Pn velocity and partial melting—discussion

The seismologically observed P_n velocity anomalies in the conterminous United States are restricted to the lithosphere, but the observed teleseismic delay-time variations are due principally to the regional variations in the physical state (i.e., thickness of lowvelocity zone and/or percent melt, etc.) of the asthenosphere. The observed low P_n velocity has been attributed to partial melting in the upper mantle, but it is shown that the partial-melting model alone cannot explain the seismologically observed Pn velocities in such an anomalous region as the Basin and Range Province. The present structure of the Basin and Range Province is possibly a result of rifting in the western conterminous United States; under it there may lie a mixed structure of old crust and mantle materials. The low-velocity zone under the Basin and Range Province would then be caused by downward chemical transition from the sub-Moho pyrolitic mantle material into a plagioclase-rich ophiolitic (old oceanic crust and upper mantle) composition and associated meltingand then into a peridotitic composition at the bottom of the lowvelocity zone. This mixed material model, with partial melting, would explain the low P_n velocity and low seismic Q in the region, as well as other geophysical observations.     

Structure of the upper mantle in the Northeastern Atlantic close to the azores-gibraltar ridge from surface-wave and body-wave observations

Phase velocities of teleseismic Rayleigh waves have been measured in the central North Atlantic on both sides of the Azores-Gibraltar Ridge (AGR) by means of a specially designed long-period station network. The dispersion data obtained were regionalized and then subjected to a “hedgehog” inversion, which gives a set of upper mantle models compatible with the observational data within specified error bounds.Reasonable model solutions were selected by using regional body-wave observations, such as P_n- and S_n-wave velocities determined from earthquakes along the AGR. The S_(itn) velocities measured indicate that the shear-wave velocity in the mantle part of the lithosphere is much higher on the northern side of the AGR. Strongly negative P-wave residuals in this area indicate faster seismic propagation than implied by the Jeffreys-Bullen travel-time tables, while propagation is much slower in the Gulf of Cadiz area. Furthermore the residuals show a clear difference for paths through oceanic and continental domains and suggest that the transition between these two domains extends much further into the ocean on the southern side of the AGR than on the northern side.The proposed model for the structure of the upper mantle in that region shows that there exists a pronounced velocity contrast across the AGR. Thickening of the lithospheric plate with increasing plate age is indicated to the south of the ridge. The greatest thickness is reached close to the continental margin within a zone about 500 km wide, whose velocity close to the Canary Islands and Madeira is significantly lower, probably due to the well-known volcanic activity there. These observations together with the travel time residuals reveal that this zone seems to be of a transitional nature somewhere between a continental and oceanic structure.     

Crustal structure in Tengchong Volcano-Geothermal Area, western Yunnan, China

Based upon the deep seismic sounding profiles carried out in the Tengchong Volcano-Geothermal Area (TVGA), western Yunnan Province of China, a 2-D crustal P velocity structure is obtained by use of finite-difference inversion and forward travel-time fitting method. The crustal model shows that a low-velocity anomaly zone exists in the upper crust, which is related to geothermal activity. Two faults, the Longling–Ruili Fault and Tengchong Fault, on the profile extend from surface to the lower crust and the Tengchong Fault likely penetrates the Moho. Moreover, based on teleseismic receiver functions on a temporary seismic network, S-wave velocity structures beneath the geothermal field show low S-wave velocity in the upper crust. From results of geophysical survey, the crust of TVGA is characterized by low P-wave and S-wave velocities, low resistivity, high heat-flow value and low Q. The upper mantle P-wave velocity is also low. This suggests presence of magma in the crust derived from the upper mantle. The low-velocity anomaly in upper crust may be related to the magma differentiation. The Tengchong volcanic area is located on the northeast edge of the Indian–Eurasian plate collision zone, away from the eastern boundary of the Indian plate by about 450 km. Based on the results of this paper and related studies, the Tengchong volcanoes can be classified as plate boundary volcanoes.     

Double-planed deep seismic zone and anomalous structure in the upper mantle beneath northeastern Honshu (Japan)

Seismicity located by using the most recent data obtained from the high-gain seismograph network of Tohoku University shows that the deep seismic zone beneath northeastern Honshu, Japan, is composed of two thin planes which are parallel to each other and are 30–40 km apart. Focal mechanisms derived from the earthquakes in the upper plane are reverse-faulting, or, some of them, down-dip compression. As a contrast, those in the lower plane are down-dip extension. The location of the upper boundary of the descending lithospheric slab, inferred from the arrival-time difference between ScS and ScSp waves and from the travel-time anomaly of intermediate-depth earthquakes observed at the small-scale seismic array, coincides exactly with the upper plane of the double-planed deep seismic zone. Anelasticity (1/Q) structure of the upper mantle consists of three distinct zones: a high-Q (Qs− 1500) inclined lithospheric slab, an intennediate-Q (Qs−350) land-side mantle between the Pacific coast and the volcanic front, and a low-Q (Qs − 100) land-side mantle between the volcanic front and the coast of the Japan Sea.The evidence obtained here provides valuable information as to the definition of the type of mechanism producing the plate motion beneath island arcs.     

The azimuthal variation of teleseismic P-residuals in southern California: Implications for upper-mantle structure

The azimuthal variation of teleseismic P-delays has been investigated for stations of the USGS-Caltech Southern California Seismographic Network. Normalized residuals show azimuthal variations as large as 1.2 s, and must be explained in terms of upper mantle structure. The observed azimuthal dependence implies the presence of a region of depressed velocity beneath the Imperial Valley, and regions of increased velocity below the Sierra Nevada, southwest Arizona, and much of the Transverse Ranges. The last is a major high velocity ridge-like structure, extending from a depth of ~40 km to over 100 km, which crosses, but is not offset by, the San Andreas Fault. This suggests that the plate boundary at depth may diverge from its surface expression. The horizontal shear resulting from the divergence of crust and mantle plate boundaries may be accommodated by a zone of decoupling associated with the regionally observed 7.8 km/s (P_n) layer.     

长白山火山下方地幔转换带中滞留的俯冲太平洋板块存在空缺吗?

针对近年来长白山火山下方地幔转换带中是否存在低波速异常指示的太平洋板块"空缺"而引起的不同科学认识的热烈辩论,本文主要回顾了我国东北地区地幔转换带的体波成像结果。使用相对走时残差的远震体波成像结果显示,长白山火山以西地幔转换带中存在低波速异常指示的太平洋板块"空缺";而使用绝对走时残差的区域成像和全球成像结果,尽管展示出长白山火山以西比以东略低的波速异常,但长白山火山以东至我国东北重力梯度带区域下方的地幔转换带均展示出明显的连续的高波速异常。在接收函数分析时,如果以全球平均值660km而非我国东北地区平均值670km作为基准,来分析660km间断面是抬升还是下沉;以全球平均值250km而非我国东北地区平均值260km作为基准,来分析地幔转换带是增厚还是减薄的话,则可以得到长白山火山以东至我国东北重力梯度带区域660km间断面下沉与地幔转换带增厚的认识。这种与绝对走时残差成像结果展示的地幔转换带为连续的高波速异常结果相一致的结果,说明太平洋板块俯冲前缘已由日本海沟抵达我国东北松辽盆地与大兴安岭交界处。结合高温高压实验、数值模拟与岩石地球化学研究结果,本文并不支持长白山火山以西的地幔转换带存在低波速异常指示的板块"空缺"和地幔转换带"减薄"的认识。长白山火山的深部起源与太平洋板块深俯冲至我国东北松辽盆地与大兴安岭交界处形成的"大地幔楔"结构动力学相关。     

P-wave tomography and origin of the Changbai intraplate volcano in Northeast Asia

We present the first seismic image of the upper mantle beneath the active intraplate Changbai volcano in Northeast Asia determined by teleseismic travel time tomography. The data are measured at a new seismic network consisting of 19 portable stations and 3 permanent stations. Our results show a columnar low-velocity anomaly extending to 400-km depth with a P-wave velocity reduction of up to 3%. High velocity anomalies are visible in the mantle transition zone, and deep-focus earthquakes occur at depths of 500–600 km under the region, suggesting that the subducting Pacific slab is stagnant in the transition zone, as imaged clearly by global tomography. These results suggest that the intraplate Changbai volcano is not a hotspot like Hawaii but a kind of back-arc volcano related to the deep subduction and stagnancy of the Pacific slab under Northeast Asia.     

Deep structure of the northeastern Japan arc and its implications for crustal deformation and shallow seismic activity

Seismic tomography studies in the northeastern Japan arc have revealed the existence of an inclined sheet-like seismic low-velocity and high-attenuation zone in the mantle wedge at depths shallower than about 150 km. This sheet-like low-velocity, high-attenuation zone is oriented sub-parallel to the subducted slab, and is considered to correspond to the upwelling flow portion of the subduction-induced convection. The low-velocity, high-attenuation zone reaches the Moho immediately beneath the volcanic front (or the Ou Backbone Range) running through the middle of the arc nearly parallel to the trench axis, which suggests that the volcanic front is formed by this hot upwelling flow. Aqueous fluids supplied by the subducted slab are probably transported upward through this upwelling flow to reach shallow levels beneath the Backbone Range where they are expelled from solidified magma and migrate further upward. The existence of aqueous fluids may weaken the surrounding crustal rocks, resulting in local contractive deformation and uplift along the Backbone Range under the compressional stress field of the volcanic arc. A strain-rate distribution map generated from GPS data reveals a notable concentration of east–west contraction along the Backbone Range, consistent with this interpretation. Shallow inland earthquakes are also concentrated in the upper crust of this locally large contraction deformation zone. Based on these observations, a simple model is proposed to explain the deformation pattern of the crust and the characteristic shallow seismic activity beneath the northeastern Japan arc.     

Deep structure of the baikal and other continental rift zones from seismic data

Abyssal variations beneath the Baikal rift zone are revealed in an irregular seismic stratification of the crust, the presence of an intracrust waveguide and by the vast (> 200,000 km²) underlying area of anomalously low velocity (P_n = 7.6−7.8 km/sec) uppermost mantle. In its abyssal structure the Baikal rift is heterogeneous along the strike, with sharp changes in crustal thickness (35–50 km).Comparison of first-arrival seismic-velocity curves and also the respective velocity columns reveals the essential similarity of upper-mantle seismic cross-sections for all continental rift zones. The anomalous upper layer of the mantle (ca. 7.7 km/sec) is relatively thin (15-13 km) and can be linked with the mantle waveguide only locally.     

Three-dimensional P- and S-wave velocity structures beneath the Japan Islands obtained by high-density seismic stations by seismic tomography

We construct fine-scale 3D P- and S-wave velocity structures of the crust and upper mantle beneath the whole Japan Islands with a unified resolution, where the Pacific (PAC) and Philippine Sea (PHS) plates subduct beneath the Eurasian (EUR) plate. We can detect the low-velocity (low-V) oceanic crust of the PAC and PHS plates at their uppermost part beneath almost all the Japan Islands. The depth limit of the imaged oceanic crust varies with the regions. High-V_P/V_S zones are widely distributed in the lower crust especially beneath the volcanic front, and the high strain rate zones are located at the edge of the extremely high-V_P/V_S zone; however, V_P/V_S at the top of the mantle wedge is not so high. Beneath northern Japan, we can image the high-V subducting PAC plate using the tomographic method without any assumption of velocity discontinuities. We also imaged the heterogeneous structure in the PAC plate, such as the low-V zone considered as the old seamount or the highly seismic zone within the double seismic zone where the seismic fault ruptured by the earthquake connects the upper and lower layer of the double seismic zone. Beneath central Japan, thrust-type small repeating earthquakes occur at the boundary between the EUR and PHS plates and are located at the upper part of the low-V layer that is considered to be the oceanic crust of the PHS plate. In addition to the low-V oceanic crust, the subducting high-V PAC plate is clearly imaged to depths of approximately 250 km and the subducting high-V PHS zone to depths of approximately 180 km is considered to be the PHS plate. Beneath southwestern Japan, the iso-depth lines of the Moho discontinuity in the PHS plate derived by the receiver function method divide the upper low-V layer and lower high-V layer of our model at depths of 30–50 km. Beneath Kyushu, the steeply subducting PHS plate is clearly imaged to depths of approximately 250 km with high velocities. The high-V_P/V_S zone is considered as the lower crust of the EUR plate or the oceanic crust of the PHS plate at depths of 25–35 km and the partially serpentinized mantle wedge of the EUR plate at depths of 30–45 km beneath southwestern Japan. The deep low-frequency nonvolcanic tremors occur at all parts of the high-V_P/V_S zone—within the zone, the seaward side, and the landward side where the PHS plate encounters the mantle wedge of the EUR plate. We prove that we can objectively obtain the fine-scale 3D structure with simple constraints such as only 1D initial velocity model with no velocity discontinuity.     

Intraplate earthquakes and their link with mantle dynamics: Insights from P-wave teleseismic tomography along the northern part of the North–South Tectonic Zone in China

The North–South Tectonic Zone (NSTZ) running across the Chinese continent is an important earthquake-prone zone. Around one third of the strong earthquakes (> 7.0) of China in the past occurred in this region. Receiver function study has imaged vertical convection in the mantle beneath the northern part of the NSTZ (NNSTZ), which might be related to stress accumulation and release as well as related earthquakes. Here we perform a P-wave teleseismic tomographic analysis of this region. Our results reveal prominent low-velocity and high-velocity perturbations in the upper mantle beneath this region, which we correlate with mantle upwelling, possibly resulting from lower crustal and (or) lithospheric delamination. Our results also reveal significant contrast in the velocity perturbation of the lithosphere along the two sides of this tectonic zone, suggesting possible material exchange between the eastern and western domains and lithosphere-scale control on the generation of earthquakes.     

The heterogeneous upper mantle low velocity zone

The upper mantle low velocity zone (LVZ) is a depth interval with slightly reduced seismic velocity compared to the surrounding depth intervals. The zone is present below a relatively constant depth of 100 km in most continental parts of the world, both in cratonic areas with high average velocity and tectonically active areas with low average velocity. Evidence for the low velocity zone arises from controlled and natural source seismology, including studies of surface waves and of primary and multiple reflections of body waves from the bounding interfaces, calculations of receiver functions, and absolute velocity tomography. The available data indicates a more pronounced reduction in seismic velocity and Q-value for S-waves than P-waves as well as high electrical conductivity in the LVZ. Seismic waves are strongly scattered by the zone, which demonstrates the existence of small-scale heterogeneity. The depth to the base of the LVZ is systematically shallower in cold, stable cratonic areas than in hot, active regions of the world. Because of its global occurrence below a relative constant depth of 100 km, the LVZ cannot be explained by metamorphic or compositional variation and rheological changes. Calculated upper mantle temperatures indicate that the rocks are close to the solidus in an interval with variable thickness below 100 km depth, provided that the rocks contain water and carbon dioxide. The presence of, even small amounts of such fluids in the mantle rocks will lower the solidus by several hundred degrees and introduce a characteristic kink on the solidus curve around 80–100 km depth. The seismic velocities and Q-values are significantly reduced of rocks, which are close to the solidus or contain small amounts of partial melt. Hence, the LVZ may be explained by upper mantle temperatures being close to the solidus in a depth interval below 100 km. Assuming that the rocks contain only limited amounts of fluids, this mechanism may explain the low velocities, Q-values, and resistivity, as well as the intrinsic scattering, and the characteristic variation in thickness of the low velocity zone.     

Synthetic seismogram and surface wave constraints on crustal models of Spitsbergen

Seismic refraction data collected on Spitsbergen in 1978 are used to obtain a crustal model assuming plane horizontal layering. The observed travel-times and wave forms are compared with those of synthetic seismograms computed for various published crustal models. The more detailed models adequately explain some, but not all, of the features of the synthetics. These models are adjusted, utilizing travel times and wave-form amplitudes until a satisfactory fit is achieved. The best-fitting model consists of a 4-layer crust having thicknesses of 4.1, 10.0 7.4 and 5.8 km and compressional velocities of 4.65, 6.21, 6.30 and 6.65 km/sec with increasing depth. The uppermost mantle has a velocity of 7.90 km/sec. A comparison of observed and synthetic P_n waveforms supports the existence of a thin low-velocity zone beginning at a depth of about 5 km beneath the Moho boundary. An inversion of seismic surface wave group velocity data yields a shear-wave model which is compatible with the compressional wave model.     

Upper-mantle velocity structure of the lower Great Lakes region

The lithospheric root beneath North America contains a prominent indentation beneath the lower Great Lakes region that is approximately aligned with the track of the New England seamounts. By combining data from the recently installed POLARIS network in southern Ontario, Canada with data acquired in 1996 during the Abitibi–Grenville teleseismic experiment, we have performed a tomographic inversion using 4543 P-wave traveltimes from 213 events (5.0 ≤ m_b ≤ 6.6), and 1860 S-wave traveltimes from 98 events (5.0 ≤ m_b ≤ 6.6), to obtain high-resolution images of the upper mantle beneath the lower Great Lakes. Two salient features of the 3-D models are: 1) a patchy, NNW-trending low-velocity region, and 2) a linear, NE-striking high-velocity anomaly. S-wave images show that the low-velocity anomaly changes from an arcuate feature at 400-km depth, to a NW-striking linear feature at 100-km depth beneath the Neoproterozoic Ottawa–Bonnechere graben. The linear high-velocity anomaly extends to at least 300-km depth and strikes parallel to surface geological belts and the Laurentian continental margin. We interpret the high-velocity anomaly as a possible relict slab associated with ca. 1.35–1.3 Ga subduction beneath the Composite Arc Belt, whereas the low-velocity anomaly is interpreted as a zone of alteration and metasomatism associated with the ascent of magmas that produced the Late Cretaceous Monteregian plutons. Our data support an interpretation of these plutons as melts generated by the passage of North America across a mantle plume, rather than a far-field response to opening of the North Atlantic.     

A dynamic model of the Hellenic Arc deduced from geophysical data

Combined gravity and seismic data from Greece and the adjacent areas have been used to explain the high seismicity and tectonic activity of this area. Computed 2-D gravity models revealed that below the Aegean region a large “plume” of hot upper-mantle material is rising, causing strong attenuation of the crust. The hot “plume” extends to the base of the lithosphere and has very probably been mobilized through compressional processes that forced the lithosphere to sink into the asthenosphere. The above model is supported by: high heat flow in the Aegean region; low velocity of the compressional waves of 7.7 km/sec for the upper mantle; lower density than normal extending to the base of the lithosphere; teleseismic P-wave travel-time residuals of the order of +2 sec for seismic events recorded at the Greek seismic stations; volcanics in the Aegean area with a chemical composition which can be explained by assuming an assimilation of oceanic crust by the upper mantle; deep seismicity (200 km) which has been interpreted by various authors as a Benioff zone.     

Crust–upper mantle seismic velocity structure across Southeastern China

One in-line wide-angle seismic profile was conducted in 1990 in the course of the Southeastern China Continental Dynamics project aimed at the study of the contact between the Cathaysia block and the Yangtze block. This 380-km-long profile extended in NW–SE direction from Tunxi, Anhui Province, to Wenzhou, Zhejiang Province. Five in-line shots were fired and recorded at seismic stations with spacing of about 3 km along the recording line. We have used two-dimensional ray tracing to model P- and S-wave arrivals and provide constraints on the velocity structure of the upper crust, middle crust, lower crust, Moho discontinuity, and the top part of the lithospheric mantle. P-wave velocity, S-wave velocity and V_P/V_S ratio are mapped. The crust is 36-km thick on average, albeit it gradually thins from the northwest end to the southeast end (offshore) of the profile. The average crustal velocity is 6.26 km/s for P-waves but 3.6 km/s for S-waves. A relatively narrow low-velocity layer of about 4 km of thickness, with P- and S-wave velocities of 6.2 km/s and 3.5 km/s, respectively, marks the bottom of the middle crust at a depth of 23-km northwest and 17-km southeast. At the crust–mantle transition, the P- and S-wave velocity change quickly from 7.4 to 7.8 km/s (northwest) and 8.0 to 8.2 km/s (southeast) and from 3.9 to 4.2 km/s (northwest) and 3.9 to 4.5 km/s (southeast), respectively. This result implies a lateral contrast in the upper mantle velocity along the 140 km sampled by the profile approximately. The average V_P/V_S ratio ranges from 1.68–1.8 for the upper crust to 1.75 for the middle and 1.75–1.85 for lower crust. With the interpretation of the wide-angle seismic data, Jiangshan–Shaoxin fault is considered as the boundary between the Yangtze and the Cathaysia block.     

华夏地块中部宽频地震剖面深部速度结构研究

华夏地块处于欧亚板块、太平洋板块和菲律宾海板块相互作用的前沿.我国著名的南岭成矿带和武夷成矿带均位于华夏地块内.已有的研究认为,南岭和武夷的成矿作用可能与中生代晚期岩浆岩的底侵有关.为研究深部速度结构,本研究在2017年7月至2020年8月期间布设了一条横跨南岭成矿带与武夷成矿带的宽频地震测线.该测线共有81个流动台站...     

http://www.sciencedirect.com/science/article/pii/S1674987112001247

The Sinai Peninsula has been recognized as a subplate of the African Plate located at the triple junction of the Gulf of Suez rift, the Dead Sea Transform fault, and the Red Sea rift. The upper and lower crustal structures of this tectonically active, rapidly developing region are yet poorly understood because of many limitations. For this reason, a set of P- and S-wave travel times recorded at 14 seismic stations belonging to the Egyptian National Seismographic Network (ENSN) from 111 local and regional events are analyzed to investigate the crustal structures and the locations of the seismogenic zones beneath central and southern Sinai. Because the velocity model used for routine earthquake location by ENSN is one-dimensional, the travel-time residuals will show lateral heterogeneity of the velocity structures and unmodeled vertical structures. Seismic activity is strong along the eastern and southern borders of the study area but low to moderate along the northern boundary and the Gulf of Suez to the west. The crustal V_p/V_s ratio is 1.74 from shallow (depth ≤ 10 km) earthquakes and 1.76 from deeper (depth > 10 km) crustal events. The majority of the regional and local travel-time residuals are positive relative to the Preliminary Reference Earth Model (PREM), implying that the seismic stations are located above widely distributed, tectonically-induced low-velocity zones. These low-velocity zones are mostly related to the local crustal faults affecting the sedimentary section and the basement complex as well as the rifting processes prevailing in the northern Red Sea region and the ascending of hot mantle materials along crustal fractures. The delineation of these low-velocity zones and the locations of big crustal earthquakes enable the identification of areas prone to intense seismotectonic activities, which should be excluded from major future development projects and large constructions in central and southern Sinai.     

Lithospheric structure of the Tornquist Zone resolved by nonlinear P and S teleseismic tomography along the TOR array

The main aim of the TOR project is to study the lithospheric–asthenospheric boundary structure under the Sorgenfrei–Tornquist Zone, across northern Germany, Denmark and southern Sweden. Relative arrival-time residuals of teleseismic P and S phases from 51 earthquakes, recorded by 150 seismic stations along the TOR array, were used to delineate the transition zone in the studied area. The effects of crustal structures were investigated by correcting the teleseismic residuals for travel-time variations in the crust based on a 3D crustal model derived from other data. The inversion was carried out for S phases. The results were then compared with the corresponding P-wave models. As expected, the derived models show that the relatively old and cold Baltic Shield has higher velocity at depth than the younger lithosphere farther South. The models show two sharp and distinct increases in depth to velocities which are low compared to our reference model, as we move from South to North. The location and sharpness of these boundaries suggests that the features resolved are, at least partially, compositional in origin, presumably related to mantle depletion. A sharp and steep subcrustal boundary is found roughly coincident with the southern edge of Sweden. This is below where the edge of the Baltic Shield is usually placed, based on surface geological evidence (the Sorgenfrei–Tornquist Zone). Another less significant transition is recognised more or less beneath the Elbe-lineament. Relatively high d(V_p / V_s) ratios under the central part of the profile (Denmark) indicate relatively low S-velocity in an area where a gravity high supports the hypothesis of extensive mafic intrusions.     

Local modification of the lithosphere beneath the central and western North China Craton: 3-D constraints from Rayleigh wave tomography

We have imaged the lithospheric structure beneath the central and western North China Craton (NCC) with Rayleigh wave tomography. The Rayleigh waveforms of 100 teleseismic events recorded by 208 broadband stations are used to yield high-resolution phase velocity maps at 13 periods from 20 s to 143 s. A 3-D S-wave velocity model is constructed based on the phase velocity maps. Our S-wave velocity model is broadly consistent with the results of previous tomography studies, but shows more detailed variations within the lithosphere. The Trans-North China Orogen (TNCO) is generally characterized by low-velocity anomalies but exhibits great heterogeneities. Two major low-velocity zones (LVZs) are observed in the north and south, respectively. The northern LVZ laterally coincides with sites of Cenozoic magmatism and extends to depths greater than 200 km. We propose that a small-scale mantle upwelling is present, confined to the north of the TNCO. A high-velocity patch in the uppermost mantle is also observed between the two LVZs adjacent to the narrow transtensional zone of the Cenozoic Shanxi–Shaanxi Rift (SSR). We interpret this as the remnant of a cratonic mantle root. The Ordos Block in the western NCC is associated with high-velocity anomalies, similarly reflecting the existence of cratonic mantle root, but a discernible low-velocity layer is observed at depths of 100–150 km in this location. We interpret that this mid-lithospheric structure was probably formed by metasomatic processes during the early formation of the NCC. Based on the observations from our S-wave velocity model, we conclude that the current highly heterogeneous lithospheric structure beneath the TNCO is the result of multiphase reworking of pre-existing mechanically weak zones since the amalgamation of the craton. The latest Cenozoic lithospheric reworking is dominated by the far-field effects of both Pacific plate subduction and the India–Eurasia collision.