Garnet amphibolites from the Ganzi–Litang fault zone,eastern Tibetan Plateau: mineralogy,geochemistry, and implications for evolution of the eastern Palaeo-Tethys Realm

The Ganzi–Litang fault zone, an outstanding tectonic element in the eastern Tibetan Plateau has been intensively debated as an in-situ suture zone marking relict of a subducted Palaeo-Tethyan oceanic crust or a failed intracontinental rift. This paper reports the garnet amphibolites discovered along the Ganzi–Litang fault zone, eastern Tibetan Plateau. These garnet amphibolites are characterized by the garnet–hornblende–rutile–sphene–plagioclase–quartz assemblage. Conventional geothermobarometry figures out the metamorphic temperature and pressure conditions at 582–626°C and 1.61–1.82 GPa, respectively. Geochemical analysis (no Nb–Ta deletions and left-inclined to flat patterns of rare-earth elements) indicates that the garnet amphibolites could represent metamorphic product of the mid-ocean-ridge (MORB)-type mafic rocks that were contaminated by a mantle plume. The protolith of the garnet amphibolites was dated at 236 Ma using in-situ U–Pb zircon method, and the retrograde metamorphism was dated at 218 Ma using in-situ U–Pb sphene method. A comprehensive analysis combined with the development of the Palaeo-Tethys Ocean and the Yidun arc through geologic time indicates a Triassic to Early Jurassic age (236–195 Ma) for the metamorphism of the garnet amphibolites. The low geothermal gradient of 9.8ºC/km and the N-MORB nature of the garnet amphibolites suggest a subduction-zone environment for the high-pressure metamorphism. Therefore, the Ganzi–Litang fault zone is a Palaeo-Tethyan suture separating the Yidun arc and the Songpan–Ganzi terrane, representing the relics of a branch of the Palaeo-Tethys Ocean that was contaminated by a mantle plume.     

Deformation along a salient-transverse zone junction: An example from the Leamington transverse zone,Utah, Sevier fold-thrust belt (USA)

Within fold-thrust belts, the junctions between salients and recesses may hold critical clues to the overall kinematic history of fold-thrust belts. The deformation history within these junctions is best preserved in areas where thrust sheets extend from a salient through an adjacent recess. We examine one such junction within the Sevier fold-thrust belt (western United States) along the Leamington transverse zone, northern Utah. The Canyon Range thrust sheet can be traced continuously from the Leamington transverse zone to its adjacent salient to the south, the Central Utah segment. Deformation within the Canyon Range thrust sheet took place by faulting and cataclastic flow. Analyses of these fault networks preserved throughout the Canyon Range thrust sheet are used to develop a kinematic history of the Leamington transverse zone. Field data is supplemented by analog sandbox experiments. This study suggests that, in detail, deformation within the overlying thrust sheet may not directly reflect the underlying basement structure. Moreover, these junctions may contain several types of accommodating structures that helped to maintain critical-taper and that serve as potential targets for natural resource exploration.     

甘孜—理塘结合带通宵—理塘沿线蛇绿岩研究

蛇绿岩以岩块形式镶嵌于甘孜—理塘结合带内,经拼合可见一个较为完整的蛇绿岩套。形成于洋中脊环境,火山岩显示张裂速率大于3cm.a-1,形成时代为晚三叠世。     

The importance of the effective intermediate principal stress (σ′2) to fault slip patterns

In normal faulting regimes, the magnitudes and orientations of the maximum and minimum principal compressive stresses may be known with some confidence. However, the magnitude of the intermediate principal compressive stress is generally much more difficult to constrain and is often not considered to be an important factor. In this paper, we show that the slip characteristics of faults and fractures with complex or nonoptimal geometry are highly sensitive to variation or uncertainty in the ambient effective intermediate principal stress (σ′₂). Optimally oriented faults and fractures may be less sensitive to such variations or uncertainties. Slip tendency (T_s) analysis provides a basis for quantifying the effects of uncertainty in the magnitudes and orientations of all principal stresses and in any stress regime, thereby focusing efforts on the most important components of the system. We also show, for a normal faulting stress regime, that the proportion of potential surfaces experiencing high slip tendency (e.g., T_s ≥ 0.6) decreases from a maximum of about 38% where σ′₂ = σ′₃, to a minimum of approximately 14% where σ′₂ is halfway between σ′₃ and σ′₁, and increases to another high of approximately 29% where σ′₂ = σ′₁. This analysis illustrates the influence of the magnitude of σ′₂ on rock mass strength, an observation previously documented by experimental rock deformation studies. Because of the link between fault and fracture slip characteristics and transmissivity in critically stressed rock, this analysis can provide new insights into stress-controlled fault transmissivity.     

黎塘岩溶区土壤铁锰结核的地球化学特征研究

对黎塘岩溶区土壤铁锰结核的地球化学特征研究表明:与土壤相比,铁锰结核的化学组成中仍以SiO2、Al2O3和Fe2O3为主,但Fe2O3的含量明显增加,成为铁锰结核中含量最多的氧化物,MnO含量也有一定程度增加,它们富集系数分别为5．19和3．40;铁锰结核对重金属具有明显的富集效应,其富集顺序为Cr（M）〉Pb〉Co〉Zn〉Ni〉Ba〉Li,它在土壤中的含量受成土母质及浅层地下水活动的影响。元素的相关分析及因子分析表明,铁锰结核在形成过程中还具有成核富集效应、专性吸附效应和成核易淋溶效应。      

青藏高原东坡理塘地区近地层湍流特征研究

利用中国气象局成都高原气象研究所在青藏高原东坡理塘地区建立的大气综合观测站观测资料, 以2006年1月和7月涡旋相关资料分别代表冬季和夏季, 分析和比较了该地区近地层包括风速、 风向、 大气稳定度在内的平均场特征, 以及湍流强度、 无量纲化风脉动方差相似性和地表通量变化特征,结果表明, 1月和7月稳定度基本集中在±0.5和±0.25之间; 湍流在<2 m·s-1的风速环境中发展最为旺盛, 随着风速的增大湍流强度减小迅速; 无量纲化三维风脉动方差符合Monin-Obukhov相似理论的“1/3”定律, 其最佳通用相似函数在稳定和不稳定条件下都可以拟合得到; 地表通量均表现出明显的日变化特征, 1月以感热为主, 潜热很小; 7月以潜热为主, 感热较小。     

TECTONIC EVOLUTION OF THE GARZE—LITANG PLATE JUNCTION, WITH PARTICULAR REFERENCE TO THE GOLD DEPOSITS

TECTONIC EVOLUTION OF THE GARZE—LITANG PLATE JUNCTION, WITH PARTICULAR REFERENCE TO THE GOLD DEPOSITS     

泌阳断陷东南缘深断裂特征及其与油藏分布关系研究

泌阳断陷断块圈闭、滚动背斜及其与砾岩体复合圈闭油藏的形成，与断陷东南缘由多条断层组成的具下道上正呈断阶状的深断裂带的演化有关。本文首先研究了泌阳断陷区域构造背景，对典型地震测线的原始采集资料进行了处理和解释。在此基础上，对断陷东南缘深断裂特征及其与油藏分布关系进行了研究。     

Structural analysis of the Brigham City-Weber segment boundary zone,Wasatch normal fault,Utah: Implications for fault growth and structure

We investigate the history, kinematics, principal stress orientations and geometry of deformation at the end of a bent normal fault segment of the Wasatch fault zone, Utah. Three fault types, developed in Archean crystalline rocks, reflect progressive uplift of fault-related footwall rocks. Chlorite-breccias and phyllonites reflect deep-level, reaction-assisted plastic deformation along the north-striking part of the segment. Planar, fretted faults which formed by cataclasis cut the phyllonites and breccias and are developed throughout the footwall of the segment. Youngest faults are hematitecoated, extremely narrow polished surfaces. Slip vectors and kinematic analyses of small faults developed in the footwall indicate oblique normal slip along the north-striking portion of the segment. Slip vectors and fault orientation along the northwest-striking portion of the segment reflect complexly oriented slip on faults which strike subparallel and at high angles to the main fault trace, yet slip is confined to a broad fault-parallel zone. Small faults at the southernmost tip of the segment indicate a strong influence of the north-striking Weber segment to the south. Inversion of fault data for principal stress orientations document complexly oriented principal stresses through the segment boundary zone and suggest that ₃ may have reoriented approximately 60° over the life of the segment. Subsurface structure combined with small fault data indicate the segment boundary is comprised of a southwest-plunging bedrock high which is reflected by a sharp bend in the Brigham City segment. The southern end of the Brigham City segment may have started, as a straight, north-striking fault which has bent due to changes in stress orientations and/or interaction with the adjacent Weber segment.     

Size-Scale Effects on Strength,Friction and Fracture Energy of Faults: A Unified Interpretation According to Fractal Geometry

Summary.  Experimental results clearly indicate that large faults involved in earthquakes possess low strength, low friction coefficient and high fracture energy, in comparison with data obtained from small scale laboratory tests on rock samples. The reasons for such an unexpected anomalous behaviour have been the subject of several studies in the past and are still under debate in the Scientific Community. In this paper we propose a unifying interpretation of these size-scale effects on the basis of fractal geometry, which represents the proper mathematical framework for the analysis of multi-scale properties of rough surfaces in contact. A rather good agreement between the proposed scaling laws and the experimental data ranging from the laboratory scale up to the planetary scale typical of natural faults is achieved. Author’s address: Marco Paggi, Politecnico di Torino, Department of Structural and Geotechnical Engineering, C.so Duca degli Abruzzi 24, 10129 Torino, Italy