STRESS ANALYSIS ON REMOTELY SENSED FRACTURE ORIENTATION AND FREQUENCY DATA IN CONJUNCTION WITH A FRACTURED ROCK AQUIFER IN NAMAQUALAND, SOUTH AFRICA

The arid Namaqualand Metamorphic Complex, forms a roughly triangular area in the Northern Cape Province, situated in the northwest of South Africa, and comprises an area of about 600 000 km2 (Joubert, 1981; Lombaard et al, 1986; SACS, 1980).

This Metamorphic Complex represents the western most extent of the Namaqua- Natal Mobile belt, which comprises part of the mid to late Proterozoic Kibaran mobile belt system between the Archaean Kaapvaal craton and similar cratonic units in South America (Blignault et al, 1983; Tankard et al, 1982).

The area is characterized by a variety of lithologies, but dominated by granitic bodies in the area of investigation. This area was subjected to several deformational phases ranging from tight isoclinal folding in the earlier of four deformation phases to predominantly brittle failure expressed in fractures and joints in the neotectonic time frame.

A Landsat Thematic Mapper (TM) image of the area, Band 2,4 and 7 has been processed and a fracture map was created. Preferred orientation and density distributions have been determined by standard statistical means applied to geological problems. From this it has been concluded that the fractures are preferentially oriented in a NW-SE and NNE - SSW direction. The maximum compressive stress direction calculated from these mutually intersecting and conjugate sets of fractures is N-S. Consequently the tensile stresses are likely to be oriented in east - west directions, explaining why the north trending fractures tend to be the water bearing structures.

A density contour map of fractures observed in the area has been created, on which the location of water yielding boreholes in the area is plotted. The bore hole yield is compared with the density contour map in order to draw conclusions concerning the relationship between fracture density, fracture orientation and relative age of the fractures as well as the tensile stresses operating in the area, facilitating fluid migration and hence water extraction.