Geophysical evidence for Early Permian igneous activity in a transtensional environment, Denmark

Tecronophysics,

193

189 (1991) 193-208

Elsevier Science

Publishers

B.V., Amsterdam

Geophysical evidence for Early Permian igneous activity in a transtensional environment, Denmark H. Thybo

and G. Schijnharting

Instiiutefor AImen Geologi, Copenhagen University, 0ster Voldgade IO, DK-I350 Copenhagen K, Denmark (Received

February

8,1989;

revised version accepted

June 15,1989)

ABSTRACT Thybo, H. and Schiinharting, G., 1991. Geophysical evidence for Early Permian igneous activity in a tramtensional environment, Denmark. In: S. Bjomsson, S. Gregersen, E.S. Husebye, H. Korhonen and C.-E. Lund (Editors). Imaging -and Understanding the Lithosphere of Scandinavia and Iceland. Tectonophysics, 189: 193-208. Integrated interpretation of gravity, magnetic and seismic data in the area of the strong Silkeborg Gravity High in Central Denmark has led to a structural geological model involving a transtensional rift feature which probably formed in connection with Permo-Carboniferous wrench faulting and the development of the Oslo-Horn Graben system. Reinterpretation of the deep seismic data from profile 2 of the EUGENO-S project in the area of the Silkeborg Gravity High detected a new thin high-velocity layer (6.25 km/s) at about 7 km deep within the Palaeozoic sedimentary sequence. In the crystalline basement a separate body of high velocity (6.9-7.0 km/s) was found at 11 km deep. Beneath thi; body the Moho reaches its shallowest position in the area. The seismic results compare well with an interpretation based on magnetism of a shallow volcanic or subvolcanil: layer at a depth of 6-8 km. This Iayer shows strong remanent and induced magnetization with a southward inclination of about 10”. Gravity reinterpretation of the area allows the separation of an upper body of density contrast 0.1 g/cm3 from a lower body of density contrast 0.2 g/cm-‘. The upper body roughly coincides with the magnetic body. The lower body, the major one, coincides with the deep seismic body and it is responsible for the main gravity anomaly in the area.

introduction

is

bing-Fyn High (RFH) and the Silkeborg-Samss Fault (SSF); the latter has been inferred from a magnetic lineament. The Ringkobing-Fyn basement high is dissected by N-S oriented grabens and troughs, including the Brande Trough (BT) which extends into the studied area. The

The area of the Silkeborg Gravity High, which situated within the Danish part of the

No~e~an-Danish to elucidate the

Basin, has been reinterpreted structural development of this

part of the basin and the possibility ic/plutonic episodes during its history.

Sorgenfrei-Tornquist

of volcan-

Zone,

which

marks

the be-

In the central part of the area the Bouguer anomaly map of Denmark shows a strong anomaly

ginning of the transition into the areas of the Baltic Shield, is located to the northeast next to the studied area.

of 50 mGa1, the Silkeborg Gravity High (SH). This feature is also associated with phase-shifted magnetic anomalies. In Fig. 1 the stronger parts of

A combined geophysical and geological study has been conducted, focusing on the area marked with the box in Fig. 1. In order to reduce the

the anomaly have been superimposed on a simplified tectonic map of Denmark and surrounding area. The approximately 150 km long and 60 km wide anomaly, which is situated in the southern part of the Norwegian-Danish Basin (NDB), trends ESE-WNW almost parallel to the Ringko-

limitations of each method, we carried out integrated inte~retation of data from existing gravity and aeromagnetic surveys, from reflection seismic surveys and available borehole data, and from a deep seismic profile of the EUGENO-S project (profile 2, Gregersen et al., 1987) which

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0 1991 - Elsevier Science Publishers

B.V.

194

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obliquely crosses the Silkeborg Gravity High. Three key profiles for the interpretation are shown in Fig. 2. The gravity and magnetic modeling was based on 2.5dimensional models and ray-tracing programs for 2-dimensional models were employed for the seismic modelling. Previous interpretations focussed on the individual data sets and only lately has it been attempted to interpret the gravity and rna~eti~ anomalies by geological models (Schiinharting, 1982; Abrahamsen and Madirazza, 1986). Inter-

r-

‘-

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\ R~NGK0BlNG-~YN .! WIGH

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ii S~‘tiONHAKTIYI~

pretation of the deep seismic EUGENO-S profiles (EUGENO-S Working Group, 1988) also took the gravity field into account and a theoretical gravity profile was calculated exclusively from the seismic velocity data. In the present study all the data sets were considered for an integrated reinterpretation in order to arrive at a reasonable tectonic model and to limit the multitude of possible models which result from using only one or two geophysical methods.

SllKEBORG

XIGH

Fig. I. Simplified tectonic map of Dwmark, sbcwixxgmajor faults and boundaries of blocks (modified from fig. 3 from EUGENE-S Working Group, 1988). The studied area is shown with tbc box, and the 20-40 mGal Bouguer gravity contours are showy as broken lines. SSF = Siiebwg-Samw Fault; BX = Brande Trough.

EARLY

PERMIAN

IGNEOUS

ACTIVITY

IN A TRANSTENSIONAL

ENVIRONMENT.

Magnetic and gravity anomaly study

mum

Magnetic and gravity anomaly maps

195

DENMARK

the magnetic the

maximum

is situated

to the

magnetic

minimuln

to

northeast

and

southwest,

thus indicating

a southward

the

declination

of the magnetization. The

Bouguer

anomaly

map (Danish

Geodetic

The

ESE-WNW

Institute,

1978) and an aeromagnetic

anomaly

map

anomaly

(courtesy

Maersk Oil and Gas) of the studied

area

km (and

are shown derived maps

in Fig. 3 (a and b) together

regional (c and

been carried

and

residual

d). Quantitative out along

gravity

with the anomaly

interpretation

the profiles

drawn

has on the

By virtue of the general and decrease

increase

in amplitudes

in wavelengths

the magnetic

anomaly

map clearly shows the deepening of the magnetic basement from the RFH in the southwest into the Norwegian-Danish The lateral extent

Basin towards the northeast. of the N-S trending Brande

Trough (BT), which divides the RFH into two separate blocks, is also easily identified in the studied area. A major linear gradient zone, interpreted to be a lineament or fault zone (SSF), dissects the area and, at the same time, separates the minimum of the Silkeborg magnetic feature from its maximum. In relation to the gravity rnaxi-

continues

positive

magnetic

over a length

of about

150

100 km further

towards

the

east-southeast) with four individual peaks arranged like a string of pearls. The magnetic minimum is most pronounced apparently, RFH

maps.

trending

is distributed

and

south of the gravity

the combined the

Silkeborg

effects

high where,

of the adjacent

anomaly

are

superim-

posed. In the Silkeborg general

Bouguer Gravity

anomaly

map

(Fi;;.

High clearly dominates

decrease

in gravity

NDB. The general

3a)

the

over the

from the RFH into the

west-northwesterly

trend of the

RFH and SH is inte~upted by N-S trends, particularly around the BT. The very strong gradient south of the SH towards the local minimum is interpreted as being the result of the Mesozoic subbasin or graben at Horsens in combination with the general feature. The regional been contoured gional

gravity

Silkeborg

gradient

of the Silkeborg

gravity

gravity anomaly map (Fig. 3c) has with the assumption that the refield is made up of the “smoothed”

gravity

anomaly

together

with

the ef-

fects from the RFH and the BT. In the following section it will be demonstrated how this albeit arbitrary premise leads to consistent ful geophysical interpretations.

and meaning-

The residual gravity anomaly map (Fig. 3d) bears a close resemblance to the magnetic anomaly map, although with some phase shift involved. Each of the two gravity maxima separates a magnetic maximum from its accompanying minimum, although with a slight shift of the magnetic axis towards the southeast. The strongest of the residual gravity maxima also corresponds with the strongest of the magnetic anomalies Hence, a common source for the gravity and magnetic anomalies is indicated by the maps. The source shows a tendency towards consisting of two sep-

Fig.

2. Map

magnetic

showing

profile;

the

G = gravity

studied profile;

circles = shotpoints

area

and

profiles.

S = seismic profile; (SP).

M = small

arate bodies which both have positive density contrasts in relation to their surroundings and strong magnetizations with resulting southward declinations.

GRAVITY

REGIONAL

MAP

camLlmrwwr smm

ANOMALY

MAP

o

‘9. ._D

‘O

‘O

‘nxrr

/_

(d) RESIDUAL -

(bl

GRAVITY

c

ANOMALY

i

‘2 MAP _-.__._I___~_-_~_

3**

‘ii-2

-~

_

_I._.._ 1

Fig. 3. Magnetic and gravity maps of the studied area. (a) Bouguer gravity map (after Danish Geodetic Institute. 1978), with magnetic, gravity and seismic profites. Contour inlervui .’ mGa1. {b) Magnetic anomaly map with the magnetic profile; redrawn on the basis of Mazrsk Oil and Gas’s aeromagnedc map. Contour interval SO nT, flight altitude 0.X km (c! Regional gravity anomaly map with location of gravity profile. Contour interval 5 mGal. fd) Residual gravity anomaly map with location of gravity profile. Contour inrerval 2 C rn(?al,

ANOMALY

BOUGUER

(a)

EARLY

PERMIAN

IGNEOUS

ACTIVITY

IN A TRANSTENSIONAL

ENVIRONMENT.

The shallow body

DENMARK

The through

from the magnetic

profiles

through

gravity Fig.

values

the

inter-

l-4

the residual

in magnetic

and

as they are drawn

3. Further

constraints

body were provided data. Reflection depth deposits. fairly

upper

body

sediments

undisturbed

km from the magnetic anomaly

with a tendency with a density same depth

towards contrast

in Fig. 4c.

magnetic body and of the of the RFH. Detailed magplate-like

body

shown in Fig. 4a. The upper surface of the plate varies from 5.8 to 6.4 km and the bottom from 7.2 to 8.2 km below the surface. The total magnetization is 8.8 A/m for the plate (with inclination 10” and declination 180”) and 4 A/m for the RFH (with inclination 70” and declination O”). Resolution is of the order of 0.5 km for the upper surface. No contribution from deeper sources was required.

the shallow I =lo”. A/m.

magnetic

Silkeborg

declination Parameters

anomaly

and observed anomaly

profile (Fig. 3b). Parameters

High body D = 180°,

I = 70”, D = 0”, J = 4 A/m.

for

(to the right): inclination

total

magnetization

for the Ringkebing-Fyn (b) Residual

J = 8.8

High (to the left): gravity interpreta-

tion along the gravity profile (Fig. 3d), showing gravity values with circles, modelled

the residual

gravity anomaly

with a

heavy line, and the modelled shallow Silkeborg High body with a density

contrast

anomalies

and observed

of 0.11 g/cm. magnetic

(c) Calculated anomaly

magnetic

(circles)

for the

shallow Silkeborg High gravity body (Fig. 4b) with magnetization values of Z = 10”. D = 180” and J = 8.8 A/m (calculated),

(heavy line)

and Z = - 30’. D = 180” and J = 8.8 A/m

line) (observed).

Magnetization

for the Ringkabing-Fyn

model as in Fig. 4a.

for the grav-

are shown

with the RFH

salt

tion of the Silkeborg induced magnetization

along the magnetic

body (Fig. 4b).

calculated

gether

which

field along the profile is interpre-

Fig. 4. (a) Modelled

two,

and in the

km,

effect of strong magnetiza-

(circles)

into

of body

- 30”) to-

this level appear

in the

divided

of

+ 10” and

ted to be the combined

resulted

Modelling

of 0.11 g/cm

anomalies

salt dome province to the northwest of the SH. Reflectors with a strong dip towards the southsouthwest are found beneath the base Zechstein.

modelling

being

runs

an offset

in a plate-like

range as the magnetic

The magnetic

have formed within the SH area, despite the considerable thickness of salt and the existence of a

netic

modelling with

profile.

resulted

and only a few salt structures

The magnetic

gravity

maximum

ity body (with inclinations

of the Zechstein

above

the

a minimum

of 5-5.5

to the bottom The

to the

by the deep seismic sounding

seismic data provided

to the

corresponds

on the maps in

on the depth

for

the gravity

along

and gravity

extremes

profile

data

The shallow body has been quantitatively preted

197

(light High

RESIDUAL r-

GRAVITY

ANOMALY

body

The inclination data

is not very well constrained

and it may vary considerably

along

the magnetic

similarity, gravity

body

cannot

be

which,

fully

by the

(up to f20”) despite

coincident

its close with

the

body. intensity sources

ues of the

remanent

The dominance

direction clearly

subvolcanic

comparison

and the high magindicate

with negative magnetization

of the remanent

to the induced

cal of extrusive

bodies

volcanic

inclination

or val-

component.

magnetization

magnetization

in

is typi-

due to the quite fast cool-

may acquire stable chemical remanence. In view of the slightly different location

of the

magnetic and gravity profiles one can further conclude that the magnetic plate with anomalously high magnetization density values, tendency

also exhibits anomalously high supporting the volcanic/sub-

origin of the residual for the denser

concentrated

the Bouguer

around

parts

gravity

feature.

of the body

The to be

the 85 km and 130 km dis-

tances along the surface is probably due to a nonuniform distribution of the subvolcanic dykes and weaker remanent magnetization tively deeper seated elements.

of these rela-

The deep feature

anomaly

ping and subsequent by features

basin.

Basin depths

aeromagnetic lies caused ment

floor”,

magnetized The major RFH include

i.e.

it

shows

seated

below

and,

only

by features which

map

anomalies

estimated

therefore,

anomaly

strip-

void with

the sedimentary

were primarily

maps

gravity

by sediment

filling of the basin

rocks;

caused

basement

ing rates which result in fine-grained magnetic minerals. However, remagnetization by oxidation is an alternative process whereby subvolcanic rocks

volcanic

recting basement

The magnetization netization

map in Fig. 5 (after Schonharting, 1082). The basement gravity anomaly is the result of cor-

from

the

regional

includes

anoma-

below the “magnetic

base-

is the level of the shallowest

rocks.

map reveals the Silkeborg anomaly as the gravity anomaly in Denmark whereas the gravity effect disappears. Minor features a positive anomaly over the Danish part of

the Central Graben, the amplitude, however, being much less than further to the northwest where the graben sediments attain maximum thickness. The Silkeborg basement-corrected anomaly, which is 70 km wide and 250 km long, trends WNW-ESE from the centre of Zealand to the middle of Jutland

from

where

it trends

towards

the

north-

northwest and finally disappears in the Skagerrak. Modelling of the SH regional basement anomaly revealed a block with the same upper surface as seen in the previous model, although with different sidewall angles (Fig. 6b). A density 0.27 g/cm3 brings the lower boundary of approximately

18 km. This depth

contrast of to a depth is somewhat

The regional gravity anomaly was interpreted with a block model of density contrast 0.20 g/cm3 (Fig. 6a). The upper surface is at 10 km deep and the lower one at 20 km deep in order to account for the steep gradient on the flanks of the SW and to agree with the arrival times of the seismic phases, including strong reflections from the top of the block. The contribution of the RFH gravity field (density contrast 0.10 g/cm3) was also taken into account. To the north of the SH, systematic differences occur between the model and the regional anomaly. They are mainly caused by further sediment thickening in this direction. To arrive at a more complete gravity interpretation in which the influence of the sediments above the crystalline basement is reduced to a minimum, we used the regional basement gravity anomaly

Fig. 5. Map of regional Schanharting,

basement

beneath the magnetic basement. anomaly

gravity

1982). This map shows

anomalies

(after

the effects of features

It clearly reveals the SiIkeborg

as the major gravity anomaly

tinuous line shows the profile employed in Fig. 6b.

in Denmark.

The con-

for the interpretation

FARLY

PERMIAN

IGNEOUS

ACTIVITY

IN A TRANSTENSIONAL

ENVIRONMENT.

S

N REGIONAL

GRAVITY

ANOMALY

DENMARK

199

good enough

to aliow

of the seismic above

the

interpreted

classical

phases.

Permian from

phase

correlation

The sedimentary

structure

Zechstein

reflection

was offset from the profile

BASEMENT

GRAVITY

static

correction

were

estimated

from

for the deeper files.

parts,

The

ANOMALY /c-y

r--v-

four

deposits

was

sections.

SP6

and this necessitated

of 50 ms. Sedimentary

record

data are shown

-‘1

salt

seismic

borehole

information

from the deep

sections

and,

seismic

of the deep

in Fig. 7, laterally

a

velocities

shifted

pro-

seismic to their

correct relative positions. SP4 is located on the RFH. The crustal refraction P, gives a very clear first arrival,

which is gradually

delayed

due to the

thickening of the sedimentary layers, to a distance of 80 km where the signal changes character. apparently in connection with an intracrustal reflection. Between 80 and 140 km there is a complex mixture of signals after the first arrival, which is very weak and which cannot be ident:ified with the scaling in Fig. 7. There is a distinct reflection Fig. 6. (a) Interpretation gravity

profile

(density

contrast

(Fig.

Fyn High (density

of the regional 3~). The

0.2 g/cm’) contrast

deep

gravity Silkeborg

field along the High

feature

lies to the right, the Ringkabing0.1 g/cm3)

tation

of the regional

basement

profile

shown in Fig. 5. Density deep Silkeborg

to the left. (b) Interpre-

gravity contrast

anomaly

along

is 0.27 g/cm3

the

for the

High body.

from the Moho (P,P) 80 km and the mantle lated

from

with a critical distance of refraction P, can be corre-

100 km and

tances beyond

as a first arrival

at dis-

140 km.

SP14 is located near the edge of the RFH. P, is distinct to the southwest, clearly showing the thinning of the sediments. To the northeast it rapidly looses energy and high velocities are encountered

poorly defined, and could be further increased by using smaller density contrasts. Assuming standard lower crust densities for the surrounding rocks, the deep block feature can well be explained by an ultramafic to mafic block with fairly steep flanks. Seismic study The location of the deep seismic profile (EUGENO-S profile 2) is shown in Fig. 2 by the line marked with four shotpoints, SP4, SP5, SP6 and SP14. The observation scheme for the data set is shown in Fig. 8a. The data used for the interpretation were plotted trace normalized after bandpass filtering (3-30 Hz). Data quality is generally

very near the source. The record section from SPS, which is situated on the maximum of the SH, is not typical of the EUGENO-S data. The sedimentary ph.ases arrive with the expected velocities out to a distance of 20-30 km, followed by crustal phases with very large apparent velocities. In the distance range 25-50 km these phases are very strcng to the southwest, and to the northeast they are of considerable amplitude, thus indicating reflections. In both directions the PMP, ~thou~ relatively weak, can be identified to a critical distance of 75 km to the northeast and 80 km to the southwest. To the southwest, relatively strong high-frequency arrivals are found approxiately 500 ms later than the P,P at distances of 50-80 km. They may be strong

200

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