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TECTONICS, VOL. 7, NO. 4, PAGES 803-821, AUGUST 1988
CRUSTAL THINNING AND NATURE OF EXTENSION
IN THE NORTHERN NORTH SEA FROM
DEEP SEISMIC REFLECTION PROFILING
Simon L. Klemperer
British Institutions' Reflection Profiling
Syndicate, Bullard Laboratories,
Department of Earth Sciences, Madingley Rise,
Madingley Road, Cambridge, England
Abstract. A regional network of deep
seismic reflection profiles recorded in
the northern North Sea has been used to
map Mesozoic and Cenozoic basin thickness
and crustal thickness•in the Viking Graben
and adjacent platform areas. Depth to the
reflection Moho varies from about 20 km in
parts of the Viking Graben to about 32 km
beneath the Shetlands and the Norwegian
margin. The shallowing of the reflection
Moho beneath the Viking Graben implies
crustal stretching factors for Mesozoic
extension greater than 2 in the center of
the North Sea basin. The axis of crustal
thinning is located directly beneath the
Viking Graben, the axis of the sedimentary
basin. Steeply dipping basin-bounding
faults are imaged in the upper crust and
several dipping reflectors are observed in
the upper mantle, but no continuous re-
flective feature extending from the sedi-
mentary basin to beneath the Moho is
observed on any of the deep profiles.
Thus these data do not support the exist-
ence of lithosphere-penetrating low-angle
detachments (zones of simple shear) as the
cause of extension in the northern North
Sea. Some of the mantle reflectors dip to
the west and some to the east, suggesting
Copyright 1988
by the American Geophysical Union.
Paper number 7T0906.
0278-7407/88/007T-0906510.00
that simple shear, if it occurs in the
upper mantle, is not of uniform sense.
Rather, these data suggest a complex,
depth-dependent pattern of brittle exten-
sional deformation in the upper crust;
pervasive, ductile extension (bulk pure
shear) in the lower crust (which is de-
coupled from deformation in the mantle);
and extension accommodated by discrete,
dipping shear zones in the lithospheric
mantle.
INTRODUCTION
A new regional network of deep seismic
reflection data which extends from the
Shetlands to Norway, the Geophysical
Company of Norway (GECO) North Sea Deep
Profiles 1984 (NSDP-84), provides for the
first time a comprehensive view of crustal
structure and thickness beneath the whole
area of the northern North Sea. These
profiles are used in this paper to map the
reflection Moho and hence the degree of
crustal thinning in the northern North
Sea, and to develop a new interpretation
of the style of extension in this area
that incorporates depth-dependent failure
mechanisms but that resembles the uniform
stretching model [McKenzie, 1978] in its
overall symmetry. Interpretation of the
four NSDP lines was carried out in con-
junction with the interpretation of a
British Institutions Reflection Profiling
Syndicate (BIRPS) deep reflection survey
around the Shetlands and a proprietary
804 Klemperer: North Sea Lithospheric Extension
4W
62 N
ERLAND
TERRACE
M•RE
BASIN
4E
FAEROE
TROUGH
SHETLAND
TERRACE
60
SHETLAND
PLATFORM
NSDP-2
EAST ORKNEY
'_'. t
o 4E
Fig. 1. Map of northern North Sea showing location of new reflection profiling
(NSDP survey, thick solid lines), previous BIRPS surveys SHET [McGeary, 1987],
MOIST [Brewer and Smythe, 1984], and DRUM [McGeary and Warner, 1985] (dot-dash
lines), and Britoil proprietary seismic profile [Beach, 1986] (dotted line). Thin
solid lines are principal fault structures. Each circled number marks the rough
location of the seismic data shown in the figure with the same number.
Britoil line across the northern Viking
Graben (Figure 1), a total da.tabase of
3000 km of deep reflection data.
Major controversy exists at present
about the mechanisms of formation of
extensional sedimentary basins. Two very
different end-member possibilities are the
uniform stretching model [McKenzie, 1978]
and the lithospheric simple-shear-zone
model [Wernicke, 1985]. In the uniform
stretching model the whole lithosphere is
uniformly extended by a factor .8, effect-
ively by pure shear, producing basins of
symmetric shape (but not necessarily
symmetric fault orientat ion) and super-
posed axes of crustal thinning, synrift
subsidence, and postrift subsidence [Mc-
Kenzie, 1978]. In the lithospheric sim-
ple-shear-zone model, extension occurs on
a single shear zone that runs from the
surface to the base of the lithosphere and
along which thinning of the mantle litho-
sphere is displaced downdip from thinning
of the crust [Wernicke, 1985]. The sim-
ple-shear-zone model predicts the exist-
ence of crustal-penetrating detachments,
asymmetric rift basin development, and
possibly the lateral displacement of the
thermal subsidence basin from the synrift
basin and axis of maximum crustal
thinning [e.g., Coward, 1986].
Several predictions of these models can
easily be tested by deep reflection
data. Reflection profiling is routinely
used in hydrocarbon exploration to delin-
eate the shapes of sedimentary basins and
in recent years has also been used to
study the deep structure of extensional
Klemperer: North Sea Lithospheric Extension 805
provinces [e.g., Allmendinger et al.,
1987]. Thus seismic reflection data can
constrain the location and degree of
crustal thinning in extended regions.
Deep reflection profiling has also proved
extremely successful in tracing structures
into the lower crust (e.g., Outer Isles
fault [Brewer et al., 1983]), and even
deep into the upper mantle (e.g., Flannan
reflector [McGeary and Warner, 1985]), and
thus can be used to test the Wernicke
[1985] hypothesis of lithosphere-penetrat-
ing shear zones.
The North Sea basin, intensively stud-
ied by hydrocarbon explorationists, has
frequently been used to test developing
ideas about extensional tectonics. In the
northern North Sea, very different models
of extension have been advanced by differ-
ent authors. Badley et al. [1988] adopt
the uniform stretching model and use the
basin subsidence history to calculate
•=1.5. Giltner [1988] also advocates the
uniform stretching model and obtains a
slightly higher estimate of extension from
subsidence curves, •=1.8, which he finds
consistent with estimates of crustal
thinning from a refraction line [Solli,
1976] and a reflection line [Beach et al.,
1987] (8=2.0). In contrast, Beach et al.
[1987] obtain from their subsidence calcu-
lations 8=3.3, greater than the 8 at which
oceanic crust might be expected to form
[Le Pichon and Sibuet, 1981] and certainly
a value which predicts large-scale basalt-
ic volcanism [Foucher et al., 1982] which
is not observed. Beach et al. try to
explain the discrepancy with their direct
estimate of crustal thinning from their
deep reflection profile (8=2.0) by a
complex model of early simple shear [Wern-
icke, 1985] and late formation of pull-
apart basins bounded by "through-crustal
strike-slip fault zones." In this inter-
pretation, Beach et al. [1987] hypothesize
the existence of a lithospheric low-angle
shear-zone and crustal-scale high-angle
faults.
In this paper •t is argued that the
available deep seismic profiles are in
fact more consistent with symmetric (uni-
form) extension than with the asymmetric,
lithospheric simple-shear-zone model.
NSDP-84: ACQUISITION AND PROCESSING
The NSDP-84 survey was acquired using
moderately large airgun arrays (around
6000 cubic inches or 100 1 at 2000 p.s.i
or 14 MPa), adequate to image the reflec-
tion Moho through all but the thickest
accumulations of sedimentary rocks but
still 30% smaller than the largest arrays
used for BIRPS deep reflection profiles
[e.g., Warner, 1986]. A 30-fold unmigrat-
ed stack was prepared by GECO, in which
there exist some problems of data quality,
including multiples and interference from
other seismic vessels. Poststack process-
ing, carried out on the VAX 11/750 at
Bullard Laboratories using Merlin Profil-
ers' Seismic Kernel System, helped allev-
iate these problems. Data shown in this
paper incorporate this poststack process-
ing (Figures 3 to 7).
Seismic profiles record reflections in
travel time and before interpretation must
be depth-migrated in order to place the
reflections in their true spatial posi-
tion. This process is particularly impor-
tant in correcting for lateral velocity
variations and velocity pull-down effects
beneath deep sedimentary basins [Peddy et
al., 1986]. Because conventional migra-
tion of deep reflection data normally
gives poor results [Warner, 1987], espec-
ially for noisy data such as the GECO NSDP
profiles, line drawings were made of the
stack sections, and these drawings were
depth-migrated by two-dimensional ray
tracing through a laterally varying veloc-
ity structure [Helbig, 1980; Raynaud,
1988]. Though only portions of the line
drawings are shown here, complete line
drawings of the NSDP survey are given by
Gibbs [1988a, b]. The velocities used for
depth migration, derived from over 50 well
logs recorded a•ong the profiles, are:
water, 1.5 kms- ; post-Palaeocene strata,
2.1 km s -1; Pala•ocene, 2.5 km s -1; Creta-
ceous, 2%• km s -l; Jurassic and Triassic,
3.2 km s ; Upper Palaeozoic, 4.5 km s-•.
The average velocity of the crystalline
crust beneath the mid-Palaeozoic (post-
Caledonian) unconformity was assumed to be
6.2 km - , based on sparse refraction
results [Smith and Bott, 1975; Solli,
1976; Christie and Sclater, 1980; Barton
and Wood, 1984] and proprietary expanding-
spread data (J.-P. Vernet, personal com-
munication, 1987). Reflections from
beneath the reflection Moho were migrated
at 8.0 km s-1 Interpretations presented
here are based largely on study of these
migrated line drawings (Figures 2b and 3c
to 7c).
SYNRIFT AND POSTRIFT SEDIMENTARY BASINS
In order to determine the amount of
extension in the North Sea, it is neces-
806 Klemperer: North Sea Lithospheric Extension
NSDP 84 LINE I TIME SECTION
Shetland Tampen
Spur Magnus Basin Shetland Terrace Spur Viking Graben Horda Platform
....... .... I
• • ' 15s
DEPTH MIGRATION
i' ............................ '..•.•.-.•;•.• ...... •...'•-...•.-..-- .•..•.•.....•.•.,,•.•--- - • -•.? .......... ?:"- ............................... •'---'/•'" ................. 1' 0
,, ' ........ > .................................... ø ......... ......-.-...-.-.-.---.-.-.-.'-'-'-'-'-'-'" ...... t•ase Mesozoic •"
L- "• , -- --.., ..... '-•-•-.•__•. __-:-....•__ . .,.•-...•.'.'::::•:-•'-:-'-t:::.-.1%_...•-:'-'-'-'-'-'-'•_...-.•,- • --- • -- - • -'- • -- I- 20
_• , • .•-•. ---• .-'.. _•i.... •- . .• .--•f"•'"" - "'---'+•+:':•"l'•.....•..:.'_,•:...• --'-- '"....' ......
F! ..•.••.. __..•.:• •.•.•:•- --.•:::•--.---:•:-:-:•::-'.'-'.':::::•:•.•::::::-.--.-:.?'•-'• ......... '•-':-.',-:-'•::.-:.• ................... .•.:•:•:-'-'-'-'.'-•'.'-:.'_:.-.-':.-':•:.¾..-.:•
[.':i"::::-*-;-::-•:::'• :'"'•:'•:: "' .... ......... :'• ......... • ...... ._• Reflection Moho -• -'• ....... •" •"q 40 km
NW 0 50 100 km SE
I • I
Fig. 2. (a) Unmigrated line drawing in travel time of NSDP line i from
Shetland Spur to Norway (for location, see Figure 1). (b) Depth-migrated
line drawing of NSDP line 1. Upper shaded zone represents interpreted base
of Mesozoic, and lower shaded zone, the reflection Moho.
sary to know the sedimentary thickness and
to subtract this from the present-day
crustal thickness in order to give the
thickness of prerift basement. The NSDP
data provide regional cross sections of
the sedimentary structure of the northern
North Sea basins (e.g., Figures 2 and 3)
which complement the far more detailed
information available from routine pro-
filing for hydrocarbon exploration [e.g.,
Ziegler, 1982; Glennie, 1986]. Generaliz-
ed stratigraphy has been mapped from
available well log control and may be
summarily divided for the purpose of this
paper into synrift sedimentary rocks
(Triassic, Jurassic, Lower Cretaceous)
that are demonstrably cut and rotated by
high-angle faults, and postrift sediment-
ary rocks (Upper Cretaceous, Cenozoic)
deposited after fault movement had es-
sentially ceased (e.g., Figure B). The
thermal subsidence basin extends across
NSDP 84-2
VIKING GRABEN
Unmigrated time section
WEST I 20 km ! EAST
Fig. 3a. Unmigrated seismic data from NSDP line 2 (to 8 s two-way travel time
only) across the Viking Graben. For location, see Figure 1.
Klemperer: North Sea Lithospheric Extension 807
TIME
8 $
NSDP 8 4- 2
VIKING GRABEN
K
Unmigrated time section
Fig. 3b. Line drawing of Figure 3a. Triangle symbols mark position of
interpreted well logs. Ng, Neogene; Eo, Eocene; upper shaded area (fine
stipple), Paleocene; K, Cretaceous; lower shaded area (coarse dots),
Lower Cretaceous; J, Jurassic; Tr, Triassic strata.
the entire northern North Sea, thinning
toward the Norwegian and Shetland coast-
lines with a characteristic, symmetric,
steer's head geometry [e.g., Dewey,
1982]. High-angle faults within the
sedimentary basins are often directly
imaged on the seismic sections (Figure 3a)
and are apparently planar after migration
(Figure 3c). Although downdip extensions
of these faults into the crystalline
basement are not imaged on the NSDP data,
a single possible example of a listric
fault plane reflection from within the
upper part of the Caledonian basement has
been reported from shallow (6 s) seismic
data [Swallow, 1986].
AMOUNT AND LOCATION OF CRUSTAL THINNING
On all the deep reflection profiles the
reflection Moho [Klemperer et al., 1986]
can be recognized as the base of the
reflective and diffractire lower crust
(Figures 2 and 4 to 7). Although the lack
of collinear reflection and refraction
profiles in the northern North Sea pre-
cludes a direct test beneath the Viking
Graben of the equivalence of the reflec-
tion Moho and the refraction Moho (the
boundary between the crust and the mantle
defined by a rapid increase in seismic
velocity), coincident BIRPS reflection
data and a refraction profile have been
used to demonstrate this equivalence in
the central North Sea [Barton, 1986].
Based on that test, and on equivalent
results worldwide [Mooney and Brocher,
1987], the reflection Moho is here assumed
to correspond to the base of the crust.
The line drawing of NSDP line 1 (Figure
2a) shows the reflection Moho at a fairly
uniform travel time (10 s ñ 1 s) across
the 300-km-long profile. However, the
depth-migrated line drawings of NSDP line
1 (Figures 2b and 4c), of NSDP line 4
(Figure 5c) and of BIRPS SHET profiles
(Figure 6c) from which the velocity pull-
down effects of the sedimentary basins
have been removed, show clear evidence for
crustal thinning beneath the Shetland
Terrace and the Viking Graben. Crustal
Migrated depth section
10 km
WEST I 20 km I EAST
Fig. 3c. Depth migration of Figure 35. Velocities are given in the text.
Arrows indicate positions of Mesozoic normal faults.
808 Klemperer: North Sea Lithospheric Extension
NSDP 84-1 VIKING GRABEN Unmigrated time section
•- ......... iii i ii
___
•_•.-.:•.:• .... -•_--- --~.•..• ___ -.•. --•._. f._ ___ . •. --. • _ -•.••. .•.•-• ••-.•_•_ •.•.,, •_: . , .•---• •-• _ •__ • ...... --•••••
NORTHWEST I 20 km I SOUTHEAST
Fig. 4a. Unmigrated seismic data from NSDP line 1 across the Viking Graben.
For location, see Figure 1.
thickness ranges from about 20 km beneath
the Viking Graben to about 30 km beneath
the Shetland Spur and the Horda Plat-
form. These values are broadly in agree-
ment with the results of a refraction
profile from the Shetlands to Norway
[Solli, 1976; Ziegler, 1982], with the
results of five proprietary expanding-
spread profiles recorded at about 61 o N
in the Norwegian sector by Elf Aquitaine
Norge and Institut Francais du Petrole
(J.-P. Vernet, personal communication,
1987), and with gravity interpretations
that assume Airy isostasy [Donato and
Tully, 1981; Zervos, 1987].
Interpretations of all the 3000 km of
deep reflection profiles shown in Figure !
have been used to prepare maps of crustal
thickness (strictly, depth to the reflec-
tion Moho) (Figure 8a) and of thickness of
pre-Mesozoic basement (one-way travel time
between interpreted base-Triassic and
reflection Moho multiplied by 6.2 km s -1)
(Figure 8b). The likely errors in these
maps can be quantified. Any pick of the
reflection Moho at a single location may
be uncertain by about ß 0.3 s (ñ ! km),
though the correlation of the reflection
Moho at over a dozen line ties and along
3000 km of profiles gives greater confi-
dence than this in general. There is
insufficient velocity information avail-
able from the northern North Sea to take
into account possible lateral variation in
mean basement velocity, but changes in the
likely range of 6.0 to 6.5 km s-! would
only produce differences of ß 1 km in
calculated crustal or basement thick-
nesses. The distribution and thickness of
Palaeozoic sedimentary rocks beneath the
Mesozoic sedimentary rocks of the Viking
Graben and Shetland Terrace is largely
unknown, but the calculated depth to the
reflection Moho is only too large by 1 km
for each 2o7-km thickness of Upper Palaeo-
zoic rocks present for the velocity struc-
ture given above. There are also potent-
ial errors in the interpretation of the
base of the Triassic section (as yet
undrilled in the Viking Graben), but int-
erpretations made from the NSDP data are
consistent with previous estimates based
on much larger volumes of seismic data
[Ziegler, 1982]. Any error in picking the
base of the Mesozoic section will produce
a smaller error in the calculated crustal
thickness but an equal error in the calcu-
lated basement (pre-Mesozoic) thickness.
The total errors are probably less than ñ
2 km in the crustal thickness and ß 3 km
in the thickness of pre-Mesozoic basement.
The maps of crustal and basement thick-
ness (Figures 8a and 8b) allow ready
calculation of the degree of extension and
show clearly the spatial relation between
crustal thinning and location of the
sedimentary basin. If the crustal thick-
ness of the Shetland Platform [Christie
and Sclater, 1980] and of western Norway
[Cassell et al., 1983] of about 32 km is
Klemperer: North Sea Lithospheric Extension 809
NSDP 84-1
VIKING GRABEN
TIME
15 s
Unmigrated time section
Fig. 4b. Line drawing of Figure 4a.
taken to represent the pre-Mesozoic crust-
al thickness of the whole North Sea re-
gion, then the thinnest basement, of about
15 km, corresponds to a maximum stretching
factor of 2.1. The mean extension across
the entire profile illustrated in Figure
2, assuming a uniform preextensional
crustal thickness of 32 km, is a factor of
1.4. These mean and maximum stretching
factors are comparable to those determined
seismically for the Central Graben (1ol at
the basin margin to 2.2 at the basin
center [Barton and Wood, 1984]) and the
Moray Firth Graben (maximum 1.7-1.8
[Christie and Sclater, 1980]).
All seismic estimates of crustal thin-
ning rely on measurements of present-day
crustal thickness and assume that the Moho
is a passive marker and that the change in
thickness of the crust is a direct measure
of the strain. However, if crustal volume
has not remained constant but has increas-
ed during extension by the addition of
basaltic melts to the crust [e.g., Keen et
al., 1983], or if the Moho is not a pas-
sive marker but has begun to revert to the
DEPTH
Migrated depth section
MOHO -
45 km
NORTHWEST I 20 km I SOUTHEAST
Fig. 4c. Depth migration of Figure 4b. Velocities are given in the text.
Note east dipping reflector in the upper mantle.
810 Klemperer: North Sea Lithospheric Extension
NSDP 84-4 SHETLAND TERRACE Unmigrated section
I:,--.-=-_ - • ___' • - •.- ---.--- ---'•'•' -" '- •- '----'• •--•-•,,•--- ' '-•'-•••••:-- -_ -• • .:-•-• 7--" -"
-•__-•_. E._..•.•-,•..•_•-..-•,,•---. -_. -----__:•_ _••• .......... ••. •.•_•.•_ _ __--.•••..•.• ::_. •
I---- _-.--.-•---_-'- '------•- •:•- ---': --'-•.•-•.. -.__._•••_i '• - -• - --:-----'-'-- -• •-•?•- -'•--• •
SOUTHWEST I 20 km I NORTHEAST
Fig. 5a. Unmigrated seismic data from NSDP line 4 across
the Shetland Terrace. For location, see Figure 1.
mechanically stable, horizontal attitude
by postextensional lower crustal flow
[Meissner et al., 1986, 1987], then the
estimates of crustal thinning given above
are minimum values. Crustal thickening by
synextensional intrusion is presumed to be
negligible for the northern North Sea,
since the moderate degree of extension, a
mean • of only 1.4 measured across the
section of Figure 2, should have been
insufficient to produce significant part-
ial melting in the mantle: Foucher et al.
[1982] calculate that partial melting only
begins at • values of 1.7 to 2.1, depend-
ing mainly on assumptions about the temp-
erature at the base of the lithosphere.
Even though the existence of stretching
factors as large as 2.1 in the center of
the Viking Graben (Figure 8b) suggests
that some partial melting may have occur-
red, the general scarcity of Mesozoic
volcanics in the northern North Sea [Dixon
et al., 1981; Ziegler, 1982] implies that
igneous additions to the crust during
extension were minor in this area unless
crustal intraplating and underplating
occurred without corresponding volcan-
ism. Some authors [e.g., Warner and
Cheadle, 1987] have suggested that layer-
ing in the lower crust corresponds to
mafic sills intruded during extension, but
if this is so, then intrusion took place
uniformly across the North Sea beneath
both rifted and unrifted areas, since the
thickness of the lower crustal reflectiv-
ity is approximately constant across the
profiles (Figure 2). Although the crustal
antiroot beneath the North Sea may have
undergone some decay since extension
ceased, the continued thermal subsidence
of the North Sea basin throughout the
Tertiary and Quaternary [Lovell, 1986],
rather than the isostatic uplift that
would be expected if crustal thickening
had occurred, suggests that this effect is
minor at the present time. It is there-
fore assumed in this paper that
the reflection Moho in the northern North
Sea is a valid strain marker for calcula-
tion of the amount of crustal thinning
with only minor errors and thus that the
maximum crustal stretching occurs beneath
the thickest synrift sedimentary deposits
in the Viking Graben and the region of
crustal thinning underlies the whole area
of Mesozoic synrift sedimentation between
the Shetland Platform and the Horda Plat-
form (Figures 2 and 8).
DIPPING REFLECTIVE ZONES
IN THE UPPER MANTLE
On all four NSDP profiles and on the
other available deep profiles, dipping
reflections or zones of diffractions are
observed at travel times of 10 to 15 s
which, after appropriate depth migration,
appear to originate from structures in the
upper mantle (Figures 2b and 4c to 7c).
The location and dip of each zone of
Klemperer' North Sea Lithospheric Extension 811
SHETLAND TERRACE
NSDP 84-4
0 i .
_
• • BASE TRI'ASSiC'"
---•..-..-..--.-•.•-••"' "'*'"•.• .............. MOHO• -
15 s
Unmigrated time section
Fig. 55. Line drawing of Figure 5a.
dipping reflections is shown in Figure
9. In common with all other reflectors
that cannot be traced to outcrop or
drilled subcrop, the age of these reflect-
ors is a matter for conjecture. One
potential test of the age of these mantle
structures is the relationship of their
trends, dips, and locations to those of
known geologic features. The northern
North Sea basin developed on crust meta-
morphosed and intruded during the Caledon-
ian orogeny [Frost et al., 1981]. The
Norwegian Caledonides have easterly ver-
gence, whereas the principal Scottish
structures are northwest vergent folds and
thrusts [e.g. Watson, 1984] or vertical
strike-slip faults [McGeary, 1987].
Devonian basins subsequently formed within
the Caledonian mountain chain, in at least
some cases by reactivation of Caledonian
faults [McClay et al., 1986]. Palaeozoic
structures in the mantle, if they exist,
may therefore dip to the west on the
Norwegian (east) side of the Viking Graben
and to the east or northeast on the west
side of the rift. Mesozoic structures
might show these dips as a result of
reactivation of Palaeozoic structures or
might alternatively have formed as new
structures with opposite dips and with
strike generally parallel to the north-
northeast trend of the Viking Graben.
Figure 9 shows that the intramantle
reflectors seen on the NSDP profiles and
on the BIRPS SHET data have apparent dips
to the west on the west side of the North
Sea basin, and, predominantly, apparent
dips to the east on the east side of the
basin. True dips can only be determined
if crossing or closely spaced seismic
lines are recorded. In the northern North
Sea such information is only available in
two places, indicating a true southeast
dip beneath the east edge of the Viking
Graben on NSDP line ! (Figure 4) and the
Britoil line, and a true west-southwest
dip beneath the west margin of the Shet-
land Terrace on NSDP lines 2 and 4 (Figure
5). Both of these structures should be
post-Palaeozoic by the criteria developed
above. Where only a single seismic l•ne
is available, true dip is constrained to
lie within 90 ø of the apparent dip, and
thus, with the possible exception of the
west dipping structure imaged om NSDP line
3 east of the graben (Figure 7), all tt•e
observed intramantle structures are prob-
ably of post-Caledonian age and related to
Mesozoic basin formation. Note the exis-
tence of crossing reflections on NSDP line
3 (Figure 7c) and on the BIRPS SHET pro-
file (Figure 6c) even after two-dimension-
al migration, suggesting either that two
generations of structures are present or
that some reflected energy has come from
out of the plane of the sections. These
possibilities can only be tested by areal
surveys, for example like those already
available in the regions from which Fig-
ures 4 and 5 are drawn, where the exist-
ence of closely spaced profiles precludes
the possibility of sideswipe from a single
structure as the cause of the mantle re-
flections.
That the dipping mantle structures are
Mesozoic rather than Palaeozoic is also
suggested by the symmetric disposition of
Oi
Migrated depth section
DEPTH
45 km.
BASE TRIASSIC
SOUTHWEST I 20 km I NORTHEAST
Fig. 5c. Depth migration of Figure 55.
Velocities are given in the text.
Note crustal thinning from southwest to
northeast, and the southwest dipping
reflector in the upper mantle.
812 Klemperer: North Sea Lithospheric Extension
BIRPS (SHET) SHETLAND PLATFORM Unmigrated section
•:-•i ---c•-•_--'f--•--_•_-= • - --'•-.------- o•_•._---L-_ .... =--_:..?--•..=..----•-•. ----•--- •-.•---•=- :.=--•- -=- - : .•_-•f--.•_--•--:_-- - =7-:•-.1•-•.•-_•- --- •
15
NORTHWEST I 20 km I SOUTHEAST
Fig. 6a. Unmigrated seismic data from BIRPS SHET profile across
the Shetland Platform. For location, see Figure 1.
the dipping reflectors about the North Sea
rift. Figure 9 shows that these struct-
ures are located directly beneath the
western margin of the Mesozoic rift ba-
sins, the boundary between the Shetland
Platform and the Shetland Terrace (Figure
1), and also between the eastern edge of
the Viking Graben and the Horda fault
zone, the eastern margin of the rift bas-
ins. This spatial association of the
dipping mantle reflectors with major
Mesozoic faults is the strongest argument
that the dipping reflectors are themselves
Mesozoic features.
Several hypotheses can •e advanced for
the geological nature of these dipping,
intramantle reflectors (and of others
observed elsewhere on BIRPS data) such as
shear zones (anisotropic, altered, or
including entrained fragments of different
rock types), relict subduct[on zones, ig-
neous intrusions, or lenses of ultramafic
rock of composition different from the
bulk upper mantle. The range of apparent
dips of the intramantle reflectors beneath
the North Sea, from 15 ø to 35 ø (Figure 9),
is not suggestive of igneous intrusions,
and the different dip directions, to west
and to east (Figure 9), would require at
least two subduction zones; but the magni-
tudes of dip and the directions of dip are
both consistent with the reflectors being
shear zones. The most intensively studied
intramantle reflector, the Flannan re-
flector northwest of Scotland, is believed
on the basis of reflectivity measurements
to represent either a thin zone of mafic
lithologies within the ultramafic mantle
or hydrated ultramafic rocks within pris-
tine ultramafic mantle [Warner and
McGeary, 1987], and thus is most probably
some sort of-fault or shear zone. The
consistent spatial association of the
mantle reflectors vertically beneath upper
crustal fault zones, both in the North Sea
(this paper), associated with the Outer
Isles fault [Matthews and Hirn, 1984], and
with the Great Glen fault and the Variscan
Front [Warner and McGeary, 1987], suggests
that all these reflectors are strain
features of some type.
The lateral extent of the dipping,
intramantle reflectors is unknown, but
their alignment beneath regionally devel-
oped upper crustal faults in the northern
North Sea suggests that related intra-
mantle structures are being imaged on the
several profiles up to 200 km apart (Fig-
ure 9). Although the Flannan reflector is
known to be of regional extent, having
been imaged to 80 km depth northwest of
Scotland [McGeary and Warner, 1985] and
traced for over 100 km along strike (un-
published BIRPS data), the continuity of
the reflections observed on the NSDP
profiles must remain unproven in the
Klemperer: North Sea Lithospheric Extension 813
TIME
15 s
BIRPS (SHET) SHETLAND PLATFORM
BASE TRIASSIC
IJnmigrated Ume section
Fig. 6b. Line drawing of Figure 6a.
M•)HO •
absence of a more complete grid of deep
seismic profiles.
NATURE OF EXTENSION IN THE NORTHERN
NORTH SEAS
The axially symmetric disposition of
rift subsidence, thermal subsidence, and
crustal thinning in the northern North Sea
is not by itself sufficient to discrimi-
nate between the uniform stretching and
the simple-shear-zone models [White,
1988]. Although the uniform stretching
model always predicts the observed axial
symmetry and vertical stacking of basins
and crustal thinning, extension along a
simple-shear zone can also produce these
features, though it may alternatively lead
to development of a thermal subsidence
basin displaced distally (downdip) from
the rift basin, depending on the dip of
the shear zone and the degree of erosion
of thermal uplift [Wernicke, 1985; Coward,
1986]. Evidence for tectonic uplift and
lateral displacement of the thermal subsi-
dence basin is not seen in the northern
North Sea [Badley et al., 1988].
The most direct way to distinguish
between different extensional models is to
image the faults or detachments involved
in deformation. As noted earlier, deep
reflection profiling is well able to image
such structures both in crust and mant-
le. Where shear zones are directly imag-
ed, it is a relatively simple matter to
interpret the structural style. Where no
continuous structures are imaged, it seems
more reasonable to presume that deforma-
tion is distributed, either homogeneously
or along anastomosing ductile shear zones,
than to presume that a single, localized
shear zone is transparent to seismic
energy.
Previous structural interpretations
based on the Britoil profile [Beach, 1986;
Beach et al., 1987] and based on a separ-
ate study of the NSDP data [Gibbs, 1988a,
b] have hypothesized the existence of
lithospheric low-angle extensional de-
tachments beneath the northern North
Sea. These conjectures are not supported
by the absence, from all the deep seismic
profiles from the northern North Sea
(NSDP: this paper and Gibbs [1988a, b];
BIRPS SHET profiles: McGeary [1987];
Britoil profile: Beach [1986] and Beach et
al. [1987]), of any continuous reflective
feature extending from the upper crustal
sedimentary basin to beneath the Moho.
Wernicke [1986] has pointed out that it is
possible to have displacement zones of
finite width running through the litho-
sphere that in part (perhaps in the duct-
ile lower crust) have shear structures
inclined rather than parallel to the shear
zone as might conventionally be expected,
and thus that the model of whole-litho-
sphere normal simple shear [Wernicke,
1985] does not require seismic evidence of
continuous, dipping reflective zones
through the whole lithosphere. However,
on the NSDP profiles the lower crust
appears to be reflective or diffractive
essentially everywhere, except where noise
trains or attenuation through very thick
sedimentary accumulations obscure all deep
reflections (Figures 2 and 4 to 7). Major
offsets of specific lower crustal reflect-
ors across the putative lower crustal
shear zones of Gibbs [1988a, b] are not
DEPTH
45 km
Migrated depth section
BASE TRIASSIC
ß ..----'"'*'--• -- MOHO
i ii i i i i i i
SOUTHEAST
NORTHWEST i , , 20 k,m , ,I
Fig. 6c. Depth miRration of Figure 6b.
Velocities are given in the text. Note
crustal thinning from northwest to south-
east, and the northwest dipping reflector
in the upper mantle.
814 Klemperer: North Sea Lithospheric Extension
1
10-
15
NSDP 84-3 HORDA PLATFORM Unmigrated time section
- _ . _ •--- • • •.---q,_.
•ORTHWEST ! :20 km I $OIJTHEAST
Fig. 7a. Unmigrated seismic data from the eastern end of NSDP line 3 across
the Horda Platform. For location, see Figure 1.
observed. There is apparently no simple
relation between development of lower
crustal reflectivity and the interpreted
positions of shear zones in either the
upper crust or the upper mantle (Figures 2
and 4 to 7). Thus, unless it is believed
that the lower crustal reflectivity both
postdates North Sea extension and is in no
way controlled by preexisting lithology or
structure, it seems special pleading to
argue for shear zones no more than a few
kilometers wide to cut, without seismical-
ly detectable effect, across lower crustal
reflectivity which extends laterally for
tens of kilometers (Figure 2).
Though whole-lithosphere normal simple
shear (Figure 10a) is a geometrical possi-
bility for crustal extension, it predicts
crustal-penetrating detachments that are
not observed, and it fails to
NSDP 84-3 HORDA PLATFORM
............ • ....................................................... BASE TRIASSIC
TIME • • .- .. __
ß ' .............................. •r- :•-•.---•_ -•• ....................... •--'--• ....... MOHO
Unrnigrated time section
Fig. 7b. Line drawing of Figure 7a.
predict the observed symmetry of mantle
structures. Whole-lithosphere normal
simple shear is not the simplest explana-
tion of the seismic data and is not here
advocated as the mechanism for extension
in the northern North Sea.
In this paper, only those features
directly imaged on the seismic data are
treated as significant, and discrete
geological structures that cannot be
recognized unambiguously in the seismic
data are not hypothesized. The model
proposed here explains the observations of
dipping reflective features in the upper
crust and the upper mantle, and the lack
of discrete, dipping, reflective struct-
DEPTH
45 km
Migrated depth section
BASE TRIASSIC
NORTHWEST • 20 km I SOUTHEAST
Fig. 7c. Depth migration of Figure 7b.
Velocities are given in the text. Note
east and west dipping reflectors in the
upper mantle.
Klemperer: North Sea Lithospheric Extension 815
.......... 26 km
+ + + +
+ + + >31 km
+ + + +
ß
ß
Basement velocity ./"' .
assumed: 6.2 km/s .. ß
ß
ß ß
+++ ß
+ +++ +
,,.'' + + + + + +
,. + + + + + + +
• + +++ + + +++ ß
,. . + + + + + +
.ß + + + + + + + + ß
+ + + +++ +++
,,,o' + + + + + + + + + + +
+ + + +++ +++
+ +++ +++ + + +++
+ + + + + + + + + +
+++ + + +++ +++++++ +
+ + + + + + +
+ + + , + + + + + + + +
+
+++ +++ + + + + +++ +
+ + + + + + + + + +
+ +++ +++ + + + + +++ + +
• + + • + + + + + + + +
• + + + + + + + + + + + +
• •_ + + + + + + + + +
• , + + ß + + +
• + + + +
+ + + +
• + + + + + + +
• + + + + + +
+ + + + + + +
+ + + + + +
+ + + + + + +
• + + + + + +
ß + -• + + + + +
• ......
0 . 4E + + + + + +
< 21 km
,ß
ß
ß
ß
Fig. 8a. Contoured map of interpreted crustal thickness, superimposed
on fault map of Figure 1. Velocities used in migration and conversion of
travel times to depths for the sedimentary rocks were averaged from over
50 well logs and for the crystalline basement were assumed to be a constant
6.2 km s -1 .
ures in the lower crust, by the presence
of brittle reflective faults in the upper
crust and discrete reflective shear zones
in the upper mantle separated by a layer
of distributed deformation in the lower
crust. Figure 10b is a cartoon based on
NSDP line 1 (Figure 2) to illustrate these
features. Faults are only imaged with
certainty on the NSDP data in the upper
crust, within or bounding Mesozoic sedi-
mentary basins. The lower crust acts to
decouple structures in the upper crust
from those in the upper mantle and allows
upper crustal faults and upper mantle
shear zones with opposite dips to act
simultaneously without locking one anoth-
er. Deformation in the lower crust is
expected to be by ductile flow [e.g.,
Meissner and Strehlau, 1982].
Although on a regional scale the neck-
ing of the lower crust observed beneath
the northern North Sea resembles the
effect of bulk pure shear, in detail the
mesostructures involved may be a complex
network of simple shear zones which anast-
omose around less-deformed blocks and
which may contribute to the many bright
reflections and diffractions from the
lower crust [e.g., Reston, 1987].
The intramantle reflectors are here
interpreted to be extensional shear zones,
but it is not possible to estimate the
amount of displacement along them. The
intramantle reflectors appear to merge at
a low angle with the reflective lower
crust, never producing apparent offsets of
the reflection Moho, though sometimes
projecting into zones where the Moho
smoothly upwarps. The lack of observable
offsets at the Moho does not preclude
significant displacement on the interpret-
ed shear zones if the structures have a
816 Klemperer: North Sea Lithospheric Extension
BASEMENT THICKNESS
(pre-Triassic)
• <16 krn
• <21 krn
.......... 26 krn
+ +++
+++ >31 km
+ + + +
, , ,
oO
+++•++++++++
+ + + + + + +
+ + + + + + +
•+++ + + + + + + + + + +
+ + + + + + + +
+ + + + + + + + +
+ + + + + + + + +
+ + + + + + + + + +
+ + + + + + + + + +
+++ +++++++++
+ + +
+ + + + + + + + +
+ + + + + + + + + +
+ + + + + + + + + +
+ + + + + + + + + +
+ + + +++ +++++ +
• + + + +
L + + + .4-
o ?
4E
r++
++++
++++
•+++++ ++
++++++
oø • ++
++
++++++
++++++++•
+
+ + + +
+ + +
Fig. 8b. Contour map of interpreted thickness of pre-Mesozoic basement,
superimposed on fault map of Figure 1. Difference between contours of
Figures 8a and 8b represents thickness of Mesozoic and Cenozoic sedimentary
rocks.
large effective width where they penetrate
into the lowermost crust [Wernicke, 1986]
or if the structures imaged as the reflec-
tive lower crust and reflection Moho have
re-formed during or after extension [Klem-
peter et al., 1986]. Alternatively,
deformation •n the upper mantle may be
pervasive, and the dipping reflectors
might represent only a minor localization
of strain. Just as, regionally, the upper
crust beneath the northern North Sea can
be characterized as a single rift extend-
ink between the Shetland Platform and the
Horda Platform but in detail consists of
many, tilted blocks and half grabens bound-
ed by faults which anastomose along
strike, so, by analogy, the upper mantle
beneath the rift basins may form a region-
al horst (Figure 10b) but in detail may
contain a complex netwo.rk of shear zones
only the most major of which are imaged on
the NSDP reflection data.
In the simplistic cartoon of Figure
10b, no thinning is shown in that part of
the mantle lithosphere directly beneath
the rift basins, and so this model should
predict less thermal subsidence over the
central part of the rift than the uniform
stretching model for an equal amount of
crustal thinning. However, there may
exist, as noted above, a more complex
pattern of discrete shear zones than is
imaged on the NSDP data, or extension may
become more penetrative with increasing
depth in the mantle part of the litho-
sphere. Deformation in the uppermost
mantle may be by either brittle or ductile
failure [e.g., Chen and Mo!nar, 1983;
Sawyer, 1985] depending largely upon the
upper mantle temperature. The maximum
depth to which any of the dipping reflec-
tive structures in the mantle has been
traced is about 45 km owing to the limited
15-s recording time of the NSDP profiles:
Klemperer: North Sea Lithospheric Extension 817
4ow
62øN
4øE
60 ø
2
15
20
4øE
20
Fig. 9. Location of dipping zones of reflections and diffractions in the
upper mantle, superimposed on fault map of Figure 1. Arrows indicate dip
direction; length of arrows indicates linear extent of dipping zones;
number shows apparent dip on the seismic section after two-dimensional
depth migration.
no statement is possible from these data
about the nature of lithospheric extension
at greater depths. The brittle layer in
the upper mantle may be very thin (or even
nonexistent), and it will be important in
future work to discover to what depth
these structures extend and whether their
seismic character changes with depth.
The model for lithospheric extension
shown in Figure 10b is symmetric rather
than asymmetric; there is no preferred
direction of dip of structures at any
level in the lithosphere. Although the
asymmetry of the surface expression of
rift zones has been stressed by many
authors [e.g., Bally, 1982; Le Pichon and
Barbier, 1987], an asymmetric sedimentary
basin can overlie a more fundamentally
symmetric pattern of lithospheric exten-
sion (Figure 10b). Figure 10b suggests a
depth dependence of extensional style
based on the expected depth-dependent
rheology of the continents, but there is
no expectation in this model of a depth
dependence of the amount of stretching or
of the position of stretching. Though
such variations may occur beneath some
rifts, in the northern North Sea there is
no evidence for significant departures
from uniform stretching and the zone of
extension in the mantle beneath the north-
ern North Sea, if represented by the
outward dipping reflectors in the mantle,
spans the region directly beneath the
major basement extension and rift basin-
formation (Figures 2, 8 and 9).
818 Klemperer: North Sea Lithospheric Extension
Whole-lithosphere normal simple shear
Fig. 10a. Cartoon to illustrate geometric
possibility for extension in which a
single continuous zone of simple shear
cuts through the whole lithosphere, after
Beach [1986] and Gibbs [1988a, b]. Dashed
line corresponds to the lithophere-pene-
trating detachment. Schematically shown
are sediment-filled half grabens, and the
reflective lower crust the base of which
is the reflection Mohoo
CONCLUSIONS
The pre-Mesozoic crust beneath the
Viking Graben has been thinned by a factor
of at least 2. If igneous inflation of
the crust was significant during extension
or if the existing crustal antiroot has
diminished since extension, then greater
crustal thinning has occurred. The mech-
anism by which the extension was accommo-
dated within the lithosphere appears to
have been variable with depth. Localized
fault zones are reflective in the upper
(brittle?) crust but apparently give way
to distributed shear in the lower (duct-
ile?) crust. The existence of dipping
Decoupled, symmetric stretching model
REFLECTIVE • • • •
LOWER CRUS'• '/ • -- • • • •
•... ,,*•.•, ,, -------- '"'--- • •
• 200 km •
V.E. • x2
Fig. [Oh. Cac•oon •o [[lus•ga•e decoupled
symme•gic s•ce•chin•, •i•h bct•[e fault-
ing in •he uppec ccus•, maccoscop[c puce
sheac in •o•ec ccus• •hich •ecoup•es
•he uppec man•[e, and disccede sheac
reflectors in the upper mantle is taken in
this paper to suggest symmetric extension
along localized shear zones beneath the
crust, over a zone as wide as and vertic-
ally beneath the region of crustal exten-
sion, to depths of at least 45 km in the
upper mantle.
Acknowledgements. The NSDP program was
shot by GECO as a speculative survey with
participation by Arco, BP, Britoil, Elf,
Esso, BIRPS, Shell, Statoil, and Norske
Hydro. BIRPS is indebted to GECO for
allowing participation in their survey and
publication of their data. This project
was assisted by other members of the BIRPS
group including D.H. Matthews, S. McGeary,
M.R. Warner, and N.J. White. C. Peddy and
B. Freeman provided useful reviews. BIRPS
is funded by the Natural Environment
Research Council. Well log information
was generously provided by BP, Conoco
(Norway), Conoco (UK), Esso Exploration
and Production UK, Kerr-McGee, Lasmo,
Mobil Exploration Norway, Mobil North Sea,
Norske Hydro, Norske Shell, Occidental,
Ranger Oil, Santa Fe Minerals, Saxon Oil,
Shell UK, Sovereign, Statoil, Total,
Tricentrol, Unocal, and Volvo Petroleum.
Britoil very kindly allowed access to
their proprietary deep seismic profile,
and Elf Aquitaine Norge A/S to results of
their expanding spread profiles. The GECO
data described in this paper are available
from Merlin Geophysical Ltd., Duke House,
1 Duke Street, Woking GU2! 5BA, U.K. The
BIRPS data referred to in this paper are
available from the Marine Geophysics
Programme Manager, British Geological
Survey, Murchison House, West Mains Road,
Edinburgh EH9 3LA, U.K. Cambridge Earth
Sciences Contribution 1034.
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(Received July 2, 1987;
revised October 12, 1987;
accepted November 16, 1987.)
