Olivier Averbuch

Magnetic fabric as a structural indicator of the deformation path within a fold-thrust structure: a test case from the Corbières (NE Pyrenees, France)

Abstract In order to define the deformation path and the distribution of internal deformation in a foreland foldthrust structure, the anisotropy of magnetic susceptibility was measured in 150 samples taken from 11 sites throughout the Vitrollian sequence of the Lagrasse structure (most external Pyrenean Zone). Whereas surrounding rocks seem completely undeformed, the Vitrollian Formation exhibits a complete set of microstructural markers thereby allowing fault striation measurements at sites also sampled for magnetic fabric analysis. Structural analysis shows that two mechanisms acted successively during thrust motion. The first one resulted from a NW-SE layer parallel shortening (LPS). It was responsible for a well defined vertical magnetic foliation parallel to the regional cleavage, in which a vertical magnetic lineation developed in conjunction with an increase in the shortening intensity towards the zone which would subsequently fail, leading to the development of an imbricate thrust system. During this second event, internal deformation was concentrated around the hinge and in the forelimb of the frontal fold. Around the hinge, the magnetic lineation rotated to an horizontal NE-SW trend parallel to the fold axis, indicating a significant component of longitudinal stretching during the development of the frontal ramp-anticline. Subsequently, the magnetic lineation swung again and became parallel to the NW-SE tectonic transport direction beneath out-of-sequence forelimb thrusts. This complex deformation history is not accompanied by a linear increase in the degree of anisotropy. On the contrary, we observe a cycle of construction and destruction of the magnetic fabric during LPS and imbricate thrusting.

First paleomagnetic evidence for a post-Eocene clockwise rotation of the Western Taurides thrust belt east of the Isparta reentrant (Southwestern Turkey)

Abstract A paleomagnetic study was carried out on 23 sites sampled in Paleocene, Eocene and Lower Miocene sedimentary formations of the Western Taurides belt east of the Isparta reentrant. The paleomagnetic data, based on twelve suitable sites, record a coherent post-Eocene ∼ 40° clockwise rotation throughout the area studied. We interpret this regional rotation as a semi-rigid rotation of the whole Western Taurides belt above a major deep decollement level. Together with previously published data, these new results suggest that the Isparta reentrant would result from diachronous opposite rotations of the two branches: first a late Eocene-Oligocene clockwise rotation of the Akseki-Beysehir Taurides initiating the Tauride arc, and then anticlockwise rotation of the Lycian Taurides, probably during Middle Miocene times.

Dynamics and inversion of the Mesozoic Basin of the Weald-Boulonnais area: role of basement reactivation

Abstract The geometry and dynamics of the Mesozoic basins of the Weald–Boulonnais area have been controlled by the distribution of preexisting Variscan structures. The emergent Variscan frontal thrust faults are predominantly E–W oriented in southern England while in northern France they have a largely NW–SE orientation. Extension related to Tethyan and Atlantic opening has reactivated these faults and generated new faults that, together, have conditioned the resultant Mesozoic basin geometries. Jurassic to Cretaceous N–S extension gave the Weald–Boulonnais basin an asymmetric geometry with the greatest subsidence located along its NW margin. Late Cretaceous–Palaeogene N–S oriented Alpine (s.l.) compression inverted the basin and produced an E–W symmetrical anticline associated with many subsidiary anticlines or monoclines and reverse faults. In the Boulonnais extensional and contractional faults that controlled sedimentation and inversion of the Mesozoic basin are examined in the light of new field and reprocessed gravity data to establish possible controls exerted by preexisting Variscan structures.

Chapter 8Productivity and bottom water redox conditions at the Frasnian-Famennian boundary on both sides of the Eovariscan Belt: constraints from trace-element geochemistry

Abtract The Late Devonian Frasnian-Famennian (F-F) mass extinction event coincides in many places with the depositionof C org -rich “Kellwasser” facies. Four F-F boundary sections representative of platform and basin environments from widely separated locations (Morocco, Germany, and France) were analysed for inorganic geochemistry, especially trace elements (redox and productivity proxies), in order to describe paleodepositional environments for the Kellwasser horizons. Ni/Co, V/Cr, U/Th, and V/(V+Ni) ratios, as well as redox trace metal concentrations indicate that oxygen-depleted conditions existed during the times of Kellwasser facies deposition. In platform settings, dysoxic conditions seem to be limited to the Late Frasnian. In basinal settings, oxygen depletion was stronger and persisted into the Early Famennian. Enrichments of Ba, Cu, Ni, that are limited to the Late Frasnian, show that surface productivity was relatively high and organic matter could accumulate, especially in the deeper environments. The stratigraphical distribution of several geochemical markers are linked with two positive excursions of the δ 13 C carb signal that result from enhanced organic matter burial. Reducing conditions likely resulted from high productivity of Late Devonian marine ecosystems. Intense nutrient supply resulted probably from the biogeochemical recycling of nutrients, and/or runoff from emerged lands. Coupled with other factors, such as rapid sea-level fluctuations and climatic changes, oxygen-depleted conditions and eutrophication would have modified Late Devonian environments and could be possible factors in the F-F mass mortality.

Fabric development and metamorphic evolution of lower Palaeozoic slaty rocks from the Rocroi massif (French–Belgian Ardennes): new constraints from magnetic fabrics, phyllosilicate preferred orientation and illite crystallinity data

Abstract This paper presents new results of a petrofabric study from Cambrian slates cropping out within the anchi–epimetamorphic core of the Rocroi massif (Ardennes Variscan fold-thrust belt of northern France and southern Belgium). This study includes measurements of the Anisotropy of the Isothermal Remanent Magnetisation (AIRM) and the phyllosilicate preferred orientation. It completes the previous results of Anisotropy of Magnetic Susceptibility (AMS) and of magnetic mineralogy which were obtained in 47 sites situated in and around this massif. These new data allow the characterisation of the different subfabrics carried by the ferromagnetic minerals (metamorphic pyrrhotite and magnetite) and the paramagnetic matrix (Fe-rich phyllosilicates). Combined with some illite crystallinity data measured throughout the massif, the petrofabric measurements evidence a polyphase tectonometamorphic evolution. A major Variscan compressional event is evidenced inducing the growth of highly anisotropic pyrrhotite, coarse-grained hematite and phyllosilicates. By contrast, the magnetite grains display a less well-organised fabric due to the existence of an inherited orientation pattern. In agreement with recent metamorphic studies, it argues for an early diastathermal metamorphic event developed during a Devonian crustal-scale extension.

The Variscan belt of Northern France–Southern Belgium: geodynamic implications of new palaeomagnetic data

Abstract Palaeomagnetic investigations were carried out in Devonian–early Carboniferous rocks of the Variscan foreland chain of Northern France–Southern Belgium in order to reveal the origin of its arcuate shape. The Brabant Parautochthon was sampled in the Boulonnais (near Calais) and near Tournai, while the Ardenne Allochthon was sampled near Maubeuge and in the Givet area. All the sampled localities yielded characteristic remanent magnetization as a result of stepwise demagnetization and component analysis. Fold or tilt tests were possible for three localities, with negative results indicating pervasive remagnetization. The tectonic position was sub-horizontal at two localities, while the tilt was monoclinal for the rest. Therefore, the acquisition time of the magnetic signals was estimated by comparing the palaeolatitude computed from each magnetic component to the palaeolatitudes of Variscan Europe calculated after Van der Voo (1993) . Three components showing: A, a southern B, a near-Equatorial, and C, a northern palaeolatitude are recognized from our data. Since a pre-Variscan age of component A (observed only in Boulonnais, at 10 sites) is not supported by data, it is assigned to an early phase of deformation. Component B (16 sites) was acquired during the peak of the Variscan tectonics (late Westphalian), while component C (five sites) originated during Permian times. Regardless of the palaeolatitudes, declinations fall between 190 and 210°, thus being conformable with the declinations expected for Variscan Europe. The declinations show no correlation with the arcuate shape of the belt, neither are they different in the Paraauthochthon and in the Allochthon, nor in the different components. Arc formation by moulding of the Allochthon on the Brabant Parautochthon is, therefore, not supported by these data, since this mechanism requires substantial (opposed) rotations on both wings of the arc. The available palaeomagnetic data are conformable with a pre-formed arc, simply docking to the Brabant obstacle of similar shape. Variable offsets along a main thrust make possible a third model, which slightly unfolds the former passive arc.

Environmental changes at the Frasnian-Famennian boundary in Central Morocco (Northern Gondwana): integrated rock-magnetic and geochemical studies.

Abstract Rock magnetic (magnetic susceptibility and hysteresis parameters) and geochemical analyses (major and trace elements) were carried out on whole rock samples of two Frasnian–Famennian boundary sections, Anajdam and Bou-Ounebdou in the Central Morocco (Western Meseta). During the Frasnian, the decreasing trend of the magnetic susceptibility signal, mainly carried by low-coercivity magnetite grains, indicates a gradual reduction of detrital influx. This decrease in detrital input parallels a Frasnian long-term sea-level rise. In the Late Frasnian Kellwasser Horizons, that are classically considered to represent highstand deposits, the magnetic signal exhibits the lowest intensities in connection with maximum diamagnetic contribution of the carbonate fraction. With respect to geochemical data, the two black carbonate-rich Kellwasser Horizons are characterized by noticeable positive anomalies of bottom-water dysoxic proxies and of marine primary productivity markers. Our data thus suggest that in Central Morocco, the Late Frasnian marine environments were marked by a relatively important biogenic productivity favouring the onset of oxygen-depleted conditions during periods of maximum transgression on the continental platforms.

Syn-folding remagnetization events in the French-Belgium Variscan thrust front as markers of the fold-and-thrust belt kinematics

New paleomagnetic studies have been carried out within the Ardennes segment of the N France - S Belgium Variscan fold-and-thrust belt to set constraints on the fold-thrust belt kinematics and reveal the casual relationships between vertical-axis rotations and major strike deviated zones localised along the general trend of the belt. Magnetite-bearing Devonian and Carboniferous limestones yielded two characteristic, secondary components of the natural remanent magnetization : a low temperature component recorded most probably during the late stages of folding and a high temperature component, acquired during incipient stages of deformation. Both post- and synfolding magnetizations were identified in the Lower Devonian hematite bearing sandstones. Ages of magnetization, inferred from the analysis of characteristic remanence inclinations compared to the reference curves for the stable parts of the Old Red Sandstones Continent (ORC), suggest the previous remagnetization event to be due to the burial of sedimentary rocks under the thick molassic foreland basin of Namurian-Westphalian age and the second to the final out-of-sequence activation of the thrust front in Stephanian times. Irrespective of the age of the magnetizations, orientations of paleomagnetic directions are dominantly governed by second-order structural trends. Clockwise rotations are observed in relatively narrow zones featuring deviated orientations of fold axes, other sites show paleomagnetic directions akin to those known from the ORC. We interpret this feature as a result of local transpressive deformations and related rotations, which occurred at lateral borders of propagating thrust-sheets. The latter deformation zones are suggested to be controlled by deep-seated discontinuities inherited from the Devonian Rheno-hercynian basin development. The Ardennes thrust belt was thus not rotated as a whole unit with respect to the ORC after the Namurian, preserving the initial orientation of the continental margin.

The ‘Basse-Normandie’ duplex (Boulonnais, N France): evidence for an out-of-sequence thrusting overprint

Abstract The thrust-sheets outcropping in the ‘Basse-Normandie’ quarry (near the Hydrequent village, Boulonnais, N France) represents an exceptionally well exposed section of the NW European Variscan thrust front. These structures, developed in the footwall of the main Hydrequent thrust, have been often described as a classic example of a duplex structure. Only the lower part of the structure satisfies, however, the geometric and kinematic criteria defining a duplex (and more precisely an intraformational hinterland dipping duplex). The upper thrust-sheets of the imbricate stack exhibit a much more complex pattern of deformation than a simple piggy-back duplication of the same rock sequence. Restoration of these thrust-sheets (based upon the definition of two marker beds as well as the analyses of fold—thrust relationships and strain markers) argues for a late NE verging thrust event that progressed within the thrust system from the tip of the upper thrust sheets towards the Hydrequent thrust in a local break-back style of thrust propagation. This out-of-sequence thrusting event induced refolding and cross-cutting of the forelimb of a hangingwall anticline developed previously above the footwall ramp of the NNE verging basal thrust of the structure. Within the whole thrust system, the lower duplex represents only a minor structure developed during the initial phase of thrusting in the foreland of the major anticline as a frontal second-order duplex. The structural data presented in this paper illustrate the tectonic processes acting within the deformed zones lying in the footwall of major thrusts and emphasize the out-of-sequence style of thrust migration that arises from the sequential blocking of thrust propagation towards the foreland.

Geometry and kinematics of the Boulonnais fold-andthrust belt (N France): implications for the dynamics of the Northern Variscan thrust front

Recent structural data obtained within the Boulonnais thrust belt along the Northern Variscan thrust front provide a new insight into the deformation history of the Variscan belt. The zone under study recorded the latest increments of inversion of the Rheno-hercynian basin in Late Westphalian—Early Stephanian times (ca 305 Ma). Thrusting within the southern margin of the “Old Red Sandstones Continent” propagated primarily in sequence up to the Brabant massif which acts as a significant buttress. This buttress effect, which we suggest to be due to the southward activation of deep-seated inherited Caledonian thrusts, resulted in the out-of-sequence dislocation of the thrust front. The associated localized thrust stack induced a sharp flexure of the underthrust block moulding the border of the molassic coal-bearing foreland basin. During this process, strain markers at the thrust front document an evolution of thrusting from a dominantly NNE to NE direction whereas more internal parts of the belt undergo dextral transcurrent deformations (e.g. the Bray fault zone). Such a partition of the deformation argues for a general transpressional setting of the SE England-Boulonnais orogenic belt acting as a relay zone between more frontal parts of the Variscan orogen (Ardenno-Rhenish and SW England segments). This mechanism highlights the segmented nature of the Variscan thrust front and points out the importance of the Devonian basin pre-structuration in the present-day arcuate shape of the Variscan belt. Late Variscan reactivation of the thrust network occured within a transtensional setting associated to a dominant NNW-SSE directed extension. This event induced the formation of local intramontane basins and contributed to the thinning and thermal restoration of the thickened external Variscan crust in Upper Stephanian—Middle Permian times.