Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021 Orogeny through time: an overview JEAN-PIERRE BURG & MARY FORD Geologisches Institut, ETH-Zentrum, Sonneggstrasse 5, CH-8092 Zurich, Switzerland Abstract: As an introduction to a diverse set of papers on orogenic studies we present a personal overview of the most notable developments in orogenic studies since the 1960s. The impact of new techniques such as deep reflection seismic profiling, geochronology, analogue and numerical modelling is discussed. The major geodynamic models and concepts which have stimulated orogenic studies in recent years are also considered. Orogenesis is the most complex of tectonic Changing approach to the study of orogens processes and interpreting ancient mountain belts is one of the greatest challenges geologists The major controversies that have arisen from face today. As originally defined by Gilbert the study of looking at orogens in the past are (1890), an orogeny (from the classical Greek documented in several works (e.g. Cady 1950; 6ros meaning 'mountain' and gen~s meaning Condie 1982; Miyashiro et al. 1982). Based on a 'stemming from') is simply a period of mountain mixture of spiritual contemplation and obser- building. To field geologists the term orogeny vation, many nineteenth century theories on the represents a penetrative deformation of the origin of mountain belts could not conceive of Earth's crust associated with phases of meta- any large movements in the Earth to produce morphism and igneous activity along restricted, orogenic belts (fixist theories). The important commonly linear zones and within a limited time concept of lateral compression gained credence interval (Dennis 1967). However our increasing as late as the middle nineteenth century. It was understanding of the rheology of lithospheres only in the twentieth century that Earth scien- allows geologists today to view orogenesis on a tists first suggested (e.g. Argand 1924; Wegener larger scale as the interaction of a series of 1912; Wilson 1966) and then established with the geodynamic processes. This volume has arisen plate tectonic paradigm (e.g. McKenzie & from a seminar series given by invited speakers Parker 1967; Isacks et aI. 1968; Le Pichon 1968; in 1994 at the Geological Institute, ETH, Morgan 1968) that large horizontal movements Zurich. The aim of this lecture series was to were responsible for Cenozoic orogens. As provoke discussion on, and greater awareness elaborated elsewhere (e.g. Condie 1982) plate of, the larger issues of orogenesis. In particular, tectonics unified several long-lived theories such is there a change in style or mode of orogeny as those of geosynclines and continental drift through geological time, or is variety of orogenic and rendered obsolete notions such as world- features rather reflecting different rheologies wide orogenic cycles and a contracting Earth. and boundary conditions in space. The wealth of Shortly after the acceptance of the plate tectonic data and ideas presented in this lecture series are theory, modern and ancient mountain belts were compiled here as a series of review-type papers. analysed in terms of global tectonics (e.g. This book can provide only scattered ex- Dewey & Bird 1970; Dickinson 1971). However, amples of orogens through time. Figure 1 shows partisans of primary vertical tectonics resisted the global distribution of orogenies of different the plate tectonics paradigm and maintained ages and those covered by this book are marked. that ancient orogens were better explained by Many of the Cenozoic orogens which have contraction of intracontinental mobile zones received considerable attention in recent years (evolved from ensialic rift zones i.e. aulacogens (Alps: Roure et al. 1990; Pfiffner et al. 1996; or geosynclines) between stable regions (cra- Schmid et al. 1996; Himalayas: Treloar & Searle tons, e.g. Weber 1984). Zwart (1967) em- 1993; Pyrenees: Choukroune et al. 1990; Oman: phasized differences between the Alpine and the Robertson et al. 1990) are not covered in this Hercynian orogens, leading to the widely used volume. classification of orogens as either Alpinotype or From Burg, J.-P. & Ford, M. (eds), 1997, Orogeny Through Time, Geological Society Special Publication No. 121, pp. 1-17. Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021 2 J.-P. BURG & M. FORD 6 ~Mesozoic and Cenozoic orogenic belts ~Palaeozoic orogenic belts Platforms ~ Proterozoic~ ~. Crat ons ~Archean ___/Shields/ Fig. 1. World map of orogens distinguished by their age. Orogens coved in this volume are boxed. 1, Archaean orogens of Choukroune et al. a, Dharwar craton and b, Superior province; 2, Mount Isa Terrain, O'Dea et al; 3, the Scandinavian Caledonides, Milnes et al. and Rey et al. ; 4, the Lachlan fold belt, Gray; 5, the Urals, Puchkov; 6, the Variscides, Rey et al.; 7, the Central Andes, Lamb et al. Adapted from Miyashiro et al. (1979) and Condie (1982). Hercynotype depending principally on the Burke 1973; Krrner 1981; Hoffman 1989). amount of ophiolites, high-pressure meta- Other researchers doubt whether Middle Pro- morphic rocks and granites. Application of the terozoic (1000 Ma) and older orogens could re- plate tectonic concept was however more fruitful sult from a plate tectonic regime (e.g. Hargraves in that geologists could show that major charac- 1976; Wynne-Edwards 1976; Reed et al. 1993). teristics of an orogen (namely deformation, It is often argued that geothermal gradients were metamorphism and igneous activity) record much higher in the Archaean than at present be- stages of the plate tectonic history of the orogen, cause heat-producing elements were much more i.e. successively subduction, obduction, collision abundant. Geological evidence for this comes and eventually post-collisional intra-continental from very high-temperature terrains, komatiitic deformation. Many articles and books, dedicated lavas in greenstoncs ~nd large plutonic bodies. to the link between plate tectonics and mountain The lithospheric plates would then have been belts (e.g. Mitchell & Reading 1969; Coney 1970; thin and their density too low to cause buoyancy- Dewey & Bird 1970; Dietz 1972; Gilluly 1973), driven subduction. However, vigorous mantle have convincingly revealed that modern orogenic convection could have resulted in folding and belts occur principally at convergent plate faulting of the thin lithosphere (reminiscent of boundaries and result from collision between the contraction theory). This heat- and gravity- continental, arc-derived or oceanic crustal driven activity would indeed have triggered blocks. intraplate orogenesis. However, the argument Today, it is generally agreed that plate may simply imply a secular variation in style of tectonics were acting throughout Phanerozoic intraplate deformation which does not exclude time, although deduction of relative directions plate boundary deformation, because if plates and rates of continental drift before 200Ma did exist in the Archaean, they would have been remains a problem. Some authors extrapolate the smaller and their motion twice as rapid as Phan- theory to the whole Precambrian (e.g. Dewey & erozoic rates (Sleep & Windley 1982). Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021 OVERVIEW 3 Recent techniques DePaolo 1981). The additional information gained from dating lithospheric plates facilitates Since the 1960s the impact of geodynamics and the definition of mineralogical and physical geophysical data on the study of orogens has differences that may develop with time. been enormous. Rapid advances in technology and increasingly powerful computers have (1) Seismicity and deep reflection seismic profiling: generated completely new data sets (e.g. geo- the layered lithosphere. The layered configur- chronology and deep seismic profiling) which ation of stable lithospheric plates verified by must be reconciled with more traditional field seismic studies stands as a fundamental concept observations and (2) allowed numerical model- that rules the mode in which an orogen may ling of complex Earth systems and processes evolve. Seismic studies have used S- and P-wave whose results can be compared with and con- velocities to recognize a layered Earth with a strained by factual data. Field geologists can thermally conductive lithosphere overlying the increasingly interpret their data in terms of large seismic low velocity zone of the convective scale lithospheric or crustal processes. We mantle. Thus a plate is colder and therefore summarize below what we feel are the more more rigid than the underlying asthenosphere. important of these modern techniques and Lithospheres are themselves seismically layered models and the impact they have had so far on and consist of an upper rigid layer and a lower the study of orogens. viscous thermal boundary layer (Parsons & McKenzie 1978). The crust is the upper part of Geochronology. Isotopic techniques permit the the 100-150km thick mechanical lithosphere. dating of crystalline rocks and therefore have Seismic velocity-depth models simplify as one become a prerequisite to understanding the level of bulk tonalitic composition the 20-35 km crystalline axes of all orogens. Our knowledge, thick continental crust, although geologists particularly in the Precambrian regions, owes know it to vary- greatly in lithological content. much to these methods. For example, many Lithospheric layering varies in young and old terranes, initially recognized as Archaean