Episodes of Cenozoic Gold Mineralization on the Eastern Margin of the Qinghai-Tibet Plateau:40Ar/39Ar Dating and Implication for Geodynamic Events

A lot of new gold deposits have been found on the eastern margin of the Qinghai-Tibet Plateau during the past two decades. Among them, three main types of gold deposits have been recognized, including quartz-vein-type, shear- zone-type and porphyry-type. The former two types of gold deposits are mainly hosted within metamorphic rocks, while the latter is related to Cenozoic magmatism. Although all of these gold deposits are believed to have been formed during the uplift process of the Qinghai-Tibet Plateau in the Cenozoic era (Wang et al., 2002b), precise isotopic age constraints have still been lacking until quite recently. This paper presents new 40Ar/39Ar data of some gold deposits on the eastern margin of the Qinghai-Tibet Plateau, which indicate that gold mineralization in the region occurred in response to the episodic stages of the orogenies. Recently obtained 40Ar/39Ar data on quartz and feldspars from several gold deposits, such as the Sandiao deposit, the Baijintaizi deposit, the Pusagang deposits, provide new constraints on gold mineralization on the eastern margin of the Qinghai-Tibet Plateau. Geochronological studies of gold deposits along the Daduhe River indicate that there are three stages of gold mineralization. The early two stages occurred as early as 65.1 Ma in the Shuibaiyang deposit and 58.95 Ma in the Ruoji deposit, while the latter stage occurred as late as 25.35 Ma in Baijintaizi and 24.70 Ma in Sandiao. Isotopic dating of three plagioclases from the Beiya deposit, Zhifanggou deposit and Luobodi deposit and a K-feldspar from the Jinchangqing deposit in Yunnan Province indicates that these deposits were formed at two stages. The Zhifanggou and Jinchangqing deposits have early stage records as old as 58.82 Ma in Zhifanggou and 55.49 Ma in Jinchangqing, but all of the above four deposits in Yunnan have late stage records of 23.18 Ma in Jinchangqing, 24.54 Ma in Zhifanggou, 24.60 Ma in Luobodi and 24.56 Ma in Hongnitang. The above results suggest that the gold deposits on the eastern margin of the Qinghai-Tibet Plateau were formed concentratedly at two main episodes, i.e. the end of the Paleocene (about 58 Ma) and the boundary between the Paleogene and the Neogene (about 25 Ma). The later episode appears to be looks like more important and was coupled with the Sichuan movement, which was extensively activated at that period. The beginning of the Cenozoic Era (about 65 Ma) might be another episode of gold mineralization, but only one deposit (Shuibaiyang) in this study has been proved to have been be formed at this stage and might be earlier than the initial collision between the Indian Plate and the Eurasia Plate. In view of geology, the above three episodes of gold mineralization are associated with three events of tectonic- magmatism and/or fluid events. Even though the gold deposits (for example, the Shuibaiyang deposit, Ruoji deposit and Pusagang deposit) were formed at different episodes, all of them are genetically related to tectonic movements in large- scale shear zones. It looks like theat tectonic events (including large-scale strike-slip) between Paleogene and Neogene had a wide influence upon gold mineralization, with new deposits formed and old deposits enriched or superimposed to be a higher grade by new stage of mineralization. The above data suggest that gold deposits were not only concentrated in some areas, but also formed mainly at different boundaries of geological times, indicating that there existed some peak stages of gold mineralization (metallogenic episodes), and that the gold deposits were formed mainly by episodic mineralization.

Neotectonics of the Eastern Margin of the Tibetan Plateau: New Geological Evidence for the Change from Early Pleistocene Transpression to Late Pleistocene-Holocene Strike-slip Faulting

We present in this paper some new evidence for the change during the Quaternary in kinematics of faults cutting the eastern margin of the Tibetan Plateau. It shows that significant shortening deformation occurred during the Early Pleistocene, evidenced by eastward thrusting of Mesozoic carbonates on the Pliocene lacustrine deposits along the Minjiang upstream fault zone and by development of the transpressional ridges of basement rocks along the Anninghe river valley. The Middle Pleistocene seems to be a relaxant stage with local development of the intra-mountain basins particularly prominent along the Minjiang Upstream and along the southern segment of the Anninghe River Valley. This relaxation may have been duo to a local collapse of the thickened crust attained during the late Neogene to early Pleistocene across this marginal zone. Fault kinematics has been changed since the late Pleistocene, and was predominated by reverse sinistral strike-slip along the Minshan Uplift, reverse dextral strike-slip on the Longmenshan fault zone and pure sinistral strike-slip on the Anninghe fault. This change in fault kinematics during the Quaternary allows a better understanding of the mechanism by which the marginal ranges of the plateau has been built through episodic activities.

Crustal flow beneath the eastern margin of the Tibetan plateau

In large continental orogens, an important research topic is the behavior of deep crustal and upper mantle deformation, and the flow styles of ductile material. The morphology of the eastern margin of the Tibetan plateau, adjacent to the Sichuan basin, is characterized by very steep relief with high mountain ranges. The crust beneath this region slows the velocities in the middle and lower crust. We have adopted a relatively dense network to inverse the detailed structure of the crust and upper mantle along the eastern margin of the Tibetan plateau and Sichuan basin, using teleseismic data via receiver function analysis. The results are in-line with the hypothesis that viscous crustal material is flowing beneath the eastern margin of the Tibetan plateau and that this process drives overlying crustal material around the strong and rigid Sichuan basin. When the viscous material hits this obstruction, flows are divided into two or more branches with different directions. The upper part of the upwelling viscous flow produces the pressure to intrude the upper crust, thereby driving uplift of mountain ranges and high peaks. In contrast, the lower part of the downwelling viscous flow produces the pressure to intrude the lower crust and upper mantle to deepen the Moho discontinuity, causing observed crustal thickening.

Crustal Motion Characteristics in the Eastern Margin of the Tibetan Plateau and Adjacent Regions after the Wenchuan Earthquake

The Wenchuan earthquake has altered the crustal motion characteristics in the eastern margin of the Tibetan Plateau and adjacent regions.Using discontinuous GPS survey data for 2008–2012, the velocity field for the Eurasia reference framework has been obtained, and the general trend of contemporary crustal motion after the occurrence of the Wenchuan earthquake has been studied.In addition, using the velocity field, the block movement velocity has been estimated by least-squares fitting.Furthermore, the properties and displacement rates of main faults have been obtained from the differences in velocity vectors of the blocks on both sides of the faults.The results reveal that there are no obvious changes in the general characteristics of crustal motion in this area after the Wenchuan earthquake.The earthquake mainly changed the rate of the movement of the Chuan-Qing block and caused variation in the movement direction of the South China block.The effect of the earthquake on faults is mainly reflected in variations in fault displacement velocity; there is no fundamental change in the properties of fault activity.The displacement rates of the Xianshuihe fault decreased by 3–4 mm/a, the Longmenshan fault increased by 9–10 mm/a, and the northern segment of the Anninghe fault increased by approximately 9 mm/a.Furthermore, the displacement rates of the Minjiang, Xueshan, Huya, Longquanshan, and Xinjin faults increased by 2–3 mm/a.This implies that the effects of the Wenchuan earthquake on crustal movement can mainly be observed in the Chuan-Qing, South China, and N-Chuan-Dian blocks and their internal faults, as well as the Xianshuihe and Longmenshan faults and the northern section of the Anninghe fault.The reason for this is that the Wenchuan earthquake disturbed the kinematic and dynamic balance in the region.

Crustal Uplift in the Longmen Shan Mountains Revealed by Isostatic Gravity Anomalies along the Eastern Margin of the Tibetan Plateau

This study examines the relationship between high positive isostatic gravity anomalies （IGA）, steep topography and lower crustal extrusion at the eastern margin of the Tibetan Plateau. IGA data has revealed uplift and extrusion of lower crustal flow in the Longmen Shan Mountains （the LMS）. Firstly, The high positive IGA zone corresponds to the LMS orogenic belt. It is shown that abrupt changes in IGA correspond to zones of abrupt change of topography, crustal thickness and rock density along the LMS. Secondly, on the basis of the Airy isostasy theory, simulations and inversions of the positive IGA were conducted using three-dimensional bodies. The results indicated that the LMS lacks a mountain root, and that the top surface of the lower crust has been elevated by 11 km, leading to positive IGA, tectonic load and density load. Thirdly, according to Watts＇s flexural isostasy model, elastic deflection occurs, suggesting that the limited （i.e. narrow） tectonic and density load driven by lower crustal flow in the LMS have led to asymmetric flexural subsidence in the foreland basin and lifting of the forebulge. Finally, based on the correspondence between zones of extremely high positive IGA and the presence of the Precambrian Pengguan-Baoxing complexes in the LMS, the first appearance of erosion gravels from the complexes in the Dayi Conglomerate layer of the Chengdu Basin suggest that positive IGA and lower crustal flow in the LMS took place at 3.6 Ma or slightly earlier.

Application of the Material Balance Method in Paleoelevation Recovery: A Case Study of the Longmen Mountains Foreland Basin on the Eastern Margin of the Tibetan Plateau

We applied the material balance principle of the denudation volume and sedimentary flux to study the denudation-accumulation system between the Longmen Mountains （Mts.） and the foreland basin. The amount of sediment in each sedimentation stage of the basin was estimated to obtain the denudation volume, erosion thickness and deposit thickness since the Late Triassic Epoch, to enable us to recover the paleoelevation of the provenance and the sedimentary area. The results show the following： （1） Since the Late Triassic Epoch, the elevation of the surface of the Longmen Mts. has uplifted from 0 m to 2751 m, and the crust of the Longmen Mts. has uplifted by 9.8 km. Approximately 72% of the materials introduced have been denuded from the mountains. （2） It is difficult to recover the paleoelevation of each stage of the Longmen Mts. foreland basin quantitatively by the present-day techniques and data. （3） The formation of the Longmen Mts. foreland basin consisted of three stages of thrust belt tectonic load and three stages of thrust belt erosional unload. During tectonic loading stages （Late Triassic Epoch, Late Jurassic-Early Cretaceous, Late Cretaceous-Miocene）, the average elevation of Longmen Mts. was lower （approximately 700-1700 m）. During erosional unloading stages （Early and Middle Jurassic, Middle Cretaceous and Jiaguan, Late Cenozoic）, the average elevation of Longmen Mts. was high at approximately 2000-2800m.

Magnetostratigraphy of a Loess-Paleosol Sequence from Higher Terrace of the Daduhe River in the Eastern Margin of the Tibetan Plateau and Its Geological Significance

The eolian deposits distributed in the river valleys in the eastern margin of the Tibetan Plateau （TP） are very useful in neotectonic and paleoclimatic studies. Firstly, the climate in the eastern margin of the TP is mainly controlled by the Indian summer monsoon, and detailed studies on the loess-paleosol sequences in this region can provide valuable terrestrial evidence of past changes in the Indian summer monsoon. Secondly, the river terraces in the eastern margin of the TP are considered to be a sensitive recorder of neotectonism to reflect the timing and amplitude of the TP uplift.

Lithospheric Electrical Structure across the Eastern Segment of the Altyn Tagh Fault on the Northern Margin of the Tibetan Plateau

Project INDEPTH （InterNational DEep Profiling of Tibet and the Himalaya） is an interdisciplinary program designed to develop a better understanding of deep structures and mechanics of the Tibetan Plateau. As a component of magnetoteUuric （MT） work in the 4th phase of the project, MT data were collected along a profile that crosses the eastern segment of the Altyn Tagh fault on the northern margin of the plateau. Time series data processing used robust algorithms to give high quality responses. Dimensionality analysis showed that 2D approach is only valid for the northern section of the profile. Consequently, 2D inversions were only conducted for the northern section, and 3D inversions were conducted on MT data from the whole profile. From the 2D inversion model, the eastern segment of the Altyn Tagh fault only appears as a crustal structure, which suggests accommodation of strike slip motion along the Altyn Tagh fault by thrusting within the Qilian block. A large-scale off-proffie conductor within the mid-lower crust of the Qilian block was revealed from the 3D inversion model, which is probably correlated with the North Qaidam thrust belt. Furthermore, the unconnected conductors from the 3D inversion model indicate that deformations in the study area are generally localized.

NEOTECTONICS ALONG THE EASTERN MARGIN OF THE TIBETAN PLATEAU: INFERENCES FROM BEDROCK RIVER INCISION PATTERNS

The topographic margin of the Tibetan Plateau adjacent to the Sichuan Basin is one of the World’s most impressive continental escarpments, rising from ～600 meters in the basin to peak elevations exceeding 6km across a horizontal distance of only 50～60km. Despite this relief, recent geodetic results (King et al., 1997; Chen et al., 1999) indicate that active shortening across this margin of the plateau is <3mm/a, and is within uncertainty of zero. Recent geodynamic models for active deformation in eastern Tibet (Royden et al., 1997) explain this discrepancy as a consequence of flow of lower crust from beneath the central plateau. These models predict relatively high rates of rock uplift in the near absence of horizontal shortening. Rivers along this margin of the plateau are actively incising into bedrock and afford the opportunity to better resolve the distribution of rock uplift along the margin.

Discovery of rhyolitic tuffaceous slate in the southwestern margin of Yangtze Craton:Zircon U-Pb ages(2491 Ma)and tectonic-thermal events

The Mesoproterozoic Dongchuan Group that is widely exposed in Yimen area,central Yunnan Province is a series of sedimentary sort of low-grade metamorphic rocks interbedded with volcanic rocks,which are closely related to the early tectonic evolution of the Earth.However,its formation era,sedimentary filling sequence,and geotectonic characteristics have always been in dispute.In this study,several rhyolitic tuffaceous slate interlayers with a centimeter-level thickness were found in the previously determined Heishan Formation of the Dongchuan Group located to the western part of Yimen-Luoci fault zone.This paper focuses on the study of the rhyolitic tuffaceous slate in Qifulangqing Village,Tongchang Township,Yimen County.LA-ICP-MS zircon dating was conducted,achieving the crystallization age of magma of 2491±15 Ma and the metamorphic ages of about 2.3 Ga,2.0 Ga,and 1.8 Ga for the first time.Meanwhile,according to in-situ Hf isotope analysis,the zirconεHf(t)values were determined to range from−3.0 to 7.6,with an average of 2.7.Furthermore,the first-stage Hf model age(TDM1)was determined to be 2513−2916 Ma,indicating that the provenance of the rhyolitic tuffaceous slate is the depleted mantle or juvenile crust between the Middle Mesoarchean and the Late Neoarchean.Therefore,it is believed that the strata of the slate were deposited in the Late Neoarchean,instead of the Mesoproterozoic as determined by previous researchers.Accordingly,it is not appropriate to group the strata into the Mesoproterozoic Dongchuan Group.Instead,they should be classified as the Maolu Formation of the Neoarchean Puduhe Group given the lithologic association and regional information.Furthermore,the magma ages of 2491±15 Ma are highly consistent with the eras of the large-scale Late Neoarchean orogenic magmatic activities on the northern margin of the Yangtze Craton,and thus reflect the orogenic process consisting of subduction and collision from Late Neoarchean to Early Paleoproterozoic.The magmatic activities during this period were possibly caused by the convergence of the supercontinent Kenorland.Meanwhile,the metamorphic ages of 2.3 Ga,2.0 Ga,and 1.8 Ga are highly consistent with three metamorphic ages of 2.36 Ga,1.95 Ga,and 1.85 Ga of the northern margin of the Yangtze Craton,indicating that the strata experienced Paleoproterozoic tectonic-thermal events.The study area is located on the eastern margin of Qinghai-Tibet Plateau,and thus was possibly re-transformed by magmatism subjected to the subduction of the Meso-Tethys Ocean during the Early Cretaceous.The discoveries made in this study will provide strong petrological and chronological evidence for analyzing the early crustal evolution of the Yangtze block.

Determination of the early Paleozoic accretionary complex in Southwestern Yunnan, China: Implications for the tectonic evolution of the Proto-Tethys Ocean

Accretionary complex study provides important knowledge on the subduction and the geodynamic processes of the oceanic plate,which represents the ancient ocean basin extinction location.Nevertheless,there exist many disputes on the age,material source,and tectonic attribute of the Lancang Group,located in Southwest Yunnan,China.In this paper,the LA-ICP-MS detrital zircon U‒Pb chronology of nine metamorphic rocks in the Lancang Group was carried out.The U‒Pb ages of the three detrital zircons mainly range from 590-550 Ma,980-910 Ma,and 1150-1490 Ma,with the youngest detrital zircons having a peak age of about 560 Ma.The U‒Pb ages of the six detrital zircons mainly range from 440-460 Ma and 980-910 Ma,and the youngest detrital zircon has a peak age of about 445 Ma.In the Lancang Group,metamorphic acidic volcanic rocks,basic volcanic rocks,intermediate-acid intrusive rocks,and high-pressure metamorphic rocks are exposed in the form of tectonic lens in schist,rendering typical melange structural characteristics of“block+matrix”.Considering regional deformation and chronology,material composition characteristics,and the previous data,this study thinks the Lancang Group may be an early Paleozoic tectonic accretionary complex formed by the eastward subduction of the Changning-Menglian Proto-Tethys Ocean,which provides an important constraint for the Tethys evolution.

Paleoseismological Study of the Late Quaternary Slip-rate along the South Barkol Basin Fault and Its Tectonic Implications,Eastern Tian Shan,Xinjiang

The easternmost Tian Shan lies in eastern Xinjiang, Central Asia. The South Barkol basin fault（SBF） in the northern part of the easternmost Tian Shan is a major tectonic structure in this orogenic region. The late Quaternary activity, paleoseismology, and deformation characteristics of the fault provide important clues for understanding the tectonic process of the eastern Tian Shan orogen and implementing seismic mitigation. Through interpretation of high-resolution satellite images, unmanned aerial vehicle measurements, and detailed geological and geomorphic investigations, we suggest that the fault exhibits clear left-lateral slip along its western segment. Paleoseismic trenches dug near Xiongkuer reveal evidence of six large paleoearthquakes. The four latest paleoearthquakes were dated： the oldest event occurred at 4663 BC–3839 BC. Data on the horizontal offsets along the probable 1842 Barkol earthquake coseismic rupture suggest clear multiple relationships between cumulative offsets and possible ~4 m of coseismic left-lateral slip per event. From the cumulative offsets and 14 C sample ages, we suggest an average Holocene left-lateral slip rate of 2.4–2.8 mm/a on the SBF, accounting for ~80% of lateral deformation within the entire eastern Tian Shan fault system. This result is comparable with the shortening rate of 2–4 mm/a in the whole eastern Tian Shan, indicating an equal role of strike-slip tectonics and compressional tectonics in this orogen, and that the SBF may accommodate substantial lateral tectonic deformation.

The Coupling Relationship between the Uplift of Longmen Shan and the Subsidence of Foreland Basin,Sichuan,China

Depending on the analysis of the coeval sedimentary geometry and subsidence mechanism in the Longmen Shan foreland basin, three models about the coupling relationship between Longmen Shan uplift and foreland basin subsidence since the Indosinian have been proposed：（1） crustal shortening and its related wide wedge-shaped foreland basin,（2） crustal isostatic rebound and its related tabular foreland basin, and（3） lower crustal flow and its related narrow wedge-shaped foreland basin. Based on the narrow wedge-shaped foreland basin developed since 4 Ma, it is believed that the narrow crustal shortening and tectonic load driven by lower crustal flow is a primary driver for the present Longmen Shan uplift and the Wenchuan（Ms 8.0） earthquake.

Three-Dimensional In Situ Stress-Field Simulations of Shale Gas Formations:A Case Study of the 5th Member of the Xujiahe Formation in the Xinchang Gas Field,West Sichuan Depression

This work established a geological model for the 5th member of the Xujiahe Formation（X5 member） in the Xinchang gas field of the West Sichuan Depression based on the lithological, structural and depositional properties, as well as logging and well completion data and drill-core observations. Rock mechanical parameters were calculated according to rock mechanic experiments and rock mechanic interpretations from logging data. We also calculated the magnitudes and orientations of the in situ stresses based on acoustic emission tests, differential strain tests, fracturing behaviour and logging interpretations as well as anisotropy logging tests, borehole-breakout measurements and well-log data. Additionally, the present stress field of the X5 member was simulated using finite element numerical（FEM） simulation methods. The numerical simulation results indicate that the distributions of lithology and fractures are key factors that influence the present stress field. The stress field in the study area is discontinuous as a result of fractures and faults in the central and eastern areas. Stress is concentrated at the end sections and bends of faults, but dissipates with distance away from both sides of the faults. A longitudinal profile clearly demonstrates the zonality and continuity of the stress field and an increase with depth. The differential stress distribution is relatively uniform; however, large deviations occur in fracture zones.

青藏高原东缘及邻区强震构造:专辑序言

青藏高原东缘由多个次级构造单元组成了独特的“多层次挤出-旋转活动构造体系”,是调节高原物质向东挤出的构造转换带,发育了高密度的活动断裂,强震频度高且强度大,而且地质地貌复杂,成为我国地震灾害问题最为突出的地区之一。近年来,伴随全国地震灾害风险普查、城市活断层探测、地震科学实验场建设、工程场地的地震安全性评价以及重大工程与城镇区的活断层鉴定与地壳稳定性评价等工作的深入开展,高精度遥感、构造地貌、古地震和第四纪年代学等方法的广泛应用,显著提升了青藏高原东缘及邻区强震构造的调查研究程度。为了及时交流这方面的最新研究成果,支撑区域防震减灾及重要工程和基础设施建设的地质安全评价等工作,《地震科学进展》编辑部组织了“青藏高原东缘及邻区强震构造”成果专辑,征集了活断层与地震、甘肃积石山地震以及相关领域的研究综述等代表性学术论文20余篇,本期《青藏高原东缘及邻区强震构造专辑Ⅰ》优选了10篇论文,后续还将推出《青藏高原东缘及邻区强震构造专辑Ⅱ》,希望这些成果可提升对区域强震活动特征与孕震构造机制的理解,并为区域强震危险性分析和有效防范强震灾害风险提供科学依据。

青藏高原东缘及邻区强震构造研究新进展:专辑序言

青藏高原东缘作为调节高原物质向东挤出的构造转换带,发育了高密度活动断裂和具有高频强震活动,而且地质地貌复杂,人口相对密集,因而成为我国地震灾害问题最突出的地区之一。近年来,伴随地震灾害风险普查和活断层探测等工作的深入,对青藏高原东缘及邻区强震构造及相关地震灾害的调查研究程度得到不断提升。为了及时交流相关的研究成果,支撑区域防震减灾和地质安全评价等工作,《地震科学进展》编辑部组织了“青藏高原东缘及邻区强震构造”成果专辑2期。此期专辑共征集优选了强震构造、地震与灾害和构造地貌相关领域的代表性学术论文11篇,希望这些成果可进一步提升对区域强震构造特征、地震活动风险及触发地质灾害规律等的认识,从而为防范强震灾害风险提供科学依据或参考。

Multi-Stage Basin Development and Hydrocarbon Accumulations： A Review of the Sichuan Basin at Eastern Margin of the Tibetan Plateau

Sichuan Basin is one of the uppermost petroliferous basins in China. It experienced three evolutionary phases which were marine carbonate platform （Ediacaran to Late Triassic）, Indosinian-Yanshanian orogeny foreland basin （Late Triassic to Late Cretaceous） and uplift and tectonic modification （Late Cretaceous to Quaternary）. The present-day tectonics of the Sichuan Ba- sin and its periphery are characterized by three basic elements which are topography, basement type and surface structure, and two settings （plate margin and interior）. Therefore, be subdivided into five units which have different structure and tectonic history. The basin contains five different sets of source rocks with thickness up to 2 500 m. These source rocks were well preserved due to the presence of Middel-Lower Triassic evaporites （〉-200 m） and thick terrestrial sediments filling in the Indosinian-Yanshanian foreland basin （〉3 000 m）. The uplift and erosion since Late Cretaceous has significant influence on cross-strata migration and accumulation of oil and gas. The multi-phase evolution of the basin and its superimposed tectonic elements, good petroleum geologic conditions and diverse petroleum systems reveal its bright exploration prospects.

Evidence of crustal ‘channel flow’ in the eastern margin of Tibetan Plateau from MT measurements

Magnetotelluric (MT) survey has been carried out in the eastern margin of the Tibetan Plateau and its neighboring Shimian-Leshan area, Sichuan Province. Analysis of this MT data reveals that the electric structure of the Tibetan Plateau differ much from that of the Sichuan block. In general, the electric re-sistivity of crust beneath the Sichuan block in the east is larger than that of the eastern margin of the Tibetan Plateau in the west. The crust of the plateau is divided into upper, middle, and lower layers. The middle crust is a low resistivity layer with minimum down to 3―10 Ωm about 10―15 km thick. It pre-sumably contains partial melt and/or salt-bearing fluids with low viscosity, prone to deform and flow, producing a "channel flow" under the southeastward squeeze of the eastern Tibetan Plateau. This low-resistivity layer makes the upper crust decoupled mechanically from the lower crust. In the brittle upper crust, faults are dominated by left-lateral strike-slip and thrust motions, leading to surface rising and shallow earthquakes. The low-resistivity layer also cut the Xianshuihe-Anninghe fault zone into two sections vertically. In this region, the thicknesses of upper, middle, and lower crust vary laterally, pro-ducing a transitional zone in the eastern margin of the Tibetan Plateau characterized by thicker crust and higher elevation in the west and thinner crust and lower elevation in the east.

Genesis of the Madang Cenozoic sodic alkaline basalt in the eastern margin of the Tibetan Plateau and its continental dynamic implications

The Madang Cenozoic sodic alkaline basalt occurred in the eastern margin of the Tibetan Plateau, where is a key tectonic transform region of Tibet, North China, and Yangtze blocks. The basalts are characterized by the variation in SiO2=42%―51%, Na2O/K2O>4, belonging to the sodic alkaline basalt series. The rocks are enriched in Ba, Th, Nb, Ta, relative to a slight depletion in K, Rb in the trace and rare earth element (REE) spider diagrams that are similar to the typical oceanic island alkaline basalt. The Sr-Nd-Pb isotopic compositions suggest that they are derived from a mixed mantle reservoir. The western Qinling-Songpan tectonic region was controlled by Tibet, North China and Yangtze blocks since Cenozoic, therefore, the region was in the stage of the substance converge from the mantle to upper crust, producing a mixed mantle reservoir in the studied area. The Madang basalts occurred in the specific tectonic background, they result from partial melting of a mixed asthenospheric mantle reservoir in the western Qinling-Songpan tectonic node.

利用双差走时成像研究青藏高原东缘地壳速度结构

青藏高原东缘断裂密布,强震频发,是研究高原侧向挤出及深部孕震环境的理想实验室.为了解龙门山次级块体及其西界龙日坝断裂带在青藏高原东缘隆升过程中的作用,我们基于四川地震台网64个宽频带地震台在2008年1月至2015年12月期间记录的震级≥3.0地震事件波形,利用双差层析成像方法揭示了四川盆地及青藏高原东缘的地壳速度结构.结果表明:夹持于龙门山断裂带(LMSF)与龙日坝断裂带(LRBF)之间的龙门山次级块体,相对东侧龙门山断裂带和四川盆地呈现明显的低速特征.结合该区域的低阻、低密度结构特征,以及块体内部、特别是龙日坝断裂带现今地震活动缺乏,我们推测这是因为该块体“相对较软”,不易脆性破裂产生地震,在青藏高原东缘与扬子块体西缘强烈相互作用过程中,该块体主要通过地壳缩短增厚和地表隆升吸收板块挤压造成的累积应变能.依据本文获得的速度等值线变化特征及已有地球物理剖面探测结果,推测龙日坝断裂带为深部向南东倾斜且向下切入基底,该断裂倾角较陡,主要以走滑运动调节应变能,而东侧龙门山断裂带倾角较缓,表现为逆冲运动导致的地壳缩短是其调节应变能的主要形式.此外,据本文多条速度剖面及已有电性剖面、重力异常的联合约束,我们推测鲜水河、安宁河断裂带均以较大倾角向南东倾斜,至少延伸至中下地壳.