Geology Reference
In-Depth Information
Figure 6.4
Basic dyke cutting through sediments in Arran, Scotland. Note how
dyke steps, influenced by the joint/faulting pattern of the rock it is intruding
through. Blown up picture shows flow alignment of vesicles in the centre of the
dyke (cf. Figure 6.10b).
mafic intrusions with width measured in terms of kilometres may also be termed
dykes. Dykes that radiate away from individual igneous centres are termed radial
dykes, and, where such dykes (and those that cannot be related to such centres)
are concentrated parallel to a regional tectonic trend, they are termed dyke
swarms. When traced along strike, an individual dyke is rarely continuous. It
may show discontinuity at outcrop, termed offlap, and may locally be offset in
such a way that parts of the dyke show overlap (Figure 6.2).
Although most dykes and sills are broadly linear features, some are related to
central igneous complexes characterised by sheet-like intrusions that may be cir-
cular in outcrop plan. These are termed
ring intrusions
and have been classified
into two types according to the attitude of the contacts (Figure 6.5).
Ring-dykes
have vertical and outward dipping contacts which are usually taken to indicate
emplacement by
subsidence
of the central cylindrical block (Figure 6.5a) in a
tensional stress field. Ring-dykes may have thicknesses of up to 1 - 2 km but
