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_Journal of Structural Geology 32 (2010) 407–422_ Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com/locate/jsg Faulting and ?ssuring in active oceanic rift: Surface expression, distribution and tectonic–volcanic interaction in the Thingvellir Fissure Swarm, Iceland L. Sonnette a,*, J. Angelier a, T. Villemin b, F. Bergerat c GEOAZUR (UMR 6526 CNRS-UNS-UPMC-IRD), Observatoire Océanographique, la Darse, B.P. 48, 06235 Villefranche-sur-Mer Cedex, France b EDYTEM (UMR 5204 CNRS-Université de Savoie), Campus scientifique, 73376 Le Bourget du lac Cedex, France c ISTeP (UMR 7193 CNRS-UPMC), Université Paris VI, Case 117, 4, Place Jussieu, F-75252 Paris Cedex 05, France Article info Article history: Received 4 July 2009 Received in revised form 18 December 2009 Accepted 2 January 2010 Available online 18 January 2010 Keywords: Icelandic rift Thingvellir Fissure Swarm Geomorphology Photogrammetry Normal fault growth Rock fracture mechanics Abstract Iceland brings exceptional opportunity for analysing extension related to rifting of the Mid-Atlantic ridge, especially revealing fresh structural patterns in active ?ssure swarms. Post-glacial fracture systems of the Thingvellir rift segment of the West Volcanic Zone (WVZ) and interaction with Holocene lava ?ow overlapping are analysed in detail in this paper. We mapped 5390 fractures at metric to kilometric scales in order to realise a precise structural map, a representative fault length distribution analysis and some statistical calculations in terms of fault length number growth rates from Holocene to recent time. Mapping and 3-D geometrical analysis of faults and ?ssures are based on use of photogrammetric techniques, GPS positioning at ground control points and validation from geological ?eld work. This approach allowed us to measure the vertical throw distribution along 52 faults with a precision around 0.5–1 m. Most of these faults have symmetric serrated fault-displacement pro?les; however some of them have pro?les offset to the north or south. Fault vertical offset as a function of the age of the hosting lava ?ows are presented too. Finally, from the study of 70 transverse topographic pro?les and the fault offset analysis, we propose a propagation model for Holocene ?ssure development, partly controlled by Pleistocene tectonic inheritance. Our model takes into special account alternating volcanic events and faulting. Simple ?ssure zones with small hangingwall monocline or more complex scarp zones with graben and larger hangingwall monocline developed. Because of lava ?ow accumulation during the rift extension, estimating the amount of extension based on the present-day morphology would have led to severe under-evaluation. ? 2010 Elsevier Ltd. All rights reserved. 1. Introduction The ongoing oceanic rifting in Iceland mainly consists of socalled ?ssure swarms containing ?ssures and faults that affect recent lava ?ows accumulated in the axial zones of the rift segments in the west, east and north volcanic zones (Fig. 1). In the volcanic zones, close relationships exist between major central volcanoes and ?ssure swarms (Johannesson and Saemundsson, 1998a,b). The recent and present-day fracture patterns can be studied at the surface over a wide range of investigation scales, from minor ?ssures to large faulted-tilted blocks. When analysed in detail, the topography reveals the mechanical consistency of the underlying extensional structure (Angelier et al., 1997; Dauteuil et al., 2001). * Corresponding author. E-mail address: sonnette@geoazur.obs-vlfr.fr (L. Sonnette). 0191-8141 $ – see front matter ? 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2010.01.003 The Thingvellir Fissure Swarm (TFS), which belongs to the West Volcanic Zone (WVZ) (Fig. 1a), provides excellent illustration of fracture patterns and magmatic activity in the bottom of an oceanic rift segment. The TFS and other Icelandic ?ssure swarms have been actively studied (Nielsen, 1930; Benauer, 1943; Kjartansson, 1964; Walker, 1964, 1965b; Thorarinsson, 1965; Saemundsson, 1978, 1992). Specific analyses have been done regarding the determination of the rate of horizontal deformation (Gerke, 1974; Brander et al., 1976; Decker et al., 1976), vertical deformation (Tryggvason, 1974, 1982), fracture geometry (Gudmundsson, 1987a,b, 2000; Villemin et al., 1994; Grant and Kattenhorn, 2004), fault growth (Gudmundsson, 1992) and relay zones (Acocella et al., 2000). However, the interaction between the volcanic activity and the development of fractures and faults in the West Volcanic Zone has not been analysed in detail. We attempt at ?lling this gap, taking advantage of the current precision and quality of GPS ground control points and photogrammetric information and processing to analyse large numbers of ?ssures and faults. In addition to observations in the ?eld, not only do these modern analyses provide accurate maps of ?ssure swarms, 408 L. Sonnette et al. Journal of Structural Geology 32 (2010) 407–422 Fig. 1. (a) The Thingvellir Fissure Swarm in the structural context of Iceland (adapted from Angelier et al., 1997). The plate velocities relative to the Iceland Mantle Plume are indicated according to the NUVEL1 kinematic model (DeMets et al., 1990, 1994). RR ? Reykjanes Ridge; KR ? Kolbeinsey Ridge; WVZ ? West Volcanic Zone; EVZ ? East Volcanic Zone; NVZ ? North Volcanic Zone; SISZ ? South Icelandic Seismic Zone; TFZ ? Tjo? rnes Fracture Zone. (b) Perspective view of the photographic mosaic of the Thingvellir Fissure Swarm projected on a Digital Elevation Model. The lava ?ows (dashed arrows) come from the Skjaldbrei?ur volcano (black triangle) and ?ssure eruptive centres between Tindaskagi and Kalfstindar (white triangles), following a general topographic slope towards the Thingvellir depression to the SSW. Gja? in Icelandic means gaping or open fracture, so the names ending by ‘‘gja?’’ refer to open fractures which correspond here to main fault scarps. Three normal fault scarps (Mjoafellagja?, Almannagja?, and Sledaasgja?) cut through the Armansfell-Lagafell Mountain and bound tilted blocks that dip towards the rift axis. they also allow 3-D analysis, including determination of vertical offsets (for vertical throw more than 0.5 m). The central segment of the TFS is limited to the north and to the south by large central volcanoes, Kjölur, Prestahnukur and Hengill respectively, where dykes provided access of the magma to the surface, giving large lava ?ows (Thordarson and Hoskuldsson, 2002; Sinton et al., 2005). Although the relationships between dykes and faults at depth have already been analysed in southwest (Forslund and Gudmundsson, 1991), in northeast Iceland (Dauteuil et al., 2001) and more precisely for Thingvellir area by Gudmundsson (2005), the problem of how lava ?ow generation may interact with fault growth has not yet been addressed in detail. In the axial zone of the TFS, where major lava ?ows of Holocene age have already been mapped and dated (Saemundsson, 1992; Sinton et al., 2005), not only does the brittle structure need to be quantitatively analysed in detail but also some important questions remain. For instance, typical fault scarps with large ?ssures, such as at the historical parliament site of Thingvellir, exhibit an along-fault monocline in the footwall. Did this particular structure originate from tectonic events exclusively, or from tectonic–volcanic interaction? Did a fault scarp already exist before the formation of the most recent lava ?ow? Knowing that erosion is negligible, how far does the scarp height re?ect the vertical offset of the fault? The answers to these questions are crucial for estimating the rates of brittle deformation, because many fault scarps are buried by more recent lava ?ows. Such problems have important inferences for the determination of vertical and horizontal relative displacements, and hence the extension rates across the rift segment. In this paper, we aim at analysing the fault characteristics, using the combination of (i) an innovative remote method, based on systematic coverage of the TFS involving geo-referencing, orthorecti?cation and 3-D photogrammetric restitution, and (ii) ?eld observation and geodetic (GPS) measurements of fractures. We could thus determine the number, density and length of 5390 individual fractures at metric to kilometric scales, characterise the shape of fault scarps in 70 across-strike pro?les, and measure the along-strike evolution of vertical throw along 52 faults. Using these data as a basis for analysing the fracture geometry, we intend to de?ne differences – if any – in fault types and characterise some major aspects of fracturing behaviour inside the TFS rift segment. We further aim at examining the relationships between fracture pattern development and volcanic activity within the TFS. L. Sonnette et al. Journal of Structural Geology 32 (2010) 407–422 409 Ключевые слова: tvh, evolution, cumulative length, geophysical, ka sk, surface, fault zone, johannesson, terra, footwall, ridge, fault movement, iceland, slope, fault dip, scale, fissure, time, einarsson, fracture nucleation, monocline, society, youngest formation, thingvellir depression, sonnette, fault journal, normal fault, offset, tfs, group, aerial photograph, volcano, rst stage, plate, wa, crustal rifting, tvh complex, hangingwall, area table, journal structural, ?ssures, villemin, structure, sonnette journal, holocene fault, elsevier, iceland journal, average length, fissure swarm, swarm, fault, power law, volcanic activity, rossi, icelandic, ?ssure, tectonicvolcanic interaction, sigvaldason, ?ow, mid-atlantic ridge, younger lava, tfh, photogrammetric model, pleistocene outcrop, marret, primary segment, fault scarp, cowie, scarp, lava owing, along-fault abscissa, model, volcanic complex, rift segment, complex, development, pattern, tryggvason, block, lava ?ows, vertical offset, open, rift, thingvellir, major, jakobsson, individual fracture, asymmetrical fault, volcanic, ?ssure swarm, large number, minor ssures, tectonophysics, distribution, oceanic ridge, icelandic rift, fracture, crustal, vertical, dip, depth, pleistocene fault, displacement, geological society, sk, major fault, fault offset, london, gordon, topography, bergerat, journal structural geology, journal, rate, lava, north, table, large fault, pleistocene, tfh complex, open ssures, average offset, data, rift zone, wvz, number, formation, year, holocene time, point, nat, jones, zone, deformation partitioning, structural geology, segment, wide range, function, gps, area, fault activity, holocene, angelier, result, thingvellir fissure, based, thingvellir area, tectonic, oldest, journal geophysical, geological map, activity, structural, throw, age uncertainty, faulting, geological, gps benchmark, ller, eds, length, dulholagja, fracture pattern, rock, main, vertical throw, kasser, gudmundsson, fault growth, fault number, watterson, walsh, gentle slope, fracture map, pro?les, reykjavik, absolute orientation, geology, scholz, letters, pre-existing fracture, rst, volcanology, ckstro, photograph, unit, maximum throw, growth, brittle deformation, saemundsson, age, average spacing, average, smooth slope, unit tfh, lava ows, nature, length scale, offset rate, ssure swarm, bulletin, tfh unit, axial zone, rifting activity, vertical fault, south, growth rate, natturufraedingurinn, ?ows, normal, open fracture, fault length, ?rst, scaling, stereoscopic analysis, ocean-ridge discontinuity, map, analysis, large, fault trace, walker, tectonic activity