Emilio González Clavijo

The late Variscan HT/LP metamorphic event in NW and Central Iberia: relationships to crustal thickening, extension, orocline development and crustal evolution

Abstract The Variscan metamorphic evolution of the autochthonous domain of NW and Central Iberia is characterized by a Barrovian gradient followed by a high-temperature–low-pressure (HT/LP) event associated with voluminous granite magmatism. The structural, metamorphic and magmatic histories of the region are described briefly and the relations between them are explained. A coherent model for evolution of the continental crust is proposed using published radiometric ages, thermal models and seismic reflection profiles. The metamorphic evolution, including the high-temperature event, is explained by crustal thickening resulting from the Gondwana–Laurussia collision followed by a period of thermal relaxation and a long-lasting extensional stage. The fact that the highest temperatures were reached in the core of the Central Iberian arc, partly occupied by remnants of a huge allochthonous nappe stack, is discussed in relation to both the emplacement of the allochthon and subsequent oroclinal bending. The overburden provided by the allochthonous pile was decisive in triggering the high-temperature event. Orocline development mostly occurred later and had no significant effect on the metamorphic evolution, although it was important for the present localization of gneiss domes and granitoids. The possible role of the mantle in supplying additional heat to explain the HT/LP event is also discussed. It would seem that little mantle contribution was needed and there are no strong arguments for mantle delamination, although some kind of mantle–crust interaction is expected beneath the hot regions presently occupying the core of the Central Iberian arc.

U–Pb detrital zircon ages in synorogenic deposits of the NW Iberian Massif (Variscan belt): interplay of Devonian–Carboniferous sedimentation and thrust tectonics

Detrital zircons from Devonian and Carboniferous synorogenic flysch deposits occurring in an imbricate stack have been dated by laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS) to: (1) obtain a maximum depositional age to constrain the maximum age limit for thrusting of exotic terranes in the NW Iberian Massif; (2) correlate the zircon age populations with published ages in nearby units to establish their possible source areas. The maximum depositional ages are Late Devonian for rocks high in the structural nappe pile (Gimonde Formation), in accordance with palynomorph dating, and around the Devonian–Carboniferous boundary for structurally lower samples (San Vitero Formation). Used in conjunction with previously published ages, the new ages are interpreted in terms of the advance of the thrust system responsible for the emplacement of exotic terranes upon the Iberian autochthon during the Variscan collision. Early Variscan zircon population ages indicate the exotic terranes as the source of synorogenic sediments, whereas their scarcity suggests derivation from the Iberian autochthon. One of the samples analysed lacks Variscan detrital zircons; this feature, together with the absence of an Early Palaeozoic zircon age population, puts into question its synorogenic character and suggests that the sample may be representative of the preorogenic parautochthon.

Stratigraphic record of preorogenic to synorogenic sedimentation, and tectonic evolution of imbricate units in the Alcañices synform (northwestern Iberian Massif)

A late Variscan structure, the Alcanices synform, preserves at its core an allochthonous stack of metasediments thrust along a decollement located on Silurian carbonaceous slates. Three internally imbricated units, separated by thrust faults, were placed on top of autochthonous sequences of the northwestern Iberian Massif. The three units consist of Silurian and Devonian clastic sediments, and subordinate carbonate and volcanic rocks. Differences in the lithologic associations and facies, together with the reconstruction of their relative predeformational positions, give information on the evolution of the continental margin of Gondwana prior to and early during its collision with Laurentia. The uppermost unit forms part of a large epizonal thrust sheet that separates the Iberian autochthon from far-traveled allochthonous terranes that include ophiolites and subducted fragments of continental crust. The Silurian deposits show abundant facies changes, evidence of mass flow and gliding, synsedimentary folds, and small basic sills and volcanic plugs. This points to an extensional regime, and correlates with the prolonged evolution of the northern Gondwanan passive margin, developed during the Ordovician. Extension was succeeded by plate convergence during the Devonian, when the approaching active Laurentian margin supplied terrigenous sequences that filled the synorogenic trough and partly overlapped the passive margin. This was followed by the Variscan thrust and nappe tectonics, characterized by ductile to brittle subhorizontal shearing that placed the allochthonous complexes on top of the former Gondwana margin, while internal deformation induced crustal thickening.

Relationships between syn-orogenic sedimentation and nappe emplacement in the hinterland of the Variscan belt in NW Iberia deduced from detrital zircons

Flysch-type, syn-orogenic deposits of Carboniferous age occur in relation to the emplacement of a large allochthonous nappe stack in the Variscan belt of NW Iberia. New U–Pb age populations of detrital zircons obtained using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) are considered together with others from previously dated samples to establish the relationships between sedimentation and thrusting. The age populations of four syn-orogenic formations are compared with those of the pre-orogenic sequence in the Autochthon and Parautochthon, representing the Gondwanan passive margin, and in the Allochthon, formed by peri-Gondwanan and oceanic terranes. In addition, a new structural study has been carried out to understand the relationships between the syn-orogenic deposits and the development of Variscan structures. The aims are to identify the sources of sediments and to establish the relationship between Variscan structural evolution and syn-orogenic sedimentation. Development of a forebulge outwards from the allochthonous front, deduced from the structural study, suggests the existence of depocentres that hosted the syn-orogenic sediments. Together with the trend shown by the more recent zircons in each formation, that are younger towards the external zones, the data suggest that sedimentation occurred in progressively migrating depocentres formed in front of the allochthonous wedge during its emplacement. The zircon age populations point to the Allochthon as the main source of detritus for the syn-orogenic basins, with perhaps a limited participation of the Parautochthon and Autochthon in the younger formations.