Geoscience Reference
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being beneath the Archaean micro-continent. The crust is highly reflective and
the Moho is very well defined beneath the orogen. The westward-dipping crustal
reflections in the west of the orogen are consistent with the geological evidence
that subduction took place beneath the Hearne craton and indicate that the arc-
derived rocks extend beneath the continental margin and Archaean rocks. There
is no apparent connection at depth between the central Archaean micro-continent
and the Hearne and Superior cratons to either side - this is consistent with the
micro-continent being exotic. The easterly dipping reflections in the east of the
orogen seem to be related to the late/post-collisional oblique deformation as
the Superior craton was thrust up and over and along the previously deformed
orogen and do not imply that there was easterly directed subduction. In addition
there is evidence for mantle structures-ahigh-P-wave-velocity (8.4 km s
−
1
)
zone in the mantle. This zone is at depths
100 km and dips westwards. Such
high P-wave velocities suggest that the material could be a remnant eclogitic
slab.
<
Ultra-high-pressure belts
Ultra-high-pressure (UHP) metamorphic belts are unusual metamorphic com-
plexes that contain relics of low-temperature, high-pressure assemblages (includ-
ing coesite and diamond). These very rare belts are associated with collisional
mountain belts and contain
10-km-thick UHP terranes of partly exhumed and
metamorphosed continental crust, tectonically juxtaposed against other, very dif-
ferent continental rocks. UHP belts are found in ancient continental collision
zones across Eurasia, which range in age from 300 Ma to
<
∼
2.3 Ga (e.g., the Dora
Maira massif, western Alps; Western Gneiss Region, Norway; Kokchetav Com-
plex, Kazakhstan; Maksyutov Complex, Urals; Qinling-Dabie-Sulu belt, China).
The wide spread of ages implies that the processes that resulted in the burial and
subsequent exhumation of these rocks were not unusual. However, their rarity
indicates that some aspects of the process must have been very uncommon. The
temperature of recrystallization of these UHP rocks was 700-900
◦
Catpres-
sures of 2.8-4.0 GPa (approximately 90-125 km). Exhumation rates were fast -
rates are estimated at 2-12 mm per year for ascent from
100 km depth to
upper/mid-crustal levels. The process responsible both for the descent of the
UHP rocks to
>
100 km and for their ascent is apparently subduction. The prob-
lems associated with an understanding of the occurrence of UHP rocks do not
concern their subduction, but rather concern (1) the way in which they have been
returned fairly rapidly to shallow levels and (2) the preservation of the relic UHP
metamorphism. It is probable that UHP rocks arise when old, cold continental
crust is part of a largely oceanic plate that is being subducted. At depth, thin
sheets of continental crust then become detached from the rest of the subducting
plate as large amounts of continental crust enter the subduction zone (Fig. 10.29).
This may occur when convergence ceases and the continents suture. Then, being
less dense than the rest of the lithosphere, these sheets, or tectonic slices, can rise
>
