Geoscience Reference
In-Depth Information
(a)
(b)
Rising UHP sheet
4
Continental crust
Continental crust
UHP
800
o
100 mm yr
−
1
−
1
Lithosphere
Lithosphere
10 mm yr
−
1
3
5 mm yr
Astheno-
sphere
Asthenosphere
2
1
0
0
200
400
600
800
1000
o
Temperature ( C)
Figure 10.29.
(a) A schematic cross section showing a rising UHP sheet during the
final stages of continent-continent collision. The thin sheet becomes detached from
the subducting plate and rises back up the subduction zone. (b) Burial and
exhumation paths for a rock that was subducted to 100 km depth and then
immediately exhumed along the same fault (now reactivated as a normal fault).
P
-
T
paths are shown for three convergence and exhumation rates, 100, 10 and
5mmyr
−1
. The shaded region indicates the ultra-high-pressure (UHP) region. (After
Ernst and Peacock, A thermotectonic model for preservation of ultra high-pressure
phases in metamorphosed continental crust, Geophysical Monograph 96, 171-8,
1996. Copyright 1996 American Geophysical Union. Modified by permission of
American Geophysical Union.)
if their upward buoyancy forces exceed the downward forces (shear at the base of
the sheet and resistance against the overriding plate). The differences in density
can be considerable - the density of basalt eclogite is
3700 kg m
−
3
∼
and that
3000 kg m
−
3
. Section 7.8 shows that, when large-scale
exhumation returns a thick layer of rock to shallow levels, the temperature of the
rising rocks will be adiabatically controlled - it will remain high (Fig. 7.23). In
such a situation any relic UHP metamorphism is unlikely to be retained since the
minerals will re-equilibrate. However, in the situation in which sheets of UHP
material rise by moving back up the subduction zone, the sheets can be cooled by
conduction both from their upper and from their lower surfaces, to the overlying
plate and to the underlying subducting plate (Fig. 10.29). If the detached sheet
is thin, cooling could be efficient enough that complete re-equilibration upon
ascent would not take place and for some relic UHP metamorphic mineralization
to be retained. Reaction rates are generally reduced in the absence of aqueous
fluids, which suggests that ascending UHP rocks may also have to have been
dry. Thus, UHP belts are very narrow linear tectonic features that can form only
under very rare specific conditions during the final stages of collision between
two continents.
of granitic gneiss is
∼
