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_Journal of Structural Geology 32 (2010) 1609-1628_ _Contents lists available at ScienceDirect_ _Journal of Structural Geology_ _journal homepage: www.elsevier.com/locate/jsg_ _Extensional faults in fine-grained carbonates: analysis of fault core lithology and thickness-displacement relationships_ _Eivind Bastesen a,b,*, Alvar Braathen b,c_ _a Centre for Integrated Petroleum Research, University of Bergen, 5020 Bergen, Norway_ _b Department of Earth Science, University of Bergen, 5020 Bergen, Norway_ _c University Centre in Svalbard, 9171 Longyearbyen, Norway_ _article info_ _Article history: Received 4 July 2009; Received in revised form 26 August 2010; Accepted 18 September 2010; Available online 25 September 2010_ _Keywords: thickness-displacement extensional faults carbonates fault core fault facies_ _abstract_ _A study of 103 extensional faults hosted by fine-grained carbonates in western Sinai, Svalbard and Oman reveals that faults vary geometrically between simple cores and cores comprising fault splays, lenses, segment linkages and overlap structures. Fault core rocks are typically carbonate breccias, carbonate and shale gouge, shale smear, secondary calcite cement and veins, and host rock lenses._ _There is a significant scatter in the core thickness for any given displacement, but the overall pattern is that the thickness increases with displacement. This increase best fits a power law function (0.29D^0.56) that describes a gradual decrease in the thickness-displacement relationship for increasing slip along faults. In more detail, the general function can be seen as the sum of two (power law) trend lines; the first representing thin localized fault cores with generally simple and planar geometry, the second representing thicker fault cores with complex geometry of lenses and overlap structures and with fault rock membranes._ _The studied faults show a significant change in composition and geometry from small (0-1 m), to moderate (1-10 m) and to large offset faults (10-400 m). The overall pattern is that fault initiates as fractures filled with calcite veins and thin shear fractures that host gouge membranes. With increased fault offset, complexity increases with breakdown of veins, more extensive fault rock membranes, and a trend towards development of lenses. When offset exceeds 100 m, cores become complex, with multiple slip zones, cemented breccia and shale smear membranes, and various types of lenses._ _We envision that the fault development as reflected by offset is dominated by forces (extension, compression) acting in the fault, mechanical heterogeneity of wall rocks, the core lithologies and their developing rheology, and especially geometric effects arising from fault irregularities._ _© 2010 Elsevier Ltd. All rights reserved._ _1. Introduction_ _Characterization and quantification of fault zones in outcrops is a fundamental requirement for modelling and forecasting structural reservoir heterogeneity in carbonate reservoirs. Key parameters for fault characterization include fault thickness, composition, geometry and displacement (e.g., Yielding et al., 1997; Manzocchi et al., 1999; Braathen et al., 2009). A fault can be defined as a zone of focused deformation that can be subdivided into domains subzones termed “core” and “damage zone(s)” (e.g., Chester and Logan, 1986). Alternatively, a fault can be considered an array of hardlinked and soft-linked fault segments of various scales that affect_ _* Corresponding author. Centre for Integrated Petroleum Research, Uni Research, 5020 Bergen, Norway. Tel.: +47 99230248._ _E-mail address: Eivind.bastesen@uni.no (E. Bastesen)._ _0191-8141 $ e see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2010.09.008_ _a restricted rock volume or “fault envelope” (e.g., Peacock, 2002; Childs et al., 2009; Braathen et al., 2009). Descriptions of fault cores (e.g., Caine et al., 1996; Childs et al., 1996; Lindanger et al., 2007; Bonson et al., 2007; Wibberley et al., 2008; Bastesen et al., 2009; Braathen et al., 2009) show a number of recurring elements such as slip surfaces, fracture deformation band sets, fault rocks (gouge, breccias and cataclasites), shale smears, and lenses of protolith or fault rock. Bulk strain of the core is semi-penetrative to penetrative, and core elements in most cases exhibit significantly altered fluid conductivity compared to the host rock from which they are derived. In contrast, bulk strain in the damage zones flanking the core is nonpenetrative and hosts discrete structures including minor faults, fractures and or deformation band sets._ _Studies addressing the width of the fault zone envelope vs. fault displacement have revealed a substantial degree of variation and uncertainty (e.g., Hull, 1988; Knott, 1994; Shipton et al., 2006; Childs et al., 2009). In most cases the thickness-displacement ratio (T/D) shows that thickness_ _1610_ _E. Bastesen, A. Braathen Journal of Structural Geology 32 (2010) 1609-1628_ _varies by up to three orders of magnitude for a given displacement, reflecting the geometric complexity arising from the presence of linked and unlinked segments (Childs et al., 2009)._ _In this paper we present characteristics and scaling laws for faults in fine-grained carbonates. The dataset includes 103 faults described using the fault facies characterization concept (Braathen et al., 2009). Fault facies refers to any feature or rock body deriving its properties from tectonic deformation (Tveranger et al., 2005), and includes the main elements such as lenses, membranes and fractures. Fault facies can be characterized in terms of dimensions, geometry, internal structure and petrophysical properties, thus facilitating quantification, pattern recognition and statistical handling of structural elements in fault envelopes (Tveranger et al., 2005; Braathen et al., 2009). In the present study we have defined fault facies according to composition and geometry. Fault core geometry and distribution of fault facies are analysed in relation to the T/D ratio as established for each studied fault._ _Fault architecture and related fluid flow properties in carbonate rocks have in the last years received increased attention (e.g., Agosta and Kirschner, 2003; Cello et al., 2003; Micarelli et al., 2003; Storti et al., 2003; Labaume et al., 2004; Agosta and Aydin, 2006; Graham Wall et al., 2006; Bonson et al., 2007; Benedicto et al., 2008; Bastesen et al., 2009; Putz-Perrier and Sanderson, 2010). However, studies addressing scaling relationships of faults in such rocks are scarce, and restricted to faults with small displacements (Billi et al., 2003; Micarelli et al., 2006; Soliva and Benedicto, 2005), or case studies (Micarelli et al., 2003; Agosta and Aydin, 2006; Bonson et al., 2007; Bastesen et al., 2009). In this paper we present a database of extensional faults from shallow buried (<2 km) carbonates from three different regions: western Sinai (Egypt), Central Spitsbergen, Svalbard (Arctic Norway), and Central Oman (Adams Foothills). The bulk of the data were collected in western Sinai, whereas data from Spitsbergen and Oman were collected for comparison purposes (i.e., different tectonic regimes and or protoliths). All three areas exhibit thick successions of sedimentary carbonates which are truncated by well exposed extensional faults at different scales, with fault displacements ranging from a few centimetres to several hundred metres. In Sinai (e.g., Moustafa, 2004) and Spitsbergen (e.g., Steel and Worsley, 1984; Maher and Braathen, in press), faulting is related to regional rifting events, whereas in Oman, extensional faults are found along the crest of regional anticlines and domes, of which the folding is controlled by deep-seated thrusting and salt movements (e.g., Hanna, 1990). Protoliths range from massive homogenous limestone to layered heterogeneous shale-rich carbonates and marls._ _2. Terminology_ _The studied fault cores exhibit several fault core facies associations (Braathen et al., 2009), which allow an identification and a description of fault facies using lithology and fault core geometry as descriptive parameters. Lithologically, fault core facies associations can be subdivided into shale smear (SS) (Lindsay et al., 1993; Yielding et al., 1997), carbonate breccia (CB) (Billi, 2005; Micarelli et al., 2003, 2006), secondary calcite (SCa) (Benedicto et al., 2008), or composite cores; the latter displaying two or all three fault core facies associations (Fig. 1)._ _The fault rocks observed in the present study formed at shallow depths and at low temperature, and in most cases bear resemblance to the primary non-cohesive breccia series described by Sibson (1977) and Braathen et al. (2004). Clast materials are fragments of carbonate formed by brittle failure. The breccia matrix consists of either finely crushed limestone fragments in a gouge (mud fraction fault rock), fine breccia (Billi, 2005) exhibiting varying degrees of cementation (CB), or shale; the latter giving rise to shale supported breccias (SCB) (Fig. 1b,c). In this study gouge was observed as very fine, crushed carbonate material and thin (<1 mm) membranes of sheared calcareous clay along faults. Shale smears are shale layers dragged into the fault, aligned parallel to fault dip and connected to a source shale layer (Lindsay et al., 1993). Secondary mineral precipitation was observed in the shape of calcite veins and void fillings and as pore space fill in breccias (described above). In many places precipitated calcite forms fault-parallel membranes and lenses, displaying the typical crack-seal vein appearance advocated by Petit et al. (1999). In the following descriptions the compositional elements are arranged into fault facies such as lenses (Childs et al., 1997; Gabrielsen and Clausen, 2001) and membranes (Braathen et al., 2009). Fault lenses are elongate pods of host rocks, fault rocks and or calcite veins which are completely separated from the surrounding fault elements by slip surfaces with wall rocks (slip zones) (Fig._ Ключевые слова: fault studied, braathen journal, surface, localized membrane, moderate, fracture, bedded limestone, petroleum geology, large fault, geological map, damage zone, benedicto, formation, zone, splay, offset fault, sudr formation, natih formation, carbonate breccia, calcite vein, deformation mechanism, sanderson, structural geology, pressure solution, offset exceeds, tanka, fault surface, clay-limestone, cb ?, small, wall rock, planar fault, calcite, minimum thickness, eds, geometry, fault core, extensional fault, larger fault, walsh, general, earth, planar geometry, bastesen braathen, increasing slip, immenhauser, poor correlation, composite core, multiple, vein, jebel quasaybah, thicker core, localized fault, society, adams foothills, case, tectonic evolution, maher, fault, data, fault displacement, pyramiden site, fault facies, carbonate, structural evolution, faults, fault zone, hammam faraoun, limestone bedded, normal fault, central, gradual decrease, breccia, structural setting, localized, lens, sca, overlap, hanging wall, developing rheology, shuaiba formation, ? ss, van, study area, shale smear, geology, clear difference, peacock, signicant change, uid, american, gabrielsen, steel, bedded, geological society, shear zone, cb, slip, observed, jebel madar, cm, prediction belt, fault rock, segment, cretaceous, thebes, geophysical, deformation, evans, journal structural, composition, layer, largest thickness, shale, smear, cement vein, evolution, structure, basin, tectonic map, samuelsberg, gouge, membrane, sharp, increasing displacement, aydin, secondary calcite, extensional, power law, bastesen, slip surface, stereographic plot, studied, thicknessdisplacement relationship, carbonate rock, thicknessedisplacement relationship, slip zone, ?ne, tanka formation, journal structural geology, rst representing, thickness varies, large, toro, planar, sinai, braathen, rock, grained, overlap structure, fault evolution, integrated petroleum, fault segment, core, calcareous, geological, shale membrane, host rock, jebel, structural, internal structure, shale layer, thickness displacement, massive, compositional facies, fault geometry, thebes formation, fault splay, rift, general function, yielding, facies, signicant scatter, fault bend, esna shale, moustafa, sedimentary, composite facies, darat formation, development, alsharhan, calcareous clay-limestone, journal, cement, limestone journal, offset, composite, relationship, childs, carbonate platform, petroleum, elsevier, generally simple, listed abbreviations, lenses, common, calcite precipitation, studied fault, normal, host, displacement, micarelli, trend, bend, thickness, oman, shale supported, relay structure, study, fossen, ss, billi, wordiekammen formation, increasing offset, limestone, fracture lled, small fault, darat, oriented nwese, area, moderate offset, billefjorden area