Crustal structure of the eastern Algerian continental margin and adjacent deep basin: implications for late Cenozoic geodynamic evolution of the western Mediterranean

Abstract

We determine the deep structure of the eastern Algerian basin and its southern margin in the Annaba region (easternmost Algeria), to better constrain the plate kinematic reconstruction in this region. This study is based on new geophysical data collected during the SPIRAL cruise in 2009, which included a wide-angle, 240-km-long, onshore-offshore seismic profile, multichannel seismic reflection lines and gravity and magnetic data, complemented by the available geophysical data for the study area. The analysis and modelling of the wide-angle seismic data including refracted and reflected arrival travel times, and integrated with the multichannel seismic reflection lines, reveal the detailed structure of an ocean-to-continent transition. In the deep basin, there is an ∼5.5-km-thick oceanic crust that is composed of two layers. The upper layer of the crust is defined by a high velocity gradient and P-wave velocities between 4.8 and 6.0 km s −1 , from the top to the bottom. The lower crust is defined by a lower velocity gradient and P-wave velocity between 6.0 and 7.1 km s −1 . The Poisson ratio in the lower crust deduced from S-wave modelling is 0.28, which indicates that the lower crust is composed mainly of gabbros. Below the continental edge, a typical continental crust with P-wave velocities between 5.2 and 7.0 km s −1 , from the top to the bottom, shows a gradual seaward thinning of ∼15 km over an ∼35-km distance. This thinning is regularly distributed between the upper and lower crusts, and it characterizes a rifted margin, which has resulted from backarc extension at the rear of the Kabylian block, here represented by the Edough Massif at the shoreline. Above the continental basement, an ∼2-km-thick, pre-Messinian sediment layer with a complex internal structure is interpreted as allochthonous nappes of flysch backthrusted on the margin during the collision of Kabylia with the African margin. The crustal structure, moreover, provides evidence for Miocene emplacement of magmatic intrusions in both the deep basin and the continental margin. Based on the crustal structure, we propose that the eastern Algerian basin opened during the southeastward migration of the European forearc before the collision, along a NW-SE elongated spreading centre that ran perpendicular to the subduction trend. Such an atypical geometry is explained by the diverging directions of the subduction rollback during the backarc opening: eastward for the Corsica-Sardinia block, and southward for the Kabylian blocks. This geometry of the forearc can be interpreted as the surface expression of a slab tear at depth, which is responsible for atypical magmatism in the overlying backarc oceanic basin.

Deep structure of the continental margin and basin off Greater Kabylia, Algeria – New insights from wide-angle seismic data modeling and multichannel seismic interpretation

“…Tomographic studies conducted in the Eastern Mediterranean spanning the Tyrrhenian basin and Calabria (Spakman and Wortel, 2004) suggest a mirror symmetric slab geometry accompanying an eastward slab rollback (Malinverno and Ryan, 1986;Rosenbaum and Lister, 2004). A one-step model of evolution of the eastern Algerian basin (EAB) before the Lesser Kabylian block collision is proposed from the eastern wide-angle seismic SPIRAL profile and the magnetic data offshore Annaba (Bouyahiaoui et al, 2015). Accordingly, the opening would be the surface expression of a slab tear at depth due to the increasing trenchlength of the subduction during its curvature.…”

The Tell-Rif orogenic system (Morocco, Algeria, Tunisia) and the structural heritage of the southern Tethys margin

“…The net result of the roll-back was the stripping-off of the Alboran, Kabylie, Peloritan and Calabria (AlKaPeCa) domains from the European margin (Alvarez, 1974;Bouillin, 1986). A two-steps scenario is currently considered for the formation of the West Mediterranean basins (Bouyahiaoui et al, 2015;Medaouri et al, 2014 and references therein). A first episode of NNW-SSE extension (Aïte and Gélard, 1997) occurred until the docking of AlKaPeCa blocks against Africa, which following radiochronologic data on granites cross-cutting the African crust, occurred during the Lower Miocene before or at 17 Ma (Abbassene et al, 2016;Aïssa et al, 1999;Chazot et al, 2017).…”

Joint modeling of potential-field data and geodynamic interpretation for northeast Algeria

“…This 380 km long profile extends a SPIRAL seismic cross-section performed in the margin north of Annaba (Bouyahiaoui et al, 2014), and ends south of Tebessa area. Fig.…”

“…Information from the last version of the geological map of Africa were used to constrain the outcropping geological units (Thiéblemont et al, 2016), while the topography of our studied area was set by the ETOPO1 model (Amante and Eakins, 2009). Additionally, a number of SPIRAL interpreted seismic cross-sections were used, in particular for the continent/ocean crustal transition (Leprêtre et al, 2013;Mihoubi et al, 2014;Bouyahiaoui et al, 2014;Aïdi et al, 2018). However, the southern extension of these seismic data is not sufficient to constrain the crustal structure of our area.…”