Reconstructing the interacting effects of base level, climate, and tectonic uplift in the lower Miٌo River terrace record: A gradient modelling evaluation

The origin of the lower Miٌo River terraces in the NW Iberian Peninsula has been a topic of debate since the 1940s. Various hypotheses about their main controlling factors have been put forward, but general consensus is still lacking. Field studies of terrace distribution and the weathering of quartzite pebbles in the terrace deposits demonstrated that terrace longitudinal correlation is not parallel to the current river bed. In addition, the longitudinal profile model FLUVER2 was used to simulate the profile evolution of the Miٌo River over the past 450 ka. A reconstructed longitudinal profile and an offshore climate record were used as inputs. Several scenarios with variable uplift rates were investigated and evaluated against properties of four selected reaches along the river for a match with the number of terraces, their relative altitude, timing, and sediment thickness. An uplift scenario with a net constant vertical uplift rate of 0.08 m ka 1 yielded the best results. The best fit scenarios indicate that base level changes from glacioeustatic changes in combination with tectonic uplift are the main driving forces of the observed terrace formation and valley incision. Contrary to most NW European fluvial systems, the timing of upstream-controlled sediment supply has a less dominant effect on terrace formation. This can be explained by the close proximity to the narrow passive Atlantic margin.