Mid-lower Crust Research Articles

Stress heterogeneity in the eastern Tibetan Plateau and implications for the present-day plateau expansion

The eastward expansion of the Tibetan Plateau has resulted in different earthquake types in the eastern Tibetan Plateau, but the mechanism remains unclear. Here, we construct a three-dimensional visco-elastoplastic finite element model considering the topography to investigate the influence of fault geometry and rheological heterogeneity on stress fields. In our best-fitting model, the minimum principal stress is nearly vertical around the southern Huya fault zone, which is adjacent to the Longmen Shan fault zone, due to the significant mid-lower crust lateral rheological heterogeneity, and the thrust stress regime accounts for the reverse fault and thrust-dominated earthquakes. In this scenario, the eastward horizontal motion of the mid-lower crust is obstructed and facilitates thrust faulting, suggesting the limited eastward expansion of the Tibetan Plateau. In contrast, the northern Huya fault zone, one of the terminal branches of the East Kunlun fault, accommodates the continuous eastward extrusion of the East Kunlun fault, where the stress regime under a more homogenized crust favors the strike-slip faulting process, along with the dominant strike-slip earthquakes. Moreover, the best-fitting of stress regime explains the thrust-dominated 2008 Ms. 8.0 Wenchuan and 2013 Ms. 7.0 Lushan earthquakes on the Longmen Shan fault zone. Combining geophysical and geodetic observations and model analyses, we propose that the hybrid deformation mode in the eastern Tibetan Plateau is accommodated by upper crustal shear and thrusting deformation and mid-lower crustal thickening driven by the gravitational potential energy gradient. Our results elucidate the mechanism for differences in strong historical earthquakes and, more importantly, isolate the effect of fault geometry from those of heterogeneous viscosity on crustal deformation and stress heterogeneity in the eastern Tibetan Plateau.

Seismotectonics and landslides of the NE border of the Calabrian Arc (Southern Italy)

ABSTRACT The north-eastern border of CA represents the accretionary system developed during its continuous collision with Apulian block. The tardive oblique tectonic component is highlighted by pervasive NW-SE crustal transpressive left-lateral strike slip faults developed from the Upper Miocene to Quaternary tectonic stages. The seismotectonic and landslides Main Map represents the update of the main faults, the style of their arrangement and kinematics, the seismotectonic features and landslide area distribution with risk implications. Seismotectonics allows us to recognise a vertical crustal zonation with the distribution of earthquakes and their focal mechanism solution. The prevalent modality of ruptures depicts a mainly transcurrent and compressive solution in mid-lower crust portions respectively. This data, together with the historical seismicity analysis of the area, lead to consider the NW-SE strike-slip faults improved on the Main Map, as recent and tectonically active. The areal distribution of landslide and susceptibility of the area is due to the interaction between seismotectonic peculiarity, lithological typologies and fault zone architectures. Landslides and flood phenomena of this sector involve many municipalities, major and minor infrastructures and economic and social activities. The landslides and the faults have been identified, mapped and classified, originally at detail scale and, then, represented at 1:50,000 scale in the Main Map, included as supplementary material. The geo-structural and geomorphological data were analysed in a geographic information system allowing data management and implementation. The work presents an updated knowledge framework of risk conditions of the study zone, available to plan and reduce the fundamental elements that determine the landslide and seismic risks in this region.

Crustal conductivity footprint of the Miocene porphyry copper polymetallic deposits in the Gangdese metallogenic belt, Tibetan Plateau

New evidence worldwide has linked the surface locations of mineral deposits and their crustal-scale electrical conductivity footprint. We examine the relationship between the Gangdese Miocene porphyry copper deposits, Tibetan Plateau, and the electrical conductivity signature from a three-dimensional model generated from 311 magnetotelluric measurements. The distribution of electrical resistivity throughout the crust and the conductance within the mid-lower crust (depth range of 25–70 km) is analyzed. The results clearly show that the large and ultra-large Miocene porphyry copper deposits coincide spatially with conductive zones and areas of very-high conductance (>10,000S) in the mid-lower crust. Computations are undertaken to determine the influence of water-bearing silicate melts and alkali-bearing (Na+ and K+) fluids on conductivity. Based on this, the bulk conductivity is interpretated to be caused by a system of alkali-rich volatile-rich partial melt. The alkali-rich volatile-rich magmatic-hydrothermal fluids facilitate the migration and concentration of metal ions originating in deep areas. The volumes necessary are much less than partial melt alone and can thus help to reconcile large conductivity variations with small seismic velocity variations. The electrical structure indicates the magma source area of anatexis in the lower crust, a multi-stage magmatic system with large mid-crustal and small upper-crustal magma reservoirs, and complex pathways related to rift zones. We determine that the conductive zones in the mid-lower crust have an influence on the development of the mineralization and the location of the mineral belt.

Crustal Deformation in Eastern Himalayan Syntaxis Constrained by Ambient Noise Tomography

ABSTRACT As the leading edge of the Indian–Asian collision, the eastern Himalayan syntaxis region has experienced extensive tectonic activities, resulting in complex crustal uplift and deformation in the corner area of the southeastern pathway for the extrusion of Tibetan plateau materials. Despite considerable efforts, the corresponding deformation mechanisms remain uncertain. This study presents a new 3D high-resolution azimuthal anisotropic shear-wave velocity model in the crust and uppermost mantle derived from ambient noise dispersion data. Results show that the upper crustal anisotropy aligns with the geological boundaries and major faults nearby, suggesting shape-preferred orientations. The upper crustal low velocity and weak anisotropy beneath the core of the eastern Himalayan syntaxis (EHS) are closely associated with the high fragmentation of shallow rocks and the upwelling of hot materials during the ongoing subduction of the Indian plate. Our model also reveals relatively complex anisotropic patterns in the midlower crust. The eastern Lhasa terrane, in particular, exhibits low velocity and strong anisotropy with a northwest–southeast-oriented fast axis, supporting the local scale midlower crustal “channel flow” model. In addition, a conspicuous, elongated low-velocity zone along the northwest–southeast direction is observed in the midlower crust and uppermost mantle beneath the Bangong–Nujiang suture. The anisotropy in this region increases with depth, and the fast directions are consistently parallel to the northeast subduction of the Indian plate. We infer that this low-velocity zone may result from partial melting under local compression driven by the Indian–Asian collision. On the basis of newly revealed anisotropic model and previous studies, we construct a new dynamic model, which reveals that the migration of mechanically weak material in the midlower crust and the significant contribution of the northeast subduction of the Indian plate jointly control the crustal deformation of the EHS region.

Decoupled anisotropy in the lower crust and upper mantle extracted by the Wave Gradiometry method resolves the differential deformation mechanisms beneath the northeastern Tibetan Plateau

The Tibet plateau is the ideal place to study how the lithospheres are deformed and how the materials move vertically and horizontally under the tectonic stress applied in an intracontinental collision region. To resolve the deformation and growth mechanism of the northeastern Tibetan Plateau, we constructed a 3-D azimuthally anisotropic shear-wave velocity (Vs) model by applying the recently developed Wave Gradiometry method based on the data recorded at 676 broadband seismic stations. Across the northeastern Tibetan Plateau, we found relatively weak low-velocity zones in the Qilian block and Bayan Har block at depths of 25 to 40 km, as well as decoupled anisotropies in the crust and mantle. In the northeastern Tibetan Plateau, the fast propagation directions (FPDs) in the mid-lower crust (20–60 km) are mostly NW-SE. Within the uppermost mantle (60–100 km), the FPDs beneath the Qilian block and the Qaidam block are nearly NE-SW, and the FPDs under the Bayan Har block are mostly in SE-NW. This result contradicts the lower crustal flow model, which expects the FPDs in the mid-lower crust to align with the assumed flow directions induced by topographic gradient (i.e. NWW-SEE beneath Bayan Har block and NE-SW beneath the Qilian block). On the contrary, our inverted 3D S-wave velocity and azimuthal anisotropy model could be well explained by pure shear deformation in the crust. That is, continuous pure shearing and shortening occur in the crust. Under the influence of asthenosphere material, the lithospheric mantle may move northeast. Crustal anisotropy is related to the strong shear deformation, whereas lithospheric mantle anisotropy is mainly due to the lattice-preferred orientation (LPO) of olivine associated with the mantle movement under the NE-SW compressional stress.

Spatial distribution of mid-lower crustal flow in the SE Tibetan Plateau revealed by P-wave velocity and azimuthal anisotropy beneath the Lijiang–Xiaojinhe fault and its vicinity

SUMMARY The Lijiang–Xiaojinhe fault (LXF) and its vicinity are located in the transition zone among the Tibetan Plateau (TP), the South China block and the Indochina block. Researchers believe that this area has acted as a key tectonic zone during the evolution of the TP. Owing to the continuous growth and SE-ward expansion of the TP, the LXF and its vicinity have experienced intense deformation. Although different models, such as the rigid block extrusion and mid-lower crustal flow models, have been proposed to explain this intense deformation, a consensus has not yet been achieved. To better understand the deformation of the LXF and its vicinity, a high-resolution image of the subsurface structure must be constructed. In this study, we construct images of P-wave velocity and azimuthal anisotropy structures by using an eikonal equation-based traveltime tomography method. We collect high-quality seismic data from 276 broad-band seismic stations and manually pick a total of 48 037 first arrivals for the tomography study. Our tomographic results reveal a strong low-velocity body below the LXF and its vicinity. In addition, a strong azimuthal anisotropy structure with an N–S-oriented fast velocity direction is distributed along the low-velocity body. These features indicate the occurrence of mid-lower crustal flow, that penetrates across the LXF and extends to the Dianzhong block (DZB). In addition, we find obvious low-velocity perturbations in the mid-lower crust and uppermost mantle beneath the DZB. The low velocities may be attributed to the upwelling of hot materials from the upper mantle. We consider the limited distribution of mid-lower crustal flow on the margin of the SE TP, and mid-lower crustal flow may not play a significant role in the expansion of the TP.

Crustal Velocity Structure of the Sichuan–Yunnan Block Revealed by High-Quality Crustal Phase Travel Time

Abstract The Sichuan–Yunnan block is located at the southeastern margin of the Tibetan Plateau, which is the key area as a transition belt from the active plate extrusion zone to the stable Yangtze Craton. Using a semiautomatic measuring method based on a graphical interface, we pick 81,585 precise travel times from 449 local earthquake records and finally obtain a crustal 3D P-wave velocity model of the Sichuan–Yunnan block. The model reveals an unexpected velocity contrast between the shallower and deeper crusts. It is summarized as weakly perturbed low-velocity belts encircling a high-velocity zone in the upper crust and strongly perturbed low-velocity anomalies in the mid-lower crust, respectively. The weak low-velocity anomalies are revealed along the major strike-slip faults, and their small perturbations may imply a slip-driven mechanism. The strong low-velocity anomalies are distributed extensively in the Sichuan–Yunnan block, and their great perturbations may be related to the partial melting of weak material extruded from Tibet. Besides, our result shows noticeable high-velocity anomalies in the core zone of the Emeishan Large Igneous Province (ELIP), which may be an indication of magma solidification from the ancient mantle plume. The result further exhibits an interesting pattern that the strong low-velocity anomalies are partially separated by the high-velocity anomalies in the ELIP. Such a specific pattern probably reflects that the stable zone in the ELIP leads to the bifurcation of weak Tibetan material.

Tectonic and metallogenic implications of a metasomatized mantle inferred from magnetotellurics and gravity data: An example of Tapajós Mineral Province, Amazon Craton

The Tapajós Mineral Province (TMP) contains evidence of subduction processes during the Paleoproterozoic. Despite well-documented arc magmatism, the lack of solid geological evidence indicating a collision raises uncertainties about whether there was an arc-continent collision in the Paleoproterozoic or the process of subduction continued, and the closing of an ocean basin occurred only in Mesoproterozoic times. Additionally, world-class deposits have been linked with lithospheric electrical conductors using magnetotelluric (MT) data. On the lithospheric scale, an assessment of the potential for more significant deposits or correlated commodities is absent in the TMP. Therefore, we acquired MT data to provide constraints for the lithosphere of the TMP. The electrical resistivity model reveals a high resistivity zone (> 10,000 Ω.m) interpreted as the emplacement zone of granitoids in the upper crust. The mid-lower crust presents resistivity between 465 and 45 Ω.m, corresponding to depths of 10–35 km. A low resistivity zone below 35 km (45–10 Ω.m) represents a hydrated upper mantle due to the long-lived arc processes. Based on gravity-forward modeling, we estimate that the degree of serpentinization is <10% due to post-subduction magmatism. The lack of a lower crust conductor and paired high and low long wavelength Bouguer anomalies corroborates that a cryptic suture is unlikely. A low-angle conductor matches the limit of different magnetic domains and reveals the exhumation of the western portion of TMP's crust. The high correlation between the position of the deposits and electrical conductors corroborates that MT is a valuable tool for detecting fossil fluid pathways relevant to finding new mineral deposits.

Pseudotachylytes in felsic lower-crustal rocks of the Calabrian Serre massif: A record of deep- or shallow-crustal earthquakes?

Pseudotachylytes (quenched frictional melts produced on a fault by seismic slip) in dry rocks exhumed from the mid-lower crust are potential indicators of earthquakes that either nucleated at, or propagated to, depths below the main shallow brittle-ductile transition zone. Establishing whether these pseudotachylytes effectively record deep-crustal earthquakes, or shallow-level earthquakes overprinting the mid-lower-crustal rocks during the exhumation path, may represent a major challenge. This challenge is mainly related to the fact that the mineral assemblage of a pseudotachylyte develops out of equilibrium during the coseismic thermal transient leading to melting and melt quenching. Here we investigate pseudotachylytes within peraluminous, sillimanite-garnet-rich, migmatitic paragneiss of the Serre Massif in Calabria (Southern Italy). These exhumed lower-crustal rocks experienced granulite-facies metamorphism (∼700–800 °C; ∼600–800 MPa), partial melting and dehydration during the late Variscan Orogeny (ca. 320–280 Ma). The crosscutting pseudotachylytes contain hercynite and sillimanite microlites, globular-shaped poikilitic cordierite and plagioclase, and rare cauliflower- to subhedral-shaped garnet. The pseudotachylytes are pristine, not affected by ductile deformation, recrystallisation or extensive alteration by fluid after their formation. A Rb-Sr isochron age of 51.4 ± 5.1 Ma is obtained for the pervasively kinked biotite in the host rock immediately adjacent to the pseudotachylyte and associated with earthquake damage, while an age of 105.3 ± 4.1 Ma is obtained for the undeformed host-rock biotite. This indicates that the granulites were cooler than the closing Rb-Sr temperature of biotite (ca. 300–400 °C) in the Cretaceous and that the studied pseudotachylytes formed by shallow seismic faulting. Therefore, sillimanite, hercynite, garnet, plagioclase, and cordierite all formed during quenching of the frictional melt well above the ambient temperature. Modelling of cordierite growth during melt quenching indicates that cordierite should have started to crystallise at T > 900 °C to achieve the grain size (up to 10 μm in diameter) observed in the pseudotachylyte. Modelling and microstructural observations allow the crystallisation sequence of microlites during melt cooling to be established. These microlites include cauliflower garnet which, in this case, did not develop in a deep-seated faulting context as commonly reported.

Evidence for partial melting and alkali-rich fluids in the crust from a 3-D electrical resistivity model in the vicinity of the Coqen region, western Lhasa terrane, Tibetan Plateau

Both low resistivity zones and low velocity zones are distributed in the middle-lower crust of the western Lhasa terrane, Tibetan Plateau, China. Some estimates from electrical resistivity data suggest large volume fractions of silicate melts that are difficult to reconcile with seismic velocity data that prefer lower volumes. A second conductive phase, such as saline fluids, that drastically reduces the conductivity but does not significantly affect the seismic velocity because of its low volume may be able to explain these differences. In this study, a 3-D model of the electrical resistivity structure is generated on a profile along longitude 85°E from a latitude of 29°N to 32.5°N. Based on experimental measurement of melts and alkali-rich fluids (e.g., H2O-NaCl), we estimate the volume fraction of each phase that is required to explain the conductive anomalies observed in the geophysical model. The model reveals that the maximum bulk conductivity of the mid-lower crust in the south (1.52 S/m) is much higher than the conductivity of the mid-lower crust in the north (0.18 S/m) when taking 31°N as a rough boundary, near Coqen region. We hypothesize that the conductive zones in the south of the Coqen region may result from a silicate melt and alkali-rich fluid (multicomponent) system. In contrast, partial melting alone can explain the conductive zones in the north. The hypothesis can reconcile the predictions from electrical resistivity data and seismic data, and it corresponds well with zircon Hf isotope data. For example, a combination such as the presence of <1% NaCl-bearing aqueous fluids in addition to 5-10% partial melt can reconcile electrical conductivity data and seismic data. We propose that the contributions from partial melt or saline fluids are controlled by the distinct tectonic dynamics in each region. Furthermore, the model compatible with the idea that the Indian lower crust subducted northwards beneath the Lhasa terrane and may not extend far beyond the Indus-Yarlung Zangbo suture (approximately 30-31°N). The widespread distribution and interconnection of crustal conductors at different depths is consistent with the lateral migration of materials. However, both geophysical data sets agree that some anomalies are discontinuous along the profile. Furthermore, the low-angle subducted Indian Plate with no obvious tearing feature and a low volume of melts may have contributed to the absence of long, continuous, N-S-trending normal faults in this region.

The deformation mechanism in the western Qiangtang terrane and its surroundings: evidence from magnetotelluric data

Located in the central part of the Tibetan Plateau, the Qiangtang terrane preserves important record of the uplift and deformation history of the Plateau, and therefore remains an attractive area of research. However, deep geophysical investigations of its western part are still limited. To further understand the deep structure of the western Qiangtang terrane and its surroundings, we use magnetotelluric array data to generate a 3D electrical structure. It reveals high resistivity anomalies in the upper crust and scattered high conductivity anomalies in the mid-lower crust. The electrical structure also suggests that the Longmu Co-Gozha Co fault once believed to be a major regional deformation boundary, may not have cut through the crust. The melt content and rheological parameters derived from the electrical structures show dominant ductile-type deformation in most of the study area, which contributes to block extrusion along the slip faults. Viscous deformation regions formed by mantle melt upwelling in the mid-lower crust may contribute to the formation of the N-S directed normal faults on the surface.

Formation of the V-shaped conjugate strike-slip faults revealed by crustal anisotropy in the central Tibetan Plateau

The dynamic processes responsible for the formation of the conjugate strike-slip faults in the central Tibet remain poorly understood. The crustal anisotropy can provide important constraints on the crustal deformation history and current state. Using data from a recently deployed seismic array (SANDWICH), we obtained crustal anisotropy results that reveal the deformation of the crust in the central Tibet with high resolution. The results show that most stations exhibit strong crustal anisotropy and nearly E-W fast orientations. Our results, when combined with previously revealed upper crustal anisotropy, indicates that the deformation in the upper and mid-lower crust is decoupled. The most prominent feature in our results is a weak crustal anisotropy belt along the BNS, suggesting the mid-lower crust has not undergone strong deformation. We propose that the combination of stable BNS and strong deformation on both sides of BNS in the mid-lower crust caused the formation of conjugate strike-slip faults.

Magnetotelluric support for edge-driven convection and shear-driven upwelling in the Newer Volcanics Province

Intraplate volcanic provinces present significant natural hazards to many populated regions globally but their origins are poorly understood. Though hypotheses involving mantle plumes are predominant, the Newer Volcanics Province of southeast Australia—a relatively young (< 4.5 Ma), EW trending collection of over 400 volcanic centres—is increasingly attributed to some combination of edge-driven convection (EDC) and shear-driven upwelling (SDU). In this paper, we provide magnetotelluric (MT) data in support of these geodynamic processes. Three-dimensional inversion of 49 new broadband MT sites, in combination with 143 previously collected broadband, long-period, and geomagnetic depth soundings, reveals an elongate zone of moderately low resistivity (∼ 10–300 Ω m) spanning the Mt Gambier subprovince at a depth of between 20 and 40 km. The newly defined Gambier Conductor is contiguous to, and orientationally aligned with, significant step in the seismically-defined lithosphere-asthenosphere boundary (LAB) presented by earlier studies. Moderately low resistivity is interpreted as fluid-catalysed alteration of iron-bearing crust resulting from percolating magmatic volatiles. We argue that localised low resistivity (< 10 Ω m) at ~ 25 km depth in the mid-lower crust is associated with 1.2–3.6% partial melt. Supporting evidence indicates possible crustal thickening from 5.8 Ma at a rate comparable to estimates of SDU-induced surface eruptions and previous NVP production rate estimates.

The elusive crustal resistive boundary beneath the Deccan Volcanic Province and the western Dharwar craton, India

The electrical properties of the boundary beneath the Deccan Volcanic Province and the western Dharwar craton are imaged by using the magnetotelluric method. The magnetotelluric study was carried out along a 150km long WNW-ESE profile from Belgaum (in the Deccan Volcanic Province) to Haveri (in the western Dharwar craton).Data from 19 magnetotelluric stations spaced 10-15km apart were used. The dominant regional geo-electric strike direction obtained is N20ºE. Two-dimensional (2-D) inversion is done by using the non-linear conjugate gradient scheme for both apparent resistivity and phase. The 2-D resistivity model shows a high electrical resistivity character (&gt;10,000ohm-m) in the western Dharwar craton. Two conductive anomalies are mapped in the crustal region. In the WNW side of the profile, a conductive feature (~200ohm-m) is imaged in the mid-lower crust and, in the central part of the profile another conductive feature is mapped in the lower crust. The conductive features robustness is tested using linear and non-linear sensitivity analyses. The conductor mapped in the WNW part of the profile is considered as a deep seated fault representing a boundary or a rift related feature beneath the Deccan Volcanic Province and the western Dharwar craton. A zone of enhanced conductivity (&lt;50ohm-m) at an approximate depth of 10-30km may represent the presence of the rift in the region. This conducting feature on the Western side of the E-W trending Kaladgi Basin can be interpreted as the extension of the Kaladgi Basin further west. A well-correlated geological cross-section is also derived to interpret the resistive features mapped in this study. The electrical resistivity nature of the crust is compared with other regions of the world.

3-D azimuthal anisotropy structure reveals different deformation modes of the crust and upper mantle in the southeastern Tibetan Plateau

The tectonic evolution and deformation process of the southeastern Tibetan Plateau has been one of the focuses of the geoscience community in recent decades. However, the crustal and mantle deformation mechanism in the southeastern Tibetan Plateau is still under debate. Seismic anisotropy inferred from surface wave tomography could provide critical insights into the deformation mechanism of the Earth’s tectonosphere. Here, we constructed a 3-D azimuthal anisotropy velocity model with Rayleigh wave phase velocity dispersion data from 132 permanent stations to analyze the deformation modes in the southeastern Tibetan Plateau. In the upper crust, the azimuthal anisotropy near the main strike-slip faults exhibits strong magnitude with fast axis subparallel to the fault strike, which is consistent with the rigid block extrusion pattern. In the mid-lower crust, two low-velocity anomalies appeared beneath the Sonpan-Ganzi Terrane and Xiaojiang Fault zone with strong azimuthal anisotropy, which may indicate ductile deformation of the weak mid-lower crust. However, the two low-velocity anomalies are separated by a high-velocity barrier with weak azimuthal anisotropy in the inner zone of the Emeishan large igneous province. In the upper mantle, the anisotropy pattern is relatively simple in the rigid Yangtze Craton and mainly represents fossil anisotropy. In the southern part, the lithosphere thinned beneath the Indochina and Cathaysia Blocks, and the anisotropy is plausibly caused by the upwelling and lateral flows of upwelling hot asthenospheric materials.

The 2022 Luding, Sichuan, China, M 6.8 earthquake: A fluid-related earthquake?

We determine a detailed 3-D crustal S-wave velocity structure in and around the Xianshuihe (XSH) fault zone using the direct ambient-noise surface-wave tomography. The continuous waveform data recorded at 22 portable seismic stations of our newly deployed around the XSH fault zone and 13 Chinese provincial stations are used in the data collection during September to December 2020. Our results show obvious low-velocity (low-V) anomalies under the Sichuan basin and high-velocity (high-V) anomalies under the Songpan-Ganzi block at 5–10 km depths but high-V anomalies under the Sichuan basin and low-V anomalies under the Chuandian block at 20–40 km depths. The XSH faut zone shows a clear boundary of low-V and high-V anomalies. Furthermore, there is a clear relationship between velocity structure and Luding mainshock. The Luding mainshock and its aftershocks occurred at the boundary of high-V anomalies in the upper crust but underlain by low-V anomalies in the mid-lower crust. These low-V anomalies indicate that the fluids have reached in the source area. These results suggest that fluids might reduce the friction force due to the decrease of both frictional coefficient and effective normal stress on the fault planes and trigger the strike-slip movement of the fault zone and the occurrence of the Luding earthquake. These fluids could be contained in the hot and wet mantle upwelling flow in the big mantle wedge caused by the deep subduction of the India slab down to the mantle transition zone, in addition to the crustal channel flow due to the Indo-Eurasian collision.

High temperature ductile deformation, lithological and geochemical differentiation along the Shagou shear zone, Qinling Orogen, China

Lithological and geochemical differentiation in ductile shear zone is essential to understanding element mobility during ductile deformation in the mid-lower crust. The Shagou ductile shear zone characterized with intensive compositional layering is investigated to understand element behaviors during high temperature ductile deformation. Geochemistry and zircon U–Pb chronology suggest the protoliths were magmatic rocks crystallized at ca. 735-723 Ma, overprinted by ductile deformation at ca. 198–184 Ma. Microstructure, quartz fabric, and thermometer coupled with barometer constrain the deformation to occur at 4.50–6.12 kbar and 633–713 °C. Field observations, geochemical and zircon U–Pb geochronological data suggest that the amphibolitic and gneissic mylonites were generated from geochemical differentiation during high-temperature ductile shearing. The geochemical contents of progressively deformed mylonites show systematic increasing or decreasing with enhancing mylonitization. Silica displays remarkably decreasing with enhancing mylonitization, whereas Mg, Fe, Mn, Ti and P tend to migrate to higher strain, more acid environment. REEs, LILEs, HFSEs, Zn and V display negative correlations with increasing mylonitization, suggesting that nearly all the trace elements migrate to lower strain in the amphibolitic mylonites. However, HREEs, HFSEs, Cr, Ni, Zn and V exhibit positive correlations with increasing mylonitization in the gneissic mylonites. It implies that these elements tend to migrate to higher strain, more acid environment during ductile deformation.

3-D electrical structure and tectonic dynamics in the Yangbajing area based on the array magnetotelluric data

The well-known N-S-trending fault in the Yangbajing area plays a crucial role in the tectonic evolution of the Tibetan Plateau. Previous researches on a few E-W geophysical profiles suggested that the eastern shear at the base of the upper crust and/or lithosphere deformation brought on by asthenosphere upwelling are the major causes of the Yadong-Gulu rift’s creation. Here we propose a 3-D electrical resistivity model derived from the magnetotelluric (MT) array data spanning the Yadong-Gulu rift (YGR), and the distribution of temperature and melt fraction is estimated by the experimental calibrated relationships bridging electrical conductivity and temperature/melt fraction. The result reveals that the Indian slab subducted steeply in the east of the Yadong-Gulu rift, while Indian slab may have delaminated with a flat subduction angle in the west. The temperature distribution shows that the upper mantle of the northern Lhasa terrane is hotter than that of the southern Lhasa terrane. This is likely the result of mantle upwelling caused by either the subduction of the Indian slab or thickened Tibetan lithosphere delamination. Moreover, the strength of the mid-lower crust is so low that it may meet the conditions of the local crust flow in the west-east direction. The local crustal flow and the pulling force from the upwelling asthenosphere jointly contributed to the formation of the Yadong-Gulu rift. These main factors exist in different stages of the evolution of the Yadong-Gulu rift.

Origin of the high conductivity anomalies in the mid-lower crust of the Tibetan Plateau: Dehydration melting of garnet amphibolites

High-conductivity layers of 0.1–1 S/m distributed in the mid-lower crust of the Tibetan Plateau are traditionally explained as the fluid/melt related crustal flow. Dehydration of amphibole-bearing rocks may play an important role in explaining these anomalies. To survey the anomalies' origin, the electrical conductivities of amphibole-bearing samples are measured at 1.5 GPa and 600–1300 K. Our experiments show that dehydration melting of amphibole occurs at about 1100 K. Before dehydration melting of amphibolites, proton conduction together with small polaron conduction dominate the conduction process, whereas ionic conduction plays a more important role after the dehydration melting of amphibolites. The dehydration melting of polycrystalline amphibole yields a conductivity of up to 1 S/m under the lower-crust conditions beneath the Tibetan Plateau. Combining the calculated thermal gradient in the crust of Tibetan Plateau with the petrology proof and seismic data, dehydration melting of garnet amphibolites likely contributes to the high conductivity anomalies within the Tibetan crust. MW and cube model simulation indicates that the melt fraction is 3.8–36 vol% in the partial molten region.

Direct surface-wave tomography under Northeast China: New insights into 3-D crustal S-wave velocity structure and dynamics of intraplate volcanism

A new three-dimensional shear-wave velocity structure of the crust beneath Northeast China is constructed by inversion of phase velocity dispersion curves between 5 and 40 s using the direct surface-wave ambient noise tomography method. In addition to 127 NECESSArray (NorthEast China Extended SeiSmic Array) seismic stations, 19 SCVDSZ (Study of Changbai Volcanoes and Deep Subduction Zone) plus 3 CDSN (China Digital Seismograph Network) stations are added in the data collection. The addition of these data greatly improves the ray path coverage and imaging results around the Zhangguangcai range and the Changbaishan mountain. Our results show an obvious low-velocity anomaly extending from the surface down to uppermost mantle under the Changbaishan, suggesting that the volcano formation could be related to mantle upwelling. One more striking feature is a widely distributed mid-lower crustal low-velocity layer under the entire Northeast China, which is connected to those in the upper crust under the volcanoes, indicating that these volcanoes could have exchanges of material and energy in the mid-lower crust. In addition, under the Shuangliao and Erke volcanoes, low-velocity anomalies are visible in the upper and middle crust but high-velocity anomalies in the lower crust, whereas under the Abaga and Halaha volcanic groups, the low-velocity anomalies extend from the crust down to uppermost mantle, suggesting their continuous mantle magma provision. Integrating with previous global and regional body-wave tomographic models, our results suggest that the volcanoes in Northeast China could be related to hot and wet mantle upwelling in the big mantle wedge caused by the deep subduction of the subducted Pacific slab in the mantle transition zone.