Page 295 - Global Tectonics

280 CHAPTER 9

NW Canada

Kamchatka Washington

California
Japan
Taiwan

Celebes

Chile

New Zealand

Figure 9.30 Paired metamorphic belts in the circum-Pacific region. Dotted lines, high pressure belts; solid lines, low
pressure belts (redrawn from Miyashiro, 1973, with permission from Elsevier)

Mesozoic times (Section 10.2.1). An example of an sion in the Taupo volcanic zone of New Zealand (Fig.
active continental backarc basin is the extensional Taupo 9.31). Further support for extension comes from the
volcanic zone on the North Island of New Zealand subsidence of remnant arcs as their dynamic support is
(Stern, 1987; Audoine et al., 2004). removed after the development of backarc basins,

Karig (1970) was one of the first to suggest that earthquake focal mechanism solutions, and the seg-
backarc basins form by the rifting of an existing island mented geometry of normal faults and spreading ridges,
arc along its length, with the two halves corresponding which also characterize continental rifts (Section 7.2)
to the volcanic and remnant arcs. This interpretation is and mid-ocean ridges. The Woodlark Rift (Fig. 7.39b),
based on observations in the Lau basin (Fig. 9.31), which records the transition from rifting to sea fl oor
which lies west of the Tonga–Kermadec arc and is spreading above a Neogene subduction zone (Section

flanked on its western side by the Lau ridge. Karig 7.8.2), illustrates this segmentation especially well.
(1970) concluded that extension was important during In general, the composition of the crust in oceanic
basin formation on the basis of the following observa- backarc basins is broadly similar to that of other ocean
tions: (i) the asymmetric cross-section of both the arc basins, although in some cases layer 1 is unusually thick.
and ridge, which are mirrored across the center of the Net accretion rates are similar to those deduced for
basin; (ii) the basin’s topographic features, which are mid-ocean ridges, and range from approximately
−1
aligned parallel to both the arc and the ridge; (iii) the 160 mm a in the northern Lau basin (Bevis et al., 1995)
−1
considerable sediment thickness present to the seaward to 70 mm a in the East Scotia Sea (Thomas et al., 2003)
−1
side of the arc and landward side of the ridge and the and 20–35 mm a in the Mariana Trough (Martínez
absence of sediment within the basin; and (iv) the con- et al., 2000). The crust in these settings commonly
tinuation of the arc–basin–ridge system to the south shows substantial thinning by normal faulting, although
where it correlates with a zone of active backarc exten- the total crustal thickness also depends upon the rate of