Terranes - accretion

Australia: The Land Where Time Began

Terranes

Terranes are fragments of continental crust that have been separated from continental landmasses as rifting is occurring, the result of the complex nature of rifting, that is usually more complex and messy than simply continents breaking cleanly away from each other. These fragments then move independently of their parent continent, but as with the parent continent, their movements are determined by the tectonic plate they are situated on. They often collide with other terranes or continents, where they fuse onto the margin of the continent. This is the mechanism leading to blocks of crust that have different geological histories being joined together as part of a continent.

New Zealand is an example of a landmass comprised of a number of different terranes. The actual makeup and origins of the terranes involved is still to be fully agreed upon. Terranes were also involved in the formation of Britain. England and Wales were attached to Morocco, as part of Gondwana, until the Early Ordovician, about 475 Ma. At that time Scotland was part of North America. England and Wales, as a sliver of continental rock called Avalonia, was rifted from Africa to fuse with Baltica (Scandinavia), on the opposite side of the Iapetus Sea from North America.

There were 4 landmasses in the Northern Hemisphere by the Late Silurian, North America (plus Scotland); Baltica (plus England & Wales), as well as 2 other fragments. At the end of the Palaeozoic, all these continental masses collected together to form Pangaea. England & Wales were brought together with Scotland by the closing of the Iapetus Ocean. In the Mesozoic, Pangaea broke up and the North Atlantic Ocean opened, Great Britain having assumed its present form, became attached to the western margin of the Euorope-Asia landmass.

Terrane accretion

Observations of many ancient orogens have suggested the processes that formed the terranes are similar to those that can be seen in action at the orogens of the present. In the southwest Pacific, regimes of active arc-continent collision are examples, analogous to the origin of a variety of tectonic and sedimentary terranes, as well as how they are emplaced on the margins of continents. The addition at subduction zones of large quantities of thick sequences of continental and oceanic, as well as island arc material, that are carried along by the subducting oceanic lithosphere, eventually choke the subduction zone as a result of their positive buoyancy. As the collision begins, thrust faults uplift the forearc and accretionary wedge, carrying, or obducting them, onto the margin of the continent. A new trench may form on the oceanward side of the choked trench if subduction is slowed or stopped, the accretionary process beginning again.

Many exotic terranes appear to have formed in processes associated with the formation and breakup of supercontinents. The many ridges, rises and plateaux that comprise about 10 % of the ocean basins at the present are thought could possibly be the origin of some terranes (Ben-Avraham et al., 1981). Most of these topographic highs are extinct island arcs, microcontinents that were submerged, and LIPs. It is believed the positive buoyancy of these structures may inhibit subduction trenches, resulting in their addition to the continental margin by accretion as exotic terranes. There are 2 other mechanisms of terrane accretion to continental margins, and continental growth - the obduction of ophiolites, and growth by magmatism, sedimentation, as well as backarc, intraarc and frorearc basins being formed and destroyed.

Accretionary tectonic processes are indicated in orogens by the presence of ophiolitic assemblages. Models of the obduction of ophiolites tend to be variable, partly as a result of the diversity of the environments these assemblages form in and their uplift and emplacement in the upper crust. One of the proposed models suggests ophiolitic material in a backarc environment, entrapped in a forearc setting before its obduction.

According to one model that has been proposed involving how ophiolitic material could be trapped in a backarc, in a forearc setting, before its subduction (Wakabayashi & Dilek, 2000). This model explains how the capture of material forming in an environment, different from the one it is emplaced in, can lead to changes in the location or polarity of subduction. At large scales it can result in the formation of a marginal sea by entrapping oceanic crust. The eastern Caribbean and Bering Sea, among a number of marginal seas of the present for which there is no convincing evidence for backarc spreading, are suggested to have possibly formed in this manner (Ben-Avraham et al., 1981; Cooper et al., 1992).

According of the model of Wakabayashi & Dilek (2000), in western North America, Coast Range ophiolite forms behind an island arc of Mesozoic age, that is located offshore above a subduction zone dipping westward. The island arc collides with a continent, later initiating a new east-dipping subduction zone, the ophiolite being captured in the developing forearc. When layers of the crust are detached and uplifted by compression, ophiolite obduction subsequently occurs. A number of mechanisms may be responsible for the compression, such as the arrival at the trench of buoyant material.

Significant continental growth may occur by the addition of magma and sedimentation, as well as by collision and accretion of exotic terranes. In southeastern Australia, the Lachlan Orogen, of Palaeozoic age, an example of an accretionary orogen, has grown by more than 700 km, mostly by the latter mechanisms (Foster & Gray, 2000; Collins, 2002a; Glen, 2005). Many features characteristic of major collisional orogens are lacking in this orogen, such as exotic terranes, high topography, deep-seated thrust faults and high pressure rock exposures. A very large volume of granitoid rock and volcanic sequences, and extensive quartz-rich turbidite, of low grade, overlying thinned continental crust and mafic lower crust of oceanic affinity (Fergusson & Coney, 1992). It records a history of ocean-continent convergence lasting about 200 million years, involving many extension and contraction cycles (Foster et al., 1999). This is similar to the Andes from the Mesozoic-Cainozoic, that also records a history of ocean-continent convergence.

Large extensional basins, up to 1000 km wide, with a floor of basalt and gabbro, formed behind 1 or more island arcs, that eventually accreted onto the margins of the continent (Glen, 2005). The accretionary part of a vast submarine dispersal system that developed along the margin of Gondwana in the Palaeozoic. Following each cycle of extension were diachronous pulses of contractional and strike-slip deformation, creating upright folds and overprinting cleavages in a series of thrust wedges in the upper 15 km of the crust. A well-defined foreland basin did not result from this type of shortening, a foreland fold and thrust belt of this type, as seen in the central Andes and the Himalayas, was also no produced. The thick succession of turbidites, 10 km thick, and locally high geothermal gradients controlled it. Orogenesis and crustal growth in the Lachlan Orogen is suggested by these relationships to be dominated by magmatism and the recycling of continental detritus during cycles of extension and contraction lasting from the Late Ordovician to the Early Carboniferous.

Backarc and intraarc extension cycles, as are seen in the Lachlan Orogen, produce thin, hot lithosphere. During subsequent contraction phases, collision and orogeny this lithosphere may localise deformation (Hyndman et al., 2005). This process was illustrated in a model of orogenesis that involved the formation and closing of autochthonous backarc basins above a subduction zone that was long-lived (collins, 2002b). According to this model, a zone of intra-arc extension evolves as a result of subducting slab roll back. The Taupo volcanic zone of the North Island of New Zealand is seen as an analogous setting. A backarc basin and remnant arc are formed as the arc splits apart and migrates away from the trench. This leads to crustal thinning and subsidence. In the upper mantle wedge below the backarc region decompression melting generates basaltic crust as mafic magma underplates and intrudes the thinned crust. The subduction zone then flattens and the upper plate of the orogen is thrown into compression, that is thought to possibly be by the arrival of an oceanic plateau or island arc at the subduction zone. It has been suggested that this stage may be an analogous regime to part of the Andes, where flat slab subduction and contraction are characteristic of the regime. The thermally-softened backarc region is the focus of contractional deformation and crustal thickening.

The backarc basins are closed by the contraction, possibly leading to the accretion of the arc and forearc to the margin of the continent. A hot, narrow orogen forms, that is short lived, about 10 million years, if a thick sediment sequence has in-filled the basin. Extension is re-established and a new arc-backarc system forms along the margin once the plateau has been subducted (Kearey et al., 2009).

Speculative models of accretionary orogens, such as this, illustrate possible mechanisms involved in the growth of the margins of continents when major collisional events are not involved. Baja California is another example of a margin that is believed to have grown by accretion during the Mesozoic (Busley, 2004). Extension above a subduction zone, in both examples, formed arc, forearc and ophiolite terranes that are buoyant, accreting them onto the upper plate during convergence, producing significant continental growth.

Sources & Further reading

Mary E. White, 1993, The Nature of Hidden Worlds: Animals and plants in prehistoric Australia and New Zealand, Reed
Kearey, Philip, Klepeis, Keith A. & Vine, Frederick J., 2009, Global Tectonics, 3rd Edition, Wiley-Blackwell.

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