Burial and structural evolution of a deformed foreland basin: the south-western Pyrenees

Deciphering the tectono-thermal evolution of deformed foreland basins is fundamental for understanding the kinematics of mountain building processes. In orogenic systems, tectonic loading during early compressional stages produces the formation of foreland basins that, as compression progresses, are folded, exhumed and incorporated into the forming fold-and-thrust belts. These exhumed foreland basins represent excellent candidates for studying the early-orogenic burial conditions and geometries. The Jaca Basin, in the south-western Pyrenees, represents the primary south Pyrenean foreland basin that was latter deformed, piggy-back thrusted and embedded into the south Pyrenean fold-and-thrust belt. The basin displays a non-cylindrical geometry and it is filled by exceptionally preserved syn-orogenic sequences: early-middle Eocene turbidites that grade upwards to late Eocene marls and late Eocene-Oligocene and Miocene continental units. Debate exists on the timing of thrusting exhuming the basin, the geometry of basement thrusts and their link to syn-orogenic sedimentation and emerging cover structures. This debate sums up to the uncertainties on the basin thermal history, with previous paleo-thermal data being heterogeneously distributed and mostly concentrated in the eastern part of the basin.

To reduce these uncertainties and contribute into the understanding of debated kinematic aspects, we carried out a combined structural and paleo-thermal study covering the eastern and central part of the deformed Jaca basin. Four sequential, seismic-based cross sections have been constructed whereas thermal and burial conditions along section traces have been constrained through Raman Spectroscopy on Carbonaceous Material (RSCM). Samples for RSCM have been collected from the Eocene turbidites and indicate maximum burial temperatures of ~200ºC at the base of the sedimentary sequence (northern part of the cross-sections) that decrease progressively to the south where younger turbidites crop-out. In the considered area, RSCM temperature estimates along specific cover thrusts indicate a westward increase of peak temperatures. Along-strike thermal variations are in line with seismic-based cross-sections that depict strong lateral changes in the geometry of the basement soling the Jaca basin. The top of the basement is at shallower positions in the central Jaca basin where the number of basement thrusts increases. Basement thrusts partly derive from the reactivation of inherited Permian-Triassic extensional faults and partition the eastern-central Jaca basin into two structural domains separated by a main, oblique basement ramp. From cross-sections and thermal estimates, this contribution allows reconstructing the tectono-thermal history of the Jaca basin from the early foreland basin stages to the advanced shortening stages both along its central and eastern segments.