Tahar Aïfa

Pliocene-Pleistocene evolution of the Tyrrhenian arc: paleomagnetic determination of uplift and rotational deformation

Abstract The Jaramillo subchron and the Brunhes/Matuyama transition have been identified in the pelagic Pleistocene marls located on the Calabrian side of the Messina Straits (Italy). Polarity of the characteristic magnetization has been deduced from an analysis of multicomponent magnetization. This magnetostratigraphic age determination combined with the paleobathymetry of the Quaternary marls implies uplift rates of fault bounded blocks of 0.7–1.0 m/1000 yr. A positive fold test on three sites from the “Trubi” Formation (4.3 Ma) shows a clockwise rotation of 36°. An antipodal test on Pliocene-Pleistocene micritic limestones (0.9–2.0 Ma) also indicates a rotation of 14° with the same trend. Both results suggest a rotation rate of about 9°/Ma. These results are in agreement with a tectonic mechanism involving simultaneous rotation and uplifts. A discussion of the possible mechanisms indicates that a compression phase oriented ENE-WSW could have played a dominant role.

Devonian-Carboniferous paleopoles for Africa: Consequences for Hercynian geodynamics

Abstract Paleomagnetic analysis of marine formations in the northwestern Sahara (Algeria) yields the following results: three sites in the lower Devonian Orthoceras-bearing limestones of Beni-Abbes (29.67° N, 2.15° W) indicate a mean direction of remanence: Dm = 149.6°; Im = 1.6°; α95 = 6.8°. This typical Permian direction implies a complete remagnetization of the sites. The Famennian griotte limestones of the Ben-Zireg anticline (31.92° N, 1.82° W) show high remanent magnetization intensities. In this structure, which has undergone polyphase tectonism, multicomponent analysis evidences a high unblocking temperature (HTb) component carried by haematite leading to a positive fold test (after partial unfolding) at the 95% confidence level with a mean direction D = 156.4°, I = 47.6°, α95 = 0.9° and attributed to the Early Carboniferous (355 Ma) with a paleopole at 25.3°S, 21.1°E, dp = 0.8°, dm = 1.2°. An intermediate unblocking temperature (ITb) component of magnetization carried probably by magnetite, also present, satisfies a positive fold test (after tilt correction), with a mean direction D = 154.9°, I = 54.3°, α95 = 3.7°. This pre-folding direction of magnetization is attributed to the Famennian. It gives a paleopole at 19.2°S, 19.8°E, dp = 3.7°, dm = 5.3°. Comparison with previous results shows a significant S-N crustal shortening of about 2500 km during Carboniferous times accompanied by a clockwise rotation of the Saharan region of about 25°. These new results give information about the tectonic phenomena involved in the collision between Gondwanaland and Laurasia: they argue for a Pangea A2 dominating model, and define mean values for shortening rate and plate velocity comparable to the present ones (3 cm/yr).

Application of feedback connection artificial neural network to seismic data filtering

The Elman artificial neural network (ANN) (feedback connection) was used for seismic data filtering. The recurrent connection that characterizes this network offers the advantage of storing values from the previous time step to be used in the current time step. The proposed structure has the advantage of training simplicity by a back-propagation algorithm (steepest descent). Several trials were addressed on synthetic (with 10% and 50% of random and Gaussian noise) and real seismic data using respectively 10 to 30 neurons and a minimum of 60 neurons in the hidden layer. Both an iteration number up to 4000 and arrest criteria were used to obtain satisfactory performances. Application of such networks on real data shows that the filtered seismic section was efficient. Adequate cross-validation test is done to ensure the performance of network on new data sets.

Application of a radial basis function artificial neural network to seismic data inversion

We investigate here the performance and the application of a radial basis function artificial neural network (RBF-ANN) type, in the inversion of seismic data. The proposed structure has the advantage of being easily trained by means of a back-propagation algorithm without getting stuck in local minima. The effects of network architectures, i.e. the number of neurons in the hidden layer, the rate of convergence and prediction accuracy of ANN models are examined. The optimum network parameters and performance were decided as a function of testing error convergence with respect to the network training error. An adequate cross-validation test is run to ensure the performance of the network on new data sets. The application of such a network to synthetic and real data shows that the inverted acoustic impedance section was efficient.

Effects of experimental deformation on the remanent magnetization of sediments

Abstract Changes in direction and in intensity of the remanent magnetization of sediments were studied during experimental deformation leading to an axial shortening of 20–30%. The remanence was produced before shortening by anhysteretic magnetization in an alternating field of 90 mT superimposed on a parallel steady field of 1 mT at various inclinations with respect to the sample. The samples were sandstones, marls and volcanic sediments. The results show that the direction of magnetization is stable during the deformation within the experimental precision. The intensity of magnetization decreases during shortening by an amount which depends on the type of rock: about 30% for sandstones and about 5% for marls and volcanic sediments. The March model does not account for the data and a discontinuous deformation must be invoked. The evolution of the Young modulus of sandstones during cyclic loading and unloading shows first a decrease in relation to crack density growth, then an increase showing a localization of the fracturation and finally a deformation by displacement of microblocks. An interpretation is given after a discussion of the results obtained by other authors with plasticine and artificial clays. The amplitude of the decrease and of the rotation of the remanent magnetization could be a function of the size of the magnetic grains compared with the scale of the fracturation during non-homogeneous processes of deformation.

Seismic noise attenuation by means of an anisotropic non-linear diffusion filter

Noise attenuation is a major seismic data processing concern. In seismic data, noise can appear as random, coherent and/or impulsive. Recently, many different techniques, ranging from relatively simple processes to extremely complex ones, have been used for noise attenuation. Image filtering techniques are relatively new methods in seismic exploration. We introduced the anisotropic non-linear diffusion filter which is an effective way to de-noise images. Since a seismic section can be considered as an image of a two-variable function, we implemented the anisotropic non-linear diffusion filter to reduce both random and Gaussian noises. This filter is shown to be effective in removing noise while preserving edges and hence reducing resolution loss in seismic data. The anisotropic non-linear diffusion filter, with Tukeys function to guide the diffusivity, was applied to synthetic and real seismic data. The results show a signal-to-noise ratio increase with reflector continuity in addition to better recovery of reflector amplitudes even when dealing with complex subsurface geological structures.

Tectonic and Hydrothermal Activities in Debagh, Guelma Basin (Algeria)

Quaternary and Pliocene travertines, deposited from hot springs, can reveal much about neotectonic and hydrothermal activity. The aim of this work is the understanding of the actual tectonic activity in the Guelma Basin and in one of its spa structures. Gravity data were collected during a field study in the Hammam Debagh (HD) area and then analyzed to better highlight the architecture of its subsurface underlying structures. This analysis was performed by means of a Bouguer anomaly, upward continuations, and residual and derivative maps. Comparison of gravity maps, field geology, geomorphic observations, and structural maps allowed us to identify the major structural features in the Hammam Debagh. As a result, we confirm the position of the Hammam Debagh active fault which is superimposed to the hydrothermal active source in the NW-SE direction characterized by a negative gravity anomaly.

Gold mineralization paragenesis to tectonic structures in the Birimian of the eastern Dialé-Daléma Supergroup, Kédougou-Kéniéba Inlier, Senegal, West African Craton

ABSTRACT New field and petrographic data from the Birimian of the Kolia-Boboti Basin in the Kédougou-Kéniéba inlier indicate two phases of gold mineralization related to Eburnean tectono-magmatic events. Syn- to late-tectonic (D2/D3) mineralization, controlled by stockwork sulphide-bearing quartz-chlorite and quartz-carbonate veins, is associated with fluid circulation related to magmatic intrusions. V2 veins and disseminated assemblages are mainly composed of quartz + chlorite + pyrite and ± gold. Haematite and arsenopyrite are added in the sediments (pelite, greywake, sandstone, quartzite, and marble) and albite in the felsic rocks (andesite, rhyolite, granodirite, and diorite). V3 veins assemblage is composed of quartz + carbonate + pyrite + chalcopyrite and ± gold. Pyrrhotite appears in the sediments (greywacke, quartzite, marble). Sericite, tourmaline, haematite, and magnetite are common in both V2 and V3 assemblages. The first sulphide-bearing quartz-chlorite assemblage is related to the hydrothermal activity of the Eburnean D2 deformation, which was focused mostly along NW- to NNE-trending tectonic structures. The second sulphide-bearing quartz-carbonate assemblage is associated with hydrothermal activity during late Eburnean D3 deformation, mainly located in NE- to E–W-trending tectonic structures. Gold is correlated with the abundance of sulphides (pyrite, chalcopyrite, arsenopyrite), and sulphide stockworks are more abundant in the veins sub-parallel (V2b) or oblique (V2c) to the N–S- to NNE-oriented S2 foliation, as well as in the N45°–N90°-oriented V3 veins. V1 veins, which are related to D1 Eburnean tectonics, are highly deformed (folded and boudinaged) and are poor in sulphides. The host structures of mineralization (V2 and V3 veins) represent the low- and medium-stress domains resulting from the Eburnean D2 and D3 tectonic phases, respectively. The intra-crystalline deformation of the quartz grains associated with these three vein types indicates relatively low temperatures. These different features suggest that most of the mineralization was associated with sulphides formed during the D2 and D3 Eburnean tectono-magmatic events dated around 2080 ± 0.9 and 2061 ± 15 Ma, respectively.

Neotectonic deformation stages in the central Ouarsenis culminating zone, Northwestern Algeria

The Tellian system in the north of Algeria results from the convergence of the African and Eurasian plates which began in the Late Cretaceous. The strong tangentional tectonics at the origin of the emplacement of nappes in its external domain occurred mainly during the Early Miocene. This major tectonic episode was followed by another important compressive deformation-oriented NNE-SSW and NNW-SSE during the Pliocene and the Quaternary, respectively. The Ouarsenis culminating zone is part of the Tellian domain and is characterized by an altogether distinct orographical structure made up of: (i) Jurassic formations which overthrust Cretaceous terrains; (ii) completely upturned series; diversely oriented faults (N40°, N70°, N120°, and N160°) of different kinds (thrust, reverse, normal, and shear faults). Triassic gypsum crops out along some of these faults. Microtectonic data analysis has shown alternation of two main compressive stresses, NW-SE and NE-SW oriented. The more recent stress, probably of Pliocene age, ∼N56° oriented, is responsible for the current face of the culminating zone. It highlights a major ∼N120° sinistral shear fault-generated deformation especially in its central part, affecting ductile material represented by Albo-Aptian turbidites. This fault also generated secondary shears accommodated according to a Riedel deformation model. The central part of this area has a complex tectonic structure squeezed between two massifs composed of hard material, along the sinistral shears. It has been extruded towards the north and has evolved as a positive “flower structure.”

Neogene rotations of the Chenoua massif, northern Algeria, from remagnetizations

The tectonics of the Chenoua massif suggests block rotations of Neogene nappes associated with the African–European plate convergence. To estimate the extent of these rotations, a Paleomagnetic study on rhyolites and andesites of Langhian–Serravallian age and sandstones of Burdigalian age was carried out on 23 sites (200 specimens). The sites are distributed in the northwestern, southeastern and southern Chenoua massif. One or two components of magnetization, mainly carried by magnetite, pyrrhotite and/or hematite, were isolated in sandstones and volcanics. The sandstone sites reveal magnetizations in sandstones from the Cap Blanc syncline that are post-folding. However, both polarities are found, which is consistent with data from Africa during the Upper Miocene. Clockwise and counterclockwise rotations were recorded, dating back to the Neogene times in volcanics and sediments. From the faulted Cap Blanc syncline counterclockwise rotations of 1 ± 4° to 18 ± 28° around a vertical axis occurred in sediments since the Miocene with respect to Africa. In fact, remagnetizations occurred at several periods of time and in different sites, providing information on the evolution of post-tectonic rotations. Some volcanics record counterclockwise rotations of about 30° since the Miocene, whereas others do not show any significant rotation. This can be explained by the direction of the principal compressive stress axis σ1 and by lateral extrusions related to an indentation model, in which we expect both clockwise and counterclockwise rotations.