Jean-Pierre Lefort

Sedimentological evidence for a limited separation between Armorica and Gondwana during the Early Ordovician

The vast extent of sedimentation deposits making up the Gres Armoricain and its equivalents over the north Gondwanan domain is used in this study to test the hypothesis of continuity between the Armorica and Gondwana plates during the Early Ordovician. The distribution of continental and marine environments, the large volume of transported elastic material, and the consistent current directions all indicate that the source of sediment supply on the north Gondwanan margin was to the south of the Sahara. Although there were local signs of incipient rifting between the two plates, Armorica was not yet detached from Gondwana by Arenigian time.

Iberian-Armorican arc and Hercynian orogeny in western Europe

Recently assembled geological and geophysical evidence from southern Brittany reveals the organization of submerged structures off the southern Brittany coast. Fragments of a high-pressure–low-temperature belt are caught between a northern high-temperature zone of middle Paleozoic age, and a southern zone of intermediate metamorphism. It is not known whether the rocks in the high-pressure–low-temperature belt belong to a middle Paleozoic allochthonous slab or whether they are parautoch-thonous and the result of Carboniferous obduction. Farther to the south a major geophysical discontinuity is interpreted as a suture, the trace of which cuts across the sedimentary facies zonation established between the Armorican Massif and Spain. It is therefore suggested that the formation of the Iberian-Armorican arc predates the Hercynian orogeny. A geodynamic model is proposed that attempts to synthesize the orogenic mechanisms in Spain and southern Brittany during middle and late Paleozoic time.

Gravity evidence for an Alpine buckling of the crust beneath the Paris Basin

Abstract The processing of the gravity data of the Paris Basin has enlightened the existence of crustal ridges. These ridges correspond to the zones of different crustal thicknesses. In plan view, the ridges are concentric with the Alpine orogen and were probably controlled by this Tertiary Alpine mountain building. The thinned zones related with the Mesozoic and pre-Mesozoic basins, were already existing before the Alpine deformations. These are of limited size and clearly associated with the basins. On a large scale, two different domains have been recognized. The first one corresponds with a zone where the crust is 37 km thick. It is also the place where the Tertiary depocentres developed. The second one relates with a zone where the crust is 29–30 km thick. It has been uplifted during the Alpine orogeny. These two observations suggest the existence of a Tertiary lithospheric buckling beneath the Paris Basin.

Topography of the Moho undulations in France from gravity data: their age and origin

Abstract The complete gravity data set from France and part of the neighboring countries has been analyzed to compute the topography of the Moho undulations. This work is based on an improved filtering technique and an appropriate assumed density contrast between the crust and the upper mantle. Comparison with deep seismic refraction data reveals that this relief map expresses the continuity and geometry of the Moho undulations better than the sparsely distributed seismic refraction data in France. This gravity Moho map, though may not give absolute depths at places, provides a far better correlation with surface geology than the result from other geophysical techniques. Four domains have been recognized: (a) the Alpine domain where all the Moho undulations are concentric with the Alps; (b) the Armorican domain in which all the undulations are north-west/south-east oriented; (c) the Pyrenean domain, in which the undulations are parallel with the Mountain chain; and (d) the Massif Central Domain which does not show clear structural orientation because of the influence of the strong heat flow located at the lower crust/upper mantle interface. Study of the topography and of the superficial structures associated with these undulations reveals that the undulations delineated in the Alpine Domain result from the Tertiary compression which shaped the Alps. The Armorican Domain was first created during the Lower to Middle Cretaceous opening of the Bay of Biscay. It is now slightly affected by the Tertiary to Quaternary closure of this Bay. The Pyrenean Domain was successively shaped by the Lower Cretaceous oblique opening of the Bay of Biscay and by the Upper Cretaceous to Eocene northward displacement of Spain. Comparison between the Moho undulations map and the stress map of France reveals that most of the undulations are perpendicular to the actual shortening directions. This observation suggests that the Mesozoic, Cenozoic and Quaternary stress directions were roughly the same. Massif Central is characterized by the convergence of these three sets of undulations. Its Post-Oligocene uplift was probably the result of the converging stresses recognized in the three surrounding domains. When the Moho undulations and the topography are compared, two types of periodic crustal instabilities can be recognized. One corresponds to the buckling of the crust developed under compression, the other to boudinage which was associated with extension. Both phenomena show a typical wavelength of 200–250 km which is in agreement with the results of the actual physical and numerical modeling currently available.

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.

Possible “Caledonian” subduction under the Domnonean domain, North Armorican area

Geological and geophysical data recently gathered in the English Channel permit delineation of the main basement structures beneath the Mesozoic and Cenozoic cover. These are, from north to south, (1) a Paleozoic trough, (2) a mafic rock complex, probably ophiolitic, and (3) a Precambrian craton, with a thin Paleozoic cover restricted to some grabens. This disposition suggests the presence of a cryptic suture. Arguments based on stratigraphy (the terranes grow older to the north), structure (faults are parallel to the mafic rock complex, and horsts and grabens are found in the craton), geochronology (the pre-Hercynian intrusions show a distinct arrangement in time and space), petrography (calcalkalic intrusions and an arc of ignimbrites are found), and paleontology (two faunal realms existed during early Paleozoic time) suggest that subduction could have occurred between late Proterozoic and middle Paleozoic time under the Domnonean domain.

Geophysical evidence for an east-dipping Appalachian subduction zone beneath Newfoundland

Geophysical data from within and adjacent to Newfoundland indicate the existence of an undulating but regionally continuous layer of ultramafic rocks generally dipping to the southeast beneath the Avalon zone of Newfoundland. This is interpreted to indicate the southeastward subduction of an early Paleozoic oceanic crust. The ophiolites of western Newfoundland (those both detached and rooted), the granites of north-central Newfoundland, and the Gander ultramafic zone occupy positions that are unique in relation to the ultramafic layer. Northeast of Newfoundland and in New Brunswick, “incomplete” continental suturing has preserved geological zones not seen in Newfoundland.

Seismic Reflection Images of the Pre-Mesozoic Crust in France and Adjacent Areas

The contribution of deep seismic profiling to our understanding of the pre-Mesozoic crust in France and adjacent areas is of the utmost importance. Despite limitations which will be discussed in Section 1.2, the method can obtain images of the crust down to and deeper than 50 km. It is the only way to observe the deep structure of the crust in some detail, to identify features such as thrusts, decollements or detachments, decoupling levels, deep faults, etc., to trace the Mohorovicic Discontinuity and image upper mantle discontinuities in favorable cases.

Contribution à l'étude d'une partie du bassin d'Essaouira (Maroc) par sismique réflexion

Abstract A large number of seismic reflection lines and boreholes have been carried out in the Essaouira Basin by the oil industry. The present study concentrates on the reinterpretation of these data in the restricted area of Khemis Meskala, in order to better characterise the structure of the Cretaceous aquiferous system. The reflector corresponding to the bottom of the Vraconian formation has been identified on the different seismic sections. This horizon, which marks the base of the aquiferous system, was first digitised on time migration sections and then converted to depth sections using a suitable linear velocity law. The isobath map of the bottom of the Vraconian resulting from this study images the 3D geometrical structure of this horizon and shows that it is slightly folded in domes and basins. This document will be useful for rationalising the future hydrogeological researches that will be undertaken in the Khemis Meskala area. To cite this article: M. Jaffal et al., C. R. Geoscience 334 (2002) 229–234.

Modelling of structures representing the South Armorican suture

Abstract The modelling of magnetic structures, which constitute a linear “marker” extending 600 km from the continental shelf off South Brittany to the northern part of the Aquitaine Basin, has led to the recognition of a major crustal fault zone; this is confirmed by the existence of other geophysical markers (gravity and seismicity). They all can be interpreted in the light of an inter-plate suture model.