Mechanisms of terrane accretion

Observations from the North American Cordillera, the Appalachians, and many other ancient orogens suggest that the accretion and dispersal of terranes involves processes that are similar to those that occur in modern orogens. The regimes of active arc-continent collision in the southwest Pacific (Section 10.5), for example, offer excellent analogues for how a variety of tectonic and sedimentary terranes originate and are emplaced onto continental margins. In general, as the subduction of oceanic lithosphere brings thick sequences of continental, oceanic, and island arc material into contact with the trench, their positive buoyancy chokes the subduction zone. Once the collision begins, the forearc and accretionary wedge are uplifted and are carried, or obducted, onto the continental margin by thrust faults. As subduction slows or stops, a new trench may develop on the oceanward side of the old one (Section 10.5) and the process of accretion may begin again.

Many exotic terranes appear to originate during rifting events associated with the formation and breakup of the supercontinents (e.g. Fig. 11.24). Others may owe their origin to the abundant oceanic ridges, rises, and plateaux that make up about 10% of the area of the present ocean basins (Ben-Avraham et al., 1981). Most of these topographic highs represent extinct island arcs, submerged microcontinents, and LIPs (Section 7.4.1). As these features are brought to a trench, their positive buoyancy also may inhibit their subduction and allow them to be accreted as exotic terranes.

In addition to the processes described elsewhere in this chapter, two additional mechanisms of terrane accretion and continental growth deserve further mention: the obduction of ophiolites (Section 2.5); and continental growth by magmatism, sedimentation, and the formation and destruction of backarc, intraarc, and forearc basins.

The presence of ophiolitic assemblages in orogens provides an important marker of accretionary tectonic processes (Sections 2.5, 10.4.3, 11.4.3). As indicated in Section 2.5, models of ophiolite obduction tend to be quite variable, in part due to the diversity of the envi ronments in which these assemblages can form and the way in which they are uplifted and emplaced in the upper crust. In one postulated model, Wakabayashi & Dilek (2000) described how ophiolitic material in a backarc environment might become entrapped in a forearc setting prior to its obduction. This model is interesting because it explains how changes in the location or polarity of subduction can result in the capture of material that originally formed in an environment different than the one in which it is emplaced. This mechanism also may occur at larger scales, where it can result in the formation of a marginal sea by the entrapment of oceanic crust. Several of the present marginal seas for which there is no convincing evidence for backarc spreading (Section 9.10), such as the eastern Caribbean and Bering Sea, may have formed in this way (Ben-Avraham et al., 1981; Cooper et al., 1992).

In the Wakabayashi & Dilek (2000) model, the Coast Range ophiolite of western North America forms behind a Mesozoic island arc located offshore and above a subduction zone that dips to the west (Fig. 10.35a). Later, the island arc collides with the continent and a new east-dipping subduction zone initiates, capturing the ophiolite in the developing forearc (Fig. 10.35b,c). Ophiolite obduction subsequently occurs in a forearc setting when layers of the crust become detached and uplifted as a result of compression (Fig. 10.35d). The compression may result from any number of mechanisms, including the arrival of buoyant material at the trench.

In addition to the collision and accretion of exotic terranes, significant continental growth may occur by magma addition and sedimentation. An example of an accretionary orogen that has grown by more than 700 km mainly by these latter mechanisms is the Middle Paleozoic Lachlan orogen of southeastern Australia (Foster & Gray, 2000; Collins, 2002a; Glen, 2005). This orogen lacks many of the features that characterize major collisional orogens, such as exotic terranes, the development of high topography, deep-seated thrust faults, and exposures of high pressure metamorphic rocks. Instead, it is dominated by a huge volume of granitoid rock (Fig. 10.36), volcanic sequences, and extensive low-grade quartz-rich turbidites that overlie thinned continental crust and mafic lower crust of oceanic affinity (Fergusson & Coney, 1992). Like the Mesozoic-Cenozoic Andes, it records a history of ocean-continent convergence that lasted some 200 million years and involved many cycles of extension and contraction (Foster et al., 1999). Large (up to 1000 km-

Coast Range ophiolite 0 -, forms by backarc spreading

(b) ca. 165—160 Ma East-dipping subduction begins: Formation of metamorphic sole and eruption of near-trench

Collided arc volcanic rocks

Island arc

120 km

Continental arc (deeper levels (d) ca 70 Ma

(b) ca. 165—160 Ma East-dipping subduction begins: Formation of metamorphic sole and eruption of near-trench

Collided arc volcanic rocks

80 km

120 km

80 km

Continental arc (deeper levels (d) ca 70 Ma preserved as Sierra Nevada batholith)

Coast Range ophiolite t

Forearc basin: Great Valley Group preserved as Sierra Nevada batholith)

Coast Range ophiolite t

Forearc basin: Great Valley Group

120 km

Figure 10.35 Possible evolution of the Coast Range ophiolite in a backarc setting offshore of California and its subsequent emplacement in a forearc setting (after Wakabayashi & Dilek, 2000, with permission from the Geological Society of America). (a) Coast Range ophiolite forms behind a Mesozoic island arc. (b,c) Island arc collides with the continent and a new east-dipping subduction zone initiates, capturing the ophiolite in forearc. (d) Ophiolite obduction occurs in a forearc setting.

Ophiolite beneath forearc basin deposits Coast Range ophiolite \ Great Valley Group c

Ophiolite beneath forearc basin deposits Coast Range ophiolite \ Great Valley Group c

120 km

120 km

120 km

Figure 10.35 Possible evolution of the Coast Range ophiolite in a backarc setting offshore of California and its subsequent emplacement in a forearc setting (after Wakabayashi & Dilek, 2000, with permission from the Geological Society of America). (a) Coast Range ophiolite forms behind a Mesozoic island arc. (b,c) Island arc collides with the continent and a new east-dipping subduction zone initiates, capturing the ophiolite in forearc. (d) Ophiolite obduction occurs in a forearc setting.

Intrusion ages of granitic plutons

360 ■

320 Ma

370 ■

365 Ma

380

370 Ma

410 ■

398 Ma

430 ■

■ 415 Ma

510 ■

490 Ma

150 km

Figure 10.36 Geologic map of the Western (WL), Central (CL), and Eastern (EL) provinces of the Lachlan orogen showing distribution of granitoids and their intrusion ages (redrawn with permission from Foster & Gray, 2000, Annual Review of Earth and Planetary Sciences 28. Copyright © 2000 Annual Reviews).

150 km

Island arc

Figure 10.36 Geologic map of the Western (WL), Central (CL), and Eastern (EL) provinces of the Lachlan orogen showing distribution of granitoids and their intrusion ages (redrawn with permission from Foster & Gray, 2000, Annual Review of Earth and Planetary Sciences 28. Copyright © 2000 Annual Reviews).

wide) extensional basins floored by basalt and gabbro were created behind one or more island arcs that eventually accreted onto the continental margin (Glen, 2005). Between the volcanic rocks are the accreted parts of a huge submarine sediment dispersal system that developed along the Gondwana margin during the early Paleozoic. Diachronous pulses of contractional and strike-slip deformation followed each extensional cycle, generating upright folds and overprinting cleavages in a series of thrust wedges in the upper 15 km of the crust. This style of shortening did not lead to the development of a well-defined foreland basin nor a foreland fold and thrust belt of the type seen in the central Andes (Fig. 10.5) and the Himalaya (Figs 10.19, 10.20). Instead, it was controlled by the thick (10 km) succession of turbi-dites and locally high geothermal gradients. These rela tionships suggest that orogenesis and crustal growth in the Lachlan orogen were dominated by magmatism and the recycling of continental detritus during cycles of extension and contraction that lasted from Late Ordovi-cian through early Carboniferous times.

Cycles of backarc and intra-arc extension, such as those that occurred in the Lachlan orogen, generate thin, hot lithosphere that may localize deformation during subsequent phases of contraction, collision, and orogeny (Hyndman et al., 2005). Collins (2002b) illustrated this process in a model of orogenesis involving the formation and closure of autochthonous backarc basins (Section 9.10) above a long-lived subduction zone. The model begins with a zone of intra-arc extension that evolves in response to the roll back (Section 9.10) of a subducting slab (Fig. 10.37a). This setting

Arc Accretion Model

Basalt intra/ underplating (granulite facies metamorphism)

Mantle counter flow

Old arc or microcontinent

Granulite terrain Lithospheric fragment

Basalt intra/ underplating (granulite facies metamorphism)

Decompression melting

Mantle counter flow

Slab flux melting

□ New accretionary prism

□ Old accretionary prism

Old arc or microcontinent

Backarc basin

Granulite terrain Lithospheric fragment

Figure 10.37 Model showing the evolution of the Lachlan Orogen of southeast Australia through accretionary tectonics involving the creation and destruction of backarc basins above an ocean-continent subduction zone (after Collins, 2002a, with permission from the Geological Society of America). (a) Intra-arc extension due to the roll-back of a subducting slab. (b) Backarc basin and remnant arc form. (c) Subduction zone flattens and the upper plate of the orogen is thrown into compression. Contraction and crustal thickening are focused in the thermally softened backarc. (d) Extension is re-established and a new arc-backarc system forms.

is analogous to the present day Taupo volcanic zone of the North Island, New Zealand. As the arc splits apart and migrates away from the trench, a backarc basin and remnant arc form (Fig. 10.37b), leading to subsidence and crustal thinning. Decompression melting (Section 9.8) in the upper mantle wedge beneath the backarc region generates basaltic crust as mafic magma underplates and intrudes the thinned crust. Next, the subduction zone flattens and the upper plate of the orogen is thrown into compression, possibly as a result of the arrival of an oceanic plateau or island arc at the subduction zone (Fig. 10.37c). This stage also may be analogous to the regime of flat slab subduction and contraction that characterizes part of the Andes (Section 10.2.2). Contractional deformation and crustal thickening are focused in the thermally softened backarc region. The contraction closes the backarc basins and may lead to the accretion of the arc and forearc onto the continental margin. If a thick sequence of sediment has infilled the basin, a hot short-lived (~10 Ma) narrow orogen forms. Once the oceanic plateau has subducted, extension is re-established and a new arc-backarc system forms along the margin (Fig. 10.37d).

Models of accretionary orogens such as this, while speculative, illustrate how some continental margins may grow in the absence of major collisional events. Another example of a margin that appears to have grown by accretionary mechanisms is preserved in the Mesozoic history of Baja, California (Busby, 2004). Here, as in the Lachlan orogen, extension above a subduction zone created buoyant arc, forearc, and ophiolite terranes that accreted onto the upper plate during convergence, resulting in significant continental growth.

Precambrian tectonics and the supercontinent cycle

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