镁橄榄石多晶体的烧结－热压合成

采用可控热力学环境高温炉和活塞圆筒式等静压高温实验装置合成矿物多晶体的烧结－热压技术，以镁橄榄石多晶体的合成为例，讨论了技术细节和样品的成岩机制，此技术可以合成得到ｃｍ量级、化学和矿物组分可控的多晶体岩石样品。这些人工合成的多晶体结构均匀，密度能够达到其理论值的９８％以上     

Garnet-ilmenite-perovskite transitions in the system Mg4Si4O12-Mg3Al2Si3O12 at high pressures and high temperatures: phase equilibria, calorimetry and implications for mantle structure

Phase relations in the system Mg₄Si₄O₁₂-Mg₃Al₂Si₃O₁₂ were examined at pressures of 19-27 GPa and relatively low temperatures of 800-1000 °C using a multianvil apparatus to clarify phase transitions of pyroxene-garnet assemblages in the mantle. Both of glass and crystalline starting materials were used for the experiments. At 1000 °C, garnet solid solution (s.s.) transforms to aluminous ilmenite s.s. at 20-26 GPa which is stable in the whole compositional range in the system. In Mg₄Si₄O₁₂-rich composition, ilmenite s.s. transforms to a single-phase aluminous perovskite s.s., while Mg₃Al₂Si₃O₁₂-rich ilmenite s.s. dissociates into perovskite s.s. and corundum s.s. These newly determined phase relations at 1000 °C supersede preliminary phase relations determined at about 900 °C in the previous study. The phase relations at 1000 °C are quite different from those reported previously at 1600 °C where garnet s.s. transforms directly to perovskite s.s. and ilmenite is stable only very close to Mg₄Si₄O₁₂. The stability field of Mg₃Al₂Si₃O₁₂ ilmenite was determined at 800-1000 °C and 25-27 GPa by reversed phase boundaries. In ilmenite s.s., the a-axis slightly increases but the c-axis and molar volume decrease substantially with increasing Al₂O₃ content. Enthalpies of ilmenite s.s. were measured by differential drop-solution calorimetry method using a high-temperature calorimeter. The excess enthalpy of mixing of ilmenite s.s. was almost zero within the errors. The measured enthalpies of garnet-ilmenite and ilmenite-perovskite transitions at 298 K were 105.2±10.4 and 168.6±8.2 kJ/mol, respectively, for Mg₄Si₄O₁₂, and 150.2±15.9 and 98.7±27.3 kJ/mol, respectively, for Mg₃Al₂Si₃O₁₂. Thermodynamic calculations using these data give rise to phase relations in the system Mg₄Si₄O₁₂-Mg₃Al₂Si₃O₁₂ at 1000 and 1600 °C that are generally consistent with those determined experimentally, and confirm that the single-phase field of ilmenite expands from Mg₄Si₄O₁₂ to Mg₃Al₂Si₃O₁₂ with decreasing temperature. The earlier mentioned phase relations in the simplified system as well as those in the Mg₂SiO₄-Fe₂SiO₄ system are applied to estimate mineral proportions in pyrolite as a function of depth along two different geotherms: one is a horizontally-averaged temperature distribution in a normal mantle, and the other being 600 °C lower than the former as a possible representative geotherm in subducting slabs. Based on the previously described estimated mineral proportions versus depth along the two geotherms, density and compressional and shear wave velocities are calculated as functions of depth, using available mineral physics data. Along a normal mantle geotherm, jumps of density and velocities at about 660 km corresponding to the post-spinel transition are followed by steep gradients due to the garnet-perovskite transition between 660 and 710 km. In contrast, along a low-temperature geotherm, the first steep gradients of density and velocities are due to the garnet-ilmenite transition between 610 and 690 km. This is followed by abrupt jumps at about 690 km for the post-spinel transition, and steep gradients between 700 and 740 km that correspond to the ilmenite-perovskite transition. In the latter profile along the low-temperature geotherm, density and velocity increases for garnet-ilmenite and ilmenite-perovskite transitions are similar in magnitude to those for the post-spinel transition. The likely presence of ilmenite in cooler regions of subducting slabs is suggested by the fact that the calculated velocity profiles along the low-temperature geotherm are compatible with recent seismic observations indicating three discontinuities or steep velocity gradients at around 600-750 km depth in the regions of subducting slabs.     

Gravity and seismicity analyses of the El Chichon volcano,Chiapas, Mexico

Subduction induced compressional stress, lateral displacement faults probably linked to the NW extension of the Polochic-Motagua fault system, and local magma emplacement create a complex geological setting for the El Chichon volcano.Gravity analysis reveals that the principal structures in the area correspond to synclines and anticlines. Downward continuation of the gravity field suggests the presence of magma emplacement SE of the volcano. This magmatic emplacement appears to be the principal cause of the truncation of the syncline in which the volcano lies.Analyses of earthquakes occurring after eruptions show that the vertical distribution of events is confined to a roughly cylindrical zone which correlates well with the region of magmatic emplacement inferred from the gravity analysis.Even though more than 350 km separate the volcano from the Middle American trench, the compressional stress regime reported for the area may correlate with subduction stress (N30°E). However, the strike direction for the principal faults in the zone, N60°W, permits us to postulate that the NW extension of the Polochic-Motagua fault system is implicated in the local tectonics of the El Chichon area.     

长白山天池火山区及邻近地区壳幔结构探测研究

对长白-敦化深地震测深剖面资料利用二维射线追踪程序包进行走时拟合及地震图计算,得到了长白山天池火山区及邻近地区地壳上地幔速度结构和深部构造. 结果表明,以C2界面为标志,研究区地壳可分为上部地壳和下部地壳. 上部地壳厚1-23km,P波速度为6.00-6.25km/s;下部地壳厚12-17km,它是由一个较均匀的速度层和一个厚6-km的壳幔过渡层构成. 地壳厚度由敦化一带31-33km向东南逐渐增厚,至天池火山区最深达3km. 在天池火山区地壳存在低速体,其速度较周围介质低约为0.15km/s. 利用地震剖面探测、地震CT和大地电磁测深等结果显示,在天池火山区地壳内存在低速、低密度及低阻异常体,该异常体可能表明壳内岩浆囊的存在.     

Two models of parameterized convection for medium-sized icy satellites of Saturn

A parameterized theory of convection is developed for 6 medium-size icy satellites (MIS) of Saturn. It is an extension of the research concerning the Mimas-Enceladus paradox. Two parameterizations of dimensionless temperature are used in the model and a new constrain for tidal heating is included. It is found that the basic results of the model are independent of particulars of the parameterizations. The new constrain considerably reduces the space of possible values of the material parameter of satellites but the two basic conclusions are unchanged, i.e.: (a) the thermal state of the considered MIS can be explained in the frame of the uniform model that includes radiogenic and tidal heating; (b) the theory indicates that endogenic activity of some MIS was (or is) a result of a specific ‘excited’, high temperature state of a given satellite. The theory could be also used for estimation of tidal heating.     

Le jeu fini-Crétacé du front nord-pyrénéen aux environs de Cucugnan (Corbières méridionales,Aude, France)

The North-Pyrenean Front overthrust in northern direction was particularly active during the Upper Cretaceous, before the Upper Eocene tectonic renewal. Indeed, in the Eastern Pyrenees (Aude), ‘Garumnian’ (continental Upper Cretaceous–Palaeocene) formations lie transgressively upon the North-Pyrenean Triassic (Keuper) formation and upon the Albian beds of the Sub-Pyrenean Cucugnan slice. Relations between the North-Pyrenean Frontal Thrust and, to the northeast, the Corbières Nappe are discussed. To cite this article: A. Charrière, M. Durand-Delga, C. R. Geoscience 336 (2004).     

Tectonic repetitions of the Early Cretaceous Agrio Formation in the Chos Malal fold-and-thrust belt,Neuquén Basin,Argentina: Geometry,kinematics and structural implications for Andean building

The Neuquén Basin, developed in a retroarc setting in the central-west of Argentina, contains more than 6000 m of Mesozoic marine and continental sedimentary rocks. These rocks were deformed during the Andean orogeny leading to several thick and thin-skinned fold-and-thrust belts. The Early Cretaceous Agrio Formation is composed by a thick marine succession predominantly of black shales in which highlights a thin fluvial-aeolian sandy interval named Avilé Member. The Avilé Member, one of the most important hydrocarbon reservoirs of the Neuquén Basin, constitutes an excellent structural marker. At the Chos Malal fold-and-thrust belt, the strong mechanical anisotropy given by the contrasting lithology of the Avilé Member within the Agrio Formation favored the location of detachments along the shales and ramps affecting the sandstones during the Andean compression. Detailed field mapping at the Chacay Melehue area allowed us to recognize tectonic repetitions of the Avilé Member, which form imbrications in the simplest case whereas in other places constitute a more complex combination of imbrications, including fault-bend folding that duplicates stratigraphic sequences and fault-propagation folding that deforms more intensely the duplicated units. Along three structural cross-sections we illustrate the geometry of these tectonic repetitions of the Agrio Formation, which in the northern area have an eastward-vergence and in the central and southern regions show a clear westward-vergence. A tear fault along the arroyo Chacay Melehue could explain this vergence change. Forward modeling of the structures at the central cross-section, where a backthrust system produced imbrication, duplication and folding of the Agrio Formation, allows us to propose a balanced kinematic reconstruction of this complex structure and to compare the features produced at different stages of the deformation sequence with field observations. Our kinematic interpretation shows that the tectonic repetitions of the Agrio Formation involve 3 km of shortening above a basal detachment within the lowermost black shales. Based on a regional balanced cross-section constructed from the basement-cored Cordillera del Viento anticlinorium toward the east, across the thin-skinned sector of the Chos Malal FTB, it is possible to connect the backthrust system with east-vergent fault-bend folds that involve the stratigraphic units below the Agrio Formation. Finally, we propose a regional structural model considering the Cordillera del Viento as a basement wedge related to a low angle Andean thrust that is inserted into the sedimentary cover producing structures of different order, which evidence a strong relationship between thick and thin-skinned structures during the Andean orogeny.     

2014年于田7.3级地震的发震构造及动力学背景的初步分析

2004年2月12日新疆维吾尔自治区于田县发生了Ms7.3级地震,其发震断裂为阿尔金断裂带西南段的贡嘎错断裂带.由于地处高山无人区,存在区域历史地震漏记,但1970年以来5级以上地震活动是完整的,近20年来强震活动增强.综合分析认为,2008年于田Ms7.3地震可能加速了本次地震的发生.根据经验统计关系估计,2014年于田地震的同震地表破裂为30-40km,最大水平位错量为1.0-1.5m,地震的复发周期为300-400年.通过阿尔金断裂上前人资料和区域构造的综合分析,认为2014年于田地震是在青藏高原向北东运动背景下左旋走滑的阿尔金断裂向南西端扩展的结果.     

Thermal evolution of the Earth: Secular changes and fluctuations of plate characteristics

The average secular cooling rate of the Earth can be deduced from compositional variations of mantle melts through time and from rheological conditions at the onset of sub-solidus convection at the end of the initial magma ocean phase. The constraint that this places on the characteristics of mantle convection in the past are investigated using the global heat balance equation and a simple parameterization for the heat loss of the Earth. All heat loss parameterization schemes depend on a closure equation for the maximum age of oceanic plates. We use a scheme that accounts for the present-day distribution of heat flux at Earth's surface and that does not depend on any assumption about the dynamics of convection with rigid plates, which remain poorly understood. We show that heat supply to the base of continents and transient continental thermal regimes cannot be ignored. We find that the maximum sea floor age has not changed by large amounts over the last 3 Ga. Calculations lead to a maximum temperature at an age of about 3 Ga and cannot be extrapolated further back in time. By construction, these calculations are based on the present-day tectonic regime characterized by the subduction of large oceanic plates and hence indicate that this regime did not prevail until an age of about 3 Ga. According to this interpretation, the onset of rapid continental growth occurred when the current plate regime became stable.