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  • 1
    ISSN: 1365-3121
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: The Syunik rhombus-like structure in the Khanarassar active dextral fault zone of Armenia is a typical pull-apart basin, formed between terminal parts of two adjacent en echelon fault segments. Some component of subsidence associated with the faults of the structure is found between the en echelon segments; nevertheless, the dextral component continues to be predominant even on the boundaries of the pull-apart basin. The late Pleistocene and Holocene lava volcanoes of the basin are also associated with those faults that have a component of extension. The relative ages of fault displacements and volcanic eruptions have been identified by the mutual correlation of lavas, moraines and topographic features and by archaeological and radiocarbon dating. According to the interpretation of rupturing and volcanism, major earthquakes and volcanic eruptions appear inter-related and three pulses of such activity during the earlier and middle Holocene have been identified.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Science Ltd
    Terra nova 9 (1997), S. 0 
    ISSN: 1365-3121
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: In the Central Mediterranean two back-arc basins, the Liguro-Provençal (LPb) and the Tyrrhenian basin (Tb), opened progressively and consecutively from the late Eocene–Oligocene to the present. Evolution in space and time of rifting and drifting processes, along three different transects across these basins, shows differences in the style of extension: LPb opened with the formation of a narrow, single rift, while in the Tb deformation and magmatism is spread over a wide area. Moreover at the Northern end of the Tb the locus of extension progressively migrated towards the east whereas in the Southern Tb the locus of extension and magmatism migrated inside the basin, inducing continental break-up and drifting of the previously formed older conjugate basins. We propose that these different styles of back-arc extension depend upon internal conditions, such as prerift rheology linked with its geological heritage, and external conditions, e.g. the style of subduction.
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  • 3
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Science Ltd
    Terra nova 9 (1997), S. 0 
    ISSN: 1365-3121
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: The Makran accretionary wedge is one of the largest on Earth. A 7-km-thick column of sands and quartzolithic turbidites are incorporated into this wedge in a series of deformed thrust sheets. We present the results of prestack depth migration and focusing-error analysis (migration velocity analysis) performed on a profile across the Makran wedge. The depth section shows the deformation style of the accreted sediments, and the migration velocities allow us to estimate porosity variations in the sediments. The thrust sheets show evidence of fault-propagation folding, with a long wavelength of deformation (≈ 12 km) and secondary thrusting in the kink bands of the folds, such that the central part of each thrust sheet is elevated to form an additional ridge. This deformation style and the 15° steep surface slope of the first ridge suggest a high degree of consolidation. Porosities were calculated from the seismic migration velocities and the ratio of fluid pressure to lithostatic pressure λ was estimated for 5 locations along the profile. Rather than being undercompacted and overpressured as in most accretionary wedges, the sedimentary input is normally compacted (exponential porosity decay) throughout almost the whole wedge. However, a slight increase in porosity and λ at depth, with respect to the normal compaction curve indicates, that the turbiditic sequence might be overpressured landward of the deformation front.
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  • 4
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Science Ltd
    Terra nova 9 (1997), S. 0 
    ISSN: 1365-3121
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: The Earth’s inner core has been the focus of much attention in recent years — its evolution, when it began to form and how quickly it grew, and what role it plays in the generation and reversals of the Earth’s magnetic field. It has also been found that it is elastically anisotropic and, even more surprisingly, that it is rotating faster than the rest of the Earth.
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  • 5
    ISSN: 1365-3121
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Peridotitic mantle xenoliths from historic and prehistoric eruptions on La Palma show many similarities. Prolonged reactions of the xenoliths with their host magmas have been used to place constraints on the magma transport system beneath the island. All xenoliths show crystalline selvages and 0.9–2.6 mm wide diffusion zones in olivine along most of their surface. Diffusion kinetics in olivine, combined with fluid inclusion barometry, document that selvages and diffusion zones formed at crustal levels within 8 to about 100 years. Some xenolith fractures lack selvages and were in contact with the host magma for less than 4 days. A multistage magma ascent is proposed: (i) peridotite wall rock was fragmented and became incorporated into the ascending magma years to decades prior to the eruption; (ii) the xenoliths were rapidly transported to, and deposited in, crustal magma reservoirs, forming selvages and diffusion zones at the xenolith rims; (iii) renewed fragmentation of the xenoliths occurred days to hours prior to eruption, possibly by decompressive strain fracturing during rapid ascent.
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  • 6
    ISSN: 1365-3121
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Internal parts of the Alps have undergone widespread extensional deformation in the course of their Neogene exhumation history. Palaeostress inversion methods are used to map the prevailing stress fields and their evolution through time. Here we present new data from 100 sites with a total of about 2000 faults/striae couples, covering a large portion of the inner north-western Alps. Palaeostress tensors are mostly extensional, although one-third of them are transcurrent. The dominant direction of minimum horizontal stress axes (σ3) is in an orogen-parallel (N30° to N70°) orientation around the bend of the north-west alpine arc. A comparison between this older (Neogene, post-metamorphic) stress field with the current stress and strain field determined from seismotectonics and geodesy indicates a change in deformation mode from early orogen-parallel extrusion to a late and ongoing orogen-perpendicular spreading.
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  • 7
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Science Ltd
    Terra nova 16 (2004), S. 0 
    ISSN: 1365-3121
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Geometric and kinematic models reveal that rotation of layers during thrusting or development of drag folds produced by movement along listric thrust faults results in the formation of dilation space at the junction of the thrust ramp with the basal decollement. This is accompanied by upwarping of the basement to occupy the available space thereby producing an antiformal structure, which restricts the thrust displacement. The structure is termed a decollement upwarp and examples are cited from the foothill belt of the Himalaya.
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  • 8
    ISSN: 1365-3121
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Textural, electron microprobe and whole rock geochemical evidence from carbonatites and associated silicate rocks on Alnö Island, Sweden, suggest that the carbonatite, at the time of emplacement, could have been an (almost) pure CaCO3 liquid with a high volatile (H2O–CO2) content and that most silicate minerals, which are ubiquitously present, are either (1) assimilated from the surrounding wall rock, by progressive and coupled fragmentation and corrosion; or (2) by-products of corrosive interaction between the carbonatite liquid and the wall rock. This interpretation is supported by balancing a reaction to describe interaction between carbonatite and a cpx + ne-bearing (ijolite) wall rock. Although our analysis does not preclude the possibility that fenitizing agents (e.g. Na, Fe) were transported by the carbonatite liquid, these components are not required to drive the observed mineralogical changes in the carbonatite.
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  • 9
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Science Ltd
    Terra nova 16 (2004), S. 0 
    ISSN: 1365-3121
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Focal mechanisms of earthquakes and fault-slip data have been collected to constrain the strain regime acting in the hydrothermal zone and surrounding areas of the Campanian Plain (southern Italy), a NW–SE elongated structural depression. The NW–SE striking faults bounding the depression move in response to a NE–SW striking regional extension. Within the depression, an extended hydrothermal circulation occurs related to the Vesuvius, Campi Flegrei and Ischia active volcanoes. In this zone, the strike of the extension is N–S. Results from a finite element model constrained by the collected data show that the presence of a lower rigidity zone due to the hydrothermal circulation may explain (a) the observed deflection of the direction of regional extension, and (b) why large magnitude earthquakes occur at the boundaries of the hydrothermal zone and not along the faults delimiting the structural depression.
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  • 10
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Science Ltd
    Terra nova 16 (2004), S. 0 
    ISSN: 1365-3121
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: A new compilation of N-isotope and abundance data for metasedimentary rocks, and hyrdothermal micas that proxy for bulk crust, show systematic patterns. (1) δ15N values of kerogen in Precambrian cherts are more negative relative to siliciclastic counterparts, probably due to a mantle hydrothermal component. (2) There is a secular trend from average δ15N 15.3 ± 1.8‰ in Archean shales, through intermediate values in the Proterozoic, to Phanerozoic counterparts where δ15N averages +3.5‰. (3) Hydrothermal micas in metamorphic hydrothermal systems of Palaeozoic and Mesozoic age that proxy for crust have δ15N within the range of contemporaneous sedimentary rocks. (4) Hydrothermal micas track the secular trend of δ15N for kerogen from 2.7 Ga to the Phanerozoic. (5) Within Precambrian datasets δ15N does not increase with decreasing N content; accordingly, high δ15N values cannot stem either from metamorphism or form Rayleigh fractionation. (6) Previous studies show isotopic shifts during metamorphism are only +1 to +3‰ up to amphibolite facies. Values of 10–24‰ are attributed to a high δ15N Archean atmosphere, a residual signature of CI carbonaceous chondrites where δ15N is +30‰ to + 42‰.
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