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Kalle Kirsime Sten Suuroja Juho Kirs Aulis Krki Maile Polikarpus Vino Puura Kalle Suuroja 《Meteoritics & planetary science》2002,37(3):449-457
Abstract— The well‐preserved Kärdla impact crater, on Hiiumaa Island, Estonia, is a 4 km diameter structure formed in a shallow Ordovician sea ?455 Ma ago into a target composed of thin (?150 m) unconsolidated sedimentary layer above a crystalline basement composed of migmatite granites, amphibolites and gneisses. The fractured and crushed amphibolites in the crater area are strongly altered and replaced with secondary chloritic minerals. The most intensive chloritization is found in permeable breccias and heavily shattered basement around and above the central uplift. Alteration is believed to have resulted from convective flow of hydrothermal fluids through the central areas of the crater. Chloritic mineral associations suggest formation temperatures of 100–300 °C, in agreement with the most frequent quartz fluid inclusion homogenization temperatures of 150–300 °C in allochthonous breccia. The rather low salinity of fluids in Kärdla crater (<13 wt% NaCleq) suggests that the hydrothermal system was recharged either by infiltration of meteoric waters from the crater rim walls raised above sea level after the impact, or by invasion of sea water through the disturbed sedimentary cover and fractured crystalline basement. The well‐developed hydrothermal system in Kärdla crater shows that the thermal history of the shock‐heated and uplifted rocks in the central crater area, rather than cooling of impact melt or suevite sheets, controlled the distribution and intensity of the impact‐induced hydrothermal processes. 相似文献
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Pattern and timing of the northwestern Barents Sea Ice Sheet deglaciation and indications of episodic Holocene deposition 总被引：1，自引：0，他引：1
Denise Christina Rüther Lilja Rún Bjarnadóttir Juho Junttila Katrine Husum Tine Lander Rasmussen Renata Giulia Lucchi Karin Andreassen 《Boreas: An International Journal of Quaternary Research》2012,41(3):494-512
The origin of two acoustic sediment units has been studied based on lithological facies, chronology and benthic stable isotope values as well as on foraminifera and clay mineral assemblages in six marine sediment cores from Kveithola, a small trough west of Spitsbergenbanken on the western Barents Sea margin. We have identified four time slices with characteristic sedimentary environments. Before c. 14.2 cal. ka, rhythmically laminated muds indicate extensive sea ice cover in the area. From c. 13.9 to 14.2 cal. ka, muds rich in ice‐rafted debris were deposited during the disintegration of grounded ice on Spitsbergenbanken. From c. 10.3 to 13.1 cal. ka, sediments with heterogeneous lithologies suggest a shifting influence of suspension settling and iceberg rafting, probably derived from a decaying Barents Sea Ice Sheet in the inner‐fjord and land areas to the north of Kveithola. Holocene deposition was episodic and characterized by the deposition of calcareous sands and shell debris, indicative of strong bottom currents. We speculate that a marked erosional boundary at c. 8.2 cal. ka may have been caused by the Storegga tsunami. Whilst deposition was sparse during the Holocene, Kveithola acted as a sediment trap during the preceding deglaciation. Investigation of the deglacial sediments provides unprecedented details on the dynamics and timing of glacial retreat from Spitsbergenbanken. 相似文献
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The H(div)-conforming approach for the Brinkman equation is studied numerically, verifying the theoretical a priori and a posteriori
analysis in K?nn? and Stenberg (2010; Math Models Methods Appl Sci, 2011). Furthermore, the results are extended to cover a non-constant permeability. A hybridization technique for the problem is
presented, complete with a convergence analysis and numerical verification. Finally, the numerical convergence studies are
complemented with numerical examples of applications to domain decomposition and adaptive mesh refinement. 相似文献
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Rudolf Vlja Kalle Kirsime Christian Koeberl Daniel Boamah Juho Kirs 《Meteoritics & planetary science》2019,54(10):2557-2572
The petrographic, mineralogical, and geochemical compositions of the incipient devitrification products in impact melt fragments found in outer suevites at the Bosumtwi impact crater were studied to reconstruct the postimpact environmental constraints on the suevite formation and to refine its cooling history. Our study shows that devitrified melt/particles contain numerous microlitic crystals and crystal aggregates of different shapes derived from rapid cooling. The matrix of melt/particles in Bosumtwi suevites contains abundant Mg‐hercynite (pleonaste)‐type spinels with sizes rarely exceeding a few micrometers. High nucleation density of microlites suggests rapid crystallization under strong undercooling in the presence of abundant volatiles. Although the Bosumtwi impact event took place in a continental environment, the possible sources for elevated fluid/volatile content could have been the groundwater in the deeply weathered and fractured‐jointed Birimian basement, dewatering of abundant hydrous phases in weathered crust or hydrothermally altered basement, and the shale/phyllite–greywacke lithologies in the target rocks. Our results show that enough volatiles were present in the target rocks at the time of impact for the effective impact melt dispersion observed in Bosumtwi impactites. 相似文献
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