X‐ray tomography of tsunami deposits: Towards a new depositional model of tsunami deposits          下载免费PDF全文

Simon Falvard  Raphaël Paris 《Sedimentology》2017,64(2):453-477

X‐ray computed microtomography is used to obtain high resolution imagery of a historical tsunami deposit in Andalusia, Spain (1755 Lisbon tsunami). The technique allows characterization of grain‐size distribution, structures, component analysis and sedimentary fabric of fine‐grained unconsolidated tsunami deposits at resolutions down to particle scale. The results are validated by comparing to data obtained using other techniques such as laser diffraction, anisotropy of magnetic susceptibility and X‐ray microfluorescence on the same deposits. Specific technical details such as sampling, scanning and image processing methods, and further improvements are addressed. The use of X‐ray computed microtomography provides new insights into the stratigraphy of the deposits and gives access to significantly more detailed view of key sedimentary features such as mudlines, rip‐up clasts, crude laminations, convolutions, floating outsized clasts and contacts between successive units. This analysis of the 1755 tsunami deposits using X‐ray computed microtomography allows the proposal of new hypotheses for the sedimentary processes forming tsunami deposits. Deposition by settling is limited and the section analysed here is dominated by a high shear stress leading to the development of traction carpets, with laminated mudlines corresponding to the basal frictional region of these carpets. The onset of the tsunami backwash is marked by a micro‐vortex resembling Kelvin–Helmoltz instabilities.  相似文献   

Geology,mineralogy and geochemistry of the Kekem dyke swarm (Western Cameroon): Insights into Paleozoic–Mesozoic magmatism and geodynamic implications     

Depesquidoux Tchato Tchaptchet  Nicole Armelle Simeni Wambo  Noël Aimé Keutchafo Kouamo  Jean-Pierre Tchouankoué  Ciro Cucciniello 《Comptes Rendus Geoscience》2017,349(4):175-185

The broadly N70°–90°E-trending dykes swarm at Kekem cut across the Paleoproterozoic-to-Achean terranes of West Cameroon remobilized during the Pan-African orogeny. They are picrite basalts and basalts with tholeiitic/transitional affinity, as shown by mineralogical and geochemical data, with variable major and trace element contents, MgO ranges from 7.3 to 12.4 wt.%, Cr from 190 to 411 ppm, Ni from 15 to 234 ppm. All the dykes are light REE enriched with La_N/Yb_N values of 5.3–8.1, suggesting a co-magmatic origin. They originated from a 2.8% partial melting of a spinel-mantle source with no or little crustal input. The geochemical features of Kekem dykes are similar to those of Paleozoic and Mesozoic dykes recorded in North and Central Africa, suggesting multiple reactivations of pre-existing fractures that resulted in the fragmentation of western Gondwana and the opening of Central and South Atlantic Oceans.  相似文献   

The mantle rotation pole position. A solar component     

Fernando Lopes  Jean-Louis Le Mouël  Dominique Gibert 《Comptes Rendus Geoscience》2017,349(4):159-164

The direction of the Earth's rotation axis with respect to the mantle has been studied for more than a century. The time variation of this direction is generally considered to be the sum of three components: the annual wobble, forced by the atmosphere, the Chandler wobble, a free oscillation with a period of 435 days, and the so-called drift of the mean pole. In the present paper, applying the singular spectrum analysis (SSA) technique, we uncover two more components, with smaller amplitude than the three first ones, but well identified, periodic with periods of 11 and 5.5 years, respectively, undoubtedly linked to solar activity. We interpret them tentatively as the result of an exchange of kinetic angular momentum between the atmosphere, in which a flow would be generated by solar activity, and the mantle. The order of magnitude of the required mean winds in the atmosphere computed in the frame of a schematic model is 1 ms^?1, compatible with the observed values of the meridional mean circulation.  相似文献