Page 362 - Global Tectonics

OROGENIC BELTS 345

is analogous to the present day Taupo volcanic zone backarc basins and may lead to the accretion of the
of the North Island, New Zealand. As the arc splits arc and forearc onto the continental margin. If a thick

apart and migrates away from the trench, a backarc sequence of sediment has infilled the basin, a hot
basin and remnant arc form (Fig. 10.37b), leading short-lived (∼10 Ma) narrow orogen forms. Once the
to subsidence and crustal thinning. Decompression oceanic plateau has subducted, extension is re-estab-
melting (Section 9.8) in the upper mantle wedge lished and a new arc–backarc system forms along the
beneath the backarc region generates basaltic crust as margin (Fig. 10.37d).

mafic magma underplates and intrudes the thinned Models of accretionary orogens such as this, while
crust. Next, the subduction zone flattens and the upper speculative, illustrate how some continental margins

plate of the orogen is thrown into compression, pos- may grow in the absence of major collisional events.
sibly as a result of the arrival of an oceanic plateau Another example of a margin that appears to have
or island arc at the subduction zone (Fig. 10.37c). This grown by accretionary mechanisms is preserved in the
stage also may be analogous to the regime of fl at slab Mesozoic history of Baja, California (Busby, 2004).
subduction and contraction that characterizes part of Here, as in the Lachlan orogen, extension above a sub-
the Andes (Section 10.2.2). Contractional deformation duction zone created buoyant arc, forearc, and ophiolite
and crustal thickening are focused in the thermally terranes that accreted onto the upper plate during con-
softened backarc region. The contraction closes the vergence, resulting in significant continental growth.