A B C KINEMATIC AXES The direction of maximum
flow or slip is termed 'a' axis, the plane ab is the slip plane and c is
the slip plane normal. The terminology first evolved at the time the
concepts for petrofabric analysis were being evolved by Bruno Sander.
Later, the same became applicable to similar folds since these were the
only folds in which the cleavage planes were not of maximum longitudinal
strain but of maximum displacement. Then the application came to the area
of plate tectonics and movement of thrust sheets. Lastly, it became
applicable to sheath folds and with the impetus on research on shear
zones, the terminology has now become applicable to sheared rocks. Though
the concept has become widely applicable, the usage of the very terms a, b
and c appears to have been
done away with. In orogenic
deformation, the movement direction of thrust sheets is
also 'a' or tectonic transport direction. Usually the thrust sheets
are emplaced and are accompanied by transpression, a term coined by
Harland in 1971. |