METAMORPHIC DEHYDRATION FROM OCEANIC CRUST PROVIDES FLUID SOURCES FOR DEEP SLOW SLIP AND TREMOR IN SUBDUCTION ZONES

Important amounts of fluids are released in subduction zones by successive dehydration reactions occurring both in the previously hydrated oceanic crust (and mantle) and overlying sedimentary cover. The release and circulation of such fluids in rocks have major consequences on both their mechanical and chemical behavior. Indeed, the presence of a free fluid phase strongly modifies the rock rheology, fracturing properties, and could be implicated in both intermediate-depth earthquake and slow slip events nucleation. Moreover, the scale of mass transfer, associated chemical changes in infiltrated rocks and element recycling in subduction zones are controlled by both the rock permeability and the amount and composition of such fluids. Thus, there is a crucial need to identify the major fluid sources, amounts and pathways to better constrain their impact on subduction dynamics.Metamorphic veins, as well as mineralized fractures and shear zones in exhumed fossil subduction zones are the best witnesses of fluid-rock interactions and fluid circulation pathways. However, their interpretation in terms of fluid sources, residence time, scale of circulation requires a good knowledge of the composition of potential fluid sources. In order to determine the composition of the fluid released by both oceanic crust and sediments at various depth along their subduction, we analyzed the composition of fluid inclusions contained in vein minerals formed at peak P-T conditions, in rock units buried at various depths in the Alpine subduction zone.The Schistes Lustr&#233;s complex is a slice-stack representing the deep, underplated part of the former Alpine accretionary wedge. These Alpine Tethys rocks are mainly composed of oceanic calcschists with fewer mafic and ultramafic rocks, buried to various depths before exhumation. From West to East, the juxtaposed Schistes Lustr&#233;s units show increasing peak P-T conditions from blueschist (300-350&#176;C - 1.2-1.3 GPa) to eclogite facies (580&#176;C - 2.8 GPa). This study focuses on the Schistes Lustr&#233;s - Monviso transect, which shows an almost continuous increase in metamorphic grade.In the Schistes Lustr&#233;s blueschist-facies sediments, fluid inclusions were analyzed in quartz from high-pressure veins, i.e. quartz that co-crystallized with prograde to peak metamorphic minerals such as lawsonite and Fe-Mg carpholite. In the metamorphosed mafic rocks, we analyzed fluid inclusions from the peak metamorphic assemblages, i.e. glaucophane +/- omphacite in blueschist facies rocks, omphacite in eclogite-facies slices. Raman spectroscopy data on these fluid inclusions suggest that fluids released during dehydration of calcschists in blueschist-facies conditions are aqueous fluids with low-salinity and small amounts of CO2 and CH4. In contrast, eclogitic fluids released from metagabbros are highly saline brines with low N2 content. These results, which will be associated with LA-ICP-MS analysis of fluid inclusions in metasedimentary quartz veins, will contribute to better constrain the evolution of composition of the fluids liberated by dehydration reactions with depth and protolith composition along the subduction interface.

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Length scales and types of heterogeneities along the deep subduction interface: Insights from exhumed rocks on Syros Island, Greece

Geosphere ◽

10.1130/ges02037.1 ◽

2019 ◽

Vol 15 (4) ◽

pp. 1038-1065 ◽

Cited By ~ 5

Author(s):

Alissa J. Kotowski ◽

Whitney M. Behr

Keyword(s):

Shear Zone ◽

Oceanic Crust ◽

Subduction Zones ◽

Greenschist Facies ◽

Coarse Grained ◽

Slow Slip ◽

Length Scales ◽

Viscous Shear ◽

The Matrix ◽

Episodic Tremor

Abstract We use structural and microstructural observations from exhumed subduction-related rocks exposed on Syros Island (Cyclades, Greece) to provide constraints on the length scales and types of heterogeneities that occupy the deep subduction interface, with possible implications for episodic tremor and slow slip. We selected three Syros localities that represent different oceanic protoliths and deformation conditions within a subduction interface shear zone, including: (1) prograde subduction of oceanic crust to eclogite facies; (2) exhumation of oceanic crust from eclogite through blueschist-greenschist facies; and (3) exhumation of mixed mafic crust and sediments from eclogite through blueschist-greenschist facies. All three localities preserve rheological heterogeneities that reflect metamorphism of primary lithological, geochemical, and/or textural variations in the subducted protoliths and that take the form of brittle pods and lenses within a viscous matrix. Microstructural observations indicate that the matrix lithologies (blueschists and quartz-rich metasediments) deformed by distributed power-law viscous flow accommodated by dislocation creep in multiple mineral phases. We estimate bulk shear zone viscosities ranging from ∼1018 to 1020 Pa-s, depending on the relative proportion of sediments to (partially eclogitized) oceanic crust. Eclogite and coarse-grained blueschist heterogeneities within the matrix preserve multiple generations of dilational shear fractures and veins formed under high-pressure conditions. The veins commonly show coeval or overprinting viscous shear, suggesting repeated cycles of frictional and viscous strain. These geologic observations are consistent with a mechanical model of episodic tremor and slow slip (ETS), in which the deep subduction interface is a rheologically heterogeneous distributed shear zone comprising transiently brittle (potentially tremor-genic) sub-patches within a larger, viscously creeping interface patch. Based on our observations of outcrop and map areas of heterogeneous patches and the sizes, distributions, and amounts of brittle offset recorded by heterogeneities, we estimate that simultaneous brittle failure of heterogeneities could produce tremor bursts with equivalent seismic moments of 4.5 × 109–4.7 × 1014 N m, consistent with seismic moments estimated from geophysical data at active subduction zones.

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Episodic stress tensor and fluid pressure cycling in subducting oceanic crust during Northern Hikurangi slow slip events

10.5194/egusphere-egu2020-12087 ◽

2020 ◽

Author(s):

Emily Warren-Smith ◽

Bill Fry ◽

Laura Wallace ◽

Enrique Chon ◽

Stuart Henrys ◽

...

Keyword(s):

Shear Zone ◽

Oceanic Crust ◽

Subduction Zones ◽

Fluid Pressure ◽

Fluid Migration ◽

Ocean Bottom ◽

Slow Slip ◽

Interface Shear ◽

Slow Slip Events ◽

Fluid Reservoir

The occurrence of slow slip events (SSEs) in subduction zones has been proposed to be linked to the presence of, and fluctuations in near-lithostatic fluid pressures (Pf) within the megathrust shear zone and subducting oceanic crust. In particular, the 'fault-valve' model is commonly used to describe occasional, repeated breaching of a low-permeability interface shear zone barrier, which caps an overpressured hydrothermal fluid reservoir. In this model, a precursory increase in fluid pressure may therefore be anticipated to precede megathrust rupture. Resulting activation of fractures during slip opens permeable pathways for fluid migration and fluid pressure decreases once more, until the system becomes sealed and overpressure can re-accumulate. While the priming conditions for cyclical valving behaviour have been observed at subduction zones globally, and evidence for post-megathrust rupture drainage exists, physical observations of precursory fluid pressure increases, and subsequent decreases, particularly within the subducting slab where hydrothermal fluids are sourced, remain elusive. Here we use earthquake focal mechanisms recorded on an ocean-bottom seismic network to identify changes in the stress tensor within subducting oceanic crust during four SSEs in New Zealand&#8217;s Northern Hikurangi subduction zone. We show that the stress, or shape ratio, which describes the relative magnitudes of the principal compressive stress axes, shows repeated decreases prior to, and rapid increases during the occurrence of geodetically documented SSEs. We propose that these changes represent precursory accumulation and subsequent release of fluid pressure within overpressured subducting oceanic crust via a &#8216;valving&#8217; model for megathrust slip behaviour. Our observations indicate that the timing of slow slip events on subduction megathrusts may be controlled by cyclical accumulation of fluid pressure within subducting oceanic crust.Our model is further supported by observations of seismicity preceding a large SSE in the northern Hikurangi Margin in 2019, captured by ocean-bottom seismometers and absolute pressure recorders. Observations of microseismicity during this period indicate that a stress state conducive to vertical fluid flow was present in the downgoing plate prior to SSE initiation, before subsequently returning to a down-dip extensional state following the SSE. We propose this precursory seismicity is indicative of fluid migration towards the interface shear zone from the lower plate fluid reservoir, which may have helped triggering slip on the megathrust. We also present preliminary results of a moment tensor study to investigate spatial and temporal patterns in earthquake source properties in SSE regions along the Hikurangi Margin. In particular, earthquakes near Porangahau &#8211; a region susceptible to dynamic triggering of tremor and where shallow SSEs occur every 5 years or so &#8211; exhibit distinctly lower double couple components than elsewhere along the margin. We attribute this to elevated fluid pressures within the crust here, which is consistent with recent observations of high seismic reflectivity from an autocorrelation study. Such high fluid pressure may control the broad range of seismic and aseismic phenomena observed at Porangahau.

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Os isotopic composition of western Aleutian adakites: Implications for the Re/Os of oceanic crust processed through hot subduction zones

Geochimica et Cosmochimica Acta ◽

10.1016/j.gca.2020.10.009 ◽

2021 ◽

Vol 292 ◽

pp. 452-467

Author(s):

Rachel Bezard ◽

Simon Turner ◽

Bruce Schaefer ◽

Gene Yogodzinski ◽

Kaj Hoernle

Keyword(s):

Isotopic Composition ◽

Oceanic Crust ◽

Subduction Zones

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Is Slow Slip in Subduction Zones for Real?

10.1002/essoar.10501097.1 ◽

2019 ◽

Author(s):

Jyoti Behura ◽

Farnoush Forghani

Keyword(s):

Subduction Zones ◽

Slow Slip

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Alteration and dehydration of subducting oceanic crust within subduction zones: implications for décollement step-down and plate-boundary seismogenesis

Earth Planets and Space ◽

10.1186/s40623-017-0635-1 ◽

2017 ◽

Vol 69 (1) ◽

Cited By ~ 7

Author(s):

Jun Kameda ◽

Sayako Inoue ◽

Wataru Tanikawa ◽

Asuka Yamaguchi ◽

Yohei Hamada ◽

...

Keyword(s):

Oceanic Crust ◽

Subduction Zones ◽

Plate Boundary ◽

Step Down

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Seismic scatterers in the lower mantle near subduction zones

Geophysical Journal International ◽

10.1093/gji/ggz241 ◽

2019 ◽

Vol 219 (Supplement_1) ◽

pp. S2-S20 ◽

Cited By ~ 3

Author(s):

Satoshi Kaneshima

Keyword(s):

Oceanic Crust ◽

Lower Mantle ◽

Subduction Zones ◽

P Waves ◽

Waveform Data ◽

Basaltic Rocks ◽

Short Period ◽

Subducted Oceanic Crust ◽

Conversion Depth ◽

Seismic Networks

SUMMARY We investigate the global distribution of S-to-P scatterers in the shallow to mid-lower mantle beneath subduction zones, where deep seismicity extends down to the bottom of the upper mantle. By array processing broadband and short period waveform data obtained at seismic networks, we seek anomalous later phases in the P coda within about 15–150 s after direct P waves. The later phases usually arrive along off-great circle paths and significantly later than S-to-P conversion from the ‘660 km’ discontinuity, often show positive slowness anomalies relative to direct P, and do not show a conversion depth that is consistent among nearby events. They are thus adequately regarded as scattered waves, rather than conversion at a global horizontal discontinuity. The S-to-P scattered waves often show amplitudes comparable to ‘S660P’ waves, which indicates that a spatial change in elastic properties by several percent occurs at the scatterers as abruptly as the post-spinel transformation and should arise from compositional heterogeneity. We locate prominent S-to-P scatterers beneath Pacific subduction zones and beneath southern Spain. Nearly half of 137 S-to-P scatterers located in this study and previous studies by the authors are shallower than 1000 km, and the number of scatterers decreases with depth. Scatterers deeper than 1800 km are rare and mostly weak. We examine relations between the locations of the scatterers and recently subducted slabs inferred from seismic tomography. The scatterers of mid-mantle depths, deeper than about 1000 km, are located distant from tomographic slabs. On the other hand, the majority of shallower scatterers are located beneath the slabs rather than near their fastest portions, which would indicate that chemically heterogeneous materials are not extensively entrained within thickened and folded slabs when the slabs impinge on the lower mantle. We also find scatterers near the locations where basaltic rocks of recently subducted oceanic crust are expected to exist, which suggests that oceanic crust is not delaminating when slabs impinge on the lower mantle.

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Frictional properties of gabbro at conditions corresponding to slow slip events in subduction zones

Geochemistry Geophysics Geosystems ◽

10.1002/2015gc006093 ◽

2015 ◽

Vol 16 (11) ◽

pp. 4006-4020 ◽

Cited By ~ 17

Author(s):

E. K. Mitchell ◽

Y. Fialko ◽

K. M. Brown

Keyword(s):

Subduction Zones ◽

Slow Slip ◽

Slow Slip Events ◽

Frictional Properties

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Bringing slow slip processes into focus

10.5194/egusphere-egu2020-18869 ◽

2020 ◽

Author(s):

Rebecca Bell

Keyword(s):

High Resolution ◽

Subduction Zones ◽

Seismic Velocity ◽

Numerical Models ◽

Fluid Pressure ◽

Geophysical Methods ◽

Rock Properties ◽

Slow Slip ◽

Velocity Models ◽

The North

The discovery of slow slip events (SSEs) at subduction margins in the last two decades has changed our understanding of how stress is released at subduction zones. Fault slip is now viewed as a continuum of different slip modes between regular earthquakes and aseismic creep, and an appreciation of seismic hazard can only be realised by understanding the full spectrum of slip. SSEs may have the potential to trigger destructive earthquakes and tsunami on faults nearby, but whether this is possible and why SSEs occur at all are two of the most important questions in earthquake seismology today. Laboratory and numerical models suggest that slow slip can be spontaneously generated under conditions of very low effective stresses, facilitated by high pore fluid pressure, but it has also been suggested that variations in frictional behaviour, potentially caused by very heterogeneous fault zone lithology, may be required to promote slow slip.Testing these hypotheses is difficult as it requires resolving rock properties at a high resolution many km below the seabed sometimes in km&#8217;s of water, where drilling is technically challenging and expensive. Traditional geophysical methods like travel-time tomography cannot provide fine-scale enough velocity models to probe the rock properties in fault zones specifically. In the last decade, however, computational power has improved to the point where 3D full-waveform inversion (FWI) methods make it possible to use the full wavefield rather than just travel times to produce seismic velocity models with a resolution an order of magnitude better than conventional models. Although the hydrocarbon industry have demonstrated many successful examples of 3D FWI the method requires extremely high density arrays of instruments, very different to the 2D transect data collection style which is still commonly employed at subduction zones.&#160;The north Hikurangi subduction zone, New Zealand is special, as it hosts the world&#8217;s most well characterised shallow SSEs (<2 km to 15 km below the seabed).&#160; This makes it an ideal location to collect 3D data optimally for FWI to resolve rock properties in the slow slip zone. In 2017-2018 an unprecedentedly large 3D experiment including 3D multi-channel seismic reflection, 99 ocean bottom seismometers and 194 onshore seismometers was conducted along the north Hikurangi margin in an 100 km x 15 km area, with an average 2 km instrument spacing. In addition, IODP Expeditions 372 and 375 collected logging-while drilling and core data, and deployed two bore-hole observatories to target slow slip in the same area. In this presentation I will introduce you to this world class 3D dataset and preliminary results, which will enable high resolution 3D models of physical properties to be made to bring slow slip processes into focus. &#160;

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Systematic characterization of slow slip events along the Mexican subduction zone from 2000 to 2019

10.5194/egusphere-egu2020-17441 ◽

2020 ◽

Author(s):

Mathilde Radiguet ◽

Ekaterina Kazachkina ◽

Louise Maubant ◽

Nathalie Cotte ◽

Vladimir Kostoglodov ◽

...

Keyword(s):

Subduction Zone ◽

Subduction Zones ◽

Scaling Laws ◽

Temporal Evolution ◽

Seismic Gap ◽

Slow Slip ◽

Slow Slip Events ◽

Grace Data ◽

Spatio Temporal ◽

Mexican Subduction Zone

Slow slip events (SSEs) represent a significant mechanism of strain release along several subduction zones, and understanding their occurrence and relations with major earthquake asperities is essential for a comprehensive understanding of the seismic cycle. Here, we focus on the Mexican subduction zone, characterized by the occurrence of recurrent large slow slip events (SSEs), both in the Guerrero region, where the SSEs are among the largest observed worldwide, and in the Oaxaca region, where smaller, more frequent SSEs occur. Up to now, most slow slip studies in the Mexican subduction zone focused either on the detailed analysis of a single event, were limited to a small area (Guerrero or Oaxaca), or were limited to data before 2012 [e.g.1-4]. In this study, our aim is to build an updated and consistent catalog of major slow slip events in the Guerrero-Oaxaca region.We use an approach similar to Michel et al. 2018 [5]. We analyze the GPS time series from 2000 to 2019 using Independent Component Analysis (ICA), in order to separate temporally varying sources of different origins (seasonal signals, SSEs and afterslip of major earthquakes). We are able to isolate a component corresponding to seasonal loading, which matches the temporal evolution of displacement modeled from the GRACE data. The sources (independent components) identified as tectonic sources of deep origin are inverted for slip on the subduction interface. We thus obtain a model of the spatio-temporal evolution of aseismic slip on the subduction interface over 19 years, from which we can isolate around 30 individual slow slip events of Mw > 6.2.&#160;The obtained catalog is coherent with previous studies (in terms of number of events detected, magnitude and duration) which validates the methodology. The observed moment-duration scaling is close to M0~T3 as recently suggested by Michel [6] for Cascadia SSEs, and our study extends the range of magnitude considered in their analysis. Finally, we also investigate the spatio-temporal relations between the SSEs occurring in the adjacent regions of Guerrero and Oaxaca, and their interaction with local and distant earthquakes.&#160;References:<ol><li>Kostoglodov, V. et al. A large silent earthquake in the Guerrero seismic gap, Mexico. Geophys. Res. Lett 30, 1807 (2003).</li> <li>Graham, S. et al. Slow Slip History for the Mexico Subduction Zone: 2005 Through 2011. Pure and Applied Geophysics 1&#8211;21 (2015). doi:10.1007/s00024-015-1211-x</li> <li>Larson, K. M., Kostoglodov, V. & Shin&#8217;ichi Miyazaki, J. A. S. The 2006 aseismic slow slip event in Guerrero, Mexico: New results from GPS. Geophys. Res. Lett. 34, L13309 (2007).</li> <li>Radiguet, M. et al. Slow slip events and strain accumulation in the Guerrero gap, Mexico. J. Geophys. Res. 117, B04305 (2012).</li> <li>Michel, S., Gualandi, A. & Avouac, J.-P. Interseismic Coupling and Slow Slip Events on the Cascadia Megathrust. Pure Appl. Geophys. (2018). doi:10.1007/s00024-018-1991-x</li> <li>Michel, S., Gualandi, A. & Avouac, J. Similar scaling laws for earthquakes and Cascadia slow-slip events. Nature 574, 522&#8211;526 (2019) doi:10.1038/s41586-019-1673-6</li> </ol>&#160;

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