Plate tectonics and orogeny: A review

Abstract

Paleomagnetic pole positions, ocean-floor magnetic anomalies and least-squares fits of continental edges show that most Phanerozoic orogenic belts have formed at or near the borders of continents and island arcs. Large areas of old ocean floor have vanished, probably along former continental margins and islands arcs. Thus most Phanerozoic orogenic belts must include subduction zone effects. Superposed on them may be effects caused by self-heating of thick ‘geosynclinal’ piles of continental margin sediment. It is not yet clear how to discriminate self-heating from slip-zone heating effects. Paleomagnetic data also show that rigid areas of the continental crust have been in relative motion for at least 2500 m.y. These data imply that plates existed and that subduction may have been active since at least the beginning of Proterozoic time. Paleomagnetic data and mapping suggest that some Proterozoic African orogenic belts may not have originated by plate-tectonic processes, though the evidence is inconclusive. There is no geophysical evidence for or against the existence of Archean plates. The causes of Archean orogenesis are speculative. Some plate-tectonic models appear to differ significantly from the actual orogenic belts they purport to describe. In particular, long-continued subduction does not necessarily give rise to continental accretion, but in some cases may actually involve tectonic erosion of the overriding plate by the slip zone. Subduction of continental crust is likely to have taken place in the Alps and probably in many other orogenic belts. Probable continental margin sequences of pre-Mesozoic age are common. Some of these could have been deposited along Atlantic-type continental margins, but in no case has the matching continental fragment been identified. Without such identification, Atlantic-type continental margin sequences cannot be readily distinguished from sequences formed at the borders of new ocean basins next to wide, migrating, active island arcs (‘marginal basins’). Additional evidence suggests that the Cambro-Ordovician sequences of northwest Scotland cannot be deposits formed on an Atlantic-type continental margin, and that the same sequence in northwest Newfoundland may not have formed along such a margin. The plate tectonic significance of most orogenic features is unknown or controversial. How andesites originate is not understood; the reasons for the systematic potash variations with depth to the slip zone in andesite suites are unknown; and the significance of foreland deformation, of slaty cleavage and of regional metamorphism are disputed. Nevertheless, the solutions to all of the above problems are considered attainable, but will require a constant interplay between field, laboratory and theoretical work.