Variscan crustal thickening, extension and late overstacking during the Namurian-Westphalian in the western Montagne Noire (France)
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NAMURIAN-WESTPHALIAN BOUNDARY IN THE NORTH-WESTERN PART OF THE INTRA SUDETIC TROUGH Summary The article presents the results of studies based on the conclusions drawn mainly from the palynological analyses of the Bialy Kamien beds, and of the upper member of the Walbrzych beds from the NW part of the Intra-Sudetic trough. The area covered with palynological sections is presented on Fig. 1. The results of the palynological examinations and previous opinions as to the stratigraphy and sedimentation of the series under consideration are given in Tab. 2. The palynological examinations have demonstrated that: 1 – top part of the Walbrzych beds should be regarded as an equivalent of the Lower Namurian B. It has been ascertained on the basis of macrofloristic studies that the Walbrzych beds correspond to the Lower Namurian (W. Gothan, W. Gropp, 1933); 2 – accumulation of the Bialy Kamien beds embraced a time interval between the Upper Namurian Band Lower Westphalian A, inclusive; 3 – microfloristic studies did not prove the presence of the so-called “floral break” between the Lower and Upper Namurian. Spore material points to a slow disappearance of certain species, and to an appearance of other ones towards the upper members of the Carboniferous deposits; 4 – the microfloristic material gathered did not attest the opinions of certain geologists that the Upper Carboniferous sedimentation had begun, within the area situated west of the Walbrzych Basin, at the Westphalian time, in connection with the Erzgebrige phase. The equivalents of the Namurian B in the Walbrzych Basin and in the western part of the lntra-Stidetic trough point to a fact that at that time both regions were an accumulation area; 5 – it appears that the disturbances corresponding to the Erzgebirge phase were rather of vertical nature; the sediment here accumulated proves that this old accumulation area did not undergo any uplifting, but was a depression filled in with terrigenous material. Although erosion connected with an increased competence of streams could have taken place in some regions, but, generally, an uninterrupted accumulation persisted from the Walbrzych beds, up to the Bialy Kamien beds, as it can be seen in both micofloristic assemblages and lithology observed in the area under consideration.
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Abstract Despite recent observations of slow earthquakes along the Nankai subduction zone, none have been reported in the central Nankai Trough between the Kii Channel and Cape Shionomisaki. In November 2018, a very dense array of 96 ocean‐bottom seismometers were deployed by JAMSTEC to acquire active‐source seismic refraction dataset (supplemented by a multichannel seismic reflection profile) from the seaward side of the subduction trough to the accretionary prism off Cape Shionomisaki. We applied traveltime tomography to the refraction data to constrain the P wave velocity down to the upper mantle, coordinating with a migrated seismic reflection profile to confirm the depth of the Moho and interpret shallower structural features. From a comparison with a transect across the Kumano basin, we conclude that structural and physical differences between these two locations, especially the geometry of the subducting plate surface, lead to different slow earthquake activities.
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The Southern Apennines chain is related to the west‐dipping subduction of the Apulian lithosphere. The strongest seismic events mostly occurred in correspondence of the chain axis along normal NW–SE striking faults parallel to the chain axis. These structures are related to mantle wedge upwelling beneath the chain. In the foreland, faulting develops along E–W strike‐slip to oblique‐slip faults related to the roll‐back of the foreland. Similarly to other historical events in Southern Apennines, the I 0 = XI (MCS intensity scale) 23 July 1930 earthquake occurred between the chain axis and the thrust front without surface faulting. This event produced more than 1400 casualties and extensive damage elongated approximately E‐W. The analysis of the historical waveforms provides the chance to study the fault geometry of this “anomalous” event and allow us to clarify its geodynamic significance. Our results indicate that the M S = 6.6 1930 event nucleated at 14.6 ± 3.06 km depth and ruptured a north dipping, N100°E striking plane with an oblique motion. The fault propagated along the fault strike 32 km to the east at about 2 km/s. The eastern fault tip is located in proximity of the Vulture volcano. The 1930 hypocenter, similarly to the 1990 (M W = 5.8) Southern Apennines event, is within the Mesozoic carbonates of the Apulian foredeep and the rupture developed along a “blind” fault. The 1930 fault kinematics significantly differs from that typical of large Southern Apennines earthquakes, which occur in a distinct seismotectonic domain on late Pleistocene to Holocene outcropping faults. These results stress the role played by pre‐existing, “blind” faults in the Apennines subduction setting.
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Two earthquake sequences that affected the Mentawai islands offshore of central Sumatra in 2005 (Mw 6.9) and 2009 (Mw 6.7) have been highlighted as evidence for active backthrusting of the Sumatran accretionary wedge. However, the geometry of the activated fault planes is not well resolved due to large uncertainties in the locations of the mainshocks and aftershocks. We refine the locations and focal mechanisms of medium size events (Mw > 4.5) of these two earthquake sequences through broadband waveform modeling. In addition to modeling the depth-phases for accurate centroid depths, we use teleseismic surface wave cross-correlation to precisely relocate the relative horizontal locations of the earthquakes. The refined catalog shows that the 2005 and 2009 "backthrust" sequences in Mentawai region actually occurred on steeply (∼60 degrees) landward-dipping faults (Masilo Fault Zone) that intersect the Sunda megathrust beneath the deepest part of the forearc basin, contradicting previous studies that inferred slip on a shallowly seaward-dipping backthrust. Static slip inversion on the newly-proposed fault fits the coseismic GPS offsets for the 2009 mainshock equally well as previous studies, but with a slip distribution more consistent with the mainshock centroid depth (∼20 km) constrained from teleseismic waveform inversion. Rupture of such steeply dipping reverse faults within the forearc crust is rare along the Sumatra–Java margin. We interpret these earthquakes as 'unsticking' of the Sumatran accretionary wedge along a backstop fault separating imbricated material from the stronger Sunda lithosphere. Alternatively, the reverse faults may have originated as pre-Miocene normal faults of the extended continental crust of the western Sunda margin. Our waveform modeling approach can be used to further refine global earthquake catalogs in order to clarify the geometries of active faults.
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