Introduction

Abstract

The topic for this Discussion Meeting arose from the apparent paradox that extensional tectonics are commonly associated with ‘compressional’ plate boundaries. Elsasser (1970) first showed how extension could be associated with convergent plate boundaries, and subsequently (1971) he added a ‘suction force’ caused by the sinker effect of the cold subducting slab (see also Forsyth & Uyeda 1975). At about the same time, Karig (1970, 1971) suggested that several ‘marginal’ basins in the western Pacific had originated by back-arc spreading, an interpretation foreshadowed by Wegener (1924, p. 123). Karig (1972) also postulated that the isolated ridges within these back-arc basins were ‘remnant arcs’ inherited from earlier stages in their development. Subsequent drilling and detailed geophysical surveys have fully vindicated the back-arc spreading - remnant arc model. Despite these initial advances, the origin of extensional features associated with convergent plate boundaries is still poorly understood, and their distribution in space and time is not well known. This Discussion Meeting was therefore organized at the suggestion of one of us (A.G.S.) and under the auspices of the British National Committee for Geodynamics to air some of these problems. The objective was not to produce a global synthesis of all extensional features associated with convergent zones, but to review basic observations from areas that illustrate the many different facets of a phenomenon for which there is almost certainly no single explanation.