Page 289 - Global Tectonics

274 CHAPTER 9

Backarc Active arc Forearc

Interbedded lava flows
Arc volcano
and volcaniclastic deposits
0 Sea level

Volcaniclastic
Distal deposits Proximal
Mafic pillow lavas and flows
Mid–crustal plutons H 2 O H 2 O Differentiating
10 magma H 2 O
chambers

Mafic cumulate CO 2
rocks and sills
Depth (km) 20 Moho CO 2 CO 2 CO 2

Lithospheric Crustal Lithospheric
mantle Mafic mantle
delamination underplating
30 Primary
partial
melts
Convecting
asthenospheric
mantle
40
0 10 No vertical exaggeration
km
Figure 9.25 Idealized section through an island arc illustrating the numerous processes involved in its construction.
Similar processes may operate beneath Andean-type arcs (redrawn from Stern, 2002, by permission of the American
Geophysical Union. Copyright © 2002 American Geophysical Union).

Compression of the arc, such as that which occurs in fi nal site of emplacement. Melt segregation from along
the Chilean Andes (Fig. 9.18a), results in deformation grain boundaries probably involves porous fl ow mecha-
that assists the thickening of arc crust (Section 10.2.4). nisms, assisted by ductile and brittle deformation. The
The mechanisms by which melts are transported ascent of melt appears to involve complex, nonvertical
through the mantle and crust are the source of a great pathways from sources located at different depths.

deal of controversy. In general, transport processes Schurr et al. (2003) identified regions of low Q (Section
operate on at least two different length scales (Petford 9.4) from P-wave arrivals beneath the central Andes that
et al., 2000): the centimeter- to decimeter-scale segrega- reveal a variety of possible sources and ascent pathways
tion of melt near its source region and the kilometer- for metamorphic fluids and partial melts (Plate 9.4(top)

scale ascent of magma through the lithosphere to its between pp. 244 and 245). A seismic refl ection profi le