Geoscience Reference
In-Depth Information
North American plate as a result of plate tectonics and subduction. Terranes that
are suspected of having originated far from the present location and of being
transported and then accreted are descriptively referred to as
suspect terranes
!
Such terranes were first identified in the early 1970s in the eastern Mediterranean
in Greece and Turkey, when it was realized that the region is composed of small
continental fragments with very different histories. The present-day subduction
and volcanism along the western edge of the North American plate is thus con-
tinuing a history that has been occurring there episodically for several hundred
million years (see also Section 3.3.4).
This chapter does not proceed in chronological order from the beginning of
the continents to the present but instead works from the present back into the
past, or, in the case of North America, from the edges to the centre. Our starting
point is to continue the discussion of subduction zones from Chapter 9.
10.2 The growth of continents
10.2.1 Volcanism at subduction zones
The geophysical setting of subduction zones has been discussed in Section 9.6.
The initial dip of the subducting plate is shallow, typically about 20
◦
for the first
100 km, as seen horizontally from the trench, or on average about 25-30
◦
in the
region from the surface to the point at which the slab is 100 km deep. Volcanic
arcs are characteristically located more than 150 km inland from their trench.
This distance is variable, but it is clear from Fig. 9.46 that, with the exception
of the New Hebrides, which is a very steeply dipping subduction zone in a very
complex region, the volcanic arcs are located above places where the top of the
subducting plate reaches a depth of about 100-125 km. The crust under volcanic
arcs is usually fairly thick, in the range 25-50 km. The volcanic arcs are regions
of high heat flow and high gravity anomalies. Despite the broad similarities, the
settings of arc volcanism vary tremendously, and the styles of volcanism and the
chemistry of the lavas vary in sympathy. The settings range from extensional
to compressional and from oceanic to continental. In each case the product is
different.
The descending slab: dehydration of the crust
The subducting plate or descending slab (both terms are used) is cooler than
the mantle. Thus, as it descends it is heated and undergoes a series of chemical
reactions as the pressure and temperature increase. The oceanic crust is heavily
faulted and cracked and water has usually circulated through it in hydrothermal
systems that became active soon after the crust formed at the ridge. The crust
of the descending slab is therefore strongly hydrated (up to several per cent
H
2
O). All the chemical reactions which take place in the descending oceanic
crust are
dehydration reactions
; that is, they involve a loss of water, usually in an
endothermic process with a reduction in volume of the residue.
