Mineralized Microbiota: A Possible Indicator of Hydrocarbon Flows in the Rift Zone of Iceland

Relationships of mineralized microbiota with the content of polycyclic aromatic hydrocarbons in hydrothermal systems are considered. It has been established that the mineralized microbiota can serve as an indicator of hydrothermal hydrocarbon flows in present-day and ancient deposits.     

Simulation of marine boundary layer characteristics using a 1-D PBL model over the Bay of Bengal during BOBMEX-99

The characteristic features of the marine boundary layer (MBL) over the Bay of Bengal during the southwest monsoon and the factors influencing it are investigated. The Bay of Bengal and Monsoon Experiment (BOBMEX) carried out during July–August 1999 is the first observational experiment under the Indian Climate Research Programme (ICRP). A very high-resolution data in the vertical was obtained during this experiment, which was used to study the MBL characteristics off the east coast of India in the north and south Bay of Bengal. Spells of active and suppressed convection over the Bay were observed, of which, three representative convective episodes were considered for the study. For this purpose a one-dimensional multi-level PBL model with a TKE-ε closure scheme was used. The soundings, viz., the vertical profiles of temperature, humidity, zonal and meridional component of wind, obtained onboard ORV Sagar Kanya and from coastal stations along the east coast are used for the study. The temporal evolution of turbulent kinetic energy, marine boundary layer height (MBLH), sensible and latent heat fluxes and drag coefficient of momentum are simulated for different epochs of monsoon and monsoon depressions during BOBMEX-99.The model also generates the vertical profiles of potential temperature, specific humidity, zonal and meridional wind. These simulated values compared reasonably well with the observations available from BOBMEX.     

Fault plane solutions of the january 26th, 2001 bhuj earthquake sequence

A 12-station temporary microearthquake network was established by the Geological Survey of India for aftershock monitoring of the January 26th, 2001 Bhuj earthquake (M _w 7.6) in the Kutch district of Gujarat state, western India. The epicentres of the aftershocks show two major trends: one in the NE direction and the other in the NW direction. Fault-plane solutions of the best-located and selected cluster of events that occurred along the NE trend, at a depth of 15–38 km, show reverse faulting with a large left-lateral strike-slip motion, which are comparable with the main-shock solution. The NW trending upper crustal aftershocks at depth <10 km, on the other hand, show reverse faulting with right-lateral strike-slip motion, and the mid crustal and lower crustal aftershocks, at a depth of 15–38 km, show pure reverse faulting as well as reverse faulting with right-lateral and left-lateral strike-slip motions; these solutions are not comparable with the main-shock solution. It is inferred that the intersection of two faults has been the source area for stress concentration to generate the main shock and the aftershocks.     

Age of Detrital Zircons from Sandstones of the Mesozoic Flysch Formation in the South Anyui Suture Zone (Western Chukotka)

The South Anyui fold zone (western Chukotka) is considered a suture zone related to closure of the South Anyui oceanic basin and collision of Eurasia with the Chukotka–Arctic Alaska microcontinent in the Early Cretaceous. The existence of a compensatory sedimentation basin (foredeep) during folding in the terminal Jurassic–initial Cretaceous remains debatable. This work presents first data on age estimates of detrital zircons from Upper Mesozoic terrigenous sequences of the South Anyui suture zone obtained by the fission-track method. The distal flysch of presumably Late Jurassic age and the proximal flysch of probably Late Triassic age were sampled in the Uyamkanda River basin. The fission-track dating showed that sandstones from the flysch sections contain detrital zircons of two different-age populations. Young zircon populations from sandstones of distal turbidites in the upper course of the Uyamkanda River (two samples) are 149 ± 10.2 and 155.4 ± 9.0 Ma old (Late Jurassic), whereas those from coarse-grained proximal turbidites sampled in the lower course of the Uyamkanda River (one sample) is 131.1 ± 7.5 Ma old (Early Cretaceous). The data obtained indicate that the Late Mesozoic folding in the South Anyui suture zone was accompanied by the formation of a marginal sedimentary basin. Sediments accumulated in this basin compose tectonic nappes that constitute a fold–thrust structure with the northern vergence.     

Compression mechanisms of coesite

 The structure of coesite has been determined at ten pressures up to a maximum of 8.68 GPa by single-crystal X-ray diffraction. The dominant mechanism of compression is the reduction of four of the five independent Si–O–Si angles within the structure. There is no evidence of the fifth linkage, Si1–O1–Si1, deviating from 180°. Some Si–O bond distances also decrease by up to 1.6% over the pressure range studied. The pattern of Si–O–Si angle reduction amounts to a rotation of the Si2 tetrahedron around the [001] direction. This rotation induces significant internal deformation of the Si1 tetrahedron. Comparison of the experimental data with rigid-unit distance least-squares simulations of coesite suggests that this pattern of compression, the anomalous positive values of both s₂₃ and K′′ in the equation of state of coesite, its high elastic anisotropy and the unusual straight Si1–O1–Si1 linkage within the structure are all consequences of the connectivity of the tetrahedral framework. Received: 11 July 2002 / Accepted: 14 January 2003 Acknowledgements The help of Christian Baerlocher of ETH Zurich in providing both the DLS-76 software and advice in its use is gratefully acknowledged, as are discussions with Paul Ribbe of Virginia Tech and the comments of two anonymous reviewers. The data analysis was supported by the National Science Foundation under grant EAR-0105864 to N.L. Ross and R.J. Angel.     

TEM study of a special grain boundary in a synthetic K-feldspar bicrystal: Manebach Twin

 A monoclinic KAlSi₃O₈ feldspar Manebach twin boundary was synthesized by diffusion bonding and examined using high-resolution transmission electron microscopy. The sharp (001) twin boundary is straight and free of strain. The boundary width is smaller than d₀₀₁. There is no rigid body shift observed at the twin boundary, and the feldspar structure is arranged symmetrically across (001). The twin boundary structure consists of bridged tetrahedral crankshafts, which are characteristic of the feldspar lattice. The grain boundary structure is in good agreement with the geometrical model of Taylor et al. (1934). The grain boundary composition of K_1/2H_1/2AlSi₃O₈ differs from their model. Received: 13 February 2002 / Accepted: 24 December 2002 Acknowledgements We thank M. Rühle, S. Hutt, J. Mayer, A. Strecker and U. Salzberger at MPI, Stuttgart, for their support and valuable advice in preparing TEM sections of bicrystals.     

Neoproterozoic accretionary and collisional events on the western margin of the Siberian craton: new geological and geochronological evidence from the Yenisey Ridge

The geological, structural and tectonic evolutions of the Yenisey Ridge fold-and-thrust belt are discussed in the context of the western margin of the Siberian craton during the Neoproterozoic. Previous work in the Yenisey Ridge had led to the interpretation that the fold belt is composed of high-grade metamorphic and igneous rocks comprising an Archean and Paleoproterozoic basement with an unconformably overlying Mesoproterozoic–Neoproterozoic cover, which was mainly metamorphosed under greenschist-facies conditions. Based on the existing data and new geological and zircon U–Pb data, we recognize several terranes of different age and composition that were assembled during Neoproterozoic collisional–accretional processes on the western margin of the Siberian craton. We suggest that there were three main Neoproterozoic tectonic events involved in the formation of the Yenisey Ridge fold-and-thrust belt at 880–860 Ma, 760–720 Ma and 700–630 Ma. On the basis of new geochronological and petrological data, we propose that the Yeruda and Teya granites (880–860 Ma) were formed as a result of the first event, which could have occurred in the Central Angara terrane before it collided with Siberia. We also propose that the Cherimba, Ayakhta, Garevka and Glushikha granites (760–720 Ma) were formed as a result of this collision. The third event (700–630 Ma) is fixed by the age of island-arc and ophiolite complexes and their obduction onto the Siberian craton margin. We conclude by discussing correlation of these complexes with those in other belts on the margin of the Siberian craton.     

Palaeomagnetic evidence for Tertiary counterclockwise rotation of Adria

With the aim of obtaining Tertiary palaeomagnetic directions for the Adriatic Foreland of the Dinaric nappe system, we carried out a palaeomagnetic study on platform carbonates from stable Istria, from the northwestern and the Central Dalmatia segment of imbricated Adria. Despite the weak to very weak natural remanences of these rocks, we obtained tectonically useful palaeomagnetic directions for 25 sites from 20 localities. All exhibit westerly declinations, both before and after tilt correction. Concerning the age of the magnetizations, we conclude that five subhorizontal and magnetite bearing Eocene localities from stable Istria are likely to carry primary remanence, whereas three tilted and hematite-bearing ones were remagnetized. In the northwestern segment of imbricated Adria the cluster of the mean directions improved after tectonic correction indicating pre-tilting magnetization. In contrast, Maastrichtian–Eocene platform carbonates from Central Dalmatian were remagnetized in connection with the late Eocene–Oligocene deformation or Miocene hydrocarbon migration. Based on the appropriate site/locality means, we calculate mean palaeomagnetic directions for the above three areas and suggest an alternative interpretation of the data of Kissel et al. [J. Geophys. Res. 100 (1995) 14999] for the flysch of Central Dalmatia. The four area mean direction define a regional palaeomagnetic direction of Dec=336°, Inc=+52°, k=107, α₉₅=9°. From these data we conclude that stable Istria, in close coordination with imbricated Adria, must have rotated by 30° counterclockwise in the Tertiary, relative to Africa and stable Europe. We suggest that the latest Miocene–early Pliocene counterclockwise rotations observed in northwestern Croatia and northeastern Slovenia were driven by that of the Adriatic Foreland, i.e. the rotation of the latter took place between 6 and 4 Ma.     

A millennial-scale record of Holocene tsunamis on the Kronotskiy Bay coast, Kamchatka, Russia

Deposits from as many as 50 large tsunamis during the last 7000 years are preserved on the Pacific coast of the Kamchatka Peninsula near the mouth of the Zhupanova River, southern Kronotskiy Bay. These deposits are dated and correlated using Holocene marker tephra layers. The combined, preserved record of tsunami deposits and of numerous marker tephras on Kamchatka offers an unprecedented opportunity to study tsunami frequency. For example, from the stratigraphy along southern Kronotskiy Bay, we estimate frequency of large tsunamis (>5 m runup). In the last 3000 years, the minimum frequency is about one large tsunami per 100 years, and the maximum about one large tsunami per 30 years; the latter frequency occurred from about 0 to 1000 A.D. This time interval corresponds to a period of increased seismicity and volcanic activity that appears to be recorded in many places on the Kamchatka Peninsula.