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Ophiolites and their modern oceanic analogues

L. M. PARSON & B. J. MURTON Institute of Oceanographic Sciences, Deacon Laboratory, Brook Road, Worrnley, Godalming, Surrey GU8 SUB, UK

Conventional studies of oceanic structure and spective on ridge geometry and processes of evolution are inevitably hampered by the diffi- axial reconfiguration. Two papers, the inter- culties with sampling the crust in the third pretations in which have largely been derived dimension. Remote sensing of surficial struc- from these remotely sensed techniques, follow. ture, topography and morphotectonics has be- Searle provides a general assessment of the come easier and of higher resolution with the style of crustal formation at mid-ocean ridges as advent of more sophisticated techniques such as recognised on sidescan and swath bathymetric GLORIA long range sidescan sonar and TOBI data, while Rusby presents a more in-depth deeptow sidescan sonar, as well as multibeam analysis of the tectonics of a complex section of swath bathymetric mapping systems. Subsurface the Easter micro-plate. Finally, Karson reviews studies, however, have enjoyed less of this prog- in some detail the types of extensional fault ress, and although deep drilling, electro- fabrics which are recognized on mid-ocean magnetic surveys and deep seismic techniques ridges, and assesses their variations along strike still provide some measure of understanding as well as across strike. below the seafloor, these data are few and relatively widely spaced. The integration of studies of ophiolites, therefore, can offer the Geochemistry and petrology marine community the benefits of a vast body of A number of regional geochemical papers fol- knowledge with regard to deeper structure and low, each providing a direct comparison of a crustal compositions, which is unattainable in present oceanic setting with studies of ophiolites the deep sea. In turn, many of the regional and of a range of ages. Taylor, Murton& Nesbitt detailed structures which have been recently collectively present data from deep sea drilling identified in active mid-ocean spreading sys- in the Western Pacific, in particular the geo- tems, such as ridge segmentation patterns and chemical variations in oceanic crust formed be- ridge axis discontinuities, now form a significant tween the arc and the trench. These data they contribution to recent studies of ophiolite com- compare with similar geochemical and geo- plexes. The complementary work of the two graphical variations of boninite sequence vol- schools is clear and ripe for development. canics described from the Troodos ophiolite, Cyprus. Kostopoulos & Murton further expand Tectonics these studies of boninite petrogenesis and en- vironments of formation, focusing in particular The first two contributions to this volume on the OIB-like component and suggesting its demonstrate the appropriate choice of theme. origin lies in the melt extraction process beneath Alexander & Harper's detailed review of the oceanic spreading centres. Ballantyne describes Josephine Ophiolite discusses the architecture the dismembered ophiolitic rocks of eastern of mid-ocean ridge magma chambers, quantifies Indonesia, notes the relatively limited MORB- the extensional tectonics, and provides a study like component of the ophiolite complex, and of the range of fault structures, both transfer discusses the mantle sequence of depleted harz- and normal, associated with slow spreading. burgites and subordinate lherzolites. Whilst They also document the alteration of the host sharing many of the petrological characteristics rock by hydrothermalism, and the relative with plutonic rocks of the Papuan and Marum chronology of deformation. MacLeod & Rothery ophiolites of the island of New Guinea, these reinforce this type of assessment of ophiolitic rocks appear to have strong affinities to others complexes with their analysis of ridge axis dis- occurring in suprasubduction environments. continuities in the 500 km long exposed section This contrasts with the likely forearc basement of the Oman ophiolite. Outcrop interpretations overthrust emplacement for the PNG com- of these types of structures, which are at present plexes. Roberts, along with Vaisami & Cann, studied only by marine structural geophysicists Girardeau & Mercier, present more detailed and geologists using remotely sensed methods, analyses of particular lithological associations offer an exciting, new and high resolution per- within Mediterranean ophiolites. Roberts con- Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

2 L.M. PARSON & B.J. MURTON tributes a study of the setting of chromite de- interpretation. Borgia & Treves, along with posits in cumulate sequences, and Valsami & Harris conclude the volume with regional Cann use the epidositic reaction zones as means studies of emplacement of oceanic crusts, with of assessing the mobility of rare earth elements. two papers describing two contrasting scales for Laurent describes intrusive peridote diapirs cut- the formation of emplacement of ophiolites. ting the ultramafic cumulates and gabbroic rocks The first uses the volcanic edifice building pro- of the Troodos ophiolite and ascribes an origin cesses associated with the Hawaiian shield vol- of formation in terms of melt percolation at the canoes as an analogue for both spreading and top of upwelling mantle columns beneath mid- overthrust (emplacement) mechanisms. This ocean spreading axes, He goes on to offer them paper is particularly thought provoking, es- as analogues of the recently drilled seamounts pecially with its controversial use of oceanic occurring in the Mariana forearc. Girardeau & dynamic terms to describe the behaviour of the Mercier conclude this section with a review of large shield volcanic systems. The second dis- plagioclase wehrlites sampled by deep drilling cusses the importance of synorogenic forearc in the eastern Pacific Ocean, by submersible extension for the formation and immediate em- and in the field in Oman and Troodos. placement suprasubduction zone ophiolites, by examining the tectonic evolution of current com- plex collision dynamics in the Banda-arc- Analogous settings Australian collision zone. The final four papers have a varied content, but We hope to have compiled in this volume a have been grouped to demonstrate the similarity suitable cross-section of current research in of methods and concepts which apply to both ophiolite geology and its bearing on studied of oceanic and ophiolitic studies. Allerton & Vine present mid-ocean ridge processes. It is our describe their paleomagnetic surveys of crustal hope that progress in the two fields of interest, rocks in the Troodos ophiolite, and report their marine and land-based, will be made together; findings of block rotation, on axes with both and that symposia such as this provide a mutual horizontal and vertical components adjacent to benefit to geologists and geophysicists alike. major fault structures. They also include details of ridge transform intersection tectonics and We would like to acknowledge the hard work and efforts of all those individuals whose names do not ridge jumps in a study which is of direct and appear as authors in this volume. They include the immense relevance to marine oceanic research referees for the manuscripts, a list of whom appear on mid-ocean ridges. Brewer, Pelling, Lovell & below, staff at Burlington House -- particularly Siam Harvey present new data from deep drilling Roblings, and at the Geological Society Publishing work in the eastern Pacific in the form of geo- House, particularly Angharad Hills. Finally thanks go chemical wireline logs. These type of data have to Yvonne Baker and Annie Williams at IOSDL for been interpreted by some geochemical petro- the endless secretarial support. physicists to provide approximate modal analy- ses of some downhole lithologies unsampled by S. Agar, J. Andrews, P. Browning, J. Cann, P. Floyd, I. Gass, A. Griffiths, J. Hall, C. Jacobs, L. coring. The methods by which some of these Johnson, J. Karson, M. Kleinrock, D. Kostopoulos, wireline log data are analysed and interpret- R. Laurent, P. Lonsdale, K. Macdonald, C. Mac- ations derived are the subject of some contro- Leod, C. Neary, R. Nesbitt, J. Pearce, H. Prichard, versy amongst 'purist' petrologists, but the D. Rickard, S. Roberts, A. Robertson, N. Rogers, authors argue convincingly for the validity of R. Searle, M. Sinha, R. Stem, P. Styles, R. Taylor, their methods, while still urging caution in the M. Wilson.