Hydrous oceanic crust hosts megathrust creep at low shear stresses

The rheology of the metamorphosed oceanic crust may be a critical control on megathrust strength and deformation style. However, little is known about the strength and deformation style of metamorphosed basalt. Exhumed megathrust shear zones exposed on Kyushu, SW Japan, contain hydrous metabasalts deformed at temperatures between ~300° and ~500°C, spanning the inferred temperature-controlled seismic-aseismic transition. Field and microstructural observations of these shear zones, combined with quartz grain-size piezometry, indicate that metabasalts creep at shear stresses <100 MPa at ~370°C and at shear stresses <30 MPa at ~500°C. These values are much lower than those suggested by viscous flow laws for basalt. The implication is that relatively weak, hydrous, metamorphosed oceanic crust can creep at low viscosities over a wide shear zone and have a critical influence on plate interface strength and deformation style around the seismic-aseismic transition

The role of compaction contrasts in sediments in décollement initiation in an accretionary prism

International audienceTo understand how décollements develop into the pristine sedimentary succession entering subduction zones, we have performed mechanical tests on samples from the sediment column entering the Nankai accretionary prism, Japan (ODP site 1173). Both poroelastic compliance and plastic shrinkage upon application of a large effective pressure sharply decrease with depth in a ~ 100 m-thick domain in the upper section of the Lower Shikoku Basin unit, i.e. in a domain stratigraphically close to the actual location of the décollement near the toe of the prism. These property contrasts provide a potential explanation for the outward migration of the décollement into the incoming sediments. When approaching the deformation front, a given material particle is affected by an increase in stress, which has a component of vertical loading due to the deposition of overburden trench sediment, and also a component of lateral compression transmitted from the accretionary wedge. Depending on its initial mechanical state, the amount of lateral shortening in the incoming Nankai sediment column varies with depth and causes horizontal velocity gradients that concentrate into the mechanical transition zone (upper section of the Lower Shikoku Basin at appx. 450-550 m depth) into which the décollement eventually propagates. Future work has to assess the role of this plastic deformation relative to other governing factors such as friction coefficient and excess pore pressure, both at Nankai and along other active margins

Viscous strengthening followed by slip weakening during frictional melting of chert

Pelagic chert is one of the major lithologies in accretionary complexes. Thus, frictional properties of chert at seismic slip rates are important for understanding of earthquake faulting in subduction zones. Here, we conducted high-velocity friction experiments on chert collected from the Jurassic accretionary complex in central Japan at a slip rate of 1.3 m/s and normal stresses of 5–13 MPa under room humidity conditions. The results show that initial slip weakening was followed by slip strengthening and subsequent second slip weakening toward a steady-state shear strength. Slip strengthening resulted from the formation of a silica-rich melt layer at lower melting temperatures than expected, which could be due to the presence of water in the illite-containing chert. The second slip weakening may be occurred due to a decrease in shear strain rate associated with the thickening of the melt layer

High-velocity frictional strength across the Tohoku-Oki megathrust determined from surface drilling torque

High-velocity frictional strength is one of the primary factors controlling earthquake faulting. The Japan Trench Fast Drilling Project drilled through the shallow plate boundary fault, where displacement was ~50 m during the 2011 Tohoku-Oki earthquake. To determine downhole frictional strength, we analyzed the surface drilling torque data acquired at rotation rates equivalent to seismic slip rates (0.8–1.3 m/s). The results show a clear contrast in high-velocity frictional strength across the plate boundary fault: the apparent friction coefficient of frontal prism sediments (hemipelagic mudstones) in the hanging wall is 0.1–0.3, while that of the underthrust sediments (mudstone, laminar pelagic claystone, and chert) in the footwall increases to 0.2–0.4. The apparent friction coefficient of the smectite-rich pelagic clay in the plate boundary fault is 0.08–0.19, which is consistent with that determined from high-velocity (1.1–1.3 m/s) friction experiments. This suggests that surface drilling torque is useful in obtaining downhole frictional strength

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

Episodic tremor and slow slip (ETS) occurs in the transition zone between the locked seismogenic zone and the deeper, stably sliding zone. Actual mechanisms of ETS are enigmatic, caused by lack of geological observations and limited spatial resolution of geophysical information from the ETS source. We report that quartz‐filled, crack‐seal shear and extension veins in subduction mélange record repeated low‐angle thrust‐sense frictional sliding and tensile fracturing at near‐lithostatic fluid pressures. Crack‐seal veins were coeval with viscous shear zones that accommodated deformation by pressure solution creep. The minimum time interval between thrusting events, determined from a kinetic model of quartz precipitation in shear veins, was less than a few years. This short recurrence time of low‐angle brittle thrusting at near‐lithostatic fluid overpressures within viscous shear zones may be explained by frequent release of accumulated strain by ETS

An explanation of episodic tremor and slow slip constrained by crack-seal veins and viscous shear in subduction mélange

Episodic tremor and slow slip (ETS) occurs in the transition zone between the locked seismogenic zone and the deeper, stably sliding zone. Actual mechanisms of ETS are enigmatic, caused by lack of geological observations and limited spatial resolution of geophysical information from the ETS source. We report that quartz‐filled, crack‐seal shear and extension veins in subduction mélange record repeated low‐angle thrust‐sense frictional sliding and tensile fracturing at near‐lithostatic fluid pressures. Crack‐seal veins were coeval with viscous shear zones that accommodated deformation by pressure solution creep. The minimum time interval between thrusting events, determined from a kinetic model of quartz precipitation in shear veins, was less than a few years. This short recurrence time of low‐angle brittle thrusting at near‐lithostatic fluid overpressures within viscous shear zones may be explained by frequent release of accumulated strain by ETS

Spectrum of slip behaviour in Tohoku fault zone samples at plate tectonic slip rates

During the 2011 Tohoku-oki earthquake, extremely extensive coseismic slip ruptured shallow parts of the Japan Trench subduction zone and breached the sea floor1, 2. This part of the subduction zone also hosts slow slip events (SSE)3, 4. The fault thus seems to have a propensity for slip instability or quasi-instability that is unexpected on the shallow portions of important fault zones. Here we use laboratory experiments to slowly shear samples of rock recovered from the Tohoku-oki earthquake fault zone as part of the Japan Trench Fast Drilling Project. We find that infrequent perturbations in rock strength appear spontaneously as long-term SSE when the samples are sheared at a constant rate of about 8.5 cm yr−1, equivalent to the plate-convergence rate. The shear strength of the rock drops by 3 to 6%, or 50 kPa to 120 kPa, over about 2 to 4 h. Slip during these events reaches peak velocities of up to 25 cm yr−1, similar to SSE observed in several circum-Pacific subduction zones. Furthermore, the sheared samples exhibit the full spectrum of fault-slip behaviours, from fast unstable slip to slow steady creep, which can explain the wide range of slip styles observed in the Japan Trench. We suggest that the occurrence of SSE at shallow depths may help identify fault segments that are frictionally unstable and susceptible to large coseismic slip propagation

Megathrust shear modulated by Albite Metasomatism in subduction mélanges

Aseismic megathrust slip downdip of the seismogenic zone is accommodated by either steady creep or episodic slow slip events (SSEs). However, the geological conditions defining the rheology of megathrust slip remain elusive. We examined exhumed subduction mélanges on Kyushu, Japan, which deformed at ∼370–500°C under greenschist to epidote‐amphibolite facies conditions, comparable to warm‐slab environments. The mélanges recorded fluid release and viscous shear localization associated with metasomatic reactions between juxtaposed metapelitic and metabasaltic rocks. Metasomatic reactions caused albitization of metapelite, resulting in depth‐dependent changes to megathrust rheology. In a mélange deformed at ∼370°C, very fine grained reaction products (metasomatic albite) facilitated grain boundary diffusion creep at stresses of ∼45 MPa, less than those in the surrounding metabasalt. Mineralogical and chemical changes during metasomatic reactions, and their field content, imply an onset of albite metasomatism at ∼350°C. Albite metasomatism therefore potentially contributed to decreased megathrust strength around the inferred thermally controlled base of the seismogenic zone. In a mélange deformed near the mantle wedge corner at ∼500°C, metasomatic reactions promoted local quartz vein formation and localized viscous shear at slow slip strain rates, during which the coarse‐grained metasomatic albite behaved as relatively rigid blocks in a viscous matrix. We suggest that albite metasomatism can facilitate changes in a megathrust slip mode with depth and may explain why slip mode changes from creep to SSEs with tremor with increasing depth

Structure and lithology of the Japan Trench subduction plate boundary fault

The 2011 Mw9.0 Tohoku-oki earthquake ruptured to the trench with maximum coseismic slip located on the shallow portion of the plate boundary fault. To investigate the conditions and physical processes that promoted slip to the trench, Integrated Ocean Drilling Program Expedition 343/343T sailed 1 year after the earthquake and drilled into the plate boundary ∼7 km landward of the trench, in the region of maximum slip. Core analyses show that the plate boundary décollement is localized onto an interval of smectite-rich, pelagic clay. Subsidiary structures are present in both the upper and lower plates, which define a fault zone ∼5–15m thick. Fault rocks recovered from within the clay-rich interval contain a pervasive scaly fabric defined by anastomosing, polished, and lineated surfaces with two predominant orientations. The scaly fabric is crosscut in several places by discrete contacts across which the scaly fabric is truncated and rotated, or different rocks are juxtaposed. These contacts are inferred to be faults. The plate boundary décollement therefore contains structures resulting from both distributed and localized deformation. We infer that the formation of both of these types of structures is controlled by the frictional properties of the clay: the distributed scaly fabric formed at low strain rates associated with velocity-strengthening frictional behavior, and the localized faults formed at high strain rates characterized by velocity-weakening behavior. The presence of multiple discrete faults resulting from seismic slip within the décollement suggests that rupture to the trench may be characteristic of this margin

Hydrogeological responses to incoming materials at the erosional subduction margin, offshore Osa Peninsula, Costa Rica

Bulk mineral assemblages of sediments and igneous basement rocks on the incoming Cocos Plate at the Costa Rica subduction zone are examined by X-ray diffraction analyses on core samples. These samples are from Integrated Ocean Drilling Program Expedition 334 reference Site U1381, ∼ 5 km seaward of the trench. Drilling recovered approximately 100 m of sediment and 70 m of igneous oceanic basement. The sediment includes two lithologic units: hemipelagic clayey mud and siliceous to calcareous pelagic ooze. The hemipelagic unit is composed of clay minerals (∼50 wt.%), quartz (∼5 wt.%), plagioclase (∼5 wt.%), calcite (∼15 wt.%) and ∼30 wt.% of amorphous materials, while the pelagic unit is mostly made up of biogenic amorphous silica (∼50 wt.%) and calcite (∼50 wt.%). The igneous basement rock consists of plagioclase (∼50–60 wt.%), clinopyroxene (∼>25 wt.%), and saponite (∼15–40 wt.%). Saponite is more abundant in pillow basalt than in the massive section, reflecting the variable intensity of alteration. We estimate the total water influx of the sedimentary package is 6.9 m³/yr per m of trench length. Fluid expulsion models indicate that sediment compaction during shallow subduction causes the release of pore water while peak mineral dehydration occurs at temperatures of approximately ∼100°C, 40–30 km landward of the trench. This region is landward of the observed updip extent of seismicity. We posit that in this region the presence of subducting bathymetric relief capped by velocity weakening nannofossil chalk is more important in influencing the updip extent of seismicity than the thermal regime.This is the publisher’s final pdf. The article is copyrighted by American Geophysical Union and published by John Wiley & Sons, Inc. It can be found at: http://agupubs.onlinelibrary.wiley.com/agu/journal/10.1002/%28ISSN%291525-2027/Keywords: Costa Rica margin, diagenesis, CRISP, seismicityKeywords: Costa Rica margin, diagenesis, CRISP, seismicit