S U M M A R YPalaeomagnetic measurements, in the Late Triassic-Liasic Zarzaytine formations of the La Recul6e area, showed the existence of superimposed and juxtaposed magnetizations. The separation of the juxtaposed magnetizations yielded Late Triassic-Liasic magnetization directions of normal and reversed polarities. The new corresponding palaeomagnetic pole agrees well with a Pangea A configuration. It confirms, from Autunian to Lias, a latitudinal displacement of stable Africa towards the north, with important anti-clockwise rotation contrary to Laurussia which did not undergo significant rotation. This transition of Pangea from A2 to A ended before the Late Triassic. It corresponds to dextral movements in the HercynoAppalachian collision area, and perhaps to part of the northwards drift of microcontinents of Gondwanian origin located north of the Arabian promontory.
International audienceThe Maâdna structure is located approximately 400 km south of Algiers (33°19' N, 4°19' E) and emplaced in Upper-Cretaceous to Eocene limestones. Although accepted as an impact crater on the basis of alleged observations of shock-diagnostic features such as planar deformation features (PDFs) in quartz grains, previous works were limited and further studies are desirable to ascertain the structure formation process and its age. For this purpose, the crater was investigated using a multidisciplinary approach including field observations, detailed cartography of the different geological and structural units, geophysical surveys, anisotropy of magnetic susceptibility, paleomagnetism, and petrography of the collected samples. We found that the magnetic and gravimetric profiles highlight a succession of positive and negative anomalies, ones that might indicate the occurrence of a causative material which is at least in part identical. Geophysical analysis and modeling suggest the presence of this material within the crater at a depth of about 100 m below the surface. Using soil magnetic susceptibility measurements, the shallowest magnetized zone in the central part of the crater is identified as a recently deposited material. Paleomagnetic and rock magnetic experiments combined with petrographic observations show that detrital hematite is the main magnetic carrier although often associated with magnetite. A primary magnetization is inferred from a stable remanence with both normal and reverse directions, carried by these two minerals. Although this is supposed to be a chemical remagnetization, its normal polarity nature is considered to be a Pliocene component, subsequent to the crater formation. The pole falls onto the Miocene-Pliocene part of the African Apparent Polar Wander Path (APWP). Consequently, we estimate then formation of the Ma^adna crater to have occurred during the time period extending from the Late Miocene to the Early Pliocene. Unfortunately, our field and laboratory investigations do not allow us to confirm an impact origin for the crater as neither shatter cones, nor shocked minerals, were found. A dissolved diapir with inverted relief is suggested as an alternative to the impact hypothesis, which can still be considered as plausible. Only a drilling may provide a definite answer
S U M M A R YDuring remagnetization, chemical changes limited to moderate grain growth from pre-existing single domain (SD) grains do not modify the palaeomagnetic direction carried by these grains. Palaeomagnetic direction from new SD grains, on the contrary, is that of the magnetic field during remagnetization. The resulting direction becomes intermediate between the directions carried by previous and new SD grains, as it appears often in case of a partial magnetic overprint. If the growth of pre-existing grains is more important, these grains become large multidomain (MD) and loose the primary magnetization. Stable magnetization is then only related to the new SD grains, which carry the total remagnetization. In the Illizi basin (Saharan platform), these different cases of partial or total magnetic overprint have been observed, resulting from palaeomagnetic studies of different Palaeozoic and Mesozoic formations.
SUMMARY
Palaeomagnetic studies have been carried out in a Stephano‐Autunian formation of the Saharan craton. Despite the presence of a strong recent magnetic overprint, some samples give a well‐defined characteristic remanent magnetization direction. Moreover, a new approach using the remagnetization circles has been used to confirm the significance of this direction. The obtained Stephano‐Autunian pole (35.3°S, 60.3°E) allows a substantial improvement of the apparent polar wander path (APWP) for stable Africa. This APWP clearly shows that the clockwise rotation of Africa, which occured during the Hercynian orogeny, was stopped during the Stephano‐Autunian period.
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