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_Journal of Structural Geology 32 (2010) 2022-2041_ _Contents lists available at ScienceDirect_ _Journal of Structural Geology_ _journal homepage: www.elsevier.com locate jsg_ _Three-dimensional characterization of a crustal-scale fault zone: The Pusteria and Sprechenstein fault system (Eastern Alps)_ _Andrea Bistacchi a,*, Matteo Massironi b, Luca Menegon b,c_ _a Dipartimento di Scienze Geologiche e Geotecnologie, Università degli Studi di Milano Bicocca, Piazza della Scienza 4, 20126 Milano, Italy_ _b Dipartimento di Geoscienze, Università degli Studi di Padova, Via Giotto 1, 35137 Padova, Italy_ _c Institutt for Geologi, Universitetet i Troms?, Dramsveien 201, 9037 Troms?, Norway_ _article info_ _Article history: Received 14 October 2009_ _Received in revised form 20 May 2010_ _Accepted 16 June 2010_ _Available online 23 June 2010_ _Keywords: Fault zone architecture 3D modelling Damage Contractional jog Cataclasite Phyllonite_ _abstract_ _The characterization and representation of fault zones is of paramount importance for studies of fault and earthquake mechanics, since their rheological and geometric complexity controls seismic aseismic behaviour and fluid circulation at depth. We present a 3D geological model of a fault system, created by integrating borehole and surface structural data, which allows us to bridge the gap between outcrop-scale descriptions and large-scale geophysical models. The model integrates (i) fault geometry and topology, (ii) fault-rock distribution, and (iii) characterization of fracturing in damage zones at the km scale. The dextral-reverse Pusteria and Sprechenstein-Mules Faults (Italian Eastern Alps) provide an opportunity to study fault rocks and damage distribution as a function of host-rock lithology and fabric, and of fault geometry. A first-order control is exerted by the composition of protoliths (quartzo-feldspathic vs. phyllosilicate-rich) and or by the presence of an inherited anisotropic fabric (massive vs. foliated), resulting in a marked asymmetry of damage zones. Interestingly, the pervasive foliation typical of some protoliths may explain both this asymmetry and the relative weakness of one of the faults. The importance of geometrical factors is highlighted when the damage zone thickness increases five times in proximity to a km-scale contractional jog. On the other hand, the type of fault rock present within the fault core does not show a direct relationship with damage intensity. In addition, the thickness of damage zones along planar fault segments does not appear to grow indefinitely with displacement, as might be envisaged from some scaling laws. We interpret both of these observations as reflecting the maturity of these large-displacement faults._ _? 2010 Elsevier Ltd. All rights reserved._ _1. Introduction and motivation_ _In recent years, a large number of papers have addressed the characterization of crustal-scale fault zone architecture, which can be reconstructed from detailed field and laboratory studies (e.g. Chester and Chester, 1998; Billi et al., 2003; Faulkner et al., 2003; Collettini and Holdsworth, 2004; Di Toro and Pennacchioni, 2005; Jefferies et al., 2006; Dor et al., 2006; Micarelli et al., 2006). All these contributions, following the pioneering work of Chester and Logan (1986) on the Punchbowl Fault, share a common approach, involving a detailed characterization, on the meso-to micro-scale (10^2-10^-6 m), of exhumed fault zones, which are considered as representative analogs of active faults at seismogenic depth (5-15 km). One important result of these studies is the conclusion_ _* Corresponding author. E-mail address: andrea.bistacchi@unimib.it (A. Bistacchi)._ _0191-8141 $ e see front matter ? 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2010.06.003_ _that deformation in fault zones develops through increasing levels of localization, and that this process leads to a characteristic architecture (e.g. Ben-Zion and Sammis, 2003), which is described with a specific nomenclature (Sibson, 1986; Chester et al., 1993; Caine et al., 1996). Damage zones are defined as large volumes (up to 1 km thick) at the margins of the fault zone, where fracturing is relatively intense and pervasive with respect to the regional background. Core zones typically refer to localized horizons (up to a few meters thick) of intense deformation, where much of the displacement is accumulated and fault rocks are developed. The term “core zone” is sometimes used in a broader or different sense. For instance, Faulkner et al. (2003) describe a 1 km thick fault core for the Carboneras Fault, comprising both fault rocks (phyllonites, cataclasites) and less deformed hectometric lozenge-shaped dolomite bodies (lithons), and Ben-Zion and Sammis (2003) show very effectively how this terminology is inherently scale-dependent._ _In any case, most papers on fault zone architecture focus on characterization of exhumed core zones, fault rocks, and related_ _A. Bistacchi et al. Journal of Structural Geology 32 (2010) 2022-2041_ _deformation processes (Chester and Chester, 1998; Faulkner et al., 2003; Di Toro and Pennacchioni, 2005; Jefferies et al., 2006; Collettini and Holdsworth, 2004). Other papers aim at characterizing exhumed damage zones on various scales (e.g. Sibson, 1986; Antonellini and Aydin, 1995; Vermilye and Scholz, 1998; Billi et al., 2003; Berg and Skar, 2005; Dor et al., 2006; Micarelli et al., 2006; Mitchell and Faulkner, 2009). A logical counterpart of this research is direct drilling of active fault zones at seismogenic depth, such as the San Andreas Fault Observatory at Depth (SAFOD), Nankai Trough and Chelungpu Fault drilling projects (www.icdponline.org), which are providing data directly comparable to those from exhumed fault zones._ _A 3D high-resolution model of exhumed fault zones at the km-scale, resulting from field observations and populated by structural and mechanical properties has never been given. Indeed fault zones have been mostly imaged in 3D on a larger scale with geophysical methods. 3D seismic surveys have been used extensively to reconstruct fault networks on the 10^4-10^3 m scale, and, in tectonically active regions, it has been shown that core zones and damage zones can be imaged with head waves and trapped waves respectively (e.g. Li and Leary, 1990; Lewis et al., 2005, 2007), whilst “mean” fault surfaces can be reconstructed from the distribution of aftershocks (e.g. Carena and Suppe, 2002). Fracturing and physical property variations in damage zones can also be imaged by means of high-resolution seismic tomography (Martí et al., 2002; Francese et al., 2009), electrical resistivity (Ritter et al., 2005; Francese et al., 2009), and magnetotelluric methods (Unsworth et al., 1997; Ritter et al., 2005). Although these methods permit characterization of fault zones only indirectly, from remotely recorded physical properties, they are very important, because they investigate in 2D or 3D a volume comparable to that of the scale of large earthquakes (10^4 m)._ _This paper presents a model of the 3D architecture of two interacting crustal-scale fault zones, the Pusteria and Sprechenstein-Mules Faults (PF and SMF respectively), both belonging to the tectonic divide between the Alpine orogenic wedge and its South alpine hinterland (e.g. Schmid et al., 1989). The model is based on field and borehole data, the latter provided by BBT SE, in charge of the Brenner Basistunnel Project. The model volume is 10 ? 5 ? 1.5 km, thus bridging the gap between classical outcrop-scale descriptions and geophysical models, and reaching the length scale of ruptures that characterize typical medium sized earthquakes. Vertical extension was obtained by means of two inclined boreholes (>2000 m of continuous cores and geophysical logs) and natural exposures in this mountainous area (almost 2000 m between the deepest valley and the highest peak). The model comprises the principal fault surfaces and the surrounding volume containing the damage zones. This paper presents and discusses structural features of the PF and SMF (fault-rock distribution, fracturing in damage zones, etc.), 3D modelling methodologies and strategies for representation of structural properties in 3D, and the results of this modelling for understanding the highly heterogeneous distribution of fracturing in damage zones and fault rocks in fault cores._ _2. Geological setting_ _The Pusteria and Sprechenstein-Mules Faults (PF and SMF) belong to the Periadriatic system, the >600 km main tectonic divide between the Europe-vergent Alpine collisional wedge and the south-propagating Southern Alps fold-and-thrust belt (Schmid et al., 1989; Fig. 1). The collisional wedge is composed of the Adriaderived Austroalpine domain and the Penninic units of the Tauern window, including ophiolites and the underlying Europe-derived continental units (Bigi et al., 1990; Transalp Working Group, 2002)._ _The AustroalpineePenninic wedge underwent complex tectonometamorphic evolution during the Alpine subduction-collision event, whereas the Southern Alps escaped Alpine metamorphism (Kurtz et al., 1998; Dal Piaz et al., 2003, and references therein)._ _The E-W sub-vertical PF represents the ca. 200 km long easternmost segment of the Periadriatic system and laterally juxtaposes the Austroalpine basement and Permo-Mesozoic covers (northern block) with the South Alpine Permian Bressanone (Brixen) Granite (southern block). The NWeSE trending SMF is characterized by an array of brittle dextral fault zones, interconnected by transpressional stepovers, linking the Brenner detachment to the PF. The eastern tip of the SMF is found in the Valles Valley, thus making the SMF trace about 20 km long._ _During the Oligocene the Periadriatic Lineament was a preferential channel for the rise and crustal emplacement of magmas from mantle sources (post-collisional magmatism; Borsi et al., 1978; Barth et al., 1989; Müller et al., 2001). This magmatism is documented in the study area by the Mules tonalite, from hereaf_ Ключевые слова: continuous, clear relationship, core, attitude, fault network, fault blocks, observation, brenner detachment, biotite, phyllonitic foliation, journal structural geology, austroalpine basement, draft model, numerical model, pollard, juxtaposition map, block, thickness, society, relative weakness, segment, hanging wall, fault block, geological boundary, damage zone, surface, veining, model, bressanone, density, stress, pennacchioni jefferies, bressanone granite, environmental condition, seismogenic depth, temperature range, tessellation algorithm, brittle, geology, fault surface, interpolation, extensive precipitation, structural, scaling parameter, pf, modelling workow, rock mechanics, main, procedure outlined, gure legend, xrpd analysis, tectonic, contractional, tectonics, order, foliated cataclasites, damage lobe, journal structural, austroalpine, borehole, extend obliquely, tectonophysics, leimgruben stepover, structural geology, constraint, tectonic unit, short time-scale, case, analysis, study, fracturing, classi?cation, eastern, fault propagation, mallet, jog, geophysical, spatial coordinate, holdsworth, quartz, alpine, geologic time, foliation, white dashed, scale, fracture, di, periadriatic lineament, volume, slip, selverstone, ?eld, coded collected, lamella, core zone, representative transects, uncertainty, geological, earth, observed, fault, boreholes, reconstructed, smf, mancktelow, frisch, lithological cross-section, rock mass, topology, international, bulletin, pervasive network, considered, epidote, sibson, crosscutting relationship, slip accumulation, ben, relationship, brittle fault, scholz, fault zone, area, tonalitic, earth planetary, network, cataclasite, cataclastic, unit, spatial resolution, natural laboratory, journal, boundary, uid circulation, magnitude larger, mechanical model, ben zion, high-temperature foliation, characterized, set, process, mature fault, damage distribution, study area, wa, basement, relay volume, discrete model, virtually absent, borehole data, bistacchi, penninic unit, mules syncline, northern boundary, structural property, slip vector, deformation, map, toro, geological society, fault segment, reverse offset, alps, tectonic evolution, journal geophysical, mass, fault-rock, continuous core, austroalpine phyllonites, cataclasite matrix, layer, rst-order control, selected outcrop, result, geophysical log, de?ned, quasi-static model, hoek, paleostress, fault geometry, schmid, zone, rectilinear segment, geometry, angelier, fault rock, equality constraint, periadriatic fault, high, tonalitic lamella, fault core, damage, zion, dal piaz, direct relationship, eld data, granite, internal structure, distribution, appendix, chlorite, bistacchi journal, leimgruben, extensional quadrant, view, architecture, contractional jog, mechanics, parameter, del, rock, graph, major segment, chester, property, structure, host-rock lithology, doi, faulting, procedure, modelled appropriately, foliated, surface data, rheological boundary, data