Mature Subduction Zones Research Articles

Magmatic fingerprints of subduction initiation and mature subduction: numerical modelling and observations from the Izu-Bonin-Mariana system

Understanding the formation of new subduction zones is important because they have been proposed as the main driving mechanism for plate tectonics and they are crucial for geochemical cycles on Earth. However, the conditions needed to facilitate subduction zone initiation and the associated magmatic evolution are still poorly understood. Using a natural case study, we conducted a series of high-resolution 2D petrological-thermomechanical (i2VIS) subduction models assuming visco-plastic rheology. We aim to model the initiation and early stage of an intra-oceanic subduction zone connected to the gravitational collapse of a weak transform zone and compare it to the natural example of the Izu-Bonin-Mariana subduction zone. We also analysed the influence of low convergence rates on magmatic evolution. We propose a viable transition from initiation to mature subduction zone divided into distinct stages that include initiation by gravitational collapse of the subducting slab, development of a near-trench spreading centre, gradual build-up of asthenospheric mantle return flow, and maturation of a volcanic arc. We further show that mantle flow variations and shear instabilities, producing thermal perturbations and depleted interlayers, influence the temporal and spatial distribution of asthenospheric mantle composition and fertility in the mantle wedge. Our modelling results are in good agreement with geological and geochemical observations of the early stages of the Izu-Bonin-Mariana subduction zone.

Tectonic slices of greenschist – epidote blueschist – epidote amphibole schist – garnet epidote amphibole schist from the Nagaland ophiolite complex, NE India: a look into their metamorphic and tectonic evolution

ABSTRACT Tectonic slices of greenschist – epidote blueschist – epidote amphibole schist – garnet epidote amphibole schist are exposed in the northern part of the NOC. Metamorphic P-T conditions and P-T paths of the garnet epidote amphibole schist and epidote blueschist were determined with the help of pseudosection calculations in the MnNCKFMASHTO system using the software Perple_X. The garnet epidote amphibole schist records a pre-peak metamorphic stage at ~12 kbar and ~360°C in the lawsonite blueschist facies field. Lawsonite and glaucophane are not preserved in the rock and are attributed to progressive metamorphic reactions during prograde metamorphism. The peak metamorphic stage is constrained at ~14 kbar and ~570°C in the hornblende eclogite facies field. Post-peak metamorphism shows slight cooling and decompression from ~550°C and ~13 kbar, possibly down to ~350°C and ~7 kbar in the greenschist facies field. These three metamorphic stages reveal a clockwise metamorphic P-T path of evolution for the garnet epidote amphibole schist. The geotherm at metamorphic peak P-T condition is ~12.5°C/km, which is hotter by ~350°C compared to thermal models of mature subduction zones but shows good agreement with models of warm subduction zones. Therefore, the low geothermal gradient of ~8°C/km during the prograde metamorphism possibly reflects the subduction of cold Neo-Tethyan ocean crust when the temperature conditions of the subduction zone were still warm. The epidote blueschist shows peak metamorphism at ~10 kbar and 380°C with a clockwise P-T path. It indicates metamorphism at a more mature stage of subduction. Buoyancy-driven exhumation along the subduction channel due to the serpentinization of the upper mantle wedge played a vital role in the detachment and exhumation of the tectonic slivers of metamorphic rocks from different depth sections in the northern margin of the NOC.

On unusual conditions for the exhumation of subducted oceanic crustal rocks: How to make rocks hotter than models

The thermal structure of subduction zones controls many important processes such as metamorphic devolatilization, arc magmatism, and seismicity. However, the thermal state of subducted oceanic slab predicted by subduction zone thermal models down to ∼80 km depth is systematically cooler than inferred from exhumed metamorphic slab rocks, raising questions about the current understanding of subduction dynamics portrayed by these models. Upon synthesizing previous petrological studies and estimating slab ages of ancient subduction zones from metamorphic terranes, we think that exhumation of subducted oceanic metamorphic rocks is extremely rare and likely requires unusual processes that are not an integral component of normal subduction dynamics. We construct simple numerical scenarios to illustrate how the thermal regime under some unusual conditions at the beginning and ending stages of a subduction margin may deviate from the normal subduction process. At the beginning stage of subduction, a shallow maximum decoupling depth (MDD) between the slab and mantle wedge enables viscous mantle wedge flow to reach a shallow depth, bringing heat to make slab rocks warmer than in a mature subduction zone. At the ending stage of subduction, if subduction cessation is in the form of slab stalling and if the stalled slab is not immediately exhumed, the slab rocks can be warmed up by the heat from the warm mantle wedge and underlying asthenosphere. In either situation, the slab rocks can be much warmer than normal before they are exhumed by other dynamic processes. Observed exhumed rocks are not expected to represent the normal subduction process and should not be directly compared with thermal models that are designed for the normal process. To extract important information about general geodynamic processes from these rocks, we must first understand the processes these rocks went through.

Progressive lawsonite eclogitization of the oceanic crust: Implications for deep mass transfer in subduction zones

Abstract Lawsonite eclogites are major hosts of H2O and trace elements and thus key for long-term deep element cycling in subduction zones. Existing cycling models suggest that the subducting oceanic crust transforms to lawsonite-eclogite assemblages; yet the scarcity of lawsonite eclogites in the rock record questions to what extent the oceanic crust transforms to lawsonite-eclogite assemblages during subduction. Here, we use petrological modeling coupled with geodynamic calculations for a typical subduction zone to show that the occurrence of lawsonite eclogites is controlled by the maturation of the subduction zone. We find that lawsonite eclogite does not form in infant subduction zones; with time, the oceanic crust forms lawsonite assemblages, but prograde heating obliterates lawsonite except in rocks exhumed prior to such heating. Lawsonite-eclogite assemblages in the oceanic crust form and survive prograde metamorphism only in mature and cold subduction zones but still necessitate specific characteristics during exhumation to preserve lawsonite. We show that the stability of lawsonite in mature subduction zones is hindered by hybridization between the mafic crust and the overlaying mantle wedge material; by contrast, lawsonite proportion increases with intense seafloor alteration and Ca-Al metasomatism. These latter processes are thus key for enhancing recycling. We argue that lawsonite-driven mass cycling to the deep mantle is important in mature subduction zones, but the role of lawsonite eclogite in carrying mass deep into Earth in the infant stage of a subduction zone is minor.

Ocean Subduction Dynamics in the Alps

The Alps preserve abundant oceanic blueschists and eclogites that exemplify the selective preservation of fragments of relatively short-lived, small, slow-spreading North Atlantic–type ocean basins whose subducting slabs reach down to the Mantle Transition Zone at most. Whereas no subducted fragments were returned during the first half of the subduction history, those exhumed afterwards experienced conditions typical of mature subduction zones worldwide. Sedimentary-dominated units were under-plated intermittently, mostly at ~30–40 km depth. Some mafic–ultramafic-dominated units formed close to the continent were subducted to ~80 km and offscraped from the slab only a few million years before continental subduction. Spatiotemporal contrasts in burial and preservation of the fragments reveal how along-strike segmentation of the continental margin affects ocean subduction dynamics.

Evidence of melt– and fluid–rock interactions in the refractory forearc peridotites and associated mafic intrusives from the Tuting–Tidding ophiolites, eastern Himalaya, India: Petrogenetic and tectonic implications

We present a comprehensive geochemical data set of whole‐rock geochemistry and mineral phases from the mantle peridotites and mafic intrusives of the Tuting–Tidding Suture Zone (TTSZ) ophiolites, eastern Himalaya, north‐east India. Modal mineralogy, low Al2O3 and CaO, high Cr# of Cr‐spinels, and forsterite number (Fo90–92) in primary olivine indicate the highly refractory nature of the TTSZ peridotites. LREE‐enriched patterns, cumulative olivines in dunites, and Cr‐spinel and olivine compositions further suggest that the peridotites were subjected to high‐temperature melt–rock interaction from a percolating boninitic melt in the nascent forearc of an intra‐oceanic subduction zone. The associated mafic intrusives are tholeiitic in nature and their geochemical characteristics [∑REE = 23.34–59.12; nearly flat rare earth elements (REE) pattern (LaN/YbN = 1.49–2.58); negative anomalies of Nb and Ti] show mid‐oceanic ridge basalt affinity. Trace elemental modelling of the mafic intrusives along with their mineralogy and geochemistry suggests that they formed due to different degrees of partial melting with the involvement of a subduction component in the spreading regime of the forearc region. Based on the presence of hydrous minerals like Cr‐chlorite and tremolite in the peridotites and P–T modelling (525–575°C, 1.05–1.09 GPa) of the mafic intrusives, it is considered that TTSZ ophiolites underwent low‐temperature metamorphism by fluid–rock interaction either during the later phase of subduction (in a cool mature subduction zone) or during Himalayan Orogeny. We also infer that the TTSZ ophiolites resemble other Neo‐Tethyan ophiolites of the Indus‐Tsangpo Suture Zone in terms of their geochemical and petrogenetic aspects.

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.

Low-Ca boninite formation by second-stage melting of spinel harzburgite residues at mature subduction zones: new evidence from veined mantle xenoliths from the West Bismarck Arc

We report the abundances of major and lithophile trace elements and volatiles (H2O, Cl and S) in orthopyroxenite veins cutting mantle-derived, spinel harzburgite xenoliths from the active Ritter volcano in the West Bismarck Arc (Papua New Guinea). The veins preserve sulfide-bearing glass coexisting with crystals. The glass formed by the quench of residual liquids left after crystallisation of abundant orthopyroxene, variable amounts of clinopyroxene, and minor olivine and spinel from silicate melts intruding spinel harzburgite. The glass compositions range from low-Ti (≤ 0.2 wt% TiO2), intermediate to magnesian (2–5 wt% MgO) and high-Mg# (0.45–0.6) andesite to dacite, with Mg# = Mg/(Mg + Fet) where Fet indicates all Fe treated as Fe2+. The glass is depleted in moderately incompatible lithophile trace elements and contains 1.2 to 2.5 wt% H2O, 400–1800 ppm Cl and 100–400 ppm S. Thermo-barometric calculations indicate that the original vein-forming melts intruded the shallow sub-arc mantle lithosphere between 0.5 and 1 GPa, where they partially crystallised (50–60%) to form residual liquids during cooling from ~ 1200 °C down to ~ 650 °C. Our data and calculations show that West Bimarck orthopyroxenite veins are of high-temperature magmatic origin and bear no relation to the carrier magmas of the xenoliths. Instead, petrological modelling demonstrates that the veins formed from the intrusion of primitive low-Ca boninite (LCB) melts in the sub-arc mantle lithosphere. These melts were originally produced by low degrees of melting (≤ 5%) of spinel harzburgite at ≥ 1360 °C and ≤ 1.5 GPa. Some key features of the parental LCB vein-forming melts from West Bismarck (e.g. lithophile trace element signatures and moderate volatile abundances and oxygen fugacity) suggest that, contrary to the widely held belief, their formation does not require persistent mantle wedge fluxing by slab-derived components. Instead, we propose that adiabatic decompression melting of spinel harzburgite residues may also form LCB in the sub-arc mantle. West Bismarck veins are very similar to those previously described in mantle xenoliths from the Kamchatka Arc; active boninite magmatism in these mature arcs shows that the generation of these melts is not as restricted as previously thought. In fact, the growing evidence for the prevalence of boninite in sub-arc mantle peridotites is consistent with the presence of boninite-like geochemical components in many island arcs.

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.

Fluid inclusions in jadeitite and jadeite-rich rock from serpentinite mélanges in northern Hispaniola: Trapped ambient fluids in a cold subduction channel

Freezing-point depression was measured in aqueous fluid inclusions to determine salinities in six samples of jadeitite and jadeite-rich rock from the Jagua Clara serpentinite mélange of the Rio San Juan Complex, Dominican Republic. The mélange represents a fossil subduction-zone channel from a cold, mature subduction zone with a geothermal gradient of ~6 °C/km. One hundred and twenty-five determinations of salinity in primary inclusions hosted in jadeite, quartz, apatite and lawsonite range between extremes of 1.2 and 8.7, but yield a well-defined mean of 4.5 ± 1.1 wt% (±1 s.d.) NaCl equiv, slightly higher than mean seawater (3.5 wt%). In one sample, eight additional fluid inclusions in quartz aligned along grain boundaries yield slightly lower values of 2.7 ± 1.3 wt% NaCl equiv. Homogenization temperatures were also measured for 47 fluid inclusions in two samples, but primary entrapment densities are not preserved. It is significant that the suite includes two types of samples: those precipitated directly from an aqueous fluid as well as examples of metasomatic replacement of a pre-existing magmatic rock. Nevertheless, the results indicate identical salinity for both types and suggest a much stronger genetic link between the two types of jadeitite and jadeite-rich rock than has previously been assumed. Based on the results of conductivity measurements in modern subduction zones, we envision a pervasive fluid in the subduction channel that evolved from salinity levels lower than those in sea-water up to the measured values due to on-going but largely completed serpentinization in the subduction channel. The present data represent a reference marker for the subduction channel of the Rio San Juan intra-oceanic subduction zone at 30–50 km depth and after 50–60 Myr of operation.

Kinematics and dynamics of the Pamir, Central Asia: Quantifying surface deformation and force balance in an intracontinental subduction zone

AbstractKinematic and dynamic models quantify deformation and force balance in the Pamir, a region undergoing the rare and poorly understood process of intracontinental subduction. We constrain a detailed kinematic model with 506 recent GPS velocities and Quaternary fault slip rates and show that the Pamir is organized like the Himalaya and Tibet, with regions of (1) localized strain rate ≥100e‐9/year along the Pamir Frontal Thrust System (the subduction interface), similar to the Himalaya, and (2) distributed north‐south compression and east‐west extension, similar to Tibet. Through standard thin viscous sheet methods we demonstrate that the lithospheric force balance in the Pamir is a combination of stresses caused by gravitational potential energy and India‐Eurasia convergence accommodated at a subduction interface, in this case the Pamir Frontal Thrust System. We find that strain rate and deviatoric stress patterns near the Pamir Frontal Thrust System are characteristic of a mature subduction zone, despite its initiation in continental lithosphere. Although the Pamir and Tibet are kinematically and dynamically similar, the Pamir is stiffer overall than Tibet, perhaps due to the presence of the highly arcuate, geometrically stiffened continental slab at depth.

The distribution of fluid mobile and other incompatible trace elements in orthopyroxene from mantle wedge peridotites

Orthopyroxene is especially suited to decode and testify to the behavior of highly immobile elements during hydrous mantle melting. Laser ablation ICP-MS analyses from orthopyroxene hosted within peridotite from the Coast Range ophiolite (CRO) demonstrates that Group A peridotites (lherzolites) have similar compositions to mid-ocean-ridge abyssal peridotite, whereas other peridotites (Groups B and C; harzburgites) retain depleted signatures, but display ‘spoon-shaped’ enrichments for the light-REE. These patterns are consistent with variable degrees of partial melting of MORB-source asthenosphere initiated within the garnet stability field (<10%) and continuing into the spinel stability field (<15%). A few samples may have been subjected to subsequent melt/rock interaction. The supra-subduction zone (SSZ) environment represented by the CRO is illustrated by enriched fluid mobile elements (Li, Be, B, Pb) in all samples - up to 200× depleted-MORB mantle (DMM). New applications of trace-element addition calculations [Shervais J. and Jean M.M. (2012) Inside the subduction factory: Modeling fluid mobile element enrichment in the mantle wedge above a subduction zone. GCA 95, 270–285] modified for orthopyroxene reveals that tens to hundreds of ppm were added to the DMM-source region. Our purpose is to demonstrate that orthopyroxene, in the absence of clinopyroxene, can be a constructive (and perhaps better) indicator of tectonic environment and magmatic processes that occurred within the North American Cordillera mantle wedge. Through this investigation we have captured all three stages of Coast Range ophiolite petrogenesis: starting with initial SSZ-coupled forearc spreading dominated by decompression melting, to a mature subduction zone with fluid-assisted partial melting, and the transition between the two.

Primary Silica-rich Picrite and High-Ca Boninite Melt Inclusions in Pyroxenite Veins from the Kamchatka Sub-arc Mantle

Island arc picrites and boninites are magnesian magmatic rocks believed to be generated by high degrees of melting of depleted mantle sources fluxed by subduction-derived, volatile-rich components. These magmas can be probes of both the mantle wedge protoliths and subduction components, but are rare among other, usually more evolved, types of arc lavas. Furthermore, many arc picrites and boninites show evidence for late-stage differentiation prior to or during eruption, masking their primary, mantle-derived geochemical signatures. We report textural and chemical data on spinel-hosted melt inclusions of mantle origin in amphibole-bearing websterite veins cross-cutting spinel harzburgite xenoliths from the active andesitic Avacha volcano (south Kamchatka, Russia). The data are used to constrain the composition and origin of melts that formed the websterite veins in the sub-arc lithospheric mantle. The melt inclusions typically contain euhedral orthopyroxene and clinopyroxene and occasionally minor amphibole in silicate glass. The melt inclusions were homogenized using heating stages and gas-mixing furnaces. The homogenized glasses range from subalkaline primitive silica-rich picrite and high-Ca boninite (>15 wt % MgO, 48–54 wt % SiO2) to rhyolite. High-Ca boninite glasses have moderate volatile and low heavy rare earth element contents and elevated Cs, Rb, Ba, U, Sr, and Li abundances, with extremely high U/Th. In turn, the glasses display no negative spikes in the high field strength elements Nb, Ta, Zr, Hf, and Ti. We show that the silica-rich picrite and high-Ca boninite liquids in this study formed by high degrees of melting (>25%), at volatile under-saturation, of hybrid melt-depleted but silica-rich mantle sources at ≥1·5 GPa. The hybrid sources formed in two stages: first, by extraction of ∼15% melt from the convecting mantle to form a refractory protolith, which was subsequently enriched in silica via interaction with subduction-derived components prior to or during remelting in the mantle wedge. The subduction-derived components were enriched in fluid-mobile elements and probably oxidized. Overall, our results suggest that silica-rich picrites and high-Ca boninites can be primary melts in mature subduction zones and differentiate within the mantle wedge and the deep arc crust to form more evolved andesite magmas.

Boninitic and tholeiitic basaltic lavas and dikes from dispersed Jurassic East Othris ophiolitic units, Greece: petrogenesis and geodynamic implications

ABSTRACTPillow lavas, massive lava flows, and sub-volcanic dikes of tholeiitic basaltic composition are found to be members of the Vrinena, Aerino, Eretria, and Velestino dispersed Middle–Upper Jurassic ophiolitic units in East Othris. The Vrinena and Eretria ophiolitic units appear to have been emplaced onto the Pelagonian continental margin during the Upper Jurassic–Lower Cretaceous, whereas the Aerino and Velestino units seem to have been finally emplaced during post-Palaeocene times. Geochemically these are divided into two groups: Group I includes subduction-related boninites and low-Ti basalts from the Vrinena and Aerino units, and Group II high-Ti basalts show spreading-type characteristics occurring in the Eretria and Velestino units. Primary magma of the Group I volcanics appears to have been formed after high partial melting degrees (~18%) of a highly depleted harzburgitic mantle source, under relatively high temperatures (mantle potential temperature ~1372°C). Petrogenetic modelling also suggests that the primary magma of the Group II volcanics were formed after lower partial melting degrees (~7%) of a moderately depleted mantle source. The petrological and geochemical data from the East Othris dispersed and diversely emplaced ophiolitic units provide evidence of a common intra-oceanic supra-subduction zone (SSZ) origin within the Pindos oceanic strand of the Western Tethys. Specifically, Group I lavas and dikes from Vrinena seem to represent the extrusive part of an almost complete fore- to island-arc ophiolitic sequence. Dikes of Aerino most likely correspond to fore-arc magmatic material that intruded within exhumed serpentinized ultramafic rocks through a subduction channel that developed close to the slab and towards the fore-arc and the accretionary prism. The Group II volcanics either corresponded to a fore-arc magmatic expression, which extruded earlier than Group I volcanics and prior to the establishment of a mature subduction zone, or represent back-arc to island-arc magmatism that was contemporaneous to the fore-arc magmatic activity during rollback subduction.

Lead transport in intra-oceanic subduction zones: 2D geochemical–thermo-mechanical modeling of isotopic signatures

Abstract Understanding the physical–chemical mechanisms and pathways of geochemical transport in subduction zones remains a long-standing goal of subduction-related research. In this study, we perform fully coupled geochemical–thermo-mechanical (GcTM) numerical simulations to investigate Pb isotopic signatures of the two key “outputs” of subduction zones: (A) serpentinite melanges and (B) arc basalts. With this approach we analyze three different geodynamic regimes of intra-oceanic subduction systems: (1) retreating subduction with backarc spreading, (2) stable subduction with high fluid-related weakening, and (3) stable subduction with low fluid-related weakening. Numerical results suggest a three-stage Pb geochemical transport in subduction zones: (I) from subducting sediments and oceanic crust to serpentinite melanges, (II) from subducting serpentinite melanges to subarc asthenospheric wedge and (III) from the mantle wedge to arc volcanics. Mechanical mixing and fluid-assisted geochemical transport above slabs result in spatially and temporarily variable Pb concentrations in the serpentinized forearc mantle as well as in arc basalts. The Pb isotopic ratios are strongly heterogeneous and show five types of geochemical mixing trends: (i) binary mantle–MORB, (i) binary MORB–sediments, (iii) double binary MORB–mantle and MORB–sediments, (iv) double binary MORB–mantle and mantle–sediments and (v) triple MORB–sediment–mantle. Double binary and triple mixing trends are transient and characterize relatively early stages of subduction. In contrast, steady-state binary mantle–MORB and MORB–sediments trends are typical for mature subduction zones with respectively low and high intensity of sedimentary melange subduction. Predictions from our GcTM models are in agreement with Pb isotopic data from some natural subduction zones.

The ‘subduction initiation rule’: a key for linking ophiolites, intra-oceanic forearcs, and subduction initiation

We establish the ‘subduction initiation rule’ (SIR) which predicts that most ophiolites form during subduction initiation (SI) and that the diagnostic magmatic chemostratigraphic progression for SIR ophiolites is from less to more HFSE-depleted and LILE-enriched compositions. This chemostratigraphic evolution reflects formation of what ultimately becomes forearc lithosphere as a result of mantle melting that is progressively influenced by subduction zone enrichment during SI. The magmatic chemostratigraphic progression for the Izu–Bonin–Mariana (IBM) forearc and most Tethyan ophiolites is specifically from MORB-like to arc-like (volcanic arc basalts or VAB ± boninites or BON) because SI progressed until establishment of a mature subduction zone. MORB-like lavas result from decompression melting of upwelling asthenosphere and are the first magmatic expression of SI. The contribution of fluids from dehydrating oceanic crust and sediments on the sinking slab is negligible in early SI, but continued melting results in a depleted, harzburgitic residue that is progressively metasomatized by fluids from the sinking slab; subsequent partial melting of this residue yields ‘typical’ SSZ-like lavas in the latter stages of SI. If SI is arrested early, e.g., as a result of collision, ‘MORB-only’ ophiolites might be expected. Consequently, MORB- and SSZ-only ophiolites may represent end-members of the SI ophiolite spectrum. The chemostratigraphic similarity of the Mariana forearc with that of ophiolites that follow the SIR intimates that a model linking such ophiolites, oceanic forearcs, and SI is globally applicable.

Comparison of gravity anomaly between mature and immature intra-oceanic subduction zones in the western Pacific

The western Pacific hosts major subduction systems such as Izu–Bonin–Mariana and Tonga–Kermadec, but also less conspicuous systems such as Yap, Mussau and Hjort trenches which constitute the young, incomplete, or ultraslow-member in the evolutionary spectrum of subduction zones. We used satellite-derived gravity data to compare well-developed and immature subduction systems. It is shown that at spatial resolution > 10–20 km or so, the satellite data have accuracy comparable to ship-board gravity measurements over intra-oceanic subduction zones. In the isostatic residual gravity anomaly map, the width of non-isostatically-compensated region of the mature subduction zones is much wider than that of immature ones. More importantly, when the gravitational attraction due to seafloor is removed, a large difference exists between the mature and immature subduction zones in the overriding plate side. Mature subduction zones exhibit broad low gravity anomalies of ~ 200–250 mGal centered at distances of 150–200 km from the trench which are not found over immature subduction zones. The cause of the broad low gravity anomalies over mature subduction zones is debatable due to lack of information on the deep crust and upper mantle structure and property. We discuss the following four causes: (1) serpentinization of the upper mantle beneath the forearc; (2) presence of partial melt in the mantle wedge caused by release of volatiles from the slab, frictional heating and distributed by mantle circulation; (3) difference in density structure between the overriding and subducting plates caused by difference in age and thermal structures with and without compositional stratification between crust and mantle; and (4) anomalous thickness of the arc not explained by isostasy. Our analysis suggests that serpentinization cannot explain the observed gravity anomaly which appears ~ 150–200 km from the trench. Although the extent and distribution of partial melt within the mantle wedge remain in question, to our best estimate, partial melting contributes little (< 50 mGal) to the total negative gravity anomaly. The difference in density structure reflecting temperature difference can only explain less than half of the low gravity anomaly. The sinking of lighter crustal material produces a large negative anomaly in the forearc but its location does not match the observed gravity anomaly. It appears that one cannot explain the total difference in gravity anomaly without invoking anomalous thickness of the arc. Although we could not identify the sole or combination of factors that give rise to the low gravity anomaly in mature subduction zones, the comparison of gravity anomalies between mature and immature subduction zones is likely to provide an important constraint for understanding the evolution and structure of subduction zones as more complementary evidences become available.

The dynamics of intra-oceanic subduction zones: A direct comparison between fossil petrological evidence (Rio San Juan Complex, Dominican Republic) and numerical simulation

Dispersed blocks of various types of metamorphic rocks in serpentinite mélanges of the northern Dominican Republic (Hispaniola) provide fossil evidence for the dynamics of the subduction zone channel in the intra-oceanic Caribbean subduction zone system between 120 and 55 Ma. Comprehensive petrological and geochronological data on three exemplary samples of eclogite and blueschist are presented that allow a series of different but interrelated pressure–temperature–time paths to be delineated. Eclogites indicate a low P/T gradient during subduction and record conditions in the nascent stages of the subduction zone. Lu–Hf data yield 103.6 ± 2.7 Ma for peak metamorphic conditions of 23 kbar/750 °C. An anticlockwise P–T path is defined. Other blocks record the continuous cooling of the evolving subduction zone and show typical clockwise P–T-paths. Omphacite blueschists reach maximum P–T-conditions of 17–18 kbar/520 °C at 80.3 ± 1.1 Ma (Rb–Sr age data). The mature subduction zone is typified by jadeite blueschists recording very high (“cold”) P/T gradients. A Rb–Sr age of 62.1 ± 1.4 Ma dates peak metamorphic P–T conditions at 16–18 kbar/340–380 °C. The array of P–T–t data allows overall cooling rates of the subduction zone at depths of c. 60 km to be constrained at 9 °C/Ma. Cooling rates and exhumation rates (i.e., vertical component of retrograde trajectories) of the metamorphic blocks are 9–20 °C/Ma and 5–6 mm/a, respectively. The derived P–T–t array is compared with a 2-D numerical subduction-zone model published by Gerya et al. [Gerya, T.V., Stöckhert, B. and Perchuk, A.L., 2002. Exhumation of high-pressure metamorphic rocks in a subduction channel: a numerical simulation. Tectonics 142, 6-1-6-19.; 45° slab dip, 40 Ma lithosphere age, convergence rates of 10–40 mm/a], which incorporates weakening of lithospheric mantle of the hanging wall by fluids emanating from the downgoing slab, resulting in an increasingly more funnel-shaped subduction channel system with time. The numerically derived array of simulated P–T–t paths as well as the calculated rates of exhumation and cooling agree well with the P–T–t data derived from the metamorphic blocks of the Rio San Juan serpentinite mélanges when convergence rates of 15 to 25 mm/a are chosen. This value is also in accord with available paleogeographic reconstructions calling for a long-term average of 22 mm/a of orthogonal convergence. On the basis of the comparison, the onset of subduction in the Rio San Juan segment of the Caribbean Great Arc can be constrained to approximately 120 Ma. This segment was thus obviously active for more than 65 Ma. An orthogonal convergence rate of 15–25 mm/a requires that a minimum amount of 975–1625 km of oceanic crust must have been subducted. Both petrological/geochronological data and numerical simulation underscore the broad spectrum of different P–T–t paths and peak conditions recorded by material subducted at different periods of time as the subduction zone evolved and matured.

Prograde and retrograde history of the Junction School eclogite, California, and an evaluation of garnet–phengite–clinopyroxene thermobarometry

Quantitative thermobarometry of inclusions in zoned garnet from a Franciscan eclogite block record a counter-clockwise P–T path from blueschist to eclogite and back. Garnet retains prograde zoning from inclusion-rich Alm52Grs30Pyp6Sps12 cores to inclusion-poor Alm62Grs25Pyp12Sps1 mantles, with overgrowths of highly variable composition. Barometry using the Waters–Martin version of the garnet–phengite–omphacite thermobarometer yields conditions of 7–15 kbar, 400–500°C (garnet cores), 18–22 kbar, ∼550°C (mantles), and 10–14 kbar, 350–450°C (overgrowths), in agreement with clinozoisite–sphene–rutile–garnet–quartz barometry. These pressures are ∼10–15 kbar less than those obtained using more recent, fully thermodynamic calibrations of the phengite–omphacite–garnet thermobarometer. Low early temperatures suggest that the block was subducted in a thermally mature subduction zone and not at the inception of subduction when prograde temperature is expected to be higher. Franciscan high-grade blocks likely represent crust subducted throughout the history of this convergent margin, rather than only at the inception of the subduction zone.

An algorithm for rapid computation of the steady-state thermal structure of mature subduction zones

An algorithm for rapid computation of the steady-state thermal structure of mature subduction zones