Page 108 - Handbook of Gold Exploration and Evaluation

88 Handbook of gold exploration and evaluation

rift valleys radiating away from the centre of the hot spot. Ideally there are three
rift valleys that form a triple junction at 120ë intervals. In practice the arms may
diverge at different angles as the continent begins to split apart.
Heat rising to the surface from convection cells continues to be concentrated
at the rifting centre and within a few million years the original continent is split
into two continents, thousands of kilometres apart on separate tectonic plates.
The newly formed continental margins are termed divergent or passive con-
tinental margins because they are no longer geologically active. They are,
however, losing heat and the continental margins become denser as they move
away from the rifting centre and cool. The crust sinks rapidly at first but more
slowly with time and in about 5±10 million years horsts that had emerged above
sea level become submerged, eventually reaching a depth of up to 14 km. The
newly formed passive continental margin is now overlaid by a wedge of clastic
sediment derived from the eroding continent and by carbonates resulting from
chemical and biological activity. The Virginia coastal region of the USA, which
was stabilised subsequent to rifting of the Atlantic Ocean almost 25 million
years ago, gives an example of a modern passive continental margin.
Axial rifts that are typically tens of kilometres wide contain numerous
smaller horsts and grabens of which the major graben is surmounted on both
sides by horst mountain highlands 4±5 km high composed of felsic and high-
grade metamorphic continental basement. The axial valley is initially on dry
land except for lakes, which are created when graben floors subside, forming
lakes in deep depressions. The Red Sea represents an early stage of continental
rifting leading to the formation of an ocean basin. The Great Rift Valley of
Africa represents an earlier stage of continental rifting.
With the East Africa Rift Zone as a model, Cas and Wright (1995) describe
the effects of several uplift-updoming phases in the Cainozoic accompanied by
widespread continental flood volcanism. At an early stage of ocean building in
the Late Cainozoic, initial rifting and extension affected the areas of principal
updoming with the development of fluvial, alluvial and lake environment within
a normal-fault controlled topography. In future, more advanced rifting stages
continued extension and axial graben subsidence should produce marked marine
incursions leading in time to development of an overall transgressive succession.
The basal parts of the succession containing volcanic materials would then
slowly disappear up-sequence as the rift widens into a narrow sea with the
formation of a mid-ocean ridge, such as in the Red Sea and Gulf of California. In
time the axial graben subsides and fan deltas will develop as the invading sea
creates a narrow marine basin. Turbidity currents will transport sediments
toward the basin centre, which is still very deep and anoxic. After 10 million
years or so the basin finishes filling, near shore deposition becomes the
dominant process, and the process of ocean basin formation begins with a
massive surge of mafic volcanic activity along one side of the axial rift.