小秦岭--熊耳山金成矿作用与区域构造的耦合

小秦岭-熊耳山地区是重要的金、多金属成矿带，金属元素高度富集，在长不足200km的矿带内，集中产出100多个大中小型金矿床(点)和10余个特大、中、小型钼矿床；成矿金属元素组合既有单一金矿床、铝矿床，也有铝-金-多金属矿床。金的矿床类型有石英脉型、构造蚀变岩型及斑岩．爆破角砾岩型和砂金矿床等。金的成矿流体为临界-超临界地幔流体，成矿物质具深源性。成矿时代主要为中生代，印支期是成矿的开始与先导，燕山期叠加其上，构成一个完整的中生代成矿旋回。金成矿作用受秦岭造山带印支期构造作用的制约，在前沿挤压，后缘滞后拉张的构造耦合作用动力学背景下产出。成矿与岩石圈拆沉作用及地幔流体上涌有关。     

Cenozoic lithospheric extension induced magmatism in Southwest Japan

Island chains off western Kyushu are the surface exposure in the northern margin of the Taiwan–Sinzi Folded Zone that spreads along the arc–trench system in the back-arc side from SW Japan to Taiwan. Intermittent igneous activity between the Middle Miocene and Holocene occurred on these islands and widely covered or intruded sedimentary rocks of Early–Middle Miocene. Geochemistry of the volcanic rocks from the Hirado, Ikitsuki and Takushima islands believed to relate to the back-arc opening along the East China and Japan Seas shows a temporal change in source material. Submarine to sub-aerial volcanism occurred on Hirado Island at 15 Ma during the final opening stage of the East China Sea producing tholeiitic basalt and associated andesite–dacite. These eruptives show low incompatible element contents and high FeO*/MgO ratios and reflect a tholeiitic differentiation trend. High Sr and Pb and low Nd isotopic ratios suggest the involvement of EM2-like lithospheric mantle and crustal material in the formation of these syn-opening volcanic rocks. Post-opening alkali basalt volcanism occurred at 9–6 Ma on the islands is characterized by OIB-like higher large ionic lithophile elements (LILE) and high field strength elements (HFSE) compared to 15 Ma basalts in this region and Quaternary basalts along the volcanic front. They have variable range of incompatible element concentrations and ratios along with variable Sr, Pb and Nd isotopic ratios suggesting the involvement of both lithospheric and asthenospheric sources at variable melting degrees (from 4% to less than 15%). The observation that the isotopic compositions of Quaternary alkali basalts south of the studied area are even more depleted suggests an increase in the involvement of asthenospheric source with time.     

Mapping the mantle lithosphere for diamond potential using teleseismic methods

Recent developments in seismic, magnetotelluric and geochemical analytical techniques have significantly increased our capacity to explore the mantle lithosphere to depths of several hundred kilometres, to map its structures, and through geological interpretations, to assess its potential as a diamond reservoir. Several independent teleseismic techniques provide a synergistic approach in which one technique compensates for inadequacies in another. Shear wave anisotropy and discontinuity studies using single seismic stations define vertical mantle stratigraphic columns. For example, beneath the central Slave craton seismic discontinuities at depths of 38, 110, 140 and 190 km appear to bound two distinct anisotropic layers. Tomographic (3-D) inversions of seismic wave travel-times and 2-D inversions of surface or scattered waves use arrays of stations and provide lateral coverage. In combination, and by correlation with electrical conductivity and xenolith petrology studies, these techniques provide maps of key physical properties within parts of the cratons known to host diamonds. Beneath the Slave craton, the discontinuity at 38 km is the base of the crust; the boundaries at 110 and 140 km appear to bound a layer of depleted harzburgite that is interpreted to contain graphite. To date, only some of these techniques have been applied to the Slave and Kaapvaal cratons so that the origin and geological history of the currently mapped mantle structures are not, as yet, generally agreed.     

Lithospheric structure and dynamic processes of the Tianshan orogenic belt and the Junggar basin

Located at the center of the Eurasian continent and accommodating as much as 44% of the present crustal shortening between India and Siberia, the Tianshan orogenic belt (TOB) is one of the youngest (<20 Ma) and highest (elevation>7000 m) orogenic belts in the world. It provides a natural laboratory for examining the processes of intracontinental deformation. In recent years, wide angle seismic reflection/refraction profiling and magnetotelluric sounding surveys have been carried out along a geoscience transect which extends northeastward from Xayar at the northern margin of the Tarim basin (TB), through the Tianshan orogenic belt and the Junggar basin (JB), to Burjing at the southern piedmont of the Altay Mountain. We have also obtained the 2D density structure of the crust and upper mantle of this area by using the Bouguer anomaly data of Northwestern Xinjiang. With these surveys, we attempt to image the 2D velocity and the 2D electric structure of the crust and upper mantle beneath the Tianshan orogenic belt and the Junggar basin. In order to obtain the small-scale structure of the crust–mantle transitional zone of the study area, the wavelet transform method is applied to the seismic wide angle reflection/refraction data. Combining our survey results with heat flow and other geological data, we propose a model that interprets the deep processes beneath the Tianshan orogenic belt and the Junggar basin.Located between the Tarim basin and the Junggar basin, the Tianshan orogenic belt is a block with relatively low velocity, low density, and partially high resistivity. It is tectonically a shortening zone under lateral compression. A detachment exists in the upper crust at the northern margin of the Tarim basin. Its lower part of the upper crust intruded into the lower part of the upper and the middle crust of the Tianshan, near the Korla fault; its middle crust intruded into the lower crust of the Tianshan; and its lower crust and lithospheric mantle subducted into the upper mantle of the Tianshan. In these processes, the mass of the lower crust of the Tarim basin was carried down to the upper mantle beneath the Tianshan, forming a 20-km-thick complex crust–mantle transitional zone composed of seven thin layers with a lower than average velocity. The thrusting and folding of the sedimentary cover, the intrusive layer in the upper and middle crust, and the mass added by the subduction of the Tarim basin into the upper mantle of the Tianshan are probably responsible for the crustal thickening of the Tianshan. Due to the important mass deficiency in the crust and the upper mantle of the Tianshan, buoyancy must occur and lead to rapid ascent of the Tianshan.The episodic tectonic uplift of the Tianshan and tectonic subsidence of the Junggar basin are closely related to the evolution of the Paleozoic, Mesozoic, and Cenozoic Tethys.     

大陆科学钻探的新发现与研究主题

大陆科学钻探是当代地球科学的重大科学工程之一,也是带动21世纪地球科学发展的关键。概括了20多年来大陆科学钻探取得的主要成就,并对其面临的科学主题作了详细阐述;最后对我国开展大陆科学钻探提出了几点建议。     

Crustal structure of seismic velocity in southern Tibet and east-westward escape of the crustal material——An example by wide-angle seismic profile from Peigu Tso to Pumoyong Tso

The Tibetan plateau, which results from continu- ous collision between India continent and southern Eurasia continent, is cross-cut by no less than three major east-west sutures[1—6]. Yarlung Zangbo suture, marked by one 1500km long ophiolite belt appearing at the southernmost Tibet, separates the Tethyan Hi- malaya to the south from the Lhasa block to the north. In order to understand the deep structure of Tibetan crust and the uplifting process of Tibetan Plateau, one wide-angle seismic…     

Sharp contrast in lithospheric structure across the Sorgenfrei–Tornquist Zone as inferred by Rayleigh wave analysis of TOR1 project data

This work is a part of the TOR1 project (1996–1997) and is devoted to determining the lithospheric structure across the Sorgenfrei–Tornquist Zone in Northern Europe. For the first time in Europe, a very dense seismic broadband array has offered the possibility of determining very sharp lateral variations in the structure of the lithosphere at small scales using surface wave analysis. We measure phase velocities for Rayleigh waves with periods ranging between 10 and 100 s, both within arrays with apertures of 40–50 km (small compared to the wavelength), and along long profiles of at least 100 km. Dispersion curves are then inverted and shear-wave velocity models down to the depth of 200 km are proposed. We show that the Sorgenfrei–Tornquist Zone is a major tectonic feature within the whole lithosphere. North–east of this feature, in Sweden beneath the Baltic Shield, no lithosphere–asthenosphere boundary is observed to exist to depths of 200 km. South–west of the Sorgenfrei–Tornquist Zone, beneath Denmark, we find a lithospheric thickness of 120±20 km. The transition across the Sorgenfrei–Tornquist Zone is sharp and determined to be very steeply dipping to the south–west. We also demonstrate the existence of a sharp discontinuity between the lithospheres beneath Denmark (120±20 km thick) and beneath Germany (characterized by thicknesses of 50±10 km in the northernmost part and 100±20 km in the southwest). This discontinuity is most likely related to the Trans-European Fault at the surface.     

Lithosphere,crust and basement ridges across Ganga and Indus basins and seismicity along the Himalayan front,India and Western Fold Belt,Pakistan

Spectral analysis of the digital data of the Bouguer anomaly of North India including Ganga basin suggest a four layer model with approximate depths of 140, 38, 16 and 7 km. They apparently represent lithosphere–asthenosphere boundary (LAB), Moho, lower crust, and maximum depth to the basement in foredeeps, respectively. The Airy’s root model of Moho from the topographic data and modeling of Bouguer anomaly constrained from the available seismic information suggest changes in the lithospheric and crustal thicknesses from ∼126–134 and ∼32–35 km under the Central Ganga basin to ∼132 and ∼38 km towards the south and 163 and ∼40 km towards the north, respectively. It has clearly brought out the lithospheric flexure and related crustal bulge under the Ganga basin due to the Himalaya. Airy’s root model and modeling along a profile (SE–NW) across the Indus basin and the Western Fold Belt (WFB), (Sibi Syntaxis, Pakistan) also suggest similar crustal bulge related to lithospheric flexure due to the WFB with crustal thickness of 33 km in the central part and 38 and 56 km towards the SE and the NW, respectively. It has also shown the high density lower crust and Bela ophiolite along the Chamman fault. The two flexures interact along the Western Syntaxis and Hazara seismic zone where several large/great earthquakes including 2005 Kashmir earthquake was reported.The residual Bouguer anomaly maps of the Indus and the Ganga basins have delineated several basement ridges whose interaction with the Himalaya and the WFB, respectively have caused seismic activity including some large/great earthquakes. Some significant ridges across the Indus basin are (i) Delhi–Lahore–Sargodha, (ii) Jaisalmer–Sibi Syntaxis which is highly seismogenic. and (iii) Kachchh–Karachi arc–Kirthar thrust leading to Sibi Syntaxis. Most of the basement ridges of the Ganga basin are oriented NE–SW that are as follows (i) Jaisalmer–Ganganagar and Jodhpur–Chandigarh ridges across the Ganga basin intersect Himalaya in the Kangra reentrant where the great Kangra earthquake of 1905 was located. (ii) The Aravalli Delhi Mobile Belt (ADMB) and its margin faults extend to the Western Himalayan front via Delhi where it interacts with the Delhi–Lahore ridge and further north with the Himalayan front causing seismic activity. (iii) The Shahjahanpur and Faizabad ridges strike the Himalayan front in Central Nepal that do not show any enhanced seismicity which may be due to their being parts of the Bundelkhand craton as simple basement highs. (iv) The west and the east Patna faults are parts of transcontinental lineaments, such as Narmada–Son lineament. (v) The Munghyr–Saharsa ridge is fault controlled and interacts with the Himalayan front in the Eastern Nepal where Bihar–Nepal earthquakes of 1934 has been reported. Some of these faults/lineaments of the Indian continent find reflection in seismogenic lineaments of Himalaya like Everest, Arun, Kanchenjunga lineaments. A set of NW–SE oriented gravity highs along the Himalayan front and the Ganga and the Indus basins represents the folding of the basement due to compression as anticlines caused by collision of the Indian and the Asian plates. This study has also delineated several depressions like Saharanpur, Patna, and Purnia depressions.     

Crustal deformation due to Alaska–Yakutat collision

The region of Alaska and adjacent northwest Canada is tectonically active and is subjected to multiple tectonic processes including plate subduction and terrane accretion. These tectonic processes and the forces originating thereof are responsible for high seismicity in the region and deformation of the crust. In the present-day tectonic setting, the Yakutat terrane is obliquely colliding with Alaska along the Aleutian Trench. Also, flat subduction due to under thrusting of a thickened crust, probably of oceanic affinity, is contributing to the tectonic evolution of this region in a basal traction collision style. This study uses the 2D, planform, thin-viscous-sheet model to investigate the effect of the Yakutat terrane colliding with Alaska and adjacent northwest Canada. Along with the obliquity and velocity of convergence, the lateral strength heterogeneities in the crust are considered in this investigation. The results of the numerical model are constrained with the observed topography and stress orientation in Alaska. It is shown that the Alaska–Yakutat collision is producing asymmetric deformation of the crust with respect to the normal to the collision boundary and that lateral strength heterogeneities contribute significantly to the deformation of the crust. Also, the influence of this collision can be observed up to a distance of 700 km inland from the collision boundary.     

Petrogenesis of the Late Triassic shoshonitic Shadegai pluton from the northern North China Craton: Implications for crust-mantle interaction and post-collisional extension

Latest Permian to Triassic plutons are widespread in the northern North China Craton(NCC); most of them show calc-alkaline, high-K calc-alkaline, or alkaline geochemical features. The Shadegai pluton in the Wulashan area has shoshonitic affinity and I-type character, and is composed of syenogranites containing abundant mafic microgranular enclaves(MMEs). LA-MC-ICP-MS U-Pb data yield weighted mean 206 Pb/238 U ages of 222 ± 1 Ma and 221 ± 1 Ma for the syenogranites and MMEs, respectively, suggesting their coeval formation during the Late Triassic. The syenogranites have high SiO_2(70.42-72.30 wt%),K_2O(4.58-5.22 wt.%) and Na_2O(4.19-4.43 wt.%) contents but lower concentrations of P_2O_5(0.073-0.096 wt.%) and TiO_2(0.27-0.37 wt.%), and are categorized as I-type granites, rather than A-type granites, as previously thought. These syenogranites exhibit lower(~(87)Sr/~(86)Sr)i ratios(0.70532-0.70547) and strongly negative whole-rock εNd(t) values(-12.54 to-11.86) and zircon εHf(t) values(-17.81 to-10.77),as well as old Nd(1962-2017 Ma) and Hf(2023-2092 Ma) model ages, indicating that they were derived from the lower crust.Field and petrological observations reveal that the MMEs within the pluton probably represent magmatic globules commingled with their host magmas. Geochemically, these MMEs have low SiO_2(53.46-55.91 wt.%)but high FeOt(7.27-8.79 wt.%) contents. They are enriched in light rare earth elements(LREEs) and large ion lithophile elements(LILEs), and are depleted in heavy rare earth elements(HREEs) and high field strength elements(HFSEs). They have whole-rock(~(87)Sr/~(86)Sr)i ratios varying from 0.70551 to 0.70564, εNd(t) values of -10.63 to -9.82, and zircon εHf(t) values of -9.89 to 0.19. Their geochemical and isotopic features indicate that they were derived from the subcontinental lithospheric mantle mainly metasomatized by slab-derived fluids, with minor involvement of melts generated from the ascending asthenospheric mantle. Petrology integrated with elemental and isotopic geochemistry suggest that the Shadegai pluton was produced by crust-mantle interactions, i.e., partial melting of the lower continental crust induced by underplating of mantle-derived mafic magmas(including the subcontinental lithospheric mantle and asthenospheric mantle), and subsequent mixing of the mantle-and crust-derived magmas. In combination with existing geological data, it is inferred that the Shadegai pluton formed in a post-collisional extensional regime related to lithospheric delamination following the collision between the NCC and Mongolia arc terranes.