Paleomagnetism and Thermochronology in Tertiary systectonic sediments of the South-central Pyrenees: Chronostratography, kinematic and exhumation constraints

• Autores: E. Beamud
• Directores de la Tesis: Josep Anton Muñoz i de la Fuente (dir. tes.), Miguel Garcés Crespo (dir. tes.)
• Lectura: En la Universitat de Barcelona ( España ) en 2013
• Idioma: inglés
• Tribunal Calificador de la Tesis: Emilio L. Pueyo Morer (presid.), Francesc Sàbat i Montserrat (secret.), Peter van der Beek (voc.)
• Materias:
  • Ciencias de la tierra y del espacio
    • Geología
      • Estratigrafía
      • Geología estructural
    • Geofísica
      • Paleomagnetismo
• Enlaces
  • Tesis en acceso abierto en:  TDX  Dipòsit Digital de la UB 
• Resumen

  • This PhD Thesis presents the Tertiary kinematic evolution of the South-Central Pyrenees from the integration of magnetostratigraphic, magnetotectonic and thermochronological analyses on its synorogenic sedimentary record. One of the main contributions of this PhD thesis is the continuous absolute dating of the syntectonic conglomerates of La Pobla de Segur, Senterada and Sis. Magnetostratigraphy of these materials establishes their deposition during Middle Eocene-Late Oligocene times (from chron 19r up to chron 9n). Integration of these results with previous magnetostratigraphic works within the Ainsa Basin has allowed the establishment of a new chronostratigraphy for the Eocene-Oligocene materials of the South-Central Pyrenees. The obtained chronostratigraphy has biochronological implications as it substantially changes the traditionally accepted ages of the European reference levels MP14 to MP17. MP14 and MP15 reference levels are proposed to correlate to Lutetian whereas MP16 and MP17 would be Bartonian in age. These results reveal that the correlation between the continental and marine Paleogene record needs further refinement and therefore, that the chronostratigraphic age attributions based on MP reference levels should be taken with caution. The magnetostratigraphic ages have been combined with detrital thermochronology on 13 granitic cobbles enclosed in the syntectonic conglomerates. The detrital apatite fission track ages obtained vary from 63 to 27 Ma. When these ages are combined with the stratigraphic ages, samples define 5 groups with thermochronological ages generally increasing down-section except in the most deeply buried ones due to post-depositional partial annealing. Thermal models reveal three periods of rapid-cooling within the Axial Zone due to movement on large-thrust sheets. A dramatic increase in exhumation rate occurred during the latest Eocene-Early Oligocene related to the onset of movement in the Rialp thrust sheet and the increase in structural relief of the Axial Zone by underthrusting. The magnetostratigraphic and thermochronological ages obtained permit the link between the sedimentation rates in the surrounding basins and the exhumation rates in the hinterland, and reveal that accommodation space exerted the main control on sedimentation rates within the piggy-back basins. Post-depositional annealing of the stratigraphically lowest samples suggests about 2 km of burial by the younger synorogenic materials during progressive burial of the South Pyrenean fold and thrust belt. Thermal models also suggest a rapid exhumation event during the Late Neogene, linked to re-excavation caused by the base level drop during the Ebro River capture to the Mediterranean Sea. Sedimentation of the studied synorogenic materials during Middle Eocene-Oligocene times occurred coeval to the development of thrust salients in the southern Pyrenees. The magnetotectonic study of 36 sites carried out in the Ainsa Oblique Zone reveal clockwise vertical-axis rotations varying from ~80° in the lower Lutetian materials of the Mediano anticline to ~20° in middle Ilerdian materials cropping out at the northern edge of the Añisclo anticline. Sites in the central part of the Montsec and Bóixols thrust sheets don’t record any significant rotation as neither do the syntectonic materials of La Pobla, Sis and Senterada. The age of the main rotation event within the Gavarnie thrust sheet is constrained to Lutetitan to Bartonian times, when all the structures of the Ainsa Oblique Zone were active. This vertical-axis rotation stage obeys to a difference of ~50 km in the amount of displacement on the Gavarnie thrust sheet controlled by the NE-SW pinch out of the Triassic salts at its basal detachment. A second rotation event of ~ 10° took place since Priabonian times, as a result of the differential displacement of about 22 km of the Serres Marginals thrust sheet, respect the Gavarnie one, on top of the upper Eocene-Oligocene evaporites. The synchronicity between thrusting and vertical-axis rotations suggests that the curved fold and thrust belt formed by progressive curvature with divergent thrust transport. The results exposed in this Thesis reveal a strong relationship between the stratigraphic record of the synorogenic materials, thrusting and exhumation in the Axial Zone and the structural evolution of the South Pyrenean thrust system. Tectonic forces controlled the observed patterns of exhumation, the evolution of the synorogenic topography of the piggy-back and foreland basins and the depositional features of the synorogenic sediments.