In order to better constrain the Mesozoic-Cenozoic evolution of the Precambrian Hoggar shield, a paleomagnetic study, combined with detailed fieldwork, was carried out to date its detrital local cover, the Serouenout Formation. Thermal demagnetization yields, only in a few samples, the characteristic remanent magnetization carried by hematite. Post-tilting remagnetization was obtained in sites located along a fault affected by intense fluids circulation. The paleomagnetic directions recorded at seven widespread other sites are on the contrary associated with a positive fold test. It provides a relatively well-defined paleomagnetic pole (A95 = 4.1°), sufficient to estimate the age of the Serouenout Formation. The comparison of this pole with the reference curve of Africa suggests two possible age windows, Triassic and Upper Cretaceous-Lower Paleocene, while the discovery in the uppermost levels of the Serouenout Formation of a fern-rich level with Weichselia reticulata (Bathonian to Cenomanian; Blanco-Moreno et al., 2018) imposes a deposition during the Cenomanian. The presence of a detrital formation at least 350 m thick, with a basal conglomerate containing large pebbles, implies the existence, during this time period, of a tectonic event that generated differential uplift. In addition, structural observations indicate that the Serouenout Formation recorded later brittle tectonics, dominated by a network of vertical N–S dextral faults. The horizontal displacement generated by one well-developed fault has been estimated to be at least 1 km. This activity is related to the known Alpine reactivations of the N–S Pan-African mega-structures, which are still at the origin of the current intraplate seismicity.

To improve the poor Gondwana paleomagnetic database for Devonian times, detailed paleomagnetic analyses were performed on red chert-like rocks and partly silicified paleosols within the Lower Devonian Ikniouen level (fine-grained sandstones including red ironstone) in conformity within the sub-horizontal Tadrart coarse white formations of the Murzuq basin. Silicification, limited to this level that is only a few meters thick, was probably due to tropical warm climatic conditions during and shortly after the rock deposition. In two sections 40 km away each other, paleomagnetic data point out a high-temperature Characteristic Remanent Magnetization (ChRM) with very well-defined mean direction, positive reversal test and relatively high (5) Q and R scores. Rock magnetic data indicate minerals of the hematite family, but the presence of a minor amount of other mineral phases remains possible. At least part of the ChRMs are Chemical Remanent Magnetizations, likely acquired during or shortly after deposition. The corresponding paleomagnetic results (paleomagnetic pole at 28.6° E and 71.1° S, with K = 1004, A95 = 1.5°) could have major geodynamical implications for the Gondwana supercontinent. In fact, ChRM acquired in this level during or shortly after deposition should imply a much-unexpected fast latitudinal continental drift of the Gondwana during the Lower Devonian or a significant and fast true polar wander. Though much more difficult to match with the ChRM and geological characteristics, the only possible alternative interpretation for the Ikniouen data should be a chemical remagnetization acquired during the Late Cretaceous–Early Paleocene times.

The late upper-level subcircular Tisselliline pluton (c. 572 Ma), emplaced in Hoggar along the major Pan-African Ounane shear zone, is NNW-SSE oriented. Its magnetic fabric is mainly characterized by dome-shaped magnetic foliations and subhorizontal magnetic lineations. When approaching the nearby Ounane shear zone, the lineations become associated with a prolate shape of the fabric and their direction evolves from NE-SW to NNE-SSW. This highlights a stretching underwent by the pluton during its late magmatic stage, in agreement with the shape of the pluton itself. These characteristics indicate a sinistral movement along the Ounane shear zone during the emplacement of the Tisselliline pluton. It is the opposite to the dextral movement that affected earlier the older Ounane batholith (c. 629 Ma) located on the other side of the shear zone. This sinistral movement can be related to the NE-SW compression that occurred in the Eastern Hoggar, western part of the Saharan metacraton, during the Murzukian event. Its age, given by the Tisselliline pluton age (c. 572 Ma), corresponds indeed to the initiation of this event (575-555 Ma). This shows that the Eastern Hoggar Murzukian event affected also the Central Hoggar, even if with a much lower intensity.

Previous paleomagnetic studies performed in the central North-Algeria (Chellif and Mitidja basins) on Neogene formations pointed out tectonic clockwise blocks rotations. This deformation pattern was interpreted as resulting from a bookshelf neotectonics, consequence of the Africa-Eurasia plates convergence. A new paleomagnetic study was conducted on the Neogene volcanic rocks outcropping in the northwestern Algeria (Marset Ben Mhidi, Aïn Temouchent, Tifaraouine area). The obtained stable remanent magnetization is mainly carried by Ti-poor titanomagnetite. The paleomagnetic data show that, since the lava emplacement, the northwestern Algeria underwent a mean moderate clockwise block rotation of 9.3° ± 4.5°. For the Algerian margin, this confirms a context of transpression and blocks rotations in a strike-slip tectonic setting. A decreasing deformation gradient from the E to the W affected the different basins of this margin, from strong rotations within the Mitidja, to the moderate ones in the Chellif and to Marset Ben Mhidi, Aïn Temouchent, Tifaraouine area, where rotation magnitudes are significantly lower.

Significant tectonic clockwise rotations were evidenced in the Tell Atlas (Neogene Algerian Cheliff basin) by previous paleomagnetic studies. For northwestern Africa and in the context of the Africa‐Eurasia convergence, they provided a new argument validating a kinematic model, based on transpression with shortening accommodated by clockwise block rotations. To corroborate this deformation pattern at a larger scale, new paleomagnetic studies were performed on 349 cores in the Mitidja basin, of Mio‐Plio‐Quaternary age, in the Algerian Tell Atlas. This intramontaneous basin is structured by two regional major E‐W to WNW‐ESE dextral shear zones. Primary magnetization data were obtained in 43 out of the 49 sampled sites. This magnetization is carried by Ti‐poor titanomagnetite. Its direction shows that significant tectonic block rotations affected this basin since 16 Ma. Zones located between the E‐W and WNW‐ESE major structures are affected by coherent clockwise rotations (average magnitude of 48°) of large blocks, compartmented by presently associated sinistral NE‐SW faults. Along the shearing structures, smaller blocks, resulting from the fragmentation of the large blocks, show various rotations, many of which are of large magnitude. These rotations, similar to those highlighted previously in the Cheliff basin, are interpreted as resulting from bookshelf, consequence of the Africa‐Eurasia plates convergence in the Tell Atlas.

Numerous paleomagnetic studies were performed in the western Saharan basins, particularly during the last decades. Primary magnetization of the sedimentary formations older than Bashkirian appeared as totally overprinted. By contrast, 23 new coherent paleomagnetic poles, mainly from Bashkirian to Autunian age and from Middle Triassic to Lias age, were determined. These new data greatly improved the Apparent Polar Wander Path (APWP) for Africa, and consequently for the whole Gondwana, especially for the Upper Carboniferous. The corresponding paleoreconstruction strongly argued for an A2 Pangea during this last period. By its comparison with paleomagnetic data from undated geological units, this new APWP provided dating of these units. Paleomagnetic data highlighted also the existence of a post-Liassic regional tectonic event having affected the Paleozoic cover in the Sahara platform. Finally, several magnetic overprints, pointed out in these studies, are of chemical origin, with likely a significant role of ground-fluids. Indeed, fluids migration phenomena often favored chemical changes and remagnetization process. Upper Carboniferous, Permian and Upper Cretaceous-Cenozoic overprinting ages were thus probably linked to regional geochemical events that occurred in the Saharan Platform.

Previous paleomagnetic studies, performed in central North-Algeria (Chelif and Mitidja basins) on Neogene formations, pointed out some tectonic clockwise rotations of large blocks, sometimes of large amplitudes. Narrow zones represent also important shear zones with strong rotations. The preliminary results of a new paleomagnetic study undertaken in Northwestern Algeria (Aïn Temouchent–Ghazaouet area) suggest the presence of clockwise block-rotations of lower magnitude. That confirms that the northern Algeria is organized as tectonic blocks, accommodating by transpression and blocks rotations, the Africa-Eurasia convergence. A gradient of “deformation” appears from the NNE to the SSW, i.e. from the Mitidja basin affected by strong rotations to Aïn Temouchent–Ghazaouet area where the rotations are limited.

The magnetic fabric obtained in the Hoggar shield on several plutons is related to various origins: simple flow, syn-deformation flow, stress field during late-magmatic stage and solid-state deformation. These results also evidenced the important role of hosting frame, acting as a more or less efficient "protection" against the effects on the magnetic fabric of the regional stress field. Combined studies, on neighboring sites of the main intrusion and of late-magmatic dykes crosscutting it, yield key-arguments about the acquisition age of this fabric. P'(K-m) diagrams clearly highlight the strain gradient in plutons very close to shear zones. The fact that K-1 axis is mainly a mineral lineation, at least in three plutons, is evidenced by the determination of the magnetic zone axis. The plutons AMS is associated with different Pan-African stages in the Hoggar: gneissification of Eburnean plutons, syn-thrust pluton emplacement, pluton emplacement under regional shearing context. Accordingly, the main movements along the major shear zones, related to the oblique collision of the Hoggar shield with the West African Craton, are contemporaneous of the first stages. Shearing context during the following period points out that this collision had still active effects, probably associated with a progressive change in orientation of the continental convergence.

Summary Paleomagnetim of the Paleozoic formations in the Illizi Basin evidenced two distinct magnetization components. Using African APWP shows remagnetizations of Cenozoic and Permian ages. The distribution of the directions of these components indicates that the Permian one is uniquely identified in three sampled sites. Its preservation may suggests a relationship between lithology and tectonic evolution. Our interpretation of potential data has allowed corroborative evidence supporting these assumptions. This interpretation is realized by cross-checking the results of each potential field transforms, which have been applied to the Bouguer gravity and total intensity aeromagnetic maps. Consequently, the obtained results allow locating horizontally the inferred zone boundaries that assuming essentially vertical contact, which is underlined globally by prevailing submeridian lineaments with higher amplitude magnetic and gravimetric anomalies. Interestingly, these submeridian uplifts are typically associated with fluids migration at origin of remagnetization and controlled by fault-related structures leading to preserve or erase the Permian component. Moreover, uplift and deformation in south of the Illizi basin, around the Hoggar area, are associated with the emplacement of thick basaltic trapps. Therefore, due to volcanism, fluids from the Hoggar could have been enriched with chemically active components that affected the sampled Paleozoic formations of the Illizi basin.

In Ordovician and Silurian sedimentary formations of the Murzuq basin (Saharan platform, Algeria), different remagnetization processes have been highlighted. These magnetic overprints totally replaced the primary magnetization. They are mainly due to chemical phenomena. Even in a site affected by contact metamorphism during Devonian, chemical changes, associated to the acquisition of the thermo-remanent overprint, were important, affecting the characteristics of the magnetite grains. In the remaining sites, remagnetizations of Cenozoic age have also a chemical origin and are carried by magnetite as well as by hematite. Contrary to what is generally deemed, these remagnetizations processes appeared limited to very short duration of acquisition, and to very local geographical extension.

Intraplate deformation is most often linked to major stress applied on plate margins. When such intraplate events are accompanied by magmatism, the use of several dating methods integrated within a multidisciplinary approach can bring constraints on the age, nature and source mobilized for generating the magma and in turn on the nature of the intraplate deformation. This study focuses on the large gabbro Arrikine sill (35 km in extension) emplaced within the Silurian sediments of the western margin of the Murzuq cratonic basin in southeastern Algeria. Its emplacement is dated during the early Devonian (415-400 Ma) through the determination of a reliable paleomagnetic pole by comparison with the Gondwana Apparent Polar Wander Path (APWP). This age can be correlated with deep phreatic eruptions before Pragian time thought to be at the origin of sand injections and associated circular structures in Algeria and Libya. For the sill, the K -Ar age of 325.6 +/- 7.7 Ma is related to a K-rich aplitic phase that has K-enriched by more than 20% the Devonian gabbro. Laser-ICP-MS U-Pb method dates only inherited zircons mostly at c. 2030 Ma with additional ages at c. 2700 Ma and younger ones in the 766-598 Ma age range. The Arrikine sill is a high-Ti alkaline gabbro having the geochemical composition of a hawaiite akin to several intraplate continental and oceanic provinces, including the contemporaneous Air ring complexes province in Niger, but also to the Mauna Loa volcano in Hawaii. This peculiar composition akin to that of the contemporaneous Air province is in agreement with a lower Devonian age for the Arrikine sill.The lower Devonian Arrikine sill emplacement is related to a "Caledonian" transtensive reactivation of the western metacratonic boundary of the Murzuq craton. This event also generated in the Saharan platform the so-called "Caledonian unconformity" of regional extension, the Air ring complexes and magmatic rocks that produced sand injections. It could be related to rifting of the Hun terranes that occurred at the plate margin to the north (Stampfli and Borel, 2002, Blackey, 2008 and references therein). The mid-Carboniferous (c. 326 Ma) reactivation corresponds to Variscan compression on NW Africa generating aplitic fluids, but also"to the major "Hercynian unconformity" of regional extension. Thegeneration of the Arrikine magma is attributed to partial melting through adiabatic pressure release of uprising asthenosphere along tectonically reactivated mega-shear zones, here bordering the relictual Murzuq craton enclosed in the Saharan metacraton. (C) 2015 Elsevier Ltd. All rights reserved.

The reliability of an Apparent Polar Wander Path (APWP) obviously depends on the paleomagnetic poles used to determine it. The APWP of Africa and South America are fairly well defined for the 330–260 Ma interval. However, this study pointed out a moderate shift between these two curves, and an incoherency of the South American data, contrary to the African ones, which are homogeneous. A number of South American pole positions were re-evaluated in an effort to better constrain the APWP for the entire continent. Most of discarded poles correspond to sites at the area of the junction of Cordillera with the stable craton. That could have structural implications for the evolution of the western margin of the Gondwana. A new criterion for the evaluation of paleomagnetic poles reliability for APWP is presented. Based on comparison of data from different continents and labeled “coherence” criterion, it is independent from Van der Voo’s ones.

The Tellian Atlas of Algeria underwent significant tectonics including compressional deformation during the Neogene time. Neotectonic features correspond to E–W to NE–SW trending folds and reverse faults affecting Quaternary deposits [1, 2, 3]. Figure 1 shows the Chelif basin with the Oued Fodda, Bou Kadir and Oued Allalah faults, which represent cases of NNE–SSW to NE–SW active structures in the E–W trending Tellian Atlas fold belt. This figure also indicates folding structures and distribution of the related faults, corresponding to imbricated thrust-and-fold structures [4]. The present paleomagnetic results confirm that the relative convergence motion between the Africa and Eurasia plates could be interpreted as a transpressional tectonic deformation model with block rotations along the Algerian continental margin. The present paleomagnetic study has been conducted on volcanic rocks outcropping on the northern border of the Neogene Chelif basin, northwest Algeria. The results show the existence of numerous small tectonic blocks, of probable size around 0.5 to 0.6 km, which underwent clockwise rotations.

(1) CRAAG, BP 63, 16340 Bouzaréah, Algeria, m.e.m.derder@gmail.com, (2) Geosciences Environnement Cergy, 5 mail gay Lussac, 95031 Cergy-Pontoise cedex France, philippe.robion@u-cergy.fr, (3) Paléomagnétisme, IPGP and CNRS, 4 avenue de Neptune, 94107 Saint-Maur cedex, France, henry @ipgp.fr, (4) GEOPS, Univ. Paris-Sud, CNRS, Université Paris-Saclay, Rue du Belvédère, Bât. 504, 91405 Orsay, France, yves.missenard@u-psud.fr, (5) Laboratoire de Géodynamique, Géologie de l’Ingénieur et Planétologie (LGGIP/USTHB) BP32, El Alia Bab Ezzouar Alger-Algérie, ouabadi@yahoo.fr

To improve paleocontinental reconstructions, paleomagnetic reference curves (Apparent Polar Wander Path: APWP) feature for large continents have to be continuously refined by adding up new high-quality data. For stable Africa, the Moscovian period was favorable for such aim, with well-dated and widespread geological formations. A new study has been conducted in the Upper "Dembaba" geological formation of Lower Moscovian age outcropping in the western part of the "Murzuq" basin (Saharan platform). Well-defined ChRMs, combined with remagnetization circles data, both constrained in age by a positive fold test, yield a new significant paleomagnetic pole (lambda = 25.2 degrees S, phi = 59.9 degrees E, K = 55, A(95) = 5.4 degrees). When joined with previous African data of the same age, it gives an improved reference pole for Africa (lambda = 28.9 degrees S, phi = 54.5 degrees E, K = 106, A(95) = 3.6). The Mean Moscovian paleomagnetic pole determined from an updated Gondwana Paleozoic APWP (lambda = 29.4 degrees S, phi = 51.5 degrees E, K= 11, A(95) = 1.8 degrees), associated with the corresponding Laurussia pole (Domeier et al., 2012), yields a more constrained paleocontinental reconstruction for 310 Ma. (C) 2013 Elsevier Ltd. All rights reserved.

The paleomagnetism is a powerful tool to date formations that have age not constrained by paleontological, stratigraphical or radiochronological data. It was applied, on the western border of the Murzuq basin in Algeria (Saharan platform), to the Zarzaitine formation, attributed to a Middle-Upper Triassic-Lower Jurassic age. Comparison of the obtained paleomagnetic pole with previous poles from the same geological formation outcropping in another basin and from other Carboniferous to Lower Mesozoic African formations yielded a clearly older age (Late Permian) than expected. That evidences a strong diachronism (at least 40 My) of the deposition of this formation on the Saharan platform. The post-Hercynian structural evolution was therefore different according to the parts of this platform, with significant differential vertical tectonic movements. The latter were at the origin of erosion, hiatus or sediments deposition according to areas. (C) 2014 Elsevier Ltd. All rights reserved.