Tectonophysics 599 (2013) Contents lists available at SciVerse ScienceDirect. Tectonophysics. journal homepage:

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1 Tectonophysics 599 (2013) Contents lists available at SciVerse ScienceDirect Tectonophysics journal homepage: The structure of an exhumed intraplate seismogenic fault in crystalline basement Steven A.F. Smith a,, Andrea Bistacchi b, Thomas M. Mitchell a,c, Silvia Mittempergher d, Giulio Di Toro a,d a Istituto Nazionale di Geofisica e Vulcanologia (INGV), Via di Vigna Murata 605, 00143, Rome, Italy b Dipartimento di Scienze Geologiche e Geotecnologie, Università di Milano Bicocca, Piazza delle Scienza 4, 20126, Milan, Italy c Department of Geosciences, Ruhr-Universität Bochum, 44780, Bochum, Germany d Dipartimento di Geoscience, Università di Padova, Via G. Gradenigo 6, 35131, Padua, Italy article info abstract Article history: Received 10 January 2013 Received in revised form 13 March 2013 Accepted 27 March 2013 Available online 6 April 2013 Keywords: Fault structure Fracture damage Fluid flow Alteration Cataclasite Adamello The 600 m-thick Gole Larghe Fault Zone (GLFZ) is hosted in jointed crystalline basement and exposed across glacier-polished outcrops in the Italian Alps. Ancient seismicity is attested by the widespread occurrence of cataclasites associated with pseudotachylytes (solidified frictional melts) formed at 9 11 km depth and ambient temperatures of C. Previous work focused on the southern part of the fault zone; here we quantitatively document fault zone structure across the full width of the GLFZ and surrounding tonalite host rocks by using a combination of structural line transects and image analysis of samples collected across fault strike. These new datasets indicate that the GLFZ has a broadly symmetric across-strike damage structure and contains distinct southern, central and northern zones distinguished by large variations in fracture density, distribution of pseudotachylytes, volume of fault rock materials, and microfracture sealing characteristics. The c. 100 m wide central zone is bound by two thick (~2 m) and laterally continuous (>1 km) protocataclastic to ultracataclastic horizons. Within and immediately surrounding the central zone, fracture density is relatively high due to cataclastic fault fracture networks that reworked earlier-formed pseudotachylytes. The fault fracture networks were associated with pervasive microcracking and fluid rock interaction, resulting in the development of a c. 200 m thick alteration zone delimited by lobate fluid infiltration fronts. In the c. 250 m thick southern and northern zones, fracture densities are much lower and pseudotachylytes systematically overprint cataclastic faults that exploited pre-existing magmatic cooling joints. Analysis of the structure of the GLFZ suggests that it shares certain characteristics with the seismogenic source responsible for the 2002 Au Sable Forks intraplate earthquake sequence in the northeastern USA, including seismicity distributed across a fault zone m thick and large (>100 MPa) static stress drops associated with frictional melting Elsevier B.V. All rights reserved. 1. Introduction Current understanding of seismogenic fault zone structure is based primarily on field and laboratory analysis of exhumed fault zones (e.g. Chester et al., 2004; Faulkner et al., 2003; Sibson, 1983), geophysical measurements of fault zone properties (e.g. seismic wave velocities, Ben-Zion, 1998; Cochran et al., 2009), and the results of drilling projects that intersect active faults at depths b3.5 km (e.g. Alpine fault drilling project, Sutherland et al., 2012; Chelungpu fault drilling project, Boullier et al., 2009; San Andreas Fault Observatory at Depth, Zoback et al., 2011; Wenchuan fault drilling project, Li et al., 2012). One of the clearest outcomes from these studies is that fault zone structure and the dominant fault rock associations vary considerably with depth, fault displacement, and host rock lithology (for recent reviews see Faulkner et al., 2010; Wibberley et al., 2008). Additionally, the physical properties of fault zones may transiently change as strain rate and Corresponding author. Tel.: address: steven.smith@ingv.it (S.A.F. Smith). fluid availability evolve during the seismic cycle (e.g. Gratier and Gueydan, 2007; Gratier et al., 2003). Documenting fault zone structure in the depth range 7 15 km is particularly relevant in terms of earthquake mechanics, as this represents the typical depth range of earthquake nucleation in normal continental crust (depending on tectonic regime, rock composition, temperature gradient etc; Scholz, 2002; Sibson, 1983). This depth range is currently out of reach of fault drilling projects, but a picture of seismogenic fault structure can be achieved by geophysical methods or by examining former seismogenic faults now exposed at the Earth's surface. Although the latter approach allows a particularly high level of resolution, it is challenging to find exhumed fault zones that meet the following criteria: exhumation from depths of 7 15 km; excellent preservation of fault-related fracture networks across extensive exposures; presence of structural markers that allows the determination of slip across individual fault strands; and the presence of solidified frictional melts (pseudotachylytes) attesting to past seismic activity. The purpose of this paper is to document the structure of the Gole Larghe Fault Zone (GLFZ), an exhumed seismogenic fault hosted in crystalline basement. Previous work on the GLFZ focused on the /$ see front matter 2013 Elsevier B.V. All rights reserved.

2 30 S.A.F. Smith et al. / Tectonophysics 599 (2013) evolution of fault rocks, including pseudotachylytes, exposed in the southern part of the fault zone (Di Toro and Pennacchioni, 2004, 2005; Di Toro et al., 2005). Here, we focus on the larger-scale structure of the fault zone, and draw conclusions regarding the evolution of fracture damage and fluid flow at seismogenic depths. Several factors combine to make the GLFZ an exceptional target to understand seismogenic fault zone structure: 1) certain areas of the fault zone (e.g. the Lobbia Glacier area studied here) are exposed across glacierpolished outcrops for kilometers both along- and across-strike; 2) the pressure and temperature conditions of faulting are well constrained from previous field observations together with microstructural and thermochronological analyses of the fault rock assemblages (Di Toro and Pennacchioni, 2005; Di Toro et al., 2005; Pennacchioni et al., 2006) and wall rocks (Reverman et al., 2012; Stipp et al., 2004; Viola et al., 2001); 3) the GLFZ cuts a young (c Ma) homogenous igneous intrusion belonging to the Adamello batholith, and; 4) there is no structural evidence for tilting of the Adamello batholith during exhumation from depth (e.g. Brack, 1981). Additionally, the fault rock assemblages formed at depths of 9 11 km show no significant structural overprint (Pennacchioni et al., 2006), and thus it is less likely that successive (and shallower) deformation events will hinder the interpretation of fault structure. The concomitance of these various factors allows a comparison to be made between the structure of the GLFZ and that of active seismogenic fault zones imaged using high-resolution seismological techniques (e.g., Au Sable Forks 2002 earthquake sequence, Seeber et al., 2002; Viegas et al., 2010). 2. Geological setting and previous work 2.1. The Adamello batholith The Adamello massif is a major batholith in the Italian Southern Alps (Fig. 1). It is composed of four main intrusions of predominantly granodiorite tonalite composition whose ages decrease progressively (42 29 Ma) from south-west to north-east (Fig. 1; Del Moro et al., 1983; Viola et al., 2001). The batholith intruded thrust nappes composed of pre-alpine metamorphic basement and its Permo-Triassic sedimentary cover belonging to the Southern Alps (Schonborn, 1992), in a region located between two major segments of the 700-km long Periadriatic lineament: the Tonale Line to the north, and the (South) Giudicarie Line to the east (e.g. Castellarin and Cantelli, 2000). Mineral assemblages preserved in the contact aureole of the Adamello batholith indicate syn-intrusive pressures in the range GPa, corresponding to an intrusion depth of ~9 11 km (assuming typical rock densities; John and Blundy, 1993; Riklin, 1985; Stipp et al., 2004). Along the northern contact of the batholith, intrusion of the Val d'avio Val di Genova and Presanella plutons was broadly synchronous with dextral strike-slip movements along the Tonale Line (Stipp et al., 2004). Post-magmatic deformation of the Adamello batholith is recorded by a down-temperature sequence of ductile and brittle structures in the Adamello and Val d'avio Val di Genova plutons, including (Di Toro and Pennacchioni, 2005; Pennacchioni, 2005; Pennacchioni et al., 2006): 1) four sets (approximately E-, W-, S-, N-dipping) of early-formed cooling joints and fractures, sometimes intruded by aplite dykes, that formed at >600 C; 2) amphibolite-facies mylonitic shear zones formed at ~500 C that nucleated mainly on E- and W-dipping cooling joints; 3) pseudotachylytes and epidote + chlorite-bearing cataclastic faults formed at C, mainly found reactivating S-dipping cooling joints, and; 4) late-stage zeolite-bearing faults and vein networks formed at b200 C, exploiting E-, W-, and S-dipping joints as well as pseudotachylyte- and cataclasite-bearing faults. Pseudotachylyte- and cataclasite-bearing faults are found predominantly within east west to NW SE trending fault zones including the Gole Larghe Fault Zone (GLFZ) discussed here, the Lares Fault Zone and Gole Strette Fault Zone to the south, and the Passo Cercen Fault Zone to the north (Fig. 1 modified from Mittempergher et al., 2009) The Gole Larghe Fault Zone The GLFZ is located mainly within the Val d'avio Val di Genova pluton, and strikes approximately east west for a distance of around 20 km (Fig. 1). The Val d'avio Val di Genova pluton is dominated by medium- to fine-grained tonalities with a bulk mineralogy consisting of 45 50% plagioclase, 25 30% quartz, 15 20% biotite and 1 5% K-feldspar (Di Toro and Pennacchioni, 2004). The GLFZ contains sub-parallel, approximately east west striking, steeply south-dipping cataclasite- and pseudotachylyte-bearing faults (Fig. 2a; Bistacchi et al., 2011; Di Toro and Pennacchioni, 2005; Griffith et al., 2010; Pennacchioni et al., 2006). Fault lineations dip shallowly to the east, and marker offsets consistently indicate dextral strike-slip movements (i.e. the faults are dextral oblique-slip; Fig. 2a). Previous observations in the Lobbia Glacier area indicated that the cataclasite- and pseudotachylyte-bearing faults define a fault zone around m thick (Figs. 2 and 3; Di Toro and Pennacchioni, 2005). The microstructural, mineralogical and geochemical characteristics of cataclasites and pseudotachylytes in the GLFZ have been extensively described by Di Toro and Pennacchioni (2004, 2005). Cataclasites are green in color and enriched in K-feldspar, epidote and chlorite with respect to the host tonalite. Grey to black in color pseudotachylytes occur as generation veins sub-parallel to the cataclasites as well as injection veins in the local host rocks (Di Toro and Pennacchioni, 2004, 2005; Di Toro et al., 2005; Griffith et al., 2010, 2012). 39 Ar 40 Ar dating of bulk pseudotachylyte indicates that faulting occurred at around 30 Ma (Pennacchioni et al., 2006), following pluton emplacement (34 32 Ma; Del Moro et al., 1983) and broadly coeval with movements along the Tonale Line (reviewed in Stipp et al., 2004). Seismic faulting occurred prior to substantial exhumation of the Adamello batholith, at depths of 9 11 km (similar to the emplacement depth of the batholith) and temperatures in the range C. These estimates arise from: 1) the observation of low-t plasticity in quartz surrounding the cataclasites (TEM investigations, Di Toro and Pennacchioni, 2004); 2) the stable mineral assemblage K-feldspar + epidote + chlorite in cataclasites (Di Toro and Pennacchioni, 2004, 2005; Pennacchioni et al., 2006), and; 3) the age of the pseudotachylytes (30 Ma) which precedes significant exhumation of the Adamello batholith (at b22 Ma) as constrained by thermochronology (zircon and apatite fission track and (U Th Sm)/He dating; Reverman et al., 2012; Viola, 2000). 3. Methodology In this work the GLFZ was studied across outcrops in front of the Lobbia Glacier, in the Upper Genova Valley (Figs. 1 and 2). In this area the fault zone is continuously exposed across steep glacier-polished terrain with almost no accumulation of debris. Elevations in the study area range from c m (at the front of the Lobbia Glacier) to 2000 m (base of the studied outcrops; Figs. 2 and 3). Structures associated with the GLFZ, most notably cataclasite- and pseudotachylyte-bearing faults, have a strong east west preferred strike orientation (stereoplots in Fig. 2a). In this case, a suitable method of Fig. 1. Geological setting of the Gole Larghe Fault Zone after Mittempergher et al. (2009), (a) simplified geological map of the Italian Southern Alps and Adamello batholith. GLFZ, Gole Larghe Fault Zone; PCFZ, Passo Cercen Fault Zone; GSFZ, Gole Strette Fault Zone; LF, Lares Fault Zone. The red rectangle shows the area covered by b, (b) structural map of the area surrounding the Gole Larghe Fault Zone, showing the main fault zones and lineaments recognizable in aerial photographs. The irregular trace of the Gole Larghe Fault Zone is due to intersection of the dipping fault with surface topography. The Gole Larghe Fault Zone is found mainly within the Val d'avio Val di Genova pluton. The red rectangle shows the area in front of the Lobbia Glacier studied here and covered in Fig. 2a.

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5 S.A.F. Smith et al. / Tectonophysics 599 (2013) base of the main outcrops (c m) there is no exposure for several hundred meters, but one transect (transect 1b) was completed across an outcrop at c m, around 500 m north of the main outcrops (Figs. 2a and3) Sample collection and microcrack analysis Fig. 3. Topographic cross section x y (no vertical exaggeration, location in Fig. 2a) showing the southern and northern margins of the GLFZ (thick dashed lines; described in Section 4), two 2 m-thick cataclasites (green lines; Section 4.4) and the margins of the alteration zone (thin dashed lines; Sections 4 and 5). The approximate locations of line transects across the topographic surface are shown in red (those in Fig. 5) and blue. The convention adopted in subsequent figures is negative distances to the south of the southern margin (moving in to the southern wall rocks) and positive distances to the north of the southern margin (moving in to the GLFZ and eventually in to the northern wall rocks). capturing fault zone structure is to use line transects approximately perpendicular to fault strike (Fig. 2a; mean fault orientation shown as blue great circle in stereoplot ii). Differential GPS units were employed to survey line transects across the GLFZ, with the aim of assessing how attributes such as fracture density vary with distance across the fault zone. Following processing, the typical precision of GPS survey points (representing the intersections of fractures with line transects) relative to the GPS base station was b1 cminxandy,and2 3 cminz Fracture transects using differential GPS The often steep terrain in the Lobbia Glacier area (Fig. 2b and c) prohibited one continuous transect across the entire fault zone. Instead, thirty-nine individual transects each between 3 m and 110 m long were carried out in areas of full exposure, both within the GLFZ and the tonalite wall rocks to the north and south (Figs. 2a and 3). Along each transect, the locations and attributes of all visible fractures were recorded (total number of fractures = 3514). Fractures were recorded as one of the following: joint, fault containing cataclasite or pseudotachylyte or both, or hairline fracture. Hairline fractures showed no opening or shear offset, and were laterally continuous for less than a few centimeters; they are not included in the data presented below and are not discussed further. For each fracture, the following attributes were noted if available: orientation of fracture, orientation of surface lineations, separation of displaced markers (e.g., aplite dykes, xenoliths or mafic inclusions), thickness and type of fault rocks, and any further comments (e.g., evidence of younger reactivation provided by presence of zeolite, crosscutting relationships etc.). Individual transects typically started and ended at prominent E W striking joints or faults that could be traced along strike for tens to hundreds of meters. Adjacent transects then used the same structures as start or end points, effectively allowing the construction of a nearcontinuous transect that spanned a north south distance of ~900 m across the GLFZ and surrounding wall rocks (Figs. 2a and 3). At the Forty-eight oriented samples were collected with roughly equal spacing across the wall rocks and fault zone, avoiding major cataclasite- and pseudotachylyte-bearing fault strands (Fig. 2a). These samples are thus representative of damage that accumulated in more intact blocks of tonalite surrounded by fault strands. Thin sections were cut in the horizontal plane, with the long axis of the thin section oriented east west and the short axis of the thin section oriented north south. Analysis of microfracture densities in quartz was performed with the optical microscope following methods outlined in Mitchell and Faulkner (2009). Quartz was selected for microfracture counts as it has little fracture anisotropy in comparison to feldspar and biotite. Our use of thin sections cut in the horizontal plane differs from the use of three orthogonal thin sections by Anders and Wiltschko (1994) and Wilson et al. (2003). However, thin sections in the horizontal plane provide the maximum visibility of microfractures related to steeply-dipping, mainly strike-slip faults (Engelder, 1974; Vermilye and Scholz, 1998). Microfractures were divided into (1) sealed microfractures (generally sealed with K-feldspar and epidote based on SEM observations in this paper and X-ray diffraction analysis in Di Toro and Pennacchioni, 2005) and,(2) open microfractures, i.e. those that did not contain a mineral fill. We infer that both sealed and open microfractures were formed coeval to activity along the GLFZ since sealed microfractures are filled by the same mineral assemblage as the cataclastic faults, and mutually crosscutting relationships are observed between the two sets Data processing The mean orientation of all faults within the GLFZ (blue great circle in stereonet ii, Fig. 2a) was used to construct a reference plane (strike 107; dip 59 S) that intersects the outcrop surface at the location where the line transects cross the southern margin of the fault zone (defined as distance = 0 m, Fig. 3). For each GPS point (i.e. for each fracture location along the transects), the orthogonal distance to the reference plane was calculated (Fig. 3). This geometric correction provides a way of comparing fracture data collected along different transects. In the text and figures below we adopt the convention of negative distances from the reference plane moving south in to the southern wall rocks, and positive distances from the reference plane moving north through the GLFZ (Fig. 3). 4. Fracture network characteristics across the Gole Larghe Fault Zone Fig. 4 summarizes variations in fracture orientations and attributes as a function of distance along the line transects. Each data point represents the average value for one of the 39 individual line transects (with lengths ranging between 3 and 110 m). For example, fracture density (Fig. 4b) was calculated for each line transect by dividing the number of fractures by transect length. The data point is then plotted at a distance corresponding to the transect mid-point. The horizontal black lines associated with the data points show the total length of each individual transect. Between 280 m and 580 m Fig. 2. Map and photographs of the survey area, (a) location of thirty-nine GPS line transects and 48 samples across the GLFZ and wall rocks. Also shown are the southern and northern margins of the fault zone (defined by large variations in fracture properties described in Section 4), the locations of the two thickest cataclasites identified in the GLFZ (Section 4.4), and the margins of the central alteration zone (Sections 4 and 5). Profile x y isshown in Fig. 3. The lower-hemisphere, equal-area stereoplots show (i) poles to joint surfaces in the southern wall rocks (n = 381), and (ii) poles to cataclasite- and pseudotachylyte-bearing faults inside the GLFZ (black circles, n = 574) and the orientation of fault surface lineations (red circles, n = 18). The mean fault surface is shown as a blue great circle (strike 107; dip 59 S). The GLFZ is a right-lateral strike slip fault with a reverse component. (b) Photograph (looking south) of the Lobbia Glacier area in the Upper Genova Valley. The GLFZ is exposed at elevations between 2000 m and 2700 m, (c) photograph overlooking the central and southern zones of the GLFZ, and the southern wall rocks. The dominant east west strike of faults in the GLFZ is visible across the polished outcrop surfaces.

6 34 S.A.F. Smith et al. / Tectonophysics 599 (2013) Fig. 4. Variations in fracture orientations and attributes with distance across the GLFZ. In parts b d each data point represents the average value for an individual line transect (39 transects in total), and the total length of each transect is shown by the horizontal black lines (transect lengths range between 3 and 110 m), (a) rose diagrams (2 sector width) showing the strikes of fractures (joints and faults) in different areas of the fault zone and wall rocks, (b) fracture density vs. distance along transect, (c) percentage of faults and joints vs. distance (red data), and the percentage of faults containing pseudotachylytes vs. distance (grey data), (d) fault rock (cataclasite + pseudotachylyte) thickness as a percentage of transect length vs. distance. Inset shows a detail from the central part of the fault zone containing the 2 m-thick cataclasites (Section 4.4).

7 S.A.F. Smith et al. / Tectonophysics 599 (2013) almost complete outcrop coverage was achieved. Between 580 m and 1100 m there is no data because of a lack of exposure. An isolated transect (transect 1b) was measured between 1100 and 1150 m. Also shown in Fig. 4 are the locations of the two thickest cataclastic faults identified in the Lobbia Glacier area to date (described in Section 4.4). Both of these faults appear as dark green, highly indurated horizons and contain a c. 2 m thick cataclasite/ultracataclasite, surrounded by up to several meters of protocataclasite and highly fractured tonalite host rocks. They are exceptional because a large majority of cataclastic faults in the GLFZ are at most a few centimeters thick, with rare examples up to a few tens of centimeters thick and only one other fault identified with a thickness of almost 1 m. Moreover, these two thick cataclastic faults are distinguished from all others in the GLFZ because they are laterally continuous along strike for distances >500 m Fracture density Fractures include both early-formed magmatic cooling joints and cataclasite- and pseudotachylyte-bearing faults. Fracture density is relatively low in the southern wall rocks, averaging between 1 and 3 fractures per meter (Fig. 4b). At the southern margin of the fault zone there is a small but abrupt increase in fracture density (Fig. 4b). Fracture density then increases within the southern zone to reach maximum values of fractures per meter in the central zone between the 2 m-thick cataclasites (Fig. 4b). In the northern zone, fracture density decreases again to 3 5 fractures per meter at a distance of m from the southern margin (Fig. 4b). At this distance, the fracture density is similar to within the southern wall rocks and the isolated transect in the northern wall rocks located at 1100 m. For this reason, we consider that the northern margin of the GLFZ is located at c. 600 m along the transects (Fig. 4b) Relative importance of faults vs. joints and distribution of pseudotachylyte In the southern wall rocks >80% of fractures are represented by conjugate sets of early-formed magmatic cooling joints (Figs. 4c and 5a). These joints strike mainly east west, with a minor set striking approximately north south (Figs. 4a and 5a). Occasionally, reworking of joints by cataclasite-bearing faults occurs in the southern wall rocks (Fig. 5a), and this increases in frequency towards the southern margin of the fault zone (Fig. 4c). Joints that are reworked by cataclastic faults are typically those that are continuous along strike for tens to hundreds of meters, and have orientations similar to the mean fault orientation (approximately east west striking, moderately south dipping; Fig. 5a). Reworked joints are spaced regularly at distances of 5 10 m (Fig. 5a). Measured values of separation across reworked joints in the southern wall rocks are up to 70 cm (Fig. 5a). Pseudotachylytes are absent in the southern wall rocks (Fig. 4c). The southern margin of the GLFZ is most clearly defined by a large and abrupt increase in the percentage of faults with respect to joints, the transition occurring across a single moderately south-dipping cataclastic fault (Figs. 4c and 5b). Although this abrupt transition has been captured by the line transects only in one location, observations along fault-strike, particularly in the area immediately to the north of the Lobbia Glacier terminus, suggest that it is characteristic of the southern fault zone margin. In both the southern and northern zones, typically between 80 and 100% of fractures are cataclasite- and pseudotachylyte-bearing faults (Figs. 4c and 5c d). Pseudotachylytes are particularly common in the southern zone, where they are present along 20 60% of faults (Fig. 4c). Faults in these areas are dominantly WNW ESE striking and dip moderately to the south (Figs. 4a and 5c d). Measured values of separation across individual faults in the southern and northern zones are up to 7.5 m, with larger separations typically occurring across faults with thicker cataclastic horizons (Fig. 5c d). Along transect 15 (Fig. 5d) separation of displaced markers could be measured across most faults, including all of those with cataclasites thicker than 2 cm. Total separation for this 15.9 m long transect was m, resulting in a separation to fault thickness ratio (γ) of 1.8. This ratio is likely a slight underestimate considering that separation could not be measured across some of the more minor faults (those with millimeterthick cataclasites). In the central zone (i.e. between the 2 m-thick cataclasites), between 50 and 80% of fractures are cataclasite- and pseudotachylyte-bearing faults (Fig. 4c). However, pseudotachylytes are relatively scarce, being present along b10% of faults. Field and microstructural observations suggest that the scarcity of pseudotachylytes in the central zone is due to pervasive reworking and alteration by dense cataclastic fault fracture networks (described in Section 4.4 and Fig. 7). The presence of these fracture networks largely accounts for the high fracture densities in the central zone (Fig. 4b). Fault orientations are much more widely distributed in the central zone, and structures forming a high angle with the mean fault strike are more common (Fig. 4a). In the northern wall rocks (represented by the transect at 1100 m distance) ~90% of fractures are cataclasite-bearing faults, despite the low overall fracture density (Figs. 4c and 5e). This highlights an asymmetry in the style of deformation to the south and north of the GLFZ. Faults in the northern wall rocks are typically clustered, each cluster being separated by intact blocks of tonalite a few meters wide (Fig. 5e). Measured separations are smaller than within the GLFZ, with maximum values of a few tens of centimeters (Fig. 5e) Fault rock thickness The total thickness of cataclasite + pseudotachylyte along each line transect is plotted as a percentage of transect length in Fig. 4d. This mirrors somewhat the fracture density (Fig. 4b), but it illustrates particularly well certain characteristics of the GLFZ. In the southern and northern zones, fault rock thickness is generally b10% of transect length. However, in the central zone between the 2 m-thick cataclasites fault rocks account for >20%, and in some cases >40% of transect length, reflecting the presence of dense cataclastic fault fracture networks (Fig. 4d, Section 4.4). The inset in Fig. 4d shows that the transects with the greatest thickness of fault rocks are immediately adjacent to the 2 m-thick cataclasites, with a slight decrease in fault rock thickness in the middle of the central zone (c. 300 m distance) Fault rocks The southern zone was previously studied by Di Toro and Pennacchioni (2005). These authors developed a model involving cataclasis along pre-existing magmatic cooling joints, followed by frictional melting and formation of pseudotachylytes. Our field observations suggest that this model is also applicable to the northern zone, where pseudotachylytes generally crosscut cataclasites and thus represent the youngest stage of deformation (Fig. 6). Alteration in these areas is mainly limited to the cataclasite fault layers and the tonalite host rocks a few centimeters either side of the faults. In most faults, a black, unaltered pseudotachylyte cuts across the cataclasites, altered tonalite and, in the most important faults, reworked pseudotachylyte (Fig. 6a c). The mineral assemblage of pseudotachylytes in this area includes high temperature mineral phases (plagioclase, biotite, dmisteinbergite) and several well preserved textures diagnostic of rapid cooling of a melt (plagioclase microlites, fluidal structures, spherulites, Fig. 6d; Di Toro and Pennacchioni, 2004; Nestola et al., 2010). The central zone includes two main characteristics that are clearly different from the southern and northern zones: (i) the occurrence of dense cataclastic fault fracture networks (Fig. 7a and b) and (ii) the presence of two 2 m-thick cataclastic faults (Fig. 7c g). The pervasive networks of fractures and faults in the central zone are responsible for

8 36 S.A.F. Smith et al. / Tectonophysics 599 (2013) Fig. 5. Line transects from different areas of the wall rocks and southern fault zone (locations shown in Figs. 2 4). Joints are shown as blue lines, and cataclasite- and pseudotachylyte-bearing faults as red lines. Values in italics show fault separation in centimeters whereitwaspossibletomeasure.thenumbersattheendsofeachtransect(e.g. 271 m at the southern end of transect 3) show the distance from the southern margin of the GLFZ. The stereoplots show the orientation of joints and faults (great circles), and the plunge and azimuth of fault lineations (black circles), (a) transect 3 from the southern wall rocks, (b) transects 8 and 9 that cross the transition from the southern wall rocks in to southern fault zone, (c) transect 13 from the southern fault zone. (d) transect 15 from the southern fault zone. Separation values were measured across most of the faults in transect 15. Total measured separation was m for the 15.9 m long transect, (e) transect 1b from the northern wall rocks.

9 S.A.F. Smith et al. / Tectonophysics 599 (2013) Fig. 6. Typical styles of faulting in the southern and northern zones of the GLFZ, (a) detailed 1 m 1 m outcrop map, located at N, E. In these areas cataclasite- and pseudotachylyte-bearing faults surround relatively intact blocks of tonalite, and pseudotachylyte overprints cataclasites (Di Toro and Pennacchioni, 2005), (b) photograph of tabular black pseudotachylyte fault vein (PST) crosscutting green cataclasites (CC). The pseudotachylyte shows injection veins that branch in to the local wall rocks, (c) photomicrograph in planepolarized light showing unaltered pseudotachylyte vein and small injection vein branchinginto adjacent cataclasites, (d) photomicrograph in plane polarized lightof a fresh spherulitic texture in pseudotachylyte. The brownish color is due to the abundance of biotite in the matrix. the relatively high fracture densities there (Fig. 4b). Many of the small cataclastic faults that form the fracture networks strike at high angles to the line transects (i.e. NNW SSE to NNE SSW), and thus a better representation of the close fault spacing can be achieved by comparing the 1 m 1 m outcrop maps in Fig. 6a (representativeofthesouthern and northern zones) and 7a (representative of the central zone). The fault fracture networks in the central zone contain cataclastic faults with both right-lateral and left-lateral offsets of up to a few centimeters (Fig. 7a). The faults systematically crosscut the dominant east west striking structures in the GLFZ (cataclasite pseudotachylyte faults; Fig. 7b) and are associated with reworking of pseudotachylytes that become violet, green or white in color (Fig. 7b). The 2 m-thick cataclasites that define the margins of the central zone strike approximately east west and dip moderately to the south (i.e. their orientations are similar to the mean fault orientation in the GLFZ; Fig. 7c). Both faults can be traced along strike for up to 500 m before running in to inaccessible areas. The northernmost of the two thick cataclasites is associated with a prominent topographic lineament that can be identified in aerial photographs on both sides of the Upper Genova valley, suggesting that it may strike for >3 km. Although displacement across these two thick faults could not be determined, we infer that displacement across each was >20 m based on the fact that two other cataclastic horizons c. 0.5 m and c. 1 m thick located in the southern zone, about 300 m to the east of the line transects, displaced a 0.5 m thick aplite dyke c.10 m [Di Toro et al., 2005] and c. 20 m (unpublished data), respectively. The southernmost 2 m thick cataclasite crops out at the bottom of a small cliff exposing a complete section through the fault (Fig. 7c and d). Several fault rock domains are distinguished (from bottom to top): (1) a 2 3 m thick horizon of greenish protocataclasite (PC in Fig. 7c and d) and highly fractured tonalite; (2) a laterally continuous, 2 3 cm thick layer of pseudotachylyte (PST), partly altered to zeolite due to later fluid circulation (evidenced by a reddish halo and violet or white color of pseudotachylyte, Fig. 7d); (3) a c. 20 cm thick layer of light

10 38 S.A.F. Smith et al. / Tectonophysics 599 (2013) Fig. 7. Typical styles of faulting in the central zone of the GLFZ, (a) detailed 1 m 1 m outcrop map, located at N, E. In the central zone the fracture density is much higher due to the presence of cataclastic fault fracture networks that give the outcrop a distinct green coloration. Legend as for Fig. 6a. (b) Photograph of E W striking altered pseudotachylyte fault vein dissected by a network of small-displacement cataclastic faults striking dominantly NNW NNE, (c) schematic of the southernmost 2 m-thick cataclasite. At the base of the fault is a 20 cm-thick layer of ultracataclasite (UC). The upper 180 cm is composed of cataclasites (CC) and protocataclasites (PC). The boundaries with the surrounding protocataclasites and fractured tonalite host rocks are marked by centimeter-thick, continuous layers of pseudotachylyte (PST), (d) photograph of the basal ultracataclasite and overlying cataclasites. The red and white layers surrounding the pseudotachylyte vein are due to late-stage zeolite mineralization, (e) photomicrograph in plane polarized light of the ultracataclastic matrix (UC) containing a reworked clast of pseudotachylyte (PST). The white dashed line marks a wavy foliation developed around the margins of the pseudotachylyte clast, (f) photomicrograph in plane polarized light of an altered spherulitic domain in a reworked pseudotachylyte slab. The matrix is white due to the leaching of biotite (compare with Fig. 6d) and the spherulites are partly obliterated by growth of chlorite and epidote, (g) photomicrograph in plane polarized light of the cataclastic matrix (CC) containing reworked fragments of protocataclasite (PC) as well as clasts of pseudotachylytes (white arrows). green ultracataclasite (UC); (4) a cm thick horizon of dark green cataclasite (CC) grading upwards to protocataclasite; (5) a cm thick horizon of greenish protocataclasite (PC); (6) a laterally continuous 2 3 cm thick layer of pseudotachylyte (PST), crosscutting all other structures and much better preserved than layer 2; and (7) a 3 5 mthickhorizon of greenish protocataclasite (PC) and highly fractured tonalite. All of the cataclastic fault rocks contain reworked clasts of pseudotachylyte (e.g. Fig. 7e). Within reworked pseudotachylyte clasts primary microstructures diagnostic of rapid cooling of a melt (e.g. spherulites) are often overprinted and obliterated by precipitation of K-feldspar, chlorite and epidote, and leaching of biotite (Fig. 7f). Under the optical microscope, the light green ultracataclasites (layer 3) are composed of a microcrystalline matrix of epidote and K-feldspar (Fig. 7e), surrounding clasts of reworked cataclasite, pseudotachylyte and, to a lesser extent, quartz and plagioclase. Plagioclase clasts are rare and overprinted by saussuritic alteration of

11 S.A.F. Smith et al. / Tectonophysics 599 (2013) microcrystals of epidote and white mica. A wavy foliation of dark, fine grained minerals is locally developed, mostly around hard objects such as pseudotachylyte clasts (Fig. 7e). The dark green cataclasites (layer 4) are composed of multiple individual cataclasite layers, bounding less deformed, protocataclastic domains (Fig. 7g). In the cataclasites, clasts of quartz, plagioclase, and reworked pseudotachylytes and cataclasites are cemented by chlorite, epidote and K-feldspar (Fig. 7g). In contact with chlorite, quartz clasts have lobate cuspate boundaries, and a wavy foliation defined by dark, fine grained layers of titanite, chlorite and opaque minerals is locally developed. 5. Microfracture density and mineral sealing in the GLFZ Fig. 8a shows variations in microfracture density across the GLFZ, as well as images of scanned thin sections that illustrate progressive changes in the mineralogy of the tonalite host rocks. We emphasize that microfracture densities in the analyzed samples are representative of damage that accumulated in blocks of tonalite between main fault strands. Microfracturing that occurred close to main fault strands as a direct consequence of slip across non-planar faults is described in Griffith et al. (2010). Total microfracture density is lowest (8 15 per millimeter) at the two ends of the transect (Fig. 8a). Here, biotite and plagioclase in the host tonalite are relatively unaltered (Fig. 8a and b). In the southern wall rocks and southern zone microfracture density increases approximately linearly to reach maximum values of ~50 per millimeter adjacent to the central zone (Fig. 8a). The only deviation in this trend is a more abrupt increase in microfracture density located at a transect distance of c. 200 m (Fig. 8a). Microfracture density is relatively high at per millimeter inside and immediately surrounding the central zone, where partial to pervasive alteration of biotite to chlorite Fig. 8. Microfracture density variations and sealing across the GLFZ. (a) Graph of microfracture density vs. distance across the fault zone. Both total microfracture density and the density of sealed microfractures are shown. Underneath the plot are images of scanned thin sections from varying distances across the fault zone. The thin sections illustrate the progressive transformation of brown biotite in the host rock to green chlorite approaching the central area of the GLFZ. Biotite is partially altered to chlorite at distances between 200 and 300 m along the transect and pervasively altered to chlorite between 300 and 400 m. Together with the presence of cataclastic fault fracture networks this defines a c. 200 m wide alteration zone between and surrounding the 2 m-thick cataclasites, (b) backscattered scanning electron microscope image of fractured tonalite at a distance of 160 m in the southern wall rocks. Qtz, Quartz; Plag, Plagioclase feldspar. Microfractures in quartz and plagioclase from this area are typically unsealed, (c) backscattered scanning electron microscope image of fractured tonalite at a distance of 334 m in the central zone of the GLFZ. Microfractures in this area are typically sealed by K-feldspar (K-feld), epidote (Epi) and minor chlorite (not visible in image).

12 40 S.A.F. Smith et al. / Tectonophysics 599 (2013) occurs in the host tonalite (Fig. 8a). Microfracture density then decreases again in the northern zone, although it is on average 30% higher than in the southern zone. In the southern wall rocks and southern zone, between 20 and 40% of microfractures are sealed (Fig. 8a and b). There is a spike in sealed microfracture density around the southern margin of the fault zone, where they account for 50 80% of the total (Fig. 8a). Within and adjacent to the central zone, sealed microfractures account for 50 85% of the total microfracture density (Fig. 8a). Backscattered scanning electron microscope images, combined with the results of powder X-ray diffraction analysis reported in Di Toro and Pennacchioni (2005),indicate that microfractures are sealed mainly by K-feldspar and epidote, with minor chlorite (Fig. 8c). Due to the combined effects of alteration of biotite to chlorite, and precipitation of K-feldspar and epidote in microfractures, the matrix of the tonalites within and surrounding the central zone (c m along the transects) has a distinct green coloration in the field (Figs. 8a and 9). This defines a region approximately 200 m wide of pervasive alteration and fluid rock interaction (Fig. 8). The southern margin of this alteration zone is visible as an irregular fluid infiltration front across particularly clean exposures (Fig. 9). The position of the infiltration front coincides with the abrupt increase in microfracture density at a distance of c. 200 m (Fig. 8a). The southern infiltration front has a distinctly lobate geometry, including isolated patches of altered tonalite detached from the main alteration zone(fig. 9). The northern front of the alteration zone is not clearly exposed, but its location can be approximated due to a change of color in the tonalite matrix (Fig. 8a). southern margin of the GLFZ and the southernmost of the 2 m-thick cataclasites) fracture densities increase progressively to the north, there is a high percentage of faults compared to joints, but the overall volume of fault rock materials (measured as fault rock thickness) is low (Fig. 10). In the southern zone it is also common to find fresh pseudotachylytes that overprint earlier-formed cataclasites (Figs.6,10; Di Toro and Pennacchioni, 2004, 2005, Di Toro et al., 2005). The c. 100 m-thick central zone is flanked by 2 m-thick cataclasites. It contains high fracture densities and a relatively high percentage of faults compared to joints. However, it is most clearly defined by a much higher volume of fault rock materials than within the southern and northern zones (Fig. 10). The central zone is distinctive in the field due to the presence of green cataclastic fault fracture networks that rework pseudotachylytes (Fig. 7). We speculate that pseudotachylytes were initially much more abundant in the central zone prior to reworking by the cataclastic fault fracture networks (Fig. 10). The c. 250 m-thick northern zone contains similar fracture properties to the southern zone (e.g. Fig. 6), the main difference being that pseudotachylytes are much less common (Fig. 10). Unlike in the central zone, the relative scarcity of pseudotachylytes in the northern zone does not reflect reworking and alteration by later stages of cataclasis and fluid rock interaction, suggesting that pseudotachylytes were not produced as frequently in the northern zone. To the north of the GLFZ the fracture density is low but faults dominate over joints (Fig. 10). Although the GLFZ has a broadly symmetric structure, this latter characteristic highlights an important difference in the style of deformation in the northern and southern wall rocks (Fig. 10). 6. Discussion 6.1. Synoptic view of the structure of the GLFZ In the southern wall rocks fracture density is relatively low and magmatic cooling joints are the dominant structure (Fig. 10). The southern margin of the GLFZ is most well defined by an abrupt increase in the number of cataclasite- and pseudotachylyte-bearing faults compared to joints (Fig. 10). In the c. 250 m-thick southern zone (i.e. between the 6.2. Evolution of fracture damage in the GLFZ In the southern and northern zones, microfractures sealed by K-feldspar and epidote are typically restricted to horizons a few centimeters thick on either side of cataclasite- and pseudotachylyte-bearing faults. Quartz in the intervening blocks of tonalite, where our samples for microfracture analysis were collected, contains mainly healed fluid inclusion surfaces and open microfractures. Sealed microfractures are much more abundant in the central zone, and for up to 50 m either Fig. 9. Photograph showing the highly irregular southern margin of the 200 m-thick alteration zone. Photograph taken looking north-west from N, E. The southern fluid infiltration front is visible as a change from white, relatively unaltered tonalite on the left of the image to green, pervasively altered tonalite on the right of the image. The front has a lobate geometry including isolated patches of altered tonalite that are detached from the main alteration zone. In the background the heavily eroded valleys containing the 2 m-thick cataclasites are visible.

13 S.A.F. Smith et al. / Tectonophysics 599 (2013) Fig. 10. Synoptic view of the structure of the GLFZ based on field surveys and microstructural analysis. (a) Schematic plots showing how the relative intensity of various fracture attributes vary with distance across the fault zone. The fine dashed line in the third plot from top shows an interpretation of the distribution of pseudotachylyte prior to its reworking by cataclastic fault fracture meshes in the central zone. (b) Simplified geological sketch of the structure of the GLFZ. The sketch highlights the main zones within the GLFZ, including the c. 200 m-wide alteration zone associated with pervasive fluid rock interaction. The γ values show the ratio of fault thickness to fault displacement measured in two areas of the southern zone (see Section 6.2 for an explanation). side of the central zone, where they are found in association with the cataclastic fault fracture networks responsible for the high fracture densities. In these areas sealed microfractures are not only found close to cataclastic horizons, but are widespread within the intervening blocks of tonalite, defining a c. 200 m-wide region of pervasive alteration and fluid rock interaction (Fig. 10). Although a large majority of pseudotachylytes within the alteration zone are reworked (Fig. 7b), there are some examples of pseudotachylytes that crosscut the cataclastic fault fracture networks and associated microfractures, including the laterally continuous, centimeter-thick pseudotachylytes found along the margins of the southernmost 2 m-thick cataclasite (Fig. 7c). This indicates that fluid rock interaction and subsequent mineralization within the alteration zone occurred at least partly at seismogenic depths. Geochemical data are not presently available to distinguish the origin(s) of fluids in the central, highly altered part of the GLFZ. However, the mineral assemblages filling sealed microfractures (K-feldspar, epidote, chlorite) in the alteration zone are the same as those found in cataclastic faults in the southern zone. In the southern zone, the abundance of large ion lithophile elements (Rb, Ba, U, Th) in the cataclasites with respect to the tonalitic wall rocks and to the older mylonites suggests that induration of the cataclasites involved hydrothermal water-rich fluids (Di Toro and Pennacchioni, 2005). Additionally, the interpretation of hydrogen isotope data (δd) suggests that the fluids are compatible with a crustal source (i.e., the involvement of surficial, meteoric fluids is excluded; Dallai et al., 2009). As in many fault zones with intense fracture damage, it is difficult to estimate how much displacement was accommodated in the southern, central, and northern zones of the GLFZ. Di Toro and Pennacchioni (2005) traced a 0.4 m thick aplite dyke for c. 25 m across fault strike in the southern zone. The aplite dyke was offset a total of c. 48 m, resulting in a fault displacement to fault thickness ratio, γ,of 1.95(Fig. 10). This is similar to γ of 1.8 measured across transect 15 (likely a slight underestimate; Figs. 5eand10). Adopting γ of 1.95 for the c. 250-m thick southern zone indicates that around 500 m of cumulative displacement was

14 42 S.A.F. Smith et al. / Tectonophysics 599 (2013) accommodated in this zone. We consider this a reasonably well constrained estimate of cumulative displacement in the southern zone because nearly complete outcrop coverage was achieved along the line transects here, and no faults were observed that individually accommodated displacements greater than a few meters. Given that fracture properties (e.g. density, percentage of faults/joints, volume of fault rocks) and the general style of faulting in the c. 250-m thick northern zone are similar (Figs. 6 and 10), we tentatively suggest that cumulative displacement in the northern zone was also on the order of several hundreds of meters, although quantitative estimates of γ in the northern zone are lacking. It is more difficult to estimate how much displacement was accommodated in the central zone because of extreme disruption to structural markers. The markedly higher fracture densities and volume of fault rock materials in the central zone (Fig. 10) maysuggestthat cumulative displacement was significantly higher than elsewhere in the GLFZ. However, it is evident from detailed mapping in the central zone that the relatively high fracture densities result from the presence of networks of small-displacement cataclastic faults (Fig. 7a and b). Displacements across these faults where measurable are typically no greater than a few centimeters (Fig. 7a and b), and no faults were identified that may have accommodated displacements larger than a few meters. From this, and also considering the thickness of the central zone (c. 100 m) with respect to the southern and northern zones (c. 250 m), we suggest that cumulative displacement in the central zone is unlikely to have been more than a few hundred meters. The most likely candidates for large-displacement structures in the GLFZ are the 2 m-thick cataclastic horizons recognized at the margins of the central zone (Fig. 10). Based on the thickness of these two structures, and the fact that they are each associated with surrounding zones of protocataclasite up to several meters thick, it seems reasonable to suggest that they individually accommodated between several tens and several hundreds of meters displacement. Even if this is the case, it is evident that, in general, displacement in the GLFZ occurred by diffuse faulting across the entire fault zone width, with relevant displacements being accommodated throughout the southern, central and northern zones. Additionally, the presence of pseudotachylytes across the entire fault zone width of c. 600 m (albeit in lower quantities in the northern zone), as well as at significant distances to the north of the GLFZ (Fig. 4), suggests that seismicity was broadly distributed over distances of several hundreds of meters to kilometers Comparison to ancient and active seismogenic sources The GLFZ shares certain characteristics with other large seismogenic strike-slip fault zones cutting crystalline basement rocks and exhumed from depths of 7 15 km, including the Fort Foster Brittle Zone in Maine (hosted in phyllitic and quartzitic metasedimentary rocks; Swanson, 1988, 2006), the Homestake Shear Zone in Colorado (hosted in Lower Proterozoic gneisses and plutonic rocks of granitic to intermediate composition; Allen, 2005) and the Glacier Lakes Fault in the Sierra Nevada (hosted in Mesozoic granites and granodiorites; Kirkpatrick et al., 2008). In each of these cases, deformation and seismicity (preserved as pseudotachylytes) were distributed amongst complex, linked zones of damage hundreds of meters to kilometers in width, rather than being localized within relatively narrow (less than a few meters) fault cores as has been documented for several large-displacement strike-slip faults exhumed from shallower (b5 km) depths (e.g. Punchbowl and North Branch San Gabriel Faults, California, Chester et al., 1993; 2004; Chester and Logan, 1986; Schulz and Evans, 2000). In the Homestake Shear Zone and Fort Foster Brittle Zone, cataclasites and pseudotachylytes exploited pre-existing, finely-spaced (mm-scale) metamorphic foliations in the host rocks. In particular, the structure of the GLFZ appears similar to the Caleta Coloso fault in northern Chile (although no direct evidence for seismic slip has been identified in this fault; Faulkner et al., 2008; Mitchell and Faulkner, 2009). This large strike-slip fault is hosted mainly in granodiorites, accommodated at least 5 km of left-lateral displacement and was exhumed from 5 to 10 km depth. The fault consists of a distinctly green in color, m thick hydrothermally altered fault core containing protocataclasites and higher-strain ultracataclasite horizons, reminiscent of the central zone in the GLFZ (Faulkner et al., 2008). The core of hydrothermal alteration in the Caleta Coloso fault was interpreted to result from extensive fluid rock interaction and precipitationstrengthening (Faulkner et al., 2008). This is surrounded by zones c. 150 m thick in which fracture and microfracture densities decrease towards background levels in the country rocks (Mitchell and Faulkner, 2009), similar to within the southern and northern zones of the GLFZ. The GLFZ, with a total displacement on the order of 1 2 km,mayberepresentative of an earlier stage in the evolution of large strike-slip faults in basement with respect to that exhibited by the Caleta Coloso fault. Field observations from the GLFZ and the other large (in some cases pseudotachylyte-bearing) strike-slip faults above indicate that faulting and seismicity in crystalline basement of the middle crust may be more widely distributed than is often assumed for faults in the upper crust. This is compatible with recent seismological observations of earthquake sequences in the middle crust that show ruptures occurring within rock volumes hundreds of meters to kilometers wide, an example of which is described in the following two paragraphs. A seismically active equivalent of the GLFZ and of the other mid-crustal pseudotachylyte-bearing fault zones above may be the fault network responsible for the 2002 Au Sable Forks M5 intraplate earthquake sequence (Kim and Abercrombie, 2006; Viegas et al., 2010). The hypocentral depths of the main shock and aftershocks in this earthquake sequence were between 9.5 and 12 km(viegas et al., 2010). Seismic reflection and tomographic studies measured high Vp/Vs ratios ( ) and high Vp (6.6 km/s) at these depths, suggesting the presence of dry rocks (low fluid pore-pressures) of mafic to tonalitic composition, consistent with lithologies documented in the area by surface geological mapping (anorthosites, tonalites, gabbros, granulites; Musacchio et al., 1997; Wiener et al., 1984). A portable seismic network installed in the week following the M5 mainshock allowed localization of aftershock hypocenters to within an error of 500 m radius, making them the most well localized earthquakes in the northeastern US to date (Kim and Abercrombie, 2006; Viegas et al., 2010). Although the mainshock fault surface could not be precisely determined (Seeber et al., 2002; Viegas et al., 2010), the aftershocks of the Au Sable Forks earthquake sequence illuminated a m thick seismically active layer, most likely containing several sub-parallel fault strands (Viegaset al., 2010). Other high-precision earthquake relocation and tomographic studies have also documented aftershock sequences in the middle crust (5 15 km) that define broad fault zones hundreds of meters to several kilometers wide (e.g. active strike-slip faults in southern California, Allam and Ben-Zion, 2012; Hauksson, 2010; active normal faults in central Italy, Valoroso et al., 2013). A second important characteristic of the Au Sable Forks earthquake sequence was the high static stress drops (187 MPa for the main shock, and 104 MPa for the median of the M > 3 earthquakes) and the high (roughly five times larger than plate boundary earthquakes) radiated energy and apparent stress (Viegas et al., 2010). Dry, mafic rocks are amongst the most prone to melting in the case of extreme localization under seismic deformation conditions (Niemeijer et al., 2011; Sibson and Toy, 2006). Frictional melting often results in melt lubrication, at least for displacements larger than a few centimeters, which can induce large dynamic and static stress drops (Di Toro et al., 2011; Niemeijer et al., 2011). Given the favorable conditions and lithologies (dry silicate rocks), we might infer that pseudotachylytes were produced during the Au Sable Forks earthquake sequence. The occurrence of seismicity distributed over wide areas and, possibly, large stress drops associated with frictional melting are characteristics that the Au Sable Forks earthquake sequence shares with the GLFZ. These characteristics might be typical of mid-crustal earthquake sequences hosted in dry crystalline basement.