Results of studying the azimuthal anisotropy of the paleozoic basement with vsp technique at the Bystrovka vibroseismic test site

At the Bystrovka vibroseismic test site (Novosibirsk region) 3-component refracted waves profiling was performed at three intersecting lines. Shear waves analysis made possible to detect anisotropy of the Paleozoic basement occurring at depth of about 10 m and to suggest symmetry elements of this medium along with their orientation. Compressional waves data were used to construct depth sections estimating head waves velocities. These velocities demonstrate significant variation in lines of different orientation. The results obtained agree with previously performed VSP.

Results of studying the azimuthal anisotropy of the Paleozoic basement with VSP technique at the Bystrovka vibroseismic test site

At present due to depletion of reserves in the majority of Western Siberia oil and gas fields, with reservoirs mainly related to lower Cretaceous and Jurassic section interval, the basement intervals present primary scientific and practical interest for prospecting and exploration. At the Bystrovka vibroseismic test site (Novosibirsk region), the Paleozoic basement occurs at depth of about 10 m. This allowed investigating its elastic properties with shallow seismic technique using VSP. Results of these investigations are presented in the present paper.

The structures of the Alai and Kichi-Karakol depressions and the latest deformations in their mountainous surroundings (Southern Tien Shan)

Research subject. A series of research studies was conducted in the Alai Range area, at the junction of the folded Tien Shan and Pamir provinces with the purpose of investigating the morphological structure of alpine deformations in the Paleozoic basement and Mesozoic–Cenozoic sediment cover.Materials and Methods. Structural and geological observations were carried out in the Alai and Kichi-Karakol inter-mountain depressions and their mountainous surroundings. These investigations included an analysis of the morphology and orientation of the deformation structures of the sedimentary cover and structural inconsistencies based on detailed geological mapping; measurements of the orientations of tectonic slickenlines and slickensides in the zones of active faults; processing the entire complex of data in terms of kinematic strain indicators using proven methods and calculation programs in order to determine the main parameters of the stress-strain state of the upper crustal rock complexes.Results. The age intervals and kinematic conditions of the main phases of structural transformation of the Southern Tien Shan at the Alpine stage of tectogenesis were determined based new structural and geological data, as well as materials published in previous works. It was demonstrated that, in the territory of Alai, the most significant deformations and activation of the Paleozoic basement faults occurred at the Neogene-Quarter boundary. Conclusions. At the present stage, the area under study constitutes a relatively stable block, which does not experience significant deformations associated with the encroachment of the upper crustal rock complexes of the Northern Pamir from the south.

Mimicking Alpine thrusts by passive deformation of synsedimentary normal faults: a record of the Jurassic extension of the European margin (Mont Fort nappe, Pennine Alps)

The Mont Fort nappe, former uppermost subunit of the Grand St-Bernard nappe system, is an independent tectonic unit with specific structural and stratigraphic characteristics (Middle Penninic, NW Italy and SW Switzerland). It consists in a Paleozoic basement, overlain by a thin, discontinuous cover of Triassic-Jurassic metasediments, mainly breccias, called the Evolène Series. The contact of this Series over the Mont Fort basement is debated: stratigraphic or tectonic? We present new observations that support the stratigraphic interpretation and consequently imply that the Evolène Series belongs to the Mont Fort nappe. We moreover show that the Mont Fort nappe was strongly affected by normal faulting during Jurassic. These faults went long unnoticed because Alpine orogenic deformation blurred the record. Alpine strain erased their original obliquity, causing confusion with an Alpine low-angle thrust. These Jurassic faults have been passively deformed during Alpine tectonics, without inversion or any other kind of reactivation. They behaved like passive markers of the Alpine strain. Detailed field observations reveal the link between observed faults and specific breccia accumulations. Areas where the Evolène Series is missing correspond to sectors where the fault scarps were exposed on the bottom of the sea but were too steep to keep the syn- to post-faulting sediments. The Mont Fort nappe thus represents an example of a distal rifted margin. The succession of synsedimentary extensional movements followed by orogenic shortening generated a situation where passively deformed normal faults mimic an orogenic thrust.

Squeezed Under the Sheet: White Mica Records High Tectonic Stresses Within a Decollement Thrust

<p>Tectonic stresses at the base of decollement thrusts are generally expected to be low due to the presence of mechanically weak evaporites. Yet, the presence of abundant micro-seismicity in the region expected to correspond to the evaporitic layer remains paradoxical. We study here a fossil thrust zone from the base of the Digne nappe (SE France) where exotic thrust slices formed by brecciated Paleozoic basement micaschists are observed within the Mio-Pliocene decollement. Petrographic investigations reveal the presence of highly-substituted phengitic rims (up to Si=3.43 apfu) around pre-alpine muscovitic cores. Similar micaschists sampled in a basement high further North do not exhibit these phengitic rims around muscovite, thus suggesting that white mica zoning relates to a younger overprint. Such high-Silica phengites are commonly found in high-pressure terranes (i.e. 7-15 kbars depending on the buffering assemblage) but are not expected in foreland regions, such as in the Digne area where the overburden has never been thicker than c.5km (i.e. approximately 1.3 kbar). We propose that the mica zoning observed reflects the former presence of non-lithostatic stresses (possibly on the order of several kilobars) related to the elastic charging of a thrust slice “squeezed” at the base of the moving nappe. This finding sheds light on stress distribution as well as on the origin of micro-seismicity along active decollement thrusts in orogenic belts.</p>

GEOTHERMAL ZONALITY OF OIL AND GAS BEARING DEPOSITS IN THE NORTH-WESTERN TERRITORY OF NOVOSIBIRSK REGION

The article is devoted to the approach to geothermal zonality. The results of geothermal studies of oil and gas bearing deposits in the northern territory of Novosibirsk region and adjacent areas are presented. Geothermal zoning was carried out, and the major geothermal anomalies were determined. The features of the geothermal zonality of Mesozoic sediments in the region under investigation are first of all defined by the material composition of the Paleozoic basement, the structural arrangement, and the warming effect of ground water during its crossflows from deeper lying horizons. Formation temperatures get leveled within the boundaries of Aptian-Albian-Cenomanian sediments, and no anomalies are distinguished. The highest geothermal gradients sup to 3,6-4,6 °C/100 m and correspond to the edge zones of the development of intrusive formations of pre-Jurassic rocks. Lower gradients sup to 2,4-3,0 °C/100 m and characterize the regions, in which Paleozoic carbonate rocks occur.