The Ebro River and margin (NW Mediterranean): transient subaereal landscapes of the Messinian from 3D seismic reflection data

Partial desiccation of the Mediterranean during the Messinian Salinity Crisis (MSC) implied the subaereal exposure of the continental margins for a relatively short time span (>1 Myr). This was enough for a complex subaereal drainage network to develop and erode continental margin sediments creating a regional unconformity known as the Messinian Erosional Surface (MES). 3D seismic data from the Ebro Margin, NW Mediterranean Sea, shed new light on the processes that formed the transient landscapes of the Messinian. The 3D seismic data provide an unprecedented and detailed view of the MES and display characteristic features of subaerial incision, including a drainage network with tributaries of at least five different orders, terraces and meandering channels. The Messinian landscape presents a characteristic stepped-like profile that allows the margin to be subdivided in three different regions roughly parallel to the coastline. No major tectonic control exists on the boundaries between these regions. The boundary between the two most distal regions marks the location of a relatively stable base level, and this is used in backstripping analysis to estimate the magnitude of sea level drop associated with the MSC on the Ebro Margin (1.3 km). The MES on the Ebro Margin is dominated by a major fluvial system, that we identify here as the Messinian Ebro River. In the relatively short time period of the MSC the Ebro River was able to carve a 1.3 km deep subaerial canyon. The meandering nature and low gradient of the Ebro river long profile suggest that equilibrium conditions were reached. The 3D seismic data, onshore geology and modeling results suggest that the Ebro River drained the Ebro Basin well in advance of the Messinian. Coupled isostasy and river incision and transport modeling offer constraints on the evolution of the Ebro River. Flexural isostatic compensation of sediment and water column changes indicate that lithospheric elastic thickness (Te) had a major role in determining the direction adopted by the main river within the exposed delta. Low values of Te imply that the exposed continental slope undergoes smaller wavelength-higher amplitude uplift, an isostatic response to the removal of the water column during the Messinian. This in turn implies a higher capability of this uplift to divert the river path. We find that for Te values lower than10 km the main river is usually diverted and reoriented parallel to the coast, whereas for values of 20 km or higher incision along the main river copes with uplift and keeps its direction perpendicular to the margin strike.