Modelling the vertical motions of the Alboran Sea since the Messinian salinity crisis: Topographic reconstruction and glacial-isostatic sea-level effects

<p>The Messinian Salinity Crisis (MSC) was caused and terminated by changes in the Atlantic-Mediterranean connectivity in the western end of the Alboran Basin, a complex tectonic area affected by the Iberia-Africa collision and the presence of a subducted lithospheric slab beneath the Betic-Rif orogen.</p><p>The isostatic, tectonic and erosional effects on surface topography work on different spatial and temporal scales, and their relative contributions to the changes in connectivity and subsequent evaporite deposition and sea-level drop are difficult to constrain.</p><p>We perform 2D-planform flexural isostatic modeling using the Messinian Erosion Surface imaged in the Alboran Basin to reconstruct the topography and vertical motions of this region since the end of the MSC. The results constrain the original depth of the Messinian erosional features to test their consistency against the various models proposed for Mediterranean sea-level changes during the MSC. <br>We apply Glacial Isostatic Adjustment theory to quantify the time response of these vertical motions to the large MSC-related mass shifts (salinification, evaporite deposition and a kilometer-scale sea-level drop),  and their gravitational effects on sea-level in the Mediterranean. In particular, models for the Strait of Gibraltar allowus to identify the potential role of these effects as feedback mechanisms influencing the rates and duration of changes in the Atlantic-Mediterranean connectivity at the straits.  We will explore the possible implications of these for the timing of the closure of the last Atlantic-Mediterranean seaway.</p>

Plio-Quaternary tectonic evolution of the southern margin of the Alboran Basin (Western Mediterranean)

Progress in the understanding and dating of the sedimentary record of the Alboran Basin allows us to propose a model of its tectonic evolution since the Pliocene. After a period of extension, the Alboran Basin underwent a progressive tectonic inversion starting around 9–7.5 Ma. The Alboran Ridge is a NE–SW transpressive structure accommodating the shortening in the basin. We mapped its southwestern termination, a Pliocene rhombic structure exhibiting series of folds and thrusts. The active Al-Idrissi Fault zone (AIF) is a Pleistocene strike-slip structure trending NNE–SSW. The AIF crosses the Alboran Ridge and connects to the transtensive Nekor Basin and the Nekor Fault to the south. In the Moroccan shelf and at the edge of a submerged volcano we dated the inception of the local subsidence at 1.81–1.12 Ma. The subsidence marks the propagation of the AIF toward the Nekor Basin. Pliocene thrusts and folds and Quaternary transtension appear at first sight to act at different tectonic periods but reflect the long-term evolution of a transpressive system. Despite the constant direction of Africa–Eurasia convergence since 6 Ma, along the southern margin of the Alboran Basin, the Pliocene–Quaternary compression evolves from transpressive to transtensive along the AIF and the Nekor Basin. This system reflects the logical evolution of the deformation of the Alboran Basin under the indentation of the African lithosphere.

A geochemical approach to reconstructing sediment dynamic and thermohaline circulation in the western Mediterranean over the last deglaciation

<p>The westernmost Mediterranean basins is an exceptional and sensitive region for reconstructing past climate and oceanographic conditions. Geochemical signatures from diverse sediment records in the Alboran Sea and the Balearic basin, in particular, Ti/ca and Fe/Ca ratios, as proxies for the relative abundance of siliciclastic vs. carbonate fraction, have been investigated. These have also been compared with other previously studied records from the western Mediterranean and the Gulf of Cadiz to elucidate the mechanisms triggering the relative variations between the carbonate and siliciclastic fraction. The lithogenic fraction represents around 70% of the sediment in the Alboran basin, which mainly derived from riverine discharge and coastal erosion. Resuspension of fine sediment particles from the slope and the sea floor by bottom-water currents is a relevant process in these basin. The studied records are located between 850 m and 2400 m below the sea level, under the influence of the Western Mediterranean Deep Water (WMDW), which is restricted to a water depth below 500-600 m and to the Moroccan margin. This deep current is formed in the Gulf of Lion, when the superficial and intermediate waters sink by a density increase, and flow out the basin through the Gibraltar Strait, contributing to the Mediterranean Outflow Water (MOW) along with the Levantine Intermediate Water (LIW). The WMDW formation is enhanced during cold and arid periods. The comparison with other previously studied records, support important variations of the mechanisms triggering the relative contribution of carbonate and siliciclastic fractions during the last 20,000 yrs. The, Ti/Ca and Fe/Ca ratios increased during cold and arid periods, such as the Heinrich Event 1 (HE1) and the Younger Dryas (YD). These changes are more prominent in the Balearic basin and the eastern Alboran basin than in the western Alboran basin and the Gulf of Cadiz. Thus, we hypothesized that the increase in the Ti/Ca and Fe/Ca ratios is rather related to the enhanced WMDW production and more remobilization of fine siliciclastic sediments.</p>

Comparison of the crust and upper mantle structure of the Alboran and Algerian domains (Western Mediterranean): Tectonic significance

<p>We present a comparison of the present-day crust to upper-mantle structure in the Western Mediterranean along two NW-SE oriented geo-transects in the Alboran and Algerian basins. The Alboran domain geo-transect traverses the Iberian Massif, the Betics, the Alboran Basin and ends in the northern margin of Africa between the Tell and Rif mountains. The Algerian domain geo-transect traverses the Catalan Coast Ranges, the Valencia Trough, the Balearic Promontory, the Algerian basin, the Greater Kabylies and ends in the Tell-Atlas Mountains in the northern margin of Africa. We model the thermal, density (i.e. compositional) and seismic velocity structure by integrating geophysical and geochemical dataset in a self-consistent thermodynamic framework. The crustal structure is constrained by seismic experiments and geological cross-sections, whereas seismic tomography models and mantle xenoliths constrain the upper mantle structure and composition. The Algerian Basin lithosphere shows a typical oceanic lithosphere composition, whereas the Alboran Basin lithosphere is slightly fertile. The lithospheric mantle beneath the Betics and Greater Kabylies are also fertile compared to the Iberian and African lithospheres showing the involvement of the fertile sublithosphere mantle during the later stages of subduction. In the Valencia Trough, the lithosphere is fertile in comparison to the Balearic Promontory lithosphere, which is similar to Iberian lithosphere. A lithosphere-scale thickening is observed in the Betics, and the Greater Kabylies, and thinning follows towards the Alboran and Algerian back-arc basins. Detached slabs with anomalous temperature of-320 <sup>o</sup>C, with oceanic lithosphere composition beneath the Greater Kabylies, and Iberian lithosphere composition beneath the Betics, are required to fit the geoid height. Our results impose important constraints for the geodynamic evolution models of the Western Mediterranean.</p><p>This work has been supported by SUBTETIS (PIE-201830E039) project, EU Marie Curie Initial Training Network ‘SUBITOP’ (674899-SUBITOP-H2020-MSCA-ITN-2015), the Agencia Estatal de Investigación through projects MITE (CGL2014-59516) and GeoCAM (PGC2018-095154-B-100), and the Agency for Management of University and Research Grants of Catalonia (AGAUR-2017-SGR-847).</p>

Morphology of the Messinian Salinity Crisis surfaces and its related deposits in the Alboran Sea: a continuous Mediterranean-Atlantic connection?

<p>The Messinian Salinity Crisis (MSC), which affected the Mediterranean region during the latest Miocene, is mainly characterized by the deposition of thick evaporites in central basins and strong fluvial erosion of margins. The subaerial fluvial erosion, known as the Messinian Erosional Surface (MES), is for instance continuously followed from the Gulf of Lions up to 360 km from the present-day shoreline upstream the Rhône Valley. Short drainage systems limited by high coastal mountain ranges, must have been significantly affected by the Messinian erosion.</p><p>The Alboran Sea is similarly characterized by a geographic context and was the first Mediterranean area concerned by connection-disconnection to the Atlantic Ocean during the last millions years. A recent study suggested that the Alboran Sea remained always connected to the Atlantic Ocean during the MSC, being the marine refuge for the Mediterranean taxa.</p><p>In this work, we have performed an extensive research of the MES and MSC-related deposits in the Alboran region, both onshore and offshore, integrating outcrop descriptions, supported by new biostratigraphic data and seismic profile analyses. This study leads to an up-to-date geological and morphological map displaying the actual contours and morphology of the MES in the whole Alboran domain. The MES has a subaerial origin and is continuously followed from land to the offshore domain, sealed by post-MSC marine sediments. Both Spanish and Morrocan sides show three different erosive morphologies: downslope trending paleocanyons cut by a large reflooding channel crossing the entire Alboran basin from the Strait of Gibraltar to Algerian Basin, and alongslope   terraces. The occurrence of all these striking erosive features with basinal extension  questions the hypothesis of the permanent connection with the Atlantic Ocean.</p>

Neogene polyphase deformation related to the Alboran Basin evolution: new insights for the Beni Bousera massif (Internal Rif, Morocco)

Located in the Internal domain of the Rif belt, the Beni Bousera massif is characterized by a stack of peridotites and crustal metamorphic units. The massif is intruded by granitic dykes and affected by several normal ductile shear zones. Structural, petrological and 40Ar–39Ar dating analyses performed on these two elements highlight that (1) the granitic dykes are emplaced within major N70° to N140° trending normal faults and shear zones, resulted from an NNE-SSW extension (2) the Aaraben fault in its NE part is characterized by N70° to N150° trending ductile normal shear zones, resulted from a nearly N-S extension and (3) the age of this extensional event is comprised between 22 and 20 Ma. Available paleomagnetic data allow a restoration of the initial orientation of extension, which was nearly E-W contemporary with the Alboran Basin opening in back-arc context, during the Early Miocene. At the onset of the extension, the peridotites were somehow lying upon a partially melted continental crust, and exhumed during this event by the Aaraben Normal Shear Zone. Afterward, the Alboran Domain suffered several compressional events.

Plio-Quaternary tectonic evolution of the southern margin of the Alboran Basin (Western Mediterranean)

Progresses in understanding the sedimentary dynamic of the Western Alboran Basin lead us to propose a model of evolution of its tectonic inversion since the Pliocene to present-time. Extensive and strike-slip structures accommodate the Miocene back-arc extension of the Alboran Basin, but undergo progressive tectonic inversion since the Tortonian. Across the Alboran Basin, the Alboran Ridge becomes a transpressive structure accommodating the shortening. We map its southwestern termination: a Pliocene rhombic structure exhibiting series of folds and thrusts. A younger structure, the Al-Idrissi fault zone (AIF), is Pleistocene to present-day active strike-slip fault zone. This fault zone crosses the Alboran Ridge and connects southward to the transtensive Nekor Basin and the Nekor fault. In the Moroccan shelf and at the edge of a submerged volcano, we date the inception of the local shelf subsidence from the 1.81–1.12 Ma. It marks the propagation of the AIF toward the Nekor Basin. Pliocene thrusts and folds and Quaternary transtension appear at first sight as different tectonic periods but reflects the long-term evolution of a transpressive system. Despite a constant direction of Africa/Eurasia convergence since 5 Ma at the scale of the southern margin of Alboran Basin, the Pliocene-Quaternary inversion evolves from transpressive to transtensive on the AIF and the Nekor Basin. This system reflects the expected evolution of the deformation of the Alboran Basin under the indentation of the African lithosphere.

Arc-related high-K magmatism in the Ceuta Peninsula (Internal Rif, Spain): discovery and consequences

We document the occurrence of micro-diorite magmatic sills, with magmatic enclaves, in the Ceuta Peninsula within metapelites from the Lower Sebtides units (Internal Rif). All magmatic rocks show a primary magmatic mineralogy and geochemical signature diagnostic for high-K calc-alkaline to shoshonitic island arc magmatism. Moreover, these rocks are significantly affected by secondary metamorphic transformations under greenschist- to amphibolite-facies conditions, regionally dated atc. 21 Ma. Geometric relationships between the sills and the main regional foliation, developed under intermediate-pressure granulite-facies conditions atc. 28 Ma, demonstrate that the sills emplaced during the late stage of this main tectonic event. New U–Pbin situanalyses of monazite performed on the micro-diorite sills provide an age of 20.64 ± 0.19 Ma, coherent with this chronological framework and interpreted as the age of greenschist-facies re-equilibration. The discovery of pre-Miocene high-K calc-alkaline to shoshonitic arc-related magmatism is clearly consistent with the subduction context proposed for the Alboran Basin evolution, according to geophysical investigations. In this framework, the Lower Sebtides units could be considered as part of the upper plate of the subduction system, while the Upper Sebtides must be regarded as the lower subducted plate.