The Alboran volcanic-arc modulated the Messinian faunal exchange and salinity crisis

Abstract

What process triggered the Mediterranean Sea restriction remains debated since the discovery of the Messinian Salinity Crisis (MSC). Recent hypotheses infer that the MSC initiated after the closure of the Atlantic-Mediterranean Betic and Rifean corridors, being modulated through restriction at the Gibraltar Strait. These hypotheses however, do not integrate contemporaneous speciation patterns of the faunal exchange between Iberia and Africa and several geological features like the evaporite distribution. Exchange of terrestrial biota occurred before, during and after the MSC, and speciation models support an exchange path across the East Alborán basin (EAB) located a few hundreds of km east of the Gibraltar Strait. Yet, a structure explaining jointly geological and biological observations has remained undiscovered. We present new seismic data showing the velocity structure of a well-differentiated 14–17-km thick volcanic arc in the EAB. Isostatic considerations support that the arc-crust buoyancy created an archipelago leading to a filter bridge across the EAB. Sub-aerial erosional unconformities and onlap relationships support that the arc was active between ~10–6 Ma. Progressive arc build-up leading to an archipelago and its later subsidence can explain the extended exchange of terrestrial biota between Iberia and Africa (~7–3 Ma), and agrees with patterns of biota speciation and terrestrial fossil distribution before the MSC (10–6.2 Ma). In this scenario, the West Alboran Basin (WAB) could then be the long-postulated open-marine refuge for the Mediterranean taxa that repopulated the Mediterranean after the MSC, connected to the deep restricted Mediterranean basin through a sill at the Alboran volcanic arc archipelago.

The Crustal Domains of the Alboran Basin (Western Mediterranean)

“…However, the lack of good 3‐D control makes it difficult to determine the real attitude of the reflectors. Similar reflections had previously been described on ESCI‐Alb line (Figure ) and interpreted as magmatic layering (Booth‐Rea et al, , ), which may be the origin of most lower crust reflectivity in the magmatic domain.…”

“…The change in basement type between the volcanic arc and the back‐arc oceanic crust is gradual in the seismic images. The volcanic arc images show a complex reflectivity in the crust of 4–6 s TWTT thick (8–18 km; Booth‐Rea et al, , and Figures ), whereas back‐arc oceanic crust—that is, formed at the spreading center—is <2–2.5 s TWTT thick (~6–7 km) and exhibits reflections typically restricted to the lower crust (Figure ), similar to images of oceanic crust from the Pacific (Ranero, Reston, et al, ) and the Atlantic (Ranero, Banda, et al, ).…”

“…The basement of the central and NE regions of the EAB is formed by Neogene magmatism (Booth‐Rea et al, , ; Giaconia et al, ; Gómez de la Peña et al, ) sampled in outcrops at the seafloor and onshore in SE Iberia (Duggen et al, , , ). The geochemical composition of these volcanic rocks is indicative of subduction‐related fluids (Duggen et al, ).…”

“…Like the Carboneras fault system, Yusuf and Alboran Ridge fault systems are located at the boundary of crustal domains and their fault activity is latemost Messinian or earliest Pliocene (Martínez‐García et al, ) and thus much younger than the basement age. Toward the east, the volcanic basement of the EAB progressively transitions to the thinner oceanic crust of the ABB (Figure , Booth‐Rea et al, , ).…”

Mantle resistance against Gibraltar slab dragging as a key cause of the Messinian Salinity Crisis

“…The three insets illustrate the timing and mechanisms of vertical motion that led to the temporary isolation (~5.33–7 Ma) of the Mediterranean, and refer to the corresponding geodynamic forces. The east Alborán Arc hypothesis suggests that an emergent, volcanic archipelago could have temporarily connected southeastern Iberia with northern Africa (Booth‐Rea, Ranero, & Grevemeyer, ). (b) Cartoon interpretation of RGB‐slab morphology across the upper mantle modified from (Spakman et al, ).…”