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_Journal of Structural Geology 32 (2010) 1656–1667_ _Contents lists available at ScienceDirect_ _Journal of Structural Geology_ _journal homepage: www.elsevier.com locate jsg _Normal-fault development during two phases of non-coaxial extension: An experimental study_ _Alissa A. Henza*, Martha O. Withjack, Roy W. Schlische_ _Department of Earth & Planetary Sciences, Rutgers University, 610 Taylor Road, Piscataway, NJ 08854-8066, USA_ _article info_ _Article history: Received 29 January 2009; Received in revised form 24 June 2009; Accepted 26 July 2009; Available online 4 August 2009_ _Keywords: Normal faults Experimental modeling Multiple phases of extension Fault reactivation_ _abstract_ _We use scaled experimental (analog) models to study the effect of a pre-existing fault fabric on fault development during extension. In our models, a homogeneous layer of wet clay undergoes two noncoaxial phases of extension whose directions differ by up to 45°. The normal faults that develop during the first phase create a pronounced fault fabric that influences normal-fault development during the second phase. In all models, pre-existing faults are reactivated during the second phase of extension. Their sense of slip depends on the angle between the first and second-phase extension directions. Specifically, the component of dip slip relative to strike slip decreases as the angle between the first and second-phase extension directions increases. New normal faults also form during the second phase of extension in all models. The number of new fault segments increases as the angle between the first and second-phase extension directions increases. The orientations of the new normal-fault segments are both orthogonal and oblique to the second-phase extension direction, indicating that both the second-phase extension direction and the pre-existing fault fabric control the orientation of new fault segments. Some of the new normal faults cut and offset the pre-existing faults, whereas others terminate against them, producing complex fault patterns and interactions. The modeling results explain fault interactions observed in the Jeanne d’Arc rift basin of offshore Newfoundland, Canada, and the reactivation of abyssal-hill normal faults at outer highs near subduction zones._ _© 2009 Elsevier Ltd. All rights reserved._ _1. Introduction_ _Many extensional provinces have undergone more than one episode of deformation. For example, researchers have recognized multiple phases of non-coaxial extension in the Jeanne d’Arc rift basin (e.g., Enachescu, 1987; Tankard and Welsink, 1987; Sinclair, 1995a, b; Sinclair and Withjack, 2008), the North Sea (e.g., Badley et al., 1988; Bartholomew et al., 1993; F?rseth, 1996), Thailand (e.g., Morley et al., 2004, 2007), and the East African rift system (e.g., Strecker et al., 1990). Although these studies provide valuable information about fault orientations and interactions in areas with complex extensional histories, several critical questions remain. How does the fault pattern that forms during an early episode of extension affect the fault patterns that form during subsequent episodes of extension? What factors determine whether faults are reactivated or new faults form during subsequent deformation? How do pre-existing normal faults affect the initiation, propagation, and geometry of newly formed normal faults? The goal of our research is to address these critical questions. Specifically, we use scaled experimental (analog) models to simulate two phases of non-coaxial extension and study how the angle between the extension directions affects the resultant deformation patterns. We also compare the modeling results with natural fault patterns in regions that have undergone multiple phases of extension._ _2. Experimental approach_ _2.1. Modeling materials_ _Most scaled experimental models of extension use either dry sand or wet clay as the primary modeling material (e.g., Eisenstadt and Sims, 2005; Withjack et al., 2007, and references therein). Deformation patterns in sand and clay models of extension have similarities and differences (Withjack and Callaway, 2000; Eisenstadt and Sims, 2005; Withjack and Schlische, 2006; Withjack et al., 2007). In both sand and clay models, normal faults form that strike 90° to the extension direction. Fault-zone widths, however, are much greater in sand models (>1.0 mm) than in clay models (<0.1 mm). Also, most deformation is localized on a few major normal faults in sand models, whereas deformation is distributed among major normal faults, minor normal faults, and folds in clay models. In this study, we use wet clay as the modeling material to provide a more detailed view of fault interactions and evolution. The wet clay is similar to that used in other experimental modeling studies (e.g., Withjack and Callaway, 2000; Eisenstadt and Sims, 2005; Withjack and Schlische, 2006; Withjack et al., 2007). It is composed mainly of kaolinite particles (<0.005 mm in diameter) and water (w40% by weight) and has a density of 1.55–1.60 g cm?3. Its coefficient of internal friction is w0.6, and its cohesive strength is w50 Pa._ _To ensure dynamic similarity between the experimental models and natural prototypes, two conditions must be satisfied (e.g., Hubbert, 1937; Nalpas and Brun, 1993; Weijermars et al., 1993). First, the modeling material must have a similar coefficient of friction to that of rocks in nature. For most sedimentary rocks, the coefficient of friction ranges between 0.55 and 0.85 (e.g., Handin, 1966; Byerlee, 1978). Thus, the wet clay in our models (with a coefficient of friction of w0.6) is a suitable modeling material. Second, the models must obey the scaling relationship:_ _C* ? r* ? L* ? g*_ _(1)_ _where C*, r*, L*, and g* are the model to prototype ratios for cohesion, density, length, and gravity, respectively. In our models, the value of r* is 0.7 and g* is 1. Thus, C* and L* must have similar magnitudes to ensure dynamic similarity. In nature, C ranges from less than 1 MPa (for loosely compacted sedimentary rocks) to more than 10 MPa (for intact igneous or metamorphic rocks) (Handin, 1966; Schellart, 2000; and references therein). Additionally, C can be significantly less than 1 MPa for fractured rocks (e.g., Byerlee, 1978; Brace and Kohlstedt, 1980). As mentioned previously, the wet clay in our models has a cohesive strength of w50 Pa, resulting in a value of C* between 10?4 and 10?6. Therefore, L* ranges between 10?4 and 10?6 in our models, depending on the cohesion of the natural prototype. If the clay simulates a layer of loosely compacted sedimentary rock, then 1 cm in the model represents w100 m in nature. Alternatively, if the clay simulates intact crystalline rock, then 1 cm in the model represents about w10 km in nature._ _2.2. Experimental set-up_ _Our experimental set-up resembles that in previous experimental models of oblique extension (e.g., Withjack and Jamison, 1986; Tron and Brun, 1991; McClay and White, 1995; Bonini et al., 1997; Keep and McClay, 1997; Clifton et al., 2000). The base of the apparatus consists of an 8-cm wide rubber sheet attached to two rigid sheets (one fixed and one mobile) (Fig. 1a). A 0.5-cm thick layer of silicone polymer, with a viscosity of w104 Pa s (Weijermars, 1986), overlies the rubber sheet. A layer of wet clay covers the layer of silicone polymer, the fixed rigid sheet, and the mobile rigid sheet (Fig. 1b). It is 3.5 cm thick above the layer of silicone polymer and 4.0 cm thick above the rigid sheets. During the experiments, the mobile rigid sheet moves outward, stretching the attached rubber sheet and the overlying silicone polymer (Fig. 1a). In response, a deformation zone develops within the clay layer above the rubber sheet and silicone polymer. The silicone polymer serves two functions in our models: it localizes deformation within the clay layer above the rubber sheet (e.g., Bellahsen et al., 2003), and it decouples the clay layer from the rubber sheet (allowing the base of the clay layer to move vertically)._ _Based on Withjack and Jamison (1986), a is the clockwise angle measured from the trend of the deformation zone within the clay layer to the displacement direction of the mobile rigid sheet (Fig. 1a). Oblique deformation with both extensional and shear components results when a s 0°, a s 90°, or a s 180° (Withjack and Jamison, 1986). Furthermore, when a s 90°, the maximum extension direction and displacement direction differ: the maximum extension direction lies midway between the displacement direction and the normal to the trend of the deformation zone (see Withjack and Jamison (1986) for details). Previous models of oblique extension (e.g., Withjack and Jamison, 1986; Tron and Brun, 1991; Clifton et al., 2000) have shown that only normal faults form when 45° a 135° (Fig. 2a). For a < 45° and a > 135°, normal, oblique-slip and or strike-slip faults develop._ _All models in this study have two phases of deformation (Fig. 2b). During the first phase, the mobile sheet moves outward at a rate of 4 cm h?1 (1.1 ?10?3 cm s?1) in a prescribed direction (a1 = 45°) for a prescribed displacement (3.5 cm). In response, a pervasive but not continuous fabric consisting of normal faults develops throughout the deformation zone in the clay layer. During the second phase, the mobile sheet moves outward at a rate of 2 cm h?1 (5.6 ?10?4 cm s?1) in a prescribed direction (a2 = 90°) for a prescribed displacement (3.5 cm). In response, new normal faults form and reactivate pre-existing faults._ Ключевые слова: reactivated, mcclay, weijermars, jamison, fault cut, sinclair, pre-existing, scaled, drawing showing, fault reactivation, nature, tectonophysics, formation, development, difference, non-coaxial extension, existing fault, cut, experimental model, trench axis, oblique extension, central, geology, modeling, fault reactivated, granger, direction, result, journal structural, enachescu, clay model, phase extension, oblique-slip fault, fault segment, central region, observation, handin, physical, billen, friction, internal friction, masson, basin, byerlee, bulletin, pattern, strike-slip component, orientation, schlische withjack, brun mcclay, model designation, bellahsen, location, fault pattern, elsevier, surface, normal, form, rutgers, journal, study, brun, orthogonal, abyssal-hill, subsequent episode, offshore newfoundland, arrows, sliding friction, jamison tron, silicone, rift, newfoundland, extension model, deformation zone, layer, extension direction, analogue modeling, zone, ?rst, clay layer, cohesive strength, rubber sheet, clay, cm, morley, dip, strike oblique, pre, normal-fault, displacement, normal fault, henza, extensional, reactivation, schellart, modeling result, rigid sheet, ?rst-phase fault, geological, abyssal-hill fault, strength, henza journal, polymer, existing, modeling material, second-phase extension, eds, dip slip, range, geological society, ?rstand, area, component, schlische, experimental, intact clay, extensional province, basal layer, growth bed, structural geology, sand, rose diagram, fault, oblique, extension affect, model, fault surface, model figs, rst phase, hubbert, arc, callaway eisenstadt, second-phase, episode, corrugation, reactivated fault, withjack, fault development, tankard, ?rst-phase, fault population, jeanne, second-phase fault, apparent dip, fabric, extension specically, ?rstand second-phase, tectonics, rst-phase fault, discussion, jsg, mcintyre, outward, sims withjack, segment, phase, strike, journal structural geology, pre existing fault, normal stress, rea, fault form, model represents, pre-existing fault, showing, interaction, rock, photograph, society, greatest adjacent, kriner, fault interaction, pre existing, wet, applied, silicone polymer, angle ?rstand, ?rst phase, wet clay, abyssal-hill normal, rigid, outer high, extension, multiple phase, trench, rst, sheet, angle, normal-fault segment, sand model, deformation, slip, displacement direction, natural prototype, structural, surface clay, stress, fault orientation, geophysical