Paleomagnetism of the Liassic member of the Zarzaïtine Formation (stable Saharan craton, Illizi basin, Algeria)

A paleomagnetic study was carried out in the carbonates and marls of the Liassic member of the Zarzaïtine Formation of the Illizi basin (SE Algeria) deposited in a continental environment. Two magnetization components were identified. The first, defined at relatively low blocking temperature, was isolated in five sites, and yields the following paleomagnetic pole (80.8°N, 20.1°E, K = 811 and A 95 = 2.2°). This magnetization is considered an overprint acquired during Cenozoic times. The second component was defined by both normal and reversed polarity. The normal polarity was identified in fourteen sites using both linear regression and great circles. The reversed one was inferred in four sites from the remagnetization circle and demagnetization path analyses. This component is mainly (it could be in part the primary magnetization) a late diagenesis magnetic overprint. It yields a new Liassic pole (71.8°S, 54.9°E, K = 91 and A 95 = 3.9°) for Africa

New Moscovian palaeomagnetic pole from the Edjeleh fold (Saharan craton, Algeria)

International audienceA palaeomagnetic study was carried out in the Moscovian (~305Ma) formation in the Edjeleh anticline, the only area where important dips can be observed in the Upper Palaeozoic series of the Illizi basin (Algeria). This study shows the existence of three magnetization components. Two of them are interpreted as Cenozoic and Permian remagnetizations; their poles are 88.8°N, 164.0°E, K=262, A95=3.3° and 43.4°S, 61.7°E, K=93, A95=5.9°, respectively. The third component is determined by both well-defined ChRMs and remagnetization circles analysis. Its associated fold test is positive and significant. Because the folding started before or during the Autunian, this third component was acquired very early and is very probably the primary magnetization. Its corresponding palaeomagnetic pole (28.3°S, 58.9°E, K=157, A95=4.2°) is close to the poles from the Saharan platform obtained from neighbouring periods. The positive fold test of this study thus validates these previous Upper Palaeozoic poles. This new result is in agreement with the geodynamical model (Matte 1986; Henry et al. 1992), which proposed the occurrence of a large clockwise rotation of Africa during the Carboniferous; such a motion agrees with the Permian Pangaea A2 reconstruction

Paleomagnetism of the Liassic member of the Zarzaïtine Formation (stable Saharan craton, Illizi basin, Algeria)

A paleomagnetic study was carried out in the carbonates and marls of the Liassic member of the Zarzaïtine Formation of the Illizi basin (SE Algeria) deposited in a continental environment. Two magnetization components were identified. The first, defined at relatively low blocking temperature, was isolated in five sites, and yields the following paleomagnetic pole (80.8°N, 20.1°E, K = 811 and A 95 = 2.2°). This magnetization is considered an overprint acquired during Cenozoic times. The second component was defined by both normal and reversed polarity. The normal polarity was identified in fourteen sites using both linear regression and great circles. The reversed one was inferred in four sites from the remagnetization circle and demagnetization path analyses. This component is mainly (it could be in part the primary magnetization) a late diagenesis magnetic overprint. It yields a new Liassic pole (71.8°S, 54.9°E, K = 91 and A 95 = 3.9°) for Africa

Composite magnetic fabric deciphered using heating treatment

International audienceAbstract In a number of AMS studies, the presence and deciphering of composite magnetic fabrics is of major importance for a correct interpretation of the data. On the basis of several examples from intrusive rocks (diorites and dolerites) we show that the use of laboratory heatings can help to extract at least one component of the composite magnetic fabrics usually present. The procedure includes comparison of the fabrics measured after stepwise laboratory heating with the fabrics determined by tensor difference and by linear regression analysis. In the diorite samples, the measured AMS results from the superimposition of different component fabrics and does not correspond exactly to any of these fabrics. In these dykes, isolated magnetic fabric during thermal treatment corresponds to that of the main magnetic mineral (Ti-poor titanomaghemite) and reveals an unknown structure. In volcanic flow or doleritic dykes, a “parasitic” fabric related to late or post-magmatic evolution superimposed to the flow fabric can produce important scattering of the AMS principal directions. Decomposition of magnetic fabric during thermal treatment allows isolation of the flow fabri

Late Pan-African Murzuq event in the Central Hoggar: Sinistral displacement along the Ounane major shear zone (Gour Oumelalen, Algeria) shown by the magnetic fabric of the Tisselliline pluton

International audienceThe 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

Enigmatic well-characterized remanent magnetization of silicified Lower Devonian rocks from the Tadrart area (Murzuq basin, SE Algeria)

International audienceTo 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

Paleomagnetic analyses on red chert-like rocks and partly silicified paleosols within the Lower Devonian Ikniouen level of the Tadrart Formation

In order 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 of the Tadrart Formation. 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 a very well-defined mean direction and a positive reversal test. Rockmagnetism data evidence minerals of the hematite family, but the presence of a minor amount of other mineral phases remains possible. These paleomagnetic results could have major geodynamical implications for the Gondwana supercontinent. In fact, ChRM acquired in this level during or shortly after deposition should imply a very unexpected fast latitudinal continental drift of the Gondwana during the Lower Devonian. That should correspond, for the APWP, to a large loop with a reversal of the drift direction from a much more southern extreme location than previously assumed. A "more realistic" interpretation should be related to a significant True Polar Wander during this period. 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