The evolution of the Indian Ocean Mega-Undation : Causing the indico-fugal spreading of gondwana fragments

Abstract

In the first section the geomechanical model of mega-undations is elaborated:

1. (1) The lower mantle may have a Newtonian viscosity, but the upper mantle, which is largely in a crystalline state, shows an Andradean viscosity, with hot-creep phenomena and the formation of lamellae separated by zones or planes of high strain rate.

2. (2) Reliable solutions of the mechanics in the fault planes of earthquake foci indicate that the spreading of the mega-undations is characterized in the outer 400 km by the farther advance of the higher structural levels with respect to the underlying ones: whereas the movements in the foci of deep earthquakes underneath the Japan Sea and South America indicate a reverse process, the lower blocks moving faster towards the Pacific than the overlying ones. This is explained by the geomechanical model of the mega-undations.

3. (3) The crest of the mega-undations shifts in the course of time, either gradually or by steps. The effects of such shifts are discussed and illustrated. 4. (4) Four stages of evolution of mega-undations are distinguished:

4.1. (a) young

4.2. (b) early mature or precocious

4.3. (c) late mature or ripe, and

4.4. (d) fossil mega-undations.

These stages are illustrated by type examples. In the section on the development of the Indian Ocean Mega-Undation this geomechanical model is tested by an analysis of the geotectonic evolution of the Indian Ocean and the surrounding shields. It appears that there is a good correspondence between the expectations according to the hypothetical model (prognoses) and the geotectonic observations (diagnostic facts). This reinforces our confidence in the adequacy of the functioning of the model. The latter might be tested further by tectonic experiments, namely, by the centrifugation of models at a reduced scale.