Thrust and detachment systems in the Ordenes Complex (northwestern Spain): Implications for the Variscan-Appalachian geodynamics

Abstract

The allochthonous complexes of northwestern Iberia consist of a pile of units of Gondwanan and peri-Gondwanan provenance, and include oceanic lithosphere. The units are classiµed into upper, intermediate (ophiolitic), and basal. We present a detailed geological map and sections across the Ordenes Complex, together with a brief description of its units and a discussion of its structures.In the upper units, two complete cycles of burial and exhumation have been identiµed. The µrst cycle, of Early Ordovician age, records a convergent plate margin, possibly in a peri-Gondwanan domain. The second is Variscan, and the structural evolution of the three groups of allochthonous units re×ects progressive accretion to an active orogenic wedge. Continuous understacking of continental and oceanic fragments toward the west began with the upper units and ended with the basal units. The latter represent the outermost margin of Gondwana, and their subduction marked the closure of the intervening ocean, and the change from subduction to a collisional regime. Terrane accretion took place in the Devonian and, during the Late Devonian and the Carboniferous, the deformation progressed inboard of the Gondwana margin. Variscan emplacement of the allochthonous units occurred in two successive thrusting episodes. The µrst placed the basal units over the sedimentary cover of the Gondwana margin in what seems to follow a normal sequence of thrusting. The second carried the upper and ophiolitic units on top of the previous nappe pile and has an out-of-sequence character. A possible correlation of the Early Ordovician convergence, early Variscan accretion, subsequent oceanic closure, continent-continent collision, and renewed thrust activity during the late Carboniferous in northwestern Iberia is established with the Taconian, Acadian, and Alleghanian orogenies in the Appalachians.Figure 2. Geological map and composite cross section of northwestern Iberia, showing allochthonous complexes and their groups of units. For location, see Figure 1. IP, intermediate pressure; HP-HT, high-pressure-high temperature.

Exhuming a cold case: The early granodiorites of the northwest Iberian Variscan belt—A Visean magmatic flare-up?

“…Most models proposed for the Variscan evolution of northwest Iberia (e.g., Martínez Catalán et al, 2002Catalán et al, , 2007Ribeiro et al, 2007;Gómez Barreiro et al, 2010;Pastor-Galán et al, 2013; agree that the continental margin of Gondwana underwent westward (in present-day coordinates) directed subduction down to ~80 km (26 kbar in eclogites; Rodríguez et al, 2003;22 kbar in blueschists;López-Carmona et al, 2013, 2014 at 370-360 Ma (Rodríguez et al, 2003;Abati et al, 2010;López-Carmona et al, 2014; was followed by the exhumation of the subducted Gondwanan rocks that were subsequently overthrust onto what is currently known as the parautochthon and autochthon domains (Fig. 10B).…”

Early Variscan HP metamorphism in the western Iberian Allochthon?A 390�Ma U?Pb age for the Bragan�a eclogite (NW Portugal)

“…The geometry and kinematics of this huge Variscan nappe tectonic, described for the first time by Ribeiro et al (1964) and Anthonioz (1972), in northern Portugal, is well established in NW Spain (Arenas et al 1986;Martinez Catalan et al 2002): The autochthonous rocks which form most of the pre-Mesozoic basement of the Iberian Peninsula are essentially represented by thick sediments (>10 km) from Upper Proterozoic to Devono-Carboniferous and by orthogneisses (Ordovician granites), with east facing recumbent folds and nappes and westward increasing metamorphism (Matte 1998). The West Iberian Allochthon is preserved as synformal klippes emplaced from west to east and probably rooted in the Badajoz-Cordoba Shear Zone, 200 km to the SW. Classically, it is composed of four main units:…”

A rootless suture and the loss of the roots of a mountain chain: The Variscan belt of NW Iberia

“…Once continental snbdnction became locked, shortening began in the inner parts of the continental platform, giving rise fi rst to recnrnbent folds (Dj), and then to large thrust sheets (D2)' 7.2, Thrust propagation and loss of the suture root Three large thrust systems are responsible for the emplacement of the allochthonons complexes, First, the Lalin-Forcarei thrust (LFT) (Fig, 3f) carried the basal nnits over the lower allochthon, Then, the npper and ophiolitic nnits moved over the basal nnits and the lower allochthon, becoming strongly imbricated in a new thrnst system, which was developed ont-of-seqnence (OST) (Fig, 3g), Dnring thrust emplacement, large recnrnbent folds developed in the npper nnits [38,42], the ophiolites [14], and the basal nnits [23,44], The OST thrusts cnt and dismembered the sntnre, which was steeper [46], So, a part of it was emplaced over the lower allochthon and snbseqnently over the antochthon, whereas its deeper, inner parts were left behind, disconnected and hidden (Fig, 3g), Shortly after, the Schistose Domain was emplaced along the lower allochthon thrust (LAT) (Fig, 3g) carrying the allochthonons nnits piggy-back and nsing the weak Silnrian carbonaceons slates to detach [26,39], The LFT moved after 346 Ma, which was the age of migmatization in one of the basal nnits [2], The S2 cleavage in the nnderlying Schistose Domain, dated at 340 Ma [21], was developed dnring emplacement of the LFT, The LAT is constrained between 340 Ma and 323 ± 11 or 317 ± 15 Ma, age of crosscntting Variscan granitoids [16,55], The Mondoiiedo basal thrust (MBT) (Fig, 3h) developed snbseqnently in the antochthon, and more thrusts developed progressively toward the foreland, 7.3, Extensional collapse, removal of the orogenic roots, and late shortening Crustal thickening was followed by thermal relaxa tion cansing increase in temperatnre and partial melting, and facilitating viscons fl ow that accommodated gravitational extension of the whole crnst (Fig. 3i).…”

“…The OST thrusts cnt and dismembered the sntnre, which was steeper [46], So, a part of it was emplaced over the lower allochthon and snbseqnently over the antochthon, whereas its deeper, inner parts were left behind, disconnected and hidden (Fig, 3g), Shortly after, the Schistose Domain was emplaced along the lower allochthon thrust (LAT) (Fig, 3g) carrying the allochthonons nnits piggy-back and nsing the weak Silnrian carbonaceons slates to detach [26,39], The LFT moved after 346 Ma, which was the age of migmatization in one of the basal nnits [2], The S2 cleavage in the nnderlying Schistose Domain, dated at 340 Ma [21], was developed dnring emplacement of the LFT, The LAT is constrained between 340 Ma and 323 ± 11 or 317 ± 15 Ma, age of crosscntting Variscan granitoids [16,55], The Mondoiiedo basal thrust (MBT) (Fig, 3h) developed snbseqnently in the antochthon, and more thrusts developed progressively toward the foreland, 7.3, Extensional collapse, removal of the orogenic roots, and late shortening Crustal thickening was followed by thermal relaxa tion cansing increase in temperatnre and partial melting, and facilitating viscons fl ow that accommodated gravitational extension of the whole crnst (Fig. 3i).…”