Relict landscapes and fluvial landforms: Catastrophic outflow following a major late Messinian base-level fall

During the Messinian Salinity Crisis (MSC), the entire Mediterranean Basins underwent dramatic canyon incision along its margins due to fluctuating sea levels and rapidly increasing salinity. However, the specific processes and water sources responsible for such profound landscape transformations have yet to be quantitatively demonstrated. In this study, we combine high resolution 3D seismic reflection data with paleo-stream network geomorphic analyses to reveal two distinct phases of fluvial activity within the Messinian Ebro Valley. The first phase is marked by an exceptional discharge of 240,000 m³/s, which rapidly carved a 700-meter-deep valley. This intense incision was followed by a period of more moderate flow, during which the newly-formed valley developed alluvial terraces. Our findings suggest that regressive erosion during the MSC lowstand breached the previously endorheic Ebro Basin, triggering a catastrophic outburst flood and the formation of an extensive drainage network. This event marks the transition from a broad, erosive system to a more confined, meandering river. By refining the timing of this outflow event to the MSC, our study challenges earlier models that positioned the Ebro Basin's opening at an earlier time. This most likely represents the first documented case of an endorheic basin complete drainage due to MSC-induced regressive erosion, providing new insights into the dynamics of this major geological period. Keywords: Messinian Salinity Crisis, Catastrophic outburst flood, Endorheic basin drainage, Seismic reflection data, Fluvial incision