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Research Institute for the Environment, Physical Sciences & Applied Mathematics
The geology and geodynamics of
the Northumberland Trough
Region:
Insights from mathematical modelling
Linda Austin1
Stuart Egan1, Stuart Clarke1 & Gary Kirby2 & Dave Millward3
1
Earth Sciences and Geography, School of Physical and Geographical Sciences, Keele University, Keele,
Staffordshire, ST5 5BG, United Kingdom.
2
British Geological Survey, Kingsley Dunham Centre, Keyworth, Nottingham, NG12 5GG, United Kingdom.
3
British Geological Survey, Murchison House, West Mains Road, Edinburgh, EH9 3LA, United Kingdom.
Introduction
The numerical modelling of the interaction of geological and geodynamic processes has proved to
be a valuable tool for explaining the causes and magnitude of regional subsidence and uplift in
response to continental tectonics. In particular, geodynamic modelling can be used to investigate
the effects of deep processes that are poorly constrained by subsurface and surface data. In this
work, we apply 2D and 3D numerical modelling techniques, combined with the analysis of surface
and subsurface data, to investigate the structural, stratigraphical and geodynamic evolution of the
Carboniferous block and basin structure of northern England. Two dimensional and three
dimensional mathematical modelling techniques combined with the analysis of surface and
subsurface data have been applied and developed to investigate the structural, stratigraphical and
geodynamic evolution of the Northumberland Trough Region. In particular, to provide insights into
the importance of deep processes, such as depth-dependent extension, and how they interact with
basin-controlling processes such as sedimentary infill.
The Northumberland Trough Region includes the Northumberland Trough, its westerly
continuation, the Solway Basin, the Alston Block, a geomorphological high situated to the south of
the Northumberland Trough, the Vale of Eden Basin to the west of the Alston Block and the
StainmoreTrough to the south of the Alston Block (Figure 1).
50Km
Alw
th
Fea
no
Gil
So
lw
a ul
eF
c ki
B
ay
as
t
into
erw
n Fa
o od
ult
lt
Fau
nd
Swi
Ha u
Fault
xley
Northumberland Trough
Hexham
in
Newcastle-Upon-Tyne
Maryport-Stublick-Ninety
Fathom Fault System
Carlisle
Pe
le
Va
Alston Block
Alston
nn
Durham
en
ul t
Fa
Ed
ine
of
Penrith
s
Ba
Lake District Block
ault
on F
in
-Lu
house
Close
wle
terkno
e-But
nedal
m
Syste
Fault
h
Troug
more
in
a
t
S
Figure 1. Location of Study area. The Northumberland Trough Region comprises a major
east-west orientated asymmetrical half-graben system that extends across northern
England into the northern Irish Sea.
Base map © 2009 Google - Imagery © 2009 TerraMetric
The region lies within the tectonic framework of the Iapetus Suture Zone, which has resulted from
continental collision between Laurentia to the north and Avalonia to the south, opposing margins of
the Iapetus or 'proto-Atlantic' ocean (Beamish and Smythe, 1986; Soper et al., 1992).
The region has subsequently experienced a number of extensional, compressional and wrench
tectonic events throughout Late Palaeozoic, Mesozoic and Cenozoic times. These events have led
to a complex subsidence-uplift history that cannot be adequately explained by basin formation due
to simple uniform lithosphere extension.
1
Campanian
Cretaceous
Upper
Santonian
Coniacian
Turonian
Cenomanian
Albian
Aptian
Lower
Barremian
Hauterivian
Berriasian
Tithonian
Upper
Kimmeridgian
Oxfordian
Callovian
Jurassic
Mesozoic
Valanginian
Middle
Bathonian
Bajocian
Aalenian
Toarcian
Lower
Pliensbachian
Sinemurian
Hettangian
Rhaetian
Triassic
Upper
Middle
Lower
Lopingian
Permian
Norian
Carnian
Ladinian
Anisian
Olenkian
Induan
Changhsingian
Wuchiapingian
Capitanian
Guadalupian
Wordian
Roadian
Kungurian
Cisuralian
Artinskian
Sakmarian
Mississippian Pennsylvanian
Carboniferous
Devonian
Paleozoic
Asselian
Upper
Gzhelian
Kasimovian
Middle Moscovian
65.5 ±0.3
Middle
Frasnian
Giventian
Eifelian
Emsian
Lower
Pragian
Pridoli
Ludlow
Ludfordian
Gorstian
89.3 ±1.0
93.5 ±0.8
99.6 ±0.9
Extension
112.0 ±1.0
125.0 ±1.0
Opening of the Atlantic Ocean in the
west and subsidence of the North Sea
Basin to the east (Ziegler, 1990).
130.0 ±1.5
136.4 ±2.0
140.2 ±3.0
Compression
145.5 ±4.0
150.8 ±4.0
155.7 ±4.0
161.2 ±4.0
164.7 ±4.0
167.7 ±3.5
Uplift of the North Sea Dome(Ziegler,
1990).
171.6 ±3.0
Erosion
175.6 ±2.0
Erosion of Permo-Triassic and younger
sediments has removed a large amount
of sedimentary cover. The thickness
and extent of rocks that have been
eroded are poorly constrained. There
are considerably more Triassic and
Jurassic sediments preserved in the
north-west of England than in the northeast of England (Chadwick et al. 1995).
183.0 ±1.5
189.6 ±1.5
196.5 ±1.0
199.6 ±0.6
203.6 ±1.5
216.5 ±2.0
228.0 ±2.0
Duration
of
Event
237.0 ±2.0
245.0 ±1.5
In late Permian to early Triassic times,
there was a transition from a
predominantly marine to a continental
environment across northern England
(Clarke, 2009).
249.7 ±0.7
251.0 ±0.4
253.8 ±0.7
260.4 ±0.7
265.8 ±0.7
268.0 ±0.7
To the west of the Pennines, east-west
orientated extension reactivated large
fault structures in the underlying
Carboniferous strata.
Uplift of the Carboniferous basins
resulted in considerable erosion of the
Carboniferous strata during the
Permian Period.
270.6 ±0.7
275.6 ±0.7
284.4 ±0.7
294.6 ±0.8
299.0 ±0.8
303.9 ±0.9
306.5 ±1.0
311.7 ±1.1
359.2 ±2.5
374.5 ±2.6
385.3 ±2.6
391.8 ±2.7
Variscan Orogeny- Collision between
Pennine Coal Measures Avalonian part of Larussia to the north
Group
and Gondwana to the south. Towards
the end of the Variscan Orogeny the
Yoredale Group
Whin Sill Suite was intruded.
Border Group
The extensional phase of the
Northumberland Trough's evolution is
characterised by a close association
between sedimentation and
contemporaneous faulting.
(Chadwick et al. 1995)
397.5 ±2.7
407.0 ±2.8
411.2 ±2.8
Basement
Emplacement of North Pennines
Batholith during the later part of the
Caledonian Orogeny (Le Bas, 1982).
416.0 ±2.8
418.7 ±2.7
421.3 ±2.6
422.9 ±2.5
Homerian
426.2 ±2.4
Sheinwoodian
428.2 ±2.3
Telychian
436.0 ±1.9
Llandovery Aeronian
439.0 ±1.8
Rhuddanian
443.7 ±1.5
Wenlock
Extension with
transtension
85.8 ±0.7
326.4 ±1.6
Famennian
Legend
East
83.5 ±0.7
Visean
Middle
345.3 ±2.1
Lower Tournasian
Upper
Remarks including Major Regional Events
Sedimentation
West
70.6 ±0.6
Lower Bashkirian
318.1 ±1.3
Upper Serpukhovian
Lockovian
Silurian
Age
Ma
Stage
Age
Series
Epoch
Erathem
Era
System
Period
Maastrichtian
Tectonics
Caledonian Orogeny- Collision
between Laurentia to the north and
Avalonia to the south resulting in the
closure of the Iapetus Ocean
Figure 2. Tectono-stratigraphic chart detailing the tectonic and stratigraphic history of the Northumberland Trough
2
Previous research conducted on the subsidence mechanism of the Northumberland Trough
Region has presented several explanations. Bott (1976) and Leeder (1976) presented theories that
attributed the subsidence to a combination of regional thinning of the crust by creep of the lower
crustal material to the south where the mid-European marginal sea was closing by subduction of the
northern continental margin, and wedge subsidence of the upper crust to form the block and trough
structures. Leeder (1982) proposed an alternative theory based on the stretching mechanism of the
McKenzie model, pure shear. This theory proposes an initial extension event, which thinned the
lithosphere by stretching during Dinantian times, resulting in the block and trough structures. The
initial stretching event caused the asthenosphere to rise, raising the temperature gradient.
Subsequently, as the lithosphere cooled during the Westphalian stage, regional thermal
subsidence affected both the block and trough regions.
Bott et al. (1984) suggested a subsidence mechanism, based on geodynamic observations, which
is a modification of these two previous hypotheses with more emphasis on the lithosphere
stretching with subordinate thermal effects. The Westphalian subsidence observed is considerably
greater than the maximum amount of subsidence predicted by the McKenzie model, indicating that
thermal subsidence was not the only factor affecting subsidence during the upper Carboniferous
Period. The Westphalian succession is almost twice as thick as that of the Namurian, indicating an
increased rate of subsidence rather than the expected exponential decay as a result of thermal
subsidence.
One of the aims of this research is to produce several end-member geological and geodynamic
models for the possible evolution of the basin which simulate these hypotheses and comment on
their feasibility.
Cross-Sections
The analyses of surface data from fieldwork and subsurface geophysical data have been used to
produce regional cross-sections showing present day structure and stratigraphy across the region.
Several north-south orientated cross-sections have been produced across the area, positioned as
shown in Figure 3, in order to show regional variations in basin depth and burial history, as well as
the position and magnitude of movement along major faults. Two east-west orientated crosssections have been drawn to tie the data together in a grid, ready for a three-dimensional
interpretation of the area.
50Km
Northumbe
w
Sol
a
a si
yB
rland Troug
h
Cross-section paths
Newcastle-Upon-Tyne
Hexham
n
Carlisle
le
Va
Alston
ck
Alston Blo
of
Durham
nB
as
Lake District Block
e
Ed
Penrith
in
c
a
b
gh
re Trou
o
Stainm
Figure 3. Location of cross-section paths within the Northumberland Trough Region. The highlighted
sections a, b and c are displayed in figure 4.
3
Figure 4. Digitised north-south cross-sections, which provide the input parameters for the modelling, including crustal
thickness, magnitude of extension, and the surface position and heave of faults. The varying shape of the North
Pennines Batholith can be seen across the sections as they move from west to east. The en-echelon fault system
bounding the Northumberland Trough on its southern margin can be seen on the sections to step from a more distal to
proximal setting as the basin developed.
4a)
NW
SE N
50Km
S
0
1
Alston Block
2
Depth Km
3
4
5
Northumberland Trough
6
Stainmore Trough
7
8
North Pennine Batholith
4b)
NW
S
SE N
50Km
0
1
Alston Block
2
Depth Km
3
4
5
Northumberland Trough
6
Stainmore Trough
7
8
North Pennine Batholith
4c)
NW
S
SE N
50Km
0
1
2
Alston Block
Depth Km
3
4
5
Northumberland Trough
6
Stainmore Trough
7
North Pennine Batholith
8
New Nomenclature
Yoredale Group
Border Group
Stainmore Formation
Alston Formation
Tyne Limestone Formation
Fell Sandstone Formation
Lyne Formation
Weardale Granite
Previous Nomenclature
Stainmore Group
Liddesdale/Alston Group
Upper Border Group
Middle Border Group
Lower Border Group Upper
Lower Border Group Lower
North Pennine Batholith
4
50Km
lecast
New -Tyne
n
Upo
ham
Hex
Durh
Carl
i sl e
am
n
Alsto
N
E
rith
Pen
Depth
Figure 5. Several cross-sections showing structural and stratigraphical components have been generated
within a 3D coordinate frame from the interpretation of seismic data. These sections have been used to
constrain the modelling. Key as in Figure 4.
Numerical Modelling
The computer modelling of the interaction of geological and geodynamic processes is a valuable
tool for explaining the causes and magnitude of regional subsidence and uplift in response to
continental tectonics.
Extensional basin formation occurs in a tectonic regime where the tensile normal stresses cause
strain in the lithosphere as a result of pulling on the vertical plane. In response to these tensile
stresses there are two opposing processes that are involved in lithospheric extension; crustal
thinning and thermally-induced uplift. Crustal thinning is the structural response to lithosphere
extension and may occur as a result of pure shear (stretching) or simple shear (faulting) (Figure 6).
Numerical models have been developed that integrate crustal thinning by simple and pure shear
mechanisms (Kusznir & Egan, 1989;Egan, 1992;Hodgetts et al., 1998;Meredith & Egan, 2002).
Within the software that has been produced as part of this research, the major processes
associated with extending the lithosphere by both faulting and pure shear mechanisms can be
combined together into a quantative model. Faulting is modelled using the Chevron (Vertical Shear)
Construction (Verral, 1981) to determine the geometry of the hanging-wall for a given amount of
extension on the fault. This method assumes that each vertical section of thickness of hanging-wall
is displaced laterally by the same amount of heave, and any section of the hanging-wall that is
Relative uplift
of footwall
post-rift
e
e
e
e
syn-rift
z
Zd
Crust
Moho
Pure shear
Model of lithosphere
deformation due to
faulting and pure
shear.
Mantle
Isotherm
Asthenosphere
Figure 6. Integrated simple and pure shear model of lithosphere extension (After Meredith and Egan, 2002).
5
unsupported following extension collapses downwards vertically onto the underlying footwall. The
major weakness associated with the Chevron Construction is that the hanging-wall is restricted to
vertical collapse following fault movement (Egan et al., 1999). The Inclined Shear Construction
(White et al., 1986) is a variation of the Chevron Construction and can also be used to model fault
geometry within the software. It assumes that collapse of the hanging-wall occurs along a definable
shear angle.
The model assumes that all of the faults have a common detachment depth, usually at mid to lower
crustal levels which represents the brittle-ductile transition, below which deformation is assumed to
be as a result of pure shear (Kusznir and Park,1987). The pure shear of the lower crust is regional
and defined in terms of a lateral position, width and a magnitude of extension that is expressed as a
sequence of beta values, all of which can be independent of the deformation by faulting in the upper
crust (Meredith and Egan,2002).
The flexural isostatic response of the lithosphere to negative loading caused by crustal thinning
generates regional uplift as the underlying lithosphere compensates for the loss of crust at the
surface. The resultant isostatically compensated lithosphere (Figure 8) shows not only uplift within
the basin but also uplift of the basin flanks, particularly uplift in the foot-walls of the basin controlling
faults.
Thermally-induced uplift is generated as the crust and mantle lithosphere are thinned raising hotter
material, i.e. the basal lithosphere boundary, closer to the surface (Figure 9). This disturbance of the
temperature field produces thermal expansion, determined by the volumetric coefficient of thermal
expansion, resulting in uplift.
Infilling a basin creates a load on the lithosphere. The density of the infill will affect the potential load
on the lithosphere (Egan, 1992). The basin will respond isostatically to the imposition of the load
and subsidence will be generated (Figure 10). This in turn created more accommodation space
which can be loaded (Figure 11).
Erosion has the combined effect of reducing the uplifted topography and unloading the lithosphere
which responds by regional isostatic uplift (Figures 12 & 13) (Egan and Urquhart, 1993).
Figures 7-23. Mathematical model of the Northumberland Trough Region. Input parameters provided by the section in
figure 4b.
0
0
Distance (km)
5
10
15
20
25
30
35
40
45
50
55
60
Depth (km)
Northumberland Trough
65
70
75
80
85
90
95
100
105
110
115
120
125
130
135
140
145
150
Stainmore Trough
Alston Block
5
10
7. Model at Time = 0My after extension. Structure only.
0
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
100
105
110
115
120
125
130
135
140
145
150
5
10
8. Model at Time = 0My after extension. Structure and isostatic compensation
6
0
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
100
105
110
115
120
125
130
135
140
145
150
5
10
9. Model at Time = 0My after extension. Structure, isostatic compensation and thermal uplift
0
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
100
105
110
115
120
125
130
135
140
145
150
5
10
10. Model at Time = 0My after extension. Structure, isostatic compensation, thermal uplift and loading
0
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
100
105
110
115
120
125
130
135
140
145
150
5
10
11. Model at Time = 0My after extension. Structure, isostatic compensation, thermal uplift, loading and further loading.
0
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
100
105
110
115
120
125
130
135
140
145
150
5
10
12. Model at Time = 0My after extension. Structure, isostatic compensation, thermal uplift, loading and erosion.
7
0
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
100
105
110
115
120
125
130
135
140
145
150
5
10
13. Model at Time = 0My after extension. Structure, isostatic compensation, thermal uplift, loading, erosion and further
erosion.
Subsequently, after extension, the geotherm re-equilibrates, as heat is lost from the surface via
convection, and subsidence occurs (Turcotte & Emerman, 1983). This re-equilibration of the
temperature field and the resultant subsidence can be calculated assuming that the lithosphere
cools by gradual heat loss due to conduction (Egan, 1992).
An algorithm has been developed to model the compaction of sediment within the basin, using the
relationship between porosity and depth (Sclater and Christie, 1980). Compaction decreases the
volume of the sediment, whilst simultaneously increasing its density, as a result there is no net
change in the mass. By reducing the volume of sediment, compaction creates new accommodation
space which can be infilled by sediment, adding a further load to the lithosphere (Figure 16).
0
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
100
105
110
115
120
125
130
135
140
145
150
80
85
90
95
100
105
110
115
120
125
130
135
140
145
150
100
105
110
115
120
125
130
135
140
145
150
5
10
14. Model at Time = 15My after extension. Thermal subsidence
0
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
5
10
15. Model at Time = 15My after extension. Thermal subsidence and loading
0
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
5
10
16. Model at Time = 15My after extension. Thermal subsidence, loading and compaction
8
The basin continues to evolve in this way over geological time. Figures 17-23 illustrate the evolution
of the Northumberland Trough, Alston Block and Stainmore Trough and the development of the
stratigraphy over several time stages from 20 to 360 million years after the initial extension event.
The PowerPoint presentation, accessible from this website provides an animation of the sequence
of events.
0
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
100
105
110
115
120
125
130
135
140
145
150
115
120
125
130
135
140
145
150
115
120
125
130
135
140
145
150
115
120
125
130
135
140
145
150
5
10
17. Model at Time = 20My after extension. Thermal subsidence, loading and compaction
0
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
100
105
110
5
10
18. Model at Time = 23My after extension. Thermal subsidence, loading and compaction
0
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
100
105
110
5
10
19. Model at Time = 27My after extension. Thermal subsidence, loading and compaction
0
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
100
105
110
5
10
20. Model at Time = 34My after extension. Thermal subsidence, loading and compaction
9
0
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
100
105
110
115
120
125
130
135
140
145
150
115
120
125
130
135
140
145
150
115
120
125
130
135
140
145
150
5
10
21. Model at Time = 47My after extension. Thermal subsidence, loading and compaction
0
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
100
105
110
5
10
22. Model at Time = 54My after extension. Thermal subsidence, loading and compaction
0
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
100
105
110
5
10
23. Model at Time = 360My after extension (present day). Thermal subsidence, loading and compaction
NW
50Km
S
SE N
0
Figure 24. Cross-section for
comparison with model
results (Same Scale).
1
Alston Block
2
Depth Km
3
4
5
Northumberland Trough
6
Stainmore Trough
7
8
North Pennine Batholith
Initial models (Figures 7-23) generate comparable amounts of subsidence to that observed in the
basin structures (Figure 24). By contrast, the amount of subsidence generated on the Alston Block
by these initial models is too great (Figure 25). The Alston Block is underlain by the North Pennines
Batholith; a non-porphyritic per-aluminous granite, intruded towards the end of the Caledonian
Orogeny, approximately 410Ma (Dunham et al., 1965). It is suggested that the additional elevation
of the Alston Block is due to the isostatic response of the lithosphere to the presence of this relatively
buoyant granite.
10
Horizontal Position (km)
0
10
20
40
30
50
60
70
Elevation (km)
-1
80
90
100
110
120
130
Cross-Section
140
150
Model
-2
-3
-4
-5
-6
Figure 25. Comparison of model results with the cross-section from which the input parameters were taken.
The shape and position of the top of the batholith is well constrained by gravity and seismic data
-3
(Figure 26). It has an average density of 2630 kgm ; this is lower than the surrounding crustal
material which has an average density of 2800 kgm-3. The North Pennines Batholith therefore acts
as a negative load upon the lithosphere, which responds by isostatic uplift, resulting in differential
subsidence between the Alston Block and the surrounding troughs.
50Km
lt
Fau
ton
aul t
ult
on F
d Fa
d
o
t
n
o
i
w
Faul
Sw
ther
xley
u
Fea
a
H
in
Alw
Depth to top batholith (km)
6.5 - 7.5
5.0 - 6.4
3.0 - 4.9
1.5 - 2.9
0.0 - 1.4
Gil
lw
So
a
au l
eF
i
k
n oc
t
Northumberland Trough
Newcastle-Upon-Tyne
Hexham
in
as
B
y
Carlisle
le
Va
Alston
Alston Block
of
Durham
Ed
en
Penrith
Ba
si n
Lake District Block
h
Troug
e
r
o
m
Stain
Figure 26. A recent reinterpretation of gravity data beneath the Alston Block provides constraint on the depth
to the top surface of the batholith. (Data courtesy of BGS/NERC)
11
Modelling of the structural and geodynamic evolution of the Northumberland Trough Region
reveals the important role played by the North Pennines Batholith in controlling the uplift of the
Alston Block. The flexural isostatic response of the lithosphere to negative loading, as generated by
a granitic batholith, produces regional uplift as the underlying lithosphere compensates for the loss
of density. Model results (Figure 27) indicate the generation of a significant amount of uplift
coincident with the presence of the batholith, and show a realistic geometry and subsidence-uplift
pattern across the Alston Block and adjacent basins. When compared to the depth to the basement
data from the cross-section that provided the input parameters, the depth to the basement
generated by the model over the Alston Block is comparable (Figure 28).
0
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
100
105
110
115
120
125
130
135
140
145
150
5
10
Figure 27. Model at Time = 360 My after extension including the buoyancy effects of the North Pennines Batholith.
There is a significant improvement in the correlation between the model results and observed data (Figure 28).
Horizontal Position (km)
0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
Cross-Section
Elevation (km)
-1
-2
Model
without
Granite
Model
with
Granite
-3
-4
-5
-6
Figure 28. Graph to compare the results of modelling with and without the granite batholith with the cross- section.
Modelling of the isostatic response of the lithosphere to the North Pennines Batholith has been
carried out to investigate the effects of various physical parameters, including volume variations
across the batholith, the density contrast between the crust and the batholith, and the effective
elastic thickness (Te) of the lithosphere. The results from these models are presented in the poster;
The influence of igneous intrusions on regional post-emplacement structural and geodynamic
evolution : Insights from numerical modelling of the North Pennines Batholith, northern England.
Model results indicate that large variations in density contrast are required, in the order of 50 kgm-3,
to significantly affect the amount of uplift generated by a granitic batholith. Varying flexural rigidity
affects the amplitude and width of the uplift generated by the granite, with increasing elastic
thickness spreading the uplift over a broader area. The most important factor affecting the isostatic
response to the batholith is the volume of the intrusion, with increasing volume initiating a greater
uplift.
12
Three-Dimensional Modelling
The project aims to produce a 3D model. 2D modelling is limited in application because flexural
isostatic compensation has a regional 3D effect. 3D flexure may modify the geometry of the entire
basin; the effect of the flexure is not concentrated solely in the plane of section as is the case with 2D
models. As a result 2D models tend to result in greater uplift, with increased footwall uplift and
shallower basin depth than those of 3D models (Hodgetts et al., 1998). In addition, loads out of the
plane of the 2D section may also have an effect on the basin profile which would not be seen in 2D
models.
Initial work on three-dimensional modelling has resulted in the creation of surfaces for each
stratigraphical horizon interpolated from the data provided by the cross-sections (Figure 29).
Crustal thickness values have been used to calculate beta values for the amount of extension
across the Northumberland Trough Region (Figure 30). These data are used within a piece of 3D
modelling software (Meredith, 2003) to produce a model of crustal thinning as a result of extension
(Figure 31).
a)
100km
N
rth
o
N
S
a
ol w
a
yB
um
la
ber
n
ro u
T
d
gh
Alston Block
si n
ain
t
S
b)
N
Northumberland Trough
re
o
m
u
Tro
gh
Stainmore Trough
Alston Block
Figure 29. Three-dimensional visualisation
of the base Carboniferous (Top Basement)
surface interpolated from data collected
from cross-sections.
Solway Basin
50km
13
100
Distance from origin (km)
80
Northu
60
Sol
B
way
nd Tr
mberla
ough
asin
lock
Alston B
40
20
Stainmore Trough
20
40
60
80
100
120
Distance from origin (km)
Figure 30. Beta Distribution Map interpolated from beta values calculated from crustal
thicknesses derived from the cross-section data.
100Km
N
um
North
w
Sol
a si n
B
ay
rough
T
d
n
berla
Alston Block
Stainmore Trough
Figure 31. Crustal thinning profile generated from three-dimensional modelling software using the beta
distribution map.
14
Summary
Cross-sections, showing structural and stratigraphical elements, have been produced from
the subsurface data provided by the British Geological Survey. These cross-sections have been
digitised in a computer aided design (CAD) environment to enhance visualisation and analysis, and
provide input parameters for the modelling component of the project. In particular, whole crustal
cross-sections have been created, which provide constraint on the magnitude of lithosphere
deformation.
Computer modelling is a useful tool for understanding how each process affects the
development of accommodation space within the basin. A software program written in the Java
programming language has been created, including algorithms to simulate faulting by vertical or
inclined shear, pure shear, flexural isostasy, thermal uplift and subsidence, sediment loading,
compaction and erosion and the isostatic effect of the presence of a body of contrasting density
within the crust.
Models that reconcile the observed amount of fault-controlled deformation with the
magnitude of overall thinning of the crust generate comparable amounts of subsidence to that
observed in the basin structures.
Model results that have included algorithms to simulate the effect of the batholith generate
decreased subsidence over the Alston Block that are equivalent to the amount observed in the
available subsurface data, whilst maintaining the volume of accommodation space created in the
basins (more details available on the poster).
These results also highlight some of the limitations of using a 2D modelling approach such
that faults are considered as 2D objects and it is not possible to consider variations in isostatic
loading outside the plane of section being considered. Further development of the modelling is
taking place to produce a realistic 3D geodynamically constrained model of the Northumberland
Trough region to provide an understanding of how regional interactions between structural,
thermal, stratigraphical infill, bathymetric and isostatic processes have controlled the development
of subsidence, and ultimately stratigraphy, within the basin system.
Future Work
Burial history analysis is being undertaken by back-stripping the cross-sections, this will
provide a further point of comparison for the models produced by the two-dimensional modelling
process.
Analysis of seismic data and borehole data is being carried out to refine the parameters,
including shear angle of the faulting and density and porosity of the sediments, used within the
modelling software to increase the accuracy of the models produced. Sensitivity testing of
parameters for which there is little control evidence is also being carried out.
Further development of the two-dimensional modelling programme is ongoing, with
algorithms being created to simulate the effects of imposing paleaobathymetry. This will affect
sediment infill and loading of the basin.
Work on the three-dimensional modelling is ongoing with a number of scenarios to be tested
and compared to the data collected from the cross-sections.
Acknowledgements
Funding for this research has been provided by the British Geological Survey and the Research
Institute for the Environment, Physical Sciences and Applied Mathematics, Keele University.
I would like to thank Dr Gary Kirby and Dr Dave Millward for their valuable input during the course of
this project.
15
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