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 + gar- net + 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 + plagio- clase 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, Ó 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 ultra- mylonites 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 ultr- amylonites along discrete, phyllonitic shear bands is attributed to strain localization because of both reac- tion and textural softening. Microseismicity may have allowed the introduction of fluids that initiated the dissolution-precipitation processes that, in turn, pro- duced discrete mylonitic shear bands. Finally, a con- ceptual model is presented for reaction-enhanced textural weakening during the development of green- schist 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 sec- tions. 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 meta- morphic 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 Ó 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