Int J Earth Sciences Geol Rundsch) 2001) 90 : 287±303 DOI 10.1007/s005310000138

ORIGINAL PAPER

Mayte Bulnes ´ Alberto Marcos Internal structure and kinematics of Variscan thrust sheets in the valley of the Trubia River Cantabrian Zone, NW Spain): regional tectonic implications

Received: 15 September 1999 / Accepted: 28 August 2000 / Published online: 17 November 2000  Springer-Verlag 2000

Abstract The Variscan Belt in western Europe shows accordance with previous regional studies of the Can- an arcuate geometry that is usually named Ibero-Ar- tabrian Zone. morican Arc. The nucleus of this arc, known as the Asturian Arc, comprises the Cantabrian Zone which is Keywords Variscan Belt ´ Asturian Ar ´ Cantabrian a foreland fold and thrust belt. The Trubia River area Zone ´ Antiform-synform pair ´ Imbricate thrust is located in the inflexion zone of the Asturian Arc, system ´ Convergent transport directions ´ Forward which is a strategic structural position for unraveling emplacement sequence the geometry and kinematics of the Variscan thrust sheets and related folds. Geological mapping, con- struction of stratigraphic and structural cross sections, Introduction analysis of kinematic indicators, and estimate of short- ening for each cross section have been carried out. The Variscan Cordillera is an orogenic belt formed This area consists of two major antiform±synform from Late Devonian to Late Carboniferous time that pairs related to two imbricate thrust systems. These involves Precambrian and Paleozoic rocks and extends folds are asymmetric, tight, and their axial traces fol- throughout Europe, from the Iberian Peninsula to the low the trend of the Asturian Arc. They have been Urals. To the south, the Variscan belt continues into interpreted as fault-propagation folds. The emplace- Morocco and in Mauritania, northwest Africa, and its ment directions measured in the Trubia River area prolongation can be found in North America, in the change from north to south and converge towards the Appalachian and Ouachita Mountains. One of the core of the Asturian Arc. The minimum shortening most remarkable structural features of the Variscan estimated ranges between 16.4 and 17.6 km, which Cordillera is the arcuate form of the main Variscan corresponds to 56.9 and 59.4%. The complex cross- structures in the westernmost European portion of the cutting relationships between folds and thrusts suggest belt, which has been traditionally called the Ibero-Ar- that, in general, the different structural units followed morican Arc. The region located in the innermost i.e., a forward-breaking sequence of emplacement, with easternmost) part of the arc, known as the Asturian some breaching and a few out-of-sequence thrusts. Arc, occupies the Cantabrian Zone in northern Spain The analysis of the transport vectors together with the Fig. 1a; Lotze 1945; Julivert et al. 1972). This Canta- disposition of the fold axes and post-thrusting faults brian Zone exhibits the main features of the external that deform the thrust stack are evidence of a late portions of compressional cordilleras deformed under deformation event that is partially or totally responsi- shallow crustal conditions i.e., a pre- and synorogenic ble for the arcuate form of the Asturian Arc. The tim- sequence involved in a thin-skinned fold-and-thrust ing of the Asturian Arc, amount of shortening, and belt in which rocks show low amounts of internal sequence of emplacement of the structures are in strain, cleavage is only locally developed, and neither metamorphism nor magmatism took place). The area investigated in this paper is the valley of the Trubia River located in the inflexion area of the M. Bulnes )) ´ A. Marcos Asturian Arc, within the Sobia and Aramo structural Departamento de Geología, Facultad de Geología, units Fig. 1a; PØrez-Esta‚n et al. 1988). The most Universidad de Oviedo, 33005 Oviedo, Spain Phone: +34-98-5103116 recent detailed maps and structural studies of this Fax: +34-98-5103103 area were carried out more than 20 years ago e.g., 288 289 u marine limestones and some conglomerates and coal Fig. 1 a Structural sketch of the Cantabrian Zone that illus- beds deposited in a foreland basin developed in front trates the main structural units and the location of the study area; b regional geological section across the Cantabrian Zone. of the growing Variscan Cordillera due to thrust load- Modified after PØrez-Esta‚n et al. 1988) ing and litospheric flexure. The preorogenic to syn- orogenic transition took place at the Devonian±Car- boniferous boundary in the westernmost part of the García Fuente 1953, 1956; Almela and Ríos 1953; Cantabrian Zone, but it is younger towards the east Almela et al. 1956; Julivert 1960; Soler 1967; Pello due to the forward propagation of the deformation 1972, 1976; Martínez Alvarez et al. 1975; Rodríguez front. Fernµndez 1978). From then on, only specific aspects The Devonian rocks that crop out in the valley of of this area have been described in regional studies the Trubia River were deposited in a marginal posi- e.g., Julivert and Arboleya 1984a; Alonso et al. 1991; tion within the sedimentary basin, relatively close to PØrez-Esta‚n et al. 1994, 1995). This area offers an the source area. This explains the significant litholog- exceptional opportunity to study in detail the geome- ical changes observed in the Devonian succession in try and kinematics of the structures developed in the an E±W direction. Two different stratigraphic sections zone of maximum curvature of an arcuate orogenic have been constructed: one in the western sector belt. In particular, the aims pursued in this paper Fig. 3a) and another one in the eastern sector involve characterizing the types of structures present Fig. 3b). The total thickness of the western column, in the study area, their relative timing, the tectonic in which the whole sequence from Cambrian to Car- transport directions, the amount of shortening, and boniferous has been represented, ranges between 2500 the sequence of deformation followed by the struc- and 3500 m, whereas the total thickness of the eastern tures which constitute this part of the Variscan column, in which the lowest part of the Paleozoic orogen. In addition, this will allow us to furnish new sequence has not been included because these rocks data regarding some structural features of the inflex- do not outcrop in this area, varies between 1500 and ion portion of the Asturian Arc, and its timing with 2000 m. The Paleozoic sequence of the valley of the respect to the main Variscan structures. Trubia River consists of Lower Paleozoic siliciclastics In order to achieve the objectives proposed, the fol- Oville, Barrios, Formigoso, and Furada formations) lowing methodology is employed: detailed geological with some carbonate beds at the base Lµncara For- mapping at 1:20,000 scale; construction of stratigraphic mation; Fig. 3a), a Devonian sequence made up of sections focusing on the mechanical stratigraphy; con- alternations of siliciclastic and carbonate rocks Ra- struction of seven geological cross sections; analysis of æeces group, Moniello and Naranco formations, and kinematic indicators; and estimate of shortening for the Upper Devonian sandstones to the east; Fig. 3), the cross sections. and a Carboniferous succession including carbonate units at the base Candamo, Alba, Barcaliente and Valdeteja formations) overlain predominantly by silici- Stratigraphy clastics and sporadic carbonate beds Carboniferous shales; Fig. 3). The geological map of the study area Fig. 2) shows Analysis of the geological map Fig. 2), together that the valley of the Trubia River is characterized by with the stratigraphic sections Fig. 3), allowed us to: an almost complete sequence from Cambrian to Car- a) ascribe a Variscan age to most of the structures boniferous rocks, which can be grouped into several that appear in the study area because, except for a stratigraphic units e.g., García Fuente 1953, 1959; few faults, they are unconformably overlain by almost Almela and Ríos 1953; Almela et al. 1956; Llopis non-deformed Mesozoic±Tertiary deposits; b) ascer- Lladó 1958; Marcos 1968; Pello 1968, 1972, 1976; MØn- tain that the Paleozoic sequence, from Lµncara to Val- dez-Bedia 1971, 1976; Vera de La Puente 1988, 1989; deteja formations, were deposited before the initiation Bulnes et al. 1999). There are also some outcrops of of the Variscan deformation in this region because Mesozoic±Tertiary rocks unconformably overlying the they are all affected by the structures, and only the Paleozoic succession to the north. Two tectonostrati- Carboniferous shales may show evidence of tectonic graphic units have been distinguished within the activity during their deposition Marcos and Pulgar Paleozoic succession of the Cantabrian Zone: a preo- 1982; Aller 1986); and c) determine the most impor- rogenic sequence and a synorogenic sequence Julivert tant mechanical properties of the rocks that control 1978; Marcos and Pulgar 1982). The preorogenic the structural style of this region. Thus, the main sequence displays an overall wedge shape, thinning detachment levels are localized where marked litho- towards the east emerged area) and thickening logical contrasts occur base of Barrios and Moniello towards the west open-sea conditions). It consists of formations) and below limestone units that overlay shallow water clastic and carbonate rocks deposited in sandstone beds base of the Lµncara and Candamo- a stable shelf environment. The synorogenic sequence Alba formations). The Lµncara Formation is the old- consists of a thick wedge that also thins towards the est stratigraphic unit in this area; however, sandstones east. It is made up of paralic sandstones and shales, of the Herrería Formation outcrop below it outside 290 291 u predominate Furada Formation, Raæeces group, and Fig. 2 Geological map of the study area modified after Bulnes Moniello Formation), and a large number of small- 1989, 1995). The location of the cross sections illustrated in Figs. 5 and 6 is indicated. The areas located to the east of the scale folds develop in shaly units Formigoso For- Pedroveya village and to the north of the Tene village have mation and Carboniferous shales). Few folds develop been enlarged to show in detail the number of small thrust in the most competent units Barrios, Barcaliente, and sheets that occur in these zones Valdeteja formations), but they are the largest ones. the study area e.g., Marcos 1968; Bastida et al. 1984; Geometry of the structures Alonso and Marcos 1992). Folds occur within all the stratigraphic formations of the Paleozoic sequence. The valley of the Trubia River is located within the Nevertheless, most cartographic-scale folds appear in northern half of the Sobia and Aramo units. It com- units formed by thin alternations of competent and prises the frontal part of the Sobia unit and most of incompetent beds, especially when incompetent rocks the Aramo unit except for its frontal part Fig. 1). A regional section across the Cantabrian Zone, from the Narcea antiform to the Ponga unit, a few kilometers Fig. 3 Synthetic stratigraphic sections constructed in the a west to the south of the study area, shows the geometry of and b east portions of the study area the Sobia and Aramo thrusts which constitute, respec- 292 tively, the basal thrusts and the eastern boundaries of section VII±VII'; Fig. 7). This method requires a the Sobia and Aramo units Fig. 1b). PØrez-Esta‚n et continuous stratigraphic horizon throughout the al. 1988) interpreted another thrust system beneath cross sections and knowing the position of its these basal thrusts. The lowermost thrust of this sys- regional datum. The horizon chosen is the top of tem constitutes the sole thrust of the Cantabrian the Moniello Formation or the base of the Can- Zone. damo Formation when the Moniello and Naranco Although deformation is predominantly accommo- formations are absent) because it is the best rep- dated by folding, both folds and thrusts are important resented horizon in all the cross sections. The large structures within the valley of the Trubia River amount of erosion in some areas, together with the Fig. 4). The macrostructure of the study area consists lack of subsurface data, pose some problems to our of four major, kilometer-scale folds and two imbricate reconstruction. The geometry of the structures thrust systems structural sketch in Fig. 4a and cross above and below the topographic surface has been sections in Figs. 5 and 6). There are other important reconstructed according to the structural style thrusts, some of them folded by the major folds and observed at surface and involving a minimum offset by the two major imbricate thrust systems sec- amount of shortening. For each cross section, the tions I±I', II±II', III±III', and VII±VII' in Fig. 5). Both regional datum has been assumed to be at the folds and thrusts are cut by oblique and transverse deepest position of the top of the Moniello For- faults that trend mainly in a NW±SE direction. These mation. The results obtained match quite well with faults are: strike-slip faults; dip-slip faults; and faults a detachment at 4±5 km depth. Nevertheless, these that involve a combination of vertical and horizontal results should be taken with caution because of: a) slip component. the amount of interpretation in the reconstruction No data are available to accurately draw the sub- of the chosen horizon above and below the topo- surface structure of the study area. In the cross sec- graphic surface; b) the assumptions inherent in the tions in Fig. 5, constructed perpendicular to the trace method; and c) the large number of factors that of the main structures, we have assumed that the influence the accuracy of the results obtained by thrusts merge into a subhorizontal basal detachment applying this method see Bulnes and Poblet 1999). located at the base of the Lµncara Formation Fig. 5) The sole thrust of the Cantabrian Zone interpreted because: by PØrez-Esta‚n et al. 1988) in a regional cross sec- 1.1The Lµncara Formation is the oldest stratigraphic tion Fig. 1a) was recognized by PØrez-Esta‚n et al. unit in the valley of the Trubia River and adjacent 1994, 1995) in a deep seismic profile at 5±5.5 s areas, and there is no evidence of thrusts cutting TWT) beneath the study area which, according to the across underlying stratigraphic sequences. The seismic velocities used by these authors 4.5±5 km/s), nearest outcrops of the stratigraphic unit located corresponds to a depth of 11±13 km. below the Lµncara Formation Herrería Formation) Alternatively, instead of the deep structure shown appear in other thrust sheets various kilometers in the cross sections in Fig. 5, the eastern thrusts away from the study area Marcos 1968; Alonso thrusts located to the east of the Sobia imbricate and Marcos 1992). thrust system) may merge into a detachment located 2.1The base of the Lµncara Formation is a well-known within the Furada formation or in the lowermost regional detachment level in the Cantabrian Zone Raæeces group Fig. 6). This alternative hypothesis e.g., Julivert 1971; PØrez-Esta‚n et al. 1988). would explain a) the lack of Lower Paleozoic rocks This subhorizontal basal detachment has been in the eastern sector of the study area because they interpreted to be at 4±5 km depth below the Aramo would not be involved in these thrust sheets, and b) unit cross sections in Fig. 5) because: the smaller size of the eastern folds compared with 1.1When plotting the dip and thickness of the strati- the western ones assuming that the folds' size is graphic sequence, the detachment appears to be related to the detachment depth. approximately at this depth. If this hypothesis is correct, the subsurface structure 2.1The interpretation of the regional-scale seismic pro- of the eastern sector should be modified Fig. 6). Nev- file ESCIN-1 by PØrez-Esta‚n et al. 1994, 1995) shows the basal thrust of the Aramo unit at 2.2 s TWT) which, according to the seismic velocities used for this deep profile 4.5 km/s according to h PØrez-Esta‚n et al. 1994; 5 km/s according to PØrez- Fig. 4 Structural sketches of the study area showing a the loca- tion of the major folds, and b the location of the major thrusts Esta‚n et al. 1995; and 4.4±5.6 km/s according to and orientation of fold axes. The areas around the Pedroveya Gallastegui et al. 1997), is approximately equivalent and Tene villages are enlarged to show in detail the large to 4.9±5.5 km depth. number of folds, thrusts, and transverse faults that occur in these 3.1The excess-area method derived by Chamberlin zones. The enlarged map around Pedroveya shows that many 1910) to calculate the detachment depth in cross thrusts affect the core of the Pedroveya antiform and the west- ern limb of the Mostayal synform. The enlarged map around sections involving folds has been applied to the Tene shows that some thrusts are folded and cut by younger cross sections illustrated in Fig. 5 except for cross folds and thrusts 293 294 295 u III±III'); b) the Feleches thrust to the north cross Fig. 5 Geological sections across the study area. The location of section VII±VII'); and c) some thrusts near Tene and these cross sections is indicated in Fig. 2 Bermiego villages, to the southeast cross section II± II'). Thrust fault/bedding relationships reveal that these thrusts have a staircase geometry with long flats ertheless, these arguments are not conclusive and, in and short ramps. The restoration of the western part our opinion, it is more reasonable to assume a similar of section II±II' across the deformed thrusts near Car- geometry for both the Sobia and Pedroveya imbricate anga and Bustiello Bulnes 1992) shows that the initial thrust systems as shown in Fig. 5. In any case, the hangingwall/footwall cutoff angles in these thrusts did occurrence of a shallower detachment beneath the not exceed 30 usually range 15±20). The cumulative eastern thrust sheets would not modify significantly displacement along the thrusts located near Caranga is the geometry, timing, and kinematic evolution tec- approximately 2.5 km Bulnes 1992). The displace- tonic transport vectors and sequence of emplacement ment along the Feleches thrust, which duplicates the of structures) presented in this paper. Moniello Formation and displays a large cartographic continuity, is 4.8 km Bulnes 1995), and the displace- ment caused by the thrusts located near Tene and Folds and thrusts Bermiego is approximately 2.2 km. The thrusts that cut and offset the structures One of the most striking features of the study area is described above have been grouped into two main the occurrence of tightly folded thrusts which are cut imbricate thrust systems, the Sobia imbricate thrust and offset by younger thrusts see map in Fig. 2 and system and Pedroveya imbricate thrust system Figs. 2, cross sections in Fig. 5). The deformed thrusts are 4b, 5). To the south, outside the study area, the Sobia with dashed lines in Fig. 4b): a) various thrusts near thrust places Cambrian rocks over Carboniferous Caranga, Bustiello, and Bandujo villages, to the south- rocks Fig. 1; e.g., García Fuente 1952, 1959; Marcos west of the study area cross sections I±I', II±II' and 1968; Marcos et al. 1980), whereas in the study area it consists of numerous splays that produce less signifi- cant tectonic superpositions Figs. 2, 4b, 5). This group Fig. 6 Alternative interpretation of cross sections III±III' and of thrusts constitutes the Sobia imbricate thrust system IV±IV' in Fig. 5, assuming the occurrence of a detachment level within the Furada Formation or Raæeces group in the eastern Fig. 4b). Similarly, the thrust that runs across the part of the study area Pedroveya village becomes a large number of splays 296

Caranga-Trubia Proaza-Caldas S. Pedroveya

La Mostayal 30.5 km 13 km 2 km 0 4 km 0 H = V Proaza-Caldas S. Section VI-VI' Caranga-Trubia A. Pedroveya

Mostayal S.

29.6 km 12.2 km 2 km 0 4 km

0 H = V Section V-V' Proaza- Caldas S. Caranga-Trubia Pedroveya A.

Mostayal S. 28.8 km 12.4 km

0 4 km 2 km Caranga-Trubia A. Proaza-Caldas S. 0 Pedroveya A.

Section IV-IV' Mostayal S.

29 km

10.9 km 2 km

0 4 km 0 Caranga-Trubia A. H = V Proaza- Caldas S. Pedroveya A. Section III-III'

Mostayal S. 29.21 km

2 km 12.4 km

0 4 km Proaza- 0 Caranga-Trubia A. Caldas S. H = V Pedroveya A. Section II-II' Mostayal S.

29.6 km 2 km 12 km

0 4 km 0 H = V

Section I-I' Section length before present-day shortening no. deformation length restored length of the top of I-I' 29.6 km 12 km 59.45% the Moniello Fm. II-II' 29.2 km 12.4 km 57.53% detachment depth III-III' 29 km 12 km 58.62% present-day length of the cross calculated by the IV-IV' 28.8 km 12.4 km 56.9% section Chamberlin method V-V' 29.6 km 12.2 km 58.7% VI-VI' 30.5 km 13 km 57.3%

Fig. 7 Estimate of the shortening and detachment depth using to the south Figs. 2, 4b, 5). This group of thrusts con- the Chamberlin 1910) method for the cross sections illustrated stitutes the Pedroveya imbricate thrust system in Fig. 5. The Moniello Formation occupies the narrow area Fig. 4b). The tectonic superposition caused by the located between the two thin lines in the cross sections Pedroveya imbricate thrust system is not significant, except for one of the thrusts which places Lower Devonian rocks on top of Carboniferous rocks ca. 1 km to the south of Pedroveya). The map traces of the thrusts that belong to the Sobia and Pedroveya 297 imbricate thrust systems trend approximately NE±SW antiform and the Proaza-Caldas synform, whereas in the northern part of the study area, N±S in the cen- they dip to the east in the case of the Pedroveya anti- tral part, and NW±SE in the southern part i.e., they form and the Mostayal synform Fig. 5). These major follow the geometry of the Asturian Arc; Figs. 2, 4b). folds are tight, asymmetric structures. Their internal The thrusts of the two imbricate thrust systems are configuration is complex due to the occurrence of mostly oblique/perpendicular to the hangingwall and smaller-scale folds and thrusts; and along-strike vari- footwall bedding at shallow levels see the geological ations in terms of number, size, and geometry of map in Figs. 2 and 4b and the shallow part of the minor structures, and orientation of fold axes and cross sections in Fig. 5). Nevertheless, they have been limbs' dip. The most striking variation in limb dip interpreted to be parallel to the hangingwall beds occurs in the backlimb of the Caranga-Trubia anti- parallel to the base of the Lµncara Formation) at form. In the northern part of the study area the aver- depth. age dip of the beds ranges between 50 and 70 to the Two fold orientations predominate within the val- west cross sections V±V', VI±VI' and VII±VII' in ley of the Trubia River: folds that follow the geometry Fig. 5), whereas to the south the beds located within of the Asturian Arc; and folds oblique or perpendicu- the westernmost thrust sheets are overturned cross lar to them, called longitudinal and transverse folds, section II±II' in Fig. 5). To a great extent, these vari- respectively, according to Julivert 1971) and Julivert ations are related to the number of thrusts in the and Marcos 1973; Fig. 4). Most of the outcrop to backlimb. For instance, cross section II±II' Fig. 5) map-scale folds appear to be related to the thrusts shows that five thrusts affect the backlimb of the Car- described above because their geometry, dimensions, anga-Trubia antiform, whereas only one thrust distribution, and orientation depend on the features of appears towards the south cross section I±I' in Fig. 5) the thrust faults Fig. 5). Thus, longitudinal fault-re- and two thrusts crop out towards the north cross sec- lated folds are associated with frontal thrust struc- tions III±III' in Fig. 5). Only cross section II±II' shows tures, whereas transverse fault-related folds are associ- overturned beds in the backlimb of the Caranga-Tru- ated with lateral thrust structures see the tectonic bia antiform progressively more overturned towards transport vectors in the next section). the west). Some of these thrusts, developed prior to The deformed thrusts, described above, exhibit folding, caused beds' dip of even 36 to the west folds related to them. Most of these fault-related folds before the antiform initiation Bulnes 1992), which are fault-bend folds formed as a consequence of the favored the occurrence of overturned beds after the staircase geometry of the thrusts. Some examples of antiform development. Two idealized sections across these structures are the gentle flexures of the beds in the southern part of the Caranga-Trubia antiform, the westernmost part of the study area due to the approximately perpendicular to cross sections I±I', II± occurrence of hangingwall and/or footwall ramps II', and III±III' in Fig. 5 and perpendicular to the tec- cross-sections I±I', II±II', and III±III' in Fig. 5), the tonic transport vector see next section below), reveal anticline±syncline pair located to the northeast of Car- that the variation in the number of thrusts in the anga Fig. 4b and cross-section II±II' in Fig. 5), and backlimb of the antiform was caused by lateral struc- some small-scale, asymmetric folds related to the tures Fig. 8). In the central-south part of the cross Feleches thrust cross section VII±VII' in Fig. 5). section A±A' several branch and cut off points result These folds are also deformed by younger folds and from the termination of the lowest thrust sheets to the thrusts like the thrusts related to them; thus, their south against a thrust that runs across the Bandujo vil- present-day geometry and orientation is partially due lage. In addition, these truncated thrust sheets pro- to tightening and/or rotation caused by the younger gressively thin to the south due to the occurrence of major folding which we describe next. hangingwall and footwall lateral ramps related to the As stated previously, the macrostructure of the thrusts. The termination of the lowermost thrust sheet study area consists of four major, kilometer-scale folds to the north is due to a footwall lateral ramp, related which follow the geometry of the Asturian Arc, and to the overlying thrust, which causes the connection of therefore, they are longitudinal folds. From west to the lowermost thrust with the basal thrust of the over- east these are: the Caranga-Trubia antiform; the Proa- lying thrust sheet north part of cross sections A±A' za-Caldas synform; the Pedroveya antiform; and the and B±B' in Fig. 8). Mostayal synform Fig. 4a). We show herein that the The large-scale folds described above exhibit inti- Caranga-Trubia antiform and the Proaza-Caldas syn- mate and linked map/cross section relations to the form are related to the Sobia imbricate thrust system, Sobia and Pedroveya imbricate thrust systems. There and the Pedroveya antiform and the Mostayal synform is a parallelism or slight obliquity) between the axial are related to the Pedroveya thrust system. The axial traces and axes of the western pair of major folds traces and axes of these folds trend approximately Caranga-Trubia antiform and Proaza-Caldas synform) parallel or slightly oblique to the traces of the thrusts and the Sobia imbricate thrust system, and between that belong to the imbricate thrust systems related to the eastern pair of major folds Pedroveya antiform them Figs. 2, 4). Their axial surfaces are subvertical and Mostayal synform) and the Pedroveya imbricate or dip to the west in the case of the Caranga-Trubia thrust system Figs. 2, 4). The Caranga and Pedroveya 298

SNsouth Bulnes 1995, 1999). In outline, the structural

1.000 ▲ features of these two tight fold pairs described above ▲ ▲ ∆▲ ∆ ∆ agree with those of fault-propagation folds in which ∆ ∆ 0 m ∆ ▲ the thrusts branch into numerous imbricates that have H = V ▲ broken through the folds as deformation progressed Section A-A' ∆ ∆ see Mitra 1990; Suppe and Medwedeff 1990). Folds not related to thrusts also appear in the val- N SN 1.000 ley of the Trubia River. The two large-scale transverse ▲ ▲ folds located in the central and northern part of the ▲ 0 m ∆ study area correspond to this group Fig. 4a). These H = V two folds deform all the thrust sheets within the study Section B-B' area. Another large-scale transverse fold, located to sheet 1 the west of Caranga, deforms various thrust sheets but not all of them. This structure is probably not totally III' III A' sheet 2 related to the thrusts. The meaning of these three B' sheet 3 transverse folds is discussed herein. II' andujo ∆ thrusts genetically related Caranga Cleavage to the Caranga-Trubia antiform B A spaced cleavage related to large-scale and mesosco- ▲ thrusts previous to the pic-scale folds developed locally. It is parallel to the I' Caranga-Trubia antiform II axial planes of folds or it displays a convergent fan. development The folds with associated cleavage correspond to both, A I folds related to deformed thrusts and folds related to the imbricate thrust systems. The cleavage is almost Fig. 8 Structural sketch and transversal sections across the restricted to the middle-western part of the valley of southern part of the study area. The lines I±I', II±II', and III±III' the Trubia River where the Raæeces group and the correspond to cross sections I±I', II±II', and III±III' in Figs. 2 and 5 Furada and Moniello formations are intensely folded. The cleavage is especially well developed in the marls of the Raæeces group and the Moniello Formation. In antiforms are located over the imbricated thrusts general, the orientation of the cleavage is NW±SE in which cause the largest displacements within the Sobia the southern part, N±S in the central part and NE±SW and Pedroveya thrust systems, respectively, whereas in the northern part similar to the orientation of the the two major synforms are located below them axial surfaces of the major longitudinal folds). It dips Fig. 5). Many of the thrusts that belong to the Sobia to the east or to the west with variable angles depend- thrust system cut through the common limbs of the ing on the position of the axial planes of the folds. Caranga-Trubia antiform and the Proaza-Caldas syn- form, whereas others seem to die out in the cores and backlimbs of the antiforms Fig. 5). Similarly, the Ped- Faults roveya imbricate thrust system cuts through the Ped- roveya antiform and the Mostayal synform. Moreover, The thrusts and folds described above are cut and off- the size and asymmetry of the four major folds are set by a large number of oblique/transverse, steeply consistent with the dimensions and sense of movement dipping to subvertical faults with dominant NW±SE see next section) of the two imbricate thrust systems strikes. These faults have a horizontal and/or vertical Figs. 2, 4, 5). In addition, most thrusts that belong to movement. In particular, most of the faults displaying the two imbricate thrust systems lose slip as they pro- a large cartographic continuity produce vertical or gressively cut younger rocks. The cross sections show oblique movements. Some of these structures produce that shortening is accommodated mainly by folding uplift of the northern blocks e.g., the transverse fault and some thrusting in the youngest rocks, whereas in located immediately to the south of Sograndio village the oldest rocks thrusting predominates. and two faults to the east of Proaza village in Figs. 2, In summary, the Sobia thrust, well defined to the 4b), whereas others cause the southern blocks to be south of the study area Fig. 1), evolves to the Sobia the uplifted ones e.g., the fault located immediately imbricate thrust system and a pair of folds the Caran- to the west of Sograndio and two faults located to the ga-Trubia antiform and the Proaza-Caldas synform) to south of Braæes village which cut the Feleches thrust the north. Similarly, the Pedroveya thrust, located to in Figs. 2, 4b). In most cases uplifted northern blocks the northeast of the study area, evolves to the Pedro- occur along faults steeply dipping to the north, veya imbricate thrust system and a pair of folds the whereas uplifted southern blocks occur along faults Pedroveya antiform and the Mostayal synform) to the steeply dipping to the south. Thus, in general, the 299 oblique/transverse faults with vertical movements are thrust; and b) the axial surface of the Mostayal syn- reverse faults. In most cases, when they involve hori- form. The eastern bounding line may involve some zontal movements strike-slip faults or oblique faults) problems because the Mostayal synform has been they are right-lateral faults. The net slip estimated interpreted as a fault-propagation fold. Using the axial along some faults ranges from a few meters to a few surface of a fault-propagation fold as a loose line leads hundred meters. to underestimate the length of the oldest beds. There- fore, to be able to compare all the sections, we meas- ured the same horizon in all of them. The horizon Fold-and-thrust kinematics chosen is the top of the Moniello Formation or the base of the Candamo Formation when the Moniello Tectonic transport vector and Naranco formations are absent) because of two reasons: a) it is the best represented horizon in all The tectonic transport directions of the thrusts have the cross sections; and b) it is the top of a competent been obtained by measuring the orientation of differ- stratigraphic unit unlikely to have undergone bed- ent linear elements axes of fault-related folds, cut off length variations during deformation. The measure- lines, and thrust branch lines) and planar elements ments carried out assume that this stratigraphic hori- axial surfaces of fault-related folds, bedding surfaces zon was horizontal before deformation. deformed by fault-related folds, and thrust surfaces). The difference between the total length of the top To estimate the correct position of these kinematic of the Moniello Formation and the length of the cross indicators at the time of formation of the structures, sections at present day ranges among 16.4 km cross we should know whether they underwent any trans- section IV±IV' in Fig. 5) and 17.6 km cross section lation and/or rotation after their formation. In the fol- I±I' in Fig. 5). Assuming that the displacement is lowing section we conclude that a deformation event equivalent to shortening, these figures imply a mini- after thrusting occurred, giving rise to the rotation of mum shortening that ranges between 56.9 and 59.4%. the thrust sheets. Nevertheless, not enough data are The values obtained are approximately constant irre- available at this stage to estimate the magnitude of spective of the along-strike variations in structural deformation involved in this late event, and therefore, style observed in the geological map Fig. 2) and geo- its effects have not been taken into account here. logical cross sections Fig. 5). Each kinematic indicator has been rotated using a rotation axis strike of the thrust to which the kine- matic indicator is related) and a rotation angle thrust Sequence of deformation dip). This correction assumes that thrusts were gener- ated approximately horizontal. Unfortunately, the cor- The sequence of emplacement of the thrusts has been rection carried out does not take into account that tentatively determined using overprinting criteria thrusts were not horizontal in ramp areas. Thus, it is because no syntectonic sediments outcrop in the study expected that corrections along flats are more precise area. Nevertheless, although geometrical relationships than in ramp areas. The results have been plotted in allow us to deduce the relative timing of the struc- Fig. 9. Each arrow is an average value of the horizon- tures, they are not definitive as a thrust cut by another tal projection of a number of measurements of differ- thrust may continue moving afterwards the whole ent kinematic indicators. thrust or only a thrust segment). As stated previously, The tectonic transport directions obtained change the Caranga-Trubia antiform and the Proaza-Caldas from south to north in a clockwise sense. This change synform are related to the Sobia imbricate thrust sys- occurs within individual thrust sheets, but the vari- tem, whereas the Pedroveya antiform and the Mos- ation is not gradual. Three main directions appear: tayal synform are related to the Pedroveya imbricate towards the ENE in the southern part, towards the thrust system. These relationships will help us to east in the Caranga-Proaza area, and towards the SE decipher the sequence of emplacement of the struc- in the northern part. The variation in the tectonic tures. transport vector is approximately 90. The oldest tectonic activity occurred probably in the westernmost sectors followed by the movement of the Sobia imbricate thrust system. This is supported Shortening by the fact that the westernmost thrusts are deformed by the Caranga-Trubia antiform and the Proaza-Cal- The amount of shortening due to the structures devel- das synform see Geometry of the structures). Later, oped in the study area has been obtained by compar- the rest of thrusts present in the study area moved, ing the lengths of six geological cross sections illus- the Pedroveya imbricate thrust system being the last trated in Fig. 5 with the total length of a particular one to emplace. This is supported by the folded stratigraphic horizon. From west to east the bounding geometry of the thrusts located in western sectors, lines chosen are Fig. 7): a) the axial surface of the which may have resulted from bending during the easternmost thrust-bend syncline related to the Sobia emplacement of younger easternmost thrusts. 300

Fig. 9 Structural sketch of the study area showing the tectonic transport directions N arrows) Brañes

Feleches

Trubia

Las Caldas

Sama de Grado Argame

Linares

Pedroveya

Proaza

Bandujo Caranga

Bermiego

Fresnedo

0 1 2 4 6 Km

In the study area, folds verge towards the east and footwall thrust ramp. This would also explain the thrusts are east-directed; however, in the easternmost ªboxº geometry of the Proaza-Caldas synform sectors the axial surfaces of the major folds and the observed in some cross sections in Fig. 5. thrust surfaces dip towards the east, i.e. they are over- The oblique/transverse faults indicate the occur- turned e.g., see cross section II±II' in Fig. 5 near rence of a late deformation event after emplacement Tene village). This has been interpreted as a result of of the thrusts and their related folds and are discussed passive rotation of these structures as the hangingwall, herein. in which they are located, moved over a west-dipping 301

The fold axes of longitudinal folds plunge to the north Discussion and conclusion in the northern area, to the south in the southern area, and are subhorizontal in the central area. Note that According to PØrez-Esta‚n et al. 1994, 1995) the locally, near Caranga village, the fold axes plunge to detailed structure within the Sobia-Aramo structural the north instead of to the south Fig. 4b). This is due units is difficult to determine from the ESCIN-1 deep to the fact that these measurements have been taken seismic reflection profile due to the quality of the seis- in the lower thrust sheets thrust sheets 1 and 2 in mic data. The structural analysis carried out in this Fig. 8) of the thrust sheet stack around Caranga. Near paper shows that, in general, beds, folds, and thrusts Caranga village, the beds within these thrust sheets are steep to vertical in the valley of the Trubia River. dip to the north because of the occurrence of lateral This disposition of the rocks does not favor imaging of thrust ramps Fig. 8). The large-scale disposition of the subsurface structure. The disrupted pattern of seis- the fold axes depicted in Fig. 4b could result from: a) mic reflections observed in the seismic profile across the occurrence of a palaeorelief in the footwall before this area may be due to the abundance of thrusts emplacement of the thrust systems that would control breaking through the Paleozoic sequence. In addition, their geometry; or b) folding after the emplacement the complex timing relationships observed between of the main structures. The first solution has been dis- folds and thrust does not help the seismic interpreta- carded as it would require notable E±W trending tion. This geological setting may complicate the inter- structures producing significant structural changes in pretation of data obtained using many geophysical N±S direction. This type of lateral structure has not techniques. Therefore, we emphasize the role of carry- been observed in the study area. The second solution ing out field work in this particular region and extrap- implies a late deformation event perhaps induced by olating the field data to depth according to the struc- N±S shortening. The movement undergone along the tural style observed at surface as a very useful tool in late, subvertical faults observed in the study area sup- predicting the subsurface structure. port this solution. At present, most of these oblique/ From the analysis of a series of kinematic indica- transverse faults trend NW±SE, and are reverse faults tors, three principal tectonic transport vectors have when they involve a vertical movement) and/or right- been deduced within the study area: towards the ENE lateral faults when they imply a horizontal move- in southern sectors, towards the east in central sectors, ment). Taking into account the location of the valley and towards the SE in northern sectors. This disposi- of the Trubia River within the Asturian Arc, these tion may be due to the following reasons: a) the faults could be interpreted as a response to accommo- orientation of the kinematic indicators was not altered dation of strain during formation or closure of the arc after the formation of the structures i.e., the original as a consequence of a N±S shortening event produced tectonic transport direction was variable); or b) the after the thrust emplacement. Similar structural fea- orientation of the kinematic indicators was altered by tures have been documented southeast of the study a subsequent deformation. In the second case the orig- area Aller 1993). inal transport direction may have been constant all Alonso et al. 1996) stated that the Alpine orogeny over the study area, so that its present disposition in the Cantabrian Mountains involved detachment and would have resulted from subsequent deformation. uplift of the Variscan basement. In a N±S section The change in orientation of the tectonic transport from the Cantabrian Sea to the Duero Basin across vectors along the study area occurs within individual the eastern part of the Central Coal Basin approx- thrust sheets, but the variation is not gradual. In our imately 40 km to the east of the valley of the Trubia opinion, this rules out a simple rotational emplace- River), they estimated a bed displacement of 25 km to ment of each thrust sheet. In addition, the variation in the south. Later, Pulgar et al. 1999) evaluated the the tectonic transport vector from north to south is effect of the Alpine shortening on the Variscan base- approximately 90 which, in such a small area, would ment in areas located to the east of the valley of the imply noticeable space problems in the internal part Trubia River, concluding that this deformation is of the arc assuming that the thrusts followed a rota- irregularly distributed, with an average value of 20% tional emplacement. Moreover, such an emplacement shortening. They suggested a passive role of the Varis- mode would require more important lateral structures can basement in the central part of the Cantabrian than the ones observed in the study area. This sug- Mountains, between Oviedo and the Leon Fault gests that the orientation of the tectonic transport vec- Fig. 1), as no reactivated Variscan structures were tors is partially or totally due to a deformation event recognized in this sector. The valley of the Trubia after the emplacement of the thrust sheets. River occupies an equivalent central position within The orientation of the fold axes in the study area the Cantabrian Mountains, slightly to the west of defines three different large domains that narrow Oviedo. The data presented here reveal that the situ- towards the nucleus of the arc Fig. 4b). The boun- ation in the study area is similar to that of Pulgar et daries between these domains converge towards the al. 1999) because no significant reactivation of the nucleus of the arc and coincide approximately with Variscan structures occurred. However, the oblique/ the map traces of the major transverse folds Fig. 4a). transverse faults that offset the main structures migth 302 have accommodated a certain amount of the N±S ern part of the study area Fig. 4b; cross-section II-II' alpine shortening. in Fig. 5). Structures developed out of sequence have The available data do not allow to ascertain been also documented in several sectors of the Canta- whether later deformation is totally responsible for brian Zone e.g., Alonso et al. 1991; Alonso and Mar- the present-day, arcuate geometry of the structures cos 1992), to the north of the study area. Asturian Arc). It seems that in some areas there was an initial curvature accentuated by late deformation. Acknowledgements We thank J. Adam and D. Brown for For instance, the abrupt change in the kinematic indi- helpful comments which substantially improved the manuscript. We also acknowledge financial support from projects cators and fold axes near Caranga, and the concavity AMB98-1012-CO2-02 ªActividad sismotectónica, estructura to the east of the structural units in this area more litosfØrica y modelos de deformación varisca y alpina en el NO remarkable in the western units than in the eastern de la Península IbØricaº) and PB98-1557 ªMecanismos de ple- ones) would be the result of a transverse fold Fig. 4b). gamiento: teoría y aplicaciones en geología económica y region- alº) funded by DGICYT ªMinisterio de Educación y Culturaº) This fold is restricted to some thrust sheets and is par- and by ªAcción Integrada Hispano-Britµnicaº HB 1999-0038 tially related to the thrust sheet stacking and a lateral ªCinemµtica de pliegues y estructuras menores asociadas a thrust ramp Fig. 8), and partially related to late cabalgamientos a partir del estudio de materiales sintectónicos y deformation that would have tightened the previous de su modelizaciónº) funded by the ªMinisterio de Educación y fold. In other areas, the arcuate disposition of the Culturaº and the British Council. M. Bulnes acknowledges financial support by a fellowship 1988±1992) within the ªFor- structures could be related solely to late deformation. mación del Personal Investigadorº F.P.I.) program funded by Thus, near Pedroveya and Soto de Ribera villages, the the ªMinisterio de Educación y Ciencia.º We are especially structural units show a slight curvature more remark- grateful to J. Aller, J.L. Alonso, A. PØrez-Esta‚n, and J. Poblet able in the eastern units than in the western ones) for their important contributions to this study. also caused by transverse folds Fig. 4b). These folds deform all the structural units in these areas and there is no evidence of lateral thrust ramps. This suggests References that these folds formed as a consequence of late deformation. In any case, our conclusions are in Aller J 1986) La estructura del sector meridional de las uni- dades del Aramo y Cuenca Carbonífera Central. Servivio de accordance with many previously published paleomag- publicaciones del Principado de Asturias, 180 pp netic and structural studies. These works conclude Aller J 1993) La estructura geológica de la Sierra del Aramo that at least some structural units of the northwest of Zona Cantµbrica, NOde Espaæa). Trab Geol Univ Oviedo the Iberian Peninsula were rotated a certain amount 19:3±13 Almela A, Ríos JM 1953) Datos para el conocimiento de la after their emplacement irrespective of the degree of geología asturiana. Valles de Riosa y Proaza. Bol Inst Geol pre-rotation curvature e.g., Julivert 1971; Matte and Min Espaæa 65:1±34 Ribeiro 1975; Ries and Shackleton 1976; Ries et al. Almela A, García Fuente S, Ríos JM 1956) Mapa Geológico de 1980; Bonhommet et al. 1981; Perroud 1982, 1986; Espaæa. Esc. 1:50,000, Hoja no. 52 Proaza). Inst Geol Min Julivert and Arboleya 1984a, 1984b, 1986; Hirt et al. Espaæa, Madrid, pp 1±81 Alonso JL, Marcos A 1992) Nuevos datos sobre la estratigrafía 1992; ParØs et al. 1994, and references therein). y la estructura de la sierra del Pedrosu Zona Cantµbrica, PØrez-Esta‚n et al. 1994) estimated that the total NW de Espaæa): implicaciones tectónicas. Rev Soc Geol accumulated displacement along the 90-km-long cross Espaæa 5:81±88 section from the east of the Narcea antiform to the Alonso JL, Aller J, Bastida F, Marcos A, Marquínez J, PØrez- Esta‚n A, Pulgar JA 1991) Mapa Geológico de Espaæa. Ponga unit Fig. 1) was approximately 150 km. Esc. 1:200,000, Hoja no. 2 AvilØs). Inst Tecn Geomin Espa- Assuming that the displacement reflects the short- æa, Madrid, pp 1±103 ening undergone by the rocks, the whole cross section Alonso JL, Pulgar JA, García-Ramos JC, Barba P 1996) Terti- shortened approximately 62.5%. The amount of short- ary basins and Alpine tectonics in the Cantabrian Mountains NW Spain). In: Friend PF, Dabrio CJ eds) Tertiary basins ening estimated in the study area is approximately of Spain: tectonics, climate and sea-level change. Cambridge constant irrespective of the along-strike variation in University Press, Cambridge, pp 214±227 geometry, size, and number of structures. It ranges Bastida F, Marcos A, PØrez-Esta‚n A, Pulgar JA 1984) Geo- among 56.9 to 59.4% which is consistent with the metría y evolución estructural del Manto de Somiedo Zona values calculated by PØrez-Esta‚n et al. 1994) using a Cantµbrica, NOde Espaæa). Bol Inst Geol Min Espaæa 95:517±539 regional cross section. Bonhommet N, Cobbold PR, Perroud H, Richardson A 1981) The structural relationships between the structures Paleomagnetism and cross folding in a key area of the Astu- observed in the study area confirm that, in outline, rian Arc Spain). J Geophys Res 86:1873±1887 deformation migrated from west to east. This is in Bulnes M 1989) La estructura del µrea situada en el entorno de las localidades de Trubia, Proaza y Sama de Grado Zona accordance with a forward sequence of thrust Cantµbrica, NOde Espaæa). Seminario de investigación, emplacement as proposed by Julivert 1978), Marcos Universidad de Oviedo, Oviedo, pp 1±23 and Pulgar 1982), and PØrez-Esta‚n et al. 1988) for Bulnes M 1992) Geometría y evolución estructural del anti- the Cantabrian Zone. However, a few structures did forme de Caranga. Actas de las sesiones Científicas. Tercero Congr Geol Espaæa 1:274±278 not follow this sequence of emplacement, such as the Bulnes M 1995) La estructura geológica del valle del río Tru- westernmost imbricated thrust within the Sobia thrust bia. Tesis doctoral. Universidad de Oviedo, Oviedo, pp system that runs through Bandujo village in the south- 1±255 303

Bulnes M 1999) Progressive thrust/fold development within large- Matte P, Ribeiro A 1975) Forme et orientation de l'ellispsoide scale fault-propagation folds of Variscan age Cantabrian de dØformation dans la virgation hercynienne de Galice: rela- Zone, NW Iberian Peninsula). Thrust Tectonics 99:146±149 tions avec le plissement et hypotheses sur la genese de l'arc Bulnes M, Poblet J 1999) Estimating the detachment depth in Ibero-Armoricain. CR Hebc Seances Acad Sci Paris cross setions involving detachment folds. Geol Mag 280:2825±2828 136:395±412 MØndez-Bedia I 1971) Nota sobre las microfacies de la For- Bulnes M, García-Alcalde J, Marcos A 1999) Litoestratigrafía mación Moniello Devónico) en la zona de Teverga Asturi- del Grupo Raæeces Devónico inferior) en el antiforme de as, NOde Espaæa). Brev Geol Ast‚rica 15:17±32 Caranga-Trubia Zona Cantµbrica, NOde Espaæa). Rev Soc MØndez-Bedia I 1976) Biofacies y litofacies de la Formación Geol Espaæa 12:239±249 Moniello-Santa Lucía Devónico de la Cordillera Cantµbrica, Chamberlin RT 1910) The Appalachian folds of central Penn- NW de Espaæa). Trab Geol Univ Oviedo 9:1±93 sylvania. J Geol 18:228±251 Mitra S 1990) Fault-propagation folds: geometry, kinematic Gallastegui J, Pulgar JA, Alvarez-Marrón J 1997) 2D seismic evolution, and hydrocarbon traps. AAPG Bull 74:921±945 modelling of the Variscan foreland thrust and fold belt crust ParØs JM, Van der Voo R, Stamatakos J, PØrez-Esta‚n A 1994) in NW Spain from ESCIN-1 deep seismic reflection data. Remagnetizations and postfolding oroclinal rotations in the Tectonophysics 269:21±32 Cantabrian/Asturian arc, northern Spain. Tectonics García Fuente S 1952) Geología del Concejo de Teverga Astu- 13:1461±1471 rias). Bol Inst Geol Min Espaæa 64:345±456 Pello J 1968) Sobre la existencia de Devónico Superior en la García Fuente S 1953) Geología de los Concejos de Proaza y región central de Asturias y los problemas que plantea el Tameza Asturias). Bol Inst Geol Min Espaæa 65:271±324 contacto Devónico-Carbonífero al Ode Oviedo.Brev Geol García Fuente S 1956) Datos para el estudio geológico del Ast‚rica 12:11±16 Concejo de Quirós Asturias). Not Com Inst Geol Min Pello J 1972) Geología de la región central de Asturias. Tesis Espaæa 41:21±32 doctoral. Univ Oviedo, Oviedo, pp 1±198 García Fuente S 1959) Mapa Geológico de Espaæa. Esc. Pello J 1976) Mapa Geológico de Espaæa. Esc. 1:50,000, Hoja 1:50,000, Hoja no. 77 La Plaza, Teverga). Inst Geol Min no. 52 Proaza). Inst Geol Min Espaæa, Madrid Espaæa, Madrid, pp 1±68 PØrez-Esta‚n A, Bastida F, Alonso JL, Marquínez J, Aller J, Hirt AM, Lowrie W, Julivert M, Arboleya ML 1992) Paleo- Alvarez-Marrón J, Marcos A, Pulgar JA 1988) A thin- magnetic results in support of a model for the origin of the skinned tectonics model for an arcuate fold and thrust belt: Asturian arc. Tectonophysics 213:321±339 The Cantabrian Zone Variscan Ibero-Armorican Arc). Tec- Julivert M 1960) La terminación septentrional de la Cuenca de tonics 7:517±537 Quirós Cordillera Cantµbrica). Brev Geol Ast‚rica 4:19±34 PØrez-Esta‚n A, Pulgar JA, Banda E, Alvarez-Marrón J, Julivert M 1971) DØcollement tectonics in the Hercynian Cor- ESCI-N Research Group Marcos A, Bastida F, Alonso JL, dillera of Northwest Spain. Am J Sci 270:1±29 Aller J, Farias P, Martínez-Catalµn JR, Comas MC, Daæo- Julivert M 1978) Hercynian orogeny and Carboniferous pal- beitia JJ, Córdoba D) 1994) Crustal structure of the Exter- aeogeography in northwest Spain: a model of deformation± nal Variscides in NW Spain from deep seismic reflection pro- sedimentation relationships. Z Dtsch Geol Ges 129:565±592 filing. Tectonophysics 232:91±118 Julivert M, Arboleya ML 1984a) A geometrical and kinematical PØrez-Esta‚n A, Pulgar JA, Alvarez-Marrón J, ESCI-N Group approach to the nappe structure in an arcuate fold belt: the Marcos A, Bastida F, Aller J, Marquínez J, Farias P, Alonso Cantabrian nappes Hercynian chain, NW Spain). J Struct JL, GutiØrrez G, Gallastegui J, Rodríguez Fernµndez LR, Geol 6:499±519 Heredia N, Bulnes M, Banda E, Martínez Catalµn JR, Cór- Julivert M, Arboleya ML 1984b) Curvature increase and struc- doba D, Daæobeitia JJ, Comas MC) 1995) Crustal structure tural evolution of the core Cantabrian Zone) of the Ibero- of the Cantabrian Zone: seismic image of a Variscan fore- Armorican arc. Sci Geol Bull 37:5±11 land thrust and fold belt NW Spain). Rev Soc Geol Espaæa Julivert M, Arboleya ML 1986) Areal balancing and estimate 8:307±319 of areal reduction in a thin skinned fold and thrust belt Perroud H 1982) Relations entre palØomagnØtisme et dØfor- Cantabrian Zone, NW Spain): constraints on its emplace- mation: exemple de la rØgion de Cabo de Peæas Espagne). ment mechanism. J Struct Geol 8:407±414 CR Acad Sci Paris Ser II 294:45±48 Julivert M, Marcos A 1973) Superimposed folding under flex- Perroud H 1986) Paleomagnetic evidence for tectonic rotations ural conditions in the Cantabrian Zone Hercynian Cordille- in the Variscan mountain belt. Tectonics 5:205±214 ra, Northwest Spain). Am J Sci 273:353±375 Pulgar JA, Alonso JL, Espina RG, Marín JA 1999) La defor- Julivert M, FontbotØ JM, Ribeiro A, Conde L 1972) Mapa tec- mación alpina en el basamento varisco de la Zona Cantµbri- tónico de la Península IbØrica y Baleares. Inst Geol Min ca. Trab Geol Univ Oviedo 21:283±294 Espaæa, Madrid, pp 1±113 Ries AC, Shackleton RM 1976) Patterns of strain variation in Llopis Lladó N 1958) Las bases estratigrµficas del devónico de arcuate fold belts. Phil Trans R Soc Lond A283:281±288 Asturias. Brev Geol Ast‚rica 1:13±21 Ries AC, Richardson A, Shackelton RM 1980) Rotation of the Lotze F 1945) Zur gliederung der Varisciden der Iberischen Iberian Arc: palaeomagnetic results from north Spain. Earth Meseta. Geotekt Forsch 6:78±92 Translated by Ríos JM Planet Sci Lett 50:301±310 1950) Observaciones respecto a la división de los varíscides Rodríguez Fernµndez LR 1978) La estructura de la prolonga- de la meseta IbØrica. Publ Extr Geol Espaæa 5:149±166 ción N de las unidades de Tameza y La Sobia Asturias, Cor- Marcos A 1968) La tectónica de la unidad de La Sobia-Bodón. dillera Cantµbrica). Trab Geol Univ Oviedo 10:389±394 Trab Geol Univ Oviedo 2:59±87 Soler M 1967) Evolución longitudinal del cabalgamiento de Marcos A, Pulgar JA 1982) An approach to the tectonostrati- Peæa Sobia Asturias). Acta Geol Hisp 2:82±84 graphic evolution of the Cantabrian Foreland thrust and fold Suppe J, Medwedeff DA 1990) Geometry and kinematics of belt, Hercynian Cordillera of NW Spain. N Jahrb Geol Palä- fault-propagation folding. Eclogae Geol 83:409±454 ontol Abh 163:256±260 Vera de la Puente C 1988) Estratigrafía, sedimentología y Marcos A, PØrez-Esta‚n A, Pulgar JA, Bastida F, Aller J, Gar- paleogeografía de los Grupos Raæeces y La Vid en la Cordil- cía -Alcalde JL, Sµnchez de Posada LC 1980) Mapa Geoló- lera Cantµbrica Asturias y León). Tesis doctoral. Univ Ovie- gico de Espaæa. Esc. 1:50,000, Hoja no. 77 La Plaza). Inst do, Oviedo, pp 1±653 Geol Min Espaæa, Madrid Vera de la Puente C 1989) Revisión litoestratigrµfica y correla- Martínez Alvarez JA, GutiØrrez Claverol M, Torres Alonso M ción de los Grupos Raæeces y La Vid Devónico Inferior de 1975) Mapa Geológico de Espaæa. Esc. 1:50,000, Hoja no. la Cuenca Astur-Leonesa). Trab Geol Univ Oviedo 18:53±65 28 Grado). Inst Geol Min Espaæa, Madrid