Search

The Owen Transform Fault (OTF) connecting the Sheba and the Carlsberg spreading ridges in the Indian Ocean currently forms the active plate boundary between India and Somalia plates. This 330-km-long transform fault is by far the longest transform fault along the India-Somalia plate boundary and its valley is buried under the thick distal turbidites of the Indus Fan with total thickness ranging from 1000 to > 5000m. A new set of seismic reflection and multibeam bathymetric data reveals remarkable transpressive structures along its entire length recorded as folds in the sedimentary cover, eruption of mud ridges at the seafloor, thrusts in the young oceanic lithosphere. Based on a new regional time-calibration of the seismic reflectors, we show that sediments in the transform valley (post 8.6 Ma) recorded a period of tectonic quiescence until the onset of a transpressive event around 1.5 − 2.4 Ma that we relate to a minor change in India-Somalia kinematics not captured by magnetic anomalies. This tectonic regime is still active based on compressive earthquakes and deformation of the most recent sediments. Transpression resulted in the formation of a proto-median ridge and the coeval propagation of the tip of the Carlsberg Ridge into the Somalian plate. These features are typically encountered at many other transform faults but rarely captured in their very early stage. Key Points A new set of seismic reflection lines allows time-calibration of the Indus Fan sediments deposited within the Owen transform valley The recent tectonic regime includes a still active transpression since 1.5-2.4 Ma that followed a million-years-long period of quiescence A nascent median ridge grows south of the fault, a feature found at many other transform faults but captured here in its very early stage Plain Language Summary Oceanic transform faults are tectonic plate boundaries along which two plates slide against each other. Often located far from sediment sources, oceanic transform fault

The Alboran Sea (Western Mediterranean) is a relatively small ocean basin connected with the Atlantic that provides a rich archive of tectonic and sedimentary processes at distinct temporal and spatial scales during the Quaternary. Since the collisional boundary of the Eurasia-Nubia plates crosses the Alboran Sea, this basin is also the locus of active geohazards: the constant seismic activity, concentrated mostly along the Al Idrissi strike-slip fault system and submarine landslides, that can cause tsunami hazards affecting the entire Alboran coasts and damages to submarine cables and infrastructures. Previous understanding of the Alboran Sea has been based on seafloor and subsurface geophysical data of differing resolution and scale, combined with very short sediment coring and IODP and industrial boreholes. In order to obtain new constrains on the geology of the Alboran Sea, the ALBACORE cruise was held in October and November 2021 onboard the R/V Pourquoi Pas? In addition to sites in the northern Alboran Sea targeting contourites, several sites in the southern Alboran Sea were selected as key study areas: the Al-Idrissi active fault zone, the Al-Hoceima shelf, the Xauen/Tofiño and the Francesc Pages banks. The scientific work of the ALBACORE campaign included the acquisition of Calypso cores (up to 28m long), sampling of consolidated strata with Cnexoville, in situ geotechnical measurements (Penfeld) with a seabed cone penetration test device (up to 50m long), heat flow measurements (up to 6m long), swath bathymetric imaging of the seafloor and water column, and sub-bottom profiling. The total length of sediments recovered reached 734m. Results from the ALBACORE cruise address the following scientific objectives: - To understand better the causal relationships between the present-day morpho-structural pattern and date Quaternary tectonic pulse and associated sedimentary systems - To determine the Late Pleistocene-Holocene stratigraphic pattern and the paleo-ocea

In the South Alboran Sea, the moderate seismicity (Mw=6.4) of the strike-slip Al Idrissi Fault Zone does not appear to control directly the landslides distribution. To provide a preliminary geohazard assessment, we characterized the spatial distribution, the volume and the ages of the submarine landslides from multibeam and seismic reflection data in the southern part of the Alboran Sea. Since the Quaternary numerous submarine landslide processes affect the marine sedimentary cover with volumes of the mass transport deposits (MTD) estimated between 0.01 to 15 km3. West of the Al Idrissi Fault Zone, along the South Alboran Ridge’s northern flank, the distribution of the MTD follows the SW-NE bank and ridge trend that correlates with blind thrusts and folds covered by a plastered contourite drift. A pockmark field, related to fluid escape, is visible near landslide scars where the contourite drift is relatively thicker. In this area, landslide scars occur on variable slopes (2-24°) and their associated MTD have variable decompacted volumes (0.01-10km3). East of the Al Idrissi Fault Zone, between the Alboran Ridge and the Pytheas Bank, the mapped MTDs have uneven volumes. The smaller ones (<1 km3) have their slide scars on steep slopes (>10°), whereas those of the largest ones (3-15 km3) occur on gentler slopes (<5°). These observations and a slope stability analysis suggest that the combination of seismic shaking, blind thrusts activity, relatively high sedimentation of contourite deposits, and fluid escape dynamics are likely the main controlling mechanisms rather than seismic shaking only. These causal factors would explain the concentration of landslide head scarps at the edge of the thickest parts of the contourite drifts (i.e. crests) may have been controlled locally by fluid overpressures in line with blind thrusts. Additionally, low to moderate seismicity potentially triggered by nearby faults might regionally have played a role in destabilising the seafloor se

Earthquakes are the most commonly cited cause of offshore slope failure, followed by high sedimentation rates and ensuing pore pressure build-up. In the South Alboran Sea, the moderate seismicity (Mw = 6.4) of the strike-slip Al Idrissi Fault Zone does not appear to control directly the landslides distribution. To provide a preliminary geohazard assessment, we characterized the spatial distribution, the volume and the ages of the submarine landslides from multibeam and seismic reflection data in the southern part of the Alboran Sea. Since the Quaternary numerous submarine landslide processes have affected the marine sedimentary cover with volumes of the mass transport deposits (MTD) estimated between 0.01 and 15 km3. West of the Al Idrissi Fault Zone, along the South Alboran Ridge's northern flank, the distribution of the MTD follows the SW-NE bank and ridge trend that correlates with blind thrusts and folds covered by a plastered contourite drift. A pockmark field, related to fluid escape, is visible near landslide scars where the contourite drift is relatively thicker. In this area, landslide scars occur on variable slopes (2–24°) and their associated MTDs show variable decompacted volumes (0.01-10 km3). East of the Al Idrissi Fault Zone, between the Alboran Ridge and the Pytheas Bank, the mapped MTDs have uneven volume. The smaller ones (<1 km3) have their slide scars on steep slopes (>10°), whereas those of the largest ones (3–15 km3) occur on gentler slopes (<5°). These observations and a slope stability analysis suggest that the combination of seismic shaking, blind thrusts activity, relatively high sedimentation rate of contourite deposits with potential weak layers, and fluid escape dynamics are likely the main controlling mechanisms. These causal factors would explain the concentration of landslide head scarps at the edge of the thickest parts of the contourite drifts (i.e. crest). Slides may have been controlled locally by fluid overpressures in line with

Automation of the throw backstripping method has proven to be an effective tool for the determination of the evolution of tectonic activity in wide fault zones. This method has been applied to the Al Hoceima Bay (southwesternmost Mediterranean, Alboran Sea) for a time period covering the last 280 kyr on 672 faults imaged on 265 high-resolution seismic reflection profiles. This area was affected by major earthquakes and corresponds to a transtensional basin deformed by growth faults. The automated application of throw backstripping allowed for a faster deciphering of the migration of tectonic activity. Results show a westward migration of the deformation with quickly increasing deformation rates in the most recent time frames near Al Hoceima, one of the most populated cities. This migration is in agreement with the current seismicity, the GPS data, and recent brittle deformation data. Vertical throw rates of up to 0.47 mm/year have been calculated, for the most recent time periods, in segments of the Bokkoya fault zone. The westward migration of the deformation fits with the reconstruction suggested by the westernmost Mediterranean geodynamic models during the Pleistocene epoch, and it might be the consequence of the interaction between the northwest (NW) movement of the South Alboran indenter and the back Rif south-westward displacement. The highly accurate constraints of the evolution of the tectonic activity offered by this automation will substantially improve the seismic hazard assessment

The tectonics of the Alboran Sea control the distribution of topographic highs and depressions, influencing the water masses' paths controlling the deep basins sedimentation rates. Altogether tectonics and deep currents shaped the seafloor on which we map active faults, contourites, pockmarks, and submarine landslides. Recent numerical models highlight that some of those landslides can generate tsunamis waves on nearby coastal areas, creating the need to describe better and understand those seabed features. Consequently, we put together bathymetric and seismic data to measure Pleistocene slides affecting the deep Alboran basin in an unprecedented collective effort. We mapped and relatively dated 66 mass transport deposits (MTDs) in the West Alboran and Pytheas fields on the north and south flank of the Alboran Ridge. We measured their surfaces, decompacted volumes, slopes, run out, scars heights, scars elevations, and described their type (debris flow or slide) qualitatively from their echo facies. When possible, we also measured the run-off from the scar. We investigated the factor of safety (FOS) and computed based seismic moments based on empirical relationship and faults geometry to characterize the preconditioning factors and triggering mechanisms. The first important result is that post-1.12 Ma MTDs mobilized the most important volumes, in line with the Alboran Sea's topographic highs. Second, seismic lines and the bathymetric images evidence blind reverse faults related to fluid escapes that could contribute to local overpressures in shallow contouritic sediments. Third, we show that local slopes are too flat to allow slopes to destabilize under gravity force only, suggesting that other causal factors need to be considered. Fourth, known seismicity on strike-slip faults in the Alboran Sea is unlikely to trigger MTDs in most investigated areas, suggesting that a combination of preconditioning factors, such as local overpressures and/or reduced strength propert

Large continental faults extend for thousands of kilometres and often form the tectonic boundaries between plates that are associated with prominent topographic features. In these active areas, well-defined faults produce large earthquakes, and thus imply a high seismic hazard. These paradigms are called into question in the Alboran Sea, which hosts an allegedly complex diffuse boundary between the Eurasia and Nubia plates, and we discovered one of the few examples worldwide of the initial stages of these key tectonic structures. On the 25th January 2016, a magnitude Mw6.4 submarine earthquake struck the north of the Moroccan coast, the largest event ever recorded in the Alboran Sea. The quake was preceded by an earthquake of magnitude Mw5.1 and was followed by numerous aftershocks whose locations mainly migrated to the south. The mainshock nucleated at a releasing bend of the poorly known Al-Idrissi Fault System (AIFS). According to slip inversion and aftershock distribution, we assume a rupture length of 18 km. Here we combine newly acquired multi-scale bathymetric and marine seismic reflection data with a resolution comparable to the studies on land, together with seismological data of the 2016 Mw 6.4 earthquake offshore Morocco – the largest event recorded in the area – to unveil the 3D geometry of the AIFS. We found that, despite its subdued relief, the AIFS is a crustal-scale boundary. We report evidence of left-lateral strike-slip displacement, characterize their fault segments and demonstrate that the AIFS is the source of the 2016 events. The occurrence of the Mw 6.4 earthquake and previous events of 1994 and 2004 supports that the AIFS is currently growing through propagation and linkage of its segments, which eventually might generate a greater rupture (up to Mw 7.6), increasing the potential hazard of the structure. The AIFS provides a unique model of the inception and growth of a young plate boundary system in the Alboran Sea (Western Mediterranean)

Since the Miocene, the thinned continental crust below the Alboran Sea and its overlying sedimentary cover have been undergoing deformation caused by both convergence of Eurasia and Africa and by deep processes related to the Tethyan slab retreat. Part of this deformation is recorded at the Xauen and Tofiño banks in the southern Alboran Sea. Using swath bathymetry and multichannel seismic reflection data, we identified different stages and styles of deformation. The South Alboran Basin is made up of Early Miocene to Pliocene sedimentary layers that correlate with the West Alboran Basin depocenter and are dominated by E‐W trending folds and thrusts. The Xauen and Tofiño Banks first recorded the phase of extension and strike‐slip movement during the slab retreat, followed by the phase of compressional inversion since the Tortonian and are now structured by tight folds, thrusts, and mud bodies. This study proposes that the Banks were located on the southern‐inherited Subduction Tear Edge Propagator (STEP) fault related to the westward migration of the Alboran domain during the Miocene. The STEP fault zone, acting as a boundary between the African block and the Alboran block, was located along the onshore Jebha‐Nekor fault and the offshore Alboran Ridge and the Yusuf fault zone. Thick‐skinned and thin‐skinned shortening occurred when slab retreat stopped, and inversion began. The present‐day style of the deformation seems to be linked to a decollement level made of undercompacted shale on top of the Ghomaride complex

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

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 5Ma 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

Large continental faults extend for thousands of kilometres to form boundaries between rigid tectonic blocks. These faults are associated with prominent topographic features and can produce large earthquakes. Here we show the first evidence of a major tectonic structure in its initial-stage, the Al-Idrissi Fault System (AIFS), in the Alboran Sea. Combining bathymetric and seismic reflection data, together with seismological analyses of the 2016 M-w 6.4 earthquake offshore Morocco - the largest event ever recorded in the area - we unveil a 3D geometry for the AIFS. We report evidence of left-lateral strike-slip displacement, characterise the fault segmentation and demonstrate that AIFS is the source of the 2016 events. The occurrence of the M-w 6.4 earthquake together with historical and instrumental events supports that the AIFS is currently growing through propagation and linkage of its segments. Thus, the AIFS provides a unique model of the inception and growth of a young plate boundary fault system.

Large continental faults extend for thousands of kilometres to form tectonic boundaries between plates, often associated with prominent topographic features. In these active areas, well-defined faults produce large earthquakes, and thus imply a high seismic hazard. These paradigms are called into question in the Alboran Sea, which hosts an allegedly complex diffuse boundary between the Eurasia and Nubia plates, and we discovered one of the few examples worldwide of the initial stages of these key tectonic structures. On the 25th January 2016, a magnitude Mw 6.4 submarine earthquake struck the north of the Moroccan coast, the largest event ever recorded in the Alboran Sea. The quake was preceded by an earthquake of magnitude Mw 5.1 and was followed by numerous aftershocks whose locations mainly migrated to the south. The mainshock nucleated at a releasing bend of the poorly known Al-Idrissi Fault System (AIFS). Here we combine newly acquired multi-scale bathymetric and seismic reflection data with a resolution, together with seismological data of the 2016 Mw 6.4 earthquake offshore Morocco –the largest event recorded in the area –to unveil the 3D geometry of the AIFS. We found that the AIFS is a crustal-scale boundary. We report evidence of left-lateral strike-slip displacement, characterize their fault segments and demonstrate that the AIFS is the source of the 2016 events. The occurrence of the Mw 6.4 earthquake and previous events of 1994 and 2004 supports that the AIFS is currently growing through propagation and linkage of its segments. The AIFS provides a unique model of the inception and growth of a young plate boundary. (Waiting for acceptance in NATURE COMMUNICATIONS, 2019)

Data files and modelling results of regional centroid moment tensors obtained for earthquakes in the 2016 South Alboran seismic crisis, and presented in the article "Earthquake crisis unveils the growth of an incipient continental fault system", by Eulàlia Gràcia et al. Large continental faults extend for thousands of kilometres to form boundaries between rigid tectonic blocks. These faults are often associated with prominent topographic features, and they can produce large earthquakes. Here we present first evidence of such a major tectonic structure in its initial growing stage, the Al-Idrissi Fault System (AIFS), in the Alboran Sea. Until now, this fault system had always been described as a complex diffuse boundary between Eurasia and Nubia plates. However, combining newly acquired high-resolution bathymetric and seismic reflection data, together with seismological analyses of the 2016 Mw 6.4 earthquake offshore Morocco – the largest event ever recorded in the area – we unveil a 3D geometry for the AIFS, which definitively correspond to a crustal-scale boundary. We report evidence of left-lateral strike-slip displacement, characterize fault segmentation and demonstrate that the AIFS is the source of the 2016 events. The occurrence of the Mw 6.4 earthquake together with historical and instrumental events supports that the AIFS is currently growing through propagation and linkage of its segments. Thus, the AIFS provides a unique model for inception and growth of a young continental fault system.

In active basins, tectonics can segment the continental shelf and control its stratigraphic architecture and physiography. Segmentation can explain the local evolution and morphology of the continental shelf because of sea-level variations, local tectonic segmentation and hydrodynamic processes. Here we investigate the tectonically active Morocco continental margin (southern Alboran Sea) using high-resolution seismic profiles and multibeam bathymetric data. The active faults bounding the transtensive Nekor basin triggered the segmentation of the shelf into three sectors showing different subsidence rates: a western sector corresponding to an extensive fault relay, a central sector corresponding to the subsiding Al-Hoceima Bay and an eastern sector corresponding to the footwall of the Trougout senestral normal fault. Results show that the staircase morphology of the shelf corresponds to successive submarine terraces at the shelf edge (ST1), mid-shelf (ST2) and inner-shelf (ST3) around −110 m,-80 m, and −40 to −20 m deep, respectively. The terraces correspond to the top of prograding wedges seaward and are erosive landward. They are correlated with stillstand from the Last Glacial Maximum to the Holocene highstand. Above the terraces, sub-aqueous dune fields are interpreted as degraded and deposited during the post-glacial transgression. In the central sector, typical delta front seafloor undulations on the shelf and crescent shaped bedforms at the head of marine incisions on the upper slope denote a fluvial influence during the Holocene. Seismic stratigraphy analysis revealed the preservation of six seismic units bounded by polygenic regional unconformities (S1, S2, S3, S4 and S5). Based on comparison with other Mediterranean margins, S1 to S5 are attributed to 4th order maximum regressive surface. We discuss the local preservation of the system tracts as a function of the vertical motion and the physiography of this tectonically active domain. This study provides us