Rock

Stereographic Projection Techniques in Structural Geology
PETER R. LEYSHON University of Glamorgan RICHARD J. LlSLE University of Wales, Cardiff
with computer programs by J. van Gool, D. van Everdingen and R.J. Lisle

BIBLIOTECA

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Geological structures of planar type
The rocks at the vas1 m-jority of outcrops possess some kind of planar structure. In most sedinientaryrocks a planar structure known as bedding is visible ( j i g a). This is a primar) feature fornied at the time of deposition and is a layering characterized by compositional, textura1 or grain-size variations. Some igneous rocks possess an equivalent structure called primary igneous layering produced by the accumulation of crystals settling out from a magma. The orientation of these primaryplanar structures reflects the mechanics of the deposition process, and measurements of their orientation can yield information about the palaeo-horizontal, the direction of flow of currents, etc. Foliation is a general term for al1 pervasively developed planar structures found in rocks. Sedimentary bedding falls under this heading as do planar structures resulting from deformational and inetamorphicprocesses. The latter are secondary foliations and include rock cleavage and schistosity. Some foliations are defined by compositional variations; others, such as slaty cleavage, by a parallel alignn~ent grains of or mineral aggregates. The directions of cleavage planes are frequently measured for the purpose of estimating the directions of geological strains in rocks. Figure b illustrates rockscontaining two foliations together; a priinary foliation (bedding) and a secondary foliation (cleavage). Gneissic banding Cfig c ) is a comrnon feature of coarse-grained metamorphic rocks such as gneisses. This secondary foliation is a compositional layering defined by the concentration of particular minerals. Other structures in rocks have planar geometry but are discrete features and are thereforenot exainples of foliation. Joints (fig d) are fractures in rocks along which little or no movement has taken place. Although they are produced by only minor tectonic strain, they represent discontinuities in the rock mass and as such are irnportant to its mechanical behaviour. Assessnient of the orientations of joints present might form an essential part of any stability analysis (see p. 86).Faults Cfig e) are planes along which the rock on one side is displaced relative to the other. By collecting, in a region, directional data on a numbei- of fault planes it is sometimes possible to estimate the directions of principal stress axes at the time of faulting (p. 58) The surfaces of contact between geological units can often be considered planar, at least on a local scale. These contactsare often parallel to the bedding in the rocks on either side of the contact, but could form the margins of intrusive igneous rocks or could be erosional, e.g. surfaces of unconforrnity. Other examples of planes are defined geometrically with respect to other features. Foi- example, the axial surface (or axial plane) of a fold (jlg f ) can be determined as the plane which bisects the angle betweentwo limbs of a fold. The profile plane of a fold is one that is perpendicular to the fold axis. Planes of this type may not appear as measurable planar surfaces at the outcrop.

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Geological - structures of planar type

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axial plane

profilc plane

Measuring and recording the orientation of planar structures
A frequently-used way of describing the attitude of a planarstructure is to measure and record:

1. The strike of the planar structure (J5g a ) . The strike is the compass direction (or bearing) of the special line in the plane which is horizontal, i.e. is not tilted at all. This horizontal line on the plane is found by means of a spirit-leve1 or device for measuring angles of tilt called a clinometer and its bearing is measured with a compass (J5g 6). 2....