(Table 1) Lead, thorium and uranium isotopic concentration and ages of zircons from Yenisey Ridge metapelites

Reconstruction of the geologic history of the Yenisey Ridge, which developed as an accretionary collision orogen on the western margin of the Siberian craton is essential to understanding the evolution of mobile belts surrounding older cratons, as well as to resolving the recently much debated problem of whether Siberia was part of the supercontinent Rodinia. Available paleotectonic models suggest that this supercontinent was assembled at the Middle-Late Riphean boundary (1100-900 Ma) as a result of the Grenville orogeny, the first long-lived mountain building event which occurred in geosynclinal areas during the Neogaea. However, the character of crustal evolution at that stage is still speculative due to the lack of reliable and conclusive isotope data. In many current geodynamic models, a common underlying assumption is that the Yenisey Ridge showed very little endogenic activity for 1 Gyr, from the time of Tarak granite emplacement (1900-1840 Ma) to the Middle Neoproterozoic (~750 Ma). On the basis of this assumption, several recent studies suggested the absence of Grenvillian collisional events within the Yenisey Ridge. The results of the SHRIMP II U-Pb analysis of rift-related plagiogranites of the Nemtikha Complex, Yenisey Ridge (1380-1360 Ma) suggest an increase in magmatic activity in the Mesoproterozoic. Interpretation of these results in terms of a supercontinent cycle may help find evidence for possible occurrence of the Grenville orogeny on the western margin of the Siberian craton. With this in mind, we attempted to reconstruct using recent geochronological constraints the evolution of metapelitic rocks from the Teya polymetamorphic complex (TPMC), which is a good example of superimposed zoning of low and medium-pressure facies series. High precision age determinations from rock complexes formed in different geodynamic settings under different thermodynamic conditions and geothermal gradients were used to distinguish several major metamorphic events and unravel their time relations with tectonic and magmatic activity in the region.

Strontium isotope ratios of middle Eocene to Oligocene sediments from Maud Rise, Antarctica

A 87Sr/86Sr isotope curve of the middle Eocene to Oligocene was produced from analysis of foraminifera in Ocean Drilling Program Hole 689B, Maud Rise, near the coast of Antarctica. Sediments from the hole are well preserved with no evidence of diagenetic alteration. The sequence is nearly complete from 46.3 to 24.8 Ma, with an average sampling interval of 166 kyr. Excellent magnetostratigraphy in Hole 689B allows calibration to the geomagnetic polarity time scale of Cande and Kent (1992). Marine strontium isotopic ratios were nearly stable from 46.3 to 35.5 Ma, averaging near 0.70773, after which they began to increase. A slow increase began after 40.4 Ma, rising at a rate of only about 8*10**-6/m.y. from base values of 0.707707. From 35.5 Ma to 24.8 Ma the average slope increased to 40*10**-6/m.y. The slope remained constant at least until 24.8 Ma, when the record becomes discontinuous owing to unconformities. We evaluate several possible controls on the marine strontium isotope curve that could have led to the observed growth in 87Sr/86Sr ratios near the Eocene/Oligocene boundary. Three mechanisms are considered, including the onset of Antarctic glaciation, increased mountain building in the Himalayan-Tibetan region, and decreased hydrothermal activity. None of the mechanisms alone seems to adequately explain the increased 87Sr/86Sr ratios during the Oligocene. Glaciation as a weathering agent was too episodic and probably began too late to explain the upturn in marine 87Sr/86Sr ratios. There is evidence that uplift in the Himalayan-Tibetan region began in the Miocene, much too late to control Oligocene strontium isotope ratios. Lastly, hydrothermal flux changes since the Eocene were apparently not great enough alone to account for the rise in marine 87Sr/86Sr ratios. We suggest that a combination of causes, such as decreased hydrothermal activity perhaps followed by increased glaciation and mountain building, might best explain the growth of the marine 87Sr/86Sr curve during the Oligocene.

Late Oligocene rapid transformations in the South China Sea

Lithobiostratigraphic data indicate that the double reflectors on the seismic profile through Ocean Drilling Program (ODP) Site 1148 represent two unconformities that coincide, respectively, with the lower/upper Oligocene boundary at ~488 mcd, and Oligocene-Miocene boundary at 460 mcd. Two other unconformities, at ~478 and 472 mcd, respectively, were also identified within the upper Oligocene section. Together they erased a sediment record of about 3 Ma from this locality in a period of very active seafloor spreading. The existence of 32.8 Ma marine sediment at the terminated depth (850 mcd) indicates that the initial breakup of the South China Sea (SCS) was probably during 34-33 Ma, close to the Eocene-Oligocene boundary. High sedimentation rates of 60-115 m/my from the much expanded, N350 m lower Oligocene section resulted from rifting and rapid subsidence between 33 and 29 Ma. The mid-Oligocene unconformity at ~28.5 Ma, which also occurred in many parts of the Indo-West Pacific region, was probably related to a significant uplift of the Himalayan-Tibetan Plateau to the west and the initial collision between Indonesia and Australia in the south. A narrowed Indonesian seaway may have accounted for the late Oligocene warming and chalk deposition in the northern South China Sea including the Site 1148 locality. The unconformities and slumps near the Oligocene-Miocene boundary indicate a very unstable tectonic regime, probably corresponding to changes in the rotation of different land blocks and the seafloor spreading ridge from nearly E-W to NE-SW, as recognized earlier at magnetic Anomaly 7. This 25 Ma event also saw the first New Guinea terrane docking at the northern Australian craton. The low sedimentation rate of ~15 m/my in the early to middle Miocene may correspond to another period of rapid seafloor spreading and rapid widespread subsidence that effectively caused sediment source areas to retreat with a rapidly rising sea level. The isostatic nature of these late Oligocene unconformities and slumps with several major collision-uplift events indicate that the rapid changes in the early evolutionary history of the South China Sea were mainly responding to regional tectonic reconfiguration including the uplift-driven southeast extrusion of the Indochina subcontinent.

Processing results from drill hole VNIIO-1988-Sht-01 on the shelf of the Barents Sea (Report 6404, Murmansk)

Interareal correlation has been carried out; composition of the deposits has been determined; sections recovered by marine drilling have been compared; reconstructed paleogeographic conditions confirm previous views on Jurassic and Cretaceous sedimentation in the area: 1. Determinate changes of continental and shallow marine mainly sandy Middle Jurassic deposits by sandy-clayey marine ones to the north and west occur. This indicates similar direction of clastic material migration and converse direction of Jurassic marine transgressions. 2. Increase of sand contents in the deposits also to the east and to the southeast indicates an important source of clastic material. It can result from incipience and development of the epiplatform orogen of Novaya Zemlya - Pai-Khoi in the Late Triassic - Early Jurassic. 3. Compositional and facial changes as well as changes in thicknesses of some Early Cretaceous lithologic-stratigraphic complexes indicate fast change of terrigenous material transport from the north to the south - south-east in the Late Valanginian - Hauterivian. Besides within the South Barents Sea region up to the Shtokman area there occurs weak variability in lithologic parameters of Neocomian avandeltaic deposits and turbidites composed of clays, claystones, and clayey siltstones. Correlation of drilling sections from the Shtokman area and from the South Basin of the Barents Sea together with paleotectonic analysis result to the conclusion about significant structure-forming movements in the Late Jurassic - Early Neocomian. During this time there occurred maximal growth of the Shtokman structure and likely of many other structures belonging to the South Basin of the Barents Sea.