Strain Migration During Multiphase Extension: 10.1002/2014TC003551 Observations from the Northern North Sea Key Points: Rebecca E

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RESEARCH ARTICLE Strain migration during multiphase extension: 10.1002/2014TC003551 Observations from the northern North Sea Key Points: Rebecca E. Bell1, Christopher A.-L. Jackson1, Paul S. Whipp2, and Benjamin Clements3 • Seismic and borehole data are used to investigate multiphase fault activity 1Basins Research Group, Imperial College London, London, UK, 2Statoil ASA, Sandsli, Norway, 3Statoil ASA, • Optimally aligned faults do not always reactivate in multiphase extension Stavanger, Norway • Lithosphere evolution and far-field stress also control multiphase rift geometry Abstract Many rifts develop through multiphase extension; it can be difficult, however, to determine how strain is distributed during reactivation because structural and stratigraphic evidence associated with earlier Supporting Information: fl • Readme rifting is often deeply buried. Using 2-D and 3-D seismic re ection and borehole data from the northern • Figure S1 North Sea, we examine the style, magnitude, and timing of reactivation of a preexisting, Permian-Triassic (Rift Phase 1) fault array during a subsequent period of Middle Jurassic to Early Cretaceous (Rift Phase 2) Correspondence to: extension. We show that Rift Phase 2 led to the formation of new N-S striking faults close to the North Viking R. E. Bell, [email protected] Graben but did not initially reactivate preexisting Rift Phase 1 structures on the Horda Platform. We suggest that at the beginning of Rift Phase 2, strain was focused in a zone of thermally weakened lithosphere associated with the Middle Jurassic North Sea thermal dome, rather than reactivating extant faults. Citation: Bell,R.E.,C.A.-L.Jackson,P.S.Whipp,and Diachronous reactivation of the Permian-Triassic fault network eventually occurred, with those faults located B. Clements (2014), Strain migration during closer to the Middle Jurassic to Early Cretaceous rift axis reactivating earlier than those toward the eastern multiphase extension: Observations from margin. This diachroneity may have been related to flexural down bending as strain became focused within the northern North Sea, Tectonics, 33, 1936–1963, doi:10.1002/2014TC003551. the North Viking Graben, and/or the shifting of the locus of rifting from the North Sea to the proto-North Atlantic. Our study shows that the geometry and evolution of multiphase rifts is not only controlled by the Received 8 FEB 2014 orientation of the underlying fault network but also by the thermal and rheological evolution of the lithosphere Accepted 12 AUG 2014 and variations in the regional stress field. Accepted article online 16 AUG 2014 Published online 16 OCT 2014

1. Introduction Continental rift basins typically develop in three key stages: (i) rift initiation stage, when extension is distributed on multiple small, geometrically and kinematically isolated fault segments, (ii) fault interaction and linkage stage,when fault segments link to form longer, larger throw fault systems, and (iii) rift climax stage, when extension becomes focused on a small number of large, basin-bounding fault systems whilst the majority of the other intrabasin faults become inactive [e.g., Prosser, 1993; Cowie, 1998; Gupta et al., 1998]. However, many rift basins develop through multiple phases of extension; rift evolution models do not yet exist to describe the evolution ofthesemultiphaserifts,whicharecharacterized by numerous, superimposed “initiation” and “climax” periods, e.g., the northern North Sea Basin [e.g., Færseth, 1996] (Figure 1a), the Gulf of Aden [Lepvrier et al., 2002], the Galicia rifted margin [e.g., Reston, 2005], and the North West Shelf of Australia [Frankowicz and McClay,2010].Our lack of understanding of how normal fault arrays behave during multiphase extension is largely due to the fact that structures and stratigraphy associated with the first phase of rifting become progressively buried, tilted, and overprinted by later extensional events, becoming difficult to observe in field, seismic, and borehole data. The propensity for preexisting faults to reactivate during later rift phases is controlled by the strength of the fault compared to intact country rock, and the dip and strike of the fault relative to the extension direction [e.g., Sibson, 1985, 1990; Keep and McClay, 1997; Henza et al., 2010]. There are a number of studies which show that reverse reactivation of a parallel set of normal faults is in fact highly selective, with faults which are seemingly optimally aligned for reactivation not reactivating or reactivating diachronously [e.g., Badley and Backshall, 1989; Hand and Sandiford, 1999; Jackson et al., 2013]. These observations, although dealing with reverse rather than normal reactivation, indicate that fault geometry is in some cases not the overriding control on fault reactivation. Alternatively, local effects, such as the physical properties of the fault zone [e.g., Sibson, 1985, 1990; Imber et al., 1997], and magmatically induced thermal weakening/strengthening associated with local plutonism [D’Lemos et al., 1997], may control the propensity for fault reactivation. At a regional scale, thermal and rheological evolution of the lithosphere associated with upper crustal extension and rift basin formation may also influence which faults are reactivated [Hand and Sandiford, 1999].

Figure 1. (a) Major tectonic elements of the northern North Sea [after Færseth, 1996]. The Horda Platform (HP), Uer Terrace (UT), and Måløy Slope (MS) area examined in this study is shown in more detail in Figure 1b. The location of the regional interpretation shown in Figure 1c is illustrated by a bold line. (b) Major faults in the Horda Platform, Uer Terrace, and Måløy Slope area interpreted in this study. Øy1 = Øygarden Fault segment 1, Øy2 = Øygarden Fault segment 2, V1 = Vette Fault segment 1, V2 = Vette Fault segment 2, V3 = Vette Fault segment 3, HP = Horda Platform, ESB = East Shetland Basin, T1 = Tusse Fault, M1 = Måløy Slope Fault 1, and M2 = Måløy Slope Fault 2. Seismic reflection and well data used in this study are also presented, alongside the onshore geology of western Norway from Gee et al. [2008]. Red lines show locations of seismic reflection profiles shown in this article, and red dotted line shows the extent of the study area. (c) Regional interpretation of the structure of the northern North Sea after Færseth [1996] and beta (β) stretching profiles along this regional transect for Rift Phase 1 and Rift Phase 2 from Odinsen et al. [2000]. G-V = Gullfaks-Visund Fault, B-S = Brent-Statfjord Fault, and H-M = Hutton-Murchison Fault.

Physical models that attempt to simulate multiphase extension generally involve the application of horizontal in-plane stresses to simulate large-scale plate movement [Sengör and Burke, 1978; Huismans et al., 2001] and do not consider the role of upwelling asthenosphere during and between rift phases [e.g., Henza et al., 2010, 2011], which may act to weaken the lithosphere and focus strain [Huismans et al., 2001; Corti et al., 2003]. Magmatic activity in a multiphase rift basin is likely to modify the thermal structure of the lithosphere between individual rift events and because they are unable to explicitly replicate this effect, physical models of multiphase rifting may not produce patterns of fault geometry and evolution observed in nature. In order to help develop a greater understanding of the processes important in multiphase extension, detailed observations of fault interaction between individual rift events from rift basins worldwide are required to test the predictions of physical and numerical models. The northern North Sea has experienced three extensional episodes in the Devonian, Permian-to-Early Triassic (Rift Phase 1), and Middle Jurassic to Early Cretaceous (Rift Phase 2) [e.g., Beach,1985;Færseth, 1996]. It is

commonly assumed that the present-day geometry of the rift is largely the result of the reactivation of Rift Phase 1 faults during Rift Phase 2 [e.g., Badley et al., 1988; Steel and Ryseth, 1990; Yielding et al., 1992]. In the East Shetland Basin area, however, some west-dipping Permian-Triassic faults have been crosscut by Middle-to-Late Jurassic east-dipping faults that developed in Rift Phase 2 (Figure 1a) [Færseth et al., 1997; Færseth and Ravnås, 1998; Tomasso et al., 2008]. The importance of fault reactivation versus new fault initiation is therefore still controversial inthenorthernNorthSea,anddespitethelargeamountof work that has been undertaken within this basin, few studies have attempted to quantify and compare fault activity within and between the individual rift phases. There is also still doubt concerning the timing and pattern of fault migration on the eastern margin of the North Viking Graben, the establishment of which could help further constrain the dynamics of North Sea rift evolution [Cowie et al., 2005] and provide insights into patterns of fault evolution during multiphase rifting in general. In this study we investigate Rift Phase 1 and Rift Phase 2 fault activity in the Horda Platform, Uer Terrace, and Måløy Slope areas on the eastern margin of the North Viking Graben, northern North Sea (Figures 1a and 1b). This is a region where previous studies have suggested that Rift Phase 2 may have mildly reactivated faults that developed in Rift Phase 1; however, the migration of strain across the fault array during reactivation and the timing of fault reactivation have never been quantiﬁed. Using a combination of 2-D and 3-D seismic reﬂection and wellbore data, we aim to investigate patterns of Permian-Triassic fault reactivation following the initiation of Rift Phase 2 by (i) quantifying the value of throw that accumulated on the fault array during Rift Phase 1 and Rift Phase 2 (section 5) and (ii) determining the timing of fault initiation, fault cessation, and fault reactivation (section 6). We compare our results (summarized in section 7) from the eastern margin of the Viking Graben with similar studies in the East Shetland Basin area [e.g., Cowie et al., 2005] to examine the importance of fault reactivation in controlling multiphase rift geometry and to assess the validity of physical models of multiphase rifting (section 8).

2. Geological Setting 2.1. Regional Tectonics 2.1.1. Pre-Permian The basement of the northern North Sea was developed by terrain accretion in the Caledonian (460–400 Ma) and Variscan (400–300 Ma) orogenies [e.g., Ziegler,1975;Gee et al., 2008]. Gravitational collapse of Caledonian nappe stacks in the Devonian, and the associated development of extensional shear zones, produced a series of intermontane basins that are exposed onshore western Norway [e.g., Fossen,1992;Fossen and Hurich, 2005; Vetti and Fossen, 2012] (Figure 1b). Two major extensional shear zones, the Nordfjord-Sogn Detachment and the Bergen Arc, have been mapped onshore western Norway, immediately to the east of the Horda Platform and Måløy Slope, although the offshore continuation of these structures remains controversial [Smethurst, 2000; Fossen and Hurich, 2005] (Figure 1b). The distribution of Devonian basins offshore in the Horda Platform area is poorly constrained due to a lack of well penetration of pre-Permian strata in deeply buried depocenters [Marshall and Hewett, 2003]. On the Måløy Slope, five wells have penetrated crystalline gneissic basement at depths of ~3 km without encountering Devonian or Permian strata [Marshall and Hewett,2003;Reeve et al., 2013] (Figure 1b). 2.1.2. Permian to Early Triassic (Rift Phase 1) The first major rift phase responsible for the formation of the North Sea Rift occurred in the Late Permian to Early Triassic (see below). Estimates of the timing of this rift event, which are derived from the dating of sedimentary rocks, fault rocks, and dykes, suggest that the rift phase initiated at 261 to 225 Ma, lasted 25–37 Myr and was most likely related to the breakup of the Pangean supercontinent [e.g., Ziegler, 1982; Ter Voorde et al., 2000]. Major Permian-Triassic basins have been reported on the Horda Platform, East Shetland Basin, and Unst Basin [e.g., Færseth, 1996; Tomasso et al., 2008] (Figure 1a), although Permian-Triassic basins have not been confidently identified beneath the Viking Graben because extreme, post-Triassic subsidence has resulted in the Permian-Triassic basins, if they ever existed, being buried to depths of >8km[Klemperer, 1988] (Figure 1c). The distribution of extension during the Permian-Triassic, described by the beta (β) stretching factor, is controversial, owing mainly to the uncertain presence and size of the Permian-Triassic basin beneath the Middle Jurassic to Early Cretaceous axis of the Viking Graben (Figure 1c). Some studies suggest higher β factors in the Horda Platform than in the area of the present-day Viking Graben [Færseth, 1996; Ter Voorde et al., 2000], while others indicate that β factors may have been fairly constant across the Permian-Triassic rift (Figure 1c) [Odinsen et al., 2000]. The extension direction in Rift Phase 1 is believed to have been E-W, supported

by the N-S orientation of Permian dykes observed onshore western Norway that crosscut preexisting, NNW-SSE striking fractures [Færseth et al., 1997; Torsvik et al., 1997]. Permian-Triassic faults strike predominantly N-S or NNW-SSE, orthogonal to the postulated extension direction, and may have been influenced to some extent by the NNW orientated basement fabrics identified onshore [Færseth et al., 1995] (Figure 1b). Major Permian- Triassic half-grabens have previously been interpreted on the Horda Platform [e.g., Badley et al., 1984; Steel and Ryseth, 1990; Nottvedt et al., 1995; Færseth,1996;Ter Voorde et al., 2000]. These basins trend N-S and are bounded by major west-dipping normal faults. Despite a number of studies observing these deep basins on 2-D seismic reflection data, details of the sediment thickness within these Permian-Triassic basins and the extent to which the Horda Platform, Uer Terrance, and Måløy Slope have experienced extension since the Permian- Triassic areas have not previously been published. 2.1.3. Middle Jurassic to Early Cretaceous (Rift Phase 2) A second major period of rifting took place in the northern North Sea from the Middle Jurassic to Early Cretaceous. Several authors have suggested that this rift phase initiated as a result of the Early Jurassic (Aalenian) rise and Middle-to-Late Jurassic deflation of a central North Sea thermal dome [Ziegler, 1990; Underhill and Partington, 1993, 1994]. Collapse of this dome is believed to have exerted regional tension and resulted in the development of the trilete North Sea rift system, which is composed of the Viking Graben, Moray Firth, and Central Graben [Davies et al., 2001]. Furthermore, other authors have argued that subsequent fault activity was related to variations in the prevailing far-field stress regime superimposed onto the trilete junction structural template [Ravnås and Steel, 1997; Davies et al., 2001; Torsvik et al., 2002; Nottvedt et al., 2008]. Based on the analysis of sediment thickness patterns adjacent to faults on the East Shetland Basin and Horda Platform, the timing of the initiation of Rift Phase 2 has been dated to the Bajocian (circa 167–170 Ma) [e.g., Færseth and Ravnås, 1998; Davies et al., 2000; Cowie et al., 2005]. The timing of the initiation and cessation of this rift episode is diachronous across the basin, with estimates of the duration of activity on individual faults ranging between 10 and 40 Myr [Cowie et al., 2005]. Some authors have suggested that rifting during the Middle Jurassic to Early Cretaceous occurred as a series of discrete rift events associated with variations in the regional extension direction [Doré and Gage, 1987; Davies et al., 2001]. It has previously been assumed that many of the large Middle Jurassic to Early Cretaceous faults in the northern North Sea represent reactivated Permian-Triassic structures [Badley et al., 1988]. However, recent studies in the East Shetland Basin, utilizing 3-D seismic volumes and abundant well data, suggest that some east-dipping, Rift Phase 2 (i.e., Middle-to-Late Jurassic) faults crosscut west-dipping, Rift Phase 1 (i.e., Permian-Triassic) faults, indicating that the initiation of new faults during Rift Phase 2 was favored over reactivation of preexisting faults [Færseth, 1996; Færseth and Ravnås, 1998; Tomasso et al., 2008]. The greatest amount of Rift Phase 2 extension in the northern North Sea took place in the Viking Graben [Odinsen et al., 2000] (Figure 1c). The strike of Rift Phase 2 faults is generally N-S, although NE-SW striking structures appear to be more prevalent during Rift Phase 2 than Rift Phase 1 [Færseth et al., 1997]. The extension direction associated with Rift Phase 2 has been debated and remains controversial; some studies suggest that the extension direction remained E-W between Rift Phase 1 and 2 [Bartholomew et al., 1993; Brun and Tron, 1993], whereas others suggest the more variable strike of faults associated with the second rift phase (i.e., both N-S and NE-SW) could indicate a change in extension direction to NW-SE or WNW-ESE [Færseth, 1996; Doré et al., 1997; Færseth et al., 1997]. Rift Phase 2 ended diachronously across the basin and was followed by a period of thermal subsidence, which was greatest in the axis of the Viking Graben in the location that underwent the greatest amount of Rift Phase 2 extension [Odinsen et al., 2000] (Figure 1c). 2.2. Stratigraphy of the Horda Platform, Uer Terrace, and Måløy Slope Devonian basins are not present on the Måløy Slope, which may indicate that this region was a Devonian structural high [Reeve et al., 2013]. Devonian basins may be present beneath Permian-Triassic basins on the eastern Horda Platform [Marshall and Hewett, 2003] (Figure 2). Devonian sediments, if they exist, were likely deposited in continental conditions, probably within intermontane basins [Marshall and Hewett, 2003] (Figure 1b). Continental deposition continued into the Permian and Triassic, resulting in the deposition of sandstones and mudstones of the Hegre Group on the Horda Platform [Lervik, 2006] (Figure 2). In the Early to early Middle Jurassic, during the postrift thermal subsidence phase following Rift Phase 1, a fluvio-deltaic to shallow marine succession was deposited that includes the Statfjord Formation and the Dunlin and Brent groups [e.g., Helland-Hansen et al., 1992] (Figure 2). The initiation of Rift Phase 2 extension was, in some

Figure 2. Stratigraphic column for the northern North Sea (modified after Whipp et al. [2014]). P = Period and E = Epoch. Names of the key Groups and Formations that are described in the text are provided. A representative section of seismic reflection data from a 2-D survey is shown to illustrate the typical seismic stratigraphy in the area. Colored squares on the black vertical line show formation top information in well 31/6-6 which crosses this seismic section. Ten horizons have been mapped regionally, which have been tied to the seismic data through synthetic seismograms (described by McAndrew [2010] and Whipp et al. [2014]). Asterisk = The timing of the initiation and cessation of Synrift Phase 2 is diachronous and addressed in this study and is summarized in Figure 10f. TD = Buildup and deflation of the central North Sea thermal dome.

locations, coincident with the deposition of the upper part of the Brent Group (upper part of the Ness and Tarbert formations) [Helland-Hansen et al., 1992]. Increased rates of fault-driven subsidence in Rift Phase 2 led to the development of fully marine conditions and the deposition of the Viking Group, which in the Horda Platform is composed of three-stacked, regressive, shallow marine clastic sequences (the Krossfjord, Fensfjord, and Sognefjord formations [Dreyer et al., 2005]). Deep marine mudstones of the Draupne Formation were deposited after ﬂooding in the Late Kimmeridgian to Late Berriasian. The Viking Group is capped by the Base Cretaceous Unconformity (BCU) [Kyrkjebo et al., 2004], the formation of which has been linked to the cessation of Rift Phase 2 extension. It is, however, more likely that the BCU formed as a result of a decrease in strain rate rather than the complete cessation of active faulting [Brun and Tron, 1993; Kyrkjebo et al., 2004]. Deep water clastics and carbonates of the Cromer Knoll and Shetland groups were deposited above the BCU, which are in turn overlain by a clastic-dominated Cenozoic succession (Figure 2).

3. Data and Methods 3.1. Seismic Reflection and Well Data The data used in this study include a regional compilation of 2-D and 3-D, time-migrated seismic reflection surveys that were collected in the Horda Platform, Uer Terrace, and Måløy Slope areas between 1980 and 2006. The coverage of these seismic surveys is presented in Figure 1b, and further details of the surveys that have been used are supplied in the supporting information (Figure S1). Line spacing for the 2-D surveys is ~5 km (Figure 1b), and these 2-D surveys image to depths of 5 to 15 s two-way traveltime (TWT). These data are suitable for regional mapping and investigation of faults which are >5 km long. Three-dimensional seismic reflection surveys image to depths of 2.7 to 5 s TWT and have 12.5 m × 12.5 m inline and crossline spacing. Data quality ranges from excellent for some of the 3-D surveys to moderate for some of the 2-D surveys. In areas where faults are imaged by high-quality 3-D seismic reflection data, a higher-resolution analysis of fault evolution (i.e., length scales of <5 km) has been undertaken. In addition to the seismic reflection data, 58 exploration wells were used in this study (Figure 1b). Interpretation of the seismic reflection profiles has been aided by the availability of stratigraphic data from these wells (i.e., biostratigraphic data and lithostratigraphic data), which have been tied to the seismic data through synthetic seismograms (described by McAndrew [2010] and Whipp et al. [2014]). 3.2. Seismic Interpretation Ten key horizons have been interpreted throughout the seismic reflection data set: acoustic basement (age unknown; see discussion below), Top Statfjord Formation (Rheatian, 200 Ma), Top Brent Group (Bathonian, 167 Ma), Top Krossfjord Formation (Bathonian, 165 Ma), Top Fensfjord Formation (Callovian, 161 Ma), Top Sognefjord Formation (Volgian, 150 Ma), Base Cretaceous Unconformity (Ryazanian, 140 Ma), Base Shetland Group (Cenomanian, 99.6 Ma), Base Cenozoic (65 Ma), and Base Quaternary (2.6 Ma). The ages of these horizons have been determined using data provided by the Norwegian Petroleum Directorate (http://www.npd.no/en/), and they have been calibrated to the absolute geological timescale of the International Stratigraphic Committee (http://www.stratigraphy.org). Intra-Cretaceous and intra-Triassic reflectors have also been locally mapped (Figure 3a). The top of the acoustic basement in the Horda Platform, Uer Terrace, and Måløy Slope region is identified as a very high amplitude reflection that separates continuous reflections from discontinuous reflections (Figures 2 and 3). This reflection is confirmed as true crystalline basement in the Måløy Slope where five wells have been drilled to basement [Reeve et al., 2013]; however, the depth to basement in the hanging wall of faults in the Horda Platform is unconstrained by drilling (Figure 1b). On some seismic profiles from the Horda Platform, semicontinuous reflections, which may be associated with the sedimentary fill of Devonian age basins, are observed beneath the acoustic basement-related reflection (e.g., Figures 2, 3a, and 3b). 3.3. Depth Conversion Eleven of the 58 exploration wells in the study area lie within 5–10 km of major faults in the Horda Platform, Uer Terrace, and Måløy Slope and have available checkshot information (Figure 1b). Checkshot data from each of these wells are plotted in the inset diagram in Figure 3c, and they reveal very similar time-depth relationships for each of the wells. A best fit, second-order polynomial relationship is fitted to these time-depth

Figure 3. (a) Uninterpreted and (b) interpreted seismic reflection profile NSR06-311182 across the Horda Platform (location shown in Figure 1b). Interpreted horizons are color coded as shown in the stratigraphic column presented in Figure 2. Bold vertical lines show wells that intersect this seismic profile. (c) Depth-converted interpretation of seismic reflection profile NSR06-311182. Colored squares on the black vertical line show formation top information in wells that intersect the seismic section and are used to quality control the depth conversion. The inset diagram shows time versus depth (checkshot) information for 11 wells in the study area. The dashed line shows a best fit second-order polynomial trend line for all of the wells except 32/4-1, and all of the time-depth information from these wells can be fit within 5% of this best fit curve (grey shading). Well 32/4-1 is an outlier (see text for discussion). (d) Depth converted interpretation of seismic reflection profile NSR06-311182 with no vertical exaggeration. Data courtesy of TGS.

curves such that, for a particular TWT value, the depth in all of the wells is within 5% of this best fitpolynomial (grey envelope in Figure 3c). Well 32/4-1, which is located in the eastern part of the Horda Platform, is the only outlier to this general relationship; compared to the best fit polynomial trend, the depth for a particular TWT is higher than expected in this well (inset in Figure 3c). Figure 3 reveals that this is due to the shallower depth to compacted Jurassic stratigraphy at the location of Well 32/4-1 compared to locations farther to the west. Depth conversion to the east of the Vette Fault has been performed using the time-depth relationship derived from well 32/4-1, whereas stratigraphy to the west has been depth converted using the best fit second-order polynomial relationship (Figure 3d). 3.4. Quantitative Analysis of Fault Evolution Isolated normal faults typically grow by radial propagation, with the maximum displacement (or throw) located at the fault center, decreasing to zero at the tips [e.g., Walsh and Watterson,1988;Cowie and Scholz, 1992b; Cartwright et al., 1995; Dawers and Anders, 1995]. Fault-scaling relationships indicate that displacement (D) and fault length (L) are related to each other by, D = cLn,wherec is a constant and n a scaling exponent [Scholz et al., 1993]. This scaling exponent is often estimated to be around 1 to 1.5 for typical normal fault systems [Walsh and Watterson, 1988; Cowie and Scholz, 1992a, 1992b; Dawers and Anders,1995;Schlische, 1995]. However, the presence of preexisting structural weaknesses in the crust may be exploited by growing faults, resulting in them having quite different D-L scaling relationships than anticipated from traditional fault growth models [Walsh et al., 2002]. It has been suggested that faults that evolve by reactivation of preexisting crustal discontinuities may obtain their maximum length relatively quickly during a rift event, with later fault growth dominated by the accumulation displacement rather than tip propagation [Walsh et al., 2002; Paton and Underhill, 2004]. Reactivated faults can appear “under displaced” relative to their length compared with new faults that grow by tip propagation [Paton, 2006]. In our study we measure throw along faults for a number of different stratigraphic horizons. We use throw versus fault length (T-x) plots to investigate the spatial evolution of faults during Rift Phase 1 and Rift Phase 2 and throw versus depth (T-z) plots to analyze the temporal evolution of faults. 3.4.1. Throw Versus Fault Length (T-x) Plots Footwall and hanging wall cutoffs for all interpreted horizons were measured in TWT on seismic sections orientated orthogonal to fault strike; these TWT values are converted to depth using the velocity model described in section 3.3, and throw is calculated. Where significant fault-parallel folding was observed immediately adjacent to a fault, the horizon cutoff value is taken from an extrapolated line that follows the regional trend of the horizon prior to drag folding, i.e., continuous deformation is incorporated in the throw calculation [Long and Imber, 2010; Whipp et al., 2014]. Sediment compaction may cause our measurements of fault throw to be 5–15% less than the true values [Taylor et al., 2008]; however, the thickness of sediment overburden above faults is fairly constant along strike of each of the faults in our study area; thus, we believe that the overall patterns of throw distribution will be valid (reviewed in Whipp et al.[2014]).Theamountof throw that accumulated during Rift Phase 1 has been calculated by subtracting throw across the Top Statfjord Formation horizon (Upper Triassic) from throw across acoustic basement, thereby effectively removing post- Triassic throw (fault displacement backstripping [e.g., ten Veen and Kleinspehn, 2000]). The amount of throw that occurred during Rift Phase 2 is given by throw across the Top Statfjord Formation horizon (a pre-Rift Phase 2 marker). This method assumes that accommodation associated with Rift Phase 1 was completely filled; this assumption seems reasonable given that continental (and likely overfilled) conditions existed during the Triassic to Early Jurassic. Throw for a particular horizon is plotted against the distance along the fault to assess the evolution of the fault system. Steep displacement gradients typically occur in relay zones when adjacent faults begin to overlap and kinematically interact with one another [e.g., Peacock, 1991]. Further displacement accumulation may result in relay ramp breaching and the formation of a single fault; these paleolinkage points or segment boundaries can typically be identified by throw minima on T-x plots and/or intrabasin highs [e.g., Peacock, 1991; Gawthorpe and Leeder, 2000; Young et al., 2001]. Regional time-thickness maps (isochrons), together with seismic stratigraphic geometries, are also used to investigate fault evolution. Isochron maps are not a direct, quantitative measure of fault-driven subsidence but instead an indirect qualitative method that tracks spatial variations in subsidence, which we suggest is due to syndepositional faulting [e.g., McLeod et al., 2000].

Figure 4. (a) Uninterpreted and (b) interpreted seismic reflection profile NVGT88-08 across the Uer Terrace and Måløy Slope (location shown in Figure 1b). Interpreted horizons are color coded as shown in the stratigraphic column presented in Figure 2. Bold vertical lines show wells that intersect this seismic profile. (c) Depth-converted interpretation of seismic reflection profile NVGT88-08. (d) Depth-converted interpretation of seismic reflection profile NVGT88-08 with no vertical exaggeration. Data courtesy of TGS.

Figure 5. (a) Depth to acoustic basement in TWT, (b) depth to Top Statfjord Formation horizon in TWT, (c) depth to Base Cretaceous Unconformity in TWT, and (d) depth to Base Shetland Group horizon in TWT. Grey lines in all of the diagrams are 2-D seismic reflection profiles, and dashed grey lines in Figures 5b–5d show the extent of 3-D seismic reflection surveys used to interpret these horizons. Hatched areas show locations where the horizon has been eroded. Contour interval 200 ms. See Figure 1 for explanation of abbreviated fault names.

3.4.2. Throw Versus Depth (T-z) Profiles and Slip Rates In addition to T-x plots, we also use throw versus horizon depth plots (T-z) to investigate the temporal growth history of faults [e.g., Hongxing and Anderson, 2007; Baudon and Cartwright, 2008; Pochat et al., 2009; Jackson and Rotevatn, 2013]. More specifically, we use these data to determine the depth at which faults nucleate and how they have propagated vertically. T-z profiles which show that throw is constant or decreases with horizon age are commonly associated with postdepositional faulting, whereas T-z profiles that indicate throw increases with horizon age are indicative of synsedimentary faulting [Hongxing and Anderson, 2007]. In areas of fault reactivation following a period of fault burial, a composite postdepositional and syndepositional T-z profile is expected [Hongxing and Anderson, 2007]. We have also calculated slip rates for several of the key faults in the study area, so their activity can be compared with other faults in the North Sea and other global rifts. Slip rate has been calculated by dividing throw values by the fault dip angle and dividing by the time period over which throw accrued. This method assumes that sedimentation rates outpaced fault slip rates and that no relief was developed across the fault (reviewed in Childs et al. [2003] and Bell et al. [2009]). We also recognize that our slip rates will be time averages, and slip rate may have varied within these time periods.

4. Structural Framework The structure of the Horda Platform, Uer Terrace, and Måløy Slope are markedly different, and in this section we describe the major structural elements, as they appear today, for each domain (Figures 3–5). The naming convention for the fault segments and systems used in this study is described below and in Figures 1b and 5. 4.1. Horda Platform Three major, basement-involved normal fault systems exist in the Horda Platform, south of 61°N; these faults are named Øygarden Fault System 1 (Øy1), Vette Fault System 1 (V1), and Tusse Fault System (T1) (Figures 1b, 3, and 5). These faults dip toward the west and bound major half-graben basins, with the basement displaced across these faults by a maximum of ~4 km (Figures 3c and 3d). Depth conversion indicates that faults Øy1, V1, and T1 have dips of ~40° where they separate Permian-Triassic sediments from crystalline basement and steepen upward to ~55° where they offset Jurassic and Cretaceous stratigraphy (Figures 3d).

Figure 6. Rose diagrams to show the orientations of fault segments (a) Øy1 and Øy2; (b) V1, V2, and V3; (c) T1; and (d) M1 and M2. Fault orientations have been calculated along 2 km long fault segments, sorted into 10° bins, and plotted in terms of the frequency of a particular orientation. The second rose plot on the right of Figure 6b, highlighted in red, shows an enlarged version of the inner circle from the rose plot on the left, to show more clearly the orientation of ~10 km long fault V2.

In the southern part of the study area, Øy1 strikes N-S to NNW-SSE and dips to the west (Figures 5a and 6a); farther north, it splays into two strands with one striking NW-SE and the other N-S (Øy1b; Figure 5a). Within the study area, Øy1 and Øy1b are 85 km and 30 km long, respectively (Figure 5). Lower Cretaceous strata have been eroded from the footwall of Øy1, and the Base Shetland Group horizon is not present; in this location Quaternary strata directly overlie Jurassic strata, and Øy1 tips out beneath the Base Quaternary unconformity (Figures 3 and 5d). V1 is 45 km long, dips to the west and strikes N-S to NNW-SSE (Figures 5a and 6b), and tips out beneath the Base Tertiary (Figure 3). Three-dimensional seismic reflection data image the northern part of V1 down to depths of 5 s TWT and reveal that at the level of the Top Statfjord Formation and BCU horizons it is composed of several linked fault segments (Figures 5b and 5c) [Whipp et al., 2014]. T1 dips to the west and is composed of 3 sections; a northern and southern section that strike N-S to NNW-SSE and a central section that strikes NE-SW (Figures 5a and 6c). It has a total length of 52 km, although, like the Vette Fault System, it likely evolved by the linkage of several smaller segments [Whipp et al., 2014]. T1 offsets the Base Shetland Group horizon and tips out in the Upper Cretaceous succession (Figure 3). 4.2. Uer Terrace The largest basement-involved fault in the Uer Terrace is the Vette Fault System 3 (V3), which offsets basement from depths of 1.8 s TWT (~1.8 km) in its footwall to depths of 2.8 s TWT (~3.5 km) in the hanging wall (Figure 4). V3 is 40 km long, dips 60° to the west, and strikes NW-SE (Figures 4, 5a, and 6b). The central part of V3 is only imaged by a few 2-D seismic profiles; however, the southern tip of V3 is imaged by 3-D seismic reflection data to a depth of 5 s and reveals that V3 bends to the SW as it approaches Øy1 but is not physically linked to it (Figure 5a). The upper tip of V3 is located within Lower Cretaceous sediments (Figures 4 and 5d). Between the Horda Platform and Uer Terrace, at around 61°N and across an ENE-WSW striking fault, the depth to the Top Statfjord Formation horizon increases abruptly from 2 s TWT (~2.2 km) in the south to 2.5 s TWT (~3 km) in the north (Figure 5b). This fault, which is 10 km long and dips steeply (60°) to the north, is named Vette Fault System 2 (V2). Three-dimensional seismic reflection data image V2 to depths of 5 s and reveal that it is physically linked to both V1 and V3 (Figure 5a). V2 tips out in Lower Cretaceous strata beneath the Base Shetland Group horizon. West of V2, there are a number of NE-SW striking faults that are 5–25 km long (Figure 5a). These faults do not have significant basement displacement and are not considered in detail in this study (see McAndrew [2010] for further details). 4.3. Måløy Slope In the Måløy Slope the basement reaches a maximum depth of 2.5 s (~3 km) in the west and has a depth of <0.5 s in the east (~0.35 km) (Figure 5a). The basement has been penetrated by five wells, which reveal that it is composed of gneiss [Slagstad et al., 2008]. The eastern margin of the Måløy Slope is bounded by a 52 km long, N-S to NNE-SSW striking fault segment we call Øygarden Fault System 2 (Øy2). Two-dimensional

Figure 7. (a) Isochron between the Top Statfjord Formation (Figure 5b) and basement (Figure 5a) horizons showing the thickness of Permian-Triassic sediments in TWT. Contour interval 200ms. (b) Isochron between the BCU (Figure 6c) and Top Statfjord Formation (Figure 6b) horizons showing Jurassic sediment thickness in TWT. Contour interval 100 ms. (c) Isochron between the Base Shetland Group (Figure 5d) and BCU (Figure 5c) showing Lower Cretaceous sediment thickness in TWT. Contour interval 100 ms. Grey lines in all of the diagrams are 2-D seismic reflection profiles, and the bold grey polygon in Figures 7b and 7c shows the extent of 3-D seismic reflection surveys used to interpret these horizons. Hatched areas show locations where the sedimentary unit has been partially eroded. See Figure 1 for an explanation of abbreviated fault names.

seismic reﬂection data indicate that Øy2 terminates to the south before it intersects Øy1 (Figure 5) and that the footwall of Øy2 has been substantially eroded and basement now crops out at the seabed (Figures 4 and 5). To the NE of Øy2 are a series of segmented faults that dip either to the east or west and have a maximum throw of ~0.5 s TWT (~1.2 km) (Figure 5). These faults are named Måløy Slope Fault 1 (M1) and Måløy Slope Fault 2 (M2). These faults strike N-S to NNE-SSW and are 25–30 km long (Figure 6d). M2 is discussed in more detail by Reeve et al. [2013] (their fault, F1).

5. Comparison of Fault Activity During Rift Phase 1 and Rift Phase 2 Having established the present-day structural framework of the eastern margin of the North Viking Graben, we now investigate how much throw accrued on faults within the Horda Platform, Uer Terrace, and Måløy Slope during Rift Phases 1 and 2. To achieve this, we use regional time-thickness maps (isochrons; Figure 7) and T-x data (Figure 8; see section 3.4). Fault activity during Rift Phases 1 and 2 is described in turn below. 5.1. Rift Phase 1: Permian to Early Triassic Figure 7a shows the Permian-Triassic sediment thickness on the eastern margin of the North Viking Graben. The greatest thicknesses of Permian-Triassic strata (up to 2.4 s TWT, ~5 km) occur in the half-graben basins controlled by Øy1, V1, and T1. To the NW, in the Uer Terrace, a ~0.5 s TWT (~1.2 km) thick succession of Permian-Triassic strata is preserved, but to the NE, in the Måløy Slope, a < 0.1 s TWT (< 0.1 km) thick succession of Upper Triassic strata is preserved, and Permian strata are absent (Figure 7a). Figures 8a–8c are T-x plots for the key faults analyzed in this study that accumulated throw during Rift Phase 1 (Øy1, V1, and T1). These measurements can only be made using 2-D seismic data because the depth to basement in the hanging wall of these faults is below the depth of recording of 3-D data. Rift Phase 1 throw on Øy1 has a typical bell-shaped proﬁle often observed along normal faults [e.g., Schlische, 1995], with

Figure 8. Throw versus length along fault (T-x) diagrams to show throw accumulation during Rift Phase 1 on faults (a) Øy1, (b) V1 and V3, and (c) T1. Asterisks indicate that throw values are minima due to erosion of stratigraphy in the footwall (see text for details). (d) Map to show the geometry of faults active during Rift Phase 1. (e) Summation of Rift Phase 1 extension across the study area in an E-W direction. Throw versus length along fault (T-x) diagrams to show throw accumulation during Rift Phase 2 on faults (f) Øy1, Øy1b, Øy2, M1, and M2 [after Sansom, 2012], (g) V1, V2, and V3, and (h) T1. (i) Map to show the geometry of faults active during Rift Phase 2. (j) Summation of Rift Phase 2 extension across the study area in an E-W direction.

maximum throw near the center of the fault of 4 km, decreasing toward the fault tips (Figure 8a). Øy1 still has ~3 km of throw at the southern limit of the study area, suggesting that the Permian-Triassic fault extends to the south of the study area and in total may be >100 km long. The linear plan-view geometry of the Permian-Triassic Øy1 fault suggests that it formed by the linkage of multiple colinear segments (Figure 8d). The northern tip of Øy1 bends to the NW, and throw decays to zero (Figure 8a). The northern splay of Øy1 and Øy1b did not accumulate any throw during Rift Phase 1. V1 and T1 also have overall bell-shaped profiles and have Rift Phase 1 throw maxima of ~3 km, although the profiles are complicated by multiple, significant throw minima (Figures 8b and 8c) [Whipp et al., 2014]. Such displacement minima are likely related to the growth of the fault system by the coalescence of smaller, colinear, initially isolated fault segments [Peacock, 1991; Cartwright et al., 1995; Young et al., 2001; Whipp et al., 2014]. V1 and T1 also continue farther south of the study area. In the Uer Terrace and Måløy Slope, faults have much lower values of Rift Phase 1 throw, and some faults show no Rift Phase 1 throw at all (i.e., they were not present during the Permian-Triassic; Figure 8b). V3 has ~0.8 km of Rift Phase 1 throw, which decreases to zero to the south as it approaches but does not intersect Øy1, and decreases to <0.1 km to the north (Figure 8b). Øy2, V2, and the Måløy Slope Faults (M1 and M2) had not yet initiated in Rift Phase 1, as no Permian-Triassic throw is documented across these faults at this time.

5.2. Rift Phase 2: Post-Late Triassic Figures 8f–8h show throw that accumulated on fault segments during Rift Phase 2 only, and Figure 8i shows the geometry of post-Triassic active faults, which have been determined from 3-D seismic reﬂection data (Figure 1b). All of the Rift Phase 1 faults on the Horda Platform were reactivated in Rift Phase 2; however, the maximum amount of displacement that accumulated on Øy1 and T1 in Rift Phase 2 was only ~0.7 km (compared to up to 4 km for Rift Phase 1) and across V1 was ~1 km (compared to up to 3 km for Rift Phase 1) (cf. Figures 8a and 8f, 8b and 8g, and 8c and 8h). Therefore, the amount of throw that accumulated on these faults during Rift Phase 2 was only 18–33% of that which accumulated during Rift Phase 1. The N-S strike of Rift Phase 2 faults largely mirrors the geometry of the underlying, reactivated Permian-Triassic structures; however, the reactivated Øy1 and V1 faults propagate farther to the north than the underlying Permian- Triassic faults (cf. Figures 8d and 8i). Along Øy1 the majority of Rift Phase 2-related throw occurred to the north of the loci of Rift Phase 1 throw accumulation, and segment Øy1b initiated farther north of the tip of Øy1 (cf. Figures 8a and 8f and 8d and 8i). Rift Phase 2 displacement on V1 increases to the north of the area of maximum Rift Phase 1 throw, indicating that the segment of V1 that initiated in Rift Phase 1 was reactivated in Rift Phase 2 and that V1 also propagated farther north during the second rift event (Figure 8g and cf. Figures 8d and 8i). Rift Phase 2 throw on the Tusse Fault largely mirrors the distribution of throw accumulated along this fault during Rift Phase 1 (Figure 8h). On the Uer Terrace, fault V3, which initiated in Rift Phase 1, underwent further activity during Rift Phase 2, resulting in throw values of up to 1.6 km (i.e., around twice as much throw accumulation as that which occurred during Rift Phase 1; Figure 8g). V2 did not exist during Rift Phase 1 and accumulated up to ~1 km of throw during Rift Phase 2. Where V1 and V2 intersect, they display almost identical throw, suggesting the faults are now physically linked. Where V2 intersects V3, fault V2 has ~0.5 km of throw and the intersection point coincides with a displacement minima (Figure 8g). During Rift Phase 2 fault Øy2 initiated in the Måløy Slope and accrued up to ~1.3 km of throw, which decays southward to zero. The fault extends outside of the study area in the north, and throw values in this location are unknown (Figure 8a). M1 and M2, which initiated during Rift Phase 2, also developed symmetrical, bell-shaped proﬁles and have maximum throw values of 0.6 km and 1.3 km, respectively (Figure 8f). 5.3. Summation of Extension Across the Study Area The extension associated with Rift Phase 1 and Rift Phase 2 can be estimated by dividing the throw values by the tangent of the fault dip angle (Figures 8e and 8j). Rift Phase 1 and Rift Phase 2 extension on fault segments at the same latitude can then be summed to provide an estimate of total extension across the study area to investigate how seismic-scale strain was distributed across the study area during each rift phase. The pattern of extension that accumulated across the study area during Rift Phase 1 is asymmetric, with 11.5 km of extension taking place across the Horda Platform south of 61°N (β = 1.26), decreasing to 2.5 km across the Uer Terrace (β = 1.05) and < 1 km across the Måløy Slope (β < 1.02) (Figure 8e). The total extension that accumulated across the study area in Rift Phase 2 is much lower (~1.5 km) than during Rift Phase 1 and is laterally more consistent from south (i.e., the Horda Platform) to the north (i.e., the Måløy Slope) (β = 1.03). A local peak in Rift Phase 2 total extension occurs slightly to the north of 61°N, where overlap between V3, Øy2, and Øy1b causes extension values to reach 2.5 km (Figure 8j).

6. Timing of Fault Activity and Fault Slip Rates In section 5 we explored how much throw accumulated on faults during Rift Phases 1 and 2. We now use seismic-stratigraphic observations from seismic sections close to the center of major faults and T-z plots to investigate in detail when these faults were active and quantify their slip rates for different time periods. 6.1. Horda Platform Faults Permian-Triassic strata thicken eastward into the hanging walls of Øy1, V1, and T1 (Figures 3, 6a, and 9). In the hanging wall of Øy1, an intra-Permian-Triassic reflection is interpreted that separates eastward diverging reflections below from reflections that are subparallel above (intra-Triassic horizon, Figures 3 and 9a). This reflection has not been directly dated from well data. However, if we assume that Triassic sediment deposition occurs at the same rate as Jurassic sedimentation (approximately 0.01mm/yr), this reflection, which

Figure 9. Seismic reflection profiles showing stratigraphy against faults, accompanied by throw-depth (T-z) plots for (a) Øy1, (b) V1, (c) T1, (d) V3, (e) V2, and (f) Øy2. The locations of these seismic reflection profiles are shown in Figure 1b. Interpreted horizons and throw values for each horizon in the T-z plots are color coded, as shown in the stratigraphic column presented in Figure 2. The depth measurement in the T-z plots is the midpoint depth between the footwall and hanging wall for a particular horizon. The oldest observed post-Triassic diverging reflections have been shaded blue if they occur in the Jurassic and yellow if they occur in the Cretaceous. PT = Permian-Triassic, J = Jurassic, C = Cretaceous, and T = Tertiary. Slip rates between two sets of horizons are shown in red text on T-z plots, and the ages of these horizons are given in section 3.2. The ages of intra-Lower Cretaceous horizons used to calculate slip rates, determined in this study assuming constant sedimentation rates (see text for discussion), are shown in brackets after the slip rate. All of the Permian-Triassic slip rates have been calculated for a rift duration of 25 Myr. Throw measurements for Øy2 are minima due to erosion of stratigraphy in the footwall. Data courtesy of TGS.

lies ~ 400 m below the Top Triassic horizon, has an age of circa 240 Ma (although we recognize that the sedimentation rate may well have been different between shallow marine Jurassic sedimentation and continental Triassic sedimentation). The age of this reflection, and by inference, the age of cessation of Rift Phase 1, is consistent with those proposed by Lippard and Liu [1992], Odinsen et al. [2000], and Ter Voorde et al. [2000]. Furthermore, this age suggests that Rift Phase 1 lasted 25 Myr, which is considerably shorter than that proposed by Ziegler [1982] and Ter Voorde et al. [2000] (~40 Myr). Slip rates on Øy1 at the location of Figure 9a are approximately 0.18 mm/yr, if a Rift Phase 1 duration of 25 Myr is assumed. T-z plots reveal that Jurassic stratigraphy has been offset by similar amounts of throw (210 to 280 m) between the Top Statfjord Formation and BCU horizons, suggesting that the fault activity associated with Rift Phase 2 occurred sometime after the Early Cretaceous (Figure 9a). This is also confirmed by the tabular nature of Jurassic sediments (0.36 s TWT, 0.55 km) in the hanging wall of Øy1 and the lack of thickness variations in this interval across the fault (Figure 9a). The first evidence of post-Triassic stratal thickening toward Øy1 is observed within Lower Cretaceous stratigraphy, where a package of distinct, eastward diverging reflections are observed approximately midway between the BCU and Base Shetland Group horizons (Figure 7c and highlighted with yellow shading in Figure 9a). If we assume that the sedimentation rate during the Early Cretaceous (i.e., between the Base Shetland Group (100 Ma) and the BCU horizons (140 Ma)) was constant, the age of the lower and upper reflections that bound this eastward thickening wedge can be estimated as circa 130 and circa 120 Ma, respectively; this implies that the unit they bound is Barremian in age. Cretaceous stratigraphy in the footwall of Øy1 has been eroded along the length of the fault (Figure 7c), and the equivalent 120–130 Ma reflections are not present. Therefore, estimates of throw in Figure 9a across these horizons and the associated calculated slip rate of approximately 0.017 mm/yr for the interval between 130 and 120 Ma are minimum estimates. Alternative interpretations are that the Lower Cretaceous stratigraphic thickening observed in Figure 9a is not related to reactivation of Øy1 but is due to activity on a fault farther east, in the area that is now onshore Norway, or that this wedge is a depositional feature and related to clinoform deposition. Whichever interpretation is correct, Øy1 must have experienced faulting post circa 120 Myr. The age of cessation of faulting on Øy1 is unknown due to the erosion of Cretaceous and Tertiary stratigraphy from its footwall; however, there is evidence that Øy1 has not truncated the Base Quaternary horizon, indicating that it has not been active since the start of the Quaternary (Figure 9a). V1 exhibits Rift Phase 1 displacement of 3.6 km at the location of the seismic profile shown in Figure 9b, located close to the center of the fault, yielding slip rates of approximately 0.15 mm/yr (assuming a 25 Myr rift duration). Throw values across the latest Triassic to earliest Cretaceous horizons are approximately constant (170–240 m; Figure 9b). This observation, together with the constant thickness of Jurassic sediments across V1 (Figure 7b) and expansion of the Lower Cretaceous succession toward the hanging wall of the fault, indicates that V1 became active during the earliest Cretaceous, circa 140 Ma. This is somewhat earlier than the first evidence for reactivation on Øy1. Average slip rates for the earliest Cretaceous fault activity associated with Rift Phase 2 are approximately 0.01 mm/yr. During Rift Phase 1, 2.8 km of throw accumulated on T1 (Figure 9c). Furthermore, and by assuming a rift duration of 25 Myr, slip rates during this period of extension are calculated to be approximately 0.11 mm/yr. Top Statfjord Formation and Top Sognefjord Formation horizons are offset by very similar amounts (670–760 km), suggesting little fault activity in the Early to Middle Jurassic. The first seismically resolvable evidence of post- Triassic, Rift Phase 2-related sediment thickening across T1 is observed in the Draupne Formation (Upper Jurassic), indicating that movement initiated at 150 Ma (Tithonian-Berriasian); calculated slip rates associated with this period of activity (i.e., between 150 and 140 Ma) are approximately 0.015 mm/yr (Figure 9c). 6.2. Uer Terrace and Måløy Slope Faults Permian-Triassic sediments thicken toward the hanging wall of V3, and Rift Phase 1 throw values of 0.75 km and slip rates of approximately 0.04 mm/yr, assuming a 25 Myr rift duration, are measured at the location of the seismic reflection profile shown in Figure 9d; it should be noted that these slip rates are significantly lower than for those documented on Rift Phase 1 faults in the Horda Platform (Figures 9a–9c). V3 appears to show evidence of fault activity at all structural levels between the Permian-Triassic and Base Tertiary, apart from a possible period of quiescence during the Early to Middle Jurassic (i.e., during the deposition of the Dunlin and Brent groups; Figure 9d). Slip rates on V3 in the Middle-to-Late Jurassic (i.e., 167–140 Ma) were approximately 0.057 mm/yr, decreasing slightly to approximately 0.05 mm/yr during the Early Cretaceous

(140–120 Ma; Figure 9d). These Rift Phase 2 slip rates are 5 times greater than the Mesozoic slip rates calculated for reactivated faults in the Horda Platform (Figures 9a–9c). V2 did not exist during Rift Phase 1 and initiated during the late Middle Jurassic (i.e., following deposition of the Brent Group at circa 167 Ma; Figure 9e). After it initiated, V2 had a slip rate of approximately 0.04 mm/yr during the latest Middle to early Late Jurassic (167–160 Ma), decreasing to approximately 0.011 mm/yr in the Late Jurassic (160–150 Ma). No sedimentary rocks are preserved in the footwall of Øy2 (Figure 9f). Virtually, no Triassic strata are present in the hanging wall of Øy2, and Jurassic stratigraphy has a constant thickness along the fault hanging wall, suggesting that the fault activity occurred after the deposition of these units (Figures 7b and 9f). The first stratigraphic interval to thicken and diverge toward Øy2 is Early Cretaceous in age and occurs directly above the BCU. Minimum estimates of slip rate for this Early Cretaceous (140–125 Ma) period of fault activity are approximately 0.015 mm/yr (Figure 9f). Quaternary sediments also thicken toward Øy2, and together with the presence of a seafloor scarp may suggest that this fault has been active in the recent past (Figure 9f). The first evidence of sedimentary thickening into the hanging walls of faults on the Måløy Slope occurs within the Brent Group (Figure 4). Middle to latest Jurassic slip rates on these faults are of the order 0.01–0.02 mm/yr.

7. Rift Evolution in the Northern North Sea In section 7.1 we describe the evolution of the normal fault array present on the Horda Platform, Uer Terrace, and Måløy Slope, on the eastern margin of the North Viking Graben (Figures 10a to 10e). A summary of the timing of initiation and cessation of activity on these faults is presented in Figure 10f. In section 7.2 we compare these patterns of fault evolution with those documented on the western margin of the North Viking Graben (Figure 10).

7.1. Fault Evolution in the Wider Horda Platform Area 7.1.1. Permian to Early Triassic (Circa 260 to Circa 220? Ma) On the eastern margin of the North Viking Graben, active normal faults (Øy1, V1, and T1) had a N-S to NNW-SSE strike, accumulated maximum throws of 3–4 km, and had mean slip rates of approximately 0.15 mm/yr (Figure 10a). Large Permian-Triassic half-grabens were restricted to the south of 61°N, and the Måløy Slope formed a structural high at this time (Figure 10a). Onshore, ﬁeld-based studies have shown that Rift Phase 1 extension was also responsible for the reactivation of faults associated with Devonian shear zones in western Norway and magmatism associated with the emplacement of alkaline dykes [Torsvik et al., 1992; Braathen, 1999]. 7.1.2. Early to Late Triassic (Circa 220 Ma to 170 Ma) The Horda Platform, Uer Terrace, and Måløy Slope areas were tectonically quiescent at this time, with a tabular package of Late Triassic sediments deposited inﬁlling remnant topography. 7.1.3. Bajocian to Oxfordian (170–155 Ma) Between 170 and 155 Ma, V3, which initiated in Rift Phase 1, was reactivated after a ~50 Myr period of quiescence. Slip rates during this period of reactivation were approximately 0.05 mm/yr, similar to the slip rates determined for the Brage Fault, located 30 km west of the study area, on the western boundary of the Horda Platform [Ter Voorde et al., 2000]. V2 and the Måløy Slope Faults initiated at this time, in locations that had not experienced prior, Permian-Triassic or Devonian extension; slip rates on these faults were approximately 0.05 mm/yr and approximately 0.01–0.02 mm/yr, respectively. Faults that initiated at this time had a predominantly N-S to NE-SW strike, as opposed to the N-S to NNW-SSE trend that dominated in Rift Phase 1 (Figure 10b). The newly initiated (V2, M1, and M2) and reactivated faults (V3) on the Uer Terrace and Måløy Slope lie within ~80 km of the Gullfaks-Visund Fault, which formed the western margin of the North Viking Graben (Figure 10b). Rift Phase 1 faults (Øy1, V1, and T1) that are located >80 km from the Gullfaks- Visund Fault Complex were not active during the Bajocian to Oxfordian (Figure 10b). 7.1.4. Oxfordian to Volgian (155–148 Ma) The Tusse Fault, which is located ~95 km to the east of the Gullfaks-Visund Fault, reactivated at circa 150 Ma and was associated with a maximum slip rate of approximately 0.015 mm/yr, this is an order of magnitude slower than that documented on the Tusse Fault during Rift Phase 1. V3 continues to slip at a rate of approximately 0.05 mm/yr between 155 and 148 Ma, whereas the slip rate on V2 decreased to approximately 0.015 mm/yr (Figures 10c and 10d).

Figure 10

7.1.5. Volgian to Berriasian (148–140 Ma) V1 is located 120 km to the east of the Gullfaks-Visund Fault, and it reactivated at circa 140 Ma and had a slip rate of approximately 0.01 mm/yr (Figure 10d). At this time, a series of relatively minor (30–100 m throw) NW-SE faults initiated on the western margin of the North Viking Graben, in the vicinity of reactivated Rift Phase 1 faults [Whipp et al., 2014]. Furthermore, Øy2, which is located 100 km to the east of the Visund Fault, initiated in an area that did not experience Permian-Triassic rifting (Figure 10d). The newly initiated Øy2 fault has a N-S to NNE-SSW strike and had a minimum slip rate of approximately 0.015 mm/yr (Figure 10d). 7.1.6. Berriasian to Barremian (140–130 Ma) Around 130 Ma, Øy1, which is located 140 km to the east of the Gullfaks-Visund Fault, was reactivated and was associated with a minimum slip rate of approximately 0.017 mm/yr (Figures 9e and 9f). Between 140 and 130 Ma, all of the large fault systems on the Horda Platform, Uer Terrace, and Måløy Slope were active (Figures 10e and 10f). Faults in the Uer Terrace, Måløy Slope, and western Horda Platform became inactive by the mid-Cretaceous; however, Øy1 and Øy2 may have remained active well into the mid-Late Cretaceous (Figure 10f). The presence of a seafloor scarp and a package of Quaternary sediments that thicken toward the hanging wall of Øy2 may indicate that this fault has been very recently active. 7.2. Fault Evolution in the East Shetland Basin This study has focused on the detailed evolution of rift-related normal faults along the eastern margin of the North Viking Graben. To fully understand the role that faults within our study area played in controlling rift-related extension, we must link our results to those from comparable analyses in the East Shetland Basin (ESB) (Figures 10a to 10e). Using seismic reflection and borehole data from the western margin of the North Viking Graben, Tomasso et al. [2008] showed that Permian-Triassic major faults in the ESB also had a N-S strike, dipped to the west, and bounded the eastern margins of eastward tilted half-graben; these basins were, therefore, of comparable size, trend, and genesis to those observed in the Horda Platform (Figure 10a) [Tomasso et al., 2008]. The geometry of the Permian-Triassic basins shown in Figure 9 of Tomasso et al. [2008] suggest that Rift Phase 1 faults had throws of 1–1.5 km, which is comparable to those observed on the Permian-Triassic V3 fault, but significantly less than those associated with Permian-Triassic faults on the Horda Platform. Tomasso et al. [2008] show syndepositional thickening toward the west-dipping faults within the Upper Lunde Formation, suggesting that west-dipping faults were active in the Late Triassic. This time period was a period of tectonic quiescence on the eastern margin of the North Viking Graben. A number of NE-SW striking, east and west-dipping faults were active in the East Shetland Basin during the Bajocian to Oxfordian; it is unclear if these faults ruptured previously unfaulted crust or if they reactivated Permian-Triassic structures (Figure 10b) [Færseth and Ravnås, 1998; Davies et al., 2000; Cowie et al., 2005]. In the Oxfordian to Volgian, strain became focused onto the east-dipping, Hutton, Murchison, Brent-Statfjord, and Gullfaks-Visund faults and coevally west-dipping Bajocian-Oxfordian age faults were abandoned and truncated [Cowie et al., 2005] (Figure 10c). In addition to strain localization, slip rates on the Brent-Statfjord

Figure 10. (a) Map view of the location of major active faults (red) in the northern North Sea during the Permian-Triassic rift phase around 260 to 220 Ma. Dashed grey lines show the Horda Platform, Uer Terrace, and Måløy Slope area described in this study, and the East Shetland Basin study area described in Cowie et al. [2005]. Light grey faults are taken from Færseth [1996] and Zanella and Coward [2003] and show faults outside of these study areas with unknown activity during the Permian-Triassic. Permian-Triassic active faults in the East Shetland Basin and onshore western Norway are drawn schematically following the work of Tomasso et al. [2008] and Torsvik et al. [1997], respectively. The thickness of lines used to draw faults is scaled to slip rate described in the legend at the bottom of the ﬁgure. Red dashed lines indicate that the slip rates on these faults are unknown. Below the map is a schematic cross section illustrating the geometry of the basin at the location of line A to A′ 260–220 Ma (not to scale). Onshore geology from Gee et al. [2008], see Figure 1b for legend. (b) Map view of the location of major active faults and their slip rates and schematic cross sections of basin geometry in the Bajocian to Oxfordian (170–155 Ma), (c) Oxfordian to Volgian (155–148 Ma), (d) Volgian to Berriasian (148–140 Ma), and (e) Berriasian to Barremian (140–130 Ma) from this study, Cowie et al. [2005], Færseth and Ravnås [1998], and Zanella and Coward [2003]. (f) Summary of the timing of initiation and cessation of activity on faults in the East Shetland Basin, Horda Platform, Uer Terrace, and Måløy Slope. Bars show the duration of fault activity, and the thickness of the bars is scaled to slip rates shown in the legend. H = Hutton Fault, M = Murchison Fault, B-S = Brent-Statfjord Fault, G-V = Gullfaks-Visund Fault, S = Snorre Fault, B = Brage Fault, T = Tusse Fault, V1 = Vette Fault segment 1, V2 = Vette Fault segment 2, V3 = Vette Fault segment 3, M = Måløy Fault Slope Faults 1 and 2, Øy1 = Øygarden Fault segment 1, and Øy2 = Øygarden Fault segment 2 fault. R1 = Rift Phase 1, PR1 = Postrift Phase 1, R2 = Rift Phase 2, PR2 = Postrift Phase 2, and TD = central North Sea thermal dome.

and Gullfaks-Visund faults increased at this time [McLeod et al., 2000], whereas slip rates on the Hutton and Murchison Faults decreased [Cowie et al., 2005]. During the Volgian to Berriasian, the Hutton and Murchison Faults became inactive, with fault activity migrating onto the Gullfaks-Visund Fault, along the western margin of the North Viking Graben, with slip rates of approximately 0.3 mm/yr at this time (Figure 10d) [Cowie et al., 2005]. During the Berriasian to Barremian, strain continued to focus onto the Gullfaks-Visund Fault [Cowie et al., 2005]. In summary, the ESB developed as predicted from rift evolution models; strain was ﬁrst distributed on multiple small faults with different polarities, before becoming focused onto a few large structures. Furthermore, the number of active faults in the ESB decreased through time as fault activity migrated eastward toward the rift axis [Cowie et al., 2005]. Tomasso et al. [2008] also suggested that Permian-Triassic faults in the ESB dipped westward [Tomasso et al., 2008]. It therefore seems that at least in this location, the underlying, Permian-Triassic rift fabric played a rather limited role in the way in which subsequent extension was accommodated, and preexisting faults may have been truncated rather than reactivated. In contrast, on the eastern margin of the North Viking Graben, Permian-Triassic faults were reactivated diachronously during Rift Phase 2, with lower slip rates (<0.01 mm/yr) than they had experienced in Rift Phase 1 (Figure 10e). Furthermore, whereas the number of active faults on the ESB decreased between the Bajocian to Barremian, the number of active faults in the Horda Platform area increased (Figure 10e). Although strain in the Berriasian to Barremian was localized over the narrow <50 km wide North Viking Graben, active faulting did continue on the Horda Platform, Uer Terrace, and Måløy Slope, up to 150 km to the east of the axis of the North Viking Graben (Figures 10e and 10f).

8. Discussion Our study has shown that (i) the distribution of strain across the Horda Platform in Permian-Triassic Rift Phase 1 and Middle Jurassic to Early Cretaceous Rift Phase 2 was markedly different; (ii) at the initiation of Rift Phase 2, new faults nucleated close to the axis of the Viking Graben and Permian-Triassic faults located at greater distances from the rift axis were not reactivated; (iii) the reactivation of Permian-Triassic faults was diachronous, with those closer to the rift axis reactivating before those located toward the rift margin; (iv) the number of active faults in the Horda Platform area increased after the initiation of Rift Phase 2; (v) reactivation of Permian- Triassic faults and the initiation of new faults continued until the middle of the Cretaceous; and (vi) at the scale afforded to us by seismic data, large faults across the ESB and Horda Platform appear to have been active throughout the Middle Jurassic to Early Cretaceous with no apparent distinct pulses of rifting or periods of regional tectonic quiescence. In this section we attempt to explain our observations by comparing them with observations from the wider northern North Sea rift, as well as other multiphase rifts, and physical and numerical modeling results. 8.1. Controls on the Geometry and Distribution of Permian-Triassic Rift Basins Strain during Permian-Triassic rifting was focused on the northern Horda Platform, with minor extension occurring across the Uer Terrace and Måløy Slope (cf. Figures 8e, 8j, and 10a). In this section we discuss the factors that may have controlled this and the geometry of associated Permian-Triassic rift basins. Prerift crustal discontinuities play a key role in the development of a number of rifts worldwide, including rift basins offshore Namibia [Maslanyj et al., 1992], the North Falklands Basin, offshore Falkland Islands [Bransden et al., 1999], and the Plemtos and Gamtoos Basins, southern South Africa [Paton, 2006]. Paton [2006] indicates that faults that form in response to reactivation of basement discontinuities have the following characteristics: (i) they can attain great lengths (i.e., >480 km), (ii) they reach their ﬁnal fault lengths very rapidly compared to faults forming in unruptured crusts, and (iii) may appear “under displaced” relative to fault systems that have grown by the progressive growth and linkage of previously isolated fault segments [e.g., Walsh and Watterson, 1988; Dawers and Anders, 1995; Kim and Sanderson, 2005]. Permian-Triassic faults on the Horda Platform, prior to post-Triassic reactivation, were relatively long (>100 km) and had throws of up to 4 km (i.e., displacements of ~5.4 km; Figure 11). To interrogate whether the Rift Phase 1 faults formed due to reactivation of preexisting structures, maximum displacement (D) versus length (L) values for Permian-Triassic faults identiﬁed in this study are plotted against equivalent measurements for a global sample of normal faults (Figure 11) [see also Kim and Sanderson, 2005; Paton, 2006]. The D/L ratios for the Permian-Triassic Øy1, V1, and T1 fault systems are similar to those documented in southern South Africa, which have demonstrably formed

due to reactivation of preexisting, basement discontinuities, and plot below the typical D/L gradient of n = 1, suggesting that they may be slightly under displaced (Figure 11) [cf. Paton, 2006]. The fact that the Permian-Triassic faults are long (>100 km) and are slightly under displaced relative to faults that have evolved by traditional fault evolution models suggest that they could be the result of reactivation of pre-Permian crustal weaknesses. The lack of age control on Permian stratigraphy means that it is difficult to assess the rate of formation of these faults; however, the classic bell-shaped nature of T-x plots for Rift Phase 1 indicates that individual fault segments had hard linked before the Late Triassic (Figure 8). On the Horda Platform major Permian-Triassic basins do not extend north of 61°N (Figures 7a and 10a). In contrast, Devonian sedimentary Figure 11. Maximum displacement versus fault length plot using basins onshore Norway are restricted to data from Kim and Sanderson [2005] (white, grey, and black circles locations north of 61°N, in the hanging wall of on the left-hand side of the figure) and Paton [2006]. We have placed the Nordfjord-Sogn detachment (Figures 1b conservative error bars on the length of faults that extend outside and 12a) [Osmundsen and Andersen, 2001; of our study area of 50%, following the method of Paton [1996]. Maximum displacement versus length measurements are shown for Johnston et al., 2007]. Devonian age, low-angle both faults following Rift Phase 1 (colored circles) and also following faults, and E-W orientated brittle structures Rift Phase 2 (colored diamonds). Results from Paton [1996] show associated with the Nordfjord-Sogn detachment the evolution in displacement versus length relationships for two onshore are known to have been active in faults in southern South Africa from 156 to 130 Ma. the Permian, confirming that at least in some locations, Rift Phase 1 reactivated fabrics formed during Devonian extension [Eide et al., 1997; Torsvik et al., 1997]. Although wells have not penetrated pre-Permian sediments on the Horda Platform, seismic reflection data suggest that Devonian sediments may underlie the area (Figure 3). Devonian extensional structures onshore dip at low angles (approximately 25°) [Souche et al., 2012], whereas Permian-Triassic faults in the Horda Platform dip moderately steeply (40–55°). We suggest that the existence of Devonian low-angle shear zones may have influenced where the Permian-Triassic faults nucleated; however, the low-angle structures themselves may not have directly reactivated; although, it is possible that the Permian-Triassic faults sole out onto low-angle Devonian structures at depth (Figure 12b). The lack of Devonian extensional structures on the Måløy Slope may explain why Rift Phase 1 extension was focused on structures farther east, currently onshore, where Devonian extensional structures are observed (Figure 12b). 8.2. Controls on the Geometry of Middle Jurassic to Early Cretaceous Rift Basins Systematic variations in the location of reactivated Permian-Triassic faults within the rift suggest that the primary controls on fault reactivation are more likely regional, rather than local (e.g., local variations in fault zone physical properties and their propensity to reactivate [cf. Sibson, 1985; Sibson, 1990; Imber et al., 1997]). Below we explore the likely control that (i) the presence and orientation of preexisting faults; (ii) thermal weakening, formation of the proto-North Viking Graben, and superimposed far-field stresses; and (iii) lithospheric flexure had on the geometry of Middle Jurassic to Early Cretaceous rift basins. 8.2.1. Presence and Orientation of Preexisting Faults Rifting at the onset of Rift Phase 2 was focused in the East Shetland Basin and North Viking Graben. Permian-Triassic faults on the Horda Platform did not contribute to defining the basin geometry at this time (Figure 10b). However, new faults did initiate on the Måløy Slope and Uer Terrace at this time (Figure 10b). These observations contrast with observations from many other multiphase rifts and analogue models, within which previous fault populations have been extensively and almost uniformly reactivated by later extension

Figure 12. Schematic block diagrams illustrating the crustal-scale evolution of the northern North Sea; no scale implied. (a) Gravitational collapse of Caledonian thrust sheets in the Devonian results in the formation of shear zones and sedimentary basins. Devonian basins are observed onshore western Norway, and also potentially in the East Shetland Basin [Platt and Cartwright, 1998], and Horda Platform (Figure 3). (b) Permian-Triassic faults may have nucleated preferentially in regions affected by Devonian extension, and steep Permian-Triassic faults may sole out onto these shear zones at depth. (c) In the Late Triassic the region experienced tectonic quiescence and thermal subsidence following Rift Phase 1. (d) The Aalenian central North Sea thermal dome resulted in extensive uplift, and the formation of the Moray Firth-Central Graben-Viking Graben trilete rift system. The proto-Viking Graben trended slightly obliquely to major Permian-Triassic basins. The collapse of the thermal dome may have formed a proto-North Viking Graben structural template. Distal-buried faults on the Horda Platform and Western Norway associated with Permian-Triassic rifting were not reactivated. (e) Regional extension associated with the opening of the Arctic Rift focused in the North Viking Graben region. Permian-Triassic faults in the Horda Platform that lie closest to the rift axis reactivate. (f) Strain continues to focus on the Gullfaks-Visund Fault into the Early Cretaceous and Permian-Triassic faults on the Horda Platform reactivate diachronously toward the basin margin.

[e.g., Morley et al., 2004; Henza et al., 2010]. One explanation for the apparent lack of reactivation of Permian- Triassic faults on the Horda Platform at the onset of Rift Phase 2 could be that they were unfavorably orientated for reactivation in the Middle-to-Late Jurassic. The extension direction or directions that resulted in the formation of the northern North Sea rift system, in particular those responsible for Rift Phase 2, remain controversial. Some authors have suggested that the extension direction remained E-W throughout Rift Phase 2 [Badley et al., 1988; Brun and Tron, 1993], whereas others suggest that it changed from E-W in the earliest part of Rift Phase 2 to NW-SE [Doré and Gage, 1987] or that it remained NW-SE throughout Rift Phase 2 [Færseth, 1996]. As discussed above, based on the results of Tomasso et al. [2008], faults that initiated at the beginning of Rift Phase 2 (170–155 Ma) in the East Shetland Basin crosscut underlying Permian-Triassic fault networks, suggesting that the orientation of these faults are a good indicator of the extension direction at this time as they do not appear to be inﬂuenced by preexisting fault fabric. These faults have a mean strike of N013°E, and this indicates an extension direction of N077°W for Rift Phase 2 (Figure 10b). The strike of Rift Phase 1 faults in the Horda Platform is N-S to NNW-SSE, and the mean extension direction determined from these faults for Rift Phase 1 is N080°E (Figures 6 and 10a). The change from N080°E extension direction in Rift Phase 1 to N077°W in Rift Phase 2 represents a 23° counterclockwise rotation in extension direction between rift phases. This variation suggested by our study is, however, fairly minor. Using scaled physical models, Henza et al. [2010] show that when the extension direction between rift events differs by <22.5°, only very few new faults initiate, and deformation is instead

accommodated by reactivation of the first-phase normal faults. Natural rift examples support this, with rifts that have experienced > 23° variations in extension direction showing significant reactivation of the first- phase fault networks [e.g., Morley et al., 2004; Bellahsen et al., 2006]. Therefore, the preference for the initiation of new faults at the onset of Rift Phase 2, rather than the reactivation of Rift Phase 1 faults, as is observed on both sides of the North Viking Graben, cannot be solely explained by this relatively small variation in extension direction. An alternative explanation is therefore required to explain the limited degree of Permian- Triassic fault reactivation at the onset of Rift Phase 2 and to account for the eventual reactivation of these faults in the Horda Platform. 8.2.2. Thermal Weakening, Formation of the Proto-North Viking Graben, and the Role of Far-Field Stresses Numerical and physical models suggest that the emplacement of magmatic bodies can weaken the lithosphere and cause the localization of rift-related extensional strain [e.g., Corti et al., 2003; Huismans and Beaumont, 2007]. For example, regional variations in the thermal structure of the lithosphere, related to the emplacement of crustal melt, influenced the pattern of fault reactivation in central Australia [Hand and Sandiford, 1999] and strain migration in Ethiopia [Daniels et al., 2014]. We argue below that the evolving thermal and rheological structure of the lithosphere, both during and between rift phases, may account for the lack of reactivation of Permian-Triassic structures on the Horda Platform at the onset of Rift Phase 2. Prior to the initiation of Rift Phase 2, thermal doming and deflation of the lithosphere occurred in the central North Sea in the Early Jurassic (Aalenian) [Ziegler, 1990; Underhill and Partington, 1993, 1994; Davies et al., 2001] (Figure 12d). The Aalenian thermal dome may be the result of a transient mantle plume [Ziegler, 1990; Underhill and Partington, 1993, 1994] or could be the result of active destabilization of the mantle lithosphere in response to Rift Phase 1 [Huismans et al., 2001]. Whatever its origin, this mantle convection would have weakened the lithosphere [Corti et al., 2003; Ziegler and Cloetingh, 2004], resulting in “active” rifting and the formation of the trilete North Sea rift system [Ziegler, 1990; Davies et al., 2001; Dyksterhuis et al., 2007] (Figure 12d). Although no evidence of volcanism has been observed this far north of the center of the proposed thermal dome, even relatively small volumes of melt may lead to lithosphere weakening [Daniels et al., 2014]. The three arms of the trilete system formed at 120° to each other, which is the geometry that requires the least energy to form in a homogeneous material [Davies et al., 2001]; there are suggestions, however, that the geometry of the trilete system may have exploited Caledonian and Tornquist structural fabrics [Fraser et al., 2002]. The Viking Graben arm of the trilete system had an overall NNE-SSW orientation and cut across the N-S Permian-Triassic fabric slightly obliquely [Davies et al., 2001] (Figure 12d). During the Callovian to Oxfordian, tensional forces associated with Arctic rifting propagated into the North Sea and initiated the most significant phase of Rift Phase 2 extension [Coward et al., 2003]. Due to the presence of the Middle Jurassic, proto-North Viking Graben arm of the trilete rift system, which formed by collapse of the thermal dome, strain may have become localized in this region during Rift Phase 2, rather than reactivating the now-buried Permian-Triassic fault population on the Horda Platform [cf. Corti et al., 2003; Dyksterhuis et al., 2007; Huismans and Beaumont, 2007] (Figure 12e). Therefore, the orientation of faults that initiated in Rift Phase 2 is related to the interplay between the geometry of the proto-North Viking Graben formed after thermal dome collapse and the regional far-field stress direction associated with Rift Phase 2 [cf. Bellahsen et al., 2006]. Whether the regional extension direction during Rift Phase 2 was E-W, NW-SE, or varied through time is difficult to constrain, as the geometry of resulting faults will be related to the interaction between the regional extension direction and the inherited proto-North Viking Graben structural template, making it difficult to constrain the true extension direction from fault geometry alone [cf. Henza et al., 2010, 2011] (Figure 12e). The interaction between thermally weakened lithosphere and far-field stresses can potentially explain why strain remained focused in the North Viking Graben at the beginning of Rift Phase 2, rather than reactivating buried Permian-Triassic faults located nearer the basin margin. However, this does not directly explain why Permian-Triassic faults were eventually reactivated systematically from west to east across the Horda Platform (Figure 10). 8.2.3. Lithospheric Flexure Although strain in the ESB became localized in the axis of the North Viking Graben during the latest Jurassic, eastward migration of active faulting across the Horda Platform caused the basin to maintain a fairly constant width of approximately 140 km (Figures 10e and 12f). This observation is similar to observations from the active western Gulf of Corinth Rift zone, central Greece. The western Gulf of Corinth has previously been cited as an example of a rift that experiences northward fault migration along its southern margin toward the rift

axis [e.g., Cowie et al., 2005]. Studies from offshore seismic reflection data, however, reveal that faults on the northern margin of the western Gulf of Corinth are in fact migrating away from the rift axis, resulting in the rift moving progressively north with time [Bell et al., 2008, 2009]. The migration of strain toward the basin margin as observed in the Horda Platform, Uer Terrace, and Måløy Slope areas may be the result of flexural bending in the hanging wall of a major normal shear zone, or in response to postrift thermal subsidence and sediment loading focused in the axis of the North Viking Graben [e.g., Vening Meinesz, 1950; Badley et al., 1984, 1988; Melosh and Williams, 1989; Scholz and Conteras, 1998] (Figure 12e). Slip on the Gullfaks-Visund Fault would have led to the formation of a flexural bulge that migrated eastward, allowing Permian-Triassic faults to reactivate, with those faults closest to the rift axis reactivating first. Postrift thermal subsidence and sediment loading in the axis of the North Viking Graben may have also contributed to ongoing flexure of the eastern margin of the North Viking Graben and reactivation of these structures into the mid-Late Cretaceous, even after the cessation of activity on the Gullfaks-Visund Fault. This mechanism is analogous to the reactivation of basement faults during the migration of the flexural wave in foreland basin settings [Bayona and Thomas, 2003], and it required that the lithosphere in the Horda Platform, Uer Terrace, and Måløy Slope area had some strength to allow flexure [Watts et al., 1982; Cowie et al., 2005]. Although flexural backstripping studies have been able to adequately reconstruct the stratigraphy in this area with a low-uniform value of elastic thickness (Te = 1.5 km) across the Viking Graben [e.g., Roberts et al., 1995], the elastic thickness would be expected to be greater in regions that have experienced less overall stretching, like the Horda Platform, Uer Terrace, and Måløy Slope areas [cf. Bell et al., 2014]. Faults on the Horda Platform remained active until at least the mid-Cretaceous (Øy1), and some may have even been active in the Holocene (Øy2). Extension in the North Viking Graben ended by the Barremian (130 Ma [Cowie et al., 2005]), and the locus of rifting shifted instead to the proto-North Atlantic with significant Early Cretaceous activity on normal faults such as the End-of-the-World and Magnus faults (Figure 10) [e.g., Ravnås and Steel, 1997; Coward et al., 2003]. The combination of flexural bending associated with postrift thermal subsidence focused in the North Viking Graben [Badley et al., 1984], and far-field stresses from rifting in the North Atlantic may have both contributed to continued activity of faulting in the Horda Platform into the Cretaceous. 8.3. Modeling Rift Reactivation Our study indicates that fault reactivation during multiphase extension can be highly selective, and even faults that are optimally orientated for reactivation can remain inactive. The geometry of a multiphase rifts is, therefore, not only controlled by the orientation of the underlying upper crustal fault network but can also be influenced by the thermal and rheological evolution of the lithosphere and regional tectonic stress field. Physical models investigating multiphase extension generally only consider homogenous extension resulting from far-field horizontal stresses. These modeling studies find that fault reactivation is related to the orientation of faults compared to the extension direction and degree of maturity of the first-phase fault fabric [Withjack and Jamison, 1986; McClay and White, 1995; Henza et al., 2010, 2011]. The influence of magmatic bodies and thermal weakening of the continental crust has been poorly explored in physical models of multiphase extension [Corti et al., 2003], and therefore, the results of physical models of multiphase rifting may not be applicable to natural rifts. Numerical models can account for complex rheological variations induced by temperature changes more easily than physical experiments [Mulugeta and Ghebreab, 2001]; however, these numerical models often lack the resolution to investigate detailed fault evolution that physical models provide. The observations in this contribution indicate that care should be taken when applying the results of physical models directly to specific multiphase rifts, without consideration of the dynamic rifting processes at a particular margin, and the thermal and rheological evolution of the lithosphere.

9. Conclusions This study has synthesized seismic and well data to investigate how a Permian-Triassic fault array in the northern North Sea has behaved during a further phase of Middle Jurassic to Early Cretaceous extension. The major conclusions of this study are: 1. Permian-Triassic faults on the Horda Platform had lengths of >100 km and accumulated throw of up to 4 km. These faults are west-dipping and during the Permian-Triassic had slip rates of approximately

0.1–0.15 mm/yr. The maximum throw versus fault length relationships for these faults are very similar to faults that have reactivated basement discontinuities in South Africa [Paton, 2006]. We suggest that the distribution of Permian-Triassic basins may have been controlled by the presence of underlying Devonian structural discontinuities. 2. In the Middle Jurassic, major rifting occurred in the East Shetland Basin and North Viking Graben; however, Permian-Triassic faults on the Horda Platform >80 km to the east of the western margin of the North Viking Graben were not reactivated. In the Middle-to-Late Jurassic new faults initiated in the Måløy Slope and Uer Terrace orientated NNE-SSW. The rise and fall of the Mid-Jurassic central North Sea thermal dome resulted in lithospheric weakening and the initiation of the NNE-SSW North Sea trilete system, which was then exploited and rifted by far-field stresses. We suggest that in the Middle-to-Late Jurassic, strain focused in this zone of thermally weakened lithosphere, rather than reactivating more distal buried upper crustal faults associated with the wide Permian-Triassic rift basin. 3. In the Late Jurassic to Early Cretaceous Permian-Triassic faults in the Horda Platform reactivated diachronously; with those closest to the North Viking Graben reactivating up to 30 Myr earlier than those at the basin margin. These reactivated Late Jurassic to Early Cretaceous faults are > 100 km long and mirror the geometry of the underlying Permian-Triassic faults. Slip rates on these faults during the Late Jurassic to Early Cretaceous are approximately 0.01 mm/yr, an order of magnitude slower than slip on these faults during the Permian-Triassic. We suggest that the reactivation may have been related to flexural down bending of the Horda Platform as strain focused in the North Viking Graben, resulting in the basin marginward migration of fault activity. There is also evidence that far-field stresses associated with North Atlantic rifting may have been influential, resulting in greater Early Cretaceous fault activity in the northern part of the Horda Platform when compared to parts of the rift located to the south. 4. The geometry of multiphase rifts is not only controlled by the orientation of the underlying upper crustal fault network but can also be influenced by the thermal and rheological evolution of the lithosphere. Care should be taken when applying the results of multiphase extension physical models directly to specific multiphase rifts, without consideration of the thermal and rheological evolution of the lithosphere at that particular location.

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