Home – Home – Jornals – Russian Geology and Geophysics 2012 number 6

Results of study of eclogite-gneiss complex of the Muya Block (East Siberia) are presented. Several structural types of the studied eclogites have been recognized. Kyanitic eclogite has been found for the first time. The host granite-gneisses are two-mica and biotite varieties, mainly garnet-bearing. The exposure of eclogites from different depths of the subducted plate at the present-day denudation level might be the reason for the wide range of the equilibrium temperatures of the Muya block eclogites (590-740?C). The Sm-Nd dating of the eclogites and host gneisses showed the Neoproterozoic age of high-pressure metamorphism (~630 Ma). The model age (T _DM ) of the eclogites (720 Ma) differs considerably from the model age of the host gneisses (>1.3 Ga). The geochemical features of the eclogites point to the mobility of LILE (Rb, Cs, Ba, K) and LREE during their interaction with fluids, whereas the gneisses in the same process showed the mobility of LILE only. The oxygen isotope composition of minerals in the eclogites varies over a narrow range (δ¹⁸O = 5.5-3.9) and is close to the average mantle value, which evidences a negligible interaction between the eclogite protoliths and meteoric or sea water. The study of fluid inclusions in quartz from the eclogites and host gneisses showed a predominance of liquid-nitrogen inclusions in the former and carbon dioxide inclusions in the latter.

A detailed study was given to the composition and structure of alkali feldspars from the pockets of the Sosedka pegmatite vein, a large source of gems within the Malkhan tourmaline deposit. The vein is of concentric-zonal structure. Three types of pockets were recognized by mineral composition: A - quartz-lepidolite-Mn-Li-Al-tourmaline (± pollucite, hambergite, borocookeite, boromuscovite, danburite, light-pink beryl); B - quartz-adularia-axinite (± laumontite); and C - quartz and laumontite (± B-containing cookeite). Each type of pockets contains feldspar of specific composition and structure. This evidences that pockets formed in strongly different conditions, though some pockets of different types are localized as close as 0.5-2.0 m from each other within a zone. The reported data disagree with the common model implying the formation of zonal pegmatite bodies as a result of crystallization differentiation within the vein.

A detailed petrographic study of a 0.4-1.2 m thick Carnian diamondiferous bed of the Angardam-Tasa Ridge (Ust'-Olenek area, northern Siberia) was performed. It showed that the bed earlier referred to as a terrigenous deposit might be interpreted as an intensely altered volcanoclastic lapilli tuff of basic and, probably, ultrabasic composition. The tuff occurs within a 100 m thick Ladinian-Carnian volcanosedimentary sequence. It consists mainly of resurgent material: altered lithoclasts (nodules of basic-ultrabasic lithology?), devitrified volcanic glass, fragments of autolithic breccia, etc. The thin volcanoclastic sheet is supposed to have formed within a sedimentation basin as a result of phreatomagmatic eruptions of kimberlite volcanoes. The substantiation of the volcanogenic origin of the Carnian diamondiferous bed might be a new approach to elucidate the primary source of widespread placer diamonds in Arctic Siberia.

The geomorphological studies and radiocarbon dating of moraine complexes and the tree line within the North Chuya Ridge, along with active slope processes, soil formation, and peat formation in southeastern Gorny Altai, constrain the age of the main glacial and climatic events in this area at 7 ka to the first half of the 19th century. It is for the first time in the history of Altai studies that 57 absolute dates were obtained for glaciation in a vast but climatically and neotectonically homogeneous area. The new data refute the conventional idea that the Holocene glaciation in this mountain land comprised eight stages of the gradual retreat of the Late Wurm (Sartan) glaciation. Also, they evidence that glaciation in the upper parts of the troughs retreated almost completely no later than 7 ka and valley glaciers in the southeastern Altai were activated many times in the second half of the Holocene. Data are given on the morphology and age of three moraine generations reflected in the topography. A combination of temperature minima and humidity maxima led to a catastrophically rapid and the largest (up to 5-6 km) ice advance at the Akkem Stage (4.9-4.2 ka). In addition to the radiocarbon data, the time limits of the Historical Stage (2.3-1.7 ka) were defined more precisely using dendrochronological and archaeological data from Scythian burials of Pazyryk culture in SE Altai. The moraines closest to the present-day glaciers formed at the Aktru Stage (late 13th-middle 19th century). During warm interglacials, the glaciers waned considerably or retreated completely and the zone of recent glaciation was reforested. As a result of progressive aridization in the Holocene, the glaciers in SE Altai waned at each successive stage, and their mass balance was not positive during the greatest temperature minimum of the last millennium (middle 19th century).

As evidenced by plentiful data, most of the large recent positive topographic features formed as a result of a dramatically accelerated crustal uplift in the Pliocene-Quaternary after a relatively stable period (~100 Myr in most of the regions). The methods used are illustrated by the well-studied large neotectonic crustal uplifts on the Tibetan Plateau and in the Himalayas. Farther north, neotectonic uplifts with amplitudes of several hundred meters to several kilometers spread over a vast area from Central and Northeast China in the south to the Taimyr Peninsula and Northeastern Asia in the north. They are often attributed to the India-Asia plate collision which began ~50 Ma.
Most of the uplifts in these regions have formed only during the last few Myr, unaccompanied by significant crustal shortening. Therefore, the large neotectonic crustal uplifts can be explained by a decrease in the lithospheric density. One of the causes was the rapid convective replacement of the lower part of the denser mantle lithosphere by the asthenosphere or mantle plume. This became possible owing to a drastic weakening of the mantle lithosphere under the influence of asthenospheric fluids. In some areas, a considerable asthenospheric top uplift is evidenced by seismic tomography data.
The lower mantle lithosphere (~50-100 km thick) was replaced by the asthenosphere underneath the neotectonic crustal uplifts of ~1.0 km in Central Asia. Areas with a thick lithosphere were affected by relatively small neotectonic uplifts, strongly nonuniform in space. They point to metamorphism with mafic-rock expansion in the lower crust upon the infiltration of an asthenospheric fluid. The large crustal uplifts which formed on the continents in the Pliocene and Pleistocene indicate large-scale quasi-synchronic supply of the mantle fluid to their lithosphere.

The importance of pre-excavation permafrost detection within ancient burial mounds in the Altai by geophysical methods is hard to overestimate. There was no way of detecting small quantities of frozen ground or ice under stone mounds, and this is a topical issue in Russian archeology. Frozen mounds, which retain organic matter owing to natural processes, are an exceptional source of information about historical and cultural processes in the Early Iron Age. Pre-excavation geophysical prospecting is especially important in the context of global warming, which might destroy a whole layer of cultural and historical information.
The integrated geophysical studies conducted in recent years focused on a group of archeological sites of the Pazyryk culture whose burial constructions are very likely to contain frozen artifacts. As a rule, such burial mounds are located at a considerable altitude and contain permafrost, which creates unique conditions for the preservation of artifacts. Such localities include the Ukok high plateau (southern Altai) and the northwestern part of Mongolian Altay. Systematic field studies were conducted on the Ukok Plateau in 2003 and 2007 and in the adjacent territory of Mongolian Altay in 2005 and 2006. The following geophysical methods were used: vertical electrical sounding (VES), electrical tomography (ET), shallow frequency scanning (SFS), georadiolocation (GR), magnetic susceptibility measurements, gamma-ray spectrometry, and chromatography. The field works were planned with a heavy reliance on the 3D mathematical modeling of electric and EM fields, which is meant for a realistic estimate of the possibilities of geoelectrics and the best ways of its application to burial-mound studies and data interpretation.
The excavations conducted in 2006 in northwestern Mongolia within the Altai Mts. confirmed the geophysical prediction for permafrost at all the sites identified by the geophysical studies in 2005. In one of the mounds, they yielded a unique intact tomb of a Scythian warrior.

Coseismic soft-sediment deformation has been studied by structural and tectononophysical methods in the Selenga Delta area shaken by the devastating М ~7.5 Tsagan earthquake in 1862. Among the documented deformation structures (seismites), clastic dikes are the most reliable paleoseismic indicators. The dikes have their sizes and extent showing proximity to the primary coseismic rupture zone and are closely associated with faults of different hierarchic levels. The Tsagan event occurred under SW-NE extension as motion on a stepped system of normal faults dipping at 300°-350°, ∠45-75?.
The amount of vertical motion measured against a reference layer in a trench reached 2.83 m, and the maximum dip displacement measured in a single fracture was 0.5 m. The earthquake was generated by the Delta Fault that dips at 60° on average to the northwest.
The distribution of quantitative parameters of brittle and brittle-plastic deformation has been analyzed along two profiles, and two new parameters were introduced: indices of mean intensity ( I ) of clastic dikes and microdikes; the new parameters were calculated by specially developed equations. Summation of significant peaks in all parameters (SUM_spp) allowed contouring the zone of most intense soft-sediment deformation near Dubinino Village.
Deformation mostly propagated in the NE-SW and N-S directions. The location of the 1862 Tsagan earthquake at 52.35° N and 106.67° E was inferred from the SUM_spp value taking into account the dip of the causative fault plane and the average origin depth of earthquakes in the Baikal rift. The approach we used is applicable to locating preinstrumental events.
The recurrence of large earthquakes in the area of Proval Bay (Lake Baikal) has been estimated to be 1120-1230 years proceeding from alternating deformed and undeformed sediments in the sections, their thicknesses and deposition rates according to radiocarbon dating. The seismic activity has been associated with the same fault which can generate M ≥ 7 events.

Software has been designed for forward and inverse modeling of natural self-potential (SP) fields which allows simulating the geoelectric patterns simultaneously for up to 29 sheet-like and prismatic electrostatically polarized conductors. The cross sections for a number of ore fields obtained by inversion with the new SPI-SV code (self-potential inversion, Siberian version) have been corroborated by later drilling and are consistent with reference geologic sections used in algorithm testing. SPI-SV simulation can provide high-quality assessment of mineral deposits and, furthermore, allows approaching global-scale investigation into the natural self potential of crust and mantle.