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_Journal of Structural Geology 32 (2010) 216–221_ _Contents lists available at ScienceDirect_ _Journal of Structural Geology_ _journal homepage: www.elsevier.com locate jsg_ _Rb–Sr systematics of fault gouges from the North Anatolian Fault Zone (Turkey)_ _Halim Mutlu a,*, I. Tongu? Uysal b, Erhan Altunel a, Volkan Karabacak a, Yuexing Feng c, Jian-xin Zhao c, Ozan Atalaya a Department of Geological Engineering, Eskis? ehir Osmangazi University, Eskis? ehir 26480, Turkey_ _b Queensland Geothermal Energy Centre of Excellence, The University of Queensland, Queensland 4072, Australia_ _c Radiogenic Isotope Facility, Centre for Microscopy and Microanalysis, The University of Queensland, Queensland 4072, Australia_ _article info_ _Article history: Received 2 April 2009 Received in revised form 13 November 2009 Accepted 18 November 2009 Available online 24 November 2009_ _Keywords: North Anatolian Fault Zone Fault gouge Illitic clays Rb–Sr dating_ _abstract_ _A combined mineralogical and Rb–Sr isotopic investigation was conducted on fault gouges from two locations at eastern part of the North Anatolian Fault Zone (NAFZ). The fault gouge samples contain no 2 M mica and consist chiefly of cryptocrystalline material, which is extensively altered to illitic clays (mixed-layered illite–smectite) and some carbonate minerals. The Rb–Sr isochrones of leachate, residue and untreated aliquots from various size fractions of samples yielded two different illite generations. The oldest illite authigenesis, which started at about 35.6 Ma might be inherited from pre-existing faults along the Tethyan suture zone. The authigenic illites from finer fractions correspond to an isochron age of 8.3 Ma, which is in agreement with findings of previous works that may suggest a middle Miocene age for the initiation of the NAFZ. Our Sr isotope data indicate that the metamorphic fluids contain some mantle components and mobilised during fault movement._ _? 2009 Elsevier Ltd. All rights reserved._ _1. Introduction_ _Dating of fault reactivations along active plate boundaries is important to understand the evolution of tectonic plate movements and interactions; however, radiometric dating of shallow crustal faulting remains highly challenging. In active tectonic zones, we can only use low temperature K-bearing authigenic minerals for age dating, which precipitate during surface or near-surface faulting in the crust’s brittle regime. Complete syntectonic crystallisation and isotopic homogenisation is, however, generally not achieved in low temperature shallow environments. Thus illitic clay minerals separated from fault gouges are composed of a mixture of authigenic 1 M illite formed during faulting and detrital 2 M illite inherited from the host rock. Recently, a number of studies have demonstrated that such clays can be dated successfully by plotting percentage detrital 2 M illite against the K–Ar or Ar–Ar dates of the clays with a range of size fractions (e.g., from <0.2 to 2–1) that allow extrapolation to apparent ages for detrital and authigenic end members (Grathoff et al., 2001; van der Pluijm et al., 2001, 2006; Haines and van der Pluijm, 2008). Similarly, using K–Ar analysis of various illite size fractions combined with X-ray diffraction quantification of detrital and authigenic endmembers we determined the age of syntectonic illite generation in a fault gouge from the central part of a plate boundary strike-slip fault system, the North Anatolian Fault Zone (NAFZ) in Turkey (Fig. 1A–B) (Uysal et al., 2006). Our finding suggests that significant strike-slip fault movements initiated at w57 Ma, immediately after the continental collision related to the closure of the Neotethys Ocean. On eastern segments of the NAFZ, clay minerals were formed through syntectonic hydrothermal alteration of fault gouges containing pseudotachylyte bands, and no detrital contaminations are present in such samples (Uysal et al., 2006). Intense hydrothermal alteration gave rise to complete recrystallisation of detrital phyllosilicate phases with precipitation of mixed-layered illite–smectite. Although such clays represent a pure authigenic mineral population, their radiometric age dating is also not straightforward. In active fault systems, newly formed illitic clays are subjected to episodic heat and fluid flow events. Heating of earlier formed authigenic illitic clays during fault reactivation to a temperature close to or above their crystallisation temperature causes partly or completely resetting of their isotopic systematics (Clauer and Chaudhuri, 1999). Mixed-layered illite–smectite as a common authigenic clay phase in fault gouges is particularly susceptible to younger thermal events because of their very small crystal sizes and hence their higher surface area to volume ratios. As a result, such clays can experience redissolution and recrystallisation a number of times during the evolution of the fault activity and thus yield decreasing K–Ar dates with decreasing grain sizes._ _H. Mutlu et al. Journal of Structural Geology 32 (2010) 216–221_ _217_ _Fig. 1. (A) Simplified Neotectonic setting of Turkey (modified from McClusky et al., 2000; Bozkurt, 2001). (B) Geological and location map of the investigated areas (modified from Herece and Akay, 2003)._ _Consequently, reliable interpretation of such age data as mixed ages due to incomplete isotopic resetting or real crystallisation ages related to episodic fault activations is difficult. For example, inclined K–Ar age spectra relative to clay particle size from fault gouges in the eastern part of the NAFZ indicate that displacements along the NAFZ have taken place continuously following the initiation of the fault movements in the latest Paleocene–Early Eocene (Uysal et al., 2006), so that the timing of significant individual fault reactivation episodes remains unknown._ _The Rb–Sr isotope system has been extensively used in geochronological studies of illite authigenesis in mudrocks and sandstones (Gilg and Frei, 1994; Clauer and Chaudhuri, 1995; Uysal et al., 2001). Three point isochrones constructed from the untreated, acid-leached residue and leachate samples have been used to evaluate the extent of Rb–Sr isotopic homogenisation and date the illite authigenesis in sedimentary basins (Clauer et al., 1993; Clauer and Chaudhuri, 1995). In the current paper, we show that the Rb–Sr isotopic system can also be used to evaluate the degree of isotopic homogenisation and determine the timing of syntectonic illite–smectite precipitation in near-surface fault gouges. Our results also add insight into the reactivation history of the North Anatolian Fault Zone, which is critical in understanding the plate tectonic evolution of the eastern Mediterranean region (e.g., S?engo? r and Yilmaz, 1981; Okay et al., 2008)._ _2. Sample locations and description_ _Location KSL is situated near the town Resadiye within a crushed zone in the Eocene sandstone-shale unit (Fig. 1B). The sandstone-shale unit exposed outside the fault zone represents the undeformed parent rock. The phyllosilicate mineral phase in the parent rocks consists of mainly chlorite with some mica, which are absent in the KSL fault rocks. The fault gouge samples consist of isotropic, glassy or cryptocrystalline groundmass (pseudotachylyte) interlayered with quartz-rich fine-grained clastic material. The glassy or cryptocrystalline material is significantly altered to illitic clays, kaolinite, and ankerite–calcite carbonate association (see Uysal et al., 2006 for more information)._ _Fig. 2. Thin section image of sample KHS showing glassy or cryptocrystalline material associated with emplacement of microscopic carbonate (ankerite–calcite) veins._ _A9000 B1600 KSL 2-1 Polytype 7000 6000 Intensity (cps) d _ 5.02 d _ 4.49 d _ 4.26 d _ 4.17 d 0 4.12 d _ 3.69 d _ 3.58 d _ 3.04 d _ 2.90 d _ 2.70 d _ 2.57 d _ 2.52 d _ 2.46 d _ 2.39 d _ 2.34 d _ 2.28 d _ 2.25 d _ 2.24 d _ 2.21 d _ 2.13 d _ 2.11 d _ 2.07 5000 4000 3000 2000 1000 0 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44_2 Theta - Scale _B1400 KHS <0.2 AD KHS <0.2 GL_1200 d _ 10.65 d _ 3.31 d _ 3.25 Intensity (cps) 1000 800 600 d _ 3.60 d _ 3.07 d _ 9.82 d _ 7.13 d _ 7.08 d _ 4.99 d _ 4.91 400 200 0_2468 C16000 KSL <0.2 AD KSL <0.2 GL_14000 12000 Intensity (cps) d _ 11.78 d _ 10.59 d _ 10.07 d _ 7.19 d _ 5.01 d _ 3.57 d _ 3.33 d _ 3.29 d _ 3.01 10000 8000 6000 4000 2000_2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32_2 Theta - Scale Fig. 3. X-ray diffraction tracings of fault gouge samples. (A) The random powder 2–1 mm fraction of sample KSL (diagnostic peaks of 2 M illite polytype (triangles) are from Grathoff and Moore (1996)); (B) Air-dried (AD) and glycolated (GL) scan for <0.2 mm fraction of sample KHS (note the shift of the 10.7-? peak towards the high angle area with ethylene glycolation); (C) Air-dried (AD) and glycolated (GL) scan for <0.2 mm fraction of sample KSL (note an asymmetrical peak at w10 ?, with a broad shoulder when glycolated)._ Ключевые слова: material, isochron age, event, fault, science, kar analysis, middle, chlorite, der, air-dried, rb–sr isochron, carbonate, closure, grade, ?uids, westward translation, yilmaz, authigenic illitesmectite, result, residue, erzincan, scale, location, leachate, khs sample, journal structural, previous, temperature, theta, suggested, engo?, akay, authigenic, initial, journal, geochronological, illite, mica, clay fraction, clays, age, illitic clay, timing, metamorphic uids, situated, anatolian fault, data, linear, analysis, geological, niksar, study, late cretaceous, clauer chaudhuri, glassy, fault dating, mixing, rb–sr, cryptocrystalline material, der pluijm, origin, provide, isochron diagram, fraction sample, vrolijk, isochron, bozkurt, location ksl, hall, uysal, pevear, isotope, leachate residue, syntectonic, untreated, linear relationship, isoplot, interaction, clauer, engo, pluijm, uids, mixture, queensland, centrifuging, fraction, clay size, older age, rbsr diagram, consistent, structural, westaway, koyulhisar, isotopic homogenisation, figs, support, location khs, dating, anatolian, leachate-residue-untreated fraction, xrd analysis, nafz, geology, consist, sr, mineral, gouge, turkey, mm, ma, rb sr, detrital, size fraction, leachate-untreated fraction, meaningful age, ksl sample, zone, suture, agreement, north, wa, north anatolian, cryptocrystalline, fault zone, x-ray, anatolian block, size, sample, faulting, leachate-residue-untreated, untreated sub-samples, diagram, town, eurasian plate, generation, tectonic, illite–smectite, evaluate, golding, time, evolution, fault gouge, illitic, van der, gouge sample, k–ar, initiation, mswd, acta, res adiye, ratio, university, illite authigenesis, mcclusky, chaudhuri, van, ludwig, sample ksl, isotopic, ksl location, khs, res?, number, plate, kar, moore, s? engo?, rock, structural geology, clay, miocene, eastern, location uysal, emplacement, mutlu, ksl, movement