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3. GEOLOGICAL FRAMEWORK
define the Galicia-Trás-os-Montes Zone (Farias
et al.
, 1987), which is emplaced by means of a
low-angle thrust over the Central Iberian Zone.
This zone and its prolongation towards the fore-
land across the West Asturian-Leonese Zone
and the Cantabrian Zone, represent an autoch-
thonous section of the margin of Gondwana
characterized by the presence of thick pre-oro-
genic sedimentary series ranging in age from the
Ediacaran to Devonian, with some magmatic
events more abundant around the Cambrian-Or-
dovician boundary (Pérez–Estaún
et al.
, 1990;
Martínez Catalán
et al.
, 1992; Díez Montes, 2007;
Rubio Pascual, 2013) (Fig. 1). The Parautochthon
consists of pre-orogenic Cambrian to Devonian
metasedimentary rock sequences with minor
metavolcanic rocks, and younger Late Devonian
and Early Carboniferous synorogenic deposits
(Farias, 1990; Díez Fernández
et al.
, 2012a and
references therein). Stratigraphic, magmatic
and faunal similarities between the Parautoch-
thon and the Iberian autochthonous sequences,
together with the absence of ophiolites within
or at their contact and the lack of evidence for
high-P metamorphism, suggest that the two do-
mains were located adjacent to each other, form-
ing part of the same section of the Gondwanan
margin. The Iberian autochthonous section and
the Parautochthon were variably deformed and
metamorphosed during Variscan times, with ac-
tivity advancing fromWest to East in present day
coordiates. In the Central Iberian Zone, the first
deformation and metamorphism was dated at
c.
359 Ma in the Alcañices Synform using
40
Ar/
39
Ar
geochronology. Progressively younger ages were
obtained in eastern sections of the Central Ibe-
rian Zone and in the West Asturian Leonese
Zone, with deformation reaching the limit with
the Cantabrian Zone at
c.
321 Ma (Dallmeyer
et
al.
, 1997). Both domains were intruded by large
massif of synkinematic and postkinematic gran-
itoids.
Terrane typology and correlation
The first studies about the terranes forming
part of the allochthonous complexes had a local
character, and were focused on the description
of the lithological and structural diversity. For a
long time the research focused on the descrip-
tion of specific areas of the complexes, without
regard to differentiation of the large individual
terranes. However, investigating the origin and
tectonothermal evolution of the terranes was
finally the main objective of later works works.
Arenas
et al.
(1986) performed the first correla-
tion and interpretation of the terranes forming
part of the allochthonous complexes in Galicia.
Such correlation is currently accepted with some
simplification and updating, and is based on the
distinction of terranes with continental or ocean-
ic nature, the latter represented by several units
interpreted as ophiolites. From bottom to top,
the terranes included in the Galician allochtho-
nous complexes are named: Basal Units, Ophiol-
itic Units and Upper Units (Fig. 2). There exists
another terrane only present in the leading edge
of the large allochthonous pile, and resting below
the Basal Units. It is a thick serpentinite mélange,
the Somozas Mélange, only described in the east-
ern part of the Cabo Ortegal Complex (Fig. 2).
Both the Basal Units and the Upper Units have a
significant proportion of metasedimentary rocks,
and they are considered te represent two different
terranes with continental affinity. Interpretations
regarding the Ophiolitic Units are more complex
as recent works have shown they are constituted
by oceanic sections with different ages generated
in diverse paleogeographic settings.
Further subdivision of the allochthonous ter-
ranes is possible considering their age, origin
and tectonothermal evolution. The Basal Units
include two different lithological successions, a
Lower Sequence and an Upper Sequence and two
distinct metamorphic groups, a Lower Metamor-
phic Group (LMG) and an Upper Metamorphic
Group (UMG). The Ophiolitic Units are formed
by two different groups, the Lower Ophiolitic
Units and the Upper Ophiolitic Units. The Up-
per Units are constituted by a lower set of units
affected by high pressure and high temperature
metamorphism (HP-HT Upper Units), and an
uppermost set characterized by an intermediate
pressure tectonothermal evolution (IP Upper
Units) (Fig. 2). Moreover, it is also convenient to
use the local name of the units in order to per-
form an accurate description of the terranes. Lo-
cation of all units described in the allochthonous
complexes of Galicia is presented in the synthetic
map shown in Fig. 2, and also in the more de-
tailed maps of the Cabo Ortegal (Fig. 3), Órdenes
(Fig. 4) and Malpica-Tui (Fig. 5) complexes.