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 ThomasMSci Geology Undergraduate

12th January 2016

Supervisors:

Dr. Rebecca E. Bell, Thomas B. Phillips and Prof Christopher A.-L. Jackson

This report is submitted as part requirement for the MSci Degree in Geology at Imperial College London and Associateship of the Royal School of Mines. It is substantially the result of my own work except where explicitly indicated in the text. The

report may be freely copied and distributed provided the source is explicitly acknowledged.

AbstractBasement 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 & 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 black line 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 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

localised areas. 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

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 Setting2.1 Pre-Permian BasementThe 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 lithology modified from (Lundmark et al., 2013). Bold black line is the study area. GGF=Great Glen Fault, HBF=Highland Boundary Fault, HSZ=Hardangerfjord Shear Zone, IS=Iapetus Suture, KSZ=Karmøy Shear, MT=Moine Thrust, MTFC=The Møre-Trøndelag Fault Complex, MS= Måløy Slope, NSDZ=Nordfjord-Sogn Detachment, ØFC=Øygarden Fault complex, SSZ=Stavanger Shear Zone, SUF=Southern Upland Fault and WBF=Walls

Boundary Fault. The boundary between Baltica and Laurentia is partly inferred as a thrust fault, highlighting that Iapetan rocks of upper allochthon are underlain to some extent by Baltican Basement.

.

2.2 Devonian Orogenic CollapseThe 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.

The Ling Depression, Åsta Graben and Stord Basin contain undefined pre-Upper Permian

sediments beneath the Acoustic Basement. These are possibly Devonian continental

deposits, such as those within detachment basins of the Nordfjord–Sogn Detachment

(Marshall & Hewett 2003; Bell et 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 called

the North and South Permian basins across the North Sea (Tvedt et al., 2013).

2.3 Regional Rift Phase 1- Permian to TriassicSubsequent 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 basin with 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 alluvial fan and fluvial environments (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 clastic mudstone 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).

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 InversionThe 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).

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.

Figure 5. (a) Borewell data from (http://factpages.npd.no/factpages/). Black wells 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.

3. Methodology and Dataset3.1 DataHigh-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.

3.2 Seismic InterpretationTen 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 ZoneThe 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

(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.

4. Structural FrameworkFigure 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

surface maps (Figure 8iv-viii) all have 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).

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.

5. Local Structural StylesFor 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

succession with equal bed thicknesses either side of the fault (Figure 9evi).

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.

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.

Jurassic strata are observed to be isopachous across the basin, indicating a lack of fault

activity 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).

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.

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 Zechstein salt was 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.

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.

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 Basin and 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 boundaryPre-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 highlight that Jurassic syn-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.

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.

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.

6. Regional Analysis 6.1 IsopachsTrue 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) exist within the Ling Depression and south of the Sele High (Figures 8ii & 14i).

Thinner deposits (450m) exist to the east of the Utsira High in the Stord Basin bounded 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 Basin boundary. Deposition occurs

on the Utsira High and is not affected by faulting.

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.

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.

6.2 Stratigraphy SummaryThe 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 StylesTwo 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 orientationsThe 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).

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. Discussion7.1 Controls on Rift EventsIn 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 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 Style7.2.1 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 lithological zone of weakness between Volcanic Arc Basement of the

Utsira High and 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 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

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 ZonesThere 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

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. ConclusionThis 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 shear zone across the basement. The geometry and timing of

these rifts is controlled by the basement heterogeneity. Reverse reactivation only

occurs on 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 basin bounding 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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