Preface
East and Southeast Asia is one of the best “natural laboratories” for the study of both extant and fossil tectonics. The complex assemblage of lithospheric continental blocks and the suture zones between them (representing former ocean basins), together with accre-tionary complexes, marginal basins and ophiolites, has attracted immense interest over the last few decades. The mineral and hydrocarbon wealth of the region has also fuelled this interest. The Gondwana origin of most of East and Southeast Asia now seems beyond doubt, and the interpretation of the evolution of East Asian continental blocks rifting and separating as successive continental slivers from northern Gondwana has received wide acceptance. The timings of separation of these slivers, their northwards drift, and amalgama-tion to form present-day East Asia are generally well constrained but still equivocal in detail.
Several extinct ocean basins are represented by the suture zones of the region, including pre-Tethyan oceans which existed prior to the assembly of Pangea, the Palaeo-, Meso- and Ceno-Tethys oceans, and a collage of minor oceanic plates related to the complex Mesozoic and Cenozoic plate kinematics of Southeast Asia.
The interest in the region’s tectonic evolution and palaeogeography led to this being the focus of three recent UNESCO International Geological Correlation Program (IGCP) projects, Pre-Jurassic evolution of
East-ern Asia (1985–1990) Gondwana dispersion and Asian accretion (1991–1996) and the current IGCP 411 Geody-namics of Gondwanaland-derived terranes in East and South Asia (1998–2002). The latest of these projects
aims at the correlation and better understanding of geodynamic and tectonic processes associated with the assembly and final emplacement of Gondwanaland-derived terranes in East and South Asia, and, through this, further evaluation of the region’s natural resource potential.
One of the critical aspects for an enhanced understanding of the geodynamic evolution of East and Southeast Asia is the study of the remnants of former ocean basins, now preserved in the various suture zones of the region. In order to identify sutures, we map out places where oceans have closed by subduction processes in the past and where pieces of continental or arc lithosphere have become
juxta-posed. A useful definition of a suture was given by Sengo¨r and Natal’in (1996) which is:
a line along which two pre-existing non-subductable pieces of lithosphere have been opposed following the demise of intervening oceanic lithosphere.
Some sutures appear to represent little or no subduction accretion, whilst others have formed as the result of a long period of subduction with substantial accretion processes. The former may be very narrow, whereas the latter may be very wide zones, especially where huge subduction accre-tion complexes have formed over which the volcanic arc has migrated oceanward turning it into an “arc massif”. In these wide suture zones, packages of oceanic lithospheric material were scraped off the downgoing slab and accreted to the overlying buoyant lithosphere along many “suture lines”. Repetition of this process can lead to a wide “suture zone” exhibiting multiple “magmatic fronts” as discussed by Sengo¨r and Natal’in (1996). Problems of suture identifica-tion, the origins of some suture zone elements, and the effects of strike-slip deformation and oblique subduction remain as ongoing concerns. The problem of identifying the origin of flysch complexes in orogens is just one complexity in studying and identifying suture zones. Another is the recognition of strike-slip deformation — prior to, during, and after the actual collision event. The extent to which oblique subduction controls distinctive deformation patterns in the overriding plate before collision takes place and during the actual collision process, is also poorly known. A common feature is partitioning of strain into components parallel to and orthogonal to the trench, and, ultimately, the collision zone.
Another problem is the role of deep crustal underthrusting during the early stages of collisional orogenies. Attenuated crustal slivers of the passive continental margin can be carried to deep levels — .100 km — to be exhumed late in the history of the orogen, commonly with the involve-ment of massive, rapid extensional shear zones. In many cases there is no problem recognising these rocks for what they are, but this is not always the case: for example, care should be taken with the interpretation of eclogites along shear zones as suture relicts. In addition, classic ophiolite nappes do not typically represent suture zones, but normally pre-date actual collision by many millions of years. For Journal of Asian Earth Sciences 18 (2000) 635–636
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example, the late Cretaceous obduction of ophiolites onto the Arabian margin occurred some 20 millions of years before the actual Arabian–Eurasian collision.
This special issue was initiated to bring together review articles and new information on the various suture zones of east and Southeast Asia in an attempt to present the current state of knowledge and to point to areas of deficiency in studies of the suture zones of the region. The issue includes ten papers which reflect the broad interdisciplinary approach necessary for the study of suture zones. This special issue forms a formal contribution to IGCP 411.
Acknowledgements
We would like to express our gratitude to all the authors for their contributions, the reviewers for their valuable positive inputs, and Dr Tony Barber for his assistance and for sharing his many years of editorial experience with us.
Reference
Sengo¨r, A.M.C., Natal’in, B.A., 1996. Palaeotectonics of Asia: fragments of a synthesis. In: Yin, A., Harrison, T.M. (Eds.). Tectonic Evolution of Asia. Cambridge University Press, Cambridge.
31 July 2000 I. Metcalfe
Asia Centre, University of New England, Armidale NSW 2351, Australia E-mail address: imetcalf@metz.une.edu.au
M.B. Allen
CASP, Department of Earth Sciences, University of Cambridge, West Building, 181A Huntingdon Road, Cambridge CB3 ODH, UK I. Metcalfe, M. Allen / Journal of Asian Earth Sciences 18 (2000) 635–636
