Page 359 - Global Tectonics

342 CHAPTER 10

deep structure exhibits the effects of processes related ronments in which these assemblages can form and the
to the release, trapping, and consumption of fl uids way in which they are uplifted and emplaced in the
above a subduction zone (Figs 10.6a, 10.20b). upper crust. In one postulated model, Wakabayashi &
Dilek (2000) described how ophiolitic material in a
backarc environment might become entrapped in a
10.6.3 Mechanisms of forearc setting prior to its obduction. This model is
interesting because it explains how changes in the loca-
terrane accretion tion or polarity of subduction can result in the capture
of material that originally formed in an environment
Observations from the North American Cordillera, the different than the one in which it is emplaced. This
Appalachians, and many other ancient orogens suggest mechanism also may occur at larger scales, where it can
that the accretion and dispersal of terranes involves result in the formation of a marginal sea by the entrap-
processes that are similar to those that occur in modern ment of oceanic crust. Several of the present marginal
orogens. The regimes of active arc–continent collision seas for which there is no convincing evidence for

in the southwest Pacific (Section 10.5), for example, backarc spreading (Section 9.10), such as the eastern
offer excellent analogues for how a variety of tectonic Caribbean and Bering Sea, may have formed in this way
and sedimentary terranes originate and are emplaced (Ben-Avraham et al., 1981; Cooper et al., 1992).
onto continental margins. In general, as the subduction In the Wakabayashi & Dilek (2000) model, the Coast
of oceanic lithosphere brings thick sequences of conti- Range ophiolite of western North America forms
nental, oceanic, and island arc material into contact behind a Mesozoic island arc located offshore and above
with the trench, their positive buoyancy chokes the sub- a subduction zone that dips to the west (Fig. 10.35a).
duction zone. Once the collision begins, the forearc and Later, the island arc collides with the continent and a
accretionary wedge are uplifted and are carried, or new east-dipping subduction zone initiates, capturing
obducted, onto the continental margin by thrust faults. the ophiolite in the developing forearc (Fig. 10.35b,c).
As subduction slows or stops, a new trench may develop Ophiolite obduction subsequently occurs in a forearc
on the oceanward side of the old one (Section 10.5) and setting when layers of the crust become detached and
the process of accretion may begin again. uplifted as a result of compression (Fig. 10.35d). The
Many exotic terranes appear to originate during compression may result from any number of mecha-
rifting events associated with the formation and break- nisms, including the arrival of buoyant material at the
up of the supercontinents (e.g. Fig. 11.24). Others may trench.
owe their origin to the abundant oceanic ridges, rises, In addition to the collision and accretion of exotic

and plateaux that make up about 10% of the area of the terranes, significant continental growth may occur by
present ocean basins (Ben-Avraham et al., 1981). Most magma addition and sedimentation. An example of an
of these topographic highs represent extinct island arcs, accretionary orogen that has grown by more than
submerged microcontinents, and LIPs (Section 7.4.1). 700 km mainly by these latter mechanisms is the Middle
As these features are brought to a trench, their positive Paleozoic Lachlan orogen of southeastern Australia
buoyancy also may inhibit their subduction and allow (Foster & Gray, 2000; Collins, 2002a; Glen, 2005). This
them to be accreted as exotic terranes. orogen lacks many of the features that characterize
In addition to the processes described elsewhere in major collisional orogens, such as exotic terranes, the
this chapter, two additional mechanisms of terrane development of high topography, deep-seated thrust
accretion and continental growth deserve further faults, and exposures of high pressure metamorphic
mention: the obduction of ophiolites (Section 2.5); and rocks. Instead, it is dominated by a huge volume of
continental growth by magmatism, sedimentation, and granitoid rock (Fig. 10.36), volcanic sequences, and
the formation and destruction of backarc, intraarc, and extensive low-grade quartz-rich turbidites that overlie

forearc basins. thinned continental crust and mafic lower crust of

The presence of ophiolitic assemblages in orogens oceanic affinity (Fergusson & Coney, 1992). Like the
provides an important marker of accretionary tectonic Mesozoic-Cenozoic Andes, it records a history of
processes (Sections 2.5, 10.4.3, 11.4.3). As indicated in ocean–continent convergence that lasted some 200
Section 2.5, models of ophiolite obduction tend to be million years and involved many cycles of extension and
quite variable, in part due to the diversity of the envi- contraction (Foster et al., 1999). Large (up to 1000 km-