THE CENTRAL IBERIAN ARC (VARISCAN BELT) Jos R. Martnez Cataln
Slide 2 Paleoposition of the Iberian Massif It is the last massif
of the European Variscan belt to the SW It includes the core and
the southern part of the Ibero- Armorican arc It consists of an
authochtonous domain and a large allochthon containing the suture
of the Rheic Ocean All units, except the ophiolitic, are of
Gondwana derivation Slide 3 The Iberian Massif consists of five
continuous parallel zones and a weirdo: Galicia-Trs-os-Montes Zone,
the allochthonous stack including the suture The Iberian Massif
Slide 4 An arc is delineated by the axial traces of D1 Variscan
folds, which converge in a covered area. D3 folds are roughly
coeval with the arc Central Iberian arc Proposed name: Central
Iberian arc because it is identified in the Central Iberian Zone
(CIZ) Slide 5 A key area in La Mancha, between Toledo and Ciudad
Real, where the arc is well delineated by upright D1 folds of the
CIZ The Central Iberian arc Slide 6 D1 folds are delineated by the
Armorican Quartzite (Early Ordovician) They rotate anticlockwise
Eastward, from an E-W to a N-S attitude Source: Google Earth Slide
7 The arc was drawn by Du Toit (Our wandering continents: An
hypothe- sis of continental drifting. Oliver and Boyd, Edinburgh,
366 pp., 1937) Slide 8 The arc is also marked by faint magnetic
anomalies Source: Aeromagnetic Map of Spain, Instituto Geogrfico
Nacional (IGN) Slide 9 Noticed by Aerden (Journal of Structural
Geology, 26, 177-196, 2004) Interpreted as due to interference of
two folding phases, the second of which related with sinistral
transpression Slide 10 The Central Iberian arc Magnetic anomalies:
High resolution aeromagnetic IGN data in Spain Slide 11 Magnetic
highs and lows delineate the arc and show that it is bounded to the
south by a straight negative magnetic anomaly The Central Iberian
arc Slide 12 The Central Iberian Arc The negative magnetic anomaly
follows the limit between the Central Iberian and Ossa-Morena
zones, and continues beneath the Betics Slide 13 seems repeated in
the Urra Fm (Portalegre area, southwestern CIZ) The allochthonous
GTMZ occupies the core of the Central Iberian arc The Central
Iberian Arc Ollo de Sapo Fm: Cambro- Ordovician (495-483 Ma) felsic
volcanism, in northern part of the CIZ. The Fm Slide 14 Allochthon:
klippen cropping out in structural basins among anatectic domes
Autochthon: a Paleozoic sequence overlying non- metamorphic or very
low-grade Cadomian basement NW Iberia and the Galicia- Trs-os-
Montes Zone Slide 15 The Galicia-Trs- os-Montes Zone consists of
three groups of allochthonous units, plus an underlying thrust
sheet: the Parautochthon, Lower allochthon or Schistose Domain Red
lines: cross sections in next slide Slide 16 The Central Iberian
arc is not evident in the allochthon : flat thrust sheets are not
prone to fold about vertical axes. Probably, there is a compromise
between oroclinal and upright folding Three groups of allochthonous
units: Above the suture, a peri-Gondwanan island arc Ophiolitic
units mark the suture of the Rheic Ocean Below the suture, the edge
of the northern Gondwana margin Slide 17 The Central Iberian Arc
The Central Iberian arc is limited to the S and W by wrench shear
zones. The Porto-Tomar shear zone overprints all previous ones
Slide 18 Strike-slip shear zones in the Variscan belt, crosscutting
relationships: The PTSZ seems to overprint the whole Central
Iberian arc (CIA) The PTSZ-SASZ-S is folded by the Ibero-Armorican
arc (IAA), but less tightly than the zones of the Iberian Massif
Slide 19 Strike-slip shear zones in the Variscan belt, crosscutting
relationships: The Central Iberian arc is older, or at least,
reached full development before than the Ibero Armorican arc Slide
20 Right-lateral wrench tectonics in the Variscan belt A model of
dominantly right- lateral strike-slip motion accounting for the
present distribution of terranes and zones was proposed by Martnez
Cataln et al. (GSA Memoir 200, 403-423, 2007) The Central Iberian
arc fits early dextral displacements on the main, large dextral
shear zone Slide 21 CONCLUSIONS The Central Iberian arc is an
orocline delineated by the trend of the first Variscan folds and by
magnetic anomalies The Central Iberian arc is an orocline
delineated by the trend of the first Variscan folds and by magnetic
anomalies in the Central Iberian Zone of the Iberian Massif. The
structure was known by Du Toit (1937) and was noticed by Aerden
(2004), but has been ignored by most due to the post-Paleozoic
cover of its hinge zone and, perhaps, the faintness of the magnetic
anomalies The arc explains the width of the Central Iberian Zone
The arc explains the width of the Central Iberian Zone (up to 400
km) by duplication of a strip initially half that width. Its sense
of curvature is opposite to that of the Ibero-Armorican arc, and it
makes more understandable the position of the allochthonous
complexes of NW Iberia, which occupy its core, and which include
the Rheic suture. The Ossa-Morena and South Portuguese zones do not
appear involved in the Central Iberian arc It is suggested that the
origin of the arc is linked to a large intracontinental,
strike-slip, dextral shear zone related with oblique convergence
between Laurussia and Gondwana. This shear zone is presently hidden
in Iberia, perhaps by Los Pedroches granitic lineament or by the
Badajoz-Crdoba shear zone, and may continue in Central Europe by
the Layale-Lubine fault
