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4. THE ECLOGITE FACIES GNEISSES
subducted complex experienced a drastic and fast
exhumation (D2 event) during the generation
of the decompressive mylonitic foliation (S2).
This constitutes the most prominent structural
feature in the HP–HT units. This pronounced
exhumation (calculated in at least
c
. 10 kbar) very
likely occurred during continuous subduction of
continental or transitional slices,
i.e.
new layers of
the same terrane that in a prograde accretionary
sequence should be presently located below the
HP–HT units. These units are not preserved, at
least in NW Iberia, because the HP–HT units rest
above ophiolitic units, avoiding the identification
of the colliding margins. The progression
of exhumation favours the generation of an
important extensional detachment, which is
represented by the contact between the Cariño
Gneisses, in the amphibolite facies and located in
the hanging wall, and the eclogite facies gneisses.
A sketch is presented showing a model for the
exhumation of the eclogite facies gneisses and in
general for the HP–HT units of the Cabo Ortegal
Complex. The tectonothermal and exhumation
history of the HP–HT units fits quite well into the
mathematical modelling (other authors) of the
exhumation of ultra-high-pressure complexes.
The progression of the deformational history
of the upper units entails the development of
large recumbent folds and a basal thrust (D3
structures). The geometry of both structures
seems to indicate that their development could be
related to each other and that they represent the
final structures developed in the exhumation of
the subducted complex. The structures predicted
in numerical models have been compared to
those explained in this article, where the large
regional structures (D3 recumbent folds and
basal thrust) and an ultra-HP buoyant plume
(HP–HT units), are represented. The progression
of the exhumation entails an important change
in the nature of the colliding plates, because
during this evolution the prograde continental
margin was replaced by the under-thrusting
of ophiolites. The tectonothermal evolution of
the eclogite facies gneisses of the Cabo Ortegal
Complex represents an illustrative example of
the complex history of these deeply subducted
complexes, which are relatively frequent in the
suture zone of the Variscan belt. The structural,
metamorphic and geochronological patterns in
the evolution of this gneissic unit follow almost
perfectly the predictions based in numerical
experiments about the exhumation of ultra-HP
complexes, and consequently, they represent a
direct confirmation of the reality of these models.
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