Aulacogens and aulacogeosynclines: Regularities in setting and evolution

Aulacogens and aulacogeosynclinal zones represent intra-continental palaeorifts and inverted palaeorifts. They were formed in the process of the break-up of continents during the Late Proterozoic. Inversion of such basins occurred variably during the Baikalian, Caledonian, Hercynian and Alpine orogenic cycles in response to the build-up of intraplate compressional stresses emanating from orogenic belts. Aulacogens were first recognized on the stable Russian Platform where they correspond to linear, graben type depressions filled with Proterozoic sediments, often buried beneath a thick cover of Palaeozoic and younger sediments. Some aulacogens have experienced a short period of reactivation during the Palaeozoic. Aulacogeosynclines differ from the genetically related aulacogens by their greater dimensions and the amount of lithospheric extension that can go to the extent of opening of small ocean basins; consequently they are characterized by a greater degree of destruction of the continental crust, the depth of subsidence and the complexity of their post-rift development which commonly terminates with their compressional deformation, giving rise to the development of intracontinental linear folded and thrusted structures, frequently associated with low-grade metamorphism and small intrusions. Many ancient platforms are characterized by aulacogens; metaplatforms are dissected by aulacogens and aulacogeosynclines, whereas geosynclinal troughs are the precursors of geosynclinal mobil belts. Aulacogens, aulacogeosynclines and geosynclinal troughs represent a group of negative, linear structures which are characterized by an increasing degree of crustal destruction during rifting processes and increasing mobility during lithospheric contraction.