3‐D structure of the Variscan Thrust Front in northern France : New insights from seismic reflection profiles

Tectonics ◽  
2021 ◽  
Author(s):  
A. Laurent ◽  
O. Averbuch ◽  
L. Beccaletto ◽  
F. Graveleau ◽  
F. Lacquement ◽  
...  
Keyword(s):  
Seismic Reflection ◽  
Northern France ◽  
Thrust Front ◽  
Seismic Reflection Profiles

3D geometry and kinematics of the Northern Variscan Thrust Front in Northern France: new insights based on reprocessing and interpretation of seismic reflection profiles.

2021 ◽  
Author(s):  
Aurore Laurent ◽  
Olivier Averbuch ◽  
Laurent Beccaletto ◽  
Fabien Graveleau ◽  
Frédéric Lacquement ◽  
...  
Keyword(s):  
Seismic Reflection ◽  
Foreland Basin ◽  
Northern France ◽  
Thrust Front ◽  
3D Geometry ◽  
Mountain Front ◽  
Thrust Zone ◽  
Seismic Reflection Profiles ◽  
Nw Europe ◽  
Geometry And Kinematics

<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 décollement 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° and N110-130°, 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>

scholarly journals Spatial Relationships between Pockmarks and Sub-Seabed Gas in Fjordic Settings: Evidence from Loch Linnhe, West Scotland

Geosciences ◽  
2021 ◽  
Vol 11 (7) ◽  
pp. 283
Author(s):  
Allan Audsley ◽  
Tom Bradwell ◽  
John Howe ◽  
John Baxter
Keyword(s):  
Water Column ◽  
Seismic Reflection ◽  
Hot Spot ◽  
Geophysical Data ◽  
Spatial Relationships ◽  
Hydrocarbon Gases ◽  
Offshore Industry ◽  
Seismic Reflection Profiles ◽  
Spot Analysis ◽  
Sedimentary Carbon

Sub-seabed gas is commonly associated with seabed depressions known as pockmarks—the main venting sites for hydrocarbon gases to enter the water column. Sub-seabed gas accumulations are characterized by acoustically turbid or opaque zones in seismic reflection profiles, taking the form of gas blankets, curtains or plumes. How the migration of sub-seabed gas relates to the origin and distribution of pockmarks in nearshore and fjordic settings is not well understood. Using marine geophysical data from Loch Linnhe, a Scottish fjord, we show that shallow sub-seabed gas occurs predominantly within glaciomarine facies either as widespread blankets in basins or as isolated pockets. We use geospatial ‘hot-spot’ analysis conducted in ArcGIS to identify clusters of pockmarks and acoustic (sub-seabed) profile interpretation to identify the depth to gas front across the fjord. By combining these analyses, we find that the gas below most pockmarks in Loch Linnhe is between 1.4 m and 20 m deep. We anticipate that this work will help to understand the fate and mobility of sedimentary carbon in fjordic (marine) settings and advise offshore industry on the potential hazards posed by pockmarked seafloor regions even in nearshore settings.

scholarly journals Inverted Basins by Africa–Eurasia Convergence at the Southern Back-Arc Tyrrhenian Basin

Geosciences ◽  
2021 ◽  
Vol 11 (3) ◽  
pp. 117
Author(s):  
Maria Filomena Loreto ◽  
Camilla Palmiotto ◽  
Filippo Muccini ◽  
Valentina Ferrante ◽  
Nevio Zitellini
Keyword(s):  
Seismic Reflection ◽  
Magnetic Data ◽  
Normal Faults ◽  
Sedimentary Sequence ◽  
Initiation Zone ◽  
Arc Setting ◽  
Seismic Reflection Profiles ◽  
Back Arc ◽  
Flower Structures ◽  
Tyrrhenian Basin

The southern part of Tyrrhenian back-arc basin (NW Sicily), formed due to the rifting and spreading processes in back-arc setting, is currently undergoing contractional tectonics. The analysis of seismic reflection profiles integrated with bathymetry, magnetic data and seismicity allowed us to map a widespread contractional tectonics structures, such as positive flower structures, anticlines and inverted normal faults, which deform the sedimentary sequence of the intra-slope basins. Two main tectonic phases have been recognised: (i) a Pliocene extensional phase, active during the opening of the Vavilov Basin, which was responsible for the formation of elongated basins bounded by faulted continental blocks and controlled by the tear of subducting lithosphere; (ii) a contractional phase related to the Africa-Eurasia convergence coeval with the opening of the Marsili Basin during the Quaternary time. The lithospheric tear occurred along the Drepano paleo-STEP (Subduction-Transform-Edge-Propagator) fault, where the upwelling of mantle, intruding the continental crust, formed a ridge. Since Pliocene, most of the contractional deformation has been focused along this ridge, becoming a good candidate for a future subduction initiation zone.

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