Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021 Continental Tectonics: an introduction P. D. RYAN, 1 & C. MAC NIOCAILL 2 1Department of Geology, National University of Ireland, Galway, Ireland (e-mail: [email protected]) 2 Department of Earth Sciences, University of Oxford, Parks Road, OX1 3PR, UK (e-mail: conallm @ ear th. ox.ac.uk) The theory of plate tectonics has proven to be versity to celebrate John F. Dewey's 60th birth- very successful in describing the behaviour of the day. This meeting addressed some of the current oceanic plates; however the study of continental developments in the field of continental tec- tectonics is complicated by the fact that con- tonics, many of which have benefited from tinental lithosphere does not behave as a rigid John's insights, provided over the past 38 years. plate. The narrow zones of seismicity that define We first review some current topics in continental plate boundaries in oceanic lithosphere become tectonics, including how the papers in this vol- diffuse in continental lithosphere (Isacks et al. ume relate to them, and the contributions them- 1968). Continental crust is thicker, more buoy- selves are then presented, arranged in a pseudo ant with respect to the asthenosphere, of dif- Wilson Cycle, as follows: processes in closing ferent composition and theology, has higher oceans; collisional orogeny; orogenic collapse; heat productivity, and is up to 20 times older and finally mechanisms of continental rifting. than oceanic crust. An understanding of con- tinental tectonics requires that these effects upon rigid plate behaviour to be taken into account. Some current controversies in A further, complicating, factor is that the prin- continental tectonics ciples of plate tectonics were derived from a remotely sensed geophysical dataset, acquired in The exact dating of ancient orogenic events is the ocean basins over some 20 years, and held in often problematic. It relies upon precise, often relatively few laboratories. Continental tectonics difficult, correlations between sedimentary or requires the reconciliation of 150 year's worth volcanic events in the upper plate with plutonic of worldwide field data involving all branches of and metamorphic events in the lower plate. The geology with these principles. The growth of recent controversy concerning the age (estimates the continents throughout time indicates that ranging from Neoproterozoic to Ordovician) of little continental lithosphere is subducted and, the Grampian event in Scotland and Ireland is an therefore, the records of past (pre-Jurassic) plate example of the difficulties that can be encoun- geometries are stored in the continental crust. tered (Soper et al. 1999). Dewey & Mange show The greater mean age of continental crust also how detailed field study can be linked to modern means that any single continental rock mass is tectonic analogues to resolve such difficulties. likely to have been affected by several cycles of The relationship between metamorphism and creation and destruction of oceanic lithosphere deformation are dealt with by several articles. (the so-called Wilson cycle). The transposed Jolivet et al. and Forster & Lister discuss the multiple fabrics associated with each cycle can role of eclogite facies phase transformations, only be elucidated by careful field study, if the which can lower the buoyancy of the lithosphere critical evidence is still preserved. As a result, (Dewey et al. 1993), in the lower crust of the continental tectonics is a complex discipline. It Aegean region. Barrovian metamorphism is gen- involves the integration of extensive geological erally taken to have occurred because of in- field and laboratory data sets with an under- creased radiogenic heating of a thickened crust standing of plate tectonic principles, their (England & Thompson 1984; Thompson & Eng- comparison with modern and past analogues land 1984). This mechanism is consistent with and often their integration in a numerical the tectonic history reported by Searle et al. simulation. for the Himalayan continent-continent colli- This volume derives from the conference held sion, but it is inconsistent with the rapid arc- on 15th-17th September 1997 in Oxford Uni- continent collisions, as described by Dewey & From: MAC NIOCAILL, C. & RYAN, P. D. (eds) 1999. Continental Tectonics. Geological Society, London, Special Publications, 164, 1-6. 1-86239-051-7/99/$15.00 ~:) The Geological Society of London 1999. Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021 2 P. D. RYAN & C. MAC NIOCAILL Mange, which also produce Barrovian assem- Two examples of reactivation along incipient blages. Dewey & Mange also address the con- plate boundaries are given in this volume. Karson troversy concerning the nature of ophiolites. Do & Brooks describe how a Precambrian fabric may they represent slices of mid-ocean ridge crust have partly-controlled ridge segmentation in the and mantle (Moores & Vine 1971) or were they northern Atlantic and Beydoun shows how a formed in supra-subduction zone environments transform plate boundary has had its geometry (Dewey & Bird, 1971; Miyashiro 1973)? Dewey modified by a pre-existing collisional structure. & Mange argue that a supra-subduction zone origin followed by emplacement during an arc collision event accounts for the complex nature Tectonic processes within arc complexes of ophiolite suites and the short interval between their formation and obduction. Ancient tectonic reconstructions rely upon the Terrane analysis (Coney et al. 1980) has identification of rock assemblages at characterize shown how laterally mobile fault-bounded frag- plate boundaries. For example, the recognition ments of the continental crust can be. The varied of ancient arcs and their polarity is critical to the nature of continental geology, however, means understanding of orogens. Ballance reviews that a fault separating two provinces of differing the history of the southwest pacific Neogene arcs geology may not have large strike slip displace- and concludes that initial complexity involving ment along it (Seng6r & Dewey 1990; Stewart three arc systems, two of NE trend and one of et al. 1999). Plant et al. adopt a novel approach NW trend, that existed for 10Ma, underwent to terrane analysis showing that tectonic linea- simplification due to the Miocene SE motion of ments do not always coincide with terrane the Pacific-Australasian pole. The proximity boundaries. of the pole in the Oligocene led to amagmatic Shallow-dipping detachment faults are widely subduction of the Pacific Plate and the opening recognized in regions where the continental crust of a back-arc basin. Late Miocene plate reorga- has undergone extensional exhumation. Where- nization was associated with ophiolite obduction as modern syntheses (e.g. Jackson & White in the North Island of New Zealand, the nuclea- 1989) indicate that seismically-active, map-scale tion of a dextral fracture zone along the earlier, extensional faults tend to have dips of 30-60 °, NW trending arc and the dextral translation and it remains controversial whether these regional rotation of the earlier NW trending fore-arc scale faults initiated at low angles. Three contri- to its present NE trend. This level of interpreta- butions Pease & Argent, Pease et al. and Karner tion, made possible by the preservation of Neo- et al. all argue for the existence of tectonically- gene oceanic lithosphere, would be impossible in active low-angle detachments. A shallow dipping an ancient orogen. Furthermore, subsequent detachment goes some of the way to resolving ocean closure could lead to removal of much the fact that whereas some detachments in the of the critical positional information (Van Staal Basin and Range have up to 70 km of dip-slip et al. 1998). motion, the highest grade metamorphic rocks Jolivet et al. use data from the Japan Sea, the exposed contain kyanite (Hamilton 1988). Tyrrhenian Sea and the Aegean Sea to review Several tectonic models have been proposed the mechanisms by which back-arc basins open. for the formation of sedimentary basins in conti- They conclude that the principal extension nental crust: pure shear (McKenzie 1978): simple mechanism is slab-pull inducing subduction shear (Wernicke 1985); flexural cantilever (Kuzs- roll-back (Dewey 1980); however, in none of nir et al. 1991); or flexural (Karner & Driseoll). the basins reviewed this is the only mechanism There is also a vigorous debate as to whether active. Retreat of the Pacific subduction zone elevated mantle temperatures are required to produced extension in the Japan Sea orthogonal generate extensive basaltic magmatism on con- to the trench and cannot explain the significant tinental margins with low /3 factors. Although dextral strike-slip component observed. Jolivet this appears to be the case in Kenya (Khan et at,.) et al. attribute this to far-field stresses caused by and east Greenland (Karson & Brooks), Karner the India-Asia collision and the formation of a & Driseoll argue that melting can also be caused trans-Asian strike-slip fault system. Such a view by extension in the lower plate. would seem to support the analysis of Tap- Finally, the reactivation of suitably orientated ponnier el al. (1982) for the expulsion of mate- older structures by new tectonic processes is well rial laterally from this collision zone, rather known (Holdsworth et al. 1997) but, perhaps, not than that of England and Molnar (1990). The well understood. The fundamental