Crustal Flow Research Articles

Syn‐ and post‐rift lower crustal flow under the Sunda Shelf, southern Vietnam: A role for climatically modulated erosion

AbstractTectonic subsidence on rifted, passive continental margins are largely controlled by patterns of extension and the nature of strain partitioning in the lithosphere. The Sunda Shelf, adjacent to the SW South China Sea, is characterized by deep basins linked to regional Cenozoic extension associated with propagating seafloor spreading caused by slab pull from the south. Analysis of seismic reflection profiles and drilled sections crossing the Nam Con Son and Cuu Long basins highlight Oligocene extension, with most of the thinning concentrated in the ductile mid‐lower crust. Upper crustal extension was modest and ductile flow is inferred to be directed northwestwards, towards the oceanic crust. Basin inversion occurred in the Mid Miocene, associated with the collision of the Dangerous Grounds Block and Borneo. Subsequent accelerated tectonic subsidence exceeded predictions from uniform extension models assumed to relate to extensional collapse after inversion. We correlate this to a period of faster erosion onshore driven by strong monsoon rains in Indochina and Peninsular Thailand at that time. Erosion of the onshore basement, inducing rock uplift and coupled with loading of the basins offshore, drives ductile mid‐lower crustal flow, likely to the northeast under Indochina, and/or to the west where Plio‐Pleistocene subsidence of the shelf is very slow. Significant sediment delivery from the Mekong River into the Cuu Long Basin began in the Late Miocene and migrated seawards as the basin filled. Mass balancing suggests that the basins of this part of the Sunda Shelf are filled through erosion of bedrock sources around the Gulf of Thailand. There is no need for sediment delivery from a major river draining the Tibetan Plateau to account for the deposited volumes.

Crustal flow driving twin domes exhumation and low-angle normal faulting in the Menderes Massif of western Anatolia

Lower crustal flow in regions of post-orogenic extension has been inferred to explain the exhumation of metamorphic core complexes and associated low-angle normal (detachment) fault systems. However, the origin of detachment faults, whether initially formed as high-angle or low-angle shear zones, and the extension is symmetric or asymmetric remains enigmatic. Here, we use numerical modeling constrained by geophysical and geological data to show that symmetric extension in the central Menderes Massif of western Anatolia is accommodated by the crustal flow. Our geodynamic model explains how opposite dipping Gediz and Büyük Menderes detachment faults are formed by ∼40° footwall rotation. Model predictions agree with seismic tomography data that suggests updoming of lower crust beneath the exhumed massifs, represented as “twin domes” and a flat Moho. Our work helps to account for the genetic relation between the exhumation of metamorphic core complexes and low-angle normal faulting in both Cordillera and Aegean orogenic regions and has important implications on crustal dynamics in extensional provinces.

Relocation of the 8 January 2022 Menyuan (Ms 6.9), China, earthquake sequence: A conjugated type of rupturing event with a seismic gap in the Qilian-Haiyuan fault in northeast Tibet

On 8 January 2022, an earthquake with Ms 6.9 struck Menyuan County, Qinghai Province, China, which provides an opportunity to investigate the seismotectonics of the Qilian-Haiyuan fault in northeast Tibet. We collect seismic observation data from China Seismic Network Center from 8 January to 6 February 2022 around the 2022 Menyuan mainshock. The double-difference relocation algorithm is applied to relocate 904 earthquakes, and 820 high-precision hypocenter locations are obtained. The mainshock epicenter was located at 10.2 km depth in the Tuolaishan fault. The aftershock sequence extended towards nearly E-W along the Tuolaishan fault and NW-SE along the Lenglongling fault, parts of the Qilian-Haiyuan fault, but there were fewer aftershocks at the beginning of the rupture near the intersection between the Lenglongling and Tuolaishan faults, suggesting a conjugated type of rupturing with a seismic gap. Integrating with focal mechanism solutions of mainshock and one large aftershock, we suggest that the rupture is a left-lateral strike-slip event with a high dip. The stress change around the faults intersection. The 1986 Ms 6.4 earthquake may trigger the 2016 Ms 6.4 earthquake, and then the extrusion stress could transfer to around the 2022 Ms 6.9 earthquake source area due to the Indo-Eurasian collision. Previous tomographic models revealed low-velocity anomalies under the Menyuan earthquakes, reflecting that fluids could decrease friction coefficient and normal stress on the fault planes and further promote the generation of these earthquakes. These fluids could be contained in the lower crustal flow and mantle upwelling flow under northeast Tibet.

Seismic velocity structure and tectonic evolution of the Continent-ocean transition in the mid-northern South China Sea

Continent-ocean transition (COT) is a key area for studying the lithospheric replacement from the thinned continental to oceanic crust. The extent and nature of the COT in the mid-northern margin of the South China Sea (SCS) have been reported recently after multiple geological and geophysical surveys, including IODP Expeditions 367/368/368X. However, the debate still remains due to limited constraints on the deep crustal structure and lateral variations. A 172 km long wide-angle reflection/refraction profile was conducted. The detailed crustal structure of the COT was mapped based on the forward and inverse modeling. The crust thickness decreased from ∼12 km in the thinned continental domain to ∼7 km in the COT, and then transitioned to ∼5 km in the oceanic domain. A ∼ 3 km thick layer with high velocity of 7.0–7.5 km/s at the bottom of the crust occurred continuously from the thinned continental to COT domains, which is interpreted as syn-rift underplating. Based on the features of crustal structure, sedimentary sequence and gravity anomaly, the landward and oceanward boundaries of the COT were determined which delimit a ∼ 20 km wide COT. The COT documents prominent tectono-magmatic interaction and rapid transition from final rifting to seafloor spreading. Eventually a model was proposed to explain the breakup processes characterized by brittle faulting in the upper crust coevally with ductile flow in the lower crust.

Iterative approaches for regional Moho determination using on-orbit gravity gradients: A case study in Qinghai-Tibet Plateau and its near zone

Summary Moho determination is an important issue in studying the Earth’s interior structure. In accordance with the isostasy-compensation hypothesis in geodesy, it is possible to recover regional or global Moho by employing gravimetric data. The non-linear property is one of the main difficulties in solving the inverse problem of isostasy. To effectively address this issue, we propose an improved iterative inversion method that combines three-dimensional integration and linear regularization to achieve an approximate non-linear solution. To estimate the contributions of different components in the gravity-gradient tensor from the Gravity field and steady-state Ocean Circulation Explorer (GOCE), other than the vertical component, we additionally develop two joint inversion scenarios that utilize diagonal horizontal components and all five non-vertical components. The validating experiments are implemented in Qinghai-Tibet Plateau and its near zone. Simulations and applications illustrate that horizontal responses of Moho undulation are also significant. Yet the off-diagonal components provide minimal contributions, adding only 0.25 km of bias to the joint inversion results. Truncation effects serve as the primary source of systematic errors, resulting in ∼1 km error in vertical inversion results and ∼2.3 km error in joint inversion results. Then, the gravimetric Moho results are compared with CRUST1.0, and they show a generally strong correlation. Differences are obvious at the northern and eastern margins of the plateau. It is maybe due to the local changes in crust-mantle density contrasts. Upwelling of asthenospheric materials and fluid flow in the middle-lower crust are the two main factors. Based on high-precision satellite gravimetry, our study could provide new insights into the tectonic structure of Qinghai-Tibet Plateau.

Stepwise Widening of the Central Andes—The Role of the Lower Crust

AbstractThe outward growth of many orogens occurs through pulsed migration of the strain front. During Cenozoic shortening of the central Andes, the strain front abruptly migrated ∼400 km eastward across the Altiplano, isolating the plateau interior from major deformation. In contrast to the traditional critical wedge model that focuses on shallow crust deformation, our lithosphere‐scale numerical models show the ductile lower crust plays a key role in the dynamics of the orogen. As the orogen grows upward, the lower crust flows outward and causes stepwise expansion of the orogen through landward‐migrating deformation. The step length of the strain‐front migration increases with decreasing lower crustal strength. Topographically, a weaker crust promotes the formation of a smooth plateau and underthrusting of stronger foreland lithosphere. Our results indicate that the seemly unrelated physiography, deep‐lithosphere structure, and plateau‐wide strain migration in the Altiplano may all be the result of a weak lower crust.

Crustal structure and the seismogenic environment in Yunnan imaged by double-difference tomography

The large-scale faulting and earthquake activities that developed extensively in the Yunnan area are associated with the collision of India and Eurasia. The fine crustal structure can provide a better understanding of the crustal deformation, seismogenic environment, and rupture processes. We performed a new 3-dimensional (3D) P wave velocity structure and seismic relocation using double-difference tomography based on seismic observations. The tomography images show that large-scale low-velocity anomalies spread around the margin of the south Chuan–Dian Block, Xiaojiang fault (XJF), and the Lijiang–Xiaojinhe fault (LJ-XJHF) in the middle and lower crust. There is an obvious high-speed anomaly in the Emeishan large igneous province (ELIP). We infer that the low-velocity anomaly under the LJ-XJHF zone may be derived from the lower crustal flow extruded from the central Tibetan plateau and obstructed by the ELIP, while the velocity anomalies around the XJF might be caused by shear heating, which is associated with the large-deep strike–slip fault and the transmission of stress in the southeast direction. The inversion results also show that the Yangbi earthquake occurred at the NW–SE boundary of high and low velocity from the upper crust to the lower crust, which coincides well with the location of the Yangbi earthquake sequence and the Weixi–Qiaohou fault. Meanwhile, the earthquake relocations show that the aftershocks are mainly distributed at low velocities. All the aforementioned research results indicate that the Yangbi earthquake might be attributed to the intrusion of the soft material flow along the Weixi–Qiaohou fault in the NW–SE direction. These low-viscosity crustal materials would cause brittle fractures and result in NW–SE sinistral strike–slip faults.

The long‐term legacy of subvolcanic intrusions on fluid migration in sedimentary basins: The Cerro Alquitrán case study, northern Neuquén Basin, Argentina

AbstractCooling subvolcanic igneous intrusions are known to have a substantial impact on fluid flow in the shallow Earth's crust, for example activation of geothermal systems, circulation of mineralized fluids in ore deposits, fast maturation of organic matter in sedimentary rocks and the potential release of large volumes of greenhouse gases, which have triggered mass extinctions during the Earth's history. However, the long‐term post‐cooling legacy of subvolcanic intrusions on fluid flow received much less attention. Here we describe a demonstrative geological example in the Andean foothills, Argentina, showing that igneous intrusions have long‐term effects on fluid flow after their emplacement and cooling. The case study is a ca. 11‐million‐year‐old, eroded subvolcanic conduit, at the rims of which large volumes of bitumen are naturally seeping out on the Earth's surface. This contribution highlights that intense syn‐emplacement fracturing of the magma has created high‐permeability pathways that affect the regional fluid circulations, even millions of years after cooling. Our observations reveal how extinct subvolcanic intrusions have long‐term consequences on subsurface fluid circulations, which need to be accounted for in the exploration of geothermal energy, drinkable groundwater, hydrocarbons and CO2 sequestration in volcanic basins and regions hosting ancient shallow magma intrusions.

The mechanism of the present-day crustal deformation in southeast Tibet: from numerical modelling and geodetic observations

SUMMARY The mechanism of present-day crustal deformation in southeast Tibet remains controversial. 3-D high-precision geodetic data can provide significant clues to analyse the key driving forces. Here, we conduct a series of 3-D finite-element modelling to investigate the influences of gravitational collapse, tectonic extrusion and mid-to-lower crustal flow on crustal deformation in southeast Tibet. The numerical results show that the gravitational collapse leads to predominant N-S extension and surface subsidence in the northern region, and predominant NW-SE compression and uplift in the southern region, which can explain the normal-faulting earthquakes in the interior. The gravity-driven horizontal velocity depends on the upper crustal viscosity, while the vertical velocity is determined by mid-to-lower crustal viscosity. The eastward tectonic extrusion causes slight southeastward rotation and predominant E-W compression in the northern region but has a little effect on the deformation in the southern region. By considering the joint effects of gravitational collapse and tectonic extrusion, we simulate the crustal deformation that reconciles with present-day geodetic observations. Both the two driving forces lead to positive shear strain rates along the major fault zones, with more contributions from the tectonic extrusion of the Tibetan Plateau. Constrained by the 3-D geodetic observations, the numerical results argue against the presence of massive fast mid-to-lower crustal flow from the Tibetan Plateau. Overall, the present-day crustal deformation in southeast Tibet is jointly driven by gravitational collapse and tectonic extrusion, which play distinct roles in shaping the faulting kinematics and regional strain partitioning.

Remnants of magma underplating by the Emeishan mantle plume within the lithosphere beneath southeastern margin of the Tibetan Plateau

The Emeishan Large Igneous Province (ELIP) is located along southeastern margin of the Tibetan Plateau, where long-wavelength crustal thickening and surface uplift have been attributed to the influx of weak crustal material from central Tibet into the lower crust beneath the Yangtze Craton. We have developed a new shear-wave velocity model for the lithosphere beneath eastern Tibet, based on a two-step linearized joint inversion technique of P-wave receiver functions (PRFs) with Rayleigh wave dispersion (10–60 s period) and Love wave dispersions (8–30 s period) for three dense seismic arrays. We applied bootstrap resampling technique to calculate the results of inversion. The resulting shear velocity model revealed two major low-velocity features in the middle–lower crust beneath southeastern Tibet. The horizontally extensive low-velocity zone (LVZ) beneath the northern Sichuan–Yunnan diamond-shaped block (SYDSB) can be interpreted as possible lower crustal flow from central Tibet, whereas the low-velocity anomaly observed beneath the Xiaojiang Fault is consistent with the presence of a partially molten layer enhanced by shear deformation along large faults. These two LVZs are separated by a lithospheric-scale high-velocity zone (HVZ) that is situated beneath the geologically inferred core of the ELIP. Based on other geophysical and geological evidence, we interpret this high-velocity anomaly body as remnants of the Emeishan plume-strengthened lithosphere that originally formed during the Permian volcanism, and we speculate that it has acted as a barrier to the hypothesized flow of lower crustal material from central Tibet since the late Cenozoic.

Measurements of Rayleigh wave ellipticity anisotropy and implications for distinct crustal deformation styles across the SE Tibet margin

Plateau growth of southeastern Tibet is often understood by applying the end-member conceptual models, such as ductile material flow in the deep crust, and lateral extrusion of coherently deforming or rigid blocks. Though recent studies suggest the local crustal flow and movements of rigid blocks may be compatible in a way, ambiguities still persist in the formation of the gently sloping margin of southeastern Tibet and interactions between the plateau and the outer forelands. Here we present a 3D crustal deformation model in southeastern Tibet and adjacent regions, by summarizing different types of seismic anisotropy measurements including a newly obtained continuous map of uppermost crustal anisotropy, which are derived from observations of the directionally dependent teleseismic Rayleigh wave ellipticity. Layered deformation is observed within the crust beneath the southeastern Tibet Plateau, while the outer foreland shows an overall coherently deforming mode. The conspicuous change of the crustal deformation styles across the southeastern Tibet margin is against the idea that the low topographic gradients of the plateau margin were produced by leakage of ductile deep crust beneath the plateau. We argue that both the crustal flow beneath the plateau interior and lateral extrusion of the outer foreland accommodate the intracontinental convergence at southeastern Tibet.

Crustal structure of the Tibetan Plateau and adjacent areas revealed from ambient noise tomography

How the Tibetan Plateau deformed in the Indian-Eurasian continental collision has been long debated, more specifically, over the relationship between the deep processes and surface structural complexity. Here, we use ambient noise tomography to obtain a high-resolution crustal S-wave velocity model beneath the Tibetan Plateau and adjacent areas involving a comprehensive dataset from over 500 stations. Our images reveal that the crustal flow should be in a limited scale according to the intermittent low-velocity zones (LVZs) observed in the middle crust at 20–40-km depth of the Tibetan Plateau. The distributions and strengths of LVZs further imply that different deep processes promote the surface deformation in various regions of the Tibetan Plateau. The LVZs in the northern plateau, collocated potassic magmatism and low velocity anomalies in the upper mantle, should be originated from the lithospheric delamination. However, in the southern plateau, the S-wave velocity showed an apparent lateral segmentation feature correlated with the north–south trending rifts. The feature indicates that the LVZs were likely controlled by the lateral tearing of the subducted Indian mantle lithosphere, which promotes the rifting deformation. Moreover, the LVZ in the central Tibet should have contributed to the formation of the conjugate strike-slip fault system. In the Tarim Basin, our model showed a high-velocity anomaly in the lower crust that may be related to ancient mantle plume activity.

Fluid-enhanced diffusive mass transfer combined with GBS as an important process for protracted weakening in the middle-lower crust

Whether dislocation creep or diffusion creep dominates the flow of the middle-lower crust remains an enigma. In this study, integrated micro-analyses on laminated ultramylonites along the Wulian detachment fault zone, North China, were performed to unravel the dominant deformation mechanisms in naturally sheared granitic rocks in the middle-lower crust. The ultramylonites consist of interlayers of quartz ribbons and polymineralic layers of feldspars and quartz. The quartz ribbons are characterized by widespread intracrystalline low-angle grain boundaries and obvious crystallographic preferred orientations (CPOs), indicating the activity of dislocation creep. In contrast, random CPOs and well-mixed phases compatible with the operation of GBS characterize the polymineralic layers. Cathodoluminescence observations reveal zoning structures in the plagioclase and K-feldspar grains from such polymineralic layers. Meanwhile, chemical composition analysis shows that K-feldspar and plagioclase grains within the layers commonly possess potassium- and calcium-enriched rims, respectively. Furthermore, no evidence for intracrystalline distortion is detected in the plagioclase and K-feldspar grains. Together with interstitial K-feldspar grains, the evidence therefore shows that fluid-enhanced diffusive mass transfer played a dominant role in accommodating GBS in the polymineralic layers. In addition, mixed phases due to cavitation-sealing and geometric phase mixing inhibit grain growth and further ensure the maintenance of GBS during progressive deformation. Consequently, we would argue that operation of fluid-enhanced diffusive mass transfer combined with GBS leads to protracted weakening of the middle-lower crust.

Fold interference pattern and crustal decoupling in northern Tanzania

High-temperature granulite facies metamorphism and deformation evolved through Neoproterozoic tectonic processes in large parts of southeastern Africa. The Eastern Granulites of Tanzania, one example for these extreme conditions, show progressive decoupling between lower and middle crust together with progressive cooling and rheology stratification. Coupled deformation during earliest stages of deformation arises from extremely small viscosity contrast of all minerals and rocks at granulite facies metamorphic conditions around 850 °C. With cooling of the orogen rheology stratification increases and deformation decouples into horizontal stretching through gravity driven channel flow in the lower crust and thickening and horizontal west-east shortening in the middle/upper crust. As soon as a stiff and cold indenter, represented by external parts of the Archean Tanzania Craton (Western Granulites) enters the system, the indenter acts as lower crustal ramp along which deep crustal units are exhumed and displaced towards the foreland. During this orogenic period the crust was fully stratified and a second phase of decoupling, detachment over crustal scale ramp, occurs. Superposition with late orogenic north-south shortening structures resulted in type 1 fold interference pattern and formation a of dome-and-basin geometry in northeastern Tanzania. Structural modelling of fold superposition helps to explain the complex structural pattern and to predict sites of decollement horizons. By comparison with fold interference pattern exposed in southern Madagascar we define different types of dome-and-basin formation. Those with neglectable rheology contrast are viscosity determined where mixing of upper and lower crustal material is enabled by vertical flow folding. Those with strong viscosity contrast are mechanically determined where domes and basins are formed by changes in regional principal stress directions.

Crustal thickening in northern Yunnan, SE Tibet, linked to the control of the regional abyssal faults on mid-lower crustal flow: evidence from ambient noise tomography

Crustal thickening in northern Yunnan, SE Tibet, linked to the control of the regional abyssal faults on mid-lower crustal flow: evidence from ambient noise tomography

Late‐Orogenic Evolution of the Southern European Variscan Belt Constrained by Fabric Analysis and Dating of the Camarat Granitic Complex and Coeval Felsic Dykes (Maures–Tanneron Massif, SE France)

AbstractThe Camarat Granitic Complex (CGC), emplaced in the migmatitic Internal Zone of the Maures–Tanneron Massif (MTM), SE Variscides, consists of the Gigaro granodiorite and the composite Camarat granite. U‐Pb dating of the latter gives crystallization ages of 304.5 ± 3.3 Ma (zircon date) and 303.5 ± 4.0 Ma (monazite date). Two representatives of late felsic dykes cutting across the MTM Internal Zone have 39Ar/40Ar muscovite ages of 302.43 ± 2.62 Ma and 298.11 ± 2.38. Magmatic lineations revealed by anisotropy of magnetic susceptibility (AMS) measurements and image analysis, aplite dykes, dyke‐like bodies of cordierite microgranite and joints in the Camarat granite, as well as the late dykes all have orientations consistent with subhorizontal, NNE‐SSW‐trending lineations in the migmatitic country rocks representing the stretching direction of a late‐Variscan transpression phase (D3). The CGC and the late dykes are therefore witness to a thermal event that affected the MTM between ∼305 and ∼298 Ma (late Pennsylvanian–earliest Permian times), at the end of D3 which initiated at ∼325 Ma. Grabens related to post‐Variscan, Permian rifting (D4 phase), which cut across the MTM are WNW‐ESE‐trending, indicating a NNE‐SSW direction of extension, parallel to the previous (D3) lateral horizontal flow. The present results and a comparison with AMS data published for the Corsica–Sardinia Batholith reveal that evolution in the SE Variscides from Devonian–early Carboniferous contraction to Permian extension, through late Carboniferous transpression is characterized by the persistence of a ca. N‐S stretching direction, supporting a strong horizontal, orogen‐parallel crustal flow.

Pressure Solution Grain Boundary Sliding as a Large Strain Mechanism of Superplastic Flow in the Upper Crust

AbstractA mechanism accommodating large strain in superplastic flow with elongation of 100%–300%, is described for fine‐grained calcareous shales deformed at temperature of 200–335°C and depths of 5–8 km up to slate schists in the Oisans massif, Western Alps, France. Using electron microscopy techniques on thin sections parallel to the principal finite strain axes, we show that the shape ratios of the slightly elongated grains (1.4–1.6), mainly calcite and quartz, do not match the finite strains recorded by the markers of the deformation (truncated belemnites, folded veins) in the maximum elongation and shortening plane (ratio 6.7) or in the maximum and minimum elongations plane (ratio 2.4). Consequently, a grain boundary sliding mechanism is required to explain the measured large finite strains. The most soluble minerals (quartz, calcite, dolomite, and albite), which represent about 95% of the rock, accommodate deformation by pressure solution grain boundary sliding whereas the least soluble minerals (muscovite, chlorite, Fe‐Ti oxides) act as indenters or are passively reoriented. Pressure solution is especially efficient in polymineralic rocks. Soluble minerals, which have been healed together in veins or fossils, are much more resistant to deformation and act as rigid objects. Models with idealized tessellation of hexagonal grains and creep laws derived from pressure solution indentation experiments provide deformation maps. We discuss the main parameters of this ductile deformation in the upper crust (thermodynamic conditions, strain rate, stress, distance of mass transfer) and show possible drastic decrease of mass transfer efficiency with decrease of stress and strain rate.

Late Miocene or older canyon incision in the northern U.S. Cordillera shown by erosion rates, incision models, and basalt flow ages

Deep canyons along the Salmon, Snake, and Clearwater rivers in central Idaho, USA suggest long-lasting transient incision, but the timing and drivers of this incision are not well understood. The perturbation of the Yellowstone hotspot, eruption of flood basalts, and drainage of Lake Idaho all occurred within or near to this region, but the relationship among these events and incision is unclear. Here, we utilized in situ 10Be cosmogenic radionuclide concentrations for 46 samples (17 new) of fluvial sediment across the region to quantify erosion rates, calibrate stream power models, and estimate incision timing. We estimate that transient incision along the Salmon River began prior to ca. 10 Ma. However, canyon age decreases to ca. 5 Ma or earlier farther to the north. For a group of tributaries underlain by basalt, we use the age of the basalt to estimate that local transient incision began between ca. 11.5 and 5 Ma. Based on these timing constraints, the canyons along the Salmon and Clearwater rivers predate the drainage of Lake Idaho. We argue that canyon incision was triggered by events related to the Yellowstone hotspot (e.g., basalt lava damming, subsidence of the Columbia Basin, reactivation of faults, and/or lower crustal flow). Furthermore, our models suggest basalt may be more erodible than the other rock types we study. We show that lithology has a significant influence on fluvial erosion and assumptions regarding river incision model parameters significantly influence results. Finally, this study highlights how geodynamic processes can exert a significant influence on landscape evolution.

Shear wave velocity structure in the middle segment of the Xiaojiang fault zone using ambient noise tomography

The middle segment of the Xiaojiang fault zone consists of strike-slip faults with frequent strong and medium–strong earthquakes under the joint influence of NE-trending faults and deep structural characteristics. The distribution characteristics of the deep subfaults of each branch need to be revealed. Two short-period dense array survey lines are deployed in the area. Shear wave velocity structures are obtained using ambient noise tomography. The results reveal that the faults in each branch dip nearly eastward, with strike-slip characteristics in the north‒south direction. Strong velocity lateral variation infer imply that the deformation characteristics of the upper crust may be brittle. NE faults are observed, which are characterized by typical compression. The high-velocity zone in the Songming Basin may be related to the deeper upwelling of high-velocity strata in the upper crustal flow.

Earthquakes evoked by lower crustal flow: Evidence from hot spring geochemistry in Lijiang-Xiaojinhe fault

Geothermal fluid is considered to be one of the important methods of earthquake forecasting, and some achievements have been made in previous studies. However, there is still a lot of work to be done before geothermal fluid can actually be used for earthquake forecasting. Lijiang-Xiaojinhe fault (LXF) which is a quake-prone region that lies on the border of Tibetan Plateau and Yangtze block is a good natural experimental field for earthquake research. In this contribution, we performed a systematic hydrogeochemistry and isotopic analysis of the geothermal waters and gases in the LXF zone. High Cl- (0.69–529.44 mg/L) concentrations in hot springs indicate that they are contaminated by deep fluid obviously. Further continuous observation (2020–2022) indicates that there are at least four times correspondence between ion concentration in two hot springs (Jinhe (94) and Xiaoshuiping (331)). Two earthquakes (ML5.2 earthquake on January 02, 2022 and ML4.6 earthquake on April 16, 2022) in Ninglang are only 50 km away from 331 hot spring, and obvious abnormal concentrations of Cl-, Na+ and SO42- were observed before the earthquakes. Combined with geological background and previous studies, we propose that lower crustal flow could be the reason why earthquake-prone in LXF. The lower crust flow through the LXF from northwest to southeast, causes internal deformation of the LXF zone. Then, a large amount of groundwater infiltrates deep underground along the fault, strengthens the pore fluid pressure, promotes the water–rock reaction, weakens fault zone, and trigger earthquakes. Ion concentrations in hot springs are sensitive to seismic activity in LXF. Unusually high or low ion concentrations in hot springs could represent the process of earthquake development, monitoring proper geothermal waters and gases in LXF will help us to study the pre-seismic hydrochemical precursors, and have a potential to provide important bases for earthquake forecasting.