The Development of Continental Margins in Plate Tectonic Theory

Abstract

Atlantic-type continental margins are formed by rifting and subsequent breakup of continents by sea-floor spreading. Large scale horizontal displacements of continental blocks on lithospheric plates are well-described by current plate tectonic theory. However, the rifting process itself entails vertical tectonics which operate prior to breakup, and such processes are not well understood within the framework of plate tectonic theory.The qualitative models, at present described in the literature involve a sequence of events illustrated by type examples: the East African Rift; the ethiopian-Afar Rift; the Red Sea Rift; and the Gulf of Aden. The thinning and subsidence of the continental crust during rifting have been considered to be the result of crustal stretching. However the writer considers the evidence ambiguous and the theory inadequate. The model presented here involves the response of the crust and the lithosphere to thermal processes implicit in plate tectonic theory. Firstly, uplift is caused by thermal expansion and phase-boundary migration in the lithosphere. which leads to crustal thinning by erosion. Secondly, subsidence is caused by metamorphism in the deep crust. These events occur out of phase and result in two cycles of uplift/erosion/and subsidence during continental margin formation.From this generalised model, theoretical time-stratigraphic and structural cross sections may be constructed to suit the different continental margins. These cross sections are composed of different litho-tectonic elements, namely: pre-rift, rift valley, and post breakup, which are in general separated by two angular unconformities. These features may be readily identified on Atlantic continental margins with only a minimum of seismic and well control.

Getting over continent ocean boundaries

“…In much tectonostratigraphic literature, the development of COBs is related to the development of regional stratigraphic discontinuities, often referred to as breakup unconformities (Falvey, 1974). As originally conceived, the breakup unconformity was thought to form at, and so pinpoint, a geological instant of strain localisation, continental rupture, failure, and wholesale subsidence of the continental margin coinciding with first emplacement of igneous crust between the diverging plates.…”

Extensional basins of the former Soviet Union — structure, basin formation mechanisms and subsidence history

“…Density increases due to basalt-eclogite phase trans- formations in the lower crust have, for example, been proposed as a possible cause of sedimentary basin formation (Falvey, 1974;Haxby et al, 1976;Artyushkov and Sobolev, 1982;Stel et al, 1993). Recent experimental data indicate (see Carswell, 1990, for a review) that the minimum pressures required for eclogite stability in rocks of lower crustal composition could be in excess of pressure estimates from linear extrapolation of the plagioclase-out reaction curves of Green and Ringwood (1967) to the low P-T range.…”

Cenozoic clay mineralogy from D.S.D.P. holes on the continental margin of the Australia—New Zealand sector of Antarctica

“…shelf. Aus tralia and Antarctica began to separate in latest Cretaceous times (Falvey 1974). Reworked Paleogene palynomorph assemblages in marine samples off East Antarctica, not far from site 268, are similar to assemblages that indicate "considerable warmth" when found in Australian Paleogene sequences (Kemp 1972).…”