Page 294 - Global Tectonics

SUBDUCTION ZONES 279

Sanbagawa
Jurassic accretionary metamorphic belt
prism Sangun belt Jurassic accretionary
Hida belt (Paleozoic) prism and Paleozoic rocks
Pleistocene Ryoke Shimanto accretionary Nankai accretionary
volcano metamorphic belt MTL prism prism
0
Cretaceous granites
Miocene 10
Upper crust granites Subducting ocean crust 20 km
Mantle lithosphere
Lower crust
30
Moho
Mantle lithosphere 40
50 0
km
Figure 9.29 Geologic cross-section of southwest Japan (modified from Taira, 2001, Annual Review of Earth and

Planetary Sciences 29, Copyright © 2001 Annual Reviews). Location of profile shown in Fig. 9.28. MTL, Median Tectonic
Line.

predicted by the model, and so it has been suggested array of processes that may or may not have accompa-
that the boundary between them, called the Median nied subduction. Taira (2001) summarizes the impor-
Tectonic Line, experienced some 400 km of strike-slip tance of terrane collision for the evolution of Japan’s
movement (Section 5.3). This transcurrent movement metamorphic belts.
has been confirmed by detailed mapping (Takagi, 1986)

and indicates that strike-slip faulting was responsible
for bringing the Sanbagawa and Ryoke belts into
juxtaposition (Fig. 9.29).
Since the work of Miyashiro (1961, 1972, 1973), 9.10 BACKARC
interpretations of paired metamorphic belts have been
attempted in both island arc and Andean type settings BASINS
around the Pacific margin (Fig. 9.30). The simplicity of

these interpretations is appealing; however, in some
examples, numerous inconsistencies exist. In the Atlan- Backarc (or marginal) basins are relatively small basins
tic region and in the Alps, many Phanerozoic metamor- of either oceanic or continental affinity that form

phic belts either lack one of the pairs or the contrast behind the volcanic arc in the overriding plate of a
between them is unclear. These patterns, and the real- subduction zone (Fig. 9.3). Oceanic varieties are most
ization that many paired metamorphic belts did not common in the western Pacific, but are also found in

form in their present positions, has led to skepticism the Atlantic behind the Caribbean and Scotia arcs. In all
about their overall significance. Brown (1998) summa- of these settings, the basins reside on the inner, concave

rized the evolution of thought that has led to a general side of the island arc and many are bounded on the side
demise of the concept of paired metamorphic belts in opposite the arc by a backarc ridge (remnant arc). Most
many convergent margins and orogens. One reason for of these basins are associated with extensional tectonics
this is that most metamorphic belts are no longer con- and high heat flow, and the majority of oceanic varieties

sidered to be characterized by a single geothermal gra- contain sea fl oor spreading centers where new oceanic
dient, mainly because the rocks record an evolution crust is generated. In continental settings, extensional
across a range of geotherms through time. In addition, backarc basins have been described in the context of
the recognition of suspect terranes and the importance Andean-type convergent margins (Section 10.2). Some
of accretionary processes (Section 10.6) suggests that of the best preserved examples of this type formed

the tectonic units along these margins reflect a complex along the western margin of South America during