Geometry, structures and evolution of the western termination of the Alpine-Pyrenean Orogen reliefs (NW Iberian Peninsula)

The geometry, structures and tectonic evolution of the western termination of the Alpine-Pyrenean Orogen relief onshore and related foreland have been studied. We present the results of structural mapping and detailed outcrop studies of Cenozoic-age structures carried out at the NW Iberian Peninsula. On the basis of this work we propose a new tectonic model for the termination. part of the Cantabrian Mountains (CM), characterised by thrusts with a south vergence, which are the continuation of the structures of the Pyrenees, (2) The Galaico-Leoneses Mountains (GLM), characterized by thrusts with a north vergence, and (3) The Rías Baixas-Terra Chá region (RBT) characterized by strike-slip faults with no relevant associated relief but with recent seismic activity, which represents the less deformed Alpine foreland. The western termination of the Alpine-Pyrenean Orogen is formed due to the superposition of two mountain ranges (CM and GLM) and a complex tectonic evolution. The CM were emplaced southwards and ! " # $ % & ' % +! % a N-S-oriented arch-shape structures (Ibias-Ancares and Rúa-Vilalba). Tectonic activity commenced during the Eocene, and in the westernmost areas during the Late Oligocene. The CM south-verging thrust activity ended at the beginning of Late Miocene, although deformation continued in the GLM with north-verging thrusts. The evidence of intense tectonic activity during the Miocene, the western migration and southward transference of tectonic activity and the interaction of two successive deformation fronts, suggests that a re-examination of the tectonic evolution of the northern Iberian microplate during the Alpine Orogeny is necessary.


Introduction
The Alpine-Himalayan Orogen has its westernmost expression onshore in the Iberian Peninsula, the Alpine-Pyrenean orogen in the north (or Pyrenean-Cantabrian) and the Betic Chains in the south (Fig. 1A) (Dewey 1977).These orogenic belts resulted from the collision of the Iberian Plate and the Eurasian and African Plates, respectively.The Alpine-Pyrenean orogen (the Pyrenees and the Cantabrian Mountains (CM)) (Fig. 1B) formed due to the oblique convergence and collision between the microcontinent Iberia and Eurasia from Late Cretaceous until Early Miocene times (Dewey et al., 1989;Srivastava et al., 1990;Muñoz, 1992;Jabaloy et al., 2002;Andeweg, 2002).Geologically speaking, their common origin and cartographic continuity would suggest that these mountain ranges are part of a continuous orogen whith reliefs extend 800 km in an E-W trend (Choukroune et al. 1990;Pulgar et al. 1996;Teixell, 1998;Muñoz, 2002;Gallastegui et al., 2002;Barnolas and Pujalte, 2004).The western part of the Alpine-Pyrenean orogen reliefs is called the Cantabrian Mountains.In Galicia the main reliefs of the Cantabrian Mountains ends just before the Atlantic Ocean, except for the Xistral Mountains (1,000 m height) a narrow range located north of this region (Fig. 1C).Tectonic structures continue westward until the Mid-Atlantic Ridge but offshore (e.g., As Pontes Fault, in Fig. 2).
Precambrian to the Cenozoic-age sediments were affected by the Alpine Orogeny (Fig. 1B), although, the sedimentary varies considerably along an eastwest transect.In the eastern part of the orogen, there is a good Mesozoic pretectonic sedimentary record, and a complete record of the Cenozoic syntectonic succession is preserved in the Ebro, Duero and Aquitaine basins.However, in the westernmost part, where the Pyrenean Orogen takes place over the Variscan basement (Iberian Massif) the lack of Mesozoic rocks and the scarcity of &HQR]RLF VHGLPHQWV )LJ % PDNH LW YHU\ GLI¿FXOW WR recognize the Alpine tectonics (Martín-Serrano et al., 1996).Moreover, existing models for the eastern part of Cantabrian Mountains and Pyrenees are not applicable in the westernmost regions (Alonso and Pulgar, 2004).This complex tectonic evolution is partly due to the result of the superposition of two orthogonal orogenic events: the Variscan and the Alpine orogenies (Gallastegi et al., 2002), but also due the fact that this region represents the lateral termination of this reliefs.Finally, due to the LQWHQVH EDVHPHQW GHIRUPDWLRQ LW LV GLI¿FXOW WR UHFRJQL]H the presence and nature of post-Variscan structures, which in many cases resulted from reactivation processes taking place on paleozoic structures.
The relief of the Alpine Orogeny in northern and northeastern Iberian Peninsula, trends east-west with a constant width of around 80 km in the Pyrenees and eastern Cantabrian Mountains.However, in the northwestern part, the Alpine relief changes direction (southwards) and separates into a series of individual mountain ranges with peaks above 2,000 m (e.g.Teleno Ranges, Segundera Ranges, etc), which can be grouped in the Galaico-Leoneses Mountains that increase the orogen width more than 200 km (Fig. 1C).These mountains determine the sharp end of the Duero basin towards the west (Fig. 1B).
The study of the Alpine relief along the NW Iberian Peninsula has implications for the evolution of the Alpine Orogeny and the Iberian microcontinent geodynamics.In have revealed the need to better understand the Cenozoic structures and clarify existing models, in order to explain the seismic activity in this region.These structures are still a subject of debate (González-Casado and Giner, 2000;Rueda and Mezcua, 2001;López-Fernández et al., 2004;Martín-González, et al., 2006;Martínez-Díaz et al., 2006;Martín-González, 2009) and despite the interest and implications of this termination, the geometry, VWUXFWXUHV DQG $OSLQH HYROXWLRQ DUH SRRUO\ GH¿QHG DQG detailed mapping of these is required (Martín-Serrano et al., 1996;Alonso and Pulgar, 2004;Barnolas and Pujalte, 2004;Martín-González, 2009).The purpose of this paper is to identify, characterize, and map the Alpine structures of the NW Iberian Peninsula EDVHG RQ WKH UHVXOWV RI VWUXFWXUDO PDSSLQJ ¿HOG VXUYH\LQJ and detailed outcrop studies) with special emphasis in this western termination, in order to propose both a tectonic subdivision and a tectonic model that might explain the major elements (geological and geomorphological features) of these areas.These elements are outlined in ¿JXUHV % & DQG DQG LQFOXGH -The abrupt end of the main E-W trending relief of the Cantabrian Mountains.
-The widening to the south of the Alpine relief.
-The structures that control limits and sedimentation in the Tertiary depressions, as well as, the distribution and dissection of the Tertiary sediments, that appear in two main trends: E-W (El Bierzo and O Barco depressions) and N-S (Sarria, Monforte and Vilalba depressions) -The relationship between the conspicuous fault corridors of the NW Iberian Peninsula (Braganza-Vilariça and Verín-Vila Real) and all the structures of the Cantabrian Mountains (CM).

Geological setting
The NW Iberian Peninsula predominantly comprises Precambrian and Paleozoic lithologies (Fig. 1 B).These rocks were mainly deformed during the Carboniferous in the Variscan Orogeny which are intruded by igneous rocks, mainly in the western region, during the Carboniferous and Permian.The overall Variscan structure shows an arch-like shape (Asturian Arch) open to the west, where the inner parts of the Varsican Orogen are located (Martínez Catalán et al.,1990;Arenas and Martínez Catalán, 2003).
The Alpine-Pyrenean Orogen is composed of the Pyrenees in the east and the Cantabrian Mountains in the west (Gallastegui et al., 2002;Muñoz, 2002;Barnolas and Pujalte, 2004).The Pyrenees form the border between Eurasia and the Iberian Peninsula, trend ESE-WNW and represent a doubly vergent continental collision orogen (Choukroune et al., 1990;Munoz, 1992;Teixell, 1998).The double vergence is asymmetric with the south Pyrenean Zone (southward vergence) being wider that the north Pyrenean Zone (northward vergence).Two continental foreland basins: the Ebro Basin in the south and Aquitaine basin in the north, formed as a result of this double vergence.
The structures of the south Pyrenean Zone continue westward across the Cantabrian Mountains, while the structures of the north Pyrenean Zone continue offshore into the Cantabrian Sea (Cámara, 1997;Gallastegui et al., 2002).Similarly, the continuation of the Ebro foreland Basin is the Duero Basin, while the Aquitaine Basin continues westward into the Landes Plateau and the Bay of Biscay abyssal plain.
The Alpine structures of the Cantabrian Mountains can be divided, based on lithology, age and deformational style of their rocks, into two regions: the Vasco-Cantabrica (VC) in the E and the Astur-Galaica (AG) in the W (Figs. 2 and 1B) The Vasco-Cantábrica region is characterized by a thick and complete succesion of Mesozoic sediments (Triassic to Cretaceous).In the north, the extensional Mesozoic structures were partially inverted during the Alpine compressional regime (Espina, 1994;Pulgar et al., 1996) whole, in the south, the Mesozoic succession appears to be detached.
The Astur-Galaica region is characterized by the absence or rarity (only in the eastern part outcrops a thin and fairly complete sequence) of Mesozoic sediments.Cenozoic sediments are found in small isolated depressions.In the eastern part of this region, the thin Mesozoic cover is undetached and deformed in a thick-skin tectonic style.In this region, the overall VWUXFWXUH )LJ $ LV D UHJLRQDO PRQRFOLQDO ÀH[XUH WKDW corresponds to the back-limb of a major fault-bend fold related to the Cantabrian Basal Thrust (CBT) (Alonso et al., 1996).This north-dipping basement thrust overturn the Mesozoic-Cenozoic sediments in the south border of the Cantabrian Mountains.
The general tectonic model proposed (Santanach, 1994;Cabrera et al., 1996;Vegas et al., 2004;Santanach et al., 1988Santanach et al., , 2005;;De Vicente and Vegas, 2009) for the termination of the Alpine-Pyrenean orogen, suggest that the shortening along the CBT was transferred south across the NE-SW left lateral corridors of Bragança-Vilariça and Vila Real-Verín.To the west of this transfer zone, residual convergence is partially accommodated by NW-SE right lateral faults.According to this model, the NE-SW corridors limits the Duero Basin to the west and generates the El Bierzo and O Barco depressions and the Sarria and Monforte as intramontane basins in the hinterland (see for details Santanach, (1994)).De Vicente and Vegas (2009) emphasize the importance of the reorientation of the CBT to the NE-SW corridors in order to explain the change in topography and the intraplate deformation of the NW Iberian Peninsula.

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LGHQWL¿FDWLRQ RI WKH $OSLQH VWUXFWXUHV LQ WKH 1: ,EHULDQ 3HQLQVXOD ZDV EDVHG RQ VWUXFWXUDO PDSSLQJ ¿HOG VXUYH\LQJ DQG GHWDLOHG RXWFURS VWXG\ RI WKH LGHQWL¿HG faults.Due to the geographical scale of the Alpine deformational style, the cartographic scale chosen was 1:200,000 for the entire mapped area.In addition, 1:50,000 and 1:25,000 cartographic scales were chosen for detailed mapping in key areas such as the Tertiaryage depressions and intersecting set of structures.The Variscan structures were compiled from IGME maps (1:200,000 and 1:50,000 scale).The characteristics of the NW Iberian Peninsula have focused the structural mapping on the conspicuous Variscan structures, but in some cases neglecting the Alpine structures in this region (e.g.Téna-Dávila, and Capdevilla, 1975).All previous Alpine structural maps in these regions were compiled (e.g.Santanach, 1994;Martín Serrano 1994;Alonso et al., 1996;Martín-González, 2009, De Vicente andVegas, 2009).However, there are conspicuous gaps in structural data especially in the Galaico-Leoneses Mountains, the Rias Baixas-Terra Cha Region and in the westernmost Cantabrian Mountains.In these areas we carried out detailed structural mapping (1:25,000 and 1:50,000), as well as a revision of the available geological maps (1:50,000 and 1:200,000).In places where no Tertiary VHGLPHQWV DOORZHG IRU WKH LGHQWL¿FDWLRQ RI $OSLQH activity, we have assessed this on the basis of analysis of geomorphic markers, such as erosion surfaces, river network patterns, position of mountain fronts, etc. (Martín-González, 2009).In the study area, a widespread planation surface has been dissected and displaced due to subsequent Cenozoic tectonics and, therefore, it can be used as a marker to measure vertical displacements (García-Abad and Martín-Serrano, 1980;Martín-González, 2009).In order to evaluate these vertical displacements on the erosion surface, longitudinal HOHYDWLRQ SUR¿OHV RYHU 6570 '(0V KDYH EHHQ FDUULHG out (90 m spatial resolution).

Structures of the western termination of the Alpine-Pyrenean Orogen reliefs
According to the present study, the western termination of the Alpine Orogen reliefs onshore and its related foreland can be subdivided in three regions based on structural and morphological characteristics (Fig. 2 and  1D).Astur-Galaica region (AG) (the westernmost part of the Cantabrian Mountains), the Galaico-Leoneses Mountains (GLM) and, in the west, the Rías Baixas-Terra Chá region (RBT).The AG is characterized by thrusts with a south vergence, the GLM is characterised by thrusts with a north vergence, while the RBT is characterized by NE-SW and NW-SE trending faults mainly reactivated as strike-slip faults during the Alpine FRPSUHVVLRQ EXW ZLWKRXW VLJQL¿FDQW DVVRFLDWHG UHOLHIV ,Q the RBT Mesozoic N-S normal faults are also observed near the Atlantic Coast, but Alpine reactivation is not recorded apparently.

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The western Astur-Galaica region (Figs.1B and 2) is bounded to the south by an imbricate thrust system that forms the continuation of the Cantabrian Basal Thrust (CBT) (Figs. 2 and 1 B).These thrusts superimpose the Cantabrian Mountains on the Duero Basin and, in the west, on the El Bierzo and O Barco depressions (Fig. 3).

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This sector (Fig. 1D) is the largest of the Astur-Galaica region and is bounded on the east by the Vasco-Cantábrica region and by Ibias-Ancares fault zone in the west.In the study area (western part) is characterized by a sequence of thrusts affecting the Late Carboniferous (Stephanian) and Tertiary sediments of the El Bierzo Basin (Figs. 3,  4B, and 6).These imbricate thrusts dissect the Tertiary basin and have a mainly south vergence (e.g.Tombrio and Noceda thrusts) (Figs.5a and 5b).Backthrusts in the north slope of this sector are common (e.g.Villablino and Sabero-Gordon thrusts, Figs.4A and 4B), and result in the formation of tectonic pop-up that uplifts the highest peaks of the western Cantabrian Mountains (Catoute-Nevadin peak, 2,100 m) (Fig. 3).In the eastern part of this sector, where the Variscan arch-like shape structure is oriented E-W (Fig. 3), Reactivation of Variscan thrust faults and a squeezing of Variscan folds is also noted (Alonso et al., 1996).However, in the western part of this sector, the Alpine structures offset the Variscan structures and only E-W structures, related to Late Carboniferous E-W sequence of imbricate thrusts (e.g.La Portela thrust) (Fig. 3).This structure is also the western limit of the Cantabrian Mountains and only a pre-Cenozoic planation surface (500 m), dissected by the recent rivers, can be LGHQWL¿HG WR WKH ZHVW 7KH 5LDV %DL[DV7HUUD &KD UHJLRQ the less-deformed foreland (Figs.1C, 1D ,and 2).The trace of the Rúa-Vilalba structure is sub-parallel to the Ibias-Ancares structure.The Rúa-Vilalba structure has a vertical component that uplifts the eastern block and superimposes basement onto the Tertiary-age Monforte, Sarria (Fig. 8) and Vilalba depressions (Figs.5D, 9B, and 9C).

El Caurel sector
The El Caurel sector (Fig. 1D) is separated from the Ancares sector by the Valcarce River and the Poio Thrust (Fig. 3).The El Caurel sector is characterized by a series of NE-SW thrusts with a NNW vergence (Fig. 3) which are responsible of the uplift of the NE-SW elongated Caurel Mountains (1,600 m).These thrusts (e.g.Folgado and Poio Thrusts) have an arched geometry with slip surfaces dipping from 22º to 70º and linking up with the structures from the southern Galaico-Leoneses Mountains region that cut the Cantabrian Mountains front (Fig. 3).Thus, several structures from the Ancares sector (E-W and NW-SE) of the Cantabrian Mountains are offset by these NE-SW thrusts with NNW vergence (e.g., Mosteiros and Villafranca thrusts).The geometry of the rivers and thrusts in this sector is quite parallel to the thrust fronts and its slope and the associated reliefs increases towards the SSE: this can be interpreted as a footwall block and lateral thrust propagation sequence (Martín-González, 2009).

Rías Altas-Xistral sector
The Rías Altas-Xistral (Fig. 1D), a narrow area with a relief close to 1,000 m in height (Xistral Mountains), is the westernmost sector of the Alpine-Pyrenean Orogen reliefs (Figs. 3 and 4C).Structures in this sector show evidences of little shortening.This sector is limited by the As Pontes and Foz faults.The As Pontes Fault is a right lateral strike-slip fault trending NW-SE (Santanach et al., 1988(Santanach et al., , 2005)).It is parallel and has the same kinematics as the Meirama and Betanzos faults (Fig. 3).In contrast, the As Pontes Fault has a relevant vertical component that uplifts the northern block.These faults superimpose basement onto Tertiary sediments (e.g., the As Pontes and Roupar faults) (Santanach et al.,1988(Santanach et al., , 2005)).
The Alpine deformation in this sector is concentrated along the southern border, while deformation in the rest of this sector is less concentrated.Imbricate thrusts are present only along the northern border of the Oviedo-,Q¿HVWR 7HUWLDU\ EDVLQ 7KH RWKHU $OSLQHDJH VWUXFWXUHV are reverse faults, originally steeply-dipping Variscan WKUXVWV ZKLFK ZHUH UHDFWLYDWHG LQ WKH QRUWKHUQ ÀDQN RI the Variscan arch.These faults have a NE-SW orientation and determine the deposit of Tertiary sediments and its deformation (e.g.Espina and Allande thrusts, Fig. 3) (De Vicente et al., 2007).Towards the south, where these structures turn to a N-S orientation, they are not reactivated.
The Fuentes Carrionas-Miravalles sector merges westward with the Ibias-Ancares structure.The Ibias-Ancares structure is a complex steeply-dipping fault zone that was already active during the Late Carboniferous (Pulgar et al., 1981;Bastida et al., 1980).The curved and anastomosed trace follows Variscan trends (Fig. 3).Therefore, as a result of the Alpine N-S horizontal compression, the northern segment of this structure is reactivated as a right lateral strike-slip fault, while to the south it is transformed into an imbricate thrust system (e.g.Brañuelas thrusts).

Ancares sector
The Ancares sector is located to the west of the Ibias-Ancares structure (Fig. 1D).Its deformation is concentrated along the southern margin within an imbricate thrust system, thrusting the northern El Bierzo depression (Figs. 3 and 4).The number of structures and the overall shortening in this sector are less than in the Fuentes Carrionas-Miravalles sector.These structures deformed and dissect the Tertiary basins of el Bierzo and O Barco (Figs. 6 and 7).In this sector the orientation of the Alpine structures are NE-SW (e.g.Paradaseca and Rubiana thrusts) (Figs.5E, 5F, and  9A) presenting a left lateral component.The change in orientation may be related to a change in orientation of the old basement structures (Asturian Arch), which run NW-SE in this sector (Fig. 3).As in the Fuentes Carrionas-Miravalles sector, backthrusts (e.g.Balouta) generate a tectonic pop-up where the highest peaks (2,000 m) are located (Martín-González, 2009).
The Rúa-Vilalba and Ibias Ancares structures partially transfer the CBT deformation to the north (Fig. 4C).So, the Rúa-Vilalba structure can be considered a lateral ramp and a transpressive structure of the CBT.The NE-SW strike-slip faults, offset the structure and dissect the related Tertiary foreland sediments, obliterating its cartographic continuity (Fig. 3).Thus, the western border of Ancares sector is the Rúa-Vilalba structure that terminates the 0DUWtQ*RQ]iOH] +HUHGLD -RXUQDO RI ,EHULDQ *HRORJ\ ).These faults produce the fragmentation of the remaining foreland basin, located near of the lateral end of the Cantabrian Mountains, which is displaced by these faults (Fig. 3), individualizing the Monforte, Sarria and Vilalba depressions (Martín-González y Heredia, 2011).NE-SW trending strike-slip faults are widespread across the NW Iberian Peninsula, with displacements being mainly horizontal with no outstanding associated relief, (except close to the El Caurel sector, where they show a reverse component, e.g.Fontaneira and Sarria faults) (Fig. 5D).Vertical displacement can be up to 300 m (Becerreá fault) and generating pivotal and hinge faults, which control the Tertiary depressions (e.g Monforte and Sarria).This tectonic interpretation agrees with the gravimetric studies of the Sarria depression, where it has been shown that the basin depocentre geometry is controlled by NE-SW faults (Martín-González et al., 2003).A vertical component is also recognized in the NE-SW faults in Portugal which were reactivated in Upper Miocene-Quaternary times (Cabral, 1989;Carvalho et al., 2006).The NE-SW faults are associated with active seismicity (González-Casado and Giner, 2000; Rueda and Mezcua, of the Tertiary deposits of the El Bierzo and O Barco depressions (Figs. 6, 7, and 9D).These depressions are also bounded in the north by the CBT, which generated a tectonic pop-down where the sediments are preserved (Fig. 9A).The north-vergent thrusts of the GLM dissect the previously emplaced thrusts of the Cantabrian Mountains front, and raise the northern part of the ancient Duero foreland basin, individualizing the El Bierzo and O Barco Tertiary depressions (Fig. 3).The other Alpine structures in the GLM are strike-slip faults, in many cases related to the thrust, conforming their lateral structures.The NE-SW-trending faults are mainly left lateral strike-slip faults (e.g.Viana and Sanabria faults) and are linked with the NE-SW corridors of the Iberian Massif (Braganza-Vilariça and Vila Real-Verín) (Fig. 3).The highest peaks of this region, such as Teleno (2,100 m) and Cabeza de Manzaneda (1,700 m), are located in the north, close to the main alpine thrusts (Figs. 3 and 4).

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The Rías Baixas-Terra Chá region (Fig. 2) has few topographic contrasts, and is mainly characterized by a pre-Cenozoic planation surface of 400-500 m (García Abad and Martín Serrano, 1980;Martín-González, 2009).In this region Tertiary-age sediments, mainly surrounding the Cantabrian and Galaico-Leoneses Mountains, are preserved.The region represents the western less- (including pebbles and boulders of quartzite) sandstones and rare lutites.The Médulas Fm overlies the basement while the other tertiary formations lies uncorformably to disconformably towards their distal parts.
The NE-SW strike-slip faults can be subdivided into WZR JURXSV 1 DQG 1 7KH ¿UVW JURXS LV widespread in the NW Iberian Peninsula and has the same orientation as a series of regional diabase dykes (Abril and Rodríguez-Fernández, 1981), and fractures related to opening of the Atlantic Ocean during Late Jurassic and Early Cretaceous times (Ancochea et al., 1992, Vegas et al., 2004).Moreover, the N 40º-60º orientation and kinematics are consistent with an E-W-oriented extension.Therefore, we consider this fault group to be mainly Mesozoic in age with fault reactivation associated with the N-S Alpine compression.Taking into account that the horizontal maximum compression calculated in the area is SSE-NNW (Herraiz et al., 2000;González-Casado and Giner, 2000), it is likely that the NE-SW strike slip faults were reactivated in the Alpine compressional regime.The second group is more consistent with a N-S Alpine compression and has hydrothermal and seismic activity associate (e.g.Orense Fault).
The N-S steeply-dipping faults are located mainly along the Atlantic coast (e.g.Pontevedra Fault).These faults are related to Mesozoic extension related with the opening of the Atlantic Ocean (Boillot and Malod, 1988).The N-S faults are offset by NE-SW and NW-SE strike slips faults (Fig. 3).

Tertiary sediments and Alpine tectonics
The aforementioned structures, mapped in the western extension of the Alpine-Pyrenean Orogen, have been developed mainly in a Paleozoic basement which was highly deformed during the Variscan Orogeny, with scarce overlain of Tertiary sediments.These sediments are found in isolated outcrops and tectonic depressions PDLQO\ ¿OOHG E\ WHUULJHQRXV FRQWLQHQWDO VHGLPHQWV transported by alluvial fans under arid or semi-arid conditions as a response to tectonic activity (Alonso et al., 1996;Alonso Gavilán et al., 2004).The sedimentary synorogenic record is similar in the main basins and three IRUPDWLRQV KDYH EHHQ LGHQWL¿HG )URP EDVH WR WRS WKHVH are the: Toral, Santalla and Médulas formations (Herail, 1981;Martín-González and Heredia, 2011).The Toral Fm is mainly composed of clay, lacustrine limestones and arkosic sandstones (Fig. 5A and 9E).The Santalla Fm comprises sandstones, gravels and polygenetic pebbles (quartzite, shales, sandstone and mudstone clasts) and lutites.The Médulas Fm is composed of gravels The structural and sedimentary data suggest that the present-day Tertiary depressions are the remains of a more extensive foreland basin that was subject to FRPSDUWPHQWDOL]DWLRQ LQ LWV ¿QDO VWDJHV 0RUHRYHU ERUGHU facies related to the faulted boundaries of this initial foreland basin have not been found (Martín-González and Heredia, 2011).
The paleontological data of the Toral Fm indicate a Oligocene to Early Miocene age in As Pontes Basin (López Martínez et al., 1993;Santanach et al., 2005), Early Oligocene in the El Bierzo (Freudenthal et al., 2010) and Late Eocene in the Oviedo Basin (Truyols et al., 1991).

Discussion
Based on detailed structural mapping and observations as presented herein, we propose a new model for the western termination of the Alpine-Pyrenean Orogen.This model connects with the currently accepted models for the eastern Cantabrian Mountains (Alonso et al., 1996;Pulgar et al., 1999;Gallastegui et al., 2002).In that part of the Cantabrian Mountains the main structure consists RI D UHJLRQDO PRQRFOLQDO ÀH[XUH RI D PDMRU IDXOWEHQG IROG of the Cantabrian Basal Thrust-CBT (Fig. 4A).Toward the west, the CBT emerges as an imbricate thrust system verging southwards (e.g.Brañuelas and Noceda thrusts) )LJ DQG DQG WKH PRQRFOLQDO ÀH[XUH LV WUDQVIRUPHG into a tectonic pop-up (Ancares and El Caurel Mountains, 2,000 m) due to a series of backthrusts (e.g.Villablino Thrust) (Fig. 4B).
No data from Alpine structures at upper crustal scales are available in the westernmost termination.Therefore, we have followed crustal constraints used by other authors in the eastern part of the studied area (Alonso et al., 1996;Pulgar et al., 1996;Gallastegui et al., 2002) (Fig. 4).These thrust, and the associated relief, decrease GLVSODFHPHQW ZHVWZDUG DQG ¿QDOO\ WKH\ OLQN XS ZLWK N-S-orientated arch-shaped structures (called Ibias-Ancares and Rúa-Vilalba).The Rúa-Vilalba structure, is a large fault zone that partially transfers the terminal CBT shortening to the north into the As Pontes Fault (Fig. 3  and 4C).The Rúa-Vilalba and Ibias-Ancares structures are the western limit for the Cantabrian Mountains (Fig. 3 and 10).Additionally the Rúa-Vilalba structure is the eastern boundary of the Tertiary-age Monforte, Sarria and Vilalba depressions.These Tertiary depressions also trend N-S, parallel to the Rúa-Vilalba structure.These lateral structures can be correlated with the western end of a high velocity intracrustal layer, which has been interpreted as the indented lower crustal wedge emplaced during the crustal thickening of the Alpine-Pyrenean Orogen (Pedreira et al., 2007).One explanation for the westernmost lateral end of this seismic and gravimetric high velocity layer may be the Ibias-Ancares and Rúa-Vilalba structures, which form the western end for the Cantabrian Mountains.Subsequently, the emplacement of north-verging thrusts resulted in uplift of the Galaico Leonese Mountains (Teleno and Cabrera peaks, 2,100 m) and displacement of the Cantabrian Mountains structures (e.g.Villafranca and Portela thrusts).Thus, these north-vergent thrusts and related NE-SW faults, generate an E-W tectonic pop-down (Fig. 7), segment the ancient Tertiary foreland basin (e.g.El Bierzo and O Barco depressions) and also divided the El Caurel sector from the Ancares sector (Fig. 3 and 10).This activity produced the vertical displacement of the Tertiary deposits (Fig. 6).The most relevant vertical displacement is recorded in Corporales (Teleno Mountains) where Tertiary sediments from the El Bierzo depressionwere uplifted a minimum of 900 m.Crosscutting relationships are in agreement with the DSDWLWH ¿VVLRQ WUDFN VWXGLHV WKDW VKRZ KRZ WKH *DODLFR Leoneses Mountains begin the uplift after the Cantabrian Mountains (Miocene vs Oligocene) (Martín-González et al., 2008).
The paleocurrents and lithologies indicate, for the earlier stages, a distal position to the northern thrusts &0 0HDQZKLOH LQWHUPHGLDWH DQG ¿QDO VWDJHV LQGLFDWH north and south provenance related with the present relief.Moreover, no fault border facies have been found in the earlier stages.Therefore, only after the emplacement of the GLM thrusts, would the El Bierzo and O Barco depressions be considered as intramontane basins.The basin evolution, proposed in this work, is similar to described in the Central Andes, where the foreland basin of the east verging main Andes (Alta Cordillera and Precordillera) is deformed and dissected by the uplift of the foreland basement of west-verging Sierras Pampeanas mountains (Rodríguez Fernández et al., 1999;Ramos et al., 2002).This type of foreland basins is called "Broken Foreland Basin" by Jordan (1995).
The shortening of the GLM north verging thrusts is compensated by the NE-SW left-lateral strike-slip faults in the west, and the right-lateral strike-slip faults in the east (Fig. 3).The kinematics and arch shape of El Caurel indicate a N and NNW tectonic transport.NE-SW oriented strike-slip faults are widely distributed on the NW Iberian Peninsula.These faults displace and in some sectors obliterate the Rúa-Vilalba structure.Their displacements are mainly horizontal with no relevant associated relief, except where they run near the El Caurel sector where they show a vertical component that generates pivot and hinge faults.The horizontal component is responsible for the dissection of the Tertiary-age sediments in the Monforte, Sarria and Vilalba depressions (Fig. 8).Vertical component is also recognized in the NE-SW trending faults in Portugal which were reactivated in Upper Miocene-Quaternary times (Cabral, 1989;Carvalho et al., 2006)).
7KH JHRORJLFDO PDS RI WKH ¿JXUH VKRZV WKDW WKH WKUXVW trace of the Cantabrian Mountains turns northward, GHFUHDVH GLVSODFHPHQW DQG WKH ¿QDO GLVSODFHPHQW LV partially transferred in this direction by N-S lateral structures (Ibias Ancares and Rúa-Vilalba) (Fig. 3, 6 and  8).In this way, the continuation of the CBT is not along the NE-SW left lateral corridors of Braganza-Vilariça and Vila Real-Verín (Santanach, 1994;Vegas et al., 2004;De Vicente and Vegas, 2009).The model described by WKHVH DXWKRUV GRHV QRW H[SODLQ WKH VLJQL¿FDQW UHOLHI RI the Galaico-Leoneses Mountains (up to 2,100 m) and the tectono-sedimentary evolution of the NW Tertiary depressions (El Bierzo, Sarria, Monforte).However, in the present model the Galaico-Leoneses Mountains are uplifted by the north vergent thrusts (Fig. 7).The proposed model agrees with the activity of these strikeslip corridors during the southward emplacement of the Cantabrian Mountains, this tectonic style is well known where thrust belts turn or curve, producing large strike-slip corridors into the foreland, as is the case in the Himalayas (e.g.Molnar and Taponier, 1975;Shen et al., 2001;Storti et al., 2003).But these corridors are not the main responsible for the Galaico Leoneses-Mountain uplift that take place later (Fig. 10).
The Variscan basement features of the NW Iberian 3HQLQVXOD PDNH GLI¿FXOW WR GDWH WKH &HQR]RLF WHFWRQLF activity (Martín-Serrano et al., 1996).However, in the light of the present work and the newly published paleontological ages, we can establish some restrictions with regards to the timing of the Alpine tectonic activity.The paleontological ages for the Toral Fm are Late Oligocene to Early Miocene in As Pontes (López Martínez et al., 1993;Santanach et al., 2005), Lower Oligocene in el Bierzo (Freudenthal et al., 2010) and Late Eocene in Oviedo (Truyols et al., 1991).On the other hand, the GLM were active during the time of deposition of Santalla Fm.This formation can be correlated with Candanedo Fm (Duero Basin), whitch is Oligocene in age (Colmenero, 1982;Corrochano, 1989) and can extend up into Upper Miocene times (Vallesian) (Herrero et al., 2004).The Oligocene age for the Toral Fm is coeval with that of the As Pontes age (Fig. 10).These observations suggest that tectonic activity began in the northeast 2YLHGR,Q¿HVWR %DVLQ GXULQJ WKH (RFHQH DQG PLJUDWHG It is worth mentioning that on the Iberian Plate, after the Middle Miocene, the maximum horizontal compression changed due to the gradual relocation of the active plate boundary to the south of Iberia (Dewey et al., 1989;Srivastava et al., 1990;Andeweg, 2002).Therefore, a part of the NW Iberian Peninsula tectonic activity was produced by intraplate propagation of the compressive deformation from the Betic Chains (SE Iberian Peninsula).

Conclusion
The geology observed along the western termination of the Alpine-Pyrenean Orogen relieves resulted from the superposition of two mountain ranges (Cantabrian Mountains and Galaico-Leoneses Mountains) and a complex tectonic evolution.The Cantabrian Mountains (CM) were emplaced southward along a basal north dipping thrust (CBT).Westward, the CBT emerges as an E-W imbricate thrust system with south vergence.The PRQRFOLQDO ÀH[XUH UHODWHG WR WKH QRUWK GLSSLQJ WKUXVW transforms into a tectonic pop-up.E-W thrusts decrease GLVSODFHPHQW ZHVWZDUG DQG ¿QDOO\ WKH\ OLQN XS ZLWK 16 trending arch structures (Ibias-Ancares and Rúa-Vilalba structures) explaining the termination of the E-Wtrending ranges of the Cantabrian Mountains and the N-S distribution of Vilalba, Monforte de Lemos and Sarria depression.These two structures partially accommodate the CBT shortening to the north.to the west (As Pontes Basin) during Late Oligocene-Early Miocene times.In the southern border of the Cantabrian Mountains, (northern Duero Basin) much of the Tertiary deposits are deformed, except the uppermost formations that onlap, and are syntectonic with the Alpine structures.The age of these syntectonic deposits ranging from Oligocene to early Upper Miocene (Colmenero et al., 1979;Corrochano, 1989;Herrero et al., 2004).However, in the western part, the youngest formation (Medulas Fm) is deformed by the north verging thrusts of the Galaico-Leoneses Mountains.These observations would suggest that tectonic activity in the Cantabrian Mountains had ceased while in the Galaico-Leoneses Mountains north verging-thrust activity continued.This observation is in agreement with the structural mapping where Galaico-Leoneses Mountains structures offset the Cantabrian Mountains structures (Fig. 3).In addition, the NE-SW strike-slip faults that are related with the north vergent thrusts, present a vertical component that have been dated in Portugal as Upper Miocene-Quaternary in age (Cabral, 1989;Carvalho et al., 2006).NE-SW faults are associated with active seismicity (González-Casado and Giner, 2000;Rueda and Mezcua, 2001;López-Fernández et al., 2004;Martínez-Díaz et al., 2006) and this together with the intense drainage reorganisation observed (Martín-González, 2009), would suggest that WKHUH ZDV D VLJQL¿FDQW /DWH 0LRFHQH4XDWHUQDU\ DFWLYLW\ along these faults.The Galaico-Leoneses Mountains (GLM) were emplaced northward by north-verging thrusts.The shortening along these thrusts was compensated by NE-SW left-lateral strike-slip faults in the west.These faults link with the NE-SW corridors of the Iberian Massif (Braganza-Vilariça and Vila Real-Verín).The GLM structures offset the CM structures.
Based on the observed structures, four main Alpine regions have been established in the NW Iberian Peninsula: (1) the Astur-Galaica region (AGR) in the western part of the Cantabrian Mountains, characterized by thrusts with south vergence, that represent the continuation of the South Pyrenean Zone structures, (2) The Galaico-Leoneses Mountains (GLM), characterized by thrusts with north vergence, (3) The Rías Baixas-Terra Chá region (RBT) characterized by strike-slip faults with no relevant associated relief but with seismicity, which represents the less-deformed foreland.
The Tertiary sediments of the El Bierzo depression, which today are preserved in small and isolated depressions, were formerly part of a widespread foreland basin located around the Cantabrian Mountains, that was dissected and eroded by the uplifting of the GLM.Thus, the Tertiary-age depressions of El Bierzo and O Barco are located in an E-W tectonic pop-down bounded by the CM and GLM thrusts.The Tertiary-age Monforte, Sarria and Vilalba depressions are limited to the east by the N-S structure of Rúa-Vilalba.Subsequently, NE-SW strikeslip faults offset the Rúa-Vilalba structure and dissected the Tertiary sediments.
The geological data presented in this work suggest that Alpine deformation in the NW Iberian Peninsula began in the northeast (Oviedo Basin) during the Eocene.In the western and southern areas, tectonic activity commenced during the Oligocene-Early Miocene (El Bierzo and As Pontes Basin).The Cantabrian Mountains tectonic activity ended at the beginning of the Late Miocene.However, deformation continued in the GLM through the Late Miocene and Quaternary.Thus, the tectonic activity in the GLM was longer lasting than in the CM, and is DOVR VXSSRUWHG E\ DSDWLWH ¿VVLRQ WUDFNV VWXGLHV 0DUWtQ González et al., 2008).In that study it was also observed that uplift of the CM began in Oligocene times, while the GLM uplift commenced during the Middle and Late Miocene.

Fig. 7 .
Fig. 7.-Detailed geological map and cross sections of the O Barco Tertiary depression.Location in Figura 4. Tertiary sediments are limited and dissected mainly by south and north verging thrusts showing a tectonic pop-down.Note the relation of north verging thrusts with the NE-SW strike slip faults and the turn northward of the Portela thrust (CBT).Fig. 7.-Mapa geológico de detalle y corte de la depresión terciaria GH 2 %DUFR /RFDOL]DFLyQ HQ ¿JXUD /RV VHGLPHQWRV WHUFLDULRV están limitados y segmentados principalmente por cabalgamientos vergentes tanto al norte como al sur, mostrando una tectónica de tipo pop-down.Nótese la relación de los cabalgamientos vergentes al norte con los desgarres NE-SW y el giro hacia el norte del cabalgamiento de la Portela (parte del Cabalgamiento Basal Cantábrico)

Fig. 8 .
Fig. 8.-Detailed geological map and cross sections of the Sarria Tertiary depression.Location in Figure 4. Tertiary sediments are limited and dissected mainly by NE-SW strike-slip faults with a vertical component and eastward by a N-S structure (Rua-Vilalba).Note the offset and displacement of the N-S structure by the strikeslip faults.Fig. 8.-Mapa geológico de detalle y corte de la depresión terciaria GH 6DUULD /RFDOL]DFLyQ HQ ¿JXUD /RV VHGLPHQWRV WHUFLDULRV HVWiQ limitados y segmentados principalmente por desgarres NE-SW con componente vertical y por el este por una estructura N-S (Rúa-Vilalba).Nótese el desplazamiento de la estructura N-S por los desgarres.

Fig. 10 .
Fig. 10.-Tectonic sketch map and cross section showing tectonic model proposed of the western termination of the Alpine-Pyrenean Orogen reliefs.Main tectonic transport has been drawn and relative age uplift.Fig. 10.-Mapa tectónico esquemático y corte mostrando el modelo tectónico propuesto para la terminación occidental de los relieves del 2URJHQR $OSLQR3LUHQDLFR /DV ÀHFKDV LQGLFDQ OD dirección de transporte tectónico y las edades del levantamiento.