Role of syn-orogenic sedimentation in the evolution of a foreland fold and thrust belt: an example from the Jaca basin, South Pyrenean Zone

Foreland fold and thrust belts (FFTB) are the locus of strong interactions between tectonics and sedimentation during both mountain building then dismantling processes. Especially, the nature and rheology of decollement layers within the basin pre and/or syn-orogenic sequence are susceptible to influence the kinematic evolution and geometry of FFTBs as well as the growth of orogenic wedges.

In the South Pyrenean Zone, the Jaca basin forms the main pro-foreland basin filled by marine to continental siliciclastic sediments during Eocene and Lower Oligocene times. From Bartonian until Rupelian at least, several alluvial fans were successively deposited from east to west along its northern margin in the footwall of emergent thrusts, where they were accompanied by the syn-sedimentary growth of shale-cored anticlines downstream.

Field surveys and “bed-to-bed” mapping of syn-orogenic formations using high-resolution DEMs were used with re-interpreted published seismic lines to build two cross-sections that highlight the structural style and evolution of the Gavarnie-Jaca FFTB. This illustrates the tectono-sedimentary evolution of a transitional, shallow-marine to alluvial foreland margin.

The Eocene-Oligocene syn-orogenic deposits are folded as a wide synclinorium that is thrusted southward over the Ebro foreland along to the Triassic detachment level. In the northern limb, the late Eocene Orosia-Cancias conglomerates exhibit a short-wavelength and small-scale fold detached within the late Middle Eocene shallow marine marls. Beneath this decoupling level, the Lower and Middle Eocene turbidites form a gently foreland ward-dipping imbricate composed of several small-scales thrust-sheets detached at the base of Upper Cretaceous. During end-Bartonian and early Priabonian, the deposition of the Orosia alluvial fan was coeval with the early growth of the Basa anticline downstream. From Priabonian, the anticline was subsequently welded and forward-tilted coevally with the development of the turbidite imbricate beneath, in response to the Gavarnie thrust-sheet emplacement. At this time, the second Cancias conglomerate unit was deposited as a large fluvial fan that progressively filled the foreland basin northern margin.

The tectono-sedimentary evolution of the north-eastern Jaca foreland margin illustrate the interactions between the regional convergence, syn-orogenic shales tectonics and alluvial sedimentation within a foreland fold and thrust belt. The South Pyrenean foreland documents a first example of a gliding alluvial fan on gravity-driven mobile shales in a convergent margin setting.