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Tensile overpressure compartments on low-angle thrust faults
© The Author(s) 2017
- Received: 8 April 2017
- Accepted: 31 July 2017
- Published: 15 August 2017
Material properties aside, the formation and reactivation of brittle faults and fractures is largely governed by the triaxial stress state within the rock mass and by the pore-fluid pressure, P f, within the rock mass (Hubbert and Rubey 1959; Jaeger and Cook 1979). Recognizing the boundary condition of zero shear stress along the Earth's free surface (taken as horizontal), Anderson (1905) postulated the existence of three basic stress regimes in the crust depending which of the principal compressive stresses (σ 1 > σ 2 > σ 3) coincides with the vertical stress, σ v. Normal faulting prevails where the crust is under extension with σ v = σ 1; strike-slip faulting occurs where σ v = σ 2; and thrust faulting develops where the crust is shortening under horizontal compression with σ v = σ 3. Information from borehole measurements (Townend and Zoback 2001), earthquake focal mechanisms (Célérier 2008), and palaeostress inversions (Lisle et al. 2006) demonstrates that ‘Andersonian’ stress states (one principal stress vertical and the other two horizontal) prevail over large areas of continental and oceanic crust (Zoback 1992). These ‘Andersonian’ stress orientations provide useful reference states for general consideration, though significant departures undoubtedly occur. For example, although a subduction interface shear zone (SISZ) is, in essence, a large-scale thrust fault accommodating underthrusting of oceanic lithosphere, significant departures from vertical and horizontal stress trajectories are expected from: (1) the kinematic control exerted by a weak plate boundary shear zone; (2) force balance analyses associated with a tapering accretionary wedge (e.g. Dahlen 1990); and (3) time variations in the stress field as a consequence of stress cycling associated with intermittent megathrust rupturing along the SISZ (Hasegawa et al. 2012).
Hubbert and Rubey (1959) were the first to apply the simple principle of effective stress to rocks whereby pore-fluid pressure reduces all normal stresses to ‘effective’ values, σ n′ = (σ n − P f), suggesting also that fluid overpressures were a means of reducing basal friction allowing emplacement of thrust sheets along low-angle overthrusts. They also defined a pore-fluid factor, λ v = P f/σ v, usefully relating the level of pore-fluid pressure with respect to the vertical stress. Where fluid pressure, P f, in pore and/or fracture space is freely interconnected up to a water table at the earth’s surface, the pore-fluid pressure is hydrostatic (λ v ~ 0.4). Pore-fluids are overpressured wherever pore-fluid pressures exceed hydrostatic values (i.e. λ v > 0.4). Fluid at a depth, z, is overpressured towards lithostatic values (λ v → 1.0) when P f approaches the lithostatic load (σ v = ρgz, where ρ is average rock density and g the gravitational acceleration). Supralithostatic overpressures (λ v > 1.0) occur where P f > σ v.
In compressional settings where σ v = σ 3, the tensile overpressure state (P f > σ 3) equates to the maximum sustainable overpressure and is close to lithostatic (Sibson 2003). It is conducive to the development and fluid activation of dilatant fault–fracture meshes developed in tabular rock volumes around low-angle thrusts and may be responsible for the range of time-dependent slow-slip activity observed around such structures.
Hydraulic extension fracturing can thus only be induced: (a) within intact rock in the absence of cohesionless faults suitably oriented for shear reactivation; (b) in settings where existing fractures are severely misoriented for reshear (containing the σ 2 axis and lying at >c. 60° to σ 1); and (c) where existing fractures have regained cohesive strength through hydrothermal cementation, etc. Rock tensile strength is thus critical in determining whether rock fails in tension or in shear under increased fluid pressure. Under the same differential stress, fluid-induced failure in heterogeneous material of mixed competence (differing tensile strength) may therefore give rise to mixed-mode brittle failure with volumetric fault–fracture meshes comprising interlinked shear and extensional fractures distributed throughout the rock mass (Sibson 1996, 2000).
Vertical and lateral extent of tensile overpressure compartments
Fault–fracture meshes in compressional regimes
Here we consider geological and geophysical evidence supporting the existence of tensile overpressure compartments associated with both active and ancient low-angle thrust faults developed within continental settings.
San Gabriel Mountains bright reflective zone
The LARSE seismic line (Ryberg and Fuis 1998; Fuis et al. 2001) runs NNE-NE from Seal Beach across the Los Angeles Basin, then crosses the upthrust crystalline massif of the San Gabriel Mountains and the bounding San Andreas fault to the Mojave Desert. The San Gabriel Mountains are made up of Proterozoic age gneisses with a metamorphosed cover sequence intruded by Mesozoic granitoid plutons, all thrust over the Pelona Schist along the Vincent thrust fault. The experiment revealed the existence of a bright reflective zone below the San Gabriel Mountains with two particularly intense bright-spots, deepening from 18 km in the SW to 23 km in the NE (average dip c. 10°), more or less coincident with the base of the seismogenic zone. Recognition of a marked negative velocity step at the top of the c. 500-m-thick low-velocity zone has led to its interpretation as a lithostatically overpressured thrust-sense ductile shear zone serving as a mid-crustal décollement. Arrays of gaping flat-lying macroscopic cracks infilled with lithostatically pressured fluid and lying subparallel to shear zone foliation are inferred to account for the markedly high reflectivity of the shear zone (Ryberg and Fuis 1998).
Cooper Basin induced microearthquake swarm
Sabie—Pilgrim’s Rest Au-quartz mineralization
The evidence for incremental precipitation coupled with the low transport solubility of gold (c. 10 ppb?) suggests that intermittent high flux episodes of hydrothermal flow passed through the system. Mineralization appears to have developed under a stratigraphic cover of c. 8.2 km (corresponding to an overburden pressure of 220–250 MPa) at a temperature of c. 320 °C, typical of a mesothermal assemblage (Boer et al. 1995). The significance of the vein assemblage occupying bedding plane thrusts with subparallel (within a few degrees) extension veins is the demonstration that thrusting occurred at very close to lithostatic fluid overpressures (Harley and Charlesworth 1996).
Glarus Overthrust, Switzerland
The Lochseiten calc-mylonite was interpreted by Schmid (1975) as resulting from superplastic deformation. However, more recently Badertscher and Burkhard (2000) documented field, petrographic, and isotopic evidence for multiple episodes of cataclasis and hydrofracturing leading to vein formation (indicative of the tensile overpressure state), overprinted by ductile crystal plastic smearing of cataclasites and veins. Most of the calcite in the Lochseitenkalk tectonite thus likely originated as vein calcite. The isotopic evidence suggests that large fluid volumes were channelled along the mylonitic shear zones with periodic build-up of fluid overpressure to near-lithostatic values inducing cataclastic deformation, hydrofracturing and vein formation, followed by renewed crystal plastic deformation once discharge and drops in fluid pressure had occurred.
Similar textures and deformation histories have been invoked for calc-mylonites associated with the Gavarnie Thrust in the Pyrenees (McCaig et al. 1995) and the McConnell Thrust in Alberta (Kennedy and Logan 1997).
A strong case can be made that the megathrusts [responsible for >90% global release of seismic moment (Pacheco and Sykes 1992)] are weakened by fluid overpressuring to near-lithostatic values (λ v → 1.0). The hosting subduction interface shear zones (SISZ) likely contain a mélange assemblage of entrained fluid-rich ocean floor sediments (muds, siliceous oozes, etc.), plus trench wall sediments (turbidite sands and muds) along with similar material from the accretionary prism, together with slivers of oceanic crust and occasional seamount volcanics (Von Huene and Scholl 1991; Kimura et al. 2012). Overpressures within the material entrained in SISZ probably arise through a combination of compaction under increasing mean stress together with metamorphic dehydration of the descending oceanic crust under rising temperature (Saffer and Tobin 2011). Force balance analyses taking account of surface and Moho topography limit depth-averaged shear stress along subduction interfaces to <40 MPa (Wang and Suyehiro 1999; Lamb 2006; Seno 2009), as does the lack of evidence for significant shear heating along the interface (Peacock 2004), and the inference of total shear stress release during large megathrust ruptures (Hasegawa et al. 2012). This equates to an ‘effective friction coefficient’ μσ′ ≈ μσ(1 − λ v) of 0.03 averaged over the full depth (c. 40 km) of the seismogenic megathrust (Sibson 2014). Even for the lowest measured friction coefficients (μσ ~ 0.1 for saponite-rich gouge) overpressures with λ v > 0.7 are required to match this effective friction coefficient.
These inferences are supported by geophysical imaging which reveals thin (several km scale) tabular shear zones, locally highly reflective and with anomalously low V P, high V P/V S and low Q (e.g. Kodaira et al. 2002; Abers 2005; Eberhardt-Phillips and Reyners 1999; Song et al. 2009) thought to represent fluid-rich low-permeability SISZ. In a profile across the Costa Rica subduction margin, Bangs et al. (2015) found a high-reflectivity interface extending to depths of c. 6 km, implying an overpressured fluid-rich interface drained by arrays of fluid-rich faults cutting through the hanging wall. MT electrical imaging provides additional constraints on fluid content. For instance, the seismogenic portion of the Cascadia subduction interface below Vancouver Island coincides with an inclined tabular zone of high electrical conductivity (Kurtz et al. 1990; Soyer and Unsworth 2006). In southwest Japan, likewise, a highly reflective low-velocity layer dips c. 7°NW below Shikoku Island, down-dip from a subducting seamount on the inner wall of the Nankai Trough, defining the subduction interface that ruptured in the 1946 Mw8.1 Nankaido megathrust earthquake. This structure also appears to represent a zone of high electrical conductivity which Kodaira et al. (2002) interpret as an overpressured fluid-rich layer. Contained fluids are inferred to be predominantly aqueous (including free water within pore/fracture space and bound water within hydrous minerals) with lesser CO2 and hydrocarbons.
Stress heterogeneity within a subduction interface shear zone (SISZ)
Within mélange formations in the Shimanto Belt of SW Japan, Kimura et al. (2012) noted the predominance of quartz ± calcite extension veins developed in sandstone phacoids, roughly orthogonal to their long axes. The stress state inside a phacoid is dominated by fibre stresses imposed by viscous drag along the boundaries of competent layers (Lloyd et al. 1982; Needham 1987) giving rise to ‘pinch and swell’ features (sometimes involving low-angle Riedel shears), boudinage, and extension fractures/veins developed orthogonal to the long axes of phacoids. Comparable systems of predominantly quartz extension veins disrupt competent phacoids in the Chrystalls Beach mélange of SE Otago, NZ, a mixed continuous–discontinuous shear zone developed in an accretionary setting of Triassic age (Fagereng 2011). However, as well as extension veins developed in the phacoids, innumerable, incrementally developed quartz (and, locally, calcite) slickenfibre veins with consistent shear sense are developed on shear surfaces lying subparallel to the flat-lying matrix foliation. Again, the extension veins are predominantly orthogonal to phacoid long axes and near-perpendicular to the flat-lying foliation developed in the surrounding mud-rich matrix. Microstructural analyses demonstrate that the flat-lying slickenfibre shears and near-orthogonal extension veins in relatively competent sandstone phacoids were coeval, together forming a fault–fracture mesh accommodating brittle shearing along the foliation (Fagereng et al. 2010). It appears that because of the large rotational strains developed within a SISZ, stress states induced within extending phacoids are locally dominant and distinct from the far-field stress field driving thrust-shearing across the SISZ (Fig. 10).
Low-permeability seals within SISZ
For near-lithostatic overpressures to be developed and maintained within a SISZ, bulk permeability must average <10−20 m2, or an order of magnitude lower if the overpressures are contained by a thin (<1 km) caprock seal (Peacock et al. 2011). Among contributing factors to the development of such low permeabilities, the first is the likely presence of a high proportion of fine-grained silicic mudrock and the second, the active hydrothermal environment (150 < T < 350 °C) within at least the lower half of the seismogenic SISZ. Under such conditions solution transfer will tend to reduce porosity and infills fractures with hydrothermal precipitates over short distance and time scales, contributing to silicification seals and adding cohesive strength (Rutter 1976; Kawabata et al. 2007; Rowe et al. 2009; Fisher and Brantley 2014). In addition, high strain shearing during progressive metamorphism will impose a foliation defined by aligned phyllosilicates oblique, but subparallel to the SISZ margins giving rise to permeability anisotropy with foliation-perpendicular permeability significantly lower than that parallel to foliation, thereby impeding vertical fluid transport. Formation of fault–fracture permeability during ongoing deformation competes with permeability reduction as a consequence of hydrothermal precipitation.
SISZ host seismogenic megathrusts which typically rupture at intervals of 100–1000 years, or so. Permeability within a SISZ thus varies with time, changing from extremely low values pre-failure to high localized values postfailure when high fracture permeability is expected within rupture zones with fractures predominantly aligned subparallel to foliation in the SISZ. Postfailure ‘seismogenic permeability’ may be in the range, 10−16–10−13 m2 (Talwani et al. 2007), but will likewise be anisotropic with higher values subparallel to the foliation and the margins of the SISZ. These high postfailure permeabilities are, however, likely to be transitory diminishing rapidly through the aftershock period as a consequence of fracture healing and cementation in the active hydrothermal environment (cf. Bosl and Nur 2002).
A potentially important additional source of silica-saturated fluid comes from serpentinization of the forearc mantle (Hyndman et al. 2015), with fluid channelled along the SISZ to create low-permeability silicification caps along the SISZ hanging wall at depths of 25–40 km characterized by high V P/V S from silica enrichment (Audet and Bürgmann 2014). In combination with the effects of solution transfer within SISZ, silicification arising from serpentinization has multiple effects—restoring cohesive strength to existing fractures, reducing porosity, precipitating low-permeability seals, and promoting overpressuring in areas of fluid release.
Association with anomalous slip behaviour
Along northern portions of the Hikurangi subduction margin offshore from Gisborne, New Zealand, Bell et al. (2010) have demonstrated an association between low-dipping (0°–9°) areas of the subduction interface at shallow depth (c. 10 ± 5 km) that are undergoing periodic slow-slip with anomalously high seismic reflectivity in regions where the interface is locally upwarped by subducting seamounts. The anomalous reflectivity is attributed to overpressured fluid-rich sediments. At depths of 25–25 km along the Tokai segment of the Nankai Trough subduction system, immediately northeast of the 1944 Mw8.1 Tonankai megathrust rupture, Kodaira et al. (2004) find an association between the 60 km × 60 km areal extent of a slow-slip event on the interface locally dipping c. 16° and a highly reflective portion of the plate interface directly overlying subducted oceanic crust with anomalously high Poisson’s ratio denoting strong fluid overpressuring. The down-dip limit to seismogenic activity in the Nankai subduction system is defined by a belt of non-volcanic tremor [NVT—equivalent to episodic tremor and slip (ETS) in Cascadia] at depths of c. 35–45 km (Obara 2002). Very-low-frequency earthquakes with extremely low stress drops located in the Nankai accretionary prism are likewise attributed to rupturing of fluid-saturated rock under extreme overpressure (Ito and Obara 2006). A high-resolution study of this belt by Shelly et al. (2006) reveals a band of low-frequency earthquakes (LFEs) along the subduction interface dipping 10° NNW overlying a band of microearthquakes within the oceanic crust of the subducting lithosphere which, from its V P/V S signature, appears to be fluid-rich from metamorphic dewatering and highly overpressured.
The hypothesis advanced here is that local attainment of the tensile overpressure state (P f > σ 3) is associated with formation and activation of distributed fault–fracture meshes that are capable of giving rise to a variety of anomalous slow-slip phenomena (LFEs, VLFEs, NVT, etc.). A great deal of circumstantial evidence supports this hypothesis, but further testing is clearly needed for full substantiation.
Geological and geophysical field evidence suggests that the tensile overpressure state (creating tensile overpressure compartments where σ 3′ = (σ 3 − P f) < 0, and hydraulic fracturing is widespread) is locally developed in both sub-greenschist and greenschist metamorphic assemblages. An association with active low-angle thrust faults (dips mostly <15°) in compressional regimes (including continental thrusts and subduction thrust interfaces) is sometimes evident from inferred geophysical characteristics (high seismic reflectivity, anomalously high V P/V S, and high electrical conductivity), suggesting that ~lithostatic levels of fluid overpressuring (i.e. P f ~ σ v) are locally achieved. In some instances, a relationship between potential tensile overpressure compartments and different varieties of anomalous slip phenomena (slow-slip events, LFEs, VLFEs, NVT, etc.) is apparent.
Circumstances contributing to the development of distributed fault–fracture meshes in tensile overpressure compartments include: (1) varying competence within the rock mass (e.g. in a mélange formation) inducing diverse modes of brittle failure (Fagereng and Sibson 2010; Fagereng 2011; Kimura et al. 2012); (2) extensive solution transfer of silica and other hydrothermal materials following dissolution along foliation which serves to reduce existing porosity, helps form low-permeability seals promoting development of overpressure, and restores cohesion across existing fractures preventing shear localization (Kawabata et al. 2007; Rowe et al. 2009). For example, the intensely veined Rodeo Cove thrust zone at Marin Headlands within the Franciscan Complex of California has many of the hallmarks of a subduction fault–fracture mesh assemblage (Meneghini and Moore 2007).
I thank the organizers and, in particular, Professor Yoshihisa Iio for making it possible for me to participate in the International Symposium ‘Crustal Dynamics 2016: Unified Understanding of Geodynamics Processes at Different Time and Length Scales’, held in Takayama City. I would also like to thank two anonymous reviewers for insightful and helpful criticism of this manuscript.
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