Geoscience Reference
In-Depth Information
Worked Example 8.2 Unravelling multiple tectonic fabrics and folding
A series of interbedded psammitic and pelitic
schists displayed numerous folds, mainly with
subhorizontal axial planes, and a dominant
micaceous schistosity, also generally fl at-lying.
In many exposures, the main foliation lay almost
parallel with the original bedding. However, in
some exposures (e.g. Figure 8.26), this
parallelism was shown to be a result of
transposition, so the main foliation formed
axial-planar to the abundant, fl at-lying folds.
areas of 10 m scale folds and near quartz vein
boudins (Figure 8.27), where the main foliation
(S3) was revealed to be a crenulation of an
earlier, coarse schistosity (S2).
8
On close inspection, this earlier schistosity
appeared to be differentiated into mica- and
quartz-rich layers, typical of fabrics formed
originally by crenulation. This hypothesis was
supported by the observation of an even earlier
fabric (S1), oblique to S2, that was only visible
as inclusion trails within albite porphyroblasts
that grew during formation of S2 (Figure 8.28).
Further detailed study of the schists highlighted
a few exposures of low-strain zones, in the hinge
Figure 8.26
Page from a
fi eld notebook with sketch
showing transposition of
lithological layer in strongly
deformed schists. (Notebook
of Tom W. Argles, The
Open University, UK.)
