Reconstruction of the Exhumed Mantle Across the North Iberian Margin by Crustal‐Scale 3‐D Gravity Inversion and Geological Cross Section

Abstract

Recent models support the view that the Pyrenees were formed after the inversion of a previously highly extended continental crust that included exhumed upper mantle rocks. Mantle rocks remain near to the surface after compression and mountain building, covered by the latest Cretaceous to Paleogene sequences. 3‐D lithospheric‐scale gravity inversion demands the presence of a high‐density mantle body placed within the crust in order to justify the observed anomalies. Exhumed mantle, having ~50 km of maximum width, continuously extends beneath the Basque‐Cantabrian Basin and along the northern side of the Pyrenees. The association of this body with rift, postrift, and inversion structural geometries is tested in a balanced cross section across the Basque‐Cantabrian Basin that incorporates a major south‐dipping ramp‐flat‐ramp extensional detachment active between Valanginian and early Cenomanian times. Results indicate that horizontal extension progressed ~48 km at variable strain rates that increased from 1 to ~4 mm/yr in middle Albian times. Low‐strength Triassic Keuper evaporites and mudstones above the basement favor the decoupling of the cover with formation of minibasins, expulsion rollovers, and diapirs. The inversion of the extensional system is accommodated by doubly verging basement thrusts due to the reactivation of the former basin bounding faults in Eocene‐Oligocene times. Total shortening is estimated in ~34 km and produced the partial subduction of the continental lithosphere beneath the two sides of the exhumed mantle. Obtained results help to pinpoint the original architecture of the North Iberian Margin and the evolution of the hyperextended aborted intracontinental basins.

The Nappe des Marbres Unit of the Basque‐Cantabrian Basin: The Tectono‐thermal Evolution of a Fossil Hyperextended Rift Basin

“…It is noteworthy that Roca et al () and Tugend et al () already proposed that both the BCB and the Parentis basins on each side of the Landes High could have been formed by a north‐dipping low‐angle detachment fault above hyperthinned crust and exhumed mantle. More recently Pedrera et al () alternatively proposed a HE model with a south dipping low‐angle fault in the BCB.…”

“…This folding may be related to salt tectonics developed in the deepest part of the basin (Figure a), with the associated rimmed depocenters. Pre‐HE and syn‐HE sediments are affected by the syn‐HE salt tectonics that controlled the structural style and sedimentary sequence deposited above the Upper Triassic series (Keuper), as highlighted by thickness and facies variations (e.g., Bodego et al, ; Iriarte, ; Pedrera et al, ), and by the development of extensional folds (Martinez‐Torres, ). The salt tectonics resulted in the formation of salt ridges and diapirs, rim‐synclines, and minibasins.…”

Comment on “Reconstruction of the Exhumed Mantle Across the North Iberian Margin by Crustal‐Scale 3‐D Gravity Inversion and Geological Cross Section” by Pedrera et al.

“…Pedrera et al () presented a new tectonic model for the Basque‐Cantabrian Basin in the Pyrenean‐Cantabrian belt, based on a geological cross section and the results of a 3‐D gravity modeling that presumably “demands the presence of a high‐density mantle body placed within the crust in order to justify the observed anomalies.” Other authors have discussed before the possibility that the strong gravimetric (and magnetic) anomalies observed over the area could be explained, totally or partially, by bodies of mantle rocks located at shallow depths beneath the sediments of the Basque‐Cantabrian Basin (e.g., Pedreira et al, ; Roca et al, ; Tugend et al, ). However, the contribution by Pedrera et al () is novel in that it suggested that the present‐day crustal‐scale structure retains largely the morphology of the hyperextended Mesozoic basin and is only slightly modified by the Cenozoic Pyrenean orogeny. In their model, the continental crust is totally removed at present beneath the Cretaceous sediments of the northern part of the Basque‐Cantabrian Basin, so that these sediments are resting on top of the mantle, with the asthenosphere at only ~15‐km depth.…”

The Early Aptian volcanic episode of Gutiolo (N Spain): Expression of the Bilbao Rift Fault Zone

“…The Bay of Biscay and surrounding areas have been surveyed in order to understand its origin and evolution. Several attempts over the years have shed light on this issue, but still many unsolved questions and discrepancies remain (e.g., Boillot, ; Boillot, Malod, Dupeuble, & Cybere Group, ; Brunet, ; Choukroune & Mattauer, ; García‐Mondéjar, ; Gong, Langereis, & Mullender, ; Jammes, Manatschal, Lavier, & Masini, ; Malod & Mauffret, ; Olivet, , ; Pedrera et al, ; Rosenbaum, Lister, & Duboz, ; Roca, Muñoz, Ferrer, & Ellouz, ; Sibuet & Collette, ; Srivastava et al, ; Thinon, ; Tugend et al, ; Ubide et al, ). In this section, we integrate the new data obtained from the Gutiolo volcanism with published data on the evolution of the BCB and the Bay of Biscay.…”

“…Another low‐angle detachment fault extensional model for the Basque–Cantabrian Basin, in this case with the major fault dipping southward, has been proposed recently (Pedrera et al, ). In this model, accumulated extension from the Berriasian to the Aptian base is only 3%, and no major steep faults cutting the whole crust are considered.…”