1
Structure of the Pyrenees at the Crustal Scale

Antonio TEIXELL1 and Pierre LABAUME2

1Departament de Geologia, Facultat de Ciències, Universitat Autònoma de Barcelona, Spain

2Géosciences Montpellier, CNRS, Université de Montpellier, France

1.1. Introduction

The Pyrenees have been the subject of intensive structural research for over a century, both at the near-surface scale and at the crustal scale. In addition to the abundant documentation of outcrop geology, the Pyrenees have long been the target of numerous geophysical studies aiming to define the deep structure of the crust and upper mantle. Refraction/wide-angle seismic studies in the early 1980s were followed by vertical-incidence seismic reflection profiling in the late 1980s and 1990s (ECORS program), which led to a series of crustal models that have become established references. Although there has been no complete agreement on the resolution of Pyrenean shortening at the level of the lower crust, models showing a persistent state of underthrusting (continental subduction) of the Iberian plate below the Eurasian plate have gained consensus over the years. Recent insights into the deep structure of the mountain chain derive from passive-source seismic data (e.g. receiver functions and tomography), which have largely supported the results of previous active-source seismic studies but have brought new controversies regarding the role and involvement of the upper mantle in the present-day Pyrenean structure.

Recent understanding of the tectonic development of the Pyrenees belt have also been strongly impacted by new reconstructions of the pre-orogenic configuration, largely derived from the application of geological concepts of modern continental margins and hyperextension. Models for the evolution of the Pyrenean orogeny must consider the feedback between a Cretaceous phase of hyperextension leading to the Pyrenean-Bay of Biscay rift and the Late Santonian to Early Miocene compressional inversion that created the mountain belt.

1.2. Tectonic framework of the Pyrenean belt

The Pyrenean belt is an asymmetric doubly vergent orogen flanked by two foreland basins (e.g. Teixell et al. 2018; Ford et al. 2022 and references therein) (Figure 1.1). The main thrust system, of southward vergence, comprises imbricated thrust units of Paleozoic basement forming the Axial Zone, connected to the south to detached thrust units of Mesozoic and Cenozoic cover forming the South Pyrenean Zone. The latter comprises syn-orogenic basins of Late Cretaceous-Paleogene age and overrides to the south of the Ebro foreland basin. The North Pyrenean Zone comprises a system of inverted Mesozoic extensional basins and Paleozoic basement massifs and is thrust northward onto the Aquitaine foreland basin. The Axial and North Pyrenean zones are separated by the sub-vertical North Pyrenean Fault (NPF) in the eastern and central Pyrenees and by the south-vergent Lakora thrust in the western part. To the west, the Basque-Cantabrian segment of the belt is mainly composed of inverted Mesozoic basins. It overrides the Duero foreland basin to the south and the offshore Landes foreland basin to the north.

1.3. Crustal investigations by deep seismic reflection surveys

The map in Figure 1.1 illustrates the numerous deep seismic profiles that have investigated the deep architecture of the Pyrenees, in this respect, a privileged mountain belt. Early seismic refraction/wide angle data presented by Gallart et al. (1981) and Daignières et al. (1982) detected a thickened crust under the basement massif of the Axial Zone, where the Mohorovicic discontinuity (the "Moho") was identified at a depth of 40-50 km. In contrast, the Moho was imaged at 30 km under the Northern Pyrenees; both domains are apparently separated by a step located under the NPF (Figure 1.1). Accordingly, the latter was interpreted as a vertical structure cross-cutting the whole crust and separating the Iberian plate from the Eurasian plate. This concept dominated the first generation of crustal models for the Pyrenees (e.g. Choukroune and Mattauer 1978; Déramond et al. 1985; Séguret and Daignières 1986) until the acquisition of the ECORS deep seismic reflection profiles in the mid-1980s.

Figure 1.1. Tectonic map of the Pyrenean orogenic belt indicating the geologic cross-section lines (red, when not coincident with the seismic traces) and the main deep seismic profiles discussed in this work

LEGEND FOR FIGURE 1.1.- Brown: seismic reflection profiles; blue: receiver function profiles, numbered after Figure 1.6 (redrawn from Teixell et al. 2018). SPZ: South Pyrenean Zone; AZ: Axial Zone; NPZ: North Pyrenean Zone; PB: Parentis basin; LH: Landes high; AM: Asturian massif; BC: Basque-Cantabrian segment; CV: Cinco Villas massif; MB: Mauléon basin; LT: Lakora thrust; GT: Gavarnie thrust; NT: Nogueres thrust; NPF: North Pyrenean fault; 3S: Trois-Seigneurs massif; AM: Agly massif.

1.3.1. Results of the ECORS profiles across the Pyrenees

The ECORS-Pyrenees seismic reflection profile, acquired in 1985-1986, crosses the Central Pyrenees (Figure 1.1), being the first deep reflection profile ever acquired across an entire orogenic belt (Choukroune et al. 1989). The profile images prominent reflections of the Mesozoic and Cenozoic sedimentary basins, the top of the Variscan basement and a set of inclined reflections that may be attributed to thrusts and normal faults in the upper to middle crust (Figure 1.2(a)). A reflective lower crust is also clearly observed, the base of which (the Moho discontinuity) is at 10 and 12 s TWT under the northern and southern forelands, respectively. Under the Axial Zone, the Iberian (southern plate) reflective lower crust clearly plunges to the north, dipping under a flat Eurasian plate down to a depth of ~20 s TWT (unmigrated) (Figure 1.2(a)). A comparable image was obtained in the ECORS-Arzacq profile across the northern half of the west-central Pyrenees (Daignières et al. 1994) (Figure 1.1), where the base of the Eurasian lower crust, which is more discontinuously imaged, is at about 10 s, whereas the edge of the Iberian plate is imaged by inclined reflections to a depth of ~17 s TWT (Figure 1.2b).

Coupled with a revisit of the surface geology, the ECORS profiles led to new conceptions of the crustal structure of the Pyrenees. The continuation of the NPF at depth as a vertical structure was disproven, being cut by prominent south-dipping reflectors (interpreted as a north-verging thrust). The reflection pattern revealed the indentation of the Eurasian plate to the south, driving the northward underthrusting of the Iberian deep crust and the formation of an orogenic prism at mid-crustal to upper crustal levels (Roure et al. 1989; Muñoz 1992; Teixell 1998) (Figure 1.3).

Figure 1.2. Line-drawing sketches of the ECORS deep seismic reflection profiles across the Pyrenees (unmigrated)

LEGEND FOR FIGURE 1.2.- The main reflective units interpreted are highlighted: the supracrustal sedimentary cover and the reflective lower crust. SPFT: South Pyrenean frontal thrust; NPFT: North Pyrenean frontal thrust; NPF: North Pyrenean fault. Moho stands for the Mohorovicic discontinuity (the base of the crust).

As an independent test for crustal structure, the Bouguer gravity anomaly of the Pyrenean region shows a long-wavelength minimum (up to -120 mgal) in the central part of the Pyrenees (Figure 1.4) (Torné et al. 1989; Bayer et al. 1996; Casas et al. 1997). This confirms the crustal thickening under the Axial Zone of the mountain belt deduced by the seismic data. The absolute value of the anomaly decreases toward the Mediterranean coast and the Bay of Biscay (Figure 1.4), attesting for thinner crust. In fact, no negative anomaly referred to as a crustal root is observed in the Basque-Cantabrian Pyrenees (Pedreira et al. 2007), which shows a progressively increasing positive value of Bouguer gravity from the South Pyrenean frontal thrust northward. However, seismic refraction/wide-angle studies acquired in 1997 still detected a crustal thickness up to ca. 40 km in the central part of this segment of the Pyrenees (Pedreira et al. 2003). To the east, the eastward thinning of the crust is related to the post-orogenic rifting of the western Mediterranean (Séranne 1999; Jolivet et al. 2020).

The geological interpretations of the ECORS profiles are illustrated in Figure 1.3. These sections seek to balance orogenic shortening at upper crustal (as deduced from surface geology) and lower crustal (as imaged in the seismic profiles) levels. The crustal shortening measured in shallow cross-sections (Roure et al. 1989; Muñoz 1992; Teixell 1998) exceeds that observed for the lower crust as imaged at depth on seismic profiles. To solve this discrepancy, additional shortening mechanisms at deep levels were envisaged, for example, a system of lower crust thrust imbrications (Roure et al. 1989) (Figure 1.3(a)) or the subduction of a continuous portion of the deep crust to a depth not recorded by seismic data (Muñoz 1992; Teixell 1998) (Figures 1.3(b) and...