Page 297 - Global Tectonics

282 CHAPTER 9

patterns. Geochemical studies also indicate that underthrusting slab (Hsui & Toksöz, 1981; Jurdy &
backarc lavas commonly display greater compositional Stefanick, 1983) or an increase in the angle of subduc-
variations, including higher water contents, than mid- tion with depth (Section 12.6). Although these and
ocean ridge basalts (Taylor & Martínez, 2003). Many other mechanisms controlling the evolution of backarc
backarc lavas are chemically related to the lavas that basins are often debated, most authors agree that basin

form the adjacent island arc. These observations suggest evolution is strongly influenced by the pattern of fl ow,
that crustal accretion in backarc basins is strongly infl u- partial melting and melt transport in the upper mantle
enced by processes related to subduction (e.g. Kitada wedge above a subduction zone. Geodynamic models
et al., 2006). increasingly have appealed to three-dimensional circula-
Tomographic images of the mantle beneath active tion patterns associated with trench migration and slab

backarc spreading centers have confirmed the impor- roll-back to explain the thermal evolution of the wedge
tant linkages that exist between backarc crustal accre- and the production of melt within it (Kincaid & Griffi ths,
tion and subduction. In one of the best-studied 2003; Wiens & Smith, 2003).
arc–backarc systems, Zhao et al. (1997) showed that Martínez & Taylor (2002) developed a model of
very slow seismic velocities beneath the active Lau crustal accretion for the Lau basin that explains the
spreading center and moderately slow anomalies under mechanism of backarc magmatism and its relationship
the Tonga arc are separated at shallow (<100 km) depths to magmatism in the Tonga arc. These authors observed
in the mantle wedge but merge below 100 km to depths that the various spreading centers in the basin (Fig. 9.32)
of 400 km (Plate 9.1 between pp. 244 and 245). The display structural and compositional patterns that
magnitude of the velocity anomalies is consistent with correlate with distance from the arc. As in most other
the presence of approximately 1% melt at depths of intra-oceanic arc systems, the crust displays a general
30–90 km (Wiens & Smith, 2003). At greater depths the depletion of certain elements relative to mid-ocean
anomalies may result from the release of volatiles orig- ridge basalt that increases from the backarc toward the
inating from the dehydration of hydrous minerals. arc. In addition, the spreading center closest to the arc
These results indicate that backarc spreading is related (the Valu Fa spreading ridge) displays a structure, depth
to convective circulation in the mantle wedge and dehy- and morphology indicating that it is characterized
dration reactions in the subducting slab. They also by an enhanced magma supply relative to other
suggest that backarc magma production is separated centers. Farther away from the arc, the East Lau and
from the island arc source region within the depth Central Lau spreading centers display diminished and
range of primary magma production. By contrast, normal magma supplies, respectively. Martínez &
below 100 km, backarc magmas originate through Taylor (2002) proposed that these variations result from
mixing with components derived from slab dehydration the migration of magma source regions supplying the
and may help to explain some of the unique features backarc spreading centers through the upper mantle
in the petrology of backarc magmas relative to typical wedge.
mid-ocean ridge basalts. The model of Martínez & Taylor (2002) begins with

A wide variety of mechanisms has been postulated the roll-back of the Pacific slab beneath the Tonga
to explain the formation of backarc basins. One trench (large white arrow in Fig. 9.33a). This motion

common view is that the extension and crustal accre- induces a compensating flow of mantle material beneath
tion that characterize these environments occur in the Lau basin (small black arrows). As the mantle
response to regional tensional stresses in the overriding encounters water that is released from the subducting
lithosphere of the subduction zone (Packham & Falvey, slab (Section 9.8), it partially melts, leaving a residual
1971). These stresses may result from the trench suction mantle depleted of a certain melt fraction. The stipple
force as the subducting slab steepens or “rolls-back” in Fig. 9.33a indicates the region of hydrated mantle.
beneath the trench (Chase, 1978; Fein & Jurdy, 1986) The region of partial melt is shown as the white back-
(Section 12.6). Such a roll-back mechanism has been pos- ground beneath the stippling. Flow induced by subduc-
tulated to occur in subduction systems where the “abso- tion drives the depleted layer toward the upper corner
lute” direction of movement of the overriding plate is of the wedge where increased water concentrations
away from the trench (e.g. Figs. 10.9b,c, 10.37). Other from the slab promote additional melting. This region
sources of the tension could include convection in the of enhanced melting (outlined area in Fig. 9.33a) sup-
upper mantle wedge induced by the descent of the plies the Valu Fa spreading ridge close to the volcanic