Abstract main rift system was the North Sea Central


The North
Sea underwent many changes in the Jurassic such as a large change in sea level
from being arid at the end of the Triassic to being deep marine by the end of
the Jurassic formation of a thermal dome due to a rising mantle plume, this
lead to an unconformity. There was a period of extension of the crust which
nucleated from the area of thermal weakness which later reactivated the
existing faults which were Permian in age. As well as a period of large scale
volcanism in the middle which occurred in 3 main areas, the volcanism lead to a
vast debate as to what caused it and how it relates to the crustal extension
which occurred.

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This report will be looking at
how the North Sea evolved during the Jurassic period, this is broadly considered
the time period when in which crustal extension in the North Sea reached its peak (P.A.Ziegler,
Tectonophysics, 30 July 1992), it will also look
at how rifting in the North Sea lead to an overall change in the sea level
during the Jurassic. As well as looking at what the main causes of uplift in
the North Sea were. The main rift system was the North Sea Central Graben, (P.A.Ziegler,
The Geological Evolution of the North Sea Area in the Tectonic Framework of
North Western Europe, 1975) this rift is broadly
considered to be an offshoot of the arctic north Atlantic rift system (Graversen O.
, 2005).
This report will look at if a majority of the extension was caused by the reactivation
of existing faults or if it was caused by the thermal lithospheric weakening
from the formation of a thermal dome, which lead to rifting. It will consider
if the rifting was a result of the fault reactivation or if it was due to the
weakened stratigraphy. As well as looking at the extent of the role; if any,
that salt played in the formation of the basin (Bell R. E.,
Jackson, Whipp, & Clements, October 2014). Also, I will be
looking at the role which volcanics played in the tectonic history of the North



The Jurassic was a period of substantial
change in the North Sea. At the start of the Jurassic it was more shallow marine
in nature, whereas by the end of the period the north sea was considered to be
more deep marine in nature, with seafloor spreading occurring in the cretaceous,
 there was subsidence occurring
throughout the Jurassic which caused the area to become deeper marine, over
time the thermal dome , which had formed due to a mantle plume close to the
surface partially melting the over lying crust, subsided due to this subsidence
it had thermally weakened the lithosphere this in turn caused rifting to occur,
this rifting caused approximately 70km of extension across the North Sea basin (it
has been calculated to be 100km by the end of the cretaceous). the extension is
thought to have nucleated at the area of the subsided dome and spread out
reactivation existing faults over time.





Early Jurassic

Looking at the changes in the North
Sea basin throughout the Mesozoic it suggests that the environment turned from
mostly arid, with sandstones and mudstones deposited in the Triassic to
becoming marine in the Jurassic, this occurred during the time when thermal subsidence
was occurring. (British Geological Survey, 2001) this lead to sea
level rise. However, it is also thought that the thermal dome which formed in
the North Sea is believed to have begun forming in the early Jurassic (Graversen O.
, 2006)
due to the formation of the dome it created an unconformity within the rock
record. The dome prevented the deposition of the early Jurassic sediment,
however the dome does allow Triassic sediments to be preserved as well as
showing the gradual sea level rise which occurred during the Jurassic. As shown
in figure 1.


Figure 1 (Vollset.
& Doré, 1984),
this shows the deposition of non-marine lithologies followed by an
unconformable period which lasts for approximately 28ma where costal material
is deposited the presence of this coastal plain material shows the sea level is
beginning to rise, following this it shows the presence of shallow marine
lithologies which are then followed by deep marine shales, the presence of
chalk in the cretaceous implies that the climate was temperate with an average


Middle Jurassic

In the Middle Jurassic, the
sediments began to accumulate on the flanks of the thermal dome following an
unconformable period. The dome was irregular in shape and size, this is due to
the fact that it is believed that the dome formed ‘by southward migrating dome
centres’ (Graversen O. , 2002) in the Central North
Sea. The origin of this is believed to be from a mantle plume head (Figure 2) (Underhill,
the volcanism which was formed as a result from the plume produced basaltic lavas, which were then extruded, the volcanics
are believed to ‘be the products of decompression-induced
partial melting of the mantle, resulting from lithospheric attenuation during
(D.B.Hendrie., N.J.Kusznir., & R.H.Hunter, 1993) due to this process the mantle began to deviate
and move upwards, hence leading to the formation of the dome. The area where
the dome formed is  ‘where the Iapetus Suture intersected the Tornquist-Teisseyre fault system
the dome area was amidst the Viking Graben, Central Graben and Moray Firth
Basin’ (Glennie & Underhill, 1998) (Torsvik &
Daniel Carlos, 2004) this is the area which rifted first
when the thermal dome began to collapse as it was thermally weakened. the
rifting that followed was in Scotland, and the Forties which are associated
with the uplifting which occurred in this area. Later When the sea level
dropped the Horda Basin and Long Forties were the volcanic centres. The presence
of the erosional unconformity is the reason which the North Sea rift dome is visible.
it shows a clear pause in the deposition so shows the region was up lifted, it
also shows the shape and size of the dome. During the middle of the Jurassic
the dome was above sea level, due to the dome rising up and being above sea level
it caused a vast amount of uplift which in turn lead to an increased level of
erosion. (Directorate, 2014)

Figure 2 (Khattak., n.d.) The figure shows the
Viking graben and how the plume caused alteration to the lithosphere, it also
shows how the faulting that occurred caused a difference in the crust.




Late Jurassic

In Late Jurassic, there was a
large amount of extensional faulting, this faulting along with subsidence of
the thermal dome lead to approximately 70km of recorded extension during the
Jurassic alone. This extensions occurred from the thermally weakened part of
the crust which was where the dome was and then propagated out to the
boundaries of the current graben systems in the North Sea, (Penny & Rosy, 1984) over time the
faulting then extended back towards the centre of the thermal dome along the
three armed rift system. (Rattey & Hayward, 1993) (Fraser, 1993) the majority of the major rifting is
believed to have occurred between 157 and155 million years ago (Underhill,
1993) (Glennie &
Underhill, 1998)
the Jurassic sediments show a trend of thickening the closer to the syndepositional
faults, the thickness of the sediment varies in a different way to how it
varied during the phase of basin development when thermal dome occurred. When
the thermal dome was formed there was an unconformable period but the sediments
that did form, formed on the flanks of the dome as the top part was being
eroded. (McKenzie, 1978) in the early
Jurassic the basin was lens shaped, this lead to the lens shaped deposits being
formed, these were thickest in the middle and lensed out towards the edges.
There were two main factors that controlled the style of the rifts, these in
turn cause the rifts to be different in different parts of the basin. In the centre
of the North Sea the rifts are more segmented, thus are more complex, the areas
where the segmentation occurs are ‘North East ‘Caledonide’ and North West
‘Trans- European Fault Zone’ (Errat, 1999) (Jones, 1999) the basement
composition is different in the northern North Sea, where it lacks the Permian
salt. There is also a major detachment in the North Sea which is caused by the
Zechstein evaporites. The evaporites which are present separate the basement
and the overlaying cover sequences (Helgeson, 1999) (Hodgson,
1992) (Smith R. H.,
the thermal subsidence also lead to further rifting to occur in the Cenozoic,
the subsidence which was caused by the dome to collapse as a result of cooling.
The large-scale block faulting lead to both uplift and extension (Directorate, 2014) the erosion shown by
the footwall relates to the erosion predicted by the domino extension model.
‘in which rigid fault- blocks rotate as extension proceeds’ (Yielding,
in the Kimmeridgian there was a pause in the rate of subsidence, this is
believed to be due to a change direction of movement exhibited in the fault,
this then lead to a period of deposition the within Norwegian Danish basin in
the north sea was shown to be both expanding and subsiding throughout the Late
Jurassic. (Andsbjerg, Nielsen, &
Johannessen, 2001)
the extension which occurred was in a North -South trend. The extension and
faulting began within the area of the lithosphere which was weakened by the
thermal dome and then propagated outwards reactivating the earlier faults.
There was multiphase faulting which occurred and it moved outwards towards the
North Viking Graben as the strain rate increased. (Bell R. E.,
Jackson, Whipp, & Clements, 2014)


Figure 3 (Partington, Copestake, Mitchener, & Underhill, 1993) (Hampson,
Sixsmith, Kieft, & Johnson, 2009) this figure shows
the south west north east chronostratigraphic cross section, it shows the
deposition of the net-transgressive upper Jurassic strata within the Central
North Sea




There are three distinct types of
magmas which propagates the magmas which are present are likely to have evolved
from a basaltic rock which is probably alkali and olivine rich. The series
which evolved are ‘an ankaramite-basaltic hawaiite-hawaiite series, analkali olivine
basalt-hawaiite series, and anintrusive hawaiite-intrusive
mugearite series.’ (Fall., Gibb., & Kanaris-Sotiriou, 1982) the volcanism which occurred
in the middle of the Jurassic is assumed to be a direct result of tensional
tectonic movements. The tension of these movements generated a high heat flow across
the spreading axis, the volcanics exhibit a general trend of thickening to the
south with approximations of the maximum thickness being up to 1500 meters,
this thickness is assumed to be due to these areas being side vents on the
flank of the dome. As the Viking graben was thickened it lead to the volcanics
being eroded so the true thickness isn’t known. There are 4 known volcanic centres
‘the Fisher Bank and Glenn centres are closely juxtaposed and form
part of the Forties volcanic province, and the Puffin centre occurs separately
on the northern flanks of the Mid North Sea High. A fourth centre, Ivanhoe, has
been tentatively located in the Outer Moray Firth.’ (Smith &
D. Ritchie, 1993)
it is suggested that there are unconformable periods of time between each of
the centres as they don’t all occur at a close time period so they were all
growing at different times and points in the Jurassic. ‘The onset of volcanism
at the Fisher Bank centre may have occurred during the late Early Jurassic; the
Puffin centre was active during the Mid-Jurassic, and the Glenn centre may be
predominantly Callovian in age.’ (Smith & D. Ritchie, 1993) there are three main
theories in relation to the formation of the Volcanics; there is the tensional
tectonic theory, which suggests that the volcanics were formed by the tensional
extension of the crust and rifting caused volcanism to occur. (Faerseth,
Cintyrre, & Naterstad, 1976) the mantle plume
hypothesis which is the theory that the mantle plume activity cause the
volcanics to occur. (Whiteman, Naylor, Pegrum, & Rees, 1975) the final theory is the
embryonic spreading axis theory this theory suggests that the spreading axis between
Iberia and newfoundland caused the volcanism to occur. (Gibb, 1973)



There is a large grey area in the
papers which I have read as to when in the Jurassic the thermal subsidence occurred,
some papers state that it had begun in the late Triassic causing the sea level
to rise which lead to the deposition of mudstones and shales in the North Sea
Basin. However, other papers state that the subsidence began in the middle to
late Jurassic, this in turn lead to the subsidence and caused rifting to begin
which then lead to the crustal extension starting to occur. Although, the time
frame at which subsidence began differs between papers and authors, the general
consensus is that the subsidence is linked to the collapse of the mantle plume;
therefore, the collapse of the dome. From reading around the subject I agree
with a majority of the literature which leans towards saying that the majority
of the subsidence occurred during the early to middle of Jurassic as even
though it could have started before I think that it had a direct impact on the
rifting and caused it to begin which started being a visible in the middle to
late Jurassic.


A further contradiction in
literature regarding this is that some texts state that the uplift transpired
due to the dome forming due to the mantle plume, yet some texts say that the
uplift was a direct product of the faulting, which caused the blocks to be
tilted therefor leading to uplift throughout the basin along the rifts which
were formed, also it leads to uplift of the Viking
Graben, Central Graben and Moray Firth Basin. I think that a majority of the
uplift occurred due to the thermal dome which formed, even though some uplift
happened as a result of rifting and fault reactivation I think the main cause
for uplift in the Jurassic was due to the formation of the thermal dome which
is reflected in the lens shaped of sediment deposition in the early Jurassic, and
the faults nucleated form this point therefore I think that the uplift form the
dome was the most significant.


The literature on
the extension all broadly relates; it all agrees that the main rifting event
occurred in the late Jurassic and began form the thermally weakened part of the
basin which is the area in which the dome had formed prior this means that
although the approximate ages all differ it shows that the area where the
rifting occurred from was where ‘the Iapetus suture intersected the Tornquist-Teisseyre
fault system’ (Glennie & Underhill, 1998) (Torsvik &
Daniel Carlos, 2004) this shows that there is substantial evidence
at this site to show that it is the area where the faulting propagated from and
that there was further faulting which radiated outwards towards the existing
faults this then lead to fault reactivation of the existing faults.


The main conflicts
in the research are due to the fact that a majority of the research is quite
old so as new technologies were developed it presented different ages and
different sequence of events. That being said, a lot of the older research
corresponds to the newer research. Another bias could be due to the fact that
the same scientists worked in teams on the research so the same team released
more than one paper at different times so there could be a bias there due to
the fact that they don’t want their point to be proven wrong as it would
discredit their previous work. Also, because they have their idea then they
might discard certain pieces of information as it wouldn’t correspond with what
their school of thought was. On the other hand, a majority of the sources which
were used were from peer reviewed journals so they were other specialists in
the field agreed with the outcomes.


The three-main theory’s
relating to volcanism all differ quite significantly the tensional tectonics
theory is shown to be broadly correct due to the volcanism relating to
lithospheric extension however there is an issue with where the magmas were
formed from as there was no evidence if the magma traveling from the
lithosphere was active of passive, it I believed that it was via a passive
nature, however because this can’t be proven it doesn’t prove the validity of the
theory. With the mantle plume theory, there is evidence which suggests that the
plume didn’t begin until the middle of the Jurassic when its widely accepted
that the plume began to form before then. The final theory which is the
Embryonic spreading axis theory is the most likely out of all the theories since
it explains the three main igneous events which occurred down to the timing and
the position of them, although the theory doesn’t explain the volcanism which occurred
in the other parts of Europe now. However, it seems like this is the most
likely as it pin points not only the location but also the timing of the events
so that they match the evidence which is observable and the ages which have
been proven to be the ages of the rocks. There is the possible explanation for
the volcanism in Europe using the embryonic theory and that is the theory that
there was a large region in the lithosphere with a high heat flow leading to a
wider spread amount of volcanism. This theory aligns with basaltic sills and
alkali dykes which are present, as well as corresponding the potassium argon
ages of the rocks to what we would expect to see.



In conclusion although some of
the material studied contradicts each other a majority of it shows the Same general trend, it shows that there was initially a thermal dome
which then subsided. The subsidence was likely due to the mantle plume reducing
in temperature, as the temperature decreased it lead to the gradual subsidence,
the subsidence led to block faulting which lead to rifting the rifts then
spread outwards from the thermally weakened lithosphere and propagated along
the existing faults which were then reactivated after they were reactivated it
caused large scale crustal extension which was in the region of 70km. There was
also volcanism which occurred the magma which was produced was and alkali
olivine basaltic magma which had 3 main types; an ankaramite-basaltic
hawaiite-hawaiite series, analkali olivine basalt-hawaiite series, and anintrusive hawaiite-intrusive mugearite series these magmas are likely
to have formed at the spreading axis between Iberia and Newfoundland this
volcanism is related to the rifting that happened and also the tectonic