Abstract Accretionary orogens form at intraoceanic and continental margin convergent plate boundaries. They include the supra-subduction zone forearc, magmatic arc and back-arc components. Accretionary orogens can be grouped into retreating and advancing types, based on their kinematic framework and resulting geological character. Retreating orogens (e.g. modern western Pacific) are undergoing long-term extension in response to the site of subduction of the lower plate retreating with respect to the overriding plate and are characterized by back-arc basins. Advancing orogens (e.g. Andes) develop in an environment in which the overriding plate is advancing towards the downgoing plate, resulting in the development of foreland fold and thrust belts and crustal thickening. Cratonization of accretionary orogens occurs during continuing plate convergence and requires transient coupling across the plate boundary with strain concentrated in zones of mechanical and thermal weakening such as the magmatic arc and back-arc region. Potential driving mechanisms for coupling include accretion of buoyant lithosphere (terrane accretion), flat-slab subduction, and rapid absolute upper plate motion overriding the downgoing plate. Accretionary orogens have been active throughout Earth history, extending back until at least 3.2 Ga, and potentially earlier, and provide an important constraint on the initiation of horizontal motion of lithospheric plates on Earth. They have been responsible for major growth of the continental lithosphere through the addition of juvenile magmatic products but are also major sites of consumption and reworking of continental crust through time, through sediment subduction and subduction erosion. It is probable that the rates of crustal growth and destruction are roughly equal, implying that net growth since the Archaean is effectively zero.

Abstract 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 °C GPa −1 ) thermal gradient type of metamorphism, or advancing trench–retreating trench cycles, associated with low (<350 °C GPa −1 ) to intermediate (350–750 °C GPa −1 ) 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 d T /d P 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.

Abstract Subduction zones are both the source of most new continental crust and the locations where crustal material is returned to the upper mantle. Globally the total amount of continental crust and sediment subducted below forearcs currently lies close to 3.0 Armstrong Units (1 AU=1 km 3 a −1 ), of which 1.65 AU comprises subducted sediments and 1.33 AU tectonically eroded forearc crust, compared with an average of c . 0.4 AU lost during subduction of passive margins during Cenozoic continental collision. Margins may retreat in a wholesale, steady-state mode, or in a slower way involving the trenchward erosion of the forearc coupled with landward underplating, such as seen in the central and northern Andean margins. Tephra records of magmatism evolution from Central America indicate pulses of recycling through the roots of the arc. While this arc is in a state of long-term mass loss this is achieved in a discontinuous fashion via periods of slow tectonic erosion and even sediment accretion interrupted by catastrophic erosion events, probably caused by seamount subduction. Crustal losses into subduction zones must be balanced by arc magmatism and we estimate global average melt production rates to be 96 and 64 km 3 Ma −1 km −1 in oceanic and continental arc, respectively. Critical to maintaining the volume of the continental crust is the accretion of oceanic arcs to continental passive margins. Mass balancing across the Taiwan collision zones suggests that almost 90% of the colliding Luzon Arc crust is accreted to the margin of Asia in that region. Rates of exhumation and sediment recycling indicate that the complete accretion process spans only 6–8 Ma. Subduction of sediment in both erosive and inefficient accretionary margins provides a mechanism for returning continental crust to the upper mantle. Sea level governs rates of continental erosion and thus sediment delivery to trenches, which in turn controls crustal thicknesses over 10 7 –10 9 years. Tectonically thickened crust is reduced to normal values (35–38 km) over time scales of 100–200 Ma.

Abstract Arc magmatism at subduction zones (SZs) most voluminously supplies juvenile igneous material to build rafts of continental and intra-oceanic or island arc (CIA) crust. Return or recycling of accumulated CIA material to the mantle is also most vigorous at SZs. Recycling is effected by the processes of sediment subduction, subduction erosion, and detachment and sinking of deeply underthrust sectors of CIA crust. Long-term (>10–20 Ma) rates of additions and losses can be estimated from observational data gathered where oceanic crust underruns modern, long-running (Cenozoic to mid-Mesozoic) ocean-margin subduction zones (OMSZs, e.g. Aleutian and South America SZs). Long-term rates can also be observationally assessed at Mesozoic and older crust-suturing subduction zone (CSSZs) where thick bodies of CIA crust collided in tectonic contact (e.g. Wopmay and Appalachian orogens, India and SE Asia). At modern OMSZs arc magmatic additions at intra-oceanic arcs and at continental margins are globally estimated at c . 1.5 AU and c . 1.0 AU, respectively (1 AU, or Armstrong Unit,=1 km 3 a −1 of solid material). During collisional suturing at fossil CSSZs, global arc magmatic addition is estimated at 0.2 AU. This assessment presumes that in the past the global length of crustal collision zones averaged c . 6000 km, which is one-half that under way since the early Tertiary. The average long-term rate of arc magmatic additions extracted from modern OMSZs and older CSSZs is thus evaluated at 2.7 AU. Crustal recycling at Mesozoic and younger OMSZs is assessed at c . 60 km 3 Ma −1 km −1 ( c . 60% by subduction erosion). The corresponding global recycling rate is c . 2.5 AU. At CSSZs of Mesozoic, Palaeozoic and Proterozoic age, the combined upper and lower plate losses of CIA crust via subduction erosion, sediment subduction, and lower plate crustal detachment and sinking are assessed far less securely at c . 115 km 3 Ma −1 km −1 . At a global length of 6000 km, recycling at CSSZs is accordingly c . 0.7 AU. The collective loss of CIA crust estimated for modern OMSZs and for older CSSZs is thus estimated at c . 3.2 AU. SZ additions (+2.7 AU) and subtractions (−3.2 AU) are similar. Because many uncertainties and assumptions are involved in assessing and applying them to the deep past, the net growth of CIA crust during at least Phanerozoic time is viewed as effectively nil. With increasing uncertainty, the long-term balance can be applied to the Proterozoic, but not before the initiation of the present style of subduction at c . 3 Ga. Allowing that since this time a rounded-down rate of recycling of 3 AU is applicable, a startlingly high volume of CIA crust equal to that existing now has been recycled to the mantle. Although the recycled volume ( c . 9×10 9 km 3 ) is small ( c . 1%) compared with that of the mantle, it is large enough to impart to the mantle the signature of recycled CIA crust. Because subduction zones are not spatially fixed, and their average global lengths have episodically been less or greater than at present, recycling must have contributed significantly to creating recognized heterogeneities in mantle geochemistry.

Abstract Eoarchaean crust in West Greenland (the Itsaq Gneiss Complex, 3870–3600 Ma) is >80% by volume orthogneisses derived from plutonic tonalite–trondhjemite–granodiorite (TTG) suites, <10% amphibolites derived from basalts and gabbros, <10% crustally derived granite, <1% metasedimentary rocks and ≪1% tectonic slices of upper mantle peridotite. Amphibolites at >3850, c. 3810 and c. 3710 Ma have some compositional similarities to modern island arc basalts (IAB), suggesting their origin by hydrous fluxing of a suprasubduction-zone upper mantle wedge. Most of the Eoarchaean tonalites match in composition high-silica, low-magnesian adakites, whose petrogenesis is dominated by partial melting of garnetiferous mafic rocks at high pressure. However, associated with the tonalites are volumetrically minor more magnesian quartz diorites, whose genesis probably involved melting of depleted mantle to which some slab-derived component had been added. This assemblage is evocative of suites of magmas produced at Phanerozoic convergent plate boundaries in the case where subducted crust is young and hot. Thus, Eoarchaean ‘subduction’ first gave rise to short-lived episodes of mantle wedge melting by hydrous fluxing, yielding IAB-like basalts±boninites. In the hotter Eoarchaean Earth, flux-dominated destructive plate boundary magma generation quickly switched to slab melting of (‘subducted’) oceanic crust. This latter process produced the voluminous tonalites that were intruded into the slightly older sequences consisting of tectonically imbricated assemblages of IAB-like pillow lavas+sedimentary rocks, gabbros and upper mantle peridotite slivers. Zircon dating shows that Eoarchaean TTG production in the Itsaq Gneiss Complex was episodic (3870, 3850–3840, 3820–3810, 3795, 3760–3740, 3710–3695 and 3660 Ma). In each case, emplacement of small volumes of magma was probably followed by 10–40 Ma quiescence, which allowed the associated thermal pulse to dissipate. This explains why Greenland Eoarchaean crustal growth did not have granulite-facies metamorphism directly associated with it. Instead, 3660–3600 Ma granulite-facies metamorphism(s) in the Itsaq Gneiss Complex were consequential to collisional orogeny and underplating, upon termination of crustal growth. Similar Eoarchaean crustal history is recorded in the Anshan area of China, where a few well-preserved rocks as old as 3800 Ma have been found including high-MgO quartz diorites. For 3800 Ma rocks, this is a rare, if not unique, situation outside of the Itsaq Gneiss Complex. The presence of volumetrically minor 3800 Ma mantle-derived high-MgO quartz diorites in both the Itsaq Gneiss Complex and the Anshan area indicates either that Eoarchaean ‘subduction’ zones were overlain by a narrow mantle wedge or that the shallow subduction trapped slivers of upper mantle between the conserved and consumed plates.

Abstract Eo- to Mesoarchaean greenstone belts (e.g. 3800–3700 Ma Isua, c . 3075 Ma Ivisaartoq, 3071 Ma Qussuk) occur within orthogneisses of the southern West Greenland Craton. Greenstone belts are composed mainly of metavolcanic rocks with minor ultramafic and sedimentary schists. Compositionally, volcanic rocks are dominantly tholeiitic basalts, boninites, and picrites, with minor intermediate to felsic volcanic rocks. These greenstone belts appear to have formed in convergent margin geodynamic settings. Detailed field observations, contrasting ages, and metamorphic and structural histories suggest that this craton was assembled in several accretionary tectonothermal events, involving accretion of arcs, back-arcs, forearcs, and continental fragments by horizontal tectonics. The Superior Province of Canada was also built by the amalgamation of oceanic and continental fragments ranging in age from 3700 to 2650 Ma, during five discrete tectonothermal events over 40 Ma between 2720 and 2680 Ma. The Neoarchaean (2750–2670 Ma) Wawa greenstone belts are composed of tectonically juxtaposed fragments of oceanic plateaux, oceanic island arcs, back-arcs, and siliciclastic trench turbidites. Following juxtaposition, these diverse lithologies were collectively intruded by syn- to post-kinematic granitoids with subduction zone geochemical signatures. Oceanic island arc lavas are easily distinguished from oceanic plateau counterparts because they possess positively fractionated rare earth element (La/Sm cn > 1 and Gd/Yb cn > 1) and high field strength element depleted (Nb/Th pm < 1; Nb/La pm < 1) patterns. In addition, the island arc association includes pyroclastic rocks that are rare to absent in the oceanic plateau volcanic association. Structural studies indicate that the Wawa greenstone belts underwent a complex history of deformation including thrusting, strike-slip faulting, and asymmetric folding. These belts constitute part of a c . 1000 km scale subduction–accretion complex that formed along an intra-oceanic convergent plate margin during trenchward migration of the magmatic arc axis. Several first-order geological observations on Archaean greenstone belts of SW Greenland and the Superior Province suggest that Phanerozoic-style plate-tectonic models can provide an elegant explanation for their structural, lithological, metamorphic and geochemical characteristics.

Abstract Based on available tectonostratigraphic, geochronological, and structural data for northeastern Canada and western Greenland, we propose that the early, upper plate history of the Trans-Hudson orogen was characterized by a number of accretionary–tectonic events, which led to the nucleation and growth of a northern composite continent (the Churchill domain), prior to terminal collision with and indentation by the lower plate Superior craton. Between 1.96 and 1.91 Ga Palaeoproterozoic deformation and magmatism along the northern margin of the Rae craton is documented both in northeastern Canada (Ellesmere–Devon terrane) and in northern West Greenland (Etah Group–metaigneous complex). The southern margin of the craton was dominated by the accumulation of a thick continental margin sequence between c . 2.16 and 1.89 Ga, whose correlative components are recognized on Baffin Island (Piling and Hoare Bay groups) and in West Greenland (Karrat and Anap nunâ groups). Initiation of north–south convergence led to accretion of the Meta Incognita microcontinent to the southern margin of the Rae craton at c . 1.88–1.865 Ga on Baffin Island. Accretion of the Aasiaat domain (microcontinental fragment?) in West Greenland to the Rae craton resulted in formation of the Rinkian fold belt at c . 1.88 Ga. Subsequent accretion–collision of the North Atlantic craton with the southern margin of the composite Rae craton and Aasiaat domain is bracketed between c . 1.86 and 1.84 Ga (Nagssugtoqidian orogen), whereas collision of the North Atlantic craton with the eastern margin of Meta Incognita microcontinent in Labrador is constrained at c . 1.87–1.85 Ga (Torngat orogen). Accretion of the intra-oceanic Narsajuaq arc terrane of northern Quebec (no correlative in Greenland) to the southern margin of the composite Churchill domain at 1.845 Ga was followed by terminal collision between the lower plate Superior craton (no correlative in Greenland) and the composite, upper plate Churchill domain in northern and eastern Quebec at c . 1.82–1.795 Ga. Taken as a set, the accretionary–tectonic events documented in Canada and Greenland prior to collision of the lower plate Superior craton constrain the key processes of crustal accretion during the growth of northeastern Laurentia and specifically those in the upper plate Churchill domain of the Trans-Hudson orogen during the Palaeoproterozoic Era. This period of crustal amalgamation can be compared directly with that of the upper plate Asian continent prior to its collision with the lower plate Indian subcontinent in the early Eocene. In both cases, terminal continental collision was preceded by several important episodes of upper plate crustal accretion and collision, which may therefore be considered as a harbinger of collisional orogenesis and a signature of the formation of supercontinents, such as Nuna (Palaeoproterozoic Era) and Amasia (Cenozoic Era).

Abstract Accretionary processes contributed to major continental growth in Fennoscandia during the Palaeoproterozoic, mainly from 2.1 to 1.8 Ga. The composite Svecofennian orogen covers c . 1×10 6 km 2 and comprises the Lapland–Savo, Fennia, Svecobaltic and Nordic orogens. It is a collage of 2.1–2.0 Ga microcontinents and 2.02–1.82 Ga island arcs attached to the Archaean Karelian craton between 1.92 and 1.79 Ga. Andean-type vertical magmatic additions, especially at c . 1.89 and c . 1.8 Ga, were also important in the continental growth. The Palaeoproterozoic crust is the end product of accretionary growth, continental collision and orogenic collapse. Preserved accretional sections are found in areas where docking of rigid blocks has prevented further shortening. The Pirkanmaa belt represents a composite accretionary prism, and other preserved palaeosubduction zones are identified in the Gulf of Bothnia and the Baltic Sea areas. In the southern segment of the Lapland–Savo orogen collision between the Archaean continent (lower plate) and the Palaeoproterozoic arc–microcontinent assembly (upper plate) produced a special type of lateral crustal growth: the Archaean continental edge decoupled from its mantle during initial collision and overrode the arc and its mantle during continued collision.

Abstract Analysis of several types of seismic and potential field geophysical data consistently indicate that the majority of the crust underlying the Canadian Cordillera and much of western Canada was originally Proterozoic sedimentary rocks shed off the Canadian Shield into rift or basin structures between 1.84 and 0.54 Ga. These variably metamorphosed strata were primarily quartz- and limestone-rich sediments and thus have distinctive geophysical signatures because of their lower density, lower magnetization, and lower Poisson’s ratio compared with more mafic rocks. The sediments formed a prograding wedge that has a distinctive, internally reflective, seismic stratigraphy. In the east, these Proterozoic sedimentary rocks thicken at a ‘hinge line’ defined by the margin of the pre-1.84 Ga crystalline basement of the Canadian Shield; previous work mapped this hinge line locally using deep reflection profiles and regionally using distinctive gravity gradients. Here we assemble previously published results of several geophysical methods to define the overall shape of the wedge along the margin and westward to where it pinches out at the modern Moho beneath the crustal collage of exotic and suspect terranes accreted onto North America during the Mesozoic. The volume of crust occupied by this wedge limits the thickness of most accreted terranes to several kilometres and suggests that deeper portions of the accreted blocks detached or underthrust the wedge during accretion and are no longer contiguous to crust exposed at the surface. This type Cenozoic accretionary orogen thus spent most of its prior geological history as a passive or extensional margin punctuated by only a few, brief convergent or accretionary events.

Abstract Palaeozoic to early Mesozoic terranes of the North American Cordillera mostly originated from three distinct regions in Palaeozoic time: the western peri-Laurentian margin, western (Asian) Panthalassa, and the northern Caledonides–Siberia. A review of geological history, fossil and provenance data for the Caledonian–Siberian terranes suggests that they probably occupied an intermediate position between northern Baltica, northeastern Laurentia and Siberia, in proximity to the northern Caledonides, in early Palaeozoic time. Dispersion of these terranes and their westward incursion into eastern Panthalassa are interpreted to result from development of a Caribbean- or Scotia-style subduction system between northern Laurentia and Siberia in mid-Palaeozoic time, termed here the Northwest Passage. Westward propagation of a narrow subduction zone coupled with a global change in plate motion, related to the collision of Gondwana with Laurentia–Baltica, are proposed to have led to initiation of subduction along the western passive margin of Laurentia and development of the peri-Laurentian terranes as a set of rifted continental fragments, superimposed arcs and marginal ocean basin(s) in mid- to late Palaeozoic time. Diachronous orogenic activity from Late Silurian in Arctic Canada, to Early Devonian in north Yukon and adjacent Alaska, Middle Devonian in southeastern British Columbia, and Late Devonian–Early Mississippian in the western USA records progressive development of the Northwest Passage and southward propagation of subduction along western Laurentia.

Abstract We present the results of geological mapping and geochronological studies of the Tally Ho shear zone (THSZ) and adjacent rocks. The shear zone crops out near the west margin of Stikinia, an oceanic arc and the largest of the accreted terranes within the Cordilleran orogen of western North America. The hanging wall of the largely flat-lying shear zone consists of coarsely crystalline leucogabbro and cumulate pyroxenite interpreted as the lower crustal and possibly lithospheric mantle roots of a magmatic arc. Rocks in the footwall consist of volcanic and volcano-sedimentary sequences of the Lewes River Arc, a Late Triassic magmatic arc characteristic of Stikinia. Because the shear zone places lower crustal plutonic rocks over a supracrustal sequence, we interpret it as a crustal-scale thrust fault. Kinematic indicators imply top-to-the-east displacement across the shear zone. The geometry of folds of the shear zone is consistent with deformation in response to displacement over ramps in deeper-seated thrust faults kinematically linked to the THSZ. Crystallization of the hanging-wall leucogabbro at 208±4.3 Ma provides a maximum age constraint for deformation, whereas a post-kinematic granitoid pluton that plugs the shear zone and that crystallized at about 173 Ma provides a lower age limit. The THSZ is, therefore, coeval with: (1) a series of latest Triassic–Early Jurassic shear and fault zones that characterize the length of the west margin of Stikinia; (2) the termination of isotopically juvenile arc magmatism of the Lewes River Arc; (3) crustal loading of Stikinia giving rise to a foreland basin that rapidly filled with westerly derived orogenic molasse that includes clasts of ultrahigh-pressure metamorphic rocks; and (4) juxtaposition of Stikinia against continental crust of the Nisling Assemblage of the Yukon–Tanana terrane to the west. These constraints are consistent with a model of deformation in response to the entry of the continental Nisling Assemblage into the trench of the west-facing Lewes River Arc, terminating subduction and imbricating the arc along a series of east-verging thrust faults, including the THSZ.

Abstract The Lachlan orogen developed as a classic accretionary orogen in an oceanic setting between the palaeo-Pacific subduction zone and the Australian craton. Direct evidence for the composition and age of the lower crust and the basement to the thick Palaeozoic turbidite fan of the Lachlan orogen is limited. Exposures of Cambrian metavolcanic rocks and geophysical data suggest that most of the basement is the mafic oceanic crust along with possible small fragments of older continental crust. The trace element compositions of Cambrian metavolcanic rocks in the western and central Lachlan orogen are similar to those of volcanic rocks formed in modern back-arc and forearc settings. Pb, Nd and Sr isotopic data from these Cambrian rocks suggest a supra-subduction zone setting with little or no influence of continental crust other than subducted sediment.

Abstract The Eurasian margin in SE Asia is a geologically complex region situated at the edge of the Sundaland continent, and is mainly within Indonesia. The external margins of Sundaland are tectonically active zones characterized by intense seismicity and volcanic activity. The region is an obvious modern analogue for older orogens, with a continental core reassembled from blocks rifted from Gondwana, and surrounded by subduction zones for much of the Mesozoic and Cenozoic. It is a mountain belt in the process of formation, and contains many features typically associated with older Pacific margin orogens: there is active subduction, transfer of material at subduction and strike-slip boundaries, collision of oceanic plate buoyant features, arcs and continents, and abundant magmatism. The orogenic belt surrounds Sundaland and stretches from Sumatra into eastern Indonesia and the Philippines. The orogen changes character and width from west to east. Its development can be tectonically described only in terms of several small plates and it includes several suture zones. The western part of the orogenic belt, where the Indian plate is subducted beneath continental crust, is a relatively narrow single suture. Further east the orogenic belt includes multiple sutures and is up to 2000 km wide; there is less continental crust and more arc and ophiolitic crust, and there are several marginal oceanic basins. The orogen has grown to its present size during the Mesozoic and Cenozoic as a result of subduction. Continental growth has occurred in an episodic way, related primarily to arrival of continental fragments at subduction margins, after which subduction resumed in new locations. There have been subordinate contributions from ophiolite accretion, and arc magmatism. Relatively small amounts of material have been accreted during subduction from the downgoing plate. In eastern Indonesia the wide plate boundary zone includes continental fragments and several arcs, but the arcs are most vulnerable to destruction and disappearance. Rollback in the Banda region has produced major extension within the collision zone, but future contraction will eliminate most of the evidence for it, leaving a collage of continental fragments, similar to the older parts of Sundaland.

Abstract Previous tectonic models (escape tectonics, topographic ooze) for SE Asia have considered that Himalayan–Tibetan processes were dominant and imposed on cool, rigid SE Asian crust. However, present-day geothermal gradients, metamorphic mineral assemblages, structural style and igneous intrusions all point to east Myanmar and Thailand having hot, ductile crust during Cenozoic–Recent times. North to NE subduction beneath SE Asia during the Mesozoic–Cenozoic resulted in development of hot, thickened crust in the Thailand–Myanmar region in a back-arc mobile belt setting. This setting changed during the Eocene–Recent to highly oblique collision as India coupled with the west Burma block. The characteristics of the orogenic belt include: (1) a hot and weak former back-arc area about 200–300 km wide (Shan Plateau) heavily intruded by I-type and S-type granites during the Mesozoic and Palaeogene; (2) high modern geothermal gradients (3–7 °C per 100 m) and heat fl ow (70–100 mW m −2 ; (3) widespread Eocene–Pliocene basaltic volcanism; (4) Late Cretaceous–earliest Cenozoic and Eocene–Oligocene high-temperature–low-pressure metamorphism; (5) c . 47–29 Ma peak metamorphism in the Mogok metamorphic belt followed by c . 30–23 Ma magmatism and exhumation of the belt between the Late Oligocene and early Miocene; (6) a broad zone of Eocene–Oligocene sinistral transpression in the Shan Plateau, later reactivated by Oligocene–Recent dextral transtension; (7) diachronous extensional collapse during the Cenozoic, involving both high-angle normal fault and low-angle normal fault (LANF) bounded basins; (8) progressive collapse of thickened, ductile crust from south (Eocene) to north (Late Oligocene) in the wake of India moving northwards; and (9) the present-day influence on the stress system by both the Himalayan orogenic belt and the Sumatra–Andaman subduction zone.

Abstract Accretionary orogens form at convergent plate boundaries and include the supra-subduction zone forearc, magmatic arc and backarc components. They can be broken into retreating and advancing types, based on their kinematic framework and resulting geological character. Accretionary systems have been active throughout Earth history, extending back until at least 3.2 Ga, and provide an important constraint on the initiation of horizontal motion of lithospheric plates on Earth. Accretionary orogens have been responsible for major growth of the continental lithosphere, through the addition of juvenile magmatic products, but are also major sites of consumption and reworking of continental crust through time. The aim of this volume is to provide a better understanding of accretionary processes and their role in the formation and evolution of the continental crust. Fourteen papers deal with general aspects of accretion and metamorphism and discuss examples of accretionary orogens and crustal growth through Earth history, from the Archaean to the Cenozoic.

Abstract We present two alternative sets of global palaeogeographical reconstructions for the time interval 615–530 Ma using competing high and low-latitude palaeomagnetic data subsets for Laurentia in conjunction with geological data. Both models demonstrate a genetic relationship between the collisional events associated with the assembly of Gondwana and the extensional events related to the opening of the Tornquist Sea, the eastern Iapetus Ocean (600–550 Ma), and the western Iapetus Ocean (after 550 Ma), forming a three-arm rift between Laurentia, Baltica, and Gondwana. The extensional events are probably plume-related, which is indicated in the reconstructions by voluminous mafic magmatism along the margins of palaeo-continents. The low-latitude model requires a single plume event, whereas the high-latitude model needs at least three discrete plumes. Coeval collisions of large continental masses during the assembly of Gondwana, as well as slab pull from subduction zones associated with those collisions, could have caused upper plate extension resulting in the rifted arm that developed into the eastern Iapetus Ocean and Tornquist Sea but retarded development of the western Iapetus Ocean. As a result, the eastern Iapetus Ocean and the Tornquist Sea opened before the western Iapetus Ocean.