Papers by Jacques DEVERCHERE
A Review of the Geodynamic Evolution of the Onshore–Offshore Central Algerian Margin Constrained by Gravity Data
1st Conference on Geophysics for Geothermal-Energy Utilization and Renewable-Energy Storage, 2019
Summary The Maghrebian margin remains one of the key questions still open to debate on its geodyn... more Summary The Maghrebian margin remains one of the key questions still open to debate on its geodynamic evolution. The geology of onshore-offshore central Algerian margin particularly records a history of accretion and/or subduction modified by the presently active transform motion between the Eurasian and African Plates. Establishing the identity and boundaries of basement blocks or terrains led to better understanding its geodynamic evolution and accretionary history. To this aim, gravity anomaly data have been used in order to determine the structure and evolution of the central Algerian margin. The gravity data set for this margin zone were compiled from available land and marine surveys. Initially, the data were reduced to complete Bouguer anomaly using an appropriate density to those carried out onshore and offshore respectively. As main results, we have reviewed in details the links between all our compiled gravity data and deep crustal structure within the complex geodynamic framework in this area. We presented a new global Bouguer anomaly map in conjunction with major structural features of this zone. It should be noted that in comparison to other margins, the gravity anomaly at this part of Algerian margin shows a number of distinctive features.
The Algerian margin resulting from a back-arc basin Algerian opening associated with a withdrawal... more The Algerian margin resulting from a back-arc basin Algerian opening associated with a withdrawal from the Tethyan break. Recent geophysical data acquired in the Algerian basin (MARADJA campaigns, 2003, 2005 (Active Margin of "el DJAzair" and SPIRAL, 2009 (Deep Seismic and Regional Investigations in Algeria) have identified signs of active or recent compressive deformation in the basin. New data from four wide-angle seismic profiles helped us image, for the first time, the deep structure of the Algerian margin and adjacent basins.We converted these velocity profiles into density models, then isostatic anomalies. This allowed us to image an isostatic imbalance (relative to a local isostasy model) scored at the margin toe. If we interpret this as the Moho depth variation over a depth of balance, then the Moho in oceanic part is too deep and Moho in continental part too shallow, on either side of a boundary located towards the margin toe. These abnormalities can be interprete...
Mapping the oceanic flexure off Algeria: along-strike changes in space and time
HAL (Le Centre pour la Communication Scientifique Directe), Apr 7, 2019
International audienc
Frontiers in Earth Science, Dec 23, 2022
The Algerian margin is located at the slow-rate convergent boundary between African and Eurasian ... more The Algerian margin is located at the slow-rate convergent boundary between African and Eurasian plates and experienced several strong earthquakes in the last centuries. Among them, the 2003 Mw 6.8 Boumerdès event has triggered large turbidity currents in the slightly concave canyons of the slope and numerous cable breaks in the abyssal plain. In this study, we explore where, how and when the tectonic inversion of the margin off Boumerdès has left witnesses in the seafloor morphology and whether the observed deformation correlates with the 2003 coseismic rupture zone and with the Plio-Quaternary sedimentation. We have performed a careful analysis of the seafloor morphology and subsurface seismic reflectors at the landscape scale and along/between canyons by combining classical GIS-based methods and Virtual Reality techniques. From the mid-slope to the deep basin off thẽ 60 km long Boumerdès-Dellys coast, we evidence large knickpoints corresponding to the development of four main cumulative fault scarps and two perched basins which are deeply incised by steep canyons and gullies. We interpret these structures to result from frontal propagation of two main southdipping thrusts by upper crustal décollement ramping, evidencing an incipient sub-thrust imbrication in a stage of initiation of an accretionary wedge. The flatramp thrust geometry and their along-strike segmentation explain the development and shape of the perched basins in the backlimb of faultrelated folds. The onset of growth strata is dated at 1.5 ± .5 Ma on the slope and .9 ± .3 Ma in the deep basin. The length, position, strike and segmentation of the older, southern thrust ramp are consistent with the coseismic characteristics of the Mw 6.8 2003 earthquake. The cumulative vertical scarp throw exceeds 1 km, supporting Quaternary shortening rates of 1.6 ± .7 mm/yr, in agreement with geodetic strain rates across the western Mediterranean basin. Virtual Reality offers powerful and promising means to correlate seismic imagery and seafloor morphology and is of great help to improve the robustness of tectonostratigraphic interpretation.
Frontiers in Earth Science, 2022
The Algerian margin is located at the slow-rate convergent boundary between African and Eurasian ... more The Algerian margin is located at the slow-rate convergent boundary between African and Eurasian plates and experienced several strong earthquakes in the last centuries. Among them, the 2003 Mw 6.8 Boumerdès event has triggered large turbidity currents in the slightly concave canyons of the slope and numerous cable breaks in the abyssal plain. In this study, we explore where, how and when the tectonic inversion of the margin off Boumerdès has left witnesses in the seafloor morphology and whether the observed deformation correlates with the 2003 coseismic rupture zone and with the Plio-Quaternary sedimentation. We have performed a careful analysis of the seafloor morphology and subsurface seismic reflectors at the landscape scale and along/between canyons by combining classical GIS-based methods and Virtual Reality techniques. From the mid-slope to the deep basin off the ∼60 km long Boumerdès-Dellys coast, we evidence large knickpoints corresponding to the development of four main cumulative fault scarps and two perched basins which are deeply incised by steep canyons and gullies. We interpret these structures to result from frontal propagation of two main south-dipping thrusts by upper crustal décollement ramping, evidencing an incipient sub-thrust imbrication in a stage of initiation of an accretionary wedge. The flat-ramp thrust geometry and their along-strike segmentation explain the development and shape of the perched basins in the backlimb of fault-related folds. The onset of growth strata is dated at 1.5 ± .5 Ma on the slope and .9 ± .3 Ma in the deep basin. The length, position, strike and segmentation of the older, southern thrust ramp are consistent with the coseismic characteristics of the Mw 6.8 2003 earthquake. The cumulative vertical scarp throw exceeds 1 km, supporting Quaternary shortening rates of 1.6 ± .7 mm/yr, in agreement with geodetic strain rates across the western Mediterranean basin. Virtual Reality offers powerful and promising means to correlate seismic imagery and seafloor morphology and is of great help to improve the robustness of tectonostratigraphic interpretation.
Mediterranean Sea evolution and present-day physiography
Oceanography of the Mediterranean Sea
New insights on the relationship between inherited structures of the opening of the Algero-Balearic basin and recent inversion of its southern margin
The Western Mediterranean Sea is a complex geological domain that has received much attention sin... more The Western Mediterranean Sea is a complex geological domain that has received much attention since the early 1970s. Several kinematic and geodynamic models have been proposed to explain the coeval birth and growth of orogenic belts (Rif-Betic cordillera, Maghrebides, Alps, Apennines, Dinarides) and extensional basins (Alboran, Algero-Balearic, Valencia, Liguro-Provençal, and Tyrrhenian) since 35 Ma in the frame of a NW-SE plate convergence of Africa with respect to Eurasia (e.g.,
Tectonophysics, 2018
During the Algerian-French SPIRAL survey aimed at investigating the deep structure of the Algeria... more During the Algerian-French SPIRAL survey aimed at investigating the deep structure of the Algerian margin and basin, two coincident wide-angle and reflection seismic profiles were acquired in central Algeria, offshore Greater Kabylia, together with gravimetric, bathymetric and magnetic data. This ~260 km-long offshore-onshore profile spans the Balearic basin, the central Algerian margin and the Greater Kabylia block up to the southward limit of the internal zones onshore. Results are obtained from modeling and interpretation of the combined data sets. Highlights ► Modeling of deep seismic data images the deep structure of the Algerian margin. ► The Algerian margin offshore Greater Kabylia is a narrow, magma-poor rifted margin. ► Margin inversion may trigger lower crust seaward flow due to isostatic disequilibrium. ► A 3-layer thin oceanic crust reveals postaccretion volcanism in the Algerian basin. ► A two-step Miocene evolution of the West Algerian backarc basin is proposed.
Crustal Structure and Miocene Geodynamic Evolution of the Easternmost Algerian Back-Arc Basin and Continental Margin (western Mediterranean Sea) from Wide-Angle and Multichannel Seismics
We use new wide-angle seismic and multichannel seismic reflection data (SPIRAL cruise, 2009) and ... more We use new wide-angle seismic and multichannel seismic reflection data (SPIRAL cruise, 2009) and additional geophysical data to study the crustal structure of the eastern Algerian back-arc basin that was born during the Miocene Tethys subduction rollback, before the collision of the European forearc (Kabylian blocks) with the northern African continent ~16-18 Ma ago. In the deep basin, the P-wave velocity model images a thin, 5.5-km-thick oceanic crust with velocity ranging between 4.8 km/s and 7.1 km/s. It is composed of two layers, with a velocity-gradient higher in the upper layer than in the lower one. S-wave modeling indicates a Poisson ratio of 0.28 in the lower crust, supporting a dominant gabbroic composition. Below the continental edge, we define two segments: (1) West of 7°45’E, a typical continental crust with P-wave velocities between 5.2 km/s and 7.0 km/s depicts a gradual seaward thinning of ~15 km over an ~35-km distance characterizing a stretched margin resulting fro...
New insights on the relationship between inherited structures of the opening of the Algero-Balearic basin and recent inversion of its southern margin
Crustal structures and salt tectonics on the margins of the western Algerian Basin (Mediterranean Region)
Marine and Petroleum Geology
We present an overview of the crustal architecture of the continental margins of the oceanic Alge... more We present an overview of the crustal architecture of the continental margins of the oceanic Algerian Basin in the westernmost Mediterranean Sea. During the Cenozoic, and with a variable oblique convergence between the African and Eurasian plates, the Western Mediterranean Sea has experienced thinning and extension behind a tight orogenic arc formed by the Betics, Rif, and Tell Cordilleras. This study is focused on the structural style affecting the Messinian salt layer, which is mostly restricted to the deep domains of the Algerian Basin, where it is floored by a thin oceanic crust of probable Miocene age. Using deep-penetrating seismic profiles and wells from offshore western Algeria to southeastern Spain, we have analyzed the crustal structures affecting the domains close to the oceanic-continent transition on the three margins of the western Algerian Basin. Since the Early Miocene, active shortening in the Tell-Atlas domain has accommodated most of the plate convergence in the basin, whereas the Alboran margin in the west and the Iberian margin in the north experienced eastward and southward crustal extension and thinning, respectively, accompanied by volcanism. The Algerian margin in the south shows incipient thrusting of African continental crust over oceanic crust. This shortening occurred since at least the Late Miocene, also promoting decoupling and contraction of the deep, sub-horizontal Messinian salt layer. The salt exhibits diapir squeezing and suprasalt folding, whereas the presalt sequence preserves partially-inverted half-grabens. Salt tectonic processes along the northern and western margins of the Western Mediterranean Basin show contrasting structural styles formed by narrow extensional and transtensional domains with gentle salt anticlines. This region shows therefore a somewhat unusual salt-tectonic style, departing from the gravity-driven model typical of continental margins that contain an initial continuous, gently-dipping salt layer. In the Algerian Basin, salt is mostly restricted to deep water domain floored by oceanic crust, so it does not participate in significant gravity-driven deformation. Instead, Messinian salt and the suprasalt sequences underwent significant shortening along the southern margin, simultaneous with thick-skinned extension involving the Messinian evaporites in the northern and eastern margins.
Deep structure of the Demerara Plateau and its two-fold tectonic evolution: from a volcanic margin to a transform marginal plateau, insights from the Conjugate Guinea Plateau
Geological Society, London, Special Publications, 2022
Transform marginal plateaus (TMPs) are large and flat structures commonly found in deep oceanic d... more Transform marginal plateaus (TMPs) are large and flat structures commonly found in deep oceanic domains, but their origin and relationship to adjacent oceanic lithosphere remain poorly understood. This paper focuses on two conjugate TMPs, the Demerara Plateau off Suriname and French Guiana and the Guinea Plateau, located at the junction of the Jurassic Central Atlantic and the Cretaceous Equatorial Atlantic oceans. The study helps to understand (1) the tectonic history of both Demerara and Guinea plateaus and (2) the relationship between the Demerara Plateau and the adjacent oceanic domains, and finally, (3) throws light on the formation of TMPs. We analyse two existing wide-angle seismic-derived velocity models from the MARGATS seismic experiment (Demerara Plateau), and adjacent composite industrial seismic lines covering the Demerara and Guinea plateaus. The Demerara Plateau displays a 30 km thick crust, subdivided into three layers, including a high-velocity lower crust. The velo...
Formation, segmentation and deep crustal structure variations along the Algerian margin from the SPIRAL seismic experiment
Journal of African Earth Sciences, 2021
The Algerian continental margin in the western Mediterranean formed as a back-arc basin and is to... more The 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.
Comptes Rendus. Géoscience, 2021
Éric Beucler et al.
Ongoing Inversion of a Passive Margin: Spatial Variability of Strain Markers Along the Algerian Margin and Basin (Mediterranean Sea) and Seismotectonic Implications
Frontiers in Earth Science, 2021
Subduction initiation is an important but still poorly documented process on Earth. Here, we docu... more Subduction initiation is an important but still poorly documented process on Earth. Here, we document one of a few cases of ongoing transition between passive and active continental margins by identifying the geometrical and structural signatures that witness the tectonic inversion of the Algerian continental margin and the deep oceanic domain, located at the northern edge of the slow-rate, diffuse plate boundary between Africa and Eurasia. We have analyzed and tied 7900 km of deep seismic reflection post-stacked data over an area of ∼1200 km long and ∼120 km wide. The two-way traveltime lines were converted into depth sections in order to reconstruct and map realistic geometries of seismic horizons and faults from the seafloor down to the acoustic basement. Along the whole length of this young transitional domain, we identify a clear margin segmentation and significant changes in the tectonic signature at the margin toe and in the deep basement. While the central margin depicts a t...
Tectonics, 2021
Tectonic inversion refers to geological processes involving the reactivation of extensional basin... more Tectonic inversion refers to geological processes involving the reactivation of extensional basin structures in response to crustal shortening or the reactivation of reverse faults during crustal extension (e.g., M.
Earth and Planetary Science Letters, 2018
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Papers by Jacques DEVERCHERE