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3. GEOLOGICAL FRAMEWORK
The Órdenes Complex
A general map and cross section of the Órdenes
Complex, the largest among the allochthonous
complexes of the NW Iberian Massif, is shown
in Fig. 4. The central part of the complex is occu-
pied by extensive exposures of the Upper Units,
which reach a thickness of
c.
12000 m. The Pon-
te Carreira Detachment, a shear zone interpret-
ed as an extensional detachment, represents the
contact between the Betanzos and O Pino units.
The Ponte Carreira Detachment has been dated
at
c.
371 Ma (
40
Ar/
39
Ar on muscovite; Gómez
Barreiro
et al.
, 2006). Below, the Monte Castelo
and Corredoiras units are characterized by high-
er metamorphic grade. Further down, the HP-
HT Upper Units are represented by the Fornás,
Belmil, Melide and Sobrado units (Figs. 2 and 4),
which are also separated from overlying units by
two extensional detachments with intense shear-
ing, the Fornás and Corredoiras detachments
(Fig. 4). The Corredoiras Detachment was dated
at
c.
375 Ma (
40
Ar/
39
Ar on hornblende; Dallmeyer
et al.
, 1997), while the Fornás Detachment seems
older and active until
c.
382 Ma (
40
Ar/
39
Ar on
hornblende; Gómez Barreiro
et al.
, 2006).
Under the HP-HT Upper Units, the Upper
Ophiolitic Units are represented by the Careón
Ophiolite, while the Lower Ophiolitic Units are
constituted by the Bazar and Vila de Cruces
ophiolites. The relative position of the two groups
of ophiolites can be observed SE of the Órdenes
Complex, where the Careón and Vila de Cruces
ophiolites are in contact. Below the ophiolites,
the Basal Units appear as a thick rather contin-
uous set of two juxtaposed sequences with intri-
cate internal structure. They are separated from
the rest of the allochthonous units by two ex-
tensional detachments, the Bembibre-Ceán and
Pico Sacro detachments (Figs. 2 and 4), which
probably were active between
c.
340-317 Ma
(Díez Fernández
et al.
, 2012b). The Upper Se-
quence of the Basal Units is represented by the
Lamas de Abade and Cercio units, and the Lower
Sequence includes the Santiago, Lalín and Forca-
rei units (Fig. 4). The Agualada Unit is the struc-
turally counterpart of the Basal Units and is con-
sidered part of its Lower Sequence thrust above
the Lamas de Abade Unit. The Campo Marzo
Unit, formed by
c.
500 m of ultramafic rocks, is
sandwiched between the Basal Units below and
the Pico Sacro detachment above (Fig. 4). The
lithological constitution and structural position
of this unit are key elements for the interpreta-
tion of the tectonothermal evolution of the Basal
Units, as discussed below.
The contact zone between the Basal Units and
the Parautochthon is well exposed at the base of
the Lalín and Forcarei units. It is a rather narrow
zone characterized by strong ductile shearing in-
terpreted as an important thrust, the Lalín-Forca-
rei Thrust. This thrust would have transported
to the E-SE the thick ensemble formed by the
allochthonous complexes over the Parautoch-
thon, and then the whole Galicia-Trás-os-Mon-
tes was transferred on top of the Central Iberi-
an Zone following a newly-formed thrust at its
base (Martínez Catalán
et al.
, 1996; Dallmeyer
et
al.
, 1997). The mylonitic fabric associated to the
basal thrust of the Parautocthon has been dated
at
c.
343 Ma (
40
Ar/
39
Ar; Dallmeyer
et al.
, 1997).
The activity of the Lalín-Forcarei thrust has been
constrained indirectly by regional data and con-
sidered older (
c.
340 Ma).
The Cabo Ortegal Complex
The Cabo Ortegal Complex is the easternmost
among the allochthonous complexes of Galicia
and therefore located at the leading edge of such
tectonic pile (Figs. 2 and 3). Despite its smaller
size, this complex includes a rather complete col-
lection of the allochthonous terranes described
in NW Iberia, with excellent outcrops on the
Cantabrian coast. The largest exposures are rep-
resented by the HP-HT Upper Units, which ac-
cording to structural and tectonothermal criteria
feature two main different units, the Capelada
Unit above and the Cedeira Unit in the lower
position. It is unclear if a third small unit, the
Peña Escrita Unit, located further below, is part
of these units or a different one (Figs. 2 and 3).
The IP Upper Units are barely represented in this
complex, the Cariño Unit being the sole example.
The contact between both types of Upper Units
is a detachment similar to those described in the
Órdenes Complex, but it has not been studied in
detail in the Cabo Ortegal Complex.
The ophiolites are only represented by the Up-
per Ophiolitic Units, which in this case include
two units with different lithological composition,
the Purrido and Moeche ophiolites. The Bas-
al Units are represented by the Espasante Unit,