Rabah Bracene

The two main steps of the Atlas building and geodynamics of the western Mediterranean

The Atlas system (Morocco, Algeria, and Tunisia) constitutes an important morphologic barrier fringing the Sahara platform. Its structural style changes along strike from a thick-skinned style in Morocco to a thin-skinned one in Algeria and Tunisia. The position relative to the Tell-Rif system is also different in eastern Algeria and Tunisia where the two systems are adjacent and in western Algeria and Morocco where they are separated by large rigid cores (Moroccan Meseta and Algerian High Plateaux). New data, as well as a reappraisal of available data, show that the Atlas build up occurred everywhere during two main phases of late Eocene and Pleistocene-lower Quaternary age, respectively. These phases are clearly distinct and do not represent end points of a progressive deformation. An additional Tortonian event exists in the eastern region where the Tell-Rif is thrusting directly over the Atlas. From Oligocene to middle Miocene the development of the Tell-Rif accretionary prism is coeval to subduction rollback of Maghrebian Tethys lithosphere and related to the opening of the western Mediterranean Sea. For kinematic and chronological reasons this process cannot account for the two specific steps of the Atlas building. They are better explained assuming that they record two jolts in the convergence of Africa with respect to Europe and correspond roughly to the initiation and the cessation of the subduction processes active in the western Mediterranean region.

The origin of intraplate deformation in the Atlas system of western and central Algeria: from Jurassic rifting to Cenozoic–Quaternary inversion

Abstract Analysis of petroleum exploration data supplemented by paleostress data enabled discussion of the origin of the deformation in the western and central Saharan Atlas (Algeria). This intraplate area has recorded the breakup of Pangea (Late Triassic), the opening of the Maghrebian Tethys (since the Dogger) and subsequently its closure (Oligocene to Present). However, the two periods of strong coupling between Europe and Africa (late Lutetian and Pleistocene), which correspond to rapid uplifts of the Atlas system and important deformations, are not collision-related. They can be correlated to the beginning and the end of the development of the western Mediterranean Sea.

Subsidence history in basins of northern Algeria

Abstract The Tellian foreland in Algeria represents a part of the southern Tethyan margin during the Mesozoic. Its tectonic evolution includes a rifting stage during the Triassic and Liassic times characterised by tilted blocks and early diapiric events during the Liassic, a post-rift regime from Middle Jurassic up to the Late Cretaceous and basin inversion during the Tertiary related to the African and European plates convergence. The subsidence modelling supported by surface and subsurface data integration emphasises different subsidence phases. (1) Liassic subsidence phase under Tethyan and Atlantic control related to crustal thinning. (2) Late Jurassic, Cretaceous and Tertiary subsidence phases, where the first one is linked to diapiric events and the second in eastern Algeria to the effects of the rifting event developed more easterly in the Gulf of Gabes and Sirte. The last one is flexural and occurs in foreland basins during the Tertiary. Taking into account the subsidence record of each structural domain, a correlation between tectonic events and the subsidence phases can be established.

Holocene turbidites record earthquake supercycles at a slow-rate plate boundary

Ongoing evidence for earthquake clustering calls for records of numerous earthquake cycles to improve seismic hazard assessment, especially where recurrence times overstep historical records. We show that most turbidites emplaced at the Africa-Eurasia plate boundary off west Algeria over the past ∼8 k.y. correlate across sites fed by independent sedimentary sources, requiring a regional trigger. Correlation with paleoseismic data inland and ground motion predictions support that M ∼7 earthquakes have triggered the turbidites. The bimodal distribution of paleo-events supports the concepts of earthquake supercycles and rupture synchronization between nearby faults: 13 paleo-earthquakes underpin clusters of 3–6 events with recurrence intervals of ∼300–600 yr, separated by periods of quiescence of ∼1.6 k.y. without major events on other faults over the study area. This implies broad phases of strain loading alternating with phases of strain release. Our results suggest that fault slip rates are time dependent and call for revising conventional seismic hazard models.

Comment on “Zemmouri earthquake rupture zone (Mw 6.8, Algeria): Aftershocks sequence relocation and 3D velocity model” by A. Ayadi et al.

Although often difficult to characterize, the relationship between a seismic rupture, its aftershock sequence, and cumulative subsurface or surface faulting or folding is an important challenge to modern seismology and seismotectonics. Among other benefits, it helps document fault length, slip, and magnitude relationships, reconstruct the evolution of the rupture process through historical and prehistorical times and identify the complexity of the deformation in its path toward the surface. This approach is a prerequisite to any seismic hazard assessment but is particularly difficult for faults whose surface trace projects offshore. A specific effort to identify and quantify the source parameters of large earthquakes in coastal areas is therefore needed, not only in subduction zones but also in areas of slow rate and/or diffuse deformation.

The thin-skinned style of the South Atlas Front in Central Algeria

Abstract Seismic lines cutting through the southern front of the Sahara Atlas show that the front is not superimposed on a major basement fault. Folded Cretaceous rocks can be observed on these lines to overlie a décollement surface climbing from Triassic to Early Cretaceous, below which flat-lying sediments can be recognized. The structural style is thus interpreted to be thin skinned and the folds underlining the front as ramp- related features. The development of duplexes in the core of some anticlines explains the apparent thickening of Cretaceous and/or Jurassic strata revealed by boreholes. This thickening was previously interpreted to be related to now-inverted extensional half-grabens, a model which cannot now be supported. This new interpretation allows a reassessment of other parts of the Sahara Atlas system, the large-scale structural model for which is that of large half-graben system that has undergone inversion because of shortening between the High Plateau massif and Sahara Platform.

Plio-Quaternary deformation pattern along the Algerian margin: insights from multibeam bathymetry and seismic reflection profiles (Maradja 1 and Samra cruises)

Los braquiopodos retzidinos son una fraccion menor de las faunas devonicas de la CordilleraCantabrica (Norte de Espana). Aparte de un par de formas raras, impublicadas, del Praguiense delDominio Palentino y del Emsiense inferior del Astur-Leones, proximas al genero Rhynchospirina, ellinaje alcanzo su maximo de diversidad en la parte superior del Emsiense, con dos especies del generoRetzia, R. adrieni y R. cf. prominula, Cooperispira subferita y, quizas, una forma impublicada dePlectospira. El grupo no es conocido en el resto del Devonico y reaparece en el Pensilvaniense con algunasformas del genero Hustedia. En este trabajo se propone un nuevo taxon de la Familia Retziidae,Argovejia n.gen., de la parte final del Emsiense superior de Asturias y Leon, constituido por su especietipo,A. talenti n.sp. y, quizas, por las formas del Emsiense superior del Macizo Armoricano (Francia)Retzia haidingeri var. armoricana y Retzia haidingeri var. dichotoma.The Ronda Depression is filled by Neogene sediments on the boundary between Subbeticreliefs, with NE-SW structural trends, and the frontal Subbetic Chaotic Complexes. The folding stylein the Subbetic Units of Western Betics is strongly controlled by the rheology of the rocks: thick andmassive beds of Jurassic limestones over Triassic marls and gypsum with plastic behaviour. Main deformationstructures in the sedimentary infill of the Ronda depression are simultaneous box folds withNNE-SSW and WNW-ESE trends that only affect its southwestern part. This distribution of folds isa consequence of the inherited fold trend that affected the basement during Early Burdigalian age.