MAPPING GEOLOGICAL FAULT STRUCTURES IN THE CAPE FOLD BELT MOUNTAINS – A CARTOGRAPHIC APPROACH

G. Brunsdon, P.W.K. Booth

Nelson Mandela Metropolitan University, Geosciences Departmant, Port Elizabeth, South Africa

gideon.brunsdon@nmmu.ac.za

Rocks of the Palaeozoic Cape Fold Belt cover the southern tip of Africa and are known as the Cape Supergroup. These Cape Fold Belt rocks were deposited between 520 – 340 million years ago and have been extensively faulted and folded due to compression forces between 280 – 215 million years ago, directed from the present day south to the north.

To gain a better understanding of where these compression forces came from and what caused them, a great emphasis has been placed on studying and understanding the structure of these rocks. In general it has been found that rock layers of the Cape Fold Belt strike in a more or less east to west direction, with individual layers either dipping to the north or the south, depending on fold structure. Many thrust faults have been described that have a more or less a similar trend, but most of these faults dip in a south-south-westerly direction. Normal faults strike in an east to west direction in general, but mostly are of a more recent deformation age and therefore usually occur after the thrust faulting. Strike-slip faults are sometimes associated with the normal faults, but strike in a north to south direction.

These structural trends are clearly visible on Landsat ETM imagery and visual interpretation of individual Landsat bands gives one an overall bird’s eye view of a large area, which enables one to see the continuation of these structures which is not possible when using aerial photographs. Individual hills and mountain ridges can be traced showing the general east-west striking trends, whereas river valleys often truncate the mountainous areas in a north-south direction along pre-existing strike-slip faults, in an uniformly spaced parallel pattern.

The mapped geology and structural data allows for further interpretation when it is combined and draped over a digital terrain model (DEM). The geological structure of the mountain ranges with large scale overturned folds, creating ridges and valleys within the mountain range, becomes clearly visible.

Including GIS and Remote Sensing into a geological study, greatly aids and enhances the interpretation and analysis of data, and it creates enhanced visualisation and cartographic output, especially with the use of three dimensional modelling. To do this properly one needs to understand and apply many cartographic principles, but the knowledge of cartography is often lacking or ignored when geological mapping is done.