We focus on the Iberian-European plate boundary (IEPB), whose nature, age, and evolution are strongly debated. In contrast to previous interpretations of the IEPB as a major lithospheric-scale left-lateral strike-slip fault, we propose a more complex deformation history. The mapping of rift domains at the transition between Iberia and Europe emphasizes the existence of spatially disconnected rift systems. Based on their restoration, we suggest that the deformation was partitioned between a set of distinct left-lateral transtensional rift systems from the Late Jurassic to Early Cretaceous. A plate kinematic reorganization at Aptian-Albian time resulted in the onset of sea-floor spreading in the western Bay of Biscay and extreme crustal and lithosphere thinning in intra-continental rift basins to the east. The formation and reactivation of the IEPB is interpreted as the result of the polyphase evolution of a diffuse transient plate boundary that failed to localize. The results of this work may provide new insights on (1) processes preceding breakup and the initiation of segmented and strongly oblique shear margins, (2) the deformation history of nascent divergent plate boundaries, and (3) the kinematics of the southern North Atlantic and Alpine domain in western Europe.
Processes that control the formation of divergent or transform plate boundaries, their locking, and potential reactivation during convergence are among the least-understood processes in tectonics. Discoveries made at present-day rifted margins have shown a complex transition between oceans and continents, characterized by extremely thinned continental crust and/or exhumed mantle (e.g., Reston, 2009), referred to as “hyperextended domains.” However, at present, little is known about the spatial and temporal evolution of hyperextended rift systems, especially how extensional deformation may migrate and eventually localize to create a new stable plate boundary.
We focus on the Iberian-European plate boundary (IEPB), characterized by Late Jurassic to mid-Cretaceous rift systems including both oceanic and hyperextended rift domains (e.g., Vergés and García-Senz, 2001; Salas and Casas, 1993; Lagabrielle and Bodinier, 2008; Jammes et al., 2010; Roca et al., 2011; Tugend et al., 2014). The onset of the northward movement of the African plate during Santonian–Campanian time (e.g., Rosenbaum et al., 2002) initiated the reactivation of the former rift systems along the IEPB, leading to the progressive formation of a new convergent plate boundary.
The tectonic setting related to the thinning and breakup of the continental lithosphere in the western Bay of Biscay remains strongly debated, resulting in controversial interpretations of the timing, kinematics, and location of the IEPB (Olivet, 1996). Based on observations on the spatial and temporal evolution of the different rift systems, we aim to provide new insights on the evolution and partitioning of the deformation at the scale of a plate boundary from its formation to its reactivation.
MAGNETIC ANOMALIES AND IMPLICATIONS FOR PLATE KINEMATICS
The amount and timing of left lateral displacement accommodated along the IEPB, and the nature of the plate boundary itself, are strongly debated (Olivet, 1996). These controversies result from contrasting interpretations and restorations of magnetic anomalies from the M series (M3–M0, 126–118.5 Ma) identified within hyperextended domains in the Bay of Biscay and the North Atlantic in general (Olivet, 1996; see contrasting restorations of Sibuet et al., 2004). They are interpreted as related either to mantle exhumation (Sibuet et al., 2007) or to an excess magmatic event during lithospheric breakup (Bronner et al., 2011). In both cases, these anomalies may not represent isochrones and may not be used as such for plate kinematic restorations.
Restorations of magnetic anomalies consider minor pre-breakup movements. Considering the widespread occurrence of hyperextended domains continentward of the first oceanic crust may lead to alternative plate kinematic models with different amounts of displacement and different ages for the formation of the proto-IEPB (see Jammes et al., 2010, and references therein). In view of the evolution of the North Atlantic and/or Alpine Tethys system, some authors proposed that the left-lateral movement of Iberia relative to Europe had already initiated in the Late Jurassic (e.g., Rosenbaum et al., 2002; Schettino and Scotese, 2002; Canérot, 2008; Jammes et al., 2010) in contrast to the mid- to late Albian onset proposed (e.g., Le Pichon et al., 1971; Choukroune and Mattauer, 1978; Olivet, 1996; Lagabrielle and Bodinier, 2008).
SPATIAL AND TEMPORAL EVOLUTION OF THE IEPB RIFT SYSTEMS
Geological and geophysical observations have been combined to map the spatial distribution of the rift systems preserved at the IEPB (Fig. 1; Tugend et al., 2014; see the GSA Data Repository1 for details on rift domain definition). Constraints on the temporal evolution of the different rift systems come from the aggradation and subsidence histories recorded in the different sub-basins (Fig. 1B; Data Repository). The array of extensional faults and transfer zones delimiting the rift systems and their reactivation as a thrust system provides first-order insights on transport direction throughout the deformation history.
The architecture of the IEPB is characterized by spatially disconnected rift systems: (1) Bay of Biscay–Parentis (BoBP), (2) Pyrenean-Basque-Cantabrian (PBC), and (3) Central Iberian (CI) rift systems (Fig. 1A; Salas and Casas, 1993; Vergés and García-Senz, 2001; Roca et al., 2011; Tugend et al., 2014). These rift systems were separated by weakly thinned continental ribbons (Lister et al., 1986), the Landes High and Ebro block, similar to those described in the southern North Atlantic (Fig. 1A; Tugend et al., 2014).
The Late Jurassic to mid-Cretaceous rifting is not recorded simultaneously at the scale of the IEPB, as indicated by subsidence analysis results in different sub-basins (Fig. 1B; see differences between the Maestrat, Cameros, Parentis, and Arzacq basins; see the Data Repository). Synrift deposits are controlled by east-west–, northwest-southeast–, and northeast-southwest–trending basement faults (e.g., BoBP: Derégnaucourt and Boillot, 1982; Thinon et al., 2003; PBC: Martín-Chivelet et al., 2002; Tavani and Muñoz, 2012; CI: Salas and Casas, 1993).
Extreme crustal thinning is evidenced in the BoBP and PBC rift systems (e.g., Thinon et al., 2003; Lagabrielle and Bodinier, 2008; Jammes et al., 2010; Roca et al., 2011; Tugend et al., 2014), whereas the CI rift system was more moderately thinned (to ∼15–20 km; see Salas and Casas, 1993). Onset of hyperextension was diachronous between the BoBP and PBC rift systems (Berriasian-Barremian to late Aptian, and Aptian to early Cenomanian, respectively; see Tugend et al., 2014; Fig. 1B). Accelerated subsidence related to extreme crustal thinning in the PBC rift system is controlled by northeast-southwest transfer zones recording the north-south to northeast-southwest divergence orientation between Iberia and Europe (Jammes et al., 2010; Roca et al., 2011; Tavani and Muñoz, 2012; Tugend et al., 2014).
Onset of convergence is recorded in Santonian to Campanian time in the BoBP and PBC rift systems (e.g., Thinon et al., 2001; Capote et al., 2002) whereas it is delayed until the middle to late Eocene in the CI rift system (Salas and Casas, 1993; Capote et al., 2002). Restorations of magnetic anomalies and the east-west–trending thrust systems in the former PBC and BoBP rift systems (Fig. 1A) suggest an almost north-south convergence orientation (e.g., Olivet, 1996; Rosenbaum et al., 2002).
HOW IS THE DEFORMATION PARTITIONED ALONG THE IEPB?
Based on the spatial and temporal evolution of the rift systems, we propose an alternative scenario for the evolution and partitioning of the deformation at the IEPB (Figs. 2 and 3). These restorations remain qualitative because of the partial underthrusting of the rift system during convergence (e.g., Vergés and García-Senz, 2001; Roca et al., 2011; Tugend et al., 2014). The amount of left-lateral offset of the Iberian plate relative to Europe is difficult to restore and may be estimated to be ∼200–500 km (Olivet, 1996).
Rift Initiation: Partitioning of Transtensional Deformation (Late Jurassic to Aptian–Albian)
The Late Jurassic initiation of the left-lateral movement of Iberia relative to Europe (e.g., Rosenbaum et al., 2002; Schettino and Scotese, 2002; Canérot, 2008; Jammes et al., 2010) is recorded along the IEPB by the formation of a wide corridor of transtensional deformation progressively shaping distinct rift systems (Figs. 1A, 2A, and 3A). The segmentation pattern of rift structures (Fig. 1A) results from the complex partitioning between strike-slip and orthogonal deformation in a strongly pre-structured basement, recorded as a local north-south extension in rift basins (Figs. 2A and 3A; e.g., Tavani and Muñoz, 2012).
From the Late Jurassic onward, fauna and/or sedimentary facies type indicate that the BoBP was opened toward the Atlantic (Durand-Delga, 1973), whereas the CI and PBC were connected to the Tethyian domain (Mas et al., 1993; Salas and Casas, 1993). In spite of the Landes High and Ebro block acting as crustal barriers between the rift systems (Figs. 2A and 3A), intermittent exchanges between the Atlantic and Tethyian seas occurred, caused by eustatic variations (e.g., Salas and Casas, 1993; Capote et al., 2002). The V-shaped nature of the BoBP rift system (Fig. 1A; Jammes et al., 2010) suggests a tentative southeast propagation, while the CI rift system may have been propagating toward the northwest (Fig. 2A) as indicated by the diachronous onset of synrift subsidence (Fig. 1B; Salas and Casas, 1993; Capote, et al., 2002). In the future PBC, discrete narrow depocenters progressively formed, only recording moderate subsidence (Figs. 2A and 3A; e.g., Martín -Chivelet et al., 2002, and references therein).
Plate Kinematic Reorganization: Tentative Localization of the Plate Boundary (Aptian–Albian to Santonian–Campanian)
The transition from left-lateral movements to north-south and northeast-southwest divergence of Iberia relative to Europe is recorded around Aptian to mid-Albian time by northeast-southwest transfer zones controlling the formation of the PBC rift system (Fig. 2B; Jammes et al., 2010; Roca et al., 2011; Tugend et al., 2014). It is difficult to determine if this change was abrupt or if the partitioning between strike-slip and orthogonal deformation evolved progressively.
Onset of sea-floor spreading processes in the western Bay of Biscay at Aptian–Albian time (Montadert et al., 1979; Figs. 2B and 3B) is related to a major change in the subsidence and deformation histories of the rift systems (Fig. 1B; see the Data Repository; Tugend et al., 2014). In the CI rift system, the decrease in tectonic subsidence in rift basins suggests a progressive cessation of rifting (Salas and Casas, 1993) leaving a network of disconnected aborted rift basins (Figs. 2B and 3B; e.g., Cameros, Maestrat). The synchronous onset of hyperextension in the PBC rift system is therefore interpreted as the migration of deformation from the CI to the PBC rift system (Figs. 1B, 2B, and 3B; see the Data Repository) consequent to the plate kinematic reorganization. Sea-floor spreading may have persisted until late Santonian to early Campanian time (chron A34; Fig. 1), resulting in north-south to northeast-southwest extension recorded in the oceanic domain of the BoBP (Figs. 2C and 3C). Eastward, this deformation seems to have been mostly transferred and partitioned between the rift basins from the PBC in a tentative development of a divergent plate boundary between Iberia and Europe (Figs. 2C and 3C).
From Subduction Initiation to Continental Collision: The Role of Rift Inheritance (Santonian–Campanian to Eocene–Oligocene)
The north-south to northeast-southwest convergence generated by the northward movement of Africa (e.g., Rosenbaum et al., 2002) is recorded diachronously at the scale of the IEPB (Figs. 2D and 3D). The first evidence of compression is documented in late Santonian to Campanian time in the BoBP (Thinon et al., 2001) and PBC rift systems (Capote et al., 2002, and references therein) while sea-floor spreading processes may have just ceased. Remarkably, this deformation is not observed in the CI rift system (Figs. 2D and 3D). This contrasting reactivation may possibly be explained by the relatively moderate thinning of the continental crust in the CI rift system (Salas and Casas, 1993) compared with the extreme lithosphere thinning of the BoBP and PBC rift systems (Fig. 2C). In particular, the occurrence of exhumed mantle seems to facilitate reactivation processes and subduction initiation (Lundin and Doré, 2011; Tugend et al., 2014). Former rift structures such as top basement detachment faults may have been reactivated using the serpentinization front of the uppermost mantle as a decoupling layer. This interpretation compares well with numerical modeling results (e.g., Burov and Poliakov, 2001; Leroy et al., 2008) suggesting that newly formed hyperextended domains are significantly weaker than moderately thinned continental crust (i.e., proximal and necking domains). The thermal state of the IEPB at the onset of convergence may therefore represent a critical factor in explaining why reactivation was initiated in the hyperextended domain.
During the late Eocene to early Oligocene, the final stage of collision in the Pyrenees (e.g., Capote et al., 2002; Vergés and García-Senz, 2001) may have resulted in a strong coupling between Iberia and Europe at the former PBC rift system. The main convergence is interpreted to have migrated progressively southward, leading to onset of inversion in the former CI rift system (Fig. 3E). Ultimately, the entire coupling of Iberia to Europe resulted in the complete migration of the convergent plate boundary between Iberia-Europe and Africa in the Miocene in the Betics (Vergés and Fernàndez, 2012).
IMPLICATIONS FOR THE NATURE AND EVOLUTION OF PLATE BOUNDARIES
The architecture and evolution of the IEBP is more complex and polyphase than previously assumed. The proposed interpretation questions the nature of the North Pyrenean fault as being the remnant of a lithospheric-scale structure representing a former transform plate boundary (e.g., Choukroune and Mattauer, 1978), and also its age. Instead, we suggest that the left-lateral displacement actually accommodated along this fault should be minimized, and we favor a partitioning of transtensional deformation between distinct rift systems (BoBP, CI, and PBC rift systems). The cause of this partitioning of the deformation is not clear and may be due to the Landes High and Ebro block representing pieces of rheologically stronger crust, difficult to thin efficiently (Fig. 2; Tugend et al., 2014). These results provide insights on the partitioning of the deformation at transform to transtensional plate boundaries and may represent an analogue to unravel the embryonic stages of the formation of segmented or strongly oblique shear margins observed worldwide.
The Aptian-Albian plate kinematic reorganization resulted in north-south and northeast-southwest divergence between Iberia and Europe. At the scale of the IEPB, the transition from localized sea-floor spreading to the west to a diffuse network of aborted rift systems to the east (PBC) is interpreted as the failed tentative localization of a divergent plate boundary (Figs. 2B and 2C) during the propagation of the North Atlantic Ocean. The subsequent reactivation of the IEPB, strongly controlled by rift-inherited architecture, initiated the formation of a convergent plate boundary. The progressive coupling between Europe and Iberia resulted in the southward migration of the plate boundary. In spite of its transient nature, the IEPB may bring new insights on the complex partitioning of extensional deformation in propagating rift systems observed at nascent plate boundary, and on their subsequent reactivation as observed in Southeast Asia (e.g., South China Sea; Franke et al., 2013; Savva et al., 2014).
Finally, it appears that pre-breakup deformation related to the formation of hyperextended domains is not negligible for plate restorations in spite of being difficult to quantify. Restorations based on magnetic anomalies alone are likely to misinterpret the amount and/or timing of movements between plates. With the IEPB being at the junction between the proto-Atlantic and Tethyian rift systems, its polyphase evolution remains to be fully integrated into the understanding of both the northward propagation of the Atlantic Ocean and the evolution of the Alpine Tethys systems.
We thank J.A. Muñoz, L. Gernigon, and an anonymous referee for constructive reviews, and G. Mohn, E. Masini, D. Frizon de Lamotte, and M. Pubellier for helpful discussions. The authors acknowledge financial support from the MM3 consortium.