Detrital zircon (U-Th)/(Pb-He) geo- and thermo-chronometric constraints on provenance and foreland basin evolution of the Ainsa Basin, south-central Pyrenees, Spain

Abstract

Synorogenic foreland basin fill of the south central Pyrenees preserves the eroded remnants of the early stages of Pyrenean fold-thrust belt evolution and topographic growth. Detailed isotopic provenance analysis allows for the reconstruction of sediment sources and boundary conditions of sediment routing systems. Detrital zircon (DZ) (U-Th)/(Pb-He) double-dating of foreland basin sediment is a powerful tool for sedimentary provenance analysis and constraining the exhumational history of mountain belts. This study integrates published structural, stratigraphic, and petrologic data, with new geochronometric and thermochronometric data in a four dimensional source-to-sink approach to decipher provenance and thermal evolution during Pyrenean orogenesis. The Ainsa Basin within the south Pyrenean foreland basin system contains the Hecho Group, a succession of turbiditic channels and levees deposited in the transition zone between the fluvial-deltaic systems of the Tremp-Graus-Ager basin in the east to the submarine fan complex of the Jaca basin in the west. This study presents 2175 new DZ U-Pb ages and 246 new DZ (U-Th)/(Pb-He) double-dated ages from 20 samples collected from the Eocene Hecho group turbidites and the overlying Sobrarbe-Escanilla fluvio-deltatic sandstone in the Ainsa Basin of the south central Pyrenees. These data along with previous DZ U-Pb studies indicate a progressive shift in sediment provenance during orogenesis. The basin was initially being fed by Cadomian/Caledonian plutonic and metamorphic rocks exposed in the eastern Pyrenees with minor sediment contribution from sources located to the south and south east of the basin. Progressive westward exhumation of the Pyrenean Axial Zone promoted a shift in the dominant sediment source to subsequently exhumed Variscan plutons and recycled Mesozoic deposits in the central Pyrenean Axial Zone. Based on DZ (U-Th)/He results, four main cooling events are identified: Pyrenean orogenesis (~56 Ma), initial inversion (~80 Ma), Cretaceous rifting (~100 Ma), and pre-Mesozoic cooling ages related to earlier tectonic phases. This study imposes new constraints on the paleogeographic evolution of the Pyrenees and illustrates that active contractional structures are the dominant control on sediment routing evolution by introducing new sources and controlling sediment pathways during orogenesis.