Intracratonic geodynamics

Geodynamic concepts of deformation and metamorphism of continental lithosphere are dominated by the effects of subduction, accretion or collision along the margins of continental lithospheric blocks. Yet it is becoming increasingly apparent that suture zones, presumably representing fossil subduction zones, but occurring far from ambient continent boundaries, play a key role in intra-cratonic deformation. In such zones the crust is strongly sheared and mantle lithosphere metasomatised. Reworking of such settings reveals a surprisingly large range of instabilities that develop in compressed/extended lithosphere with lateral heterogeneities inherited from fossil subduction settings. Structural complexity arises which is quite sensitive to the pre-existing geometry and tectonic setting. This influences localization of deformation, topographic evolution, melt generation and fluid flow patterns. We recognise a class of instabilities, labelled acceleration instabilities, of which the classical Rayleigh–Taylor instability is one example. In many cases shown in this paper such instabilities are responsible for triggering most of the response of the lithosphere. In an elastic-plastic material a necessary condition for instability is that the material reaches the yield point; thus not only density contrasts between media drive instability but also processes induced by other forces normal to the interface. As a geological example the Petermann orogeny in central Australia is given.

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► Lateral heterogeneities in the intra-plate setting lead to growth of instabilities.
► Width of the weak zone controls symmetry/asymmetry of the structures.
► Removal of the base of the lithosphere leaves imprint at the surface.