William J. Collins

Accretionary orogens through Earth history

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.

Hot orogens, tectonic switching, and creation of continental crust

Many granulite terrains were too hot to have formed during continental collision. Rather, along with many high-grade metamorphic terrains that typify continental crust, most formed in accretionary orogens during tectonic switching, when prolonged lithospheric extension was interrupted by intermittent, transient contraction. Based on modern and ancient examples, tectonic switching occurs when slab retreat induces upper plate extension, causing arc splitting, formation of microcontinent slivers, and backarc basins; then intermittent arrival of buoyant oceanic plateaus induces transient flat subduction (or slab flip) and crustal thickening. During slab retreat, basaltic magmas produced from decompressed asthenosphere advect into the extending orogen, causing granulite facies metamorphism and granite generation, but subsequent thickening during flat subduction cools the region. Thickening is focused in the thermally softened backarc region and, if sediment filled, a hot, short-lived (~10 m.y.), narrow (50– 100 km) orogenic belt forms. Such thickening is often misleadingly ascribed to arc or microcontinent collision. Once slab-retreat mode is reestablished, lithospheric extension recommences and a new arc-backarc system forms, generally outboard. Arrival of another plateau will reverse the procedure, and another short-lived, hot orogen will form within the orogenic system. Cycles of tectonic switching efficiently produce continental crust.

Depositional features and stratigraphic sections in granitic plutons: implications for the emplacement and crystallization of granitic magma

Abstract Many granitic plutons contain sheet-like masses of dioritic to gabbroic rocks or swarms of mafic to intermediate enclaves which represent the input of higher temperature, more mafic magma during crystallization of the granitic plutons. Small-scale structures associated with these bodies (e.g. load-cast and compaction features, silicic pipes extending from granitic layers into adjacent gabbroic sheets) indicate that the sheets and enclave swarms were deposited on a floor of the magma chamber (on granitic crystal mush and beneath crystal-poor magma) while the mafic magma was incompletely crystallized. These structures indicate ‘way up, typically toward the interior of the intrusions, and appear to indicate that packages of mafic sheets and enclave concentrations in these plutons are a record of sequential deposition. Hence, these plutons preserve a stratigraphic history of events involved in the construction (filling, replenishment) and crystallization of the magma chamber. The distinctive features of these depositional portions of plutons allow them to be distinguished from sheeted intrusions, which usually preserve mutual intrusive contacts and ‘dike–sill’ relations of different magma types. The considerable thickness of material that can be interpreted as depositional, and the evidence for replenishment, suggest that magma chamber volumes at any one time were probably much less than the final size of the pluton. Thus, magma chambers may be constructed much more slowly than presently envisaged. The present steep attitudes of these structures in many plutons may have developed gradually as the floor of the chamber (along with the underlying solidified granite and country rock) sank during continuing episodes of magma chamber replenishment. These internal magmatic structures support recent suggestions that the room problem for granites could be largely accommodated by downward movement of country rock beneath the magma chamber.

Nature of extensional accretionary orogens

Extensional accretionary orogens form by creation and destruction of large arc/back arc basin systems, generated by extension and sediment infilling during prolonged slab retreat, but episodically thickened by basin inversion during short-lived (∼10 Ma), orogenic contraction events. They are characterised by widespread, syntectonic, silicic, and minor basaltic magmatism, regional low-P, variable-T metamorphism, and by the enigmatic development of rift basins throughout the peak orogenic history. These orogens have features associated with retreating subduction boundaries and contrast markedly with those formed by terrane accretion, such as the Canadian Cordillera. The Paleozoic Lachlan orogen example from eastern Australia shows that Silurian-Devonian synorogenic basalts and gabbros were intimately associated with rifting and granite emplacement, but they formed during a period of repeated orogenic contraction. Moreover, primitive basaltic compositions have oceanic affinities, indicating generation under lithosphere that was <30 km thick. Only in the final stages of orogeny (Middle Devonian), after at least three major crustal contraction events, did the lithosphere thicken to ∼80 km or more, leading to stabilization of the orogen. Extensional accretionary orogens grow by magmatic and sedimentary additions during extension, caused mainly by asthenospheric melting and rift basin formation/sedimentation, augmented by localized and repeated crustal thickening events. Orogenic contraction leaves an indelible structural imprint which may obliterate the prior-formed extensional structures. The orogen remains hot, despite repeated thickening events, because of ongoing extension, which promotes advective heat transfer into the crust by basalt injection and crustal melting. Rapid switching to contraction, possibly during intermittent arrival of buoyant oceanic plateaus, inverts the thermally softened basins and forms localized fold-thrust belts in which the penetrative foliations record the peak metamorphism. Their most diagnostic features are the presence of basaltic rocks and rift basins throughout the orogenic contraction history.

Evaluation of petrogenetic models for Lachlan Fold Belt granitoids: Implications for crustal architecture and tectonic models

The restite (one source‐component) model suggests that granitoids are derived from contrasting source rocks and that the typical linear chemical variation of Lachlan Fold Belt granitoids is produced by restite separation. However, it cannot explain the general chemical and isotopic similarity of S‐ and I‐type granitoids in the eastern Lachlan Fold Belt, the similarity of zircon inheritance patterns between the two granite types, nor their apparently simple eNd‐Sr isotopic array. A two source‐component mixing model, based on the eNd‐Sr isotopic array, suggests that linear chemical variation of the granitoids reflects variable incorporation of deeply buried Ordovician sedimentary rocks and basaltic magmas. However, isotopically defined mixes do not match the predicted chemical mixes. Also systematic and sympathetic isotopic and chemical variations are observed for both the felsic and mafic granites within suites across the Bega Batholith. For a simple two‐component model to apply, both the crustal and mantl...

Geodynamic significance of S-type granites in circum-Pacific orogens

In Phanerozoic circum-Pacific orogenic belts, most post-collisional S-type granites and associated high-temperature, low-pressure metamorphic complexes formed during early arc extension. The granites are part of a tripartite association consisting of (1) inboard S-type granite, (2) outboard oceanic arc, and (3) intervening, turbidite-filled backarc basin. S-type granites herald the formation of new outboard oceanic arc and extensional backarc systems, but thickening of a preexisting, sediment-dominated back-arc basin is a prerequisite for their generation. In these environments, S-type plutonism is triggered by renewal of arc magmatism following thickening, when hot basaltic magmas are intruded into the thickened backarc crust once slab retreat is reestablished. With ongoing extension during retreat, the crust becomes progressively thinned, the sedimentary contribution is diminished, and the granites lose their S-type character. Such tripartite associations involving S-type granite are probably diagnostic of repeated slab-retreat episodes, and the Jurassic U.S. cordillera might be an example.

A three-component Sr-Nd isotopic mixing model for granitoid genesis, Lachlan fold belt, eastern Australia

Previous Sr-Nd isotopic models of two-component crust-mantle mixing (based on an apparently simple hyperbolic ϵNd-Sri array) have failed to account for all the geochemical and isotopic features of granitoids from the Lachlan fold belt, eastern Australia. Here we show that a three-component mixture of mantle-derived magma and two contrasting crustal components successfully explains the isotopic compositions of the granitoids. New isotopic data from the Moruya granitoids of the fold belt reveal that a separate isotopic trend exists at the most primitive end of the ϵNd-Sri array, defined by a depleted mantle component (ϵNd = +8.0; Sri = 0.7032) and a mafic crustal component (ϵNd = +2.7; Sri = 0.7049). This crustal component has isotopic characteristics similar to Cambrian greenstones within the fold belt, and appears to be widespread in the lower crust. After also considering data from the ubiquitous Ordovician turbidites which host the granitoids (ϵNd = −9.61 to −10.64; Sri = 0.7204 to 0.7273), three-component isotopic mixing curves are constructed that enclose all the granitoids of the fold belt, indicating that contributions from the Ordovician turbidites, Cambrian greenstones, and a depleted mantle component could produce the isotopic characteristics of the granitoids. Important implications of the model are that (1) S- and I-type granites appear to be mixtures of, rather than unique products from, contrasting sources and so Lachlan granitoids do not directly “image” their source-rock compositions; (2) the chemical variation lines defining granitoid suites are neither mixing lines nor lines reflecting restite separation; (3) previously inferred Precambrian continental basement beneath the fold belt is unnecessary; and (4) Lachlan granitoids may have formed in an environment related to subduction, where arc magmatism promoted crustal fusion and crust-mantle mixing.

Geochronological constraints on orogenic events in the Arunta Inlier: a review

Early workers subdivided the Arunta Inlier, central Australia, into three major tectonic provinces, classified the principal tectonostratigraphic units into three “Divisions”, and separated the tectonomagmatic history into five discrete “Events”, ranging in age from ∼ 1800 Ma to ∼ 300 Ma. n nA review of the evidence for tectonic boundaries within the Arunta Inlier suggests that no major structural discontinuity exists between the northern and central tectonic provinces, but that the Redbank Thrust Zone is, at least in part, a province boundary. The tectonostratigraphic “Division” concept also requires revision as Division 3 rocks of the northern and southern Arunta cannot be time equivalents, Division 1 rocks grade into Division 2 in the northern region, and Division 3 of the northern region may be equivalent to Division 1 in the central region. Rocks originally assigned to divisions are better grouped into a larger number of lithological assemblages until better stratigraphic and isotopic correlations can be made. n nCompilation of Rbue5f8Sr and 40Ar39Ar age determinations obtained before 1984 indicate an almost complete age spectrum from 1800 Ma to 1000 Ma, but age clusters exist at ∼ 1400 Ma, ∼ 1100 Ma and 400-300 Ma, which have geological significance. Subdivision of older ages into discrete tectonic events was not justified from this data alone, but more recent Uue5f8Pb isotopic analysis of zircons, in rocks designated to particular structural/metamorphic events, has revealed a complex tectonic history in the 1880-1600 Ma interval. The northern and central provinces underwent major tectonism between 1780-1730 Ma, but much of the southern province was not deformed until ∼ 1600 Ma. n nThe revised tectonic history nomenclature is proposed for the Arunta Inlier: following deposition of a widespread turbiditic unit at or before 1880 Ma, possibly on thinned Archaean crust, the northern and central provinces underwent localized tectonism/magmatism at ∼ 1880 Ma (Yuendumu tectonic event) and 1820 Ma (Stafford tectonic event), prior to the widespread Strangways orogeny, which is subdivided into an early phase at 1780-1770 Ma and late phase at 1745-1730 Ma. It is not clear whether the Strangways orogeny is a single, Arunta-wide event or a series of separate short-lived, smaller-scale events, although the major deformation in all areas appears to have involved W- to SW-directed thrusting. In the southern province, the major events occurred at ∼ 1680 Ma (Argilke tectonic event) and ∼ 1600 Ma (Chewings orogeny), the latter involving N-directed thrusting that was penecontemporaneous with post-tectonic pegmatite intrusion in the northern province, suggesting linkage of both regions at this stage. n nThe younger history of the Inlier is characterised by several stages of uplift and isolated thermal (magmatic) events. Major Mesoproterozoic crustal exhumation (Anmatjira uplift phase) has been interpreted from widespread 1500-1400 Ma ages, including: regional-scale closure of 40Arue5f839Ar systems in hornblende from mafic granulite; outcrop-scale Rbue5f8Sr total rock isotopic systems in mylonitized granitoids; and Smue5f8Nd garnet-hornblende pairs in mylonitized amphibolites. A ∼ 1150 Ma thermal event in the southern part of the Inlier (Teapot magmatic event) is recorded by intrusion of granite, pegmatites, and the alkaline Mordor Igneous Complex, and by isotopic resetting of Rbue5f8Sr mineral systems. Dolerite dyke injection at ∼ 1050 Ma reflects limited extensional tectonism that may have related to formation of the Amadeus Basin. The second major uplift event was the Alice Springs orogeny at 400-300 Ma, an unusual intracratonic, thick- and thin-skinned thrust event that involved limited retrograde amphibolite facies, but extensive greenschist facies metamorphism, and restricted pegmatite intrusion.

Partial convective overturn of Archaean crust in the east Pilbara Craton, Western Australia: driving mechanisms and tectonic implications

Strain varies systematically from weakly-developed, outward-dipping, S-tectonites in the ∼3320–3310±10 Ma Mount Edgar Batholith to intensely deformed, subvertical, L-tectonites in greenstones of the Warrawoona syncline. A consistent ‘greenstone-down/batholith-up’ sense of shear is recorded in batholithic domal margins and adjacent high-grade supracrustal rims: lineations converge to a central zone of subvertical extension (zone of sinking) along the synclinal axis. At domal margins, early kinematic granitoid sheets and ‘intrusive diatexites’ are subconcordant to a well-developed, dome-parallel schistosity, but late- to post-kinematic intrusives are discordant, high-level plutons. All granitoids are the same age, within analytical error. n nThese syn-doming features conform with structural tests for diapirs, and differ from those expected during metamorphic core complex formation and cross-folding. Diapirism is part of a larger process involving partial convective overturn of the crust. Based on strain patterns and kinematic criteria, we argue that deformation was initiated by sinking of greenstones, which rapidly subsided; the domes then rose passively as accommodation structures. Ongoing doming (D4) partitioned strain to the southern rim of the batholith and uplifted a wedge of the Warrawoona Syncline keel. We consider that crustal overturn occurred in response to mantle plume activity: Eruption of a 5–10 km thick, ∼3325 Ma, mafic–ultramafic greenstone pile onto an older granite–greenstone terrain created a negatively buoyant crust, but convective overturn occurred some 5–20 Ma later, triggered by widespread anatexis (thermal softening). Convective overturn may have been a common Archaean process, occurring in response to a hotter mantle, and represents an end-member deformation mechanism that includes thrust-accretion of lithotectonic assemblages in other Archaean terranes, such as the Superior Province of Canada.

Slab pull, mantle convection, and Pangaean assembly and dispersal

Two global-scale mantle convection cells presently exist on Earth, centred on upwelling zones in the South Pacific Ocean and northeast Africa: one cell (Panthalassan) contains only oceanic plates, the other (Pangaean) contains all the continental plates. They have remained fixed relative to one another for >400 Ma. A transverse (Rheic–Tethyian) subduction system splits the Pangaean cell. Poloidal plate motion in the oceanic cell reflects circumferential pull of Panthalassan slabs, but toroidal flow in the Pangaean cell, reflected by vortex-type motion of continents toward the Altaids of central-east Asia throughout the Phanerozoic, has resulted from the competing slab-pull forces of both cells. The combined slab-pull effects from both cells also controlled Pangaean assembly and dispersal. Assembly occurred during Palaeozoic clockwise toroidal motion in the Pangaean cell, when Gondwana was pulled into Pangaea by the NE-trending Rheic subduction zone, forming the Appalachian–Variscide–Altaid chain. Pangaean dispersal occurred when the Rheic trench re-aligned in the Jurassic to form the NW-trending Tethyside subduction system, which pulled east Gondwanan fragments in the opposite direction to form the Cimmerian–Himalayan–Alpine chain. This re-alignment also generated a new set of (Indian) mid-ocean ridge systems which dissected east Gondwana and facilitated breakup. 100–200-Myr-long Phanerozoic Wilson cycles reflect rifting and northerly migration of Gondwanan fragments across the Pangaean cell into the Rheic–Tethyian trench. Pangaean dispersal was amplified by retreat of the Panthalassan slab away from Europe and Africa, which generated mantle counterflow currents capable of pulling the Americas westward to create the Atlantic Ocean. Thermal blanketing beneath Pangaea and related hotspot activity were part of a complex feedback mechanism that established the breakup pattern, but slab retreat is considered to have been the main driving force. The size and longevity of the two cells, organised and maintained by long-lived slab-pull forces, favours deep mantle convection as the dominant circulation process during the Phanerozoic.