Page 300 - Global Tectonics

SUBDUCTION ZONES 285

transport of heat within the mantle wedge (England & occurred without the formation of a basaltic basin fl oor
Wilkins, 2004). Thus, many of the fundamental differ- (Mpodozis & Allmendinger, 1993). Only in Colombia
ences between crustal accretion in backarc and mid- and southernmost Chile did extension proceed to a
ocean ridge settings can be explained by the structure stage where complete rupture of continental crust

and dynamics of flow in the upper mantle wedge. (Section 7.5) occurred and oceanic-type spreading
However, it is important to realize that, in addition to centers developed. A possible modern analogue of

processes related to subduction, it is probable that some these oceanic basins is the Bransfield basin, located
backarc basins are infl uenced by the specifi c confi gura- behind the South Shetland trench near the Antarctic
tions of plate boundaries in their vicinity. An example Peninsula (Fig. 9.1). This latter basin is asymmetric and
of this may be the North Fiji basin where the backarc displays evidence for having opened by rift propagation
spreading direction is oriented at unusually low angles through pre-existing arc crust beginning 4–5 million
(10–30°) to the trend of the arc. Schellart et al. (2002) years ago (Barker et al., 2003). Mora-Klepeis &
suggested that this unusual spreading direction could be McDowell (2004) discuss the geochemical signatures of
related to an asymmetric opening of the basin and to rocks that record a similar transition from arc volcanism
collisional processes occurring along the plate to rifting in the Baja California region of northwestern
boundary. The evolution of the Woodlark Rift (Section México.
7.8.2) also is strongly influenced by local boundary Although common, not all continental backarcs are

conditions, including rheological weaknesses in the associated with extension or rifting. Many zones of
lithosphere. ocean–continent convergence, including the modern
Variability in the structure and magmatic character- Andean margin (Section 10.2), record shortening and
istics of backarc basins also is common in continental orogenesis in the backarc environment. Regardless of
settings. Along the western margin of South America, the style of deformation they record, most continental
for example, a series of extensional basins formed backarcs are characterized by relatively thin, hot litho-
during a period of Mesozoic extension above a sphere (Hyndman et al., 2005) (e.g. Fig. 10.7) whose
long-lived subduction zone (Fig. 9.34) (Section 10.2.1). properties greatly affect the mechanical evolution of the
In most of these basins, extension and backarc rifting convergent margin (e.g. Sections 10.2.5,10.4.6).