Full Description

Special Publication 81

Geoscientists need to familiarize themselves with the characteristics of actual plate motions. A popular, current thesis relates the state of stress to the degree of coupling of the two convergent plates. Recent two-dimensional, finite element modelling has demonstrated how the stress in the overriding plate is critically dependent on whether the subduction zone is locked (i.e. high degree of coupling) or unlocked. Thus the compressional stress regime occurs in locked systems, and extensional stress in unlocked systems; a gradient from compression in the fore-arc to extension in the back-arc is said to characterize partially locked system. However, studies of the strain field in real cases (modern arc regions) clearly indicate the prevalence of subhorizontal extension orientated nearly perpendicular to the arc. Implicit in these real observations is the suggestion that stresses resulting from plate coupling are either not transmitted to the volcanic arc, or else locked subduction zones are uncommon and extension may be the normal mode of behaviour.

The rationale and evidence for extension and coeval volcanism at consuming plate margins is presented. Using mainly Cenozoic case histories for oceanic and continental margin arcs, structural evidence is presented, which demonstrates that volcanism in most arc systems is contemporaneous with normal faulting and subsidence as a general case. Representative, modern geochemical data sets are provided and integrated in a petrological discussion for each case history. The western Pacific, now widely regard as a type region for the study of convergent plate margin processes, is particularly well represented. Also included is evidence from less commonly investigated regions, where subduction has ceased or is no longer obvious but an association with an arc is unequivocal. By these means, it is hoped that this volume contains a representative cross-section of current research on extension-related arc volcanism in broadest sense, and that it will stimulate further discussion and research into the complex tectonics and petrology of convergent plate margins.

This is a valuable book … I heartily recommend it to anyone working in any arc terrain, modern or ancient, and to anyone with an interest in volcanism and magmatic processes. Mineralogical Magazine.