Flexural behaviour of the north Algerian margin and tectonic implications

Abstract

The Algerian margin formed through back-arc opening of the Algerian basin (Mediterranean Sea) resulting from the roll-back of the Tethyan slab. Recent geophysical data acquired along the Algerian margin showed evidence of active or recent compressive deformation in the basin due to the ongoing Africa-Eurasia convergence. Published data from four wide-angle seismic profiles have allowed imaging the deep structure of the Algerian margin and its adjacent basins. In this study, we converted these velocity models into density models, then into isostatic anomalies. This allowed us to image an isostatic disequilibrium (relative to a local isostasy model) reaching a maximum amplitude at the margin toe. Converting isostatic anomalies into Moho depth variations shows that the Moho extracted from wide-angle seismic data is deeper than the one predicted by a local isostasy model in the oceanic domain, and shallower than it in the continental domain. These anomalies can be interpreted by opposite flexures of two plates separated by a plate boundary located close to the margin toe. We use a finite element model to simulate the lithospheric flexure. The amplitude of the equivalent vertical Moho deflection is larger in the central part of the study area (6-7 km) than on the easternmost and westernmost profiles (3 km). The effective elastic thickness used to best match the computed deflection is always extremely low (always less than 10 km) and probably reflects the relatively low strength of the lithosphere close to the plate boundary. Comparison with other wide-angle seismic profiles across an active and a passive margin show that the North Algerian margin displays isostatic anomalies close to that of an active margin. Finally, plate flexure is highest at the southern tip of the ocean-continent transition, possibly indicating that a former passive margin detachment is reactivated as a crustal scale reverse fault pre-dating a future subduction.

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

“…Onland, the tectonic inversion is expressed by N75° trending, 20 km-wide, folds in the Miocene series in the Dellys and Tizi Ouzou area (Aïté and Gélard, 1997) whose asymmetry suggests that they are connected at depth to southdipping, blind thrust faults, and by an uplift of at least 900 m of the Miocene series since their deposition (Strzerzynski et al, 2010 and references therein). The inversion is also emphasized by a large-scale downward flexure of the oceanic plate and an isostatic overcompensation at the margin toe that might indicate possible southward dipping nascent subduction (Hamai et al, 2015). The crustal flexure is the largest off Greater Kabylia that is in the study area (Hamai et al, 2015).…”

Style of Atlassic tectonic deformation and geodynamic evolution of the southern Tethyan margin, Tunisia

“…A C C E P T E D ACCEPTED MANUSCRIPT 13 1997;Vergés and Sàbat, 1999;Déverchère, et al, 2005;Zouaghi et al, 2005;Frizon de Lamotte et al, 2009;Billi et al, 2011;Roure et al, 2012;van Hinsbergen et al, 2014, Hamai et al, 2015. In the study area, field data together with previous studies allow us to define the unconformities and syntectonic deposits recording the tectonic inversion of the inherited structures (Permian (?)…”

The large Algerian earthquakes (267 A.D.-2017)

“…Indeed, the focal mechanism given by Harvard CMT displays a thrust mechanism with an azimuth of 57°, dip 44° and rake 71° (Table 1). This earthquake revealed the uplift of the coastline of about 0.5m and also the likely incipient subduction zone in the central part of the Algerian margin (Auzende, 1975, Yelles-Chaouche et al, 2009c, Hamai et al, 2015. From an analysis of the more complete aftershock's sequence of one year recording, , propose a more precise vision of this earthquake.…”

“…The Algerian earthquakes are superficial, no deeper than 20 kilometers. Hamai et al, 2015 demonstrate from geophysical profiles analysis during the Spiral project (Figure 11 a,b) that seismic events occurrence is linked with the existence of an incipient subduction zone along the slope of the Algerian margin.…”

“…By now, the several plate kinematics studies demonstrate that the velocity rate of the collision in the western Mediterranean region is estimated to 0.5 cm/year in northern Algeria along the NNW-SSE direction (Stich et al, 2006;Nocquet et al, 2012). The recent Maradja and Spiral projects (Yelles-Chaouche et al, 2010) analyzing the superficial and deep seismic structures of the Algerian margin demonstrate that the deflection of the oceanic lithosphere of the Algerian basin near the continental slope supposes the existence of an incipient subduction zone in the central segment of the margin (Hamai et al, 2015).…”