Tectonic Inversion and Geomorphic Evolution of the Algerian Margin Since Messinian Times: Insights From New Onshore/Offshore Analog Modeling Experiments

International audienceTectonic inversion of passive margins is a common but poorly documented process preceding subduction inception. We perform here a comprehensive land-sea experimental modeling of this key process by reproducing the morphotectonic and sedimentary evolution of the central Algerian margin over the last 6 Myr. Our approach is based on scaled analog models integrating interactions between crustal shortening and surface processes, including erosion, water transport, sedimentation, gravitational instabilities, and base-level changes. A challenge was to simulate the effects of the Messinian Salinity Crisis (MSC) through a major sea-level oscillation and halite deposition. By using realistic boundary conditions, adapted analog material, and robust, first-order parameters for physiography setups, we successfully reproduce the morphotectonic domains and the time-dependent geometrical relationships between fluvio-deltaic sedimentary systems, erosional surfaces, and thrust faults as observed since Messinian times. Our results highlight (1) the key role played by the MSC sea-level oscillation on an ultra-fast building, destruction and re-sedimentation of fans and deltas from the upper slope to the abyssal plain; (2) the development of a large popup structure subparallel to the coastline, with progressive strain migration from the backthrust on land toward a frontal thrust of opposite vergence at mid-slope and the margin toe; and (3) the importance of lateral changes in initial wedge shape and strain distribution for determining the non-cylindrical geometry of the margin and progradation of piggy-back basins during tectonic inversion. Our results support that the central Algerian margin is witnessing the early building of an accretionary wedge combining thin-skinned and thick-skinned tectonic styles

Use of the ESI-2007 scale to evaluate the 2003 Boumerdès earthquake (North Algeria)

In this study, we applied the environmental seismic intensity (ESI-2007) scale to a major recent Algerian earthquake. The ESI-2007 scale is an effective tool to assess the seismic hazard and has been applied to onshore earthquakes. Here we applied the scale to a recent earthquake (Mw 6.8, 2003) that took place offshore in the province of Boumerdès in the north of Algeria along the boundary between African and Eurasian plates. The main shock was associated to an unknown submarine structure. No surface ruptures were observed on the onshore domain, but many earthquake environmental effects (EEEs) were reported during several field investigations. In addition to onshore ground effects, this event triggered turbidity currents responsible for 29 submarine cable breaks. Mapping and describing coseismic ground effects allowed us to distinguish primary and secondary effects like coastal uplifts, liquefaction phenomena, tsunami waves, turbidity currents, cracks, rock falls, slope movements and hydrological anomalies. Considering the total area affected and the distribution of ground effects, we suggest intensity X that appears in agreement with intensity calculated in previous study with the EMS-98 scale. Thus, this method is validated even in the case of a coastal earthquake, and could be applied in the future to Algerian historical earthquakes that have affected scarcely inhabited zones but where EEEs were listed and located

Coastal uplift west of Algiers (Algeria): pre- and post-Messinian sequences of marine terraces and rasas and their associated drainage pattern

(IF 1.50; Q2)International audienceThe North Africa passive margin is affected by the ongoing convergence between the African and Eurasian plates. This convergence is responsible for coastal uplift, folding, and reverse faulting on new and reactivated faults on the margin. The active deformation is diffuse and thus rather difficult to locate precisely. We aim to determine how a coastal landscape evolve in this geodynamic setting and gain insights into active tectonics. More particularly, we evidence and quantify coastal uplift pattern of the Chenoua, Sahel, and Algiers reliefs (Algeria), using sequences of marine terraces and rasas and computing several morphometric indices from the drainage pattern. Upper and Middle Pleistocene uplift rates are obtained by fossil shoreline mapping and preliminary U/Th dating of associated coastal deposits. Extrapolation of these rates combined to analyses of sea-level referential data and spatial relationships between marine terraces/rasas and other geological markers lead us to tentatively propose an age for the highest coastal indicators (purported the oldest). Values of morphometric indices showing correlations with uplift rate allow us to analyze uplift variation on area devoid of coastal sequence. Geological and geomorphological data suggest that coastal uplift probably occurred since the Middle Miocene. It resulted in the emergence of the Algiers massif, followed by the Sahel ridge massif. The Sahel ridge has asymmetrically grown by folding from west to east and was affected by temporal variation of uplift. Compared to previous study, the location of the Sahel fold axis has been shifted offshore, near the coast. The Chenoua fault vertical motion does not offset significantly the coastal sequence. Mean apparent uplift rates and corrected uplift rates since 120 ka are globally steady all along the coast with a mean value of 0.055 ± 0.015 mm/year (apparent) and of 0.005 ± 0.045 mm/year (corrected for eustasy). Mean apparent coastal uplift rates between 120 and 400 ka increase eastward from 0.045 ± 0.025 to 0.19 ± 0.12 mm/year (without correction for eustasy) or from 0.06 ± 0.06 to 0.2 ± 0.15 mm/year (with correction for eustasy). In addition, the combination of structural and geomorphic data suggests a low uplift rate for the southern part of the Algiers massif

Plio-Quaternary reactivation of the Neogene margin off NW Algiers, Algeria: The Khayr al Din bank

International audienceThe Algiers region, northern Algeria, is known to be seismically active, with recurrent large (M>6) earthquakes. Because of the lack of high-resolution bathymetry, the offshore structures remained for a long time poorly known. Thanks to a new marine data base (MARADJA 2003 cruise), the offshore part of the margin is accurately mapped, and new active and recent structures are described. West of the bay of Algiers, the margin enlarges, forming the Khayr al Din bank, interpreted as a tilted block of the passive margin born during the opening of the Algero-Provençal basin. At the slope break, a 80 km-long fault-tip Quaternary fold, namely the Khayr al Din fault, extends at the foot of the margin off NW Algiers and represents the largest active structure of the coastal area, together with the Sahel anticline. We also map for the first time a set of overlapping, en echelon active folds in the upper part of the Khayr al Din bank, located off previously known active structures on land. Most of these faults represent actually a threat for the Algiers region in terms of seismic hazard but also geological hazards, such as tsunamis, as most of them depicts significant dimensions and slip rates. The highest long-term horizontal shortening rate is found on the Khayr al Din fault and is estimated at 0.5 ± 0.1 mm/yr, with a maximal magnitude of 7.3, which provides one of the highest seismogenic potential in the region. A new tectonic framework for the Algiers region is proposed, in which the main south-dipping offshore structure, of opposite vergence relative to most thrusts on land, appears to be nowadays the main driving fault system, as also found further east in the Boumerdès (M 6.8) 2003 rupture zone. The overall apparent pop-up structure of the recent and active faults may result from a progressive migration of the plate limit from the Late Miocene, north-dipping suture zone on land, to the Quaternary, south-dipping main Khayr al Din fault at sea, suggesting a process of subduction inception