In NW Europe, the late Carboniferous Variscan collision between the Laurussia and the Armorica-Gondwana continental blocks led to the development of a crustal-scale north-verging thrust system along the southern Laurussian margin. In northern France, the 3-D geometry and kinematics of the Variscan deformation front have been investigated on the basis of reprocessing and interpreting 532 km of industry-level seismic reflection profiles. This extensive seismic imaging provides new constraints on the structural and kinematic features of the orogenic front. It particularly emphasizes the localization of displacement along the main frontal thrust zone that accommodated more than 50 km of total displacement of the allochthonous units above the foreland. It also highlights the induced large underthrusting of the foreland basin below the frontal thrust zone, and its truncation in a general outof-sequence mode of thrust propagation. We built structural maps that led to better delineating a major NW-SE lateral ramp along the main frontal thrust. The Mid-Upper Devonian series within the flexured foreland were deformed at depth by N060-080° trending and N110-130° trending syn-sedimentary normal faults that led to their south-to southwestward thickening. These pre-existing structures along the margin have exerted a primary control on the ensuing dynamics and geometry of the Northern Variscan Front by localizing both the frontal and lateral ramps during thrust wedge growth.LAURENT ET AL.
<p>In the general context of the development of renewable energy in the Hauts-de-France region (N France), some growing interest has been focused recently on the potential of deep geothermy. This area displays favourable conditions due to the burial of a regionally well-defined reservoir, i.e. the Dinantian karstic and brecciated limestones (Lower Carboniferous, 360-330 Ma), below the Nord-Pas-de-Calais coal-bearing Upper Carboniferous basin, developed by flexural subsidence in the foreland of the Northern Variscan frontal thrust system. The predominance of shales within the molassic basin as well as within the basal units at the floor of the thrust wedge (the Lower Devonian clastic units) are furthermore likely to form a large-scale permeability barrier potentially favouring the localization of hot waters within the underlying carbonate reservoir. The occurrence of a Dinantian regional geothermal resource has already been proven in Southern Belgium in the Hainaut coal basin area (the eastern prolongation of the Northern France coal basin) where the temperature in three geothermal wells reaches about 70&#176;C.</p> <p>To provide further constraints on such potential deep geothermal field in a structurally complex setting (a laterally segmented thrust front), the geometry of the Dinantian reservoir in northern France has been investigated through the integration and interpolation in a 3-D model of a large database including 1&#160;128 boreholes and 532 km of reprocessed, interpreted and depth-converted seismic reflection profiles. The results of the 3-D modelling indicate that the Dinantian reservoir is present at depth over a large area covering approximately 7675 km&#178; in northern France-southern Belgium. It extends at least 30 to 40 km south of the coal mining district area, underneath the Ardennes Allochthonous Unit of the Northern Variscan Front. The Dinantian reservoir is less than 200 m deep in the Lille metropolitan area and strongly deepens southward through a sharp flexure. It reaches 1000-3000 m depth underneath the coal basin and a maximum of 6944 m depth at the southern end of the study area. Overall, the Dinantian reservoir is structured along two main directions oriented N70-80&#176; and N110-130&#176;, related respectively to deep frontal Variscan thrusts and lateral-oblique ramps. The Dinantian reservoir ends west of Douai against a major complex lateral ramp system forming a first-order transfer zone within the Northern France Variscan thrust front. The latter localizes a set of strongly dipping N110-N130 faults (the Artois faults) representing second-order structures produced during subsequent deformation periods i.e. the Late Carboniferous-Permian rifting event and the Tertiary inversion related to the far-field accommodation of the Alpine-Pyrenean shortening.</p>
<p>In NW Europe, the Upper Carboniferous Variscan collision between Avalonia and the Armorica-Gondwana accretion complex led to the progressive tectonic inversion of the southern Avalonian margin and the development of a crustal-scale north-vergent thrust system propagating outward from the Late Mississippian to the Middle Pennsylvanian (330-305 Ma). The northern Variscan thrust front spreads over 2,000 km across NW Europe. In the Nord-Pas-de-Calais (NPC) coal district area (northern France), its 3D geometry and kinematics have been investigated through the reprocessing and interpretation of 532 km in length of industrial seismic reflection profiles acquired in the 1980s. The seismic interpretations point out the major compressional and extensional tectonic features affecting this fossil, deeply eroded, mountain front, highlighting its very atypical structure and kinematics.</p><p>The deformation front is characterized by a main frontal thrust zone localizing most of the northward displacement (i.e. several tens of kilometers) of the Ardennes Allochthonous Unit above the slightly-deformed part of the Avalonian margin, referred to as the Brabant Para-autochthonous Unit. This large displacement induced the underthrusting of the molassic foreland basin (NPC coal basin) over nearly 20 km and was associated to the out-of-sequence dislocation of the mountain front. The underthrust Brabant Para-autochthonous Unit, made of both the Namurian-Westphalian (330-305 Ma) molassic foreland basin and the underlying Mid-Upper Devonian (390-360 Ma) and Dinantian (360-330 Ma) carbonate platform, is deformed by a series of second-order north-vergent thrust faults, often associated with ramp-related folds. These thrust faults are rooted in d&#233;collement zones located either at the transition between the Namurian shales and the Dinantian carbonates or in the Famennian shales.</p><p>The 3D integration of the seismic interpretations led to the characterization of a major lateral ramp oriented NW-SE, affecting both the main frontal thrust zone and the basal thrust of some Overturned Thrust Sheets developed at its footwall. This lateral ramp represents a major zone of relay along the thrust front, in between two major segments, oriented respectively ENE-WSW to the east and WNW-ESE to the west. At the base of the underthrust Brabant Para-autochthonous Unit, the Mid-Upper Devonian platform is shown to be structured by synsedimentary normal faults responsible for the southward deepening and thickening of the southern Avalonian margin. These faults are oriented along two main directions i.e. N060-080&#176; and N110-130&#176;, that is the general orientation of the future Variscan structures. Overall, the results indicate that the Devonian pre-structuration of the southern Avalonian margin exerted a primary control on the dynamics and segmentation of the Northern Variscan Front in northern France by localizing both the frontal and lateral ramps within the thrust wedge.</p>
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