Crustal stress field in Yunnan: implication for crust-mantle coupling

We applied the g CAP algorithm to determine 239 focal mechanism solutions 3：0≤MW≤ 6：0） with records of dense Chin Array stations deployed in Yunnan,and then inverted 686 focal mechanisms（including 447 previous results） for the regional crustal stress field with a damped linear inversion. The results indicate dominantly strike-slip environment in Yunnan as both the maximum（r1） and minimum（r3） principal stress axes are sub-horizontal. We further calculated the horizontal stress orientations（i.e., maximum and minimum horizontal compressive stress axes： S H and S h, respectively） accordingly and found an abrupt change near ＊26°N. To the north, S H aligns NW-SE to nearly E-W while S h aligns nearly N-S. In contrast, to the south, both S H and S h rotate laterally and show dominantly fan-shaped patterns. The minimum horizontal stress（i.e., maximum strain axis） S h rotates from NW-SE to the west of Tengchong volcano gradually to nearly E-W in west Yunnan, and further toNE-SW in the South China block in the east. The crustal strain field is consistent with the upper mantle strain field indicated by shear-wave splitting observations in Yunnan but not in other regions. Therefore, the crust and upper mantle in Yunnan are coupled and suffering vertically coherent pure-shear deformation in the lithosphere.

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.

Constructing the latest Neoproterozoic to Early Paleozoic multiple crust-mantle interactions in western Bainaimiao arc terrane,southeastern Central Asian Orogenic Belt

Identifying the crust-mantle interactions in association with the evolution of the Precambrian microcontinents provides critical constraints on the accretionary evolution in the Central Asian Orogenic Belt(CAOB).The Bainaimiao arc terrane(BAT)is one of the most important Precambrian microcontinents in southeastern CAOB,however,few studies have paid attention to the types and the evolving processes of the crust-mantle interactions that occurred before its final accretion onto the northern North China Craton.This study presents an integrated study of geochronology,zircon Hf isotope and whole-rock geochemistry on the latest Neoproterozoic diabases and the Early Paleozoic arc intrusions in the western BAT.The latest Neoproterozoic(ca.546 Ma)diabases display low SiO2(46.52-49.24 wt.%)with high MgO(8.23-14.41 wt.%),Cr(66-542 ppm)and Ni(50-129 ppm),consisting with mantle origin.Their highly negative zirconεHf(t)(-12.0 to-24.7)and high Fe/Mn ratios(62.1-81.7)further indicate a significantly enriched mantle source.Considering that the BAT maybe initially separated from the Tarim Craton with a thickened crustal root,we propose that these diabases were generated through partial melting of an enriched lithospheric mantle source that had been hybridized by lower-crustal eclogites during foundering of the BAT lower crust.The Early Paleozoic(ca.475-417 Ma)arc intrusions in western BAT can be divided into PeriodsⅠandⅡat approximately 450 Ma.The PeriodⅠ(>450 Ma)intrusions contain abundant mafic minerals like hornblende and pyroxene,and show positive zirconεHf(t)(+1.5 to+10.9).They are predominantly medium-K calc-alkaline with broad correlations of SiO2 versus various major and trace elements,which correlate well with the experimental melts produced by the fractional crystallization of primitive hydrous arc magmas at 7 kbar.We assume they were formed through mid-crustal differentiation of the mantle wedge-derived hydrous basaltic melts.By contrast,the PeriodⅡ(≤450 Ma)intrusions are characterized by variable zircon eHf(t)(-15.0 to+11.5)with irregular variations in most major and trace elements,which are more akin to the arc magmas generated in an open system.The general occurrence of elder inherited zircons,along with the relatively high Mg#(>45)of some samples,call upon a derivation from the reworking of the previously subduction-modified BAT lower crust with the input of mantle-derived mafic components.In combination with the Early Paleozoic tectonic melanges flanking western BAT,we infer that the compositional transition from PeriodⅠtoⅡcan be attributed to the tectonic transition from south-dipping subduction of Solonker ocean to north-dipping subduction of South Bainaimiao ocean in southeastern CAOB.The above results shed light not only on the latest Neoproterozoic to Early Paleozoic multiple crust-mantle interactions in western BAT,but also on the associated crustal construction processes before the final arc-continent accretion.

Upper crustal anisotropy from local shear-wave splitting and crust-mantle coupling of Yunnan,SE margin of Tibetan Plateau

The upper crustal anisotropy of Yunnan area, SE margin of Tibetan Plateau, is investigated by measuring the shear wave splitting of local earthquakes. The mean value of the measured delay times is 0.054 s and far less than that from Pms splitting analysis, indicating that the crustal anisotropy is contributed mostly from mid-lower crust. The fast polarization directions are mostly sub-parallel to the maximum horizontal compression directions while the stations near fault zones show fault-parallel fast polarization directions, suggesting both stress and geological structure contribute to the upper crust anisotropy.Comparing fast polarization directions from shear wave splitting of local earthquakes and Pms, large angle differences are shown at most stations, implying different anisotropy properties between upper and mid-lower crust. However, in southwestern Yunnan, the fast polarization directions of Pms and Swave splitting are nearly parallel, and the stress and surface strain rate directions show strong correlation, which may indicate that the surface and deep crust deformations can be explained by the same mechanism and the surface deformation can represent the deformation of the whole crust. Therefore,the high correlation between surface strain and mantle deformation in this area suggests the mechanical coupling between crust and mantle in southwestern Yunnan. In the rest region of Yunnan, the crustmantle coupling mechanisms are supported by the lack of significant crustal anisotropy with Ne S fast polarization directions from Pms splitting. Therefore, we conclude that the crust and upper mantle are coupled in Yunnan, SE margin of Tibetan Plateau.

Study of crust-mantle transitional zone along the northeast margin of Qinghai-Xizang plateau

This study deals with complexity, frequency spectrum and velocity model of the crust-mantle transitional zone in different tectonic units along the northeast margin of Qinghai-Xizang plateau, based on PmP waveform data from two deep seismic sounding profiles passing through the area. It reveals that Moho has stable tectonic features in Ordos and Lingzhong basins, where crust and mantle are coupled as first-order discontinuity. Moho shows obvious signs of activity in Haiyuan seismic region and in the contact zone between Bayanhar block and Qaidam block. Crust and mantle in these two areas are coupled as complicated crust-mantle transitional zone consisting of multiple laminae with alternate high and low velocities, totaling 20 km in thickness. The difference between Moho of different tectonic units reflects heterogeneity of the coupled crust-mantle zone; the difference between fine structures of Haiyuan seismic region and Maqin fault zone reflects different deep material composition of the two continent-continent collision zones and the interaction between blocks.

A model of crust-mantle differentiation for the early Earth

The Archean continents,primarily composed of the felsic tonalite-trondhjemite-granodiorite(TTG)suite,were formed or conserved since~3.8 Ga,with significant growth of the continental crust since~2.7 Ga.The difficulty in direct differentiation of the felsic crustal components from Earth’s mantle peridotite leads to a requirement for the presence of a large amount of hydrated mafic precursor of TTG in Earth’s proto-crust,the origin of which,however,remains elusive.The mafic proto-crust may have formed as early as~4.4 Ga ago as reflected by the Hf and Nd isotopic signals from Earth’s oldest geological records.Such a significant time lag between the formation of the mafic proto-crust and the occurrence of felsic continental crust is not reconciled with a single-stage scenario of Earth’s early differentiation.Here,inspired by the volcanism-dominated heat-pipe tectonics witnessed on Jupiter’s moon Io and the resemblances of the intensive internal heating and active magmatism between the early Earth and the present-day Io,we present a conceptual model of Earth’s early crust-mantle differentiation,which involves an Io-like scenario of efficient extraction of a mafic proto-crust from the early mantle,followed by an intrusion-dominating regime that could account for the subsequent formation of the felsic continents as Earth cools.The model thus allows an early formation of the preTTG proto-crust and the generation of TTG in the continent by providing the favorable conditions for its subsequent melting.This model is consistent with the observed early fractionation of the Earth and the late but rapid formation and/or accumulation of the felsic components in the Archean continents,thus sheds new light on the early Earth’s differentiation and tectonic evolution.

The complexity feature of crust-mantle boundary in Zhangbei seismic region and its tectonic implication

The ProP waveform data obtained from a deep seismic sounding profile, which ran through Zhangbei seismic region, were processed by means of both seismic wave complexity coefficient and frequency spectrum analysis methods, and the complexity characteristics of crest-mantle boundary beneath the studied area and its adjacent region were determined. The results show that the place below epicenter can be taken as boundary, the northern side of which is Inner Mongolia axis with small complexity coefficient and the southern side of which is Huai＇an basin with large complexity coefficient. The different spectrum patterns at the two sides of the epicenter were inferred from spectrum analysis. In the epicentral area, there have been multi-period magmatic eruptions since Meso-Cenozoic and craters exist at the surface. From the velocity imaging of middle and upper crust in Zhangbei seismic region it can be found that there are crustal low velocity bodies around the craters and also there are low velocity zones, which went into deep crust. It is suggested that the distinct zones of crust-mantle boundary complexity may be the margin, where the magma had intruded due to magma activity in Meso-Cenozoic. The southern side with large complexity coefficient is deep magmatic activity area and the northern side with small complexity coefficient is stable crust-mantle tectonics. The difference of crust-mantle complexity provides deep background for the development of strong earthquake.

Crust-mantle structure under South Pole and Ross Sea Beach in Antarctica

A comparison of theoretical seismograms under discussion with four teleseismograms recorded by WWSSN of SBA(New Zealand) and SPA (U.S.A.),shows that the crustal thickness beneath South Pole is a double-layered structure, about 45 km ; crustal thickness below the Ross Sea Beach,a single structure, is about 24 km ;and at the depth of 350～450 km there exists a low velocity layer. The above results indicate the different tectonic characteristics on both sides of Antarctic Transantarctic Mountains.

Seismic anisotropy of upper mantle in eastern Tibetan Plateau and related crust-mantle coupling pattern

By using the polarization analysis of teleseismic SKS waveform data recorded at 116 seismic stations which respectively involved in China National Digital Seismograph Network, and Yunnan, Sichuan, Gansu and Qinghai regional digital networks, and portable broadband seismic networks deployed in Sichuan, Yunnan and Tibet, we obtained the SKS fast-wave direction and the delay time between fast and slow waves of each station by use of the stacking analysis method, and finally acquired the fine image of upper mantle anisotropy in the eastern Tibetan Plateau and its adjacent regions. We analyzed the crust-mantle coupling deformation on the basis of combining the GPS observation results and the upper mantle anisotropy distribution in the study area. The Yunnan region out of the plateau has dif-ferent features of crust-mantle deformation from the inside plateau. There exists a lateral transitional zone of crust-mantle coupling in the eastern edge of the Tibetan Plateau, which is located in the region between 26° and 27°N in the west of Sichuan and Yunnan. To the south of transitional zone, the fast-wave direction is gradually turned from S60°―70°E in southwestern Yunnan to near EW in south-eastern Yunnan. To the north of transitional zone in northwestern Yunnan and the south of western Sichuan, the fast-wave direction is nearly NS. From crust to upper mantle, the geophysical parameters (e.g. the crustal thickness, the Bouguer gravity anomaly, and tectonic stress direction) show the feature of lateral variation in the transitional zone, although the fault trend on the ground surface is inconsis-tent with the fast-wave direction. This transitional zone is close by the eastern Himalayan syntaxis, and it may play an important role in the plate boundary dynamics.

Late Mesozoic crust-mantle interaction in southeastern Fujian——Isotopic evidence from the Pingtan igneous complex

UNDERPLATING Of mantle-derived basaltic magma and interacting with the lower crust materials are believed to be an important mechanism for the growth and transformation of the crust.

Subduction-zone peridotites and their records of crust-mantle interaction

Subduction is the core process of plate tectonics. The mantle wedge in subduction-zone systems represents a key tectonic unit, playing a significant role in material cycling and energy exchange between Earth's layers. This study summarizes research progresses in terms of subduction-related peridotite massifs, including supra-subduction zone(SSZ) ophiolites and mantle-wedge-type(MWT) orogenic peridotites. We also provide the relevant key scientific questions that need be solved in the future. The mantle sections of SSZ ophiolites and MWT orogenic peridotites represent the mantle fragments from oceanic and continental lithosphere in subduction zones, respectively. They are essential targets to study the crust-mantle interaction in subduction zones. The nature of this interaction is the complex chemical exchanges between the subducting slab and the mantle wedge under the major control of physical processes. The SSZ ophiolites can record melt/fluid-rock interaction, metamorphism,deformation, concentration of metallogenic elements and material exchange between crust and mantle, during the stages from the generation of oceanic lithosphere at spreading centers to the initiation, development, maturation and ending of oceanic subduction at continental margins. The MWT orogenic peridotites reveal the history of strong metamorphism and deformation during subduction, the multiple melt/fluid metasomatism(including silicatic melts, carbonatitic melts and silicate-bearing C-HO fluids/supercritical fluids), and the complex cycling of crust-mantle materials, during the subduction/collision and exhumation of continental plates. In order to further reveal the crust-mantle interaction using subduction-zone peridotites, it is necessary to utilize high-spatial-resolution and high-precision techniques to constrain the complex chemical metasomatism, metamorphism,deformation at micro scales, and to reveal their connections with spatial-temporal evolution in macro-scale tectonics.

Two types of the crust-mantle interaction in continental subduction zones

Plate subduction is an important mechanism for exchanging the mass and energy between the mantle and the crust,and the igneous rocks in subduction zones are the important carriers for studying the recycling of crustal materials and the crust-mantle interaction.This study presents a review of geochronology and geochemistry for postcollisional mafic igneous rocks from the Hong’an-Dabie-Sulu orogens and the southeastern edge of the North China Block.The available results indicate two types of the crust-mantle interaction in the continental subduction zone,which are represented by two types of mafic igneous rocks with distinct geochemical compositions.The first type of rocks exhibit arc-like trace element distribution patterns(i.e.enrichment of LILE,LREE and Pb,but depletion of HFSE)and enriched radiogenic Sr-Nd isotope compositions,whereas the second type of rocks show OIB-like trace element distribution patterns(i.e.enrichment of LILE and LREE,but no depletion of HFSE)and depleted radiogenic Sr-Nd isotope compositions.Both of them have variable zircon O isotope compositions,which are different from those of the normal mantle zircon,and contain residual crustal zircons.These geochemical features indicate that the two types of mafic igneous rocks were originated from the different natures of mantle sources.The mantle source for the second type of rocks would be generated by reaction of the overlying juvenile lithospheric mantle with felsic melts originated from previously subducted oceanic crust,whereas the mantle source for the first type of rocks would be generated by reaction of the overlying ancient lithospheric mantle of the North China Block with felsic melts from subsequently subducted continental crust of the South China Block.Therefore,there exist two types of the crust-mantle interaction in the continental subduction zone,and the postcollisional mafic igneous rocks provide petrological and geochemical records of the slab-mantle interactions in continental collision orogens.

Crust-mantle transitional zone of Tianshan orogenic belt and Junggar Basin and its geodynamic implication

The traveling time of the reflection waves of each shot point from the crust-mantle transitional zone has been obtained by data processing using wavelet transform to the waves reflected from the crust-mantle transitional zone. The crust-mantle transitional zone of the Xayar-Burjing geoscience transect can be divided into three sections: the northern margin of the Tarim Basin, the Tianshan orogenic belt and Junggar Basin. The crust-mantle transitional zone is composed mainly of first-order discontinuity in the Tarim Basin and the Junggar Basin, but in the Tianshan orogenic belt, it is composed of 7–8 thin layers which are 2-3 km in thickness and high and low alternatively in velocity, with a total thickness of about 20km. The discovery of the crust-mantle transitional zone of the Tianshan orogenic belt and Junggar Basin and their differences in tectonic features provide evidence for the creation of the geodynamic model “lithospheric subduction with intrusion layers in crust” for the Tianshan orogenic belt.

Garnet-Rich Granulite Xenoliths from the Hannuoba Basalts, North China: Petrogenesis and Implications for the Mesozoic Crust-Mantle Interaction

Garnet-rich granulite xenoliths collected from the Hannuoba basalts, the North China craton （NCC）, were studied to reveal the Mesozoic crnst-mantle interaction. These xenoliths are characterized by low SiO2 （37.7 wt.%-46.0 wt.%） and high Al2O3 （10.8 wt.%-17.9 wt.%） contents. Their Mg# （60-75, Mg#=100×Mg/（Mg＋Fe）, atomic number） are relatively low for their low SiO2 contents. They have low rare-earth element （REE） contents and LREE-rich REE patterns, and show remarkable enrichments in Sr relative to the adjacent REE. Some of them exhibit convex RISE patterns with a maximum at Nd and remarkably positive Eu anomalies. Taking into account their high garnet mode （generally 〉30%）, these features suggest that they are high-pressure metamorphic products of lowpressure cumulates （e.g., gabbro） after it had been depressed into the garnet stability field. They have evolved Nd and Sr isotopic compositions （143Nd/144Nd=0.511 763-0.512 173, STSr/86Sr=0.705 34-0.706 99） and fall in the trend defined by the 〉110 Ma Mesozoic basalts and high-Mg# andesites from the NCC. Zircon U-Pb dating by LA-ICP-MS shows a wide age range from 83 to 2 581 Ma, most of which cluster in 83-134 Ma. CL images of some Mesozoic zircons from the granulites show typical features of igneous zircons, providing direct evidence for the Mesozoic underplating event in this area. Neither peridotite-derived basaltic underplating model nor residue model of ancient lower crust after lithospheric thinning alone can reasonably explain the above features of the garnet-Hch granulite xenoliths. Combined with the previous research, we propose that most of the granulite xenoliths from the Hannuoba basalts are products of the Mesozoic magmatie underplating and mixing with the pre-existing lower crust （i.e., AFC process）. However, the melts could be mostly derived from partial melting of basaltic layers that were previously subducted （a fossil oceanic slab） or underplated into the base of the lithospheric mantle, or from partial melting of Archean lithospheric mantle that was variably hybridised by melts derived from foundered lower crustal edogite, although it cannot be excluded that some of the melts were derived from depleted man. tie peridotite. In other words, parent melts of most granulite xenoliths could share the same petrogenesis as the〉110 Ma Mesozoic basalts from the NCC.

Partial melting and crust-mantle interaction in subduction channels:Constraints from experimental petrology

It is proposed in the subduction channel model that the plate interface interaction is a basic mechanism for the mass and energy exchange between Earth’s surface and interior.The significant difference in composition and nature between continental lithosphere and oceanic lithosphere inevitably leads to variations in deep physical and chemical processes as well as crust-mantle interaction products in these two settings.Many studies of experimental petrology have provided constraints on the potential partial melting and crust-mantle interaction in oceanic subduction channels for silicate and carbonate rocks.The partial melts of mafic and felsic compositions are adakitic or non-adakitic granitic melts depending on melting pressure or depth.A trivial amount of CO2 can lower significantly the melting temperature of peridotites and lead to pronounced enrichment of incompatible elements in carbonate melt.The silica saturated or unsaturated melts can react with mantle-wedge peridotites in subduction channels to generate complex products.However,the existing experiments are mostly dedicated to island arc settings above oceanic subduction zones rather than dehydration melting above continental subduction zones.It is crucial to conduct high pressure and high temperature experiments to investigate all possible reactions between peridotites and crustal materials and their derivatives under the conditions responsible for the slab-mantle interface in continental subduction channels.Experimental results,combined with natural observations,are possible to elucidate the processes of metamorphic dehydration,partial melting and mantle metasomatism in continental subduction channels.

U-Pb isotopic geochemistry of the post-collisional mafic-ultramafic rocks from the Dabie Mountains——Crust-mantle interaction and LOMU component

The U-Pb isotope geochemical study of the pyroxenite-gabbro intrusion in the Dabie Mountains shows that the post-collisional mafic-ultramafic rocks of the Dabie Mountains are char-acterized by relative high Pb contents, low U contents and low U/Pb ratios. These characters may be results of interaction between lithosphere or depleted asthenospheric mantle (DMM) and lower crust, but have nothing to do with mantle plume and subducted continental crust. It was first ob-served that some samples with lower 206Pb/204Pb and higher 207Pb/204Pb ratios show typical char-acters of the LOMU component. The Pb, Sr, and Nd isotopic tracing shows that three components are needed in the source of the Zhujiapu pyroxenite-gabbro intrusion. They could be old enriched sub-continental lithospheric mantle (LOMU component), lower crust and depleted asthenospheric mantle. The crust-mantle interaction process producing primitive magma of post-collisional ma-fic-ultramafic rocks in the Dabie Mountains could be described by a lithospheric delamination and magma underplating model. After continent-continent collision, delamination of the thickened lithosphere induced the upwelling of depleted asthenospheric mantle, which caused partial melting of asthenospheric mantle and residual sub-continental lithospheric mantle. The basaltic magma produced in this process underplated in the boundary between the crust and mantle and interacted with lower crust resulting in the geochemical characters of both enriched lithospheric mantle and lower crust.

Golmud-Ejin heat flow profile and straightforward calculation of crust-mantle temperature

The Golmud-Ejin heat flow profile consists of 18 observed heat flow values. They arelimited in data number, distributed unevenly and vary from 55 mW·m-2 to 79 mW·m-2

A review of geophysical studies on the Mongolian Plateau

The Mongolian Plateau in Central Asia is an intracontinental tectonic system far from active plate boundaries.Despite its distance from these boundaries,the plateau is characterized by intense crustal deformation accompanied by voluminous Cenozoic volcanism and active modern seismicity.However,the intraplate deformation mechanism has long been debated owing to the scarcity of observations and contradictions between different results.In recent years,growing geophysical studies have been conducted on the Mongolian Plateau,providing constraints on its lithospheric structure and dynamics.Here,we review the geophysical research on the Mongolian Plateau over the last decade,including seismological,geodetic,gravity,magnetotelluric,and geodynamic aspects.This review aims to(a)describe crustal and mantle structures based on multiscale seismic images;(b)describe deformation patterns based on seismic anisotropy,focal mechanisms,and global positioning system(GPS)observations;and(c)discuss the mechanisms behind intraplate deformation,volcanism,and seismic activity across the Mongolian Plateau.Seismic images show that the crustal structure of the plateau has significant east-west differences.Several blocks in the western Mongolian Plateau have thick crusts,including the Altai Mountains,Hovsgol Rift,and Hangay Dome.The lithospheric deformation across the Mongolian Plateau has strong lateral variation,with NE-SW shortening in the Altai Mountains and W-E or NW-SE shear deformation in the Hangay Dome region and the eastern part.The varied deformation may result from the superposition of multiple mechanisms,including far-field stress in the Altai Mountains,mantle upwelling,and mantle flow in the Hangay Dome region.However,it is difficult to identify the geodynamics of the formation of the entire Mongolian Plateau because the deformation is too complicated,and the present models are not sufficient and are always partial.Overall,this review encompasses recent advances in seismic observations of the Mongolian Plateau,illuminates the heterogeneities in the crust and mantle structure and deformation of the plateau,and discusses the mechanisms behind the deformation,magmatism,and seismicity.

Geology and mineralization of the Daheishan supergiant porphyry molybdenum deposit(1.65 Bt),Jilin,China:A review

The Daheishan supergiant porphyry molybdenum deposit(also referred to as the Daheishan deposit)is the second largest molybdenum deposit in Asia and ranks fifth among the top seven molybdenum deposits globally with total molybdenum reserves of 1.65 billion tons,an average molybdenum ore grade of 0.081%,and molybdenum resources of 1.09 million tons.The main ore body is housed in the granodiorite porphyry plutons and their surrounding inequigranular granodiorite plutons,with high-grade ores largely located in the ore-bearing granodiorite porphyries in the middle-upper part of the porphyry plutons.Specifically,it appears as an ore pipe with a large upper part and a small lower part,measuring about 1700 m in length and width,extending for about 500 m vertically,and covering an area of 2.3 km^(2).Mineralogically,the main ore body consists of molybdenite,chalcopyrite,and sphalerite horizontally from its center outward and exhibits molybdenite,azurite,and pyrite vertically from top to bottom.The primary ore minerals include pyrite and molybdenite,and the secondary ore minerals include sphalerite,chalcopyrite,tetrahedrite,and scheelite,with average grades of molybdenum,copper,sulfur,gallium,and rhenium being 0.081%,0.033%,1.67%,0.001%,and 0.0012%,respectively.The ore-forming fluids of the Daheishan deposit originated as the CO_(2)-H_(2)O-NaCl multiphase magmatic fluid system,rich in CO_(2)and bearing minor amounts of CH4,N2,and H2S,and later mixed with meteoric precipitation.In various mineralization stages,the ore-forming fluids had homogenization temperatures of>420℃‒400℃,360℃‒350℃,340℃‒230℃,220℃‒210℃,and 180℃‒160℃and salinities of>41.05%‒9.8%NaCleqv,38.16%‒4.48%NaCleqv,35.78%‒4.49%NaCleqv,7.43%NaCleqv,and 7.8%‒9.5%NaCleqv,respectively.The mineralization of the Daheishan deposit occurred at 186‒167 Ma.The granites closely related to the mineralization include granodiorites(granodiorite porphyries)and monzogranites(monzogranite porphyries),which were mineralized after magmatic evolution(189‒167 Ma).Moreover,these mineralization-related granites exhibit low initial strontium content and high initial neodymium content,indicating that these granites underwent crust-mantle mixing.The Daheishan deposit formed during the Early-Middle Jurassic,during which basaltic magma underplating induced the lower-crust melting,leading to the formation of magma chambers.After the fractional crystallization of magmas,ore-bearing fluids formed.As the temperature and pressure decreased,the ore-bearing fluids boiled drops while ascending,leading to massive unloading of metal elements.Consequently,brecciated and veinlet-disseminated ore bodies formed.

Mechanism of Structure Variations at Rifted Margins in the Central Segment of South Atlantic:Insights from Numerical Modeling

Rifted margins in the central South Atlantic portray spatial variability in terms of preserved width and thickness,which relates to complex rift-related fault activities.However,there is still a lack of systematic and quantitative explanations for the causes of the variations that are observed along the paired rifts.To elucidate this issue,2D viscous-plastic thermomechanical numerical models are applied to capture the behavior of deformation,in which we investigate the effects of extensional rate,crustal strength and thickness on crust-mantle coupling,and timing of transition from rifting to breakup.Our numerical experiments demonstrate that crust-mantle decoupling accounts for crustal hyperextension,and that incorporating moderate-intensity rheology into lower crust may yield insights into the hyper-extended crust and asymmetric architecture observed in the central South Atlantic.The results also suggest that undulations in lithospheric basement cause asymmetric mantle upwelling.The lower crust of fold belts takes priority to be thermally weakened over craton and induces rift migration simultaneously.A new mechanism for the formation of failed rift is described,where the mechanical decoupling derived from thermally weakened lower crust gives access to dual rift migration.These results reinforce the interpretation on how crustal rheology shapes margins architectures and highlight the first-order effects of crust-mantle coupling.