B. Le Gall

40Ar/39Ar geochronology and structural data from the giant Okavango and related mafic dyke swarms, Karoo igneous province, northern Botswana

Abstract In NE Botswana, the Karoo dykes include a major N110° dyke swarm known as the Okavango giant dyke swarm (ODS/N110°) and a second smaller set of N70° dykes belonging to the Sabi-Limpopo dyke swarm (SLDS/N70°). New 40Ar/39Ar plagioclase dating of Karoo dolerites of the giant ODS/N110° and the SLDS/N70° in NE Botswana yield plateau ages between 179.6±1.2 and 178.4±1.1 Ma. Our data are concordant with previous 40Ar/39Ar ages for Northern Karoo dykes and lava flows exposed in western Zimbabwe. The data are tightly clustered, indicating a short-lived (179–181 Ma) flood basalt magmatism in this region. The new radiometric dates allow the definition of a diachronous Jurassic flood basalt activity in southern Africa. A significant south to north younging at the scale of the Karoo igneous province correlates with a chemical zonation from low-Ti (south) to high-Ti (north) mafic rocks. Structural measurements on the ODS/N110° and SLDS/N70° Karoo dykes of NE Botswana suggest that: (1) most of the host fractures are inherited Precambrian structures; (2) dyke emplacement occurred under unidirectional tensional stresses; (3) significant syn- and post-volcanic extensional tectonics are lacking. Combined with regional geology, these geochronological and structural data do not confirm unambiguously the triple-junction hypothesis usually put forward to support a mantle plume model for the evolution of the Karoo igneous province, prior to Gondwana breakup.

Superimposed Variscan, Caledonian and Proterozoic features inferred from deep seismic profiles recorded between southern Ireland, southwestern Britain and western France

Abstract From southern Ireland to the northern Bay of Biscay, subduction was active from the Proterozoic to the late Palaeozoic. They led to the Cadomian, Caledonian and Variscan collisions. Features characterizing these three orogenies have been crossed by the WINCH, SWAT, WAM and Bay of Biscay deep seismic profiles recorded by the BIRPS and ECORS groups. In the Celtic Sea, the Variscan front is characterized by a S-dipping ramp rising directly from the middle crust with much less northward flattening than shown by the front in the northern France and West Germany profiles. South of the Variscan front, other S-dipping reflections occur above a possible deep detachment. The Lizard ophiolitic suture marks S-dipping events traversing the entire crust and the present Moho in the English Channel. Several sets of prominent N-dipping reflections occurring in the Celtic Sea and south Irish Sea are interpreted as Caledonian features running parallel to the Caledonian Iapetus suture and the pattern of folds and faults recognized in Wales and southeastern Ireland. Proterozoic S-dipping subduction is recorded by geological data in northern Wales and the English Channel. Related crustal structures correspond to deep seismic events cutting across the lower crust and the Moho and show the same apparent dip as the Variscan features. Parallelism between dips and/or strikes of structural features with different ages suggests a tectonic heritage. The crust thus appears to be characterized by N and S-dipping features which cut across large parts of it and may be assigned to Variscan, Caledonian or Candomian thrusts or shear zones. The lower part of the crust shows a band of high-amplitude subhorizontal reflections and a flat Moho reflection, which seem to have overprinted the above-mentioned dipping features in a later period. The lack of evidence of any shallow detachment in the Caledonian and Variscan belts together with the presence of dipping crustal thrust-like features suggest that the region may have reached the very latest collisional stage, the end of both of these orogenies.

Palaeo-stress fields in the Variscan foreland during the Carboniferous from microstructural analysis in the British Isles

Abstract North of the Variscan front the Carboniferous series of the British Isles is affected by mainly N-S trending folds which are almost perpendicular to this E-W oriented front. Flat and unconformable overlying Permian strata demonstrate a Variscan age for the deformation. A microtectonic survey made in Dinantian (and to a lesser extent Namurian) calcareous layers has revealed two successive events. According to chronological criteria, the first event (maximum stress E-W trending) led to the formation of N-S folds with local reorientation in the vicinity of major pre-existing faults. This compression may have been active since the Namurian. It pre-dates a second compressional event orientated N-S which is related to the final emplacement of the Variscan nappes.

New palynological data from the Gramscatho Group at the Lizard Front (Cornwall): palaeogeographical and geodynamical implications

Palynological investigation of the terrigenous Gramscatho Group Formations dates the uppermost part of the Porscatho Formation (lower Gramscatho) as being of Frasnian age. This suggests a late Devonian age (Famennian?) for the chaotic Meneage Formation (mid-Gramscatho) which overlies the dated fossiliferous levels. During the Upper Devonian, the parautochthonous Gramscatho basin thus appears as a fault controlled basin which registers, by both a chaotic sedimentation and a volcanic activity, the earlier Variscan tectonic events active in more southern internal zones. The presence of reworked Ordovician acritarchs, in some lower Gramscatho horizons, constitutes a valuable tool for the palaeogeographical evolution pattern of the Devonian of this Cornwall area.

The Karoo Large Igneous Province: brevity, origin and relation with mass extinction in question from new 40Ar/39Ar age data.

[Riley et al. (2006a)][1] question our interpretations of 40Ar/39Ar data obtained on the Karoo flood basalts ([Jourdan, et al., 2005][2]). We believe their arguments are partially based on (1) poorly reliable 40Ar/39Ar age data (e.g., analyses of whole rocks that might have suffered hardly

Traces of Caledonian and Proterozoic crustal features in deep seismic profiles recorded between France and the British Isles

Abstract The area between France and the British Isles has undergone three major orogenies since Proterozoic times, the Cadomian-Monian (late Proterozoic), Caledonian (late Silurian-early Devonian) and Variscan (Devonian-Carboniferous). This paper shows that crustal structures related to the first two orogenies can be recognized in deep seismic profiles in the English Channel, the Celtic Sea and the Irish Sea. In the northernmost area. remarkable north-dipping reflections in the deep crust of the Celtic and Irish Seas may be correlated with surface structures and interpreted as Caledonian south-verging thrusts parallel to the lapetus suture. These features occur as far south as the Cornubian ridge (Cornwall), within the northern segment of the Variscan north-verging thrust belt. The latter is characterized by south-dipping reflections interpreted as Variscan thrusts. Caledonian thrusts were probably re-activated by the Variscan compression. Evidence of Caledonian thin-skinned detachment cannot be recognized in the seismic profiles. It is possible that either detachment structures were strongly deformed and obliterated by late Caledonian thrusting, or that detachment may never existed. Caledonian shortening was probably dominated by deep-rooted crustal thrusts as a result of the overall obliquity of the collision. Caledonian thrusts are regarded as very late features formed when the crust between the colliding continents was already relatively thick. They are distributed throughout the southern Caledonides including the lapetus suture area. Deep south-dipping reflections in the lower crust in the southern Irish Sea and the English Channel may be related to the Cadomian-Monian orogenic event. The Cadomian, Caledonian and Variscan features have probably been re-activated a number of times.