the leading side (proside), and a divergent wedge on the trailing side (retroside). In this sense, the wedges are bound by two plates. The subducting plate is familiar. It creates a thrust-sense traction beneath the proside of the wedge. The second plate is an “educting” plate, which is creates a normal-sense traction beneath the retroside of the wedge. The

Subduction in the Southern Caribbean

NASA Astrophysics Data System (ADS)

Levander, A.; Schmitz, M.; Bezada, M.; Masy, J.; Niu, F.; Pindell, J.

2012-04-01

The southern Caribbean is bounded at either end by subduction zones: In the east at the Lesser Antilles subduction zone the Atlantic part of the South American plate subducts beneath the Caribbean. In the north and west under the Southern Caribbean Deformed Belt accretionary prism, the Caribbean subducts under South America. In a manner of speaking, the two plates subduct beneath each other. Finite-frequency teleseismic P-wave tomography confirms this, imaging the Atlantic and the Caribbean subducting steeply in opposite directions to transition zone depths under northern South America (Bezada et al, 2010). The two subduction zones are connected by the El Pilar-San Sebastian strike-slip fault system, a San Andreas scale system. A variety of seismic probes identify where the two plates tear as they begin to subduct (Niu et al, 2007; Clark et al., 2008; Miller et al. 2009; Masy et al, 2009). The El Pilar system forms at the southeastern corner of the Antilles subduction zone by the Atlantic tearing from South America. The deforming plate edges control mountain building and basin formation at the eastern end of the strike-slip system. In northwestern South America the Caribbean plate tears, its southernmost element subducting at shallow angles under northernmost Colombia and then rapidly descending to transition zone depths under Lake Maracaibo (Bezada et al., 2010). We believe that the flat slab produces the Merida Andes, the Perija, and the Santa Marta ranges. The southern edge of the nonsubducting Caribbean plate underthrusts northern Venezuela to about the width of the coastal mountains (Miller et al., 2009). We infer that the underthrust Caribbean plate supports the coastal mountains, and controls continuing deformation.

Cenozoic tectono-thermal history of the Tordrillo Mountains, Alaska: Paleocene-Eocene ridge subduction, decreasing relief, and late Neogene faulting

USGS Publications Warehouse

Benowitz, Jeff A.; Haeussler, Peter J.; Layer, Paul W.; O'Sullivan, Paul B.; Wallace, Wes K.; Gillis, Robert J.

2012-01-01

Topographic development inboard of the continental margin is a predicted response to ridge subduction. New thermochronology results from the western Alaska Range document ridge subduction related orogenesis. K-feldspar thermochronology (KFAT) of bedrock samples from the Tordrillo Mountains in the western Alaska Range complement existing U-Pb, 40Ar/39Ar and AFT (apatite fission track) data to provide constraints on Paleocene pluton emplacement, and cooling as well as Late Eocene to Miocene vertical movements and exhumation along fault-bounded blocks. Based on the KFAT analysis we infer rapid exhumation-related cooling during the Eocene in the Tordrillo Mountains. Our KFAT cooling ages are coeval with deposition of clastic sediments in the Cook Inlet, Matanuska Valley and Tanana basins, which reflect high-energy depositional environments. The Tordrillo Mountains KFAT cooling ages are also the same as cooling ages in the Iliamna Lake region, the Kichatna Mountains of the western Alaska Range, and Mt. Logan in the Wrangell-St. Elias Mountains, thus rapid cooling at this time encompasses a broad region inboard of, and parallel to, the continental margin extending for several hundred kilometers. We infer these cooling events and deposition of clastic rocks are related to thermal effects that track the eastward passage of a slab window in Paleocene-Eocene time related to the subduction of the proposed Resurrection-Kula spreading ridge. In addition, we conclude that the reconstructed KFATmax negative age-elevation relationship is likely related to a long period of decreasing relief in the Tordrillo Mountains.

Structure and Tectonics of the Andaman Subduction Zone from Modeling of Seismological and Gravity Data

NASA Astrophysics Data System (ADS)

Nemalikanti, P. R.; Rao, N.; Hazarika, P.; Tiwari, V. M.; Mangalampally, R.; Singh, A.

2012-12-01

The 10 August 2009 Andaman earthquake of Mw 7.5 occurred to the north of the Andaman and Nicobar Islands at 14o N and 93o E which interestingly, coincides with the northern periphery of the rupture of the Sumatra-Andaman giant mega-thrust earthquake of Mw 9.1 that occurred on 26 December 2004. The event was followed by aftershocks with a peculiar vertical distribution at the same location which was earlier devoid of any significant seismicity. Waveform modeling of five of these events recorded by ISLANDS - the broadband seismic network deployed along the Andaman and Nicobar Islands, indicates that the main shock and two of its aftershocks have a normal fault mechanism with shallow focal depths within 18 km while two others have a strike-slip mechanism occurring deeper, down to 26 km. The computed Bouger gravity anomalies in this region indicate the steepest gradient of 1.5 mgal/km exactly centered over this zone of vertical seismic distribution that characterizes a region of lithospheric split or tear which is devoid of a subducting slab. This is in contrast to a clear subduction trend visible in the southern Andaman and Sunda arcs further south, as evidenced by tomographic images. Joint inversion of waveforms of these five events simultaneously, provides the best fitting P wave velocity structure of this region, given by a Moho at a depth of 30 km and a high crustal Vp/Vs ratio of 1.81. We infer an oceanic double crustal column corresponding to a thickness of about 21 km of Burmese crust including a 5 km thick sedimentary column, underlain by a thinner Indian crust which apparently has a thickness of about 9 km, a model that is also confirmed independently by gravity modeling. We interpret the mechanism of shallow normal fault earthquakes as an intra-plate relaxation phenomenon following the buckling of the overriding Burmese plate in the accretionary wedge of the fore-arc basin, in response to the 2004 mega-thrust subduction event. The deeper strike slip events

Array-Based Receiver Function Analysis of the Subducting Juan de Fuca Plate Beneath the Mount St. Helens Region and its Implications for Subduction Geometry and Metamorphism

NASA Astrophysics Data System (ADS)

Mann, M. E.; Abers, G. A.; Creager, K. C.; Ulberg, C. W.; Crosbie, K.

2017-12-01

westward increase in wavespeed of the continental crust. The seismically-imaged surface of the subducting JdF slab at 68 km beneath MSH is the shallowest yet documented beneath an arc volcano. Combined with the inference of serpentinization in the mantle wedge, this geometry presents a problem in that vertical mantle melt migration seems unfeasible, yet mantle melts contribute to erupted MSH magmas.

Adakitic (tonalitic-trondhjemitic) magmas resulting from eclogite decompression and dehydration melting during exhumation in response to continental collision

NASA Astrophysics Data System (ADS)

Song, Shuguang; Niu, Yaoling; Su, Li; Wei, Chunjing; Zhang, Lifei

2014-04-01

Modern adakite or adakitic rocks are thought to result from partial melting of younger and thus warmer subducting ocean crust in subduction zones, with the melt interacting with or without mantle wedge peridotite during ascent, or from melting of thickened mafic lower crust. Here we show that adakitic (tonalitic-trondhjemitic) melts can also be produced by eclogite decompression during exhumation of subducted and metamorphosed oceanic/continental crust in response to continental collision, as exemplified by the adakitic rocks genetically associated with the early Paleozoic North Qaidam ultra-high pressure metamorphic (UHPM) belt on the northern margin of the Greater Tibetan Plateau. We present field evidence for partial melting of eclogite and its products, including adakitic melt, volumetrically significant plutons evolved from the melt, cumulate rocks precipitated from the melt, and associated granulitic residues. This “adakitic assemblage” records a clear progression from eclogite decompression and heating to partial melting, to melt fractionation and ascent/percolation in response to exhumation of the UHPM package. The garnetite and garnet-rich layers in the adakitic assemblage are of cumulate origin from the adakitic melt at high pressure, and accommodate much of the Nb-Ta-Ti. Zircon SHRIMP U-Pb dating shows that partial melting of the eclogite took place at ∼435-410 Ma, which postdates the seafloor subduction (>440 Ma) and temporally overlaps the UHPM (∼440-425 Ma). While the geological context and the timing of adakite melt formation we observe differ from the prevailing models, our observations and documentations demonstrate that eclogite melting during UHPM exhumation may be important in contributing to crustal growth.

Why does continental convergence stop

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hynes, A.

1985-01-01

Convergence between India and Asia slowed at 45 Ma when they collided, but continues today. This requires that substantial proportions of the Indian and/or Asian lithospheric mantle are still being subducted. The resulting slab-pull is probably comparable with that from complete lithospheric slabs and may promote continued continental convergence even after collision. Since descending lithospheric slabs are present at all collision zones at the time of collision such continued convergence may be general after continental collisions. It may cease only when there is a major (global) plate reorganization which results in new forces on the convergent continents that may counteractmore » the slab-pull. These inferences may be tested on the late Paleozoic collision between Gondwanaland and Laurasia. This is generally considered to have been complete by mid-Permian time (250 Ma). However, this may be only the time of docking of Gondwanaland with North America, not that of the cessation of convergence. Paleomagnetic polar-wander paths for the Gondwanide continents exhibit consistently greater latitudinal shifts from 250 Ma to 200 Ma than those of Laurasia when corrected for post-Triassic drift, suggesting that convergence continued through late Permian well into the Triassic. It may have been accommodated by crustal thickening under what is now the US Coastal Plain, or by strike-slip faulting. Convergence may have ceased only when Pangea began to fragment again, in which case the cause for its cessation may be related to the cause of continental fragmentation.« less

Horizontal mantle flow controls subduction dynamics.

PubMed

Ficini, E; Dal Zilio, L; Doglioni, C; Gerya, T V

2017-08-08

It is generally accepted that subduction is driven by downgoing-plate negative buoyancy. Yet plate age -the main control on buoyancy- exhibits little correlation with most of the present-day subduction velocities and slab dips. "West"-directed subduction zones are on average steeper (~65°) than "East"-directed (~27°). Also, a "westerly"-directed net rotation of the lithosphere relative to the mantle has been detected in the hotspot reference frame. Thus, the existence of an "easterly"-directed horizontal mantle wind could explain this subduction asymmetry, favouring steepening or lifting of slab dip angles. Here we test this hypothesis using high-resolution two-dimensional numerical thermomechanical models of oceanic plate subduction interacting with a mantle flow. Results show that when subduction polarity is opposite to that of the mantle flow, the descending slab dips subvertically and the hinge retreats, thus leading to the development of a back-arc basin. In contrast, concordance between mantle flow and subduction polarity results in shallow dipping subduction, hinge advance and pronounced topography of the overriding plate, regardless of their age-dependent negative buoyancy. Our results are consistent with seismicity data and tomographic images of subduction zones. Thus, our models may explain why subduction asymmetry is a common feature of convergent margins on Earth.

Paleomagnetic Constraints on the Tectonic History of the Mesozoic Ophiolite and Arc Terranes of Western Mexico

NASA Astrophysics Data System (ADS)

Boschman, L.; Van Hinsbergen, D. J. J.; Langereis, C. G.; Molina-Garza, R. S.; Kimbrough, D. L.

2017-12-01

The North American Cordillera has been shaped by a long history of accretion of arcs and other buoyant crustal fragments to the western margin of the North American Plate since the Early Mesozoic. Accretion of these terranes resulted from a complex tectonic history interpreted to include episodes of both intra-oceanic subduction within the Panthalassa/Pacific Ocean, as well as continental margin subduction along the western margin of North America. Western Mexico, at the southern end of the Cordillera, contains a Late Cretaceous-present day long-lived continental margin arc, as well as Mesozoic arc and SSZ ophiolite assemblages of which the origin is under debate. Interpretations of the origin of these subduction-related rock assemblages vary from far-travelled exotic intra-oceanic island arc character to autochthonous or parautochthonous extended continental margin origin. We present new paleomagnetic data from four localities: (1) the Norian SSZ Vizcaíno peninsula Ophiolite; (2) its Lower Jurassic sedimentary cover; and (3) Barremian and (4) Aptian sediments derived from the Guerrero arc. The data show that the Mexican ophiolite and arc terranes have a paleolatitudinal plate motion history that is equal to that of the North American continent. This suggests that these rock assemblages were part of the overriding plate and were perhaps only separated from the North American continent by temporal fore- or back-arc spreading. These spreading phases resulted in the temporal existence of tectonic plates between the North American and Farallon Plates, and upon closure of the basins, in the growth of the North American continent without addition of any far-travelled exotic terranes.

Trench curvature and deformation of the subducting lithosphere

NASA Astrophysics Data System (ADS)

Schettino, Antonio; Tassi, Luca

2012-01-01

The subduction of oceanic lithosphere is generally accompanied by downdip and lateral deformation. The downdip component of strain is associated with external forces that are applied to the slab during its sinking, namely the gravitational force and the mantle resistance to penetration. Here, we present theoretical arguments showing that a tectonic plate is also subject to a predictable amount of lateral deformation as a consequence of its bending along an arcuate trench zone, independently from the long-term physical processes that have determined the actual curvature of the subduction zone. In particular, we show that the state of lateral strain and the lateral strain rate of a subducting slab depend from geometric and kinematic parameters, such as trench curvature, dip function and subduction velocity. We also demonstrate that the relationship between the state of lateral strain in a subducting slab and the geometry of bending at the corresponding active margin implies a small component of lateral shortening at shallow depths, and may include large extensional lateral deformation at intermediate depths, whereas a state of lateral mechanical equilibrium can only represent a localized exception. Our formulation overcomes the flaws of the classic 'ping-pong ball' model for the bending of the lithosphere at subduction zones, which lead to severe discrepancies with the observed geometry and style of deformation of the modern subducting slabs. A study of the geometry and seismicity of eight modern subduction zones is performed, to assess the validity of the theoretical relationship between trench curvature, slab dip function, and lateral strain rate. The strain pattern within the eight present-day slabs, which is reconstructed through an analysis of Harvard CMT solutions, shows that tectonic plates cannot be considered as flexible-inextensible spherical caps, whereas the lateral intraslab deformation which is accommodated through seismic slip can be explained in terms

Varying Structure and Physical Properties of the Lithosphere Subducting Beneath Indonesia, Consequences on the Subduction

NASA Astrophysics Data System (ADS)

Jacob, J.; Dyment, J.

2013-12-01

We make inferences on the structure, age and physical properties of the subducting northern Wharton Basin lithosphere by (1) modeling the structure and age of the lithosphere subducted under the Sumatra trench through three-plate reconstructions involving Australia, Antarctica, and India, and (2) superimposing the resulting fracture zones and magnetic isochrons to the geometry of the subducting plate as imaged by seismic tomography. The model of Pesicek et al. (2010) was digitized and smoothed in order to get a realistic topography of the subducting plate. The fracture zone and magnetic isochron geometry was draped on this topography assuming a N18°E direction of subduction. This model provides an effective means to study the effect of varying physical properties of the subducting lithosphere on the subduction along the Sumatra trench. 1) The age of the oceanic lithosphere determines its thickness and buoyancy, then its ability to comply with or resist subduction. We define the "subductability" of the lithosphere as the extra weight applied on the asthenosphere by the part of the bulk lithospheric density exceeding the asthenospheric density. A negative subductability means that the bulk lithospheric density is lower than the asthenospheric density, i.e. the plate will resist subduction, which is the case for lithosphere less than ~23 Ma. The area off Sumatra corresponds to oceanic lithosphere formed between 80 and 38 Ma, with a lower subductability than other areas along the Sunda Trench. 2) The spreading rate at which the oceanic lithosphere was formed has implications of the structure and composition of the oceanic crust, and therefore on its rheology. In a subduction zone, the contact between the subducting and overriding plates is often considered to be the top of the oceanic crust and the overlying sediments. The roughness of this interface and the rheology of its constitutive material are essential parameters constraining the slip of the down going plate in

Shallow depth of seismogenic coupling in southern Mexico: implications for the maximum size of earthquakes in the subduction zone

NASA Astrophysics Data System (ADS)

Suárez, Gerardo; Sánchez, Osvaldo

1996-01-01

Studies of locally recorded microearthquakes and the centroidal depths of the largest earthquakes analyzed using teleseismic data show that the maximum depth of thrust faulting along the Mexican subduction zone is anomalously shallow. This observed maximum depth of about 25 ± 5 km is about half of that observed in most subduction zones of the world. A leveling line that crosses the rupture zone of the 19 September 1985 Michoacan event was revisited after the earthquake and it shows anomalously low deformation during the earthquake. The comparison between the observed coseismic uplift and dislocation models of the seismogenic interplate contact that extend to depths ranging from 20 to 40 km shows that the maximum depth at which seismic slip took place is about 20 km. This unusually shallow and narrow zone of seismogenic coupling apparently results in the occurrence of thrust events along the Mexican subduction zone that are smaller than would be expected for a trench where a relatively young slab subducts at a rapid rate of relative motion. A comparison with the Chilean subduction zone shows that the plate interface in Mexico is half that in Chile, not only in the down-dip extent of the seismogenic zone of plate contact, but also in the distance of the trench from the coast and in the thickness of the upper continental plate. It appears that the narrow plate contact produced by this particular plate geometry in Mexico is the controlling variable defining the size of the largest characteristic earthquakes in the Mexican subduction zone.

­­New Finite-Frequency Teleseismic P-wave Tomography of the Anatolian Sub-continent and the Fate of the Subducted Cyprean Slab

NASA Astrophysics Data System (ADS)

Portner, D. E.; Biryol, C. B.; Delph, J. R.; Beck, S. L.; Zandt, G.; Özacar, A.; Sandvol, E. A.; Turkelli, N.

2016-12-01

The eastern Mediterranean region is characterized by active subduction of Tethyan lithosphere beneath the Anatolian sub-continent at the Aegean and Cyprean trenches. The subduction system is historically characterized by slab roll-back, detachment, and slab settling in the mantle transition zone. Prior mantle tomography studies reveal segmentation of the subducted Tethyan lithosphere, which is thought to have a strong control on surface volcanism and uplift across Anatolia. However, tomographic resolution, particularly in central Anatolia, has been limited, thus making detailed delineations of the subducted slab segments difficult. To improve resolution, we combine two years of seismic data from the recent Continental Dynamics - Central Anatolia Tectonics (CD-CAT) seismic deployment and Turkey's national seismic network ( 33,000 residuals) to 33,000 travel time residuals from Biryol et al. (2011, GJI) in a new finite-frequency teleseismic P-wave tomographic inversion. Our new images reveal with detail a complicated geometry of fast velocity anomalies associated with subducted Tethyan lithosphere. At shallow depths, slow velocities separate the fast anomalies connected to the Aegean and Cyprean trenches. The fast anomaly connected to the Cyprean trench has an arcuate shape in map view, following the trace of the Central Taurus Mountains. This anomaly is separated from a high-amplitude block to the north that appears to dip sub-vertically throughout the upper mantle (200-660 km depth). Other blocks of fast material that may represent subducted Tethyan lithosphere appear down-dip of the vertical block. Additionally, our images indicate that some of the fast velocity anomalies previously seen to flatten in the mantle transition zone may continue into the lower mantle. Thus, our new images provide a more detailed picture of the fate of the Cyprean slab and suggest that some of the fast anomalies associated with the slab continue into the lower mantle, bringing to

Modelling of sea floor spreading initiation and rifted continental margin formation

NASA Astrophysics Data System (ADS)

Tymms, V. J.; Isimm Team

2003-04-01

Recent observations of depth dependent (heterogeneous) stretching where upper crustal extension is much less than that of the lower crust and lithospheric mantle at both non-volcanic and volcanic margins plus the discovery of broad domains of exhumed continental mantle at non-volcanic rifted margins are not predicted by existing quantitative models of rifted margin formation which are usually based on intra-continental rift models subjected to very large stretching factors. New conceptual and quantitative models of rifted margin formation are required. Observations and continuum mechanics suggest that the dominant process responsible for rifted continental margin formation is sea-floor spreading of the young ocean ridge, rather than pre-breakup intra-continental rifting. Simple fluid flow models of ocean ridge processes using analytical iso-viscous corner-flow demonstrate that the divergent motion of the upwelling mantle beneath the ocean ridge, when viewed in the reference frame of the young continental margin, shows oceanward flow of the lower continental crust and lithospheric mantle of the young rifted margin giving rise to depth dependent stretching as observed. Single-phase fluid-models have been developed to model the initiation of sea-floor spreading and the thermal, stretching and thinning evolution of the young rifted continental margin. Finite element fluid-flow modelling incorporating the evolving temperature dependent viscosity field on the fluid flow also show depth dependent stretching of the young continental margin. Two-phase flow models of ocean ridges incorporating the transport of both solid matrix and melt fluid (Spiegelman &Reynolds 1999) predict the divergent motion of the asthenosphere and lithosphere matrix, and the focusing of basaltic melt into the narrow axial zone spreading centre at ocean ridges. We are adapting two-phase flow models for application to the initiation of sea-floor spreading and rifted continental margin formation. i

Anatomy of an ancient subduction interface at 40 km depth: Insights from P-T-t-d data, and geodynamic implications (Dent Blanche, Western Alps)

NASA Astrophysics Data System (ADS)

Angiboust, Samuel; Glodny, Johannes; Oncken, Onno; Chopin, Christian

2014-05-01

An exhumed metamorphic suture zone over 40 km long is exposed in the Dent Blanche Region of the Western Alps belt, along the Swiss-Italian border. In this region, the metasediment-bearing ophiolitic remnants of the Liguro-Piemontese ocean (Tsaté complex) are overthrusted by a continental, km-sized complex (Dent Blanche Tectonic System: DBTS) of Austro-Alpine affinity. The DBTS represents a strongly deformed composite terrane with independent tectonic slices of continental and oceanic origin. In order to better understand the nature and the geodynamic meaning of the shear zone at the base of the DBTS (Dent Blanche Thrust, DBT) we re-evaluated the pressure-temperature-time-deformation (P-T-t-d) history of these two units using modern thermobarometric tools, Rb/Sr deformation ages and field relationships. Our results show that the Tsaté complex is formed by a stack of km-thick calcschists-bearing tectonic slices, having experienced variable maximum burial temperatures of between 360°C and 490°C at depths of ca. 25-40 km, between 41 Ma and 37 Ma. The Arolla gneissic mylonites constituting the base of the DBTS experienced a continuous record of protracted high-pressure (12-14 kbar), top-to-NW D1 deformation at 450-500°C between 43 and 55 Ma. Some of these primary, peak metamorphic fabrics have been sheared (top-to-SE D2) and backfolded during exhumation and collisional overprint (20 km depth, 35-40 Ma) leading to the regional greenschist facies retrogression particularly prominent within Tsaté metasediments. The final juxtaposition of the DBTS with the Tsaté complex occurred between 350 and 500°C during this later, exhumation-related D2 event. Although some exhumation-related deformation partially reworked D1 primary features, we emphasize that the DBT can be viewed as a remnant of the Alpine early Eocene blueschist-facies subduction interface region. The DBT therefore constitutes the deeper equivalent of some shallower portions of the Alpine subduction

Middle Ordovician subduction of continental crust in the Scandinavian Caledonides: an example from Tjeliken, Seve Nappe Complex, Sweden

NASA Astrophysics Data System (ADS)

Fassmer, Kathrin; Klonowska, Iwona; Walczak, Katarzyna; Andersson, Barbro; Froitzheim, Nikolaus; Majka, Jarosław; Fonseca, Raúl O. C.; Münker, Carsten; Janák, Marian; Whitehouse, Martin

2017-12-01

The Seve Nappe Complex of the Scandinavian Caledonides is thought to be derived from the distal passive margin of Baltica which collided with Laurentia in the Scandian Phase of the Caledonian Orogeny at 430-400 Ma. Parts of the Seve Nappe Complex were affected by pre-Scandian high- and ultrahigh-pressure metamorphism, in a tectonic framework that is still unclear, partly due to uncertainties about the exact timing. Previous age determinations yielded between 505 and 446 Ma, with a general trend of older ages in the North (Norrbotten) than in the South (Jämtland). New age determinations were performed on eclogite and garnet-phengite gneiss at Tjeliken in northern Jämtland. Thermodynamic modelling yielded peak metamorphic conditions of 25-27 kbar/680-760 °C for the garnet-phengite gneiss, similar to published peak metamorphic conditions of the eclogite (25-26 kbar/650-700 °C). Metamorphic rims of zircons from the garnet-phengite gneiss were dated using secondary ion mass spectrometry and yielded a concordia age of 458.9 ± 2.5 Ma. Lu-Hf garnet-whole rock dating yielded 458 ± 1.0 Ma for the eclogite. Garnet in the eclogite shows prograde major-element zoning and concentration of Lu in the cores, indicating that this age is related to garnet growth during pressure increase, i.e. subduction. The identical ages from both rock types, coinciding with published Sm-Nd ages from the eclogite, confirm subduction of the Seve Nappe Complex in Northern Jämtland during the Middle Ordovician in a fast subduction-exhumation cycle.

A revised dislocation model of interseismic deformation of the Cascadia subduction zone

USGS Publications Warehouse

Wang, Kelin; Wells, Ray E.; Mazzotti, Stephane; Hyndman, Roy D.; Sagiya, Takeshi

2003-01-01

CAS3D‐2, a new three‐dimensional (3‐D) dislocation model, is developed to model interseismic deformation rates at the Cascadia subduction zone. The model is considered a snapshot description of the deformation field that changes with time. The effect of northward secular motion of the central and southern Cascadia forearc sliver is subtracted to obtain the effective convergence between the subducting plate and the forearc. Horizontal deformation data, including strain rates and surface velocities from Global Positioning System (GPS) measurements, provide primary geodetic constraints, but uplift rate data from tide gauges and leveling also provide important validations for the model. A locked zone, based on the results of previous thermal models constrained by heat flow observations, is located entirely offshore beneath the continental slope. Similar to previous dislocation models, an effective zone of downdip transition from locking to full slip is used, but the slip deficit rate is assumed to decrease exponentially with downdip distance. The exponential function resolves the problem of overpredicting coastal GPS velocities and underpredicting inland velocities by previous models that used a linear downdip transition. A wide effective transition zone (ETZ) partially accounts for stress relaxation in the mantle wedge that cannot be simulated by the elastic model. The pattern of coseismic deformation is expected to be different from that of interseismic deformation at present, 300 years after the last great subduction earthquake. The downdip transition from full rupture to no slip should take place over a much narrower zone.

Earthquake hazards on the cascadia subduction zone.

PubMed

Heaton, T H; Hartzell, S H

1987-04-10

Large subduction earthquakes on the Cascadia subduction zone pose a potential seismic hazard. Very young oceanic lithosphere (10 million years old) is being subducted beneath North America at a rate of approximately 4 centimeters per year. The Cascadia subduction zone shares many characteristics with subduction zones in southern Chile, southwestern Japan, and Colombia, where comparably young oceanic lithosphere is also subducting. Very large subduction earthquakes, ranging in energy magnitude (M(w)) between 8 and 9.5, have occurred along these other subduction zones. If the Cascadia subduction zone is also storing elastic energy, a sequence of several great earthquakes (M(w) 8) or a giant earthquake (M(w) 9) would be necessary to fill this 1200-kilometer gap. The nature of strong ground motions recorded during subduction earthquakes of M(w) less than 8.2 is discussed. Strong ground motions from even larger earthquakes (M(w) up to 9.5) are estimated by simple simulations. If large subduction earthquakes occur in the Pacific Northwest, relatively strong shaking can be expected over a large region. Such earthquakes may also be accompanied by large local tsunamis.

Tear geometry at active STEPs: an analogue model approach

NASA Astrophysics Data System (ADS)

Broerse, Taco; Sokoutis, Dimitrios; Willingshofer, Ernst; Govers, Rob

2017-04-01

At the lateral end of a subduction zone, tearing of lithosphere is the result of subduction of oceanic lithosphere while adjacent buoyant continental lithosphere stays at the surface. The location of lithospheric tearing is called a Subduction-Transform-Edge-Propagator (STEP), which continuously extends the plate boundary between overriding plate and continental lithosphere. One of our areas of interest is the southern Caribbean where Atlantic lithosphere subducts below the Caribbean plate. Mantle tomography suggests a clear southern edge of the Lesser Antilles slab, which makes the boundary between the Caribbean and South America a clear STEP candidate. At the surface, the San Sebastián/El Pilar fault zone forms the plate boundary between the Caribbean and South America and the active STEP is located near Trinidad. For the deeper part of the damage/shear zone, some information is available from a recent 3D gravity study: significant lateral variability in densities of the lithospheric mantle to the south of the STEP fault zone. The low-density zone may result from higher sub-crustal temperatures, such as would arise from an asthenospheric window resulting from detachment. Interpreted in this way, the mantle part of the damage zone may be 200-250 km wide. So, while the location of the plate boundary at the surface is relatively well resolved, little is known about the deeper continuation of the active STEP in the mantle lithosphere. We study the evolution of the tearing process at a STEP using analogue models. In our models we use silicone putty (lithosphere) and glucose (asthenosphere). Solely gravitational forces resulting from density differences between oceanic lithosphere and asthenosphere drive our model. Lithospheric tearing commences after subduction has initiated. The geometry of the tear varies with the rheology of the lithosphere and asthenosphere, particularly Newtonian versus power-law. We investigate the dependence on model parameters of the width of

High-pressure metamorphic age and significance of eclogite-facies continental fragments associated with oceanic lithosphere in the Western Alps (Etirol-Levaz Slice, Valtournenche, Italy)

NASA Astrophysics Data System (ADS)

Fassmer, Kathrin; Obermüller, Gerrit; Nagel, Thorsten J.; Kirst, Frederik; Froitzheim, Nikolaus; Sandmann, Sascha; Miladinova, Irena; Fonseca, Raúl O. C.; Münker, Carsten

2016-05-01

The Etirol-Levaz Slice in the Penninic Alps (Valtournenche, Italy) is a piece of eclogite-facies continental basement sandwiched between two oceanic units, the blueschist-facies Combin Zone in the hanging wall and the eclogite-facies Zermatt-Saas Zone in the footwall. It has been interpreted as an extensional allochthon from the continental margin of Adria, emplaced onto ultramafic and mafic basement of the future Zermatt-Saas Zone by Jurassic, rifting-related detachment faulting, and later subducted together with the future Zermatt-Saas Zone. Alternatively, the Etirol-Levaz Slice could be derived from a different paleogeographic domain and be separated from the Zermatt-Saas Zone by an Alpine shear zone. We present Lu-Hf whole rock-garnet ages of two eclogite samples, one from the center of the unit and one from the border to the Zermatt-Saas Zone below. These data are accompanied by a new geological map of the Etirol-Levaz Slice and the surrounding area, as well as detailed petrology of these two samples. Assemblages, mineral compositions and garnet zoning in both samples indicate a clockwise PT-path and peak-metamorphic conditions of about 550-600 °C/20-25 kbar, similar to conditions proposed for the underlying Zermatt-Saas Zone. Prograde garnet ages of the two samples are 61.8 ± 1.8 Ma and 52.4 ± 2.1 Ma and reflect different timing of subduction. One of these is significantly older than published ages of eclogite-facies metamorphism in the Zermatt-Saas Zone and thus contradicts the hypothesis of Mesozoic emplacement. The occurrence of serpentinite and metagabbro bodies possibly derived from the Zermatt-Saas Zone inside the Etirol-Levaz Slice suggests that the latter is a tectonic composite. The basement slivers forming the Etirol-Levaz Slice and other continental fragments were subducted earlier than the Zermatt-Saas Zone, but nonetheless experienced similar pressure-temperature histories. Our results support the hypothesis that the Zermatt-Saas Zone and the

The influence of regional extensional tectonic stress on the eruptive behaviour of subduction-zone volcanoes

NASA Astrophysics Data System (ADS)

Tost, M.; Cronin, S. J.

2015-12-01

Regional tectonic stress is considered a trigger mechanism for explosive volcanic activity, but the related mechanisms at depth are not well understood. The unique geological setting of Ruapehu, New Zealand, allows investigation on the effect of enhanced regional extensional crustal tension on the eruptive behaviour of subduction-zone volcanoes. The composite cone is located at the southwestern terminus of the Taupo Volcanic Zone, one of the most active silicic magma systems on Earth, which extends through the central part of New Zealand's North Island. Rhyolitic caldera eruptions are limited to its central part where crustal extension is highest, whereas lower extension and additional dextral shear dominate in the southwestern and northeastern segments characterized by andesitic volcanism. South of Ruapehu, the intra-arc rift zone traverses into a compressional geological setting with updoming marine sequences dissected by reverse and normal faults. The current eruptive behaviour of Ruapehu is dominated by small-scaled vulcanian eruptions, but our studies indicate that subplinian to plinian eruptions have frequently occurred since ≥340 ka and were usually preceded by major rhyolitic caldera unrest in the Taupo Volcanic Zone. Pre-existing structures related to the NNW-SSE trending subduction-zone setting are thought to extend at depth and create preferred pathways for the silicic magma bodies, which may facilitate the development of large (>100 km3) dyke-like upper-crustal storage systems prior to major caldera activity. This may cause enhanced extensional stress throughout the entire intra-arc setting, including the Ruapehu area. During periods of caldera dormancy, the thick crust underlying the volcano and the enhanced dextral share rate likely impede ascent of larger andesitic magma bodies, and storage of andesitic melts dominantly occurs within small-scaled magma bodies at middle- to lower-crustal levels. During episodes of major caldera unrest, ascent and

Coupling of Oceanic and Continental Crust During Eocene Eclogite-Facies Metamorphism: Evidence From the Monte Rosa Nappe, Western Alps, Italy

NASA Astrophysics Data System (ADS)

Lapen, T. J.; Johnson, C. M.; Baumgartner, L. P.; Skora, S.; Mahlen, N. J.; Beard, B. L.

2006-12-01

Subduction of continental crust to HP-UHP metamorphic conditions requires overcoming density contrasts that are unfavorable to deep burial, whereas exhumation of these rocks can be reasonably explained through buoyancy-assisted transport in the subduction channel to more shallow depths. In the western Alps, both continental and oceanic lithosphere has been subducted to eclogite-facies metamorphic conditions. The burial and exhumation histories of these sections of lithosphere bear directly on the dynamics of subduction and the stacking of units within the subduction channel. We address the burial history of the continental crust with high precision U-Pb rutile and Lu-Hf garnet geochronology of the eclogite-facies Monte Rosa nappe (MR), western Alps, Italy. U-Pb rutile ages from quartz-carbonate-white mica-rutile veins that are hosted within eclogite and schist of the MR, Gressoney Valley, Italy, indicate that it was at eclogite-facies metamorphic conditions at 42.6 +/- 0.6 Ma. The sample area (Indren glacier, Furgg zone; Dal Piaz, 2001) consists of eclogite boudins that are surrounded by micaceous schist. Associated with the eclogite and schist are quartz-carbonate-white mica-rutile veins that formed in tension cracks in the eclogite and along the contact between eclogite and surrounding schist. Intrusion of the veins occurred at eclogite-facies metamorphic conditions (480-570°C, >1.3-1.4 GPa) based on textural relations, oxygen isotope thermometry, and geothermobarometry. Lu-Hf geochronology of garnet from a chloritoid-talc-garnet-phengite-quartz-calcite-pyrite - chalcopyrite bearing boudin within talc-chloritoid whiteschists of the MR, Val d'Ayas, Italy (Chopin and Monie, 1984; Pawlig, 2001) yields an age of 40.54 +/- 0.36 Ma. The talc-chloritoid whiteschists from the area record pressures and temperatures of 1.6-2.4 GPa and 500-530°C (Chopin and Monie, 1984; Le Bayon et al., 2006) indicating near UHP metamorphic conditions. Based on the age, P-T, and textural

High-Mg subduction-related Tertiary basalts in Sardinia, Italy

NASA Astrophysics Data System (ADS)

Morra, V.; Secchi, F. A. G.; Melluso, L.; Franciosi, L.

1997-03-01

The Oligo-Miocene volcanics (32-15 Ma), which occur in the Oligo-Miocene Sardinian Rift, were interpreted in the literature as an intracontinental volcanic arc built upon continental crust about 30 km thick. They are characterized by a close field association of dominantly andesites and acid ignimbrites, with subordinate basalts. In this paper we deal with the origin and evolution of recently discovered high-magnesia basalts aged ca. 18 Ma occurring in the Montresta area, northern Sardinia, relevant to the petrogenesis of the Cenozoic volcanics of Sardinia. The igneous rocks of the Montresta area form a tholeiitic, subduction-related suite. Major-element variation from the high-magnesia basalts (HMB) to high-alumina basalts (HAB) are consistent with crystal/liquid fractionation dominated by olivine and clinopyroxene. Proportions of plagioclase and titanomagnetite increase from HAB to andesites. Initial {87Sr }/{86Sr } ratios increase with differentiation from 0.70398 for the HMB to 0.70592 for the andesites. This suggests concomitant crustal contamination. The geochemical characteristics of the high-magnesia basalts are typical of subduction-related magmas, with negative Nb, Zr and Ti spikes in mantle-normalized diagrams. It is proposed that these high-magnesia basalts were produced by partial melting of a mantle source characterized by large-ion lithophile elements (LILE) enrichment related principally to dehydration of subducted oceanic crust. Chondrite-normalized rare earth elements (REE) patterns indicate that the lavas are somewhat enriched in light rare earth elements (LREE), with flat heavy rare earth elements (HREE) patterns. This evidence is consistent with a spinel-bearing mantle source. The sub-parallel chondrite-normalized patterns show enrichment with differentiation, with a greater increase of LREE than HREE. The occurrence of high-magnesia basalts at 18 Ma in Sardinia appears to be correlated with and favoured by pronounced extensional tectonics at

The Gibraltar subduction: A decade of new geophysical data

NASA Astrophysics Data System (ADS)

Gutscher, M.-A.; Dominguez, S.; Westbrook, G. K.; Le Roy, P.; Rosas, F.; Duarte, J. C.; Terrinha, P.; Miranda, J. M.; Graindorge, D.; Gailler, A.; Sallares, V.; Bartolome, R.

2012-10-01

The Gibraltar arc, spans a complex portion of the Africa-Eurasia plate boundary marked by slow oblique convergence and intermediate and deep focus seismicity. The seemingly contradictory observations of a young extensional marine basin surrounded by an arcuate fold-and-thrust belt, have led to competing geodynamic models (delamination and subduction). Geophysical data acquired in the past decade provide a test for these models and support a narrow east-dipping, subduction zone. Seismic refraction studies indicate oceanic crust below the western Gulf of Cadiz. Tomography of the upper mantle reveals a steep, east-dipping high P-wave velocity body, beneath Gibraltar. The anisotropic mantle fabric from SKS splitting shows arc-parallel "fast directions", consistent with toroidal flow around a narrow, westward retreating subducting slab. The accompanying WSW advance of the Rif-Betic mountain belt has constructed a thick pile of deformed sediments, an accretionary wedge, characterized by west-vergent thrust anticlines. Bathymetric swath-mapping images an asymmetric embayment at the deformation front where a 2 km high basement ridge has collided. Subduction has slowed significantly since 5 Ma, but deformation of recent sediments and abundant mud volcanoes suggest ongoing activity in the accretionary wedge. Three possible origins for this deformation are discussed; gravitational spreading, overall NW-SE convergence between Africa and Iberia and finally a WSW tectonic push from slow, but ongoing roll-back subduction. In the absence of arc volcanism and shallow dipping thrust type earthquakes, evidence in favor of present-day subduction can only be indirect and remains the object of debate. Continued activity of the subduction offers a possible explanation for great (M > 8.5) earthquakes known to affect the area, like the famous 1755 Great Lisbon earthquake. Recent GPS studies show SW motion of stations in N Morocco at velocities of 3-6 mm/yr indicating the presence of an

Tectonic evolution and crustal-scale structure of Kyrgyz Central Asian Orogenic Belt: new insights from the Darius programme

NASA Astrophysics Data System (ADS)

Rolland, Yann; Loury, Chloé; Guillot, Stéphane; Mikolaichuk, Alexander

2014-05-01

Mechanisms and history of the Late Palaeozoic accretion followed by formation of trunscurrent strike-slip faults were studied in the southern segment of the Central Asian Orogenic Belt (CAOB) within Kyrgyz South Tianshan. 1. South Tianshan Suture: ending accretion process after docking of Tarim craton This study gives insights into the crustal-scale structure and Upper Paleozoic history of this mountain belt, currently intensely reactivated by the India-Asia collision. Structural, petrological and geochronological studies were carried out within South Tianshan suture east of the Talas-Ferghana Fault (TFF). New data highlight a south-dipping structure featured by a HP metamorphic core complex comprised of c. 320 Ma continental and oceanic eclogites exhumed by top-to-North motion. A large massif (10 x 50 km) of continental HP rocks in the Atbashi Range is comprised of hectometric boudins of eclogites embedded in metapelites and gneissesMetamorphic units exhibit blueschist to eclogite facies conditions, with oceanic (MORB) rocks in the blueschist facies representing the accretionary oceanic prism being thrusted by oceanic rocks and a continental unit in the eclogite facies (510 ± 50°C and 24 ± 2 kbar). Evidence for eclogite facies both in metasediments and mafic lithologies and geological structure are in agreement with a previously thinned continental margin. Subduction of this thinned COT (Continent-Ocean Transition) probably occurred by slab pull in a south-dipping subduction zone, while another north-dipping subduction was active below Middle Tianshan. Final stacking of Middle and South Tianshan occurred at 320-310 Ma. These opposite subduction zones are still reflected in the main structures of Tianshan. Reactivation of the South-dipping structures since 30-25 Ma is ascribed to explain the current Tianshan intra-continental subduction from seismology. 2. Talas-Ferghana Fault (TFF) activity & Basin formation After this accretionary episode, the South Tianshan

Heterogeneous coupling along Makran subduction zone

NASA Astrophysics Data System (ADS)

Zarifi, Z.; Raeesi, M.

2010-12-01

The Makran subduction zone, located in the southeast of Iran and southern Pakistan, extends for almost 900 km along the Eurasian-Arabian plate boundary. The seismic activities in the eastern and western Makran exhibit very different patterns. The eastern Makran characterized by infrequent large earthquakes and low level of seismicity. The only large instrumentally recorded earthquake in the eastern Makran, the 27 Nov. 1945 (Mw=8.1) earthquake, was followed by tsunami waves with the maximum run-up height of 13 m and disastrous effects in Pakistan, India, Iran and Oman. The western Makran, however, is apparently quiescent without strong evidence on occurrence of large earthquakes in historical times, which makes it difficult to ascertain whether the slab subducts aseismically or experiences large earthquakes separated by long periods exceeding the historical records. We used seismicity and Trench Parallel Free air and Bouguer Anomalies (TPGA and TPBA) to study the variation in coupling in the slab interface. Using a 3D mechanical Finite Element (FE) model, we show how heterogeneous coupling can influence the rate of deformation in the overriding lithosphere and the state of stress in the outer rise, overriding, and subducting plates within the shortest expected cycle of earthquake. We test the results of FE model against the observed focal mechanism of earthquakes and available GPS measurements in Makran subduction zone.

Crustal Structure of the Yakutat Microplate: New Constraints for Understanding the Evolution of Subduction and Collision in southern Alaska

NASA Astrophysics Data System (ADS)

Worthington, L. L.; van Avendonk, H. J.; Gulick, S. P.; Christeson, G. L.; Pavlis, T. L.

2010-12-01

Flat-slab subduction and accretion of the Yakutat (YAK) microplate in southern Alaska characterizes the most recent iteration in the process of terrane accretion that has built the tectonic assemblage of the Canada-Alaska Cordillera since the Mesozoic. Despite the potentially pivotal role of the Yakutat collision in the evolution and deformation of the North American Cordillera, major questions regarding locations of active faults and velocity structure and thickness of the Yakutat block itself have gone unanswered. We present results of a 2008 marine seismic reflection/refraction survey acquired as part of the St. Elias Erosion and Tectonics Project (STEEP), a multi-disciplinary NSF-Continental Dynamics project aimed at structural evolution and geodynamics related to the YAK collision. An onshore-offshore wide-angle refraction profile shows YAK crustal thickness ranging from ~15 km near the Bering Glacier to ~35 km east of the Dangerous River Zone (DRZ), with calculated lower crustal velocities potentially >7km/s. Crustal velocity and structure are continuous across the DRZ on the YAK shelf, which is historically described as a vertical boundary between continental crust on the east and oceanic basement on the west. Instead, we observe a gradual shallowing of elevated crustal velocities associated with a basement high observed on coincident marine reflection data near the DRZ. Crustal velocity and thicknesses are comparable to the Kerguelen oceanic plateau and the Siletz terrane, thus supporting the oceanic plateau theory for the origin of the YAK microplate. The observed variable crustal thickness indicates that the YAK slab may be slightly wedge-shaped, thinning in the direction of subduction. The thickest portion of the offshore YAK is entering the orogen near the eastern syntaxis, where the Fairweather fault system encounters a restraining bend as its orientation changes from north-south to east-west. It follows that observations of elevated exhumation rates

Large scale obduction of preserved oceanic crust: linking the Lesser Caucasus and NE Anatolian ophiolites and implications for the formation of the Lesser Caucasus-Pontides Arc

NASA Astrophysics Data System (ADS)

Hassig, Marc; Rolland, Yann; Sosson, Marc; Galoyan, Ghazar; Sahakyan, Lilit; Topuz, Gultelin; Farouk Çelik, Omer; Avagyan, Ara; Muller, Carla

2014-05-01

During the Mesozoic, the Southern margin of the Eurasian continent was involved in the closure of the Paleotethys and opening Neotethys Ocean. Later, from the Jurassic to the Eocene, subductions, obductions, micro-plate accretions, and finally continent-continent collision occurred between Eurasia and Arabia, and resulted in the closure of Neotethys. In the Lesser Caucasus and NE Anatolia three main domains are distinguished from South to North: (1) the South Armenian Block (SAB) and the Tauride-Anatolide Platform (TAP), Gondwanian-derived continental terranes; (2) scattered outcrops of ophiolite bodies, coming up against the Sevan-Akera and Ankara-Erzincan suture zones; and (3) the Eurasian plate, represented by the Eastern Pontides margin and the Somkheto-Karabagh Arc. The slivers of ophiolites are preserved non-metamorphic relics of the now disappeared Northern Neotethys oceanic domain overthrusting onto the continental South Armenian Block (SAB) as well as on the Tauride-Anatolide plateform from the north to the south. It is important to point out that the major part of this oceanic lithosphere disappeared by subduction under the Eurasian Margin to the north. In the Lesser Caucasus, works using geochemical whole-rock analyses, 40Ar/39Ar dating of basalts and gabbro amphiboles and paleontological dating have shown that the obducted oceanic domain originates from a back-arc setting formed throughout Middle Jurassic times. The comprehension of the geodynamic evolution of the Lesser Caucasus supports the presence of two north dipping subduction zones: (1) a subduction under the Eurasian margin and to the south by (2) an intra-oceanic subduction allowing the continental domain to subduct under the oceanic lithosphere, thus leading to ophiolite emplacement. To the West, the NE Anatolian ophiolites have been intensely studied with the aim to characterize the type of oceanic crust which they originated from. Geochemical analyses have shown similar rock types as in

Active and long-lived permanent forearc deformation driven by the subduction seismic cycle

NASA Astrophysics Data System (ADS)

Aron Melo, Felipe Alejandro

I have used geological, geophysical and engineering methods to explore mechanisms of upper plate, brittle deformation at active forearc regions. My dissertation particularly addresses the permanent deformation style experienced by the forearc following great subduction ruptures, such as the 2010 M w8.8 Maule, Chile and 2011 Mw9.0 Tohoku, Japan earthquakes. These events triggered large, shallow seismicity on upper plate normal faults above the rupture reaching Mw7.0. First I present new structural data from the Chilean Coastal Cordillera over the rupture zone of the Maule earthquake. The study area contains the Pichilemu normal fault, which produced the large crustal aftershocks of the megathrust event. Normal faults are the major neotectonic structural elements but reverse faults also exist. Crustal seismicity and GPS surface displacements show that the forearc experiences pulses of rapid coseismic extension, parallel to the heave of the megathrust, and slow interseismic, convergence-parallel shortening. These cycles, over geologic time, build the forearc structural grain, reactivating structures properly-oriented respect to the deformation field of each stage of the interplate cycle. Great subduction events may play a fundamental role in constructing the crustal architecture of extensional forearc regions. Static mechanical models of coseismic and interseismic upper plate deformation are used to explore for distinct features that could result from brittle fracturing over the two stages of the interplate cycle. I show that the semi-elliptical outline of the first-order normal faults along the Coastal Cordillera may define the location of a characteristic, long-lived megathrust segment. Finally, using data from the Global CMT catalog I analyzed the seismic behavior through time of forearc regions that have experienced great subduction ruptures >Mw7.7 worldwide. Between 61% and 83% of the cases where upper plate earthquakes exhibited periods of increased seismicity

Structure of the Lithosphere-Asthenosphere Boundary Onshore and Offshore the California Continental Margin from Three-Dimensional Seismic Anisotropy

NASA Astrophysics Data System (ADS)

Gomez, C. D.; Escobar, L., Sr.; Rathnayaka, S.; Weeraratne, D. S.; Kohler, M. D.

2016-12-01

The California continental margin, a major transform plate boundary in continental North America, is the locus of complex tectonic stress fields that are important in interpreting both remnant and ongoing deformational strain. Ancient subduction of the East Pacific Rise spreading center, the rotation and translation of tectonic blocks and inception of the San Andreas fault all contribute to the dynamic stress fields located both onshore and offshore southern California. Data obtained by the ALBACORE (Asthenospheric and Lithospheric Broadband Architecture from the California Offshore Region Experiment) and the CISN (California Integrated Seismic Network) seismic array are analyzed for azimuthal anisotropy of Rayleigh waves from 80 teleseismic events at periods 16 - 78 s. Here we invert Rayleigh wave data for shear wave velocity structure and three-dimensional seismic anisotropy in the thee regions designated within the continental margin including the continent, seafloor and California Borderlands. Preliminary results show that seismic anisotropy is resolved in multiple layers and can be used to determine the lithosphere-asthenosphere boundary (LAB) in offshore and continental regions. The oldest seafloor in our study at age 25-35 Ma indicates that the anisotropic transition across the LAB occurs at 73 km +/- 25 km with the lithospheric fast direction oriented WNW-ESE, consistent with current Pacific plate motion direction. The continent region west of the San Andreas indicates similar WNW-ESE anisotropy and LAB depth. Regions east of the San Andreas fault indicate NW-SE anisotropy transitioning to a N-S alignment at 80 km depth north of the Garlock fault. The youngest seafloor (15 - 25 Ma) and outer Borderlands indicate a more complex three layer fabric where shallow lithospheric NE-SW fast directions are perpendicular with ancient Farallon subduction arc, a mid-layer with E-W fast directions are perpendicular to remnant fossil fabric, and the deepest layer

Source and fractionation controls on subduction-related plutons and dike swarms in southern Patagonia (Torres del Paine area) and the low Nb/Ta of upper crustal igneous rocks

NASA Astrophysics Data System (ADS)

Müntener, Othmar; Ewing, Tanya; Baumgartner, Lukas P.; Manzini, Mélina; Roux, Thibaud; Pellaud, Pierre; Allemann, Luc

2018-05-01

The subduction system in southern Patagonia provides direct evidence for the variability of the position of an active continental arc with respect to the subducting plate through time, but the consequences on the arc magmatic record are less well studied. Here we present a geochemical and geochronological study on small plutons and dykes from the upper crust of the southern Patagonian Andes at 51°S, which formed as a result of the subduction of the Nazca and Antarctic plates beneath the South American continent. In situ U-Pb geochronology on zircons and bulk rock geochemical data of plutonic and dyke rocks are used to constrain the magmatic evolution of the retro-arc over the last 30 Ma. We demonstrate that these combined U-Pb and geochemical data for magmatic rocks track the temporal and spatial migration of the active arc, and associated retro-arc magmatism. Our dataset indicates that the rear-arc area is characterized by small volumes of alkaline basaltic magmas at 29-30 Ma that are characterized by low La/Nb and Th/Nb ratios with negligible arc signatures. Subsequent progressive eastward migration of the active arc culminated with the emplacement of calc-alkaline plutons and dikes 17-16 Ma with elevated La/Nb and Th/Nb ratios and typical subduction signatures constraining the easternmost position of the southern Patagonian batholith at that time. Geochemical data on the post-16 Ma igneous rocks including the Torres del Paine laccolith indicate an evolution to transitional K-rich calc-alkaline magmatism at 12.5 ± 0.2 Ma. We show that trace element ratios such as Nb/Ta and Dy/Yb systematically decrease with increasing SiO2, for both the 17-16 Ma calc-alkaline and the 12-13 Ma K-rich transitional magmatism. In contrast, Th/Nb and La/Nb monitor the changes in the source composition of these magmas. We suggest that the transition from the common calc-alkaline to K-rich transitional magmatism involves a change in the source component, while the trace element ratios

Exploring Interactions Between Subduction Zone Earthquakes and Volcanic Activity in the South Central Alaskan Subduction Zone

NASA Astrophysics Data System (ADS)

Lanagan, K. M.; Richardson, E.

2012-12-01

Although great earthquakes such as the recent moment-magnitude (M) 9 Tohoku-Oki earthquake have been shown to trigger remote seismicity in volcanoes, the extent to which subduction zone earthquakes can trigger shallow seismic swarms at volcanoes is largely unexplored. Unknowns in this relationship include the upper limit of distance, the lower limit of magnitude, the upper time limit between events, and the effects of rupture directivity. We searched the Advanced National Seismic System earthquake catalog from 1989 - 2011 for correlations in space and time between M > 5.0 earthquakes in the south central Alaskan subduction zone (between 58.5°N and 62.5°N, and 150.7°W and 154.7°W) and volcanic activity at Mt. Redoubt, Mt. Iliamna, and Mt. Spurr volcanoes. There are 48 earthquakes M > 5 in this catalog; five of these are M > 6. The depths of the 48 M>5 events range from 49km to 220km, and they are all between 100km and 350km of the three volcanoes. Preliminary analysis of our catalog shows that four of the five M > 6 earthquakes are followed by a volcanic earthquake swarm at either Redoubt or Spurr within 100 days, and three of them are followed by a volcanic earthquake swarm within a month. None of these events correlated in space and time with swarms at Mt. Iliamna. We are also searching for swarms and moderate earthquakes occurring in time windows far removed from each other. The likeliest case of remotely triggered seismicity in our search area to date occurred on January 24 2009, when a magnitude 5.8 earthquake beneath the Kenai Peninsula at 59.4°N, 152.8°W, and 95km depth was immediately followed by an increase of volcanic activity at Mt. Redoubt approximately 153km away. The first swarm began on Jan 25 2009. On Jan 30 2009, volcanologists at the Alaskan Volcano observatory determined the increased volcanic seismicity was indicative of an impending eruption. Mt. Redoubt erupted on March 15 2009. Proposed mechanisms for triggering of volcanoes by

Slab1.0: A three-dimensional model of global subduction zone geometries

NASA Astrophysics Data System (ADS)

Hayes, Gavin P.; Wald, David J.; Johnson, Rebecca L.

2012-01-01

We describe and present a new model of global subduction zone geometries, called Slab1.0. An extension of previous efforts to constrain the two-dimensional non-planar geometry of subduction zones around the focus of large earthquakes, Slab1.0 describes the detailed, non-planar, three-dimensional geometry of approximately 85% of subduction zones worldwide. While the model focuses on the detailed form of each slab from their trenches through the seismogenic zone, where it combines data sets from active source and passive seismology, it also continues to the limits of their seismic extent in the upper-mid mantle, providing a uniform approach to the definition of the entire seismically active slab geometry. Examples are shown for two well-constrained global locations; models for many other regions are available and can be freely downloaded in several formats from our new Slab1.0 website, http://on.doi.gov/d9ARbS. We describe improvements in our two-dimensional geometry constraint inversion, including the use of `average' active source seismic data profiles in the shallow trench regions where data are otherwise lacking, derived from the interpolation between other active source seismic data along-strike in the same subduction zone. We include several analyses of the uncertainty and robustness of our three-dimensional interpolation methods. In addition, we use the filtered, subduction-related earthquake data sets compiled to build Slab1.0 in a reassessment of previous analyses of the deep limit of the thrust interface seismogenic zone for all subduction zones included in our global model thus far, concluding that the width of these seismogenic zones is on average 30% larger than previous studies have suggested.

Early Jurassic Volcanism in the South Lhasa Terrane, Southern Tibet: Record of Back-arc Extension in the Active Continental Margin

NASA Astrophysics Data System (ADS)

Wei, Y.; Zhao, Z.; Zhu, D. C.; Wang, Z.; Liu, D.; Mo, X.

2015-12-01

Indus-Yarlung Zangbo Suture Zone (IYZSZ) represents the Mesozoic remnants of the Neo-Tethyan Ocean lithosphere after its northward subduction beneath the Lhasa Terrane. The evolution of the Neo-Tethyan Ocean prior to India-Asia collision remains unclear. To explore this period of history, we investigate zircon U-Pb geochronology, geochemistry and Nd-Hf isotopes of the Early Jurassic bimodal-like volcanic sequence around Dagze area, south Tibet. The volcanic sequence comprises calc-alkaline basalts to rhyolites whereas intermediate components are volumetrically restricted. Zircons from a basaltic andesite yielded crystallization age of 178Ma whereas those from 5 silicic rocks were dated at 183-174Ma, which suggest that both the basaltic and the silicic rocks are coeval. The basaltic rocks are enriched in LREE and LILE, and depleted in HFSE, with Epsilon Nd(t) of 1.6-4.0 and zircon Epsilon Hf(t) of 0.7-11.8, which implies that they were derived from a heterogenetic mantle source metasomatized by subduction components. Trace element geochemistry shows that the basaltic rocks are compositionally transitional from normal mid-ocean ridge basalts (N-MORB) to island arc basalts (IAB, e.g. Zedong arc basalts of ~160-155Ma in the south margin of Lhasa Terrane), with the signature of immature back-arc basin basalts. The silicic rocks display similar Nd-Hf isotopic features of the Gangdese batholith with Epsilon Nd(t) of 0.9-3.4 and zircon Epsilon Hf(t) of 2.4-17.7, indicating that they were possibly generated by anatexis of basaltic juvenile lower crust, instead of derived from the basaltic magma. These results support an Early to Middle Jurassic (183-155Ma) model that the back-arc extension tectonic setting were existing in the active continental margin in the south Lhasa Terrane.

Numerical modeling of fluid migration in subduction zones

NASA Astrophysics Data System (ADS)

Walter, M. J.; Quinteros, J.; Sobolev, S. V.

2015-12-01

It is well known that fluids play a crucial role in subduction evolution. For example, mechanical weakening along tectonic interfaces, due to high fluid pressure, may enable oceanic subduction. Hence, the fluid content seems to be a critical parameter for subduction initiation. Studies have also shown a correlation between the location of slab dehydration and intermediate seismic activity. Furthermore, expelled fluids from the subduction slab affect the melting temperature, consequently, contributing to partial melting in the wedge above the down-going plate and extensive volcanism. In summary, fluids have a great impact on tectonic processes and therefore should be incorporated into geodynamic numerical models. Here we use existing approaches to couple and solve fluid flow equations in the SLIM-3D thermo-mechanical code. SLIM-3D is a three-dimensional thermo-mechanical code capable of simulating lithospheric deformation with elasto-visco-plastic rheology. It has been successfully applied to model geodynamic processes at different tectonic settings, including subduction zones. However, although SLIM-3D already includes many features, fluid migration has not been incorporated into the model yet. To this end, we coupled solid and fluid flow assuming that fluids flow through a porous and deformable solid. Thereby, we introduce a two-phase flow into the model, in which the Stokes flow is coupled with the Darcy law for fluid flow. Ultimately, the evolution of porosity is governed by a compaction pressure and the advection of the porous solid. We show the details of our implementation of the fluid flow into the existing thermo-mechanical finite element code and present first results of benchmarks and experiments. We are especially interested in the coupling of subduction processes and the evolution of the magmatic arc. Thereby, we focus on the key factors controlling magma emplacement and its influence on subduction processes.

Role of local to regional-scale collisions in the closure history of the Southern Neotethys, exemplified by tectonic development of the Kyrenia Range active margin/collisional lineament, N Cyprus

NASA Astrophysics Data System (ADS)

Robertson, Alastair; Kinnaird, Tim; McCay, Gillian; Palamakumbura, Romesh; Chen, Guohui

2016-04-01

Active margin processes including subduction, accretion, arc magmatism and back-arc extension play a key role in the diachronous, and still incomplete closure of the S Neotethys. The S Neotethys rifted along the present-day Africa-Eurasia continental margin during the Late Triassic and, after sea-floor spreading, began to close related to northward subduction during the Late Cretaceous. The northern, active continental margin of the S Neotethys was bordered by several of the originally rifted continental fragments (e.g. Taurides). The present-day convergent lineament ranges from subaqueous (e.g. Mediterranean Ridge), to subaerial (e.g. SE Turkey). The active margin development is partially obscured by microcontinent-continent collision and post-collisional strike-slip deformation (e.g. Tauride-Arabian suture). However, the Kyrenia Range, N Cyprus provides an outstanding record of convergent margin to early stage collisional processes. It owes its existence to strong localised uplift during the Pleistocene, which probably resulted from the collision of a continental promontory of N Africa (Eratosthenes Seamount) with the long-lived S Neotethyan active margin to the north. A multi-stage convergence history is revealed, mainly from a combination of field structural, sedimentological and igneous geochemical studies. Initial Late Cretaceous convergence resulted in greenschist facies burial metamorphism that is likely to have been related to the collision, then rapid exhumation, of a continental fragment (stage 1). During the latest Cretaceous-Palaeogene, the Kyrenia lineament was characterised by subduction-influenced magmatism and syn-tectonic sediment deposition. Early to Mid-Eocene, S-directed thrusting and folding (stage 2) is likely to have been influenced by the suturing of the Izmir-Ankara-Erzincan ocean to the north ('N Neotethys'). Convergence continued during the Neogene, dominated by deep-water terrigenous gravity-flow accumulation in a foredeep setting

Numerical model of the transition from continental rifting to oceanization: the case study of the Ligure-Piemontese ocean.

NASA Astrophysics Data System (ADS)

Roda, M.; Marotta, A. M.; Conte, K.; Spalla, M. I.

2015-12-01

The transition from continental rifting to oceanization has been investigated by mean of a 2D thermo-mechanical numerical model in which the formation of oceanic crust by mantle serpentinization, due to the hydration of the uprising peridotite, as been implemented. Model predictions have been compared with natural data related to the Permian-Triassic thinning affecting the continental lithosphere of the Alpine domain, in order to identify which portions of the present Alpine-Apennine system, preserving the imprints of Permian-Triassic high temperature (HT) metamorphism, is compatible, in terms of lithostratigraphy and tectono-metamorphic evolution, with a lithospheric extension preceding the opening of the Ligure-Piemontese oceanic basin. At this purpose age, petrological and structural data from the Alpine and Apennine ophiolite complexes are compared with model predictions from the oceanization stage. Our comparative analysis supports the thesis that the lithospheric extension preceding the opening of the Alpine Tethys did not start on a stable continental lithosphere, but developed by recycling part of the old Variscan collisional suture. The HT Permian-Triassic metamorphic re-equilibration overprints an inherited tectonic and metamorphic setting consequent to the Variscan subduction and collision, making the Alps a key case history to explore mechanisms responsible for the re-activation of orogenic scars.

Volcanism and Subduction: The Kamchatka Region

NASA Astrophysics Data System (ADS)

Eichelberger, John; Gordeev, Evgenii; Izbekov, Pavel; Kasahara, Minoru; Lees, Jonathan

The Kamchatka Peninsula and contiguous North Pacific Rim is among the most active regions in the world. Kamchatka itself contains 29 active volcanoes, 4 now in a state of semi-continuous eruption, and I has experienced 14 magnitude 7 or greater earthquakes since accurate recording began in 1962. At its heart is the uniquely acute subduction cusp where the Kamchatka and Aleutian Arcs and Emperor Seamount Chain meet. Volcanism and Subduction covers coupled magmatism and tectonics in this spectacular region, where the torn North Pacific slab dives into hot mantle. Senior Russian and American authors grapple with the dynamics of the cusp with perspectives from the west and east of it, respectively, while careful tephrostratigraphy yields a remarkably precise record of behavior of storied volcanoes such as Kliuchevskoi and Shiveluch. Towards the south, Japanese researchers elucidate subduction earthquake processes with unprecedented geodetic resolution. Looking eastward, new insights on caldera formation, monitoring, and magma ascent are presented for the Aleutians. This is one of the first books of its kind printed in the English language. Students and scientists beginning research in the region will find in this book a useful context and introduction to the region's scientific leaders. Others who wish to apply lessons learned in the North Pacific to their areas of interest will find the volume a valuable reference.

The role of hydrothermal fluids in the production of subduction zone magmas: Evidence from siderophile and chalcophile trace elements and boron

NASA Astrophysics Data System (ADS)

Noll, P. D.; Newsom, H. E.; Leeman, W. P.; Ryan, J. G.

1996-02-01

In order to evaluate the processes responsible for the enrichments of certain siderophile/ chalcophile trace elements during the production of subduction-related magmas, representative lavas from seven subduction zones have been analyzed for Pb, As, Sb, Sn, W, Mo, Tl, Cu, and Zn by inductively coupled plasma-mass spectrometry (ICP-MS), radiochemical epithermal neutron activation analysis (RENA), and atomic absorption (AA). The siderophile/chalcophile elements are compared to the highly fluid-mobile element B, the light rare earth elements (LREEs), U, and Th in order to place constraints on their behavior in subduction zones. Boron, As, Sb, and Pb are all enriched in arc lavas and continental crustal rocks more so than expected assuming normal magmatic processes (melting and crystallization). Tin, W, and Mo show little evidence of enrichment. Correlations of Pb/Ce, As/Ce, and Sb/Ce with B/La are statistically significant and have high correlation coefficients (and, more importantly, slopes approaching one) suggesting that Pb, As, and Sb behave similarly to B (i.e., that they are fluid-mobile). In addition, across-arc traverses show that B/La, As/Ce, Pb/Ce, and Sb/Ce ratios decrease dramatically with distance towards the back-arc basin. W/Th, Tl/La, Sn/Sm, and Mo/Ce ratios and Cu and Zn concentrations have much less systematic across-arc variations and correlations with B/La are not as strong (and in some cases, not statistically significant) and the regression lines have much lower slopes. Mixing models between upper mantle, slab-derived fluid, and sediment are consistent with a fluid-derived component in the arcs displaying extra enrichments of B, Pb, As, and Sb. These observations imply efficient mobilization of B, Pb, As, Sb, and possibly Tl into arc magma source regions by hydrothermal fluids derived from metamorphic dehydration reactions within the slab. Tin, W, and Mo show little, if any, evidence of hydrothermal mobilization. Copper appears to be slightly

A Review on Forearc Ophiolite Obduction, Adakite-Like Generation, and Slab Window Development at the Chile Triple Junction Area: Uniformitarian Framework for Spreading-Ridge Subduction

NASA Astrophysics Data System (ADS)

Bourgois, Jacques; Lagabrielle, Yves; Martin, Hervé; Dyment, Jérôme; Frutos, Jose; Cisternas, Maria Eugenia

2016-10-01

This paper aggregates the main basic data acquired along the Chile Triple Junction (CTJ) area (45°-48°S), where an active spreading center is presently subducting beneath the Andean continental margin. Updated sea-floor kinematics associated with a comprehensive review of geologic, geochemical, and geophysical data provide new constraints on the geodynamics of this puzzling area. We discuss: (1) the emplacement mode for the Pleistocene Taitao Ridge and the Pliocene Taitao Peninsula ophiolite bodies. (2) The occurrence of these ophiolitic complexes in association with five adakite-like plutonic and volcanic centers of similar ages at the same restricted locations. (3) The inferences from the co-occurrence of these sub-coeval rocks originating from the same subducting oceanic lithosphere evolving through drastically different temperature-pressure ( P- T) path: low-grade greenschist facies overprint and amphibolite-eclogite transition, respectively. (4) The evidences that document ridge-jump events and associated microplate individualization during subduction of the SCR1 and SCR-1 segments: the Chonos and Cabo Elena microplates, respectively. The ridge-jump process associated with the occurrence of several closely spaced transform faults entering subduction is controlling slab fragmentation, ophiolite emplacement, and adakite-like production and location in the CTJ area. Kinematic inconsistencies in the development of the Patagonia slab window document an 11- km westward jump for the SCR-1 spreading segment at ~6.5-to-6.8 Ma. The SCR-1 spreading center is relocated beneath the North Patagonia Icefield (NPI). We argue that the deep-seated difference in the dynamically sustained origin of the high reliefs of the North and South Patagonia Icefield (NPI and SPI) is asthenospheric convection and slab melting, respectively. The Chile Triple Junction area provides the basic constraints to define the basic signatures for spreading-ridge subduction beneath an Andean

Subducted seamounts and recent earthquakes beneath the central Cascadia forearc

USGS Publications Warehouse

Tréhu, Anne M.; Blakely, Richard J.; Williams, Mark C.

2012-01-01

Bathymetry and magnetic anomalies indicate that a seamount on the Juan de Fuca plate has been subducted beneath the central Cascadia accretionary complex and is now located ∼45 km landward of the deformation front. Passage of this seamount through the accretionary complex has resulted in a pattern of uplift followed by subsidence that has had a profound influence on slope morphology, gas hydrate stability, and sedimentation. Based on potential-field data and a new three-dimensional seismic velocity model, we infer that this is the most recent of several seamounts subducted over the past several million years beneath this segment of Cascadia. More deeply subducted seamounts may be responsible for recent earthquake activity on the plate boundary in this region and for along-strike variations in the thickness of the subduction channel, which may affect coupling across the plate boundary.

Subduction and collision processes in the Central Andes constrained by converted seismic phases.

PubMed

Yuan, X; Sobolev, S V; Kind, R; Oncken, O; Bock, G; Asch, G; Schurr, B; Graeber, F; Rudloff, A; Hanka, W; Wylegalla, K; Tibi, R; Haberland, C; Rietbrock, A; Giese, P; Wigger, P; Röwer, P; Zandt, G; Beck, S; Wallace, T; Pardo, M; Comte, D

The Central Andes are the Earth's highest mountain belt formed by ocean-continent collision. Most of this uplift is thought to have occurred in the past 20 Myr, owing mainly to thickening of the continental crust, dominated by tectonic shortening. Here we use P-to-S (compressional-to-shear) converted teleseismic waves observed on several temporary networks in the Central Andes to image the deep structure associated with these tectonic processes. We find that the Moho (the Mohorovicić discontinuity--generally thought to separate crust from mantle) ranges from a depth of 75 km under the Altiplano plateau to 50 km beneath the 4-km-high Puna plateau. This relatively thin crust below such a high-elevation region indicates that thinning of the lithospheric mantle may have contributed to the uplift of the Puna plateau. We have also imaged the subducted crust of the Nazca oceanic plate down to 120 km depth, where it becomes invisible to converted teleseismic waves, probably owing to completion of the gabbro-eclogite transformation; this is direct evidence for the presence of kinetically delayed metamorphic reactions in subducting plates. Most of the intermediate-depth seismicity in the subducting plate stops at 120 km depth as well, suggesting a relation with this transformation. We see an intracrustal low-velocity zone, 10-20 km thick, below the entire Altiplano and Puna plateaux, which we interpret as a zone of continuing metamorphism and partial melting that decouples upper-crustal imbrication from lower-crustal thickening.

78 FR 75905 - Credit for Increasing Research Activities: Intra-Group Gross Receipts

Federal Register 2010, 2011, 2012, 2013, 2014

2013-12-13

... CFR Part 1 [REG-159420-04] RIN 1545-BE14 Credit for Increasing Research Activities: Intra-Group Gross... Internal Revenue Code (Code) relating to the treatment of qualified research expenditures (QREs) and gross... trades or businesses under common control (intra-group transactions) for purposes of determining the...

Three-dimensional structure and seismicity beneath the Central Vanuatu subduction zone

NASA Astrophysics Data System (ADS)

Foix, O.; Crawford, W. C.; Koulakov, I.; Regnier, M. M.; Pelletier, B.; Garaebiti, E.

2017-12-01

The 1 400 km long Vanuatu subduction zone marks the subduction of the oceanic Australia plate beneath the North-Fijian microplate. Seismic and volcanic activity is high, and several morphologic features enter into subduction, affecting seismicity and probably plate coupling. The Northern d'Entrecasteaux Ridge, West-Torres plateau, and Bougainville seamount currently enter into subduction below the forearc islands of Santo and Malekula. This subduction/collision coincides with a strongly decreased local convergence velocity rate at the trench (35 mm/yr compared to 120-160 mm/yr to the north and south) and significant uplift on the overriding plate. Two large forearc islands located 20-30 km from the subduction front Santo and Malekula to the trench allow excellent coverage of the megathrust seismogenic zone for a seismological study. We use data from the 10 months, 30-station amphibious ARC-VANUATU seismology network to construct a 3D velocity model and locate 11 617 earthquakes. The 3D model reveals low P and S velocities in the uppermost tens of kilometers in front of the Northern d'Entrecasteaux Ridge and the Bougainville Guyot. These anomalies may be due to heavy faulting of related subducted features, possibly including important water infiltration. We also identify a possible seamount entered into subduction beneath a smaller uplifted island between the two main islands. The spatial distribution of earthquakes is highly variable, as is the depth limit of the seismogenic zone, suggests a complex interaction of faults and stress zones related to high and highly variable stress that may be associated with the subducted features.

Alpine inversion of the North African margin and delamination of its continental lithosphere

NASA Astrophysics Data System (ADS)

Roure, FrançOis; Casero, Piero; Addoum, Belkacem

2012-06-01

This paper aims at summarizing the current extent and architecture of the former Mesozoic passive margin of North Africa from North Algeria in the west up to the Ionian-Calabrian arc and adjacent Mediterranean Ridge in the east. Despite that most paleogeographic models consider that the Eastern Mediterranean Basin as a whole is still underlain by remnants of the Permo-Triassic or a younger Cretaceous Tethyan-Mesogean ocean, the strong similarities documented here in structural styles and timing of inversion between the Saharan Atlas, Sicilian Channel and the Ionian abyssal plain evidence that this portion of the Eastern Mediterranean Basin still belongs to the distal portion of the North African continental margin. A rim of Tethyan ophiolitic units can be also traced more or less continuously from Turkey and Cyprus in the east, in onshore Crete, in the Pindos in Greece and Mirdita in Albania, as well as in the Western Alps, Corsica and the Southern Apennines in the west, supporting the hypothesis that both the Apulia/Adriatic domain and the Eastern Mediterranean Basin still belong to the former southern continental margin of the Tethys. Because there is no clear evidence of crustal-scale fault offsetting the Moho, but more likely a continuous yet folded Moho extending between the foreland and the hinterland beneath the Mediterranean arcs, we propose here a new model of delamination of the continental lithosphere for the Apennines and the Aegean arcs. In this model, only the mantle lithosphere of Apulia and the Eastern Mediterranean is still locally subducted and recycled in the asthenosphere, most if not all the northern portion of the African crust and coeval Moho being currently decoupled from its former, currently delaminated and subducted mantle lithosphere.

Chronobiology of deep-water decapod crustaceans on continental margins.

PubMed

Aguzzi, Jacopo; Company, Joan B

2010-01-01

Species have evolved biological rhythms in behaviour and physiology with a 24-h periodicity in order to increase their fitness, anticipating the onset of unfavourable habitat conditions. In marine organisms inhabiting deep-water continental margins (i.e. the submerged outer edges of continents), day-night activity rhythms are often referred to in three ways: vertical water column migrations (i.e. pelagic), horizontal displacements within benthic boundary layer of the continental margin, along bathymetric gradients (i.e. nektobenthic), and endobenthic movements (i.e. rhythmic emergence from the substrate). Many studies have been conducted on crustacean decapods that migrate vertically in the water column, but much less information is available for other endobenthic and nektobenthic species. Also, the types of displacement and major life habits of most marine species are still largely unknown, especially in deep-water continental margins, where steep clines in habitat factors (i.e. light intensity and its spectral quality, sediment characteristics, and hydrography) take place. This is the result of technical difficulties in performing temporally scheduled sampling and laboratory testing on living specimens. According to this scenario, there are several major issues that still need extensive research in deep-water crustacean decapods. First, the regulation of their behaviour and physiology by a biological clock is almost unknown compared to data for coastal species that are easily accessible to direct observation and sampling. Second, biological rhythms may change at different life stages (i.e. size-related variations) or at different moments of the reproductive cycle (e.g. at egg-bearing) based on different intra- and interspecific interactions. Third, there is still a major lack of knowledge on the links that exist among the observed bathymetric distributions of species and selected autoecological traits that are controlled by their biological clock, such as the

Orogenic inheritance and continental breakup: Wilson Cycle-control on rift and passive margin evolution

NASA Astrophysics Data System (ADS)

Schiffer, C.; Petersen, K. D.

2016-12-01

Rifts often develop along suture zones between previously collided continents, as part of the Wilson cycle. The North Atlantic is such an example, formed where Pangaea broke apart along Caledonian and Variscan sutures. Dipping upper mantle structures in E. Greenland and Scotland, have been interpreted as fossil subduction zones and the seismic signature indicates the presence of eclogite and serpentinite. We speculate that this orogenic material may impose a rheological control upon post-orogenic extension and we use thermo-mechanical modelling to explore such effects. Our model includes the following features: 1) Crustal thickness anomalies, 2) Eclogitised mafic crust emplaced in the mantle lithosphere, and 3) Hydrated mantle peridotite (serpentinite) formed in a pre-rift subduction setting. Our models indicate that the inherited structures control the location and the structural and magmatic evolution of the rift. Rifting of thin initial crust allows for relatively large amounts of serpentinite to be preserved within the uppermost mantle. This facilitates rapid continental breakup and serpentinite exhumation. Magmatism does not occur before continental breakup. Rifts in thicker crust preserve little or no serpentinite and thinning is more focused in the mantle lithosphere, rather than in the crust. Continental breakup is therefore preceded by magmatism. This implies that pre-rift orogenic properties may determine whether magma-poor or magma-rich conjugate margins are formed. Our models show that inherited orogenic eclogite and serpentinite are deformed and partially emplaced either as dipping structures within the lithospheric mantle or at the base of the thinned continental crust. The former is consistent with dipping sub-Moho reflectors often observed in passive margins. The latter provides an alternative interpretation of `lower crustal bodies' which are often regarded as igneous bodies. An additional implication of our models is that serpentinite, often

Petrologic evolution of Miocene-Pliocene mafic volcanism in the Kangal and Gürün basins (Sivas-Malatya), central east Anatolia: Evidence for Miocene anorogenic magmas contaminated by continental crust

NASA Astrophysics Data System (ADS)

Kocaarslan, Ayça; Ersoy, E. Yalçın

2018-06-01

This study discusses the geochemical features of the Early-Middle Miocene and Pliocene basaltic (SiO2 = 46-52; MgO = 6-10 wt%) to andesitic (SiO2 = 59; MgO = 4 wt%) rocks exposed in the Gürün and Kangal basins (Sivas, eastern part of central Anatolia), respectively. The basaltic rocks are characterized by alkaline to tholeiitic affinities, while the more evolved andesitic samples show calc-alkaline affinity. Trace element variations reveal that they can be evaluated in three sub-groups, each represented by different contents of trace elements for given Nb contents. Primary magmas of each groups were likely produced by different degrees of partial melting ( 1-2, 2-3, 7-10% respectively) from a common mantle source, subsequently underwent different degrees of fractionation and crustal contamination. Derivation from a common mantle source of the primitive magmas of each group is supported by similar Sr, Nd and Pb isotopic ratios. Increasing degrees of partial melting seem to be responsible for the alkaline to tholeiitic variation among the basaltic samples, while higher degrees of crustal contamination (AFC) resulted in calc-alkaline affinity of the more evolved samples. Most primitive Pliocene samples show intra-plate (anorogenic) geochemical features, while the more evolved Miocene calc-alkaline samples resemble geochemically subduction-related (orogenic) magmatic rocks. However, on the basis of detailed geochemical models, we propose that the calc-alkaline affinity among the Miocene samples can also be gained by crustal contamination of their primary magmas which were also anorogenic in character. If this is true, overall, the Miocene and Pliocene basaltic to andesitic rocks in the Gürün and Kangal basins appear to may have formed by variable degrees of partial melting of a common anorogenic mantle that had not been subject to subduction-related metasomatism. This is an alternative approach to the general view assuming the Early-Middle Miocene magmatic activity

Porosity and Salt Content Determine if Subduction Can Occur in Europa's Ice Shell

NASA Astrophysics Data System (ADS)

Johnson, Brandon C.; Sheppard, Rachel Y.; Pascuzzo, Alyssa C.; Fisher, Elizabeth A.; Wiggins, Sean E.

2017-12-01

Motivated by recent evidence for subduction in Europa's ice shell, we explore the geophysical feasibility of this process. Here we construct a simple model to track the evolution of porosity and temperature within a slab that is forced to subduct. We also vary the initial salt content in Europa's ice shell and determine the buoyancy of our simulated subducting slab. We find that porosity and salt content play a dominant role in determining whether the slab is nonbuoyant and subduction in Europa's ice shell is actually possible. Generally, we find that initially low porosities and high salt contents within the conductive lid are more conducive to subduction. If salt contents are laterally homogenous, and Europa has a reasonable surface porosity of ϕ0 = 0.1, the conductive portion of Europa's shell must have salt contents exceeding 22% for subduction to occur. However, if salt contents are laterally heterogeneous, with salt contents varying by a few percent, subduction may occur for a surface porosity of ϕ0 = 0.1 and overall salt contents of 5%. Thus, we argue that under plausible conditions, subduction in Europa's ice shell is possible. Moreover, assuming that subduction is actively occurring or has occurred in Europa's recent past provides important constraints on the structure and composition of the ice shell.

Discontinuous character of the Wadati-Benioff zone in the Banda Arc region: a consequence of a cyclic character of the process of subduction

NASA Astrophysics Data System (ADS)

Matejkova, R.; Spicak, A.; Vanek, J.

2010-12-01

Our former investigation into heterogeneous distribution of earthquakes at convergent plate margins led us to an idea of discontinuities in the process of subduction (e.g. Hanuš and Vaněk, 1978; Špičák et al., 2007). This idea suggests the existence of subduction cycles lasting several million years. A fade-out of a subduction process should be caused e.g. by a collision of the slab with the 670 boundary, convergence of hardly subductable seamount provinces, collision with a thick continental crust etc. Such a fade-out of subduction may be followed by an onset of a new subduction cycle, with important consequences to, e.g., position of the volcanic arc. In this contribution, we analyse spatial distribution of intermediate-depth and deep earthquakes in the southern part of the Banda Arc region (5°-10°S and 127°-132°E). The EHB global hypocentral determinations (Engdahl et al., 1998) covering the period 1964-2007 have been used. To visualize depth distribution of earthquake foci, we covered the region of interest by narrow (25 km width) swaths, oriented perpendicular to the plate margin, and displayed earthquake foci in vertical sections. The vertical sections show concentration of earthquake foci of the Wadati-Benioff zone (hereafter denoted as WBZ) in four distinct domains that do not fit a continuous plate-like body. These domains can be clearly distinguished from each other by a distinct gap in seismicity between them and/or a noticeable change in dip angle of neighbouring domains. This observation casts doubt on a generally accepted idea that the seismically active domains of the Banda WBZ represent one continuous slab. The deepest domain D1 of earthquakes (400-700 km depth) is probably associated with deep earthquakes north of Java and corresponds to a subduction cycle that faded about 8 Ma ago. Our estimate of the age of the cycle is based on the assumption of a steady convergence rate of about 7 cm/yr. The domain D2 of earthquakes at depths between

MT2D Inversion to Image the Gorda Plate Subduction Zone

NASA Astrophysics Data System (ADS)

Lubis, Y. K.; Niasari, S. W.; Hartantyo, E.

2018-04-01

The magnetotelluric method is applicable for studying complicated geological structures because the subsurface electrical properties are strongly influenced by the electric and magnetic fields. This research located in the Gorda subduction zone beneath the North American continental plate. Magnetotelluric 2D inversion was used to image the variation of subsurface resistivity although the phase tensor analysis shows that the majority of dimensionality data is 3D. 19 MT sites were acquired from EarthScope/USArray Project. Wepresent the image of MT 2D inversion to exhibit conductivity distribution from the middle crust to uppermost asthenosphere at a depth of 120 kilometers. Based on the inversion, the overall data misfit value is 3.89. The Gorda plate subduction appears as a high resistive zone beneath the California. Local conductive features are found in the middle crust downward Klamath Mountain, Bonneville Lake, and below the eastern of Utah. Furthermore, mid-crustal is characterized by moderately resistive. Below the extensional Basin and Range province was related to highly resistive. The middle crust to the uppermost asthenosphere becomes moderately resistive. We conclude that the electrical parameters and the dimensionality of datain the shallow depth(about 22.319 km) beneath the North American platein accordance with surface geological features.

Numerical simulations of water transport in subduction zone: Influences of serpentinized layer in oceanic slabs on subduction dynamics

NASA Astrophysics Data System (ADS)

Nakao, A.; Hikaru, I.; Nakakuki, T.; Suzuki, Y.; Nakamura, H.

2017-12-01

Water liberated from subducting oceanic slabs can affect the subduction dynamics such as mantle wedge flows and plate motion (e.g., Gerya & Meilick, 2011; Horiuchi & Iwamori, 2016; Nakao et al., 2016). However, how water liberated from the slabs, in particular a hydrated part within the oceanic lithosphere (e.g., Fujie et al., 2013), is transported and affects the subduction dynamics has not been fully understood. In order to clarify the roles of water in subduction dynamics, we conducted 2-D dynamical simulations of water transport and mantle convection without imposing the geometry and velocity of subducting slabs. Using the simulations with various thicknesses (0-20 km) of a partially serpentinized layer (hereafter referred to as "SL") underlaying the altered oceanic basalt crust (AOC) in the subducting oceanic lithosphere, we estimate the subduction rate, back-arc spreading, trench migration, and slab geometry. The simulations show that the plate motion significantly changes depending on the amount of liberated water. When the SL is absent (0 km thick), the AOC mostly dehydrates at shallow depths (< 70 km). In this case, the plate subducts slowly, the trench is stationary, and the slab penetrates the 660-km boundary. If the SL is 7.5 km in thickness, it dehydrates at a greater depth compared to AOC, and more water enters the mantle wedge and the back-arc region. The liberated water reduces the viscosity of mantle wedge, and consequently, the subduction rate increases, the trench migrates seaward, and the slab stagnates on the 660-km. If the SL is 20 km in thickness, the upper SL releases much water into the mantle wedge and the back-arc region, whereas the lower SL does not dehydrate because of water uptake by phase A and phase D. In this case, because buoyancy of the subducting slab increases, the subduction is slow, back-arc spreading is weakened, and the slab penetrates the 660-km. Our results imply that the observed variety of subducting slabs reflects

Subduction-Related Structure in the Mw 9.2, 1964 Megathrust Rupture Area Offshore Kodiak Island, Alaska

NASA Astrophysics Data System (ADS)

Krabbenhoeft, A.; von Huene, R.; Klaeschen, D.; Miller, J. J.

2016-12-01

Some of the largest earthquakes worldwide, including the 1964 9.2 Mw megathrust earthquake, occurred in Alaskan subduction zones. To better understand rupture processes and their mechanisms, we relate seafloor morphology from multibeam and regional bathymetric compilations with sub-seafloor images and seismic P-wave velocity structures. We re-processed legacy multichannel seismic (MCS) data including shot- and intra-shotgather interpolation, multiple removal and Kirchhoff depth migration. These images even reveal the shallow structure of the subducting oceanic crust. Traveltime tomography of a coincident vintage (1994) wide angle dataset reveals the P-wave velocity distribution as well as the deep structure of the subducting plate to the ocean crust Moho. The subducting oceanic crust morphology is rough and partly hidden by a thick sediment cover that reaches 3 km depth at the trench axis. Bathymetry shows two major contrasting upper plate morphologies: the shallow dipping lower slope consists of trench-parallel ridges that form the accreted prism whereas the steep rough middle and upper slopes are composed of competent older rock.Thrust faults are distributed across the entire slope, some of which connect with the subducted plate interface. A subtle change in seafloor gradient from the lower to the middle slope coincides with a thrust fault zone marking the boundary between the margin framework and the frontal prism. It corresponds to the most prominent lateral increase in seismic P-wave velocities, 25 km landward of the trench axis.Major thrusts in several MCS-lines are correlated with bathymetric data, showing their > 100 km lateral extent, which might also be tsunamigenic paths of earthquake rupture from the seismogenic zone to the seafloor.

Transfer of subduction fluids into the deforming mantle wedge during nascent subduction: Evidence from trace elements and boron isotopes (Semail ophiolite, Oman)

NASA Astrophysics Data System (ADS)

Prigent, C.; Guillot, S.; Agard, P.; Lemarchand, D.; Soret, M.; Ulrich, M.

2018-02-01

The basal part of the Semail ophiolitic mantle was (de)formed at relatively low temperature (LT) directly above the plate interface during "nascent subduction" (the prelude to ophiolite obduction). This subduction-related LT deformation was associated with progressive strain localization and cooling, resulting in the formation of porphyroclastic to ultramylonitic shear zones prior to serpentinization. Using petrological and geochemical analyses (trace elements and B isotopes), we show that these basal peridotites interacted with hydrous fluids percolating by porous flow during mylonitic deformation (from ∼850 down to 650 °C). This process resulted in 1) high-T amphibole crystallization, 2) striking enrichments of minerals in fluid mobile elements (FME; particularly B, Li and Cs with concentrations up to 400 times those of the depleted mantle) and 3) peridotites with an elevated δ11B of up to +25‰. These features indicate that the metasomatic hydrous fluids are most likely derived from the dehydration of subducting crustal amphibolitic materials (i.e., the present-day high-T sole). The rapid decrease in metasomatized peridotite δ11B with increasing distance to the contact with the HT sole (to depleted mantle isotopic values in <1 km) suggests an intense interaction between peridotites and rapid migrating fluids (∼1-25 m.y-1), erasing the initial high-δ11B subduction fluid signature within a short distance. The increase of peridotite δ11B with increasing deformation furthermore indicates that the flow of subduction fluids was progressively channelized in actively deforming shear zones parallel to the contact. Taken together, these results also suggest that the migration of subduction fluids/melts by porous flow through the subsolidus mantle wedge (i.e., above the plate interface at sub-arc depths) is unlikely to be an effective mechanism to transport slab-derived elements to the locus of partial melting in subduction zones.

Continental collision with a sandwiched accreted terrane: Insights into Himalayan-Tibetan lithospheric mantle tectonics?

NASA Astrophysics Data System (ADS)

Kelly, Sean; Butler, Jared P.; Beaumont, Christopher

2016-12-01

Many collisional orogens contain exotic terranes that were accreted to either the subducting or overriding plate prior to terminal continent-continent collision. The ways in which the physical properties of these terranes influence collision remain poorly understood. We use 2D thermomechanical finite element models to examine the effects of prior 'soft' terrane accretion to a continental upper plate (retro-lithosphere) on the ensuing continent-continent collision. The experiments explore how the style of collision changes in response to variations in the density and viscosity of the accreted terrane lithospheric mantle, as well as the density of the pro-lithospheric mantle, which determines its propensity to subduct or compress the accreted terrane and retro-lithosphere. The models evolve self-consistently through several emergent phases: breakoff of subducted oceanic lithosphere; pro-continent subduction; shortening of the retro-lithosphere accreted terrane, sometimes accompanied by lithospheric delamination; and, terminal underthrusting of pro-lithospheric mantle beneath the accreted terrane crust or mantle. The modeled variations in the properties of the accreted terrane lithospheric mantle can be interpreted to reflect metasomatism during earlier oceanic subduction beneath the terrane. Strongly metasomatized (i.e., dense and weak) mantle is easily removed by delamination or entrainment by the subducting pro-lithosphere, and facilitates later flat-slab underthrusting. The models are a prototype representation of the Himalayan-Tibetan orogeny in which there is only one accreted terrane, representing the Lhasa terrane, but they nonetheless exhibit processes like those inferred for the more complex Himalayan-Tibetan system. Present-day underthrusting of the Tibetan Plateau crust by Indian mantle lithosphere requires that the Lhasa terrane lithospheric mantle has been removed. Some of the model results support previous conceptual interpretations that Tibetan

Intra-Extra Vehicular Activity Apollo Spacesuits

NASA Technical Reports Server (NTRS)

Thomas, Kenneth S.

2016-01-01

Kenneth Thomas will discuss the Apollo Intra-Extra Vehicular Activity (IEVA) spacesuits, which supported launch and reentry and extra-vehicular activity. This program was NASA's first attempt to develop a new suit design from requirements and concepts. Mr. Thomas will chronicle the challenges, developments, struggles, and solutions that culminated in the system that allowed the first human exploration of the Moon and deep space (outside low-Earth orbit). Apollo pressure suit designs allowed the heroic repair of the Skylab space station and supported the first U.S. and Russian spacecraft docking during the Apollo Soyuz Test Project. Mr. Thomas will also discuss the IEVA suits' successes and challenges associated with the IEVA developments of the 1960s.

IODP Expedition 366 Reveals Widespread Seamount Subduction Effects in the Mariana Forearc

NASA Astrophysics Data System (ADS)

Fryer, P. B.; Wheat, C. G.; Williams, T.

2017-12-01

Numerous studies of the subduction of seamounts at accretionary convergent plate margins show considerable vertical tectonic deformation in the forearc region. This includes embayment of the trench axis, steepening of the inner trench slope, the creation of troughs in the wake of the seamount track beneath the forearc sediment wedge, but hypotheses regarding the seismogenic consequences of these processes are frequently at odds. In the nonaccretionary Mariana convergent plate margin, it is clear that ridges crosscut the entire forearc region in commensurate dimensions with thicker areas of subducting Pacific plate. Furthermore, to-date deep-sea drilling results on ODP Legs 125 and 195 and on IODP Expedition 366 recovered seamount materials from 5 serpentinite mud volcanoes over a 640 km along-strike distance, within 90 km west of the trench axis, and from 13 to 19 km depth to slab. The location of the serpentinite mud volcanoes is always associated with fault lineaments. The faulting creates the conduits for eruption of mixtures of fluids from the subduction channel and fault gouge from both the subduction channel and the forearc lithosphere. Cores from IODP 366 confirm that seamount subduction and deformation is a temporally and spatially pervasive process on the Mariana forearc. The new findings provide windows on a continuum of the evolution of plate and seamount subduction from the trench to nearly 20 km depth within the subduction channel. Cased boreholes were deployed at the summits of three active serpentinite mud volcanoes (Yinazao (Blue Moon), Asùt Tesoro (Big Blue), and Fantangisña (Celestial) Seamounts) during Expedition 366. These, plus the existing borehole observatory at ODP Site 1200C on the active summit of Conical Seamount provide a means to monitor processes of subduction related to serpentinite mud volcanism of the Mariana forearc. Such drilling results and borehole observations impact current paradigms of lithospheric deformation, mass cycling

Numerical Modelling of Subduction Zones: a New Beginning

NASA Astrophysics Data System (ADS)

Ficini, Eleonora; Dal Zilio, Luca; Doglioni, Carlo; Gerya, Taras V.

2016-04-01

Subduction zones are one of the most studied although still controversial geodynamic process. Is it a passive or an active mechanism in the frame of plate tectonics? How subduction initiates? What controls the differences among the slabs and related orogens and accretionary wedges? The geometry and kinematics at plate boundaries point to a "westerly" polarized flow of plates, which implies a relative opposed flow of the underlying Earth's mantle, being the decoupling located at about 100-200 km depth in the low-velocity zone or LVZ (Doglioni and Panza, 2015 and references therein). This flow is the simplest explanation for determining the asymmetric pattern of subduction zones; in fact "westerly" directed slabs are steeper and deeper with respect to the "easterly or northeasterly" directed ones, that are less steep and shallower, and two end members of orogens associated to the downgoing slabs can be distinguished in terms of topography, type of rocks, magmatism, backarc spreading or not, foredeep subsidence rate, etc.. The classic asymmetry comparing the western Pacific slabs and orogens (low topography and backarc spreading in the upper plate) and the eastern Pacific subduction zones (high topography and deep rocks involved in the upper plate) cannot be ascribed to the age of the subducting lithosphere. In fact, the same asymmetry can be recognized all over the world regardless the type and age of the subducting lithosphere, being rather controlled by the geographic polarity of the subduction. All plate boundaries move "west". Present numerical modelling set of subduction zones is based on the idea that a subducting slab is primarily controlled by its negative buoyancy. However, there are several counterarguments against this assumption, which is not able to explain the global asymmetric aforementioned signatures. Moreover, petrological reconstructions of the lithospheric and underlying mantle composition, point for a much smaller negative buoyancy than predicted

The structure of the Calabrian subduction system from the fore-arc to the back-arc: new insights from wide-angle seismic data

NASA Astrophysics Data System (ADS)

Prada, M.; Sallares, V.; Ranero, C. R.; Grevemeyer, I.; Zitellini, N.

2017-12-01

The Calabrian arc is a Neogene-Quaternary arcuate orogen result from the subduction of the Ionian Lithosphere under Calabria. The SE migration of this subduction system, triggered by slab rollback, caused the opening of the Tyrrhenian back-arc basin. The large-scale lithospheric structure of the subduction system is mostly imaged by regional earthquake tomography studies. The limited resolution of these studies, however, hinders the definition of smaller-scale details on the location, nature and transition of different lithospheric domains, which are crucial to study the geodynamic evolution of the system. Here we perform travel-time tomography of offshore and onshore active-source wide-angle seismic data to define the 2D Vp structure of the entire Calabrian subduction system. The data were acquired along a 550 km-long transect that extends from the Tyrrhenian back-arc domain to the fore-arc in the Ionian Sea, across Calabria. From NW to SE, the tomographic model shows abrupt variations of the velocity structure. In the back-arc system, particularly in the Vavilov and Marsili basins, OBS sections lack PmP-like arrivals and the velocity structure shows a continuous and strong vertical velocity gradient of 1 s-1. These results strongly support the presence of a basement made of exhumed mantle rocks. Between the Vavilov and Marsili basins, a relatively thick, low-velocity block is interpreted to be of continental affinity. The transition between Marsili Basin and Calabria is marked by a steep Moho geometry that shallows from SE to NW, revealing a dramatic crustal thinning along the N Calabrian margin. The lower crust of the margin has localized Vp of 7 km/s under the submarine volcanic arc. SE Calabria, the model shows a strong horizontal velocity gradient that is interpreted as the backstop of the subduction. In the Ionian, a 3-5 km thick sedimentary wedge thickens towards the NW. The frontal part of the wedge shows sub-vertical low-velocity anomalies indicating the

Spatial analysis of toxic emissions in LCA: a sub-continental nested USEtox model with freshwater archetypes.

PubMed

Kounina, Anna; Margni, Manuele; Shaked, Shanna; Bulle, Cécile; Jolliet, Olivier

2014-08-01

This paper develops continent-specific factors for the USEtox model and analyses the accuracy of different model architectures, spatial scales and archetypes in evaluating toxic impacts, with a focus on freshwater pathways. Inter-continental variation is analysed by comparing chemical fate and intake fractions between sub-continental zones of two life cycle impact assessment models: (1) the nested USEtox model parameterized with sub-continental zones and (2) the spatially differentiated IMPACTWorld model with 17 interconnected sub-continental regions. Substance residence time in water varies by up to two orders of magnitude among the 17 zones assessed with IMPACTWorld and USEtox, and intake fraction varies by up to three orders of magnitude. Despite this variation, the nested USEtox model succeeds in mimicking the results of the spatially differentiated model, with the exception of very persistent volatile pollutants that can be transported to polar regions. Intra-continental variation is analysed by comparing fate and intake fractions modelled with the a-spatial (one box) IMPACT Europe continental model vs. the spatially differentiated version of the same model. Results show that the one box model might overestimate chemical fate and characterisation factors for freshwater eco-toxicity of persistent pollutants by up to three orders of magnitude for point source emissions. Subdividing Europe into three archetypes, based on freshwater residence time (how long it takes water to reach the sea), improves the prediction of fate and intake fractions for point source emissions, bringing them within a factor five compared to the spatial model. We demonstrated that a sub-continental nested model such as USEtox, with continent-specific parameterization complemented with freshwater archetypes, can thus represent inter- and intra-continental spatial variations, whilst minimizing model complexity. Copyright © 2014 Elsevier Ltd. All rights reserved.

Initiation of continental accretion: metamorphic conditions

NASA Astrophysics Data System (ADS)

Clement, Conand; Frederic, Mouthereau; Gianreto, Manatschal; Adbeltif, Lahfid

2017-04-01

The physical processes involved at the beginning of the continental collision are largely unknown because they are transient and therefore hardly identifiable from the rock record. Despite the importance of key parameters for understanding mountain building processes, especially the formation of deep mountain roots and their impacts on earthquakes nucleation, rock/fluid transfers and oil/gas resources in the continental crust, observations from the earliest collision stages remain fragmentary. Here, we focus on the example of Taiwan, a young and active mountain belt where the transition from oceanic subduction, accretion of the first continental margin to mature collision can be followed in space and time. We present preliminary results and provide key questions regarding the reconstruction of time-pressure-temperature paths of rocks & fluids to allow discriminating between rift-related thermal/rheological inheritance and burial/heating phases during convergence. Previous studies have focused on peak temperatures analyzed by Raman Spectrometry of Carbonaceous Matter from the deeper structural layers exposed in the Central Range of Taiwan. In the pre-rift sediments, these studies reported a positive gradient from West to Est, and values from <330 to 520 °C. We detect from our preliminary analyses of the Miocene "post-rift" sediments, a trend of increasing temperature from 170 to 340 °C northwards. These temperature data are discussed against key structural features recognized in the field and available low-temperature thermochronological constraints. We show that our RSCM temperatures cannot directly be interpreted in terms of syn-convergence nappe stacking only and must reflect a component of initial (pre-collisional) high-geothermal gradients (up to 60°C/km) known in the region, and higher temperature closer to the pre-rift units. Cross sections and maps with high resolution peak temperatures are in process as well as pressure estimations to determine how the

Growth of the Pamir

NASA Astrophysics Data System (ADS)

Gloaguen, R.; Ratschbacher, L.

2009-04-01

We aim to establish the Late Cenozoic deformation field of the Pamir by localizing and characterizing active and neotectonic deformation structures, and setting up the drainage-basin, river-capture, river- reversal, and regional erosion history. The project thus aims to record the short-term, upper crustal response to active intra-continental subduction, orocline formation, and erosion. Our hypothesis is that the neotectonics is governed by subduction beneath the frontal part of the orocline, E-W extension in the intra-plateau Karakul-lake rift, and transtension (east) and transpression (west) along the lateral margins of the orocline, a result of oroclinal formation, rotation of the Indian indenter, and focused precipitation caused by the Westerlies. The model for the evolution of the drainage system involves: growth of the Pamir by N-ward propagating deformation, establishing E-trending belts of shortening and rivers/drainages; diversion and blocking of these rivers by the development of the lateral boundaries of the orocline that resulted in river capture and reversal. Even the present-day Panj (Amu Darya) is affected by ongoing uplift: tilted river terraces, wind gaps, and abnormal intersection of streams of different order indicate that large parts of the river have changed flow direction. The determination of a number of geomorphic indices with remote sensing techniques help us to identify areas experiencing tectonic deformation.

NRC Continental Margins Workshop

NASA Astrophysics Data System (ADS)

Katsouros, Mary Hope

The Ocean Studies Board of the National Research Council is organizing a workshop, “Continental Margins: Evolution of Passive Continental Margins and Active Marginal Processes,” to stimulate discussion and longterm planning in the scientific community about the evolution of all types of continental margins. We want to coordinate academic, industry, and government agency efforts in this field, and to enhance communication between sea-based and land-based research programs.The continental margins constitute the only available record of the long-term dynamic interaction of oceanic and continental lithosphere. Of great interest are the unique structures and thick sedimentary sequences associated with this interaction. A major focus of the workshop will be to define strategies for exploring and understanding the continental margins in three dimensions and through geologic time. The workshop will be divided into 7 working groups, each concentrating on a major issue in continental margins research. A background document is being prepared summarizing recent research in specific continental margin fields and identifying key scientific and technical issues.

Oxygen Isotopes in Intra-Back Arc Basalts from the Andean Southern Volcanic Zone

NASA Astrophysics Data System (ADS)

Parks, B. H.; Wang, Z.; Saal, A. E.; Frey, F. A.; Blusztajn, J.

2013-12-01

The chemical compositions of volcanic rocks from the Andean Southern Volcanic Zone (SVZ) reflect complex and dynamic interactions among the subducting oceanic lithosphere, the mantle wedge, and the overlying continental crust. Oxygen isotope ratios of olivine phenocrysts can be a useful means to identifying their relative contributions to the arc magmatism. In this study, we report high-precision oxygen-isotope ratios of olivine phenocrysts in a set of intra-back arc basalts from the SVZ. The samples were collected from monogenetic cinder cones east of the volcanic front (35-39 degrees S), and have been geochemically well-characterized with major and trace element contents, and Sr-Nd-Pb isotope compositions. Compared to lavas from the volcanic front, these intra-back arc lavas have similar radiogenic isotope, and a more alkalic and primitive (higher MgO content) chemical composition. We determined the oxygen-isotope ratios using the CO2-laser-fluorination method set up at the Department of Geology and Geophysics, Yale University following the techniques reported in Wang et al (2011). The samples were analyzed with standards of Gore Mountain Garnet (5.77×0.12‰ 1σ; Valley et al., 1995) and Kilbourne Hole Olivine (5.23×0.07‰ 1σ; Sharp, 1990) in order to account for minor changes in the vacuum line during analyses. The obtained δ18OSMOW values of olivine phenocrysts from the intra-back arc basalts vary from 4.98×0.01 to 5.34×0.01‰. This range, surprisingly, is similar to the δ18O values of olivines from mantle peridotites (5.2×0.2‰). Preliminary results indicate significant correlations of 87Sr/86Sr, 143Nd/144Nd and trace element ratios of the basaltic matrix with the δ18O values of olivine phenocrysts, indicating at least three components involved in the formation of the arc volcanism. By comparing the δ18O with the variations of major and trace element contents (e.g., MgO, TiO2 and Ni), and trace element ratios (e.g. Ba/Nb), we evaluate the effects

Identifying coseismic subsidence in tidal-wetland stratigraphic sequences at the Cascadia subduction zone of western North America

USGS Publications Warehouse

Nelson, Alan R.; Shennan, Ian; Long, Antony J.

1996-01-01

Tidal-wetland stratigraphy reveals that great plate boundary earthquakes have caused hundreds of kilometers of coast to subside at the Cascadia subduction zone. However, determining earthquake recurrence intervals and mapping the coastal extent of past great earthquake ruptures in this region are complicated by the effects of many sedimentologic, hydrographic, and oceanographic processes that occur on the coasts of tectonically passive as well as active continental margins. Tidal-wetland stratigraphy at many Cascadia estuaries differs little from that at similar sites on passive-margin coasts where stratigraphic sequences form through nonseismic processes unrelated to coseismic land level changes. Methods developed through study of similar stratigraphic sequences in Europe provide a framework for investigating the Cascadia estuarine record. Five kinds of criteria must be evaluated when inferring regional coastal subsidence due to great plate boundary earthquakes: the suddenness and amount of submergence, the lateral extent of submerged tidal-wetland soils, the coincidence of submergence with tsunami deposits, and the degree of synchroneity of submergence events at widely spaced sites. Evaluation of such criteria at the Cascadia subduction zone indicates regional coastal subsidence during at least two great earthquakes. Evidence for a coseismic origin remains equivocal, however, for the many peat-mud contacts in Cascadia stratigraphic sequences that lack (1) contrasts in lithology or fossils indicative of more than half a meter of submergence, (2) well-studied tsunami deposits, or (3) precise ages needed for regional correlation. Paleoecologic studies of fossil assemblages are particularly important in estimating the size of sudden sea level changes recorded by abrupt peat-mud contacts and in helping to distinguish erosional and gradually formed contacts from coseismic contacts. Reconstruction of a history of great earthquakes for the Cascadia subduction zone will

Permeability anisotropy of serpentinite and fluid pathways in a subduction zone

NASA Astrophysics Data System (ADS)

Katayama, I.; Kawano, S.; Okazaki, K.

2011-12-01

Subduction zones are the only sites where water is transported into the Earth's deep interior. Although the fluid released into the mantle wedge is generally believed to ascend under buoyancy, it is possible that fluid movement is influenced by anisotropic permeability in localized shear zones. The mantle rocks at the plate interface of a subducting slab are subjected to non-coaxial stress and commonly develop a strong foliation. Indeed, the existence of foliated serpentinite is indicated by strong seismic anisotropy in the forearc mantle wedge (e.g., Katayama et al., 2009; Bezacier et al., 2010). Therefore, fluid pathways in the mantle wedge may be controlled by the preferred orientation of highly anisotropic minerals. In this study, we measured the permeability of highly foliated natural serpentinite, in directions parallel and perpendicular to the foliation, and we discuss the influence of permeability anisotropy on fluid flow in subduction zones. The permeability was measured by an intra-vessel deformation and fluid flow apparatus housed at Hiroshima University. In the measurements, we used nitrogen gas as a pore fluid and maintained constant pore pressure during the measurements (Pp < 6 MPa). The obtained gas permeability was then converted to intrinsic permeability using the Klinkenberg effect, which is known to be insensitive to the type of pore fluid. Under low confining pressure, all the experiments show similar permeability, in the order of 10-19 m2. However, permeability anisotropy appears under high confining pressures, with the specimens oriented parallel to the foliation having higher permeability than those oriented normal to the foliation. At a confining pressure of 50 MPa, the difference in permeability between the samples with contrasting orientations reaches several orders of magnitude, possibly reflecting the pore tortuosity of the highly sheared serpentinite, as indicated by the Kozeny-Carman relation. The present experimental data show that

Thermal buoyancy on Venus - Underthrusting vs subduction

NASA Technical Reports Server (NTRS)

Burt, Jeffrey D.; Head, James W.

1992-01-01

The thermal and buoyancy consequences of the subduction endmember are modeled in an attempt to evaluate the conditions distinguishing underthrusting and subduction. Thermal changes in slabs subducting into the Venusian mantle with a range of initial geotherms are used to predict density changes and, thus, slab buoyancy. Based on a model for subduction-induced mantle flow, it is then argued that the angle of the slab dip helps differentiate between underthrusting and subduction. Mantle flow applies torques to the slab which, in combination with torques due to slab buoyancy, act to change the angle of slab dip.

Role of H2O in Generating Subduction Zone Earthquakes

NASA Astrophysics Data System (ADS)

Hasegawa, A.

2017-03-01

A dense nationwide seismic network and high seismic activity in Japan have provided a large volume of high-quality data, enabling high-resolution imaging of the seismic structures defining the Japanese subduction zones. Here, the role of H2O in generating earthquakes in subduction zones is discussed based mainly on recent seismic studies in Japan using these high-quality data. Locations of intermediate-depth intraslab earthquakes and seismic velocity and attenuation structures within the subducted slab provide evidence that strongly supports intermediate-depth intraslab earthquakes, although the details leading to the earthquake rupture are still poorly understood. Coseismic rotations of the principal stress axes observed after great megathrust earthquakes demonstrate that the plate interface is very weak, which is probably caused by overpressured fluids. Detailed tomographic imaging of the seismic velocity structure in and around plate boundary zones suggests that interplate coupling is affected by local fluid overpressure. Seismic tomography studies also show the presence of inclined sheet-like seismic low-velocity, high-attenuation zones in the mantle wedge. These may correspond to the upwelling flow portion of subduction-induced secondary convection in the mantle wedge. The upwelling flows reach the arc Moho directly beneath the volcanic areas, suggesting a direct relationship. H2O originally liberated from the subducted slab is transported by this upwelling flow to the arc crust. The H2O that reaches the crust is overpressured above hydrostatic values, weakening the surrounding crustal rocks and decreasing the shear strength of faults, thereby inducing shallow inland earthquakes. These observations suggest that H2O expelled from the subducting slab plays an important role in generating subduction zone earthquakes both within the subduction zone itself and within the magmatic arc occupying its hanging wall.

Tectonic slicing and mixing processes along the subduction interface: The Sistan example (Eastern Iran)

NASA Astrophysics Data System (ADS)

Bonnet, G.; Agard, P.; Angiboust, S.; Monié, P.; Jentzer, M.; Omrani, J.; Whitechurch, H.; Fournier, M.

2018-06-01

Suture zones preserve metamorphosed relicts of subducted ocean floor later exhumed along the plate interface that can provide critical insights on subduction zone processes. Mélange-like units are exceptionally well-exposed in the Sistan suture (Eastern Iran), which results from the closure of a branch of the Neotethys between the Lut and Afghan continental blocks. High pressure rocks found in the inner part of the suture zone (i.e., Ratuk complex) around Gazik are herein compared to previously studied outcrops along the belt. Detailed field investigations and mapping allow the distinction of two kinds of subduction-related block-in-matrix units: a siliciclastic-matrix complex and a serpentinite-matrix complex. The siliciclastic-matrix complex includes barely metamorphosed blocks of serpentinized peridotite, radiolarite and basalt of maximum greenschist-facies grade (i.e., maximum temperature of 340 °C). The serpentinite-matrix complex includes blocks of various grades and lithologies: mafic eclogites, amphibolitized blueschists, blue-amphibole-bearing metacherts and aegirine-augite-albite rocks. Eclogites reached peak pressure conditions around 530 °C and 2.3 GPa and isothermal retrogression down to 530 °C and 0.9 GPa. Estimation of peak PT conditions for the other rocks are less-well constrained but suggest equilibration at P < 1 GPa. Strikingly similar Ar-Ar ages of 86 ± 3 Ma, along 70 km, are obtained for phengite and amphibole from fourteen eclogite and amphibolitized blueschist blocks. Ages in Gazik are usually younger than further south (e.g., Sulabest), but there is little age difference between the various kinds of rocks. These results (radiometric ages, observed structures and rock types) support a tectonic origin of the serpentinite-matrix mélange and shed light on subduction zone dynamics, particularly on coeval detachment and exhumation mechanisms of slab-derived rocks.

Mantle dynamics following supercontinent formation

NASA Astrophysics Data System (ADS)

Heron, Philip J.

This thesis presents mantle convection numerical simulations of supercontinent formation. Approximately 300 million years ago, through the large-scale subduction of oceanic sea floor, continental material amalgamated to form the supercontinent Pangea. For 100 million years after its formation, Pangea remained relatively stationary, and subduction of oceanic material featured on its margins. The present-day location of the continents is due to the rifting apart of Pangea, with supercontinent dispersal being characterized by increased volcanic activity linked to the generation of deep mantle plumes. The work presented here investigates the thermal evolution of mantle dynamics (e.g., mantle temperatures and sub-continental plumes) following the formation of a supercontinent. Specifically, continental insulation and continental margin subduction are analyzed. Continental material, as compared to oceanic material, inhibits heat flow from the mantle. Previous numerical simulations have shown that the formation of a stationary supercontinent would elevate sub-continental mantle temperatures due to the effect of continental insulation, leading to the break-up of the continent. By modelling a vigorously convecting mantle that features thermally and mechanically distinct continental and oceanic plates, this study shows the effect of continental insulation on the mantle to be minimal. However, the formation of a supercontinent results in sub-continental plume formation due to the re-positioning of subduction zones to the margins of the continent. Accordingly, it is demonstrated that continental insulation is not a significant factor in producing sub-supercontinent plumes but that subduction patterns control the location and timing of upwelling formation. A theme throughout the thesis is an inquiry into why geodynamic studies would produce different results. Mantle viscosity, Rayleigh number, continental size, continental insulation, and oceanic plate boundary evolution are

Geology is the Key to Explain Igneous Activity in the Mediterranean Area

NASA Astrophysics Data System (ADS)

Lustrino, M.

2014-12-01

Igneous activity in tectonically complex areas can be interpreted in many different ways, producing completely different petrogenetic models. Processes such as oceanic and continental subduction, lithospheric delamination, changes in subduction polarity, slab break-off and mantle plumes have all been advocated as causes for changes in plate boundaries and magma production, including rate and temporal distribution, in the circum-Mediterranean area. This region thus provides a natural laboratory to investigate a range of geodynamic and magmatic processes. Although many petrologic and tectonic models have been proposed, a number of highly controversial questions still remain. No consensus has yet been reached about the capacity of plate-tectonic processes to explain the origin and style of the magmatism. Similarly, there is still not consensus on the ability of geochemical and petrological arguments to reveal the geodynamic evolution of the area. The wide range of chemical and mineralogical magma compositions produced within and around the Mediterranean, from carbonatites to strongly silica-undersaturated silico-carbonatites and melilitites to strongly silica-oversaturated rhyolites, complicate models and usually require a large number of unconstrained assumptions. Can the calcalkaline-sodic alkaline transition be related to any common petrogenetic point? Is igneous activity plate-tectonic- (top-down) or deep-mantle-controlled (bottom-up)? Do the rare carbonatites and carbonate-rich igneous rocks derive from the deep mantle or a normal, CO2-bearing upper mantle? Do ultrapotassic compositions require continental subduction? Understanding chemically complex magmas emplaced in tectonically complex areas require open minds, and avoiding dogma and assumptions. Studying the geology and shallow dynamics, not speculating about the deep lower mantle, is the key to understanding the igneous activity.

Paleo movement of continents, mantle dynamics and large wander of the rotational pole

NASA Astrophysics Data System (ADS)

Greff-Lefftz, M.; Besse, J.

2010-12-01

Polar wander is known to be mainly linked to mass distribution changes in its mantle or surface, and more particularly to subductions evolution. On one hand, the peri-pacific subductions seem to be a quite permanent feature of the earth's history at least since the Paleozoic, while the "Tethyan" subductions have a complex history with successive collisions of continental blocs (Hercynian, Kimmerian, Indian) and episodically rebirth of E-W subduction zones. We investigate plate motion during the last 350 million years in a reference frame where Africa is fixed, this last plate being a central plate from which most continents diverged since Pangea break-up. The exact amount of subduction is unknown before 120 Ma and we try to estimate it from the study of the subduction volcanism in the past and plate motion history, when available. Assuming that the subducted slabs sink vertically into the mantle and taking into account large-scale upwellings derived from present-day tomography and intra-plate volcanism in the past, we compute the time variation of mantle density heterogeneities since 350 Ma. By conservation of the angular momentum of the Earth, the temporal evolution of the rotational axis, with respect to the fixed Africa, is computed and compared to the Apparent Polar Wander (APW) observed by paleomagnetism since 280 Ma. We find that a major trend of the computed APW can be described as successive oscillatory clockwise or counter-clockwise motions and that the cusps (around 230 Ma and 170 Ma), both in the observed Africa APW and in the computed pole, are essentially due to the Hercynian (340-300 Ma) and Kimmerian (270-230 Ma) continental collisions.

Constraints on the Amount of deeply subducted Water from numerical Models in comparison with natural Samples

NASA Astrophysics Data System (ADS)

Konrad-Schmolke, M.; Halama, R.

2014-12-01

The subduction of hydrated slab mantle to beyond-arc depths is the most important and yet weakly constrained factor in the quantification of the Earth's deep geologic water cycle. During subduction of hydrated oceanic lithosphere, dehydration reactions in the downgoing plate lead to a partitioning of water between upper and lower plate. Water retained in the slab is recycled into the mantle where it controls its rheology and thus plate tectonic velocities. Hence, quantification of the water partitioning in subduction zones is crucial for the understanding of mass transfer between the Earth's surface and the mantle. Combined thermomechanical and thermodynamic models yield quantitative constraints on the water cycle in subduction zones, but unless model results can be linked to natural observations, the reliability of such models remains speculative. We present combined thermomechanical, thermodynamic and geochemical models of active and paleo-subduction zones, whose results can be tested with independent geochemical features in natural rocks. In active subduction zones, evidence for the validity of our model comes from the agreement between modeled and observed across-arc trends of boron concentrations and isotopic compositions in arc volcanic rocks. In the Kamchatkan subduction zone, for example, the model successfully predicts complex geochemical patterns and the spatial distribution of arc volcanoes. In paleo-subduction zones (e.g. Western Gneiss Region and Western Alps), constraints on the water budget and dehydration behavior of the subducting slab come from trace element zoning patterns in ultra-high pressure (UHP) garnets. Distinct enrichments of Cr, Ni and REE in the UHP zones of the garnets can be reconciled by our models that predict intense rehydration and trace element re-enrichment of the eclogites at UHP conditions by fluids released from the underlying slab mantle. Models of present-day subduction zones indicate the presence of 2.5-6 wt.% of water