北萨哈林盆地的热演化特征与影响因素

北萨哈林盆地作为俄罗斯远东地区最大的含油气盆地,其热流分布和演化过程对油气勘探有重要意义。本文通过单井和区域热流分析,恢复了北萨哈林盆地和鄂霍茨克海域各时期热流值的分布范围;对沉积盆地热演化各期次进行数值模拟,改进了简单沉降模型,使其适用于弧后拉分盆地;利用EASY%Ro 方法对热演化过程进行恢复,并与理论计算的热流值进行对比分析。研究表明,北萨哈林盆地各时期热流分布极为不均,热演化可以分成两个阶段：1） 古近纪,盆地内陆和海域具有各自的热流中心,其热演化过程相对独立;2） 晚渐新世后,盆地受弧后拉张和断裂剪切的共同作用,热流范围发生改变,构造拼合带成为新的热流中心。本文认为岩浆活动强弱和沉降速率差异是造成该现象的内因和外因。     

1995年上半年世界灾害地震综述

１９９５年上半年的世界灾害地震活动不同以往，主要特点是：日本、俄国地震毁城，经济损失巨大，地震社会影响深远；两次灾难地震又是前所未料。日本阪神大地震将成为今年世界重大事件之道。本文综合介绍了今年上半年的世界灾害地震活动。灾情及地震灾害特点，指出了地震预测，工程抗震研究的前途仍属光明。     

The limited suitability of silica diagenetic boundaries as isothermal markers: Insights from seismic reflection imaging,Offshore Sakhalin,Russian Far East

In many sedimentary basins the seismic reflections that mark the conversion of opal-A to opal-CT and the subsequent conversion of opal-CT to quartz, are parallel to the present-day seabed. As the reactions are in part thermally activated these boundaries have been proposed as potential isothermal markers and could have utility for hydrocarbon exploration. We describe opal-A to opal-CT and opal-CT to quartz diagenetic boundaries using 2D seismic data from the North Sakhalin Basin (NSB). These are not parallel to the present-day seabed, but for 80% of the area of the basin are parallel to an unconformity of Late Miocene age and may represent palaeo-isotherms that were parallel to the Late Miocene seabed. This characteristic has been identified in other basins and may indicate silica diagenetic boundaries do not make reliable present-day isothermal boundaries. We propose diagenetic boundaries, which are not parallel to the seabed, are not present-day isothermal boundaries. This characteristic could result from: (1) temperature decrease, such as through a declining geothermal gradient or erosion of the overburden, which will cause the rate of conversion to slow; (2) variations in the burial rate that would cause changes to the rate of conversion; and (3) a change in the rate of conversion as a result of variation in the physico-chemical factors influencing the silica diagenetic reactions.     

Palaeotemperature curve for the Late Cretaceous of the northwestern circum-Pacific

In the northwestern circum-Pacific, two main trends in Late Cretaceous temperatures can be recognized. (1) In general, a recurrent warming trend is thought to have begun in the Turonian–Campanian, reaching temperature maxima in the early Late Santonian and early Late Campanian, and temperature minima in the earliest Santonian and perhaps early Campanian. (2) During the Maastrichtian, temperatures dropped sharply, with only a slight warming in the early Late Maastrichtian. The existence of a thermal maximum at the Coniacian–Santonian transition has previously been expected, but is not confirmed by new isotopic results.     

Winter Oceanographic Conditions in the Southwestern Part of the Okhotsk Sea and Their Relation to Sea Ice

In the southwestern part of the Okhotsk Sea, oceanographic and sea-ice observations on board the icebreaker Soya were carried out in February 1997. A mixed layer of uniform temperature nearly at the freezing point extending down to a depth of about 300 m was observed. This is much deeper than has previously been reported. It is suggested that this deep mixed layer originated from the north (off East Sakhalin), being advected along the shelf slope via the East Sakhalin Current, accompanied with the thick first-year ice (average thickness 0.6 m). This vertically uniform winter water, through mixing with the surrounding water, makes the surface water more saline (losing a characteristic of East Sakhalin Current Water) and the water in the 100–300 m depth zone less saline, colder, and richer in oxygen (a characteristic of the intermediate Okhotsk Sea water). The oceanographic structure and a heat budget analysis suggest that new ice zone, which often appears at ice edges, can be formed through preconditioning of thick ice advection and subsequent cooling by the latent heat release due to its melting. This revised version was published online in August 2006 with corrections to the Cover Date.     

A late diagenetic (syn-folding) magnetization carried by pyrrhotite: implications for paleomagnetic studies from magnetic iron sulphide-bearing sediments

Paleomagnetic, rock magnetic, and sedimentary micro-textural data from an early Miocene mudstone sequence exposed in Okhta River, Sakhalin, Russia, indicate the presence of pyrrhotite and magnetite at different stratigraphic levels. Sites that contain only magnetite have a reversed polarity characteristic remanent magnetization (ChRM) with a low-coercivity overprint, which coincides with the present-day geomagnetic field direction. Pyrrhotite-bearing sites have stable normal polarity ChRMs that are significantly different from the present-day field direction. After correction for bedding tilt, the ChRM data fail a reversals test. However, the normal polarity pyrrhotite ChRM directions become antipodal to the tilt-corrected magnetite ChRM directions and are consistent with the expected geocentric axial dipole field direction at the site latitude after 40% partial unfolding. These data suggest that the pyrrhotite magnetization was acquired during folding and after lock-in of the magnetite remanences. Electron microscope observations of polished sections indicate that fluid-associated halos surround iron sulphide nodules. Pyrrhotite is present in randomly oriented laths in and around the nodules, and the nodules do not appear to have been deformed by sediment compaction. This observation is consistent with a late diagenetic origin of pyrrhotite. Documentation of a late diagenetic magnetization in pyrrhotite-bearing sediments here, and in recent studies of greigite-bearing sediments, suggests that care should be taken to preclude a late origin of magnetic iron sulphides before using such sediments for geomagnetic studies where it is usually crucial to establish a syn-depositional magnetization.     

Time variations in the geophysical fields above a gas field on Sakhalin Island

Results of repeated geophysical observations above the Yuzhno-Lugovskoye gas field (southern Sakhalin) carried out from 2003 to 2006 are considered. It is shown that the gas field is a natural indicator of geodynamic processes occurring in the gas field and beyond it. A complex of methods and observations is offered to estimate the degree of productivity of the objects involved in prospecting drilling. They provide further development of the short-term forecast of seismic events by geophysical methods, because seismic pulses propagating from an external source cause sharp changes of the geophysical fields above the gas-containing structure.     

Active rifting in the Japan and Okhotsk seas and the tectonic evolution of the Central Sakhalin Fault zone in the Cenozoic

Large-scale geological maps available for individual areas in the Central Sakhalin Fault zone and geological-geophysical maps of Sakhalin and surrounding sea areas were analyzed to elucidate the tectonic evolution of the fault zone determined by movements of crustal blocks due to the opening of rift basins. Changes in the direction of horizontal compression in the Sakhalin fold system from diagonal (NW-SE) to near-latitudinal resulted in the transformation of near-meridional right-lateral strike-slip faults into reversed faults in the Late Miocene. This allows Sakhalin faults to be interpreted as a zone of recent right-lateral shear between Eurasian and Sea of Okhotsk plates.     

Metamorphic evolution of the Susunai metabasites in southern Sakhalin, Russian Republic

The Susunai Complex of southeast Sakhalin represents a subduction-related accretionary complex of pelitic and basic rocks. Two stages of metamorphism are recognized: (1) a local, low- P / T  event characterized by Si-poor calcic amphiboles; (2) a regional, high- P / T  event characterized by pumpellyite, actinolite, epidote, sodic amphibole, sodic pyroxene, stilpnomelane and aragonite. The major mineral assemblages of the high- P / T  Susunai metabasites contain pumpellyite+epidote+actinolite+chlorite, epidote+actinolite+chlorite, epidote+Na-amphibole+Na-pyroxene+chlorite+haematite. The Na- amphibole is commonly magnesioriebeckite. The Na-pyroxene is jadeite-poor aegirine to aegirine-augite. Application of empirically and experimentally based thermobarometers suggests peak conditions of T  =250–300 °C, P= 4.7–6 kbar. Textural relationships in Susunai metabasite samples and a petrogenetic grid calculated for the Fe³⁺-rich basaltic system suggest that pressure and temperature increased during prograde metamorphism.