Metamorphic patterns in orogenic systems and the geological record

Regional metamorphism occurs in plate boundary zones. Accretionary orogenic systems form at subduction boundaries in the absence of continent collision, whereas collisional orogenic systems form where ocean basins close and subduction steps back and flips (arc collisions), simply steps back and continues with the same polarity (block and terrane collisions) or ultimately ceases (continental collisions). As a result, collisional orogenic systems may be superimposed on accretionary orogenic systems. Metamorphism associated with orogenesis provides a mineral record that may be inverted to yield apparent thermal gradients for different metamorphic belts, which in turn may be used to infer tectonic setting. Potentially, peak mineral assemblages are robust recorders of metamorphic P and T, particularly at high P–T conditions, because prograde dehydration and melting with melt loss produce nominally anhydrous mineral assemblages that are difficult to retrogress or overprint without fluid influx. Currently on Earth, lower thermal gradients are associated with subduction (and early stages of collision) whereas higher thermal gradients are characteristic of back-arcs and orogenic hinterlands. This duality of thermal regimes is the hallmark of asymmetric or one-sided subduction and plate tectonics on modern Earth, and a duality of metamorphic belts will be the characteristic imprint of asymmetric or one-sided subduction in the geological record. Accretionary orogenic systems may exhibit retreating trench–advancing trench cycles, associated with high (.750 8C GPa) thermal gradient type of metamorphism, or advancing trench–retreating trench cycles, associated with low (,350 8C GPa) to intermediate (350–750 8C GPa) thermal gradient types of metamorphism. Whether the subducting boundary advances or retreats determines the mode of evolution. Accretionary orogenic systems may involve accretion of allochthonous and/or para-autochthonous elements to continental margins at subduction boundaries. Paired metamorphic belts, sensu Miyashiro, comprising a low thermal gradient metamorphic belt outboard and a high thermal gradient metamorphic belt inboard, are characteristic and may record orogen-parallel terrane migration and juxtaposition by accretion of contemporary belts of contrasting type. A wider definition of ‘paired’ metamorphism is proposed to incorporate all types of dual metamorphic belts. An additional feature is ridge subduction, which may be reflected in the pattern of high dT/dP metamorphism and associated magmatism. Apparent thermal gradients derived from inversion of age-constrained metamorphic P–T data are used to identify tectonic settings of ancient metamorphism, to evaluate the age distribution of metamorphism in the rock record from the Neoarchaean Era to the Cenozoic Era, and to consider how this relates to the supercontinent cycle and the process of terrane export and accretion. In addition, I speculate about metamorphism and tectonics before the Mesoarchaean Era. Forty years ago, the introduction of the plate tectonics paradigm provided a robust framework within which to understand the tectonics of the lithosphere, the strong outer layer of Earth above the softer asthenosphere (Isacks et al. 1968), during the Cenozoic and Mesozoic Eras (the maximum lifespan of the ocean floors before return to the mantle via subduction). Orogenesis, the process of forming mountains, was one of a number of fundamental geological processes that became understandable once placed within a plate tectonics context (Dewey & Bird 1970). Within a few years, Dewey et al. (1973) had demonstrated that the evolution of young orogenic systems could be unravelled by inverting geological data in combination with ocean-floor magnetic anomaly maps by following the kinematic principles of plate tectonics. At the same time, the relationship between plate tectonics and metamorphism was addressed by Ernst (1971, 1973, 1975), Oxburgh & Turcotte (1971), Miyashiro (1972) and Brothers & Blake (1973). Currently, the circum-Pacific and Alpine–Himalayan–Indonesian orogenic systems define two orthogonal great circle distributions of the continents (Fig. 1), each of which has a different type of orogenic system along the convergent plate boundary zone (Dickinson 2004). These orogenic systems record the two main zones of active subduction into the mantle, the circum-Pacific and the Alpine–Himalayan–Indonesian subduction systems (Collins 2003). Complementary to these are two major Pand S-wave low-velocity structures (superswells or superplumes) in the lower mantle, under southern Africa and the South Pacific From: CAWOOD, P. A. & KRÖNER, A. (eds) Earth Accretionary Systems in Space and Time. The Geological Society, London, Special Publications, 318, 37–74. DOI: 10.1144/SP318.2 0305-8719/09/$15.00 # The Geological Society of London 2009. (Montelli et al. 2006; Tan & Gurnis 2007). They are interpreted to record upwelling responsible for advective heat transport through the lower into the upper mantle (Nolet et al. 2006). This arrangement of subduction and upwelling reflects a simple pattern of long-wavelength mantle convection, a view that is supported by a variety of geophysical data (Richards & Engebretson 1992). Accretionary orogenic systems form above a subduction boundary during continuing plate convergence in the absence of continental collision, as exemplified by the evolution of the Pacific Ocean rim during the Phanerozoic Eon (Coney 1992). These systems vary according to whether the subduction boundary or trench is retreating, neutral or advancing. They may exhibit cyclic behaviour in which a retreating trench changes to an advancing trench or in which an advancing trench changes to a retreating trench (Lister et al. 2001; Lister & Forster 2006). Accretion of arcs and/or allochthonous terranes, some of which may be far-travelled, is a common feature of accretionary orogenic systems, making the distinction from collisional orogenic systems somewhat arbitrary with the exception of terminal continent–continent collisions. Previously these orogenic systems have been called ‘Pacific-type’ (e.g. Matsuda & Uyeda 1971) or ‘Cordilleran-type’ (e.g. Coney et al. 1980), and more recently Maruyama (1997) has proposed that they should be called ‘Miyashiro-type’. Collisional orogenic systems are those in which an ocean is closed and arcs and/or allochthonous terranes and/or continents collide, as exemplified by the Tethysides (Dercourt et al. 1985, 1986; Savostin et al. 1986). The collision of an island arc with a rifted continental margin involves choking the subduction zone, subducting slab breakoff and a reversal or flip in the subduction polarity if subduction is initiated behind the arc. These ‘soft’ collisions generate only a short period of orogenesis because the forces opposing shortening are relieved by the renewed subduction (Dewey 2005). A similar process occurs where allochthonous blocks and terranes are sutured to an active continental margin, except that the subduction boundary steps back and continues to subduct towards the overriding plate and the associated orogenic event is minimal (e.g. van Staal et al. 2008). In contrast, orogens in which two continents become sutured involve significant thickening of the continental lithosphere over a wide zone, perhaps up to 1500 km across, and far-field shortening of the crust, which generates a mountain front and a wide orogenic plateau surrounded by internal basins, which themselves are bordered by thrust belts (Dewey 2005). Features such as deformation, metamorphism and magmatism may vary in intensity along and across the length and breadth of these systems because the continental margin being subducted need not be rectilinear. Previously these orogenic systems have been called ‘Himalayan-type’ (Liou et al. 2004) or ‘Turkic-type’ (Şengör & Natal’in 1996). Turkictype collisional orogenic systems are characterized by incorporation into the suture of large subduction– accretion complexes with associated magmatic arcs and terrane elements that evidence a complex history prior to terminal collision (Şengör et al. 1993; Moix et al. 2008). In some orogenic systems, such as the Alpine–Himalayan–Cimmerian orogenic system, the principal suture, in this case the Palaeo-Tethyan suture, divides the system into two parts on either side of which the subducting boundary zones, which were active accretionary orogens prior to collision, had very different character (Şengör 1992). Accretionary orogenic systems may experience multiple ‘soft’ collisions. The modern example of such a system involves the Cenozoic evolution of southeastern Asia and the southwestern Pacific (Hall 2002). Here there were three system-wide events, at c. 45, 25 and 5 Ma, each involving changes in plate boundaries and motions as a result of collisions along some part of the subduction boundary system that led to subducting slab breakoff (Cloos et al. 2005). In the older geological record, the early history of the Appalachian–Caledonian orogenic system involved peri-Laurentian arcs accreted during the Taconic (e.g. Newfoundland: Lissenberg et al. 2005; Zagorevski et al. 2006) or Grampian (e.g. western Ireland: Friedrich et al. 1999; Dewey 2005) orogenies. In these cases, the period from initial Fig. 1. Distribution of continents in relation to the Alpine–Himalayan and Circum-Pacific orogenic belts (Cordilleran orogen is cross-hatched) in ‘circular’ projection. BI, British Isles; F, Fiji; G, Greenland; GA, Greater Antilles; J, Japan; NZ, New Zealand; PI, Philippine Islands. Reprinted, with permission, from Dickinson, W. R. 2004. Evolution of the North American Cordillera. Annual Review of Earth and Planetary Sciences, 32, 13–45. # 2004 by Annual Reviews (www. annualreviews.org). M. BROWN 38