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_Journal of Structural Geology 32 (2010) 727–740_ _Contents lists available at ScienceDirect_ _Journal of Structural Geology_ _journal homepage: www.elsevier.com locate jsg_ _Structural analysis of fault populations along the oblique, ultra-slow spreading Knipovich Ridge, North Atlantic Ocean, 73°14'30"–78°14'40"N_ _Daniel Curewitz a,*, Kyoko Okino b, Miho Asada b, Boris Baranov c, Evgeny Gusev d, Kensaku Tamaki b,e_ _a Syracuse University, Department of Earth Sciences, 204 Heroy Geology Laboratory, Syracuse, NY 13244, USA_ _b Ocean Research Institute, University of Tokyo, 1-15-1 Minamidai, Nakano, Tokyo 164-8639, Japan_ _c Shirshov Institute of Oceanology RAS, 36 Hakhimovsky, Moscow 117851, Russia_ _d VNIIOkeangeologia, 1 Anglyiiski Av., St. Petersburg 190121, Russia_ _e University of Tokyo, School of Engineering, 3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan_ _article info_ _Article history: Received 7 November 2008 Received in revised form 9 July 2009 Accepted 7 August 2009 Available online 18 August 2009_ _Keywords: Fault populations Oblique rifting Segmentation Mid-Ocean Ridge_ _abstract_ _The Knipovich Ridge (73°14'30"–78°14'40"N) is an extreme end-member of the mid-ocean ridge spreading system, both in terms of spreading rate (<1.5 cm yr) and angle between the ridge and the spreading direction (40°–53°). Structural analysis of side-scan sonar images obtained along 400 km of the ridge axis reveals systematic relationships between fault population parameters, obliquity, and axial segmentation along the ridge._ _Fault population characteristics conform to observational and experimental analyses of oblique rifting and spreading systems. For the ridge as a whole, faults in the axial region are short, straight, isolated (not linked into complex fault zones), and exhibit length-scaling relationships characteristic of young and active fault systems. Faults are generally oblique to both the ridge axis and the spreading direction, and orientations vary systematically with angle between ridge and spreading direction._ _Along-axis analysis reveals the influence of axial segmentation on fault population characteristics. Segment centers are dominated by faults perpendicular and sub-perpendicular to plate motion with longer characteristic length and generally lower fracture density. Conversely, segment ends are dominated by faults striking oblique to plate motion with shorter characteristic length and generally higher fracture density. We infer that faulting in segment centers is strongly influenced by the mechanical effects of dike intrusion perpendicular to plate motion, while faulting in segment ends is controlled by the mechanics of oblique rifting, non-transform discontinuities, and or accommodation zones. The contrasts between these distinct structural and mechanical settings along the ridge axis are accentuated by the high obliquity and ultra-slow spreading rate of this spreading system._ _2009 Elsevier Ltd. All rights reserved._ _1. Introduction_ _1.1. Study area_ _The Knipovich Ridge lies in the northern Norwegian-Greenland Sea, Northeast Atlantic, just off the coast of the Eurasian continental margin (Fig. 1a). It is situated asymmetrically between Svalbard and NE Greenland, and extends from 73°14'30"N to 78°14'40"N, from the junction with the Mohns Ridge to the Molloy Transform. It is one of the slowest and most obliquely spreading mid-ocean ridges on the planet. The ridge trends ~347° from 74°14'30" to 75°14'50"N, and ~000° from 75°14'50" to 78°14'40"N. The NUVEL-1A and REVEL (DeMets et al., 1990, 1994; Sella et al., 2002) global circuit plate-tectonic models predict a plate motion vector of ~307° and a full spreading rate of ~1.4 cm yr (Fig. 1b). The ridge axis is highly oblique to the spreading direction, with an angle between the spreading vector and the ridge axis (noted as “alpha” after Withjack and Jamison, 1986) ranging from 37° in the north to 50° in the south._ _Magnetic anomaly data are sparse, and interpretations vary drastically (e.g. Crane et al., 1991; Vogt et al., 1998; Skogseid et al., 2000), however Vogt et al. (1998) provide a tentative identification of magnetic anomaly lineations in the Northeast Atlantic based on compiled ship and aeromagnetic surveys (Olesen et al., 1997). These interpretations reveal discrete packages of short, discontinuous anomalies that trend oblique to the ridge axis and nearly perpendicular to the relative plate motion vector. These packages are bounded by approximately linear, plate-motion parallel regions of low magnetization and are interpreted as possible traces of spreading discontinuities (Fig. 1b, after Vogt et al., 1998)._ _Previous work on the Knipovich Ridge (Crane et al., 1991, 2001; Crane and Solheim, 1995; Vogt et al., 1998; Gusev and Shkarubo, 2001; Okino et al., 2002) has delineated a 3000–3500 m deep, 10–15 km wide rift valley punctuated by widely spaced (20–40 km), en echelon volcanic highs ranging from 0.5 to 1 km in height above the seafloor (Fig. 2a, 2b). The axial seamounts commonly lie along the trend of chains of off-axis highs (Fig. 2b), suggesting that they represent relatively stable loci of magmatic accretion (Okino et al., 2002). The most prominent axial volcanic highs are located at the intersection of the discrete packages of magnetic anomalies and the ridge axis (shaded circles in Fig. 1b)._ _Sediment thickness on the axial valley floor (up to 100 m) and flanks (up to 1 km or more) is high owing to the proximity of the Eurasian continental shelf (Vogt and Sundvor, 1996; Vogt et al., 1998; Sundvor et al., 2000; Gusev and Shkarubo, 2001). Seismic activity is generally low, and earthquakes are widely scattered in the rift valley and on the eastern (Eurasian) flank of the ridge (Crane et al., 1991; Gusev and Shkarubo, 2001)._ _1.2. Knipovich Ridge morphology and segmentation_ _Mid-ocean ridge segmentation is defined in terms of axial depth profiles and the mantle Bouguer anomaly (MBA) profile. MBA profiles are used to investigate density variations in the mantle in order to determine the pattern of mantle upwelling and melt distribution beneath a spreading segment. High and low MBA values are interpreted to represent denser colder mantle with lower magma production potential, or lighter warmer mantle with higher magma production potential, respectively. Ridge axis discontinuities generally occur at local depth maxima and correspond to high MBA values (Kuo and Forsyth, 1988; Lin et al., 1990), while spreading segment centers generally occur at local depth minima and correspond to low MBA values. General categories of ridge axis discontinuities include transform faults (1st order), overlapping spreading centers (OSCs) or non-transform_ _D. Curewitz et al. Journal of Structural Geology 32 (2010) 727–740_ _78°N a 78°N b 77°N 77°N 76°N 76°N 307° 1.47 cm yr 75°N 75°N 6°E 7°E 8°E 9°E 6°E 7°E 8°E 9°E Fig. 2 a) Compiled IBCAO, SeaMARCII, 7.5 and 3.5 kHz echo-sounder data from the Knipovich Ridge axis and near axis crust. Track line is shown in white. Contour interval is 200 m. b) Interpretation showing the location of the rift valley walls (hatched), oblique, en echelon axial highs (dark gray) and elongate axial lows (light gray). The axial highs anchor symmetric chains of off-axis highs (medium gray) that are parallel to plate motion (arrows)._ _discontinuities (NTDs) (2nd order), and deviations from axial linearity (DEVALs), small OSCs, or other minor discontinuities (3rd order) (Macdonald et al., 1988, 1991; Macdonald, 1982)._ _Bathymetric data from the Knipovich Ridge indicate that axial highs take the form of isolated en echelon ridges whose long axes are approximately perpendicular to the plate motion direction (Fig. 2a and b). Off-axis seamount chains aligned parallel to plate motion meet the ridge at several of these en echelon axial highs. Similar features and relationships are identified on the Mohns Ridge, the Reykjanes Ridge, and the Southwest Indian Ridge (Murton and Parson, 1993; Geili et al., 1994; McAllister et al., 1995; Dauteuil and Brun, 1996; Tuckwell et al., 1996; Searle et al., 1998b; Abelson and Agnon, 2001; Sauter et al., 2002). The Knipovich Ridge is uninte_ Ключевые слова: spreading, fault orientation, rifting, knipovich ridge, nature, oblique rifting, axial valley, mid-ocean ridge, tectonophysics, margin, difference, analysis, depth, mcallister, fault population, parallel, experimental model, experimental study, clifton, survey, fault data, geology, fault-scaling relationship, gravity data, ridge trend, based, direction, journal structural, planetary, macdonald, smaller fault, experimental data, mohns ridge, dauteuil, axial segmentation, observational result, cowie, map area, letters, length, marrett ackermann, magmatic, mid, clifton schlische, eds faulting, azimuth, exponential, oblique spreading, exponential relationship, wright, knipovich, orientation, chadwick, rift valley, re?ect, fault number, center, elsevier, normal, searle, intrusion, journal, study, brun, crane, obliquity, fault length, experimental analysis, white, mantle, fracture density, ocean, rift, rift trend, magmatic activity, ridge axis, dike-induced faulting, fault greater, earth planetary, spreading direction, norway, zone, curewitz journal, axial trend, valley, volcanic construction, measured, along-axis, eld study, region, segment center, relationship, sea?oor, dike intrusion, distribution, image, trace, axial high, long, sanderson, longer fault, discontinuity, ridge, fault growth, geological, earth, highly oblique, sonar data, fault trace, institute university, norwegian, en, echelon, trend, area, sonar image, perpendicular plate, schlische, reect difference, experimental, structural geology, scaling relationship, volcanic, density, fault, plate, axial, population, oblique, curewitz, measurement error, fault longer, generally, angular difference, model, reches, abelson, journal geophysical, gra cia, rubin, doi, fault density, interpretation, fault azimuth, characteristic, high angle, fault strike, rift normal, sonar, withjack, ge li, rift axis, geophysical letters, relative, mba, norwegian margin, tectonics, embley, bathymetric, characteristic length, plate motion, accommodation zone, discussion, olesen, segment, number, journal structural geology, faulting, data, geometry, segmentation, anomaly, rock, plan view, axis, axial region, general trend, eruption, sella, rift obliquity, parson, order, vogt, american, high, klingelho fer, extension, magma, sundvor, angle, mid-ocean, gray, motion, displacement direction, dike, structural, geophysical, science, perpendicular