The flow pattern and jointing mechanism of Abu Zaabal doleritic basalt

Abstract

Abu Zaabal doleritic basalt, a volcanic extrusion present in the form of an extensive sheet, is a coarse holocrystalline rock exhibiting an ophitic texture. Petrofabric techniques were used in order to reveal the flow pattern of this rock using the (010) composition plane of the albite and carlsbad twins in the plagioclase crystals as the fabric element. The results of the fabric analysis on two basalt samples showed the tendency of the poles of this planar fabric element to describe a girdle pattern of orientation. It was also found that the plagioclase crystals tend to be oriented with their longest dimension parallel to the girdle axis. The motion was found to have been accomplished through the rotation of the plagioclase crystals about an axis normal to the direction of flow. Field studies showed the presence of two groups of joints; primary highly dipping joints and secondary nearly horizontal joints. Correlation between the orientation of the primary joints and the results of the fabric analysis showed that they are tension fractures developed parallel to the (ac) plane of the fabric. The mechanism of development of these tensile fractures are discussed in terms of the possible thermal anisotropy of the plagioclase crystals in such an anisotropic rock mass.