Young Subduction Zone Research Articles

Subduction without volcanic arc magma: Insights from two young subduction zones in the western Pacific

Missing volcanic arcs in young subduction zones are commonly attributed to an insufficient subduction depth of the downgoing slab. The North Sulawesi subduction zone (NSS) in the western Pacific is deeper than the average volcanic sub-arc depth; however, no corresponding volcanoes or magmas are along this arc. To investigate this apparent contradiction, we conducted a comparative analysis of the NSS with the middle section of the Philippine subduction zone (MPS) examining the plate flexure, slab dehydration, and mantle wedge thermal structure. Based on our results, the subducting plates of the NSS exhibit a shallow maximum depth of the yield zone, indicating a lower water content. Additionally, the dehydration depth in the NSS system is shallower compared to MPS. The wedge temperature corresponding to this dehydration depth is exceptionally low, which restricts the production of a significant amount of arc magma. Therefore, we propose that the absence of arc magma in the NSS could be explained by the little water influx and the shallow water dehydration of the subducting slab. Our findings also suggest that, when the angle between a young slab and the overriding plate is small, a long arc magma history will not be observed during early subduction stage.

Strike‐Slip Enables Subduction Initiation Beneath a Failed Rift: New Seismic Constraints From Puysegur Margin, New Zealand

AbstractSubduction initiation often takes advantage of previously weakened lithosphere and may preferentially nucleate along pre‐existing plate boundaries. To evaluate how past tectonic regimes and inherited lithospheric structure might lead to self‐sustaining subduction, we present an analysis of the Puysegur Trench, a young subduction zone with a rapidly evolving tectonic history. The Puysegur margin, south of New Zealand, has experienced a transformation from rifting to seafloor spreading to strike‐slip, and most recently to incipient subduction, all in the last ∼45 million years. Here we present deep‐penetrating multichannel reflection and ocean‐bottom seismometer tomographic images to document crustal structures along the margin. Our images reveal that the overriding Pacific Plate beneath the Solander Basin contains stretched continental crust with magmatic intrusions, which formed from Eocene‐Oligocene rifting between the Campbell and Challenger plateaus. Rifting was more advanced to the south, yet never proceeded to breakup and seafloor spreading in the Solander Basin as previously thought. Subsequent strike‐slip deformation translated continental crust northward causing an oblique collisional zone, with trailing ∼10 Myr old oceanic lithosphere. Incipient subduction transpired as oceanic lithosphere from the south forcibly underthrust the continent‐collision zone. We suggest that subduction initiation at the Puysegur Trench was assisted by inherited buoyancy contrasts and structural weaknesses that were imprinted into the lithosphere during earlier phases of continental rifting and strike‐slip along the plate boundary. The Puysegur margin demonstrates that forced nucleation along a strike‐slip boundary is a viable subduction initiation scenario and should be considered throughout Earth's history.

Subduction initiation terranes exposed at the front of a 2 Ma volcanically-active subduction zone

The development of ideas leading to a greater understanding of subduction initiation is limited by the scarcity of present-day examples. Furthermore, the few examples identified so far unfortunately provide few insights into the nature of magmatism at the inception of subduction. Here we report new observations from the Matthew and Hunter (M&H) subduction zone, a very young subduction zone located in the South-West Pacific. Tectonics of the area show it is younger than 2 Ma, making the M&H the youngest known volcanically-active subduction system and hence providing unique insights into the earliest stages of subduction initiation. Volcanism in this area comprises an exceptionally diverse range of contemporaneously erupting magma compositions which are spatially juxtaposed. Pb isotopic compositions and abundance of LILE and REE strongly suggest melting of upwelling asthenospheric mantle (Indian MORB) and subducted oceanic crust (Pacific MORB of the South Fiji Basin) and the mixing of these two components. Volcanism occurs much closer to the trench compared to volcanism in more mature subduction zones. We demonstrate that the M&H subduction zone is a modern example of an immature subduction system at the stage of pre-arc, near-trench magmatism. It is not yet building an arc but what we propose to call a Subduction Initiation Terrane (SITER). Today, the proto-forearc of the M&H subduction zone is a collage of these SITERs, coeval back-arc domains and remnants of pre-existing terranes including old Vitiaz Arc crust. The M&H area represents a modern analog of a Supra Subduction Zone setting where potentially a majority of ophiolites have formed their crustal and lithospheric components. Present-day magmatism in the M&H area therefore provides clues to understanding unforeseen distribution of contrasted magmatic rock types in fossil forearcs, whether they are at the front of mature subduction zones or in ophiolites.

Fluid‐Assisted Deformation and Strain Localization in the Cooling Mantle Wedge of a Young Subduction Zone (Semail Ophiolite)

AbstractThis study reports on feedback mechanisms between fluid migration, ductile deformation, and strain localization processes in an incipiently forming mantle wedge: the basal banded unit of the Semail ophiolite. These peridotites were located right above the plate interface during intraoceanic subduction infancy that ultimately led to ophiolite obduction. During this stage, they were affected by coeval ductile deformation, forming (proto)mylonites at ~900–800 °C to ultramylonites at ~700 °C, and interaction with subduction fluids. From the petrological and microstructural study of these hydrated peridotites and their protolith (preserved lenses of porphyroclastic tectonites), we show that peridotite interaction with hydrous fluids triggered dissolution/precipitation processes. Dissolution of coarser grains and precipitation of new and smaller ones (mainly pyroxenes, spinel, and amphibole) resulted in a drastic grain size reduction, phase mixing and a switch from olivine dislocation to grain size sensitive creep in (proto)mylonites. Data also evidence a feedback process of fluid focusing in actively deforming shear zones. This rapid switch in olivine deformation mechanism, driven by subduction fluid‐peridotite interaction, triggered an intense weakening of the peridotites at ~900–800 °C and a transition from a mechanically decoupled to coupled plate interface (as witnessed by the detachment and underplating of a high‐temperature metamorphic sole to the base of the ophiolite). It also explains the intense strain localization (<1‐km‐thick shear zone) along this ductile portion of the plate interface. Considering that similar mechanisms take place in mature subduction zones, they may explain plate interface coupling at subarc depths in worldwide subduction zones.

The subduction initiation stage of the Wilson cycle

Abstract In the Wilson cycle, there is a change from an opening to a closing ocean when subduction begins. Subduction initiation is commonly identified as a major problem in plate tectonics and is said to be nowhere observable, yet there are many young subduction zones at the west Pacific margins and in eastern Indonesia. Few studies have considered these examples. Banda subduction developed by the eastwards propagation of the Java trench into an oceanic embayment by tearing along a former ocean–continent boundary. The earlier subducted slab provided the driving force to drag down unsubducted oceanic lithosphere. Although this process may be common, it does not account for young subduction zones near Sulawesi at different stages of development. Subduction began there at the edges of ocean basins, not at former spreading centres or transforms. It initiated at a point where there were major differences in elevation between the ocean floor and the adjacent hot, weak and thickened arc/continental crust. The age of the ocean crust appears to be unimportant. A close relationship with extension is marked by the dramatic elevation of land, the exhumation of deep crust and the spectacular subsidence of basins, raising questions about the time required to move from no subduction to active subduction, and how initiation can be identified in the geological record.

Forearc Peridotites from Tonga Record Heterogeneous Oxidation of the Mantle following Subduction Initiation

The elevated oxygen fugacity recorded by subduction-related lavas and peridotites, relative to their mid-ocean ridge counterparts, fundamentally influences the petrogenesis of arc magmas. However, the timing, process, and spatial extent of oxidizing mass transfer at subduction zones remain unknown. Forearc peridotites, which are sometimes exposed on the trench wall of the overriding plate, record chemical fingerprints of the melting and melt–rock interaction processes that occur during and following subduction initiation, and thus provide insight into the spatial and temporal evolution of this oxidized signature. In this study, we present new major element, trace element, and oxygen fugacity data for a suite of forearc peridotites recovered from the Tonga Trench, in addition to a new assessment of literature data for previously studied forearc peridotites. For Tonga samples and literature data for forearc, ridge, and subduction-zone peridotites, we calculate oxygen fugacity (fO2) using an updated method. In contrast to previous studies, we find that spinel Cr#, a proxy for extent of melt extraction, does not correlate with oxygen fugacity, such that many forearc peridotites with high spinel Cr# do not record oxygen fugacity higher than the mid-ocean ridge peridotite array. Combining these observations with trace element modeling, we conclude that forearc peridotites are less pervasively influenced by oxidation owing to subduction processes than previously reported. The oxygen fugacity recorded by Tonga forearc peridotites is heterogeneous between dredges and homogeneous within dredges. To explore these variations, we grouped the dredges into two categories. Group I peridotites have high spinel Cr#, extremely depleted trace element compositions and oxygen fugacity values consistent with the mid-ocean ridge peridotite array. We interpret these to be the residues of large degrees of fractional melting, with little influence from arc-like melts or fluids, formed during the first stages of subduction initiation. Group II peridotites have lower spinel Cr#, enriched light rare earth elements, and oxygen fugacity elevated by ≥1 log unit above the mid-ocean peridotite array. We interpret these peridotites to be the residues of flux melting, initiated once corner flow is established in the young subduction zone. We conclude that the forearc mantle is not pervasively oxidized relative to mid-ocean ridge mantle, and that the asthenospheric mantle in the proto-subduction zone region is not oxidized prior to subduction initiation. As the oxidized signature in Group II peridotites accompanies geochemical evidence of interaction with subduction-related fluids and melts, this suggests that the sub-arc mantle is oxidized concurrently with addition of subduction fluids to the mantle wedge.

Deciphering 3-D structure of young subduction zone from spatial variation of conditions of magma generation: Miocene volcanism in the SW Japan

Deciphering 3-D structure of young subduction zone from spatial variation of conditions of magma generation: Miocene volcanism in the SW Japan

Microxenoliths from the Slave craton: Archives of diamond formation along fluid conduits

A suite of 23 diamondiferous microxenoliths from the Ekati kimberlites in the central Slave craton was studied to elucidate the tectonothermal evolution of the lithosphere during craton formation and modification along craton margins, and associated diamond formation. Major- and trace-element abundances of eclogites (n = 16) reveal the presence of two compositionally distinct suites: a group of three eclogites with typical gabbroic trace-element patterns (flat REE N and low ΣREE, positive Sr N and Pb N, negative Zr N anomalies) and a group of 13 eclogites that are characterised by lower Mg#, Cr, Sc and Sr combined with higher Al 2O 3, TiO 2 and Na 2O, and a conspicuously stepped REE pattern with lower LREE/HREE. Inclusions in diamonds from the central Slave craton consistently plot with the latter group. The unusual trace-element characteristics of the second group are strikingly similar to those produced along fluid conduits in younger subduction zones, where infiltration of a metasomatic fluid induced eclogitisation. Eclogitic diamonds contain an average of 540 at.ppm nitrogen that is poorly aggregated (12% in the B centre), similar to eclogitic diamonds recovered from the A154 kimberlite (Diavik Mine, ~ 30 km southeast) that were dated to 1.85 Ga and linked to subduction at the western craton margin. This suggests that (1) by 1.85 Ga subduction-related reactive fluid transport and element mobilisation similar to younger examples were established, and (2) this fluid flow not only catalysed eclogitisation of subducting oceanic crust, but also was a prerequisite for the majority of eclogitic diamond formation beneath the central Slave craton. Harzburgitic and lherzolitic diamondiferous microxenoliths (n = 4) may document diamond formation in the Slave craton prior to emplacement of eclogites. Peridotitic garnets have sinusoidal REE-patterns with strong depletions in Ti, consistent with carbonate-melt or fluid metasomatism that may have also prompted diamond growth. Their temperatures of last equilibration (~ 1020 to 1300 °C), combined with diamond N characteristics are consistent with precipitation precontemporaneously to ~ 3.3 Ga formation of the deep Slave lithosphere from carbonatitic melts or fluids derived from below the lithosphere–asthenosphere boundary.

Seismic Evidence for Possible Slab Melting from Strong Scattering Waves

Slab melting in young and hot subduction zones has been studied using geochemical observations and thermal modelling, but there are few data from seismic studies to confirm slab melting. Also the detailed geometry in the deep part of the melting slab is often ambiguous in that the intraslab earthquakes within the Wadati-Benioff zone are only limited to shallower depths. To improve our understanding of both the seismic features and geometry found in a young and hot subducted slab, I analyzed anomalous moonquake-like seismograms that were generated by an intermediate-depth earthquake recorded in central Japan. For this study, possible reflected (or scattered) sources were examined using detailed analyses of particle motions and a grid search for travel-time differences between the direct and later P-waves. The results show that using strong seismic scattering, slab melting is likely occurring in the deeper, flexing part of the subducted Philippine Sea plate. Because the subducted Philippine Sea plate in central Japan is young and therefore hot, partial melting might have taken place to produce abundant melting spots in the subducted slab. Melting spots, identified as ”bright spots,” could efficiently reflect or scatter seismic energy and generate many later phases with large amplitudes.

Thermodynamic calculation on the phase transformation and water release of subducted sediment from 10 to 35 kbar

Thermodynamic calculation results of a clastic sediment composition (K 2O–Na 2O–CaO–MgO–FeO–Fe 2O 3–Al 2O 3–TiO 2–SiO 2–H 2O) at 10 to 35 kbar and 300–900 °C show that H 2O release of subducted sediment in subduction zone is mainly controlled by the metamorphic dehydrations of hydrous minerals: chlorite, muscovite, carpholite, paragonite, lawsonite and chloritoid. At lower (≤70 km) and deeper (≥70 km) depths in subduction zone, the dehydration temperature of sediment varies from 350–800 to 420–600 °C, respectively. Above 780 °C, hydrous minerals are broken down entirely in the system except for muscovite. The phase transition and dehydration of sediment may be responsible for the H 2O source of the shallow level fluid in young (hot) subduction zone and the deeper fluids in elder (cold) subduction zone.

Fifty‐five‐million‐year history of oceanic subduction and exhumation at the northern edge of the Caribbean plate (Sierra del Convento mélange, Cuba)

AbstractPetrological and geochronological data of six representative samples of exotic blocks of amphibolite and associated tonalite‐trondhjemite from the serpentinitic mélange of the Sierra del Convento (eastern Cuba) indicate counterclockwise P–T paths typical of material subducted in hot and young subduction zones. Peak conditions attained were ∼750 °C and 15 kbar, consistent with the generation of tonalitic partial melts observed in amphibolite. A tonalite boulder provides a U‐Pb zircon crystallization age of 112.8 ± 1.1 Ma, and Ar/Ar amphibole dating yielded two groups of cooling ages of 106–97 Ma (interpreted as cooling of metamorphic/magmatic pargasite) and 87–83 Ma (interpreted as growth/cooling of retrograde overprints). These geochronological data, in combination with other published data, allow the following history of subduction and exhumation to be established in the region: (i) a stage of hot subduction 120–115 Ma, developed upon onset of subduction; (ii) relatively fast near‐isobaric cooling (25 °C Myr−1) 115–107 Ma, after accretion of the blocks to the upper plate lithospheric mantle; (iii) slow syn‐subduction cooling (4 °C Myr−1) and exhumation (0.7 km Myr−1) in the subduction channel 107–70 Ma; and (iv) fast syn‐collision cooling (74 °C Myr−1) and exhumation (5 km Myr−1) 70–60 Ma.

Evolution of shallow, crustal thermal structure from subduction to collision: An example from Taiwan

We study crustal thermal evolution by examining heat flow patterns along a convergent boundary from a young subduction zone to a more structurally mature collision zone. More than 8000 km of seismic profiles covering an offshore region of 45,000 km 2 in southern Taiwan show widespread bottom-simulating reflectors (BSRs). We derived 1107 BSR-based heat flows before combining 42 additional, published, offshore thermal probe data and 86 on-land heat flow data to document the shallow forearc thermal structures from the subduction zone to the collision zone. In the subduction zone, the geothermal gradient ranges mostly within 30–80 °C/km, and decreases toward the arc due to slab cooling, intensive dewatering at the toe, sediment blanketing, topographic effects, and other processes. The geothermal gradient ranges mostly from 30 to 90 °C/km in the collision zone, and increases, instead of decreases, toward the arc, possibly caused by exhumation, erosion, topographically induced ground-water circulation, and some upper mantle processes related to collision. Heat flow in the collision zone ranges from 80 to 250 mW/m 2 . The high heat flow in the collision zone correlates with a shallower seismicity zone and high seismic attenuation, while the lower heat flow in the subduction zone might allow the earthquakes to rupture to greater depth. The heat flow increases along the topographic high from subduction to collision zone due to increasing geothermal gradients and higher thermal conductivities of the exhumed basement rocks. This heat flow variation may generate an artificial exhumation rate pattern, if a conventional 30 °C/km geothermal gradient was used in fission-track studies.

Did the character of subduction change at the end of the Archean? Constraints from convergent-margin granitoids

Large ion lithophile and high field strength element distributions in juvenile upper continental crust are controlled chiefly by the abundance of tonalite-trondhjemite-granodiorite (TTG) in the Archean shifting to a combination of TTG, calc-alkaline granitoid, and graywacke control thereafter. Geochemical differences between TTG and high-silica adakites do not require production of most TTG magmas in descending slabs. Changes in the ratio of TTG to calc-alkaline granitoids after 2.5 Ga indicate that Archean subduction zones must have differed from younger subduction zones in two very important ways: (1) a deep mafic crust served as a TTG magma source (either as thickened crust or in descending slabs), and (2) they did not give rise to significant volumes of calc-alkaline magma. Thickened mafic crust in the Late Archean may have resulted from plate jams in subduction zones caused by thicker oceanic crust and oceanic plateaus produced during Late Archean mantle thermal events.

Shear stresses on megathrusts: Implications for mountain building behind subduction zones

Shear stresses τ on a subduction megathrust play an important role in determining the forces available for mountain building adjacent to a subduction zone. In this study, the temperatures and shear stresses on megathrusts in 11 subduction zones around the Pacific rim (Hikurangi, Tonga, Izu‐Ogasawara, western Nankai, northeastern Japan, Aleutians, western Alaska, Cascadia, northern Chile, southern Chile) and SE Asia (northern Sumatra) have been determined. The main constraint is that vertical normal stresses beneath the highlands behind the subduction zone are nearly equal to horizontal normal stresses, in the plane of a trench‐ or arc‐normal section. For a typical brittle and ductile megathrust rheology, frictional shear stress τ = μρgz, for depth z, and ductile shear stress τ = A exp (B/RT) at temperature T, where μ, A, B are rheological parameters treated as constants. Rheological constants common to all the megathrusts (μcrust, μmantle, B) are determined by simultaneously solving for the force balance in the overlying wedge and megathrust thermal structure, using a simplex minimization algorithm, taking account of the induced mantle corner flow at depth (65 ± 15 km (2σ)) and constant radiogenic heating (0.65 ± 0.3 μW m−3 (2σ)) throughout the crust. The A constants are solved individually for each subduction zone, assuming that the maximum depth of interplate slip earthquakes marks the brittle‐ductile transition. The best fit solution shows two groupings of megathrusts, with most subduction zones having a low mean shear stress in the range 7–15 MPa (μcrust = 0.032 ± 0.006, μmantle = 0.019 ± 0.004) and unable to support elevations >2.5 km. For a typical frictional sliding coefficient ∼0.5, the low effective coefficients of friction suggest high pore fluid pressures at ∼95% lithostatic pressure. Tonga and northern Chile require higher shear stresses with μcrust = 0.095 ± 0.024, μmantle = 0.026 ± 0.007, suggesting slightly lower pore fluid pressures, at ∼81% lithostatic. Ductile shear in the crust is poorly resolved but in the mantle appears to show a strong power law dependency, with B = 36 ± 18 kJ mol−1. Amantle values are sensitive to the precise value of B but are in the range 1–20 kPa. The power law exponent n for mantle flow is poorly constrained but is likely to be large (n > 4). The brittle‐ductile transition in the crust occurs at temperatures in the range 370°C–512°C, usually close to the base of the crust and in the mantle at much lower temperatures (180°C–300°C), possibly reflecting a marked change in pore fluid pressure or quasi ductile and subfrictional properties. In subduction zones where the subducted slab is older than 50 Ma, a significant proportion of the integrated shear force on the megathrust is taken up where it cuts the mantle and temperatures are ≤300°C. In much younger subduction zones, the stress transmission is confined mainly to the crust. The shear stresses, particularly in the crust, may be kept low by some sort of lubricant such as abundant water‐rich trench fill, which lowers the frictional sliding coefficient or effective viscosity and/or raises pore fluid pressure. The unusual high stress subduction zone in northern Chile lacks significant trench fill and may be poorly lubricated, with a mean shear stress ∼37 MPa required to support elevations >4 km in the high Andes. However, where the crust is thin in sediment‐starved and poorly lubricated subduction zones, such as Tonga, the mean shear stress will still be low. Sediment may lubricate megathrusts accommodating underthrusting of continental crust, such as in the Himalayas or eastern central Andes, which have a low mean shear stress ∼15 MPa.

Offshore geodetic data conducive to the estimation of the afterslip distribution following the 2003 Tokachi-oki earthquake

A cable-based seafloor observatory located offshore of Hokkaido, Japan, provides the world's first direct observation of offshore post-seismic displacement. After removing thermal noise, this data is used to constrain the distribution of offshore afterslip from a great interplate earthquake (Mw 8.0) that occurred during 2003 along the southern Kuril Trench. A checkerboard resolution test showed a great contribution of the offshore data to improving the spatial resolution on the plate interface slip near the trench. The 1-yr afterslip is distributed in a U-shaped pattern encircling its co-seismic slip area. This result indicates that the stick-slip frictional properties of the plate interface are distributed in a patch-like pattern as opposed to a banding pattern as previously assumed in young subduction zones.

Evolving force balance during incipient subduction

Nearly half of all active subduction zones initiated during the Cenozoic. All subduction zones associated with active back arc extension have initiated since the Eocene, hinting that back arc extension may be intimately associated with an interval (several tens of Myr) following subduction initiation. That such a large proportion of subduction zones are young indicates that subduction initiation is a continuous process in which the net resisting force associated with forming a new subduction zone can be overcome during the normal evolution of plates. Subduction initiation is known to have occurred in a variety of tectonic settings: old fracture zones, transform faults, and extinct spreading centers and through polarity reversal behind active subduction zones. Although occurring within different tectonic settings, four known subduction initiation events (Izu‐Bonin‐Mariana (IBM) along a fracture zone, Tonga‐Kermadec along an extinct subduction boundary, New Hebrides within a back arc, and Puysegur‐Fiordland along a spreading center) were typified by rapid uplift within the forearc followed by sudden subsidence. Other constraints corroborate the compressive nature of IBM and Tonga‐Kermadec during initiation. Using an explicit finite element method within a two‐dimensional domain, we explore the evolving force balance during initiation in which elastic flexure, viscous flow, plastic failure, and heat transport are all considered. In order to tie theory with observation, known tectonic settings of subduction initiation are used as initial and boundary conditions. We systematically explore incipient compression of a homogeneous plate, a former spreading center, and a fracture zone. The force balance is typified by a rapid growth in resisting force as the plate begins bending, reaching a maximum value dependent on plate thickness, but typically ranging from 2 to 3 × 1012 N/m for cases that become self‐sustaining. This is followed by a drop in stress once a shear zone extends through the plate. The formation of a throughgoing fault is associated with rapid uplift on the hanging wall and subsidence on the footwall. Cumulative convergence, not the rate of convergence, is the dominant control on the force balance. Viscous tractions influence the force balance only if the viscosity of the asthenosphere is >1020 Pa s, and then only after plate failure. Following plate failure, buoyancy of the oceanic crust leads to a linear increase with crustal thickness in the work required to initiate subduction. The total work done is also influenced by the rate of lithospheric failure. A self‐sustaining subduction zone does not form from a homogeneous plate. A ridge placed under compression localizes subduction initiation, but the resisting ridge push force is not nearly as large as the force required to bend the subducting plate. The large initial bending resistance can be entirely eliminated in ridge models, explaining the propensity for new subduction zones to form through polarity reversals. A fracture zone (FZ) placed in compression leads to subduction initiation with rapid extension of the overriding plate. A FZ must be underthrust by the older plate for ∼100–150 km before a transition from forced to self‐sustaining states is reached. In FZ models the change in force during transition is reflected by a shift from forearc uplift to subsidence. Subduction initiation is followed by trench retreat and back arc extension. Moderate resisting forces associated with modeled subduction initiation are consistent with the observed youth of Pacific subduction zones. The models provide an explanation for the compressive state of western Pacific margins before and during subduction initiation, including IBM and Tonga‐Kermadec in the Eocene, and the association of active back arcs with young subduction zones. On the basis of our dynamic models and the relative poles of rotation between Pacific and Australia during the Eocene, we predict that the northern segment of the Tonga‐Kermadec convergent margin would have initiated earlier with a progressive southern migration of the transition between forced and self‐sustaining states.

Dehydration and earthquakes in the subducting slab: empirical link in intermediate and deep seismic zones

In this study, a close link between the depth-distribution of subduction-zone earthquakes and dehydration events in the hydrated slab-peridotite is established. A phase diagram up to 27 GPa in the MgO–Al 2O 3–SiO 2–H 2O (MASH) system was constructed using a combination of thermodynamic calculation and Schreinemakers analysis on previous experimental data. At intermediate-depths (<250 km), dehydration of antigorite, talc, brucite, clinochlore, and Mg-sursassite occur. In the 200–500 km depth-range, dehydrations of Mg-sursassite and brucite+phase D are possible. Dehydration of brucite and dense hydrous Mg-silicates (phase E, D and superhydrous phase B) occurs in the mantle-transition zone. Since conditions for dehydration are dependent on slab temperature, we constructed diagrams illustrating the distribution patterns of dehydration events at several model slab-temperatures. In parallel to the mineralogical study, the depth-distribution of global subduction-zone earthquakes are compiled into a thermal parameter-depth diagram and compared with the dehydration event distributions determined previously. The distribution of dehydration events and earthquakes are comparable in terms of: (1) non-linear correlation between the maximum depth of earthquake and temperature of the slab, (2) an aseismic zone at 300–600 km depth in medium- T subduction zones, and (3) lack of deep-earthquakes in young subduction-zones. Considering those links, we propose an extended dehydration-induced earthquake (EDIE) hypothesis that considers any dehydration event has a potential to induce earthquakes in the subducting slab peridotite.

A complex, young subduction zone imaged by three-dimensional seismic velocity, Fiordland, New Zealand

SUMMARY The Fiordland subduction zone, where subduction developed in the late Miocene, has been imaged with P and S‐P arrival-time data from 311 earthquakes in a simultaneous inversion for hypocentres and 3-D VP and VP/VS models. The three-month microearthquake survey, recorded with 24 portable seismographs, provides excellent coverage, and, since earthquakes to depths of 130 km are included, parts of the model are well-resolved to depths of 100 km. The crustal features are generally consistent with geology. The low velocity in the upper 10 km is associated with the Te Anau and Waiau basins. The Western Fiordland Orthogneiss is associated with a prominent feature from nearsurface to over 40 km depth, which includes the residue from the basaltic source rocks. It is defined by high VP (7.4 km s x1 at 15 km depth) and slightly low VP/VS, and has distinct boundaries on its southern and eastern margins. Adjacent to the deepest earthquakes, there is high-velocity Pacific mantle below 80 km depth, inferred to be the mantle expression of ongoing shortening since the early Miocene. As the subducting slab moves down and northeast, it is hindered by the high-velocity body and bends to nearvertical. Bending is accommodated by distributed fracturing evidenced by high VP/VS and persistent deep earthquake activity. Buckling of the subducted plate pushes up the Western Fiordland Orthogneiss. In the transition to the Alpine fault in northern Fiordland, a prominent low-velocity crustal root is consistent with ductile thickening in combination with downwarp of the subducted plate.

Latent heat effects of the major mantle phase transitions on low-angle subduction

Very low to zero shallow dip angles are observed at several moderately young subduction zones with an active trenchward moving overriding plate. We have investigated the effects of latent heat for this situation, where mantle material is pushed through the major mantle phase transitions during shallow low-angle subduction below the overriding plate. The significance of the buoyancy forces, arising from the latent heat effects, on the dynamics of the shallowly subducting slab is examined by numerical modeling. When a 32-Ma-old slab is overridden with 2.5 cm/yr by a continent, flat subduction occurs with a 4–5 cm/yr convergence rate. When latent heat is included in the model, forced downwellings cause a thermal anomaly and consequently thermal and phase buoyancy forces. Under these circumstances, the flat slab segment subducts horizontally about 350 km further and for about 11 Ma longer than in the case without latent heat, before it breaks through the 400-km phase transition. The style of subduction strongly depends on the mantle rheology: increasing the mantle viscosity by one order of magnitude can change the style of subduction from steep to shallow. Similarly, an overriding velocity of less than 1 cm/yr leads to steep subduction, which gradually changes to flat subduction when increasing the overriding velocity. However, these model parameters do not change the aforementioned effect of the latent heat, provided that low-angle subduction occurs. In all models latent heat resulted in a substantial increase of the flat slab length by 300–400 km. Varying the olivine–spinel transition Clapeyron slope γ from 1 to 6 MPa/K reveals a roughly linear relation between γ and the horizontal length of the slab. Based on these results, we conclude that buoyancy forces due to latent heat of phase transitions play an important role in low-angle subduction below an overriding plate.

Tonga slab deformation: The influence of a lower mantle upwelling on a slab in a young subduction zone

There are fundamental geographic variations in the deformation of slabs in the transition zone. The seismic energy release and morphology of the Tonga slab show that it is deforming faster and has accumulated more deformation than any other slab. We show that Tonga overlies the edge of the large‐scale Pacific superplume. There is no substantial aseismic penetration into the lower mantle beneath Tonga, consistent with initiation of subduction during the Eocene. Other major subduction systems overlay seismically fast structures. For long‐lived subduction systems, the lower mantle tends to pull down on slabs while in Tonga the lower mantle pushes upward, partially accounting for the intense deformation. The perturbation to the state of slab stress due to large‐scale mantle flow is 10 to 40 MPa — nearly as large as that expected from slab pull.