Gateway to a vanishing ocean: the tectono-sedimentary evolution of the Rifian Corridor (Morocco) and the Late Miocene isolation of the Mediterranean

Abstract

The Mediterranean Sea needs a connection to the world ocean otherwise it would evaporate. If the Gibraltar Straits were closed, the Mediterranean would desiccate in about 3000 to 8000 years. Because the volume of water lost by evaporation exceeds the volume received by rain and rivers, constant exchange is crucial between the Mediterranean and the ocean via key marine gateways. The geometry and location of these gateways have changed over time thus influencing the environments and climate of the circum-Mediterranean region. For example, during the Messinian Salinity Crisis (5.97 – 5.33 million years ago), the Mediterranean-Atlantic exchange became sufficiently restricted to isolate the Mediterranean and gross evaporation caused the deposition of Earth’s youngest salt giant. Unlike today's single narrow Mediterranean–Atlantic connection through the Strait of Gibraltar, before 5.33 Ma, there were multiple gateways. Most important of these ancient gateways was the Rifian Corridor, through what is now Morocco, and the Betic Corridor in what is now Spain. This PhD thesis focusses on the timing, causes and consequences of the closure of the Rifian Corridor. Applying techniques of sedimentology, stratigraphy and structural geology, we show that the progressive restriction of the Rifian Corridor was caused by a switch in tectonic regime that favoured enhanced uplift in the region. High-angle faults that cross-cut the seaway axis created narrow constrictions that funnelled bottom-currents in the seaway. We found that distinctive, sandy contouritic sediments denote emanation of saline plumes out of the Mediterranean; these plumes might have influenced thermohaline circulation at a time of pronounced global cooling and transition from a greenhouse to icehouse system. Finally, this thesis proposes four new palaeogeographic maps showing the evolution of the Rifian Corridor, whose closure occurred at 7 ±0.1 Ma. These results, combined with Mediterranean hydrological models and recent dating of the Betic Corridor sediments, indicate that both seaway systems were closed by 7 Ma. Since the Mediterranean requires at least one open seaway to the Atlantic in order to exist until 5.5 Ma, we conclude that the Strait of Gibraltar was already the sole Mediterranean-Atlantic seaway 2 million years earlier than previously thought.