Igneous intrusions - Introducing Tectonics, Rock Structures and Mountain Belts - page 73

of Arran, Skye and Mull will encounter
numerous dykes belonging to these
Cenozoic swarms, varying from perhaps

one to several metres in width individu-
ally, but collectively representing a sig-
nificant degree of crustal extension.

8

72 73

cone
sheets

ring dykes

cone
sheets

P1

P1 P1

P1

P1

P1

P1

sill

sill

Figure 8.7 Mode of formation of cone sheets, ring dykes and
radial dykes. A. Vertical cross-section through an idealised central
igneous complex. The magma pressure acting at right angles
to the pluton margin produces a radial arrangement of the lines
of maximum principal stress, P1; the lines of minimum principal
stress, P3, which in this case is likely to be an extension, are
perpendicular to P1 and follow an elliptical path. As explained
in Chapter 5, shear fracture planes (shown here in red) make
an acute angle with P1 (see Figure 5.9). It is thought that the
most likely method of generating cone sheets is that a sudden
increase in magma pressure within the pluton causes shear
fractures to form, which, because they make a high angle with P3,
become filled with magma. Some sheets may follow a shallower
path to form sills. Ring dykes, shown here in purple, may form
along steeper fracture planes inclined steeply outwards. B. Plan
view showing how the lines of maximum principal stress, P1,
radiate outwards from the pluton; the minimum principal stress,
P3, follows a set of circular paths around the pluton. If P3 is
extensional, as is likely if the pluton is expanding, this may create
the conditions for radial dykes to be intruded. C. Dyke swarms are
sets of parallel dykes that originate from an igneous centre; when
away from the local stress field generated by the pluton, they
come under the influence of the regional stress field and become
more parallel.

pluton

P3 P3 P3

P3

radial
dyke

P1

A

P1

P1

P3

P3

radial
dyke

P3

P3

P1

P1 pluton

radial
dyke

P3 P3

P3

P3

P1

P1

B

radial
dyke

P1

regional
stress

regional
stress

local
stress
control

dyke
swarm

pluton

C