Inhomogeneous rift inversion and the evolution of the Pyrenees

Abstract

The kinematic evolution of the Pyrenees was largely controlled by structural and thermal heterogeneities inherited from a precursor rift system, resulting in complex along-strike variations in structural style and amount of shortening that remain a matter of debate. Based on a comprehensive compilation of published geological and geophysical data, this paper presents four newly-built sequentially restored cross-sections that make it possible to quantify the cumulative deformation along the Pyrenees and to assess the factors behind inhomogeneous lith ospheric shortening. In map-view, the cumulative shortening estimates are further constrained by regional vertical-axis rotations inferred from an extensive paleomagnetic dataset. Total shortening estimates reach maximum values of 125-122 km in the Central Pyrenees (Lannemezan and ECORS cross-sections, respectively) and diminish sideways, becoming 102 km in the Andorra cross-section, to the east, and 94 km in the Oloron ´ cross-section, to the west. To explain this shortening gradient, a new tectonic model with three key stages is proposed. (1) The onset of convergence took place during the Late Cretaceous, with the squeezing of a shallow asthenosphere and the extrusion of thinned crust pieces along the rift axis. (2) The dominant mechanism of deformation transitioned from squeezing to indentation during most of the Eocene, with the northern rift margin (European crust), including the thick exhumed lithospheric mantle, indenting into the weaker southern rifted margin (Iberian crust). The resulting bivergent orogenic wedge was heavily influenced by the lateral variability of the southern rifted margin geometry, with a long necking taper that narrows westward. (3) Crustal under thrusting, mostly operating in Priabonian-Aquitanian times, advanced steadily in the four studied cross-sections, with the strain gradually localizing into a single frontal basement thrust, which promoted incipient subduction. Frictional variations were greatly enhanced by thickness changes of the basal Upper Triassic salt unit, which favored a greater basement-cover decoupling in the Central Pyrenees, and explain the large-scale 20◦ clockwise vertical-axis rotations documented in the South-Western Pyrenees.