Reactivation in the Ringvassøy-Loppa Fault Complex - the role of detachments

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Reactivation of the

Ringvassøy-Loppa Fault Complex, SW Barents Sea –

the role of detachments

Heleen Zalmstra

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Loppa Fault Complex, SW Barents Sea – the role of detachments

Heleen Zalmstra

Master Thesis in Geosciences Discipline: Structural Geology

Department of Geosciences

Faculty of Mathematics and Natural Sciences

University of Oslo

June 2013

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© Heleen Zalmstra, 2013

Tutor(s): Prof. Roy Helge Gabrielsen and Prof. Jan Inge Faleide

This work is published digitally through DUO – Digitale Utgivelser ved UiO http://www.duo.uio.no

It is also catalogued in BIBSYS (http://www.bibsys.no/english)

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The Ringvassøy-Loppa Fault Complex is an extensional fault complex with old zones of weakness, in the SW Barents Sea. It separates the Tromsø Basin in the west from the Hammerfest Basin in the east. It has been reactivated during several tectonic events, and it is associated with both distinct and di↵use detachments.

Detailed 3D and 2D seismic interpretation has been carried out, with emphasis on detachment zones and reactivation of the fault systems. In addition to this, analogue modeling has been done to investigate the role of detachments during multiple phases of reactivation and apply this knowledge to the study area.

After comparison of fault maps of di↵erent interpreted levels, and structural analysis of seismic data, it was found that three detachment levels are very likely to be present in the study area. These are a distinct detachment in the Early Permian evaporites, and two di↵use detachments, in the Triassic shales and in the Cretaceous fine clastic sediments.

Five main tectonic events have a↵ected the study area: (1) middle Carboniferous, (2) Late Permian - Early Triassic, (3) late Middle Jurassic - earliest Cretaceous, (4) Early Cretaceous (Aptian), and (5) Cenozoic (Eocene). Evidence for these events can be seen in the seismic data in the form of structures related to growth faulting.

The change in stress regime in the Cenozoic, related to the breakup of the NE Atlantic, can be seen when comparing the fault orientation of the Cretaceous and the Cenozoic levels.

The set-up used in the analogue experiment, using strata of di↵erent materials, can recreate the structures seen in the seismic data. The structures that were seen in the analogue models support the conclusion of the existence of multiple detachments in the Ringvassøy-Loppa Fault Complex. The similarities between the analogue models and seismic section shows that the models are a good representation of the situation in nature.

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First of all, I would like to thank my supervisors, Roy Helge Gabrielsen and Jan Inge Faleide for their invaluable support and discussions during the writing of this thesis.

I’m also very grateful to Michel Heeremans, for setting up my project in Petrel and for always being available for help with computer-issues.

For the analogue experiments, I would like to thank Dimitrios Sokoutis, Ernst Willing- shofer and everybody at the TecLab at the University of Utrecht for their welcome and for all the help on the experiments.

Special thanks go out to the IMPACT project in CIPR, University of Bergen, and sponsors, for cooperation, discussion and financial support for the experimental work. In particular the head of the project, dr. Anita Torabi. I would like to acknowledge Statoil for providing the 3D seismic data and TGS and Fugro for access to the 2D seismic lines.

Last, but certainly not least, I want to thank all my friends and family in both Norway and Holland for all their support, motivation and help throughout my study. I couldn’t have done it without you all.

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1 Introduction

The Barents Sea is the northwestern part of the Eurasian continental shelf. It is bounded by Svalbard and Franz Josef Land in the north, the northern Norwegian coast and Russia to the south, the Norwegian-Greenland Sea to the west and Novaya Zemlya in the east (Faleide et al., 1984). It is bounded by two passive contintental margins, the Norwegian-Greenland Sea in the west and the Eurasia Basin in the north (fig. 1).

The Barents Sea consists of a number of fault complexes, with old zones of weakness, some of which date back to the Caledonian orogeny (Gudlaugsson et al., 1998). They have been reactivated during a number of post Caledonian tectonic events, most of which were extensional. The fault complexes are associated with di↵erent structural styles, and in some cases they are associated with distinct or di↵use detachments. This study is focused on the southern part of one of these fault complexes, the Ringvassøy-Loppa Fault Complex. The role of detachments during multiple phases of reactivation in this fault complex is studied by carrying out detailed seismic interpretation and analogue modeling.

The aim of this study is to determine whether or not di↵use and/or distinct detachments are present in the Ringvassøy-Loppa Fault Complex, and to study the role of these detachments in the reactivation of the fault system. Detailed 3D seismic interpretation is done to analyze the fault systems in the study area, in order to find evidence for the presence of detachments, and the reactivation of the faults.

Then, analogue modeling is used to investigate the role of detachments during reactivation, and apply this knowledge to the Ringvassøy-Loppa Fault Complex.

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1 INTRODUCTION

Figure 1: Location of the Barents Sea. The black square marks the location of the study area.

Modified from Faleide et al., 2008

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2 Geological framework

2.1 Regional geology

The western Barents Sea is a epicontinental sea, formed by a series of tectonic events.

It can be divided into three geological provinces: an east-west trending basinal province between 74 degrees north and the coast of northern Norway, an elevated platform area in the north and the western continental margin (Faleide et al., 1984).

Figure 2: Main structural elements of the Barents Sea. Modified after Gabrielsen, 1984 and Faleide et al., 1984

The metamorphic basement of the area is most likely made up of rocks of Caledonian age, which were deformed during the Caledonian orogeny in Late Silurian to Early Devonian times, when the Precambrian Baltic shield and Laurentia collided (Roberts and Gee, 1985).

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2.1 Regional geology 2 GEOLOGICAL FRAMEWORK

It is believed that the structural grain of this Caledonian basement influenced the later structural development of the Barents Sea (Ziegler, 1988, Dor´e, 1991). The Caledonides were eroded in the Early Devonian, when there were arid, continental conditions in the western Barents Sea (Faleide et al., 1984).

After the compressional regime of the Caledonian orogeny, two extensional phases oc- cured, in Late Devonian to Middle Carboniferous and in Permian to Early Triassic times.

This first phase led to the formation of interconnected basins, separated by highs. (Lippard and Roberts, 1987, Gabrielsen, 1990, Dengo and Røssland, 1992, Jensen and Sørensen, 1992, Nøttvedt et al., 1990). Basins formed at this time include the Tromsø, Bjørnøya, Nordkapp, Fingerdjupet, Maud and Ottar basins, with north-east to north trends (Dengo and Røssland, 1992, Jensen and Sørensen, 1992, Bugge et al., 1995, Breivik et al., 1995).

Also the Hammerfest Basin is believed to have been formed during this phase (Dengo and Røssland, 1992, Jensen and Sørensen, 1992). The second phase a↵ected mostly the western areas (Gudlaugsson et al., 1998). In the eastern parts, faulting stopped at the end of the Carboniferous, and a Late Carboniferous-Permian platform succession filled in the structural relief (Stemmerik and Worsley, 1989, Gabrielsen, 1990, Dengo and Røssland, 1992, Stemmerik and Larssen, 1992, Bruce and Toomey, 1993, Cecchi, 1993, Nilsen et al., 1993, Stemmerik et al., 1995). Block-faulting, uplift and erosion a↵ected the western parts during the Permian to Early Triassic (Berglund et al., 1986, Riis et al., 1986, Stemmerik and Worsley, 1995, Gabrielsen, 1990, Johansen et al., 1994).

The Triassic up to Early Jurassic was a relatively quiet time. Significant rifting of the area started again in the Middle Jurassic. This lasted until the uppermost Upper Jurassic (Faleide et al., 1984). At the transition from Jurassic to Cretaceous, another extensional phase was initiated. Along zones of weakness that existed in the Caledonian basement, large normal faults were created. This includes the tilted fault blocks in the Ringvassoy- Loppa Fault Complex (Faleide et al., 1984). After this rifting event, subsidence in the Barents Sea and Norwegian Sea increased, with a maximum during Aptian-Albian times.

The whole basin subsided, with the exception of the Svalbard Platform, which was uplifted and eroded. In the western Barents Sea, the main structural elements formed due to di↵erential subsidence (Faleide et al., 1984). West of the Ringvassoy-Loppa Fault Complex, the subsidence was significantly faster than on the east side of the fault complex. In the

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Figure 3: Schematic diagram of the lithostratigraphy in the western Barents Sea and main tectonic events. Modified from Glørstad-Clark et al., 2010

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2.1 Regional geology 2 GEOLOGICAL FRAMEWORK

the Laramide phase close to the Cretaceous-Tertiary transition (Faleide et al., 1984). In the Paleocene and Eocene, there was a lot of magmatic activity in the western part of the Barents Shelf, related to the break-up of the North Atlantic and subsequent sea floor spreading in the Norwegian-Greenland Sea. Erosion and uplift of most of the Barents Sea followed in the Neogene (Berglund et al., 1986), most importantly during the Late Pliocene and Pleistocene, due to glaciation (Eidvin and Riis, 1989).

2.1.1 Stratigraphy

The area moved from the equatorial zone, in the middle Devonian, to the Arctic where it is today. This resulted in major climatic changes throughout time, which is reflected in the sedimentation (Worsley, 2008). The sedimentation was controlled by several tectonic events along the margins of the Barents Shelf. In the late Paleozoic, there was compressive Uralide development to the east and proto-Atlantic rifting to the west. This was followed by the opening of the polar Euramerican Basin to the north in the Mesozoic, and transpression and transtension during the opening of the Norwegian-Greenland Sea to the west, in the Cenozoic (Worsley, 2008).

During the late Devonian to mid-Permian, a large carbonate platform developed, stretch- ing westwards up to Alaska. In the large basins that developed during the Late Devonian- Middle Carboniferous rifting event, several kilometers of evaporites were deposited. The platform areas between the basins were the sites of thinner sabkha deposits and warm-water carbonates (Worsley, 2008).

In the mid-Permian, the evaporitic deposition ceased, and shifted to cool-water carbonates and later to clastic deposition. Throughout the Triassic, there were major deltaic progradations, followed by a stabilization of the area in the late Triassic to mid-Jurassic. As the Middle Jurassic extensional phase formed the present platform and basin pattern, the sedimentation rates decreased. During the rest of the Jurassic and Cretaceous, the area was dominated by fine clastic deposition, while moving northwards through temperate latitudes (Worsley, 2008).

The opening of the Norwegian-Greenland Sea in the Paleogene formed a new margin to the western shelf. In the Neogene, thick clastic wedges were shed from these margins, due to depression and uplift of the hinterland caused by glaciation and deglaciation phases on

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2.2 Geological setting

This study is focused on the southern part of the Ringvassøy-Loppa Fault Complex. To be able to discuss and understand the geological history of this area, it is important to have knowledge of the surrounding structural elements. The major structural elements in the southwestern Barents Sea are the Nordkapp Basin, Hammerfest Basin, Bjørnøy Basin, Loppa High, Tromsø Basin and the Senja Ridge (Faleide et al., 1984).

2.2.1 Hammerfest Basin

The ENE-WSW striking Hammerfest Basin is bordered in the east by the Bjarmeland Platform and in the west by the Tromsø Basin. The Ringvassøy-Loppa Fault Complex is the transition from the Hammerfest to the Tromsø Basin. It is a relatively shallow basin which can be subdivided in a western and eastern subbasin. These subbasins are separated by the extension of the Trolfjord-Komagelv fault trend (Gabrielsen, 1990). The western part of the Hammerfest Basin dips towards the Tromsø Basin in the west. The depth of the basement in the Hammerfest Basin is 6-7 km (Roufosse, 1987).

There is an internal fault system with E-W, ENE-WSW and WNW-ESE trending faults.

Gabrielsen (1984), named this the Hammerfest Basin fault system. Next to this, there are also deep, high-angle faults along the margins and listric normal faults which are detached above the Permian sequence. These listric faults are located in the more central part of the basin. The deformation in the basin has been dominated by extension (Gabrielsen, 1990).

The Hammerfest Basin separated from the Finnmark Platform in the south in the Middle Carboniferous. The boundaries of the basin as they are now formed from the Mid Jurassic (Gabrielsen, 1990). The main subsidence of the basin happened during the Early Cretaceous.

2.2.2 Tromsø Basin

The NNE-SSW trending Tromsø Basin is bordered in the west by the Senja Ridge. In the east, the Ringvassøy-Loppa Fault Complex separates it from the Hammerfest Basin. In the southeast, it has a border with the Troms-Finnmark Fault Complex and in the north the Veslemøy High separates it from the Bjørnøya Basin. The depth is calculated to be 10-13 km (Roufosse, 1987).

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2.2 Geological setting 2 GEOLOGICAL FRAMEWORK

The Tromsø Basin contains a series of salt diapirs, associated with a system of detached faults. Faulting along the eastern margin of the basin started in the Middle Jurassic and separated the basin from the Hammerfest Basin in the Early Cretaceous. In the north, the Tromsø could have been a separate basin in the Paleozoic, but was united with the Bjørnøya Basin later on. They were not separated again until the Late Cretaceous (Gabrielsen, 1990).

2.2.3 Ringvassøy-Loppa Fault Complex

The northern part of the complex is the western boundary of the Loppa High. The southern part comprises the transition between the Hammerfest Basin and the Tromsø Basin, before merging with the Troms-Finnmark Fault Complex in the south. The Ringvassøy-Loppa Fault Complex is characterised by major faults in the west, across which the Middle Jurassic reflector drops from 2,5 to more than 5 seconds TWT. The faults in the east of the complex cut backwards into the Hammerfest Basin and have a strongly concave shape (Gabrielsen, 1990).

The southern part of this fault complex is dominated by normal faulting (Øvrebø and Talleraas, 1976, 1977, Gabrielsen, 1984, Faleide et al., 1984 Berglund et al., 1986). The geometry of the complex is interpeted as two levels of detached listric normal faults and a possible deeper zone of weakness (Gabrielsen, 1984).

The main subsidence along the southern part of the complex was from Mid Jurassic time to Early Cretaceous. This was due to large-scale extensional rifting (Talleraas, 1979).

It was reactivated during the Late Cretaceous and also Cenozoic strata have been a↵ected by faulting.

2.2.4 Loppa High

The Loppa High consists of a platform in the east and a crestal margin in the west and northwest. It is located north of the Hammerfest Basin and southeast of the Bjørnøya Basin.

In the south it is bounded by the Asterias Fault Complex, in the east and southeast by a monocline towards the Hammerfest Basin and the Bjarmeland Platform. The Ringvassoy- Loppa and Bjornoyrenna Fault Complexes bound the Loppa High to the west (Gabrielsen, 1984). A relatively shallow metamorphic Caledonian basement in the western part causes positive gravity and magnetic anomalies.

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Tertiairy tectonism. It was part of a regional cratonic platform from Middle Triassic to Middle Jurassic. In most of the Cretaceous times, the high was an island with deep canyons cutting into the Triassic sequence. In the Palaeogene, it was covered with shales, most of which were eroded during uplift in the Late Tertiary (Gabrielsen, 1984).

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2.2 Geological setting 2 GEOLOGICAL FRAMEWORK

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3 Seismic Interpretation

3.1 Data

The data set for the study comprised a 3D seismic cube of survey ST09M03 and several 2D lines. The 3D seismic data is provided by Statoil and was shot in 2009. It covers the southern part of the Ringvassøy-Loppa Fault Complex, and parts of the Tromsø and Ham- merfest basins (figure 4). The additional 2D lines were selected to aid in the interpretation of the 3D data. They have a deeper range and some of them extend outside the area of the 3D seismic cube in either western or eastern direction.

Figure 4: Location of the seismic cube in the study area. The red circles mark the key wells. The numbered lines indicate the locations of the key profiles.

3.2 Horizons and Seismic/well tie

Six key reflections of regional extent and with good and continuous seismic expression were selected for interpretation in the 3D and 2D lines. These correspond to, from top to bottom, the Top of the Cretaceous, Intra Cretaceous, Middle Jurassic, Middle Triassic, Top of the Permian and Intra Permian sequences. The first five are calibrated to the Top

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3.2 Horizons and Seismic/well tie 3 SEISMIC INTERPRETATION

Nygrunnen Group, Top Kolje Formation Top Stø Formation, Top Klappmyss Formation and Top Ørret Formation, respectively (fig. 5). These reflections were chosen to get the best understanding of the rifting events and the reactivation of faults in the area and to test the hypothesis that there are several levels of detachment in the study area. A major rifting phase occurred in the late Permian, which would have a↵ected the strata between the Top Permian and mid Permian reflections (Glørstad-Clark et al., 2010). The Top Stø reflection was chosen because of the start of the Mesozoic rifting at this time. This also a↵ects the Intra Cretaceous level. Finally, the Top Cretaceous reflection was chosen to see if the area is a↵ected by the opening of the Norwegian-Greenland Sea in the Eocene.

By looking at the interpreted reflections throughout the area, the e↵ects of these di↵erent rifting events can be investigated.

The reflections are correlated to the tops of the previously mentioned formations and groups by the use of well data (table 1). Several wells are available in the study area (NPD Factpages) and four of these were selected for tying to the seismic data. The selected wells are located both in the Hammerfest Basin and in the transition zone to the Tromsø Basin (fig.4, table 2).

Wells 7119/12-1 and 7119/12-3 are located in the transition zone between the Ham- merfest and Tromsø basins and both penetrate the Early Jurassic sequence. Well 7120/9-2 is located in the Hammerfest Basin and is the only well penetrating the Permian. It is therefore an important well for calibrating the seismic data at the Permian level. Figure 5 shows how the reflections of the seismic data are correlated to the formation tops of wells 7120/9-2 and 7120/7-1.

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Figure 5: Formation tops of wells 7120/9-2 and 7120/7-1 tied to the seismic data (modified from Glørstad-Clark et al., 2010)

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3.2 Horizons and Seismic/well tie 3 SEISMIC INTERPRETATION

Table 1: Formation tops of the selected wells

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Table2:Generalinformationoftheselectedwells,fromNPDFactpages Well7119/12-17119/12-37120/9-27120/7-1 NSUTM(m)7889443.987904727.317932809.507912388.54 EWUTM(m)456424.42454909.86489425.03471011.63 UTMzone34343434 Drilledinlicense060060078077 DrillingoperatorDenNorskeStatsOljeselskapDenNorskeStatsOljeselskapNorskHydroProduksjonDenNorskeStatsOljeselskap Drillingdays11911618670 Entrydate14.06.198020.05.198318.04.198431.07.1982 Completiondate10.10.198012.09.198320.10.198408.10.1982 TypeExplorationExplorationExplorationExploration StatusP&AP&AP&AP&A DiscoveryOilshowsGas/CondensateGasGas KB(m)25292325 Waterdepth(m)200211293233.5 TD(MD)[mRKB]3088331450722839 OldestpenetratedlevelEarlyJurassicEarlyJurassicLatePermianLateTriassic OldestformationStFm.NordmelaFm.RyeFm.Tub˚aenFm.

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3.3 Interpretation procedure 3 SEISMIC INTERPRETATION

Figure 6: 2D line compared to the 3D data of the same area.

3.3 Interpretation procedure

First, the chosen reflections were interpreted in the 3D data with the use of Petrel software (www.slb.com). This was done by interpreting the reflections in the cross-lines and in-lines at fixed intervals. The spacing between the lines was smaller in the cross-lines, since they are oriented perpendicular to the Ringvassøy-Loppa Fault Complex. After that, autotracking was done, to fill in the spaces between the interpreted lines and get time-structure maps for the horizons. For the Intra Permian horizon, the interpretation was done with the aid of the 2D-lines, because of poor resolution of the seismic data at that depth. Next, a structural interpretation was done, by interpreting and mapping of the faults in the area. This was done in both the cross- and in-lines and the created time-structure maps to characterize the lateral geometry of the structures. The 2D lines were particularly used to focus more on the structures in the deeper parts of the section, since the 2D data extends further down than the 3D data (fig.6). The structures at Permian level were interpreted in the 2D lines and later integrated in the 3D interpretation to get a full overview. Three key profiles were chosen to be described, which give a good overview of the data.

Time-structure maps made in Petrel, and fault maps made manually, were then used to analyze the fault patterns at the di↵erent levels and also to identify possible detachments levels in the area.

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3.4 Key Profile 1

In the description of the key profiles and fault maps, a nomenclature is used in which faults in the Hammerfest Basin are denoted with a small ’f’ and the faults in the Ringvassøy- Loppa Fault Complex and Tromsø Basin with a capital ’F’. The faults related to the Troms-Finnmark Fault Complex in the south of the study area are also denoted with a capital ’F’.

Figure 4 shows the location of key profile 1. The line is located in the north of the study area and strikes NE-SW. It shows the Hammerfest Basin and the transition zone towards the Tromsø Basin, as well as the eastern part of the Tromsø Basin. The six reflections mentioned above (see fig. 5) were interpreted in the Hammerfest Basin and throughout the Ringvassøy-Loppa Fault Complex (fig. 7). In the Tromsø Basin, only the Top Cretaceous reflection could be traced, due to bad resolution of the data. The reflection defining the top of the Cretaceous is mostly una↵ected by the major faults in the line and dips slightly to the SW. The Intra Cretaceous and Top Stø Formation reflections are displaced by the normal faults with top-to-SW displacement in the Ringvassy-Loppa Fault Complex. They dips stronger towards the SW once in the deeper Tromsø Basin. The Intra Cretaceous reflection dips towards the SW in between the faults on the Ringvassøy-Loppa Fault Complex. In contrast, the Top Stø Formation reflection dips towards the NE between the faults, creating a wedge shape between the two reflections. This indicates activity along these faults in this period (Late Jurassic - Early Cretaceous). The Top Klappmyss Formation and Top Ørret Formation reflections are cut by the main fault mf1 in the Hammerfest Basin and by two faults in the western boundary of the Ringvassøy-Loppa Fault Complex. The deepest horizon, the Intra Permian reflection, is not a↵ected by the faults in the Ringvassøy-Loppa Fault Complex, but it is displaced by mf1 in the Hammerfest Basin.

The main fault in the Hammerfest Basin is mf1. It has a listric geometry and is concave downward towards the southwest. It consists of two segments. Mf1-a penetrates down to the Jurassic level. Mf1-b cuts through the Jurassic strata down to at least the mid Permian. Along mf1-b, fault drag is visible, most clearly for the Intra Permian reflection.

In the Jurassic succession, a smaller fault f1 branches out of mf1-a, which a↵ects the Base to Intra Cretaceous sequence. For mf1-a , there is a varying amount of vertical o↵set with depth: the older strata have a larger o↵set than the younger. This is evidence of at least two reactivations. One in the Cretaceous and one in the Cenozoic. The interval between

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3.4 Key Profile 1 3 SEISMIC INTERPRETATION

Figure 7: Interpretation of Key Profile 1. For location, see figure 4

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the Intra Permian reflection and the Top Ørret Formation reflection is thicker west of fault mf1-b than east of it. This shows that there has been activity along this fault in this period.

To the SW, in the Ringvassøy-Loppa Fault Complex, the Top Klappmyss Formation and Top Ørret Formation reflections are relatively undisturbed by faulting, whereas both the Intra Cretaceous and Top Stø Formation reflections are cut by multiple planar normal faults. These faults have a top-to-SE extensional displacement and are synthetic to mf1.

They demonstrate a fault system with a staircase geometry which gradually deepens towards the centre of the Tromsø Basin. Fault F9, is antithetic and cut o↵ by F8. All faults have a larger vertical o↵set in the older strata than in the younger, proving that there has been reactivation of the faults.The first activity along faults F8 and F9, which a↵ect also the top Klappmyss Formation and Top Ørret Formation reflections, was at the latest in the late Permian. The first activity along the rest of the faults of the Ringvassøy-Loppa Fault Complex was at the latest in the Jurassic. Reactivation for all faults, except F8, was in the Cretaceous, and faults F4 to F7 were also reactivated in the Cenozoic. NE of mf1, several planar normal faults, antithetic to mf1, show a staircase geometry, with a dip-slip component to the NE. Four normal faults, F10 to F13, a↵ect only the Intra Permian reflection and deeper. They have a top-to-SW displacement. F11 has a listric geometry and is concave downward towards the southwest. F10, F12 and F13 are planar faults. F12 has a low dip-angle, and is significantly less steep than the other faults in the area.

3.5 Key Profile 2

Key profile 2 is located in the central part of the study area and has a NW-SE orientation (fig. 4). It displays the Ringvassøy-Loppa Fault Complex, the southern part of the Hammerfest Basin and the Troms-Finnmark Fault Complex in the easternmost part of the line.

In this line, the six interpreted reflections are una↵ected by faults in the Hammerfest Basin (fig. 8). The Top Cretaceous reflection dips gradually towards the Tromsø Basin. The Intra Cretaceous reflection also dips towards the Tromsø Basin, but is displaced by several normal faults with dip-slip towards the NW in the Ringvassøy-Loppa Fault Complex. These faults also a↵ect the Top Stø Formation reflection, but between the faults, the reflection dips towards the SE. This creates a wedge-shape between the Intra Cretaceous and Top Stø Formation reflections, which indicates activity of these faults between the Late Jurassic and

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Early Cretaceous. The Top Klappmyss Formation reflection is not a↵ected by the normal faults in the Ringvassøy-Loppa Fault Complex as much as the shallower horizon, but at least two faults do displace the reflection. The Top Ørret Formatio reflection is horizontal in the eastern part of the section, but dips towards the SE in the western part. Of the shallower faults of the Ringvassøy-Loppa Fault Complex, only the westernmost fault a↵ects the reflection. Two deeper faults also displace the reflection. The Intra Permian reflection dips towards the NW in the eastern part and is displaced by two normal faults. In the western part of the fault complex, the reflection dips towards the SE. In the Tromsø Basin, the reflections older than the Cenozoic dip steeply towards the NW.

The main fault in the profile is MF1, the easternmost fault of the Ringvassøy-Loppa Fault Complex. It is a listric normal fault, concave downwards towards the NW. It cuts down to the Permian strata and is divided into two segments. MF1-a goes down to the Triassic, before it flattens out into the Triassic strata. MF1-b takes over and cuts down to the Permian strata, where it flattens out. There is fault drag of the Intra Permian reflection against mf1-b. The vertical o↵set of the fault is variable. It is largest on the Top Stø Formation reflection, and gets less in the younger reflections. This means that it has been reactivated at least twice, in the Cretaceous and in the Cenozoic. The interval between the Intra Permian and Top Ørret Formation reflection is significantly thicker west of mf1-b than east of the fault. That shows that there has been activity along this fault in this period.

Three planar normal faults, synthetic to MF1, with a dip-slip component to the NW, F1, F3 and F5, are present in the Ringvassøy-Loppa Fault Complex. They a↵ect the Cenozoic strata and cut down to the Jurassic strata. Fault drag of the Intra Cretaceous reflection can be seen along F1. F5 also probably a↵ects the top Klappmyss Formation reflection.

Activity along these faults has started in the Jurassic at the latest. F2 and F4 are planar normal faults which are antithetic to MF1. They have a top-to-SE displacement. They only a↵ects the Top Stø Formation reflection. F4 is a branch of F5.

Normal fault F6 has a top-to-NW displacement and only a↵ects the Intra Permian reflection and deeper. The Intra Permian reflection shows normal drag with this fault.

In the east of the profile, a dome-like feature is present at the approximate position of the Troms-Finnmark Fault Complex. Its e↵ect decreases upwards and it is no longer visible at the Cenozoic level.

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3.6 Key Profile 3

Key Profile 3 is parallel to Profile 1, oriented NE-SW, as can be seen in figure 4. This line shows the western part of the Hammerfest Basin, the Ringvassøy-Loppa Fault Complex and the easternmost part of the Tromsø Basin.

The Top Cretaceous reflection gradually dips towards the Tromsø Basin (fig. 9). It is cut by six normal faults, but there is only minimal o↵set. The Intra Cretaceous reflection is horizontal and is displaced by several normal faults. In between the faults, it dips slightly towards the Tromsø Basin. In the Tromsø Basin the dip becomes steeper. The Top Stø Formation reflection is horizontal in the Hammerfest Basin. It is displaced by several normal faults in the Ringvassøy-Loppa Fault Complex and dips towards the Hammerfest Basin in between these faults. This creates a wedge shape between the Intra Cretaceous and Top Stø Formation reflections. This geometry indicates activity of these faults during this period (Late Jurassic - Early Cretaceous). Both the Top Klappmyss Formation and Top Ørret Formation reflections are displaced by the main fault in the Hammerfest Basin.

They dip slightly towards the Tromsø Basin. In the Ringvassøy-Loppa Fault Complex, both of the reflections are cut by fault F3-a. The Intra Permian reflection is displaced by the main fault in the Hammerfest Basin, as well as by several older faults of mid Permian and older age. The Intra Permian reflection is a horizontal reflection in the eastern part of the section but further to the west it dips towards the Tromsø Basin. West of mf1, the interval between the two oldest reflections is thicker than east of the fault.

The main fault in this profile is mf1, in the Hammerfest Basin. It consists of three segments. Fault mf1-a a↵ects the youngest three interpreted reflections and flattens out in the Triassic strata. Fault mf1-b cuts through the Triassic and top of the Permian before flattening out in the Early Permian strata. The last segment, mf1-c, displaces the Intra Permian reflection. Normal fault f1 is antithetic to the main fault, a↵ecting only the Upper Jurassic and Lower Cretaceous sediments. It ends at mf1-a. There is slight normal drag visible, of the Intra Permian reflection against mf1-c and the Top Ørret Formation reflections against mf1-b.

In the Ringvassøy-Loppa Fault Complex, there are two planar normal faults (F1 and F2-a) that are synthetic to the main fault and have top-to-SW displacement. They a↵ect the strata down to the Late Triassic, and have a larger o↵set in the older sediments than

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3.7 Time-structure maps and fault maps 3 SEISMIC INTERPRETATION

the latest and reactivation happened in both the Cretaceous and Cenozoic. F3-a, F4-a, F4-b and F6 are normal faults with a listric geometry. They a↵ect all reflections down to the early Permian and flatten out there. There is fault drag visible of the Top Stø Formation reflection with fault F6. Since the amount of vertical displacement of the top Ørret Formation and top Klappmyss Formation reflections is not visible, it is not possible to say if activity along these faults started in the Triassic or Jurassic at the latest. The variable o↵set along the fault from the Top Stø Formation reflection and up shows that there was reactivation in the Cretaceous and Cenozoic. F3-b is a branch of F3-a, and only displaces the Intra Cretaceous and Top Stø Formation reflections. The latter is also true for two faults that are antithetic to the main fault, F2-b and F5.

Three listric normal faults (F7, F8 and F9) in the deeper part of the section only a↵ect the Intra Permian reflection and deeper. They have a top-to-SE displacement. There is fault drag of the Intra Permian reflection against faults F7 and F8.

Directly SW of F9, a dome-like feature is visible (red square in fig. 9). It seems to only a↵ect the Cretaceous and younger sediments and the e↵ect decreases upwards. This structure can be interpreted as a roll-over anticline.

3.7 Time-structure maps and fault maps

In this section, the time-structure and fault maps of the six interpreted horizons will be described. This will give a clear view of the fault pattern at the di↵erent levels. Compari- son of these fault patterns can give important information about the possible detachment levels in the study area. The Intra Cretaceous horizon will be discussed first, since the nomenclature of the faults is based on this horizon. The other horizons will be discussed from young to old.

3.7.1 Intra Cretaceous

Figure 10(a) shows the time-structure map of the Intra Cretaceous horizon. In the Ham- merfest Basin area, the horizon stays relatively flat, with a slight dip towards the north.

In the Ringvassøy-Loppa Fault Complex, the reflection is displaced stepwise by the N-S oriented faults in the area. Once in the Tromsø Basin, it drops to an even deeper level.

The main fault in the Hammerfest Basin is mf1 (fig. 10(b)). It is curved and does not

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(a)

(b)

Figure 10: a) Time-structure map of the Intra Cretaceous horizon. b) Fault map of the Intra25

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dies out towards the south. Multiple linear N-S oriented smaller faults are present to the south of mf1. The Ringvassøy-Loppa Fault Complex is characterized by large N-S oriented normal faults. It is bordered in the east by the slightly curved main fault MF1. It consists of two segments, MF1 and MF1-a, of which MF1 is the largest. The displacement along this fault is largest in the north. The western border of the Ringvassøy-Loppa Fault Complex is the linear fault F3. It consists of three segments (F3-a, F3 and F3-b) and a smaller fault that branches o↵ F3. In between the borders of the fault complex, there are two more faults, F2 and F4. F2 is a curved fault that consists of two segments and the northern part of segment F2 has the most o↵set in the area. F4 is a small linear fault in the north of the area, which dies out towards the south. In the northern part of the study area, several small, W-E oriented faults are present in both the Ringvassøy-Loppa Fault Complex and the Hammerfest Basin.

3.7.2 Top of the Cretaceous sequence

The time-structure map of the Top Cretaceous (fig. 11(a)) shows that the horizon is relatively una↵ected by faulting. In the Hammerfest Basin, it gradually deepens towards the NW. In the south, the Troms-Finnmark Fault Complex cuts o↵ the reflection, which can- not be traced any further south of the fault complex. In the northeast, in the Hammerfest Basin, the curved fault mf1 displaces the reflection (fig. 11(b)). The N-S oriented faults of the Ringvassøy-Loppa Fault Complex that can be seen in the Intra Cretaceous fault map (fig. 10(b)) are not present at this level. Instead, a NE-SW oriented normal fault (MF1) is present. It consists of two segments. MF1-a is curved and has the largest o↵set. Segment MF1-b is more linear and has significantly less o↵set than MF1-a. In the west, a slightly curved normal fault with NNE-SSW orientation (F2) forms the western boundary of the fault complex. Multiple faults with little o↵set are present in the Hammerfest Basin. The slightly curved fault f1 is E-W oriented. North of f1, the two linear faults f2 and f3 are N-S and NW-SE oriented, respectively. There are three faults in the southern part of the fault map. The two curved faults f4 and f5 are SW-NE and NW-SE oriented, respectively. East of these, f6 is N-S oriented and has an irregular shape.

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3.7.3 Top of the Stø Formation

Figure 12(a) shows the time-structure map of the Top Stø Formation horizon. In the Hammerfest Basin, the horizon is shallowest in the central part and in the northwest.

It shows deepening towards the SW and the NE. Also in the northernmost part of the basin it is deeper. In the south, around the Troms-Finnmark Fault Complex, an array of linear WSW-ENE oriented faults displaced the horizon, moving it up. The faults in the Ringvassøy-Loppa Fault Complex stepwise deepen the horizon and in the Tromsø Basin it drops further down. In the Hammerfest Basin, the fault with the most o↵set is mf1 (fig. 12(b)). It is strongly curved. In the north, it cuts o↵three slightly curved faults with WSW-ENE orientation. In the southeastern corner of the study area, three linear faults radiate out of a shallower part directly north of the Troms-Finnmark Fault Complex. MF1 marks the eastern boundary of the Ringvassøy-Loppa Fault Complex. The o↵set along this fault is largest in the northern part of the area and decreases towards the south. It is mostly linear, but curves towards the west in the north. MF1-a is a branch of MF1, east of the fault, in the northern part of the area. West of MF1, to faults branch out towards the NW. More to the west, multiple normal faults form a staircase towards the Tromsø Basin. F2 consists of two segments, both of which are curved. The vertical displacement along the fault is larger in the south than in the north. F3 is the western boundary of the Ringvassøy-Loppa Fault Complex. It is curved, and has one branch (F3-a) towards the north. F3-a is linear, but its northern part bends to the east. Throughout the study area, multiple smaller W-E oriented faults are present. Fault f2 marks the northern boundary of the Troms-Finnmark Fault Complex.

3.7.4 Top of the Klappmyss Formation

As with the other stratigraphic levels, the top of the Klappmyss Formation is relatively flat in the Hammerfest Basin area (fig. 13(a)). In the Hammerfest Basin, It is shallowest in the east and in the center and deepens towards the northwest and the southwest.

In the north of the study area, a large, curved, N-S oriented fault (mf1) cuts through the reflection (fig. 13(b)). It has the most vertical displacement in the center of the fault.

The displacement lessens towards the north, and towards the south the fault gradually dies out. It reaches farther to the south than in the shallower interpreted reflections.

In the Ringvassøy-Loppa Fault Complex, NNW-SSE oriented normal faults displaced the

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reflection, deepening it towards the Tromsø Basin. The eastern boundary of the fault complex is the curved fault MF1. The o↵set along this fault is largest in the north and decreases towards the south. Two smaller faults (F2 and F3) run parallel to MF1. In the south of the study area, a WNW-ENE oriented fault of the Troms-Finnmark Fault Complex cuts through the reflection. The top of the Klappmyss Formation reflection can not be traced south of the Troms-Finnmark Fault Complex and in the Tromsø Basin.

3.7.5 Top of the Ørret Formation

Figure 14(a) shows the time-structure map of the top of the Ørret Formation The shallowest part of this reflection in the Hammerfest Basin area is in the southeast and northeast. It deepens towards the west. In the south, the reflection is displaced by the Troms-Finnmark Fault Complex. South of the fault complex the reflection can no longer be traced. Multiple NNE-SSW oriented normal faults stepwise deepen the reflection in the transition between the Hammerfest Basin and the Tromsø Basin.

The fault map of this reflection (fig.14(b)) shows two faults in the Hammerfest Basin area. The main fault mf1 in the northern part and f1 in the south. Mf1 is N-S oriented and it is linear in its southern part, but bends towards the west in the northern part.

It is more linear, and goes farther to the south than in the fault maps of the shallower interpreted reflections. The fault more to the south, f1, is also N-S oriented and mostly linear, except for a curve in the middle of the fault. The vertical displacement along both faults is uniform.

The Ringvassøy-Loppa Fault Complex consists of several NNE-SSW oriented normal faults. MF1 is the main fault in the fault complex and consists of three segments. All segments are curved. The southernmost segment, MF1, has the largest vertical displacement and MF1-b in the north the least.

3.7.6 Intra Permian

The intra Permian reflection is shallowest in the northeast and southeast of the study area (fig.15(a)). It gradually deepens towards the west. The southern boundary of the area is the Troms-Finnmark Fault Complex. In the Ringvassøy-Loppa Fault Complex, NNE- SSW oriented normal faults displace the reflection and form a staircase geometry from the Hammerfest Basin to the deeper Tromsø Basin. The reflection can not be traced south of

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the Troms-Finnmark Fault Complex.

Figure 15(b) shows the fault map of the intra Permian reflection. In the Hammerfest Basin, two N-S oriented normal faults (mf1 and f1) can be seen, and a NW-SE oriented fault (f2) in the middle of the area. Mf1 is more or less linear, with a slight curve in the southern part. It is more linear and reaches farther to the south of the area than in the shallower interpreted reflections. Both f1 and f2 are linear. Multiple normal faults and their branches form the Ringvassøy-Loppa Fault Complex. The eastern boundary of the fault complex is marked by normal fault MF1.

3.8 Comparison of the fault maps

In this section, the fault maps of four of the interpreted horizons will be compared. Com- paring the fault maps will show the di↵erence in fault pattern at the di↵erent levels which could indicate the presence of one or multiple detachments in the area.

In the previous section where the fault maps have been described, the following main di↵erences and similarities can be seen. The only fault that can be seen at all levels in mf1 in the Hammerfest Basin. It changes orientation with depth and varies between NW-SE and N-S orientation. Also the location changes slightly with depth. The Ringvassøy-Loppa Fault Comlex is characterized by N-S oriented segmented normal faults, at all levels except for the youngest, the top of the Cretaceous. In this fault map, the faults are NE-SW oriented. Small E-W oriented faults in the Hammerfest Basin are present only in the younger horizons, and can not be seen in the Triassic and Permian maps.

Figure 16 displays the four fault maps positioned on top of each other to compare the location and amount of faults.

The main di↵erences between the four fault maps are in the Hammerfest Basin and in the western part of the Ringvassø-Loppa Fault Complex. To get the best overview of the di↵erences between the horizons, the fault maps are first compared with just the layer above or below it.

Figure 17(a) displays the fault maps of the Top Cretaceous (TC) and the Intra Creta- ceous (IC) superimposed.

Especially in the Hammerfest Basin, the fault pattern between these two stratigraphic levels is very di↵erent. The IC faults in the basin are mostly N-S oriented, whereas the TC faults have a W-E orientation. The main fault in the basin, mf1, is present in both

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3.8 Comparison of the fault maps 3 SEISMIC INTERPRETATION

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Figure 16: The fault maps of the four horizons. The figure in the middle shows the di↵erent fault maps together

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3.8 Comparison of the fault maps 3 SEISMIC INTERPRETATION

(a) (b)

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Figure 17: Fault maps of: a) the Top Cretaceous (yellow) and the Intra Cretaceous (green), b)the Intra Cretaceous and Base Cretaceous (pink) and c) the Base Cretaceous and Top Permian (purple) horizons together

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the fault maps, although its location is more to the northeast in the Top Cretaceous map.

In the Ringvassøy-Loppa Fault Complex, a fault in the south coincides with the southern part of MF1. F3-a is present in both fault maps, as well as F3, although the latter lies more to the west in the TC map. W-E oriented faults in the TC cross the IC faults in the eastern part of the Ringvassøy-Loppa Fault Complex.

In figure 17(b), the fault maps of the Intra Cretaceous and Base Cretaceous (BC) horizons are shown together. Most of the larger faults in the area coincide in the two maps.

The main di↵erences are found in the Hammerfest Basin, where mf1 turns more towards the west in the BC map than in the IC. It also continues further to the south in the BC map. In the BC horizon, in the southern part of the Hammerfest Basin, four faults radiate out of the Troms-Finnmark Fault Complex area in NE and NW directions. These faults are not seen on the IC fault map, where the faults in the basin have a N-S orientation. In the Ringvassøy-Loppa Fault Complex, there are more similarities between the two horizons.

Although the segmentation is shows some di↵erences, the main fault MF1 is present in both maps. The same is true for F2. The southern part of F3 is similar in both maps, but in the BC map it does not splay in the northern part, and a branch of the BC fault goes more to the east. The eastern part of the main fault of the Troms-Finnmark Fault Complex is located more to the south in the BC fault map, and in the western part it crosses the IC fault twice.

An important comparison is that of the Base Cretaceous and the Top Permian (TP) (fig. 17(c)). Di↵erences in fault pattern between these two maps can give information about the possible presence of a detachment between the two levels. A distinct di↵erence is that mf1 in the Hammerfest Basin is not present in the TP fault map. The main fault in the basin in the TP is N-S oriented an not present in the BC map. Also the smaller radiating faults in the south of the basin are not present in the TP. In the Ringvassøy- Loppa Fault Complex MF1 is present in both horizons. However, in the TP map it does not continue as far to the south and it deviates from the BC fault in the northern part, with less segmentation. F2 is not present in the Top Permian. There is a fault in the Top Permian horizons which partly coincides with F3 in the BC map, but as the BC fault continues northwards, the fault in the TP turns towards the northwest.

Going back to the comparison of all horizons together (fig. 16), it is clear that the main deviations are the Top Cretaceous and Top Permian horizons. The general fault pattern

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3.9 Time-thickness maps 3 SEISMIC INTERPRETATION

Figure 18: Time-thickness map of the interval between the top Klappmyss Formation reflection and top Ørret Formation reflection

of the Top Cretaceous is more W-E oriented, compared to the mainly N-S orientation of the faults at the other levels. The fault-orientation of the Top Permian faults is generally the same as for the other horizons, but one of the main Permian faults is not present in the other levels, and in general there seems to be less faulting in this horizon.

3.9 Time-thickness maps

Time-thickness maps were created of intervals between the three deepest reflections. Dif- ference in thickness between two levels can mean that there has been tectonic activity between these two times, creating more or less accommodation space for sediments.

Figure 18 shows the time-thickness map of the interval between the top of the Klappmyss Formation and the top of the Ørret Formation Several areas stand out in being thicker than average. The northwestern part of the Hammerfest Basin area, west of the main fault in the basin, is thicker than the rest of the basin. This can also be seen in key profile 1 (fig. 9).

This is evidence of activity along the main fault in the Hammerfest Basin between the late Permian and the early Triassic. Also in the north and middle of the Ringvassøy-Loppa Fault Complex, the interval is thicker.

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Figure 19: Time-thickness map of the interval between the top Ørret Formation reflection and Intra Permian reflection

of the Ørret Formation and the Intra Permian reflection (fig.19). The area west of the main fault in the Hammerfest Basin is thicker than the rest of the basin, showing activity along this fault in the late Permian. In the Ringvassøy-Loppa Fault Complex, the interval becomes thicker towards the west in the Ringvassøy-Loppa Fault Complex.

Hence, the time-thickness maps shows activity along the main faults in the Hammerfest Basin and the Ringvassøy-Loppa Fault Complex both in the late Permian and early Triassic.

3.10 Summary

From the seismic interpretation, it can be concluded that well defined as well as di↵use detachments are present in the area. This can be concluded by the di↵erent structural styles between the Permian and Jurassic levels and between the Cretaceous and Cenozoic levels.

Also the fault geometry in the cross-sections shows possible detachment levels. Comparing the fault maps shows the possibility of detachments in the Early Cretaceous and in the Triassic. The cross-sections also show evidence of a possible well defined detachment in the Early Permian. To better understand the mechanisms behind these detachments, analogue modeling is done.

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3.10 Summary 3 SEISMIC INTERPRETATION

Evidence of several extensional phases can be seen, both in the form of reactivation of faults and in thickening of intervals. These phases, and how they have been determined, are:

- Late Carboniferous: growth faulting along fault mf1, below the Intra Permian reflection.

-Late Permian - Early Triassic: growth faulting along fault mf1, between the Intra Permian reflection and the Top Ørret Formation reflection.

-Late Middle Jurassic - earliest Cretaceous: wedge shape of the interval between the Top Stø Formation reflection and the Intra Cretaceous reflection in the Ringvassøy- Loppa Fault Complex.

- Early Cretaceous: Reactivation of the faults in the Ringvassøy-Loppa Fault Com- plex.

-Cenozoic (Eocene): Reactivation of the faults in the Ringvassøy-Loppa Fault Com- plex.

From analysis of the faults in the key profiles, it can be concluded that activity in the Ringvassøy-Loppa Fault Complex started at the latest in the Triassic. After this, the faults were reactivated at least three times, in the Jurassic, in the Cretaceous and in the Cenozoic.

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4 Analogue experiments

The Ringvassøy-Loppa Fault Complex is a zone with long-lived extensional faults that have been reactivated in several stages. The geometry of the faults in this area has been interpreted as two levels of detached listric normal faults and a possible deeper detachment zone (Gabrielsen, 1984). The seismic interpretation has shown the presence of both di↵use and well defined detachments. Analogue models were used to recreate this geometry and test the viability of the conclusion of the seismic interpretation. A series of analogue models was run at the Tectonic Laboratory (TecLab) at the VU University in Amsterdam to reproduce and analyze the fault geometry in an extensional fault zone with multiple detachments and several stages of reactivation. Quartz sand and silicon putty were used to simulate layers of brittle and ductile rheology respectively to investigate the e↵ect of strata rheology on the style of faulting. Di↵erent model setups were used, with di↵erent amounts of deformation and one or multiple deformation phases. At the end of every model run, the experiments were covered with a thin layer of sand to stabilize the structures and then saturated by water to facilitate slicing of the model. Finally, cross sections were cut perpendicular to the structures.

Several model setups where utilized in the study. Figure 20 shows the general setup for all models. A 1 mm thick plastic sheet was placed on a table with a thin layer of glass beads beneath it to reduce the friction between the sheet and the table. Two plastic straps were attached to this sheet, to facilitate pulling of the hanging wall block of the experiment. A sand sequence was sieved on the sheet, with di↵erent layer colors to show the deformation in the cross-sections. When the setup contained just sand, the sheet was pulled by hand, one centimeter at a time. When silicon putty was involved, the sheet was attached to a machine that pulled with a constant speed of 1 cm/hr. The deformation was done in either one or two phases, to simulate reactivation of the fault systems. If during deformation a graben formed, this was filled with layers of quartz sand to simulate syn-tectonic sedimentation.

The models are divided into three di↵erent groups with increasing complexity: I (reference): The models in this group were run to act as a reference to compare the other models to. They contained just sand, and no weak layers as possible detachments. In model IB, two di↵erent types of sand were used, to test if a detachment would form due

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4 ANALOGUE EXPERIMENTS

Figure 20: Top view of the general model setup, with D the total displacement of the plastic sheet.

Table 3: Summary of the setups and general geometry of the models

to the di↵erent strengths of the sand. II (one detachment): These models contained a 0,8 cm thick silicon putty layer in the middle of the sequence to simulate a weak layer and zone of possible detachment. In model IIC, glass beads were used to investigate if reducing the friction between two sandlayers would create a detachment zone. III (two detachments): These models contained two weak layers to simulate two possible detachments. The weak layers consisted of one of two types of silicon putty. The stronger type of silicon putty was used to better simulate the situation as it is in the Ringvassoy-Loppa Fault Complex, with the deepest detachment zone consisting of Permian salt and the upper layer of unconsolidated clay.

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Table 4: Properties of the sand and silicon putty used in the experiments Material Grain size (.m) Critical

angle of repose

Internal friction Density (kg m-3) Viscosity (Pa s)

Quartz 300 42 0.9 1510

Feldspar 100

Silicon putty 1 970 5x104

Silicon putty 2 1383 2.045x104

4.1 Model scaling and rheology

Sand and silicon putty were used as brittle material and ductile detachment zones, respectively. The properties of the materials used are listed in table 4. These material have often been used as a analog for brittle and viscous rheologies in analogue modeling studies (e.g.

Leever et al., 2011, Bonini, 2001, Casas et al., 2001, Davy and Cobbold, 1991, Smit et al., 2003). They obey the scaling rules of dynamic similarity (Hubbert, 1937) which states that if a scaled model is to be representative to its counterpart in nature, it has to have similar distributions of stresses, rheologies and densities.

4.2 Model results

In this section, the results of the analogue models are discussed and interpreted. The cardinal directions will be used to describe and compare the models (fig. 21). In the top views, black and red lines represent normal faults and reverse faults respectively. The green lines represent morphological features that are not related to faults. The numbering of the faults is done from west to east, in the top views and in the cross-sections. The faults numbered in the cross-sections are not necessarily the same as the faults in the top views. Table 5 shows the abbreviations used in the figures. The top-view of the model after deformation is discussed first, to define the main structures. Then a cross-section that best represents the structures in the models is interpreted. If another section shows significant di↵erences to this main section, this will be discussed next. Finally, the development of the model is discussed by use of top-view pictures taken during deformation.

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4.2 Model results 4 ANALOGUE EXPERIMENTS

Table 5: List of abbreviations used in the model descriptions Abbreviation Meaning

G Graben

NF Normal Fault

BF Branching Fault

RF Reverse Fault

MF Main Fault

MFl Morphological Flat

Table 6: Syn-graben sediments model IA Model IA

Deformation(cm) Color

1 Black

2 Pink

3 Black

4 Yellow

5 Black

6 Pink

7 Black

8 Yellow

9 Black

4.2.1 Group I: Reference models 4.2.1.1 Model IA

This model is the main reference model, with the simplest setup (table 3). Table 6 shows the order and colors of sand used for the syn-tectonic sedimentation.

Figure 21 shows model IA at the end of deformation. An asymmetrical graben (G1) has formed in the middle of the model, with its deepest part west of the centre of the graben. The graben was bordered by two normal faults, named the main faults (MF1 and MF2). On the east side of the graben, four more normal faults (NF2-5) were visible, two of which were continuous throughout the model (NF4 and NF5). On the west side, a normal fault (NF1) was initiated. There were small landslides on both sides of the graben. On the western slope there was a morphological flat, caused by a more stable part of the sand layer.

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Figure 21: Top view of the first model after deformation

Figure 22: Geometry of model IA. a) Top-view of deformation after 1 cm. b) After 3 cm of deformation. c) After 5 cm of deformation. d) After 7 cm of deformation; the arrow indicates the direction of movement

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Figure 23: Development of model IA. Solid lines are periods of active faulting

Development of Model IA

The structural development can be subdivided into five stages, each stage defined by the initiation and termination of master faults that could be seen at the surface (fig. 23).

Stage I (1-3 cm of extension): The graben (G1 in fig. 2)was formed by the initiation of one master normal fault (MF1 and MF2) on each margin (fig. 22a). These faults matured during the rest of the stage. Another normal fault developed on each margin (NF1 on the western margin and NF1a). MF1 and MF2 developed more o↵set along the entire length of the graben and NF1 and NF1a developed only in the lower part of the model. Small cracks formed on the lower part of the western side (fig. 22b) and a morphological flat developed on the eastern fault escarpment.

Stage II (3-4 cm of extension): The western margin of the graben was more active than the eastern side. On the eastern margin, the MF1 and the lower and upper part of NF1 developed more o↵set. The displacement associated with MF2 and NF1a also continued to increase and a new fault (NF1b) formed from the already present cracks.

Stage III (4-5 cm of extension): NF1b connected to the existing fault (NF1) to constitute a continuous fault strand. A new fault (NF2) formed in the upper western part of the graben. MF2 and NF1a developed a little more o↵set on the lower and upper parts of the model. A gravity slide destroyed the middle part of the morphological flat on the eastern escarpment.

Stage IV (5-6 cm of extension): There was another gravity slide in the middle of the eastern escarpment. On the western side, a new fault (NF2a) formed in the middle and developed outwards and NF2, formed in the previous stage, developed towards the lower

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Figure 24: Uninterpreted (a) and interpreted (b) cross-section B of model IA. See table 3 for the model set-up. The location of the section is shown in fig. 21. The arrow indicates the direction of deformation

Stage V (6-9 cm of extension): During this stage NF2a connected to NF2 to form a continuous fault. A new fault, NF3, was initiated in the middle of the graben. The morphological flat wss again continuous along the length of the slope. NF4 formed in the lower part of the western side of the graben. After developing more o↵set on the main fault of the eastern side, large parts of the eastern slope collapsed. The development of more o↵set on the present fault caused small gravity slides. At the end of the final stage, a last new fault (NF5)was initiated and movement along MF2 on the eastern margin of the graben resulted in multiple small gravity slides.

Cross-sections of model IA

The structures seen in cross-section B of model IA, are described in table 7. On the eastern side of the graben, the deformation was divided over seven faults while on the western side it was focused in one fault zone(MF2). On the western side of the graben, landslides could be seen between the layers of syn-tectonic sedimentation. The branching faults MF1 and

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Table 7: Structures of cross-section B of model IA

Figure 25: Uninterpreted (a) and interpreted (b) cross-section A of model IA. See table 3 for the model set-up. The location of the section is shown in fig. 21. The arrow indicates the direction of deformation

NF4, formed a horse.

Cross-section A was made in the most southern part of the model (fig. 25). The structures are described in table 8. A branch of MF1 (BF1) formed a horse. NF6 splayed into three faults near the bottom of the model.

4.2.1.2 Model IB

The setup of this model can be seen in table 3. Two types of sand with di↵erent strength were used, to create a rheological contrast. Table 9 shows the order and colors of sand used

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Table 8: Structures of cross-section A of model IA

Table 9: Colors of syn-rift sedimentation during deformation of model IB 1st stage 2nd stage

Deformation (cm)

Color Deformation (cm)

Color

1 Black 1 Pink

2 Pink 2 Blue

3 White 3 Pink

4 Yellow 4 Blue

5 Blue 5 White

6 Pink 6 Blue

Figure 26: Topview of model IB after two stages of deformation

Reactivation in the Ringvassøy-Loppa Fault Complex - the role of detachments

Reactivation of the

Ringvassøy-Loppa Fault Complex, SW Barents Sea –

the role of detachments

Heleen Zalmstra

Loppa Fault Complex, SW Barents Sea – the role of detachments

Heleen Zalmstra

Master Thesis in Geosciences Discipline: Structural Geology

Department of Geosciences

Faculty of Mathematics and Natural Sciences

University of Oslo

June 2013

Contents

1 Introduction

2 Geological framework

2.1 Regional geology

2.2 Geological setting

3 Seismic Interpretation

3.1 Data

3.2 Horizons and Seismic/well tie

3.3 Interpretation procedure

3.4 Key Profile 1

3.5 Key Profile 2

3.6 Key Profile 3

3.7 Time-structure maps and fault maps

3.8 Comparison of the fault maps

3.9 Time-thickness maps

3.10 Summary

4 Analogue experiments

4.1 Model scaling and rheology

4.2 Model results