A model of forced folding controlled by syn-orogenic clastic deposits in a foreland fold and thrust belt. The case of the Jaca Basin, South Pyrenean Zone.

The nature and rate of syn-orogenic sedimentation play an important role in the tectono-stratigraphic evolution of foreland basins. Recent works have shown syn orogenic evaporites at the transition between underfilled and overfilled conditions in the Sivas basin (Turkey) have strongly influenced the subsequent structural and sedimentary evolution of the foreland fold and thrust belt (Legeay et al., 2020).

In the western South Pyrenean Zone, the Jaca basin exhibits the classical stratigraphic succession of a pro-foreland basin systems. This was filled during Eocene-Oligocene with siliciclastic sediments derived from the erosion of the Pyrenees and deposited in a marine to continental environment. During Bartonian, a derivation in the sediment routing system and decreasing subsidence rate resulted in the deposition of a km-thick shale unit forming the transition between marine and continental conditions.

The “bed-to-bed” mapping of the syn-orogenic units at the basin-scale using high-resolution DEM together with the reinterpreted seismic lines were used to build three cross-sections that highlight the structural style of the western South Pyrenean Zone. These illustrate the tectono-sedimentary evolution of a transitional shallow-marine to continental foreland basin.

From end-Bartonian, the deposition of the Orosia-Cancias conglomerates in the north-eastern basin margin is coeval with the onset of the folding of the Basa anticline forward. During Priabonian, this were subsequently welded and forward-tilted coevally with the development of an imbricate in Hecho turbidites beneath, that can be linked to the emplacement of the Gavarnie thrust-sheet. At this time, the Orel alluvial fan was deposited westward at the termination of the Basa anticline and was associated with the growth of the second, shale-cored Atarès anticline folding ahead. This was in turn welded during Rupelian at the time of deposition of the San Juan conglomerates at its western termination, coevally to the Botaya anticline folding downstream.

The tectono-sedimentary evolution of the Jaca fold-and-thrust belt is thus resulting from complex interactions between the deformation of mobile shale and syn-orogenic alluvial sedimentation both at local and regional scale. This example highlights the influence of the syn-orogenic sedimentation and especially the deposition of a decoupling layer at the transition between marine and continental conditions during the evolution of foreland fold and thrust belts as well as thein mountain building processes.