Multidisciplinary constraints on the Cadomian compression and early Cambrian extension in the Iberian Chains, NE Spain

Tectonophysics 461 (2008) 215–227

Contents lists available at ScienceDirect

Tectonophysics j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / t e c t o

Multidisciplinary constraints on the Cadomian compression and early Cambrian extension in the Iberian Chains, NE Spain J. Javier Álvaro a,b,⁎, Blanca Bauluz a, Andrés Gil Imaz a, José Luis Simón a a b

Dpto. Ciencias de la Tierra, Universidad de Zaragoza, 50009-Zaragoza, Spain UMR 8157 CNRS, Université de Lille I, Cité Scientifique, 59655-Villeneuve d'Ascq, France

A R T I C L E

I N F O

Article history: Received 24 May 2007 Received in revised form 31 March 2008 Accepted 2 April 2008 Available online 11 April 2008 Keywords: Low-grade metamorphism Fold Cleavage Cadomian orogeny Iberia West Gondwana

A B S T R A C T This paper presents a multidisciplinary approach (stratigraphic, sedimentologic, structural, and mineralogic) to document the evidences for Cadomian and earliest-Cambrian tectonic processes recorded in the Iberian Chains, which were strongly overprinted during Variscan and Alpine deformation episodes. The contact of the Neoproterozic Paracuellos Group and the lower Cambrian Bámbola Formation is commonly identifiable by the presence of a distinct erosive unconformity related to the onset of alluvial plain sequences, or associated with synsedimentary faults. A meso-structural analysis of the deformation recorded in the Paracuellos slates reveals the existence of an earlier slaty cleavage (S1N) overprinted by Variscan contractional structures, some of them related to positive inversion of pre-Variscan normal faults. The comparative geometrical study with structures observed in the Bámbola Formation (essentially, a single, NW–SE to N–S striking cleavage S1C) is not conclusive about the record of the Cadomian orogeny. However, micro-scale deformation associated with the earliest cleavage is much more intense in the Paracuellos slates. In addition, the metamorphic grade and the pressure conditions of the lower Cambrian anchizonal shales contrast with the Neoproterozoic epizonal slates. Both observations are consistent with a Cadomian contractional deformation associated with a low-grade, intermediatepressure metamorphism, absent in the overlying rocks. Postdating both processes, stratigraphically marked by a tectono-thermal discontinuity located at the Paracuellos/Bámbola contact, the onset of the lowermost Cambrian Bámbola Formation is accompanied by strong palaeogeographic transformations in the sedimentary basin, including: (i) a sharp relative sea-level fall leading to deposition of alluvial plain sediments; and (ii) an increase in sedimentation rates and tectonically induced subsidence at fault-bounded depressions. All these changes, together with the presence of normal or transtensional faults at the base of Bámbola Formation, point to an earliest-Cambrian extensional regime similar to that described for other neighbouring post-Cadomian basins. © 2008 Elsevier B.V. All rights reserved.

1. Introduction The structure of the Cambrian intra-cratonic and passive-margin basins that bordered West Gondwana was primarily controlled by the nature of the Cadomian basement. The effects of the Cadomian orogeny are recognizable in some tectonostratigraphic units of the Iberian Peninsula, whereas in others, like the Iberian Chains, the overprinting of severe Variscan and Alpine deformation episodes and the lack of magmatic activity across the Neoproterozoic–Cambrian transition precludes a clear picture of Cadomian events. The Cadomian orogeny was firstly associated with the record of an unconformity separating the Neoproterozoic ‘Brioverian’ volcanosedimentary complex and the Cambrian molasse of the northern Armorican Massif, and dated between ca. 645 and 540 Ma (Miller et al., 1999; ⁎ Corresponding author. Dpto. Ciencias de la Tierra, Universidad de Zaragoza, 50009Zaragoza, Spain. E-mail address: [email protected] (J.J. Álvaro). 0040-1951/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2008.04.006

Samson and D'Lemos, 1999). Subsequently, the Cadomian orogeny has been extended to encompass an extensive orogenic cycle lasting ca. 700–450 Ma, recorded in a belt running from the northern Appalachians to the Iberian, Armorican and Bohemian massifs (D'Lemos et al., 1990; Quesada, 1991; Von Raumer and Neubauer, 1993; Linnemann et al., 2007). As the tectonostratigraphic subdivision (or zonation) in which the pre-Variscan relics have been subdivided directly depends on the Variscan structure, the relics of the Cadomian orogeny do not necessarily follow the Variscan subdivision. The effects of the Cadomian orogeny are recognisable in several areas of the Iberian Peninsula. In the Narcea antiform, NW Spain, the Narcea Slate Formation records a Cadomian deformation truncated by a lower Cambrian angular unconformity (Díaz García, 2006). In the Central Iberian Zone, some authors favoured also the existence of map-scale angular unconformities due to Cadomian deformation (López-Díaz, 1995; Fernández-Suárez et al., 1998; Eguíluz et al., 2000; RodríguezAlonso et al., 2004; and references therein), whereas others have

216

J.J. Álvaro et al. / Tectonophysics 461 (2008) 215–227

defined only sedimentary unconformities related to extensional events and sea-level changes (Valladares et al., 2002). The Cadomian imprint in the Ossa-Morena Zone mostly consists of widespread, though not voluminous, Neoproterozoic calc-alkaline magmatism, which has been related to subduction. A problem in Ossa-Morena relates to the timing of the switch from Neoproterozoic calc-alkaline volcanism related to compression to the middle Cambrian tholeiiticalkaline volcanism related to rifting (e.g., Bandrés et al., 2004; Salman, 2004; Simancas et al., 2004; Pereira et al., 2006). Both in the Narcea antiform and the Ossa-Morena Zone two Cadomian deformation phases have been identified. The larger and most conspicuous structures are recumbent folds with well-developed axial-plane cleavage belonging to the second phase. In the Narcea antiform, these folds are NW-vergent, transverse to Variscan structures, and developed under low-grade metamorphic conditions (Díaz García, 2006). In the Ossa-Morena Zone, folds are E–W to ESE–WNW trending, southward verging; evidence of medium- to high-grade metamorphism (both of low- and high-pressure types) do exist, although radiometric ages provide few constraints (Eguíluz et al., 2000). Finally, in the Iberian Chains (NE Spain), an inlier of Variscan basement occurs surrounded by Mesozoic and Cenozoic strata, and represents the south-eastward prolongation of the Cantabrian and West Asturian-Leonese zones (Carls, 1983; Gozalo and Liñán, 1988). The existence of a Cadomian event here had been suggested on the basis of: (i) a distinct erosive unconformity capping Neoproterozoic strata (Tejero, 1986; Liñán and Tejero, 1988; Álvaro and Blanc-Valleron, 2002); and (ii) the abundance of granules and pebbles bearing predepositional microstructures within Cambrian sedimentary rocks (Álvaro and Vennin, 1998a,b). The aim of this paper is to document and discuss the arguments for and against the existence of Cadomian compressive and early Cambrian extensive events in the Iberian Chains, based on a multidisciplinary approach using stratigraphic, sedimentologic, mineralogical, and structural arguments. This discussion will help to improve our understanding of the basement setting of the Cambrian intra-cratonic and passive-margin basins that subsequently bordered West Gondwana.

2. Structural setting The core of the Iberian Chains represents a relic of the deeply eroded Variscan orogen in NE Spain (Fig. 1A). The pre-Variscan basement consists of a mosaic of crustal elements fragmented and deformed during the Variscan and Alpine orogenies, and characterized by a dominant NW–SE trend. The late Carboniferous (postWestphalian A; Villena and Pardos, 1983) Variscan structures are attributable to, at least, three major deformational phases (Capote and González Lodeiro, 1983; Tejero, 1986; Tejero and Capote, 1987), which developed under low to very low grades of metamorphism (Bauluz et al., 1998). The first and second phases produced NW–SE-trending folds and thrusts, the former being accompanied by conspicuous and penetrative axial-plane cleavage. The third phase is characterized by WNW–ESE to E–W trending folds and associated crenulation cleavage. The Alpine compressional architecture of the Iberian Chains is strongly controlled by the superposition of the basement (Variscan, Permian and lower Triassic) and cover (Jurassic, Cretaceous, and Tertiary) structural elements. Both are separated by a regional detachment located within the upper-Triassic Keuper, which constitutes, in many areas, the sole thrust of the cover (Guimerà and Álvaro, 1990). Kinematic indicators and map relationships of pre-Variscan outcrops generally show dextral movement on NW–SE striking planes and sinistral movement on NE–SW faults. Other inherited NW–SE transpressive dextral-reverse faults, such as the Embid fault (Fig. 2A), are distinct, some of which cut across Triassic (Morés trough) and Tertiary (Calatayud–Teruel trough) strata. Compression is likely to have started by early–middle Eocene times, and reached a peak by the late Oligocene, when compressive structures of different orientations developed, especially NNW–SSE to WNW–ESE folds and thrusts, ending across the Oligocene–Miocene transition. Some thrusts were reactivated affecting middle Miocene strata, coeval with ENE–WSW trending fold axes and thrusts (Capote et al., 2002; and references therein). The Neoproterozoic Paracuellos Group is exposed within the cores of two disconnected structures: the Paracuellos antiform and the

Fig. 1. A. Pre-Hercynian outcrops of the Iberian Peninsula showing their main tectonostratigraphic units: CZ = Cantabrian Zone, WALZ = West Asturian-Leonese Zone, CIZ = Central Iberian Zone, OMZ = Ossa-Morena Zone, SPZ = South Portuguese Zone, BR = Betic Ranges, DR = Demanda Range, IC = Iberian Chains, CCC = Coastal Catalonian Chains, and P = Pyrenees; boxed area is Fig. 1B. B. Geological sketch of the pre-Variscan outcrops of the Iberian Chains (after Álvaro and Vennin, 1998a); boxed areas in Figs. 2 and 9.

J.J. Álvaro et al. / Tectonophysics 461 (2008) 215–227

217

Fig. 2. A. Geological sketch of the Paracuellos antiform (modified from Aragonés et al., 1981; Liñán and Tejero, 1988). B. Interpreted geological section of the Paracuellos antiform based on seismic profiles, boreholes, and surface mapping (modified from Álvaro and Blanc-Valleron, 2002).

Codos inlier (Fig. 1B). The structure of the Paracuellos antiform was widely described by Capote and González Lodeiro (1983), Tejero (1986), and Tejero and Capote (1987). A cross-section through its axial core, constructed from surface maps, seismic exploration and extensive drilling, was reported by Álvaro and Blanc-Valleron (2002). The Neoproterozoic basement forms a broad anticline, limited by two prominent tectonic structures (Figs. 1B and 2): (i) the eastern reverse Datos fault that flanks the Triassic Morés trough; and (ii) the western reverse Embid fault, marked in seismic profiles as a thin NEdipping reflector. The Embid fault (sensu Aragonés et al., 1981; named Paracuellos fault by Álvaro and Blanc-Valleron, 2002) is flanked to the west by, at least, four distinct Variscan thrust systems bearing Cambrian strata; their style is thin-skinned and the thrust faults end downward in a main detachment level that does not involve the cratonic basement on which the sedimentary prism was transported (Fig. 2). The Codos inlier displays a monoclinal structure of Neoproterozoic strata, bounded to the NW by a NNE–SSW striking fault of probable synsedimentary origin, which is described below. Both the Paracuellos antiform and the Codos inlier were proposed by Gozalo and Liñán (1988) as the south-eastward prolongation of the aforementioned Narcea antiform, separating two Iberian flanks with different Variscan styles of deformation. 3. Stratigraphy of the Neoproterozoic–Cambrian transition 3.1. Neoproterozoic Paracuellos Group The core of the Paracuellos and Codos structures consists of a slatedominated unit, the Paracuellos Group (Lotze, 1956). The group lacks

coeval igneous rocks and a Neoproterozoic age is suggested due to the presence of the ichnospecies Torrowangea aff. rosei Webby, 1970 (Liñán and Tejero, 1988) and Cloudina-like skeletonized microfossils (Streng, 1996; Álvaro and Blanc-Valleron, 2002). The Paracuellos Group has been subdivided into three formations, in ascending order, the Sestrica, Saviñán, and Aluenda formations (Liñán and Tejero, 1988). The two youngest formations are separated by the Frasno chert bed in the Paracuellos antiform and the Codos dolostone bed in the Codos inlier (Álvaro and Blanc-Valleron, 2002; Fig. 3). The Sestrica Formation has been identified both in subsurface and exposure. Its subsurface part, drilled in the western limb of the Paracuellos antiform, is more than 100 m thick and consists of darkgrey to black, thinly laminated slates with common intercalations of either thin- to medium-bedded, very fine- to medium-grained feldspathic metagreywackes, or laterally discontinuous beds of coarse-grained metasiltstone. Dark colour is associated with the presence of finely dispersed iron sulfides (mainly pyrite). The upper part of the Sestrica Formation, up to 60 m thick, consists of centimetre- to decimetre-thick alternations of dominant, grey to green slates and fine-grained metarenites. These are predominantly metagreywackes, which petrographically contain quartzofeldspathic grains set in a matrix rich in chlorite and fine-grained muscovite (sericite). The Saviñán Formation, 200 to 300 m thick, consists of alternating grey to brownish, fine- to medium-grained subfeldspathic metagreywackes and green slates. Clay minerals in the sandstone matrix and slaty intercalations are fine-grained muscovite (sericite) and chlorite. The Frasno chert bed, up to 4 m thick, consists of an alternation of grey to black, bedded chert and grey to green slates. Alternating silicified and slaty laminae, 1–2 cm thick, are abundant at the bottom

218

J.J. Álvaro et al. / Tectonophysics 461 (2008) 215–227

Fig. 3. Summarized stratigraphic and palaeoenvironmental framework of the Neoproterozoic and lowermost Cambrian strata from the Iberian Chains (modified from Álvaro and Vennin, 2001).

of the formation, which passes gradually into the underlying Saviñán Formation, whereas the top of the Frasno chert bed is sharp. The chert is traceable along strike for several tens of metres along the SE edge of the Paracuellos antiform (Liñán and Tejero, 1988), and is sharply underlain by Stephanian–Permian rhyolitic sills (Lago et al., 1992). The mineralogy of this chert, determined by X-ray diffraction and thinsection observations, is quartz, muscovite, albite and chlorite, in decreasing order of abundance. Silicification was seemingly sourced from the Variscan rhyolitic sills (Álvaro and Blanc-Valleron, 2002). The Codos dolostone bed occurs at the base of the Codos inlier, and consists of a dark-grey phosphatic dolostone, up to 4 m thick. Sedimentary structures include dominant low-angle cross-beds, and subsidiary ripple cross-laminae, low-angle and parallel lamination. The dolomitized ooidal packstones occur as bed and lenticular sets, less than 60 cm thick. They consist of moderate-sorted, fine to medium sand-sized ooids and small amounts of skeletal debris (such as Cloudina-like microfossils), commonly flattened and distorted by compaction. The terrigenous component mainly consists of sand-sized, subrounded, monocrystalline quartz, mica flakes and rock fragments. Both phosphate grains and bioclasts, in the present form of fluorapatite, reach 10% of the whole rock. Dolomitization of allochems and matrix (microsparite) is nearly complete (Álvaro and Blanc-Valleron, 2002). The Aluenda Formation, which has yielded the trace fossil Torrowangea aff. rosei Webby, 1970, consists of alternating grey to white, feldspathic metagreywackes and brownish to green slates (90–200 m thick). 3.2. Lower Cambrian Bámbola and Embid formations The Paracuellos/Bámbola contact is commonly marked by an erosive unconformity underlying the lowermost conglomerate and breccia channels of the Bámbola Formation. The Bámbola Formation, 200–400 m thick, can be subdivided into three units or members, in ascending order: (i) quartzitic conglomerates and breccias bearing dispersed granules and pebbles, up to 100 m thick, and arranged in amalgamated metre-thick channels, lenses, and sheets; (ii) quartzite lenticular beds, up to 60 m thick, episodically interrupted by

subsidiary centimetre-thick shale interbeds; and (iii) alternating centimetre- to decimetre-thick, green shales and quartzite tabular and lenticular beds, 100 to 300 m thick (Fig. 3). The base of the upper unit is marked by the occurrence of burrowed quartzites yielding Skolithos, Arenicolites, and Diplocraterion; Valenzuela et al. (1990) and Liñán et al. (1993) reported the presence of Phycodes, Bergaueria and Treptichnus in the shale interbeds of the uppermost part, an ichnofossil assemblage considered as Cambrian in age. The lower member displays sharp changes in facies and thickness: e.g. in the western limb of the Paracuellos antiform, it is possible to recognize the presence of conglomerate and breccia channels either directly eroding the Neoproterozoic Aluenda shales or separated by a transitional arenite/shale alternation, which would reflect a gradational transition between both lithostratigraphic units. The amalgamated channel system changes from 4 to 100 m in thickness in less than 1 km. Although the contact with the underlying Aluenda Formation can be somewhat gradational (Teyssen, 1980; Tejero, 1986), the overlying sandstone/shale alternations are truncated upward by erosive channels or cut by synsedimentary faults. The Embid Formation (Fig. 3), 200–400 m thick, consists of alternating sandstones and green shales with rare conglomerates. Its base is marked by the disappearance of white quartzites, and a progressive change from quartz arenites to arkoses with a matrix dominated by sericite. The formation has yielded a diversified ichnofossil assemblage, including Astropolithon, Psammichnites, and Rusophycus, which belong to the Corduban stage (earliest-Cambrian stage defined for the Iberian Peninsula; Álvaro et al., 1993; Liñán et al., 1993). 3.3. Neoproterozoic–early Cambrian sedimentary evolution The late Neoproterozoic sedimentary evolution of the Iberian platform displays a large-scale, composite, regressive–transgressive depositional sequence recorded in a passive margin (Álvaro BlancValleron, 2002). A first regression ranges from offshore-hemipelagic, black and green shales (Sestrica Formation) to progradational shoaling trends recorded during episodes of rapid sediment influx (Saviñán

J.J. Álvaro et al. / Tectonophysics 461 (2008) 215–227

219

marine conditions and the re-appearance of marine trace fossils (Liñán and Tejero, 1988; Gabaldón, 1990; Álvaro and Vennin, 2001). 4. Meso- and micro-scale tectonic structures The structures affecting the materials below and above the Paracuellos/Bámbola contact were the target of a specific study with the purpose of searching for possible evidences of pre-Variscan compressive and extensive structures. Several high-quality outcrops were studied in the vicinity of Sestrica (Fig. 2A), at the northwest edge of the Paracuellos antiform, where both the Paracuellos Group and the Bámbola Formation are well exposed showing a variety of meso- and micro-scale structures. Fig. 4A shows the location of three selected sites, while the outcrop sketches and stereoplots of Figs. 5–8 show the geometry and orientation of the observed structures. Another interesting outcrop neighbouring the Datos fault was studied in the Codos inlier (Figs. 1B and 9) 4.1. Compressive structures

Fig. 4. A. Geological sketch of the Sestrica area with location of the studied mesostructural sites. B. Schematic cross-section of the Paracuellos antiform.

Formation), presumably in response to a low standing sea-level. This regressive siliciclastic succession was locally punctuated by deposition of phosphatic carbonates (Codos dolostone bed), rich in ooids and skeletonized microfossils, representing a major shoaling event and demarcating a short-time development of carbonate productivity. Diagenetically induced bedded cherts (Frasno chert bed) occur in the Paracuellos antiform, and are interpreted as being the product of Variscan silicification episodes. Both the Codos and Frasno beds are overlain by the transgressive shales of the Neoproterozoic Aluenda Formation, the latter dominated by offshore features (Álvaro and Blanc-Valleron, 2002). The Neoproterozoic Paracuellos Group is unconformably overlain by an earliest-Cambrian transgressive depositional sequence, which comprises the Bámbola and lower part of the Embid formations (Gabaldón, 1990; Álvaro et al., 1993). The two first units of the Bámbola Formation reflect a sharp change in the sedimentary evolution of the Iberian platform, with deposition of alluvial wedges and fluvial deposits. Thick alluvial fan sequences have not been recognized at the lower part of the Bámbola Formation, so that these deposits fit better with the development of alluvial plain sequences. Evidence of syntectonic deposition occurs in the form of rapid lateral thickness variations, wedge-like down-dip geometry of amalgamated channel systems, and the high diversity of grain size and clast angularity shown by the alluvial conglomerates and breccias, derived from highrelief areas and deposited on fault-bounded blocks. The Bámbola– Embid transition reflects the onset of a transgressive delta front and a wave-dominated open-sea platform marked by the recovery of

The Neoproterozoic Paracuellos slates exhibit a conspicuous and penetrative slaty cleavage (S1N) homogeneously developed in the whole study area (Fig. 4B). At a microscopic scale, it is defined by submillimetrescale bands of recrystalized phyllosilicates alternating with quartz-rich bands, the latter showing a minor degree of preferred orientation (Fig. 6A). Following the morphological classification by Powell (1979), it can be described as a zonal cleavage. The presence of large, elongated quartz grains showing (i) undulose extinction, (ii) subgrains with preferred orientation, and (iii) related pressure shadows, surrounded by aggregates of small elongated grains of uniform size, strongly suggests very intense internal deformation accompanied by dynamic recrystallization. The S1N planes usually dip gently and are nearly parallel to bedding, which is locally manifested by a compositional layering. However, a slight obliquity between them has been observed in a few cases, both at mesoscopic and microscopic scale, which allows us to exclude a diagenetic origin for this foliation. In addition, the presence of microfolded quartz veins with axial traces parallel to the S1N cleavage unequivocally demonstrates its tectonic origin (Fig. 6B). Folds related to such a cleavage have not been observed, either at meso- or macro-scale. Nevertheless, a NW–SE-trending intersection lineation between the average S0 and S1N planes at site E1 (Fig. 5C) suggests the existence of hypothetic earlier folds (F1N) with such an orientation. At site E1, S1N is overprinted by both later metre-scale folds (F2N) and related axial-plane crenulation cleavage (S2N) at a millimetre- to submillimetre-scale (Fig. 5E,H). Microscopic observation shows symmetrical, extremely closed microfolding of S1N surfaces (Fig. 6C) with a closely spaced (102 µm) axial-plane foliation. The presence of dark seams parallel to the limbs indicates that their development was accompanied by pressure solution. Mesoscopic fold axes and crenulation lineations L2N are locally parallel to each other, but show diverse trends in non-homogeneous structural domains within the outcrop: an E–W trend at the eastern part (Fig. 5C), and ENE–WSW and NNE–SSW trends at the western part (Fig. 5A) (in the footwall and the hanging wall of fault F1, respectively). The bimodal distribution observed at the western sector represents a true interference of two separate crenulations, which is clearly visible at a centimetre-scale. At sites E2 and E3 (Figs. 7 and 8), the style of structures overprinting S1N reveals deformation developed at shallower crustal levels. Post-S1N folds do not show axial-plane crenulation cleavage. They are buckle, locally detached folds (F3N) trending NW–SE at site E2 (Fig. 7B,C) and NE–SW at site E3 (Fig. 8C). At site E2, S1N is also affected by conjugate kink-bands at a millimetre- to centimetre-scale (Fig. 7E). The kink-band system and F3N folds show the same movement plane (Fig. 7B). Orientation, vergence and distribution of this structural assemblage are compatible with a contractional Variscan deformation associated with fault F2 (Fig. 7D). Actually, this fault shows a normal

220

J.J. Álvaro et al. / Tectonophysics 461 (2008) 215–227

Fig. 5. Mesostructures at site E1. A. Stereoplot (equal area projection, lower hemisphere) of structural elements in the Paracuellos Group at the hanging wall of fault F1. B. Idem in the Bámbola Formation at the footwall of F1. C. Idem in the Paracuellos Group at the footwall of F1; directional data plotted using Stereonet 4.9.5 (Allmendinger, 2004). D. Geological sketch of site E1; abbreviations correspond to those used in the stereoplot legend. E. Detail of a post-S1N fold. F. Field view of the erosive unconformity between the Bámbola Formation (up) and the Paracuellos Group (down); bedding trace (S0) and traces of cleavages S1N and S1C are indicated. G. Detail of S1C in conglomerates of the Bámbola Formation. H. Detailed photograph and sketch of crenulation lineation L2N in the Paracuellos Group.

displacement. Nevertheless, it is near orthogonal to the aforementioned movement plane, so we consider the contractional deformation to have resulted from buttressing related to incipient positive inversion of the fault that did not fully counteract a previous normal offset. The quartzitic conglomerates and quartzites of the Bámbola Formation show a more simple tectonic fabric (S1C) determined by two different elements: (i) one or two closely oriented sets of discrete surfaces with centimetre-scale spacing; and (ii) preferred orientation

of flattened clasts (Fig. 5G). Planes of S1C dip steeply and trend close to N–S in all the studied outcrops, with minor deviations to either NNW or NNE (Figs. 5B, 7A and 8B). Within some thin pelitic layers interbedded with quartzites at site E3, the dominant fabric takes the appearance of a slaty cleavage (S1C') showing a reverse fan pattern associated with small NW–SE-trending cuspate-lobate folds (Fig. 8A). The same NW–SE trend also characterizes the folds that affect the rest of Cambrian and Ordovician units exposed in the surrounding area, as was mentioned in Section 2.

J.J. Álvaro et al. / Tectonophysics 461 (2008) 215–227

221

Fig. 6. Photomicrographs of cleavage in the Neoproterozoic slates at site E1. A. Cleavage S1N parallel to bedding S0 in a highly deformed metasiltstone. Note the intense recrystallisation of phyllosilicates and the elongated quartz subgrains linked to dynamic recrystallisation. B. Detail of a microfolded quartz vein (dashed line) with axial traces parallel to cleavage S1N. C. Aspect of highly penetrative crenulation cleavage S2N.

In summary, a succession of three main deformation episodes is recorded in the Paracuellos slates of the study area: (i) highly penetrative slaty cleavage S1N; (ii) penetrative crenulation cleavage S2N related to E–W and NE–SW-trending folds F2N developed at a deep structural level; and (iii) flexural folding and kinking related to Variscan inversion of normal faults at a shallower structural level (as interpreted in site E2). This complex structural pattern contrasts with the single, non-refolded cleavage found in neighbouring exposures of Cambrian strata, everywhere striking N–S except in the Bámbola quartzites where it strikes nearly N–S. As a first approach, the greater number of superposed meso- and microstructure sets in the Paracuellos Group with respect to Cambrian units may suggest the structures are partly the result of a Cadomian compressive event. Nevertheless, this could be as well a consequence of the high competence of the Bámbola quartzites, which sharply diminishes their ability for recording micro-scale deformation episodes. In fact, other younger slate units within the Palaeozoic series of the West Asturian-Leonese Zone show deformation patterns as complex as those found in the Paracuellos Group (e.g. Pulgar, 1980; Martínez-Catalán, 1985; Simón, 1986; Tejero, 1986; Tejero and Capote, 1987).

The results of the mesostructural analysis in the Sestrica area are not conclusive, on its own, about the occurrence of a Cadomian folding pre-dating the deposition of the early Cambrian Bámbola Formation. Evidences about a hypothetical genetic relationship of S1C with either S1N or S S2N are contradictory: (i) A nearly horizontal, NW–SE to NNW–SSE-trending fold axis (the usual orientation of large Variscan folds in the region) could be compatible with S1N and S1C cleavages, since both the S0/S1C and the S0/S1N intersection lineations at site E1 are very close to such orientation (Fig. 5B,C). (ii) In contrast, the extremely gentle dip of S1N at site E1 does not fit the apparent axial-plane of the Variscan macro-scale fold represented in Fig. 4B. The overprinting Variscan multiphase and pluri-mechanism deformation makes difficult to ascertain whether S1N is related to such fold. For example, S1N could be linked to an earlier Variscan recumbent fold with an anticline hinge located eastward of Sestrica, then refolded by the visible upright anticline. Furthermore, the dip of S1N could have been modified by shearing or dragging (with a top-to-NE sense) produced by detachment along the Paracuellos/Bámbola

222

J.J. Álvaro et al. / Tectonophysics 461 (2008) 215–227

Fig. 7. Mesostructures at site E2. A. Stereoplot (equal area projection, lower hemisphere) of structural elements in the Bámbola Formation. B. Idem in the Paracuellos Group. C. Geological sketch of site E2. D. Detail of conjugate kink-bands in parasiltstones of the Paracuellos Group; symbols and abbreviations from Fig. 5.

contact, a deformation mechanism feasible owing to their high competence contrast. 4.2. Extensional structures In addition to the described compressive structures, meso-scale deformation recorded across the Paracuellos/Bámbola transition also suggests the occurrence of an extensional or transtensional tectonic activity, partly coeval with the early Cambrian sedimentation. In the Sestrica area, we have mentioned the existence of high-angle faults cutting the contact. One of them (site E2, Fig. 7C) has been interpreted as an originally normal fault subsequently inverted during the Variscan compression. A larger extensional fault has been identified at the Paracuellos/ Bámbola boundary in the Codos inlier (Fig. 9A). Some 0.5 km to the East of Codos, a high-angle, NNE–SSW striking fault bounds the Neoproterozoic outcrop and offsets the base of the early Cambrian Bámbola Formation with a minimum normal dip separation of 50– 60 m. A normal–sinistral movement has been inferred for this fault on the basis of oblique striations exhibited by minor planes within the fault zone (Fig. 9B). An unusual, more than 80 m thick sequence of coarse-grained (granule to cobble) quartz conglomerates and breccias,

arranged as amalgamated metre-scale channels, appears in the hanging wall (Fig. 9C). These show a sharp increase in grain size as approaching the fault, the angular clasts reaching in some cases up to 30 cm in size (Fig. 9D). The thickness of this sequence is reduced in the footwall, where the base of the Bámbola Formation only contains some 20–30 m of quartz conglomerates overlying a relatively sandstone-rich Neoproterozoic Aluenda Formation. This setting strongly suggests synsedimentary activity of the Codos fault, the tectonically induced subsidence being responsible for sharp differences in thickness and facies within the lower unit of the Bámbola Formation. As in the Sestrica area, attitudes of bedding and cleavage in the Neoproterozoic beds of the Codos inlier are compatible with Variscan folding (Fig. 9B). Both S0 and S1N tend to become nearly parallel to the fault in its vicinity, following a reverse-drag pattern. This is not consistent with a hypothetic post-Variscan normal faulting; on the contrary, it suggests, as in site E2, the occurrence of contractional Variscan deformation conditioned by a pre-existing normal fault. 5. Metamorphism Low-grade metamorphic processes give rise to rocks characterized by a lack of chemical and textural equilibrium and minerals with a

J.J. Álvaro et al. / Tectonophysics 461 (2008) 215–227

Fig. 8. Mesostructures at site E3. A. Field view of cuspate–lobate folds affecting interfaces between quartzite (Q) and thin interbedded lutite (L) within the Bámbola Formation. B. Stereoplot (equal area projection, lower hemisphere) of structural elements in the Bámbola Formation. C. Idem in the Paracuellos Group; symbols and abbreviations from Fig. 5.

very small grain size that currently cannot be recognized under light microscopy. Clastic lithologies are commonly characterized by the absence of changes in the mineral paragenesis, making difficult or impossible to apply the criteria normally used for higher grades, based on the petrogenetic grid of mineral association equilibria or on true geothermometers or geobarometers. Micas are key minerals to characterize very low and low-grade metamorphic processes involved from diagenesis to regional deformation. Because of the sensitivity of the crystal–chemical parameters of micas to temperature, pressure and deformation changes, they can be traced along complex, multiphase deformation events (see e.g., Frey et al., 1980; Johnson and Oliver, 1990; Gutiérrez-Alonso and Nieto, 1996; Abad et al., 2003). The mineralogical characterization of the Neoproterozoic Paracuellos slates (n = 21) was determined by X-ray diffraction (XRD), following Bauluz et al.'s (1998) method, and by transmission electron microscopy (TEM). The crystallinity index of the mica was measured in the fraction b2 μm, as recommended by the IGCP 294 IC Working Group (Kisch, 1991), and the obtained values were transformed using the interlaboratory standards (C.I.S.) provided by Warr and Rice (1994). This standardization allows us to consider as anchizonal limits the following values: 0.42–0.25° 2θ. The b0 cell parameter of the micas, a semi-quantitative geobarometer, was also determined by XRD. The texture of the phyllosilicates in the slates was characterized using a transmission electron microscopy (TEM), JEOL-2000F, with an Oxford EDS detector. Electron-transparent sections were prepared with surfaces normal to slaty cleavage and initially examined by optical microscopy. The chemical composition of the physllosilicates was also obtained by TEM. K-factors were determined analysing albite, sodalite, biotite,

223

muscovite, wollastonite, and benitoite standards (Cliff and Lorimer, 1975). The analytical values displayed a margin of error of ~ 5–7%. The slates of the Neoproterozoic Sestrica Formation are rich in micas (muscovite) and chlorites (chamosite). Larger phyllosilicates display orientations parallel to subparallel to the slaty cleavage direction. The mica crystallinity values range from 0.21 to 0.30° 2θ. Despite these variations, the 0.23–0.25° values are common, implying an epizonal grade (Fig. 10). In contrast, the crystallinity values determined in the lower Cambrian shales of the Bámbola and Embid formations (Bauluz et al., 1998; new analyses of the lowermost Bámbola shale interbeds have been undertaken) are larger (lower crystallinity) and more heterogeneous, indicating a lower metamorphic grade, typical of the anchizone. The larger heterogeneity of the crystallinity indexes reported in the lower Cambrian shales probably indicates a mixture of detrital and newly formed phases. When the grade increases, like in the case of the Sestrica slates, the phyllosilicates not only display higher crystallinity indexes, but also more homogenous values. The b0 cell parameter of the micas from the Neoproterozoic Sestrica Formation is, on average, a value of 9.020 Å (±0.004). This parameter, under some circumstances, may reflect averaged values between detrital and recrystalllized white micas. However, the low standard deviation yielded by the values of the Sestrica slates, by comparison with those determined for the micas in lower Cambrian shales (Bauluz et al., 1998), supports a significant metamorphic reequilibrium of the micas under epizonal conditions. According to Guidotti and Sassi (1986), the values obtained (9.020 Å) indicate intermediate-pressure conditions, although, as pointed out by the same authors, in some cases the b0 parameter may be influenced by rock composition. TEM images obtained from the Neoproterozoic slates show both packets of well-crystallized mica (Fig. 11A,D) and parallel to subparallel packets of strain-free micas. High-resolution TEM images (HRTEM) illustrate that mica crystals consist of straight lattice fringes with uniform contrast; the fringes are continuous over hundred of angstroms (Fig. 11B). Selected area electron diffraction (SAED) patterns show well-defined reflections, periodic non-00l reflections with a 20 Å periodicity corresponding to a two-layer polytype, probably 2M1 (Fig.11C). The images also show quartz grains, characterized by distinct contrast and elongated shape indicative of tectonic stress-induced deformation. Low-resolution TEM images display variations in mica crystal thickness (Fig. 11B–D); the most common size ranges from 35 to 55 nm (Fig. 10B). These values are clearly higher than those reported for the micaceous phases documented in the lower Cambrian shales (20–30 nm in Bauluz et al., 2000). The chemical analyses of micas from the Neproterozoic slates indicate that they are dioctahedral K-micas with a phengite component. On average, they show the following formula: (Si3.13Al0.87) (Al1.68Fe0.16 Mg0.22Ti0.01) O10 (OH)2 K1.03. In contrast, Bauluz et al. (1998) reported that the phyllosilicates of the lower Cambrian shales are a mixture of illites and micas. This is in agreement with the lower metamorphic degree inferred by XRD. As stated by Bauluz et al. (2000), the lower Palaeozoic shales of the Iberian Chains display a gradual increase in the aforementioned crystallinity index, reaching an anchizonal peak in the lowermost Cambrian. This could be achieved by increasing lithostatic pressure and temperature, related to ca. 7 km of lower Palaeozoic materials and temperatures close to 250 °C (Frey, 1987). By contrast, the epizonal grade reached by the slates of the Paracuellos Group cannot be related to deep burial, and another factor, such as tectonic stress and/or rapid thermal anomalies, is necessary to accelerate the clay mineral reactions at these low grades (Merriman and Peacor, 1999). Thus, both processes may be invoked: the metamorphic grade was related either to tectonic stress accelerating mica crystal growth (Merriman and Peacor, 1999), or to a short-term thermal flux associated with extensional settings and crustal thinning (Robinson and Bevins, 1989). However, the K-white mica b0 cell dimensions in extensional setting

224

J.J. Álvaro et al. / Tectonophysics 461 (2008) 215–227

Fig. 9. A. Field view of the Codos fault showing the tectonic contact between the channelized conglomerates and breccias of the Bámbola Formation, and the interbedded metagreywackes and slates of the Aluenda Formation (see boxed areas of next figures). B. Stereoplot (equal area projection, lower hemisphere) of the structural and palaeocurrent elements. C. Subrounded clasts, up to 4 cm in size, filling the distal low-relief conglomeratic channels of the Bámbola Formation. D. Angular clasts, up to 30 cm across, filling the proximal high-relief breccia channels of the Bámbola Formation.

are smaller (8.98–9.01 Å) than the values reported in the epizonal Paracuellos slates, which fit better with intermediate-pressure facies (Guidotti and Sassi, 1986). In summary, we suggest that the microtexture of the phyllosilicates and the crystallochemical parameters of Neoproterozoic slates and lower Cambrian shales reflect a metamorphic discontinuity across the Paracuellos/Bámbola transition, which is compatible with compressive tectonics under intermediate-pressure conditions. 6. Discussion: evidences for Cadomian compressive and earliest-Cambrian extensional tectonics As previously mentioned, both compressive and extensional tectonic structures are recorded in the Neoproterozoic and lowermost Cambrian strata of the Iberian Chains. Some of them can be reasonably attributed to the Cadomian orogeny and to an earliest-Cambrian rifting episode, respectively, both well known at a large regional scale (e.g., FernándezSuárez et al., 2000; Linnemann et al., 2007; and references therein). The occurrence of Cadomian compression is based on two main arguments. The first one deals with the magnitude of strain and deformation mechanisms inferred from microfabric of the earliest cleavage affecting the Paracuellos slates: very intense recrystallization of phyllosilicates and the presence of elongate quartz subgrains related to dynamic recrystallization. These features have not been

described in the Cambrian-to-Carboniferous rocks that crop out in the Variscan massifs of the Iberian Chains (Tejero, 1986; Tejero and Capote, 1987). The second one is linked to the metamorphic conditions inferred from the microtexture and crystallochemical parameters of the S1N-related Paracuellos phyllosilicates, which sharply disappear at the Paracuellos/Bámbola contact. The metamorphic grade of the Neoproterozoic slates is clearly higher than that of the lower Cambrian shales (epizonal vs. anchizonal). According to the b 0 cell parameter of micas in Neoproterozoic slates, the epizonal metamorphism took place under intermediate-pressure conditions, which allows us to reject the alternative hypothesis of a metamorphic process induced by crustal extension, as that found in some intraplate domains (Ter Voorde and Bertotti, 1994; Mata et al., 2001). Low-grade, intermediate-pressure metamorphic conditions probably provided an adequate deformational environment for the mechanism of dynamic recrystallization to progress giving rise to great finite strain. As a result, the microfabric and metamorphic features exhibited by the Paracuellos slates document a sharp tectono-thermal discontinuity that pre-dated the erosive unconformity that underlies the Bámbola Formation. Such tectono-thermal event is stratigraphically associated with the Paracuellos/Bámbola contact. The main evidences of extensional tectonics recorded in the lower unit of the Bámbola Formation refer to: (i) sharp differences in

J.J. Álvaro et al. / Tectonophysics 461 (2008) 215–227

225

reliable model. In other areas of the Iberian Peninsula, as in other regions of the Avalonian/Cadomian belt (Murphy and Nance, 1991), the scarcity or even absence of deformational, volcanic, and metamorphic indicators related to the Cadomian orogeny suggest that the termination of the subduction event was not due to final continent–continent collision. Murphy and Nance (1989), Vidal et al. (1994) and Fernández-Suárez et al. (2000) proposed that the Cadomian subduction was followed by extensional activity, which eventually could have included transcurrent components. As a result, broad shallow intracratonic and passive margin, epeiric seas occupied extensive areas of West Gondwana. Both in Morocco (Gasquet et al., 2005) and Ossa-Morena (Quesada, 1991; Sánchez-García et al., 2003; Pereira et al., 2006), a phase of intra-continental extension took place, which led to development of multi-stage rifting that reflects the diachronous opening (latest Neoproterozoic to late Cambrian in the former, and continuing into the Ordovician in the latter) of finally aborted rifts. In other neighbouring areas, i.e. in Central Iberia, extensional tectonics were related to differential block subsidence and syn-sedimentary deformation (Ortega et al., 1988).

Fig. 10. A. Mica crystallinity (° 2θ) determined by XRD in the b 2 μm fraction of the Sestrica slates; data from the lower Cambrian Embid shales, given for comparison, are taken from Bauluz et al. (1998) along with new analyses of the lowermost Bambola shale. B. Mica thickness distribution (in nanometers) measured on TEM images obtained from the Sestrica slates.

thickness and facies exhibited by this unit, similar to that found in other block-faulted sedimentary platforms; and (ii) observation of normal or transtensional, synsedimentary faults associated with the former. Such an extensional event in the Iberian platform, similar to that described in other basins post-dating the Cadomian orogeny (Vidal et al., 1994), would be able to explain the new topographic setting at the beginning of Cambrian times, with slopes created by fault scarps and tilted blocks controlling the accommodation space for sediments of the Bámbola Formation. This new scenario agrees with the strong sedimentary changes occurred in the Iberian platform during the Neoproterozoic–Cambrian transition (Álvaro and BlancValleron, 2002): (i) sharp increase of grain size from the metarenites and slates of the Neoproterozoic Paracuellos Group to the early Cambrian conglomerates and breccias of the Bámbola Formation; and (ii) dramatic relative sea-level fall, leading to deposition of alluvial plain sediments overlying an offshore-dominated succession. The complex assemblage of folds and cleavages observed in the Neoproterozoic materials, though sharply contrasting with those found in Cambrian units, is not strictly incompatible with Variscan deformation on the basis of their geometric relationships. If we had found any positive evidence of overprinting of the earliest cleavage S1N by the early Cambrian normal faults, the complete meso-structural sequence would have been demonstrated: Cadomian cleavage, early Cambrian extensional faulting, and Variscan folding and cleavage. Unfortunately, this is not the case: all the observed structures overprinting S1N are genetically linked to folding or fault inversion of Variscan age. Therefore, the results of our geometric and kinematic analysis of meso-structures do not provide, on their own, conclusive evidences for a Cadomian contractional deformation. The inferred compressive and extensional events, although not completely characterized in their geometric, kinematic and dynamic aspects, fit well with a regional tectonic framework across the Neoproterozoic–Cambrian transition. A tectonic sequence of orogenic compression followed by moderate extension represents the most

Fig. 11. TEM images of the Sestrica slates. A. Representative low-resolution TEM image showing parallel to subparallel mica crystals. B. Lattice-fringe images of a segment of a mica crystal. C. SAED pattern of a 2-layer polytype mica crystal. D. Low-resolution TEM image showing intergrowths of mica crystals and quartz grains.

226

J.J. Álvaro et al. / Tectonophysics 461 (2008) 215–227

7. Conclusions The presence or absence of Cadomian deformations in the Iberian Chains has been a recurrent matter of debate since the identification of the Neoproterozoic Paracuellos Group fifty years ago. We have tried to characterize the tectonic setting of the Neoproterozoic–Cambrian transition by means of a multidisciplinary approach which pays attention to: (i) stratigraphy and sedimentary evolution in the Iberian platform; (ii) key structures (both in the ductile and the brittle domain) affecting Neoproterozoic materials and overlain by lower Cambrian strata, including conventional structural analysis; and (iii) significant changes in the P–T conditions of the metamorphism. The Paracuellos/Bámbola contact, which broadly coincides with the Neoproterozoic–Cambrian boundary interval, represents a sharp tectono-thermal discontinuity. This is characterized by: (i) a sharp change in the sedimentary evolution of the Iberian platform, marked by a severe regression that culminated with the record of earliestCambrian alluvial sedimentary conditions; (ii) a change in style of the micro-scale deformation associated with an early penetrative cleavage which is more intense in the Paracuellos slates (S1N); (iii) a discontinuity in the metamorphic evolution from Neoproterozoic epizonal to lowermost Cambrian anchizonal, intermediate-pressure metamorphic rocks; and (v) the onset of earliest-Cambrian synsedimentary extensional or transtensional faulting. The fabric style of the Paracuellos slates and the metamorphic discontinuity support the onset of a tectono-thermal event related to orogenic compression pre-dating the deposition of the Bámbola Formation. The record of extensional tectonics at the beginning of the early Cambrian is also evident. A compressive-extensional sequence agrees with previous models that suggest that the termination of the Cadomian subduction was not due to continent–continent collision throughout West Gondwana, and was followed by extensionaltranscurrent activity, responsible for the breakdown of platforms, with tectonically induced subsidence and concomitant increase in sedimentation rates at fault-bounded depressions. Acknowledgements The authors thank the critical revision by two anonymous referees and the associated editor. This paper is a contribution to IGCP project 497 “The Rheic Ocean: its origin, evolution and correlatives” and French projects ECLIPSE and ANR. Financial support has been obtained from the regional government of Aragón (Geotransfer and Recursos Minerales research groups) and project CGL 2006-13533 of the Spanish Ministerio de Educación y Ciencia and FEDER. We appreciate the cooperation of T. Arrufat, V. Hernández, L. Lafuente, L. Palazón, J. Parrilla, P. Prado, and A. Serrat in field surveys. References Abad, I., Gutiérrez-Alonso, G., Nieto, F., Gertner, I., Becker, A., Cabero, A., 2003. The structure and the phyllosilicates (chemistry, crystallinity and texture) of Talas AlaTau (Tien Shan, Kyrgyz Republic): comparison with more recent subduction complexes. Tectonophysics 365, 103–127. Allmendinger, R.W., 2004. Stereonet for Macintosh. Stereonet for Windows. http:// www.geo.cornell.edu/geology/faculty/RWA/maintext.html. Álvaro, J.J., Blanc-Valleron, M.M., 2002. Stratigraphic and structural framework of the Neoproterozoic Paracuellos Group, Iberian Chains, NE Spain. Bull. Soc. Géol. Fr. 173, 27–35. Álvaro, J.J., Vennin, E., 1998a. Stratigraphic signature of a terminal Early Cambrian regressive event in the Iberian Peninsula. Can. J. Earth Sci. 35, 402–411. Álvaro, J.J., Vennin, E., 1998b. Petrografía y diagénesis de las calizas cámbricas del Grupo Mesones (Cadenas Ibéricas, NE de España). Bol. R. Soc. Esp. Hist. Nat. (Sec. Geol.) 93, 33–53. Álvaro, J.J., Vennin, E., 2001. Benthic marine communities recorded in the Cambrian Iberian platform, NE Spain. Palaeontographica, Abt. A 262, 1–23. Álvaro, J.J., Liñán, E., Gozalo, R., Gámez, J.A., 1993. Estratigrafía del tránsito CordubienseOvetiense (Cámbrico Inferior) en la Cadena Ibérica Occidental (España). Cuad. Lab. Xeol. Laxe 18, 147–162. Aragonés, E., Hernández, A., Aguilar, M., Ramírez, J., 1981. Mapa Geológico de España. E. 1:50.000. Calatayud (no. 409). IGME ed., Madrid, 44 p.

Bandrés, A., Eguíluz, L., Pin, C., Paquette, J.L., Ordóñez, B., Le Fèvre, B., Ortega, L.A., Gil Ibarguchi, J.I., 2004. The northern Ossa-Morena Cadomian batholith (Iberian Massif): magmatic arc origin and early evolution. Int. J. Earth Sci. 93, 860–885. Bauluz, B., Fernández Nieto, C., González López, J.M., 1998. Diagenesis—very low grade metamorphism of clastic Cambrian and Ordovician sedimentary rocks in the Iberian Range (Spain). Clay Miner. 33, 373–393. Bauluz, B., Peacor, D.R., González López, J.M., 2000. Transmission electron microscopy study of illitization in pelites from the Iberian Range, Spain: layer-by-layer replacement? Clay Miner. 48, 374–384. Capote, R., González Lodeiro, F., 1983. La estructura hercínica de los afloramientos paleozoicos de la Cordillera Ibérica. In: Comba,, J.A (Ed.), Libro Jubilar J.M Ríos. IGME, Madrid, pp. 513–528. Capote, R., Muñoz, J.A., Simón, J.L., Liesa, C.L., Arlegui, L.E., 2002. Alpine tectonics I: the Alpine system north of the Betic Cordillera. In: Gibbons, W, Moreno, T (Eds.), The Geology of Spain. The Geological Society, London, pp. 367–400. Carls, P., 1983. La Zona Asturoccidental-Leonesa en Aragón y el macizo del Ebro como prolongación del Macizo Cantábrico. In: Comba, JA (Ed.), Contribuciones sobre temas generales. Libro Jubilar J.M. Ríos, 3. IGME, Madrid, pp. 11–32. Cliff, G., Lorimer, G.W., 1975. The quantitative analysis of thin specimens. J. Microscopy 103, 203–207. Díaz García, F., 2006. Geometry and regional significance of Neoproterozoic (Cadomian) structures of the Narcea Antiform, NW Spain. J. Geol. Soc., London 163, 499–508. D'Lemos, R.S., Strachn, R.A., Topley, C.G., 1990. The Cadomian Orogeny. Geol. Soc., Spec. Publ. 51 423 p. Eguíluz, L., Gil Ibarguchi, J.L., Ábalos, B., Apraiz, A., 2000. Superponed Hercynian and Cadomian orogenic cycles in the Ossa-Morena Zone and related areas of the Iberian Massif. Geol. Soc. Am. Bull. 112, 1398–1413. Fernández-Suárez, J., Gutiérrez Alonso, G., Jenner, G.A., Jackson, S., 1998. Geochronology and geochemistry of the Pola de Allaude granitoids (northern Spain). The bearing on the Cadomian/Avalonian evolution of NW Iberia. Can. J. Earth Sci. 35, 1–15. Fernández-Suárez, J., Gutiérrez-Alonso, G., Jenner, G.A., Tubrett, M.N., 2000. New ideas on the Proterozoic–Early Palaeozoic evolution of NW Iberia: insights from U–Pb detrital zircon ages. Precambrian Res. 102, 185–206. Frey, M., 1987. Low Temperature Metamorphism. Blackie, Glasgow. 51 p. Frey, M., Teichmüller, M., Teichmüller, R., Mullis, J., Kuenzi, B., Breitschmid, A., Gruner, U., Schwizer, B., 1980. Very low grade metamorphism in external parts of the Central Alps, illite ‘‘crystallinity’’, coal rank and fluid inclusion data. Ecl. Geol. Helv. 73, 173–203. Gabaldón, V., 1990. Plataformas siliciclásticas externas: facies y su distribución areal (plataformas dominadas por tormentas). Parte II: Análisis de cuencas. Bol. Geol. Min. 101, 827–857. Gasquet, D., Levresse, G., Cheilletz, A., Rachid Azizi-Samiz, M., Mouttaqi, A., 2005. Contribution to a geodynamic reconstruction of the Anti-Atlas (Morocco) during Pan-African times with the emphasis on inversion tectonics and metallogenic activity at the Precambrian–Cambrian transition. Precambrian Res. 140, 157–182. Gozalo, R., Liñán, E., 1988. Los materiales hercínicos de la Cordillera Ibérica en el contexto del Macizo Ibérico. Est. Geol. 44, 399–404. Guidotti, C.V., Sassi, F.P., 1986. Classification and correlation of metamorphic faces by means of muscovite b0 data from low-grade metapelites. N. Jb. Miner., Abh. 153, 363–380. Guimerà, J., Álvaro, M., 1990. Structure et evolution de la compression alpine dans la Chaîne ibérique et la Chaîne côtière catalane (Espagne). Bull. Soc. Géol. Fr. 6, 33–348. Gutiérrez-Alonso, G., Nieto, F., 1996. White-mica ‘‘crystallinity’’, finite strain and cleavage development across a large Variscan structure, NW Spain. J. Geol. Soc. 153, 287–299. Kisch, H.J., 1991. Development of slaty cleavage and degree of very-low grade metamorphism: a review. J. Metam. Geol. 9, 735–750. Johnson, M.R.W., Oliver, G.J.H., 1990. Precollision and postcollision thermal events in the Himalaya. Geology 18, 753–756. Lago, M., Álvaro, J.J., Arranz, E., Pocoví, A., Vaquer, A., 1992. Condiciones de emplazamiento, petrología y geoquímica de las riolitas calco-alcalinas y stephaniense-pérmicas en las Cadenas Ibéricas. Cuad. Lab. Xeol. Laxe 17, 187–198. Linnemann, U., Gerdes, A., Drost, K., Buschmann, B., 2007. The continuum between Cadomian Orogenesis and opening of the Rheic Ocean: Constraints from LA-ICP-MS U–Pb zircon dating and analysis of plate-tectonic setting (Saxo-Thuringian Zone, NE Bohemian massif, Germany). In: Linnemann, U, Nance, D, Kraft, P, Zulauf, G (Eds.), The Evolution of the Rheic Ocean: From Avalonian–Cadomian Active Margin to Alleghenian–Variscan Collision. Geol. Soc. Am., Spec. Paper, 423, pp. 61–96. Liñán, E., Tejero, R., 1988. Las formaciones precámbricas del antiforme de Paracuellos (Cadenas ibéricas). Bol. R. Soc. Esp. Hist. Nat. (Sec. Geol.) 84, 39–49. Liñán, E., Perejón, A., Sdzuy, K., 1993. The Lower–Middle Cambrian stages and stratotypes from the Iberian Peninsula: a revision. Geol. Mag. 130, 817–833. López-Díaz, F., 1995. Late Precambrian series and structures in the Navalpino Variscan anticline (Central Iberian Peninsula). Geol. Runds. 84, 151–163. Lotze, F., 1956. Das Präkambrium Spaniens. N. Jb. Geol. Paläont., Mh. 8, 373–380. Martínez-Catalán, J.R., 1985. Estratigrafía y estructura del Domo de Lugo (sector oeste de la Zona Asturoccidental-leonesa). Corpus Geologicum Gallaeciae 2 (PhD Thesis, University of Salamanca, 1981). Mata, M.P., Casas, A.M., Canals, A., Gil Imaz, A., Pocoví, A., 2001. Thermal history during Mesozoic extension and Tertiary uplift in the Cameros Basin, northern Spain. Basin Res. 13, 91–111. Merriman, R.J., Peacor, D.R., 1999. Very low-grade metapelites: mineralogy, microfabrics and measuring reaction progress. In: Frey, M, Robisnon, D (Eds.), Low-Grade Metamorphism. Blackwell Sci., Oxford, pp. 10–60. Miller, B.V., Samson, S.D., D'Lemos, R.S., 1999. Time span of plutonism, fabric development, and cooling in a Neoproterozoic magmatic arc segment: U–Pb age

J.J. Álvaro et al. / Tectonophysics 461 (2008) 215–227 constraints from syntectonic plutons, Sark, Channel Islands, UK. Tectonophysics 312, 79–95. Murphy, J.B., Nance, R.S., 1989. Model for the evolution of the Avalonian–Cadomian belt. Geology 17, 735–738. Murphy, J.B., Nance, R.S., 1991. Supercontinent model for the contrasting character of Late Proterozoic orogenic belts. Geology 19, 469–472. Ortega, E., Hernández Urroz, J., Fernández Lodeiro, F., 1988. Distribución paleogeográfica y control estructural de los materiales anteordovícicos en la parte suroriental del autóctono de la Zona Centro Ibérica. Simp. Cinturones Orogénicos, II Congr. Geol. Esp. 85–89. Pereira, M.F., Chichorro, M., Linnemann, U., Eguíluz, L., Silva, J.B., 2006. Inherited arc signature in Ediacaran and Early Cambrian basin of the Ossa-Morena Zone (Iberian Massif, Portugal): Paleogeographic link with European and North African Cadomian correlatives. Precambrian Res. 144, 297–315. Powell, C.M.A., 1979. A morphological classification of rock cleavage. Tectonophysics 58, 21–34. Pulgar, J.A., 1980. Análisis e interpretación de las estructuras originadas durante las fases de replegamiento en la Zona Asturoccidental-Leonesa (Cordillera Herciniana, NW de España). PhD, Oviedo Univ. (unpubl.). Quesada, C., 1991. Geological constraints on the Paleozoic tectonic evolution of the tectonostratigraphic terranes in the Iberian Massif. Tectonophysics 185, 225–245. Von Raumer, J.F., Neubauer, F. (Eds.), 1993. Pre-Mesozoic Geology in the Alps. SpringerVerlag, Berlin. 672 p. Robinson, D., Bevins, R.E., 1989. Diastathermal (extensional) metamorphism at very low grades and possible high grade analogues. Earth Planet. Sci. Lett. 92, 81–88. Rodríguez-Alonso, M.D., Peinado, M., López-Plaza, M., Franco, P., Carnicero, A., Gonzalo, J.C., 2004. Neoproterozoic–Cambrian synsedimentary magmatism in the Central Iberian Zone (Spain): geologic, petrologic and geodynamic significance. Int. J. Earth Sci. 93, 897–920. Salman, K., 2004. The timing of the Cadomian and Variscan cycles in the Ossa-Morena Zone, SW Iberia: granitic magmatism from subduction to extension. J. Iberian Geol. 30, 119–132. Samson, S.D., D'Lemos, R.S., 1999. A precise Late Neoproterozoic U–Pb zircon age for the syntectonic Perelle quartz diorite, Guernsey, Channel Islands, UK. J. Geol. Soc., London 156, 47–54.

227

Sánchez-García, T., Bellido, F., Quesada, C., 2003. Geodynamic setting and geochemical signatures of Cambrian–Ordovician rift-related igneous rocks (Ossa-Morena Zone, SW Iberia). Tectonophysics 365, 233–255. Simancas, F., Expósito, I., Azor, A., Martínez Poyatos, D., González Lodeiro, F., 2004. From the Cadomian orogenesis to the Early Palaeozoic Variscan rifting in Southwest Iberia. J. Iberian Geol. 30, 53–71. Simón, J.L., 1986. Sobre las deformaciones del Paleozoico en el macizo del Desierto de las Palmas (Castellón). Est. Geol. 42, 407–411. Streng, M., 1996. Erläuterungen zur geologischen Karte des Gebietes NE und SW Codos (Östliche Iberische Ketten, NE Spanien). Diss., Univ. Würzburg, 150 p. (unpubl.). Tejero, R., 1986. Tectónica de los macizos paleozoicos al NE de Calatayud, Rama Aragonesa de la Cordillera Ibérica (Prov. Zaragoza). Tesis Doct., Univ. Complutense Publ., 300 p. Tejero, R., Capote, R., 1987. La deformación hercínica de los macizos paleozoicos nororientales de la Cordillera Ibérica. Est. geol. 43, 425–434. Ter Voorde, M., Bertotti, G., 1994. Thermal effects of normal faulting during rifted basin formation, 1. A finite difference model. Tectonophysics 240, 133–144. Teyssen, T., 1980. Acerca del problema de una discordancia asíntica en las Cadenas Ibéricas (NE España). Est. geol. 36, 403–407. Valenzuela, J.I., Gámez, J.A., Liñán, E., Sdzuy, K., 1990. Estratigrafía del Cámbrico de la región de Brea, Cadena Ibérica oriental. Bol. R. Soc. Esp. Hist. Nat. (Sec. Geol.) 85, 45–54. Valladares, M.I., Barba, P., Ugidos, J.M., 2002. Precambrian. In: Gibbons, W, Moreno, T (Eds.), The Geology of Spain. Geol. Soc. London, pp. 7–16. London. Vidal, G., Palacios, T., Gámez-Vintaned, J.A., Díez Balda, M.A., Grant, S.W.F., 1994. Neoproterozoic–early Cambrian geology and palaeontology of Iberia. Geol. Mag. 131, 729–765. Villena, J., Pardo, G., 1983. El Carbonífero de la Cordillera Ibérica. In: Martínez Díaz, C (Ed.), Carbonífero y Pérmico de España. IGME, Madrid, pp. 189–206. eds. Warr, L.N., Rice, A.H.N., 1994. Interlaboratory standardization and calibration of clay mineral crystallinity size data. J. Metam. Geol. 12, 141–152. Webby, B.D., 1970. Late Precambrian trace fossils from New South Wales. Lethaia 3, 79–109.