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SUBDUCTION ZONES 281

2000m bathymetric contour
Trench
Active spreading center
10 S
Samoa
Coral
Sea Vanuatu
Basin
New Fiji
20 S Caledonia

South Lau Ridge Lau Basin
New Fiji Kermadec Ridge
Caledonia Basin
Australia Basin Kermadec-Tonga Trench
Three
30 S Kings
Lord Rise
Howe Havre
Havre
Rise Trough
Trough
Tasman
Sea
New
40 S Zealand Taupo
Volcanic Zone
Chatham
Rise
0 1000 2000

50 S km Campbell
Plateau
140 E 150 E 160 E 170 E 180 E
Figure 9.31 Map showing the location of backarc basins in a part of the southwest Pacific, including the Lau basin,

the South Fiji basin, the New Caledonian basin, and the Taupo volcanic zone (modified from Collins, 2002b, by
permission of the American Geophysical Union. Copyright © 2002 American Geophysical Union). Box shows location of
Fig. 9.32.

magma addition and the age of the crust. In the Mariana Structures corresponding to a mid-ocean ridge are
Trough (Fig. 9.1), for example, crustal thicknesses range not always present in backarc basins, and magnetic
from 3.4 to 6.9 km, with the thinnest values correspond- lineations may be poorly developed (Weissel, 1981).
ing to either slow spreading centers or magma-starved Those lineations that are present can be correlated with
regions, and the thickest crust corresponding to regions the magnetic polarity timescale (Section 4.1.6), although
of high magmatic activity (Kitada et al., 2006). they tend to be shorter, of lower amplitude and less
These observations and the evidence for extension clearly defined than oceanic anomalies. In the southern

and sea floor spreading imply that backarc crust in Lau Basin, for example, individual magnetic anomaly

oceanic settings is generated in a manner similar to that lineations cannot be traced for more than 30 km along
occurring at mid-ocean ridges (Section 6.10). However, the strike of the basin (Fujiwara et al., 2001). This short

there are many important differences in the processes length probably reflects the small size of basement
that form basaltic crust in these two environments. faults, which typically display segmented, en echelon