Proterozoic orogens and the break-up of Gondwana: why did some orogens notrift?

Ancient orogens appear to localise rifting of (super)continents and many ex amples of alignment between old orogens and rifts during Gondwana break-up have been described. in this paper, these correlations are critically asses sed. Of the similar to 25,000 km of rifted Gondwana margin, 45% lies parall el with old orogens, similar to 46% cuts across previous structures or crat ons and 9% is uncertain. Furthermore, more than 20,000 km of pre-existing o rogens did not rift. This begs the question: why did some old orogens form the locus of rifting while others did not? It is suggested that some ancien t orogens form lithospheric weak zones; others, however, form strong zones, having an integrated shear strength higher than that of normal lithosphere . Convective removal of the thickened thermal boundary layer of the mantle lithosphere (TBL removal) and resulting orogenic collapse is a very efficie nt way of weakening the lithosphere. Thermal modelling suggests that small orogens (thickening factor < 1.5) that do not suffer TBL removal may remain lithospheric zones of strength long after orogenesis. Large, thick orogens may result in lithospheric zones of weakness, whether TBL removal occurs o r not. It is suggested that most Neoproterozoic belts that rifted during Go ndwana break-up may have suffered TBL removal or were very thick orogens, a nd that most Neoproterozoic belts that did not rift may not have suffered T BL removal. (C) 2000 Elsevier Science Limited. All rights reserved.