Palaeozoic structural development along the Tornquist Zone, Kattegat area, Denmark

TECTONOPHYSICS ELSEVIER

Tectonophysics

240 (1994) 19 I-214

Palaeozoic structural development along the Tornquist Zone, Kattegat area, Denmark Tommy Geologisk Institut. Anrhw Received

15 January

E. Mogensen Unitwsitrt,

’

DK-8000 irhus

1994; revised version

accepted

C, Drnrnurk 17 March

lYY4

Abstract The interpretation of newly released commercial 2D reflection seismic data in the Kattegat area, Denmark. has provided us with a better understanding of the Palaeozoic tectonic processes along the Tornquist Fault Zone. A Base Palaeozoic time structure map, a Lower Palaeozoic TWT isopach map. a “true” Lower Palaeozoic TWT isopach map, an Upper Carboniferous/Lower Permian syn-rift TWT isopach map, a Top pre-Zechstein time structure map and a Zechstcin combined TWT isopach and Palaeogeography map have been generated. The uniform Lower Palaeozoic sequence thickness in the Kattegat, both inside and outside the Tornquist Zone indicates only minor lateral movements if any, whereas the extensive Upper Silurian sequence, increasing in thickness to the north, indicates a relatively fast regional subsidence. The Base Palaeozoic time structure map and the Late Palaeozoic syn-rift isopach map show a clear Late Palaeozoic extension in the area. The syn-rift isopach map, in combination with the time-equivalent opening of the Skagerrak graben at right angles to the Tornquist Zone in the Kattegat, indicates that this extensional tectonic event had a dextral slip component. Measurements on internal extensional faults in the Tornquist Zone, give a minimum right-lateral displacement of 10.4 km. The footwall blocks wcrc deeply eroded during the Early Permian rifting, and at Zechstein times the area became a peneplane. The Tornquist Zone was later exposed to several tectonic phases, where dextral slip played a role, indicated by the “push up” and “pull down” structures caused by restraining and releasing bends of the Borglum Fault, The dcxtral displacement along the Borglum Fault since the beginning of the Permian is in the order of S-7 km based on the displacement of a Lower Palaeozoic local depocentre. Early Permian depocentres and faults. which gives a total amount of right-lateral displacement since the Early Palaeozoic in the order of 15-20 km. The continuously rcpcatcd tectonic episodes along the Tornquist Zone throughout most of the Phanerozoic, show that the zone was easily reactivated, implying deep-seated basement faults. The Tornquist Zone can be seen as a “buffer Lone”, bctwecn continental blocks, whenever changes in the regional stress field arc induced.

1. Introduction The Tornquist Zone in the Kattegat. and the surrounding onshore areas of Denmark and Swe__’ Present address: Stabekk,

Norsk

Hydro

a.s., P.O. Box 200, N-1321

Nonvay.

0040- 1% I /94/$07.00 8 1994 Elsevier s.sn/ 0040-1951~94~00105-1

Science

B.V. All rights

den, have been investigated through the interprctation of commercial retlection seismic surveys since the mid-sixties (field work in Scania, Sweden), resulting in several papers concerning the structure of the Fennoscandian Border Zone (Baartman and Christensen, 1975; Liboriussen et al., 1987; Michelsen and Nielsen, 1991, 1993) and reserved

the Tornquist Zone (Bergstriim et al., lYX2; Pegrum, 1984; Symposium on Tornquist Zone Geology, 1984; Norling and BergstrGm. 1987: Ziegler, 1987; Aubert, 1988; EUGENO-S Working Group, 1088; Franke, 1990; Bergstriim et al., 19YO;BABEL Working Group, 1991; Mogensen, 1902, lYY4; Mogensen and Korstglrd, 19Y3; Mogensen and Jensen, 1994). Only a few of these papers have dealt with the Palaeozoic structural/ stratigraphic history of the Kattegat area, primarily due to the scarcity of well and reflection seismic data, but also because of the poor resolution of the available seismic. Mapping of the Palaeozoic sequence was first carried out by Aubert (1988) based on seismic 10 km spaced with a IO-km line spacing. This gave a first approximation of the Palaeozoic sequence geometry. The structural nomenclature used in this study is from Aubert (1988). A more specific picture of the structural/ stratigraphic history of the Palaeozoic in the Kattegat area was given in the study by Michelsen and Nielsen (1993). This tied the seismic to the new deep wells in the area and gave a timing for Late Palaeozoic rifting. A relationship between this rifting in the Kattegat area Tornquist Zone and contemporaneous rifting in the Skagerrak Graben-Oslo Graben system in the Late Palaeozoic. was suggested by both Ro et al. (199Oa,b) and Michelsen and Nielsen (1993). Based on the configuration of geological structures, Pegrum (1984) and Liboriussen et al. (1987) postulated Palaeozoic to Late Cretaceous-Early Tertiary strike-slip movements along the Tornquist Zone. Using regional evidence they estimated strike-slip displacement in the order of several hundred kilometres. More recent papers (Aubert, 1988; Mogensen, 1992; Mogensen and Korstg%rd, 1993; Mogensen and Jensen, 1994) also indicate strike-slip motion, but based on more local observations, suggest post-rift lateral displacement of less than 10 km. Based on the interpretation of closely spaced newly released seismic surveys, the main objective of this study is to reveal the three-dimensional structural-stratigraphic framework of the complex Tornquist Zone in the Kattegat area during the Palaeozoic. Particular attention is given to

Fig. 1. The Tornquist Bgrglum Fault.

investigated

Zone (T-Z.), Fault,

area (box) in the Kattcgat

the Greni-Helsingborg

O.G. = Oslo Graben.

Farsund

with the

and main faults in thr Palaeozoic: S.G

Basin, EB. = Egersund

Fault

= Skagerrak

the

and the Sazby Grabcn.

F./i =

Basin.

the sense and amount of any lateral fault displacement, the depositional pattern and the relationship between neighbouring Late Palaeozoic rift systems. The two main faults in the Kattegat area, crossing from Scania, Sweden to North Jutland (Fig. I) are the B@rglum Fault and the GrenlHelsingborg Fault. These faults are considered two separate strands of the Tornquist Zone, whereas the Szby Fault is considered to be a branch of the BBrglum Fault (Fig. 1). The Tornquist Zone joins up with the Skagerrak Graben to the northwest of Denmark (Fig. 1) (Ro et al., 199Oa,b), but whether it terminates here as a Palaeozoic structure or extends further to the north into the Farsund Basin area, is still a matter of discussion.

2. Data The present study of the Tomquist Zone and Kattegat area is primarily based on newly re-

T.E. Mnfienserr / T~~ctonophysrcs240 (1994) 191-ZIJ Table I Seismic surveys su wey

Year

Fold

SSL6267 WGC67A PRKL7374A GS17SB

64-67 h? 73-74 75

h h-12 12

DNJ8183D RTDXIK DCSXlK GYX2K GEC083AK DN84D DKX4K AMX4K TXXJK A0851 TXXSK

82-83 82 82 x3 83 84 x4 84 84 x.5 HS

I2 48 4x 4x 4x 24 4x ho 3x 24 4x

1

Filtered/ Migrated

Onshor Offshore

Filtered Filtered Filtered Repro. and Migr. 1983 Migrated Migrated Migrated Migrated Migrated Migrated Migrated Migrated Migrated Migrated Migrated

Onshore Offshore Onshore Offshore Onshore Offshore Offshore Offshore Offshore Onshore Offshore Offshore Offshore Onshore Offshore

leased commercial 2D reflection seismic data, shot during a period of renewed hydrocarbon exploration in the early eighties. This resulted in the drilling of the first two deep wells, Hans-l and Terne-1 in the central part of Kattegat, and the Szby-l well in North Jutland (Fig. 21, all of which encountered the Palaeozoic. The reflection seismic surveys used in this study (Table l), vary in quality from the single fold seismic, shot in 1967 to the 60 fold data shot in 1985. Only survey GSI75B (Table 1) has been reprocessed. Seismic resolution is higher in the offshore data, but in a few onshore regional lines shot in 1980, good continuous events down to 4 s TWT can be seen. Data from all wells in the area (Table 2), augmented by information on geology of onshore Sweden, have been used in the study. In the

193

central Kattegat area and onshore Denmark, the well data were sufficient to provide good ties to the reflection seismic surveys. Interpretati~~n in the area in between might be more speculative. because of the sparseness of high resolution seismic data (Fig. 2). The portion of four key lines reproduced in Fig. 3 is indicated on all maps. The TWT thicknesses on all isopach maps have been measured at a right angle to the bedding plane, except for the mobile Zechstein isopach map where thicknesses have been measurea vertically. Measuring perpendicular to the bedding plane of tilted beds, makes it impossible to image a true position of the contour on map view. In this study the midpoint of the thickness of a sequence has been used for the map view position (Fig. 4). The pick for the Near Base Palaeozoic TWT structure map is the top of a package of three strong reflections. These are evenly separated and the unit is approximately 200 ms thick. This unit has partly been encountered in the Terne-I well, where the TD of the well is in the top of the Hardeberga Sandstone Equivalent and close to the middle reflection. The Hardeberga Sandstone Equivalent ~~g~~ have a thickness of approximately 100 ms (Bergstrom et al., 1982), and the bottom reflection could therefore be the true base of the Palaeozoic, but if the thickness is smaller, the bottom reflection could also be a multiple. The uncertainty of the true base of the Palaeozoic was the reason why the top reflection has been chosen for the Base Palaeozoic. It is also the most easy to recognize of the three reflections.

3. Structural development 3. I. Early Palaeozoic

Tdblc 2 Wells Well

Terminates in

Frederikshavn-1 Gassum- I Hans- 1 Ronde- I S&Iy-I Terne- 1

Precambrian Zechstein Upper Carboniferous Upper Silurian Rotliegend Cambrian

The Palaeozoic pre-rift TWT isopach map (Fig. 6) outlines the Tornquist Zone, by the thickest Lower Palaeozoic interval between the GrenlHelsingborg Fault and the Borglum Fault. The Near Base Palaeozoic TWT structure map (Fig. 5) again shows the Tornquist Zone and also outlines the later structural development in the Kat-

L__

i

AL..

j-

‘..

1

I

Fig. 2. Seismic and well coverage of the area investigated. Main Palaeozoic faults are indicated, and the four interpreted (Lines 1-4, Fig. 3) are shown. For survey names and well terminations see Tables 1 and 2.

lines

m Mesozotc [ZFj

Postrift

Mobile Zechstein /@j@j

Syn-rrft

Upper Carbon / Lower Permian \ ,‘\‘\~,‘\‘\‘\‘\’ ~\l~.~\l\~\.~\~\l\i\).

I >z_), , I I \..‘, ,, ‘,‘,‘,‘,.‘(\.‘(‘,

Lower Paleozorc

f--F=

Prerift

Basement Fig. 3. Four Interpreted lines across the Tornqurst Zone (for location, see Fig. 2 and maps). Note the Late Palaeozoic half-graben formation and the outline of the Greng-Helsingborg Fault and Borglum Fault. Note also the peneplane at Zechstein rimes and the apparent dip-slip separation of the Upper Carboniferous/Lower Permian depocentre on Fault A (Line 3). C;.-l-f.F‘ = GremI~elsinborg Fault, R.F. = Borglum Fault, SF. = &by Fault. Faults A and R on Line 3 are shown in Figs. 13-15. The post-rift, syn-rift and me-rift sequences are also rndicated.

Fig. 4. Method applied to measure pre-rift tilted strata. Measuring perpendicular to the bedding plane of tilted beds, it is impossible to image a true position of the contour in map view. In this study the midpoints of the thickness of a tilted sequence have been used for the map view position. Note the “true” pre-rift sequence thickness to the right of the erosion/ no erosion line.

tegat as this surface has experienced all the Phanerozoic tectonic events. The deepest present burial of the Base Palaeozoic is to the southwest, in contrast to the northeast where the Palaeozoic remnants are found at shallow levels (see line 3, Fig. 3). This is primarily due to the Mesozoic and Cenozoic events. In the investigated area the Lower Palaeozoic has been encountered in the Ronde-1 and Terne-1 wells. In Ronde-1 the well terminated after drilling ca. 300 m of Upper Silurian volcanic rock and claystone. In the Terne-1 well rocks ranging from the Cambrian to the Upper Silurian were encountered (Michelsen and Nielsen, 1991). The Upper Silurian in the Terne-1 well is not complete. The reflection seismic data clearty show a deep erosional truncation of the Lower Palaeozoic sequence close to the Terne-1 well. A similar deep erosion of the Lower Palaeozoic is seen in line 1 (Fig. 31, south of the WEby- well. The Upper Silurian in the Terne-1 well is overlain by marine elastics of Zechstein age. This implies

that the age of the missing Palaeozoic section in Terne-1 covers the interval from Late Silurian to Rotliegende. The missing section, approximately 1500 m in the Terne-1 well, is believed to be of Late Silurian age, as was also proposed by Michelsen and Nielsen ( 1993), although deposits of Devonian and/or Early Carboniferous age cannot be excluded. In this study the missing section will be referred to as Lower Palaeozoic. All strata from the base of the Palaeozoic to the Late Palaeozoic syn-rift sequence are parallel and conformable. Deep erosion. however. of the Lower Palaeozoic, caused by the Late Palaeozoic tectonics. has given rise to a very variable present-day thickness of the Lower Palaeozoic in the area (Fig. 6). The Palaeozoic pre-rift WT isopach map does, therefore, not give a good indication of the initial thickness of the Lower Palaeozoic. Only where the Lower Palaeozoic sequence is preserved beneath the Late Paiaeozoic syn-rift sequences, can a more realistic Palaeozoic pie-rift thickness be estimated. In this study this is called the “true” Palaeozoic prc-rift sequence (Fig. 71. This isopach map shows the pre-rift thickness beneath the syn-rift sequence (Fig. 4), where the most complete pre-rift sequence has been preserved, and where no erosional truncation can be seen. Based on the map in Fig. 7, the thickness of the complete Palaeozoic pre-rift sequence ranges from 2.5 km to 4.0 km, based on the interval velocity information in Terne-1 (Nielsen and Japsen, 1991). The Cambrian to Mid Silurian sequence has a fairly constant thickness of approximately 700 m in the region (Bergstrom et al., 1982; Michelsen and Nielsen, 19911, which would give 1.8-3.3 km of Upper Silurian sediments, that have been deposited over the area. Limitations of the “true” Palaeozoic pre-rift map (Fig. 7) is that the syn-rift sequence is only preserved as isolated and scattered patches close to major faults (Fig. 8). Consequently the complete section thickness of the Lower Palaeozoic is preserved in the same scattered way, which reduces information about the subsidence pattern. Furthermore as interpreted in this study, the pre-rift sequence comprises both the Lower Palaeozoic and the uppermost Carboniferous se-

73. Mogensen / Ttmm~physics 240 (1994) IYI-214

.

lil “E

Kattegat map 1

Near Base Paleozoic TWT Time Structure Map <’ %=- - Normal Fault 7: -- _ T- _=- Reverse Fault s”pv - 1 Well ,

*c--

Limit of Lower Paleozoic Seq. Depth in msec TWT 4000 _ 4500 ~~2000 3500 - 4000 E-2

Fig. 5. Near Ihe Paiaeozoic R.E = Htirglum Fault.

TWT structure

map. Two major fault directions

are present.

NW-SE

- 2500

1500 - 2000

and a N-S to NNW-fjSE

trend.

T.E. Mo~enser~ / Tectonophyxs

240 (1994) I91 -214

1V’E Kattegat map 2

‘--P;==” Syn Rift journal Fault

:i

* ,\:

g

)$

I

,;

s”pv-’

Well

,.*--

Limit of Lower Paleozoic Seq.

I

I Ii, ]

Thickness in msec TWT

Fig. 6. Palaeozoic pre-rift TWT isopach map. Note the preserved thick pre-rift sequence present internally in the Tornqu~st Zone. Note also that Fault A (arrow) close to Line 3 seems to offset the Paiaeozoic pre-rift sequence late&y. G.-H.F. = @en%-Helsingborg Fault, RF. = Berglum Fault, S.F. = Saeby Fault.

T.E. Mogensrn

/ Tuctonophysics

240 (1994)

IYl-214

Kattegat map 4

“True” Paleozoic Pre Rift TWT lsopach Map Limit of Syn Rift Sequence and therefore most complete Pre Rift Sequence

f/‘_\

‘SPY- ’

Well

Thickness in msec TWT 1400 - 1600 1200 - 1400 57”N

;E:y I.._ :I

1000 _ ,200 900-1000

Anholt (I- 4)

Rende - 1 .

Fig. 7. “True” Palaeozoic pre-rift TWT isopach map. Note the general increase in thickness to the northwest and the fittlc variation of thickness across the margins of the Tornquist Zone (at Line 4). Note also the apparent right-lateral separation, of the order of 5 km (arrow). of the local depocentre just south of Anholt.

200

T.E. Mogensen / Tecfonophysic’s 240 (19941 191-214

Kattegat map 3 ./’ I

I

“.e*,, *,,,,,,

Syn and/or Post Rift Fault With Wrench Component

I”-\

Limit of Syn Rift SequencsS Thickness in msec TWT

i’ *I, , “, J’

:

am
f$ggj

m-400

-MO-800

m

O-200

‘5, ‘h, ‘5, ‘*,, ‘r.,

%,

‘l.‘)

I

Fig. 8. Palaeozoic syn-rift TWT isopach map. Note the NW-SE and N-S orientations of the fault-reMed dupoce~ntres. In an E-W extewionai setting, the NW-SE-oriented depocentres might have been formed by o~i~e~siip/s~ke-8~ fautting. Tbe area highlighted is location of Figs. 13 and 15.

T. E. Mogensen / Tecronophysics 240 (1994) 191-214

quences at the Hans-l well (Michelsen and Nielsen, 1993, fig. 7), so at least 180 ms of the pre-rift sequence might be Upper Palaeozoic deposited parallel to the Lower Palaeozoic. In spite of this, some interesting aspects concerning the Lower Palaeozoic sequence can be addressed. (1) The thickness of the Lower Palaeozoic prc-rift sequence, increases towards the northwest. The thickness varies from 1 s in the southeast to 1.5 s TWT to the northwest. Further to the south in the northern part of Zealand a Lower Palaeozoic sequence thickness of 0.9-l .O s, can be interpreted (survey A08.51, see Table 11, whereas further to the north, in the Skagerrak Graben a pre-rift sequence thickness close to 2 s TWT (Ro et al., 1990a,bI and even 3 s TWT (I.E. Lie, pers. commun., 1993) has been found. If a uniform thick Cambrian-Mid Silurian sequence thickness is assumed (see above), this indicates that the Upper Silurian and/or Upper Carboniferous sequence thickness increases to the northwest. (2) The Lower Palaeozoic sequence thickness seems not to vary significantly from inside the Tornquist Zone to the surrounding areas, indicating no large-scale lateral offset. (31 A third aspect is small preserved Lower Palaeozoic local depocentres (Fig. 7). Most of these depocentres seem to have evolved as saucepan shaped holes, at some distance from later major faults. These faults might have existed or been initiated at this time, although no relationships of fault/depocentre or age has so far been found. Whether these depocentres are fault-related Upper Silurian local depocentres, rift-initiating Upper Carboniferous volcanics or perhaps just a seismic artifact has still to be resolved. The pre-rift depocentre just southwest of Anholt (Fig. 7) is of particular interest, as this has been cut by a younger fault (Fig. 6). The isopach map data (Fig. 7) indicate approximately 5 km of right-lateral displacement along this fault since the evolution of the pre-rift depocentre. 3.2. Late Palaeozoic An Upper Carboniferous section of 467 m, around 180 ms, was encountered in the Hans-l

201

well (for location see Fig. 81, below the Rotliegende syn-rift sequence (Michelsen and Nielsen, 1991). The Upper Carboniferous sequence is conformable with the suggested Lower Palaeozoic sequence, and no sequence boundary between these sequences can be identified on the seismic sections. This indicates that only minor tectonic activity had taken place before the Late Palaeozoic rift phase, and that no large differential erosion seems to have happened before the rifting. In the Hans-l well Upper Carboniferous extrusive volcanic rocks, approximately timeequivalent with the volcanic activity in the Oslo Graben (Michelsen and Nielsen, 19931, heralds the dramatic change in tectonical and depositional style that took place during the rifting in Rotliegende time (Lines 1-4, Fig. 31. The Palaeozoic syn-rift TWT isopach map (Fig. 8) shows the extent of the rift sequence, but the extent of the rifting itself is probably better delineated by the Palaeozoic pre-rift TWT isopach map (Fig. 61, where all proposed syn-rift faults are shown. This map suggests a much broader zone of rifting than the syn-rift isopach map (Fig. 8). Both the pre-rift and the syn-rift isopach maps (Figs. 6 and 81 clearly ilhrstrate the down-faulting, subsidence and thereby the preservation of the Lower Palaeozoic interval internally in the Tornquist Zone. The trough-like preservation of the thickest Lower Palaeozoic sequences continues towards Scania, and seems to coincide with the Colonus Trough, indicating a Rotliegende age ot this trough. The pre-rift isopach map (Fig. 6) also shows the half-graben pattern that came into existence during the Kotliegende, with hangingwall subsidence and footwall uplift and erosion (see also Lines 1-4, Fig, 3). The syn-rift depocentres (Fig. 8) arc clearly fault-related, and depocentre axes typically strike NW-SE and N-E. The NW-SE orientation follows the border faults of the Tornquist Zone. whereas the N-S-oriented depocentres are lo cated internally and to the southeast of the Tornquist Zone. These fault-related depocentre dircctions indicate a NE-SW to E-W orientation of the regional extension during Rotliegende rifting. Approximately 5 km right-lateral displacement

T.E. Mogensen / Tectonophysics 240 (I 994) 191-214

202

is indicated by the separation of the local Lower Palaeozoic depocentre just southwest of Anholt. This separation also seems to exist between synrift depocentres at the same position southwest of Anholt (Fig. 81, which indicate that the lateral offset of these depocentres primarily must be post-rift movements. At the end of the rifting the entire investigated area had been exposed to erosion and a major discordant hiatus was formed (Lines 1-4, Fig. 3). This introduced a new tectonic regime during the Late Permian Zechstein, where only minor faulting took place (Fig. 9). A regional subsidence took place to the southwest of the Tornquist

I

Zone, and the zone became more or less the northeastern margin of the North Zechstein Basin, with deposition of marginal Zechstein deposits. This tectonic pattern continued into the Early Mesozoic (Mogensen, 1995).

4. Discussion 4.1. Early Palaeozoic structural development The suggested increase in thickness of the Upper Silurian sequence to the north, and the small variation of the “true” pre-rift sequence

Kattegat map 5

/I

Z@-tdain~~

lsepmh

Thickrws of Mot& Zechstein in msec TWT

~300-400

, , , I SI aI,

Limit of Zechstein

- - - -

Limit of Mobile Zechstein

--llc

Fault Active During Zechstein

-,__

Deliniatian of GrenC Helsingborg Fault

‘“.”

_’

Wetl

~200-300

100-200 m

()_l()O

Non Mobile 2 _. .

Fig. 9. Zechstein TWT Isopach Map and Palaeogeography. Note the very limited fault activity.

tic 01 ‘w-r

TX. Mogenserz / Tectonophysic:~ 240 (1994) 191-214

thickness from inside of the Tornquist Zone to the surrounding area (Fig. 7), indicate a wide Late Silurian foreland deep to the approaching Scandinavian Caledonides. This foreland deep was possibly delineated to the south by the Tram-European Fault Zone (TEF), a wide zone of shear, and according to EUGENO-S Working Group (1988) and Coward (1990) the plate bounda~ between the Scandinavian and northwest European biocks. EUGENO-S Working Group (19881, Neugebauer (1989) and Coward (1990, 1993) proposed oblique closure between these blocks during the Caledonian orogeny, resulting in right-lateral strike-slip. Because the “true” pre-rift sequence thickness in the Kattegat area is relatively constant in thickness across the margins of the Tornquist Zone (Fig. 7). lateral displacement in the investi-

Sho~ening Axis P

P Fracture

(R’

Syntetic and Antithetic Shears

:z,

Angie of Internal Friction

QZ Principal i

Displacement

Zone

Orientation of Extension Fractures

-“._. Orientation of Fold Axes TEM-9:

Fig. I I. Dextra-shear components (modified from C‘ristieBlick and Biddle, 1985: Sylvester, 198X). The riedel shears R and R’ in combination with extension fractures and principal displacement shears PDZ, are the most likely to occur in a transtensional setting (Woodcock and Fisher. 1YMf

Fig. 10. The Late Carboniferous/Early Permian OsloSkagerrak-Kattegat rift and fault system. Arrows indicate one possible (orientation of the movement of the crustal block to the east of the riftsystem. This hypothetical direction gives dextral movement in the Kattegat rift and a sinistral opening of the Skagerrak part of the Skagerrak-Oslo rift. Note also the Sazby Fault trying to adjust the system by breaking up the eastern block.

M-SP Fig. 12. Dextral-shear components are superimposed on the faults cutting the Base Palaeozoic surface. This comparison shows a possible connection between the existing faults and ;I dextral-shear environment.

204

ll"30‘E --73?dtR

’ ----._

'I

c

Ni -

1oKm

.“‘..‘...*. Eroded Synrift Sequence - * * Limit of Synrift Sequence

/

Seismic Line

Thickness in trsec TWT

) 13J P

Fig. 13. A detailed (RF.),

map of the syn-rift sequence (for location

as well as depocentres

restraining

bend. are indicated.

K83”Olaf

/

“Push UP”

see Fig. 8). Lines across the Faults A and B of the B&urn

D and u’ (Fig. 14) arc shown. Also the interpreted

G.-H.F. = Gren%-Helsinghorg

gated area in the order of several hundred kiiometres, as proposed in previous papers (Pegrum 1984; Liboriussen et al., 1987) is unlikely to have taken place during the Phanerozoic. Strike-slip cannot be excluded, but large-scale strike-slip would supposedly have juxtaposed Lower Palaeozoic sequences of unequal thicknesses. Any large-scale strike-slip in the Palaeozoic in the Polish part of the Tornquist Zone would therefore have to follow the TEF, whereas the Tornquist Zone in the Swedish-Danish area probably was either slightly active to inactive. The Kattegat area as part of a wide Late Silurian foreland deep is also more or less in accordance with the work of Michelsen and Nielsen (1993) and EUGENO-S Working Group (1988), and favours a primarily post-Early Palaeo-

lines in Fig. 19 across a “push up” structure,

Fault in a

Fault.

zoic, probably Late Carboniferous/ Rotliegende, evolution of the Scanian, Colonus Trough, where a thick Lower Palaeozoic layer has been preserved. This trough have earlier been proposed to having evolved in the Late Silurian (BergstrGm et al., 1982). 4.2. Late Palaeozoic structural development Following the large and rapid deposition of the Late Silurian, only small differential subsidence or erosion seems to have happened in the Kattegat area during the Devonian and Early Carboniferous. This period is therefore suggested to have been tectonically quiescent. The investigated area could have been a subaerial exposed plateau, between the old Caledonian erogenic

T. E. Mogensm

/ Tcctorwphysics

belt to the north and the evolving Variscan orogenie belt to the south, illustrating the Caledonian post-erogenic situation of the Kattegat area. The onset of rifting in the Kattegat area introduced a new tectonic framework, and the Tornquist Zone in the Kattegat became more directly related to the Polish part of the Tornquist Zone for the rest of the Phanerozoic (EUGENO-S Working Group, 1988). The rift phase in the Kattegat along the NWSE-oriented Tornquist Zone was contemporaneous with or slightly older than the rift phase in the NNE-SSWto NE-SW-oriented OsloSkagerrak grabens (Ro et al., 1990a), which suggests that the most likely direction of regional extension was E-W (Fig. 10). An opening of the two graben systems in a strict normal sense would

Fig. 14. Lines across Fault A, syn-rift depocentres D and D’ and Fault B; for location see Fig. 12. Note the flip in fault displacement from normal to reverse to normal, along Fault A. B.F. = Borglum Fault.

230 (19941 191-214

‘0

otherwise have created a space problem close to the intersection of the graben systems. This is not seen, and the basement block between the two graben systems appears to have acted as one coherent block. This may imply sinistral obliqucness in the extension tectonics of the Skagerrak Graben and Oslo Graben (Fig. 10). East-west extension would give normal dip-slip of the N-Soriented faults in the Tornquist Zone and rightlateral oblique dip-slip of the NW-SE-oriented faults (Figs. 6 and 8). The Szby Fault and the other faults to the northeast of the Btirglum Fault try to adjust the orientation of the Tornquist Zone with respect to the regional extension, into a more N-S-oriented zone (Figs. 6 and IO).

Dextral shear creates different sorts of fractures and folds (Fig. 11) (Cristic-Blick and Biddle. 1985; Sylvester, 1988). Such structures as cxtcnsion fractures, riedel shears (R) and fractures in the principal displacement direction (PDZ) (Fig. 111, arc the most likely to occur in a transtensional setting (Woodcock and Fisher, I986). These structures are superimposed on a surface which has experienced all tectonic episodes in the Phanerozoic, namely the Near Base Palaeozoic TWT structure map (Fig. 5). to produce a comparison map (Fig. 12). This comparison shows that several faults in the Tornquist Zone could have been formed in such a dextral-shear environment, indicating dextral strikc-slip (see also Aubert, 198X). Another explanation for dextral movements on the main faults, instead of a regional dcxtral-shear environment, could be the existence of NW-SEtrending deep-seated crustal weakness zones, that first became reactivated during the Late Palaeozoic rifting event. E-W extension would reactivate such weakness zones in a dextral fashion, but instead of a regional dextral-shear environment, this dextral shear would be localized at those faults that were in a favourable orientation. In other words, the existence of prc-rift wcaknchs zones determines the sense of movement on the faults, when they are exposed to a certain stress field.

206

T. E. Mogensen / Tectonophysics 240 flWf/

A detailed map of the syn-rift sequence south of Anholt is shown in Fig. 13. The fault that disptaces right-laterally a Lower Palaeozoic (Figs. 6 and 7) and a Rotliegende depocentre, is Fault A, a branch of the Borglum Fault. Such laterai separation of a body is the best indication of strike-slip (Sylvester, 19881, and the separation of the Lower Palaeozoic depocentre (Figs. 6 and 7) indicates post-Early Palaeozoic dextral movements along Fault A of the order of 5 km. Three sections across Fault A, the separated depocentres and Fault B (Fig. 14, see also Line 3, Fig. 3) show the present-day configuration. Experiments on reactivation of normal basement

IYI-214

faults in oblique slip (Richard, 19901, with strikeslip/dip-slip ratios between 1 and 3.5, show good agreement with the observed configuration of Fault A, where both steep normal and steep reverse separation along strike of the fault are seen. While Fault A primarily seems to take up strike-slip components, Fault B primarily takes up the dip-slip components (Figs. 6 and 141. A reconstruction to the beginning of the Rothegende of the detailed area in Fig. 13 is shown in Fig. 15. Approximately 7 km of “inverse” dextral displacement has been used, which would more or less fit both the Lower Palaeozoic depocentre (Figs. 6 and 7) and the syn-rift de-

.F.

Releasing Bend \

Fig. 15. A reconstruction of Fig. 13 to the beginning of the Rotliegende. The separated syn-rift depocentres D and D’ have been juxtaposed (Fig. 131, as well as the underlying Lowei Palaeozoic depocentre (Fig. 7). The juxtaposition requires 7 km of “inverse” dextrai displacement. Releasing and restraining beilds in a dextrai-shear environment are indicated along the curvilinear B&urn Fault (BE).

pocentre. Most of the lateral activity of Fault A is of Mesozoic age (primarily EarIy Triassic, Mogensen, 19951, but minor right-lateral syn-rift movements might aiso have taken place, since the syn-rift sequence is bending upwards close to the fault and eroded before the first Mesozoic sediments were deposited (Figs. 13 and 15). The reconstructed map (Fig. 151 also indicates both releasing bends and a restraining bend, intimately reIated to strike-slip fauiting (Cristie-Blick and Biddle, 1985; Aydin and Nur, 1985; Harding, 1985, 1990; Sylvester, 1988), along the curvilinear Borglum Fault. The schematic structural history of the Lower Permian syn-rift depocentres in the middle of the Kattegat (Fig. 16) clearly outlines the syn-rift releasing bends, The restraining bend (Figs. 13 and lS), uplifted (“pushed up”) and exposed to erosion, indicated by local thinning of the Lower Palaeozoic sequence (Fig. 61, might be of syn-rift age, but is primariiy a Mesozoic structural feature (Mogensen, 1995). Fault A is probably trying to take a short cut at the westernmost releasing bend (Fig. 151, and straighten out the

orientation of extension Fig. 16. A schematic evolution of the Lower Prrmian syn-rift. depocentres. This evolution is a mixture of N-S-trending normal fault activity in combination with right-lateral transtensional faulting along the NW-SE-trending marginal faults.

strike of the Borglum Fault (Figs. 6, 15 and 16). Note also the local Lower Palaeozoic depocentre at this releasing bend position (Fig. 7). This might be indicative of minor right-lateral movements during the Early Palaeozoic. The Top pre-Zechstein sequence boundary (Fig. 171, is a major regional unconformity, and in the study area a peneplane in Zechstein times. The present-day configuration of the Top preZechstein surface is, therefore, the cumulative result of all younger structural movements. Taking a closer Iook at the Top pre-Zechstein surface in the same area as above (Fig. 181, the former easternmost releasing bend of Fig. 15 is still a releasing bend, resulting in a “puIf down”, whereas the former restraining bend now depicts a very clear “push up”. Three dip lines and one strike line across this structure (Fig. 19) show the configuration of the “‘push up” structure and the increased deformation over the centrc part (Fig. 19, Lines K83-010 and K83-001). Erosion of both the syn-rift sequence and the Lower Paiaeozoic sequence to the east of the “push up” structure (Fig, 13 and Fig. 19, Line K83-001) indicates the existence of the restraining bend in the Rotlicgende, and supports, therefore, the above suggested dextral displacement along the B@rglum Fault during the rifting. Note also that Fault A (Fig. 18) succeeded in taking the short cut and straightens out the strike of the B0rglum Fault, a tectonic phase that primarily took place in the Early Triassic ~Mogensen, 1995). The postulated 5-7 km of dextral strike-slip along the B@rglum Fault is lateral displacement accumulated primarily from the post-Rotliegende to the present. The amount of dextral strike-slip along this fault during the rifting is not easy to quantify since there are no pin points. But one way of estimating a minimum lateral displacument is by examining the fault pattern carefuIly. The Tornquist Zone is bounded by the Bgrglum and wrens-Helsingborg fautts
208

T.E. Mogensen / Tectonophysics

Estimating the heave of these three faults gives 10.4 km of heave, which would move the block north of the Barglum Fault 10.4 km to the right relatively to the block south of the GreniHelsingborg Fault. So a minimum estimate of the dextral syn-rift displacement is 10.4 km. Possible N-S-oriented normal faults further to the west in the Tornquist Zone would add to this number, but maximum right-lateral syn-rift displacement is unlikely to exceed 20 km. The post-rift 5-7 km of dextral displacement along the Borglum Fault and a possible 11-20 km of dextral syn-rift offset, indicate approximately 16-27 km of dextral strike-slip over the entire

240 (I 994) 191-214

Tornquist Zone from the Late Carboniferous/ Rotliegende to the present. 4.4, The Tomquist Zone, a crurtal buffer zone A decrease of the crustal thickness towards the southwest across the Scandinavian part of the Tornquist Zone (EUGENO-S Working Group, 1988; Lie and Husebye, 1992) might indicate an old Precambrian block boundary, perhaps an east to northeastward accretion of a minor continental block to the Baltic Shield, during the Sveconorwegian orogeny. This potential block boundary seems to show little activity in the Early Palaeo-

Surface Rocks Onshore Sweden

U. Cretaceout

Fig. 17. Top pre-Zechstein TWT structure map. Highlighted area is Fig. 18. Note the Post-Rotliegende At Zechstein times this surface was a peneplane.

reactivation of older faults.

zoic, but it became very active in the Late Carboniferous/ Rotliegende during the Late Hercynian wrench tectonics. Reactivations of this old NW-SE-trending lineament from the Late Palaeozoic to the present, indicate the weakened nature of the proposed crustal block boundary. These reactivations formed the NW-SE-oriented Borglum Fault and GrenA-Helsinborg Fault, which probably came into existence during the Rotliegende E-W-oriented extension, giving the former mentioned dextral displacement. To the south of the Tornquist Zone in the Kattegat area, NNW-SSEtrending Rotliegende faults indicate a more independent development. These fauits were only slightly reactivated during the Mesozoic-Cenozoic. In North Germany primarily N-S-oriented faults developed during this E-W extension, also indicating only minor control from crustal weakness zones (Gast, 1988). Deep seated crustal lineaments controlling the Late Palaeozoic, Mesozoic and Cenozoic fault patterns have been described from the Oslo Graben fswenson, 1990) and lately from the North Sea region, supporting the genesis of the fault pattern in the Kattegat

I

Depth in msec TWT

Fig. 18. A detailed a releasing bend indicated.

1500-2000

(Coward, 1993; Bartholomew et al., 1993; Sears et al., 1993) Being such an old weak block boundary. the Tornquist Zone in the Kattegat area is sensitive to changes in the regional stress field (Fig. 21). Exposed to both tensive/ transtensive and compressive/ transpressive forces, faults in the zone became repeatedly reactivated. The Tornquist Zone can, therefore, be considered as a “bufferzone” between more coherent crustal blocks whenever changes in the stress field wcrc induced. The dextral-shear environment from the Rotliegende seems to have continued up through the Mesozoic, with both right-lateral transtcnsion and right-lateral transpression (Mogensen, 199%. The cross-section in Fig. 21 shows the configuration of the zone after the Late Cretaceous/ Early Tertiary compression/ transpression.

5. Conclusion In the study area of the Kattegat the “true” Lower Palaeozoic sequence, possibly the Upper Silurian, increases its thickness to the northwest

1000-l

500

~~~~1 5oo_, ~&&::::‘:,

o(.)*

I

Top pre-Zechstein TWT structure map (for location see Fig. 171, in the same area as Figs. 13 and IS. Here both and the restraining bend “push up” are clearly shown. Lines across the “push up” structure arc

“pull down”

indicating a foreland deep in front of the Scandinavian Caledonides. As only minor “true” Lower Palaeozoic thickness variations occur from inside the Tornquist Zone to the surrounding area, large-scale strikeslip, in the range of hundreds of kilometres, is unlikely to have happened in the area investigated.

All major faults in the Kattegat seem to have come into existence during the Late Palaeozoic rift phase. Based on detailed investigations of the pre- and syn-rift sequences, and the time-equivalent opening of the Skagerrak Graben perpendicular to the Tornquist Zone in the Kattegat. this rift phase had a dextral strike-slip component.

Fig. 19. Lines across the “push up” structure (for location see Figs. 18 and 13). Note the stronger deformation over the crest of the structure. which is closest to the restraining bend. Note also the erosion of the syn-rift sequence on Line K83-001. B.F. = B0rglum Fault.

T.E. Mogensen / Tectonophysics 240 (1994) 191-214

.

11”‘O

Syn-rift dextral strike-slip 1 displacement across the Tornquist Zone >

/

if-rederikshavn

i f 1 Transtensional strain ellipsoid with 11stnke of syn-rift faults superimposed.

-1

Faults with most transtensional . . .-

Faults with most dip-slip

,

i/r

More

71

strike-slip

I Less

Fig. 20. Amount Zone.

of syn-rift

dextral

strike-slip,

measured

by the heave of the N-S-oriented

normal

faults, internally

in the Tornquist

Zechstein

Early Paleozoic

Upper Carbon./ Rotliegendes syn rift sequencr

Upper Silurian Devonian?

Late CarbonJEarly

i

.J ,’

,’

2-y

Permian

m Cambrian Lower Silurian [m, Basement

Fig. 22. A summary of the structural development during the Palaeozoic, along the Tornquist Zone in the Kattegat area. The Late Carboniferous/Early Permian picture indicate? right-lateral movements along the marginal faults of the Tornquist Zone.

Fig. 21. The Tornquist Zone acting as a “bufferzone” between more coherent crustal blocks, whenever changes in the stress field is induced.

Both a local Lower Palaeozoic and a syn-rift depocentre is separated 5-7 km right-laterally along the Bsrglum Fault. This offset is a cumulative displacement from post-Rotliegende rifting to the present day. An analysis of the syn-rift fault orientations and movements indicates 11-20 km of dextral displacement during the rifting. This would give a right-lateral displacement in the order of 16-27 km from the Late Palaeozoic to the present. Easy reactivation of the Tornquist Zone faults since the Late Palaeozoic indicates a weak basement zone, that might be an old crustal block boundary. This weak zone becomes a “bufferzone” whenever changes in the regional stress field take place. Fig. 22 summarizes the Palaeozoic structural evolution.

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The Oslo Rift-its evolution on the basis of geological and geophysical observations. In: E.-R. Neumann (Editor), Rift Zones in the Continental Crust of Europe-Geophysical, Geological and Geochemical Evidence: Oslo-Horn Graben. Tectonophysics, 178: 11-28. Sears, R.A., Harbury, A.R., Protoy, A.J.G. and Stewart, D.J., 1993. Structural styles from the Central Graben. In: J.R. Parker (Editor), Petroleum Geology of North West Europe. Proc. 4th Conf. Geol. Sot. London, pp. 1231-1245. Swenson, E., 1990. Cataclastic rocks along the Nesodden Fault, Oslo Region, Norway: a reactivated Precambrian shear zone. In: E.-R. Neumann (Editor), Rift Zones in the Continental Crust of Europe-Geophysical, Geological

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