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- Almqvist, Bjarne, et al.
(författare)
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Seismic anisotropy in the Morcles nappe shear zone : Implications for seismic imaging of crustal scale shear zones
- 2013
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Ingår i: Tectonophysics. - : Elsevier BV. - 0040-1951 .- 1879-3266. ; 603, s. 162-178
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Tidskriftsartikel (refereegranskat)abstract
- Microstructures and textures of calcite mylonites from the Morcles nappe large-scale shearzone in southwestern Switzerland develop principally as a function of 1) extrinsic physical parameters including temperature, stress, strain, strain rate and 2) intrinsic parameters, such as mineral composition. We collected rock samples at a single location from this shear zone, on which laboratory ultrasonic velocities, texture and microstructures were investigated and quantified. The samples had different concentration of secondary mineral phases (<5 up to 40 vol.%). Measured seismic P waveanisotropy ranges from 6.5% for polyphase mylonites (similar to 40 vol.%) to 18.4% in mylonites with <5 vol.% secondary phases. Texture strength of calcite is the main factor governing the seismic P wave anisotropy. Measured S wave splitting is generally highest in the foliation plane, but its origin is more difficult to explain solely by calcite texture. Additional texture measurements were made on calcite mylonites with low concentration of secondary phases (<= 10 vol.%) along the metamorphic gradient of the shear zone (15 km distance). A systematic increase in texture strength is observed moving from the frontal part of the shear zone (anchimetamorphism: 280 degrees C) to the higher temperature, basal part (greenschist facies: 350-400 degrees C). Calculated P wave velocities become increasingly anisotropic towards the high-strain part of the nappe, from an average of 5.8%in the frontal part to 13.2% in the root of the basal part. Secondary phases raise an additional complexity, and may act either to increase or decrease seismic anisotropy of shear zone mylonites. Inlight of our findings we reinterpret the origin of some seismically reflective layers in the Grone-Zweisimmen line in southwestern Switzerland (PNR20 Swiss National Research Program). We hypothesize that reflections originate in part from the lateral variation in textural and microstructural arrangement of calcite mylonites in shear zones.
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- Balashova, Anna, et al.
(författare)
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New tephrostratigraphic data from Lake Emakat (northern Tanzania) : Implications for the eruptive history of the Oldoinyo Lengai volcano
- 2018
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Ingår i: Journal of African Earth Sciences. - : PERGAMON-ELSEVIER SCIENCE LTD. - 1464-343X .- 1879-1956. ; 147, s. 374-382
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Tidskriftsartikel (refereegranskat)abstract
- The northern Tanzanian sector of the Gregory Rift is an area of an active continental rifting, in which sedimentation processes are strongly affected by volcanism. Due to limited stratigraphic exposure, the volcanic record of the region is rather sparse, and assigning volcanic centres for the individual eruptions is difficult. This study presents new data on the tephrostratigraphy of the sedimentary sequence of Lake Emakat, Empakaai Crater, northern Tanzania. Seven volcanic ash layers are identified and described from a 1.1-m core of lake sediments. Geochemical, mineralogical, petrographic and magnetic analyses show that: (1) all ash layers are products of highly explosive eruptions of melilite-bearing magmas; (2) most of the eruptions originate from a complex magmatic system; (3) all ash horizons are very well preserved in the lake environment; and (4) there are significant fluctuations of the bulk magnetic susceptibility of the lacustrine sediments which is related to microtephra from additional eruptions, the result of detritus, washed from the shore during periods of strong lake level fluctuations or periods of high erosion rates, or simply by the contamination by the material from the ash layers. Based on geochemistry and mineralogy of the seven identified ash layers in Lake Emakat, combined with the eruption ages from C-14 datings, we can pinpoint Oldoinyo Lengai volcano as the source of these specific layers. The combination of this new data with existing chronological data from Ryner et al. (2007), retrieved from the same core, provides precise ages of the voluminous highly explosive eruptions in this region of East Africa during the Pleistocene-Holocene transition.
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- Balashova, Anna, et al.
(författare)
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The Lake Natron Footprint Tuff (northern Tanzania) : volcanic source, depositional processes and age constraints from field relations
- 2016
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Ingår i: Journal of Quaternary Science. - : Wiley. - 0267-8179 .- 1099-1417. ; 31:5, s. 526-537
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Tidskriftsartikel (refereegranskat)abstract
- The Engare Sero Footprint Site, situated on the southern shore of Lake Natron in northern Tanzania, has been reported to host one of the best preserved sets of fossilized hominid footprints in the world. However, until now there has been no detailed characterization and age determination of the footprint-bearing strata (the Footprint Tuff). Here, we combine field observations with geochemical and mineralogical analyses and measurements of the anisotropy of magnetic susceptibility to constrain depositional processes, the role of reworking and the volcanic source for the Footprint Tuff. We find that the footprint-bearing horizon consists of volcanic ash-fall that has been slightly reworked by water, and that this was produced during a voluminous eruption of the Oldoinyo Lengai volcano. The unit, which covered the footprints and helped to preserve them, consists of the wind-blown material from the same eruption, mixed together with locally derived detrital material. We can constrain the ash horizon to be of Holocene age, based on: (i) the location of the Footprint Tuff within the regional stratigraphy, (ii) previous age determination of an ash layer that can be correlated with the Footprint Tuff and (iii) the regional climatological history of the area. The ash horizon was probably deposited around 11000-10500 years ago, but could potentially be even younger than this.
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