Geoscience Reference
In-Depth Information
Figure 10.8.
Cross
sections of subduction
zones, showing the
metamorphism of the
downgoing slab,
associated earthquakes,
loss of volatiles and arc
magmatism. (a) Thermally
mature subducting plate -
high thermal parameter.
Intermediate-depth
earthquakes result from
the dehydration and
transformation to eclogite
of the oceanic crust and
reactivation of faults.
(b) Young and/or slowly
subducting plate - low
thermal parameter. Since
the dehydration,
formation of eclogite and
reactivation often take
place at shallow depths,
the amount of partial
melting in the mantle
wedge and hence arc
magmatism may be
reduced. (Based on Kirby
et al
.(1996) and Peacock
(1996).)
(Fig. 10.9). There is still uncertainty about the exact location of melt genera-
tion, but it is probable that much of the melting occurs in the warm overlying
wedge immediately above the locus where the slab reaches 100-120 km depth.
The
238
U-
230
Th disequilibrium means that, for the Mariana arc, the time delay
between dehydration of the slab and eruption of the lavas is less than 30 000 yr.
Such a short time interval implies that the fluids are the primary cause of mantle
melting and that the melt migration is rapid. In contrast, the timescale for trans-
port of the signature of subducted melted sediment to the melt source region is
