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included right-lateral components. Continental
layers were imbricated below previously accret-
ed ophiolites, while a variety of C-type eclogites,
blueschists and lawsonite bearing metabasites
developed. The Basal Units can be subdivided in
two main groups according to their tectonother-
mal evolution: an upper group with high-P and
medium-high-T metamorphism (Agualada and
Espasante units); and a lower group with high-P
and low-medium-T metamorphism (Ceán, Mal-
pica-Tui, Lamas de Abade, Santiago, Cercio, Lalín
and Forcarei units). The upper group is consid-
ered the closest section to the overlying mantle
wedge during Variscan subduction, whereas the
lower group accounts for the cooler sections of
the subduction wedge. The arrival of thicker,
more buoyant continental crust blocked the sub-
duction, leading to several thrusting events that
transported the subduction complex onto the ad-
jacent, inner sections of the margin, represented
by the parautochthonous sequences of the Schis-
tose Domain.
The Somozas Mélange is a piece of the Vari-
scan continental subduction channel developed
between the section of the Gondwanan margin
represented by the Basal Units and their respec-
tive overlying mantle wedge. The subducted
continental margin was exhumed later on and
emplaced over the mélange zone. The mélange
appears as a unique element at the easternmost
contact between the allochthonous terranes and
their relative autochthon. The mélange unit con-
sists of
c.
500 m of serpentinite showing block-
in-matrix texture. The blocks are variable in size
and include metasedimentary rocks, volcanics,
gabbros, granitoids and high-P rocks. As a major
plate boundary, and given the nature and sensi-
tive structural position of this unit, multiple tec-
tonic events have left strong imprint on it.
Continental convergence did not decline after
Variscan subduction and early Variscan nappe
tectonics. The allochthonous pile and the suture
zone were transferred onto the Gondwana main-
land, thus triggering the thermal and gravitation-
al collapse of the collisional wedge. The conver-
gence continued during the Pennsylvanian, when
the entire allochthonous pile was subjected to
heterogeneous reworking in strike-slip systems.
Considering the allochthonous character of
the nappe pile and the strong deformation asso-
ciated to the Variscan collision, there are prob-
lems to identify the original tectonic setting of
some terranes and thence, it is almost impossi-
ble to reconstruct the paleogeographic setting
during the Variscan and pre-Variscan times in
detail. Key features to perform any evolving
model for the Variscan convergence should con-
sider the existence of two different high-P meta-
morphic events, both of them affecting continen-
tal or transitional crustal sections that belonged
to the margin of Gondwana. On the other hand,
the Ophiolitic Units provide evidence for two
stages of generation of oceanic or transitional
crust precisely within the paleogeographic do-
main that separated the two sections recording
the high-P events. Previous models developed in
NW Iberia suggested that the Upper Units rep-
resent a peri-Gondwanan terrane drifted away
from the main continent during the opening of
the Rheic Ocean. The Lower and Upper Ophiol-
itic units would be generated respectively during
the opening (rifting) and the beginning of the
closure (by intraoceanic subduction) of this Pa-
leozoic ocean. In those models, the two high-P
metamorphic events would be related first to the
accretion of the drifted terrane to the southern
margin of Laurussia (Upper Units), and then to
the subduction of the thinned Gondwanan mar-
gin after complete closure of the Rheic Ocean
(Basal Units).
The previous models have important problems
to explain the high/ultra-high-P metamorphic
event and the exhumation of deeply subducted
transitional-type sections. On the other hand,
the recently discovered participation of an old-
er continental crust in the generation of some
protoliths belonging to both types of ophiol-
itic units (Purrido, Moeche and Vila de Cruces
units), along with their highly depleted Sm-Nd
isotopic signature, make difficult their relation-
ship to open wide oceanic domains. A two-stage
collisional model affecting a wide Gondwanan
platform may explain most of the evidences in
NW Iberia. This platform would contain Cam-
brian back-arc sections with transitional crust (
c.
500 Ma) filled by siliciclastic material, and also
the remnants of a previous Ediacaran-Cambrian
magmatic arc. Collision of this platform with the
southern margin of Laurussia, in a dextral con-
vergence setting, would have caused imbrication
13
3. GEOLOGICAL FRAMEWORK