This document provides a summary of a student report on the multiphase evolution of the Ling Depression offshore Southern Norway. It was influenced by the underlying Hardangerfjord Shear Zone, a major basement structure. The study utilized seismic and borehole data to map the shear zone and overlying fault network. It found two styles of rifting along the shear zone, with faults in different areas undergoing independent rifting episodes in the Upper Triassic-Middle Upper Jurassic and Middle-Upper Jurassic-Lower Cretaceous. This indicates the Ling Depression did not conform to regional rift timing models. Characteristics like fault initiation style, shear zone steepness, and basement lithology determined the type of rift formed

1. DEPARTMENT OF EARTH SCIENCE & ENGINEERING, ROYAL SCHOOL OF MINES, IMPERIAL
COLLEGE LONDON
Multiphase evolution of the
Ling Depression: influence of a
large-scale heterogeneity on
rift evolution
Christopher Thomas
MSci Geology Undergraduate
12th January 2016
Supervisors:
Dr. Rebecca E. Bell, Thomas B. Phillips and Prof Christopher A.-L. Jackson
This report is submittedas part requirement for the MSci Degree in Geologyat Imperial College London and Associateship
of the Royal School ofMines. It is substantiallythe result of myownwork except where explicitlyindicatedinthe text. The
report may be freely copied and distributed provided the source is explicitly acknowledged.

2. Abstract
Basement shear zones are major controls on rift geometry worldwide; however they are
currently poorly understood due to a lack of high quality deep seismic data as the basement
structures are deeply buried. This study utilises 2D seismic reflection data and borehole data
from the Hardangerfjord Shear Zone and the overlying Ling Depression, located offshore
Southern Norway. The extent of the shear zone basement expression as well as the overlying
fault array is mapped out. Here I show that there are two styles of rifting present aligned NW-
SE along the offshore projection of the Hardangerfjord Shear Zone, in two discrete areas. The
two domains underwent independent rifting episodes. Faults that are reactivated directly by
the shear zone have Upper Triassic and Middle-Upper Jurassic rift episodes compared to
faults that are not reactivated directly by the shear zone which have Middle-Upper Jurassic
and Lower Cretaceous rift episodes. This means that Ling Depression does not conform to the
published regional models for the timing of extensional events in the northern North Sea. The
major controlling factors determining the style of rift formed in relationship to the shear zone
are suggested to be the style of fault initiation, the steepness of the shear zone and the
lithology of the crystalline basement. This study shows the style, geometry and timing of
rifting is affected by characteristics of the Hardangerfjord Shear Zone, with similar examples
worldwide.
1. Introduction
Pre-existing deep seated basement heterogeneities, such as shear zones, play a major role in
the evolution and geometry of overlying rift systems worldwide (Fossen & Rykkelid 1992;
Morley et al., 2004; Fossen & Hurich 2005; Kirkpatrick et al., 2013). These heterogeneities act
as long lived zones of weakness which can in some cases lead to repeated reactivation of the
basement structure (Holdsworth et al., 1997) creating a control of the location, trend and
structural style of the overlying rifts (Daly et al., 1989).
Notable studied examples of such basement influenced rifts include the: (i) Gamtoos Basin in
South Africa (Paton & Underhill 2004; Paton 2006), (ii) NE Brazilian rift margin (Kirkpatrick et
al., 2013), (iii) West Orkney basin (Bird et al., 2015), (iv) Namibian passive margin (Clemson et
al., 1997) and (v) Gulf of Thailand (Morley et al., 2004). However, some recent studies have
highlighted that some basement structures may not always play a significant role in
determining the geometry of later faults, with examples from Northern Scotland (Roberts &

3. Holdsworth 1999) and offshore western Norway (Reeve et al., 2014) where younger faults
cross-cut_basement_structures.
Figure 1. Major tectonic elements of the northern North Sea. Bold blackline highlights the location of the study area. Offshore
geology modified from (Bell et al. 2014). Offshore structural domains are based upon base Upper Permian surface. Onshore
geology modified from (Fossen 2014). Well data obtained from (http://factpages.npd.no/factpages/). Location of major
Devonian shear zones are shown by bold green lines from (Fossen & Hurich 2005) and (Fossen et al., 2014). Shear zone

4. nomenclature HSZ=Hardangerfjord Shear Zone. KSZ= Karmøy Shear Zone. SSZ= Stravenger Shear Zone. Faults from (Bell., et
al., 2014) and this study.
Assessing the relationship between the basement structures and overlying rifts is often
challenging due to the depth and the thickness of the syn and post-rift sediments (Kirkpatrick
et al., 2013; Bell et al., 2014). The lack of high resolution 3D seismic data at depth leads to
poor quality of basement imaging. Therefore the understanding of this relationship remains
poorly understood worldwide with recent studies restricted to localisedareas. Previous works
highlight that reactivation potential of shear zones is related to at least four properties; (i)
thickness (Kirkpatrick et al., 2013), (ii) steepness, (Sibson 1985) (iii) orientation to regional
stress (Morley et al., 2004; Henza, A et al., 2011) and (iv) previous reactivations of the rift
faults (Henza, A et al., 2011; Phillips et al., in prep). In this study extra criteria, the effect of
the depth and strength of the surrounding country rock (Keep & McClay 1997), the spatial
relationship between the initiation of the faults and the shear zone, and the effect of salt are
discussed.
The study area (Figure 1) is c. 14500 Km2 centred on the Ling Depression bounded by the
Utsira High and Sele High to the north and south respectively, with the Stord Basin and Åsta
Graben to the East. The Ling Depression offers an ideal opportunity to further study the
relationships between basement shear zones and overlying rift sediments, situated along-
strike of the Hardangerfjord Shear Zone (HSZ) in Southern Norway. Currently, the area is
poorly understood due to a lack of study. The HSZ is believed to be over 600 Km long and is a
low angle ductile extensional feature with an onshore and offshore expression (Figure 1)
(Fossen & Hurich 2005; Fossen et al., 2014). It coincides with the transition from thin-skinned
to thick-skinned tectonics from the Caledonian Orogeny (Fossen & Hurich 2005), has 10-15
km displacement and reaches 25 Km depths onshore (Fossen & Hurich 2005; Fossen et al.,
2014). The HSZ’s offshore extension follows the same trend as the Highland Boundary Fault,
Southern Upland Fault and Laerdal-Gjende Fault (Figure 2) (Heeremans & Faleide 2004;
Fossen & Hurich 2005).
Previous studies suggest that shear zones affect overlying sediments by reactivating through
multiple tectonic events, generating overlying rifts and fault networks, often oblique to the
regional stress field (Fossen & Rykkelid 1992; Fossen 1993; Fossen & Hurich 2005). This
study’s aims are to further investigate this by constraining: (i) the relative timing of fault

5. activity, (ii) the relationship between depocentre geometry and the faults with the basement
structure, (iii) reconstruct the area’s geological history, (iv) analyse reasons for these
relationships between the basement structures and the overlying rift and (v) assess the HSZ’s
impact on regional geology outside the study area.
Surprisingly, considering its size and prominence, this is the first specific study on the HSZ’s
offshore expression in the Ling Depression.
2. Regional Tectonic Setting
2.1 Pre-Permian Basement
The Caledonian Orogeny (500-390 Myr) involved the subduction of the Iapetus ocean beneath
Laurentia and Baltica followed by the subduction of Baltica under Laurentia (Lundmark et al.,
2013) (Figure 2). Microcontinents, island arcs and oceanic crust from the Iapetus Ocean were
accreted onto Laurentia (Lundmark et al., 2013), later categorised into four allochtons (Figure
2), with the uppermost removed by erosion (Fossen & Hurich 2005; Fauconnier et al., 2014).
In the late Proterozoic, the Pre-Cambrian basement was peneplained with subsequent
sedimentation. These sediments were metamorphosed into intensely sheared phyllites and
micaschists (Fossen&Hurich 2005), utilised as the décollement horizon during the Caledonian
Orogeny_(A.G_Milnes_et_al.,_1997;_Fossen_&_Hurich_2005).
Figure 2. Regional Crystalline Basement lithologymodified from(Lundmark et al., 2013). Boldblack line is the studyarea.
GGF=Great GlenFault, HBF=HighlandBoundaryFault, HSZ=HardangerfjordShear Zone, IS=Iapetus Suture, KSZ=Karmøy

6. Shear, MT=Moine Thrust, MTFC=The Møre-TrøndelagFault Complex, MS= MåløySlope, NSDZ=Nordfjord-Sogn
Detachment, ØFC=Øygarden Fault complex, SSZ=Stavanger Shear Zone, SUF=SouthernUplandFault andWBF=Walls
BoundaryFault. The boundarybetween Baltica andLaurentia is partlyinferredas a thrust fault, highlightingthat Iapetan
rocks of upper allochthonare underlainto some extent byBaltican Basement.
.
2.2 Devonian Orogenic Collapse
The Early Devonian (410-402 Myr) marked the onset of extensional tectonics (Fossen &
Dunlap 1998; Dewey & Strachan 2013). The resulting formation of extensional basins and
large extensional shear zones which include the HSZ, Nordfjord–Sogn Detachment, Karmoy
Shear Zone and Stravanger Shear Zone (Norton 1987; Osmundsen & Andersen 1994;
Andersen 1998; Lundmark et al., 2013; Fossen et al., 2014). Devonian extension is split into
two stages (Figure 3) (Fossen 1992). Mode 1 extension involved kilometre-scale back
movement of the Caledonian Nappes along the active décollement due to orogenic collapse
(Fossen 1992; Fossen & Hurich 2005) (Fossen 2010). Late Devonian continued extension
initiated Mode 2 extension where oblique shear zones cross-cut and monoclinicaly fold the
original décollement (Fossen 2010) (Fossen & Dunlap 1998).
Figure 3. Schematic evolution of the Hardangerfjord Shear Zone modified from (Fossen 1992); (i) Caledonian thrust wedge,
(ii) Mode 1 extension, the Hardangerfjord Shear Zone initiates with backward movement of extensional faults along
Caledonian thrusts décollement and (iii) Mode 2 where continued extension of the Hardangerfjord Shear Zone cross cuts and
deactivates the décollement which is monoclinally folded with the Caledonian nappes.

7. The Ling Depression, Åsta Graben and Stord Basin contain undefined pre-Upper Permian
sediments beneath the AcousticBasement. These are possiblyDevonian continental deposits,
such as those within detachment basins of the Nordfjord–Sogn Detachment (Marshall &
Hewett 2003; Bellet al., 2014). No wellbore data penetrates these units in the Ling Depression
(Figure 5).
Late Carboniferous crustal thickening resulted in the collapse of the Variscan Orogen, causing
crustal thinning (Khan 2013). During this period two major N-S basins formed calledthe North
and South Permian basins across the North Sea (Tvedt et al., 2013).
2.3 Regional Rift Phase 1- Permian to Triassic
Subsequent predominantly E-W rifting overprinted pre-existing basins, forming smaller sub
basins including the Ling Depression (Faerseth et al., 1997; Torsvik et al., 1997; Heeremans &
Faleide 2004; Jackson et al., 2010; Jackson & Lewis 2015; Reeve et al., 2015).
During the Upper Permian salt (Zechstein group) was deposited by cycles of basin flooding
and desiccation across the central and southern North Sea (Jackson & Lewis 2013; Jackson &
Lewis 2015). It is unclear whether salt deposition is (i) restricted to structural lows during the
Upper Permian or (ii)deposited throughout the whole basinwith subsequent erosion on post-
Permian structural highs (Underhill & Partington 1993; Jackson & Lewis 2015). Triassic
sedimentation consists of interbedded conglomerates, sandstones, siltstones and shales
(Skagerrak Fm), deposited in non-marine alluvialfanand fluvialenvironments (Deegan& Scull
1977; Lervik 2006).
2.4 Regional Rift Phase 2- Late Jurassic-Early Cretaceous
The Mid-North Sea salt dome formed during the Lower Jurassic and collapsed during the
Middle Jurassic causing regional rifting and rapid subsidence (Jackson & Lewis 2015) and the
deposition of deep marine clasticmudstone dominated sediments (Bryne, Sandnes, Egersund
and Tau Formations) (Jackson et al., 2013; Jackson & Lewis 2013; Tvedt et al., 2013). The
extension orientation is not fully constrained by previous studies, ranging from NW-SE to
WNW-ESE (Færseth, 1996, Dore et al., 1997; Færseth et al., 1997) or E-W (Bartholomew et
al., 1993; Brun and Tron, 1993). Top Jurassic is marked by the prograding then retrograding
shelf slope margin clinoform sequence (Sauda and Flekkerfjord Formations), informally the
Hardangerfjord unit (Sømme et al., 2013).

8. A marine transgression dominated Cretaceous sedimentation. The Cromer Knoll group
(Asgard, Sola and Rødby formations) consists of marine mudstone and marl (Jackson & Lewis
2015) deposited as deep water clastics and carbonates (Bell et al., 2014). Upper Cretaceous
sediments consist of the chalk dominated Shetland group (Hod and Tor formations)
representing basin sea level rise and a reduction in clastic input (Jackson et al., 2013).
The Late Cretaceous saw the continental break up of Norway and Greenland and the onset of
seafloor spreading (Brekke et al., 1999; Eldholm et al., 2002). Inversion structures are
observed in neighbouring basins during this period (Brekke et al., 1999).
2.5 Cenozoic- Basin Inversion
The Alpine Orogeny caused basin inversion during the Late Cretaceous (Ziegler 1992)
leading to the reactivation of rift related faults and low amplitude regional folding (Jackson
Lewis 2015).

9. Figure 4. Stratigraphic column for the Ling Depression used throughout this study. Representative seismic section is correlated to
borewell 17/4-1. P = Period and E = Epoch. The main horizons considered in this study are in bold and highlighted in the seismic
stratigraphic framework section. In ‘Tectonic section’ bold horizontal line means certain, dashed horizontal line means approximate
for constraining rift timings.

10. Figure 5. (a) Borewell data from (http://factpages.npd.no/factpages/). Blackwells do not penetrate basement where red well
penetrate basement. Major faults highlighted by fault names. Green linesare shear zones - see figure 1. Study area shown by
bold black line. Fault Nomenclature: L1- Ling Depression segment 1, L2- Ling Depression segment 2, L3- Ling Depression
segment 3, L4- Ling Depression segment 4, S1- Stord Basin segment 1, S2- Stord Basin segment 2, S3- Stord Basin segment 3,
LS- Ling Depression Stord Basin transition, K1- Karmøy Shear Zone segment 1, K2- Karmøy Shear Zone segment 2, Øy2-
Øygarden fault segment 2, Øy3- Øygarden fault segment 5 further split into Ø3a, Ø3b, Ø3c, Ø3d and Ø3e. (b) Location,
lithology depth and age of basement rocks modified from (Lundmark et al., 2011). Borewell 8/3-1 is located outside of the
study area. All subsequent maps show full extent of the HSZ from this study.

11. 3. Methodology and Dataset
3.1 Data
High-quality pre-stacked time migrated 2D seismic reflection data is used (7-9 sec two-way
travel time, TWT) utilising 7 data sets of different vintages; NS, NSR06 (Stord), CNST, GLD92,
HPS98, SBGS, GNSR-01 and NSR surveys (Figure 6), collected between 1992 and 2006.
Generally, the data quality is high and is evenly distributed across the area at an average line
spacing_of_5Km.
Figure 6. A map of 2D seismic lines (black), the study area (red) and the Norwegian coastline.

12. 3.2 Seismic Interpretation
Ten key horizons have been mapped out across the study area (Figure 4); Acoustic Basement
(Capitanian, 259 Ma), Top Zechstein (Lopingian, 252 Ma), Top Skagerrak (Rhaetian, 201 Ma),
Top Tau (Kimmeridgian, 152 Ma), Top Sauda (Berriasian, 140 Ma), Top Asgard (Aptian, 113
Ma), Top Cromer Knoll (Albian, 101 Ma) and top Tor (Maastrichtian, 66 Ma). Borehole data is
used to constrain the age of these horizons using the Norwegian Petroleum Directorate
(http://www.npd.no/en/).
The top acoustic basement is defined as the very high amplitude reflection that separates
continuous from discontinuous reflections (Bell et al., 2014). Mapped out horizons are not all
continuous across the study area (Figure 8).
Due to a lack of well and seismic stacking velocity data an exact velocity model cannot be
created. A simple depth conversion is built using suitable velocities for likely lithologies in the
basin obtained from the literature; sea c. 1500 ms-1, sediments c. 3000 ms-1 and basement c.
6100 ms-1 (Abramovitz & Thybo 2000). The intrabasement dominant frequency is c. 20Hz with
a vertical resolution of c. 80m.
3.4 Mapping the Hardangerfjord Shear Zone
The HSZ has been mapped by identifying the high amplitude basement reflection, continued
along strike from its marked onshore location (Figure 1). Typical lithospheric reflectivity for
the North Sea is that the upper crust is fairly transparent and the lower crust is highly
reflective (Færseth et al., 1995). In this study area it is consistently possible to observe the
top and the bottom of the shear zone structures in the upper crust with high acoustic
impedance contrasts (Figure 12a-b). The differing orientation of the reflections compared to
any other reflections in the basement highlights that the reflection is not an artefact or
multiple.
The onshore projection of the HSZ has been identified by onshore mapping by Fossen &
Hurich (2005) and platform areas have been mapped out using deep ILP seismic lines by
Fossen et al., (2014) (Figure 1). Previous studies have identified the HSZ onshore as a linear
(NNE-SSW) low angle basement mylonitic structure (22-230) which continues 20-25 Km
down to the lower crust where it flattens and merges with general deformation fabrics

13. (Fossen & Hurich 2005).
Figure 7. Depth converted diagrams from this study of the sea bed surface, acoustic basement surface and HSZ surface of (a)
line GNSR-91-149, (b) GNSR-91 00400 42 and (c) NSR06 41135.0021389. For depth conversion see section 3.2.

14. 4. Structural Framework
Figure 8i shows the surface map of the depth to the Top Accoustic Basement Level
highlighting the rift geometry of the study area. Three prominent basins shown on the
structure TWT map which are the Ling Depression, Åsta Graben and Stord Basin trending NE-
SW, N-S and N-S respectively. The Ling Depression is 25-30 Km wide and the eastern boundary
of the Ling Depression terminates against the Åsta Graben and the Stord Basin. A 12 Km wide
graben is present to the west of the Stord basin.
The Upper Permian surface map (Figure 8ii) shows the depositional range of the Zechstein
Fm, which is restricted to structural lows in the Ling Depression and western boundaries of
the Stord Basin and Åsta Graben. The Upper Triassic surface (Figure 8iii) highlights a bowl
shape geometry of the unit between the Ling Depression and Åsta Graben. The post-Triassic
surfacemaps (Figure 8iv-viii)allhave regional gradients deepening offshore unaffected by the
overall rift geometry.
The surface map of the top HSZ surface (Figures 8xi) highlights that it dips to the NW to a
depth of c. 20 Km where it flattens. This map shows the extent of the HSZ mapped during this
study. Further offshore the HSZ is shallower dipping (400) than nearer the coastline (550)
(Figure 7). The HSZ strikes along the boundary of the Åsta Graben, dips underneath the Stord
Basin and passes beneath the Ling Depression extending out of the study area (Figure 8ix).
The basin bounding Øygarden Fault Complex runs approximately 265Km along the Norwegian
coastline (Figure 2) (Charoenpun 2013).

16. Figure 8. Time-structure maps showing (i) Acoustic Basement Surface (Structural Framework)- Age Unknown, (ii) Salt Surface- Top Permian,
(iii) Skagerrak Surface- Top Triassic, (iv) Tor Surface- Near Top Jurassic, (v) Sauda Surface- Top Jurassic, (vi) Inter Cromer Knoll- Age Unknown,
(vii) Top Cromer Knoll- Top Lower Cretaceous, (viii) Tor Surface- Top Cretaceous and (ix) Hardangerfjord Shear Zone Surface. Bold red lines
represent fault activity, bold blue lines represent faulting related to salt, bold green lines are shear zones. HSZ= Hardangerfjord Shear Zone,
KSZ= Karmoy Shear Zone and SSZ= Stravenger Shear Zone. Velocities used for depth conversion are highlighted in section 3.2. For the HSZ is
calculated estimating 5000 ms TWT depth to the Acoustic Basement. Min = Mininum depth to surface and Max = Maximum depth to surface.
For Figure (ix) red dashed line= zone 1, blue dashed line = zone 2. This figure represents the extent of the HSZ mapped in this study.

17. 5. Local Structural Styles
For the rest of the study the study area is split into zone 1 and zone 2, which exhibit different
structural characteristics (Figure 8ix).
5.1 Examples of rift not affected by the presence of a Shear Zone (Zone 1)
5.1.1 Structural evolution of the Ling Depression
The region is bounded by the Utsira High and Sele High to the north and south respectively
(Figure 9).
The first sedimentary oldest sediments in the area are Pre-Permian under the Acoustic
Basement reflection (Figure 9eii). Divergent reflections towards the fault planes indicate syn-
rift deposition within the graben.
Late Permian and Triassic sediment had isopachous geometries indicating that there was no
fault activity within the Ling Depression during this time (Figure 9eiii).
Jurassic sediments wedge into the faults L1 and L4 highlighting syn-rift deposition (Figure
9eiv). The anticlinal structure in the centre of the graben was created by differential loading
at the faults causing salt mobilisation towards the centre of the basin. Fault L2 also initiated
within the graben during the Jurassic.
Isopachous Volgian sediments were the first to deposit on the Utsira High by spilling over the
Ling Depression (Figure 9ev) during a period of faulting quintessence.
Lower Cretaceous sediments wedge into the faults highlighting that minor syn-rift
sedimentation of fault L4 occurred as well as regional preferential subsidence with thickening
to SW (Figure 9b). The isopachous geometries of upper cretaceous formations highlight that
no faulting activity. Cenozoic faults L4 and L3 cut the whole sedimentary successionwithequal
bed thicknesses either side of the fault (Figure 9evi).

19. Figure 9. (a) An uninterpreted seismic section (profile HPS-98-104), (b) an interpretation 2D seismic line (profile HPS-98-104), (c) graphs showing the timings of area, solid line= confident, dashed line=
inferred/ unclear and (d) map of region of case study and the seismic line and (e) schimatic reconstruction of seismic line HPS-98-104 split up into geological periods (i) Devonian, (ii) Pre Permian, (iii) Upper
Permian, (iv) Upper Triassic, (v) Middle Upper Jurassic and (v) Upper Cretaceous/ Recent.

20. 5.1.2 East Utsira High
The region is situated bounding the Utsira High and the Stord Basin at an oblique angle to the
HSZ (Figure 10).
Pre-Permian sediments exist under the Acoustic Basement between Faults S1 and S3 (Figure
10ei). Lack of wedging highlights that sediments were deposited into a pre-formed basin.
Subsequent wedging into fault S3 indicated that syn- rift sedimentation occurred during the
late stage of the pre-Permian basin formation.
Upper Permian salt is deposited in the area as a thin isopachous bed between faults S1 and
S3, indicating no fault activity during deposition (Figure 10eii).
Triassic sediments wedge into fault S1 highlighting that syn-rift deposition occurred. Permian
and Lower Triassic sediments are faulted by faults S2 and S3. Wedging of Lower Triassic
sediments indicates that syn-rift deposition occurred during this time.
Jurassicstrataare observed to be isopachous across the basin,indicating alackof faultactivity
during this interval (Figure 10eiii).
The basin first over spilled in the Volgian with sediments deposited on top of the Utsira High
(Figures 10 b & eiv). Thickening to the ENE is related to the Hardangerfjord unit which
prograded across the study area during the Upper Jurassic to Lower Cretaceous.
Lower Cretaceous reactivation occurs of fault S1, shown by minor folding and wedging of the
Asgard formation (Figure 10eiv).
Upper Cretaceous and Cenozoic sediments are isopachous, indicating that these are post-rift
units. Cenozoic low amplitude regional folding of all units is probably related to inversion
(Figure 10ev).

22. Figure 10. (a) An uninterpreted 2D seismic section (GLD-92-205), and (b) an interpretation 2D seismic line (GLD-92-205), (c) graphs showing the fault activity timings of area, solid line= confident, dashed
line= inferred/ unclear (d) map of region of case study and the seismic line and (e) schematic reconstruction of seismic line GLD-92-205 split up into geological periods (i) Pre Permian, (ii) Upper Triassic
(iii) Upper Triassic, (iv) Near Upper Jurassic and (iv) Middle Upper Cretaceous and (v) Upper Cretaceous/Recent.

23. 5.1.3 Ling Depression/ Åsta Graben Transition
This region is at the intersection between the Ling Depression and the Sele High, oblique to
major fault trends (Figure 11) linking the NE-SW trending Ling Depression faults with the N-S
trending Åsta Graben.
Fault LS cuts the Acoustic Basement forming a Pre-Permian basin (Figure 11). Wedging into
the fault plane indicates syn-rift sedimentation.
No Upper Permian Zechsteinsaltwas deposited (Figure 11). This is either due to (i) deposition
was limited to structural lows elsewhere or (ii) salt has been eroded elsewhere.
Isopachous Triassic sediments are restricted to structural lows with onlap against the sides of
the basin (Figure 11) not associated with faulting.
Jurassic sediments onlap onto the paleo topography and wedge onto the fault LS, indicating
syn-rift deposition (Figure 11). The geometry of the strata mirrors the Acoustic Basement due
to Upper Jurassic/ Lower Cretaceous blind faults within the basement which cross-cut the
HSZ. An angular unconformity forms the Tau- Sauda transition highlighting an erosional top
surface.
Berriasian sediments thicken to the East due to the prograding Hardangerfjord unit. Wedging
of Top Lower Cretaceous sediments highlights syn-rift reactivation of the fault LD occurs.
Anticlines in the hanging wall indicate inversion of normal faults during the Middle
Cretaceous. Upper Cretaceous/Cenozoic sediments have isopachous geometries and are
post-rift deposits.

24. Figure 11. An uninterpreted 2D seismic section (HPS-98-110), and (b) an interpretation 2D seismic line (HPS-98-110, (c) graphs showing the fault
activity timings of area, solid line= confident, dashed line= inferred/ unclear and (d) map of region of case study and the seismic line. Fault names
explained by Figure 5.

25. 5.2 Examples of parts of the rift affected by shear zones (Zone 2)
Zone 2 is located in the Åsta Graben, bounded the Stord Basinand Stravanger Platform (Figure
8ix). The HSZ splits into five splays beneath the Åsta Graben, including the Karmøy Shear Zone
(KSZ) (Figure 12). The HSZ and KSZ are a décollement for the rift faults to detach from as listric
synthetic and antithetic faults (Figure 12). Direct reactivation of the shear zones also happens
by extending the shear plane into the overlying rift sediments from the root of the overlying
faults. Similar structures have been identified by (Phillips et al., in prep) in the neighbouring
Stravanger and Egersund basins. The interaction of the Shear Zones with the acoustic
basement does not always cause the initiation of overlying rift faults.
5.2.1 Åsta Graben/ Stord Basin boundary
Pre-Permian sediments under the acoustic basement reflection, wedge into the faults Ø3b,
Ø3c, K1 and K2 (Figure 15ei). Syn-rift sedimentation is caused by reactivation of the HSZ and
KSZ as the faults directly sole onto the shear zones.
No Upper Permian salt was deposited in the area either due to (i) absence of a suitable pre-
formed basin or (ii) salt has been eroded. There is no evidence for Permian rifting (Figure
15eii).
Fault movements highlightthat Jurassicsyn-rift deposition occurs on faults Ø3b and Ø3c,with
no reactivation of the KSZ which has moved into post-rift sedimentation (Figure 15eiii).
The Hardangerfjord delta (Sauda Fm) floods the area during the Volgian to Beriasian, marking
the transition from syn-rift to post-rift phases of the Øygarden Fault Complex (Figure 15eiv).
Lower Cretaceous strata are isopachous as continued post-rift (Figure 15ev). Upper
Cretaceous basin inversion is highlighted by regional low amplitude folding of beds and
reactivation of fault Ø3a in a reverse sense, which detaches off of the main Øygarden fault
(Figure 15evi). Figure 18 highlights the Øygarden Fault Complex reactivating and inverting,
the schematic reconstruction is shown by Figure 19.

27. Figure 12. (a) An uninterpreted 2D seismic section (NRR06-11144.0015963), and (b) an interpretation 2D seismic line (NRR06-11144.0015963), (c) graphs showing the fault activity timings of area, solid
line= confident, dashed line= inferred/ unclear and (d) map of region of case study and the seismic line and (e) Schematic reconstruction of seismic line NRR06-11144.0015963 split up into geological
periods (i) Pre Permian, (ii) Top Triassic, (iii) Near Top Jurassic, (iv) Top Jurassic, (v) Top Lower Cretaceous, (vi) Near Top Cretaceous and (vii) Top Cretaceous. Fault names explained by Figure 5.

29. Figure 13. (a) An uninterpreted 2D seismic section (NSR06 41143.0023078), and (b) an interpretation 2D seismic line (NSR06 41143.0023078), (c) graphs showing the fault activity timings of area, solid
line= confident, dashed line= inferred/ unclear and (d) map of region of case study and the seismic line. Bottom left schematic reconstruction of seismic line NSR06 41143.0023078 split up into geological
periods (i) Pre Permian, (ii) Near Top Jurassic, (iii) Top Triassic, (iv) Near Top Cretaceous and (iv) Top Cretaceous/ Recent Cretaceous. Fault names explained by Figure 5.

30. 6. Regional Analysis
6.1 Isopachs
True stratigraphic thickness maps (isochrones) have been created highlighting thicknesses
between mapped out surfaces (Figure 14). Red areas indicate regions with no deposition or
that erosion has occurred.
6.1.1 Upper Permian- Salt Thickness
Salt occurrences are limited to within the pre-Permian basins. Relatively thick deposits
(1200m) existwithin the Ling Depression and south of the SeleHigh (Figures 8ii &14i). Thinner
deposits (450m) existto the eastof the Utsira High in the Stord Basinbounded by pre-existing
faults S1 and S3. Salt is not present within the rest of the Åsta graben or Stord Basin
highlighting the northern depositional limit of Zechstein Fm. Equally no salt is present on the
Utsira High or Sele High structural highs.
6.2 Triassic (AB to Top Skagerrak)
Major depocentres up to 3000m thick exist in the Åsta Graben and Stord Basin (Figure 14ii),
deposited due to rifting of the Øygarden Fault Complex. No deposits exist on the Utsira High
or Sele High as these were Triassic structural highs; however sedimentation occurred as a
thinner unit (600m) elsewhere within the study area.
6.3 Near Top Jurassic
Triassic depocentres form in the transition from the Ling Depression to the Åsta Graben, up
to 1500m thick (Figure 14iii & v). Deposition in the Ling Depression is controlled by faults L1
and L4. A regional depocentre (600m thick) forms trending E/W orientation with no apparent
fault control across the Åsta Graben, restricted to structural lows.
6.4 Top Jurassic
Sedimentation style changes in the Top Jurassic (Figure 14vi) as prograding sediments cover
the study area. The progradational limit of sediments aligns with the change of thickness from
1000-300 metres, roughly at the Åsta Graben-Stord Basinboundary. Deposition occurs on the
Utsira High and is not affected by faulting.

31. 6.5 Lower Cretaceous
A 1300 metre thick depocenter formed during the Lower Cretaceous in the Åsta Graben
(Figure 14 vii). Its western boundary is partially controlled by faults and the rest probably by
regional subsidence. Deposition is very limited over the Utsira High region.
6.6 Middle Cretaceous
Middle Cretaceous depositional patterns change with a linear 750m thick depocenter striking
N-S throughout the study aligned with the Åsta Graben (Figure 14viii). This is caused by
faulting, instead accommodation developed probably due to regional subsidence.
6.7 Top Cretaceous
Another regional gradient develops into the Top Cretaceous with the stratigraphic horizon
thickening to the SE with a roughly linear gradient from 1000-600 metres thick (Figure 14ix).
This unit is unaffected by faulting, and is probably influenced by regional subsidence.

33. Figure 14. Thickness maps for (i) Acoustic Basement to Top Zechstein Salt, (ii) Acoustic Basement to top Top Skagerrak, (iii) Acoustic
Basement to Top Tau, (iv) Top Zechstein Salt to Top Skagerrak, (v) Top Skagerrak to Top Tau, (vi) Top Tau to Top Sauda, (vii) Top Sauda
to Top Inter Cromer Knoll, (viii) Top Inter Cromer Knoll to Top Cromer Knoll and (ix) Top Cromer Knoll to Top Tor. Bold red lines highlights
fault activities, bold blue lines is faulting due to salt, bold green lines are shear zones. HSZ= Hardangerfjord Shear Zone, KSZ= Karmoy
Shear Zone and SSZ= Stravenger Shear Zone.

34. 6.2 Stratigraphy Summary
The stratigraphic analysis highlights that Permian to Boknjford sediments are limited to Pre-
Permian basin structural lows (Figure 14). It is unlikely that the sediments were deposited on
structural highs and subsequently eroded as the deposition is controlled by faults. This
answers the question presented by Jackson & Lewis (2015) highlighted in section 3.2. Post-
rift phase regionally coincides with the formation of the Hardangerfjord unit throughout the
study area. The western boundary of the clinoforms marks out the depositional range of the
main prograding sequences (figure 14vi). All Upper Cretaceous sediments are deposited as
post-rift.
6.3 Observed Structural Styles
Two distinct rifting styles present within the study area, typified by zones 1 and 2. In zone 2
the shear zone is reactivated and is steeper dipping than in zone 1. The Øygarden Fault
Complex directly detaches onto the HSZ in zone 2 and it forms shear zone splays, including
the KSZ (Figure 13). In Zone 1 the faults often are often near to the HSZ, however do not
directly exploit the shear zone (Figure 9). Reasons for this are discussed below.
The relative timing of rifting events is different between zones 1 and 2. Both areas
experienced Pre-Permian fault activities, creating the structural framework of the study area.
The rifting in zone 1 has rift-phase 1 in the Middle- Upper Jurassic and rift phase 2 is in the
Early Cretaceous where in zone 2 rift-phase 1 is during the Upper Triassic and rift phase 2 is
Middle-Late Jurassic. Reasons for this are not entirely clear; however have to be associated
with the reactivation of the HSZ in relation to the regional stress.
Top Cretaceous/ Cenozoic both compressional and extensional faults are present.
6.4 Fault orientations
The faults in zone 1 predominantly trend NW-SE aligned with the HSZ and Ling Depression
(Figure 15b). The zone 2 faults also predominantly trend NW-SE (Figure 15a), including the
bottom of the Øygarden Fault Complex (Øy3c). This is different to the predominant regional
tectonics in which faults strike N-S. During the different rifting episodes the overall
orientations of fault becomes less clear, with the general trend orientated N-S (Figures 15c-
g).

35. Figure 15. Rose diagrams to show the orientations of faults in trend and plunge direction for (a) Zone 2, (b) Zone 1, (c) Pre-
Permian Structural Framework, (d) Triassic, (e) Jurassic, (f) Near Top Cretaceous and (g) Top Cretaceous/ Recent. Principle
compressive stress orientation is perpendicular to the trend of the faults. The average trend and plunge of each fault counts
as one value on the graphs. Regional extensional directions were E/W during the Permian- Triassic (Reeve et al 2015) and
unclear for the Late Jurassic- Early Cretaceous. Number of input points is n=x.
7. Discussion
7.1 Controls on Rift Events
In the study area it is not always possible to apply the regional model of rift phase 1 (Permian-
Triassic) and rift phase 2 (Mid-Jurassic to Earliest Cretaceous) as rifting episodes occur during
the Upper Triassic and Lower Jurassic which are periods of quintessence in neighbouring
basins (Figures 9c, 10c, 11c & 12c) (Jackson et al., 2010; Bell et al., 2014). This is because the

36. basement heterogeneity makes this area more susceptible to rifting events than these areas,
resulting in multiple reactivations occurring on the same fault over different discrete time
intervals. The shear zone anisotropy acts as a basement weakness which the faults exploit in
extension and compression. The HSZ is cut by Upper Jurassic basement faults in some areas
of the Ling Depression, highlighting the inactivity of the shear zone during this period in this
location.
The difference in fault activity between zones 1 and 2 is apparent. This is not due to
differences in fault orientations as they are similar (Figure 15). Instead, the HSZ takes up the
strain, directly reactivating faults in zone 2 until this area cannot accommodate further
extension faults. At this point rifts develop in the same orientation in the adjacent zone 1. In
some cases Mesozoic basement faults in zone 1 cross-cut the HSZ highlighting its inactivity.
7.2 Controls on Rifting Style
7.2.1 Øygarden Fault Complex
This study shows that the Øygarden Fault Complex detaches from the HSZ and is reactivated
from it. Its southernmost point terminates on top of the HSZ at the boundary between zone
1 and 2 causing the Øygarden Fault changes its trend form N/S to NNE/SSW at its bottom tip
(Figure 2). The Øy3c formed completely separately from the rest of the Øygarden Fault
Complex during the Early Permian and joined by a soft link by the Late Triassic (Charoenpun
2013). Analysing T/Z plots from Charoenpun (2013) reveals that the growth of Øy3c initiated
on top of the HSZ and subsequent throw increases to the north. This indicates that the
formation of Ø3 exploited the HSZ weakness, leading to the direct reactivation of the
Øygarden Fault Complex by the HSZ. Similar fault initiations could be a controlling factor for
rifts worldwide.
7.2.2 Crystalline Basement
The HSZ exploits a lithologicalzone of weakness between VolcanicArc Basement of the Utsira
Highand Iapetun rock units with Upper Allochton of the Sele High forming the Ling Depression
(Lundmark et al., 2013) (Figure 2). The basement lithological boundary acts as a mechanical
weakness which will preferentially accommodate extension compared to basement solid
rock. There is no current evidence of equivalent basement weaknesses existing on the
Nordfjord-Sogn detachment (Lundmark et al., 2013), further study is required. Equivalent

37. models are accepted for the formation of the Scottish terranes which major faults separating
different lithologies (Figure 2) (Kinny et al., 2005), where the Southern Uplands Fault exploits
the same lithological weakness as the HSZ (Lundmark et al., 2013). This concept could an
impact on further shear zone formations worldwide.
The change in basement lithology could have had an impact on the different structural styles
in the Ling Depression, due to the different mechanical strength of the basement (Figure 2).
7.2.3 Shear Zone Geometry
Theoretical studies of fault evolution highlight that the steepness of shear zone affects the
reactivation potential (Sibson 1985). These criteria show that reactivation of a thrust faults as
a low angle normal faults is unlikely. There is a notable change in the steepness of the shear
zone, which is steeper in zone 1 (550) than zone 2 (400), and coincides with the change in style
of rifts (Figure 7). Therefore, the steeper sections of the HSZ appear to be preferentially
reactivated compared to the more shallow sections.
7.2.4 Salt
There is a relationship between the lack of salt present within the stratigraphy of rifts and the
rift being reactivated by the shear zone (Figures 10-13). This is not likely to be a control on
the reactivation, instead being a result of the area being a topographic high during the Upper
Permian. The presence of salt within the Ling Depression had limited impact on rift evolution
as no salt is used as a slip horizon for the major faults.
7.3 Controls on Anomalous Fault Orientations
The study area has major faults trending in different orientations to the broad regional
extensional orientation through all rift phases (Figure 15). The HSZ has had a major effect on
controlling the fault orientations either with faults striking along or obliquely to it. This is
caused by fault linkages between the shear zones (Figure 16). The faults join as relay ramps
as strain increases causing faults to form obliquely to the HSZ (Figure 1) (Crider 2001). As
these faults get further away from the shear zone there strikes change to N-S back to the

38. orientation of the regional stress field, typified by the Øygarden Fault (Figure 2).
Figure 16. Block diagram of fault linkages at an oblique angles under 90 degrees, modified from (Cridler 2001). (a) Upper
ramp breach. (b) Lower ramp breach.
7.4 Comparison to Other Shear Zones
There are numerous examples of rifts reactivated by shear zones exhibiting similar rifting
characteristics this study area. In the Gamtoos offshore basin, southern South Africa, strain
is taken up by the shear zone to such an extent that intra-basin faults did not form, and the

39. overlying rift follows the shear zone round a 900 bend (Paton & Underhill 2004; Paton
2006). Similar features are observed in Thailand where the Uttardit Fault zone follows
basement fabrics rather than regional extension directions (Morley et al., 2004). These both
have direct comparisons to the Øygarden fault and the zone 2 rifting style.
Recent studies also highlight faults that do not have same trend as the basement shear
zone. Reeve et al., (2015) documents Late Jurassic faults cross cutting the basement shear
zone on the Måløy Slope (Figure 2). This is highly comparable to the Ling Depression/ Asta
Graben transition (Figure 11) with the same faulting timings and orientations. Equally
(Roberts & Holdsworth 1999) questions the influence of Caledonian faults on the Mesozoic
faults in the Scottish Highlands, exhibiting similar characteristics to zone 1 with changing
rifting styles along strike of the basement heterogeneity.
8. Conclusion
This study has analysed 2D seismic data from offshore Southern Norway in order to
reconstruct the geological history of the study area and analyse the relationship between the
basement heterogeneity and the overlying Permian-Cretaceous sediments. This involved the
mapping of basement structures and the overlying rift sequences. The main conclusions of
this study are:
1. The HSZ extends offshore over 175 Km into the Central North Sea. Its basement
projection intersects the Asta Graben and Stord Basin and passes beneath the Ling
Depression, dipping NW and striking 3150. It splits into 5 splays beneath the Åsta
Graben, including the KSZ. The HSZ exploits the lithological basement weakness
between Iapetan rock (Upper Allochton) and Baltica (Lower and Middle Allochton).
2. Overlying faults exploit the plane of weakness created by the HSZ forming the NNE-
SSW orientated rift margin, which is highly susceptible to multiple reactivations.
Linkage faults form between the major basin bounding faults at oblique angles.
3. Two structural characteristics of rifting are present in two adjacent discrete areas.
Further offshore under the Åsta Graben, the Øygarden Fault Complex and other
associated faults such as the Åsta Fault are directly reactivated from the HSZ and its
splays, either by merging with the shear zone at depth using it as a detachment or the
faults root into the shearzone across the basement. The geometry and timing of these
rifts is controlled by the basement heterogeneity. Reverse reactivation only occurs on

40. the Øygarden Fault Complex where it directly detaches onto the HSZ. Further offshore
rifts of the Ling Depression do not directly reactivate from the HSZ. In the localised
areas of the Ling Depression Upper Jurassic basement fault cross cut the HSZ
highlighting its inactivity during this period.
4. Differences in the characteristics of the two areas are documented. The steepness of
the shear zone onshore is greater onshore (550) than that offshore (400) with
variations in basement lithology. Both transitions coincide with the changes in rift
structural styles. Fault Øy3 initiated on top of the HSZ in the Permian, making it more
prone to reactivation.
5. The area does not comply with the accepted regional model of rift phases. Rift
episodes in zone 1 are during the Middle-Upper Jurassic with a minor phase during
the Lower Cretaceous where in zone 2 main episodes are during the Upper Triassic
and the Middle-Upper Jurassic, due to modifications in the regional strain by the
basement structure.
6. Throw-length should be generated for the Ling Depression to further constrain growth
and linkage of basinbounding faults in order to compare with the Øygarden Fault. This
will enhance the understanding of the control on rifting styles.
7. Shear zones are a major control on rift geometries, timings, influencing basin
evolution and the style of rift faulting.
9. Acknowledgements
I would like to thank my supervisors Rebecca Bell, Thomas Phillips and Christopher Jackson
for all of the support they have provided throughout this project. I would also like to thank
the rest of the Basins Research Group for their general support throughout and provision of
seminars, and my fellow MSci students, in particular Jonathan Jones, Kishan Patel and
Joanne Kadi for making the experience particularly memorable.

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