The rotation of rigid objects within a flowing viscous medium is a function of several factors
including the degree of noncoaxiality. The relationship between the orientation of such objects and their
aspect ratio can be used in vorticity analyses in a variety of geological settings. Method for estimation of vor
ticity analysis to quantitative of kinematic vorticity number (Wm) has been applied using rotated rigid objects,
such as quartz and feldspar objects. The kinematic vorticity number determined for high temperature mylo
nitic Abt schist in Al Amar area, extreme eastern Arabian Shield, ranges from ~0.8 to 0.9. Obtained results
from vorticity and strain analyses indicate that deformation in the area deviated from simple shear. It is con
cluded that nappe stacking occurred early during an earlier thrusting event, probably by brittle imbrications.
Ductile strain was superimposed on the nappe structure at highpressure as revealed by a penetrative subho
rizontal foliation that is developed subparallel to tectonic contacts versus the underlying and overlying
nappes. Accumulation of ductile strain during underplating was not by simple shear but involved a compo
nent of vertical shortening, which caused the subhorizontal foliation in the Al Amar area. In most cases, this
foliation was formed concurrently with thrust sheets imbrications, indicating that nappe stacking was associ
ated with vertical shortening. |
