Investigating the Whole-lithosphere Structure of a Mineral System——Pathways and Source of Ore-forming Fluids Imaged with Magnetotelluric Modeling

Whole-lithosphere structure has direct implications for both the genesis of minerals and the locations of mineral emplacement;thus knowledge of the deep structural framework of the lithosphere can advance understanding of the development and evolution of mineral systems.

P-wave Velocity Structure of Crustal and Upper Mantle beneath South China and its Tectonic Implications

The 3D P-wave velocity structure beneath the South China Block was determined by applying arrival times from 269 teleseismic events recorded by 240 seismic stations within the study region. Our tomographic results reveal the deep structural characteristics of major tectonic units and ore concentration areas. There are distinct high velocity anomalies beneath the ancient Yangtze and Cathaysia blocks, with the lithosphere of the Cathaysia Block being thinner than the Yangtze Block;the Jiangnan orogenic belt, located in the combined zone of two blocks, is a high and low velocity anomaly conversion zone;the famous metallogenic belts of Edongnan, the Youjiang Basin and the Cathaysia Block are obviously low velocity areas with different metallogenic mechanisms. The deep ore-forming material source in the Edongnan metallogenic belt is different from that of the Cathaysia Block. The low velocity anomaly under the Cathaysia Block related to mineralization results from the upwelling of mantle material, caused by the joint action of the Paleo-Tethys tectonic domain, the Paleo-Pacific tectonic domain and the Hainan mantle plume migration and erosion, which has been occurring from northeast to southwest since 80 Ma. The low-temperature mineralization mechanism of Youjiang Basin should be considered not only in terms of the influence of the Emeishan mantle plume in the west and the Paleo-Tethys tectonic domain in the south, but also in the context of the influence of the upwelling of asthenospheric material from the PaleoPacific tectonic domain in the east.

The Lithospheric Structure in the South China Block from Receiver Function: Implications for the Source of Mineral System

The South China block(SC),composed of Cathaysia(CA),Jiangnan Orogenic belt(JNB),and Yangtze block(YB),is one of the most important poly-metallic metallogenic provinces in the world(Zhang et al.,2013),containing of four famous major Mesozoic metallogenic belts,involving the Middle-Lower Yangtze Fe-Cu-Au metallogenic belt(MYMB).

Lithospheric structure and deformation in SE Tibet revealed by ambient noise and earthquake surface wave tomography: Recent advances and perspectives

High-resolution lithospheric structure is essential for understanding the tectonic evolution and deformation patterns of the southeastern Tibetan plateau. This is now possible due to recent advances in ambient noise and earthquake surface wave tomography, and great improvements in data coverage from dense portable array stations deployed in SE Tibet. In this review paper, I first give a brief overview of the tomographic methods from ambient noise and earthquake surface waves, and then summarize the major findings about the lithospheric structure and deformation in SE Tibet revealed by ambient noise and earthquake surface wave tomography as well as by other seismic and geophysical observations. These findings mainly include the 3-D distribution of mechanically weak zones in the mid-lower crust, lateral and vertical variations in radial and azimuthal anisotropy, possible interplay of some fault zones with crustal weak zones, and importance of strike-slip faulting on upper crustal deformation. These results suggest that integration of block extrusion in the more rigid upper-middle crust and channel flow in the more ductile mid-lower crust will be more compatible with the current geophysical observations. Finally I discuss some future perspective researches in SE Tibet, including array-based tomography, joint inversion using multiple seismic data, and integration of geodynamic modeling and seismic observations.

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.

Lithospheric structure in NW of Africa:Case of the Moroccan Atlas Mountains

This study presents the outcomes of the local earthquake tomography applied in the Moroccan Atlas domains. A seismic data collected by 36 seismic and a linearized inversion technics are used for determination of local velocity structure.The interpretation of tomography images results emphasizes a new and detailed lithosphere structure： a remaining subducted zone beneath the Souss Basin located from 20-to 45-km depth dipping to the North is detected and interpreted as a body that marks the border between the Moroccan Anti-Atlas and the Meseta-Atlas domains.A subduction zones is detected in the SW of the High Atlas, beneath the Hercynian Tichka massif from 10 to 50-km inclined away from Anti Atlas and in the eastern part of Anti Atlas, dipping northward from Jbel Ougnat at 15e40 km.The junction of the western and middle High Atlas is depicted by two high velocity blocks subducting from 10 to 50 km depth. The first is dipping SW beneath the High Atlas and the second is dipping SE beneath the Ouarzazate Basin.In the northern part of the southwestern High Atlas, a high velocity body dipping towards the north beneath the Essaouira Basin from 15 to 45 km depth.In northeastern part of the High Atlas in the Mougeur zone, a high velocity body is detected from 10 to 45 km depth, dipping to the Se E beneath the eastern High Atlas.The negative lithospheric anomalies found in the upper and in the lower crust are interpreted as a hot asthenospheric material upwelling from deep and gradually replacing the part of crust detached in the High Atlas. The occurrence magmatic activities in these regions testify the existence of a remaining subduction process. This paper argues the implication of these deep structures in the evolution of the Moroccan Atlas Mountain.

Lithospheric structure and continental geodynamics

This paper briefly reviews main progress in the research on lithospheric structure and continental geodynamics made by Chinese geophysicists during last 4 years since 22nd IUGG general assembly in July 1999. The research mainly covers the following fields: investigations on regional lithospheric structure, DSS survey of crust and upper mantle velocity structure, study on present-day inner movement and deformation of Chinese mainland by analyzing GPS observations, geodynamics of Qingzang plateau, geophysical survey of the Dabie-Sulu ultra-high pressure metamorphic belt and probing into its formation mechanism, geophysical observations in sedimentary basins and study on their evolution process, and plate dynamics, etc.

Regional characteristics of the lithospheric structure in the southeast of North China revealed by explosion seismology

Based on the blasting seismic detection data obtained in the southeast of North China in recent years,this paper comprehensively analyzes and studies the crust-mantle lithospheric structure and seismological characteristics of different tectonic regions,such as offshore basins,west Shandong uplift,Tanlu fault zone and Jiangsu-Shandong orogenic belt.The low-velocity Pg waves in Dongying depression and Northern Jiangsu basin reveal the unstable basement structure with extremely thick sediments.The travel time of Pg wave is characterized by relatively low propagation velocity and small crustal thickness of offshore continental margin;the first break time and high apparent velocity of Pg wave in west Shandong uplift indicate that the sedimentary basement is relatively thin.The Pm wave shows the characteristic of dominant wave in the first-order velocity discontinuity of the crust-mantle interface,which reflects the high crustal velocity and stable structure in west Shandong uplift.The Pm and Pl wave are obviously complicated,which can reflect the crust-mantle lithospheric structure of the transitional zone between Tanlu fault zone and Jiangsu-Shandong orogenic belt.The small time difference between Pn and PL waves can be regarded as the highly destructive seismological manifestation of Tanlu fault zone on the crust-lithosphere scale.Based on many geophysical phenomena such as electrical structure,density structure and terrestrial heat flow,it is believed that the lithospheric destruction degree of Tanlu fault zone and Jiangsu-Shandong orogenic belt was high during the destruction of the North China Craton.

Structural features of crust-mantle assemblage in the lithosphere along the Longitudinal Seismic Belt of China and their tectonic effect

The 1°×1° distribution map of crustmantle structural ratio R for the lithosphere along the Longitudinal Seismic Belt of China has been compiled using computer based on the results of geophysical prospecting by previous researchers, and the latest results by the present authors. Based on this map, an insight into the structural features of the crustmantle assemblage along the Longitudinal Seismic Belt has been gained, while their relation to seismic activity and the distributions of geothermal flux and intracrustal high conductivitylow velocity layers, as well as their tectonic effect to seismicity have been discussed.

Crust and Upper Mantle Density Structures beneath the Eastern Tianshan Region and Its Tectonic Implications

The deformation mechanisms of the Tianshan orogenic belt(TOB)are one of the most important unresolved issues in the collision of the Indian and Eurasian plates.To better understand the lithospheric deformation of the eastern Tianshan orogenic belt,we combined the S-wave tomography and gravity data to develop a three-dimensional(3D)density model of the crust and upper mantle beneath the eastern Tianshan area.Results show that the crust of the eastern Tianshan is mainly characterized by positive density anomalies,revealing widespread subduction-related magmatism during the Paleozoic.We however have also observed extensive low-density anomalies beneath the eastern Tianshan at depths deeper than~100 km,which is likely linked to a relatively hot mantle.The most fundamental differences of the lithosphere within the eastern Tianshan occur in the uppermost mantle.The uppermost mantle layers in the Bogda Shan and Harlik Shan are relatively dense.This is likely associated with an eclogite body in the uppermost mantle.The most significant negative anomaly of the uppermost mantle is however found in the Jueluotage tectonic belt and the central Tianshan Block and is possibly associated with depleted mantle material.We suggest that these differences related to compositional changes may control the strength of the lithospheric mantle and have affected the uplift of the northern and southern segments of the eastern Tianshan after the Permian.

Progress in the Study of Deep Profiles of Tibet and the Himalayas (INDEPTH)

This paper introduces 8 major discoveries and new understandings with regard to the deep structure and tectonics of the Himalayas and Tibetan Plateau obtained in Project INDEPTH, They are mainly as follows. (1) The upper crust, lower crust and mantle lithosphere beneath the blocks of the plateau form a 'sandwich' structure with a relatively rigid-brittle upper crust, a visco-plastic lower crust and a relatively rigid-ductile mantle lithosphere. This structure is completely different from that of monotonous, cold and more rigid oceanic plates. (2) In the process of north-directed collision-compression of the Indian subcontinent, the upper crust was attached to the foreland in the form of a gigantic foreland accretionary wedge. The interior of the accretionary wedge thickened in such tectonic manners as large-scale thrusting, backthrusting and folding, and magmatic masses and partially molten masses participated in the crustal thickening. Between the upper crust and lower crust lies a large detachment (e.g. the Main Himalayan Thrust in southern Tibet, 5-8 km thick) or a very thick shear-schistose zone (e.g. the Main Qiangtang Thrust-MQT in northern Tibet, up to 20 km thick), which causes the decoupling of the upper crust and lower crust and separation of tectonic activities. (3) During the collision-compression, the Indian mantle lithosphere was delaminated into two layers from where the crust thickened most rapidly (beneath the High Himalayas). The upper layer extends to 34.5°N and the lower layer to 33.5°. They have been underthrust to depths of 250-300 km into the asthenosphere. Meanwhile the Asian lithosphere (possibly the Qaidam terrane) has also been subducted southwards. Very thick mantle lithosphere does not exist beneath the plateau. (4) The oceanic lithosphere, in light of its lithology and dynamic behaviour, might be close to those of the continental lithosphere and its front might enter the asthenosphere before the continental lithosphere. (5) A 150-200 km deep low-velocity body below 35°N and a wide low-velocity zone below the area between 33.5° and 35°N dip north at very steep angles. Volcanism took place frequently in northern Tibet and anisotropy variations are prominent at depths, which might indicate a zone of large-scale eastward transfer of deep-seated materials.

Crustal Structural and Tectonic Evolution and Oil Prospect Evaluation in the Songpan-Zoigê Block

Recent study of magnetotelluric （MT） inversion indicates that the basement of the Songpan- Zoige area could be a stable continental crust. There has developed quite thick and stable Paleozoic continental shelf margin-platform clastic and carbonate sediments during the Triassic. Preliminary field geologic investigation and hydrocarbon potential study show that good-quality source rocks, mainly argillaceous and carbonaceous shale, were deposited in the Cambrian and Silurian in this region, while diverse reservoirs of platform facies carbonate and clastic rocks were in the Carboniferous and Permian. The good vertical source-reservoir-seal configuration might indicate that there exists a certain potential for oil and gas exploration in this area.

An Asthenospheric Upwelling Beneath Central Mongolia——Implications for Intraplate Surface Uplift and Volcanism

Intraplate processes,such as continental surface uplift and intraplate volcanism,are enigmatic and the underlying mechanisms responsible are not fully understood.Central Mongolia is an ideal natural laboratory for studying such processes because of its location in the continental interior far from tectonic plate boundaries.

A review on developments in the electrical structure of craton lithosphere

Cratons have a long history of evolution.In this paper,applications of the magnetotelluric method used in the study of craton lithosphere over the past 30 years were reviewed,examining case studies of cratons in North America,South America,Asia,Australia,and Africa.The nuclei of the Archean cratons,for example the Kalahari Craton and Rae Craton,are usually characterized by thick and highly resistive lithospheric roots.During or after the formation of the cratons,tectonothermal events,such as collision,mantle plume,and asthenosphere upwelling led to the formation of high-conductivity zones in the craton lithosphere,which could be attributed to the increased hydrogen content(of nominally anhydrous minerals),higher iron content,and formation of graphite films or sulfides along the grain boundary of minerals.These conductive zones are characterized by resistivity discontinuities in craton lithosphere.In particular,the conductive zones include(1)large-scale lithospheric mantle conductors beneath the Slave Craton,Gawler Craton,and central part of North China Craton(Trans-North China Orogen);(2)near-vertical high-conductivity zone associated with the fossil subduction zone beneath the Dharwar Craton and Slave Craton;and(3)regional lateral electrical discontinuities,such as a conductive anomaly under the Bushveld Complex of the Kaapvaal Craton.The eMoho refers to the electrical discontinuity in the crust-mantle boundary.In existing research,this has been detected under the condition of extremely high lithospheric resistivity with only a slight decrease in the lower crust,and in the case of a very thin conductive lower crust or the lack thereof.In the resistivity model,the unique"mushroom-like"lower crust-lithosphere mantle conductor and very thin lower crust layer of the North China Craton may represent lithosphere destruction and/or thinning.We also find that some of the cratons are still not well understood.Therefore,extensive three-dimensional inversion and joint interpretation of geochemical,geophysical,and geologic data are necessary to understand the tectonic evolutionary history of craton lithosphere.

Bouguer Gravity Anomaly in the Andean Orogenic Belt and its Dynamic Implications for Regional Tectonic Evolution

Calculated Bouguer gravity anomalies from the Andean orogenic belt interpreted as derived from regional gravity data to aid understanding of the lithospheric structure and tectonic evolution of the belt.These anomalies reveal lithospheric structures distributed throughout the belt,including linear and circular structures.NE-trending structures reflect sinistral transpression across the northern part of the belt,and NW-trending structures represent dextral transtension in the southern part.These results are supported by gravity-anomaly patterns that demonstrate mantle flow in a trench-parallel direction both northward and southward away from the stagnation band that is beneath the subducting Nazca slab.This mantle flow has served as an important driving force in the evolution of the Andean orogenic belt.Features of the modified tectonic model of the Andean orogenic belt are consistent with the spatial variation in and interpretation of Bouguer gravity anomalies.

Multiple phases analysis of deep seismic sounding records in North China Craton

The North China Craton(NCC)is a key region to study the destruction of the ancient craton.Two groups of phases(denoted as"Pw1"and"Pw2"),which are parallel to the PmP phase reflected from the Moho discontinuity and the PLP phase reflected from the Lithosphere and Asthenosphere Boundary(LAB)respectively,are found on the record section of the Rongcheng-XinzhouAlxa long-range deep seismic sounding profile.The nature of the two phases is still unclear,although they are clearly observable and reverberant.In this paper,we use travel time inversion and amplitude forward modelling to fit the reflected and refracted phases in the lithosphere.The results show:(1)the Pw1 is a multiple reflected phase which is successively reflected by the crystalline basement,the surface,the Moho and then finally received on the surface;(2)the Pw2 phase is also a multiple reflected phase successively reflected by the crystalline basement,the surface,the LAB interface and then received on the surface.We conclude that the significant velocity difference between the thick sedimentary cover and the crystalline basement in the North China rifted basin may be the main reason for generating the multiple reflections.Furthermore,the two multiple reflections provide potent constraints on the lithospheric velocity model,and constitute seismological evidence for the lithospheric thinning in the eastern NCC.

Three-dimensional lithospheric density distribution of China and surrounding regions

In this paper, we analyze lithospheric density distribution of China and surrounding regions on the basis of 30＇ × 30＇ gravity data and 1°×1 °P-wave velodty data, Firstly, we used the empirical equation be- tween the density and the P-wave velocity difference as the base of the initial model of the Asian lithospheric density. Secondly, we calculated the gravity anomaly, caused by the Moho discontinuity and the sedimentary layer discontinuity, by the Parker formula. Thirdly, the gravity anomaly of the spherical harmonics with 2 40 order for the anomalous body below the lithosphere is calculated based on the model of EGM96. Finally, by using Algebra Reconstruction Techniques （ART）, the inversion of 30＇ ~ 30＇ residual lithospheric Bouguer gravity anomaly caused by the lithosphere yields a rather detailed struc- tural model. The results show that the lithospheric density distribution of China and surrounding regions has a certain connection with the tectonic structure. The density is relatively high in the Philippine Sea plate, Japan Sea, the Indian plate, the Kazakhstan shield and the Western Siberia plain, whereas the Tibetan Plateau has low-density characteristics. The minimum value of density lies in the north of Philippines, in the Taiwan province and in the Ryukyu island arc.

Lithospheric structure beneath Ordos Block and surrounding areas from joint inversion of receiver function and surface wave dispersion

Ordos Block has undergone rapid uplift,and a series of rift basins have been formed around the block since the Cenozoic,but the formation mechanisms remain controversial.A high-resolution 3 D velocity structure of crust and mantle is important for understanding the lithospheric deformation and deep dynamic process.A here we present a 3 D S-wave velocity structure of the crust and upper mantle in the Ordos Block and surrounding regions by joint inversion of receiver functions and surface wave data from a dense broadband seismic deployment.The lithosphere of the Ordos Block exhibits an obvious highvelocity anomaly.In the east and north of the Ordos and the southwestern part of the Tibetan Plateau,obvious low-velocity anomalies are detected in the upper mantle and extend into the Ordos.The lithosphere of the Ordos Block is thick in the center and thin at the edge,while the crust is relatively thin in the center and thick in the southwest and northeast.The crustal thickness of the tensional basin in the north is greater than that in the central Ordos.We suggest that the outward expansion of the mantle thermal materials in eastern Tibet and the upper mantle thermal upwelling in the eastern part of the North China Craton lead to the non-uniform lithospheric thinning,temperature rise and density reduction of the Ordos Block.The additional buoyancy and thermodynamic effects provided by them contributed to the continuous uplift of the Ordos Block since the Cenozoic.Influenced by the extrusion of Tibetan Plateau,the crustal thickening and rapid uplift occur in the southwestern and northern parts of the Ordos Block.The lithospheric structures of the Alxa and Ordos Blocks are different,and they may belong to different independent blocks before the Mesozoic.

Numerical Analysis and Simulation Experiment of Lithospheric Thermal Structures in the South China Sea and the Western Pacific

The asthenosphere upwelled on a large scale in the western Pacific and South China Sea during the Cenozoic, which formed strong upward throughflow and caused the thermal structure to be changed obviously. The mathematical analysis has demonstrated that the upward throughflow velocity may have varied from 3×10^11 to 6×10^12 m/s. From the relationship between the lithospheric thickness and the conductive heat flux, the lithospheric heat flux in the western Pacific should be above 30 mW/m^2, which is consistent with the observed data. The huge low-speed zone within the upper mantle of the marginal sea in the western Pacific reflects that the upper mantle melts partially, flows regionally in the regional stress field, forms the upward heat flux at its bottom, and causes the change of the lithospheric thermal structure in the region. The numerical simulation result of the expansion and evolution in the South China Sea has demonstrated that in the early expansion, the upward throughflow velocity was relatively fast, and the effect that it had on the thickness of the lithosphere was relatively great,resulting in the mid-ocean basin expanding rapidly. After the formation of the ocean basin in the South China Sea, the upward throughflow velocity decreased, but the conductive heat flux was relatively high, which is close to the actual situation. Therefore, from the heat transfer point of view, this article discusses how the upward heat flux affects the lithospheric thermal structure in the western Pacific and South China Sea. The conclusions show that the upward heat throughflow at the bottom of the lithospheric mantle resulted in the tectonic deformation at the shallow crust. The intensive uplifts and rifts at the crust led to the continent cracks and the expansion in the South China Sea.

Lithospheric structures across the Qiman Tagh and western Qaidam Basin revealed by magnetotelluric data collected using a self-developed SEP system

Geophysics offers an important means to investigate the physical processes occurring inside the earth.In particular,since the 1960s,electromagnetic(EM)methods have played important roles in mineral exploration and engineering investigation.Such investigation requires extensive data acquisition and experimental analysis based on geophysical techniques.However,high-performance geophysical equipment,particularly EM exploration equipment,has been dominated by large geophysical companies from the United States,Canada,Germany,and other European countries for decades.This has limited the development of deep exploration technology in China.Recently,we have developed a high-resolution acquisition system with a wireless control unit and a high-power transmitting system for surface EM prospecting(SEP).The new system has been tested in the high-intensity,noisy environment in Jian-sanjiang area,Heilongjiang Province.We then conducted a field survey on the western edge of Qaidam Basin,Qinghai Province.A highly conductive anomaly was found in the upper mantle below the Qinmantage Mountains,which indicates a possible northward fluid channel that runs from below the Qinmantage Mountains to the bottom of the western crust of the Qaidam Basin.Identification of this significant feature was made possible by the new SEP for its better resolution than the previous systems.Also,geophysical analysis confirmed that the thick Cenozoic sediments of the Qaidam Basin are underlain by rigid Precambrian basement rocks and are characterized by a series of folds.The resistivity profile indicates that the Qaidam Basin was formed due to the folding structures in the northern part of the Qinghai-Tibet Plateau which provided an additional evidence for the uplifting of the Qinghai-Tibet Plateau.