Stable carbon isotope geochemistry of adsorbed alkane gases in near-surface soils of the Saurashtra Basin,India

The stable carbon isotopic compositions of light hydrocarbon gases adsorbed in near-surface soil and sediments from the Saurashtra basin were characterized for their origin and maturity. Saurashtra is considered geologically prospective for oil and gas reserves; however, a major part of the basin is covered by the Deccan Traps, hindering the exploration of Mesozoic hydrocarbon targets. Surface geochemical prospecting, based on micro-seepage of hydrocarbons from subsurface accumulations, could be advantageous in such areas. In light of this, 150 soil samples were collected from the northwestern part of Saurashtra, around the Jamnagar area, where a thick sedimentary sequence of about 2–3 km exists under 1–1.5 km of Deccan basalt. The concentration of acid desorbed alkane gases from soil samples was found to vary (in ppb) as: methane (C₁) = 3–518; ethane (C₂) = 0–430; propane (C₃) = 0–331; i-butane (iC₄) = 0–297; n-butane (nC₄) = 2–116; i-pentane (iC₅) = 0–31 and n-pentane (nC₅) = 0–23, respectively.Fifteen samples with high concentrations of alkane gases were measured for their δ¹³C₁; δ¹³C₂ and δ¹³C₃ compositions using gas chromatography–combustion-isotope ratio mass spectrometry (GC–C-IRMS). The values for methane varied from ? 27 to ? 45.4‰, ethane from ? 20.9 to ? 27.6‰, and propane from ? 20.4 to ? 29.1‰ versus the Vienna PeeDee Belemnite (VPDB). The carbon isotope ratio distribution pattern represents isotopic characteristics pertaining to hydrocarbon gases derived from thermogenic sources. Comparisons of carbon isotopic signatures and compositional variations with the standard carbon isotopic models suggest that hydrocarbon gases found in the shallow depths of the study area are not of bacterial origin but are formed thermally from deeply buried organic matter, likely to be mainly a terrestrial source rock with a partial contribution from a marine source. These gases may have migrated to the near-surface environment, where they represent an admixture of thermally generated hydrocarbon gases from mixed sources and maturity. The maturity scale (δ¹³C versus Log Ro %) applied to the surface sediment samples of the Jamnagar area indicated the source material to be capable of generating oil and gas. The detection of thermogenic alkane gases in near-surface sediments offers the possibility of hydrocarbons at depth in Saurashtra.     

Frictional properties of saponite-rich gouge from a serpentinite-bearing fault zone along the Gokasho-Arashima Tectonic Line,central Japan

We studied a serpentinite-bearing fault zone in Gokasho-Arashima Tectonic Line, Mie Prefecture, central Japan, characterizing its internal structures, mineral assemblage, permeability, and frictional properties. The fault core situated between the serpentinite breccia and the adjacent sedimentary rocks is characterized by a zone locally altered to saponite. The clayey gouge layer separates fault rocks of serpentinite origin containing talc and tremolite from fault rocks of sedimentary origin containing chlorite but no quartz. The minerals that formed within the fault are the products of metasomatic reaction between the serpentinite and the siliceous rocks. Permeability measurements show that serpentinite breccia and fault gouge have permeability of 10⁻¹⁴–10⁻¹⁷ m² and 10⁻¹⁵–10⁻¹⁸ m², respectively, at 5–120 MPa confining pressure. Frictional coefficient of the saponite-rich clayey fault gouge ranged between 0.20 and 0.35 under room-dry condition, but was reduced to 0.06–0.12 when saturated with water. The velocity dependence of friction was strongly positive, mostly ranging between 0.005 and 0.006 in terms of a–b values. The governing friction law is not constrained yet, but we find that the saponite-rich gouge possesses an evolutional behavior in the opposite direction to that suggested by the rate and state friction law, in addition to its direct velocity dependence.     

37 ka以来日本海沉积物有机质碳和氮稳定同位素变化及其古海洋学意义

海洋沉积物有机质碳氮稳定同位素(δ¹³C、δ¹⁵N)广泛用于有机质来源示踪、古生产力和古海洋环境重建。日本海沉积物δ¹³C和δ¹⁵N值一个显著特征是在末次冰盛期(LGM)同步负偏,但是对这一现象产生的原因以及他们的演化过程的认识仍然存在明显不足。在本研究中,我们详细调查了37 ka以来日本海中部LV53-23-1岩心沉积物δ¹³C和δ¹⁵N演化历史。结果显示,沉积物δ¹³C和δ¹⁵N分别介于-26.3‰至-22.5‰和1.6‰至6.1‰,低值出现在LGM(26.5~17 ka)暗色层状泥发育时期,指示较强的陆源输入贡献。在Heinrich冰阶1时期(17~14.5 ka),δ¹³C和δ¹⁵N快速正偏,表明日本海海洋环境发生了明显的转换,对应于对马海峡淹没及对马暖流入侵。14.5 ka之后,沉积物δ¹⁵N值恢复到5‰,与开阔大洋海水硝酸盐的δ¹⁵N值近似。我们采用二端员混合模型粗略地估算了有机质来源的相对贡献。LGM时期陆源有机质贡献介于65%至80%,14.5 ka以后海源有机质贡献介于60%至80%。除了增加的陆源有机质贡献以外,LGM时期沉积物δ¹⁵N亏损还涉及如下过程:(1)较高的含Fe沙尘供给提高日本海表层海洋生物固氮效率;(2)缺氧环境盛行减弱成岩作用对沉积物δ¹⁵N影响。37 ka以来,日本海沉积物δ¹³C和δ¹⁵N变化与有机质来源、营养盐的供给、表层生产力和沉积物氧化还原条件相关,实际受海平面和全球气候制约。     

2008年汶川8.0级地震的发震构造:沿龙门山断裂带新生的地壳深部断裂

在区域地质构造研究中,龙门山断裂带也称为龙门山褶皱-冲断带或推覆构造带。许多研究者认为,2008年汶川8级地震的发震构造是这条断裂带或其中央映秀—北川断裂。笔者在深入分析龙门山断裂带的构造演化和岩石圈结构构造特征的基础上,着重探讨8级地震的发震构造,提出不同的认识。龙门山断裂带经历了松潘—甘孜造山带的前陆褶皱-冲断带(T3-J)、造山带(K-E)和青藏高原边缘隆起带(N-Q)3个动力学条件不同的演化阶段,在前两个阶段断裂带递进发展,第三阶段断裂带则被改造。从三维空间看,龙门山断裂带位于松潘—甘孜地块东南缘的上地壳内,并被推覆到扬子陆块上;而松潘—甘孜地块的中—下地壳和岩石圈地幔发生韧性增厚,而且向扬子陆块壳下俯冲,从而使浅、深部构造在垂向上形成吞噬扬子地块的鳄鱼嘴式结构。虽然在平面上汶川8级地震的主余震分布与映秀—北川断裂一致,但从剖面上看其震源所构成的震源破裂体位于龙门山断裂带之下的扬子陆块内。这种不一致性表明,8级地震的发震构造不是龙门山断裂带,而是扬子陆块内新生的高角度断裂,其走向基本与龙门山断裂带一致。推测这一震源断裂的形成过程是:当松潘—甘孜地块向东南推挤时,其前缘鳄鱼嘴构造咬合并错断被吞噬的扬子陆块部分,形成具有右旋逆平移性质的新断裂,导致汶川8级地震的发生。     

Diffusion and mass flow of dissolved carbon dioxide, methane, and dissolved organic carbon in a 7-m deep raised peat bog

In 65 samples, we got values (unusually replicable and consistent for this type of work) of concentration, ¹⁴C/¹³C (AMS) age, and δ¹³C for: peat, dissolved organic carbon (DOC), peat fractions, and dissolved CO₂ and CH₄ at 50-cm intervals down to 700 cm in Ellergower Moss, a rainwater-dependent raised (domed) bog in southwest Scotland. (1) We attribute the consistency of the results to Ellergower Moss being unusually homogeneous, with unusually low hydraulic conductivity, and containing only a few gas spaces; and to the sampling methods including 18-month equilibration of in situ samplers. (2) The dissolved gas concentration depth profiles are convex and very similar to each other, though CO₂ is 5-10 times more concentrated than CH₄, while the profile of DOC is concave. (3) The age profile of peat is near linearly proportional to depth; that for DOC is about 500-1000 yr younger than the peat at the same depth; the dissolved gases are 500-4300 years younger than the peat. The age of the operational peat fractions humic acid and humin is similar to that of whole peat. (4) The δ¹³C profile for deep peat is almost constant; δ¹³C-CO₂ is more enriched than the peat (δ¹³C-CO₂ 35‰ more); δ¹³C-CH₄ is the same amount more depleted. Nearer the surface both dissolved gases become steadily more depleted, δ¹³C is about 20‰ less at the surface. (5) A simulation shows that mass flow can account for the concentration and age profiles of DOC, but for the gases diffusion and an additional source near the surface are needed as well, and diffusion accounts for over 99% of the dissolved gas movements. (6) The same processes must operate in other peatlands but the results for Ellergower should not be extrapolated uncritically to them.     

Dissolved organic carbon and methane in a regional confined aquifer,southern Ontario,Canada: Carbon isotope evidence for associated subsurface sources

Information regarding the origin, composition and transport of natural dissolved organic carbon (DOC) in groundwater is necessary to understand the transport of metals and organic pollutants, as well as for the use of¹⁴C in DOC as an isotopic groundwater dating method. Previous research in several groundwater systems has suggested soil organic C is the predominant source of high molecular weight DOC to the subsurface. Through the use of stable isotopes,¹⁴C and geochemical analyses, this study shows that significant concentrations of DOC and CH₄ in a regional confined aquifer can be generated in situ from subsurface sedimentary organic sources. The DOC and CH₄ produced is a combined result of degradation of buried peats and bacterial action, resulting in high DOC concentrations and strongly methanogenic conditions in the aquifer. The DOC and CH₄ comprise, on average, nearly 50% of the total dissolved C pool in the central part of the aquifer. Methanogenic conditions complicate isotopic groundwater dating by the conventional dissolved inorganic carbon (DIC) method. Estimates of isotopic groundwater residence time using DOC¹⁴C data are proposed by the application of¹⁴C isotope and mass balance corrections.     

Geological,geophysical and geochemical structure of a fault zone developed in granitic rocks: Implications for fault zone modeling in 3-D

The structure of a fault zone developed in granitic rocks can be established on the basis of the spatial variability of geological, geophysical and geochemical parameters. In the North Fault of the Mina Ratones area (SW Iberian Massif, Spain), fault rocks along two studied traverses (SR-2 and SR-3 boreholes) exhibit systematic changes in mineralogy, geochemistry, fabrics and microstructures that are related to brittle deformation and alteration of granite to form cataclasite and subsequent gouge. The spatial distribution and intensity of these changes suggest a North Fault morphology that is consistent with the fault-core/damage-zone model proposed by Chester et al. (1993) to describe a fault zone architecture. North Fault damage zone thickness can be defined by the development of mechanically related mesoscopic faults and joints, that produce a Fracture Index (FI)>10. High FI values are spatially correlated with relative low seismic velocity zones (V_P<5 km/s and V_S<2.5 km/s in the well-logs), more probably related to a high concentration of fractures and geochemical alteration produced by meteoric water-granite interaction along fault surfaces. This correlation is the base of a geostatistical model proposed in the final part of this study to image the fault zone architecture of a granitic massif.     

Stable carbon isotope composition and concentrations of CO2 and CH4 in the deep catotelm of a peat bog

Vertical profiles of concentration and C-isotopic composition of dissolved methane and carbon dioxide were observed over 26 months in the catotelm of a deep (6.5 m) peat bog in Switzerland. The dissolved concentrations of these gases increase with depth while CO₂ predominates over CH₄ (CO₂ ca. 5 times CH₄). This pattern can be reproduced by a reaction-advection-ebullition model, where CO₂ and CH₄ are formed in a ratio of 1:1. The less soluble methane is preferentially lost via outgassing (bubbles). The isotopic fractionation between CO₂ and CH₄ also increases with depth, with α_C values ranging from 1.045 to 1.075. The isotopic composition of the gases traces the passage of respiration-derived CO₂ (from the near surface) through a shallow zone with methanogenesis of low isotopic fractionation (splitting of fermentation-derived acetate). This solution then moves through the catotelm, where methanogenesis occurs by CO₂ reduction (large isotopic fractionation). In the upper part of the catotelm the C-13-depleted respiration-derived CO₂ pool buffers the isotopic composition of CO₂; the δ¹³C of CO₂ increases only slowly. At the same time strongly depleted CH₄ is formed as CO₂ reduction consumes the depleted CO₂. In the lower part of the catotelm, the respiration-derived CO₂ and shallow CH₄ become less important and CO₂ reduction is the dominant source of CO₂ and CH₄. Now, the δ¹³C values of both gases increase until equilibrium is reached with respect to the isotopic composition of the substrate. Thus, the δ¹³C values of methane reach a minimum at intermediate depth, and the deep methane has δ¹³C values comparable to shallow methane. A simple mixing model for the isotopic evolution is suggested. Only minor changes of the observed patterns of methanogenesis (in terms of concentration and isotopic composition) occur over the seasons. The most pronounced of these is a slightly higher rate of acetate splitting in spring.     

Geochemical behaviour of LREE,Y and Zr in uranium mineralized and non-mineralized granite from Darshanapur area,in the Gogi-Kurlagere fault zone,Bhima basin,Yadgiri district,Karnataka

Granite core samples (n=14) from the Gogi-Kurlagere fault zone in the central part of the Bhima basin were studied in terms of LREE, Y and Zr mobility during uranium mineralization. LREE, Zr and Y along with LILE (Ba, Rb) and P show behavioral differences in the mineralised and the non-mineralised samples. Average ΣLREE in mineralised granite (240 ppm) is higher than in non-mineralised samples (157 ppm). The average Zr and Y in the mineralised granite are 193 ppm and 17 ppm, while the corresponding abundances of these elements in non-mineralised portion are 148 ppm and 11 ppm respectively. Besides enrichment of U, Th, Ba, Pb and Rb and depletion of Sr are observed in mineralized granite in comparison to non-mineralized granite. Hydrothermal alteration has led to the mobility of these elements, which again dependent on the overall geochemical behavior of the migrating fluid. REE and Y in association with uranyl [(UO₂)²⁺] ion were transported as carbonate complexes like [UO₂(CO₃)₃]^4- and [REE (CO₃)₃]^3- and were later incorporated into favourable structural loci by precipitating minerals like pitchblende and coffinite.     

贵州深切峡谷区典型岩溶地下河水文水化学特征

黔中水利枢纽水源工程——平寨水库位于贵州高原三岔河深切峡谷型岩溶区,是保障黔中地区水资源安全的重要水利基础设施。文章采用水文水化学自动监测技术,对前期水文地质基础研究较为薄弱的平寨水库区间流域内的重要支流——三塘地下河系统的水文水化学变化规律进行了研究,在此基础上探讨了深切峡谷型岩溶地下河系统的岩溶发育规律及管道模式。研究结果表明,该地下河系统水化学动态主要受覆被CO₂效应、有效降雨稀释效应和径流-排泄通道开放效应三者的控制。在不同时间尺度和大气降水条件下,水化学动态的总体变化特征不同,起主导作用的效应也有所不同。水温在年尺度上呈夏季高冬季低的变化规律,日尺度上呈昼高夜低的变化规律。电导率和水中CO₂分压的动态变化,年尺度下,在覆被CO₂效应与稀释效应的共同作用下总体呈现平水期较高丰水期较低;月尺度下,降雨初期可见明显的覆被CO₂效应,降雨后则为有效降雨的稀释效应主控;在旱季无雨条件下的日尺度上,可见由径流-排泄通道的开放效应造成的水化学日动态。径流-排泄通道开放效应的识别,可为今后在岩溶水系统模型化研究中判断岩溶管道的承压状态提供水化学方面的依据。本研究对今后该区开展地表与地下水库联合调度、水资源合理利用研究提供了一定的水文地质基础。     

Water-rock interaction in a zone of lateral flow: A natural example from the active geothermal field and gold-mineralized zone of Beppu (Kyushu Island,Japan)

Summary Beppu is a water-dominated active geothermal system in Kyushu (Japan). Gold mineralization exists in places within altered Pleistocene volcanites in the southernmost part of this field. A shallow 300 m well was drilled in an alluvial fan deposit made up of unsorted volcanic clasts of andesite and dacite. Water and core samples were collected and a temperature log was carried out. In the depth range of 150-230 m and at temperatures of 126-148° C the main authigenic minerals are quartz, calcite, K-feldspar, mixed layers illite/smectite, stilbite and pyrite. The chemical data of the waters indicate that mixing occurs between shallow bicarbonate waters and deep Na-Cl geothermal waters in the outflow zone. The authigenic mineral assemblages found in the well are analogous to those found in the epithermal gold deposits of southern Kyushu. Au concentration data for the hydrothermal springs of Beppu (0.53 g/kg) indicate that the fluids found in the borehole at 148° C could theoretically deposit gold. It is reasonable to hypothesise that a process of mixing between deep Na-Cl fluids and shallow Ca-HCO₃-SO₄ fluids were responsible for massive silicate deposition, accompanied in places by Au mineralizations.

---

Wasser-Gesteins-Wechselwirkung in einer Zone lateraler Fließbewegung: ein natürliches Beispiel aus der aktiven Geothermal-Golderz-Zone von Beppu (Kyushu, Japan)

Zusammenfassung Beppu ist ein geothermales System auf der Insel Kyushu, Japan, in der Wasser die dominante Rolle spielt. Goldvererzungen treten stellenweise in umgewandelten pleistozänen Vulkaniten im südlichsten Teil des Gebietes auf. Ein 300 m tiefes Bohrloch wurde in einem alluvialen Sedimentations-Fächer, der aus unsortierten vulkanischen Klasten, aus Andesit und Dazit besteht, niedergebracht. Wasserproben und Kernproben wurden genommen und die Temperaturen im Bohrloch systematisch gemessen. In Tiefen von 150–230 m und bei Temperaturen von 126–148°C sind die wichtigsten autigen gebildeten Minerale Quarz, Calcit, K-Feldspat, Illit/Smectit, Stilbit und Pyrit. Der Chemismus der Wässer zeigt, daß eine Mischung zwischen seichten Bikarbonat-Wässern und aus der Tiefe stammenden Na-Cl geothermalen Wässern erfolgt. Die autigenen Mineralassoziationen in diesem. Bohrloch sind vergleichbar mit denen in epithermalen Goldlagerstätten des südlichen Kyushu. Auf der Basis von existierenden Goldkonzentrationsdaten für die hydrothermalen Quellen auf Beppu (0.53 g/kg) läßt sich zeigen, daß die Fluide, die in dem Bohrloch bei 148 angetroffen wurden, theoretisch Gold absetzen könnten. Daher läßt sich die Theorie vertreten, daß Mischungsvorgänge zwischen einem aus der Tiefe stammenden Na-Cl Fluid und einem oberflächennahen Ca-HCO₃-SO₄ Wasser für die massive Silikatausscheidung verantwortlich waren, die stellenweise von Goldmineralisation begleitet wurden.

---

---

Contribution No 4. Foreign Visiting Scientist Section, Beppu Geophysical Research Laboratory, Kyoto University.

---

This paper was presented at the IGCP 291 Project Symposium Metamorphic Fluids and Mineral Deposits, ETH Zurich, March 21–23, 1991.     

冷俯冲带深部的板块断裂过程——以日本中部侏罗纪燧石-碎屑岩杂岩体中的断裂带为例

近年来,深海大洋钻探已证实在洋陆俯冲带的浅部,板块边界断裂高度集中在特征性岩层中,然而对于这种断裂高度集中化现象是否发生在俯冲带深部仍然缺乏充分的认识,因此对日本中部侏罗纪燧石-碎屑岩杂岩体中一个露头宽度约50 m的断裂带进行了调查。该断裂带沿古俯冲带深部的板块边界延伸而出的逆冲断层发育,断裂带中的剪切变形集中在数层黑色有机质黏土岩中,表现为沿交织鳞片状叶理的分散剪切,剪切方向分为左行走滑和逆冲走滑,可能形成于俯冲带中的滑动分异或左行斜俯冲过程。体现滑动集中化的拆离面切割了黑色有机质黏土岩层,拆离面表现为左行滑动。利用碳物质拉曼光谱测量了拆离面上及黑色有机质黏土岩层中的碳物质。结果表明,相对于黑色有机质黏土岩,拆离面中的碳物质具有更高的成熟度,表明在左行滑动过程中产生了快速的摩擦升温。野外观测和实验结果表明,这一古断裂带记录的俯冲带深部板块边界断裂高度集中在黑色有机质黏土岩层中,发育于其中的拆离面的左行剪切可能产生于深部地震性滑动。     

琼东南盆地深水区新生代海底扇沉积演化——以宝南断阶带为例

琼东南盆地深水区是油气勘探的重要领域,大型储集体的发育条件及预测研究对该领域下一步的勘探至关重要。以钻井、测井、高精度三维地震等资料为基础,应用井震精细标定、骨架砂体雕刻和高精度三维地貌叠合等技术手段,对琼东南盆地深水区宝南断阶带的物源方向和海底扇展布规律进行了精细研究。结果表明:①宝南断阶带的主物源来自南部隆起;②研究区内主要发育永乐C-1和永乐D-1两个大型海底扇,扇体纵向上多期叠置,扇体内部沟通下伏烃源岩的断裂发育,成藏条件优越,具有重要的勘探意义。     

Growth, linkage, and termination processes of a 10-km-long strike-slip fault in jointed granite: the Gemini fault zone, Sierra Nevada, California

Field-based structural analysis of an exhumed, 10-km-long strike-slip fault zone elucidates processes of growth, linkage, and termination along moderately sized strike-slip fault zones in granitic rocks. The Gemini fault zone is a 9.3-km-long, left-lateral fault system that was active at depths of 8–11 km within the transpressive Late-Cretaceous Sierran magmatic arc. The fault zone cuts four granitic plutons and is composed of three steeply dipping northeast- and southwest-striking noncoplanar segments that nucleated and grew along preexisting cooling joints. The fault core is bounded by subparallel fault planes that separate highly fractured epidote-, chlorite-, and quartz-breccias from undeformed protolith. The slip profile along the Gemini fault zone shows that the fault zone consists of three 2–3-km-long segments separated by two ‘zones’ of local slip minima. Slip is highest (131 m) on the western third of the fault zone and tapers to zero at the eastern termination. Slip vectors plunge shallowly west-southwest and show significant variability along strike and across segment boundaries. Four types of microstructures reflect compositional changes in protolith along strike and show that deformation was concentrated on narrow slip surfaces at, or below, greenschist facies conditions. Taken together, we interpret the fault zone to be a segmented, linked fault zone in which geometrical complexities of the faults and compositional variations of protolith and fault rock resulted in nonuniform slip orientations, complex fault-segment interactions, and asymmetric slip-distance profiles.     

Irregular western margin of the Yangtze block as a cause of variation in tectonic activity along the Longmen Shan fault zone,eastern Tibet

On 12 May 2008 and 20 April 2013, respectively, the devastating magnitude 7.9 (Wenchuan) and magnitude 7.0 (Ya’an) earthquakes struck the southwestern Longmen Shan fault zone (LMSFZ), the eastern margin of the Tibetan Plateau. These events were notable because they occurred in a heavily populated area and resulted in severe damage and loss of life. Here we present an integrated analysis of potential field anomalies and a crustal-scale seismic reflection image to investigate the crustal structure and some tectonic relationships associated with these devastating events. Our results show that the western margin of the Yangtze crustal block possesses an irregular margin that extends westward beyond the LMSFZ to the northeast and merges gradually with the LMSFZ to the southwest. We interpret this variation in deep structure to create a lateral heterogeneity in the local stress regime that explains the observed variations in fault geometry and slip distribution, as well as seismicity, of the LMSFZ. This structural complexity results in a differential build-up of stress as the Tibetan Plateau is being extruded eastward. Thus, the results of this research can help identify potential natural hazard zones and focus efforts on hazard mitigation.     

古天然气渗漏沉积型锰矿床成矿系统与成矿模式——以黔湘渝毗邻区南华纪“大塘坡式”锰矿为例

黔湘渝毗邻区"大塘坡式"锰矿床形成于南华纪裂谷盆地中,矿床主要分布于裂谷盆地中的次级断陷沉积盆地中心,受古断裂控制。矿物成分主要是菱锰矿和钙菱锰矿,具有低δ13C值(-7‰～-10‰)、高δ34S值(+40‰～+63‰)的特征。含锰岩系中,碳质页岩的Mn/Cr比值在40左右。结合菱锰矿体及相邻岩层中普遍发育由古天然气泄漏沉积形成的气泡状构造、底辟构造、渗漏管构造、帐篷构造、泥火山构造及相关的软沉积变形纹理等特征,提出该地区南华纪"大塘坡式"锰矿床是古天然气渗漏沉积成矿成因,并建立了古天然气渗漏沉积型锰矿床成矿系统与成矿模式。     

新疆柯坪地区寒武纪-奥陶纪碳酸盐岩沉积旋回的碳氧同位素研究

野外观测发现,在新疆柯坪地区蓬莱坝剖面,中寒武统阿瓦塔格组(C2a)至下奥陶统蓬莱坝组(O1p)的碳酸盐岩在剖面上存在不同尺度的沉积旋回:体系域级、米级旋回级、藻纹层级.对49件样品进行微区取样,共得到98件样品,对其进行了精细的不同尺度沉积旋回碳氧同位素组成的分析研究.结果表明,碳酸盐岩样品的δ13C值(PDB)分布在-1.6‰～1.6‰之间,平均为-0.5‰,δ18O值(PDB)为-12.7‰～-4.3‰,平均-6.2‰,因此,除了明显受到后期成岩蚀变作用(退白云岩化)的藻碳酸盐岩外,其余样品的碳氧同位素组成总体位于正常的海相碳酸盐岩沉积范围内.在体系域尺度上,对于Ⅲ2旋回层序高水位体系域(C2a1),海平面变化是控制碳酸盐岩碳氧同位素组成的主要因素;对于Ⅲ3旋回层序(C2a2-O1p),伴随海平面的升降变化,岩石沉积微环境复杂多变.在米级旋回尺度上,米级旋回的形成除了受高频海平面变化影响外,还受到古构造、古气候的叠加影响.在藻纹层旋回尺度上,δ13C值比δ18O值更能反映藻纹层的旋回性变化特征,藻纹层的形成受控于有机质沉积速率的季节性变化.     

Paleoceanographic changes of surface and deep water based on oxygen and carbon isotope records during the last 130 kyr identified in MD179 cores,off Joetsu,Japan Sea

We reconstructed the paleoenvironmental history of surface and deep water over the last 130 kyr from oxygen and carbon isotope ratios of planktonic and benthic foraminifera in two cores (MD179-3312 and MD179-3304) from the Joetsu Basin, eastern margin of the Japan Sea. Our data showed that paleoceanographic changes such as influx of surface currents and vertical circulation were associated with global glacial–interglacial sea level change. Surface water conditions were influenced by the influx of Tsushima Current, East China Sea coastal or off-shore waters through the Tsushima Strait during interglacial or interstadial stages, and strongly affected by freshwater input during the glacial maximum. During interglacial maximums such as Marine Isotope Stages 1 and 5e, development of well-oxygenated bottom water was indicated. A density-stratified ocean with weak ventilation was inferred from the isotopic records of benthic foraminifera during the Last Glacial Maximum. Local negative excursions in carbon isotopes during deglacial or interglacial periods may suggest the dissolution of gas hydrates or methane seep activities.     

Variations in the kinematics of deformation along the Zagros inclined transpression zone,Iran: Implications for defining a curved inclined transpression zone

The combination of inclined collision and plate boundary shape can control the nature of deformation and the sense of shear along a transpression zone. The present study investigated the effects of a boundary zone with curvilinear shape along a transpression zone on the kinematics of deformation. The kinematics of the Zagros transpression zone varies with the orientation of the zone boundary. Detailed structural and microstructural studies showed sinistral sense of shear on the southeastern part of the Zagros inclined transpression zone (Fars Arc), but dextral sense of shear on the northwestern part of the zone. It is inferred that the both senses of shear were developed coevally under a bulk general shear, regional-scale deformation along a curved inclined transpression miming the shape of the Fras Arc of the Zagros and the reentrant of the Bandar Abbas Syntaxis. The Zagros transpression zone formed by inclined continental collision between the Afro-Arabian continent and Iranian microcontinent.