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2008 Vol. 19, No. 2

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Geochronology Constraints on Transformation Age from Ductile to Brittle Deformation of the Shangma Fault and Its Tectonic Significance, Dabieshan, Central China
Guocan Wang, Po Wang, Chao Liu, An Wang, Runqing Ye
2008, 19(2): 97-109.

By a detailed investigation of geometry and kinematics of the Shangma (商麻) fault in Dabieshan (大别山), three different crust levels of extension movement have been recognized in sequence from the deep to the shallow: ① low-angle ductile detachment shearing with top to the NW; ② low-angle normal fault with top to the NW or NWW in brittle or brittle-ductile transition domain; ③ high-angle brittle normal fault with top to the W or NWW. Two samples were chosen for zircon U-Pb age dating to constrain the activity age of the Shangma fault. A bedding intrusive granitoid pegmatite vein that is parallel to the foliation of the low-angle ductile detachment shear zone of the country rock exhibits a lotus-joint type of boudinage deformation, showing syn-tectonic emplacing at the end of the ductile deformation period and deformation in the brittle-ductile transition domain. The zircon U-Pb dating of this granitoid pegmatite vein gives an age of (125.9±4.2) Ma, which expresses the extension in the brittle-ductile transition domain of the Shangma fault. The other sample, which is collected from a granite pluton cutting the foliation of the low-angle ductile detachment shear zone, gives a zircon U-Pb age of (118.8±4.1) Ma, constraining the end of the ductile detachment shearing. Then the transformation age from ductile to brittle deformation can be constrained between 126-119 Ma. Combined with the previous researches, the formation of the Luotian (罗田) dome, which is locatedto the east of the Shangma fault, can be constrained during 150-126 Ma. This study gives a new time constraint to the evolution of the Dabie orogenic belt.

Zircon U-Pb Age, Trace Element, and Hf Isotope Evidence for Paleoproterozoic Granulite-Facies Metamorphism and Archean Crustal Remnant in the Dabie Orogen
Nengzhong Lei, Yuanbao Wu
2008, 19(2): 110-134.

Zircon U-Pb age, trace elements, and Hf isotopes were determined for granulite and gneiss at Huangtuling (黄土岭), which is hosted by ultrahigh-pressure metamorphic rocks in the Dabie (大别) orogen, east-central China. Cathodoluminescence (CL) images reveal core-rim structure for most zircons in the granulite. The cores show oscillatory zoning, relatively high Th/U and 176Lu/177Hf ratios, and high rare earth element (HREE)-enriched pattern, consistent with magmatic origin. They gave a weighted mean 207Pb/206Pb age of (2 766±9) Ma, dating magma emplacement of protolith. The rims are characterized by sector or planar zoning, low Th/U and 176Lu/177Hf ratios, negative Eu anomalies and flat HREE patterns, consistent with their formation under granulite-facies metamorphic conditions. Zircon U-Pb dating yields an age of (2 029±13) Ma, which is interpreted as a record of metamorphic event during the assembly of the supercontinent Columbia. The gneiss has a protolith age of (1 982±14) Ma, which is similar to the zircon U-Pb age for the granulite-facies metamorphism, suggesting complementary processes to granulite-facies metamorphism and partial melting, A few inherited cores with igneous characteristics have 207Pb/206Pb ages of approximately 3.53, 3.24, and 2.90 Ga, respectively, suggesting the presence of Mesoarchean to Paleoarchean crustal remnants. A few Triassic and Cretaceous metamorphic ages were obtained, suggesting the influences by the Triassic continental collision and postcollisional collapse in response to the Cretaceous extension. Comparing with abundant occurrence of Triassic metamorphic zircons in ultrahigh-pressure eclogite and granitc gneiss from the Dabie-Sulu (苏鲁) orogenic belt, however, very limited availability of aqueous fluid or hydrous melt is evident for zircon growth in the Huangtuling granulite and gneiss during the continental collision. The magmatic protolith zircons from the granulite show a large variation in 176Hf/177Hf ratios from 0.280 809 to 0.281 289, corresponding to εHf(t) values of -7.3 to 6.3 and Hf model ages of 2.74 to 3.34 Ga. The 2.90 Ga inherited zircons show the similar Hf isotope features. These indicate that both growth of juvenile crust and reworking of ancient crust took place at the time of zircon formation. It is inferred that the Archean basement of the Yangtze block occurs in the north as the Dabie orogen, with ca. 2.90-2.95 Ga and 2.75-2.80 Ga as two major episodes of crustal formation.

Response of Glacier and Lake Covariations to Climate Change in Mapam Yumco Basin on Tibetan Plateau during 1974-2003
Qinghua Ye, Tandong Yao, Feng Chen, Shichang Kang, Xueqin Zhang, Yi Wang
2008, 19(2): 135-145.

The study of spatial and temporal covariances of glaciers and lakes would help us to understand the impact of climate change within a basin in Tibet. This study focuses on glacier and lake variations in the Mapam Yumco (玛旁雍错) basin (covering 7 786.44 km2) by integrating series of spatial data from topographic maps and digital satellite images at four different times: 1974, 1990, 1999, and 2003. The results indicate that: (1) decreased lakes, retreated glaciers, enlarged lakes and advanced glaciers co-exist in the basin during the last 30 years; (2) glacier recession was accelerated in recent years due to the warmer climate; (3) lake areas in the basin are both reduced and enlarged by an accelerated speed with more water supplies from speeding melt glaciers or frozen ground in the last three decades.

In Situ Resonance Raman Spectra of Organic Pigments in Momo Coral
Luwei Fan, Mingxing Yang
2008, 19(2): 146-151.

In this study, Raman scattering measurements were obtained for momo corals covering their typical range of colors. Three different excitation wavelengths (785, 633, 514 nm) are used for the same samples at the same points. All the samples show the two major Raman features of polyenic compounds assigned to double carbon-carbon (C=C) stretching vibration at approximately 1 500 cm-1 and single carbon-carbon (C—C) stretching vibration at approximately 1 130 cm-1 bond stretching mode. These peaks are not detected in the corresponding white parts of momo corals. However, some changes in intensities, shape, and position of C=C stretching vibrations of the same point are observed by using different excitation wavelengths. The exact position of C—C stretching vibration of polyenic molecules depends strongly on the number of double bonds contained in their polyenic chain. In addition, the number of double bonds contained in the polyenic chains shows that different colors of the red momo coral are caused by different mixtures of polyenic compounds.

Bottom Simulating Reflector and Gas Seepage in Okinawa Trough: Evidence of Gas Hydrate in an Active Back-Arc Basin
Xiwu Luan, Kelin Wang, Roy Hyndman, Willoughby Eleanor
2008, 19(2): 152-161.

To look for gas hydrate, 22 multi-channel and 3 single-channel seismic lines on the East China Sea (ECS) shelf slope and at the bottom of the Okinawa Trough were examined. It was found that there was indeed bottom simulating reflector (BSR) occurrence, but it is very rare. Besides several BSRs, a gas seepage was also found. As shown by the data, both the BSR and gas seepage are all related with local geological structures, such as mud diapir, anticline, and fault-controlled graben-like structure. However, similar structural "anomalies" are quite common in the tectonically very active Okinawa Trough region, but very few of them have developed BSR or gas seepage. The article points out that the main reason is probably the low concentration of organic carbon of the sediment in this area. It was speculated that the rare occurrence of gas hydrates in this region is governed by structure-controlled fluid flow. Numerous faults and fractures form a network of high-permeability channels in the sediment and highly fractured igneous basement to allow fluid circulation and ventilation. Fluid flow in this tectonic environment is driven primarily by thermal buoyancy and takes place on a wide range of spatial scales. The fluid flow may play two roles to facilitate hydrate formation: to help gather enough methane into a small area and to modulate the thermal regime.

Influence of Salt Beds on the Segmentation of Structure and Hydrocarbon Accumulation in Qiulitag Structural Belt, Tarim Basin, China
Qiuhai Fan, Xiuxiang Lü, Minghui Yang, Huiwen Xie
2008, 19(2): 162-173.

Seismic information and balanced profile technology were used to reveal the influence of the salt bed in segmentation of structure and hydrocarbon accumulation in Qiulitag structural belt in Tarim basin. From west to east, the shortening of strata above the salt beds gradually decreases, while, the shortening below the salt beds gradually increases, which shows that the segmentation of structure integrated the seismic profile. There is great difference of the deformation of strata below and above the salt beds between the west segment and the east segment. The analysis of the distribution of oil/gas fields and the hydrocarbon properties indicates the similar segmentation to the structure segmentation. The salt beds in relatively shallow layers change the stress condition from basement of Kuqa foreland basin, which leads to the segmentation of Qiulitag structural belt. Because the salt beds in the west segment came into being earlier than those in the east segment, the west segment captures hydrocarbon from two sets of source rock, while the east segment can only capture hydrocarbons from one set of source rock. So, the salt beds play an important role in the segmentation of structure and hydrocarbon accumulation.

Tephra Discovered in High Resolution Peat Sediment and Its Indication to Climatic Event
Shenggao Cheng, Xumei Mao, Fenglin Wang, Yetang Hong, Yongxuan Zhu, Qi An
2008, 19(2): 174-183.

Floating tephra was deposited together with ice core, snow layer, abyssal sediment, lake sediments, and other geological records. It is of great significance to interpret the impact on the climate change of volcanic eruptions from these geological records. It is the first time that volcanic glass was discovered from the peat of Jinchuan (金川) Maar, Jilin (吉林) Province, China. And it is in situ sediments from a near-source explosive eruption according to particle size analysis and identification results. The tephra were neither from Tianchi (天池) volcano eruptions, Changbai (长白) Mountain, nor from Jinlongdingzi (金龙顶子) volcano about 1 600 aBP eruption, but maybe from an unknown eruption of Longgang (龙岗) volcano group according to their geochemistry and distribution. Geochemical characters of the tephra are similar to those of Jinglongdingzi, which are poor in silica, deficient in alkali, Na2O content is more than K2O content, and are similar to distribution patterns of REE and incompatible elements, which helps to speculate that they originated from the same mantle magma with rare condemnation, and from basaltic explosive eruption of Longgang volcano group. The tephra, from peat with age proved that the eruption possibly happened in 15 BC–26 AD, is one of Longgang volcano group eruption that was not recorded and is earlier than that of Jinglongdingzi about 1 600 aBP eruption. And the sedimentary time of tephra is during the period of low temperature alteration, which may be the influence of eruption toward the local climate according to the correlativity of eruption to local temperature curve of peat cellulose oxygen isotope.

Application of Three-Dimensional Laser Scanning and Surveying in Geological Investigation of High Rock Slope
Runqiu Huang, Xiujun Dong
2008, 19(2): 184-190.

The appearance of 3D laser scanning technology is one of the most important technology revolutions in surveying and mapping field. It can be widely used in many interrelated fields, such as engineering constructions and 3D measurements, owing to its prominent characteristics of the high efficiency and high precision. At present its application is still in the initial state, and it is quite rarely used in China, especially in geotechnical engineering and geological engineering fields. Starting with a general introduction of 3D laser scanning technology, this article studies how to apply the technology to high rock slope investigations. By way of a case study, principles and methods of quick slope documentation and occurrence measurement of discontinuities are discussed and analyzed. Analysis results show that the application of 3D laser scanning technology to geotechnical and geological engineering has a great prospect and value.