Geoscience Reference
In-Depth Information
You may be able to distinguish with a hand lens between
stretched grains (stretching lineation) and elongate grains that
have grown or been rotated into parallel orientations
(alignment lineation). Some igneous rocks show weak
alignments of elongate phenocrysts due to fl ow of magma
(magmatic lineation).
Care is needed to distinguish stretching lineations from other
lineations such as intersection lineations (Section 8.3.5) or
striations (Figure 8.6c, p. 170). Examine the mineral grains with
a hand lens to confi rm their stretched nature, and corroborate
your initial impression of the stretching direction with other
clues, such as stretched pebbles, fossils, quartz lenses,
boudinaged veins or layers, tension gashes and pressure
shadows (e.g. around porphyroblasts or porphyroclasts). Some
examples of features to look for as clues for stretching
directions are given in Figure 8.19.
8
hard nodules
with strain shadow
stretching
direction
boudinaged
competent
layer
deformed
fossils
pebble
bed
stretched pebbles
(
(
Figure 8.19
Clues to the stretching direction. (a) Dilational fractures perpendicular to a mineral lineation on
this mylonitic foliation plane confi rm the stretching direction, northwest India. (b) Composite sketch depicting
clues to the stretching direction. ((a)Tom W. Argles, The Open University, UK.)
8.3.3 Sense of shear: Kinematic indicators
One of the most critical observations in zones of ductile
deformation is the shear sense. Consistent shear indicators
constrain the sense of motion on major ductile shear zones in
the same way as faults, but the shear strain is commonly
distributed across a wider area.
When determining sense of
shear, always observe
features in a plane that cuts
the foliation at a high angle,
and
parallel
to the stretching
lineation.
With kinematic indicators, it's always a case of 'the more the
merrier'! You may fi nd that not all the indicators share the
same shear sense. Only if you fi nd that
most
indicators are
