The classic model of orogeny in the Wilson cycle is of a closing ocean basin producing a continent–continent collision, such as India and Asia in the Himalyan orogeny or the closing of Rheic ocean to produce the Allegenian–Variscan orogeny. However, not all orogenies follow this pattern, the most obvious example being the western Cordillera of the Americas. As this recent Geological Society Special Publication explains, orogens consist of three end members—collisional, intracratonic and accretionary. Accretionary orogens are produced when subduction is not interrupted by a major continental collision. Instead, subduction is ongoing and smaller terranes are episodically accreted to the overriding plate. It is this type of orogeny that is theme of this volume. The notion of accretionary orogens has been around since 1980, originally as terrane tectonics, but the concepts (and terminology) have subsequently evolved.
Cawood and Kröner's volume emerges from a working group formed in 2003, and two subsequent field workshops in Taiwan and Japan. The volume consists of 14 papers, four of which are conceptual, with the remaining ten consisting of case studies of different regions and geologic time periods. They are organized in order of decreasing geologic time, rather than geographic regions, with half the papers covering Precambrian examples.
The excellent opening paper by Cawood et al. sets the scene. It reviews the major themes that occur throughout the rest of the book (and will be essential reading for my students). Long-lived periods of subduction are essential for accretionary orogens, which may be retreating, where roll-back of the hinge of the downgoing plate retreats oceanward faster than the overriding plate, causing extension behind the active arc, or advancing, where roll-back is less rapid than the oceanward motion of the overriding plate, thus causing shortening behind the arc. The latter is the case for the present day central and southern Andes. Over the life of the orogen, these two modes may alternate.
The metamorphic signature of accretionary orogens is the dual high P/low T - high T/low P of Miyashiro's paired metamorphism. As the chapter by Brown points out, this dual thermal regime is a characteristic of modern subduction processes and may be used to evaluate whether subduction operated in Archean systems.
An essential feature of accretionary orogens is, of course, the accretion of buoyant elements of the subducting crust, such as seamounts, arcs and microcontinents. The large proportion of Precambrian case studies in this volume is a reflection of the widespread opinion that terrane accretion was an important process in the early development of the continents.
Crustal material can be returned to the mantle in accretionary sytems by sediment subduction and tectonic ‘erosion’ of the overlying plate in strongly coupled, advancing systems. Continental and arc crust are produced by the magmatic arc outputs. Scholl and von Huene suggest that these two processes have been in balance since about 3 Ga and that the total flux of material through the mantle in that time is equal to the present volume of the continental crust.
Cawood and Kroner's volume is another successful addition to the Geological Society's stable of publications. It points out the enormous importance that accretionary orogenic systems have for the formation of the continents and for geochemical cycling through the mantle. This volume should be on the shelves of all serious students of tectonics.