progressive fault triggering and fluid flow in aftershock domains...

tters 250 (2006) 318–330www.elsevier.com/locate/epsl

Earth and Planetary Science Le

Progressive fault triggering and fluid flow in aftershock domains:Examples from mineralized Archaean fault systems

Steven Micklethwaite a,⁎, Stephen F. Cox a,b

a Research School of Earth Sciences, The Australian National University, ACT 0200, Australiab Department of Earth and Marine Sciences, The Australian National University, ACT 0200, Australia

Received 21 October 2005; received in revised form 28 July 2006; accepted 28 July 2006Available online 1 September 2006

Editor: R.D. van der Hilst

Abstract

In strike-slip fault systems, Coulomb failure stress changes due to mainshocks can trigger large aftershocks or further earthquakes.The combination of static stress changes from mainshocks and large aftershocks potentially has a profound influence on the finaldistribution of aftershocks and crustal-scale fluid redistribution. Because mineralization acts as a high fluid flux indicator theinteraction of static stress changes, fault triggering and fluid flow can be studied frommineralized fossil fault systems. Two examplesare presented from separate fault systems in the Kalgoorlie greenstone terrane, Western Australia (the Black Flag and Boulder-LefroyFault systems). Using mapped fault geometries, slip directions and the known distribution of fault-hosted gold mineralization weshow that the repeated arrest of mainshock ruptures, at both dilational and contractional fault step-overs, controlled aftershock-relatedfluid flow. Importantly, the largest aftershocks or subsequent triggered earthquakes exerted a very strong control on where the highestfluid fluxes occurred through small-event aftershock fault networks (at distances up to∼15 km away from the step-overs). Fluid flowthrough mid-crustal fault systems in crystalline rock is spatially localised in regions where repeated clusters of aftershocks causepermeability enhancement. It is dependent on the seismogenic behaviour of the system, rather than a passive exploitation of theinternal structure and fabrics developed by faults or damage zones. Field evidence implies that high pore fluid factors were repeatedlyattained in the aftershock-related mineralized faults and that the fluids were derived from deep-level, overpressured reservoirs, ratherthan local wall rock porosity. It is apparent that high-pressure fluids, possibly released in a pulse after a mainshock, contribute to therupture of structures already promoted towards failure from static stress changes.© 2006 Elsevier B.V. All rights reserved.

Keywords: Faults and fluids; Mineralization; Coulomb failure stress change; Aftershocks; Triggering

1. Introduction

Seismogenic processes that operated in ancientmineralized fault systems have long been thought to

⁎ Corresponding author. Tel.: +61 2 61255169; fax: +61 2 62518253.E-mail addresses: [email protected]

(S. Micklethwaite), [email protected] (S.F. Cox).

0012-821X/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.epsl.2006.07.050

have a first-order control on fluid flow and gold pre-cipitation [1–3]. On this basis, it has been recognised thatthe permeability enhancement associated with after-shocks controls the flow of mineralizing fluids aftermainshock rupture arrest [4,5]. Significantly, there ispotential to gain insights on fault behaviour, plus theimpact on crustal-scale fluid redistribution, from studiesthat link the rock record to fault processes observed inmodern, active fault systems.

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http://dx.doi.org/10.1016/j.epsl.2006.07.050

Fig. 1. Schematic map and location of the two case study sites in theKalgoorlie greenstone terrain, Western Australia. Inset shows locationof Yilgarn Craton.

319S. Micklethwaite, S.F. Cox / Earth and Planetary Science Letters 250 (2006) 318–330

Active fault processes are notoriously complex.Nonetheless, co-seismic static stress change calculationsfor fault-slip events have had substantial success at ex-plaining some aspects of earthquake behaviour. Positivestatic stress changes correlate with 60–80% of after-shocks after an earthquake [6,7] and are consistent withthe stepwise triggering of earthquakes along the SanAndreas and Anatolian fault systems [6,8]. Faults aremodelled in 3D as dislocations with an imposed slip, andthe resulting change in Coulomb failure stress is obtained[6,9]. A feature of stress transfer modelling is that it relieson parameters that, in some cases, are accessible from therock record. Indeed parameters, such as fault geometryor slip direction, can be well constrained.

Outstanding questions remain however, becausestatic stress changes do not explain observations of spa-tial and temporal evolution in aftershock sequences.Dynamic stress changes, which result from the passageof seismic waves, seem to have a role in triggering failureof unstable faults close to the mainshock rupture plane[10]. It is also possible that asthenospheric and lowercrustal stress relaxation following major rupturestransfers stress to the upper crust [11,12]. Finally, fluidredistribution after mainshocks potentially has a pro-found influence on rupture-related stress changes, whichmay explain spatial and time-dependent behaviour ob-served in some aftershock sequences [13–15]. Hydro-mechanical models of this latter behaviour tend toconsider the effects of fluid pressure changes on stressstates due to fluid redistribution from porosity surround-ing a mainshock. Alternative models consider a scenariowhere high-pressure fluids are released into a fault sys-tem from overpressured reservoirs, breached by ruptures[1,14].

Some Archaean greenstone terranes provide anopportunity to investigate fluid flow in fault systems.Gold deposits are commonly hosted on faults and occur inspaced clusters known as goldfields, commonly ∼30 kmapart. High fluid fluxes are associated with mineralization[3,16] and the fluids are localised within complexnetworks of small-displacement faults, shear zones,related veins, plus their alteration haloes [17,18]. Suchfaults are kinematically related to adjacent, large-displacement, crustal-scale faults. In low porosity, mid-crustal environments the overwhelming control on fluidflow is co-seismic fracture permeability, followed byrapid interseismic hydrothermal sealing and permeabilitydestruction [16]. Thus, with regard to goldfields, thosestructures that activated the most over the lifetime of afault system experienced enhanced fluid flow and min-eralization. These fault systems represent a novel op-portunity to test the long-term cumulative influence of

static stress changes on the behaviour of fault–fracturenetworks and their impact on fluid redistribution betweencrustal fluid reservoirs.

We describe two ancient mineralized fault networksfrom near Kalgoorlie, Western Australia (Fig. 1), whichare associated respectively with dilational and contrac-tional fault step-overs between large-displacement faultsegments. In an earlier work Cox and Ruming [4] andMicklethwaite and Cox [5] demonstrated for the firsttime that the clustered distribution of gold deposits onsmall-displacement faults can be understood broadly interms of static stress changes associated with mainshockrupture events on adjacent large-displacement compo-nents of the fault systems. Results supported the hypoth-esis that aftershocks on small-displacement faults wereresponsible for extended periods of fracture-inducedpermeability enhancement, fluid flow and mineraliza-tion. However, it was also found that significant areasdid not contain mineralization where it may be expected.

Here, we explore the impact that large aftershocks ortriggered earthquakes can have on static stress changesgenerated by the mainshock rupture. In particular weexamine how one or two triggered aftershocks or earth-quakes, on adjacent medium- to large-displacement faultscan modify the static stress change distribution from the

320 S. Micklethwaite, S.F. Cox / Earth and Planetary Science Letters 250 (2006) 318–330

mainshock rupture. We examine the correlation betweenthe modified distribution of static stress change and thedistribution of fault-hosted gold deposits, to constrainhow fluid pathways are affected by aftershock seismicityand permeability enhancement. Our field observationsargue that high-pressure fluids, from breached over-pressured reservoirs, were also involved in inducing fail-ure on those small-displacement faults already promotedto failure by static stress changes.

2. Structural architecture and gold mineralization:case studies

The Mt Pleasant and St Ives goldfields are associatedwith two fault networks in the Kalgoorlie greenstoneterrane, Western Australia [4,5] (Fig. 1). They weremineralized at different times relative to the regionalstructural sequence but share many similarities in con-ditions of formation and mineralization style. Mafic vol-canic and intrusive rocks comprise the majority of hostrock in both goldfields. Alteration assemblages, plus fluid

Fig. 2. The Black Flag fault system and Playa–Lefroy fault systems, active durthe Black Flag fault (proxy for displacement) vs. fault length; modified fromnormalised over the maximum strike separation measured on the Black Flagprofile. (B) Map of the Black Flag fault system. TheMt Pleasant goldfield is lowhich coincides with a marked minima in the displacement-length profile of talong fault traces indicate dip direction. Note, the Lefroy fault is also referre

inclusion and isotopic constraints, indicate temperaturesduring mineralization were 250–300 °C at Mt Pleasantand ∼400 °C at St Ives [19,20], at mid-to upper-crustaldepths.

Gold mineralization is hosted by small-displacementfaults and shears, with associated breccias and vein ar-rays (Fig. 2). The faults that are mineralized compriseonly small portions of the two fault systems. They areassociated with large-displacement, poorly mineralizedstrike-slip faults, which have irregular and complex faulttraces (Fig. 2). Most mineralized faults identified inmines are b1.0 km long, with 1–100mmaximum offsetsand tend to be spatially linked with one another. Theyoccur over areas N15 km2 and can exhibit a range oforientations relative to the adjacent large-displacementfault segments. Zones of intense hydrothermal alterationsurround lode-hosting faults and shear zones.

In the following descriptions the term step-over refersto the region between and around understepping oroverstepping fault segments [21] that show evidence ofinteraction (step-overs are synonymous with fault jogs).

ing mineralization. (A) Profile of strike separation of stratigraphy alongMicklethwaite and Cox [5]. Strike separation measured at any point isfault of 2.4 km. Separation on secondary faults is not integrated in thiscated adjacent to a 1 km-long dilational fault step (see Fig. 3 for detail),he Black Flag fault. (C) Map of the Lefroy and Playa faults. Tick marksd to as the Boulder-Lefroy fault [35].


Accordingly, soft-linked fault segments show evidence ofmechanical interaction even though there is no evidenceof connecting faults or fractures between the segments.Hard-linked fault segments are connected across the step-over, often by a network of small-displacement faults orfractures. Soft-linked fault segments are expected toevolve to hard-linked fault segments as displacementincreases along a fault system [21].

The case study areas enable us to compare anunderstepping, hard-linked, dilational step-over on theBlack Flag fault system (Mt Pleasant), with an under-stepping, hard-linked, contractional step-over on thePlaya–Lefroy fault system (St Ives). At Mt Pleasant twodextral strike-slip fault segments with maximumdisplacements N2.5 km cut across an open regionalanticline and transfer displacement across an extension-al step-over [5]. The fault segments are understeppingby ∼2.0 km, and hard-linked by faults, narrow strike-slip duplexes, silicified breccias and subvertical,semicontinuous quartz veins (Figs. 2 and 3a). Strike

Fig. 3. (A) Simplified map of the gold-bearing quartz vein and fault network athe fault and associated extension vein is ∼40°. Inset: Map of fault and veinssericite rich shear zones are flanked by steep extension veins with consistentshear zone boundaries. (B) Intensely silicified breccia from the Black Flagclasts of earlier vein material and silicified breccia. (C) Thrust-related extensigoldfield; after Cox and Ruming [4].

separation of stratigraphy across the step is ∼0.4 km,which represents a minimum in the offset-fault lengthprofile for the Black Flag fault (Fig. 2a; see alsoMicklethwaite and Cox [5]). A network of syntheticbranch faults and a few apparently unlinked faults aredeveloped throughout the goldfield. Quartz–carbonateextension veins (i.e. mode I fractures with quartz–carbonate infill) form dense arrays connected to faults,or en-echelon sets near faults. Extension veins arepredominantly subvertical, strike 060° and have mutualcrosscutting relationships with fault-fill veins or brec-cias. The geometric relationships between faults andextension veins confirm a dextral slip on the Black Flagfault system. Silicified wear breccias contain clastscomposed of pre-existing breccia and vein fill (Fig. 3b)as well as wall rocks. Development of foliation adjacentto some faults, pressure solution of veins as well as shearzone fabrics record a significant component of non-brittle deformation in the area. There is also an apparentasymmetry present in the fault network, with the

ssociated with the Mt Pleasant goldfield. The dominant angle betweenin two drives at the Quarters mine, Mt Pleasant goldfield. Carbonate–orientations. Shear veins also occur within the shear zones, parallel topit, Mt Pleasant goldfield, crosscut by extension veins and containingon and shear veins, mutually crosscutting one another, from the St Ives


majority of known branch faults splaying off thesouthern segment of the Black Flag fault and not thenorthern.

The St Ives goldfield stretches north from a contrac-tional step on the Playa fault, which links to the regionalLefroy fault system. The Playa–Lefroy fault system is asinistral-reverse strike-slip system, spatially associatedwith a complex network of small-displacement faults ata contractional step [4,18]. Imbricate thrusts connectacross the contractional step and are associated withspectacular arrays of mutual cross-cutting, subhorizon-tal extension veins and fault-fill veins and breccias (Fig.3c). Widespread development of shear zone foliationrecords a component of non-brittle deformation in thearea [18]. Northwest from the contractional step, golddeposits occur in a narrow corridor between the northernPlaya fault segment and a thrust (the Delta fault). Animbricate array of east- or west-dipping, connectingfaults link the Playa and Delta faults and hostmineralization. These are predominantly small-displace-ment thrusts and sinistral-reverse faults, associated withzones of foliation (one to hundreds of metres thick), plusbrecciation and veining. Low-grade gold mineralizationis also present on some E–W very low-displacementnormal faults and shear zones within the mineralizedcorridor [4]. At the northernmost part of the goldfield, asmall number of gold deposits are hosted on small-displacement sinistral faults and shear zones.

Mineralization in both goldfields was coeval withdeformation and involved the migration of substantialvolumes of hydrothermal fluids that were out ofchemical equilibrium with the local host rocks. Thefield examples are associated with hard-linked, under-stepping, step-over zones between large-displacementsegments of the fault systems. We infer that such largestep-overs were long-lived rupture arrest features [4,5],which probably evolved from soft-linked to hard-linkedin the latter stages of fault system development. Theobserved mutually overprinting fault-fill and extensionvein geometries are indicative of a fault-valve failuremechanism for many of the mineralized faults. For thismechanism to operate, the transient and repeatedattainment of suprahydrostatic fluid pressures is re-quired [1,18,22]. Herein, fluid pressure is described interms of the pore fluid factor (λ), which is the ratio offluid pressure relative to the lithostatic overburdenpressure (σv, equivalent to σ2 in these studies).

This study examines ancient fault systems in whichslip events are inferred rather than observed. Therefore itis necessary to establish the nature of the fault-slipevents before we proceed. Faults can fail in stable slipevents (frictional creep) as well as unstable seismic slip

events. Both modes of failure generate static stresschanges that can trigger subsequent failure [12].Abundant breccias observed in both case study sitesare cemented by silica and contain clasts of pre-existingbreccia and vein material (Fig. 3). These observationsrequire multiple episodes of fault slip and brecciation,with intervening periods of fluid flow, cementation andsealing, in which faults become healed. Thus, althoughfault creep may have been a component of deformation,it is apparent that the small-displacement faults wererepeatedly triggered as aftershocks, following slipevents on adjacent large-displacement fault segments.

3. Coulomb stress change and the apparent frictionapproximation

Changes in static stress, resulting from earthquakes,have been used extensively to explain the distribution ofaftershocks and the triggering of subsequent earth-quakes [6–8]. We can also relate stress changes to thefossil fault systems described [4,5]. The arrest of faultslip can be strongly influenced by features such as faulttips or step-overs between segments [23–25]. We inferthat over the lifetime of a fault system, features like faulttips and fault step-overs act as the starting and stoppingpoints for many ruptures, even though some rupturescan jump across step-overs (e.g. Landers earthquake).This inference allows us to calculate static stresschanges from ancient fault-slip events.

In stress transfer modelling, fault slip is simulated asa dislocation in an elastic half-space [9]. A modifiedCoulomb failure criterion is used to quantify static stresschanges on fault planes adjacent to a fault rupturesurface [6,26],

DrF ¼ Dsþ l VðDrnÞ ð1Þ

where ΔσF is the change in Coulomb failure stress, Δτthe change in shear stress and Δσn the change in normalstress (negative in compression). The apparent coeffi-cient of friction (μ′) uses a simple poroelasticassumption to incorporate the effect of changing normalstress on fluid pressure, where during and immediatelyafter slip, stress changes are too rapid for pore fluidmigration to occur. Thus strictly, Coulomb stresstransfer modelling examines the influence of instanta-neous static failure stress changes on the triggering offault–fracture networks or subsequent ruptures. Afterrupture events, any changes in fluid pressures, ductileflow of lower crust, or frictional fault creep will alter theinitial static stress changes [10–14]. Various stresstransfer studies have found that low values for the

Fig. 5. Calculations of Coulomb failure stress change for optimally oriented fthe two major segments of the Black Flag fault (white lines) with the distribgoldfield. (A) Model setup. (B) Static stress change from slip on northernCombined-slip stress change (resulting from slip on both fault segments). Achange and the distribution of mineralization. (E) Model with lower value ofsignificantly affected by variations in this parameter. (F) Model with low Poistatic stress change is not significantly affected by variations in this paramet

Fig. 4. Relationship between Skemptons coefficient (B) and apparentfriction (μ′) for differing values of static friction (μ) measured inlaboratory experiments. Shaded region represents the domain in whichreported experimental values of panel B lie.


apparent friction coefficient of μ′=0.2 are mostconsistent with aftershock distributions [13,27–29].The parameter μ′ relates confining stress to pore fluidpressure by Skempton's coefficient, B, such that,

l V¼ l½1−B� ð2Þ

B is the ratio of the change in fluid pressure to thechange in normal stress due to fault slip [30], withexperimental values ranging from 0.5 to 0.9 [31]. Thefriction coefficient μ typically has values 0.55≤μ≤0.90 [32]. It follows that values of μ′=0.2 are to beexpected (Fig. 4) and do not imply a fault is weak, whilstvalues of μ′=0.4 are at the upper end of the expe-rimentally determined range (Fig. 4). Nonetheless, evenhigher values of apparent friction, up to μ′=0.8, havebeen found for continental thrusts [e.g. 33]. Suchelevated values of μ′ may not be unusual in fractured

aults (dextral strike-slip faults). Results are compared from fault slip onution of fault-hosted gold deposits (grey polygons) in the Mt Pleasantsegment. (C) Static stress change from slip on southern segment. (D)n excellent correlation exists between the combined-slip static stressapparent friction. The distribution of positive static stress change is notsson's ratio appropriate to dry mafic rocks. The distribution of positiveer.


rock masses because fracture geometry should have acontrol on B. B is expected to tend towards zero in rockcontaining fractures parallel to the maximum principalstress [34], in which case Eq. (2) indicates that μ′becomes equivalent to μ. Thus, as stress increases priorto rupture, microcracks and fractures opening parallel tothe maximum principal stress drive B towards zero. Thisscenario may be particularly true for fluid-saturatedrocks in high λ regions, and for this reason we prefer touse a slightly elevated value of μ′=0.4. A more

Fig. 6. Calculations of Coulomb failure stress change (ΔσF) comparing stressystem with the distribution of the St Ives goldfield deposits (grey polygons).Lefroy fault segment (white line). ΔσF is resolved on strike-slip faults paorientation). Dashed line shows positive stress change at the 1 bar contour, reand northern Playa fault segments are both brought closer to failure. (C) Δσsouthern segment of the Playa fault, plus triggered large aftershocks on the nthrust faults. The ΔσF pattern is the result of mainshock slip on the southenorthern Playa fault segment and the Delta fault. An excellent correlation exfault segments and the distribution of mineralization.

thorough treatment of apparent friction is given byBeeler et al. [31].

4. Coulomb stress changes around the Mt Pleasantand St Ives fault systems

For both case studies we have calculated ΔσF

generated by mainshock events, then we have examinedthe static stress changes from large aftershocks ortriggered earthquakes (fault lengths ≥5 km) and how

s change from fault slip on major segments of the Playa–Lefroy fault(A) Model setup. (B) ΔσF generated by slip on the 30 km-long Playa–rallel to the northern Playa fault segment (close to the optimal faultsolved for faults parallel to the Delta fault. Results show that the Delta

F resolved for strike-slip faults, resulting from mainshock slip on theorthern Playa fault segment and the Delta fault. (D) ΔσF resolved onrn segment of the Playa fault, plus triggered large aftershocks on theists between the cumulative static stress change due to slip on all these

Table 1Coulomb model parameters

Parameter Value

BF slip 1.0 m right-lateralPL slip 1.0 m left-lateral (0.5 m on northern-most

plane)P slip 0.2 m left-lateral, 0.1 m reverseD slip 0.2 m left-lateral, 0.05 m reverseApparent friction,

μ′0.4

Young's modulus,E

7.0×104 MPa

Poisson's ratio, ν 0.4dσ1/dz 36.5 MPa/kmdσ2/dz 29.0 MPa/kmdσ3/dz 21.5 MPa/km

BF—Black Flag fault (north and south segments), PL—Playa–Lefroyfault, P—northern Playa fault segment, D—Delta fault. Parameters μ′and ν were modified in tests to examine their influence on results.


they modify the mainshock stress change distribution(Figs. 5 and 6). Thus, in the Black Flag fault case studywe account for stress triggering of one large Black Flagfault segment by slip on the other. In the Playa–Lefroyfault case study we account for triggering of the Deltafault and northern Playa fault segment by large slipevents on the southern Playa fault segment. Herein werefer to these as “combined-slip” calculations. A similarapproach was employed by Stein et al. [8], where staticstress changes were correlated with the historictriggering of progressive ruptures along the NorthAnatolian fault, Turkey, since 1939. Although modelcalculations were carried out in 3D, results are presentedin 2D map view for ease of comparison with the mappedfault configuration. ΔσF was calculated on a 100 m gridspacing. For each goldfield the results from the com-bined-slip calculations are compared with ΔσF patternsdue solely to single mainshock ruptures (e.g. Fig. 6band c). Previous stress transfer studies of the Mt Pleasantand St Ives goldfields considered single mainshock slipevents only [4,5]. However, although there was a cor-relation between the distribution of mineralization andpositive ΔσF, it was found that significant areas did notcontain mineralization where it may be expected. Inaddition, understanding of the geometry and kinematicsof the fault systems in the two case study areas hasimproved since the earlier studies [4,5].

We model the structures shown in Fig. 2 as verticaland dipping faults, using COULOMB 2.5 [9]. Estimatesof the orientations for regional stress during minerali-zation were gained from consistent vein orientations andfault kinematics measured at both case study sites. Theinferred stress fields are in broad agreement with thestructural evolution of the entire terrane as identifiedfrom pervasive fabrics, fold geometries and fault-shearzone orientations elsewhere [35]. Both sets of far-fieldstress are appropriate to Andersonian-type wrench faultsystems, and the magnitudes of vertical stress gradientswere estimated on this basis (Table 1). The imposedmaximum principal stress (σ1) is horizontal and strikes060° at Mt Pleasant and 100° at St Ives. Co-seismic slipdistributions for the faults at both case study sites cannotbe known. The normalised slip profile, shown in Fig. 2for the Black Flag fault system, is the cumulative resultof multiple slip events and for that reason the directapplication of this slip distribution is avoided in ourcalculations. Instead, for ease of comparison betweenboth sites we simplify slip distributions to first-orderuniform slip models (Table 1). This has the effect ofamplifying static stress changes at the rupture surfacetips. However, the location and distribution of the staticstress changes are not significantly altered, as these

factors are more dependent on the geometry and slipdirection of the faults, which are known. In addition,tests were carried out on the Mt Pleasant goldfield, usingtapered slip distributions and it was found that resultswere not significantly affected.

Hydrothermal alteration assemblages in our casestudy areas indicate faulting took place at mid-crustaldepths; accordingly we sample stresses in the models at10 km depth. In any case, because the Black Flag fault isa vertical strike-slip fault system, with no oblique-slipcomponent, the model results are similar for the entiredepth-extent of the modelled dislocations (2–15 km).This is not true of the St Ives case study (Fig. 2) whereboth the northern Playa fault segment and the Delta faultare moderately to steeply dipping, oblique-slip struc-tures—the latter dominantly dip-slip (Table 1).

4.1. Results

Results for the Mt Pleasant goldfield show thatcombined-slip events on the large-displacement north-ern and southern segments of the Black Flag fault,induce cumulative positive static stress changes with anexcellent correlation with the distribution of mineralizedsmall-displacement faults (Fig. 5d). Those few areaswhere there is no apparent correlation, such as faultbends (Fig. 5d), have not yet been well-explored.

Field evidence, such as multiple vein types and hy-drothermal alteration, indicates that the Mt Pleasant andSt Ives fault networks were fluid-saturated. For thisreason both goldfields were modelled using a highPoisson's ratio (ν=0.4), representative of fluid-saturatedupper crustal rocks. However, we used the Mt Pleasantcase study to explore in a simple way the effect of


varying μ′ and ν. Fig. 5e–f shows that there is very littlesensitivity to variations in these parameters in termsof the resulting distribution of positive static stresschanges.

At St Ives, the northern Playa and Delta fault seg-ments can be regarded as relatively large faults (≥5 kmin length) within the Playa–Lefroy fault system. Thelargest structure in the system is the southern Playa–Lefroy fault segment (∼30 km-long). The combined-slip static stress changes, from slip on all three faults,match well the distribution of mineralized faults andalso the orientation of faults that are mineralized(Fig. 6c–d). Where mineralized thrust faults dominatein the central corridor at St Ives (between the Delta faultand northern Playa fault) positive stress changes occuron optimally oriented thrusts (Fig. 6c), but ΔσF is alsopositive for strike-slip faults. Where there is a dominantlobe of positive ΔσF resolved on strike-slip structures inthe north of the goldfield (Fig. 6d), some mineralizationis indeed hosted on strike-slip faults in the field.

Two final observations are relevant. Firstly, Figs. 5and 6 show that, in both case studies, the domains ofpositive ΔσF obtained from combined-slip calculationsfit the distribution of mineralization far better than thosethat consider just a single major slip event on the large-displacement fault segments [e.g. Figs. 5b–c and 6b; seealso Figs. 4 and 5]. Secondly, in the Mt Pleasant casestudy, stress changes from a single fault-slip event on thenorthern fault segment increase the proximity to failureon the southern segment, and vice versa (Fig. 5b–c),indicating such events would have had the potential totrigger one another. Similarly, in the St Ives case study,modelling indicates the northern Playa fault and Deltafault segments would have been brought closer tofailure, and therefore prone to triggering, followingfault-slip events on the large-displacement, southernPlaya fault segment (Fig. 6b).

5. Discussion

Field observations have been combined with staticstress change calculations, in two examples of fossilfault systems. Both fault systems feature stronglymineralized small-displacement faults developedaround understepping fault step-overs, between large-displacement fault segments. The patterns of static stresschanges associated with large fault-slip events mimicthe distribution of those small-displacement faults thathost gold mineralization. In particular, we have foundthat combined-slip calculations accounting for slip fromall the moderate-to large-displacement faults in agoldfield, have an excellent correlation with the

distribution of mineralized small-displacement networks(Figs. 5d and 6c–d). The correlation exists overdistances in excess of 10 km away from the large-displacement fault step-overs. In this section we discusshow the distribution of gold deposits around crustal-scale fault systems reflects not just static stress changesdriven by mainshocks, but is also critically dependenton how static stress changes evolve during triggeredrupture sequences. We explore the implications for themigration of fluids through fault systems and theconcomitant impact of changes in fluid pressure.

5.1. Effects of rupture sequences on evolution of staticstress changes and fluid flow

A number of features make the case studies detailedhere particularly interesting: (1) Our observations onfossil fault systems affirm the concept that static stresschanges exert a first-order control on subsequent near-field faulting and fracturing. (2) Static stress changesfrom single slip events on large-displacement faultsegments would have brought other large-displacementand moderate-displacement faults closer to failure,potentially triggering them. (3) Static stress changesfrom the resulting combined-slip calculations have thebest correlation with the distribution of small-displace-ment faults that are mineralized. (4) Gold mineralizationpreferentially hosted in small-displacement faultsimplies that those faults experience high fluid fluxesrelative to adjacent, poorly mineralized large-displace-ment faults [3]. (5) Where stress shadows are predicted,mineralization is absent.

Overall, our results are consistent with crustal-scalefault systems in the late Archaean operating in much thesame way as modern day fault systems. The Black Flagand Playa fault systems record behaviour such asearthquakes triggering subsequent earthquakes [6–8]and secondary aftershock behaviour (the triggering ofsmall aftershocks by large aftershocks [36,37]). At MtPleasant, it could also be argued that similar ΔσF

patterns could result from ruptures propagating alongone segment of the Black Flag fault and jumping thestep-over. However, at St Ives the best fit betweenmineralization and cumulative static stress changerequired triggering of the Delta fault in addition to thePlaya fault segments. The important point to emerge isthat there is potential for radical reorganisation of after-shock distributions when large aftershocks or ruptureson adjacent faults are triggered by initial large-displace-ment ruptures. As a consequence there is also radicalreorganisation of fluid flow pathways within an after-shock sequence when this happens.


Fig. 7 illustrates the above point for the St Ives casestudy. In the first stage of the rupture sequence, a largerupture propagating along the southern Playa–Lefroyfault segment is arrested at the contractional step,promoting aftershocks, permeability enhancement andfluid migration over a broad lobate area. However, theresulting changes in static stress promote failure on boththe northern Playa fault segment and the Delta thrustfault. The triggering of slip on these structures has asignificant impact on the distribution ofΔσF, modifyingthe domain of positive ΔσF so that it becomes restrictedto a central corridor between the northern Playa faultand the Delta fault. Accordingly, the distribution ofaftershock activity is expected to be correspondinglyrestricted. Outside of the central corridor, initialaftershock porosity, triggered by the mainshock, israpidly sealed by migrating hydrothermal fluids becausethose structures do not continue to fail repeatedly.Therefore the permeability enhancement due to initialaftershock activity is lost. Fluid migration and ulti-mately gold mineralization becomes confined to themodified aftershock distribution, where repeated failuremaintains active percolation networks over the life spanof the aftershock sequence (months to decades incontemporary active fault systems).

It is worth noting in both examples that a consistentcorrelation exists between the distribution of minerali-zation and positive ΔσF resulting from rupturesequences, in excess of 10 km from the large faultstep-overs (Figs. 5 and 6). Thus aftershock-related fault

Fig. 7. Potential evolution of static stress change during a compound rupture schange are for optimally oriented faults. (A) Map of the Playa–Lefroy fault syat a contractional step-over. Thus, early after the mainshock, fluid flow andPlaya and Delta faults are triggered as large aftershocks and modify the initialsequence, fluid flow and mineralization are controlled by the cumulative stresstriggered events modify the positive ΔσF pattern, the region of fluid flow an

networks can control fluid flow at large distances awayfrom fault step-overs, or other rupture arrest locations. Aclose spatial association between fluid flow, minerali-zation and fault step-overs is well known [e.g. 38],although there has been a tendency in the literature toemphasise the importance of dilational fault steps overcontractional fault steps [e.g. 39]. The studies describedhere show that fluid flow need not be associated justwith permeability enhancement in fault step-overs. Thisstudy and previous work [4,5] indicate that anassociation exists because repeated triggering of after-shocks around such features can generate large domainsof permeability enhancement. Contractional and exten-sional fault steps are both important. Mineralization atMt Pleasant and St Ives must have occurred in thepotentially short period that their extensional andcontractional fault steps acted as effective geometricbarriers to repeated ruptures.

5.2. Fluid flow in aftershock networks

A number of hydromechanical models have consid-ered the interaction between fluid migration, fluidpressure evolution and static stress changes resultingfrom an earthquake [see 40]. In one class of model, wallrock porosity and fractures local to the triggering faultsare considered to be fluid-saturated, with a drainedresponse to co-seismic stress changes [13,15,41]. Fluidsmigrate, pore pressures evolve and seismicity is pro-moted, in an essentially diffusive process, controlled by

equence on the fault system at St Ives. Domains of positive static stressstem, (B) Rupture propagating along Playa–Lefroy fault and arrestingassociated Au-mineralization might occur over a large region. (C–D)domain of positive static stress change. (D) After the complete rupturechange and related distribution of secondary aftershocks. Thus as larged mineralization shrinks significantly.


poroelastic responses to a fault-slip event. A secondclass of model recognises that aftershock failure canbe driven by migration of fluid pressure pulses frombreached overpressured reservoirs [e.g. 14]. Failure offaults in response to increases in fluid pressure may takeprecedence over co-seismic static stress changes, oncehigh pore fluid factor fluids migrate in to an aftershockfault network [14,16].

Our observations show static stress changes, mostlikely from repeated compound rupture sequences, con-trol the distribution of structures that experienced en-hanced fluid flow. However, fluids evidently attainedhigh pore fluid factors in the mineralized fault networks(e.g. the occurrence of extension veins), whilst hydro-thermal alteration in and around the mineralized faultsdemonstrates that the fluids were out of chemicalequilibrium with the wall rocks. Likewise, gold andsilica solubilities are such that very large finite volumesof fluid are required to explain the gold deposits and theirrelated fault-vein networks [3]. For example, theobserved gold resource of N100 tonnes at Mt Pleasantrequires a minimum fluid volume of 108 to 1012 m3, onthe basis of the solubility of Au inmid-crustal fluids [42].This value is several times larger for the St Ives goldfield.We propose that fluid-pressure driven failure of faults inthe aftershock zone exploited those faults alreadybrought closer to failure by static stress changesassociated with large rupture events (Fig. 8). Themineralizing fluids were not stored in local wall rock

Fig. 8. (A) Schematic map and longitudinal-section of a strike-slip fault sys(small circles) depicted to illustrate the concept only. A migrating pulse of higto a recently ruptured large-displacement fault segment. If fluid migrates vdistribution of aftershocks with time. Grey arrow—movement vector of theand down temperature gradients leads to rapid sealing of the mainshock faultmaintain connectivity and permeability for longer. Over repeated mainshaftershock zones relative to the mainshock rupture [5]. Note, in some cases, evolumetrically extensive and deeply penetrating aftershock network may do

or fracture porosity but instead infiltrated the faultnetworks from overpressured fluid reservoirs, whichwere breached initially by the rupture sequence. As such,fault-valve failure mechanisms, triggered by migratinghigh fluid pressure pulses, may be characteristic of fluid-rich aftershock sequences. Similar fluid-rich aftershocksequences associated with active fault systems maydisplay both double-couple and non double-coupleseismic signals, and anomalous seismic velocity ratios.

At the St Ives and Mt Pleasant goldfields golddeposition was localised in repeatedly reactivated after-shock networks, rather than in the nearby large-dis-placement faults. This implies that transient increases inpermeability and fluid redistribution between crustalreservoirs are preferentially enhanced in aftershocknetworks relative to mainshock rupture surfaces (Fig. 8).Fluid reservoirs are expected to be common in the crustand could include magmatic or metamorphic volatilestrapped beneath a low-permeability carapace, beneathfold closures, beneath low permeability seals withincrustal stratigraphy, or even trapped in sealed compart-ments within the fault system. A conceptual model pre-sented in Fig. 8 shows amainshock rupture breaching theupper part of a fluid reservoir, with a fluid pressure wavemigrating upwards along the mainshock rupture, as wellas exploiting the triggered aftershock domain. Hydro-thermal sealing rapidly destroys permeability andinhibits fluid flow along the mainshock rupture, whereasaftershock networks are active for substantial periods

tem, with a sealed, overpressured fluid reservoir at depth. Aftershocksh-pressured fluid (grey) penetrates the aftershock fault network adjacentertically from depth, there will be no marked evolution in the mapfluid pulse. (B) The cementation potential of fluids migrating up faults. Aftershock fault networks, with their extended period of activity, canock events, much larger time-integrated fluid fluxes are attained inven if a mainshock rupture does not breach the fluid reservoir, the moreso.


following mainshock rupture and are likely to sustainlonger-lived, fracture-enhanced permeability than main-shock fault planes [5]. Additionally, aftershock faultnetworks develop over substantial crustal volumes, indonut-like domains around the tip of the mainshock slippatch. Accordingly, the network drains a larger, hydrau-lically accessible portion of the fluid reservoir than asingle mainshock rupture surface (Fig. 8). The after-shock network may also penetrate to a deeper level in thefluid reservoir. As the fluid pressure pulse migratesupward through the aftershock network, elevated porefluid factors promote fluid-driven aftershock nucleation(Fig. 8). Furthermore, the greater fault-related surfacearea affords much greater potential for fluid–rock inter-action and gold deposition relative to a single mainshocksurface.

6. Conclusions

It has been shown that the geometry and distributionof domains of positive Coulomb failure stress change(ΔσF) from large fault-slip events can be substantiallymodified by the subsequent triggering of other medium-and large-displacement faults within the initial zone ofpositive static stress change. This process happens oftenenough that a record of the process is preserved inArchaean fault systems that localised high fluid fluxduring gold mineralization.

In two case studies of Archean fault systems, wefound that the modification to patterns of mainshock-induced positive ΔσF, due to the triggering of largeaftershocks, very closely matches the distribution ofsmall-displacement faults hosting gold mineralization.Thus, large aftershocks or subsequent triggered earth-quakes can exert a strong control on where the highestfluid fluxes occur through small-event aftershock faultnetworks. We infer that the repeated triggering ofaftershock networks plays a critical role in inducingpermeability enhancement and controlling fluid redis-tribution between crustal reservoirs.

The case studies provide evidence that fluids in theaftershock networks were at high pressure and camefrom deep-level, overpressured fluid reservoirs. Fluidswere out of equilibrium with the host rocks and werenot sourced from local wall rock or fracture porosity.

The field evidence indicates that higher fluid fluxesare preferentially localised in aftershock networksrelative to adjacent, mainshock, large-displacementfaults. We interpret this to be due to the extended periodof activity of aftershocks following mainshock rupture,which sustains longer-lived, fracture-enhanced perme-ability. The permeability of the mainshock fault is

rapidly lost due to hydrothermal sealing. Additionally,aftershock networks can tap deeper in to fluid reservoirsthan mainshock faults and generate enhanced transientpermeability over larger volumes of the crust.

Applying stress transfer modelling to the rock recordprovides a novel opportunity to study how the dynamicsof seismogenic fault systems may influence fluidredistribution in the continental crust. In a similar man-ner, stress transfer modelling is a powerful tool for goldexploration and may also afford insights in to petroleumreservoir leakage during active faulting.

Acknowledgements

This research was supported by an AMIRA Interna-tional Ltd. and Australian Research Council Linkagegrant. Placer Dome Ltd. andGold Fields Ltd. are thankedfor geological, financial and logistical support, includingopen access to mine sites and company data. Inparticular, Alan Goode, Bob Morrison, Karen Connors,Gerard Tripp and Brett Davis provided excellentlogistical support, geological input and numerousdiscussions. We are also indebted to Ross Stein and ananonymous reviewers whose comments greatly im-proved the manuscript. The U.S. Geological Surveyand S. Toda are thanked for making the Coulombmodelling package freely available.

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