Fernando Bastida

Diachronous Variscan tectonothermal activity in the NW Iberian Massif: Evidence from 40Ar/39Ar dating of regional fabrics

Abstract Multigrain concentrates of hornblende and muscovite together with whole-rock slate/phyllite samples have been dated (27 analyses) using 40Ar/39Ar incremental-release methods along a systematic traverse across the various lithotectonic structural elements which comprise northwestern sectors of the Variscan Iberian Massif. Hornblende concentrates from amphibolites in the allochthonous Ordenes Complex yield plateau isotope-correlation ages of 425 Ma and 377 Ma. Muscovite concentrates and whole-rock slate/phyllite from this and the Cabo Ortegal Complex yield plateau ages which range from 367 Ma to 295 Ma. Analyses of similar material from the relative autochthon yield plateau ages between 359 Ma and 275 Ma. Muscovite concentrates from three late- to post-kinematic granitic stocks yield plateau ages between 309 Ma and 274 Ma. At least seven of the 40Ar/39Ar analyses from metamorphic rocks record variable thermal rejuvenation of intracrystalline argon systems associated with emplacement of proximal granitic stocks. The remaining analyses may be used to constrain the local age of various Variscan tectonothermal events. The oldest fabric ages are recorded in allochthonous units whereas the youngest fabric ages occur along the boundary between internal and external zones. Middle Devonian ages are recorded in the allochthon and suggest a chronological continuity with deformational events in the relative autochthon, where Variscan deformation initiated in the Upper Devonian and diachronously prograded eastward. The first deformational events recorded in the limit within the internal and external zones occured ca. 20–25 Ma later (lower Namurian). Variscan deformation systematically prograded diachronously eastward across the orogen as new crustal material was added along the front of the developing orogenic wedge. However, the entire orogen remained tectonically active with different structural features forming at different times and at different places. An average propagation rate of ca. 5 km/m.y. is suggested by consideration of a 20–25 Ma difference in correlative fabric ages and present separations.

Crustal thickening and deformation sequence in the footwall to the suture of the Variscan belt of northwest Spain

Abstract The sequence of formation and propagation of structures, as well as the evolution of crustal thickening, in the Variscan belt of northwest Spain are proposed in this paper. The foreland of this mountain belt shows thin-skinned tectonics. A polyphase deformation with associated regional metamorphism is recognized in the hinterland zones. Two important culminations, the Narcea and Lugo antiforms, are present in an E-W cross-section through the footwall to the suture of this orogen. Such culminations are derived from the stacking of several thrust systems generated during two successive periods. The sequence of thrust emplacement helps to clarify the formation of different types of structures at the same time in different places in the orogen.

SEISMIC STRUCTURE OF THE NORTHERN CONTINENTAL MARGIN OF SPAIN FROM ESCIN DEEP SEISMIC PROFILES

Abstract By the end of the Carboniferous, the crust of the continental shelf in northwestern Spain was made up of deeply rooted structures related to the Variscan collision. From Permian to Triassic times the tectonic setting had changed to mainly extensional and the northern Iberian continental margin underwent rifting during Late Jurassic-Early Cretaceous times, along with sea-floor spreading and the opening of the Bay of Biscay until the Late Cretaceous. Subsequently, the northern Iberian margin was active during the north-south convergence of Eurasia and Iberia in the Tertiary. A multichannel seismic experiment, consisting of two profiles, one north-south (ESCIN-4) crossing the platform margin offshore Asturias, and another (ESCIN-3) crossing the platform margin to the northwest of Galicia, was designed to study the structure of the northern Iberian margin. The ESCIN-4 stacked section reveals inverted structures in the upper crust within the Le Danois Basin. North of the steep continental slope, ESCIN-4 shows a thick sedimentary package from 6 to 9.5 s, two-way travel time (TWT). Within this latter package, a 40-km-long, north-tapering wedge of inclined, mainly south-dipping reflections is thought to represent a buried, Alpine-age accretionary prism. In the north western part of the ESCIN-3 (ESCIN-3-1) stacked section, horizontal reflections from 6.5 to 8.5 s correspond to an undisturbed package of sediments lying above oceanic-type basement. In this part of the line, a few kilometres long, strong horizontal reflection at 11.2 s within the basement may represent an oceanic Moho reflection. Also, a band of reflections dips gently towards the southeast, from the base of the gently dipping continental slope. The part of ESCIN-3 line that runs parallel to the NW-Galicia coast (ESCIN-3-2), is characterized by bright, continuous lower crustal reflections from 8 to 10 s. Beneath the lower crustal reflectivity, a band of strong reflections dips gently toward the southwest from 10 to 13.5 s. The part of ESCIN-3 that parallels the northern margin (ESCIN 3-3), shows good reflectivity in all levels. Upper crustal reflections image the sedimentary fill of probable Mesozoic to recent basins. Mid-crustal reflectivity is characterized by dipping reflections until 8 s that are probably related to compressional Variscan features. The lower crustal level shows ‘layered’ reflections between 8 and 12 s. Dipping reflections are found below the continental Moho.

Structural, metamorphic and magmatic history of the Mondon̄edo nappe (Hercynian belt, NW Spain)

Abstract The Mondonedo nappe is one of the main units in the internal zone of the Hercynian belt in the Iberian Peninsula. It consists of low to medium grade metasediments, late Precambrian and Palaeozoic in age, folded by large D 1 E-verging recumbent structures, and thrusted several tens of km towards the east during a D 2 deformation episode. At the base of the thrust sheet, a shear zone of up to 3 km thick was developed. Granitoid bodies, which were intruded after D 1 and before or during D 2 , were deformed in this basal zone. The geometry of D 1 and D 2 structures and that of the isograds are described, as well as the relationships between metamorphism and deformation. A structural evolution is inferred which shows an episode of generalized ductile deformation ( D 1 ) followed by another in which the deformation tended to concentrate in a subhorizontal ductile shear zone ( D 2 ) and culminated with the thrusting along an important fracture. The shear zone originated in a thermally softened level which, according to metamorphic data, was at temperatures in the range 550–600°C. The structural, metamorphic and magmatic features of the Mondonedo nappe suggest a relationship with a crustal scale overthrusting of the continental mass to which the nappe belongs. This could be a consequence of the collision against another continental mass to the west.

Transition from Diagenesis to Metamorphism Near the Front of the Variscan Regional Metamorphism (Cantabrian Zone, Northwestern Spain)

Distribution of paleotemperatures has been studied in Paleozoic rocks of the Variscan fold and thrust belt of the NW Cantabrian Zone (Spain) by a combined application of illite crystallinity (IC) and conodont color alteration index (CAI). Results show that the diagenetic‐metamorphic grade increases with the increase in rock age, as marked by decreasing IC values and increasing CAI values. Moreover, the CAI isogrades roughly reflect the structural traits of the area, indicating that maximum temperatures were attained during sedimentary burial previous to folding. The geothermal paleogradient obtained suggests conditions near the boundary between low and middle P/T conditions. There is evidence of at least two thermal events during the tectonothermal development of the belt. The main stage is interpreted as the result of the burial of the rocks characterized by late diagenesis conditions that occurred under a geothermal gradient of ∼35°C km−1. The narrow zone with low anchizonal conditions in the western part of the area could represent the front of the Variscan orogenic metamorphism. This stage was followed by thrusting that juxtaposed higher‐grade rocks of the internal zone (Westasturian‐Leonese Zone) over lower‐grade rocks of the external one (Cantabrian Zone), giving rise to an inverted metamorphic pattern. Subsequently, rocks in the southern part of the area were affected by contact metamorphism associated with emplacement of granitoid bodies. This late‐Variscan thermal event led to local enhancement of CAI values and to changes in the clay mineralogy by metasomatism.

GEOMETRICAL ANALYSIS OF FOLDED SURFACES USING SIMPLE FUNCTIONS

Abstract Several functions have been chosen in order to approximate fold profile geometry. Some of them are valid mainly for alloclinal folds (interlimb angle>0), whereas others are mainly valid for isoclinal folds (interlimb angle=0). In all cases, a fold profile can be characterised by an aspect ratio (y0/x0) between the height and the width of a limb (fold amplitude), and a shape parameter characteristic of the considered function. The shape parameters have been mutually linked through the area beneath the fold profile. The geometrical analysis enables a graphical classification based on a shape–amplitude diagram in which the most common types of folded surfaces are represented: cuspate, chevron, sinusoidal, parabolic, elliptic and box folds. Any of the shape parameters can be used as x-axis of the diagram in order to approximate the geometries commonly exhibited by natural folds. In the diagram presented in this paper two shape parameters have been combined: the exponent n of a power function for alloclinal folds, and a parameter C/y0, defined from a function composed of an elliptic part and a line segment of length C for isoclinal folds. In order to show the suitability of the classification method, it has been applied to some examples of finite-element, experimental and natural folds.

FoldModeler: a tool for the geometrical and kinematical analysis of folds ☆

Abstract FoldModeler is a system constructed in the Mathematica ™ environment that enables strain analysis in the profile of layers folded by the simultaneous or successive superposition of several strain patterns (layer shortening, tangential longitudinal strain, flexural flow and flattening). The fundamentals of the system involve the deformation of an initial grid of quadrilaterals according to the folding mechanisms considered. The main inputs to the system define the number, shape and size of the quadrilaterals, the characteristics of the sequences of incremental strain patterns involved, and the successive variations in form of a reference line named the ‘guideline’. The main outputs of the program are the parameters defining the form of the folded guideline, the drawings of the folded layer with several markers showing the strain distribution, graphics showing the variations in the orientation of the principal directions and the aspect ratio of the finite strain ellipse as functions of the layer dip, and Ramsays classification of the folded layer. FoldModeler has two main geological applications: (a) to predict the geometrical properties of folds produced by the combination of several types of strain patterns and (b) to analyse the strain state in specific natural quasi-symmetrical folded layer profiles and the possible combinations of strain patterns that could give rise to such a fold. This can be done by searching, with a fit and error method, a theoretical fold with the same geometrical characteristics as a given natural fold. The second application requires the existence of cleavage in the natural folded layer, and the best results are obtained when some strain measures are available.

A cross-section through the Zilair Nappe (southern Urals)

A structural transect in the Zilair-Kugarchi area involves the western part of the Suvanyak Complex, the Zilair Nappe and the eastern part of the foreland thrust and fold belt. This section has been analyzed using field, microstructural and seismic data. The cross-section shows the transition from the hinterland to the foreland in the footwall to the suture of the southern Urals. The rocks involved range from early Palaeozoic to Permian in age. A characteristic of the Zilair Nappe is the dominance of a succession of volcanic greywackes and mudrocks of Late Devonian age (Zilair Formation). The metamorphic grade decreases from east to west, from greenschist facies to diagenetic conditions. The structure of the cross-section mainly comprises west-directed thrusts and thrust-related folds with an associated cleavage. Fold vergence changes along the section depending on of the distance to the associated thrust and its geometry. The Zilair thrust which separates the Zilair Nappe from the foreland thrust and fold belt accommodated ca. 10 km displacement and the characteristics of the deformation are similar on both sides of it. The contact between the Zilair Nappe and Suvanyak Complex is a west-dipping normal fault that does not represent a major tectonic boundary.

Analysis of folding by superposition of strain patterns

Abstract Two methods have been developed in this paper to model the strain state and the layer geometry of folds. These methods analyse the superposition of strain patterns due to layer shortening, tangential longitudinal strain, flexural-flow and fold flattening. The first method multiplies the deformation gradients of these strain patterns to model the successive superposition of mechanisms. The second method is more general and is based on the transformation of points from the initial configuration to deformed points according to the geometrical properties of the folding mechanisms involved. This method simulates the simultaneous and successive superposition of strain patterns. Both methods generate graphic outputs that describe the strain variation through the folded layer. Another application of these methods is to attempt to find theoretical folds that fit natural or experimental folds and to perform a geometric and kinematical analysis of these folds. Knowledge of the shape of the folded layer and the cleavage pattern is the most common basic information available in natural folds that can be used to perform the analysis. Additional strain data from the folded rocks are valuable for improving knowledge of the kinematical mechanisms involved in the folding.

Anatomy of the Mondoñedo Nappe basal shear zone (NW Spain)

This paper analyses a major shear zone from the Iberian Hercynian belt which forms the basal thrust of the Mondonedo Nappe. The shear zone developed by ductile deformation under amphibolite facies metamorphic conditions and later by brittle-ductile deformation in greenschists facies. Folds in the shear zone are asymmetric, very tight, 1C or similar class and frequently developing sheath geometries. The sheath folds originated by non-coaxial flow superimposed on earlier irregularities. The fabric of quartzitic rocks in the shear zone changes from bottom to top from ultramylonites through blastomylonitic rocks to non-mylonitic tectonites. c-axis fabrics vary across the shear zone, but show a dominant monoclinic symmetry. The blastomylonitic rocks include the fabrics representing the highest temperatures. The main foliation of the schists results from flattening of an earlier foliation, recording occasional microfolds. The use of different kinematic criteria has allowed an analysis of their validity as well as an assessment of movement direction towards the foreland of the orogen.