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

Geosphere ◽  
2019 ◽  
Vol 15 (4) ◽  
pp. 1038-1065 ◽  
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.

Hydration- and dehydration-induced rheological heterogeneities on the deep subduction interface, and possible relationships to episodic tremor and slow slip

2020 ◽  
Author(s):  
Whitney Behr ◽  
Carolyn Tewksbury-Christle ◽  
Alissa Kotowski ◽  
Claudio Cannizzaro ◽  
Robert Blass ◽  
...  
Keyword(s):  
Subduction Zones ◽  
Shear Zones ◽  
Ductile Deformation ◽  
Coarse Grained ◽  
Slow Slip ◽  
Brittle Deformation ◽  
Interface Shear ◽  
Dehydration Reactions ◽  
Viscous Shear ◽  
Episodic Tremor

<p>Episodic tremor and slow slip (ETS) is observed in several subduction zones down-dip of the locked megathrust, and may provide clues for preparatory processes before megathrust rupture. Exhumed rocks provide a unique opportunity to evaluate the sources of rheological heterogeneity on the subduction interface and their potential role in generating ETS-like behavior. We present data from two subduction interface shear zones representative of the down-dip extent of the megathrust: the Condrey Mountain Schist (CMS) in northern CA (greenschist to blueschist facies conditions) and the Cycladic Blueschist Unit (CBU) on Syros Island, Greece (blueschist to eclogite facies). Both complexes highlight the propensity for fluid-mediated metamorphic reactions to produce strong rheological heterogeneities:</p><p>In the CMW, hydration reactions led to progressive serpentinization of peridotite bodies that were entrained from the overriding plate and underplated along with oceanic-affinity sediments. The margins of each peridotite-serpentinite lens show extreme strain localization accommodated by dislocation glide and minor pressure solution in antigorite, whereas lens interiors show evidence for more distributed, alternating, frictional-viscous deformation, with abundant crack-seal veins occupied by antigorite, brucite and oxides that are in some places also ductilely sheared. Deformation in the surrounding metasedimentary matrix was purely viscous.</p><p>In the CBU on Syros Island, dehydration reactions in MORB-affinity basalts, subducted and underplated with oceanic and continental-affinity sediments, led to progressive development of strong eclogitic lenses within a weaker blueschist and metasedimentary matrix. The eclogite lenses are commonly coarse-grained and massive and show brittle deformation in the form of dilational and shear fractures/veins filled with quartz, white mica, glaucophane and/or chlorite. Brittle deformation in the eclogites is coeval with ductile deformation in the surrounding blueschist and metasedimentary matrix, indicating concurrent frictional-viscous flow.</p><p>Although we cannot easily distinguish transient deformation processes in exhumed rocks, we can use the following three approaches to assess whether these heterogeneities could have generated deformation behaviors similar to deep ETS: 1) We measure displacements within, and dimensions of the heterogeneities in outcrop/map-scale to estimate the maximum possible seismic moment that would be released when the frictional heterogeneities slip;  2) We compare deformation mechanisms inferred from field and microstructural observations to their expected mechanical behavior from rock deformation experiments; and 3) We use seismo-thermo-mechanical modeling to examine expected slip velocities and moment-duration ratios for frictional-viscous shear zones that are scaled to observations from nature and the lab.  </p><p>All three approaches suggest that frictional-viscous heterogeneities of the types and length-scales we observe in the exhumed rock record are compatible with ETS as documented in modern subduction zones.</p>

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

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

<p><span>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 (P</span><sub><span>f</span></sub><span>) 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. </span></p><p><span>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’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 ‘valving’ 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.</span></p><p><span>Our model is further supported by observations of seismicity preceding a large SSE in the northern Hikurangi Margin in 2019, captured by ocean-bottom seismometer</span><span>s</span><span> and </span><span>absolute </span><span>pressure </span><span>recorders.</span> <span>O</span><span>bservations of microseismicity </span><span>during this period </span><span>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</span><span> down-dip</span><span> 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. </span></p><p><span>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 – a region susceptible to dynamic triggering of tremor and where </span><span>shallow </span><span>SSEs occur every 5 years or so – exhibit distinctly lower double couple components than elsewhere along the margin. We </span><span>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. </span></p>

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

2020 ◽  
Author(s):  
Cailey Condit ◽  
◽  
Victor Guevara ◽  
Jonathan R. Delph ◽  
Melodie French
Keyword(s):  
Oceanic Crust ◽  
Subduction Zones ◽  
Slow Slip ◽  
Fluid Sources ◽  
Metamorphic Dehydration

Boron isotope systematics of Higashi-akaishi mantle wedge peridotites (Sanbagawa belt, Japan): implications for fluid recycling in subduction zones

2021 ◽  
Author(s):  
Cees-Jan De Hoog ◽  
Keiko Hattori ◽  
Eleri Clarke
Keyword(s):  
Isotopic Composition ◽  
Subduction Zones ◽  
Isotope Fractionation ◽  
Mantle Wedge ◽  
Greenschist Facies ◽  
Coarse Grained ◽  
High Concentration ◽  
Fine Grained ◽  
Dynamic Recrystallisation ◽  
Mantle Olivine

<p>Boron provides an efficient tracer of fluids in subduction zones, due to its high concentration in surface reservoirs, low concentration in the mantle, and large isotope fractionation. The Higashi-akaishi peridotite body in Sanbagawa UHP belt, Japan, is composed of partially serpentinised dunites and harzburgites, which are interpreted to be exhumed mantle wedge peridotites. Compositions of olivine (Fo90-94, NiO 0.28-0.48 wt%, MnO 0.10-0.16 wt%) and chromite (Cr# >0.7, TiO<sub>2</sub> <0.4 wt%) confirm its origin as highly refractory fore-arc mantle. Several generations of olivine and serpentine can be recognised in the samples, and were analysed in-situ for their B content and B isotopic composition by SIMS. Coarse-grained primary mantle olivine has low [B] (1-3 µg/g), but is still significantly B-enriched compared to typical mantle olivine, and has δ<sup>11</sup>B of -10 to -3 ‰. Lower B contents in olivine cores compared to rims suggests diffusive incorporation of B from slab-derived fluids at high temperature.  Later fine-grained olivine neoblasts, products of dynamic recrystallization, have higher [B] (3-11 µg/g) and higher δ<sup>11</sup>B (-7 to +2‰). Platy antigorite associated with the olivine neoblasts have similar [B] (4-12 µg/g) but higher δ<sup>11</sup>B (-4 to +6‰). Late-stage greenschist-facies overprint resulted in lizardite veining with high [B] (18-52 µg/g) and a narrow range of δ<sup>11</sup>B (-2 to -1‰).</p><p>We envisage the following scenario. Coarse-grained mantle olivine acquired B from slab-derived fluids when the peridotites were dragged down by mantle corner flow and positioned near the slab-mantle interface. The values of δ<sup>11</sup>B (-10 to -3‰) are consistent with fluids from dehydrating slab at ca. 110-150 km depth, but are potentially affected by diffusion-controlled kinetic isotope fractionation. High temperatures (> 650-700°C) prevented the peridotites from serpentinisation. Subsequently the rocks were down-dragged in a subduction channel where olivine neoblasts formed first and platy antigorite crystallized later when temperature dropped below 650°C. Both phases show heavier δ<sup>11</sup>B than coarse-grained olivine; the values are consistent with fluids from dehydrating slab at ca. 70-100 km depth. Finally, the peridotites were exposed to crust-derived B-rich fluids with low δ<sup>11</sup>B during exhumation and amalgamation with crustal units, forming lizardite veining during greenschist-facies overprint.</p><p>This study shows that mantle olivine may scavenge significant amounts of B from percolating fluids by diffusive re-equilibration or dynamic recrystallisation, lowering the B content of such fluids and potentially modifying their B isotopic composition.</p>

scholarly journals An Explanation of Episodic Tremor and Slow Slip Constrained by Crack‐Seal Veins and Viscous Shear in Subduction Mélange

2018 ◽  
Vol 45 (11) ◽  
pp. 5371-5379 ◽  
Author(s):  
Kohtaro Ujiie ◽  
Hanae Saishu ◽  
Åke Fagereng ◽  
Naoki Nishiyama ◽  
Makoto Otsubo ◽  
...  
Keyword(s):  
Slow Slip ◽  
Viscous Shear ◽  
Episodic Tremor

scholarly journals What’s down there? The structures, materials and environment of deep-seated slow slip and tremor

2021 ◽  
Vol 379 (2193) ◽  
pp. 20200218 ◽  
Author(s):  
Whitney M. Behr ◽  
Roland Bürgmann
Keyword(s):  
Subduction Zones ◽  
Source Region ◽  
Fluid Pressure ◽  
Plate Boundary ◽  
Seismogenic Zone ◽  
Slow Slip ◽  
Earthquake Dynamics ◽  
Plate Interface ◽  
Viscous Shear ◽  
Geophysical Imaging

Deep-seated slow slip and tremor (SST), including slow slip events, episodic tremor and slip, and low-frequency earthquakes, occur downdip of the seismogenic zone of numerous subduction megathrusts and plate boundary strike-slip faults. These events represent a fascinating and perplexing mode of fault failure that has greatly broadened our view of earthquake dynamics. In this contribution, we review constraints on SST deformation processes from both geophysical observations of active subduction zones and geological observations of exhumed field analogues. We first provide an overview of what has been learned about the environment, kinematics and dynamics of SST from geodetic and seismologic data. We then describe the materials, deformation mechanisms, and metamorphic and fluid pressure conditions that characterize exhumed rocks from SST source depths. Both the geophysical and geological records strongly suggest the importance of a fluid-rich and high fluid pressure habitat for the SST source region. Additionally, transient deformation features preserved in the rock record, involving combined frictional-viscous shear in regions of mixed lithology and near-lithostatic fluid pressures, may scale with the tremor component of SST. While several open questions remain, it is clear that improved constraints on the materials, environment, structure, and conditions of the plate interface from geophysical imaging and geologic observations will enhance model representations of the boundary conditions and geometry of the SST deformation process. This article is part of a discussion meeting issue ‘Understanding earthquakes using the geological record’.

Seismic and transient slip characteristics of frictional-viscous shear zones in subduction environments

2021 ◽  
Author(s):  
Whitney Behr ◽  
Taras Gerya
Keyword(s):  
Shear Zone ◽  
Numerical Models ◽  
Low Frequency ◽  
Shear Zones ◽  
Finite Width ◽  
Slow Slip ◽  
Deep Roots ◽  
Slow Slip Events ◽  
Aspect Ratios ◽  
Viscous Shear

<p>The deep roots of subduction megathrusts exhibit aseismic slow slip events, commonly accompanied by tremor and low-frequency earthquakes. Observations from exhumed rocks suggest that the deep subduction interface is a shear zone in which frictional lenses are embedded in a weaker, distributed viscous matrix deformed under high fluid pressures and low stresses. Here we use numerical models to explore the transient slip characteristics of finite-width frictional-viscous shear zones. Our model formulation utilizes an invariant form of rate- and state-dependent friction (RSF) and simulates earthquakes along spontaneously evolving faults embedded in a 2D continuum. The setup includes two elastic plates bounding a viscoelastoplastic shear zone (subduction interface) with inclusions (clasts) of varying sizes, aspect ratios, distributions and viscosity contrasts with respect to the surrounding matrix. The entire shear zone exhibits the same velocity-weakening RSF parameters, but the low viscosity matrix in the shear zone has the capacity to switch between RSF and linear viscous creep as a function of its local viscosity and stress state. Results show that for a range of matrix viscosities near a threshold viscosity (representative of the frictional-viscous transition), viscous damping and stress heterogeneity in these shear zones both 1) sets the ‘speed limit’ for earthquake ruptures that nucleate in clasts such that they propagate at velocities similar to observed slow slip events; and 2) simultaneously permits the transmission of slow slip from clast to clast, allowing slow ruptures to propagate substantial distances over the model domain. For reasonable input parameters, modeled events have moment-duration statistics, stress drops, and rupture propagation rates that match natural slow slip events. Events resembling very low-frequency earthquakes appear to be favored at high clast densities and low matrix viscosities, whereas longer duration, higher-magnitude slow slip events are favored at intermediate clast densities and near-threshold viscosities. These model results have potential to reconcile geophysical constraints on slow slip phenomena with the exhumed geological record of the slow slip environment.</p>

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

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

Amphibole and mica 40Ar/39Ar ages from the Kaipokok and Aillik domains, Makkovik Province, Labrador: towards a characterization of back-arc processes in the Paleoproterozoic

2002 ◽  
Vol 39 (5) ◽  
pp. 749-764 ◽  
Author(s):  
Nicholas Culshaw ◽  
Peter Reynolds ◽  
Gavin Sinclair ◽  
Sandra Barr
Keyword(s):  
Thermal Effect ◽  
Shear Zone ◽  
Second Order ◽  
Greenschist Facies ◽  
Metamorphic Rocks ◽  
Arc Magmatism ◽  
First Order ◽  
Back Arc ◽  
Order Pattern

We report amphibole and mica 40Ar/39Ar ages from the Makkovik Province. Amphibole ages from metamorphic rocks decrease towards the interior of the province, indicating a first-order pattern of monotonic cooling with progressive migration of the province into a more distal back-arc location. The amphibole data, in combination with muscovite ages, reveal a second-order pattern consisting of four stages corresponding to changing spatial and temporal configurations of plutonism and deformation. (1) The western Kaipokok domain cooled through muscovite closure by 1810 Ma, long after the cessation of arc magmatism. (2) The Kaipokok Bay shear zone, bounding the Kaipokok and Aillik domains, cooled through amphibole closure during 1805–1780 Ma, synchronous with emplacement of syn-tectonic granitoid plutons. (3) Between 1740 and 1700 Ma, greenschist-facies shearing occurred along the boundary between the Kaipokok domain and Nain Province synchronous with A-type plutonism and localized shearing in the western Kaipokok domain, cooling to muscovite closure temperatures in the Kaipokok Bay shear zone, and A-type plutonism and amphibole closure or resetting in the Aillik domain. (4) In the period 1650–1640 Ma, muscovite ages, an amphibole age from a shear zone, and resetting of plutonic amphibole indicate a thermal effect coinciding in part with Labradorian plutonism in the Aillik domain. Amphibole ages from dioritic sheets in the juvenile Aillik domain suggest emplacement between 1715 and 1685 Ma. Amphibole ages constrain crystallization of small mafic plutons in the Kaipokok domain (reworked Archean foreland) to be no younger than 1670–1660 Ma. These ages are the oldest yet obtained for Labradorian plutonism in the Makkovik Province.

Sign in / Sign up

Export Citation Format

Share Document