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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-Estan 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-Estan 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-Estan 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).

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