Contribution a l'etude paleomagnetique du Permien du sud de la France : evolution de la Pangee du Permien au lias

CNRS T Bordereau / INIST-CNRS - Institut de l'Information Scientifique et TechniqueSIGLEFRFranc

New paleomagnetic data on dykes to assess the accretions to West African Craton to Columbia and Rodinia

International audienceA new Apparent Polar Wander Path has been calculated for the West African Craton (WAC) using paleomagnetic data obtained on the Yetti-Eglab intrusions and stromatolite-bearing formations. As no contact test was available, the magnetic stability of carriers was demonstrated thanks to rock magnetic measurements. One, two or three magnetization components on 159 oriented specimens (14 sites) have been isolated using principal component analysis or great circles technique, mainly on doleritic dykes. High and low unblocking temperature components were isolated in six dykes but also in four sites of the well dated Hank stromatolite-bearing formation (875-890 Ma). For the two kinds of rocks, the main components show stable remanent magnetization mainly carried by magnetite (or titanomagnetite) as described using thermomagnetic or Isothermal Remanent magnetization curves, where no significant mineralogical transformation occurred at high temperatures.Five groups of paleopoles have been calculated: they witness the existence of a large circum-terrestrial loop with a drift from the high latitudes of the lower hemisphere to the high southern latitudes of the upper hemisphere followed by a re-entry in the high latitudes of the lower hemisphere. This is in agreement with the previous loop computed by Sabaté and Lomax (1975) between 2.1 Ga and 1.7 Ga. The Yetti and Eglab blocks amalgamated around 1.9 Ga and participated to the formation of Columbia Supercontinent. The paleopole computed for the stromatolite-bearing formation corresponds with the location of Rodinia Supercontinent at its early stages of amalgamation.The geodynamic evolution of the WAC respect with that of Columbia and Rodinia Supercontinents suggests that the Eglab and Yetti were clearly separated by a large oceanic crust before 1.9 Ga. A volcanic arc developed during the subduction of this ocean followed by crustal thickening which generated an intrusive suite (Aftout granites) when it was at the northern low latitudes. Fennoscandia and Rio de la Plata Cratons were close to the WAC after 1.7 Ga. Fennoscandia probably followed the same loop as the WAC between 1.7 and 0.9 Ga. In Africa, the collapse of the Birimian orogen was probably contemporaneous with the fragmentation of Columbia. At 0.9 Ga, Fennoscandia was probably not attached together with the WAC since the latter was not affected by the Grenvilian orogen, so did not necessarily have the same loop

New paleomagnetic data on dykes to assess the accretions to West African Craton to Columbia and Rodinia

International audienceA new Apparent Polar Wander Path has been calculated for the West African Craton (WAC) using paleomagnetic data obtained on the Yetti-Eglab intrusions and stromatolite-bearing formations. As no contact test was available, the magnetic stability of carriers was demonstrated thanks to rock magnetic measurements. One, two or three magnetization components on 159 oriented specimens (14 sites) have been isolated using principal component analysis or great circles technique, mainly on doleritic dykes. High and low unblocking temperature components were isolated in six dykes but also in four sites of the well dated Hank stromatolite-bearing formation (875-890 Ma). For the two kinds of rocks, the main components show stable remanent magnetization mainly carried by magnetite (or titanomagnetite) as described using thermomagnetic or Isothermal Remanent magnetization curves, where no significant mineralogical transformation occurred at high temperatures.Five groups of paleopoles have been calculated: they witness the existence of a large circum-terrestrial loop with a drift from the high latitudes of the lower hemisphere to the high southern latitudes of the upper hemisphere followed by a re-entry in the high latitudes of the lower hemisphere. This is in agreement with the previous loop computed by Sabaté and Lomax (1975) between 2.1 Ga and 1.7 Ga. The Yetti and Eglab blocks amalgamated around 1.9 Ga and participated to the formation of Columbia Supercontinent. The paleopole computed for the stromatolite-bearing formation corresponds with the location of Rodinia Supercontinent at its early stages of amalgamation.The geodynamic evolution of the WAC respect with that of Columbia and Rodinia Supercontinents suggests that the Eglab and Yetti were clearly separated by a large oceanic crust before 1.9 Ga. A volcanic arc developed during the subduction of this ocean followed by crustal thickening which generated an intrusive suite (Aftout granites) when it was at the northern low latitudes. Fennoscandia and Rio de la Plata Cratons were close to the WAC after 1.7 Ga. Fennoscandia probably followed the same loop as the WAC between 1.7 and 0.9 Ga. In Africa, the collapse of the Birimian orogen was probably contemporaneous with the fragmentation of Columbia. At 0.9 Ga, Fennoscandia was probably not attached together with the WAC since the latter was not affected by the Grenvilian orogen, so did not necessarily have the same loop

New paleomagnetic data on dykes to assess the accretions to West African Craton to Columbia and Rodinia

International audienceA new Apparent Polar Wander Path has been calculated for the West African Craton (WAC) using paleomagnetic data obtained on the Yetti-Eglab intrusions and stromatolite-bearing formations. As no contact test was available, the magnetic stability of carriers was demonstrated thanks to rock magnetic measurements. One, two or three magnetization components on 159 oriented specimens (14 sites) have been isolated using principal component analysis or great circles technique, mainly on doleritic dykes. High and low unblocking temperature components were isolated in six dykes but also in four sites of the well dated Hank stromatolite-bearing formation (875-890 Ma). For the two kinds of rocks, the main components show stable remanent magnetization mainly carried by magnetite (or titanomagnetite) as described using thermomagnetic or Isothermal Remanent magnetization curves, where no significant mineralogical transformation occurred at high temperatures.Five groups of paleopoles have been calculated: they witness the existence of a large circum-terrestrial loop with a drift from the high latitudes of the lower hemisphere to the high southern latitudes of the upper hemisphere followed by a re-entry in the high latitudes of the lower hemisphere. This is in agreement with the previous loop computed by Sabaté and Lomax (1975) between 2.1 Ga and 1.7 Ga. The Yetti and Eglab blocks amalgamated around 1.9 Ga and participated to the formation of Columbia Supercontinent. The paleopole computed for the stromatolite-bearing formation corresponds with the location of Rodinia Supercontinent at its early stages of amalgamation.The geodynamic evolution of the WAC respect with that of Columbia and Rodinia Supercontinents suggests that the Eglab and Yetti were clearly separated by a large oceanic crust before 1.9 Ga. A volcanic arc developed during the subduction of this ocean followed by crustal thickening which generated an intrusive suite (Aftout granites) when it was at the northern low latitudes. Fennoscandia and Rio de la Plata Cratons were close to the WAC after 1.7 Ga. Fennoscandia probably followed the same loop as the WAC between 1.7 and 0.9 Ga. In Africa, the collapse of the Birimian orogen was probably contemporaneous with the fragmentation of Columbia. At 0.9 Ga, Fennoscandia was probably not attached together with the WAC since the latter was not affected by the Grenvilian orogen, so did not necessarily have the same loop

Rock magnetic study on the Yetti-Eglab intrusions, Sahara: Contribution to the West African Craton geology.

International audienceNew palaeopoles have been calculated for the West African Craton (WAC) using palaeomagneticdata obtained on 178 cores of the Yetti–Eglab intrusions and stromatolite-bearing formation. One, two orthree components of magnetization on 172 (14 sites) and 37 (6 sites) oriented specimens or cores were isolatedmainly on doleritic dykes. High and low unblocking temperature components were evidenced in the dykes butalso at four sites of the well-dated Hank stromatolite-bearing formation (875–890 Ma). Rock magnetic analysesshow stable remanent magnetizations mainly carried by magnetite (or titanomagnetite) but may also reveal thepresence of hematite, pyrrhotite and goethite. Regarding the recently constructed apparent polar wander path ofthe WAC until 500 Ma, these newly computed remanent components seem to be mainly older, some of whichcould be remagnetizations at different periods. However, according to geological dating and literature, the Yettiand Eglab terranes belonging to theWACwere amalgamated around 1.9 Ga and involved in the formation of theColumbia Supercontinent. The palaeopole computed for the stromatolite-bearing formation corresponds with thelocation of Rodinia Supercontinent at its early stages of amalgamation. The geodynamic evolution of the WACwith respect to both supercontinents suggests that the Eglab and Yetti could be separated by oceanic crust before1.9 Ga. A volcanic arc developed during the subduction of this ocean followed by crustal thickening that generatedan intrusive suite (Aftout granites) when it was at low northern latitudes. Baltica and Rio de la Plata cratonswere close to the WAC after 1.7 Ga, following a nearly similar path between 1.7 and 0.9 Ga. In Africa, the collapseof the Birimian orogen was probably contemporaneous with the fragmentation of Columbia. At 0.9 Ga,Baltica was probably not attached to the WAC since the latter was not affected by the later Grenvillian orogen

Contraintes magnétostratigraphiques concernant la durée de l'interruption des communications marines en Méditerrannée occidentale pendant le Messinien supérieur

Une coupe stratigraphique du Messinien supérieur, épaisse de 72 m et composée essentiellement de grès fins et marnes bleues, a été échantillonnée près du village de Bouzghaïa (bassin du Chélif, Algérie). L\u27étude magnétique des 51 échantillons récoltés a permis d\u27isoler, selon les échantillons, une ou deux composantes d\u27aimantation portées par la pyrrhotite et/ou par la magnétite. Quatre zones à polarité normale ont été isolées. La base de la coupe est composée d\u27une zone à polarité normale " polluée " par des composantes mixtes. La limite Messinien-Zancléen est caractérisée par une zone de polarité normale assez longue (210 Ka), suivie par des zones à polarité inverse. Le sommet de la coupe est formé par les marnes du " Trubi ", caractéristiques du Pliocène inférieur. Cette étude a permis de préciser en outre la limite Messinien-Pliocène et la durée de l\u27interruption des apports marins dans le bassin méditerranéen occidental qui est ici de l\u27ordre de 700 Ka. La vitesse de sédimentation déduite de la coupe est d\u27environ 4 cm/Ka.An upper Messinian stratigraphic section, about 72 m thick, and composed mainly of fine-grained sandstones and of blue marls, has been sampled near the Bouzghaïa village (Chelif basin, Algeria). A magnetic study conducted on 51 samples shows, according to the samples, one or two components of magnetization carried either by pyrrhotite and/or magnetite. Four normal polarity zones were isolated. The base of the section is composed of a normal polarity zone disturbed by mixed components of magnetization. The Messinian-Zanclian limit is characterized by a long normal polarity zone (about 210 Ka), followed by a reversed polarity zone. The top of the section constituted by blue marls of the "Trubi" Formation is characteristic of the lower Pliocene. This study allowed to precise the Messinian-Pliocene limit and the duration of the marine communications interruption in the Western Mediterranean during the upper Messinian which is here around 700 Ka. In the section the sedimentation rate is close to 4 cm/Ka.</p

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

International audienceQuaternary and Pliocene travertines, deposited from hot springs, can reveal much about neotectonic and hydrothermal activity. Theaimof 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

Paleomagnetic and magnetostructural study of Jurassic formations of Gara Djebilet (Tindouf Basin, Southwestern Algeria)

International audienceRecent geochemical analyzes and 40Ar/39Ar datings of dolerite sills and dykes and basalticlava flows from Southwestern Algeria (Tindouf, Reggane, Bechar and Hank basins) haveshown that these rocks are related to the Central Atlantic Magmatic Province (CAMP). TheCAMP is one of the largest among the Mesozoic basaltic provinces identified and formed about200 Ma ago as a preamble to the breakup of Pangea. These data were solid arguments for undertakinggeological observations and sampling for paleomagnetic studies and magnetic fabrics onthe CAMP formations. A network of three long doleritic dykes (198.9 ± 1.8 Ma) located in theTindouf Basin were targeted to clarify the structural context of their emplacement (magneticfabrics) and to determine a new reliable Mesozoic pole.The magnetic fabric, in almost all the sections sampled, is defined mainly through groupingof k1 and k2 axes on the dyke plane whereas axis k3 is almost perpendicular to the dykeplane. This fabric can therefore be interpreted as the magma flow direction. The new Jurassicpaleomagnetic pole, of excellent quality, is very close to those obtained on detrital formationsof the Algerian Sahara and close to those recently determined on igneous formations of Morocco.This new pole is also very close to the North African pole at 200 Ma. These results representa new contribution for a better knowledge of the geodynamic context during this period

Paleomagnetic and magnetostructural study of Jurassic formations of Gara Djebilet (Tindouf Basin, Southwestern Algeria)

International audienceRecent geochemical analyzes and 40Ar/39Ar datings of dolerite sills and dykes and basalticlava flows from Southwestern Algeria (Tindouf, Reggane, Bechar and Hank basins) haveshown that these rocks are related to the Central Atlantic Magmatic Province (CAMP). TheCAMP is one of the largest among the Mesozoic basaltic provinces identified and formed about200 Ma ago as a preamble to the breakup of Pangea. These data were solid arguments for undertakinggeological observations and sampling for paleomagnetic studies and magnetic fabrics onthe CAMP formations. A network of three long doleritic dykes (198.9 ± 1.8 Ma) located in theTindouf Basin were targeted to clarify the structural context of their emplacement (magneticfabrics) and to determine a new reliable Mesozoic pole.The magnetic fabric, in almost all the sections sampled, is defined mainly through groupingof k1 and k2 axes on the dyke plane whereas axis k3 is almost perpendicular to the dykeplane. This fabric can therefore be interpreted as the magma flow direction. The new Jurassicpaleomagnetic pole, of excellent quality, is very close to those obtained on detrital formationsof the Algerian Sahara and close to those recently determined on igneous formations of Morocco.This new pole is also very close to the North African pole at 200 Ma. These results representa new contribution for a better knowledge of the geodynamic context during this period

Paleomagnetic and magnetostructural study of Jurassic formations of Gara Djebilet (Tindouf Basin, Southwestern Algeria)

International audienceRecent geochemical analyzes and 40Ar/39Ar datings of dolerite sills and dykes and basalticlava flows from Southwestern Algeria (Tindouf, Reggane, Bechar and Hank basins) haveshown that these rocks are related to the Central Atlantic Magmatic Province (CAMP). TheCAMP is one of the largest among the Mesozoic basaltic provinces identified and formed about200 Ma ago as a preamble to the breakup of Pangea. These data were solid arguments for undertakinggeological observations and sampling for paleomagnetic studies and magnetic fabrics onthe CAMP formations. A network of three long doleritic dykes (198.9 ± 1.8 Ma) located in theTindouf Basin were targeted to clarify the structural context of their emplacement (magneticfabrics) and to determine a new reliable Mesozoic pole.The magnetic fabric, in almost all the sections sampled, is defined mainly through groupingof k1 and k2 axes on the dyke plane whereas axis k3 is almost perpendicular to the dykeplane. This fabric can therefore be interpreted as the magma flow direction. The new Jurassicpaleomagnetic pole, of excellent quality, is very close to those obtained on detrital formationsof the Algerian Sahara and close to those recently determined on igneous formations of Morocco.This new pole is also very close to the North African pole at 200 Ma. These results representa new contribution for a better knowledge of the geodynamic context during this period