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Thèse de Chafik Aidi

La marge dans la région centrale de Boumerdès - Tigzirt fait l’objet de la thèse de Chafik Aidi dont, en attendant les publications, des résumés de présentations à des congrès synthétisent quelques résultats ici, et ci-dessous pour deux d’entre eux :

(1) Aidi, C., M.O. Beslier, K. Yelles-Chaouche, A. Ribodetti, R. Bracene, L. Schenini, H. Djellit, F. Sage, J. Déverchère, M. Medaouri, F. Klingelhoefer, A. Abtout, P. Charvis, and A. Bounif (2014), Tectonic Inversion of the Algerian Continental Margin off Great Kabylia (North Algeria) - Insights from new MCS data (SPIRAL cruise), Session TS7.4/G6.4/GD6.7/SM6.2 "Dynamics and Structure of the Mediterranean Alpine Collision and Back-arc", in EGU General Assembly Conference Abstracts, vol. 16, EGU2014-14570 (Poster). EGU General Assembly 2014, 27 April - 02 May, Vienna, Austria.

Sub-marine active faulting threatens the coastline of Algeria, as shown by the major Mw 6.9 May 21, 2003 earthquake that occurred in Great Kabylia close to Boumerdes. We present here the structures associated to the Plio-Quaternary (P-Q) tectonic inversion of the central part of the Algerian margin offshore Great Kabylia using new deep multichannel seismic (MCS) lines. Five MCS lines were acquired in the study area during the Algerian-French SPIRAL cruise (September 2009, R/V Atalante). Four lines were acquired using a 3040 cu. in. air-gun array and a 4.5 km 360 channel digital streamer and a 8350 cu. in. source favoring deep penetration was used for one coincident WAS profile and the fifth MCS line. All profiles are pre-stack time migrated and additional pre-stack depth migration was performed in key areas. The MCS lines crosscut the margin from the upper slope to the deep Algero-Provençal Basin either in a N-S direction sub-perpendicular to the structural trend of the margin, or in a NW-SE direction parallel to the actual convergence between Africa and Eurasia plates.
Tectonic inversion is expressed on all profiles at the deep margin. The eastern line displays a flat-ramp compressive system in the deep sedimentary series, which emerges at the foot of the continental slope and marks the seaward limit of a P-Q basin perched at mid-slope. The south-dipping ramps are neo-formed structures, whereas the flats useinherited lithologic discontinuities (base of the Messinian evaporitic series, top of the acoustic basement). Westward in the Boumerdes area, the compressive deformation is expressed deeper in the acoustic basement where a southward dipping reflector is interpreted as a blind thrust on top of which all the sedimentary series (Miocene to P-Q) are bent in an antiform that uplifts the base of the Messinian series. A second antiform prolongates this uplift 20 km northward although no clear reverse structure is imaged underneath. These antiforms delimit two asymmetric sub-basins filled with a southward thickening P-Q wedge. As a whole, the geometry of the reverse structures supports fault-propagation or fault-bent fold models, as previously inferred from HR seismic interpretation. They are likely to participate to large uplifts such as the coastal one related to the Boumerdes earthquake.
Reverse structures in the thinned continental crust co-exist with transtensional deformation at the transition with the oceanic domain 50 km northward of the margin toe, where a narrow asymmetric basin shows a downward offset of the base of the Messinian series and a southward thickening Miocene to P-Q wedge. A transcurrent component on crustal faults playing since the Miocene may explain both basin geometry and lateral variations in width and depth.

(2) Aidi, C., F. Klingelhoefer, A. Yelles-Chaouche, M.-O. Beslier, R. Bracene, P. Schnürle, H. Djellit, A. Galve, A. Bounif, L. Schenini, F. Sage, and P. Charvis (2013), Deep structure of the Algerian continental margin in the region of the Great Kabylies - Insights from wide-angle seismic data modelling, AGU Fall Meeting Abstracts, 21, #T21A-2522. American Geophysical Union, Fall Meeting 2013, San Francisco, Calif., 9-13 december.

During the Algerian-French SPIRAL cruise (Sismique Profonde et Investigation Régionale du Nord de l’Algérie) conducted
onboard R/V Atalante (September-October 2009), one deep reflection and wide-angle seismic profile with total length of 140 km was acquired on the Algerian margin, offshore Greater Kabylia. 40 ocean bottom seismometers (OBS) were deployed on the profile, located perpendicular to the margin and it was additionally extended on land using 26 seismological stations. A 8350 in3 tuned air-gun array consisting of 10 Bolt air-guns was used to generate deep frequency shots to allow for a good penetration. A coincident multi-channel seismic profile was acquired using a 3040 in3 seismic source and a 4.5 km 360 channel digital seismic streamer. Underway geophysical measurements included gravimetric and magnetic data. The combined profile with a total length of about 260 km, crosses from north to south the Algero-Provençal basin, the central Algerian margin and onshore the crystalline basement of the Kabylides bloc up to the southward limit of the internal zones.
We present results concerning the sedimentary and crustal structures in the study area using tomographic inversion, forward and gravimetric modelling. Modelling of the wide-angle and multi-channel seismic data reveals that the thickness of the sedimentary cover along the profile varies from several hundreds of metres onland in Tizi-Ouzou basin (Bracène, 2001), to 4 km at the foot of the margin and then decreasing northward to less than 3 km. The Messinian evaporitic units have been modelled by a high velocity layer, representing a velocity inversion with underlying pre-Messinian Miocene sedimentary layers. Progressive thinning of the continental crust towards the North is observed, with thicknesses decreasing from 20 km at the foot of the margin to 4-5 km in the deep basin. Seismic velocities range between 6.2 and 6.6 km/s in the continental domain and 5.2 - 6.8 km/s in the deep basin. The uppermost crust of the deep margin is characterised by low velocities of only 4.5-5.0 km/s probably due to fracturing during the thinning of the crust.
The transition between continental crust and crust of oceanic origin is located about 60 km from the coast. Its extension is very narrow (< 20 km) with a possibility of it being absent in this region. The crust underlying the basin at the foot of the continental slope is characterised by a thickness of only 3-5 km which is about 2 km thinner than normal oceanic crust. Seismic velocities however indicate that the crust is of oceanic origin and does not represent exhumed and partly serpentinised mantle material, although the presence of small amounts of mantle material in an otherwise igneous crust cannot be ruled out. Similar thin oceanic crust has been imaged in other Mediterranean Basins, such as the Liguro-Provençal basin (Gailler et al., 2009).

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