The influence of igneous intrusions on regional post-emplacement structural and geodynamic evolution:
Insights from numerical modelling of the North Pennines Batholith, northern England.
1
1
1
2
Linda Austin , Stuart Egan , Stuart Clarke & Gary Kirby
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.
Research Institute for the Environment, Physical Sciences & Applied Mathematics
Hexham
sin
a
yB
Newcastle-Upon-Tyne
Maryport-Stublick-Ninety
Fathom Fault System
Carlisle
Alston Block
Pe
le
Va
nn
u
Fa
e
Ed
lt
tem
lt Sys
u
a
F
le
rknow
e
t
t
u
-B
edale
n
u
L
ugh
u se
o
o
r
h
T
e
s
e
o
in
as
nB
Lake District Block
Durham
i ne
of
Penrith
Alston
A geomorphological high underlain
by the North Pennines Batholith.
mor
Stain
Cl
DENSITY CONTRAST
FLEXURAL RIGIDITY
The density variation across the batholith ranges from 2628 kg.m-3 to 2634 kg.m-3.
An average value of 2800 kg.m-3 is used for the density of the crust. The contrast in
density between the granite intrusion and the surrounding crustal material is
responsible for the isostatic response of the lithosphere such that the higher the
density contrast the greater the uplift (Figs. 6a and 6b).
The rigidity of the lithosphere is controlled by its effective elastic thickness (Te),
which determines both the wavelength and amplitude of the flexural deflection of
the lithosphere in response to loading. Figures 7a and 7b show the variation of this
flexural deflection for selected Te values and assuming a granite density of 2630
kg.m-3.
400
400
350
350
300
Uplift (m)
250
300
250
250
200
The shape and position of the top of the batholith is well constrained by gravity and seismic data 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.
ick ult
There is a linear relationship
between the density contrast
and the resultant uplift.
200
200
FIGURE 2. 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)
50Km
Northumberland
Trough
N ew
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
Northumberland Trough
am
Hexh
lw
So
a
a
St
Newcastle-Upon-Tyne
Hexham
in
as
B
y
e
or
i nm
g
ou
r
T
h
2632-2634kg.m
The granite modelled
has a constant shape
and volume.
2630-2632kg.m
2628-2630kg.m
-3
am
Durh
lw
So
Carlisle
n
Alsto
Carli
Alston Block
N
E
50Km
S o l w a y Basin
Batholith
Depth
Lake District Block
sin
Ba
Sta
h
Troug
e
r
o
inm
Penrith
Lyne Formation
North Pennines Batholith
Faults
Durham
en
in
as
nB
North
Alston Block
Ed
Stainmore Formation
Alston Formation
Tyne Limestone Formation
Fell Sandstone Formation
ith
Penr Pennines
Alston
of
e
Ed
Lake District Block
Carlisle
sle
Durham
of
Penrith
a
Newcastle-Upon-Tyne
Hexham
le
Va
le
Va
Alston
Northumberland Trough
in
as
B
y
FIGURE 3: Several cross-sections showing structural and stratigraphic components have been
generated within a 3D coordinate frame from the interpretation of seismic data. These sections
have been used to constrain the modelling.
100
50
h
Troug
e
r
o
m
Stain
50Km
The granite modelled
has a constant shape
and volume.
200
300
250
200
0
Contrast = 200 kg m
2
3
6
5
4
10
20
30
40
50
60
70
80
90
100
110
120
130
140
Te = 0
Te = 1
Te = 2
Te = 3
Te = 4
Te = 5
-3
Contrast = 190 kg m
Contrast = 180 kg m-3
Contrast = 170 kg m-3
Contrast = 160 kg m-3
Contrast = 150 kg m-3
150
-50
10
KEY
Te = 6
-3
0
9
8
7
A Te of 0, indicates zero
resistance to bending and
responds to loading according
to the theory of Airy Isostasy
(Airy, 1855) and uplift or
subsidence would only occur
beneath the load itself.
150
50
KEY
1
Effective Elastic Thickness (km)
A high Te generates a long
wavelength, low amplitude
100 upwards deflection of the
lithosphere.
0
0
10
20
40
30
50
60
90
80
70
100
110
120
130
150
140
Te = 7
Te = 8
Te = 9
-50
Te = 10
Horizontal Position (km)
VOLUME AND SHAPE
-3
-3
150
Density Contrast (kg m )
150
Horizontal Position (km)
FIGURE 4. Density variations across the batholith.
With slightly higher densities in the cupolas to the
main batholith (After Kimbell et al.2006).
castle
160
170
180
190
-3
0
350
300
-3
l a
tub m F
S
tor atho
p
ry F
Ma inety m
N ste
ne
Sy
n-Ty
-Upo
With a low Te, the deflection
would be more localised,
shorter wavelength, but
would have higher amplitude.
350
50Km
THE NORTH PENNINES BATHOLITH
7a) The effect of varying Te across the basin 7b) The relationship between Te and uplift
6b) Relationship between density contrast and uplift
6a) The effect of varying density contrast
Uplift (m)
Northumberland Trough
The volume of the batholith is important because the greater the volume of granite
emplaced, the greater the negative loading of the lithosphere. The shape of the
batholith is responsible for the distribution of the volume of lower density material
within the crust and may effect where the batholith has the largest effect on
overlying uplift. The top of the batholith is well constrained by gravity and seismic
data. Estimates of the depth to the base of the batholith have been made using the
available subsurface data. Seismic data across the region shows the base of the
reflective lower crust beneath the granite to be at a depth of approximately 10km,
which can be assumed to be the base of the batholith.
FIGURE 9a. Module results showing 9a)
350
the change in isostatic deflection due to
varying the shape of the top of the
300
granite (see inset figure). The uplift
250
profile across the two skewed shapes
has a shorter wavelength and greater
200
amplitude compared to that generated
150
by the 'Average' shape. The peak of the
uplift profile is also skewed towards the
100
thickest part of the batholith where the
50
volume is greatest.
KEY
Decreasing
left to right
Increasing
left to right
Average
Uplift (m)
lwa
o
S
t
Research into the evolution of the Northumberland Trough region has
provided insights into the influence of igneous intrusions on the postemplacement structural development of the area. A kinematic modelling
approach that includes structural, thermal and isostatic processes has
been applied to the Northumberland Trough region. Initial models
generate comparable amounts of subsidence to that observed in the basin
structures. In contrast, the amount of subsidence generated on the block
structures by these initial models is too great. The Alston Block is
underlain by the North Pennines Batholith; a non-porphyritic peraluminous 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.
The flexural isostatic response of the lithosphere to negative loading, as generated by a granitic batholiths, produces regional
uplift as the underlying lithosphere compensates for the loss of density.Model results 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. 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.
Uplift (m)
Gil
a ul
F
e
ki
n oc
INTRODUCTION
ul t
n Fa
o
t
n
i
Alw
lt
Fau
aul t
F
n
o
d
t
nd
oo
Faul
Sw i
erw
y
h
t
e
l
a
x
Fe
H au
Uplift (m)
FIGURE 1. The Northumberland
Trough region comprises a major
east-west orientated asymmetrical
half-graben system that extends
across northern England into the
northern Irish Sea.
MODELLING INISIGHTS INTO THE ISOSTATIC EFFECTS GENERATED BY THE NORTH PENNINES BATHOLITH
0
0
FIGURE 9b. The isostatic effect from
-50
varying the shape of the top of the
batholith such that it is narrowest in the
9b)
FIGURE 8a. The isostatic effect of varying volume of intrusion. The varied volume was achieved using a fixed top centre and thickest towards the
at 4km and varying the depth to the base of the intrusion. Increasing volume results in a distribution of uplift with a margins. The results is a peak with a
greater amplitude than the 'Average'
greater amplitude. A 1km increase in thickness is responsible for approximately 50m of uplift.
shape.
FIGURE 8b. The relationship between batholith volume and the resultant uplift. There is a linear relationship
FIGURE 9c.The isostatic effect from an
between increasing the volume of the batholith and the uplift generated as a result ofisostatic adjustments.
intrusion with the shape as modelled
from the gravity data. The resulting
8b) Relationship between volume and uplift
8a) Effect of varying thickness (volume)
distribution has a smaller wavelength
600
and greater amplitude than the
600
'Average' shape and the peak is
500
skewed towards the greatest volume.
500
This deviation from the average can be
400
observed in Figure 9d.
10
20
30
40
50
60
70
90
80
100
110
130
120
140
150
Horizontal Position (km)
450
KEY
Decreasing
to middle
400
Average
350
Uplift (m)
300
Lyne Formation
North Pennines Batholith
The depths to the pre-Carboniferous
basement within the basins are
greatest near the Alston Block where
the faults that controlled their formation
are situated.
5b)
The amount of subsidence and therefore
accommodation space generated on the
block by this model is too great. There are
no Lyne Formation deposits present on
the block in the cross-section yet they
form one of the thickest deposits on the
model.
50
0
10
9
11
13
12
Depth to Base of Intrusion (km)
200
0
10
20
30
40
50
60
70
80
90
Horizontal Position (km)
Border Group
Lyne Formation
North Pennines Batholith
60
80
70
90
100
110
120
130
140
150
FIGURE 9d. The deviation from the
‘Average’ that results from the shape of
the top of the North Pennines Batholith.
KEY
Average
-100
Stainmore Formation
Alston Formation
Tyne Limestone Formation
Fell Sandstone Formation
50
300
100
Yoredale Group
40
Modelled
Batholith
200
8
0
30
Horizontal Position (km)
350
300
20
9c)
KEY
New Nomenclature
10
-50
300
400
0
100
110
120
130
140
150
14
15
9d)
250
40
200
150
Base = 8km
Base = 9km
Base = 10km
Base = 11km
Base = 12km
Base = 13km
Base = 14km
Base = 15km
100
20
0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
-20
50
0
Deviation from Average (m)
Border Group
Stainmore Group
Liddesdale/Alston Group
Upper Border Group
Middle Border Group
Lower Border Group Upper
Lower Border Group Lower
Weardale Granite
100
Uplift (m)
Yoredale Group
Stainmore Formation
Alston Formation
Tyne Limestone Formation
Fell Sandstone Formation
Previous Nomenclature
200
150
Uplift (m)
New Nomenclature
FIGURE 5a. A digitised N-S cross-section, which provides the
input parameters for the modelling, including crustal
thickness, magnitude of extension, and the surface position
and heave of faults.
FIGURE 5b. A model based upon the parameters extracted
from the section in figure 5a. There is a close match between
model results and observed data within the basin regions, but
the Alston Block exhibits too much subsidence.
FIGURE 5c. Model including the buoyancy effects of the North
Pennines Batholith. There is a significant improvement in the
correlation between the model results and observed data.
Associated Uplift (m)
MODELLING
5a)
250
0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
-40
-50
Horizontal Position (km)
Previous Nomenclature
-60
Horizontal Position (km)
Stainmore Group
Liddesdale/Alston Group
Upper Border Group
Middle Border Group
Lower Border Group Upper
Lower Border Group Lower
Weardale Granite
CONCLUSIONS
5c)
The presence of the batholith generates a
significant amount of uplift. Less
accommodation space is created early in
the extension and the Lyne Formation is
not deposited.
New Nomenclature
Yoredale Group
Border Group
Stainmore Formation
Alston Formation
Tyne Limestone Formation
Fell Sandstone Formation
Lyne Formation
North Pennines Batholith
Previous Nomenclature
Stainmore Group
Liddesdale/Alston Group
Upper Border Group
Middle Border Group
Lower Border Group Upper
Lower Border Group Lower
Weardale Granite

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.

Model results indicate that large variations in density contrast are required, in the order of 50 kg.m-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.
REFERENCES
Airy, G.B., 1855, On the computations of the effect of the attraction of the mountain masses as disturbing the apparent astronomical latitude of station in geodetic surveys. Philosophical Transactions of the Royal Society, London, 145, 101-104.
Dunham, K. C., Dunham, A. C., Hodge, B. L., & Johnson, G. A. L. 1965, "Granite beneath Viean sediments with mineralization at Rookhope, northern Pennines", Quarterly Journal of the Geological Society, vol. 121, no. 1-4, pp. 383-414.
Kimbell, G.S., Carruthers, R.M., Walker, A.S.D. And Williamson, J.P., 2006, Regional Geophysics of Southern Scotland and Northern England, Version 1.0 on CD-ROM (Keyworth, Nottingham, British Geological Survey).