Placing limits to shortening evolution in the Pyrenees: Role of margin architecture and implications for the Iberia/Europe convergence

Abstract

International audienceEstimating shortening in collision belts is critical to reconstruct past plate motions. Balanced cross-section techniques are efficient in external domains but lack resolution in the hinterland. The role and the original extent of the continental margins during the earliest stages of continental convergence are debated. Here we combine existing and new sequentially restored cross sections in the central Pyrenees, with Iberia/Europe (IB/EU) plate kinematic reconstructions and new apatite fission track, zircon (U-Th)/He, and U/Pb ages to discuss higher and lower bounds of crustal shortening and determine the amount of distal margin sutured during collision. We show that after extension in the Albian (~110 Ma), a 50 km wide extremely thinned crustal domain underwent subduction at 83 Ma. Low-temperature data and thermal modeling show that synorogenic cooling started at 75–70 Ma. This date marks the transition from suturing of the highly extended margin to collision of the more proximal margin and orogenic growth. We infer a relatively low crustal shortening of 90 km (30%) that reflects the dominant thick-skinned tectonic style of shortening in the Pyrenees, as expected for young (Mesozoic) and weak lithospheres. Our proposed reconstruction agrees with IB/EU kinematic models that consider initially rapid convergence of Iberia, reducing from circa 70 Ma onward. This study suggests that plate reconstructions are consistent with balanced cross sections if shortening predicted by age-dependent properties of the continental lithosphere is taken into account

Thick-Skinned and Thin-Skinned Tectonics: A Global Perspective

“…As the two margins collided, the basin was inverted and a bivergent system of thrust faults developed. [254] give a detailed review of the lithosphere scale evolution of the Pyrenees. They invoke a 50 km wide highly thinned continental margin that was subducted between 83 and 71 Ma, followed by the arrival of the thicker proximal margin.…”

“…The two traverses through the Eastern and west Central Pyrenees give a complete image for the internal structure of collision orogen, the one through the west Central Pyrenees being more simple for two reasons: (1) total shortening for the west Central Pyrenees amounts to 75 -80 km [259], while it is a minimum of 90 km, resp. 147 km for the Eastern Pyrenees ( [254,257], resp. [261]).…”

“…[261]). (2) In the Eastern Pyrenees the Iberian crystalline basement was updomed much more in the axial zone, such that the imbricate thrusts in the basement were rotated into plunging antiforms (see cross-section by [257] and [254]). In addition a balanced section including the structure of the cover sediments based on fieldwork and seismics by [259] is available for the west Central Pyrenees.…”

“…Thus some of the thrust faults are reactivated synsedimentary normal faults ( [36] and references therein). [254] emphasize the role of the margin architecture and thermal regime of the basin upon closure and collision.…”

“…The thrust faults affecting the upper crustal basement are in part linked to thrust faults within the cover sequence spatially, but the varying displacements along these thrust faults inhibits a direct kinematic correlation. The structural asymmetry of the Pyrenees reflects the asymmetry of the rift basin [253,254]: the thick basement thrust sheets of the Southern Pyrenees (Guarga and Gavarnie) stem from the relatively undeformed lower plate margin of the rift basin, the highly dismembered units of the Northern Pyrenees from the hyper-extended upper plate margin. As described by [253], the inversion of the basin onto of the hyper-extended crust reactivated some of the synsedimentary normal faults and resulted in a bivergent pop-up structure now situated just north of the Axial Zone.…”