Geophysical data demonstrate elevated seismic activity in subduction zones. Here dehydration and fluid pressure cycling as a function of increasing compaction and metamorphic grade are closely linked to deformation over a multitude of spatial and time scales. The highly anisotropic and initially fluid saturated marine sediments and altered oceanic crust dehydrate, while being incorporated into the accretionary wedge and subducted under the upper plate. Under high tectonic stresses, fluid overpressure eventually results in mechanical instabilities, promoting either hydrofracturing or ductile failure giving way for fluids to circulate. Collection of these fluids at the micron-scale and propagation along pathways up to the deca-kilometre scale are probably in charge for phenomena such as episodic tremor and slow slip. Increasing evidence from geophysical and seismic studies suggest that accumulation of slow slip events and fluids may even trigger devastating high-energy megathrust earthquakes. Quantitative understanding about (i) the release of fluids from their host rocks, (ii) the effect of localisation of both fluid flow and deformation and (iii) their effect on seismic activity are therefore crucial to understand the complex feedback processes. This system can only be fully understood by a close collaboration between experts from structural geology, metamorphic petrology and geophysics. In this interdisciplinary session, we therefore invite contributions from natural, experimental- and numerical modelling-based studies focussing on both exhumed (paleo) and active subduction zones.

Co-organized by GMPV2/SM6
Convener: Ismay Vénice AkkerECSECS | Co-conveners: Francesco GiuntoliECSECS, Marco Herwegh, Christoph Schrank, Emily Warren-Smith
| Attendance Wed, 06 May, 10:45–12:30 (CEST)

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Chat time: Wednesday, 6 May 2020, 10:45–12:30

D1227 |
| Highlight
Whitney Behr, Carolyn Tewksbury-Christle, Alissa Kotowski, Claudio Cannizzaro, Robert Blass, and Taras Gerya

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:

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.

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.

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.  

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.

How to cite: Behr, W., Tewksbury-Christle, C., Kotowski, A., Cannizzaro, C., Blass, R., and Gerya, T.: Hydration- and dehydration-induced rheological heterogeneities on the deep subduction interface, and possible relationships to episodic tremor and slow slip, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3373, https://doi.org/10.5194/egusphere-egu2020-3373, 2020.

D1228 |
Zoe Braden and Whitney Behr

The plate interface in subduction zones accommodates a wide range of seismic styles over different depths as a function of pressure-temperature conditions, compositional and fluid-pressure heterogeneities, deformation mechanisms, and degrees of strain localization. The shallow subduction interface (i.e. ~2-10 km subduction depths), in particular, can exhibit either slow slip events (e.g. Hikurangi) or megathrust earthquakes (e.g. Tohoku). To evaluate the factors governing these different slip behaviors, we need better constraints on the rheological properties of the shallow interface. Here we focus on exhumed rocks within the Chugach Complex of southern Alaska, which represents the Jurassic to Cretaceous shallow subduction interface of the Kula and North American plates. The Chugach is ideal because it exhibits progressive variations in subducted rock types through time, minimal post-subduction overprinting, and extensive along-strike exposure (~250 km). Our aims are to use field structural mapping, geochronology, and microstructural analysis to examine a) how strain is localized in different subducted protoliths, and b) the deformation processes, role of fluids, and strain localization mechanisms within each high strain zone. We interpret these data in the context of the relative ‘strengths’ of different materials on the shallow interface and possible styles of seismicity.  

Thus far we have characterized deformation features along a 1.25-km-thick melange belt within the Turnagain Arm region southeast of Anchorage.  The westernmost melange unit is sediment poor and consists of deep marine rocks with more chert, shale and mafic rocks than units to the east. The melange fabric is variably developed (weakly to strongly) throughout the unit and is steeply (sub-vertical) west-dipping with down-dip lineations. Quartz-calcite-filled dilational cracks are oriented perpendicular to the main melange fabric.

Drone imaging and structural mapping reveals 3 major discrete shear zones and 6-7 minor shear zones within the melange belt, all of which exhibit thrust kinematics. Major shear zones show a significant and observable strain gradient into a wide (~1 m) region of high strain and deform large blocks while minor shear zones are generally developed in narrow zones (~10-15 cm) of high strain between larger blocks. One major shear zone is developed in basalt and has closely-spaced, polished slip surfaces that define a facoidal texture; the basalt shear zone is ~1 m thick. Preserved pillows are observable in lower strain areas on either side of the shear zone but are deformed and indistinguishable within the high strain zone. The other two major shear zones are developed in shale and are matrix-supported with wispy, closely-spaced foliation and rotated porphyroclasts of chert and basalt; the shale shear zones are ~0.5-2 m thick.  

Abundant quartz-calcite veins parallel to the melange fabric and within shale shear zones record multiple generations of fluid-flow; early veins appear to be more silicic and later fluid flow involved only calcite precipitation. At the west, trench-proximal end of the mélange unit there is a 5-10 m thick silicified zone of fluid injection that is bound on one side by the basalt shear zone. Fluid injection appears to pre-date or be synchronous with shearing.

How to cite: Braden, Z. and Behr, W.: Rheological properties of the shallow subduction interface: insights from the Chugach Complex, Alaska, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5646, https://doi.org/10.5194/egusphere-egu2020-5646, 2020.

D1229 |
Carolyn Tewksbury-Christle, Whitney Behr, and Mark Helper

The low velocity layer (LVL) in modern subduction zones is a 3-5 km thick region that parallels the top of the downgoing slab and is characterized by anomalously high Vp/Vs ratios (1.8-2.5) consistent with 2.5-4% fracture porosity at near-lithostatic pore fluid pressures. The LVL has been previously interpreted as partially hydrated, relatively undeformed oceanic crust at the top of the downgoing slab, but collocation of the LVL with episodic tremor and slow slip events (ETS) in modern subduction zones suggests that the LVL may alternatively represent the seismic signature of a subduction interface shear zone. 

To test this hypothesis, we use field & structural observations, geochronology, and seismic velocity calculations to compare and contrast the bulk seismic properties of a fossil subduction interface shear zone (Condrey Mountain Schist, CMS, northern CA) to properties of modern LVLs. Specifically, we 1) determined thicknesses of underplated packages (interpreted to represent the maximum thickness of the actively deforming interface) using depositional age discontinuities and high resolution structural mapping, 2) averaged the bulk rock seismic velocities weighted by mapped lithologic proportions and corrected for pressure-temperature effects, and 3) used field evidence of modifying factors (e.g., microcracks, fluid-filled veins, mineral anisotropy) to further refine the possible range of seismic velocities and effects on Vp/Vs ratio.

The CMS greenschist- to blueschist-facies units were subducted to ~25-35 km (450°C, 0.8-1.0 GPa) with limited retrogression or exhumational overprint. These rocks were underplated episodically at depth in three packages individually up to 4.5 km thick from 155-135 Ma, based on detrital zircon data. Each package is dominantly composed of metasedimentary rocks with m- to km-scale metamafic and serpentinized ultramafic lenses. Strain localization to ~1 km thick ductile shear zones between underplating episodes is collocated with km-scale serpentinized ultramafic lenses at the base of each package. Deformation was distributed and ductile with rare macro- or micro-scale prograde brittle failure in the metasedimentary or metamafic units. In the serpentinized ultramafics, ductile shear zones wrap massive blocks with prograde brittle fracture. Maximum fracture porosity estimated from relict veins is ~10%. Average Vp/Vs for the CMS is ~1.6 (lithology alone) but up to 3.0 (accounting for maximum fracture porosity).

The fossil subduction interface shear zone preserved in the CMS is consistent in both thickness and seismic signature with the LVL in modern subduction zones. Estimated Vp/Vs is higher than the LVL but assumes that all fractures are simultaneously open. The total thickness of the CMS (10+ km) is greater than the LVL, however, so previously underplated material must lose its anomalous seismic signature during underplating (e.g., due to fluid loss and transport up the slab during or after underplating). Our results support the hypothesis that LVLs in modern subduction zones represent the seismic signature of the subduction interface shear zone.

How to cite: Tewksbury-Christle, C., Behr, W., and Helper, M.: Rock record constraints on the seismic signature of subduction interface shear zones, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3285, https://doi.org/10.5194/egusphere-egu2020-3285, 2020.

D1230 |
Hugues Raimbourg, Vincent Famin, Kristijan Rajic, Saskia Erdmann, Benjamin Moris-Muttoni, and Donald Fisher

Veins that form contemporaneously with deformation are the best recorders of the fluids circulating in the depths of orogenic and subduction zones. We have analyzed syn-kinematic quartz veins from accretionary prisms (Shimanto Belt in Japan, Kodiak accretionary prism in Alaska) and tectonic nappes in collisional orogens (Flysch à Helminthoïdes in the Alps, the southern domain of the variscan Montagne Noire), which formed at temperature conditions between 250 and 350°C, i.e. spanning the downdip limit of large subduction earthquakes and the generation of slow slip events. In all geological domains, veins hosted in rocks with the lower temperature conditions (~250-300°C) show quartz grains with crystallographic facets and growth rims. Cathodoluminescence (CL) imaging of these growth rims shows two different colors, a short-lived blue color and a brown one, attesting to cyclic variations in precipitation conditions. In contrast, veins hosted in rocks with the higher temperature conditions (~350°C), show a homogeneous, CL-brown colored quartz, except for some very restricted domains of crack-seal structures of CL-blue quartz found in Japan, Kodiak and Montagne Noire. Based on laser ablation and electron microprobe mapping, the variations in CL colors appear correlated with the trace element content of quartz, the short-lived CL-blue being associated with the substitution of Si4+ by Al3++Li+/H+.

Due to their ubiquitous presence in various settings, the variations in CL colors in the lower T range reflect a common, general process. We interpret these cyclic growth structures as a reflection of deformation/fracturing events, which triggered transient changes in (1) the fluid pressure through fluid flow and (2) the chemistry of the fluid due to enhanced reactivity of the fractured material. The CL-blue growth rims delineate zones where quartz growth was rapid and crystals incorporated a large proportion of Al and Li. Crystal growth continued at a lower pace after fluid pressure and composition evolved to equilibrium conditions, leading to the formation of CL-brown quartz with few substitutions of tetrahedral Si. The variations in fluid pressure fluctuated at values close to lithostatic conditions, as indicated by growth in cavities that remained open.

The crack-seal microstructures have been interpreted as the result of slow-slip events near the base of the seismogenic zone (Fisher and Brantley, 2014; Ujiie et al., 2018). Our observations on quartz composition suggest that the quartz in crack-seal microstructures records episodic variation in fluid pressure and composition, similar to vein quartz at T<~300 °C. In contrast to the cooler and shallower domain, the variations are significantly smaller, as recorded by the very limited extent of the CL-blue domains, and most if not all of the quartz growth occurred under constant physico-chemical conditions, including a near lithostatic fluid pressure. 

We conclude that quartz trace element content is a useful tool to track variations in fluid conditions. In particular, at seismogenic depths (i.e. near 250°C), fluid pressure varies significantly around a lithostatic value. In contrast, deeper, near the base of the seismogenic zone where slow slip events occur (i.e. near 350°C), the variations in fluid pressure conditions are smaller.

How to cite: Raimbourg, H., Famin, V., Rajic, K., Erdmann, S., Moris-Muttoni, B., and Fisher, D.: Record by quartz veins of earthquakes and slow slip events, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5472, https://doi.org/10.5194/egusphere-egu2020-5472, 2020.

D1231 |
Ismay Vénice Akker, Christoph E. Schrank, Michael W.M. Jones, Cameron M. Kewish, Alfons Berger, and Marco Herwegh

In plate-convergent settings, fluid-saturated sediments dehydrate during subduction. The sediments are subsequently accreted to the upper plate. Along their dehydration-deformation path, the initial unconsolidated soft marine sediments become thick, foliated, impermeable meta-sedimentary sequences. Fluid flow through such ‘non’-porous low-permeability rocks is concentrated in fracture networks, ranging from the mm- to the km-scale. We study the interplay between ductile and brittle deformation processes and fluid flow by investigating calcite veins in slates from the exhumed European Alpine accretionary wedge across scales (µm to km). These slates experienced peak metamorphic temperatures between 200°C and 330°C and represent the transition between the upper aseismic and seismic zone. With the use of Synchrotron X-ray Fluorescence Microscopy (SXFM), we investigate the slates by visualizing trace-element distributions. This technique shows that alternating cycles of slow pressure-dissolution processes and brittle fracturing persist over long time scales. At the micron-scale, pressure solution of the initial carbonate-rich slates is indicated by an enrichment of newly recrystallized phyllosilicates on cleavage planes and in pressure shadows. These ductile deformation features are mutually overprinted by calcite veins (aperture 10 µm), which are nicely visualized with Sr-SXFM maps. Increasing compaction and recrystallization in the slate-rich matrix leads to progressed dehydration resulting in an increased pore fluid pressure and subsequent hydrofracturing. The micron-sized fractures are immediately filled in with minerals, which are oversaturated at that time in the fluid, resulting in the formation of (i) micron-veinlets. Micron-veinlets collect (ii) into mm-cm sized veins, which themselves form (iii) vein arrays and (iv) mega-arrays, respectively at the 50-100 m and 300-400 m scale. This upscaling of fluid pathways indicates a localised fluid transport through the accretionary wedge, which has important implications for the understanding of the mechanical stability of the accretionary wedge and related seismic activity.

How to cite: Akker, I. V., Schrank, C. E., Jones, M. W. M., Kewish, C. M., Berger, A., and Herwegh, M.: The coupling of dehydration and deformation results in localised fluid flow in the accretionary wedge – a novel study of calcite veins, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6845, https://doi.org/10.5194/egusphere-egu2020-6845, 2020.

D1232 |
Carolyn Boulton, Marcel Mizera, Maartje Hamers, Inigo Müller, Martin Ziegler, André Niemeijer, and Timothy Little

The Hungaroa Fault Zone (HFZ), an inactive thrust fault along the Hikurangi Subduction Margin, accommodated large displacements (~4–10 km) at the onset of subduction in the early Miocene. Within a 40 m-wide high-strain fault core, calcareous mudstones and marls display evidence for mixed-mode viscous flow and brittle fracture, including: discrete faults; extensional veins containing stretched calcite fibers; shear veins with calcite slickenfibers; calcite foliation-boudinage structures; calcite pressure fringes; dark dissolution seams; stylolites; embayed calcite grains; and an anastomosing phyllosilicate foliation.

Multiple observations indicate a heterogeneous stress state within the fault core. Detailed optical and electron backscatter diffraction-based texture analysis of syntectonic calcite veins and isoclinally folded limestone layers within the fault core reveal that calcite grains have experienced intracrystalline plasticity and interface mobility, and local subgrain development and dynamic recrystallisation. The recrystallized grain size in two calcite veins of 6.0±3.9 µm (n=1339; 1SD; HFZ-H4-5.2m_A;) and 7.2±4.2µm (n=406; 1SD; HFZ-H4-19.9m) indicate high differential stresses (~76–134 MPa). Hydrothermal friction experiments on a foliated, calcareous mudstone yield a friction coefficient of μ≈0.35. Using this friction coefficient in the Mohr-Coulomb failure criterion yields a maximum differential stress of 55 MPa at 4 km depth, assuming a minimum principal stress equal to the vertical stress, an average sediment density of 2350 kg/m3, and hydrostatic pore fluid pressure. Interestingly, calcareous microfossils within the foliated mudstone matrix are undeformed. Moreover, calcite veins are oriented both parallel to and highly oblique to the foliation, indicating spatial and/or temporal variations in the maximum principle stress azimuth.

To further constrain HFZ deformation conditions, clumped isotope geothermometry was performed on six syntectonic calcite veins, yielding formation temperatures of 79.3±19.9°C (95% confidence interval). These temperatures are well below those at which dynamic recrystallisation of calcite is anticipated and exclude shear heating and the migration of hotter fluids as an explanation for dynamic recrystallisation of calcite at shallow crustal levels (<5 km depth).

Our results indicate that: (1) stresses are spatiotemporally heterogeneous in crustal fault zones containing mixtures of competent and incompetent minerals; (2) heterogeneous deformation mechanisms, including frictional sliding, pressure solution, dynamic recrystallization, and mixed-mode fracturing accommodate slip in shallow crustal fault zones; and (3) brittle fractures play a pivotal role in fault zone deformation by providing fluid pathways that promote fluid-enhanced recovery and dynamic recrystallisation in the deforming calcite at remarkably low temperatures. Together, field geology, microscopy, and clumped isotope geothermometry provide a powerful method for constraining the multiscale slip behavior of large-displacement fault zones.

How to cite: Boulton, C., Mizera, M., Hamers, M., Müller, I., Ziegler, M., Niemeijer, A., and Little, T.: Heterogeneous stresses and deformation mechanisms at shallow crustal conditions, Hungaroa Fault Zone, Hikurangi Subduction Margin, New Zealand, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11961, https://doi.org/10.5194/egusphere-egu2020-11961, 2020.

D1233 |
Claudio Petrini, Luca Dal Zilio, and Taras Gerya

Slow slip events (SSEs) are part of a spectrum of aseismic processes that relieve tectonic stress on faults. Their occurrence in subduction zones have been suggested to trigger megathrust earthquakes due to perturbations in fluid pressure. However, examples to date have been poorly recorded and physical observations of temporal fluid pressure fluctuations through slow slip cycles remain elusive. Here, we use a newly developed two-phase flow numerical model — which couples solid rock deformation and pervasive fluid flow — to show how crustal stresses and fluid pressures within subducting megathrust evolve before and during slow slip and regular events. This unified 2D numerical framework couples inertial mechanical deformation and fluid flow by using finite difference methods, marker-in-cell technique, and poro-visco-elasto-plastic rheologies. Furthermore, an adaptive time stepping allows the correct resolution of both long- and short-time scales, ranging from years to milliseconds during the dynamic propagation of earthquake rupture.

Here we show how permeability and its spatial distribution control the degree of locking along the megathrust interface and the interplay between seismic and aseismic slip. While a constant permeability leads to more regular seismic cycles, a depth dependent permeability contributes substantially to the development of two distinct megathrust zones: a shallow, locked seismogenic zone and a deep, narrow aseismic segment characterized by SSEs. Furthermore, we show that without requiring any specific friction law, our model shows that permeability, episodic stress transfer and fluid pressure cycling control the predominant slip mode along the subduction megathrust. Specifically, we find that the up-dip propagation of episodic SSEs systematically decreases the fault strength due to a continuous accumulation and release of fluid pressure within overpressured subducting interface, thus affecting the timing of large megathrust earthquakes. These results contribute to improve our understanding of the physical driving forces underlying the interplay between seismic and aseismic slip, and demonstrate that slow slip events may prove useful for short-term earthquake forecasts.

How to cite: Petrini, C., Dal Zilio, L., and Gerya, T.: Episodic fluid pressure cycling controls the interplay between Slow Slip Events and Large Megathrust Earthquakes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17557, https://doi.org/10.5194/egusphere-egu2020-17557, 2020.

D1234 |
Emily Warren-Smith, Bill Fry, Laura Wallace, Enrique Chon, Stuart Henrys, Anne Sheehan, Kimihiro Mochizuki, Susan Schwartz, Katherine Woods, John Ristau, and Spahr Webb

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’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.

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 – a region susceptible to dynamic triggering of tremor and where shallow SSEs occur every 5 years or so – 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.

How to cite: Warren-Smith, E., Fry, B., Wallace, L., Chon, E., Henrys, S., Sheehan, A., Mochizuki, K., Schwartz, S., Woods, K., Ristau, J., and Webb, S.: Episodic stress tensor and fluid pressure cycling in subducting oceanic crust during Northern Hikurangi slow slip events, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12087, https://doi.org/10.5194/egusphere-egu2020-12087, 2020.

D1235 |
Effat Behboudi, Ivan Lokmer, David McNamara, Tom Manzocchi, Laura Wallace, Demian Saffer, Philip Barnes, Ingo Pecher, Hikweon Lee, Gil Young Kim, Hung-Yu Wu, Katerina Petronotis, Leah LeVay, Iodp Expedition372 Scientists, and Iodp Expedition375 Scientists

The Hikurangi Subduction Margin (HSM) of New Zealand is well-known for its variable seismic behaviour along strike, and across the Pacific-Australian subduction interface. Pacific-Australian plate motion is accommodated by a combination of slow slip and normal seismic events. The mechanics of slow slip earthquakes and their relationship to normal earthquakes are not well constrained, and so they represent a challenge to the development of hazard models for this region of New Zealand.Variability in a number of aspects of the stress state along the HSM may play a role in controlling the observed spatially variable seismic behaviour. Here we present preliminary analysis of stress orientation information from borehole image logs acquired from oil and gas exploration wells, and scientific wells drilled as part IODP Expedition 372. Orientations of borehole breakouts (BOs) and drilling-induced tensile fractures (DITFs) from these image logs are used to determine orientations of the minimum and maximum horizontal stress directions respectively within the upper 3 km of the over-riding Australian Plate (hanging wall of the HSM subduction interface). Our analysis reveals that present day maximum horizontal compressive stress orientation (SHmax) within the subduction hangingwall varies along the HSM strike, from NE-SW (oblique to Pacific-Australian plate motion, parallel to HSM strike) in the northern HSM, to NW-SE (oblique to Pacific-Australian plate motion and HSM strike) in the southern HSM. Some deviation from this trend can be observed in both regions. At the frontal thrust in the northern HSM, SHmax is oriented N-S, and in the southern HSM one well shows a shallow E-W oriented SHmax. The borehole image log SHmax orientations in the northern HSM are consistent with SHmax orientations derived from shallow focal mechanism inversion. In contrast, borehole image logs SHmax orientations in the south of HSM are oblique to focal mechanisms derived shallow SHmax orientations. Borehole SHmax orientations are compatible with the maximum horizontal contraction strain rate direction determined from campaign GPS surface velocities, though in the southern HSM multiple directions are determined from this technique. The difference in stress orientations along the HSM is consistent with the variation in seismic behaviour linked to changes in coupling at the subduction interface. Interestingly, it is the southern HSM, where the subduction interface is considered strongly coupled, that stress field orientations are not consistent with depth and show variability. In the northern HSM, where the subduction interface is considered to be weakly coupled, stress orientations appear broadly consistent with depth. 

How to cite: Behboudi, E., Lokmer, I., McNamara, D., Manzocchi, T., Wallace, L., Saffer, D., Barnes, P., Pecher, I., Lee, H., Kim, G. Y., Wu, H.-Y., Petronotis, K., LeVay, L., Expedition372 Scientists, I., and Expedition375 Scientists, I.: Stress orientation variability along the Hikurangi Subduction Margin: Insights from borehole image logging, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20603, https://doi.org/10.5194/egusphere-egu2020-20603, 2020.

D1236 |
Abhishek Prakash, Caleb W Holyoke III, Peter B Kelemen, William M Lamb, Stephen H Kirby, and Andreas K Kronenberg

Seismicity of subduction zones at upper-mantle depths is commonly explained by dehydration reactions of serpentine and hydrous silicates and reductions in effective pressure. However, the conditions of Wadati-Benioff zone seismicity do not strictly correspond to temperatures and depths of serpentine dehydration, and there is no independent evidence that seawater penetrates the lithosphere to form serpentine at depths >30km below the seafloor. Altered lithosphere may contain magnesian carbonates in addition to hydrous silicates, both at the top of plates, where CO2 of seawater reacts with mantle rocks and at the base of plates where CO2 is introduced by mantle plumes.

Adapting the thermal softening model of Kelemen and Hirth (2007), we model the strain localization and shear heating within magnesite horizons embedded within an olivine host using flow laws determined experimentally for dislocation creep and diffusion creep of the carbonate layer and olivine host (Hirth and Kohlstedt, 2003; Holyoke et al., 2014). Strain rates predicted within carbonate-rich layers of downgoing slabs are much higher than those of the surrounding olivine at all conditions. However, shearing may be either stable or unstable depending on the relative rates of shear heating and conductive heat loss from the shear zone. Localized strain rates reach a steady state when shear heating and heat flow are balanced, while unstable strain rates are calculated where shear heating exceeds heat flow. Modeled strain rates accelerate to 10+1 s-1, as temperatures reach melting conditions, and stresses drop, corresponding to a seismic event. Applications of this model to the double Benioff zones of the NE Japan trench predict unstable seismic shear for both upper and lower seismic zones to subduction depths of ~300 km. For cold downgoing slabs, such as the Tonga subduction system, unstable seismic shear is predicted for carbonate horizons of altered downgoing slabs to depths exceeding 400 km.

How to cite: Prakash, A., Holyoke III, C. W., Kelemen, P. B., Lamb, W. M., Kirby, S. H., and Kronenberg, A. K.: Stable and Unstable Shear in Altered Downgoing Slabs: Predicted Strain Localization in Magnesian Carbonates and Wadati-Benioff Seismicity, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12631, https://doi.org/10.5194/egusphere-egu2020-12631, 2020.

D1237 |
Jade Dutilleul, Marianne Conin, Sylvain Bourlange, and Yves Géraud and the Expedition 358 Science Party

In subduction zones, megathrust seismicity depends on the hydrogeological, thermal, physical and mechanical properties of the sediments before they enter the subduction zone and how these properties evolve through the subduction process. In particular, fluids are progressively released by compaction and/or mineral dehydration reactions as burial increases, resulting in the build-up of pore fluid pressure in low-permeability sediments that strongly affects fault behavior through its control on effective normal stress.

We use porosity, logging and chemical data to characterize compaction state and bound water content of sediments at Site C0024. This site was drilled in March 2019 during International Ocean Discovery Program Expedition 358 in the anticline overlying the frontal thrust of the Nankai margin, a few kilometers landward of Sites C0006 and C0007 that were previously drilled during the NanTroSEIZE project. Sites C0024, C0006 and C0007 transect the décollement and overlying accreted Upper Shikoku Basin and wedge slope deposits. At Site C0024, the main frontal thrust at ~820 mbsf, the top of its footwall (up to ~870 mbsf) and its hanging-wall were logged. Two intervals were cored in the hanging-wall (~0-320 mbsf and ~510-652 mbsf). Four sedimentary facies were identified : (1) the slope apron (~0-4 mbsf) composed of silty clay to clayey silt hemipelagites, (2) accreted trench wedge sediments (~4–519 mbsf) composed of hemipelagites with volcanic ash layers, silt and sand turbidites, (3) outer trench-wedge sediments (~519-555 mbsf) mainly composed of hemipelagites with rare silt turbidites and volcanic ash layers and (4) accreted Upper Shikoku Basin (>555mbsf) composed of hemipelagites and volcanic ash layers. The Pliocene to Miocene accreted Upper Shikoku Basin deposits at the frontal thrust are correlated with undeformed Upper Shikoku Basin deposits at reference Sites C0011 and C0012 seaward in the incoming Shikoku Basin.

Following previous studies, we use Cation Exchange Capacity (CEC) to determine bound water content and interstitial porosity in the cored interval. Unlike total porosity commonly measured onboard, interstitial porosity is representative of the compaction state of sediments. We use interstitial porosity data to calibrate resistivity-derived porosity through the hanging-wall, the décollement and below. Resistivity-derived porosity is obtained with a resistivity model accounting for the high surface conductivity of clays based on CEC, exchangeable cation composition, LWD resistivity and gamma ray logs. We also document the evolution of the structure of micro- to macropores with depth using low pressure nitrogen adsorption/desorption, mercury injection capillary pressure and nuclear magnetic resonance.  Finally, we compare the porosity dataset at Site C0024 with that of Sites C0006 and C0007 in the frontal thrust and reference Sites C0011 and C0012 in the entering Shikoku Basin to characterize the evolution of the compaction state of sediments during accretion.

How to cite: Dutilleul, J., Conin, M., Bourlange, S., and Géraud, Y. and the Expedition 358 Science Party: Characterization of porosity from Cation Exchange Capacity and resistivity data at IODP Expedition 358 Site C0024: insights on compaction state, stress and deformation history at the frontal thrust of the Nankai margin, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11685, https://doi.org/10.5194/egusphere-egu2020-11685, 2020.

D1238 |
Yusuke Yokota, Tadashi Ishikawa, Shun-ichi Watanabe, and Yuto Nakamura

Our research group has been studying advanced GNSS-A (Global Navigation Satellite System – Acoustic ranging combination) technique over two decades. In recent years, detection sensitivity of GNSS-A observations has been sophisticated by improving the accuracy and frequency of analysis technology and acoustic systems [e.g., Yokota et al., 2018, MGR; Ishikawa et al., in prep]. The current observation frequency is more than 4 times/year, the observation accuracy for each observation is less than 2 cm, and it can detect a steady deformation rate of 1 cm/year or less and an unsteady fluctuation of 5 cm or less. Also, efforts are being made to strengthen the observation network.

GNSS-A observations for the 2011 Tohoku-oki earthquake and its postseismic field revealed the details of the crustal deformation field on the Japan Trench side [Sato et al., 2011, Science; Watanabe et al., 2014, GRL]. The long-term observation data in the Nankai Trough region revealed the strain accumulation process at the interseismic period [Yokota et al., 2016, Nature; Watanabe et al., 2018, JGR; Nishimura et al., 2018, Geosphere]. Furthermore, detection and monitoring of large-scale slow slip events (SSEs) in the shallow part of the Nankai Trough was achieved by recent sensitivity improvements [Yokota & Ishikawa, 2020, Science Advances]. The detected postseismic fields, coupling condition and shallow SSEs contain universal features that should be shared in many subduction zones. Here, along with the latest observations, we discuss spatial and temporal relationships of these events, strain accumulations and releases along subduction zones around Japan by GNSS-A and its impact on slow earthquake science.

Recently, because of the need for continuous monitoring a shallow SSE, the monitoring ability of GNSS-A was also investigated. It was confirmed that relatively large-scale shallow SSE (surface deformation: > 5 cm) could be monitored. However, the ability to determine the time constant of an SSE is poor. For monitoring the detail of an SSE, it is essential to improve the observation frequency in the future. Here, we also discuss the technical issues to be considered and their solution plans (e.g., new platform and system).

How to cite: Yokota, Y., Ishikawa, T., Watanabe, S., and Nakamura, Y.: Recent advances in GNSS-A observation technology and networks and latest observation results around Japan Islands, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3231, https://doi.org/10.5194/egusphere-egu2020-3231, 2020.

D1239 |
Homa Ghadimi Moghaddam, Alireza Khodaverdian, and Hamid Zafarani

Long term crustal flow of the Makran subduction zone is computed with a kinematic finite element model based on iterated weighted least squares fits to data. Data include 91 fault traces, 56 fault offset rates, 76 geodetic velocities, 1962 principal stress azimuths, and velocity boundary conditions. Model provides long-term fault slip rates, velocity, and distributed permanent strain rates between faults in the Makran region from all available kinematic data. Due to low seismicity of western Makran compared to its eastern part we defined two models to evaluate the possibility of creep in the Iranian Makran subduction. One model assumes that geodetic velocities measured adjacent to the Makran subduction zone reflect a temporarily locked subduction zone will be referred to as the “seismic deformation model”. In contrast, another model called the “half creeping deformation model” assumes that the western part of Makran may creep smoothly without any locking. In order to verify the models, the estimates of fault slip rates are compared to slip rates from merely analysing geodetic benchmark velocities or paleoseismological studies or published geological rates which have not been used in the model. Our estimated rates are all in the range of geodetic rates and are even more consistent with geological rates than previous GPS-based estimates. Another verification for the model is comparison of the computed interseismic velocities at GPS benchmarks to GPS measurements. While neither model accurately predicts these interseismic velocities at benchmarks, the half creeping deformation model is more accurate for Chabahar station than the seismic deformation model. These results have important earthquake and tsunami hazard implications. For example, Fault slip rates are the main component of time-dependent seismic hazard studies and can be used to estimate activity rates for more sophisticated earthquake models.

How to cite: Ghadimi Moghaddam, H., Khodaverdian, A., and Zafarani, H.: An investigation on possibilty of creep on the Makran subduction zone based on deformation data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-864, https://doi.org/10.5194/egusphere-egu2020-864, 2020.

D1240 |
Gaspard Farge, Nikolai Shapiro, Claude Jaupart, and Édouard Kaminski

The activity of slow-earthquakes in subduction zones have been closely linked to fluid circulation processes — like hydro fracturation and pore-pressure pulses — on the one hand by geological observations and on the other hand by slow-earthquake triggering and interaction models. In deep fault zone environments, where slow slip events and various regimes of tremor are observed, fluids coming from metamorphic dehydration of slab sediments are channeled towards the surface. Geological observations indicate that fluid transport conditions vary significantly on short time scales, and along dip, strike and width of the fault zone. In homogeneously permeable systems where fluids transit under stable conditions, pore-pressure can be described by a diffusion equation. We use a time and space bimodal description of the transport properties to model a tremor generating, permeable fault zone. Thin zones of low permeability acting as valves are distributed along the 1D channel with a higher background permeability. When a threshold pore-pressure differential is reached, the valve permeability is brought to background levels, until the barrier is healed and closes again. In this model, the opening of a valve occurs at the same time as the source of a low-frequency earthquake (LFE) is triggered. In such a set up, sources interact uniquely due to the channeling of stress through pore-pressure diffusion, and the interaction characteristics in time/space are described in the framework of a diffusive system. When the number of sources is high, the model can reproduce clustering behaviours observed for LFE activity in subduction zones. The transition from a Poisson process description of seismicity to highly clustered, cascading events is governed by the source interaction distances, directly relating to the transport properties of the medium. In time, such a model is meant to diagnose the transport conditions in a subduction zone or a magmatic system, provided that it can be characterized by clustering statistics on the low-frequency seismicity it generates.

How to cite: Farge, G., Shapiro, N., Jaupart, C., and Kaminski, É.: Sounds of the deep subduction zone plumbing system: modeling non-volcanic tremor activity in a fault-valve, pore-pressure diffusive system, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8315, https://doi.org/10.5194/egusphere-egu2020-8315, 2020.

D1241 |
Francesco Turco, Andrew Gorman, Gareth Crutchley, Leonardo Azevedo, Dario Grana, and Ingo Pecher

Geophysical data indicate that the Hikurangi subduction margin on New Zealand’s East Coast contains a large gas hydrate province. Gas hydrates are widespread in shallow sediments across the margin, and locally intense fluid seepage associated with methane hydrate is observed in several areas. Glendhu and Honeycomb ridges lie at the toe of the Hikurangi deformation wedge at depths ranging from 2100 to 2800 m. These two parallel four-way closure systems host concentrated methane hydrate deposits. The control on hydrate formation at these ridges is governed by steeply dipping permeable strata and fractures, which allow methane to flow upwards into the gas hydrate stability zone. Hydrate recycling at the base of the hydrate stability zone may contribute to the accumulation of highly concentrated hydrate in porous layers.
To improve the characterisation of the hydrate systems at Glendhu and Honeycomb ridges, we estimate hydrate saturation and porosity of the concentrated hydrate deposits. We first estimate elastic properties (density, compressional and shear-wave velocities) of the gas hydrate stability zone through full-waveform inversion and iterative geostatistical seismic amplitude versus angle (AVA) inversion. We then perform a petrophysical inversion based on a rock physics model to predict gas hydrate saturation and porosity of the hydrate bearing sediments along the two ridges.
Our results indicate that the high seismic amplitudes correspond to the top interface of highly concentrated hydrate deposit, with peak saturations around 35%. Because of the resolution of the seismic data we assume that the estimated properties are averaged over layers of 10 to 20 meters thickness. These saturation values are in agreement with studies conducted in other areas of concentrated hydrate accumulations in similar geologic settings.

How to cite: Turco, F., Gorman, A., Crutchley, G., Azevedo, L., Grana, D., and Pecher, I.: Methane hydrate saturations at the Southern Hikurangi margin (New Zealand) estimated from seismic and rock physics inversion, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11486, https://doi.org/10.5194/egusphere-egu2020-11486, 2020.