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
1Earth Sciences and Geography, School of Physical and Geographical Sciences, Keele University, Keele, Staffordshire, ST5 5BG, United Kingdom. 2British Geological Survey, Kingsley Dunham Centre, Keyworth, Nottingham, NG12 5GG, United Kingdom.
3British 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).
ult 50Km n Fa into Alw lt ault Fau n F ood indo lt erw Sw Fau ath xley Fe Hau
lt au e F Northumberland Trough cki no Gil
Hexham in Newcastle-Upon-Tyne as B ay Maryport-Stublick-Ninety lw Carlisle om Fault System So Fath
V a P l e e n Alston Alston Block n o i Durham f n e E F d a e u tem n lt lt Sys Fau Penrith B nowle a utterk s le-B Lake District Block neda in e-Lu gh ehous Trou Clos more Stain
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
m d s
h m
e
e
e o e c
e
e
a a
g
i i
t
h
r
g r r o
g t
a
s Remarks including Major Regional Events Legend M
t Tectonics Sedimentation
e e
E p
A a
A
y r
S
P S E
S E 65.5 ±0.3 West East Maastrichtian Extension with 70.6 ±0.6 Campanian transtension 83.5 ±0.7 Santonian Upper 85.8 ±0.7 Coniacian
s 89.3 ±1.0
u Turonian 93.5 ±0.8 o Cenomanian e 99.6 ±0.9
c Albian Extension a
t 112.0 ±1.0
Aptian e
r 125.0 ±1.0
C Barremian Lower 130.0 ±1.5 Opening of the Atlantic Ocean in the Hauterivian 136.4 ±2.0 west and subsidence of the North Sea Valanginian Basin to the east (Ziegler, 1990). 140.2 ±3.0 Berriasian 145.5 ±4.0 Compression
c
i Tithonian
o 150.8 ±4.0
z Upper Kimmeridgian 155.7 ±4.0 o Oxfordian s 161.2 ±4.0 e Callovian
c 164.7 ±4.0 i M Bathonian s 167.7 ±3.5
s Middle Bajocian Uplift of the North Sea Dome(Ziegler, a
r 171.6 ±3.0 1990). Erosion
u Aalenian
J 175.6 ±2.0 Toarcian 183.0 ±1.5 Erosion of Permo-Triassic and younger Pliensbachian sediments has removed a large amount Lower 189.6 ±1.5 Duration Sinemurian of sedimentary cover. The thickness 196.5 ±1.0 and extent of rocks that have been of Hettangian Event 199.6 ±0.6 eroded are poorly constrained. There Rhaetian are considerably more Triassic and 203.6 ±1.5 Upper Norian Jurassic sediments preserved in the
c 216.5 ±2.0 north-west of England than in the north- i Carnian s 228.0 ±2.0 east of England (Chadwick et al. 1995).
s Ladinian a i Middle 237.0 ±2.0 r Anisian
T 245.0 ±1.5 Olenkian Lower 249.7 ±0.7 In late Permian to early Triassic times, Induan there was a transition from a 251.0 ±0.4 Changhsingian predominantly marine to a continental Lopingian 253.8 ±0.7 environment across northern England Wuchiapingian 260.4 ±0.7 (Clarke, 2009). Capitanian
n 265.8 ±0.7
a Guadalupian Wordian i 268.0 ±0.7
m Roadian r 270.6 ±0.7 To the west of the Pennines, east-west e Kungurian orientated extension reactivated large
P 275.6 ±0.7 Artinskian fault structures in the underlying Cisuralian 284.4 ±0.7 Sakmarian Carboniferous strata. 294.6 ±0.8 Asselian Uplift of the Carboniferous basins 299.0 ±0.8 resulted in considerable erosion of the n Gzhelian a i Carboniferous strata during the n s Upper 303.9 ±0.9 a
u Kasimovian v Permian Period. l
o y 306.5 ±1.0
r s
n Middle Moscovian
e Variscan Orogeny- Collision between n
f
i 311.7 ±1.1 e Pennine Coal Measures Avalonian part of Larussia to the north P n Lower Bashkirian Group
o 318.1 ±1.3
n and Gondwana to the south. Towards
c a
b
i Upper Serpukhovian i r p the end of the Variscan Orogeny the p 326.4 ±1.6 i Yoredale Group
a
o s
s Middle Visean
i Whin Sill Suite was intruded. z C
s 345.3 ±2.1 s i Border Group
o Tournasian
M Lower The extensional phase of the e 359.2 ±2.5
l Famennian Northumberland Trough's evolution is
a Upper 374.5 ±2.6 characterised by a close association
Frasnian P
n 385.3 ±2.6 b e t w e e n s e d i m e n t a t i o n a n d a i Giventian contemporaneous faulting.
n 391.8 ±2.7 Middle (Chadwick et al. 1995)
o Eifelian
v 397.5 ±2.7
e Emsian Emplacement of North Pennines 407.0 ±2.8 Basement D Lower Pragian Batholith during the later part of the 411.2 ±2.8 Caledonian Orogeny (Le Bas, 1982). Lockovian 416.0 ±2.8 Pridoli 418.7 ±2.7 Ludfordian Ludlow 421.3 ±2.6 Gorstian Caledonian Orogeny- Collision
n 422.9 ±2.5 between Laurentia to the north and a
i Homerian
r Avalonia to the south resulting in the
Wenlock 426.2 ±2.4 u
l Sheinwoodian closure of the Iapetus Ocean i 428.2 ±2.3
S Telychian 436.0 ±1.9 Llandovery Aeronian 439.0 ±1.8 Rhuddanian 443.7 ±1.5 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 cross- sections have been drawn to tie the data together in a grid, ready for a three-dimensional interpretation of the area.
50Km
gh erland Trou Northumb Cross-section paths
n Hexham Newcastle-Upon-Tyne asi y B lwa So Carlisle
V a lock le Alston lston B A o Durham f E d e Penrith n B a Lake District Block s in c a b ugh ore Tro 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.
SE N S 4a) NW 50Km
0
1
2 Alston Block
3
m
K
h 4
t
p
e
D 5
Northumberland Trough 6 Stainmore Trough
7
8 North Pennine Batholith
SE N S 4b) NW 50Km
0
1
Alston Block 2
3
m
K
h 4
t
p
e
D 5 Northumberland Trough
6
7 Stainmore Trough
8 North Pennine Batholith
SE N S 4c) NW 50Km
0
1
2 Alston Block
3
m
K
h 4
t
p
e
D 5
Northumberland Trough 6 Stainmore Trough 7
8 North Pennine Batholith
New Nomenclature Previous Nomenclature Stainmore Formation Stainmore Group Yoredale Group Alston Formation Liddesdale/Alston Group Tyne Limestone Formation Upper Border Group Fell Sandstone Formation Middle Border Group Border Group Lyne Formation Lower Border Group Upper Lower Border Group Lower Weardale Granite North Pennine Batholith
4 50Km le- cast New ne n-Ty Upo
ham Hex
am Durh
isle on Carl Alst
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 post-rift Relative uplift e e e e of footwall syn-rift z Model of lithosphere Zd Crust deformation due to faulting and pure Moho Pure shear 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. Distance (km)
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 0 Northumberland Trough Alston Block Stainmore Trough
)
m
k
( 5
h
t
p
e
D
10
7. Model at Time = 0My after extension. Structure only.
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 0
5
10
8. Model at Time = 0My after extension. Structure and isostatic compensation
6 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 0
5
10
9. Model at Time = 0My after extension. Structure, isostatic compensation and thermal uplift
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 0
5
10
10. Model at Time = 0My after extension. Structure, isostatic compensation, thermal uplift and loading
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 0
5
10
11. Model at Time = 0My after extension. Structure, isostatic compensation, thermal uplift, loading and further loading.
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 0
5
10
12. Model at Time = 0My after extension. Structure, isostatic compensation, thermal uplift, loading and erosion.
7 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 0
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 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 0
5
10
14. Model at Time = 15My after extension. Thermal subsidence
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 0
5
10
15. Model at Time = 15My after extension. Thermal subsidence and loading
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 0
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 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 0
5
10
17. Model at Time = 20My after extension. Thermal subsidence, loading and compaction
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 0
5
10
18. Model at Time = 23My after extension. Thermal subsidence, loading and compaction
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 0
5
10
19. Model at Time = 27My after extension. Thermal subsidence, loading and compaction
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 0
5
10
20. Model at Time = 34My after extension. Thermal subsidence, loading and compaction
9 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 0
5
10
21. Model at Time = 47My after extension. Thermal subsidence, loading and compaction
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 0
5
10
22. Model at Time = 54My after extension. Thermal subsidence, loading and compaction
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 0
5
10
23. Model at Time = 360My after extension (present day). Thermal subsidence, loading and compaction
SE N S NW 50Km
0 Figure 24. Cross-section for 1
Alston Block 2 comparison with model
3 results (Same Scale).
m
K
h 4
t
p
e
D 5 Northumberland Trough
6
7 Stainmore Trough
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)
10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 0
-1 Cross-Section Model
) -2
m
k
(
n
o i
t -3
a
v
e
l E -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 (Figure 26). It has an average density of 2630 kgm-3; 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.
ult 50Km n Fa into Alw Depth to top batholith (km) lt ault Fau n F ood indo lt erw Sw Fau ath xley 6.5 - 7.5 Fe Hau 5.0 - 6.4 3.0 - 4.9 1.5 - 2.9 lt 0.0 - 1.4 au e F Northumberland Trough cki no Gil
Hexham Newcastle-Upon-Tyne in as B ay lw Carlisle So
V a Alston le Alston Block
o Durham f E d e n Penrith B a Lake District Block s in gh Trou more 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 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 0
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)
10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 0
Cross-Section -1
Model with
) Model -2 Granite
m without k
( Granite
n
o i
t -3
a
v
e
l E -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
gh ou Tr nd rla be um rth Alston Block No
gh n ou si Tr Ba e ay or olw m S ain St
Stainmore Trough b) Northumberland Trough N 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
80 rough rland T
) e rthumb
m No
k
(
n
i
g i
r 60 o in m as o B r y f a lw e So c lock n ton B a Als
t 40
s
i D
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
gh Trou rland umbe North
Alston Block sin Ba way Sol
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 References
Beamish, D. and Smythe, D.K. 1986. Geophysical images of the deep crust: the Iapetus suture. Journal of the Geological Society. 143. 3. 489-497. Bott, M.H.P. 1976. Formation of sedimentary basins of graben type by extension of the continental crust. Tectonophysics. 36. 1-3. 77-86. Bott, M.H.P., Swinburn, P.M., and Long, R.E. 1984. Deep Structure and Origin Of The Northumberland And Stainmore Troughs. Proceedings of the Yorkshire Geological Society. 44. 32. 479-495. Clarke, S.M. 2009. Permian, Triassic and Jurassic. in Northern England Regional Guide. British Geological Survey. Chadwick, R.A., Holliday, D.W., Holloway, S., and Hulbert, A.G. 1995. The structure and evolution of the Northumberland-Solway Basin and adjacent areas. HMSO. Dunham, K.C., Dunham, A.C., Hodge, B.L., and Johnson, G.A.L. 1965. Granite beneath Viean sediments with mineralization at Rookhope, northern Pennines. Quarterly Journal of the Geological Society. 121. 1-4. 383-414. Egan, S.S. 1992. The Flexural Isostatic Response of the Lithosphere to Extensional Tectonics. Tectonophysics. 202. 2-4. 291-308. Egan, S.S. and Urquhart, J.M. 1993. Numerical Modeling of Lithosphere Shortening - Application to the Laramide Orogenic Province, Western Usa. Tectonophysics. 221. 3-4. 385-411. Egan, S.S., Kane, S., Buddin, T.S., Williams, G.D., and Hodgetts, D. 1999. Computer modelling and visualisation of the structural deformation caused by movement along geological faults. Computers & Geosciences. 25. 3. 283-297. Hodgetts, D., Egan, S.S., and Williams, G.D. 1998. Flexural modelling of continental lithosphere deformation: a comparison of 2D and 3D techniques. Tectonophysics. 294. 1-2. 1-20. Kusznir, N.J. and Park, G. 1987. The extensional strength of the continental lithosphere: its dependence on geothermal gradient, crustal composition and thickness. in Continental Extensional Tectonics. Coward, M.P. and Dewey, J.F. Ed. Geological Society of London Special Publication No.28. 35-52. Kusznir, N.J. and Egan, S.S. 1989. Simple-Shear and Pure-Shear Models of Extensional Sedimentary Basin Formation: Application to the Jeanne d'Arc Basin, Grand Banks of Newfoundland. in Extensional Tectonics and Stratigraphy of the North Atlantic Margin. Tankard, A.J. and Balkwill, H.R. Ed. American Association of Petroleum Geologists. 305- 322. Le Bas, M.J. 1982. The Caledonian granites and diorites of England and Wales. in Igneous Rocks of the British Isles. Sutherland, D.S. Ed. John Wiley and Sons. 191-202. Leeder, M.R. 1976. Sedimentary facies and the origins of basin subsidence along the northern margin of the supposed hercynian ocean. Tectonophysics. 36. 1-3. 167-179. Leeder, M.R. 1982. Upper Palaeozoic basins of the British Isles--Caledonide inheritance versus Hercynian plate margin processes. Journal of the Geological Society. 139. 4. 479-491. Meredith, D.J. and Egan, S.S. 2002. The geological and geodynamic evolution of the eastern Black Sea basin: insights from 2-D and 3-D tectonic modelling. Tectonophysics. 350. 2. 157-179. Sclater, J. G. and Christie, P. A. F., 1980, Continental stretching: an explanation of the post-Mid- Cretaceous subsidence of the Central North Sea Basin, J. Geophys. Res., 85, B7, 3711- 3739. Soper, N.J., England, R.W., Snyder, D.B., and Ryan, P.D. 1992. The Iapetus suture zone in England, Scotland and eastern Ireland: a reconciliation of geological and deep seismic data. Journal of the Geological Society. 149. 5. 697-700. Turcotte, D.L. and Emerman, S.H. 1983. Mechanisms of active and passive rifting. Tectonophysics. 94. 1-4. 39-50. Verral, P., 1981, Structural interpretation with application to North Sea Problems. Course notes No. 3, Joint Association For Petroleum Exploration Courses (UK). White, N.J., Jackson, J.A. and McKenzie, D.P., 1986, The relationship between the geometry of normal faults and that of the sedimentary layers in their hanging walls, Journal of Structural Geology, 8, 897-909. Ziegler, P.A. 1990. Tectonic and palaeogeographic development of the North Sea rift system. in Tectonic evolution of the North Sea Rifts. Blundell, D.J. and Gibbs, A.D. Ed. Clarendon 16