Comparative study of the orogenic mechanisms for main orogenic belts in China (Project Leader: CAO Hui)
The project mainly focused on studying the composition, structure, dynamic evolution and deep driving mechanism for the main orogenic belts in China (Tianshan orogenic belt, Songpan Ganzi-Qinling Dabie orogenic belt, Qiangtang-Sanjiang orogenic belt, Himalayan orogenic belt and southern China orogenic belt), based on the theory of “continental dynamics”. The main innovations of this project are as follows:
(1) We proposed a 3D tectonic model for the Greater Himalayan Complex
(2) PWe poposed a three-dimensional drawer-tectonics escape model for the southeastern Tibetan plateau
(3) We summarized the evolution of the Paleo-Tethys system and accretionary orogenesis in the Tibet Plateau and the mechanism of the Indosinian orogenic system
(4) We proposed two possible models for the Indian/Asian subduction collision system.
(5) We discussed the late Paleozoic accretionary orogenesis in the middle East Tianshan Mountains
(6) We confirmed the nature of the early Paleozoic continental orogenic belt in South China.
Deep seismic reflection experiments and the study of crustal structures (Project Leader: GAO Rui)
After four years of hard work (2013-2016), the project group has overfulfilled all proposed objectives of the project, and great achievements were obtained. According to the objectives of this project and the corresponding implementing plans, we have carried out supplementary experiments in certain critical areas, and the seismic reflection data obtained were systematically processed. In the end, based on a structural interpretation of the dominant data from a long and deep seismic reflection profile, together with other geological and geophysical data sets, we conducted an integrated analysis and the following results were achieved:
(1) We completed data collection of a 403 km-long deep seismic reflection profile; we integrated processing of a 3294.6 km-long deep seismic reflection profile; cross section data were acquired along a 1010 km long wide-angle reflection/refraction profile; a 2150 km-long crustal velocity structure profile was obtained; data acquisition and processing was carried out in a 1830 km-long magnetotelluric (MT) sounding profile; we carried out grid drawing of a fence diagram for a 2500 km-long deep seismic reflection profile; geological mapping of a 220 km-long profile in the western Himalayas was carried out; we conducted multiscale waveform inversion experiments; we obtained a 400 km-long crust-mantle velocity structure profile across the Great Xing’an Range; we carried out an integrated 3D analysis of the long deep seismic reflection profile, the gravity anomaly, the magnetic anomaly and the geological data for an area of ca. 192000 km2.
(2) We have tested and developed several advanced techniques regarding imaging of the structural geometry of the lower crust and the Moho by near-vertical single shots, as well as techniques to conduc integrated exploration between deep seismic reflection and refraction.
(3) We have tested and developed a series of special processing techniques for geophysical profiles and techniques for joint-constraint processing and interpretation.
(4) The fine crustal structure and deep deformation in the research area have been well documented. In order to address the key geological problems under the complicated geological conditions of China, fine processing and supplementary exploration have been conducted to explore the deep crustal structure of typical regions. Interaction between the crust and mantle has been illustrated. Consequently, based on data acquisition and convincing evidence, together with surface structural investigations, the relationship between the crustal structure and deep deformation has been reconstructed after systematic analysis. The following results were obtained: 1) A reflective Moho has become evident for the first time beneath the Yarluang-Zangbo suture zone in the western Himalayas, and a nearly flat Moho does not support the hypothesis of a 10-20 km offset of the Moho beneath this suture zone. In addition, seismic reflection exploration indicates a crustal-scale duplex structure on both sides of the Yarlung-Zangbo suture zone, a process that thickened the crust. The leading edge of the subducting Indian continental crust becomes thinner from south to north and is thinned to less than 12 km beneath the Yarlung-Zangbo suture zone; 2) The Yangtze crystalline basement extends beyond the Longmen Shan fault zone to the west into areas adjacent to the Longriba fault zone. This indicates that the Longriba fault zone, rather than the Longmen Shan fault zone, marks the westernmost edge of the Yangtze block (Fig. 3.2.3); 3) A fossil subduction zone was identified beneath the western Sichuan basin, indicating that structural differentiation exists in the lower-middle crust between the western and eastern segments of the Sichuan crystalline basement. Thus, the Yangtze craton was composed of two sub-continents in the Proterozoic and these two sub-continents merged to become one craton during the Neoproterozoic (1000-850 Ma) (Fig. 3.2.4); 4) Different crustal-scale structural features were identified on both sides of the Minjiang fault zone and the Huya fault zone. In addition, the Huya fault zone cuts through the Moho and is recognized as a new active block boundary in the eastern Tibetan Plateau. This significant finding provides strong support for revealing the mechanism regarding growth and lateral escape of the Tibetan Plateau; 5) Decoupled crustal deformation with respect to lateral escape of the northeastern Tibetan Plateau was identified. This model not only accounts for the interaction and accommodation among large-scale intrablock strike-slip faults but throws new light on the eastward extent of large-scale strike-slip faults and their relationship to northward growth of the plateau (Fig. 3.2.5).
A total of 55 peer-reviewed papers were published with support of this project. Among theseare 36 papers listed in the SCI index (27 papers in foreign journals and 9 in Chinese journals). Three published papers belong to the International EI index. Additionally, there are 46 conference abstracts and 7 of these were for international conferences. This project has supported 5 postdoctoral researchers and 8 PhD students. Three project members were promoted to Associate Researcher and two were promoted to Full Professor.
Test of multi-scale imaging technology and study of crust and upper mantle velocity and density imaging in Central Asia/East Asia (Project Leader: HE Rizheng)
This is the 5th task “Multiscale Imaging Technology Test and Study on Crust & Upper Mantle Velocity & Density Imaging in Central Asia/East Asia” (SinoProbe-02-05), established in SinoProbe-02 of the Crust Exploration Plan. As the only regional research task in SinoProbe-02, its objective was to experimentally investigate the imaging technology of multiscale seismic velocity and gravity density according to the deep physical properties of the crust and upper mantle of the Asian continent and integrate a set of high-tech imaging technologies. This project has studied the crust-mantle velocity structure and density structure in the research area by passive seismic observation data in Central and East Asia and the global gravity field data, which have provided a basis for deepening the understanding and interpretation of plate tectonic movements and continent dynamics.
Through five years of research and calculations from 2008 to 2012, all participating organizations completed the designed tasks, reached the expected objectives and obtained the anticipated achievements. The main results are as follows:
(1) We collected and sorted out seismic wave event records and seismic phase reports. We collected, sorted out and formulated a sedimentary formation density and thickness chart for Central Asia/East Asia and surrounding regions, a bouguer gravity anomaly chart, an isostasy gravity anomaly chart and an upper mantle density distribution chart.
(2) New methods were
developed such as a full-wave finite-frequency tomography method
with topographic relief, a spherical-coordinate double-difference travel time
tomography method based on eikonal equation and teleseismic P-wave anisotropic
tomography method, and we have verified their reliability and validity through
examples; we also developed a sequential inversion method for seismic-gravity
joint inversion, based on the ART technology, and we have documented a quick
solving method for a full-mantle convection model under large Lateral Viscosity
Variation (LVV).
(3) We studied the Moho depth and
crustal average Vp/Vs specific values beneath the China Mainland and obtained a
P-wave velocity model and Sv-wave velocity model by using a time travel
tomography method. We obtained the 1-D velocity structure of part of the
stations by applying the finite-frequency full-wave inversion method and the
convection image of the shallow part of the mantle and the convection stress
field at the bottom of the lithosphere.
(4) At the conclusion of this project, 41 papers were published, of which 23 are listed in the SCI/EI (11 papers from the first funding of the project, 4 papers from the secondary funding and 8 papers from other fundings) and 18 of these are listed in GCJC. We attended domestic and overseas conferences for more than 50 person-times and promoted 11 doctoral and 11 masters theses.
This is part of the Sinoprobe Project, led by the Chinese Academy of Geological Sciences and co-led by Nanjing University, Institute of Geology and Geophysics of the Chinese Academy of Sciences, the Institute of Geophysics of the China Earthquake Administration, the Institute of Tibetan Plateau of the Chinese Academy of Sciences and the Institute of Mineral Resources of the Chinese Academy of Geological Sciences, in the period 2008 – 2014.
The Digital Acquisition System (DAS) comes from several world-class providers of seismological instrumentation, such as REFTEK (US), Kinematics (US) and Nanometrics Inc. (CA); and the Sensor is mostly from Guralp (CMG-3T or 3ESP) and a few Trillium 120 and STS-2. Nearly 600 broadband seismographs were deployed in the Qinghai Tibet Plateau and adjacent areas, in southern China and northeast China (Fig. 3.2.6)
We focused on the following scientific issues：
(1) Deep structural features and boundary positions of the main blocks in southern China.
(2) The deep and widespread distribution of magmatic rocks in southern China and the response of the southeastern continental margin to subduction of the Philippines plate.
(3) The leading edge of the subducted lithospheric mantle of the Indian continent and convergence of the lithospheric mantle in India and the deep state of the North-South rift.
(4) The deep structure of the Songliao Basin and the dynamics of western Pacific subduction.
We examined linear patterns to understand the fundamental characteristics and relationships of tectonic blocks in the seismic profiles. The temporary station spread was 10 to 25 km and earthquake events were recorded continuously at a sample rate of 50 sps for one and a half years. There is a total of 5 TB of raw data in the Sinoprobe database for analysis.
Current seismological methods, such as receiver function, body wave and surface wave tomography and anisotropy analysis (shear wave split) were used together or individually, and the highlight are as follows:
(1) The North China Craton lithospheric mantle was underthrust beneath the entire Qilian orogenic belt (LI Qiusheng Group).
As only dense observation profiles of receiver functions were available through the Qinling-Qilian-Alxa region at that longitude, our result is adding new evidence for southward Asian subduction. It is clearly demonstrated in a receiver function CCP profile that the North China Craton lithospheric mantle was underthrust beneath the entire Qilian orogen. Accordingly, there developed thick-skinned crustal accretionary wedges above a mid-lower crustal decollement at the northeastern margin of the Tibetan Plateau.
We also added evidence to observations of Moho complications such as doubling below major surface faults in the north, similar to comparable observations in south-central Tibet.
The results of shear wave splitting analysis indicates that crustal decoupling occurred primarily in the Qilian orogen and vertically coherent deformation is dominant within the Tibetan Plateau.
Overall, our results provide critical observational evidence for a dynamic model of the Asian lithopheric mantle wedging southwards into the Tibetan Plateau and growth and outward expansion of its northeastern margin.
(2) Development of a tearing model of the Indian lithospheric slab beneath the southern Lhasa block (ZHANG Zhongjie Group)
We observed that shear wave (SKS) splitting delay-times systematically increased from 0.2 seconds (East) to 1 seconds (West) along the profile of broadband seismic data the in Gangdise-Southern Lhasa block; the distribution characteristics of SKS delay-time can be explained by slab tearing or breaking due to angular changes as the Indian plate subducts snorthwards ( Fig. 3.2.8).
(3) The boundary of the Cathays block and the Yangtze craton has been clearly defined (CHEN Ling Group)
The crustal and upper mantle structure through the whole of southern China was imaged by teleseismic receiver function and shows that the crustal thickness and Moho sharpness of the Yangtze Craton is distinctly different from that of the Cathays Block. The boundary belt was clearly defined along the Xuefeng fault.
(4) New observations on lithospheric structure along the southeastern margin of the Eurasia plate (LI Qiusheng Group, WANG Liangshu Group and SHI Danian Group)
Crustal thinning towards the coast
The southeastern margin of the Chinese Mainland, eastwards towards the western Pacific, is the most active continental margin in the world.
The results of teleseismic receiver function analysis show that the crust tends to thin from inland to the coastal area at a rate of 2 km per 100 km (33~29 km), and the average is 31.3 km, the present characteristics of the transition from continental to oceanic crust.
Min River fault
The Min River fault extends deep to the Moho and has a obvious controlling effect on the internal activities of the crust such as the earthquakes, geothermal patterns, etc..
Thin lithosphere
A negative phase was found at 7 seconds of the P wave receiver function, which also appears in the data of a regional permanent station. Although the negative phase arrived earlier than the global model (IASP91) and very close to the Moho, we are sure it is not a side lobe of the Moho phase, because it does not change and move up and down with varying filter parameters. It also does not have antisymmetric features against the main lobe of the Moho phase (positive). We propose that the phase is the converted wave from the lithosphere/asthenosphere interface (LAB). The negative seismic phase is clearer in CCP profile. Although the propagation velocity of S waves is obviously different from that of P waves, the negative phase is imaged at the same average depth of 60-80 km as the migration image of P and S wave receiver function. This observation indicates that not only the crust but also the lithosphere of southeastern China is thinning towards the east.
Dynamics of the southeastern margin of the Eurasian plate
We have not seen clear uplifts or downdips of the 410 km and 660 km discontinuities in depth migration images of P wave receiver functions (along the long section across the whole of southern China or the coastal sections), although the absolute depth of the two discontinuites is slightly greater than the values of the IASP91 model. The thickness of the transition zone (depth 660 to depth 410) is normal and shows no indications of plume or plate subduction beneath the upper mantle of southern China.
The driving forces may come from the upper mantle. Tomography research shows that the upper mantle in the southeastern coastal region of China and the Taiwan Strait is well developed with low velocity anomalies from 150 km to 400 km, but this is not obvious in inland areas. It is suggested that the formation of high temperature and thin lithosphere in southeastern of China is caused by deeper mantle dynamics.
Body wave tomography data show that the Eurasian plate with a high velocity appears at a depth of 60 to 100 km and is underthrust gently beneath the western edge of Taiwan Island. Affected by steep angle subduction of the opposite Philippine plate, the Eurasia plate slab breaks off beneath the Taiwan Strait and hot material from the deep mantle is upwelling along the gap and leads to high heat flow on both sides of the Taiwan Strait.
(5) Observation on deep structure of the Songliao Basin and the influnce of western Pacific subduction (WU Qingju Group)
60 broadband seismic instruments have been deployed in a long profile across the Greater Xing’an Range and the Songliao Basin, and Pn wave imaging, body wave traveltime tomography, receiver function analysis, shear wave splitting and so on have been undertaken.
The regional average of Pn is 7.95 km/s, lower than the average 8.0 km/s of the Chinese continent. The southern Songliao Basin, the Liaohe Basin and the Bohai Basin are characterized by low velocity anomalies (7.6-7.8 km/s) and they are surrounded by areas of relative high velocity (7.95-8.2 km/s) as shown in Fig. 3.2.11.
Body wave travel-time tomography
Beneath the volcanic areas of Changbai, Arxan and Wudalianchi velocity anomalies extend to the mantle transition zone, but the low velocity anomaly beneath the Wudalianchi volcano only extends down only about 100 km; the low velocity anomaly maybe a cooling magma body; the low velocity anomalies beneath the Songliao Basin is connected below 400 km with low velocity anomalies beneath Changbai and Arxan and extends to the lower mantle as a channel for the upwelling of hot material of the lower mantle to the upper mantle.
P and S wave receiver function
The results of P wave and S wave receiver function form H-K stack and CCP are in good agreement (Fig. 3.2.12). In the west of the Greater Xing'an Range, the Moho depth is about 40 km. In the transition zone of the Songliao basin to the small Xing'an Ridge, the Moho becomes shallow to about 29~33 km; the Moho deepens and dips eastwards east of the Small Xing'an Range and is deepest at F2 and forms an overlapping structure with the crust.
The Moho interface of S wave receiver function is consistent with the P waves. The lateral variation of lithosphere and crustal thickness is also consistent. This indicates considerable rigidity of the lithosphere of northeastern China. The LAB in the west of the Greater Xing'an Range is buried at about 140-160 km in the east and becomes gradually shallower towards the Songliao Basin where it appears at about 100 km. The LAB interface is weak in the east of the Small Xing'an Range. Compared with the surrounding orogenic belts, the thickness of the lithosphere and crust of the Songliao basin are obviously reduced, but the lithosphere thickness is thinned more significantly.
The shear wave polarization direction of fast waves in northeastern China is NNW, the velocity of wave delay is 0.8~1.4s. The stable and fast polarization direction in the Greater Khingan Range area, Songliao Basin is less, the fast direction of the Songliao basin is consistent with the Greater Khingan Range. East of the Tanlu fault it is difficult to distinguish the fast polarization direction.
The fast direction in the Greater Khingan Range Songliao Basin is mainly affected by the anisotropy of the lithosphere. The complex anisotropy to the east of the Tanlu fault may be caused by the influence of the subducting plate, the overlying plate and mantle flow.
The study of geological cross-sections along a deep seismic reflection profile and experimental research on the integrated interpretation and fusion technique using regional and sectional data were carried out in this project. The project integrated the combined reflection and refraction techniques, established a technical guide for deep exploration, produced a popular science video regarding deep exploration technology and achievements, and obtained the following results:
(1) An experiment combining the reflection and refraction exploration techniques was done successfully and provides new technical support for deep exploration. The experiment achieved multiple aims including diminishing the costs of field data acquisition, increasing the folds of refraction, improving the resolution and the quality of observations, obtaining a single-fold reflection profile of the Moho discontinuity and improving the processing accuracy by means of constrained refraction and reflection processing.
(2) A corridor geological map and a 0-50 km section geological map were compiled along the deep seismic reflection profile. The regional and sectional data were integrated and an interpretation was researched. Research on field data from five study areas located, respectively, in the central region of the Qinghai-Tibet Plateau, the eastern margin of the Qinghai Tibet Plateau, the northern margin of North China, South China, the Songliao Basin was completed. Comprehensive research on non-seismic data from the northeastern margin of the Qinghai Tibet Plateau, on broad-band seismic data from Longmenshan and on a seismic reflection profile and geological data from northeastern China (Songliao Basin) provides surface geological data and regional geophysical field data for an integrated interpretation of the seismic profile.
(3) Based on the integration of experimental research of other research groups on detecting technology and processing and interpretation technology, this project established a deep exploration technical guide (proposed draft), which consists of 4 parts, namely a guide for deep seismic reflection profile acquisition and processing technology, a guide for artificial source deep seismic exploration technology, and a guide for broad-band seismic observation technology and magnetotelluric sounding method technology. The guide constitutes the technical preparation for further deep exploration experiments.
(4) A popular science video concerning deep exploration technology and achievements was made in this project. The content of the video includes earth structural characteristics, gravity deep exploration technology, magnetic force deep exploration technology, magnetotelluric sounding deep exploration technology and seismic deep exploration technology. The purpose of this video is to introduce deep geophysical exploration and its results to the public.
Metallogenic systematics and exploration model for Cu-Mo polymetallic systems in the Anji-Dexing ore belt (Project Leader: PAN Xiaofei)
The Anji-Dexing metallogenetic belt is about 500 km long and is located in the northeastern part of the well-known NE-trending QinHang suture. In order to complete the ore-exploration task in the unknown and deep area, four aspects on the reginal metallogenetic systematics, typical ore deposit genetic models and exploration methods have been studied during the past 5 years.
It is confirmed that most of the known large deposits are of magmatic-hydrothermal types and are located at the intersection of NE-, NW-, and nearly W-E-striking faults under control of the Wan-Zhe-Gan deep fault in the Anji-Dexing metallogenetic belt. All typical deposits have no specific wall rock except that skarn-type deposits are related to carbonate or carbonate-bearing formations. The ore-forming time can be divided into four phases from old to young (183~170 Ma, 162~141 Ma, 141~134 Ma, 125~117 Ma). All intrusions correlated with magmatic-hydrothermal types of deposits are high-K calcium alkaline felsic rocks including granite, granitic porphyry, dioritic porphyry and orthoclase granite porphyry, and some of these show features of adakitic rocks, and the Sr, Nd and Pb isotopes indicate that they are derived from both crust and mantle. Material from the mantle is more voluminous when the deposits contain large volumes of copper and gold.
The study of the metallogenic mechanism for the Dexing porphyry Cu (Mo, Au) deposit, the Zhuxi skarn W-Cu deposit, the Tianjingshan gold deposit and the Anji Mo, Pb, Zn, and Ag deposit on alteration, mineralization, ore-forming fluid and H-O-S-Pb isotopes has been completed, and four metallogenetic models for these deposits have been developed. Combined with the reginal setting and deep magma evolution, models for porphyry, subvolcanic and hydrothermal Cu (Mo, Au) metallogenetic series related to I-type magmas have been developed and show that the mineralization originated from juvenile crust. In contrast, skarn and hydrothermal W (Cu), Au (Pb, Zn) deposits related to S-type magmas resulted from crustal melting. Porphyry and W-Mo (Pb, Zn) mineralization related to I-S transition or A-type magmas evolved from variable degreed of crustal-mantle interaction.
Previous research on gravity profile measurement, high-precision magnetic measurement, geochemical data of stream sediments and soil have been combined with our research results of different exploring methods including remote sensing, gravity profile measurement high-precision magnetic measurement and comprehensive logging measurement. These were carried out by our project team in the Zhuxi, Tianjingshan and Anji typical ore fields during the past five years, to predict areas for metallogenic prospecting. Geological, geophysical and geochemical prospecting indicators and remote sensing information were summarized to develop prospecting models. Some exploration drilling was carried out to verify the above research results. More than 200 million tons of WO3 resources have been identified.
With accretion, superposition of collision orogeny and its metallogenic theory and metallogenic tectonic specificics as theoretical guidance, employing aeromagnetic interpretation, large-scale geological mapping and laboratory analysis, we studied the combination and distribution of ore-bearing porphyry rocks. By the comprehensive research on emplacement sequences, geochronology and geochemistry, we illuminated the petrogenesis and tectonic environment of ore-bearing intrusive rocks and further analyzed the coupling relationship between structure and magmatism. Through detailed field investigation, we systematically studied the ore-controlling structures of the Cu-Mo-Au deposits and the tectonic style of the ore-concentration area. Based on exact U-Pb and Ar-Ar ages, we identified a two phase structurural evolution in the Shanyang-Zhashui area. In addition, through detailed research on petrology, geochronology, geochemistry, D-O-S-Pb and He-Ar isotopes, we confirmed the metallogenic epoch, evolution of ore-forming fluids and the source for the ore-forming material in typical Cu Mo Au deposits. In combining our results, we discussed the metallogenic environment and coupling relationship bezween magma, structure, fluid and mineralization. Furthermore, by collection and summarizing the existing geological material, we selected favorable areas for gravity, aeromagnetic interpretation mapping and structural mapping, and summarized the metallogenic regularity and analysis of the main ore-controlling factors. Comparing alteration mapping, geophysical anomaly features, geochemical indicator elements and remote sensing alteration information, we assessed the geology, remote sensing, as weel as geophysical and geochemical prospecting and established a prospecting model. Through geological, geophysical, geochemical, alteration mineral mapping and rock composition, we established a fast positioning technology for concealed rocks and ore bodies, and developed a mineral deposit exploration model. Using new technology we analyzed the original data, and selected abnormal areas for prospecting. Based on deep drilling in the abnormal area, we further optimized the combination of prospecting methods and improved our exploration model. On this basis we proposed the best combined method for the deep prospecting of porphyry Cu-Mo deposits in composite orogenic belts.
Institute Of Geology, Chinese Academy Of Geological Sciences
No. 26 Baiwanzhuang Street 100037 Beijing,China, Email:geoinst@cags.ac.cn