Crustal uplift in the southern Superior Province, Canada, revealed by paleomagnetism

A change in the polarity of magnetization with depth in the 2.45 Ga Matachewan dyke swarm is used to document vertical crustal movements that occurred at 1.9–2.3 Ga along the Kapuskasing Structural Zone, a 500-km-long fault zone that transects the Archean Superior Province of Canada. At shallow crustal levels dykes have a primary magnetization dominantly of one polarity, but at greater depths (20 km down) a polarity change occurs associated with the growth of exsolved magnetite in feldspar due to slow crustal cooling after cessation of Matachewan igneous activity. Regions of the dyke swarm with one dominant polarity are separated from those with opposite polarity by major faults. Using this polarity distribution and associated variations in the intensity of feldspar clouding and hydrous alteration, maps of the southern Superior Province are produced that display regional crustal tilting on which are superimposed more local fault-bounded blocks associated with the Kapuskasing zone. Some of these blocks have been recognized for the first time as a result of this study.The paleomagnetic work has also shown that the Matachewan swarm is regionally distorted both within and north of the Kapuskasing zone, and originally had a more radial disposition. This widespread distortion suggests that the lower crust was still relatively ductile at the time of deformation, perhaps due to high heat flow associated with the waning stages of the Matachewan mantle plume beneath.     

西倾山隆起带龙家沟组的厘定及其沉积特征

通过大比例尺区域地质调研,对西倾山地区的一套粗碎屑岩地层,依据植物化石及含煤线等特征,厘定为下侏罗统龙家沟组(Jlj);根据岩石、岩性组合、沉积构造及沉积环境等特征的研究,认为该套地层为晚印支期西秦岭褶皱成山后,山间断陷盆地中由曲流河所形成的产物;近年来发现,该套地层中有金矿产出。客观地认识和识别该套地层,对拓宽该区金矿勘查,具有现实意义。     

Detection and Evaluation of Gas Hydrates in the Eastern Nankai Trough by Geochemical and Geophysical Methods

Abstract: Interstitial waters extracted from the sediment cores from the exploration wells, “BH‐1” and “MITI Nankai Trough”, drilled ～60 km off Omaezaki Peninsula in the eastern Nankai Trough, were analyzed for the chloride and sulfate concentrations to examine the depth profiles and occurrence of subsurface gas hydrates. Cored intervals from the seafloor to 310 mbsf were divided into Unit 1 (～70 mbsf, predominated by mud), Unit 2 (70–150 mbsf, mud with thin ash beds), Unit 3 (150–250+ mbsf, mud with thin ash and sand), and Unit 4 (275–310 mbsf, predominated by mud). The baseline level for Cl “concentrations was 540 mM, whereas low chloride anomalies (103 to 223 mM) were identified at around 207 mbsf (zone A), 234–240 mbsf (zone B), and 258–265 mbsf (zone C) in Unit 3. Gas hydrate saturation (Sh %) of sediment pores was calculated to be 60 % (zone A) to 80 % (zones B and C) in sands whereas only a few percent in clay and silt. The total amount of gas hydrates in hydrate‐bearing sands was estimated to be 8 to 10 m³ of solid gas hydrate per m², or 1.48 km³ CH₄ per 1 km². High saturation zones (A, B and C) were consistent with anomaly zones recognized in sonic and resistivity logs. 2D and high‐resolution seismic studies revealed two BSRs in the study area. Strong BSRs (BSR‐1) at ～263 mbsf were correlated to the boundary between gas hydrate‐bearing sands (zone C) and the shallower low velocity zone, while the lower BSRs (BSR‐2) at～289 mbsf corresponded to the top of the deeper low velocity zone of the sonic log. Tectonic uplift of the study area is thought to have caused the upward migration of BGHS. That is, BSR‐1 corresponds to the new BGHS and BSR‐2 to the old BGHS. Relic gas hydrates and free gas may survive in the interval between BSR‐1 and BSR‐2, and below BSR‐2, respectively. Direct measurements of the formation temperature for the top 170 m interval yield a geothermal gradient of ～4.3d?C/ 100 m. Extrapolation of this gradient down to the base of gas hydrate stability yields a theoretical BGHS at～230 mbsf, surprisingly ～35 m shallower than the base of gas hydrate‐bearing sands (zone C) and BSR‐1. As with the double BSRs, another tectonic uplift may explain the BGHS at unreasonably shallow depths. Alternatively, linear extrapolation of the geothermal gradient down to the hydrate‐bearing zones may not be appropriate if the gradient changes below the depths that were measured. Recognition of double BSRs (263 and 289 mbsf) and probable new BGHS (～230 mbsf) in the exploration wells implies that the BGHS has gradually migrated upward. Tectonically induced processes are thought to have enhanced dense and massive accumulation of gas hydrate deposits through effective methane recycling and condensation. To test the hypothetical models for the accumulation of gas hydrates in Nankai accretionary prism, we strongly propose to measure the equilibrium temperatures for the entire depth range down to the free gas zone below predicted BGHS and to reconstruct the water depths and uplift history of hydrate‐bearing area.     

塔里木盆地塔中隆起构造演化与油气关系

笔者结合塔里木盆地形成的构造背景对塔中隆起的主要构造演化过程进行了研究，并具体分析了各构造演化阶段与志留系油气的关系。研究表明，塔中隆起源于早古生代拉张背景下的正断层，寒武-奥陶纪为其烃源岩形成的重要时期；早奥陶世末-晚奥陶世末的构造反转是塔中隆起的主要形成期，同时也为志留系的形成及复合圈闭提供了古地理背景；泥盆纪末塔中鼻状隆起基本定型，古油藏遭受破坏，形成了沥青砂岩；早二叠世晚期的构造变形使古油藏遭受进一步的破坏；二叠纪后构造变动主要以调节为主。伴随整个构造的演化，志留系的油气聚集也是分期次、不同规模的进行。     

Tertiary to recent oblique convergence and wrenching of the Central Dinarides: Constraints from a palaeostress study

The late Eocene to Neogene tectonic evolution of the Dinarides is characterised by shortening and orogen-parallel wrenching superposed on the late Cretaceous and Eocene double-vergent orogenic system. The Central Dinarides exposes NW-trending tectonic units, which were transported towards the Adria/Apulian microcontinent during late Cretaceous–Palaeogene times. These units were also affected by subsequent processes of late Palaeogene to Neogene shortening, Neogene extension and subsidence of intramontane sedimentary basins and Pliocene–Quaternary surface uplift and denudation. The intramontane basins likely relate to formation of the Pannonian basin. Major dextral SE-trending strike-slip faults are mostly parallel to boundaries of major tectonic units and suggest dextral orogen-parallel wrenching of the whole Central Dinarides during the Neogene indentation of the Apulian microplate into the Alps and back-arc type extension in the Pannonian basin. These fault systems have been evaluated with the standard palaeostress techniques. We report four palaeostress tensor groups, which are tentatively ordered in a succession from oldest to youngest: (1) Palaeostress tensor group 1 (D₁) of likely late Eocene age indicates E–W shortening accommodated by reverse and strike-slip faults. (2) Palaeostress tensor group 2 (D₂) comprises N/NW-trending dextral and W/WSW-trending sinistral strike-slip faults, as well as WNW-striking reverse faults. These indicate NE–SW contraction and subordinate NW–SE extension related to Oligocene to early Miocene shortening of the Dinaric orogenic wedge. (3) Palaeostress tensor group 3a (D_3a) comprises mainly NW-trending normal faults, which indicate early/middle Miocene NE–SW extension related to syn-rift extension in the Pannonian basin. The subsequent palaeostress tensor group 3b (D_3b) includes NE-trending, SE-dipping normal faults indicating NW–SE extension, which is likely related to further extension in the Pannonian basin. (4) Palaeostress tensor group 4 (D₄) is characterised by mainly NW-trending dextral and NE-trending sinistral strike-slip faults. Together, with some E-trending reverse faults, they indicate roughly N–S shortening and dextral wrenching during late Miocene to Quaternary. This is partly consistent with the present-day kinematics, with motion of the Adriatic microplate constrained by GPS data and earthquake focal mechanisms. The north–north-westward motion and counterclockwise rotation of the Adriatic microplate significantly contribute the shortening and present-day wrenching in the Central Dinarides.     

粒度统计分析方法在青藏高原隆升研究中的运用及效果--以昌马洪积扇为例

通过运用粒度统计方法对昌马洪积扇沉积物进行研究，结果显现自早更新世晚期以来存在7次沉积物粒度变粗事件，说明青藏高原东北缘自早更新世晚期以来发生了7次隆升，其间为稳定期.该结论与通过青藏高原东北缘盆地沉积分析、活动断裂及河流阶地活动时代对比而厘定的青藏高原东北缘构造活动时段基本一致，同时与整个青藏高原自早更新世晚期以来的隆升具有很好的耦合.由此为山前洪积扇研究提供了新的方法和线索。     

新疆博格达山初始隆升时间的地层学标定

博格达山位于东天山北缘,是准噶尔盆地与柴窝堡凹陷的分界岭。它的初始隆升时间是一个意义重大而又长期争论的基础地质问题。地表地质调查和人工地震勘探结果证实,石炭纪以来最早的区域性角度不整合发育于白垩系与前白垩纪地层之间。博格达地区斜层理、叠瓦状砾石迎水面等古流向示向构造研究结果证实,博南、博北沉积物扩散方向在早二叠世均指向南、侏罗纪时指向博格达主脊、而白垩纪及其后则背离博格达主脊。博格达山南、北砾石成分统计结果显示,侏罗系具双物源特征,而白垩纪物源区则为博格达山。据此推论,博格达山初始隆升发生在侏罗纪末-白垩纪初。基于博格达山初始隆升发生在侏罗纪、白垩纪转换之际,有理由相信博格达山地区在海西期未卷入天山造山带,在早中生代期间柴窝铺凹陷与准噶尔盆地同属一个盆地。     

A numerical modelling study on the processes of uplift and planation of the Tibetan Plateau

The Tibetan Plateau has experienced a number of processes of uplift and planation alternately since about 45 Ma B. P. when it began to raise. A differential equation model for describing the Plateau altitude variation with time is formulated on the basis of previous field studies and a theoretical hypothesis: if palaeomagnetic polarity is positive, the convective activity in the earth is strong; orogenic movement is violent; and the raising velocity of the Plateau is high andvice versa. The analytical solution of the equation is obtained. The altitude variation from the beginning of the Plateau uplift to present is computed through using the geomagnetic polarity reversals timing series and interstellar atomic hydrogen concentration data. A comparison between the model results and the field studies indicates that the former is quite similar to the latter. The model results are able to basically reproduce the alternating processes of uplift and planation of the plate geological history. In the present model, the influences of the denudation and the geomagnetic polarity and interstellar atomic hydrogen concentration on the raising velocity of the Plateau altitude are mainly considered. Project supported by the National Climbing Project “A Study on the Uplift, Evolution and Ecosystem of the Tibetan Plateau” and the CAS’ (Chincse Academy of Sciences) Foundations for Returned Scholars.     

天山地区冰川物质平衡的突变探测

据世界冰川监测处收集，编辑和出版的全球冰川物质平衡序列资料，用Ｍａｎｎ－Ｋｅｎｄｅｌｌ与滑动平均ｔ检测法探测了天山地区６条冰川１０ａ尺度的物质平衡突变，结果表明，７０年人或它们曾发生过突变，约三分之一的突变量系年积累减少引起的，另外三分之二由消融量加剧导致的，所以，本次突变系夏季温度升高，消融剧增为主导，叠加年降雪量递减的综合结果。