2012 Vol. 23, No. 6
Two fresh types of eclogites, namely the massive eclogite and foliated eclogite, are discernible in large eclogite bodies surrounded by country rock gneisses from the Dabie (大别)-Sulu (苏鲁) UHP metamorphic zone. They are different in mineral assemblage, texture and structure at various scales. The massive eclogite has a massive appearance with a metamorphic inequigranular and granoblastic texture, which consists mainly of nominally anhydrous minerals such as garnet, omphacite, rutile with inclusions of coesite and rare microdiamond. Massive eclogites which formed at the peak UHP metamorphic conditions (~3.1–4.0 GPa, 800±50 ℃) within the coesite to diamond stability field recorded the deep continental subduction to mantle depths greater than 100 km during the Triassic (~250–230 Ma). The diagnostic UHP minerals, mineral assemblages and absence of notable macroscopic deformation indicate the peak metamorphic' forbidden-zone' P-T conditions, an extremely low geothermal gradient (≤7 ℃·km−1) and low differential stress. The foliated eclogite is composed of garnet+omphacite+rutile+phengite+kyanite+zoisite+talc+nyböite±coesite/quartz pseudomorphs after coesite. It is quite clear that the foliated eclogite bears relatively abundant hydrous mineral, and shows well-developed penetrative foliation carrying mineral and stretching lineation reflecting intense plastic deformation or flow of eclogite minerals. The foliated eclogite occurred at mantle levels and recorded the earliest stages of exhumation of UHP metamorphic rocks. At a map scale, the foliated eclogites define UHP eclogite-facies shear zones or high-strain zones. Asymmetric structures are abundant in the zones, implying bulk plane strain or dominant non-coaxial deformation within the coesite stability field. The earliest stages of exhumation, from mantle depths to the Moho or mantle-crust boundary layering, were characterized by a sub-vertical tectonic wedge extrusion, which occurred around 230-210 Ma. The three-dimensional relationship between the massive and foliated eclogites is well displayed a typical 'block-in-matrix' rheological fabric pattern indicating the partitioning of deformation and metamorphism in the UHP petrotectonic unit. The existing data support the now widely accepted concept of deep continental subduction/collision and subsequent exhumation between the Yangtze and Sino-Korean cratons. The pressure is a constitutive geological variable. The influence of tectonic overpresure on UHP metamorphism is rather limited.
The formation time of the Fengshuigouhe (风水沟河) Group in the northwestern Lesser Xing'an Range (小兴安岭), NE China, remains controversial owing to the lake of the precise dating data. This article reports zircon U-Pb ages for the leptynite and gneissic granitoids from the Fengshuigouhe Group in the northwestern Lesser Xing'an Range. The aim is to constrain the formation time and provenance of Fengshuigouhe Group. Field observation indicates that the Fengshuigouhe Group consists of a suit of metamorphic rocks (leptynite) and gneissic granitoids intruding the leptynite, and that both of them are cut by late granitic pegmatite. Zircons from two leptynites are euhedral-subhedral in shape and display oscillatory zoning in CL (cathodeluminescence) images. These detrital zircons give weighted mean ages of 255, 291, 321, 361, 469, and 520 Ma. The youngest age of them is interpreted to maximum depositional age of the protoliths of these leptynites. Zircons from gneissic granites are euhedral and subhedral in shape and exhibit typical oscillatory zoning in CL images. The dating results indicate that the gneissic granites were formed in the Early Jurassic (185±2 Ma). Zircons from the late granitic pegmatite are subhedral in shape and exhibit two types in CL images: structureless and oscillatory zoning. The former gives a weighted mean age of 143±1 Ma, considered to represent the timing of crystallization of the pegmatite, the latter yield several groups of ages: 178, 273, 319, 482, 611, and 788 Ma, representing the crystallization age of inherited or captured zircons entrained by the pegmatite. Taken together, we conclude that the Fengshuigouhe Group in the northwestern Lesser Xing'an Range formed between Late Paleozoic (255 Ma) to Early Mesozoic (185 Ma), rather than Neoproterozoic as previously believed, and that the sediments in the Fengshuigouhe Group were sourced directly from geological bodies in the study area and adjacent regions.
A wide variety of tectonic models have been invoked to explain the exhumation of the world's largest ultrahigh pressure (UHP) orogenic belt, the Qinling (秦岭)-Dabieshan (大别山)-Sulu (苏鲁) belt in China, and its correlatives in Korea. Most of these models assume that the orogen contains one main collisional suture between the North and South China cratons that collided in the Mesozoic. New field data reveal that this model is too simplistic, and that the collision involved an additional microplate, which initially rifted off the Yangtze craton. This continental microplate was partially subducted beneath an active margin on the North China craton, and subsequently an additional active Andean-style margin developed on the southern margin of the Qinling microplate after collision, leaving the near-vertical microplate wedged between the two thickened and thermally softened margins. The thermo-mechanical environment of collision thus left a cold, thick, and buoyant microplate wedged between two easily deformed margins, which acted as power-law creep channels, accommodating rapid buoyancy-driven rise of a 2 000 km long wedge of the subducted microplate, which became intimately involved with the collisional process. An additional segment of the northern Yangtze craton was subducted to > 100 km, and formed a separate wedge that rose alongside the thermally softened margin of the Qinling microcontinent, and was bordered on the south by the recently thermally-softened rift zone where the Qinling microcontinent broke off the Yangtze craton between Late Devonian and Permian times. Recognizing the dual active margins in Qinling-Dabieshan-Sulu orogen and the thermally-softened power-law creep channels sheds new light on understanding exhumation of the world's largest ultrahigh pressure belt. We propose that this model is generally applicable to other UHP belts worldwide.
In order to constrain temperature during subduction and subsequent exhumation of felsic continental crust, we carried out a Ti-in-zircon thermometer coupled with zircon internal structure and U-Pb age on migmatitic gneisses from the Weihai (威海) region in the Sulu (苏鲁) ultra-high pressure (UHP) metamorphic terrane, eastern China. The Weihai migmatitic gneisses are composed of intercalated compositional layers of melanosome and plagioclase (Pl)-rich leucosome and K-feldspar (Kfs)-rich pegmatite veins. Four stages of zircon growth were recognized in the Weihai migmatitic gneisses. They successively recorded informations of protolith, prograde metamorphism, decompressional partial melting during early stage exhumation and subsequent fractional crystallization of primary melt during later stage cooling exhumation. The inherited cores in zircon from the melanosome and the Pl-rich leucosome suggest that the protolith of the migmatitic gneiss is Mid-Neoproterozoic (~780 Ma) magmatic rock. Metamorphic zircons with concordant ages ranging from 243 to 256 Ma occur as overgrowth mantles on the protolith magmatic zircon cores. The estimated growth temperatures (625–717 ℃) of the metamorphic zircons have a negative correlation with their ages, indicating a progressive metamorphism in HP eclogite-facies condition during subduction. Zircon recrystallized rims (228±2 Ma) in the Pl-rich leucosome layers provide the lower limit of the decompresssional partial melting time during exhumation. The ages from 228±2 to 219±2 Ma recorded in the Pl-rich leucosome and the Kfs-rich pegmatite vein, respectively, suggest the duration of the fractional crystallization of primary melt during exhumation. The calculated growth temperatures of the zircon rims from the Pl-rich leucosome range from 858 to 739 ℃, and the temperatures of new growth zircon grains (219±2 Ma) in Kfs-rich vein are between 769 and 529 ℃. The estimated temper atures have a positive correlation with ages from the Pl-rich leucosome to the Kfs-rich pegmatite vein, strongly indicating that a process of fractional crystallization of the partial melt during exhumation.
It is known from macrocomparisons and microresearches of bioherm reservoirs in main sedimentary basins of the South China Sea through deep-water petroleum explorations and by means of 2D/3D seismic data and a whole-coring core from the Xisha (西沙) Islands that there are great differences between deep-sea oil and gas fields in the world and those in the South China Sea, as reservoir systems of the former are mainly clastic rocks, whereas the latter have organic reefs that act as reservoirs of their largest oil and gas fields, which are represented by large Liuhua (流花) 11-1 reef oilfield in the north and super-large L reef gas field in the south of the South China Sea. Therefore, it is of great significance to study deep-water bioherm reservoirs in the South China Sea. Comparisons of organic reefs in the four large islands of the South China Sea give evidences that such reefs in main sedimentary basins came into being during Cenozoic, especially in Neogene, and mainly occur as tower (point) reef, massive reef, platform-edge reef, and patch reef in shape, which show different reservoir physical properties and seismic reflection configurations and make up carbonate rock-bioherm formations in the island reef and sedimentary basin areas. Generally, the south and north parts differ from the east and the west of the South China Sea in geologic conditions, as their corresponding continental shelf/island shelf areas are relatively wide/narrow, large stream current systems are well developed/not so well developed, and terrigenous sediments are relatively sufficient/insufficient. The southeast and south parts of the South China Sea had organic reefs built up earlier than the north and the reef building mainly took place in Neogene; these Neogene organic reefs all belong to plant algal reef rocks. Liuhua oilfield in the Pearl River Mouth basin is found to mainly have red algal bindstone, Malampaya reef in the northern Palawan basin is rich in both red algal bindstone and green algal reef segmented rock, and especially Miocene red algal framestone and green algal segmented rock are discovered in the Xisha Islands. These algal reefs created different sedimentary microfacies as well as various rock structures and types, and through recent researches on the mechanism of dolomitization, freshwater dolomite was discovered and grouped under products from dolomitization in mixed water that was regression reefal dolomite of good reservoir properties.
Coalbed gas (CBG) in Enhong (恩洪) syncline, eastern Yunnan (云南), China, is characterized by high concentration of heavy hydrocarbon with the highest content of ethane, which is more than 30%. Some previous investigators paid much attention to the abnormal concentration of heavy hydrocarbon in the CBG of Enhong, but few have researched on its origin. This article describes the characteristics of abnormal high concentration of heavy hydrocarbon in Enhong syncline and analyzes its reason from the aspects of origin and evolution of heavy hydrocarbon by carbon isotope, coal petrography, and coal rank. Features of gas carbon isotope composition display that there is no inorganic gas or oil components in the CBG, which is classified to thermogenetic gas produced by humic material, with characteristic of secondary biogenic gas in shallow coal seam. The concentration of heavy hydrocarbon in Enhong syncline increases with the increase of vitrinite, vitrinite/inertinite ratio, and hydrogen/carbon ratio and decreases with the increase of inertinite, so hydrogen-rich vitrinite may be a very important factor resulting in the abnormal concentration of heavy hydrocarbon. It also increases with the increase of degree of coalification of coking to lean coals during which the peak of heavy hydrocarbon generation is reached. Therefore, we think that high concentration of heavy hydrocarbon originated from the coupling effect of higher content of the hydrogen-rich vitrinite in the coal and the coal rank of coking to lean coals.
The Dauki fault is a major fault along the southern boundary of the Shillong plateau that may be a source of destructive seismic hazards for the adjoining areas, including northeastern Bangladesh. In this study, we simulated the present stress distribution and the ongoing convergence deformation within the study area using numerical simulation. This simulation attempts to determine the proper dip angle of the Dauki fault from a variety of differently established models but with the same realistic boundary conditions. The simulated stress field helped to predict the type of tectonic environment and the faulting patterns of the study area. The best-fit model can account for the compressive stress regime in the study area, except in the uppermost part of the Shillong plateau, the Bengal basin, and Assam areas where extensional stress regimes exists. In the model, the elements for which Mohr-Coulomb's failure occurs correspond with the observed earthquake data that refer to the Dauki fault as a north-dipping thrust fault.
The solution of static deformation of two homogeneous, isotropic, and perfectly elastic half-spaces in welded contact due to a long dip-slip fault with uniform slip is well known. The aim of the present article is to obtain the static deformation of two homogeneous, isotropic, and perfectly elastic half-spaces in welded contact due to nonuniform slip along a long dip-slip fault. The fault is vertical and lies entirely in the lower half-space, extending up to the interface. Four slip profiles are considered: linear, parabolic, elliptic, and cubic. Closed-form expressions for the elastic residual field have been obtained for the different slip profiles. The displacement field due to four nonuniform slip profiles is compared with the displacement field due to uniform slip. For comparison, we have assumed the source potency for different slip profiles to be the same, which is achieved by taking the fault slip at the interface constant and varying the fault width. It is found that the displacement field varies significantly at the fault width rather than at the interface. Moreover, uniform slip along the fault makes the edges singular. This singularity at the lower edge is not present in the case of nonuniform slip along a long dip-slip fault.
Lineaments in the southeastern Jordan plateau are mapped using gravity data and field studies in order to understand the tectonic origin of these lineaments, especially in relation to the Dead Sea transform (DST) and the Red Sea opening. Four sets trending E-W, NW-SE, NE-SW, and N-S are identified in gravity data. Field studies generally reveal similar orientations. Field and gravity studies indicate that most of the lineaments are extensional features that correspond to normal faults. Most of these were subsequently reactivated into strike-slip shear fractures. The NW-SE and N-S lineaments represent dilatational fractures. The N-S trending lineaments are the oldest. The E-W lineaments form conjugate shear fractures and are younger than the N-S lineaments. These conjugate shear fractures are also older than other set of conjugate shear fractures oriented NE-SW. The evolution of all these fractures is attributed to the DST and the Red Sea spreading. Kinematic and dynamic analysis of the two, older and younger, pairs of conjugate strike-slip fractures revealed, respectively, broadly NW-SE and N-S oriented transpressional stress (σ1) with corresponding transtensional stress (σ3) oriented NE-SW and E-W.
This study presents an integrated study of expansion process of salinized land in Basrah Province, a typical salt-affected area in southern parts of Iraq, by using geo-information techniques. Satellite images (Landsat TM 1990) and thematic maps (ETM 2003) were used to provide comprehensive views of surface conditions such as vegetation cover and salinization detection. With ERDAS software, the normalized differential salinity index (NDSI) and salinity index (SI) were computed and then evaluated for soil degradation by salinization. ARC/INFO software was used along with field observation data (global positioning system) for analysis. During the past 13 years, the salinized land in study area increased by 6 579.1 km2 and in 2003 covered 34.5% of the total area; in the meantime, vegetation cover has decreased by 4 595.9 km2 and in 2003 covers only 24.1% of the study area. Environmental changes show that, between 1990 and 2003, 37.5% of vegetation cover and 45.9% of marshlands were transformed into salty meadow and wet salty crust, respectively. In addition, there was 16.6% of sand lands converted into dry puffy salty crust. Results using spatial analysis methods showed that 7 894.9 km2 (41.4%) of land had no risk of environment degradation by soil salinity, 4 595.9 km2 (24.1%) had slight risk, 4 042.8 km2 (21.2%) had moderate risk, and 2 536.3 km2 (13.3%) of the total land area was at a high risk of environment degradation by soil salinity. In conclusion, the study area was exposed to a high risk of soil salinity.
The badland topography of Basanputti Village, Ganjam District, Odisha, possesses red sediments. The typical red sediment deposit, on average, consists of 71.8% total heavy minerals (THM), out of which 62.1% are ilmenite. The other heavy minerals sillimanite, zircon, garnet, monazite, and pyriboles are in the order of abundance identified. In this present study, Mozley mineral separator has been used to recover THM from red sediment. Mineral separator results indicate that a product obtained contains 72.2% by weight with 94.4% THM and 95% recovery. The ilmenite concentrate recovered using dry low-intensity magnetic separator from the THM concentrate can be used in pigment industries after suitable pyrometallurgical/chemical processing methods.
The present study was carried out to track the path of Vellar estuary over a period of 38 years (1970–2008) and also to detect the impact of tsunami on estuarine complex of the Vellar estuary. Visual interpretation techniques were employed by using multispectral data of Landsat TM (1991) and IRS-P6 LISS III (2004, 2006, and 2008) to delineate shoreline changes in the Vellar estuarine complex. Results clearly revealed the changes that occurred along the estuarine path over the period, and severe erosion was noticed in the seaward side of the MGR Thittu and accretion, along the estuarine mouth. It is also visible that there is clear river path shift in the river course. Although the sandbar formation was seen with only little morphologic modification up to 2004, the Indian Ocean tsunami that struck this coast (26 December, 2004) distorted the sandbar of the Vellar estuarine mouth; however, the 2006 satellite images confirmed that the sandbar was formed again to its original structure as that of before the tsunami.
The estimation of shear strength of rock mass discontinuity is always a focal, but difficult, problem in the field of geotechnical engineering. Considering the disadvantages and limitation of existing estimation methods, a new approach based on the shadow area percentage (SAP) that can be used to quantify surface roughness is proposed in this article. Firstly, by the help of laser scanning technique, the three-dimensional model of the surface of rock discontinuity was established. Secondly, a light source was simulated, and there would be some shadows produced on the model surface. Thirdly, to obtain the value of SAP of each specimen, the shadow detection technique was introduced for use. Fourthly, compared with the result from direct shear testing and based on statistics, an empirical formula was found among SAP, normal stress, and shear strength. Data of Yujian (鱼简) River were used as an example, and the following conclusions have been made. (1) In the case of equal normal stress, the peak shear stress is positively proportional to the SAP. (2) The formula for estimating was derived, and the predictions of peak-shear strength made with this equation well agreed with the experimental results obtained in laboratory tests.
A browser/server system has been developed for the purpose of marine geophysical data sharing based on WebGIS technology, which uses MapServer open source system and ORACLE Database Management System. The main steps during development, including system design and implementation, are discussed in this article. The system can provide convenient, efficient, and interactive marine geophysical information sharing and visualization services through Internet or Intranet to improve data exchange and utilization.