A three-dimensional model of the Pyrenean deep structure based on gravity modelling, seismic images and petrological constraints

Abstract

Summary The present study aims at building a 3‐D density model of a whole collisional mountain belt at the lithospheric scale. The geometry of internal structures is constrained by seismic studies, while density values are imposed by petrological considerations. The model must fit both the Bouguer and geoid anomalies. This approach is applied to the Pyrenees, for which numerous geophysical data are available. The crustal structure is constrained from the results of seismic profiles. The most important crustal feature is the large Moho jump which coincides with the surface expression of the North Pyrenean Fault, considered as the suture between the Iberian and Eurasian plates. The Eurasian crust is about 30 km thick, whereas the Iberian crust is 50 km thick in the central part of the range. Thick sedimentary basins at the Piedmont, and two large heavy intracrustal bodies located in the western and central parts of the range, are also important features of the crustal model. At the lithospheric scale, a low‐velocity strip under the North Pyrenean Fault down to 80–100 km depth, interpreted as the subduction of the Iberian lower crust, has been revealed by tomographic models. The density of this structure has been constrained using petrological arguments, which assume that the crustal material is transformed into eclogite at about 12–15 kbar (40–50 km depth). An eclogite‐type density value, much higher than the mantle density, is thus imposed from 50 to 100 km depth. Theoretical Bouguer and geoid anomalies are computed for this 3‐D density model using a summation of density block anomalies. A good agreement is found with observations, giving strong support to the proposed model. The crustal structure, as seen by seismic profiles, explains most of the observed long‐wavelength negative Bouguer anomaly, as well the local anomalies related to the intracrustal bodies. A possible mantle origin of these bodies is ruled out. On the other hand, the transformation of the subducted lower crust into dense eclogite provides a successful fit to the long‐wavelength positive geoid anomalies. In addition, the water released during eclogite formation may help to explain the high conductivity zone detected down to 80 km south of the North Pyrenean Fault. The 3‐D model proposed here thus reconciles petrological considerations and geophysical data.

Deep-rooted “thick skinned” model for the High Atlas Mountains (Morocco). Implications for the seismic Eurasia-Africa plate boundary region

“…The continental collision of Iberia and Europe produced the formation of the Pyrenean orogen with a partial subduction of the Iberian lithosphere to the north (Muñoz, 1992;Roure and Choukroune, 1998;Beaumont et al, 2000). This possibility was confirmed by the magnetotelluric survey along the ECORS profile (Vacher and Souriau, 2001). These studies shows a steep north-dipping, high conductivity body, which reaches a depth of about 70 km and has been interpreted as subducted partly melted lower crust and lithospheric mantle.…”

Crust and mantle lithospheric structure of the Iberian Peninsula deduced from potential field modeling and thermal analysis

“…Thus, Torne et al (1989) interpreted the anomaly as either a relatively shallow (from 1 to 12 km) narrow highdensity body (2930 kg m -3 ) corresponding to a slice of lower crust, based on seismic fabrics, or to various intracrustal mantle slices in agreement with the presence of outcropping lherzolites. Vacher and Souriau (2001), from 3D gravity modeling and considering previous results from Souriau and Granet (1995), concluded that the Labourd Massif anomaly may be explained by the presence of a high-density body (2930 to 2950 kg m -3 ) that extends to 12 km depth, becoming laterally wider below 7 km and then narrowing again until a depth of 12 km. The Saint-Gaudens anomaly is attributed to the presence of a high density block about 60 km long, 25 km wide and extending from 7 to 15 km (Vacher and Souriau, 2001).…”

“…10a). Vacher and Souriau (2001) pointed out the extent of these positive anomalies in the Bouguer anomaly map (Labourd Massif and Saint-Gaudens in the east) (see Fig. 5) as was already defined in tomography studies carried out by Souriau and Granet (1995).…”

The Pyrenean architecture as revealed by teleseismic P-to-S converted waves recorded along two dense transects

“…The thick crustal root beneath the axial zone also provides an explanation for the pronounced negative Bouguer anomaly observed along most of the Pyrenean range, except in its eastern termination close to the Mediterranean Sea (Daignières et al 1982). The Labourd and Saint Gaudens positive anomalies have been attributed to the presence of dense materials at shallow levels , but their exact nature and geometry are still uncertain (Torné et al 1989;Casas et al 1997;Vacher & Souriau 2001).…”