Menchu Comas

2D seismic reflection data tied to biostratigraphical and log information from wells in the central and southeastern Alboran Sea have allowed us to constrain the spatial and temporal distribution of rifting and inversion. Normal faults, tilted basement blocks, and growth wedges reveal a thinned continental crust that formed in response to NW‐SE extension. To the east, a secondary SW‐NE trend of extension affects the transitional crust adjacent to the oceanic Algerian Basin. The maximum thickness of syn‐rift sediments is ~3.5 km, and the oldest recorded deposits are Serravallian. The WNW‐ESE Yusuf fault formed a buttress separating and accommodating variable extension between two different tectonic domains: the thinned continental crust of Alboran and the oceanic spreading of the Algerian Basin. Late Tortonian to present‐day NW‐SE Africa/Eurasia plate convergence drove shortening and reactivation of some of the earlier extensional structures as reverse and strike‐slip faults, forming complex, compartmentalised subbasins. Tectonic inversion coexisted with the formation of new faults and folds. Inversion was partial along the Habibas Basin and Al‐Idrisi fault, but complete along the Alboran Ridge, where some SW‐NE trending faults were perpendicular to the recent NW‐SE plate convergence and were reactivated as thrusts. The WNW‐ESE Yusuf fault is oblique to the convergence vector, and therefore, reactivation is mainly expressed as transpressional deformation. Volcanic rocks intruded along the Alboran Ridge and Yusuf faults during the latest stages of extension formed rheological anisotropies that localised the later inversion.

The continental shelf of NW Iberia is of particular interest due to the complex North Atlantic rifting tectonics and modern oceanographic processes that led to its current geomorphological configuration and sediment dynamics. The shelf forms a narrow slow-dipping north-bearing geomorphological structure with a well-defined shelf break located at water depths of 160–180 m. It is a continental margin with sedimentation rates of about 1.5±2.0 mm a⁻¹(the highest values in recent bottom sediments of the ría) subjected to a highly energetic seasonal regime of waves and tides, seasonal upwelling and coast-parallel currents, significantly modulated by the North Atlantic Oscillation. Sediment provenance is dominated by inputs of continental sediments via runoff and riverine discharges and, to a lesser extent, by an upwelling-enhanced bioclastic input. Present-day sedimentation in the area occurs closely associated with two geomorphologically distinct sedimentary environments: the rías inner shelf sedimentary systems and the open continental shelf, and their subsequent evolution since the last glacial.

Mud volcanoes (MVs) are the most prominent indicators of active methane/hydrocarbon venting at the seafloor on both passive and active continental margins. Their occurrence in the western Mediterranean is patent at the West Alboran Basin, where numerous MVs develop overlaying a major sedimentary depocentre containing overpressured shales. Although some of these MVs have been studied, the detailed biogeochemistry of expelled mud so far has not been examined in detail. This work provides the first results on the composition and origin of organic matter, anaerobic oxidation of methane (AOM) processes and general characteristics on MV dynamics using lipid biomarkers as the main tool. Lipid biomarker analysis was performed on MV expelled material (mud breccias) and interbedded hemipelagic sediments from Perejil, Kalinin and Schneider's Heart MVs located in the northwest margin of the Alboran Sea. The n alkane distributions and n alkane-derived indices (CPI and ACL), in combination with the epimerization degree of hopanes (22S/(22S+22R)) indicate that all studied mud breccia have a similar biomarker composition consisting of mainly thermally immature organic matter with an admixture of petroleum-derived compounds. This concordant composition indicates that common source strata must feed all three studied MVs. The past or present AOM activity was established using lipid biomarkers specific for anaerobic methanotrophic archaea (irregular isoprenoids and dialkyl glycerol diethers) and the depleted carbon isotope composition (δ13C) of crocetane/phytane. The presence of these lipid biomarkers, together with the low amounts of detected glycerol dialkyl glycerol tetraethers, is consistent with the dominance of anaerobic methanotrophs of the ANME-2 over ANME-1, at least in mud breccia from Perejil MVs. In contrast, the scarce presence or lack of these AOM-related lipid biomarkers in sediments from Kalinin and Schneider's Heart MVs, suggests that no recent active methane seepage has occurred at these sites. Moreover, the observed methane concentrations support the current activity of Perejil MV, and the very low methane seepage activity in Kalinin and Schneider's Heart MVs.

A comprehensive interpretation of single and multichannel seismic reflection profiles integrated with biostratigraphical data and log information from nearby DSDP and ODP wells has been used to constrain the late Messinian to Quaternary basin evolution of the central part of the Alboran Sea Basin. We found that deformation is heterogeneously distributed in space and time and that three major shortening phases have affected the basin as a result of convergence between the Eurasian and African plates. During the Messinian salinity crisis, significant erosion and local subsidence resulted in the formation of small, isolated, basins with shallow marine and lacustrine sedimentation. The first shortening event occurred during the Early Pliocene (ca. 5.33–4.57 Ma) along the Alboran Ridge. This was followed by a major transgression that widened the basin and was accompanied by increased sediment accumulation rates. The second, and main, phase of shortening on the Alboran Ridge took place during the Late Pliocene (ca. 3.28–2.59 Ma) as a result of thrusting and folding which was accompanied by a change in the Eurasian/African plate convergence vector from NW‐SE to WNW‐ESE. This phase also caused uplift of the southern basins and right‐lateral transtension along the WNW‐ENE Yusuf fault zone. Deformation along the Yusuf and Alboran ridges continued during the early Pleistocene (ca. 1.81–1.19 Ma) and appears to continue at the present day together with the active NNE‐SSW trending Al‐Idrisi strike‐slip fault. The Alboran Sea Basin is a region of complex interplay between sediment supply from the surrounding Betic and Rif mountains and tectonics in a zone of transpression between the converging African and European plates. The partitioning of the deformation since the Pliocene, and the resulting subsidence and uplift in the basin was partially controlled by the inherited pre‐Messinian basin geometry.