Subducting Plate Interface Research Articles

Characteristic activities of slow earthquakes in Japan.

Slow earthquakes are a recently discovered phenomenon that mainly occur updip and downdip of the seismogenic zones of great earthquakes along the subducting plate interface. The spatiotemporal activity of various slow earthquakes occurring in the Nankai subduction zone is characterized by along-strike heterogeneity and along-dip systematic changes. Various slow earthquakes are horizontally distributed at their own depths and along-strike segments can be observed with respect to this distribution downdip of the locked zone; however, slow and great earthquakes occur in the same depth range near the Nankai Trough and Japan Trench axes. The frequently observed spatiotemporal interactions between different slow earthquakes can be attributed to their sensitivity and the stress transfer of the surrounding areas. This stress transfer is expected to extend to the adjacent sections in the seismogenic zone. Therefore, precise monitoring of slow earthquakes is important for future evaluations of great earthquakes, which requires the long-term maintenance and continuous improvement of the high-quality observation networks.

Elastic Fault Interactions and Earthquake Rupture Along the Southern Hellenic Subduction Plate Interface Zone in Greece

AbstractThe importance of splay‐thrust faults in subduction seismogenesis is increasingly acknowledged; however, their elastic interaction with the plate interface remains unclear. Here, we use GPS velocities, constrained by millennial fault slip rates, to study elastic fault‐interactions between the plate interface and its upper‐plate splay‐thrust faults from the southern Hellenic Subduction System (HSS). We find that, despite its largely aseismic character, the HSS plate interface zone is kinematically segmented, with slip rate deficits locally reaching ~85% and ~45% of the plate convergence rate on the western and eastern segments, respectively, and on structures different from those that ruptured historically. Although western Crete has been more active seismically during late Holocene, we find that the eastern HSS has higher seismic potential for large‐magnitude (M > 6) earthquakes and its interface zone is closer to failure. Elastic fault interactions are responsible for both significant intersegment variability in strain accumulation and uniformity in earthquake rupture segmentation along the HSS over millennial timescales.

Secular Variation of Early Cretaceous Granitoids in Kyushu, SW Japan: The Role of Mélange Rocks as a Possible Magma Source

The Early Cretaceous volcanic-arc granitic rocks from Kyushu, SW Japan are contemporaneous with the granitic rocks of the Yanshan Orogeny (SE China) along the eastern Eurasian continental margin. The secular geochemical variations of the whole-rock major elemental and Sr-Nd isotope data of the Early Cretaceous granitic rocks from Kyushu, SW Japan, as well as the zircon and apatite saturation temperatures, shows distinct changes during the Albian (~115 Ma to ~100 Ma) as: (1) the mASI value of the rocks (i.e. Shiraishino granodiorites) decreases below 1, (2) the Sr-Nd isotopic data are relatively constant (87Sr/86Sri = 0.70471 to 0.70573; Nd(t) = +0.2 to +1.9) within different rock types including granites, granodiorites tonalites and adakitic rocks (i.e. the Shiraishino granodiorites), following the increase of 87Sr/86Sri and decrease of Nd(t) from Berriasian, and (3) higher maximum temperatures at ~105 Ma. The secular changes indicate that important geodynamic changes occurred in the arc system of SW Japan as it changed from subduction-accretion during the Jurassic to continental arc during the Early Cretaceous. Thermodynamic partial melting modelling demonstrates that the Albian granitic rocks can be derived from melange rocks, such as chlorite-actinolite schists, at moderate depth and variable redox conditions. It is concluded that the genesis of the Early Cretaceous granitic rocks from Kyushu, SW Japan, may be related to upwelling of the asthenosphere and hot corner flow into the mantle wedge caused by slab rollback, which followed a shallowing of the subduction angle and subsequent flat-slab subduction during the Late Jurassic. The resultant heat induced the partial melting of the melange rocks that formed on and were transported from the subducted plate interface.

Subduction megathrust heterogeneity characterized from 3D seismic data

Megathrust roughness and structural complexity are thought to be controls on earthquake slip at subduction zones because they result in heterogeneity in shear strength and resolved stress. However, because active megathrust faults are difficult to observe, the causes and scales of complexity are largely unknown. Here we measured the in situ properties of the megathrust of the Middle America subduction zone in a three-dimensional seismic reflection volume to determine how fault properties vary. We quantify spatial variability in the megathrust roughness, overburden and rock physical properties. Heterogeneity in the megathrust roughness exists at length scales of a few kilometres because the megathrust is dissected by active lower-plate normal faults, which offset the megathrust and renewed fault roughness. Spatial variations in the rock physical properties at the plate interface are characterized by correlation length scales of hundreds of metres. Frontal prism taper, historical seismicity and the variation in earthquake stress drop values local to the megathrust are all affected by the heterogeneity at these length scales. Both geometric and rheological complexities may therefore control the mechanical behaviour of the subduction plate interface, which includes earthquake rupture characteristics. Geometric and rheological complexities may control the mechanical behaviour of megathrusts, according to an analysis of the heterogeneity in roughness and rock properties of the Middle America megathrust from 3D seismic reflection data.

Along‐Arc Heterogeneity in Local Seismicity across the Lesser Antilles Subduction Zone from a Dense Ocean‐Bottom Seismometer Network

AbstractThe Lesser Antilles arc is only one of two subduction zones where slow‐spreading Atlantic lithosphere is consumed. Slow‐spreading may result in the Atlantic lithosphere being more pervasively and heterogeneously hydrated than fast‐spreading Pacific lithosphere, thus affecting the flux of fluids into the deep mantle. Understanding the distribution of seismicity can help unravel the effect of fluids on geodynamic and seismogenic processes. However, a detailed view of local seismicity across the whole Lesser Antilles subduction zone is lacking. Using a temporary ocean‐bottom seismic network we invert for hypocenters and 1D velocity model. A systematic search yields a 27 km thick crust, reflecting average arc and back‐arc structures. We find abundant intraslab seismicity beneath Martinique and Dominica, which may relate to the subducted Marathon and/or Mercurius Fracture Zones. Pervasive seismicity in the cold mantle wedge corner and thrust seismicity deep on the subducting plate interface suggest an unusually wide megathrust seismogenic zone reaching ∼65 km depth. Our results provide an excellent framework for future understanding of regional seismic hazard in eastern Caribbean and the volatile cycling beneath the Lesser Antilles arc.

Concordant pulse in Mn, Y and HREEs concentrations during UHP eclogitic garnet growth: Transient rock dynamics along a cold subduction plate interface

We herein report strikingly concordant Mn, Y, and HREE garnet zoning in ultrahigh-pressure low-temperature oceanic eclogites found in the fossil Devonian-Carboniferous subduction plate interface in SW-Tianshan. Zr-in-rutile thermometry, thermodynamic modeling and diffusion-controlled garnet growth modeling show that this type of zoning can be explained by a short-lived transient, most likely a ∼80°C thermal pulse (from 490 to 570°C). This event could be as transient as ∼300 Ka, assuming a prograde to peak burial duration of 5 Ma. Such garnet zoning is observed for grains sampled meters apart (Fig. 1d) and was detected in other eclogites in the region. This transient event could reflect the short-lived movement of UHP oceanic slices, once detached, towards the hotter mantle wedge, and/or deformation-induced remobilization of fluids enhancing diffusion-controlled garnet growth.

Distributed deformation along the subduction plate interface: The role of tectonic mélanges

Recent geophysical monitoring of subduction zones has unraveled a complete spectrum of plate coupling behaviors, from coupled portions rupturing during earthquakes to decoupled portions slipping aseismically. However, the deformation mechanisms and the exhumed rock corresponding to these contrasted behaviors are not yet identified. Tectonic mélange zones are thought to play a major role in the deformation of the plate interface as they represent remnants of the subducted plate scraped off by the overriding plate. In this work we examine several tectonic mélange zones (Hyuga, Okitsu, Mugi) from the Shimanto Belt, an accretionary prism in southwest Japan connecting to the active Nankai subduction zone. These tectonic mélange zones have a block-in-matrix structure, with lenses of sandstones and basalts within a metapelitic matrix, and their deformation is distributed over zones of hundreds of meters in thickness. In addition, the examples of mélange considered here are bounded by sharp faults, some of them bearing pseudotachylyte layers, so that distributed deformation within the mélange and localized deformation on its boundary are juxtaposed. Distributed deformation involves the development of a foliation, as well as of a pervasive network of macroscopic and microscopic shear zones. Along with slip on this network, strain proceeds by fracturing and precipitation of quartz, in long veins parallel to the foliation or in smaller cracks perpendicular to stretching and forming in the neck of competent lenses of sandstones or former quartz veins. The analysis of shear band kinematics shows in all three examples a dominant, top-to-the-trench sense of shear, consistent with deformation along the plate boundary during subduction. Moreover, most shear zones, when foliation is restored back to syn-subduction position, are extensional structures. Finally, the geometry and kinematics of the mélange-bounding faults, as well as radiometric constraints, show that in most cases the faults (=localized structures) were formed during a later stage than mélange internal deformation. These findings bear several consequences on the structure and dynamics of the subduction plate boundary at seismogenic depths. First, there is no support for a model of plate boundary fault zone composed simultaneously of localized slip zones and domains of more distributed deformation. Second, rather than proposed models of underplating, where all deformation is localized into the thrusts bounding the tectonic sheets, we suggest that underplating was to a large extent accommodated by distributed deformation within the mélange sheets. This underplating model accounts for (i) the large amount of strain within the mélange, (ii) the absence of contractional structures during underplating, (iii) thinning of the mélange required by the network of extensional shear bands and stretched boudins. Third, mélanges appear as likely candidates for portions of the plate interface deforming by aseismic slip. The seismic vs. aseismic character of the plate interface might depend on the ability of sediments on top of the subducting plate to undergo distributed strain, which in turn depends on the efficiency of pressure solution to operate.

Enhanced Resolution of the Subducting Plate Interface in Central Alaska From Autocorrelation of Local Earthquake Coda

AbstractSubducting plates below 10‐km depth are primarily imaged using phases from teleseismic earthquakes at frequencies dominantly below 1 Hz, resulting in low‐resolution images compared to fault zone thickness. Here we image the plate boundary zone in Alaska using scattered body wave arrivals in local earthquake coda to produce a higher‐resolution image of the slab. An autocorrelation method successfully extracts coherent arrivals from the local earthquake signals. Our autocorrelation results image interfaces associated with the subducting oceanic plate at higher resolution than our teleseismic receiver functions, with increased coherence and sharper boundaries. Our results provide one of the first coherent structural images of the seismogenic zone using scattered local body waves. Amplitudes suggest that seismic wave speed decreases with increasing depth within the low‐velocity zone, supporting lithologic rather than purely overpressure models for the zone in our region. Similar methodologies using dense stations could provide higher‐resolution images to characterize crustal and uppermost mantle boundaries globally.

Paleo-stress orientations and magnitudes from triaxial testing and stress inversion analysis in Nankai accretionary prism sediments

Combined knowledge on the magnitude as well as the orientation of paleo-stress is usually not given. Based on drilling investigations and laboratory experiments, we are able to provide both, constraints on paleo-stress magnitude as well as orientation in the shallow part of the Nankai accretionary wedge at Sites C0001 and C0002 of the Integrated Ocean Drilling Program (IODP) Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE). Slip data of core-scale faults indicate multiple paleo-stress geometries, including normal, strike-slip, and reverse faulting regimes. Combining new and previously obtained results from triaxial compressional tests that constrain internal friction angles at critical state with the paleo-stress inversion results, we examine paleo-stress magnitudes for each stress state at each site. We observe that the paleo-stress magnitude for the normal faulting regime is smaller than that for the reverse faulting regime, although the paleo-stress magnitude for Site C0002 is not well-constrained. The change in stress magnitude may reflect changing horizontal tectonic stresses transmitted to the accretionary wedge from changing friction conditions along the subduction plate interface and in turn may have implications for the evolution of the accretionary wedge and/or the seismic cycle.

Intermediate-depth brecciation along the subduction plate interface (Monviso eclogite, W. Alps)

The Monviso meta-ophiolite complex (Northern Italy, Western Alps) represents an almost intact fragment of Tethyan oceanic lithosphere metamorphosed at ~80 km depth (~2.6GPa–550 °C) during the Alpine subduction. We focus our study on a major shear zone cutting across this slab fragment at low angle (the Lower Shear Zone; LSZ). Here, in its talc and tremolite-rich serpentinite matrix, are embedded (together with metasedimentary lenses) variously brecciated Fe-Ti and Mg-Al metagabbro blocks. The latter were either interpreted as eclogitic breccias resulting from intermediate-depth rupture or as inherited, overprinted oceanic core complex features. Our new field, structural and petrographic data testify the genesis of this metagabbro breccia blocks at eclogite-facies conditions. Three types of eclogitic blocks can be distinguished, with non-random distribution (and decreasing size from top to base) throughout the ~200-m-thick and ~15 km-long LSZ: (1) Fe-Ti-metagabbros, brecciated and scattered in the upper to intermediate levels of the LSZ; (2) meter-size blocks and decameter-scale slivers of intact Mg-Al metagabbros, locally brecciated; (3) dm- to m-scale blocks of intact Fe-Ti metagabbros without breccia fabrics. Brecciation at eclogite facies conditions (at ~80 km depth) is documented by: i) the eclogitic foliation of intact Mg-Al-rich metagabbros (composed of omphacite + clinozoisite ± rutile and locally garnet) cut by breccia planes (cemented by omphacite + garnet ± lawsonite) and ii) the occurrence in breccia clasts of minerals that are fractured and offset along peak P-T omphacite-bearing planes. Rupture preferentially affected the Fe-Ti metagabbros, suggesting that rheological contrasts controlled the locus of brecciation. The occurrence of a first omphacite-rich matrix (M1, ~2.7GPa - 580 °C) crosscut by omphacite + garnet-bearing matrix M2 (~2.4GPa - 560 °C), witnesses multiple brittle rupture events, prior to a stage of eclogite facies fluid ingression marked by massive lawsonite recrystallization (matrix M3).

Crustal earthquakes in the Cook Inlet and Susitna region of southern Alaska

Several large (M ≥ 6) earthquakes have occurred in the vicinity of Anchorage, Alaska, within the past century. The presence of the underlying subducting Pacific plate makes it difficult to determine the origin of these older earthquakes as either crustal, slab, or the subduction plate interface. We perform a seismological study of historical and modern earthquakes within the Cook Inlet and Susitna region, west of Anchorage. We first estimate hypocenters for historical large earthquakes in order to assess their likelihood of origin as crustal, slab, or plate interface. We then examine modern crustal seismicity to better understand the style of faulting and the location of active structures, including within (and beneath) the Cook Inlet and Susitna basins. We perform double-couple moment tensor inversions using high frequency body waves (1–10 Hz) for small to moderate (M ≥ 2.5) crustal earthquakes (depth ≤ 30 km) occurring from 2007 to 2017. Our misfit function combines both waveforms differences as well as first-motion polarities in order to obtain reliable moment tensor solutions. The three focus regions—Beluga, upper Cook Inlet, and Susitna—exhibit predominantly thrust mechanisms for crustal earthquakes, indicating an overall compressive regime within the crust that is approximately consistent with the direction of plate convergence. Mechanisms within upper Cook Inlet have strike directions aligned with active anticlines previously identified in Cook Inlet from active-source seismic data. Our catalog of moment tensors is helpful for identifying and characterizing subsurface faults from seismic lineaments and from faults inferred from subsurface images from active-source seismic data.

P–T–time (phengite Ar closure) history of spatially close-outcropping HP and UHP oceanic eclogites (southwestern Tianshan): implication for a potential deep juxtaposing process during exhumation?

ABSTRACTThe Chinese southwestern Tianshan HP–UHP/LT metamorphic complex possesses well-preserved mafic layers, tectonic slices/blocks, boudins/lens of different sizes, and lithology embedded within dominant metavolcanoclastics. A recent study on the ultra-high pressure (UHP) eclogite revealed a short timescale of exhumation (≤10 Ma, ~315 ± 5 Ma). However, controversies still exist on some key questions: (1) the reasonable interpretation of spatially close-outcropped high pressure (HP) and UHP slices with respect to regional geodynamics, and (2) if the previous regional scatter Ar–Ar ages proved the existence of internally coherent sub-belts or troubled by dating on samples with notable 40Ar retention. This study focusses on detailed P–T–time (phengite Ar closure) recovery of samples from a HP eclogite lens and its host rock, the UHP thick-layered eclogite. Based on data from bulk–rock, microprobe analysis, and muscovite Ar–Ar chronological dating, we link phengite growth to potential garnet growth stages via thermodynamic modelling. Facilitated by the P–T–Ar retention% graph, we collect all the regional muscovite Ar–Ar data together with results in this study for evaluating the significance of regional muscovite Ar–Ar ages and set back to geodynamics. According to modelling results, the HP lens eclogite reached peak metamorphism at ~550°C, 2.50 GPa with an Ar–Ar muscovite plateau age of 316.9 ± 1.0 Ma that could date the mass phengite growth event during prograde metamorphism. In contrast, the UHP layered eclogite experienced UHP peak burial at ~510°C, 2.95 GPa, and then to HP peak metamorphism at ~560°C, 2.60 GPa with ~311.6 ± 0.7 Ma plateau age that may constrain the cooling age during early exhumation. Noteworthy, both of them share a quite similar early exhumation path despite bearing contrasting prograde metamorphic experiences. With considering updated regional exhumation pattern, this might imply the existence of a potential deep juxtaposing (capture) process between HP slices and exhumating UHP complex, at about 45–60 km depth along subduction plate interface.

Does subduction of mass transport deposits (MTDs) control seismic behavior of shallow–level megathrusts at convergent margins?

We present a critical appraisal of the role of subducted, medium (10–1000 km2) to giant (≥1000 km2) and heterogeneous, mud-rich mass transport deposits (MTDs) in seismic behavior and mechanisms of shallow earthquakes along subduction plate interfaces (or subduction channels) at convergent margins. Our observations from exhumed ancient subduction complexes around the world show that incorporation of mud-rich MTDs with a “chaotic” internal fabric (i.e., sedimentary mélanges or olistostromes) into subduction zones strongly modifies the structural architecture of a subduction plate interface and the physical properties of subducted material. The size and distribution of subducted MTDs with respect to the thickness of a subduction plate interface are critical factors influencing seismic behavior at convergent margins. Heterogeneous fabric and compositions of subducted MTDs may diminish the effectiveness of seismic ruptures considerably through the redistribution of overpressured fluids and accumulated strain. This phenomenon possibly favors the slow end-member of the spectrum of fault slip behavior (e.g., Slow Slip Events, Very Low Frequency Earthquakes, Non-Volcanic Tremors, creeping) compared to regular earthquakes, particularly in the shallow parts (T < 250 °C) of a subduction plate interface.

Slow-slip events in semi-brittle serpentinite fault zones

Slow-slip events are earthquake-like events only with much lower slip rates. While peak coseismic velocities can reach tens of meters per second, slow-slip is on the order of 10−7±2 m/s and may last for days to weeks. Under the rate-and-state model of fault friction, slow-slip is produced only when the asperity size is commensurate with the critical nucleation size, a function of frictional properties. However, it is unlikely that all subduction zones embody the same frictional properties. In addition to friction, plastic flow of antigorite-rich serpentinite may significantly influence the dynamics of fault slip near the mantle wedge corner. Here, we show that the range of frictional parameters that generate slow slip is widened in the presence of a serpentinized layer along the subduction plate interface. We observe increased stability and damping of fast ruptures in a semi-brittle fault zone governed by both brittle and viscoelastic constitutive response. The rate of viscous serpentinite flow, governed by dislocation creep, is enhanced by high ambient temperatures. When effective viscosity is taken to be dynamic, long-term slow slip events spontaneously emerge. Integration of rheology, thermal effects, and other microphysical processes with rate-and-state friction may yield further insight into the phenomenology of slow slip.

Fast intraslab fluid-flow events linked to pulses of high pore fluid pressure at the subducted plate interface

A better understanding of the subduction zone fluid cycle and its chemical–mechanical feedback requires in-depth knowledge about how fluids flow within and out of descending slabs. Relicts of fluid-flow systems in exhumed rocks of fossil subduction zones allow for identification of the general relationships between dehydration reactions, fluid pathway formation, the dimensions and timescales of distinct fluid flow events; all of which are required for quantitative models for fluid-induced subduction zone processes. Two types of garnet–quartz–phengite veins can be distinguished in an eclogite-facies mélange block from the Pouébo Eclogite Mélange, New Caledonia. These veins record synmetamorphic internal fluid release by mineral breakdown reactions (type I veins), and infiltration of an external fluid (type II veins) with the associated formation of a reaction selvage. The dehydration and fluid migration documented by the type I veins likely occurred on a timescale of 105–106 years, based on average subduction rates and metamorphic conditions required for mineral dehydration and fluid flow. The timeframe of fluid–rock interaction between the external fluid and the wall-rock of the type II veins is quantified using a continuous bulk-rock Li-diffusion profile perpendicular to a vein and its metasomatic selvage. Differences in Li concentration between the internal and external fluid reservoirs resulted in a distinct diffusion profile (decreasing Li concentration and increasing δ7Li) as the reaction front propagated into the host rock. Li-chronometric constraints indicate that the timescales of fluid–rock interaction associated with type II vein formation are on the order of 1 to 4 months (0.150−0.08+0.14 years). The short-lived, pulse-like character of this process is consistent with the notion that fluid flow caused by oceanic crust dehydration at the blueschist-to-eclogite transition contributes to or even dominates episodic pore fluid pressure increases at the plate interface, which in turn, may trigger slip events reported from many subduction zones.

Lawsonite-bearing eclogite from a tectonic mélange in the Ligurian Alps: new constraints for the subduction plate-interface evolution

AbstractLawsonite eclogites are rare rocks and have been described from only a few localities in the world. Lawsonite-bearing assemblages are highly unstable and physico-chemical processes linked to exhumation may destroy them; only aggregates interpreted as pseudomorphs after lawsonite could be often recognized. In this paper, we present a detailed structural and petrological study of an area in the northwestern sector of the metaophiolitic high-pressure Voltri Massif (Ligurian Western Alps, Italy). The study area is characterized by a lawsonite-bearing eclogitic metagabbro associated with carbonated serpentinites and glaucophanic metasediments. The metagabbro body reached eclogitic metamorphic peak conditions atT= 465–477°C andP= 20.9–24.4 kbar, with H2O continuously supplied to the system. H2O under-saturated conditions, with the occurrence of both lawsonite and epidote, characterized the exhumation path. Both the low temperature recorded by the body and the occurrence of variously carbonated serpentinites led us to interpret this area as a portion of the top of the subducted slab, coupled with a ‘cool’ mantle wedge, where both aqueous fluids and carbonate-rich fluids were present. The occurrence of rocks belonging to different paleogeographic domains (e.g. continent versus ocean) and the multiple deformations recorded by the metagabbro suggest that this area was nearby the slab–mantle interface. This sector was thus affected by a shear regime that acted in a low-viscosity serpentinite channel, bringing these high-pressure rocks back to the surface.

Linking serpentinite geochemistry with tectonic evolution at the subduction plate-interface: The Voltri Massif case study (Ligurian Western Alps, Italy)

Recent geochemical work shows that subduction-zone serpentinites are repositories for fluid-mobile elements absorbed during interaction with sediment-derived fluids. Unraveling the geochemical fingerprint of these rocks helps to define timing of tectonic accretion of sediments along the subduction interface and the role of serpentinite in element recycling to volcanic arcs. Here we present the trace element and isotopic composition (B–O–H, Sr, Pb) of high-pressure serpentinites from the Voltri Massif (Ligurian Western Alps, Italy), to discuss their role as incompatible element carriers and their contribution to recycling of sediment-derived components in subduction zones.The serpentinites presented here record metamorphic olivine growth during eclogite-facies metamorphism and show undeformed and mylonitic textures. Field relations show that undeformed rocks are enclosed in deformed ones and that no metasedimentary rocks are present nearby. Undeformed serpentinite has very high δ11BSRM951 (from +26‰ to +30‰), low Sr and Pb isotope ratios (87Sr/86Sr=0.7053–0.7069; 206Pb/204Pb=18.131–18.205) and low As and Sb contents (0.1 and 0.01μg/g, respectively). Oxygen and hydrogen isotope compositions are +4.5‰ and −67‰, respectively. In contrast, mylonitic serpentinite shows lower δ11B (from +22‰ to +17‰), significant enrichment in radiogenic Sr and Pb isotopes (87Sr/86Sr up to 0.7105; 206Pb/204Pb up to 18.725), and enrichment in As and Sb (1.3 and 0.39μg/g, respectively). δ18O of the mylonitic serpentinites reaches values of +5.9‰, whereas δD is comparable with that of undeformed rocks (approximately −70‰). In mylonitic serpentinites, the B and Sr isotopic values and the fluid-mobile element (FME) concentrations are near those for the Voltri metasedimentary rocks (calc- and mica-schists). Pb systematics also reveal influx of a crust-derived component.Our dataset shows that undeformed serpentinite still preserves an oceanic geochemical fingerprint, whereas mylonitic serpentinite is reset in its concentrations of FME and its B, Sr and Pb isotope compositions, due to interaction with sediment- and crust-derived fluids. The environment of this interaction is either compatible with (i) an outer-rise zone setting, with percolation of seawater-derived fluids enriched in sedimentary components into bending-related fault structures, or with (ii) subduction channel domains, where ascending sediment-derived slab fluids infiltrate slices of former oceanic serpentinite accreted to the plate interface domain. Influx of sediment-derived subduction fluids along major deformation zones in serpentinite modifies the element budget of the rocks, with important implications for element recycling and the tectonic history of serpentinite. The B, Sr and Pb isotopic systematics, coupled with FME concentration in serpentinites are particularly helpful geochemical tracers of interaction between different reservoirs in subduction-interface environments, and are more sensitive than the traditionally applied stable oxygen and hydrogen isotope compositions.

Probabilistic tsunami hazard assessment for the Makran region with focus on maximum magnitude assumption

Abstract. Despite having been rather seismically quiescent for the last decades, the Makran subduction zone is capable of hosting destructive earthquakes and tsunami. In particular, the well-known thrust event in 1945 (Balochistan earthquake) led to about 4000 casualties. Nowadays, the coastal regions are more densely populated and vulnerable to similar events. Furthermore, some recent publications discuss rare but significantly larger events at the Makran subduction zone as possible scenarios. We analyze the instrumental and historical seismicity at the subduction plate interface and generate various synthetic earthquake catalogs spanning 300 000 years with varying magnitude-frequency relations. For every event in the catalogs we compute estimated tsunami heights and present the resulting tsunami hazard along the coasts of Pakistan, Iran and Oman in the form of probabilistic tsunami hazard curves. We show how the hazard results depend on variation of the Gutenberg–Richter parameters and especially maximum magnitude assumption.

Three‐dimensional numerical modeling of temperature and mantle flow fields associated with subduction of the Philippine Sea plate, southwest Japan

AbstractWe investigated temperature and mantle flow distributions associated with subduction of the Philippine Sea (PHS) plate beneath southwest Japan, by constructing a three‐dimensional parallelepiped model incorporating a past clockwise rotation, the bathymetry of the Philippine Sea plate, and distribution of the subducting velocity within its slab. The geometry of the subducting plate was inferred from contemporary seismic studies and was used as a slab guide integrated with historical plate rotation into the 3‐D simulation. Using the model, we estimated a realistic and high‐resolution temperature field on the subduction plate interface, which was constrained by a large number of heat flow data, and attempted to clarify its relationship with occurrences of megathrust earthquakes, long‐term slow slip events (L‐SSEs), and nonvolcanic low‐frequency earthquakes (LFEs). Results showed that the oblique subduction coupled with the 3‐D geometry of subducting PHS plate was a key factor affecting the interplate and intraplate temperature distributions, leading to a cold anomaly in the plate interface beneath western Shikoku, the Bungo Channel, and the Kii Peninsula. Temperatures in the slab core in these regions at a depth near the continental Moho were nearly 200°C lower than that in eastern Shikoku, indicating a high thermal lateral heterogeneity within the subducting plate. The geothermal control of the LFEs beneath western Shikoku was estimated to be within a range from 400 to 700°C, and the interplate temperature for the L‐SSEs with a slip larger than 15 cm beneath the Bungo Channel was estimated to be approximately 350–500°C. A large horizontal temperature gradient of 2.5 ~ °C/km was present where the LFEs occurred repeatedly. The steep temperature change was likely to be related to the metamorphic phase transformation from lawsonite or blueschist to amphibolite of hydrous minerals of the mid‐ocean ridge basalt of the subducting PHS plate.

Heterogeneous rheology controlled postseismic deformation of the 2011 Tohoku‐Oki earthquake

AbstractUsing two‐dimensional finite element modeling, we reproduced the observed postseismic deformation of the 2011 Tohoku‐Oki earthquake. Our model, which accounts for the lithosphere‐asthenosphere boundary and weak zones beneath volcanoes, was able to reproduce small‐scale (&lt;20 km) perturbations in postseismic deformation observed by the dense geodetic network, such as local subsidence around Quaternary volcanoes. The inverted afterslip has a peak at the downdip limit of the main rupture region on the subducting plate interface, consistent with physical predictions. The combination of afterslip and viscoelastic relaxation in a heterogeneous rheology model explains the observations well, even on small scales.