Structural studies are probably the type of
geological investigation that most often complements the indirect
methods of geophysics. Together with seismic reflection and other
geophysical data, structural studies contribute directly to an understanding
of how continental crust has been thickened or thinned. This is
determined by establishing the geometry and style of folding that
has affected the rocks, and the geometry and nature of the faults
that cut them, especially the amount and direction of displacement
along them.
These structures include strain markers such as deformed pebbles,
crystals, vesicles (irregularly shaped spaces in the rocks), fossils,
and various kinematic indicators which show the degree and directions
in which the rocks have been deformed. In turn, these observations
permit some inference of the direction of paleostresses, that is
the stresses on the rocks at the time they were being deformed.
When combined with isotopic dating methods, such strain markers
make it possible to establish the time at which movement took place
and hence the age and duration of paleostresses. Estimates of the
temperatures, pressures and hence depths at which rocks deformed
in the crust, and sometimes even the magnitude of the stresses required,
are possible when the rocks contain certain co-existing metamorphic
mineral assemblages, for which equilibrium temperatures and pressures
have been determined experimentally.
Here we see an interpretative diagram showing geological features
of the Montreal - Val d'Or geotraverse (across the central Grenville
corridor). Major terrane boundaries are indicated, and allochthonous
slices (or packages of rocks which have been moved onto other rocks),
as well as shear zones (along which movement took place) and intrusive
complexes (plutons). The red lines show seismic lines along which
data were acquired in 1993.
The collaborative interaction of structural geologists with seismologists
that is planned for all transects is fundamental to achieving the
most meaningful and complete interpretations of the seismic sections.
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