The slip behavior of major faults depends largely on the frictional and hydrologic properties of fault gouge. We report on laboratory experiments designed to measure the strength, friction ...constitutive properties, and permeability of a suite of saturated clay‐rich fault gouges, including: a 50:50% mixture of montmorillonite‐quartz, powdered illite shale, and powdered chlorite schist. Friction measurements indicate that clay‐rich gouges are consistently weak, with steady state coefficient of sliding friction of <0.35. The montmorillonite gouge (μ = 0.19–0.23) is consistently weaker than the illite and chlorite gouges (μ = 0.27–0.32). At effective normal stresses from 12 to 59 MPa, all gouges show velocity‐strengthening frictional behavior in the sliding velocity range 0.5–300 μm/s. We suggest that the velocity‐strengthening behavior we observe is related to saturation of real contact area, as documented by the friction parameter b, and is an inherent characteristic of noncohesive, unlithified clay‐rich gouge. Permeability normal to the gouge layer measured before, during, and after shear ranges from 8.3 × 10−21 m2 to 3.6 × 10−16 m2; permeability decreases dramatically with shearing, and to a lesser extent with increasing effective normal stress. The chlorite gouge is consistently more permeable than the montmorillonite and illite gouge and maintains a higher permeability after shearing. Permeability reduction via shear is pronounced at shear strains ≲5 and is smaller at higher strain, suggesting that shear‐induced permeability reduction is linked to fabric development early in the deformation history. Our results imply that the potential for development of excess pore pressure in low‐permeability fault gouge depends on both clay mineralogy and shear strain.
Slow earthquakes represent an important conundrum in earthquake physics. While regular earthquakes are catastrophic events with rupture velocities governed by elastic wave speed, the processes that ...underlie slow fault slip phenomena, including recent discoveries of tremor, slow-slip and low-frequency earthquakes, are less understood. Theoretical models and sparse laboratory observations have provided insights, but the physics of slow fault rupture remain enigmatic. Here we report on laboratory observations that illuminate the mechanics of slow-slip phenomena. We show that a spectrum of slow-slip behaviours arises near the threshold between stable and unstable failure, and is governed by frictional dynamics via the interplay of fault frictional properties, effective normal stress and the elastic stiffness of the surrounding material. This generalizable frictional mechanism may act in concert with other hypothesized processes that damp dynamic ruptures, and is consistent with the broad range of geologic environments where slow earthquakes are observed.
In subduction zones, high fluid content and pore pressure are thought to promote aseismic creep, whereas well‐drained conditions are thought to promote locking and failure in earthquakes. However, ...observations directly linking fluid content and seismic coupling remain elusive. Heise et al. (2017, https://doi.org/10.1002/2017GL074641) use a magnetotelluric survey to image the electrical resistivity structure of the northern Hikurangi subduction thrust to ~30 km depth, as an indicator of interconnected fluid content. The authors document a clear correlation between high resistivity and a distinct geodetically locked patch and between conductive areas and weak coupling. Their study, together with other recent geophysical investigations, provides new evidence for the role of fluids in governing subduction thrust locking.
Key Points
Electrical resistivity maps fluid content along the subduction megathrust
Patterns of resistivity correlate with geodetic locking
The MT investigation provides support for hypotheses invoking variations in fluid content to explain fault slip behavior
Recent seismic reflection and ocean bottom seismometer (OBS) studies reveal broad regions of low seismic velocity along the Nankai subduction plate boundary megathrust offshore SW Japan. These low ...velocity zones (LVZ's) extend ∼55 km landward from the trench, corresponding to depths of >∼10 km below sea floor. Here, we estimate the in‐situ pore pressure and stress state within these LVZ's by combining P‐wave velocities obtained from the geophysical surveys with new well‐constrained empirical relations between P‐wave velocity, porosity, and effective mean stress defined by laboratory deformation tests on drill core samples of the incoming oceanic sediment. We document excess pore pressures of 17–87 MPa that extend ∼55 km into the subduction zone, indicating that trapped pore fluids support ∼45–91% of the overburden stress along the base of the upper plate and surrounding major fault zones. The resulting effective stresses in the LVZ are limited to ∼1/3 of the values expected for non‐overpressured conditions. These low effective stresses should lead to a mechanically weak and predominantly aseismic plate boundary. The region of lowest effective stress coincides with precisely located very low frequency earthquakes, providing the first quantitative evidence linking these anomalous slip events to low stress and high pore pressure.
Key Points
High pore fluid pressure and low effective stress develop in low velocity zones
Low effective stresses lead to a mechanically weak and aseismic plate boundary
The location of VLFEs coincides with the region of lowest effective stress
The discovery of slow earthquakes has revolutionized the field of earthquake seismology. Defining the locations of these events and the conditions that favor their occurrence provides important ...insights into the slip behavior of tectonic faults. We report on a family of recurring slow-slip events (SSEs) on the plate interface immediately seaward of repeated historical moment magnitude (M
w) 8 earthquake rupture areas offshore of Japan. The SSEs continue for days to several weeks, include both spontaneous and triggered slip, recur every 8 to 15 months, and are accompanied by swarms of low-frequency tremors. We can explain the SSEs with 1 to 4 centimeters of slip along the megathrust, centered 25 to 35 kilometers (km) from the trench (4 to 10 km depth). The SSEs accommodate 30 to 55% of the plate motion, indicating frequent release of accumulated strain near the trench.
Along plate boundary subduction thrusts, the transformation of smectite to illite within fault gouge at temperatures of ~150 degree C is one of the key mineralogical changes thought to control the ...updip limit of seismicity. If correct, this hypothesis requires illite-rich gouges to exhibit frictionally unstable (velocity-weakening) behavior. Here, we report on laboratory experiments designed to investigate the frictional behavior of natural and synthetic clay-rich gouges. We sheared 5-mm-thick layers of commercially obtained pure Ca-smectite, a suite of smectite-quartz mixtures, and natural illite shale (grain size ranging from 2 to 500 mu m) in the double-direct shear geometry to shear strains of ~7-30 at room humidity and temperature. XRD analyses show that the illite shale contains dominantly clay minerals and quartz; within the clay-sized fraction (<2 mu m), the dominant mineral is illite. Thus, we consider this shale as an appropriate analog for fine-grained sediments incoming to subduction zones, within which smectite has been transformed to illite. We observe a coefficient of friction ( mu ) of 0.42- 0.68 for the illite shale, consistent with previous work. Over a range of normal stresses from 5 to 150 MPa and sliding velocities from 0.1 to 200 mu m/s, this material exhibits only velocity-strengthening behavior, opposite to the widely expected, potentially unstable velocity-weakening behavior of illite. Smectite sheared under identical conditions exhibits low friction ( mu =0.15-0.32) and a transition from velocity weakening at low normal stress to velocity strengthening at higher normal stress (>40 MPa). Our data, specifically the velocity-strengthening behavior of illite shale under a wide range of conditions, do not support the hypothesis that the smectite-illite transition is responsible for the seismic-aseismic transition in subduction zones. We suggest that other depth- and temperature-dependent processes, such as cementation, consolidation, and slip localization with increased shearing, may play an important role in changing the frictional properties of subduction zone faults, and that these processes, in addition to clay mineralogy, should be the focus of future investigation.
The role of phyllosilicate fabrics in fault gouge is a poorly understood component of the mechanical and hydrologic behavior of brittle fault zones. We present 90 fabric intensity measurements using ...X‐ray texture goniometry on 22 natural clay‐rich fault gouges from low‐angle detachment faults (Death Valley area detachments, California; Ruby Mountains, Nevada; West Salton Detachment Fault, California) and the Peramola thrust in NE Spain. Natural fault gouges have uniformly weak clay fabrics (multiples of a random distribution (MRD) = 1.7–4.5, average MRD = 2.6) when compared to phyllosilicate‐rich rocks found in other geologic settings. Clay fabric intensities in natural gouges do not vary significantly either as a function of tectonic environment or of dominant clay mineralogy in the gouge. We compare these natural samples with 69 phyllosilicate fabric intensities measured in laboratory experiments on synthetic clay‐quartz mixtures. Clay fabric intensities from laboratory samples are similar to those in natural gouges (MRD = 1.7–4.6), but increase systematically with increasing shear strain and normal stress. Total phyllosilicate content does not significantly affect clay fabric intensity. Shear strain is important for developing stronger fabrics; samples subjected solely to compression exhibit uniformly weak fabrics (MRD = 1.6–1.8) even when compressed at high normal stresses (150 MPa). The weak fabrics found in natural fault gouge indicate that if anisotropic and overall low fault zone permeability allow elevated pore fluid pressures and fault weakening, this anisotropy must be a transient feature that is not preserved. Our data also reinforce the idea that clay fabric development in sedimentary rocks is primarily a function of authigenic mineral growth and not of compaction‐induced particle rotation.
Although specific organs in some plant species exhibit helical growth patterns of fixed or variable handedness, most plant organs are not helical. Here we report that mutations in Arabidopsis ...RHAMNOSE BIOSYNTHESIS 1 (RHM1) cause dramatic left-handed helical growth of petal epidermal cells, leading to left-handed twisted petals. rhm1 mutant roots also display left-handed growth. Furthermore, we find that RHM1 is required to promote epidermal cell expansion. RHM1 encodes a UDP-L-rhamnose synthase, and rhm1 mutations affect synthesis of the pectic polysaccharide rhamnogalacturonan-I. Unlike other mutants that exhibit helical growth of fixed handedness, the orientation of cortical microtubule arrays is unaltered in rhm1 mutants. Our findings reveal a novel source of left-handed plant growth caused by changes in cell wall composition that is independent of microtubule orientation. We propose that an important function of rhamnose-containing cell wall polymers is to suppress helical twisting of expanding plant cells.
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•Loss of RHM1 activity results in left-handed helical growth of roots and petals•RHM1 is required for synthesis of the pectic polysaccharide rhamnogalacturonan-I•Loss of RHM1 activity does not affect microtubule orientation•Helical pattern of cell expansion is associated with altered cell wall composition
Saffer et al. demonstrate that decreased levels of pectic polysaccharides in the cell wall lead to helical growth of plant cells and organs. Unlike other alterations resulting in helical growth, microtubule orientation is unaffected, so this finding describes a novel role for pectin in the control of plant cell patterning.
Permeability controls fluid flow in the Earth's crust and affects a wide range of processes including advective transport and pore pressure generation. However, in situ measurements of permeability ...are few, especially in active tectonic settings or at scales relevant to regional flow. We analyze formation fluid pressure records from oceanic boreholes in the Nankai accretionary prism offshore southwest Japan, focusing on unexpected responses to drilling operations conducted at boreholes ~100 m to the northeast. We develop a 2‐D numerical model of transient fluid flow and conduct a parametric grid search to define hydraulic diffusivity. A value of 0.19–0.46 m2/s (corresponding to a permeability of 9.8 × 10−13 to 2.4 × 10−12 m2) yields the best fit to observed pressure responses. Together with laboratory measurements on core samples and drillstrem tests reported in previous studies, our analysis indicates a strong scale dependence of permeability, likely reflecting the presence of permeable faults and fractures.
Plain Language Summary
Despite the importance of permeability to a wide range of geological and geomechanical processes in subduction zones, in situ permeability at scales of tens to several hundreds of meters is generally not well constrained, particularly in deforming, active tectonic and structural system. In this work, we analyzed formation fluid pressure variations recorded in a borehole observatory in the Nankai accretionary prism offshore southwest Japan, in response to drilling at nearby boreholes. These responses provide a set of inadvertent cross‐hole experiments, and thus represent a unique opportunity to quantify formation hydraulic properties at a scale of ~100 m. Together with permeabilities reported in previous studies from laboratory experiments, down‐hole tests and regional scale numerical simulations, our results define a clear scale dependence of permeability over scales ranging from cm to > 100 m. We attribute this to the presence of permeable fractures and faults in the accretionary prism across a range of scales.
Key Points
Inadvertent cross‐hole experiments enable determination of the in situ hydraulic diffusivity of the active Nankai Trough accretionary wedge
Permeability of the accretionary prism at the ~100‐m scale is 9.8 × 10−13 to 2.4 × 10−12 m2
Compilation of values determined using a range of methods at different measurement scales indicates a scale dependence of permeability
At subduction zones, continuous influx of fluids drives a dynamic system in which fault slip, fluid flow, and advective transport are tightly coupled. Field and numerical modeling studies have ...provided insight into the nature and rates of flow in these systems and illustrate that active subduction faults, including the master décollement and splay faults cutting the upper plate, are important conduits. Observations of in situ fracture dilation, modeling studies, and direct measurements documenting strong pressure dependence of fault permeability collectively suggest that permeability varies in time, perhaps due to pore pressure cycling. However, mechanical and fluid budget considerations dictate that increased fault permeability cannot be sustained, nor can it be present across the entire fault surface at a given time. The emerging conceptual model is that permeable patches or channels occupy only a fraction of the fault surface and shift transiently. Fault zone permeabilities obtained by several approaches are consistent between margins, with time‐averaged values of approximately 10−15 to 10−14 m2, several orders of magnitude higher than for the sediment matrix. Higher, transiently increased values of approximately 10−13 to 10−11 m2 are required to explain geochemical and thermal signals and observed focused flow rates. Although faults accommodate significant fluid fluxes from dewatering of the surrounding sediment, they have little effect on pore pressures within the wall rock, where drainage is limited by low matrix permeability. However, fault permeability is a key control on the transport and preservation of localized geochemical and thermal anomalies from depths where temperatures are higher and low‐temperature metamorphic reactions are underway. Despite significant recent progress, several key aspects of hydrologic behavior in these active faults remain incompletely understood, including the nature and timescale of transience, the causes of permeability enhancement and its relationship to fault slip and pore pressure fluctuations, and the depths and distances from which deeply sourced fluids are captured, mixed, and transported up‐dip.
Subduction zone faults are key conduits for fluid flow and advection of heat and solutes. Drilling, numerical modeling, and laboratory studies all suggest that their permeability also varies through time, likely due to pore pressure cycling and dilation of fracture networks. Detailed studies of several active subduction zones illustrate that both time‐averaged and transiently elevated fault permeabilities are remarkably consistent between margins, ranging from approximately 10−15 to 10−14 m2 and approximately 10−11 to 10−13 m2, respectively, and are up to six orders of magnitude higher than that of the surrounding sediment matrix.
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