Tectonostratigraphic terranes and the geodynamic evolution of the Iberian Variscan Fold Belt

438 SELECTED ABSTRACTS

Tectonostratigraphic terranes and the geodynamic evolution of the Iberian Variscan Fold Belt

A. Ribeiro a, C. Quesada b and R.D. Dallmeyer ’ a Rua Escola Politecnica, 1294 Lisbon, Portugal

b Institute Geol6gico y Miner0 de Espaiia, 23 Rios Rows, 28003 Madrid, Spain

’ Department of Geology, University of Georgia, A them, GA, USA

(Received November 27, 1989)

Abstract

The Iberian Variscan orogen is represented by

four distincly different tectonostratigraphic ter-

ranes, including the following:

(1) A variably allochthonous, Early-Late Pale-

ozoic sedimentary sequence and unconformably

underlying Precambrian basement. Within the

Cantabrian and West Asturian-Leonese zones

these sequences display NE-vergent imbrication as

a result of Late Paleozoic erogenic activity. Within

the Ossa-Morena Zone these sequences show

SW-vergent deformation of Middle(?) and Late

Paleozoic age. Similar sequences within interven-

ing portions of the Central Iberian Zone appear to

be autochthonous/parautochthonous.

(2) A series of allochthonous, composite struc-

tural units comprised of variably deformed and

metamorphosed (locally to eclogite) oceanic se-

quences and structurally overlying continental

crustal rocks (Cabo Ortegal, Santiago de Com-

postela, Morais, Braganqa and Malpica-Tuy com-

plexes). Initial high-grade metamorphism and sub-

sequent abduction appears to have occured before

the Middle Devonian in a possible palinspastic

setting removed from the Iberian miogeocline. Fi-

nal emplacement into present structural settings

occurred in the Late Paleozoic and involved imbri-

cation with Iberian miogeoclinal sequences.

(3) The Beja-Acebuches ophiolite and Pulo do

Lobo succession, which are represented by units

of oceanic affinities located along the structural

boundary between the Ossa-Morena and South

Portuguese Zones.

(4) A series of intracontinental, transtensional

basin rocks (sedimentary and volcanic) which oc-

cur within the South Portuguese Zone. These over-

stepped the contact between the Ossa-Morena

and Pulo de Lobo Zones, and were subsequently

deformed in a transpressional regime during the

Late Carboniferous.

The present disposition of the Iberian terranes

suggests that the northwest allochthonous crystal-

line complexes were most probably initially de-

rived from a zone along the northern extension of

the boundary between the Iberian miogeoclinal

sequences of the Ossa-Morena Zone and the

Beja-Acebuches ophiolite. The oceanic sequences

within the northwest crystalline complexes and the

Beja-Acebuches ophiolite are probably the rem-

nants of a once continuous ocean, despite their

contrasting tectonothermal evolution. The com-

plexes are interpreted to contain components

which initiated within an oceanic realm, and were

imbricated with structural units of continental

crust under high-pressure metamorphic conditions

prior to abduction of the Beja-Acebuches ophio-

lite in the Middle(?) Devonian. This oceanic re-

gime was sequentially reduced throughout the De-

vonian along an E-NE-dipping subduction com-

plex. Development of an associated accretionary

complex resulted in metamorphism, imbrication

Elsevier Science Publishers B.V.

SELECTED ABSTRACTS 439

and eventual abduction of the Beja-Acebuches

ophiolite. This was followed by later imbrication

of the previously metamorphosed crystalline com-

plexes and sequences within the Iberian miogeo-

cline. Continual oceanic closure led to lithospheric

delamination and development of a detached

tectonic “flake”. The amplitude of continental

overthrusting increases toward the hinge of the

Ibero-Armorican Arc, suggesting that dextral

transpression developed because of an oblique an-

gle of collision between the continental layers.