Numerical modelling of Quaternary terrace staircase formation in the Ebro foreland basin, southern Pyrenees, NE Iberia

摘要

The southern foreland basin of the Pyrenees (Ebro basin) is an exorheic drainage basin since LateudMiocene times. Remnants of an early exorheic Ebro drainage system are not preserved, but morphologyudprovides evidence for the Pliocene–Quaternary drainage development. The incision history ofudthe Ebro system is denoted by (i) extensive, low gradient pedimentation surfaces which are associatedudwith the denudation of the southern Pyrenean piedmont around the Pliocene–Quaternary transitionudand (ii) deeply entrenched Quaternary river valleys. Presumably since the Middle Pleistocene fluvialudincision intensified involving the formation of extensive terrace staircase in the Ebro basin. Terraceudexposure dating in major Ebro tributary rivers indicates climate-triggered terrace formation inudresponse to glacial–interglacial climate and glacier fluctuations in the Pyrenean headwaters. Theudoverall (semi)parallel longitudinal terrace profiles argue for progressive base level lowering for theudwhole Ebro drainage network. The landscape evolution model, TISC, is used to evaluate climatic,udtectonic and base level scenarios for terrace staircase formation in the Ebro drainage system. Modeludsimulations are compared with morpho-climatic, tectonic and chronologic data. Results show that climaticudfluctuations cause terrace formation, but the incision magnitudes and convergent terrace profilesudpredicted by this climate model scenario are not consistent with the (semi)parallel terraces in theudEbro basin. A model including previous (late Pliocene) uplift of the lower Ebro basin results in rapidudbase-level lowering and erosion along the drainage network, small late stage incision magnitudes andudterrace convergence, which are not in agreement with observations. Instead, continuous Quaternaryuduplift of both the Pyrenees and the Ebro foreland basin triggers (semi)parallel terrace staircase formationudin southern Pyrenean tributary rivers in consistency with the observed longitudinal terrace profilesudand Middle–Late Pleistocene incision magnitudes. Forward model simulations indicate that theudpresent Ebro drainage system is actively incising, providing further evidence for uplift.