Tensile Overpressure Compartments on Low-Angle Thrust Faults

Sibson Earth, Planets and Space (2017) 69:113 DOI 10.1186/s40623-017-0699-y

FULL PAPER Open Access Tensile overpressure compartments on low‑angle thrust faults Richard H. Sibson*

Abstract Hydrothermal extension veins form by hydraulic fracturing under triaxial stress (principal compressive stresses, σ1 > σ2 > σ3) when the pore-fuid pressure, Pf, exceeds the least compressive stress by the rock’s tensile strength. Such veins form perpendicular to σ3, their incremental precipitation from hydrothermal fuid often refected in ‘crack- seal’ textures, demonstrating that the tensile overpressure state, σ3′ (σ3 Pf) < 0, was repeatedly met. Systematic arrays of extension veins develop locally in both sub-metamorphic and= metamorphic− assemblages defning tensile overpressure compartments where at some time Pf > σ3. In compressional regimes (σv σ3), subhorizontal extension veins may develop over vertical intervals <1 km or so below low-permeability sealing= horizons with tensile strengths 10 < To < 20 MPa. This is borne out by natural vein arrays. For a low-angle thrust, the vertical interval where the tensile overpressure state obtains may continue down-dip over distances of several kilometres in some instances. The overpressure condition for hydraulic fracturing is comparable to that needed for frictional reshear of a thrust fault lying close to the maximum compression, σ1. Under these circumstances, especially where the shear zone material has varying competence (tensile strength), afecting the failure mode, dilatant fault–fracture mesh structures may develop throughout a tabular rock volume. Evidence for the existence of fault–fracture meshes around low-angle thrusts comes from exhumed ancient structures and from active structures. In the case of megathrust ruptures along subduction interfaces, force balance analyses, lack of evidence for shear heating, and evidence of total shear stress release during earthquakes suggest the interfaces are extremely weak (τ < 40 MPa), consistent with weakening by near- lithostatically overpressured fuids. Portions of the subduction interface, especially towards the down-dip termination of the seismogenic megathrust, are prone to episodes of slow-slip, non-volcanic tremor, low-frequency earthquakes, very-low-frequency earthquakes, etc., attributable to the activation of tabular fault–fracture meshes at low σ3′ around the thrust interface. Containment of near-lithostatic overpressures in such settings is precarious, fuid loss curtailing mesh activity.

Introduction σv. Normal faulting prevails where the crust is under Material properties aside, the formation and reactiva- extension with σv = σ1; strike-slip faulting occurs where tion of brittle faults and fractures is largely governed by σv = σ2; and thrust faulting develops where the crust is the triaxial stress state within the rock mass and by the shortening under horizontal compression with σv = σ3. pore-fuid pressure, Pf, within the rock mass (Hubbert Information from borehole measurements (Townend and Rubey 1959; Jaeger and Cook 1979). Recognizing the and Zoback 2001), earthquake focal mechanisms (Célé- boundary condition of zero shear stress along the Earth’s rier 2008), and palaeostress inversions (Lisle et al. 2006) free surface (taken as horizontal), Anderson (1905) pos- demonstrates that ‘Andersonian’ stress states (one princi- tulated the existence of three basic stress regimes in the pal stress vertical and the other two horizontal) prevail crust depending which of the principal compressive over large areas of continental and oceanic crust (Zoback stresses (σ1 > σ2 > σ3) coincides with the vertical stress, 1992). Tese ‘Andersonian’ stress orientations provide useful reference states for general consideration, though signifcant departures undoubtedly occur. For exam- *Correspondence: [email protected] Department of Geology, University of Otago, Dunedin 9054, New Zealand ple, although a subduction interface shear zone (SISZ)

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is, in essence, a large-scale thrust fault accommodating Controls on brittle failure of rock underthrusting of oceanic lithosphere, signifcant depar- Criteria for all modes of brittle failure in rock include tures from vertical and horizontal stress trajectories are the levels of diferential stress and pore-fuid pressure ′ expected from: (1) the kinematic control exerted by a [through the principle of efective stress, σn = (σn − Pf)]. weak plate boundary shear zone; (2) force balance anal- Te two drivers to failure, therefore, are (σ1 − σ3) and Pf, yses associated with a tapering accretionary wedge (e.g. which may act separately or together. In ‘classical’ rock Dahlen 1990); and (3) time variations in the stress feld mechanics (e.g. Secor 1965; Jaeger and Cook 1979; Sib- as a consequence of stress cycling associated with inter- son 2000), the composite Grifth–Coulomb failure curve ′ mittent megathrust rupturing along the SISZ (Hasegawa for intact rock in τ/σn space can be normalized to nomi- et al. 2012). nal rock tensile strength, To (half the cohesive strength). Hubbert and Rubey (1959) were the frst to apply the Te ratio of diferential stress to rock tensile strength simple principle of efective stress to rocks whereby (σ1 − σ3)/To then determines the potential mode of brit- pore-fuid pressure reduces all normal stresses to ‘efec- tle failure (Fig. 1). For ‘generic’ internal friction, μi 0.75 ′ = tive’ values, σn = (σn − Pf), suggesting also that fuid in the compressional feld, extension fractures form only overpressures were a means of reducing basal fric- when (σ1 − σ3)/To < 4, hybrid extensional-shear fractures tion allowing emplacement of thrust sheets along low- require 4 < (σ1 − σ3)/To < 5.66, and compressional shear angle overthrusts. Tey also defned a pore-fuid factor, fractures (faults) form when (σ1 − σ3)/To > 5.66 (Secor λv = Pf/σv, usefully relating the level of pore-fuid pres- 1965). Additionally, as noted above, the presence of exist- sure with respect to the vertical stress. Where fuid ing fractures appropriately oriented for reshear within pressure, Pf, in pore and/or fracture space is freely inter- the stress feld suppresses the formation of any new connected up to a water table at the earth’s surface, the extensional or hybrid extensional-shear fractures. Fluid pore-fuid pressure is hydrostatic (λv ~ 0.4). Pore-fuids circulation through a fractured rock mass may, how- are overpressured wherever pore-fuid pressures exceed ever, restore cohesive and tensile strength by hydrother- hydrostatic values (i.e. λv > 0.4). Fluid at a depth, z, is mal cementation (e.g. silicifcation), efectively restoring overpressured towards lithostatic values (λv → 1.0) when ‘intact’ character. Pf approaches the lithostatic load (σv = ρgz, where ρ is Hydraulic extension fracturing can thus only be average rock density and g the gravitational acceleration). induced: (a) within intact rock in the absence of cohe- Supralithostatic overpressures (λv > 1.0) occur where sionless faults suitably oriented for shear reactivation; (b) Pf > σv. in settings where existing fractures are severely misori- Formation and opening of extension fractures per- ented for reshear (containing the σ2 axis and lying at >c. pendicular to least compressive stress, σ3, in intact rock 60° to σ1); and (c) where existing fractures have regained occurs by hydraulic fracturing when cohesive strength through hydrothermal cementation, Pf = σ3 + To (1) etc. Rock tensile strength is thus critical in determining whether rock fails in tension or in shear under increased where To is rock tensile strength, provided (σ1 − σ3) < 4 fuid pressure. Under the same diferential stress, fuid- To inhibiting shear failure of the rock (Secor 1965; Ether- induced failure in heterogeneous material of mixed com- idge 1983). For competent sedimentary rocks and crys- petence (difering tensile strength) may therefore give talline rocks with To ~ 10 MPa and ~20 MPa, respectively rise to mixed-mode brittle failure with volumetric fault– (Lockner 1995), diferential stresses are restricted to <40 fracture meshes comprising interlinked shear and exten- and <80 MPa. A local absence of existing fractures suit- sional fractures distributed throughout the rock mass ably oriented for shear reactivation is also a prerequi- (Sibson 1996, 2000). site. Te tensile overpressure state (Pf > σ3) thus defnes portions of the rock mass with the potential for hydrau- Tensile overpressure compartments lic fracturing leading to the formation of extension vein Extension veins [often with ‘crack-seal’ infllings of arrays. quartz, calcite, etc., refecting incremental precipita- In compressional settings where σv = σ3, the tensile tion (Ramsay 1980)] thus diagnose the tensile overpres- ′ overpressure state (Pf > σ3) equates to the maximum sure state (σ3 = (σ3 − Pf) < 0, or Pf > σ3) once having sustainable overpressure and is close to lithostatic (Sib- been achieved within the rock mass. In extensional son 2003). It is conducive to the development and fuid (σv = σ1) and strike-slip (σv = σ2) stress regimes where activation of dilatant fault–fracture meshes developed in σ3 is horizontal, vertical extension veins may form under tabular rock volumes around low-angle thrusts and may hydrostatic levels of fuid pressures in the near-sur- be responsible for the range of time-dependent slow-slip face (z < 1–2 km), but suprahydrostatic fuid pressures activity observed around such structures. are required for their formation at depths more than a Sibson Earth, Planets and Space (2017) 69:113 Page 3 of 15

Extension Fractures Shear Fractures (Faults)

3 3

1 1

( 1 3)/ < 4(1 3)/ > 5.66

Extensional-Shear Fractures

4 < ( 1 3)/ < 5.66

Fig. 1 Potential modes of brittle failure within intact isotropic rock under horizontal compression (σ σ ), in relation to axes of principal compres- v = 3 sive stress (σ1 > σ2 > σ3)

kilometre or so (Secor 1965). In compressional regimes form, below which, provided pore/fracture space is inter- (σv = σ3), however, supralithostatic overpressures are connected, the pressure gradient reverts to hydrostatic required at all depths for the formation of subhorizon- (Sibson and Scott 1998). Te supralithostatic overpres- tal extension veins unless signifcant stress heterogeneity sure condition, Pf > σv, will be maintained below the base exists such that σv < σ3. Systematic arrays of hydrother- of the seal over a vertical interval: mal extension veins are widely developed in both sub- �z = Ts/(ρ−ρf)g (2) greenschist and greenschist metamorphic assemblages (e.g. Fig. 2) and can be used to defne former tensile over- (where ρ is rock density, ρf is fuid density, and g is gravi- pressure compartments where Pf > σ3. tational acceleration) to a depth where the pore-fuid Compressional stress regimes (σv = σ3) are particularly pressure drops below lithostatic. efective at containing fuid overpressure because: (1) activated faults and fracture tend to be fat-lying, inhibit- Vertical and lateral extent of tensile overpressure ing vertical fow and (2) under the same diferential stress, compartments faults and fractures in compressional regimes are gener- Variations in rock tensile strength allow horizontal hydro- ally capable of sustaining higher overpressures before fracturing throughout the vertical interval of supralitho- activation to provide drainage paths than in other tec- static overpressure (Fig. 3). Sealing horizons may also tonic regimes (Sibson 2003). As with compartmentalized migrate vertically with time. Tensile rock strength ranges overpressures developed in sedimentary basins (Hunt from 1 to 10 MPa for sedimentary rocks of low to mod- 1990), development of overpressure generally requires erate competence, to 20 MPa or so for high-competence a low-permeability sealing horizon perhaps associated crystalline rocks (Lockner 1995). For Ts = 10–20 MPa, with silicifcation or some other form of hydrothermal Δz = 600–1200 m. Tese values compare with the verti- cementation. If the sealing horizon has a tensile strength, cal extent of horizontal vein arrays observed in Panasque- Ts, overpressure will increase across it to a maximum ira Mine, Portugal (Foxford et al. 2000) and Sigma Mine, value, Pf = σ3 + Ts, at its base, where hydrofractures Val d’Or (Robert and Brown 1986). If the seal to a tensile Sibson Earth, Planets and Space (2017) 69:113 Page 4 of 15

Fig. 2 Systematic arrays of hydrothermal extension veins: a quartz vein array inflling subvertical extension fractures in Carboniferous sandstone (strike-slip regime), Millook, North Devon; b array of subhorizontal gold-bearing quartz veins, Damang Mine, Ghana (Tunks et al. 2004; photograph courtesy Andrew Tunks)

FLUID PRESSURE

Pf = 3 + Ts x = z cot

seal ? ? > lithostatic ? z z Pf > 3 ? overpressure ?

hydrostatic

Shear zone with residual DEPTH cohesive strength

? Fig. 3 Vertical and lateral extent of hydrofracture arrays below a sealing horizon in a compressional stress regime. Potential hanging-wall seal discussed in text indicated by dashed shading

overpressure compartment extends across a low-angle laterally for a distance, L = cotδΔz. For Δz = 1 km and thrust-sense shear zone with dip, δ (perhaps retaining low δ = 10°, 5°, and 1°, L = 5.7, 11.4, and 57 km, respectively. cohesive strength through hydrothermal cementation— Tensile overpressure compartments may thus extend Fig. 3), the tensile overpressure compartment may extend laterally for considerable distances around low-dipping Sibson Earth, Planets and Space (2017) 69:113 Page 5 of 15

thrust faults that have regained some cohesion. In real- being restricted to discrete planar slip zones. An impor- ity the permeability structure is likely to be more complex tant issue (not further considered) is whether dilatation than that pictured in Fig. 3. Upward fow of overpressured accompanying opening of extension fractures and their fuids along a thrust (cf. Safer and Tobin 2011) is likely inflling with precipitates is compensated in the long run to transport solutes derived from deformation involving by volume loss in the surrounding rock mass from disso- pressure solution within fne-grained fault rocks as well as lution and solution transfer. solutes from prograde metamorphism of the footwall—all may contribute to cementation (especially silicifcation) of Frictional reshear of low‑angle thrusts the hanging wall. In a simple compressional regime with σv = σ3, a low- dipping planar discontinuity (e.g. an existing fracture or Fault–fracture meshes in compressional regimes fault, a bedding plane, or foliation) may lie at a very low Dispersed fault–fracture meshes may develop in com- angle to σ1. Te stress condition for frictional reactivation pressional regimes in a number of confgurations (Fig. 4). of an existing cohesionless plane with a friction coef- Possibilities to be considered include: (1) refraction of cient, μs, containing the σ2 axis is: ′ ′ thrust faults across competence layering (e.g. bedding) σ1/σ3 = (σ1 − Pf)/(σ3 − Pf) creating localized sites of dilatation [factors afecting the (3) = (1 + µs cot θr)/(1 − µs tan θr) degree of fault refraction across competence boundaries are discussed by Ferrill and Morris (2003)]—the overall where θr is the angle of reactivation with respect to dip envelope then depends on the ratio of extension frac- σ1 (Sibson 1985). For the situation where σv = σ3, this tures to thrust-sense shear fractures; (2) dilational thrust expression may be rewritten as: stepovers/jogs arising from stress heterogeneity associ- (σ1 − σ3)/(σ3−Pf) = (σ1 − σ3)/(σv−Pf) ated with en echelon thrust segmentation; (3) distributed = µs[(tan θr + cot θr)/(1 − µs tan θr)] fault–fracture meshes developing within a mixed com- (4) petence mélange shear zone (Sibson 1996; Fagereng and Sibson 2010). To a greater or a lesser extent, fault–frac- defning the reactivation condition plotted in Fig. 5 ture meshes occupy a substantial rock volume rather than for μs = 0.6 [the lower end of Byerlee’s (1978) range for

Fig. 4 Development of fault–fracture meshes in a compressional thrust regime: a low-angle thrust refracting across more competent (high tensile strength) layers which fail by extension fracturing; b extension fracturing localized in a dilatational stepover (jog) between en echelon thrust faults; c extension fracturing concentrated in an aggregate of high-competence pods in a mélange formation Sibson Earth, Planets and Space (2017) 69:113 Page 6 of 15

San Gabriel Mountains bright refective zone = v 3 Te LARSE seismic line (Ryberg and Fuis 1998; Fuis 15 et al. 2001) runs NNE-NE from Seal Beach across the 1 Los Angeles Basin, then crosses the upthrust crystalline r massif of the San Gabriel Mountains and the bounding - ) 1 3 San Andreas fault to the Mojave Desert. Te San Gabriel ( v - Pf) Mountains are made up of Proterozoic age gneisses with a metamorphosed cover sequence intruded by Meso- zoic granitoid plutons, all thrust over the Pelona Schist 10 along the Vincent thrust fault. Te experiment revealed the existence of a bright refective zone below the San Gabriel Mountains with two particularly intense bright- spots, deepening from 18 km in the SW to 23 km in the s = 0.6 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 optimal orientation 5 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 fat-lying macroscopic cracks inflled with lithostatically pressured fuid and lying subparallel to shear zone foliation are inferred to account for the markedly high refectivity of the shear zone (Ryberg and Fuis 1998).

Cooper Basin induced microearthquake swarm 0 10° 20° 30° 40° Te Cooper Basin in north-eastern South Australia con- r tains a cover sequence of Late Carboniferous—Triassic Fig. 5 Reactivation factor (σ σ )/(σ P ) plotted against the 1 − 3 3 − f sediments, some 3.6 km thick, overlying a mid-Carbonif- reactivation angle, θr (in accordance with Eq. 4), for shallow-dipping thrust faults with σ σ erous granitic basement. Te contemporary stress regime v = 3 is one of regional compression with σ3 vertical and regional σ1 oriented 082° ± 5° (Holl and Barton 2015). →∞ Te basement assemblage below the insulating blanket hard-rock friction]. Te ratio (σ1 σ3)/(σv Pf) − − of cover sediments is a signifcant source of radiogenic as θr → 59° or 0°, requiring Pf → σv. In the case of a thrust fault that retains (or has acquired) some cohesive heat, and the region has been assessed as an enhanced strength, c, the fuid pressure requirement for reactiva- geothermal system (EGS) with a temperature of c. 250 °C tion of faults approaching frictional lock-up becomes encountered at the base of a borehole some 4.25 km deep (Baisch et al. 2006). A series of injection experiments supralithostatic (Pf = σv + c/μs) (Sibson 2009). Te essen- tial point is that the overpressure condition needed to down diferent wells produced a tabular, fat-lying micro- induce thrust reshear along thrusts at very low angles earthquake swarm <100–150 m thick extending initially over an area of 2 km 1.5 km, but expanding signif- to σ1 (Pf → σv) is near-lithostatic or even supralithos- × tatic and very close to the hydraulic fracturing criterion. cantly with further injection experiments (Baisch et al. Under such circumstances, slight heterogeneities in 2009, 2015; Cox 2016), and dipping very gently WSW the stress feld or in rock material properties may allow (Fig. 6). Of the 525 focal mechanisms obtained from the hydraulic extension veins to form subparallel to thrust- induced microearthquake swarm, the great majority yield reactivated planes. thrust solutions with one very low-angle nodal plane dipping c. 10° W parallel to the sheet dip of the swarm, but a Tensile overpressure compartments few have more moderately dipping nodal planes (Baisch along low‑angle intracontinental thrusts et al. 2015). Fluid pressure during injection at the bottom Here we consider geological and geophysical evidence of the wells approached lithostatic or slightly supralitho- supporting the existence of tensile overpressure com- static (Holl and Barton 2015). Te activated structure partments associated with both active and ancient hosting the swarm is generally interpreted as a simple low-angle thrust faults developed within continental planar thrust fault, but the volumetric character of the settings. swarm, together with its very low-angle attitude, suggests Sibson Earth, Planets and Space (2017) 69:113 Page 7 of 15

Fig. 6 Schematic of Cooper Basin geothermal feld with gently inclined tabular belt of induced microearthquakes (not to scale) and inferred fault– fracture mesh developed within a compressional stress feld with σ σ v = 3 it may represent a distributed fault–fracture mesh (Sib- carbonate-siliciclastic sequence exposed along the west- son 1996) comprising interlinked minor thrusts and erly dipping rim of the Palaeoproterozoic Transvaal Basin extension fractures developed by hydrofracturing (Fig. 6). (Harley and Charlesworth 1992, 1996). Au-quartz min- Note that this swarm was induced by comparatively eralization is largely hosted along bedding plane thrusts rapid overpressuring, and the length of time for which and is likely coeval with early stages of the emplacement overpressuring may be maintained in comparison with of the Bushveld igneous complex (c. 2060 Ma) (Boer et al. natural settings, given the possibility of subcritical crack 1995). In the Elandshoogte Mine, thin (0.2–0.5 m, locally growth, etc., is questionable. Nonetheless, at the very <2 m) zones of laminated, incrementally deposited Au- least the swarm activity provides an example of thrusting quartz mineralization resulting from multiple episodes activity on low-angle planes almost perpendicular to σ3 of brecciation and hydrothermal cementation are con- induced by close-to-lithostatic fuid overpressures. centrated along bedding plane thrusts dipping c. 5°W. Te mineralized bedding plane shears are sometimes Sabie—Pilgrim’s Rest Au‑quartz mineralization imbricated in thrust duplexes and are also fanked by Te Sabie—Pilgrim’s Rest goldfeld of the Eastern Trans- bands of silicifcation in which fat-lying extension veins vaal in South Africa is hosted within the lower Transvaal occur subparallel to the bedding plane thrusts (Fig. 7). Sibson Earth, Planets and Space (2017) 69:113 Page 8 of 15

Fig. 7 a Setting of bedding plane thrusts hosting Au-quartz mineralization in the Sabie—Pilgrim’s Rest goldfeld, Eastern Transvaal, South Africa; b fat-lying hydraulic extension veins in silicifed hanging wall to thrust; c mineralized thrust fault showing repeated fracturing with brecciation and cementation

Subordinate steep (sometimes subvertical) leader veins Glarus Overthrust, Switzerland also occur locally throughout the goldfeld. Plausibly, Te Glarus Overthrust of Oligocene–Miocene age is well they formed from time to time as a consequence of tran- exposed at elevations approaching 3000 m in the Swiss sitory switching to an extensional stress feld following Alps as a generally fat-lying but arched thrust contact total shear stress release along the low-angle thrusts. (Fig. 8) between overlying Mesozoic carbonates and the Te evidence for incremental precipitation coupled Permian Veruccano formation emplaced over Tertiary with the low transport solubility of gold (c. 10 ppb?) Flysch (Herwegh et al. 2008). Te present fat-lying atti- suggests that intermittent high fux episodes of hydro- tude of the Glarus Trust difers from its likely low-angle thermal fow passed through the system. Mineralization (c. 15°S) orientation when active (Pffner 1986). Over appears to have developed under a stratigraphic cover of 35 km of northward translation has occurred across an c. 8.2 km (corresponding to an overburden pressure of ~1-m-thick shear zone of shear zone of highly deformed 220–250 MPa) at a temperature of c. 320 °C, typical of a carbonate—the Lochseiten tectonite—with crystal plas- mesothermal assemblage (Boer et al. 1995). Te signif- tic deformation occurring over temperatures ranging cance of the vein assemblage occupying bedding plane from 230° to 360 °C with increasing palaeodepth. thrusts with subparallel (within a few degrees) exten- Te Lochseiten calc-mylonite was interpreted by sion veins is the demonstration that thrusting occurred Schmid (1975) as resulting from superplastic deforma- at very close to lithostatic fuid overpressures (Harley and tion. However, more recently Badertscher and Burkhard Charlesworth 1996). (2000) documented feld, petrographic, and isotopic Sibson Earth, Planets and Space (2017) 69:113 Page 9 of 15

Fig. 8 Cartoon of Glarus overthrust, Switzerland, illustrating ~35 km NW translation of hanging wall (after Badertscher and Burkhard 2000)

evidence for multiple episodes of cataclasis and hydro- Similar textures and deformation histories have been fracturing leading to vein formation (indicative of the invoked for calc-mylonites associated with the Gavarnie tensile overpressure state), overprinted by ductile crys- Trust in the Pyrenees (McCaig et al. 1995) and the tal plastic smearing of cataclasites and veins. Most of the McConnell Trust in Alberta (Kennedy and Logan 1997). calcite in the Lochseitenkalk tectonite thus likely origi- nated as vein calcite. Te isotopic evidence suggests that Tensile overpressure compartments large fuid volumes were channelled along the mylonitic along subduction interfaces shear zones with periodic build-up of fuid overpressure Megathrust ruptures hosted within subduction interface to near-lithostatic values inducing cataclastic deforma- shear zones (SISZ) (Fig. 9) are also low-angle structures, tion, hydrofracturing and vein formation, followed by dipping <5° in the vicinity of the trench inner wall to c. renewed crystal plastic deformation once discharge and 15° around the maximum down-dip limit of the rup- drops in fuid pressure had occurred. tures at depths of 40 ± 5 km (governed by isotherms in

50 km

c. 5 km ! ! ! ! ! ! ! ! S I S Z. ! !

oblique shear dilatant extension fractures zone foliation

from high competence ! ! solution transfer boudins

Fig. 9 a Schematic of megathrust rupture lying within a subduction interface shear zone (SISZ) based on the 2011 Tohoku-Oki M w9.0 earthquake, northern Japan; b possible structural features infuencing fast- and slow-slip rupturing within a SISZ (schematic and not to scale) Sibson Earth, Planets and Space (2017) 69:113 Page 10 of 15

the 350–450 °C range), perhaps 150–200 km inboard subduction interface below Vancouver Island coincides from the trench axis (Hyndman 2007; Lay et al. 2012). with an inclined tabular zone of high electrical conduc- Details of the internal structure of SISZ and the physi- tivity (Kurtz et al. 1990; Soyer and Unsworth 2006). In cal conditions prevailing therein are inevitably specula- southwest Japan, likewise, a highly refective low-velocity tive, but are constrained by geophysical observations layer dips c. 7°NW below Shikoku Island, down-dip from and the structural characteristics of analogous exhumed a subducting seamount on the inner wall of the Nankai thrust zones. Mélange formations so typical of exhumed Trough, defning the subduction interface that ruptured SISZ (Fagereng 2011; Kimura et al. 2012) are of particular in the 1946 M w8.1 Nankaido megathrust earthquake. interest because of the likelihood of extreme stress and Tis structure also appears to represent a zone of high competence (i.e. tensile strength) heterogeneity within electrical conductivity which Kodaira et al. (2002) inter- such formations (Fagereng and Sibson 2010). pret as an overpressured fuid-rich layer. Contained fuids A strong case can be made that the megathrusts are inferred to be predominantly aqueous (including free [responsible for >90% global release of seismic moment water within pore/fracture space and bound water within (Pacheco and Sykes 1992)] are weakened by fuid over- hydrous minerals) with lesser CO2 and hydrocarbons. pressuring to near-lithostatic values (λv → 1.0). Te hosting subduction interface shear zones (SISZ) likely contain Stress heterogeneity within a subduction interface shear a mélange assemblage of entrained fuid-rich ocean foor zone (SISZ) sediments (muds, siliceous oozes, etc.), plus trench wall Subduction mélange formations are typically charac- sediments (turbidite sands and muds) along with similar terized by relatively competent phacoids of sandstone, material from the accretionary prism, together with sliv- cherts, and metavolcanics set in a mud-rich matrix ers of oceanic crust and occasional seamount volcanics (Kimura et al. 2012). In deeper portions of the seismo- (Von Huene and Scholl 1991; Kimura et al. 2012). Over- genic zone (T > 150 °C) where ductile deformation mech- pressures within the material entrained in SISZ prob- anisms (e.g. solution transfer) come into play, progressive ably arise through a combination of compaction under simple shear leads to the development of an oblique increasing mean stress together with metamorphic dehy- foliation along X–Y planar trajectories of the fnite dration of the descending oceanic crust under rising tem- strain ellipsoid in the fne-grained mud matrix (Fig. 10), perature (Safer and Tobin 2011). Force balance analyses the degree of obliquity to the shear zone walls decreas- taking account of surface and Moho topography limit ing with increasing shear strain (Ramsay and Graham depth-averaged shear stress along subduction interfaces 1973). As noted by Bridgewater et al. (1973), the fnite X to <40 MPa (Wang and Suyehiro 1999; Lamb 2006; Seno stretching direction then ends up lying subparallel to the 2009), as does the lack of evidence for signifcant shear regional shortening direction. Where present, such folia- heating along the interface (Peacock 2004), and the infer- tion anisotropy may become a preferential failure plane ence of total shear stress release during large megathrust for propagating earthquake ruptures. ruptures (Hasegawa et al. 2012). Tis equates to an ‘efec- Within mélange formations in the Shimanto Belt of SW ′ tive friction coefcient’ μσ ≈ μσ(1 − λv) of 0.03 averaged Japan, Kimura et al. (2012) noted the predominance of over the full depth (c. 40 km) of the seismogenic megath- quartz ± calcite extension veins developed in sandstone rust (Sibson 2014). Even for the lowest measured friction phacoids, roughly orthogonal to their long axes. Te coefcients (μσ ~ 0.1 for saponite-rich gouge) overpres- stress state inside a phacoid is dominated by fbre stresses sures with λv > 0.7 are required to match this efective imposed by viscous drag along the boundaries of compe- friction coefcient. tent layers (Lloyd et al. 1982; Needham 1987) giving rise Tese inferences are supported by geophysical imag- to ‘pinch and swell’ features (sometimes involving low- ing which reveals thin (several km scale) tabular shear angle Riedel shears), boudinage, and extension fractures/ zones, locally highly refective and with anomalously low veins developed orthogonal to the long axes of phacoids. VP, high VP/VS and low Q (e.g. Kodaira et al. 2002; Abers Comparable systems of predominantly quartz exten- 2005; Eberhardt-Phillips and Reyners 1999; Song et al. sion veins disrupt competent phacoids in the Chrystalls 2009) thought to represent fuid-rich low-permeability Beach mélange of SE Otago, NZ, a mixed continuous– SISZ. In a profle across the Costa Rica subduction mar- discontinuous shear zone developed in an accretion- gin, Bangs et al. (2015) found a high-refectivity interface ary setting of Triassic age (Fagereng 2011). However, as extending to depths of c. 6 km, implying an overpres- well as extension veins developed in the phacoids, innu- sured fuid-rich interface drained by arrays of fuid-rich merable, incrementally developed quartz (and, locally, faults cutting through the hanging wall. MT electrical calcite) slickenfbre veins with consistent shear sense imaging provides additional constraints on fuid content. are developed on shear surfaces lying subparallel to the For instance, the seismogenic portion of the Cascadia fat-lying matrix foliation. Again, the extension veins are Sibson Earth, Planets and Space (2017) 69:113 Page 11 of 15

stress

trajectories of maximum 1 S I S Z. ! !

! !

quartz- shear surfaces

stress state inside stretched phacoid

Fig. 10 Schematic of subduction interface shear zone (SISZ) with oblique foliation following X–Y planar trajectory of fnite strain ellipsoid and bou- din trains of stretched competent layers. While boudins elongate parallel to the maximum fnite extension, X, the internal stress state within boudins (shown in inset) arising from fbre stresses difers markedly from the far-feld stress feld driving thrust-sense shear across the SISZ

predominantly orthogonal to phacoid long axes and near- half of the seismogenic SISZ. Under such conditions perpendicular to the fat-lying foliation developed in the solution transfer will tend to reduce porosity and inflls surrounding mud-rich matrix. Microstructural analyses fractures with hydrothermal precipitates over short dis- demonstrate that the fat-lying slickenfbre shears and tance and time scales, contributing to silicifcation seals near-orthogonal extension veins in relatively compe- and adding cohesive strength (Rutter 1976; Kawabata tent sandstone phacoids were coeval, together forming et al. 2007; Rowe et al. 2009; Fisher and Brantley 2014). a fault–fracture mesh accommodating brittle shearing In addition, high strain shearing during progressive met- along the foliation (Fagereng et al. 2010). It appears that amorphism will impose a foliation defned by aligned because of the large rotational strains developed within phyllosilicates oblique, but subparallel to the SISZ mar- a SISZ, stress states induced within extending phacoids gins giving rise to permeability anisotropy with foliation- are locally dominant and distinct from the far-feld stress perpendicular permeability signifcantly lower than that feld driving thrust-shearing across the SISZ (Fig. 10). parallel to foliation, thereby impeding vertical fuid transport. Formation of fault–fracture permeability during Low‑permeability seals within SISZ ongoing deformation competes with permeability reduc- For near-lithostatic overpressures to be developed and tion as a consequence of hydrothermal precipitation. maintained within a SISZ, bulk permeability must aver- SISZ host seismogenic megathrusts which typically age <10−20 m2, or an order of magnitude lower if the rupture at intervals of 100–1000 years, or so. Perme- overpressures are contained by a thin (<1 km) caprock ability within a SISZ thus varies with time, changing seal (Peacock et al. 2011). Among contributing factors from extremely low values pre-failure to high localized to the development of such low permeabilities, the frst values postfailure when high fracture permeability is is the likely presence of a high proportion of fne-grained expected within rupture zones with fractures predomi- silicic mudrock and the second, the active hydrothermal nantly aligned subparallel to foliation in the SISZ. Post- environment (150 < T < 350 °C) within at least the lower failure ‘seismogenic permeability’ may be in the range, Sibson Earth, Planets and Space (2017) 69:113 Page 12 of 15

10−16–10−13 m2 (Talwani et al. 2007), but will likewise Discussion and conclusions be anisotropic with higher values subparallel to the folia- Te hypothesis advanced here is that local attainment tion and the margins of the SISZ. Tese high postfailure of the tensile overpressure state (Pf > σ3) is associated permeabilities are, however, likely to be transitory dimin- with formation and activation of distributed fault–frac- ishing rapidly through the aftershock period as a conse- ture meshes that are capable of giving rise to a variety of quence of fracture healing and cementation in the active anomalous slow-slip phenomena (LFEs, VLFEs, NVT, hydrothermal environment (cf. Bosl and Nur 2002). etc.). A great deal of circumstantial evidence supports A potentially important additional source of silica-sat- this hypothesis, but further testing is clearly needed for urated fuid comes from serpentinization of the forearc full substantiation. mantle (Hyndman et al. 2015), with fuid channelled Geological and geophysical feld evidence suggests that along the SISZ to create low-permeability silicifcation the tensile overpressure state (creating tensile overpres- ′ caps along the SISZ hanging wall at depths of 25–40 km sure compartments where σ3 = (σ3 − Pf) < 0, and hydrau- characterized by high VP/VS from silica enrichment lic fracturing is widespread) is locally developed in both (Audet and Bürgmann 2014). In combination with the sub-greenschist and greenschist metamorphic assem- efects of solution transfer within SISZ, silicifcation aris- blages. An association with active low-angle thrust faults ing from serpentinization has multiple efects—restoring (dips mostly <15°) in compressional regimes (including cohesive strength to existing fractures, reducing poros- continental thrusts and subduction thrust interfaces) is ity, precipitating low-permeability seals, and promoting sometimes evident from inferred geophysical character- overpressuring in areas of fuid release. istics (high seismic refectivity, anomalously high VP/VS, and high electrical conductivity), suggesting that ~lithos- Association with anomalous slip behaviour tatic levels of fuid overpressuring (i.e. Pf ~ σv) are locally Along northern portions of the Hikurangi subduction achieved. In some instances, a relationship between margin ofshore from Gisborne, New Zealand, Bell et al. potential tensile overpressure compartments and dif- (2010) have demonstrated an association between low- ferent varieties of anomalous slip phenomena (slow-slip dipping (0°–9°) areas of the subduction interface at shal- events, LFEs, VLFEs, NVT, etc.) is apparent. low depth (c. 10 ± 5 km) that are undergoing periodic Circumstances contributing to the development of dis- slow-slip with anomalously high seismic refectivity in tributed fault–fracture meshes in tensile overpressure regions where the interface is locally upwarped by sub- compartments include: (1) varying competence within ducting seamounts. Te anomalous refectivity is attrib- the rock mass (e.g. in a mélange formation) inducing uted to overpressured fuid-rich sediments. At depths of diverse modes of brittle failure (Fagereng and Sibson 25–25 km along the Tokai segment of the Nankai Trough 2010; Fagereng 2011; Kimura et al. 2012); (2) extensive subduction system, immediately northeast of the 1944 solution transfer of silica and other hydrothermal mate- Mw8.1 Tonankai megathrust rupture, Kodaira et al. (2004) rials following dissolution along foliation which serves fnd an association between the 60 km × 60 km areal to reduce existing porosity, helps form low-permeabil- extent of a slow-slip event on the interface locally dipping ity seals promoting development of overpressure, and c. 16° and a highly refective portion of the plate interface restores cohesion across existing fractures prevent- directly overlying subducted oceanic crust with anoma- ing shear localization (Kawabata et al. 2007; Rowe et al. lously high Poisson’s ratio denoting strong fuid overpres- 2009). For example, the intensely veined Rodeo Cove suring. Te down-dip limit to seismogenic activity in the thrust zone at Marin Headlands within the Franciscan Nankai subduction system is defned by a belt of non-vol- Complex of California has many of the hallmarks of a canic tremor [NVT—equivalent to episodic tremor and subduction fault–fracture mesh assemblage (Meneghini slip (ETS) in Cascadia] at depths of c. 35–45 km (Obara and Moore 2007). 2002). Very-low-frequency earthquakes with extremely Incremental shear displacement across fault–frac- low stress drops located in the Nankai accretionary prism ture meshes necessarily involves dilatation of extension are likewise attributed to rupturing of fuid-saturated fractures coupled to increments of shear displacement rock under extreme overpressure (Ito and Obara 2006). so that they have a tendency to act as viscous ‘dashpots’ A high-resolution study of this belt by Shelly et al. (2006) slowing down slip transfer (Fig. 11). Tis allows a spec- reveals a band of low-frequency earthquakes (LFEs) trum of time-dependent behaviour depending on the along the subduction interface dipping 10° NNW overly- proportion of shear to extensional fractures, fuid viscos- ing a band of microearthquakes within the oceanic crust ity, rate of overpressure generation, etc. Such structures of the subducting lithosphere which, from its VP/VS sig- may plausibly be responsible for the range of observed nature, appears to be fuid-rich from metamorphic dewa- slow-slip behaviour including LFEs, VLFEs, NVT (or tering and highly overpressured. ETS). Maintenance of near-lithostatic fuid overpressures Sibson Earth, Planets and Space (2017) 69:113 Page 13 of 15

Fig. 11 Possible ‘volumetric’ source model for anomalous slow-slip (LFEs, VLFEs, NVT, etc.). Local development of a thrust fault–fracture mesh within a ‘log-jam’ of competent boudins developed within a SISZ. Thrust ruptures tend to follow anisotropy from oblique shear zone foliation. Slip transfer involves dilatational opening of linking extension fractures and associated fuid difusion. Equivalent rheological model whereby rate-and-state frictional instability is damped by viscous dashpot

is precarious, and it is likely that fault–fracture meshes Baisch S, Vörös R, Weidler R, Wyborn D (2009) Investigation of fault mechanisms during geothermal reservoir stimulation experiments in the overpressured to near-lithostatic levels are extremely Cooper Basin (Australia). Seismol Soc Am Bull 99:148–158 unstable. Loss of fuid from an overpressured mesh Baisch S, Rothert E, Stang H, Vörös R, Koch C, McMahon A (2015) Continued will cause frictional strengthening and a change in slip geothermal reservoir stimulation experiments in the Cooper Basin (Aus- tralia). Seismol Soc Am Bull 105:198–209. doi:10.1785/0120140208 behaviour. Bangs NL, McIntosh KD, Silver EA, Kluesner JW, Ranero CR (2015) Fluid accumu- lation along the Costa Rica subduction thrust and development of the seismogenic zone. J Geophys Res 120:67–86. doi:10.1002/2014JB01265 Acknowledgements Bell R, Sutherland R, Barker DHN, Henrys S, Bannister S, Wallace L, Beavan J I thank the organizers and, in particular, Professor Yoshihisa Iio for making (2010) Seismic refection character of the Hikurangi subduction interface, it possible for me to participate in the International Symposium ‘Crustal New Zealand, in the region of the repeated Gisborne slow slip events. Dynamics 2016: Unifed Understanding of Geodynamics Processes at Diferent Geophys J Int 180:34–48 Time and Length Scales’, held in Takayama City. I would also like to thank two Boer RH, Meyer FM, Robb LJ, Graney JR, Vennemann TW, Kesler SE (1995) anonymous reviewers for insightful and helpful criticism of this manuscript. Mesothermal-type mineralization in the Sabie-Pilgrim’s Rest gold feld, South Africa. Econ Geol 90:865–876 Bosl WJ, Nur A (2002) Aftershocks and pore fuid difusion following the 1992 Publisher’s Note Landers earthquake. J Geophys Res 107(B12):2366. doi:10.1029/200 Springer Nature remains neutral with regard to jurisdictional claims in pub- 1JB000155 lished maps and institutional afliations. Bridgewater D, Escher A, Watterson J (1973) Tectonic displacements and thermal activity in two contrasting Proterozoic mobile belts from Greensland. Received: 8 April 2017 Accepted: 31 July 2017 Phil Trans R Soc Lond A 273:513–533 Byerlee JD (1978) Friction of rocks. Pure Appl Geophys 116:615–626 Célérier B (2008) Seeking Anderson’s faulting in seismicity: a centennial cel- ebration. Rev Geophys 46:RG4001. doi:10.1029/2007RG000240 Cox SF (2016) Injection-driven swarm seismicity and permeability enhance- References ment: implications for the dynamics of hydrothermal ore systems in high Abers GA (2005) Seismic low-velocity layer at the top of subducting slabs: fuid-fux overpressured faulting regimes—an invited paper. Econ Geol observations, predictions, and systematics. Phys Earth Planet Int 149:7–29 111:559–587 Anderson EM (1905) The dynamics of faulting. Trans Edin Geol Soc 8:387–402 Dahlen FA (1990) Critical taper model of fold-and-thrust belts and accretionary Audet P, Bürgmann R (2014) Possible control of subduction slow-earthquake wedges. Ann Rev Earth Planet Sci 18:55–99 periodicity by silica enrichment. Nature 510:389–392 Eberhardt-Phillips D, Reyners M (1999) Plate interface properties in the north- Badertscher NP, Burkhard M (2000) Brittle-ductile deformation in the Glarus east Hikurangi subduction zone, New Zealand, from converted seismic thrust Lochseiten (LK) calc-mylonite. Terra Nova 12:281–288 waves. Geophys Res Lett 26:2565–2568 Baisch S, Weidler R, Vörös R, Wyborn D, DeGraaf L (2006) Induced seismicity Etheridge MA (1983) Diferential stress magnitudes during regional deforma- during the stimulation of a geothermal HFR reservoir in the Cooper Basin tion and metamorphism: upper bound imposed by tensile fracturing. (Australia). Seismol Soc Am Bull 96:2242–2256 Geology 11:231–234 Sibson Earth, Planets and Space (2017) 69:113 Page 14 of 15

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