Structural styles and Neogene petroleum system around the Yusuf-Habibas Ridge (Alboran Basin, Mediterranean Sea)

International audienceThe Algerian offshore is part of the southern margin of the western Mediterranean Sea. The western part of this offshore area represents the transitional margin between the South Algero-Balearic Basin and the Alboran Basin. The study area includes the southern and eastern parts of the Alboran Basin and the northwestern part of the Algerian margin and is in the western part of the plate boundary between Eurasia and Africa (Figure 1). The Yusuf-Habibas Ridge is a major EW-striking structure of this complex plate boundary, separating the eastern and southern parts of the Alboran Basin from the South Algero-Balearic Basin (Martinez-Garcia et al., 2011, and references therein). The ridge played an important role during the Neogene Alboran westward block migration between the Africa and Iberia plates, while the Kabylies blocks migrated southward and accreted to Africa. Furthermore, the ongoing NW-SE convergence between Africa and Iberia has induced a new stress field, since 7 Ma ago, replacing an earlier stress field (Fernandez-Ibañez et al., 2007) and leading to reactivation and polyphased deformation on the main structures in the basin, including the Yusuf-Habibas Ridge

Evidence for a developing plate boundary in the western Mediterranean

Acknowledgements The authors acknowledge support from the Spanish Ministry of Economy and Competitiveness through the Complementary Action ESF TopoEurope TOPOMED (CGL2008- 03474-E/BTE) and national project EVENT (CGL2006-12861-C02-02). L.G.P. was funded by the Spanish Ministry of Education, Culture and Sport through the FPU fellowship AP2012-1579 and a Short-Term Scientific Mission (COST-STSM-ECOST-STSM-ES1301- 180814-045667) inside the COST Action ES1301. This study benefited from a Marie Skłodowska-Curie Individual Fellowship to L.G.P. (H2020-MSCA-IF-2017 796013), the project “MORPHOMED” (PID2019-107138RB-I00) funded by MCIN/SRA (State Research Agency/10.13039/501100011033), FEDER/Junta de Andalucía-Consejería de Transformación Económica, Industria, Conocimiento y Universidades/Projects (B-RNM-305-UGR18, A-RNM-508-UGR20 and P18-RT-3632), and acknowledges the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S) and C.R.R. project PID2019-109559RB-I00 of the Spanish “Ministerio de Ciencia e Innovación”.We thank the reviewers (JoaoDuarte and anonymous) for their insightful comments. This work has been carried out within Grup de Recerca Consolidat de la Generalitat de Catalunya “Barcelona Center for Subsurface Imaging” (2017 SGR 1662).Additional information Supplementary information The online version contains supplementary material available at https://doi.org/10.1038/s41467-022-31895-z.The current diffuse-strain model of the collision between Africa and Eurasia in the western Mediterranean predicts a broad region with deformation distributed among numerous faults and moderate-magnitude seismicity. However, the model is untested because most deformation occurs underwater, at poorly characterized faults of undetermined slip. Here we assess the diffuse-strain model analysing two active offshore fault systems associated with the most prominent seafloor relief in the region. We use pre-stack depth migrated seismic images to estimate, for the first time, the total Plio-Holocene slip of the right-lateral Yusuf and reverse Alboran Ridge structurally linked fault system. We show that kinematic restoration of deformational structures predicts a slip of 16 ± 4.7 km for the Alboran Ridge Fault and a minimum of 12 km for the Yusuf Fault. Thus, this fault system forms a well-defined narrow plate boundary that has absorbed most of the 24 ± 5 km Plio-Holocene Africa-Eurasia convergence and represents an underappreciated hazard.MCIN Projects A-RNM-508-UGR20, B-RNM-305-UGR18, P18-RT-3632H2020 Marie Skłodowska-Curie Actions H2020-MSCA-IF-2017 796013, PID2019-107138RB-I00Society for Research on AdolescenceGeneralitat de Catalunya 2017 SGR 1662Ministerio de Educación, Cultura y Deporte AP2012-1579, COST-STSM-ECOST-STSM-ES1301-180814-045667, ES1301 MECDMinisterio de Economía y CompetitividadMinisterio de Ciencia e InnovaciónSevero Ochoa Centre of Excellence CEX2019-000928-S, PID2019-109559RB-I0

Flexural behaviour of the north Algerian margin and tectonic implications

International audienceThe Algerian margin formed through back-arc opening of the Algerian basin (Mediterranean Sea) resulting from the roll-back of the Tethyan slab. Recent geophysical data acquired along the Algerian margin showed evidence of active or recent compressive deformation in the basin due to the ongoing Africa–Eurasia convergence. Published data from four wide-angle seismic profiles have allowed imaging the deep structure of the Algerian margin and its adjacent basins. In this study, we converted these velocity models into density models, then into isostatic anomalies. This allowed us to image an isostatic disequilibrium (relative to a local isostasy model) reaching a maximum amplitude at the margin toe. Converting isostatic anomalies into Moho depth variations shows that the Moho extracted from wide-angle seismic data is deeper than the one predicted by a local isostasy model in the oceanic domain, and shallower than it in the continental domain. These anomalies can be interpreted by opposite flexures of two plates separated by a plate boundary located close to the margin toe. We use a finite element model to simulate the lithospheric flexure. The amplitude of the equivalent vertical Moho deflection is larger in the central part of the study area (6–7 km) than on the easternmost and westernmost profiles (3 km). The effective elastic thickness used to best match the computed deflection is always extremely low (always less than 10 km) and probably reflects the relatively low strength of the lithosphere close to the plate boundary. Comparison with other wide-angle seismic profiles across an active and a passive margin show that the North Algerian margin displays isostatic anomalies close to that of an active margin. Finally, plate flexure is highest at the southern tip of the ocean-continent transition, possibly indicating that a former passive margin detachment is reactivated as a crustal scale reverse fault pre-dating a future subduction

Global tomography from ambient seismic noise cross-correlation

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The 1856 Tsunami of Djidjelli (Eastern Algeria): Seismotectonics, Modelling and Hazard Implications for the Algerian Coast

International audienceOn August 21st and 22nd 1856, two strong earthquakes occurred off the seaport of Djidjelli, a small city of 1000 inhabitants, located 300 km east of Algiers (capital of Algeria). In relation to these two earthquakes, an important tsunami (at least one) affected the western Mediterranean region and the eastern Algerian coastline between Algiers and La Calle (Algero-Tunisian border). Based on historical information as well as on data recently collected during the Maradja 2 survey conducted in 2005 over the Algerian margin, we show that the tsunami could have been generated by the simultaneous rupture of a set of three en echelon faults evidenced off Djidjelli. From synthetic models, we point out that the area affected along the Algerian coast extended from Bejaia to Annaba. The maximum height of waves reached 1.5 m near the harbor of Djidjelli

Turbidite chronostratigraphy off Algiers, central Algerian margin: A key for reconstructing Holocene paleo-earthquake cycles

Northern Algeria is threatened by moderate to large magnitude earthquakes resulting from the slow convergence between the African and European plates. Main active faults are located offshore along the Algerian coast, as exemplified by the 2003 Mw 6.9 Boumerdès earthquake. This event triggered numerous and widespread turbidity currents over ∼ 150 km along strike in the Algerian basin (reaching 2800 m of water depth) and demonstrates the multi-source and multi-path characteristics of earthquake-triggered turbidity flows along this margin segment. We rely on the sedimentological analysis of five cores located at the toe of the Algiers margin, close to the 2003 cable break sites, to explore the potential for Holocene turbidite paleoseismology. Radiocarbon measurements provide age models for hemipelagic sediments. Based on sedimentary facies identification, analysis of depositional sequences (stacking pattern) and a stratigraphic framework established by age models, a first correlation of turbidites between the 5 cores is attempted. The number of turbidites is constant at the base of the continental slope and decreases seawards (over 80 km away from the coast). From turbidite correlations, 36 synchronous events are identified along the Algiers margin segment over the last 9 kyr, and are tentatively interpreted as seismically triggered, providing a 250 yr long mean recurrence interval. The main historical earthquakes in the Algiers area (2003, 1716 and 1365 AD) reasonably correlate with three out of the four last turbidites, strengthening the hypothesis that turbidites are suitable markers for Holocene paleoseismology. Recurrence intervals of turbidites range between 50 and 900 yr, defining quiescence periods exceeding 450 yr. Three quiescence periods lasting about 800, 1400 and 500 yr (7–6.2 ka BP, 5.4–4 ka BP, and 1.5–1 ka BP, respectively) support irregular earthquake cycling. Earthquake-triggered turbidites are more frequent in the study area than in the western adjacent margin segment (offshore El Asnam). This higher frequency could arise from the location of the seismogenic faults beneath the continental slope, whereas they are located several tenths of kilometers onland in the El Asnam area, implying less instabilities of the submarine slope

Towards subduction inception along the inverted North African margin of Algeria? Insights from thermo-mechanical models

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East Algerian margin

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Formation, segmentation and deep crustal structure variations along the Algerian margin from the SPIRAL seismic experiment

International audienceThe Algerian continental margin in the western Mediterranean formed as a back-arc basin and is today reactivated by the convergence between the African and Eurasian plates. It is one of the very rare examples of passive margins undergoing inversion expressed in a moderate seismicity of the margin and is possibly giving way for future subduction. With the objective to better know the deep structure of the margin and its associated basin, the origin of its seismicity and to study the mechanism of reactivation, five existing wide-angle seismic profiles along the margin are revisited. They were located offshore Mostaganem, Tipasa, Greater Kabylia, Jijel and Annaba. These profiles show that the basin is underlain by a 5 km thick crust of oceanic magmatic origin, possibly created at non-continuous small accretionary ridge segments. The continent-ocean transition zone is narrow, except at the easternmost profile, possibly due to an opening including a shear movement. No high velocity zone in the lower crust corresponding to mantle rocks has been imaged at the Algerian margin. The continental crust is thinned in a narrow and strongly segmented manner. It is widest (70 km) in the central segment offshore Greater Kabylia where there is a wider zone of distal thinned continental crust than on the other margin segments. The thickest crust detected during this survey corresponds to the African continental crust and the Kabylides blocks and is about 22-25 km thick. This reduced thickness in comparison with unthinned continental crust might be due to the influence of earlier subduction at the margin, in form of erosion by the subducting slab