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The oldest orogenic events associated with
the assembly of Pangea and the distribution of
the oceanic domains closed during the collision,
can only be deduced from the study of some
terranes involved in the Variscan-Appalachian-
Alleghanian Orogen. However, the first orogenic
stages usually appear strongly overprinted by
subsequent events. It is therefore fundamental
to identify the terranes and domains where these
oldest events can still be recognized. One of the
main characteristics of the Variscan Orogen is the
presence of a long suture zone outlined by several
allochthonous terranes including ophiolites and
high-P units (Fig. 1). They preserve the most
significant information in relation to the oldest
orogenic events and the evolution of the oceanic
domains that preceded the collision. Therefore
a clear understanding of these terranes is a
key issue to increase the knowledge about the
characteristics of the initial interaction between
Gondwana and Laurussia. On the other hand, the
allochthonous terranes also provide information
about the Proterozoic and Paleozoic evolution of
the continental margins before orogenesis that is
valuable for paleogeographic reconstructions.
Different orogenic models have been proposed
in the last years for the Variscan Belt, based on
the study of the origin and tectonothermal
evolution of terranes involved in the orogen
(Matte, 1991, 2001; Stampfli
et al.
, 2002;
Winchester
et al.
, 2002; Gómez Barreiro
et
al.
, 2007; Ballèvre
et al.
, 2009, 2014; Martínez
Catalán
et al.
, 2009; Kroner and Romer, 2013;
Arenas
et al.
, 2014a). However, an integrated
description of the terranes that define the suture
zone has not been presented so far. Considering
that these terranes are mainly continuous along
the whole orogen (Fig. 1), their description for a
specific region can be considered as a reference
framework that would greatly aid correlations
along the Variscan belt. This paper presents an
updated and detailed revision of the origin and
tectonothermal evolution of the terranes making
up the allochthonous complexes of Galicia (NW
of the Iberian Massif; Fig. 2). These complexes
have been the subject of extensive research in
recent decades, with publication of many detailed
papers, granting a comprehensive review. The
main conclusions reached in this paper can be
considered a solid foundation for future progress
in the understanding of the Variscan Orogen
and, consequently, the assembly of Pangea.
Allochthonous complexes and terranes from
Galicia (NW Iberian Massif)
Three allochthonous complexes exist in Gali-
cia in the most internal part of the Variscan
Orogen, from East to West the Cabo Ortegal,
Órdenes and Malpica-Tui complexes (Fig. 2).
These large structures were considered litholog-
ical complexes because they contain a stacking
of allochthonous units with different origin and
contrasted tectonothermal evolution. The Mal-
pica-Tui Complex is included among the alloch-
thonous complexes both for historical reasons, as
it was denominated Ancient Complex in the first
geological maps of Galicia (Parga Pondal, 1963),
and simplicity. However, this complex is different
to the others because it is apparently constituted
by a single terrane. The three allochthonous com-
plexes show a general synformal structure which
has favored their preservation. It is a wide, open,
upright synform in the Órdenes Complex, and
two narrow upright synforms in the Cabo Orte-
gal and Malpica-Tui complexes (Fig. 2).
The allochthonous complexes are constitut-
ed by a thick stacking of terranes which include
ophiolitic units and high-P metamorphic rocks.
They are equivalent to other terrane assemblages
described in the Variscan Orogen (Fig. 1). These
complexes contain the remnants of a Variscan su-
ture, but since this suture is rootless and thrust
above an autochthonous-parautochthonous sub-
stratum, it is unclear whether it accounts for a
primary suture separating large continental land-
masses. The complexes were first described as al-
lochthonous structures by Ries and Shackleton
(1971), who interpreted them as remnants of a
Variscan thrust plate. Some later works proposed
an alternative interpretation, according to which
the complexes would represent rooted structures
related to the activity of a mantle plume devel-
oped in an extensional setting (Van Calsteren
et
al.
, 1979). However, new research after 1980 has
fully confirmed the allochthonous nature of these
complexes and their equivalence with residual
klippen of a giant nappe pile.
The Parautochthon (also referred to as Schis-
tose Domain) and the allochthonous complexes
16
3. GEOLOGICAL FRAMEWORK