Jacques DEVERCHERE | Université de Bretagne Occidentale (original) (raw)

Papers by Jacques DEVERCHERE

Frontiers in Earth Science, Dec 23, 2022

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.

Oceanography of the Mediterranean Sea

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...

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.

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...

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

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...

Earth and Planetary Science Letters, 2018

Geophysical Journal International, 2018

Marine and Petroleum Geology, 2016

Abstract The eastern Algerian basin, part of the Western Mediterranean domain, is a back-arc basi... more Abstract The eastern Algerian basin, part of the Western Mediterranean domain, is a back-arc basin resulting from the SE drift of the Lesser Kabylia block gave rise to continental extension and birth of a new oceanic domain. We aim here to define the structure and tectonostratigraphic evolution of this basin and the adjoining continental margin. Indeed, the absence of wells and the lack of published MCS data kept this area mostly unknown. The crustal architecture (down to the Moho) and seismo-stratigraphy of the sedimentary infill are detailed, emphasizing particularly the pre-Messinian series, by combining a new set of offshore deep penetrating seismic, conventional MCS sections, and wide angle seismic data. Because no well exist offshore we realized new field descriptions of the Late Oligocene and Miocene deposits on land and propose onshore-offshore extrapolation and age correlations. The pre-Messinian series display four main units (PMSU1 to 4) above which Messinian units are consistent with stratigraphic models already proposed for the Mediterranean basin, the deep basin displays units as old as the OMK (“Oligo-Miocene Kabyle”) series on land. The basement is shaped by conjugate transcurrent faults striking N120°- N40°, forming a series of rhombic troughs resulting from transtensional tectonics. The coeval E-W extension and NW-SE stretching of the continental crust suggest a radial opening of the Algerian basin during Late Oligocene- Lower Miocene, followed by a progressive uplift of the upper margin after the collision of Lesser Kabylia with Africa. Besides, steep slope of the eastern Algerian margin, downward flexure of the oceanic basin near the margin toe and growth of S-dipping thrusts within the margin evidence a significant Quaternary stress inversion. Overall, this study highlights the complex stress evolution of the East Algerian margin since Oligocene times, as evidenced by significant upward/downward vertical motions and crustal extensional/compressional successions. We summarize this evolution into three main stages: (1) rifting (including sea-floor spreading) at Late Oligocene- Burdigalian in the context of back-arc, (2) collision between AlKaPeCa blocks and Africa at 17 Ma, and post-rift sag basin infill, till Late Tortonian to Messinian, and (3) inversion of the margin and development of flexural basins at Quaternary times.