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.