Flexural to broken foreland basin evolution as a result of Variscan collisional events in northwestern Spain
Published:January 01, 2007
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M. Keller, H. Bahlburg, C.D. Reuther, A. Weh, 2007. "Flexural to broken foreland basin evolution as a result of Variscan collisional events in northwestern Spain", 4-D Framework of Continental Crust, Robert D. Hatcher, Jr., Marvin P. Carlson, John H. McBride, José R. Martínez Catalán
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The Variscan Cantabrian Zone of northern Spain records the transition from passive-margin sedimentation to foreland basin deposition during the Late Devonian. Initiation of the foreland basin system is reflected in the evolution of a hinge line that separated the foredeep from the adjacent peripheral bulge. Bulge erosion provided siliciclastic detritus that was deposited in the evolving foredeep. After cessation of sedimentary input, the foredeep achieved starved conditions during the earliest Tournaisian. These events were a long-distance effect of initial subduction of the Iberian continental margin prior to collision between northwestern Gondwana and North America. During the Namurian, propagation of the deformation front and the rise of the orogen led to increasing input of detritus that began to fill the foredeep.
During the late Namurian and the early Westphalian, deformation started to affect the Cantabrian Zone. The Palencian nappes were emplaced onto the Valsurvio Dome, after which thick wedge-top deposits formed (the Curavacas Conglomerate). When the Esla nappe and the Valsurvio Dome became involved in deformation and approached the rigid lithosphere of the Cantabrian Block, anticlinal stacks began to form, and the lithosphere broke. Two major faults developed, the León fault and the Ruesga fault, which dissected the lithosphere to accommodate further compression. The resulting topography led to the gravitationally induced gliding of the Palencian nappes into the Pisuerga Carrión unit, where sedimentation and deformation continued under broken foreland basin conditions. The Cantabrian Mountains preserve a foreland basin system that in its early stage was formed under flexural deformation of the lithosphere induced by subduction. When deformation affected the autochthonous continental lithosphere, the size of the flexurally induced basins changed from a single, large-scale depocenter to smaller-scale, interconnected basins. In its late stage of development, the Cantabrian foreland basin system records the transition to broken foreland conditions as the thrust front approached the rigid Cantabrian Block, a basement feature that has influenced the tectonostratigraphic evolution of the Cantabrian Zone since Silurian times. Incipient oroclinal bending during the main stage of thrusting might have been an additional factor triggering the failure of the lithosphere.
Foreland basins systems (DeCelles and Giles, 1996) are depositional centers that form in response to compressional platetectonic events and the resulting deformation of the continental lithosphere. The foreland basin system discussed in this paper developed during the late Variscan formation of Pangea, which included the collision between Gondwana and Laurussia. During this collision, several smaller-scale continental fragments were trapped between the colliding landmasses, and each of them has a distinct deformational history (Franke, 2000; Stampfli et al., 2002). In many cases, neither the exact timing nor the interactions between the terranes during accretion are clear (Franke et al., 2000; Robardet, 2000, 2003; McKerrow et al., 2000).
The foreland basin system in the Cantabrian Mountains (Figs. 1 and 2) developed during the Late Devonian (Keller, 2000) and persisted until the close of the Variscan orogeny during the Stephanian. In the Cantabrian Mountains, recognition of foreland basin initiation is based on the geometry and stratigraphy of the Upper Devonian deposits and on a basinwide unconformity that is interpreted to represent the erosion of the peripheral bulge (Keller, 2000). The first episode of foreland basin sedimentation continued for several millions of years and is characterized by pelagic, condensed successions.
During the Namurian and Westphalian, a depocenter developed in the southeastern Cantabrian Zone that accumulated thick successions of synorogenic detritus. This basin is known as the Pisuerga Carrión Unit (Figs. 2 and 3). Its basin fill traditionally has been interpreted to represent syntectonic sedimentation in a foreland basin, the “flysch basin” of the Cantabrian Mountains sensu Julivert (1978), Reuther (1977), and Maas (1974). In recent models (Rodríguez Fernández, 1993, 1994; Rodríguez Fernández and Heredia, 1987), unconformity-bounded sedimentary units are attributed to the advance of discrete nappe piles, and a succession of individual foreland basins has been reconstructed.
The intentions of this paper are (1) to document the Late Devonian through Pennsylvanian foreland basin evolution of the Cantabrian Mountains as represented by the sedimentary basin fill; (2) to document that there was a single, long-lived foreland basin, which reflects flexural bending of the lithosphere in its early evolution and the abrupt change to broken foreland conditions during its late stage; and (3) to propose a new model for the interaction between sedimentation and tectonics in the Pisuerga Carrión unit.
Our interpretation of the Carboniferous sedimentation in the Cantabrian Zone is based on different models that were developed for individual depocenters to explain sedimentary patterns during the Variscan orogeny. Together with our own observations, these models are combined in a single comprehensive model that tries to duplicate the events that created a foreland basin system during the Late Devonian and that are responsible for a complex architecture of basin-fill sediments.
The Northern Iberian Massif
East-west cross sections through the Variscan rocks of northwestern Spain (Julivert, 1971, 1979, 1981; Marcos and Pulgar, 1982; Fernández Viejo et al., 2000; Gallastegui et al., 1997; Pérez-Estaún et al., 1988, 1994) show the Central Iberian Zone (Julivert, 1971) in the core of the Iberian Massif, where Variscan deformation started during the Devonian (Dallmeyer et al., 1997; Martínez Catalán et al., 1997). During the Late Devonian, west-directed subduction of the northwestern Iberian margin beneath an accretionary wedge preceded the main collisional event that formed the Iberian Variscan orogen (Rodríguez et al., 2003). During the Mississippian, several allochthonous complexes including oceanic crust were obducted onto the autochthonous Central Iberian Zone, where they are preserved as klippen in northwestern Spain and northern Portugal (Fig. 1; Arenas et al., 2000).
The Central Iberian Zone (Julivert, 1971) is the most internal zone of the Iberian Massif. It is composed of Upper Proterozoic terrigenous sediments and volcanic rocks (today mainly gneisses) overlain by Lower Ordovician marine metasediments. The youngest pre-orogenic rocks are Silurian and Lower Devonian platform deposits. The first deformation affected the Central Iberian Zone at ca. 359 Ma (Dallmeyer et al., 1997), shortly after the Devonian-Carboniferous boundary.
The West Asturian Leonese Zone (Fig. 1) shows many characteristics of thick-skinned tectonics (Martínez Catalán et al., 1990). Stratigraphically, it is composed of a very thick succession of Cambrian through Silurian sedimentary rocks (Julivert, 1983; Pérez-Estaún et al., 1990) that locally accumulated to almost 10 km of strata. In contrast, Lower Devonian rocks are very thin and are only present in a few localities (Drot and Matte, 1967; Truyols Massoni, 1986). Emsian through Westphalian sediments are nowhere preserved. Stephanian molasse deposits unconformably overlie the Lower Paleozoic rocks (Pérez-Estaún et al., 1990). Carls (1988) claimed that many of the intervening deposits had been stripped off and transported toward the Cantabrian Zone as the Palencian nappes.
During the Mississippian, at around 336 Ma (Dallmeyer et al., 1997), deformation began to affect strata close to the eastern edge of the West Asturian Leonese Zone. During the early Namurian, the boundary between the West Asturian Leonese zone and the Cantabrian Zone was incorporated in the deformation. The accompanying metamorphism is estimated to have taken place post–early Emsian and pre–late Westphalian (Suárez et al., 1990).
THE CANTABRIAN ZONE
By Westphalian B time, thrusting and deformation had migrated farther east, and the westernmost parts of the Cantabrian Zone (the Somiedo Correcilla unit; Fig. 2) experienced their first deformation (Pérez-Estaún et al., 1988; Dallmeyer et al., 1997). Today, the Cantabrian Zone is the northeastern thin-skinned foreland fold-and-thrust belt of the external Iberian autochthon (Quesada, 1992). There is still considerable discussion about the arcuate pattern (Iberian-Armorican or Cantabrian-Asturian Arc) of the Variscan rocks in northwestern Spain (see Aller et al., 2002): whether it is a primary feature or of secondary origin and whether the curvature was initiated by oroclinal bending or by strike-slip deformation (Van der Voo et al., 1997; Hirt et al., 1992; Parés et al., 1994; Steward, 1995; Weil et al. 2000, 2001; Shelley and Bossiere, 2000). The most controversial issues are the age of the presumed bending and the intensity of rotation that affected the Cantabrian Zone between the Westphalian and the Permian. The most radical reconstruction was proposed by Weil et al. (2000, 2001) and Kollmeier et al. (2000), who claimed that the original extension of the shelf was essentially north-south and that all of the curvature is of secondary origin.
Tectonosedimentary Subdivision of the Cantabrian Zone
1. The Fold-and-Nappe Province is composed of three major thrust complexes, the Somiedo Correcilla unit, the Sobia Bodón unit, and the Áramo unit (Fig. 2). East of the Porma fault (Fig. 2), the Esla nappe, the Valsurvio “Dome,” and the Sierra del Brezo are the southeastern extensions of the Somiedo Correcilla unit. Still farther east, a small fragment of the Fold-and-Nappe Province is preserved in the Revilla nappe (Fig. 3).
To the south and west, Precambrian sediments separate the Fold-and-Nappe Province from the West Asturian Leonese Zone (Fig. 2) and act as a rheologic boundary between the zones (Gutiérrez Alonso, 1992, 1996). Along the southern branch of the Cantabrian Zone, the León fault (Fig. 2) and the Ruesga fault (Fig. 3) separate the Fold-and-Nappe Province from the Central Coal Basin and the Pisuerga Carrión unit. There is ongoing debate about the role of these faults during sedimentation, their age, and their origin (Marcos, 1968a, 1968b, 1979; Kullmann and Schönenberg, 1978; Wagner et al., 1984; Espina et al., 1996; Weh, 2006). The Fold-and-Nappe Province (Fig. 2) has a continuous succession of Silurian and Devonian rocks along its western and southern margin. These sediments were deposited on a shelf in a passive-margin setting. Toward the northeast, there is a systematic change toward proximal depositional environments and an increase in number and magnitude of unconformities and hiatuses.
A profound change in depositional style took place during the Frasnian, when the shelf became dissected along a hinge line into a southern basinal area with rapid subsidence and a tilted northern area of uplift and erosion. The corresponding unconformity is mantled by uppermost Devonian rocks of the Ermita Formation and will further on be referred to as “pre- Ermita unconformity.” These tectonically induced changes reflect the initiation of the Cantabrian foreland basin. The area of uplift and erosion corresponds to the peripheral bulge, the area south of the hinge line to the foredeep. This hinge line, called “Intra-Asturo Leonesian faciesline” by Raven (1983), is recognized from the Asturian coast to the Esla area. From there, the hinge can be traced to the Valsurvio Dome and the Sierra del Brezo into a major shear zone, which today separates these two units.
Soon after the formation of the pre-Ermita unconformity, and close to the Devonian-Carboniferous boundary, most of the Cantabrian Zone became deeply submerged, and a very thin and strongly condensed succession represents the entire Mississippian. This succession is composed of Tournaisian black shales with phosphorite nodules (Vegamian Formation), and Visean nodular limestones, red siliceous shales, and abundant debrisflow deposits (Genicera or Alba Formation). Together with the sandstones and crinoidal limestones above the pre-Ermita unconformity, this succession will further on be called the Mississippian condensed succession.
2. The Pisuerga Carrión unit is the relative autochthon to all the surrounding nappe piles (Julivert, 1978; Aller et al., 2002; Rodríguez Fernández, 1993, 1994; Rodríguez Fernández and Heredia, 1987). Successive emplacement of the different nappes led to the deposition of discrete clastic wedges in front of the individual thrust piles and the formation of multiple unconformities between the individual wedges (Fig. 4B; Alonso and Rodríguez Fernández, 1983; Rodríguez Fernández and Heredia, 1987). The age of the oldest sediments in the autochthon is not well constrained. Heredia et al. (1990) described Tournaisian to Namurian deposits, which in their interpretation form the pre-orogenic succession. Recent field work suggests that these deposits are contained within allochthonous complexes and that the oldest preserved autochthonous successions (Perapertú Formation and equivalents) are of late Namurian age (Fig. 4B). The siliciclastic strata of the Pisuerga Carrión unit are generally poorly dated, and their spatial relationships are not clear, mainly because differentiation of strata within the allochthon and the autochthon is controversial (Fig. 4B).
3. The Palencian nappes (Fig. 3) are contained within the Pisuerga Carrión unit as allochthonous, apparently gravitationally induced structures (Wagner, 1971; Ambrose, 1974; Frankenfeld, 1983, 1984; Marquínez and Marcos, 1984; Rodríguez Fernández, 1993, 1994; Rodríguez Fernández and Heredia, 1987). Middle and Upper Devonian sediments are represented by pelagic facies not known from elsewhere within the Cantabrian Zone. These sediments have been called the “Palencian Facies” (Brouwer, 1964). The allochthonous units comprise strata from the Silurian through the Mississippian. They are present within the Alto Carrión unit (Fig. 3; Rodríguez Fernández, 1994), the Gildar-Montó area (Marquínez and Marcos, 1984; Frankenfeld, 1983), the San Julian “klippen” and Mudá area (Wagner, 1971), and the Liébana area, where the Palencian Facies was redeposited as large olistoliths (Maas, 1974). Rocks of the Palencian Facies are unconformably overlain by a thick succession of the Curavacas Conglomerate (Kanis, 1956; Brouwer and Van Ginkel, 1964; Colmenero et al., 1988; Fig. 4B). According to our field work, this angular unconformity is only present within the allochthonous units. It was attributed to the Palencian folding phase (Wagner 1959, 1965) of early Westphalian age and used to confine the emplacement of the Palencian nappes into the Pisuerga Carrión flysch basin to the late Namurian or early Westphalian.
Wagner (1971), Ambrose (1974), and Wagner and Winkler Prins (2000) claimed that the Palencian Facies was transported from the north toward the south either from within the Pisuerga Carrión unit (Ambrose, 1974) or from outside (Wagner, 1971; Wagner and Winkler Prins, 2000). This would place the Palencian nappes into a position now occupied by the Picos de Europa nappes (Fig. 2) or still farther north. Frankenfeld (1983, 1984) argued in favor of a southern provenance for the Palencian Facies within the West Asturian Leonese Zone. Structural and sedimentologic evidence supports this view (Marquínez and Marcos, 1984; Henn and Jahnke, 1984; Carls, 1988; Rodríguez Fernández and Heredia, 1987; Rodríguez Fernández, 1993, 1994).
4. The Ponga nappe unit (Fig. 2) was thrust eastward onto the Pisuerga Carrión unit between late Westphalian D and Stephanian A time. It consists of a large number of individual thrust sheets, which show strong interference patterns of the various deformations due to their position close to the core of the Asturian Arc.
5. The Picos de Europa unit (Fig. 2) was the last major thrust unit to be emplaced. There is general agreement that thrusting occurred during the Stephanian B and that transport was essentially south directed (Rodríguez Fernández and Heredia, 1990; Aller et al., 2002). The corresponding deformation is recognized in the Ponga nappe unit, the Pisuerga Carrión unit, and the Valsurvio Dome.
Stratigraphically, the Ponga nappes, the Picos de Europa nappes, and the Central Coal Basin (Fig. 2) are characterized by the absence of Silurian and most of the Devonian strata. Very thin, uppermost Devonian conglomerates, sandstones, and crinoidal limestones mantle the pre- Ermita unconformity, which in this area spans the Middle and Late Ordovician, the Silurian, and the Devonian. This major hiatus led to the postulation of the Cantabrian Block (Fig. 2; Radig, 1962) or the Asturian geanticline (Van Adrichem Boogaert, 1967) as a structural high with no sedimentation or, alternatively, erosion during Silurian-Devonian times. The Cantabrian Block was the source area for most of the siliciclastic rocks deposited on the Asturo Leonese shelf. After deposition of the Mississippian condensed succession, the Central Coal Basin and the Ponga nappe unit received abundant and thick successions of synorogenic detritus from the Westphalian onward. In contrast, the Picos de Europa province has an almost exclusively calcareous succession that spans most of the Carboniferous.
FORELAND BASIN RESTORATION
Relative Position of Tectonic Units Prior to Thrusting
The paleogeography of the convex side of the Cantabrian Zone prior to thrusting and the organization of the foreland basin system are reflected in the stratigraphy and sedimentology of the Silurian through Namurian strata within the major thrust sheets (Fig. 4).
Alba Syncline (Figure 5)
Located south of the hinge line, the Silurian-Devonian stratigraphy shows the most complete record on the shelf and the most distal depositional environments. During the Late Devonian, several individual prograding clastic successions record a northern source area (Van Loevezijn, 1986; Raven, 1983). After deposition of the Mississippian condensed succession, siliciclastic input from a southern source is interpreted to reflect the approach of the orogenic front (Reuther, 1977; Julivert, 1978). This evolution of the Carboniferous succession is unique to the Alba syncline (Fig. 6).
In the eastern Las Palomas syncline and in the northern Pedroso syncline, a similar succession is observed during the Carboniferous. However, the absence of most of the Upper Devonian rocks (Fig. 6) indicates that these sections were located north of the hinge line. Likewise, the autochthon of the Esla nappe is characterized by the absence of Upper Devonian deposits and the pre-Ermita erosion cut into Givetian limestones.
Valsurvio Dome (Figure 6)
The oldest strata in the Valsurvio Dome are of late Emsian age. Despite its greenschist facies metamorphic overprint, all stratigraphic equivalents of the Lower and Middle Devonian succession in the Esla area are recognized. The Upper Devonian is represented by a very thick succession of coarse-grained siliciclastic deposits. The facies of these pre-Ermita sediments is similar to that of the Somiedo area (Las Palomas syncline and adjacent areas). This very thick Upper Devonian succession of the Valsurvio Dome is interpreted to have been deposited south of the Upper Devonian hinge line. Above the Mississippian condensed succession, Namurian turbiditic limestones (Barcaliente Formation) are observed. However, there is a marked loss in thickness across the Valsurvio Dome from south to north beneath the overlying Valdeteja carbonate platform (Reuther, 1977; Wagner and Winkler Prins, 2000). Unconformable siliciclastic turbidites with intercalated quartz pebble conglomerates of Late Namurian and Westphalian A age (“upper Namurian turbidites”: Carmen Formation or Cervera Formation) represent the youngest pre-Stephanian rocks preserved in the Valsurvio Dome.
Pre–Upper Devonian strata are equivalent to those of the Valsurvio Dome. In the Sierra del Brezo, however, the Ermita unconformably rests on the Givetian limestones with a well-developed karst surface (Raven, 1983; Wagner and Winkler Prins, 2000). Recognition of the pre-Ermita unconformity in the Sierra del Brezo points to an original position north of the Upper Devonian hinge line. The overall Carboniferous sedimentation resembles that of the Valsurvio Dome, but the cut-out of strata beneath the Valdeteja carbonates is much more pronounced (Wagner and Winkler Prins, 2000).
Revilla Nappe (Figure 3)
The oldest strata exposed are of Givetian age and are unconformably overlain by the Visean nodular limestones (Fig. 6; Wagner et al., 1984). Another erosional unconformity separates the Visean rocks from the Valdeteja Formation; no Barcaliente Formation is preserved. Yet another erosional unconformity brings the upper Namurian turbidites, the youngest strata preserved in this nappe, on top of the Valdeteja carbonates (Fig. 6).
Sobia Bodón Unit (Figure 3)
In the Sobia Bodón unit, the proximity to the Cantabrian Block during Silurian–Devonian time is manifested by abundant erosional unconformities and proximal facies in both carbonate and siliciclastic systems (Van den Bosch, 1969; Evers, 1967; Rupke, 1965; Van Loevezijn, 1986; Buggisch et al., 1982; Keller, 1988; 1997). In vast areas, the pre-Ermita unconformity is cut into Lower–Middle Devonian limestones. At the eastern end of the Sobia Bodón unit (sections 5 and 6 in Fig. 4A), the Ermita locally rests on Ordovician rocks (Fig. 4). Above the Mississippian condensed succession, there is the Barcaliente Formation and then the Valdeteja carbonate platform (Fig. 4A). This platform interfingers with and is finally destroyed by the turbidites of the San Emiliano Formation. The interfingering, the successive onlap onto the platform, and its final destruction are evident in the eastern Sobia Bodón unit (near the village of Oville; Frankenfeld, 1976). Locally, no carbonate platform could develop, and siliciclastic turbidites rest on the Barcaliente Formation.
If the Palencian Facies are restored into a position just south or west of the convex boundary of the Cantabrian Zone (Fig. 7), as originally proposed by Frankenfeld (1983, 1984), then Silurian to Lower Emsian strata have a very close match in sections in the southern Alba syncline and the Las Palomas syncline of the Fold-and-Nappe Province. The differences between the Palencian Facies and the distal shelf facies in their restored position are much smaller than the facies changes that are observed if the shelf strata are traced from the distal sections to sections proximal to the Cantabrian Block. After a drowning event during the early Emsian (Keller, 1997), the shelf returned to carbonate-platform environments during the late Emsian, whereas the Palencian Facies remained in pelagic environments. Why the two areas that formerly formed a single shelf became separated during the late Emsian remains unclear.
Distribution of sedimentary successions and the position of unconformities within the restored paleogeographic units indicate that a simple north-south or east-west arrangement of transects is an oversimplification of the likely paleogeography. During the Silurian through Devonian, the Cantabrian Block acted as the dominant sediment source. Paleocurrent measurements in the Upper Devonian siliciclastic successions indicate a sediment transport away from the Cantabrian Block toward the south and west (Raven, 1983; Van Loevezijn, 1986). Proximal-distal trends in the older formations support these polarities (e.g., Buggisch et al., 1982; Suarez de Centi, 1988; García Ramos, 1978; Keller, 1988, 1997). It has also been demonstrated that the boundary between the shelf and the Cantabrian Block was not a linear feature but that there was a succession of promontories and reentrants, which exerted major control on the sedimentary patterns and erosional unconformities. Two well-documented examples are the Pardamino High (Fig. 5; Rupke, 1965; Evers, 1967) and the Somiedo High (Fig. 5; Raven, 1983).
This polarity around the Asturian Arc disappeared with the deposition of the Mississippian condensed succession, and, subsequently, a different orientation of the sedimentary surfaces developed. This is first evident during the deposition of the Visean nodular limestones, which unconformably rest on uppermost Devonian siliciclastic rocks of the Ermita Formation in the Valsurvio Dome area and farther east. In the Revilla nappe, finally, the nodular limestones are found on top of Givetian sedimentary rocks (Fig. 6).
The thickness of the Barcaliente Formation also decreases toward the east as it is successively cut out beneath the overlying Valdeteja Formation (Fig. 6). The Barcaliente Formation is not preserved in the Revilla nappe. Similarly, from the Valsurvio Dome toward the east, the erosional contact beneath the upper Namurian turbidites (Wagner, 1971) is always marked by paleokarst wherever these rest on older limestones. In contrast, in the Sobia Bodón unit, both the Barcaliente and the Valdeteja Formations are very thick, and no major unconformities are reported from there. During the Namurian and Lower Westphalian, a west-east gradient developed from a rapidly subsiding area close the advancing thrust sheets in the west toward an area in the east in which several subaerial unconformities testify to repeated uplift (Reuther, 1977; Wagner and Winkler Prins, 2000).
The Flexural Basin
Along the (present-day) western margin of Iberia, subduction of the Iberian autochthon under an accretionary wedge to the west or northwest started around 385–380 Ma (Rodríguez et al., 2003) and had ceased between 374 and 365 Ma (Martínez Catalán et al., 2002), subsequently giving way to collisional events during the Mississippian (Martínez Catalán et al., 2004a). Concomitantly, a foreland basin system developed that extended from the Central Iberian Zone across the West Asturian Leonese Zone into the Cantabrian Zone (Keller, 2000). Polarities across the subduction/collision zone, with west-directed subduction of the Gondwana margin and a foreland basin system to the east, indicate the evolution of a peripheral foreland basin (sensu Dickinson, 1974; Miall, 1995).
Distal remnants of the foredeep are preserved in the Palencian nappes, which at that time were part of the West Asturian Leonese Zone, and along the southern margin of the Somiedo Correcilla unit. The flysch-like deposits of San Clodio at the western margin of the West Asturian Leonese Zone (Riemer, 1963; Pérez-Estaún, 1974) have been attributed to this episode (Frankenfeld, 1984; Eichmüller and Seibert, 1984; Ábalos et al., 2002). However, as these deposits recently have been dated as early Namurian (Martínez Catalán et al., 2004b), they are younger than the events discussed here, so that there is no record of this episode in the northern Iberian massif outside of the Cantabrian Zone.
A hinge line separated the distal foredeep from the peripheral bulge, which was subject to erosion almost immediately after its rise (Keller, 2000). Current directions in the Upper Devonian rocks (Raven, 1983; Van Loevezijn, 1986) indicate that sediment export was toward the foredeep, perpendicular to the basin axis, from the uplifted area in the north and east into the newly formed basin. In the uplifted area north of the hinge line, the pre-Ermita unconformity developed because of forebulge erosion. Several prograding wedges are recognized, and one of them extended into the Palencian Facies area, where it is preserved as the Murcia Quartzite. The unconformity is not recognized in the Palencian Facies, which was deposited farther outboard and under deeper bathymetrical conditions.
There are no indications of Late Devonian activity along the Sabero Gordón fault or the León fault (Fig. 5) that would have accommodated the differential movements between uplift and subsidence, although synsedimentary activity along these faults was repeatedly postulated in the older literature (e.g., de Sitter, 1962; Kullmann and Schönenberg, 1978; Reijers, 1974; Krans, 1982; Raven, 1983; Van Loevezijn, 1986). Instead, it was the Late Devonian hinge line (Fig. 7) that acted as a flexural node of the newly established foreland basin. With the initiation of the foreland basin system during the Late Devonian, the Cantabrian Block was transformed from a sediment shedding hinterland into the foreland of the evolving Variscan orogen of northern Spain, and the former shelf was transformed into the distal foredeep. With the migration of the peripheral bulge toward the foreland, the entire region became successively submerged, and starved sedimentation prevailed during the Mississippian. A diachronous base of the lithostratigraphic units documents migration of the forebulge and the foredeep during Tournaisian and Visean times (Frankenfeld, 1984; Van Adrichem Boogaert, 1967). These pelagic environments extended from the West Asturian Leonese Zone, where they had persisted since the Middle Devonian, across the entire Cantabrian Zone. Sedimentation is characterized by strongly condensed successions of thin crinoidal grain-stones, pelagic carbonates, red and green shales, siliceous shales, and black shales with phosphorite nodules.
During the Tournaisian, the sedimentary basin was dissected into a succession of ridges and troughs (Raven, 1983). On the ridges, shallow-water carbonates (Baleas/Cándamo Limestones) were deposited; in the troughs, black shales with phosphorite nodules were deposited (Vegamián Formation). This interval has been interpreted as a “levelling stage” by Kullmann and Schönenberg (1975) or as a stage of tectonic quiescence by Reuther (1977), Heredia et al. (1990), and Rodríguez Fernández (1993, 1994). Marcos and Pulgar (1982) regarded the uppermost Devonian–Mississippian succession as transitional between stable shelf sedimentation and synorogenic deposition.
Stockmal et al. (1986, 1992) showed that under the assumption of a relatively old and thus rigid continental crust, deep-water, starved conditions may prevail in a foreland basin for several tens of millions of years. This is especially true if the corresponding margin was a passive margin, which is basically able to accommodate thick overthrust loads without creating substantial subaerial relief (Stockmal et al., 1986; Stockmal and Beaumont, 1987). Northwestern Iberia is interpreted to be an integral part of the northern Gondwana passive margin (e.g., Martínez Catalán et al., 1997, 2004a): as thrusting propagated toward the foreland relatively slowly (∼5 km/m.y.; Dallmeyer et al., 1997), several millions of years had to pass before the thrust sheets of the Central Iberian Zone and the West Asturian Leonese Zone finally developed a topographic expression above sea level. Until that time, the peripheral foreland basin remained in an underfilled stage (Flemings and Jordan, 1989) and the Mississippian condensed succession was deposited. Underfilled basins are often characterized by a submarine topographic depression caused by thrust loads, where the basin axis is oriented parallel to the thrust front. Consequently, away from the margins of the basin, sediment transport is more or less parallel to the basin axis. This general pattern is recognizable in the Cantabrian Zone (Sanchez de la Torre et al., 1983; Raven, 1983; Van Loevezijn, 1986) up to the basal Namurian.
Rheologically, this stage of foreland basin evolution as interpreted for the Cantabrian Zone can be explained by flexural bending of the lithosphere under elastic or viscoelastic conditions (see Jordan  and Beaumont  for details and differences in the models). From a tectonosedimentary viewpoint, a foreland basin and a forebulge of the northern Iberian foreland basin system (DeCelles and Giles, 1996) are identified. Wedge-top deposits are not preserved, whereas the potential back-bulge area on the opposite side of the Cantabrian Block is unknown.
In the Mississippian condensed succession, many contacts between the stratigraphic units are unconformable, and slumping and sliding are common phenomena (Keller, 2000). Toward the end of the Visean and during the basal Namurian, breccia beds or debris-flow deposits become increasingly abundant. A well-documented example is the Brezo Breccia of Reuther (1977), which is a widespread deposit in the Valsurvio Dome–Sierra del Brezo. It might have a correlative in a debris-flow horizon close to the top of the Genicera Formation in the Alba syncline. Another well-known example is the Porma Breccia (Reuther, 1977) of Arnsbergian (Namurian A) age, which is found in almost all sections of the Barcaliente Formation.
Compartmentalization of the Foreland Basin
In the Cantabrian Zone, the Namurian through Westphalian A is the time of major lateral changes of facies (Figs. 8 and 9). Along the southern limb of the Alba syncline, there is a thick succession of siliciclastic sediments (Cuevas Formation) composed of turbiditic, coarsening- and thickening-upward cycles (Boschma and Van Staalduinen, 1968; Wagner et al., 1971). Internally, several higher-order successions with similar themes are recognized (Moser, 2001). Thick conglomerate horizons in the upper part of the succession are composed of siliceous shales that yielded a Late Devonian–Mississippian radiolarian fauna. Petrographic analysis also shows the input from a magmatic-metamorphic hinterland (Moser, 2001). These siliciclastic sediments were shed from the south and west during the Namurian A through Namurian C (?) (Reuther, 1977; Julivert, 1978; Sanchez de la Torre et al., 1983). Lithologically, many of the clasts can be attributed to the Upper Devonian of the Palencian Facies and to the Mississippian condensed succession (Moser, 2001) where siliceous shales are abundant. Within this succession, there are a few tongues of clastic carbonates that are regarded as southern extensions of a carbonate turbidite system (Barcaliente Formation; Wagner et al., 1971). These turbidites apparently were shed from the north and east (Hemleben and Reuther, 1980). In this direction, the depositional environments of the Barcaliente Formation shallow (González Lastra, 1978), and these shallow-marine environments apparently provided the carbonate debris for the calcareous turbidites. With the progradation of the siliciclastic system toward the foreland, the carbonate turbidites became buried (Fig. 9).
After deposition of the Mississippian condensed succession, subsidence in the Somiedo Correcilla unit must have increased considerably to accommodate the thick siliciclastic detritus provided by the rising orogen and to create a relief for the shedding of turbiditic carbonates from the foreland side of the basin. Increasing subsidence in the Somiedo Correcilla unit is contrasted by relative uplift in the Sobia Bodón unit, where the carbonate platform of the Valdeteja Formation was established above the carbonate turbidites (Fig. 9). Some of the prograding siliciclastic sediments bypassed this platform and were deposited in an area otherwise characterized by starved basin conditions. Typical sediments of these starved areas in the form of red and brown siliceous shales and marlstones with manganese nodules are found in the Ponga nappes (Ricacabiello Formation; Sjerp, 1967; Bahamonde and Colmenero, 1993) and the Central Coal Basin (Fernández, 1993; Salvador, 1993).
A characteristic feature of the carbonate platforms is their marked diachronism, with strata older in the south and west and younger to the north and east (Eichmüller, 1985; Eichmüller and Seibert, 1984). Restoration of the Palencian nappes to a southern and western position provides an elegant solution for a long-lived controversy: Is there a succession of individual carbonate platforms of different age or do the scattered outcrops of carbonates represent one long-lived depositional system that interfingers with the carbonate turbidites? The contradicting interpretations are based on carbonate-platform deposits along the southern margin of the Palencian nappes (within the Espigüete and Santa Lucia thrust sheets), which were dated as Namurian A (Frankenfeld, 1983) and thus considered coeval to the Barcaliente Formation (Barba et al., 1990; Rodríguez Fernández et al., 1985). Similarly, the limestones in the Valsurvio Dome and the Sierra del Brezo are older than the Valdeteja Formation but resemble their lithology and facies (Wagner, 1971; Wagner and Winkler Prins, 2000), and the carbonate turbidite system is very thin.
The reconstruction presented here offers a solution to these contradictory interpretations of the carbonate platforms. With the approach of the orogenic front and the siliciclastic turbidites from the west, subsidence increased in the Somiedo Correcilla unit, generating a depocenter for the detritus. This subsidence was compensated by a flexural bulge in the Sobia Bodón unit, where thick carbonate platforms developed. With the migration of subsidence toward the foreland, the site of carbonate-platform construction on the corresponding bulge also had to migrate to escape burial by detritus. This migration caused a pronounced diachronism of the carbonate-platform deposits (Eichmüller, 1985; Eichmüller and Seibert, 1984). As the Palencian nappes were located closer to the orogenic front at that time, the carbonates preserved there are older than those of the shelf. Local uplift by bulge migration is reflected in the Valsurvio Dome, the Sierra del Brezo, and the Revilla nappe, where large parts of the carbonate turbidite system are missing and where there are laterally extensive breccias at the base of the Valdeteja Formation (Brezo Breccia of Reuther, 1977). We interpret this erosion to reflect bulge movement and erosion.
A major event of Namurian–Westphalian A time is the onset of autochthonous synorogenic sedimentation in the Pisuerga Carrión unit during the Bashkirian. In the Liébana area (Figs. 3 and 4B), the Cosgaya and Vejo Formations (combined in the Potes Group; Fig. 4B) are a succession of conglomerates, breccias, sandstones, and shales of turbiditic origin. Lithostratigraphically and chronostratigraphically, they are equivalents of the upper Namurian turbidites in the southern Pisuerga Carrión unit and in the Palencian nappes (Fig. 4B). During the Visean, thrusting had prograded toward the eastern parts of the West Asturian Leonese Zone (Dallmeyer et al., 1997) and it, together with the increasing morphologic relief, is held to be responsible for the formation and deposition of breccias well into the Namurian. It is also responsible for the subdivision into local troughs and highs.
The rising orogen began to shed siliciclastic detritus into the foredeep from approximately the Visean-Namurian boundary. In the Alba syncline, this is documented in the prograding siliciclastic turbidites overlying the Visean nodular limestones. Depositional environments in the Las Palomas syncline and corresponding settings show northerly derived carbonate turbidites on top of the Mississippian condensed succession and only thereafter the onset of siliciclastic turbiditic sedimentation (Fig. 6).
This sedimentary evolution was caused by increased subsidence close to the orogenic front, which enabled the accommodation of thick siliciclastic successions. From the foreland side of the basin, carbonate debris was shed and deposited probably in a ramp-like setting. This morphology was enhanced by uplift so that the depositional environment was brought to depths favorable for carbonate production (Valdeteja Formation and equivalents). The paleogeography (Fig. 8), with an orogen-derived siliciclastic belt, a carbonate belt migrating toward the foreland, and a basin behind the reef belt, is here interpreted to reflect the formation of a subsequent peripheral bulge with a foredeep and a back-bulge area (Fig. 9). Rodríguez Fernández (1993), Eichmüller (1985), Bahamonde and Colmenero (1993), and Fernández (1993) have presented similar interpretations of this Namurian-Westphalian paleogeography. With the migration of the basin, the bulge, too, was forced to migrate, and so was the carbonate factory. As a result, the carbonate systems display a marked diachronism, where the oldest sediments of Namurian A age are in the Palencian nappes and the Valsurvio Dome–Sierra del Brezo–Revilla nappes.
Several pathways for the siliciclastic detritus dissected the belt of carbonate platforms (Fig. 8). In the west, siliciclastic detritus that bypassed the platform in the Sobia Bodón unit was trapped in the Central Coal Basin (Salvador, 1993). In other areas of the Sobia Bodón unit, the carbonate barrier effectively protected the back-bulge area from major siliciclastic deposition. As a result, the Ponga Nappe area became a starved basin during deposition of the Ricacabiello Formation (Sjerp, 1967; Maas, 1974; Bahamonde and Colmenero, 1993). Farther to the southeast, the barrier was more permeable, and sediment began to spill over into the Pisuerga Carrión basin and finally buried the carbonate belt there. From this time onward (late Namurian), the Pisuerga Carrión unit is recognized as a sedimentary basin, which began to accommodate thick successions of synorogenic detritus.
The debris shed from the rising mountain chain became increasingly coarse grained. It is found over vast areas of the Valsurvio Dome, the Sierra del Brezo, and the Pisuerga Carrión unit. It is also present in the Palencian nappes. Conglomerate horizons in these successions are often almost exclusively composed of metamorphosed quartz arenites (Kanis, 1956; Aller et al., 1985; Colmenero et al., 1988). A likely source for these conglomerates is the thrust sheets containing the Lower Paleozoic section of the West Asturian Leonese Zone (Aller et al., 1985), which is dominated by quartz arenites. These successions had already experienced deformation and metamorphism by Namurian time (Dallmeyer et al., 1997).
In the Namurian–basal Westphalian foreland basin system (Fig. 9), we identify a foredeep, a migrating forebulge area with a reef belt, and a back-bulge area. In contrast to the Upper Devonian peripheral bulge, the Namurian-Westphalian forebulge formed on a relatively local scale and was a narrow but elongated feature. Its rise reflects the approach of the orogenic front to the boundary between the West Asturian Leonese Zone and Cantabrian Zone during the Namurian (Dallmeyer et al., 1997). The bulk of the detritus was delivered to the Pisuerga Carrión area. Its spatial relations to the Ponga nappe area, where unusually condensed sediments were deposited, are not clear. The starved nature of the sediments in some areas of the back-bulge area testifies to maximum protection against siliciclastic detritus by the carbonate belt but also to minimal sediment input from the foreland side of the basin. This might have been the result of a high eustatic sea level during the Mississippian (Ross and Ross, 1988) and the absence of a hinterland due to peneplanation of the preceding bulge.
In comparison with the preceding episode, the evolution of the foreland basin system during the Namurian records a dramatic change in size of the tectonosedimentary units. Two thrust sheets, the Trones and the La Espina thrust sheets, mark the tectonic boundary between the West Asturian Leonese Zone and the Cantabrian Zone (Gutiérrez Alonso, 1992, 1996). Syntectonic movements along the Trones thrust have been dated at ca. 321 Ma (Dallmeyer et al., 1997), which corresponds to the early Namurian. We correlate the onset of accelerating siliciclastic turbidite deposition to the emplacement and morphologic rise of the easternmost thrust units of the West Asturian Leonese Zone. The actual foredeep fill (San Emiliano Formation; Fernández, 1993) is relatively well known, as it was constrained to the area between the rising thrust sheets and the peripheral bulge with the evolving carbonate platforms. Back-bulge deposits are found in the Ponga nappes, the Central Coal Basin, and the Pisuerga Carrión unit. The evolution and the propagation of the sedimentary environments and their strong diachronism are well documented (Eichmüller, 1985; Bahamonde and Colmenero, 1993; Fernández, 1993).
Fill of the foredeep during the Namurian B through Westphalian A includes the transition from the underfilled to the overfilled stage sensu Flemings and Jordan (1990), at least in the Sobia Bodón unit and the Áramo unit (Fernández, 1993), where depositional environments evolved from deep-marine turbidite systems during the late Namurian to fluvial-deltaic deposits during the early Westphalian, which finally overwhelmed the carbonate platforms. As thrusting propagated toward the foreland, the Palencian Facies were incorporated into deformation, and this area was transformed into the Palencian nappes (Fig. 10B). They were transported onto the Valsurvio Dome–Sierra del Brezo area, where the Pisuerga Carrión unit was the adjacent foredeep. Soon after the emplacement, the rising orogen began to shed increasing amounts of quartzite-clast conglomerates (the Curavacas Conglomerate), which were unconformably deposited across the deformed and thrusted Palencian Facies. This unconformity represents the Palencian folding phase of Wagner (1960) and Wagner and Bowman (1983). The conglomerate succession represents a proximal fan-delta deposit and is many hundred meters thick (Colmenero et al., 1988). It is here interpreted to represent wedge-top deposits (sensu DeCelles and Giles, 1996) that originated close to the rising mountain chain (Colmenero et al., 1988). The fluvial-deltaic sediments laterally grade into and interfinger with shales and turbiditic sandstones of the Lechada Formation (Fig. 4). Some of the boulders and pebbles that constitute the Curavacas Formation were transported beyond the limit of the wedge top and deposited in the Pisuerga Carrión basin (Fig. 10B). There, they are associated with turbiditic sequences.
The presence of these tongues of conglomerate with the (assumed) same age and lithology as the Curavacas Conglomerate led Wagner (1960), Rodríguez Fernández and Heredia (1987), and Rodríguez Fernández (1994) to the assume that the Palencian folding phase affected the entire Pisuerga Carrión unit and that the Palencian nappes must had been emplaced by Westphalian B time (the age of the basal Curavacas Formation). However, we are not aware of any deformation similar in style and intensity to the type-Palencian deformation that affected the Pisuerga Carrión unit prior to the deposition of the Curavacas Conglomerate tongues. In contrast, maps and cross sections (Maas, 1974; Savage, 1977; Rodríguez Fernández et al., 1994) show a more or less uninterrupted Namurian-Westphalian succession, an aspect also mentioned by Alonso and Rodríguez Fernández (1983). The erosional contact observed in some localities at the base of the conglomerate units is a purely depositional feature. Sedimentation continued with thick turbiditic successions. Locally, however, some isolated carbonate platforms were able to persist.
The Broken Foreland
Breaking of the foreland crust has been documented from a variety of orogens (see summary in Jordan, 1995), notably from the Laramide orogen of the western United States (Dickinson et al., 1988; McQueen and Beaumont, 1989) and from the Andes of South America (Jordan and Almendinger, 1986; Fielding and Jordan, 1988). The characteristics of a broken foreland are major deep-seated faults dipping at an angle of 30° to 45°. These faults bound small rigid blocks or intermediate blocks with an additional flexural component at the block margins. The faults may evolve from mid-crustal or even deeper levels. Finally, differential movements and tilting of the blocks may create a synoptic topographic relief of several kilometers (Jordan, 1995; McQueen and Beaumont, 1989).
During the Westphalian B, the Somiedo Correcilla unit and the Esla nappe were incorporated into thrusting (Pérez-Estaún et al., 1988). Emplacement of the Esla nappe and the Valsurvio Dome–Sierra del Brezo seems to have been coeval (Rodríguez Fernández, 1993), so that the onset of thrust deformation there can also be attributed to the Westphalian B. However, Pérez-Estaún et al. (1988) pointed out the differences between deformational styles in the Somiedo Correcilla unit and the Esla nappe. Whereas the Somiedo nappe shows little imbrication or duplex formation, the Esla nappe is characterized by the formation of three superimposed thrust sheets and three duplexes that together accommodated almost 90 km of horizontal displacement. The León fault, across which there is an abrupt change in deformational style, limits this anticlinal stack (Alonso, 1987; Pérez-Estaún et al., 1988) to the north. Similarly, the Valsurvio Dome and the Sierra del Brezo were subject to important horizontal shortening after emplacement of and together with the Palencian nappes (Fig. 10C).
The dynamic metamorphism observed in the Valsurvio Dome during its first deformation (Koopmans, 1962; Weh, 2006) may be attributed to the combined thickness of the Palencian Facies plus the overlying conglomerates which had been thrust upon the Valsurvio Dome. Here, the Ruesga fault is the tectonic boundary to the adjacent Pisuerga Carrión unit. Another anticlinal stack was described by Marcos (1968a) from the Sobia Bodón unit close to the León fault, where the Cueto Negro anticlinal stack implies severe crustal shortening compensated by vertical piling of thrust sheets.
The association of anticlinal stacks and major fault zones, the León fault and the Ruesga fault, is here interpreted to indicate a genetic link between their structures. As the three areas migrated toward the north-northeast (Arboleya, 1981), they finally approached the zone in which, during the Silurian and Devonian, the Cantabrian Block was bounded by the adjacent subsiding shelf. In addition, close to this transition, the Upper Devonian hinge line had developed.
In the foreland basin system of the Cantabrian Zone, the approach of the thrust front toward the rigid Cantabrian Block involved high in-plane stress and the formation of several anticlinal stacks in the Sobia Bodón unit, the Esla area, and the Valsurvio Dome area. It is likely that during the intended thrusting of the Valsurvio Dome and the Esla nappe, with their thick and competent Upper Devonian succession, toward the rigid Cantabrian Block, the zone of crustal weakness between the Cantabrian Block and the shelf was transformed into deep-seated faults: the foreland broke (Fig. 10C) along the León and Ruesga faults. Shortening was accomplished not by thrusting alone but by folding and uplift along these newly formed faults, which gave rise to internal deformation in all areas. In the Esla area, much of the shortening was transformed into the formation of an anticlinal stack (Alonso, 1987; Pérez-Estaún et al., 1988) where the stratigraphic succession was at least triplicated. Thrusting also occurred in the Valsurvio Dome, and there, almost all stratigraphic contacts were converted into thrust planes (Bittner, 1992; Weh, 2006).
Whereas the southern fault block began to rise, the foreland basin deepened dramatically. The growing topographic relief together with the buckling of the lithosphere in the Valsurvio Dome area during the Westphalian seem to have triggered the gravitational gliding of the Palencian nappes into the foredeep of the Pisuerga Carrión unit with the Curavacas Conglomerate “riding piggyback” (Fig. 10D). In this scenario, the wedge-top deposits together with parts of the wedge itself (the Palencian nappes) were overriding their distal equivalents of the Pisuerga Carrión unit. This probably happened during the late Westphalian, since some olistoliths within the Lechada Formation of the Palencian nappes yield Westphalian C ages (Alonso and Rodríguez Fernández, 1983).
During the late Westphalian, the Pisuerga Carrión unit was strongly compartmentalized by a mosaic of deep-seated faults so that small carbonate platforms developed on the tilted and uplifted edges of blocks. They shed abundant debris into the adjacent foredeep. Several of the faults, e.g., the Peñas Matas and Polentinos faults, served as pathways for ascending magmas. Gabbroic and gabbrodioritic sills are reported from the Ruesga and León faults, and the main granitic stocks in the Pisuerga Carrión unit are also confined to this system of faults (Corretgé and Suárez, 1990). The subsidence history of the Central Coal Basin shows a dramatic increase from the Mississippian into the late Westphalian (Salvador, 1993). Sedimentation rates increased from 1 cm/1000 yr during the Mississippian to 10 or 15 cm/1000 yr during the Bashkirian to 40 cm/1000 yr during the Westphalian A and B, and finally to almost 150 cm/1000 yr during the Upper Westphalian (Salvador, 1993). We interpret this increase not only to be the result of flexural bending by emplacement of the Somiedo Correcilla and the Sobia Bodón nappes but also of mechanical failure of the foreland.
A deep-seated origin of the Ruesga fault is an explanation for the concentration of small magmatic intrusions and the high heat fluxes and metamorphism observed in small Stephanian basins (Raven and van der Pluijm, 1986; Frings, 2002) along their traces. Uplift of the southern block and stripping off of the Palencian nappes exposed relatively lower crustal levels in the Valsurvio Dome. This might be an explanation for the higher degree of syndeformational metamorphism exposed in this area when compared to other regions. It also explains the strong thickening of the crust to the north of the fault, which led to underplating and abundant magmatism in the SE Cantabrian Zone (Fernández-Suárez et al., 2000; Valverde Vaquero et al., 1999).
The presumed age of both faults is in good agreement with the model presented here. The León fault is interpreted to have formed during the Westphalian, shortly after emplacement of the Sobia Bodón unit but prior to the Stephanian (Marcos, 1968a, 1968b, 1979). The Ruesga fault is attributed to the Westphalian A or B (Wagner and Winkler Prins, 2000). Both faults were apparently reactivated as strike-slip or tear faults during the Upper Westphalian emplacement of the Ponga nappes onto the Pisuerga Carrión unit (Marcos, 1979; Heredia, 1998; Wagner et al., 1984). The complex history of both faults, including several reactivations, continued well into the Mesozoic and Cenozoic (Alonso et al., 1996; Pulgar et al., 1999) and underlines their structural importance.
During the Westphalian D, a basin reorganization is recognized in the Pisuerga Carrión unit and adjacent areas. This reorganization is known as the “Leonian phase” or “Leonian movements” (Wagner, 1959; Wagner and Martínez García, 1974). The main characteristic of this Leonian phase in the Pisuerga Carrión unit is the dominance of faulting. First, reverse faults developed, which were rapidly followed by normal faults. Folding and thrusting were not involved in this deformation (Wagner et al., 1984; Wagner and Martínez García, 1974; Heredia et al., 1990; Weh, 2006). Concomitantly, steep valleys filled with coarse continental debris formed. Laterally, they grade into marine deltaic complexes. Locally, carbonate platforms were able to survive. Other local basins were filled with turbiditic sandstones and debris-flow deposits. Many of the basin-fill interpretations invoke tilting and differential subsidence to explain the rapid lateral changes of facies and thickness on either side of the Leonian unconformity, e.g., Wagner et al. (1984), Rodríguez Fernández (1994), Van de Graaff (1971a, 1971b, 1971c) and Wagner and Varker (1971). We conclude from our data and observations in the Pisuerga Carrión unit that there seems to be continuity in the overall tectonosedimentary evolution from the higher Westphalian into the Cantabrian. Local unconformities (Leonian unconformity) are present and probably testify to tilting of individual blocks or to increasing activity along normal faults.
The entire evolution as interpreted here indicates that breaking of the foreland lithosphere occurred on a regional scale and that the León and Ruesga faults are only two of the more prominent representatives. Other faults dissected the Pisuerga Carrión unit, and their activation-reactivation led to the tilting and the formation of tilt-block basins. Following Rodríguez Fernández (1994), Rodríguez Fernández and Heredia (1987), and Weh (2006), the mainly west-east–directed deformation during this time may have been related to the formation of the Central Coal Basin and the emplacement of the Ponga nappes unit. Clastic wedges associated with their emplacement are confined to relatively narrow depocenters. This gives further indication of a strong compartmentalization of the basin.
The next major tectonic event was a south-directed compression, which led to a strong overprint of the former structures. There is general agreement that the emplacement of the Picos de Europa nappe pile onto the Pisuerga Carrión unit induced this event. The sedimentary record of this event is relatively sparse in the southern Pisuerga Carrión unit, so that a detailed analysis of the relationship between sedimentation and tectonics is not possible. The subsequent late stages of foreland basin deformation are recorded mainly in the Picos de Europa unit. Wagner and Martínez García (1974), Wagner et al. (1984), and Martínez García (1990) have described these final events of the Variscan orogeny in the Cantabrian Zone of northern Spain in detail, and the reader is referred to these publications for further information. One problem that we could not address in this paper is the effect of oroclinal bending on the evolution of the broken foreland basin system. Neither its timing nor the prebending paleogeography is clear. However, Hirt et al. (1992) and Steward (1995) emphasized that bending began relatively early during the compressional history. If so, this early bending must have created additional space problems in the core of the Asturian Arc (the Pisuerga Carrión unit). These problems might have contributed to the breaking of the lithosphere.
The Cantabrian Zone of northwestern Spain records the transition from a passive-margin setting into the peripheral foreland basin system of the Variscan orogen during the late Paleozoic. Evolution of the foreland basin system started during the Late Devonian with the formation of a foredeep and the adjacent peripheral bulge. These large-scale features developed as a response to subduction prior to collision along the western margin of Iberia. After erosion of the bulge and its subsequent submergence, the entire basin system migrated toward the foreland. This is documented in the overall diachronism, facies, and areal distribution displayed by the Upper Devonian and Mississippian lithostratigraphic units. Erosional debris from the rising orogen reached the Cantabrian Zone during the Namurian.
In contrast to previous models, we propose a succession of events that started with the emplacement of the Palencian nappes onto the Valsurvio Dome and the Sierra del Brezo without the postulated immediate gravitational transport of these nappes into the Pisuerga Carrión flysch basin. In our model, and in agreement with the general style of deformation in the Cantabrian Zone (Pérez-Estaún et al., 1988; Pérez-Estaún and Bastida, 1990), deformation followed a simple forward-propagating style. After emplacement of the Palencian nappes onto the Valsurvio Dome, erosion of the rearward thrust sheets with rocks of the West Asturian Leonese Zone provided large amounts of detritus. This debris, mainly composed of quartzites, was deposited in a wedge-top setting on the Palencian nappes but also in the adjacent foredeep.
During the Westphalian B, thrusting began to affect the Esla area and the Valsurvio Dome area. Their proximity to the rigid Cantabrian Block prevented a simple forward propagation of thrusting. Instead, the lithosphere began to buckle and finally broke. A system of fractures originated that was rooted in deeper crustal levels. The León fault and the Ruesga fault are two of the most important faults of this system. The concomitant uplift along these faults provided the topographic relief necessary for the gravitationally induced emplacement of the Palencian nappes in the Pisuerga Carrión unit, including part of the wedge-top system. Further on, broken foreland conditions controlled sedimentation in the Pisuerga Carrión unit. With the successive emplacement of the Central Coal Basin, the Ponga nappes unit, and the Picos de Europa unit, many of the faults became reactivated, so that today they exhibit complex superposition of structural indicators.
The Variscan Cantabrian foreland basin system is a long-lived system that originated under flexural bending of the lithosphere during the Late Devonian. It was transformed into a broken foreland basin during the Westphalian and achieved its final geometry through oroclinal bending, which might have had additional influence on the breaking of the crust underlying the basin, and through the formation of large-scale faults.
We thank J.R. Martínez Catalán (Salamanca) for inspiring discussions about the orogenic events and their timing in northwestern Spain. D. Barbeau (Columbia), J.R. Martínez Catalán (Salamanca), and an anonymous referee made constructive reviews on an initial version of the manuscript. These comments and suggestions were greatly appreciated and helped to improve the paper considerably. Field work was supported by the Deutsche Forschungsgemeinschaft through various grants, which are gratefully acknowledged.
Figures & Tables
4-D Framework of Continental Crust
- Asturias Spain
- Cantabrian Mountains
- Castilla y Leon Spain
- depositional environment
- fore-arc basins
- foreland basins
- Galicia Spain
- Iberian Massif
- Iberian Peninsula
- Lugo Spain
- orogenic belts
- passive margins
- plate tectonics
- Southern Europe
- thrust faults
- Upper Devonian
- Variscan Orogeny
- Esla Nappe
- Alba Syncline
- Leon Fault
- Ponga Nappes
- Aramo Unit
- Sierra del Brezo
- Valsurvio Dome
- Palencian Nappes
- Curavacas Conglomerate
- Pisuerga Carrion Unit
- Cervera de Pisuerga Spain
- Sobia Bodon Unit
- Ruesga Fault
- Picos de Europa Nappes
- Somiedo Correcilla Unit