Reaction Localization and Softening of Texturally Hardened Mylonites in a Reactivated Fault Zone, Central Argentina

J. metamorphic Geol., 2005, 23, 411–424 doi:10.1111/j.1525-1314.2005.00588.x

Reaction localization and softening of texturally hardened mylonites in a reactivated fault zone, central Argentina

S. J. WHITMEYER1 * AND R. P. WINTSCH2 1Department of Earth Sciences, Boston University, Boston, MA 02215, USA ([email protected]) 2Department of Geological Sciences, Indiana University, Bloomington, IN 47305, USA

ABSTRACT The Tres Arboles ductile fault zone in the Eastern Sierras Pampeanas, central Argentina, experienced multiple ductile deformation and faulting events that involved a variety of textural and reaction hardening and softening processes. Much of the fault zone is characterized by a (D2) ultramylonite, composed of fine-grained biotite + plagioclase, that lacks a well-defined preferred orientation. The D2 fabric consists of a strong network of intergrown and interlocking grains that show little textural evidence for dislocation or dissolution creep. These ultramylonites contain gneissic rock fragments and porphyroclasts of plagioclase, sillimanite and garnet inherited from the gneissic and migmatitic protolith (D1) of the hangingwall. The assemblage of garnet + sillimanite + biotite suggests that D1-related fabrics developed under upper amphibolite facies conditions, and the persistence of biotite + garnet + sillimanite + plagioclase suggests that the ultramylonite of D2 developed under middle amphibolite facies conditions. Greenschist facies, mylonitic shear bands (D3) locally overprint D2 ultramylonites. Fine-grained folia of muscovite + chlorite ± biotite truncate earlier biotite + plagioclase textures, and coarser-grained muscovite partially replaces relic sillimanite grains. Anorthite content of shear band (D3) plagioclase is c. An30, distinct from D1 and D2 plagioclase (c. An35). The anorthite content of D3 plagioclase is consistent with a pervasive grain boundary fluid that facilitated partial replacement of plagioclase by muscovite. Biotite is partially replaced by muscovite and/or chlorite, particularly in areas of inferred high strain. Quartz precipitated in porphyroclast pressure shadows and ribbons that help define the mylonitic fabric. All D3 reactions require the introduction of H+ and/or H2O, indicating an open system, and typically result in a volume decrease. Syntectonic D3 muscovite + quartz + chlorite preferentially grew in an orientation favourable for strain localization, which produced a strong textural softening. Strain localization occurred only where reactions progressed with the infiltration of aqueous fluids, on a scale of hundreds of micrometre. Local fracturing and microseismicity may have induced reactivation of the fault zone and the initial introduction of fluids. However, the predominant greenschist facies deformation (D3) along discrete shear bands was primarily a consequence of the localization of replacement reactions in a partially open system. Key words: Argentina; reaction localization; reactivation; textural softening; ultramylonite.

At crustal depths near the brittle–ductile (frictional– INTRODUCTION viscous) transition, locally brittle (microseismic) The characterization of fault zones through the full processes may initiate deformation and enable fluid range of crustal depths is an important goal for infiltration prior to a switch to aseismic, creep- understanding the rheology of the crust and, in par- dominated (ductile) processes (Imber et al., 2001; ticular, the brittle to ductile transition. Fault zones are Holdsworth, 2004). Thus, knowledge of the specific typically depicted as narrow in the upper crust where deformation mechanisms defining the brittle–ductile softening processes such as cataclasis (Sibson, 1977; transition is critical to this issue. Brittle deformation is Passchier & Trouw, 1998) or foliation strengthening dominated by fracture (cataclastic) processes, which (Shea & Kronenberg, 1993; Wintsch et al., 1995) are variably preserved and incompletely understood localize deformation. In the deeper crust, below the because of the uncertainty of fluid pressures. Ductile brittle–ductile transition, fault zones are thought to processes are typically dominated by dislocation creep, widen (e.g. Sibson, 1977; Sibson 1986; Scholz, 1990), dissolution creep, or both. These processes may pro- perhaps because ductile processes, especially crystal duce rocks with strong crystal lattice-preferred orien- plasticity, but also solution creep, may strengthen fault tations (e.g. Lister & Williams, 1979; Etchecopar & zones in relation to the surrounding host rocks. Vasseur, 1987), or with strongly zoned mineral grains (e.g. Wintsch et al., 2005). However, such processes by *Present address: Department of Earth and Planetary Sciences, themselves cannot adequately explain the initiation of University of Tennessee, Knoxville, TN 37996-1410, USA. ductile deformation at the brittle–ductile transition,

2005 Blackwell Publishing Ltd 411 412 S. J. WHITMEYER & R. P. WINTSCH nor do they completely account for the widening of ductile fault zones with increasing depths. In this contribution we report on an ultramylonite zone in central Argentina that reaches a thickness of at least 15 km and is locally cut by narrow, centimetres to metres thick phyllonites (Simpson et al., 2003; Whit- meyer & Simpson, 2003). Mineralogy of the ultramylonites indicates that the predominant deformation fabric developed at temperatures above 500 C, certainly warm enough to be in the ﬁeld of crystal plasticity for most crustal minerals including the plagioclase and biotite that dominate the zone. Therefore, a fundamental question that we address is how deformation in this zone expanded from what are presumed to have been initially localized shear zones into a kilometres-wide ultramylonite zone. A textural hardening process is proposed that induced migration of the high-strain zone into adjacent, well-foliated host gneisses. Reactivation of the texturally hardened ultramylonites along discrete, phyllonitic shear bands is attributed to strain localization because of both reaction and textural softening. Microseismicity may have allowed the introduction of ﬂuids that initiated the dissolution-precipitation processes that, in turn, produced discrete mylonitic shear bands. Finally, a conceptual model is presented for reaction-enhanced textural weakening during the development of greenschist facies mylonites and its relevance for other ductile fault systems is discussed.

GEOLOGICAL SETTING AND BACKGROUND The Tres Arboles fault zone marks the western margin of migmatitic rocks of the Sierras de Co´rdoba in cen- Fig. 1. Simplified geological map of the Sierras de Co´rdoba and tral Argentina (Fig. 1). This east dipping fault zone is north-eastern portion of the Sierra de San Luis, Argentina. exposed discontinuously for 250 km and contains Sample location within Tres Arboles fault zone is indicated by amphibolite facies mylonitic and ultramylonitic rocks arrow. Map modified from Whitmeyer & Simpson (2003). that reach a thickness of 10–15 km in southern sections. The fault accommodated the westward thrusting of upper amphibolite to granulite facies gneisses and margin of Gondwana that initiated in the Early migmatites over biotite + muscovite quartzites and Cambrian (Rapela et al., 1998; Simpson et al., 2003). quartzofeldspathic rocks (Whitmeyer & Simpson, Previous work on the Tres Arboles fault zone suggests 2003). Discrete greenschist-facies shear bands over- that the most significant phase of convergence print the ultramylonite along localized, metres-wide occurred after the regional intrusion of Middle Ordo- zones and contain abundant east-dipping mineral vician plutons (Whitmeyer & Simpson, 2004), and elongation lineations and east-over-west kinematic in- ceased by the Late Devonian (Dorais et al., 1997) dicators (Simpson et al., 2003; Whitmeyer & Simpson, when undeformed plutons intruded the mylonite zone 2003). Samples were collected from the interior of the (Fig. 1). southern portion of this fault zone (Fig. 1) with the goal of understanding the deformational and metamorphic processes responsible for producing such a RESULTS wide fault zone and which facilitated subsequent The Tres Arboles fault zone is dominated by ultra- greenschist facies local reactivation. mylonitic schists containing biotite + plagioclase ± The Tres Arboles fault zone is interpreted as the quartz in a poorly foliated aggregate of 15–50 lm Early Palaeozoic suture between the Sierras de Co´r- grains. This schist contains ovoid clasts of coarse doba and the Sierra de San Luis (Whitmeyer & grained garnet and plagioclase, and fragments of Simpson, 2003, 2004). These two terranes are compo- quartzofeldspathic and pelitic rocks (Figs 2 & 3) relict nents of the Eastern Sierras Pampeanas and contain of a precursor rock. Well-defined mylonitic shear evidence of east-directed subduction along the western bands defined primarily by muscovite ± chlorite with

2005 Blackwell Publishing Ltd REACTION SOFTENING IN REACTIVATED MYLONITES 413 abundant, elongated quartz ribbons overprint the in garnet have aspect ratios of 1:2 to 1:4 and exhibit ultramylonite matrix. Understanding the relationship rounded margins and well-defined crystal cleavage among successive generations of fabric-forming min- planes. Biotite also occurs floating in a plagioclase host erals within the Tres Arboles fault zone is important in a box-work texture that forms rims around garnet for determining the metamorphic and microstructural porphyroclasts. Here randomly oriented 50–150 lm evolution of these rocks. Therefore, the petrology and flakes with aspect ratios of >1:10 together with plag- textures of each of the major minerals is discussed ioclase strongly embay garnet porphyroclasts, isolating below. them from the surrounding ultramylonite matrix. Finally, biotite also occurs as 10–50 lm long grains in well-oriented and overprinting mylonitic shear bands. Plagioclase These grains may be straight or curved with aspect Plagioclase in the most abundant mineral in these ratios from 1:2 to 1:5, intergrown with muscovite, and mylonitic rocks. In the ultramylonite matrix it occurs locally chlorite. as 10–50 lm grains, in a poorly aligned aggregate with Chemical compositions of biotite reveal a clear dif- biotite. Grains are typically sub-equant and blocky, ference between coarse-grained biotite inclusions in with aspect ratios of 1:1 to 1:2 (Fig. 3d). A more garnet, and finer-grained rim and matrix biotite. Bio- conspicuous occurrence of plagioclase is as porphyro- tite inclusions in garnet define a distinctly low Ti and clasts or rock fragments. Here 100–300 lm grains are high AlVI field relative to fine-grained matrix biotite rounded to elliptical with width-to-length aspect ratios and medium-grained biotite in garnet porphyroclast between 1:1 and 1:3 (Fig. 2a). Some plagioclase por- rims (Fig. 5a). Biotite inclusions in garnet also tend to phyroclasts occur as elliptical theta and sigma grains be richer in Mg than rim and matrix biotite (Fig. 5b). (Passchier & Simpson, 1986), the latter with quartz tails Little chemical variation is apparent between matrix that typically contain muscovite + quartz ± chlorite biotite and box-work biotite. (Fig. 3b,c). Other grains with similar shapes and sizes are composed of a patchwork intergrowth of plagioclase and quartz (Fig. 3b). Margins of plagioclase Garnet porphyroclasts are commonly intergrown with 10– Garnet within the Tres Arboles fault zone occurs as 20 lm biotite (Fig. 3c). Finally, plagioclase occurs as 1–5 mm grains as rock fragments or porphyroclasts that the major component of well-defined mantles that range from elliptical to elongated and often preserve an isolate garnet porphyroclasts from the ultramylonite internal gneissic foliation defined by aligned sillimanite matrix (Fig. 2d,e). and biotite grains (Fig. 2b,c). Coarse, 50–100 lm, The compositions of plagioclase vary systematically biotite ± sillimanite inclusions are common in garnet with mode of occurrence. Anorthite content of matrix cores, with rare quartz and plagioclase inclusions near plagioclase is rather uniform at An30–32 (Table 1). the garnet margins (Fig. 2d). Garnet is mantled by a This is in contrast to the porphyroclasts that tend to be 100–200 lm rim of plagioclase + randomly oriented zoned, with cores slightly more calcic (‡An 35%) than biotite flakes, which frequently fill embayments into the rims (32–33%; Fig. 4). This normal zoning pattern is garnet margin (Fig. 2d,e). In some occurrences, these reversed in the few relict feldspar that are now com- plagioclase + biotite mantles have almost completely posed of a patchwork of plagioclase + quartz replaced garnet porphyroclasts, leaving only a small (Fig. 3b). In these grains cores are relatively Na-rich core of original garnet (Fig. 2f). Garnet is almandine- (c. An30) and rims are slightly more calcic (c. An33; rich (Table 2) and shows no systematic zoning. Garnet is Table 1). Anorthite content of plagioclase that not present in the fine-grained ultramylonite matrix or in armours garnet porphyroclasts decreases from the overprinting shear band fabrics. interior garnet margin (c. An40) to the outer matrix boundary (c. An30). Thus plagioclase compositions on the margins of zoned grains all approach the matrix Sillimanite composition of c. An33 (Fig. 4). Sillimanite has several modes of occurrence. It is most abundant in rock fragments where coarse-grained sillimanite (up to 20 lm in cross-section) is present Biotite within and adjacent to garnet grains (Fig. 2c). Finer- Biotite is the major mineral in all of the rocks of the grained (20–60 lm long) sillimanite needles also occur Tres Arboles fault zone. In the ultramylonitic schists as aligned inclusions within garnet porphyroclasts. that dominate the zone, it occurs as weakly oriented These record an earlier fabric that is not preserved in 10–50 lm grains associated with plagioclase and loc- the enclosing ultramylonite matrix (Fig. 2c). Sillima- ally quartz. It is much coarser-grained in included nite is also locally present within the ultramylonitic porphyroclasts and rock fragments. Here grains up to matrix as fibrolite bundles in some porphyroclast 200–300 lm occur as inclusions within garnet por- pressure shadows, and as individual coarser porphyr- phyroclasts (Fig. 2d), or associated with garnet + sil- oclasts preserved within 200–500 lm muscovite grains limanite + quartz (Fig. 2c). Coarse biotite inclusions (Fig. 3a).

2005 Blackwell Publishing Ltd 414 S. J. WHITMEYER & R. P. WINTSCH

(a) (b) fsp

gar plag

gar

qtz

gar

(e) plag (f) gar

plag qtz

D3 plag

qtz

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with quartz ribbons and well-aligned muscovite-rich Quartz bands to define shear band folia. In the latter chlorite Quartz occurs as rare inclusions in garnet and plagio- composes c. 10–25% of shear band, with minor quartz clase porphyroclasts and as a significant component of or biotite. Chlorite also occurs as discrete aggregates of the overprinting shear band fabric. Quartz inclusions in helicitic 200–500 lm grains in pressure shadows garnet grains are rounded to slightly elongated, 100– (Fig. 2e) and as pseudomorphic replacements of bio- 200 lm in length and occur near the garnet margins or tite. Small, 20–50 lm chlorite grains are common as rarely within plagioclase rims around garnet (Fig. 2d). partially to completely altered biotite grains in narrow Some plagioclase porphyroclasts contain 50–100 lm shear bands around porphyroclasts (Fig. 6). The quartz grains along their margins (Fig. 3c). Less com- resulting biotite–chlorite intergrowth is too fine- mon patchwork plagioclase + quartz porphyroclasts grained to be resolved on the microprobe, but the contain abundant quartz inclusions and exhibit quartz percentage of chlorite alteration can be monitored with tails in pressure shadow regions (Fig. 3b). Small, 50– the K cations of the aggregate. In altered regions, K 100 lm quartz grains occur within the ultramylonitic cations of the aggregate range between 1.5 (mostly matrix, but constitute a small percentage of the mode of biotite) and 0.0 (pure chlorite) and reveal the extent and the ultramylonites. Abundant, elongated quartz rib- distribution of biotite-to-chlorite alteration. Alteration bons wrap around garnet porphyroblasts, and are is most extensive near porphyroclast margins in regions commonly parallel to the shear band fabric (Fig. 3e). of higher strains, whereas little to no alteration occurs Quartz also occurs in pressure shadows and forms tails in porphyroclast pressure shadows (Fig. 6). on plagioclase porphyroclasts (Figs 3b & 6). Quartz grains within such tails exhibit sutured boundaries and undulatory extinction (Fig. 3f). DISCUSSION Protolith of the ultramylonites Muscovite Rock fragments and porphyroclasts of plagioclase, Muscovite (and chlorite) are retrograde minerals garnet and sillimanite in Tres Arboles ultramylonites whose occurrence is restricted to overprinting shear and mylonites show that these rocks were derived from bands, or to sites that can be related to shear band a high-grade pelitic gneiss. In fact the gneisses imme- generation. In well foliated domains fine-to-medium- diately east of, and in the hangingwall of, the Tres grained (20–100 lm) muscovite defines mylonitic shear Arboles fault zone (Fig. 1) contain migmatitic mineral bands, where it commonly interfingers with biotite and assemblages and structures indistinguishable from quartz ribbons (Figs 2e & 3e). Less well-oriented those in rock fragments within the fault zone. These grains occur in replacement settings. This is most hangingwall rocks consist of well-foliated, upper conspicuous where 200–300 lm muscovite grains amphibolite facies quartz + plagioclase + biotite + embay porphyroclasts of sillimanite (Fig. 3a,b). Loc- garnet gneisses and K-feldspar + sillimanite migma- ally, very fine-grained muscovite appears to have tites, which record temperatures and pressures of 650– replaced biotite and plagioclase in ultramylonite 950 C and 7–8 kbar (Otamendi et al., 1999). The (upper right and centre, Fig. 3d), or nucleated along identical mineralogy, fabrics (here termed D1), and internal fractures in plagioclase porphyroclasts appropriate structural setting make these hangingwall (Fig. 3c) and in extension sites between biotite clasts rocks compelling candidates as the protoliths of the (Fig. 3d). The compositions of muscovite are sensitive Tres Arboles fault zone ultramylonites (Whitmeyer & to associated mineral assemblages. Coarse muscovite Simpson, 2003). replacing sillimanite is richer in Al and Na and poorer in Mg than matrix muscovite (Table 2). Modification and digestion of included fragments of hangingwall rocks Chlorite In spite of the apparent preservation of clasts of D1 Chlorite is another retrograde mineral restricted to rocks, these inclusions in the ultramylonitic rocks have overprinting shear bands. Chlorite typically interleaves undergone some changes. Temperatures obtained from

Fig. 2. Transmitted polarized light and back scattered electron images of fault rocks viewed perpendicular to foliation and parallel to lineation. (a) Photomicrograph of plagioclase porphyroclast, showing bent crystallization twins cross-cut by dagger-shaped deformation twins, surrounded by a (D3) fabric of quartz + muscovite + biotite. Crossed polarized light. (b) Photomicrograph of garnet porphyroclast with plagioclase + biotite rim, in ﬁne-grained ultramylonite (D2) matrix. Plane light. (c) Photomicrograph of a gneissic rock fragment dominated by garnet surrounded by and including a quartz–biotite sillimanite schist. Crossed polarized light. (d) Back scattered electron image of a garnet porphyroclast with a coarse biotite inclusion and a mantle composed of an intergrowth of plagioclase + randomly oriented biotite. (e) Back scattered electron image of a garnet porphyroclast with plagioclase + biotite rim, surrounded by a mylonitic matrix (D3) of biotite + muscovite + quartz. D3 shear bands indicated; note the chlorite + quartz in pressure shadow of the garnet porphyroclast. (f) Back scattered electron image of an intergrowth of plagioclase + randomly oriented biotite around a relict garnet core. Scale bars: (a) 100 lm; (b), (d), (e), (f) 200 lm; (c) 60 lm.

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(a) (b) bio

qtz qtz

sill plag

qtz bio qtz qtz

sill plag

bio plag

plag bio

musc

bio

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(e) (f)

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Table 1. Representative chemical analyses of plagioclase. Cati- (a) 1.8 ons based on 8 oxygen. Matrix biotite 1.6 Porphyroclast Biotite in garnet rim Patchwork Garnet rim 1.4 Biotite inclusions in garnet Weight % Core Margin core next to gar Matrix 1.2 SiO2 59.67 61.82 61.66 57.16 61.25 Al2O3 25.67 25.22 25.13 27.45 25.12 1.0

FeO 0.04 0.05 0.03 0.38 0.13 Al MnO 0 0 0.03 0.01 0 VI 0.8 CaO 6.52 6.34 6.19 8.15 6.17 Na O 6.53 7.64 7.51 6.70 7.73 2 0.6 K2O 0.11 0.14 0.14 0.06 0.12 Total 98.52 101.21 100.68 99.91 100.52 0.4 Cations based on 8 oxygen Si 2.68 2.71 2.71 2.56 2.70 0.2 AlIV 1.36 1.30 1.30 1.45 1.31 2+ Fe 0.00 0.00 0.00 0.01 0.01 0.0 Mn 0 0 0.001 0 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Ca 0.31 0.30 0.29 0.39 0.29 Na 0.57 0.65 0.64 0.58 0.66 Ti (cation) K 0.01 0.01 0.01 0.00 0.01 Total 4.93 4.97 4.96 5.00 4.98 (b) 1.8 2.0 Siderophyllite Eastonite 1.6 IVAl 1.4

0.0 Annite Phlogopite 1.2 0 Mg# 100

1.0 Al VI 0.8

0.6

0.4 Matrix biotite Biotite in garnet rim 0.2 Biotite inclusions in garnet 0.0 30 40 50 60 70 Mg/(Mg+Fe2+)

Fig. 5. (a) AlVI v. Ti plot of matrix biotite, biotite needles on garnet rims, and coarser-grained biotite inclusions in garnet porphyroblasts. Note the low-Ti ﬁeld deﬁned by biotite inclu- VI 2+ Fig. 4. Plot of Al cations across representative plagioclase por- sions. (b) Al v. Mg/(Mg + Fe ) plot of matrix biotite, biotite phyroclast and patchwork plagioclase, showing chemical zona- needles on garnet rims, and coarser-grained biotite inclusions in tion from core region to the margins. Note: Anorthite content is garnet porphyroblasts. Inset shows analysed biotite samples equivalent to Al (cations) )1. (hatched area) plot on a chart of the end-member biotite components. coarse-grained sillimanite and biotite in garnet por- clase + quartz in the pressure shadows of some garnet phyroclasts from the Tres Arboles fault zone (Fig. 2c) porphyroclasts (Whitmeyer & Simpson, 2003) suggests suggest re-equilibration of the garnet–biotite exchange that at least some early deformation occurred at these thermometer at lower temperatures between 540 middle amphibolite facies metamorphic conditions. and 590 C (Whitmeyer & Simpson, 2003). The oc- Plagioclase porphyroclasts are chemically zoned currence of sillimanite needles with biotite + plagio- with the cores more calcic (up to An40) than the

Fig. 3. Transmitted polarized light and back scattered electron images of fault rocks viewed perpendicular to foliation and parallel to lineation. (a) Back scattered electron image of sillimanite porphyroclast partially replaced by muscovite, in a matrix of D3 muscovite + biotite + quartz. (b) Back scattered electron image of D2 matrix hosting plagioclase and sillimanite porphyroclasts and a patchwork grain of plagioclase + quartz with quartz beards developed in pressure shadows. D2 matrix is cut and partially replaced by muscovite + quartz, and overprinted by muscovite + biotite as D3 shear bands. (c) Back scattered electron image of plagioclase porphyroclast in a D2 biotite + plagioclase + quartz matrix. This matrix is locally cut by muscovite-dominated D3 shear bands (dashed line), and very fine-grained muscovite locally replaces plagioclase in a fracture in the porphyroclast. (d) Back scattered electron image of a D2 aggregate of c.20lm grains of biotite and plagioclase, set in a 2–5 lm matrix of muscovite (of D3 generation). D3 muscovite also fills the gap in the biotite grain. (e) Photomicrograph of quartz ribbons around plagioclase porphyroclasts and parallel to D3 shear band foliation. D3 matrix consists of fine-grained muscovite + chlorite + plagioclase ± biotite. Plane light. (f) Pho- tomicrograph of quartz in pressure shadow region, showing sutured grain boundaries and deformation bands. Scale bars for (a) 100 lm; (b), (c) 200 lm; (d) 20 lm; (e) 300 lm; (f) 50 lm.

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Table 2. Representative chemical analyses of phyllosilicates: onite. However, the textures marking the margins biotite, chlorite and muscovite. Biotite and muscovite cations of these grains suggest that they were not in based on 22 oxygen, and chlorite cations based on 28 oxygen. chemical equilibrium with the P–T-ﬂuid environment

Biotite Muscovite Coarse muscovite that produced the ultramylonitic host (here termed mantling D2). Weight % Matrix Inclusionchlorite In matrix In plagioclase sillimanite

SiO2 34.56 34.39 25.85 46.66 47.78 44.98 Garnet Al2O3 17.38 20.72 20.96 34.55 36.67 37.96 TiO2 2.11 0.25 0.37 0.13 0.02 0.06 Most garnet porphyroclasts are rimmed and isolated FeO 21.90 16.96 24.34 2.29 0.69 1.19 MnO 0.08 0.18 0.13 0 0 0 from the D2 matrix by a plagioclase + biotite rim. MgO 10.45 13.02 17.25 1.22 0.18 0.47 Deep embayments of biotite that invade the garnet CaO 0 0.05 0.02 0.10 0.02 0.51 mantles are strong evidence for the replacement of Na2O 0.05 0.01 0 0.13 0.55 0.23

K2O 7.64 9.42 0.02 10.73 9.95 10.55 garnet by plagioclase + biotite. Further support is F 0 0.13 NA 0 0 0.02 gained from the variable width of the plagioclase– H2O 3.87 3.91 11.62 4.51 4.58 4.52 Total 98.04 99.05 100.55 100.32 100.42 100.48 biotite rims, which in the extreme can leave only a very Total-H2O 94.17 95.14 88.94 95.81 95.84 95.96 small garnet in the centre of the texture (Fig. 2f). This Cations based on 22 oxygen (biotite, musc), 28 oxygen (chlorite) suggests that plagioclase and biotite grew at the Si 5.36 5.20 5.34 6.21 6.25 5.95 expense of garnet by the reaction: AlIV 2.64 2.80 2.66 1.79 1.75 2.05 AlVI 0.53 0.89 2.44 3.62 3.91 3.87 garnet þ quartz ¼ biotite þ plagioclase ð1Þ Ti 0.25 0.03 0.06 0.01 0.00 0.01 Fe2+ 2.84 2.14 4.20 0.25 0.08 0.13 Using representative analyses for the garnet and Mn 0.01 0.02 0.02 0 0 0 Mg 2.42 2.93 5.31 0.24 0.04 0.09 associated rim and D2 matrix biotite and plagioclase Ca 0 0.01 0.00 0.02 0.00 0.07 (Table 3), one balanced reaction that could explain the Na 0.02 0.00 0 0.03 0.14 0.06 K 1.51 1.82 0.01 1.82 1.66 1.78 texture is: F00NA00 0 Total 15.57 15.84 20.04 14.00 13.82 14.01 31ðMg,Fe,MnÞ2:9Ca0:1Al2:0Si3:0O12 þ 11SiO2 þ 2H2O þ þ þþ þ þ 2TiO2 þ 18K þ 12Na þ 4:9Ca þ 36H 20K Mg,Fe Ti Al Si O OH margins (Fig. 4). This composition is similar to that ¼ 0:9ð Þ2:6 0:1 1:7 2:6 10ð Þ2 þþ within granulite gneisses east of the fault zone (Ota- þ 20Na0:6Ca0:4Al1:4Si2:6O8 þ 37:9ðMg,Fe,MnÞ mendi et al., 1999). However, the normal zoning that ð2Þ converges to c. An33 suggests exchange with the pla- gioclases in the ultramylonite matrix. While the reaction is not unique, it does show that SiO2 is a reactant, consistent with the absence of quartz from the biotite–plagioclase intergrowth. It also Replacement of D1 clasts + shows that the reaction consumes both H2O and H , Clasts of rock fragments and of single crystals of suggesting that the reaction required the introduction garnet and plagioclase are common in the ultramyl- of aqueous ﬂuids into the fault zone.

bio

plag Fig. 6. Composite backscatter electron image of plagioclase and patchwork por- qtz phyroclasts, with a region of random D2 matrix above the plagioclase porphyroclast K cations qtz just right of centre. D3 shear bands are vis- 1.50 ible in the top left and bottom right, and are 1.40 1.30 plag indicated by dashed white lines; sinistral movement direction is indicated by white 1.20 qtz 1.10 arrows. Biotite matrix grains are preferen- 1.00 tially altered to chlorite in high-strain 0.80 regions (indicated by yellow arrows); K 0.3–0.4 cations chart indicates compositional range 0.00 between predominantly biotite grains (red) 200 µm and predominantly chlorite grains (green).

2005 Blackwell Publishing Ltd REACTION SOFTENING IN REACTIVATED MYLONITES 419

Table 3. Representative chemical analyses for principal minerals best preserved within strain shadows adjacent to in the D2 reaction: garnet ¼ biotite + plagioclase. plagioclase and garnet porphyroclasts (e.g. Figs 2 & 3;

Weight % Garnet Biotite Plagioclase the top-right region of Fig. 6), where D2 textures are protected from later strain and reactions related to SiO2 36.17 33.71 57.93 overprinting shear bands. The mechanisms that led to Al2O3 21.75 18.60 26.97 the formation of these fine-grained and poorly aligned TiO2 0.04 2.14 NA FeO 32.46 19.92 0.17 matrix fabrics are not well understood at present, but MnO 4.32 0.18 0.01 may have resulted from grain boundary sliding at MgO 3.69 10.34 NA CaO 1.13 0.04 8.03 significant strain rates (Whitmeyer & Simpson, 2003). Na2O NA 0.05 6.67 The D2 reaction above (Eq. 2) shows that both quartz K2O NA 9.26 0.13 and H2O are consumed, which is supported by the H2O NA 3.86 NA Total 99.56 98.08 99.90 almost complete absence of quartz within garnet man- Total-H O 94.22 2 tles and D2 matrix fabrics. The consumption of H2Oand Cations based on 24 oxygen (garnet), 22 oxygen (biotite), 8 oxygen (plagioclase) alkalis indicates that the system was open, at least to Si 6.09 5.23 2.59 AlVI – 2.77 1.42 these components, and is consistent with the reactions AlVI 3.96 0.64 – occurring during active deformation, where faults and Ti 0.01 0.25 NA shear zones often facilitate fluid migration (Sibson, Fe2+ 4.26 2.59 0.01 Mn 0.59 0.02 0 1977; Scholz, 1990; Wintsch et al., 1995). The fact that Mg 0.89 2.39 NA these reactions are arrested indicates that the activities of Ca 0.16 0.01 0.39 the reactants became too low to completely consume Na NA 0.02 0.58 K NA 1.83 0.01 garnet. Thus, the low activity of SiO2 and possibly H2O Total 15.95 15.74 4.99 may indicate the local cessation of D2 deformation. The abundance of biotite in the D2 matrix and around Garnet cations based on 24 oxygen, biotite cations based on 22 oxygen and plagioclase cations based on 8 oxygen. garnet rims would normally be interpreted as representative of a mechanically weak fabric (e.g. Boullier & Orthoclase Gueguen, 1975; Schmid, 1982). However, plagioclase within the ultramylonitic matrix and within rims around The plagioclase + quartz patchwork feldspar garnet grains appears to pin the elongate biotite needles (Fig. 3b) may also have been modified, but more in variable orientations (Fig. 2d,f) to create a poorly to completely. Similar patchwork grains are not found in non-aligned fabric, which would not easily facilitate hangingwall rocks, but cm-size orthoclase grains are grain boundary sliding (Schmid, 1982; Shea & Kro- common in both the hangingwall (Otamendi & Rab- nenberg, 1993). The absence of quartz within D2 fabrics bia, 1996) and in less deformed, marginal regions of would also result in a texturally strong fabric at the fault zone (Whitmeyer & Simpson, 2003). It is thus deformation temperatures below the onset of feldspar possible that the patchwork plagioclase + quartz ductility (at least 450 C; De Paor & Simpson, 1993; pattern resulted from the consumption of original K- Rosenberg & Stu¨ nitz, 2003). The development of a feldspar porphyroclasts by the reaction: strong, somewhat interlocked biotite + plagioclase K-feldspar þ Naþ þ Caþþ ¼ plagioclase þ quartz þ Kþ matrix devoid of quartz may have facilitated the widening of the Tres Arboles zone, as surrounding well- ð3Þ foliated and favourably oriented gneisses were likely Using a representative analysis for patchwork plag- mechanically weaker than reaction-hardened, poorly ioclase from the core region (Table 1), the balanced foliated shear zone fabrics. This suggestion is supported reaction is: by the presence of metres-thick, discrete ductile defor- þ þþ 13KAlSi3O8 þ 7Na þ 3Ca mation zones that lie east of, but in close proximity to, the eastern margin of the Tres Arboles fault zone proper 10Na Ca Al Si O 12SiO 13Kþ 4 ¼ 0:7 0:3 1:3 2:7 8 þ 2 þ ð Þ (Simpson et al., 2003). The implication is that defor- In support of the operation of this reaction, volu- mation became less continuous, and therefore less metric proportions of plagioclase and quartz as reac- intense, in marginal regions where it was infiltrating the tion products calculated using 1 atmosphere molar host gneisses. Although appropriate equilibrium com- volumes of the solids is 100:27, qualitatively similar to positions during D2 deformation are not available for that found in patchwork grains (e.g. Fig. 3b). Ortho- geothermometry, the presence of fibrolite needles in clase is not preserved in cores as they are in the garnet porphyroclast pressure shadows (Whitmeyer & Simp- replacements, and so it is likely that this reaction has son, 2003) suggests that deformation at least initiated at gone to completion, consuming all inherited orthoclase. lower to middle amphibolite facies.

Development of D2 ultramylonite Overprinting D3 shear band fabrics and reaction processes Most of the matrix surrounding D1 clasts consists of The above D2 ultramylonitic schists that include clasts poorly aligned biotite and plagioclase. This texture is of D1 gneiss have further been overprinted by a

2005 Blackwell Publishing Ltd 420 S. J. WHITMEYER & R. P. WINTSCH greenschist facies mylonitic fabric (D3). These are de- sitions used above ignore small amounts of Ti in fined primarily by strongly oriented chlorite and biotite (Table 2). However, rutile occurs locally in D3 muscovite in discrete mylonite shear bands. Quartz shear bands, suggesting that rutile is a minor by- ribbons are aligned with the long axes of phyllosilicate product of this reaction. The low solubility of TiO2 minerals and contribute to defining the fabric (Figs 2e probably restricted its transport such that it was & 3e). Growth of muscovite and chlorite in a preferred precipitated locally as isolated needles during D3 alignment within mylonitic shear bands probably deformation. occurred through several reactions in locally open fluid Muscovite is also produced at the expense of plagi- systems. Muscovite was likely produced by three oclase, both within mylonitic shear bands and within locally independent reactions, involving the consump- rare plagioclase porphyroclasts where muscovite nuc- tion of sillimanite, plagioclase and biotite. leates along fractures (Fig. 3c). The reaction balanced on Al using theoretical muscovite and plagioclase of An40 composition is: Muscovite þ þ Crystallization of muscovite and chlorite in a preferred 15Na0:6Ca0:4Al1:4Si2:6O8 þ 7K þ 14H þ þþ alignment within mylonitic shear bands probably ¼ 7KAl3Si3O10ðOHÞ2 þ 18SiO2 þ 9Na þ 6Ca occurred via several locally metasomatic reactions. ð8Þ Muscovite was produced by three separate reactions, involving the consumption of sillimanite, biotite and This reaction consumes K+ and H+ and pro- plagioclase. Individual coarse grains of sillimanite duces muscovite and quartz, with a 9% volume within the mylonitic matrix are always mantled by decrease. muscovite (Fig. 3a), suggesting the reaction: sillimanite + SiO muscovite (e.g. Carmichael, 1969). 2 ¼ Chlorite Using the Ômuscovite mantling sillimaniteÕ analysis from Table 2, the balanced reaction is: Chlorite occurs with muscovite as 200–500 lm þ þ aggregates (Fig. 2e), and as partial alteration prod- 59Al2SiO5 þ 59SiO2 þ 67H2O þ 36K þ 2Na ucts of biotite grains (Fig. 6). In both of these cases, þ 2Caþþ þ 6ðMg,FeÞþþ chlorite crystallizes at the expense of biotite, particularly in locations of high strain. The balanced 20K Na Ca Mg,Fe Al Si O OH ¼ 1:8 0:1 0:1ð Þ0:3 5:9 5:9 20ð Þ4 reaction using theoretical biotite and chlorite formu- þ 54Hþ ð5Þ lae is: þ This reaction can be rewritten with theoret- 2KðMg,FeÞ3AlSi3O10ðOHÞ2 þ 4H ical mineral compositions with no loss of applica- ¼ðMg,FeÞ Al Si O ðOHÞ þ 3SiO bility as: 5 2 3 10 8 2 þ þþ þ þ 2K þðMg,FeÞ ð9Þ 3Al2SiO5 þ 3SiO2 þ 3H2O þ 2K + þ The reaction requires the introduction of H (but ¼ 2KAl3Si3O10ðOHÞ2 þ 2H ð6Þ not H2O) and produces quartz at 25 volume % of This reaction requires the local introduction of K+, chlorite. This is in qualitative agreement with propor- SiO2, and H2O, and results in a 30% solid volume tions of quartz and chlorite in some intergrowths (e.g. increase. Fig. 2e). The total reaction produces an c. 30% volume Fine-grained muscovite in D3 folia cuts poorly loss showing a strong theoretical dependence on local oriented D2 biotite (Fig. 3a,b), or grows in cracks high stress. This dependence is in agreement with the between biotite grains (Fig. 3d). These truncating degree of reaction progress of biotite replacement structures suggest the replacement of biotite by mus- shown in Fig. 6. Here the replacement is most com- covite. The reaction balanced on Al and using theor- plete along the margins of the plagioclase grain facing etical mineral compositions is: the shortening direction (yellow arrows). þ 3KðMg,FeÞ3AlSi3O10ðOHÞ2 þ 20H Quartz ¼ KAl3Si3O10ðOHÞ2 þ 6SiO2 þ 12H2O þþ SiO is a product of most D3 reactions (Eqs 7–9), þ 2Kþ þ 9ðMg,FeÞ ð7Þ 2 however quartz grains are not always present next to This metasomatic reaction requires the local intro- muscovite or chlorite in D3 shear bands (e.g. Fig. 3a). + duction of H , and produces H2O, SiO2 and other The high solubility of quartz probably means that aqueous ions. It also results in a 39% mineral volume some of the SiO2 was free to migrate and accumulate decrease. The reaction produces muscovite and quartz into porphyroclast pressure shadows (Fig. 3b) and as in nearly equal volumes, explaining the high propor- largely monomineralic ribbons (e.g. Figs 2e & 3e). This tion of quartz in some patchy intergrowths (Fig. 3a) is supported by the local absence of quartz from high and in shear bands (Fig. 3e). The simplified compo- strain regions (Fig. 6), which suggests that SiO2 reac-

2005 Blackwell Publishing Ltd REACTION SOFTENING IN REACTIVATED MYLONITES 421

+ 3H2O + 2K 18H+

3 sillimanite 3 biotite = 2 muscovite 2H+ = muscovite

2 ++ iO 9(Mg,Fe) 6S 9H2O quar 2 tz ribbon +

14H 3SiO 5K+ 2 3SiO + 2 4H Fig. 7. Diagram after Carmichael (1969), 15SiO showing the four principal D3 reactions. Local open system behaviour imports H2O and H+ ions and enables the aqueous transfer of SiO2, alkalis and other cations. 15 plagioclase 2 biotite = chlorite Excess SiO2 precipitates as quartz ribbons = 7 muscovite ++ and beards. Note that most aqueous cations (Mg,Fe) and minerals in solution are components 9Na+ 6Ca++ within a larger chemical system, which is + mostly closed on a scale of centimetres. 2K tion products migrated during deformation. The sure shadows, including some porphyroclast tails abundance of quartz in D3 shear bands suggests that (Figs 3b & 6). Thus, the net reaction of sillima- far more quartz was produced by D3 reactions (Eqs nite + biotite + plagioclase ¼ muscovite + chlorite + 7–9) than was consumed in reaction (Eq. 6). quartz ± rutile involved the net introduction of H2O and H+, and the removal of Fe2+,Mg2+,K+,Na+ and Ca2+ (Fig. 7) and could not have operated in a com- Significance of D3 reactions pletely closed system. Several ions, such as (Fe,Mg)++ The D3 reactions discussed above (Eqs 5–9) depend are not balanced in Fig. 7, but could easily be compo- on chemical systems that are locally open to the nents in a chlorite replaces plagioclase reaction. This transfer of SiO2,H2O and aqueous cations. However, reaction probably did occur, but there are no textures many of the ionic reactants external to the local sufficiently compelling to propose it directly. replacement site can be provided by products from other D3 reactions at the centimetre scale (Fig. 7). Centimetre-scale diffusive transfer of aqueous ions is Other D3 deformation processes well known in metamorphic and reaction-enhanced Deformation-induced chemical reactions were signifi- deformation processes (e.g. Carmichael, 1969; Wintsch cant in forming the D3 shear band fabric, however other & Yi, 2002; Wintsch et al., 2005). Most of these reac- deformation processes were likely operating simulta- tions involve a local volume decrease, particularly if neously. Evidence for dislocation creep and grain SiO2 does not precipitate locally as quartz. The volume boundary migration is seen in quartz ribbons and tails, decrease would have produced an increase in porosity in the form of deformation bands and sutured grain and permeability, which in turn would have facilitated boundaries (Tullis, 1977; Hirth & Tullis, 1992; Fig. 3f). the infiltration of fluids. Therefore, a local positive Biotite apparently is not produced by D3 reactions; feedback loop may have existed that enhanced the however aligned biotite grains are common within D3 open system behaviour required by these syntectonic shear bands. This suggests that dissolution-precipitation reactions. (pressure solution) of biotite (e.g. Mares & Kronenberg, D3 reactions are typically concentrated in regions of 1993; Shea & Kronenberg, 1993) was active during D3 high strain and therefore were likely driven by local deformation, although the similarity in grain size compressive stresses. This is supported in the samples by between D2 and D3 biotite suggests that some mech- the predominance of reaction products, such as mus- anical rotation of existing D2 biotite grains was likely. covite and chlorite, along porphyroclast margins in regions of apparent maximum shortening (Fig. 6). Quartz is produced by three of four D3 reactions for Approximate temperature during D3 deformation which there is strong textural evidence, but is not Chemical reactions active during D3 deformation abundant in local high strain sites. This can be explained produced both muscovite and chlorite at the expense of by an open system distribution of aqueous ions and biotite and plagioclase. The breakdown and con- fluids that would facilitate the accumulation of sumption of biotite and growth of chlorite suggests quartz ± chlorite in micro-extensional sites and pres- that temperatures were below the biotite stability field

2005 Blackwell Publishing Ltd 422 S. J. WHITMEYER & R. P. WINTSCH

(less than c. 400 C; Spear & Cheney, 1989). However, for early microseismicity was quickly obliterated by some biotite may have dissolved and reprecipitated subsequent ductile deformation. within D3 shear bands, which would indicate temperatures at or above 400 C. Quartz microstructures within D3 ribbons and tails are consistent with Regime SUMMARY AND CONCEPTUAL MODEL FOR I recrystallization (Hirth & Tullis, 1992) at tempera- REACTION-ENHANCED FABRIC DEVELOPMENT tures of c. 250 ± 50 C (De Paor & Simpson, 1993). IN THE TRES ARBOLES FAULT ZONE Stipp et al. (1998) suggested that quartz deforms The Tres Arboles shear zone is defined by a wide zone by bulging recrystallization (roughly equivalent to of biotite + plagioclase ultramylonite. Relic D1 Regime I recrystallization) up to temperatures of protolith fabrics from hangingwall gneisses are c. 400 C. Therefore, D3 deformation is estimated preserved within rock fragments and include a silli- to have occurred at greenschist facies conditions at manite + biotite foliation (likely D1-related). Incor- temperatures of 300–400 C. These temperatures are poration of these fragments into a poorly aligned, consistent with typical estimates for faulting at or fine-grained biotite + plagioclase ultramylonitic mat- near the brittle–ductile transition in the crust rix (D2) was followed by reactions of the clasts with (e.g. Scholz, 1990; Imber et al., 2001; Gueydan et al., their new matrix. This included the development of 2004). plagioclase + biotite rims around garnet grains, and the modification of feldspar porphyroclasts. Strain rates must have been quite high, and the intrusion of Initiation of D3 deformation H2O was limited, such that reactions that could have The above sections argue for reaction- and texture- produced weak phyllosilicates did not go to comple- enhanced, greenschist facies deformation that was tion. The development of poorly oriented D2 fabrics driven by local compressive stresses and largely self- was a textural and possibly a reaction-hardening pro- sustained through feedback loops of interchanging cess that probably caused the migration of active SiO2,H2O and free ions in solution. We suggest that deformation zones into well-foliated (and thus weaker) most of the reactants necessary for these chemical gneissic rocks of the hangingwall, thus widening the reactions are available as products from other simul- zone of ultramylonite. taneously occurring reactions as part of a system that The D3 reactivation of the Tres Arboles fault zone is open on the scale of tens of millimetres, but largely may have initiated with local fracturing and microseis- closed on a scale of several centimetres. However, the micity. However, if this occurred, it was quickly over- processes that caused D3 deformation to initiate in printed by ductile deformation processes that enabled zones that were at least partially reaction-hardened slip along discrete mylonitic shear bands at greenschist- during earlier D2 deformation are not readily appar- facies conditions. Local, metasomatic reactions pro- ent. Imber et al. (2001) suggested that the formation of duced muscovite and chlorite that principally formed in greenschist facies phyllonites (mica-rich mylonites) is regions of high strain, and also produced quartz that often preceded by brittle microseismicity. In their accumulated in ribbons and tails. Thus the distribution model initially brittle faulting at the grain scale can of reaction products was strongly influenced by local produce microfractures that weaken the rock and compressive stresses that especially led to the diffusive facilitate the infiltration of fluids. Subsequent work by mass transfer of SiO2 into quartz ribbons and por- Gueydan et al. (2003) suggested that feldspar-to-white phyroclast pressure shadows. Preferential alignment of mica reactions which occur near the brittle–ductile muscovite, biotite and chlorite within shear bands transition nucleate along pre-existing fractures in presumably facilitated grain boundary sliding, and the feldspar grains. Evidence for such a feldspar-to-mica alignment of biotite within shear bands may have reaction in these rocks is apparent in Fig. 3c, and is been enhanced by dissolution–precipitation processes. described by reaction (Eq. 7), discussed above. This Quartz ribbons and tails also exhibit evidence of dislo- reaction requires the input of H2O and ions in solution cation creep and grain boundary migration. Thus, and appears to have nucleated along an internal frac- simultaneous chemical and mechanical processes acted ture in a feldspar porphyroclast. Therefore, although to produce softening of the Tres Arboles rocks and en- not conclusive, our data are consistent with the sug- abled at least local reactivation of the fault zone along a gestion that greenschist facies ductile deformation network of discrete shear bands under greenschist facies processes may initiate with fracturing and microseis- metamorphic conditions. micity at the grain scale, and that such a process may have provided local pathways for early fluid migration, reaction and thus strain localization. However, we IMPLICATIONS OF THIS WORK FOR DUCTILE contend that initially brittle processes were superceded DEFORMATION PROCESSES by ductile reaction-enhanced processes that incorpor- The classic Sibson–Scholtz crustal model for fault ated a reaction feedback loop that was largely closed at zones (Sibson, 1977; Sibson 1986; Scholz, 1990) depicts the several centimetre scale, except for the introduction a ductile deformation zone of increasing thickness + of H and H2O. We suspect that much of the evidence below the brittle–ductile transition. However, this

2005 Blackwell Publishing Ltd REACTION SOFTENING IN REACTIVATED MYLONITES 423 model does not fully address the mechanisms and Zone Weakening. Geological Society of London, Special Pub- processes that facilitate such an increase in fault zone lications, 186, 344. Imber, J., Holdsworth, R. E. & Butler, C. A., 2001. A width. More recent work has re-interpreted the prob- reappraisal of the Sibson-Scholtz fault zone model: the nature able depth of the brittle–ductile transition (Imber of the frictional to viscous (ÔÔbrittle-ductileÕÕ) transition along et al., 2001) and addressed the driving mechanisms for a long-lived, crustal-scale fault, Outer Hebrides, Scotland. fault zone reactivation (Imber et al., 2001; Holds- Tectonics, 20, 601–624. worth, 2004). In this contribution we have focused on Lister, G. S. & Williams, P. F., 1979. Fabric development in shear zones: theoretical controls and observed phenomena. syntectonic reaction processes and provided a con- Journal of Structural Geology, 1, 283–297. ceptual model that may at least partially explain fault Mares, V. M. & Kronenberg, A. K., 1993. Experimental de- zone widening at mid-crustal depths and the formation formation of muscovite. Journal of Structural Geology, 15, of discrete overprinting shear bands near the brittle– 1061–1076. Otamendi, J. E. & Rabbia, O. M., 1996. Petrology of high-grade ductile transition. Given the abundant documented gneisses from Macizo Rio Santa Rosa: evidence of decom- evidence of fault zone reactivation in many locations pression in the Eastern Sierras Pampeanas. XIII Congreso around the world (e.g. Holdsworth et al., 2001) we Geolo´gico Argentino y III Congreso de Exploracioń de Hi- suggest that reaction-facilitated deformation processes, drocarburos, Actas V, 527. Mendoza, Argentina. such as those described here, are significant factors in Otamendi, J. E., Patinõ Douce, A. E. & Demichelis, A. H., 1999. Amphibolite to granulite transition in aluminous greywackes the formation of most mylonitic fault zones. The ideas from the Sierra de Comechingones, Co´rdoba, Argentina. expressed in this contribution are testable in a variety Journal of Metamorphic Geology, 17, 415–434. of exhumed fault zone settings, and it is suggested that Passchier, C. W. & Simpson, C., 1986. Porphyroclast systems as continued investigation into syntectonic reaction pro- kinematic indicators. Journal of Structural Geology, 8, 831– 844. cesses will provide an important addition to the more Passchier, C. W. & Trouw, R. A. J., 1998. Microtectonics. extensive historical focus on mechanical deformation Springer-Verlag, Heidelberg. processes. Rapela, C. W., Pankhurst, R. J., Casquet, C. et al. 1998. The Pampean Orogeny of the southern proto-Andes: Cambrian continental collision in the Sierras de Co´rdoba. In: The Proto- ACKNOWLEDGEMENTS Andean Margin of Gondwana (eds Pankhurst, R. J. & Rapela, C. W.). Geological Society of London Special Publication, 142, The authors thank C. Simpson for helpful discussions 181–218. and introduction to the Tres Arboles fault zone. Rosenberg, C. L. & Stu¨ nitz, H., 2003. Deformation and re- Microprobe analyses were conducted on a Cameca crystallization of plagioclase along a temperature gradient: an example from the Berger tonalite. Journal of Structural Geol- SX50 electron microprobe at Indiana University with ogy, 25, 389–408. assistance from C. Li. This manuscript benefited from Schmid, S. M., 1982. Microfabric studies as indicators of reviews by G. Solar, R. Holdsworth and an anony- deformation mechanisms and flow laws operative in mountain mous reviewer. The research was partially supported building. In: Mountain Building Processes (ed. Hsu, K. J.), pp. by NSF grants EAR-9628158 to C. Simpson and 95–110, Academic Press, London. Scholz, C. H., 1990. The Mechanics of Earthquakes and Faulting. EAR-9909410 to R. P. Wintsch. Cambridge University Press, Cambridge. Shea, W. T. & Kronenberg, A. K., 1993. Strength and aniso- tropy of foliated rocks with varied mica contents. Journal of REFERENCES Structural Geology, 15, 1097–1121. Sibson, R. H., 1977. Fault rocks and fault mechanisms. Journal Boullier, M. T. & Gueguen, Y., 1975. SP-mylonites: origin of of the Geological Society of London, 133, 191–213. some mylonites by superplastic flow. Contributions to Miner- Sibson, R. H., 1986. Earthquakes and rock deformation in alogy and Petrology, 50, 93–104. crustal fault zones. Annual Review of Earth and Planetary Carmichael, D. M., 1969. On the mechanism of prograde Sciences, 14, 149–175. metamorphic reactions in quartz bearing pelitic rocks. Con- Simpson, C., Law, R. D., Gromet, L. 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