Abstract:
Conjugate Shear fractures are pair of fractures which were developed during brittle
failure of rocks. Presence of shear fractures signs faulting or relative movement of
rocks in a particular area. This study aimed at investigating the shear fractures
present in the Charnockite at Galgoda, Balangoda in order to understand the sense
of slip and brittle deformation history in the area. Orientation data of fractures
(n=136) and slickenside striations were collected using geological compass. Samples
were collected from selected locations for laboratory studies. Fracture orientation
data were plotted and analyzed using Rock Ware Stereo Stat software. Poles plot for
shear fracture planes were contoured and best fit great circles were constructed.
Principal compressive stresses (1, 2, 3) were then calculated using the software.
A thin film of fault gouge is present in the shear fractures. Slickenside striations are
distinct in some planes while slickenside striations in some planes with moderate
plunge seem to have overprinted. Steps are also visible in some planes. A vertical to
near vertical shear fracture is present where slickenside striations are parallel to the
strike of the shear plane. These, slickenside striations show moderate to horizontal
plunge with NW, W-NNW and NNW-N trends. Orientation of shear fractures shows
four directions showing presence of two generations of conjugate fault systems. The
principal stress directions of one conjugate fault system are 1(089°/63°), 2
(282°/26°) and 3 (189°/06°). The principal stress directions of the other conjugate
fault system are 1(170°/74°) and 2 (329°/15°) and 3 (060°/06°). Because the 1
direction is nearly vertical, the sense of slip on both conjugate faults is dominantly
vertical. The sense of shear in vertical to near vertical shear fractures could indicate
strike-slip on tensile fracture. Thus, the Galgoda Charnockite shows at least two
generations of normal faulting and a subsequent strike-slip suggesting change of the
stress regime acting on these rocks.
