Reconstruction of Holocene Environmental Changes in North-Western Pacific in Relation to Paleorecord from Shikotan Island

Doklady Earth Sciences - Results of a paleolimnological investigation of a well-dated lake sediment section from Shikotan Island (Southern Kurils) showed that from ca 8.0 to 5.8 cal ka BP a warm...     

Evolution of east coast of India — A plate tectonic reconstruction

The evolution of east coast of India sis discussed within the ambit of clearly identifiable four major tectonic stages which had a profound effect in shaping the tectonic grain of the east coast basins. The evolutionary process began with rift related crustal extension between India and Sri Lanka as a consequence of Africa-Antarctica rifting and development of Natal Basin. An arm of this rift led to initial extension in the Cauvery Basin and failed. Later, the India-West Australia rift propagated further in southwesterly direction initiating Mahanadi and Krishna-Godavari Basins. This extension was an oblique one with Nayudupeta high acting as pivot. The oblique extension followed by asymmetric seafloor spreading developed transpression along India-Sri Lanka and Antarctica junction, resulting in a NNW-SSE trending transcurrent fault along which Antarctica moved southward. Subsequently, entire east coast evolved through a more or less uniform post rift stage.     

Zirconology of dunite of the Nizhnii Tagil massif (middle Urals)

Doklady Earth Sciences -     

Stars with discrepant <Emphasis Type="Italic">υ</Emphasis> sin <Emphasis Type="Italic">i</Emphasis> as derived from Ca II 3933 and Mg II 4481 Å lines. II. Multiple star HD 90569

Axial rotation of a star plays an important role in its evolution, physical condition in its atmosphere and appearance of its spectrum. Methods of determinations of υ sin i are based on comparison of the observed profiles of spectral lines with theoretical ones. Their accuracy depends on the kind and quality of spectrograms as well as on the algorithms used. A frequently used method is the simple comparing of one line, e.g. the Ca II at 3933 Å or Mg ii at 4481 Å. This however, may result in a false value of υ sin i if low dispersion spectra are used. In this work we studied contemporary CCD as well as older photographic spectra of the multiple star HD90569. We determined the projected rotational velocity value to be υ sin i = 11 km/s. Besides formerly reported enhancing of lines of Cr, Fe, Mn and Sr, we found also large overabundances of rare earths, gallium and platinum. Helium, carbon, nitrogen, oxygen, aluminium, calcium, scandium and nickel are in deficit. The spectrum of the occultation double was not identified to be of the SB2-type, however, there are some observable evidences that the pair creates a binary with a long orbital period. Despite this there are also observations that leave such interpretation uncertain.     

Nature and age of metamorphic rocks from the basement of the West Siberian megabasin (according to U-Pb isotopic dates)

The SHRIMP-II zircon U-Pb dates for metamorphic rocks from the West Siberian basement are determined for the first time. It is established that the major protolith of the metamorphic strata from the Shaimsk-Kuznetsovsk meganticlinorium is composed of sedimentary Late- and Middle-Devonian rocks (395–398 Ma). It is likely that the greywackes, whose strata were mainly formed under erosion of ophiolitic rocks, served as a substrate for the metamorphic rocks. The metamorphic transformations of the rocks occurred under conditions of greenschist and occasionally lower amphibolite facies of metamorphism during the Late Carboniferous-Early Permian period.     

Revisiting the abandoned shorelines of Lake George,Australia: a refined optical dating framework

Abandoned shorelines are an important archive used to constrain past fluctuations in the hydrological balance of lakes around the globe. Within Australia, the shorelines preserved at Lake George, NSW, form one of the few shoreline archives in the south-east of the continent that record palaeoenvironmental conditions throughout the late Quaternary. Here, we examined and tested the lake-level record for Lake George constructed in the 1970s by dating a well-preserved shoreline sequence at Luckdale, on the lake's eastern shore, using single-grain optically stimulated luminescence (OSL) dating. Ten stratigraphic units were identified, and these suggest a late Quaternary highstand for Lake George in MIS 3, with fluctuations superimposed upon an overall drying trend throughout MIS 2 and into the present. At Luckdale, the highest four shoreline-associated units were deposited ~13 to 19 m above lake base and date to between 39 ± 2 and 29 ± 1 ka ago. Our study pushes back the timing of maximum lake depth at Lake George to at least MIS 3, rather than MIS 2. The overall drying trend is supported by similar reductions in both Riverine Plain fluvial activity and other associated lake-level records from within the Murray basin.     

Development of New Zealand according to the fore-arc model of crustal evolution

Analysis of New Zealand geology using a fore-arc model (Crook, 1980a) leads to the recognition of four arc terrains. The west facing Tuhua volcanic arc was active from the Late Proterozoic until the Middle or Late Cambrian. Post-subduction sediments, neritic in the east and flysch in the west, accumulated on the Tuhua accretionary prism from the Late Cambrian until the Early Devonian. Thermal equilibration, metamorphism, granitoid plutonism and penetrative deformation occurred in the Middle to Late Devonian. A small area of Permian platform cover has escaped later erosion. The east-facing Rangitata Terrain records subduction from Early Permian to late Early Cretaceous. Much of its accretionary prism consists of a submarine fan complex derived from Western Antarctica and carried sideways into the trench. The accretionary prism is thick and completely kratonized in southern New Zealand, but the thickness is more variable northwards. There the overlying Upper Cretaceous to Upper Oligocene post-subduction sequence comprises shelf sediments (implying an intermediate-thickness prism) or flysch followed by shelf sediments (implying a thin prism). During the accumulation of this sequence the Rangitata Terrain was a passive continental margin. The south-facing Jurassic-late Oligocene Northland Terrain collided with this passive margin in northern New Zealand at the end of the Oligocene, forming the Northland Allochthon. Subduction then flipped and the oldest part of the Kaikoura Terrain volcanic arc formed on the outer part of the Northland Terrain. Originally this terrain faced northeast and consumed the southwestern part of the South Fiji Basin crust, but during the Miocene the arc migrated clockwise to assume its present northeastern orientation. The fore-arc model employed here satisfactorily explains most first-order and many second-order features of New Zealand geology without requiring modification, thus attesting to the model's versatility and robustness. New Zealand provides a basis for elaborating some aspects of the model, particularly the transition from the syn- to post-subduction phases of fore-arc evolution. Combination of this study with a similar study of the southeastern Australian Paleozoic yields insights into the Phanerozoic evolution of the Australian: Pacific Plates' active margin.     

Diamond formation and source carbonation: mineral associations in diamonds from Namibia

Mineral inclusions in diamonds from Namibia document a range of mantle sources, including eclogitic, websteritic and peridotitic parageneses. Based on unusual textural features a group of inclusions showing websteritic, peridotitic and transitional chemical features is assigned to an 'undetermined suite' (12% of the studied diamonds). The mutual characteristic of this group is the occurrence of lamellar intergrowths of clinopyroxene and orthopyroxene. In addition, the 'undetermined suite' is associated with a number of uncommon phases: in one diamond MgCO₃ is enclosed by clinopyroxene. Other minerals that form touching inclusions with the pyroxene lamellae are (1) a SiO₂ phase observed in three diamonds, together with CaCO₃ in one of them, (2) phlogopite and a Cr-rich 'titanate' (probably lindsleyite). The inclusions document a metamorphic path of decreasing pressures and temperatures after entrapment in diamond. First, homogeneous low-Ca clinopyroxenes were entrapped at high temperatures. They subsequently exsolved orthopyroxene and probably also SiO₂ (coesite) on cooling along a P,T trajectory that did not allow garnet to be exsolved as well. Phlogopite, carbonates and LIMA phases are the result of overprint of a peridotitic source rock by a carbon-rich agent. The resulting unusual, olivine-free mineral association and the host diamonds are interpreted as products of extensive carbonation of the peridotite.     

Late Holocene estuarine–inner shelf interactions; is there evidence of an estuarine retreat path for Tampa Bay, Florida?

The purpose of this study was to determine if and how a large, modern estuarine system, situated in the middle of an ancient carbonate platform, has affected its adjacent inner shelf both in the past during the last, post-glacial sea-level rise and during the present. An additional purpose was to determine if and how this inner shelf seaward of a major estuary differed from the inner shelves located just to the north and south but seaward of barrier-island shorelines. Through side-scan sonar mosaicking, bathymetric studies, and ground-truthing using surface grab samples as well as diver observations, two large submarine sand plains were mapped – one being the modern ebb-tidal delta and the other interpreted to be a relict ebb-tidal delta formed earlier in the Holocene. The most seaward portion of the inner shelf studied consists of a field of lobate, bathymetrically elevated, fine-sand accumulations, which were interpreted to be sediment-starved 3D dunes surrounded by small 2D dunes composed of coarse molluscan shell gravel. Additionally, exposed limestone hardbottoms supporting living benthic communities were found as well. This modern shelf sedimentary environment is situated on a large, buried shelf valley, which extends eastward beneath the modern Tampa Bay estuary. These observations plus the absence of an incised shelf valley having surficial bathymetric expression, and the absence of sand bodies normally associated with back-tracking estuarine systems indicate that there was no cross-shelf estuarine retreat path formed during the last rise in sea level. Instead, the modern Tampa Bay formed within a mid-platform, low-relief depression, which was flooded by rising marine waters late in the Holocene. With continued sea-level rise in the late Holocene, this early embayment was translated eastward or landward to its present position, whereby a larger ebb-tidal delta prograded out onto the inner shelf. Extensive linear sand ridges, common to the inner shelves to the north and south, did not form in this shelf province because it was a low-energy, open embayment lacking the wave climate and nearshore zone necessary to create such sand bodies. The distribution of bedforms on the inner shelf and the absence of seaward-oriented 2D dunes on the modern ebb-tidal delta indicate that the modern estuarine system has had little effect on its adjacent inner shelf.