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_Journal of Structural Geology 32 (2010) 1085-1100_ _Tip to midpoint observations on syntectonic veins, Ouachita orogen, Arkansas: Trading space for time_ _Pablo Cervantes 1, David V. Wiltschko*_ _Texas A&M University, Center for Tectonophysics, Department of Geology and Geophysics, Spence Street, College Station, TX 77843-3115, United States_ _Article info_ _Article history: Received 25 November 2008; Received in revised form 22 June 2010; Accepted 24 June 2010; Available online 14 July 2010_ _Keywords: Veins, Veinlets, Fracture, Fracturing, Boudinage, Boudins, Ouachitas, Mazarn shale, Benton uplift, Arkansas_ _Abstract_ _By examining a vein from its tip to center, we have established the transition from a single filled fracture at the vein tip to typical 'crack-seal' textures observed in fibered, laminated veins. The vein is contained in the boudin neck of a sandstone layer within the Lower Ordovician Mazarn Formation, Benton Uplift, Ouachita orogen. The tip of the vein is composed of one or more isolated veinlets, defined as quartz-filled narrow (5-25 mm) fractures parallel to the larger vein's long dimension. Scanned SEM-based cathodoluminescence shows that quartz laminae of the same orientation and thickness are found throughout the vein. Wall-normal fibers first appear in the vein where detrital grains are cut by multiple veinlets, each veinlet mimicking the crystallographic orientation of the detrital grain, whereas later veinlets reflect the established crystallographic orientation of the fiber. Fibers throughout the vein retain evidence of having been formed by repeated fracturing and filling of a pre-existing grain (at the vein walls) or fiber. However, recrystallization later modified the fibers by obliterating some evidence of the veinlets and moving fiber walls. Boudin formation provided the extension site that localized fracturing and vein filling. The vein grows by the repeated addition of veinlets in the neck region. Recrystallization altered the shape of previously formed fibers._ _© 2010 Elsevier Ltd. All rights reserved._ _1. Introduction_ _Despite a wealth of observations on syntectonic veins from a variety of tectonic regimes, fundamental questions remain as to the interplay of fracturing and vein filling. For example, although the origin of veins as fractures is largely established (e.g., Ramsay, 1980; Cox, 1987; Fisher and Brantley, 1992; Bons, 2000; Hilgers and Urai, 2002; Laubach et al., 2004a), it's not always clear why fractures initiate where they do and how the vein propagates once started. For sample, is fracture initiation followed immediately by fracture-filling or is the process of precipitation in part responsible for the fracture initiating (e.g., Taber, 1916, 1918, 1920; Misik, 1971; Li, 2000; Means and Li, 2001; Wiltschko and Morse, 2001). In those fractures that show 'crack-seal' textures or vein-parallel bands of precipitate, what controls the localization of repeated fracturing? How do wall-normal fibers develop by wall-parallel fracturing? What role does cementation have in altering the mechanics of fracture propagation as the vein grows?_ _* Corresponding author. E-mail addresses: pablo.cervantes@bp.com (P. Cervantes), d.wiltschko@tamu.edu (D.V. Wiltschko). 1 Present address: BP America, Inc._ _0191-8141 $ e see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2010.06.017_ _The problem of understanding the evolution of veins is in part one of scale. Veins that are meters long may be made up of grains that are microns in size, a range of 6 orders of magnitude. To date, fabric observations have been largely confined to small portions of veins, usually their centers (although see Laubach et al., 2004b). This approach has not yielded the observations directly applicable to the initiation of vein fabrics. In this paper, we report on a study of a vein from tip to midpoint in order to trace the evolution of vein formation from initiation to appreciable aperture. By choosing a vein that is of moderate length yet representative of fibered veins, our purpose is to trace the textures from tip to midpoint to understand the transition from a single fracture to a well-developed fibered, banded 'crack-seal' vein. Our idea is that if the tip represents the earliest stages of vein formation and the midpoint the most advanced stage that a particular vein displays, then one can observe the progression of fabric development. Therefore, this paper is an exercise in examining the evolution of a single vein from tip to center under the assumption that it is possible to trade space for time, although how much time is not known. With this assumption, we can begin to answer some of the questions raised above._ _The vein sampled for this study came from the Ouachita Mountains in Arkansas. This vein is typical in that it is fibrous and shows 'crack-seal' textures or banding parallel to the vein wall._ _1086_ _P. Cervantes, D.V. Wiltschko Journal of Structural Geology 32 (2010) 1085-1100_ _2. Geological setting_ _The veins of interest are found in the Benton Uplift of the Ouachita fold and thrust belt. The latter orogenic belt trends east-west and extends from central Arkansas to southeastern Oklahoma (Fig. 1A). To the east, the Ouachita system plunges beneath Mesozoic and Tertiary sediments of the Mississippi embayment. Based on COCORP reflection profiles, Lillie et al. (1983) and Nelson et al. (1982) proposed that during the lower Paleozoic the area was a south-facing passive continental margin followed in the Mississippian by collision with a southern, unknown block with North America. A thick Carboniferous marine shale facies at least 12 km thick (e.g., Houseknecht and Matthews, 1985) was emplaced on coeval shelf carbonates. Late motion on a crustal-scale ramp resulted in the northward and vertical movement of a portion of the continental margin producing the Benton and Broken Bow uplifts (Fig. 1A)._ _The frontal portion of the Ouachitas consists of north to northwest verging folds and thrust faults, although in places the most exterior structure is a triangle zone (e.g., Arbenz, 1989). Uplift of the core of the orogen (Broken Bow and Benton uplifts) has resulted in south-vergent folds and north-dipping cleavage on the south flank of the core. The deeper structures causing the core uplift displace margin sediments and/or continental crust on thrusts that most likely crop out at the thrust front. This deep thrusting was a late event because sediments are continuous both north and south of the uplifts until late Pennsylvanian or earliest Permian._ _Paleo-thermal gradients, while cut by thrust faults, are lower than would be predicted from burial of each thrusted sequence suggesting that thrusting was synchronous with peak metamorphism (Underwood et al., 1988). Vitrinite reflectance values increase to the southeast, leading Houseknecht and Matthews (1985) to suggest that postorogenic Cretaceous plutons influenced the thermal maturation of the core of the orogen. However,_ _Fig. 1. Geologic map of the Ouachitas. (A) Geologic map of the Ouachitas after Miser (1959). The white square shows the location of the Hot Springs area. (B) Regional geology of the Hot Springs, AR, area with the location of the Lake Pineda dam outcrop (black arrow pointing to solid black rectangle). After Haley and Stone (1996)._ _1087_ _P. Cervantes, D.V. Wiltschko Journal of Structural Geology 32 (2010) 1085-1100_ _Apatite fission-track analysis results show that regional cooling of the Ouachitas took place in Late Paleozoic, before emplacement of post-tectonic Cretaceous plutons within the Mississippi Embayment (Arne, 1992). Two proposals for the origin of the Cretaceous apatite fission-track ages are passage of the Bermuda hotspot (Cox and Van Arsdale, 1997, 2002) and postorogenic burial followed by Cretaceous exhumation (Corrigan et al., 1998)._ _The greatest concentration of quartz veins in the Ouachitas is found in a region 50-65 km wide at the center of the Benton and Broken Bow Uplifts (Miser, 1959). The rocks containing veins are also the rocks with highest metamorphic grade. The central part of the Benton Uplift is characterized by both north-dipping thrust faults and south-verging overturned folds (Fig. 1B). Miser (1959) observed that the veins in this region follow faults, bedding planes and fractures with the latter cutting across folds. The structural setting of the veins in the Ouachitas suggests that the veins are synorogenic (Nielsen et al., 1989; Viele, 1989). Radiometric dating of adularia in commercial quartz veins yield Late Pennsylvanian to Early Permian ages (Bass and Ferrara, 1969; Denison et al., 1977; Shelton et al., 1986)._ _3. Outcrop geology_ _All samples come from an outcrop located at Hot Springs Village, 13 miles north of Hot Springs, AK (Stone et al., 1994). The rocks exposed at the outcrop are from the Lower Ordovician Mazarn Shale, an interbedded banded green and black shale, laminated fine-grained gray siltstone, and minor lenses of fine-grained brownish-gray quartzitic sandstone. This formation has been interpreted to have been deposited as density and marine currents from north or northeast sources (Stone and McFarland, 1981; Lowe, 1989)._ _These rocks were deformed and metamorphosed up to greenschist facies as part of the Benton uplift. The outcrop is crossed by two thrust faults oriented N60E (Fig. 2). Fold wavelengths vary from cm's to m's in length. Small (cm to dm) folded sandstone layers are contained within shale beds while larger wavelength folds involve thicker (dm to m) sandstone beds. All shale units display cleavage in outcrop whereas the sandstone units do not._ _Bedding (Fig. 3A) strikes NEeSW. Fold axes (Fig. 3B) are consistent with those observed in other areas of the Benton uplift, namely NEeSW trending axes as reported by Viele (1989)._| Ключевые слова: e, r, o