The evolving continents: understanding processes of continental growth – introduction

We have organized and edited this Special Publication of the Geological Society of London to honour the career of Brian F. Windley, who has been hugely influential in helping to achieve our current understanding of the evolution of the continental crust, and who has inspired many students and scientists to pursue studies on the evolution of the continents. Brian has studied processes of continental formation and evolution on most continents and of all ages, and has educated and inspired two generations of geologists to undertake careers in studies of continental evolution. The contributions in this volume represent only a small percentage of studies that Brian has influenced, yet the scope and significance of these papers are clear, and stand as a testimony to Brian’s contributions to understanding processes of continental evolution, growth, and stabilization. The volume is organized into six sections: oceanic and island arc systems and continental growth; tectonics of accretionary orogens and continental growth; growth and stabilization of continental crust: collisions and intraplate processes; Precambrian tectonics and the birth of continents; active tectonics and geomorphology of continental collision and growth zones. The first section, oceanic and island arc systems and continental growth, begins with a paper by Stern, who summarizes the current state of knowledge about intra-oceanic arc systems from petrological, geophysical, and tectonic viewpoints and emphasizes that these systems have been the most important sites of juvenile continental crust formation for as long as plate tectonics has operated (the time of the start of plate tectonics is a matter of debate between some geologists). Stern describes the main components and zonation of intra-oceanic arc systems, including the trench, forearc, volcanic–magmatic arc, and back-arc, typically forming a system about 200 km wide, and strongly influenced by hydrous melting process in the underlying mantle. He then describes differences between the various stages of intra-oceanic arc systems, including juvenile arc lithosphere preserved in many forearcs, to mature arc systems where magmatism is concentrated along the magmatic–volcanic front. Mature intra-oceanic arc systems are typically extensional with volcanism and sea-floor spreading developing in the back-arc, and also show a transition in mantle types from serpentinized harzburgite beneath forearc sections, pyroxene-rich low-Vp mantle beneath the magmatic front, and lherzolite– harzburgite mantle beneath back-arc basins. In the second paper in this section, Xiao et al. describe the major differences between the types of arc systems found in the circum-Pacific region, including Mariana-, Japan-, Cordillera-, and Alaska-type arcs, and compare these systems with accreted terranes in accretionary orogens. They show how arcs are complex systems that can be different along strike (such as in the Alaska-type systems), and change with time. They suggest that some unresolved issues in accretionary and collisional orogens may be related to geologists not appreciating some of the complexities in modern arc systems, and use examples from the Altaids and other systems to demonstrate their points. Part 2 of the book, tectonics of accretionary orogens and continental growth, highlights a common thread of Brian Windley’s multi-year efforts of trying to work out the framework and evolution of accretionary orogens. His studies followed through early studies on the Tien Shan, Altai, and Central Asian Orogenic Belt in Mongolia, the Solonker suture in China, the Qilian Shan, the western Kun Lun Mountains and the Bei Shan in China. His most recent work in Asia has all been done in collaboration with Xiao Wenjiao, and in the Mona Complex in Anglesey and Lleyn in Wales with Shigenori Maruyama. Appropriately, the first paper in this section is by Maruyama et al. They recognize three types of strongly imbricated oceanic plate stratigraphy in the Neoproterozoic accretionary orogen on the island of Anglesey and the Lleyn Peninsula.