Libyan Desert Glass (LDG) is a SiO2-rich natural glass whose origin, formation mechanism, and target material are highly debated. We here report on the finding of a lens-shaped whitish inclusion within LDG. The object is dominantly composed of siliceous glass and separated from the surrounding LDG by numerous cristobalite grains. Within cristobalite, several regions rich in mullite often associated with ilmenite were detected. Mineral assemblage, chemical composition, and grain morphologies suggest that mullite was formed by thermal decomposition of kaolinitic clay at atmospheric pressure and T ≥ 1600 °C and also attested to high cooling rates under nonequilibrium conditions. Cristobalite contains concentric and irregular internal cracks and is intensely twinned, indicating that first crystallized β-cristobalite inverted to α-cristobalite during cooling of the SiO2-rich melt. The accompanied volume reduction of 4% induced the high density of defects. The whitish inclusion also contains several partly molten rutile grains evidencing that at least locally the LDG melt was at T ≥ 1800 °C. Based on these observations, it is concluded that LDG was formed by high-temperature melting of kaolinitic clay-, rutile-, and ilmenite-bearing Cenozoic sandstone or sand very likely during an asteroid or comet impact onto Earth. While melting and ejection occurred at high pressures, the melt solidified quickly at atmospheric pressure. 相似文献
To cope with water scarcity in drylands, stormwater is often collected in surface basins and subsequently stored in shallow aquifers via infiltration. These stormwater harvesting systems are often accompanied by high evaporation rates and hygiene problems. This is commonly a consequence of low infiltration rates, which are caused by clogging layers that form on top of the soil profile and the presence of a thick vadose zone. The present study aims to develop a conceptual solution to increase groundwater recharge rates in stormwater harvesting systems. The efficiency of vadose-zone wells and infiltration trenches is tested using analytical equations, numerical models, and sensitivity analyses. Dams built in the channel of ephemeral streams (wadis) are selected as a study case to construct the numerical simulations. The modelling demonstrated that vadose-zone wells and infiltration trenches contribute to effective bypassing of the clogging layer. By implementing these solutions, recharge begins 2250–8100% faster than via infiltration from the bed surface of the wadi reservoir. The sensitivity analysis showed that the recharge rates are especially responsive to well length and trench depth. In terms of recharge quantity, the well had the best performance; it can infiltrate up to 1642% more water than the reservoir, and between 336 and 825% more than the trench. Moreover, the well can yield the highest cumulative recharge per dollar and high recharge rates when there are limitations to the available area. The methods investigated here significantly increased recharge rates, providing practical solutions to enhance aquifer water storage in drylands.
We present a synoptic overview of the Miocene-present development of the northern Alpine foreland basin (Molasse Basin), with
special attention to the pattern of surface erosion and sediment discharge in the Alps. Erosion of the Molasse Basin started
at the same time that the rivers originating in the Central Alps were deflected toward the Bresse Graben, which formed part
of the European Cenozoic rift system. This change in the drainage direction decreased the distance to the marine base level
by approximately 1,000 km, which in turn decreased the average topographic elevation in the Molasse Basin by at least 200 m.
Isostatic adjustment to erosional unloading required ca. 1,000 m of erosion to account for this inferred topographic lowering.
A further inference is that the resulting increase in the sediment discharge at the Miocene–Pliocene boundary reflects the
recycling of Molasse units. We consider that erosion of the Molasse Basin occurred in response to a shift in the drainage
direction rather than because of a change in paleoclimate. Climate left an imprint on the Alpine landscape, but presumably
not before the beginning of glaciation at the Pliocene–Pleistocene boundary. Similar to the northern Alpine foreland, we do
not see a strong climatic fingerprint on the pattern or rates of exhumation of the External Massifs. In particular, the initiation
and acceleration of imbrication and antiformal stacking of the foreland crust can be considered solely as a response to the
convergence of Adria and Europe, irrespective of erosion rates. However, the recycling of the Molasse deposits since 5 Ma
and the associated reduction of the loads in the foreland could have activated basement thrusts beneath the Molasse Basin
in order to restore a critical wedge. In conclusion, we see the need for a more careful consideration of both tectonic and
climatic forcing on the development of the Alps and the adjacent Molasse Basin. 相似文献
Monometamorphic metasediments of Paleozoic or Mesozoic age constituting Schneeberg and Radenthein Complex experienced coherent deformation and metamorphism during Late Cretaceous times. Both complexes are part of the Eoalpine high-pressure wedge that formed an intracontinental suture and occur between the polymetamorphosed Ötztal–Bundschuh nappe system on top and the Texel–Millstatt Complex below. During Eoalpine orogeny Schneeberg and Radenthein Complexes were south-dipping and they experienced a common tectonometamorphic history from ca. 115 Ma onwards until unroofing of the Tauern Window in Miocene times. This evolution is subdivided into four distinct tectonometamorphic phases. Deformation stage D1 is characterized by WNW-directed shearing at high temperature conditions (550–600°C) and related to the initial exhumation of the high-pressure wedge. D2 and D3 are largely coaxial and evolved during high- to medium-temperature conditions (ca. 450 to ≥550°C). These stages are related to advanced exhumation and associated with large-scale folding of the high-pressure wedge including the Ötztal-Bundschuh nappe system above and the Texel–Millstatt Complex below. For the area west of the Tauern Window, F2/F3 fold interference results in the formation of large-scale sheath-folds in the frontal part of the nappe stack (formerly called “Schlingentektonik” by previous authors). Earlier thrusts were reactivated during Late Cretaceous normal faulting at the base of the Ötztal–Bundschuh nappe system and its cover. Deformation stage D4 is of Oligo-Miocene age and accounted for tilting of individual basement blocks along large-scale strike-slip shear zones. This tilting phase resulted from indentation of the Southern Alps accompanied by the formation of the Tauern Window. 相似文献
The 2006 western Java tsunami deposited a discontinuous sheet of sand up to 20 cm thick, flooded coastal southern Java to
a depth of at least 8 m and inundated up to 1 km inland. In most places the primarily heavy mineral sand sheet is normally
graded, and in some it contains complex internal stratigraphy. Structures within the sand sheet probably record the passage
of up to two individual waves, a point noted in eyewitness accounts. We studied the 2006 tsunami deposits in detail along
a flow parallel transect about 750 m long, 15 km east of Cilacap. The tsunami deposit first becomes discernable from the underlying
sediment 70 m from the shoreline. From 75 to 300 m inland the deposit has been laid down in rice paddies, and maintains a
thickness of 10–20 cm. Landward of 300 m the deposit thins dramatically, reaching 1 mm by 450 m inland. From 450 m to the
edge of deposition (around 700 m inland) the deposit remains <1 mm thick. Deposition generally attended inundation—along the
transect, the tsunami deposited sand to within about 40 m of the inundation limit. The thicker part of the deposit contains
primarily sand indistinguishable from that found on the beach 3 weeks after the event, but after about 450 m (and roughly
coinciding with the decrease in thickness) the tsunami sediment shifts to become more like the underlying paddy soil than
the beach sand. Grain sizes within the deposit tend to fine upward and landward, although overall upward fining takes place
in two discrete pulses, with an initial section of inverse grading followed by a section of normal grading. The two inversely
graded sections are also density graded, with denser grains at the base, and less dense grains at the top. The two normally
graded sections show no trends in density. The inversely graded sections show high density sediment to the base and become
less dense upward and represents traction carpet flows at the base of the tsunami. These are suggestive of high shear rates
in the flow. Because of the grain sorting in the traction carpet, the landward-fining trends usually seen in tsunami deposits
are masked, although lateral changes of mean sediment grain size along the transect do show overall landward fining, with
more variation as the deposit tapers off. The deposit is also thicker in the more seaward portions than would be produced
by tsunamis lacking traction carpets. 相似文献
This study demonstrates a relationship between changes of magnetic susceptibility and microstructure developing in minerals of a magnetite‐bearing ore, experimentally shocked to pressures of 5, 10, 20, and 30 GPa. Shock‐induced effects on magnetic properties were quantified by bulk magnetic susceptibility measurements while shock‐induced microstructures were studied by high‐resolution scanning electron microscopy. Microstructural changes were compared between magnetite, quartz, amphibole, and biotite grains. In the 5 GPa sample, a sharp drop of magnetic susceptibility correlates with distinct fragmentation as well as with formation of shear bands and twins in magnetite. At 10 GPa, shear bands and twins in magnetite are accompanied by droplet‐shaped nanograins. In this shock pressure regime, quartz and amphibole still show intensive grain fragmentation. Twins in quartz and foam‐shaped, highly porous amphibole are formed at 20 and 30 GPa. The formation of porous minerals suggests that shock heating of these mineral grains resulted in localized temperature spikes. The identified shock‐induced features in magnetite strongly advise that variations in the bulk magnetic susceptibility result from cooperative grain fragmentation, plastic deformation and/or localized amorphization, and probably postshock annealing. In particular, the increasing shock heating at high pressures is assumed to be responsible for a partial defect annealing which we suggest to be responsible for the almost constant values of magnetic susceptibility above 10 GPa. 相似文献