The papers in the following thematic set arise from the 1991 William Smith Lecture Meeting of the Geological Society, convened by J. M. Cubitt and held at Burlington House on 3 April 1991.
The purpose of this meeting was to review our current understanding of passive margin subsidence. To this end an international cast of speakers was invited to a meeting centred on the William Smith Lecture by A.B. Watts of Oxford University. Four of the papers delivered are published here.
Passive margins form just over half of the present-day continental margin system and, although like the world's oceans all are relatively young (Mesozoic-Tertiary), former passive margins of Palaeozoic and Precambrian age are preserved in orogenic belts and plate interiors. It is now generally accepted that passive margin subsidence is a direct consequence of litho-spheric thinning, most of which took place prior to final separation of the continents and emplacement of an oceanic spreading centre. Intracontinental rifts that failed to develop oceanic crust are geodynamically very similar to passive margins, and in recent years lessons learned in one environment have been successfully applied in the other.
In both cases, subsidence is isostatically driven and results from thinning of the continental crust and its replacement by higher density mantle. Because extension and thinning tends to involve the whole lithosphere (125 km or so in thickness), and not just the top 30 km or so of continental crust, higher density lithospheric mantle is displaced by lower density astheno-sphere during stretching, partially canceling