Mantle Velocity Research Articles

High‐resolution uppermost mantle velocity structure beneath central Tibet and its implications for geodynamics

High‐resolution uppermost mantle velocity structure beneath central Tibet and its implications for geodynamics

S-wave velocity structure of the upper mantle beneath the Arctic region from Rayleigh wave dispersion data

A new model of the upper mantle structure of the Arctic region based on a representative dataset of Rayleigh wave group velocity dispersion curves from 10 to 250 s period is presented. A surface wave tomography procedure is used to image the group velocity distributions. Local dispersion curves are constructed and inverted to vertical S-wave velocity sections up to the depth of 500 km. Results are presented in two forms: as horizontal slices of S-wave velocity variations relative to the average velocities at several mantle depths and as 2D velocity sections along three profiles crossing different tectonic units. The model shows lateral and vertical velocity variations over the whole depth range within the study area. The most prominent velocity heterogeneities are localized in the uppermost mantle up to the 200 km depth. In this depth range, the velocity anomalies are strongly correlated with the main tectonic structures. The calculated S-wave velocity distribution demonstrates that the upper mantle of the continental shields is characterized by high S-wave velocities and contains regional velocity anomalies associated with small-scale blocks. Low mantle velocities are attributed to the areas with high level of tectonic and seismic activity. Significant difference in deep velocity structure of the marginal seas and deep-water oceanic basins of various ages is revealed. The obtained model suggests some insights into the geodynamical evolution of the lithosphere of the Arctic region.

Heterogeneous upper mantle structure beneath the Ross Sea Embayment and Marie Byrd Land, West Antarctica, revealed by P-wave tomography

We present an upper mantle P-wave velocity model for the Ross Sea Embayment (RSE) region of West Antarctica, constructed by inverting relative P-wave travel-times from 1881 teleseismic earthquakes recorded by two temporary broadband seismograph deployments on the Ross Ice Shelf, as well as by regional ice- and rock-sited seismic stations surrounding the RSE. Faster upper mantle P-wave velocities (∼+1%) characterize the eastern part of the RSE, indicating that the lithosphere in this part of the RSE may not have been reheated by mid-to-late Cenozoic rifting that affected other parts of the Late Cretaceous West Antarctic Rift System. Slower upper mantle velocities (∼−1%) characterize the western part of the RSE over a ∼500 km-wide region, extending from the central RSE to the Transantarctic Mountains (TAM). Within this region, the model shows two areas of even slower velocities (∼−1.5%) centered beneath Mt. Erebus and Mt. Melbourne along the TAM front. We attribute the broader region of slow velocities mainly to reheating of the lithospheric mantle by Paleogene rifting, while the slower velocities beneath the areas of recent volcanism may reflect a Neogene-present phase of rifting and/or plume activity associated with the formation of the Terror Rift. Beneath the Ford Ranges and King Edward VII Peninsula in western Marie Byrd Land, the P-wave model shows lateral variability in upper mantle velocities of ±0.5% over distances of a few hundred km. The heterogeneity in upper mantle velocities imaged beneath the RSE and western Marie Byrd Land, assuming no significant variation in mantle composition, indicates variations in upper mantle temperatures of at least 100°C. These temperature variations could lead to differences in surface heat flow of ∼±10 mW/m2 and mantle viscosity of 102 Pas regionally across the study area, possibly influencing the stability of the West Antarctic Ice Sheet by affecting basal ice conditions and glacial isostatic adjustment.

Subdividing the tectonic elements of Aegean and Eastern Mediterranean with gravity and GPS data

Western Anatolia has been formed by the motions of the African plate, Arabian plate and Hellenic Subduction zone. The Hellenic Subduction zone, which has high seismicity, is the main tectonic feature of the eastern Mediterranean Sea related to the subduction of the African Plate beneath the Aegean Sea Plate. The Hellenic Subduction zone has a complex lithospheric structure and shows complex differences in the Aegean Sea in terms of continental crust thickness and mantle velocity. In the study area, the directions of Global Positioning System (GPS) velocity vectors which are towards SE change towards S from North of Western Anatolia to Hellenic Subduction zone. It is thought that the factor which controls this mechanism is the shear force or subduction zone located in Aegean Sea. Western Anatolia region, which is located in Western Anatolia Extensional province, includes several morphologically significant N–S trending active normal faults. Besides, the NE–SW and NW–SE trending faults, which their kinematic features change from north to south, are very effective on the tectonic regime of the region. Additionally, for determining the boundaries of these tectonic elements, the Complete Spherical Bouguer (CSB) gravity anomaly of study area was calculated by using World Gravity Map (WGM2012) model. Moreover, in historical and instrumental studies, the high seismicity of the study area is remarkable. It is thought that this case is also related with the mechanism which oriented the GPS velocity vectors to southward. Consequently, the dominant kinematic structure of the region was classified by combining the GPS velocity vectors computed for Izmir and its surroundings bounded by Western Anatolia, Aegean Sea and Eastern Mediterranean and the CSB gravity anomaly. Finally, the results were interpreted together with focal depth distributions of earthquakes and Bouguer gravity data.

Uppermost mantle velocity and anisotropy structure beneath the North China Craton and its adjacent regions

The North China Craton has experienced significant tectonic reactivation and destruction, and it is a natural place to study structural heterogeneities and their influences on the lithospheric evolution. In order to have a better understanding of the lithosphere modification of the North China Craton, we collected as many as possible bulletin and exceptionally high-quality handpicked Pn arrival times, yielding a well-distributed dataset with high lateral resolution, to conduct a new uppermost mantle velocity model of the North China Craton. The present tomographic results show distinct lateral heterogeneities in both velocity and anisotropy, which are well correlated with the major active tectonic units. Obvious high Pn velocity anomalies with E-W fast directions are imaged beneath the Ordos Block, suggesting the Ordos Block lithosphere may remain a strong Archean craton. The high-velocity North China Basin may indicate a thick detached lithospheric structure, which could have been preserved during multiple tectonic episodes. Prominent low Pn velocity anomalies are observed beneath the central North China Craton and along the Tanlu fault. These low velocity anomalies suggest complex lithospheric deformation due to the upwelling of hot material triggered by the northwestward subduction of the Pacific plate. The Pn fast directions beneath the central North China Craton are generally parallel to the strike of mountains, suggesting that the mountains may be formed in response to the deformation of the uppermost mantle velocity structure. The distribution of large earthquakes and the pattern of velocity anomalies may imply that the uppermost mantle velocity structure could increase seismicity and impact the corresponding active tectonic faults beneath the North China Craton. These observations provide new geophysical insights on the lithospheric deformation and mantle dynamic processes of the North China Craton.

Ambient Noise Tomography Across the Taiwan Strait, Taiwan Island, and Southwestern Ryukyu Arc: Implications for Subsurface Slab Interactions

AbstractThe subduction flipping in the Taiwan region leads to complexities in the lithospheric structure. To better understand the arc‐continental collision process in this zone, we construct a 3‐D lithospheric model of the Taiwan Strait, Taiwan Island, and southwestern Ryukyu Arc. In the middle to lower crust, low velocities are mainly located beneath the Ryukyu Trough and the rifted basins in the Taiwan Strait, and high velocities are imaged in the Kuanyin and Penghu Platforms. Likely, the Kuanyin and Penghu Platforms act as relatively strong barriers that block the persistent westward advance of the western deformation front of the Taiwan mountain belt, as manifested by shape of the deformation front that withdraws to the east in the basement highs. In the mantle, high velocities are observed beneath the Central Range and Ryukyu Arc, which is interpreted as the two oppositely oriented slab subductions, that is, subduction of the Philippine Sea slab in the northeast Taiwan and Eurasian slab subduction in south Taiwan. Based on the vertical transects through our 3‐D model, we find that the subduction polarity has flipped in north Taiwan, and the tectonic framework in central Taiwan is in the process of subduction polarity reversal. We also image significant low mantle velocities beneath the North Taiwan Volcanic Zone and Taihsi Basin, which may be related to the breakoff of the subducting Eurasian slab. In addition, we propose that the inherited weakness of the Taihsi Basin may have facilitated the slab breakoff in central Taiwan.

Investigation of the crustal and upper-mantle structure of the eastern Pontides orogenic belt (NE, Turkey): a receiver-function study

In this study, we use teleseismic P and S receiver functions (i.e., S-to-P and P-to-S converted signals) and their joint inversions to determine seismic discontinuities in the crust and upper mantle. Eight permanent broadband stations from the KOERI (Kandilli Observatory and Earthquake Research Institute) that are distributed along the eastern Pontides orogenic belt (EPOB), NE, Turkey, comprise our database. Inversion is performed by using a simulated annealing technique with and without travel time residuals. Our inversion results reveal the Moho depth, a high S velocity lid, a low-velocity zone, and the underlying upper mantle layer. The studied area is divided into two regions based on the station locations: (a) a northern region and (b) a southern region. The inversion results from the northern area produce crustal models that indicate that the uppermost crust is represented by a low P and S wave velocity (Vp = ~ 5.0 km/s and Vs = ~ 2.8 km/s). These velocities are clear evidence of (1) the sedimentary and volcanic rocks that widely crop out in the region and (2) a thinner uppermost crust, whereas the velocities of the southern region (Vp = ~ 6.0 km/s and Vs = ~ 3.1 km/s) indicate a thicker uppermost crust (~ 7 km). Our calculated Vp/Vs velocity ratio in the lower crust is approximately 1.90 and 1.80 in the northern and southern regions, respectively. These ratios are generally attributed to mafic rocks. Beneath the northern stations, the crustal thicknesses are 30, 33, 37, and 40 km from east to west, while the depths of the Moho are 46, 42, 39, and 44 km beneath the southern stations. Some velocity histograms show a transition from the high S velocity mantle lid to the low-velocity zone, which is known as the lithosphere–asthenosphere boundary. A representative value of the boundary’s depth is around 83 km in the north and ~ 88 km in the south. The hypothesis of some researchers regarding the existence of southward subduction beneath the eastern Pontides orogenic belt during the Late Mesozoic–Cenozoic is supported by our 2-D and 3-D velocity-depth models.

Verification of the seismic P-wave velocities under Moho boundary: Central Poland case study, LUMP profile

The tectonic settings investigated by several seismic projects in previous research targeting the structure in Central Poland mainly focused on the Earth’s crust. In this paper, we present P-wave velocity verification in the uppermost mantle beneath LUMP profile towards SSE-NNW. Using recordings of 36 DATA-CUBE recorders from ca. 300–490 km far earthquake in coal mine “Janina” in southern Poland, we calculated travel times to verify P-wave velocity below the Moho boundary from previous studies. It shows that a significantly lower mean velocity value should be used for the upper mantle while counting these offsets of travel times in the SSE-NNW direction than that used on previous profiles. We present two possible models: first, the most simple one that fits the observed first arrivals, and the second with a low-velocity layer beneath the Moho boundary. In both cases, we used a priori crustal model focusing only on P-wave velocity in the uppermost mantle. Both of them significantly improved adjustment of travel times to the observed data. To evaluate the tendency of adopting too high velocities beneath the Moho, we used also 11 broadband stations, Reftek 151-121 “Observer”, from “13 BB Star” passive experiment and 6 STS-2 seismometers from permanent stations of the Polish Seismological Network (PLSN).

Water input into the Mariana subduction zone estimated from ocean-bottom seismic data.

The water cycle at subduction zones remains poorly understood, although subduction is the only mechanism for water transport deep into Earth. Previous estimates of water flux1-3 exhibit large variations in the amount of water that is subducted deeper than 100 kilometres. The main source of uncertainty in these calculations is the initial water content of the subducting uppermost mantle. Previous active-source seismic studies suggest that the subducting slab may be pervasively hydrated in the plate-bending region near the oceanic trench4-7. However, these studies do not constrain the depth extent of hydration and most investigate young incoming plates, leaving subduction-zone water budgets for old subducting plates uncertain. Here we present seismic images of the crust and uppermost mantle around the central Mariana trench derived from Rayleigh-wave analysis of broadband ocean-bottom seismic data. These images show that the low mantle velocities that result from mantle hydration extend roughly 24 kilometres beneath the Moho discontinuity. Combined with estimates of subducting crustal water, these results indicate that at least 4.3 times more water subducts than previously calculated for this region3. If other old, cold subducting slabs contain correspondingly thick layers of hydrous mantle, as suggested by the similarity of incoming plate faulting across old, cold subducting slabs, then estimates of the global water flux into the mantle at depths greater than 100 kilometres must be increased by a factor of about three compared to previous estimates3. Because a long-term net influx of water to the deep interior of Earth is inconsistent with the geological record8, estimates of water expelled at volcanic arcs and backarc basins probably also need to be revised upwards9.

Velocity Model of Western Volgo-Uralia from Receiver Functions

The results of studying the deep structure of the Earth’s crust and upper mantle in the central part of the Russian platform from receiver functions are presented. The records of teleseismic waves by the Monakovo small-aperture seismic array in the region of the northwestern slope of the Tokmovskii Arch of the Volga–Kama anteclise are used. The modification of the P-receiver function method (Vinnik, 1977) suggested in (Sanina et al., 2014) for analyzing the receiver functions in the regions with a complexly structured upper part of the section and the presence of a thick sedimentary cover is applied. The method is based on separating the high- and low-frequency components of the seismic record and successive reconstruction of the V-s velocity section in the upper part of the crust, which is performed first and, next, the entire deep section of the crust and the mantle down to a depth of ~300 km. The positions of the seismic conversion boundaries in the crust and upper mantle beneath the Monakovo array are determined. The upper mantle velocity section constructed based on the observations at the Mikhnevo array (Sanina et al., 2014) is compared with the world data on the ancient Precambrian platform.

The Changing Face of the Lithosphere‐Asthenosphere Boundary: Imaging Continental Scale Patterns in Upper Mantle Structure Across the Contiguous U.S. With Sp Converted Waves

AbstractJuxtaposed terranes of highly varied tectonic history make up the contiguous U.S.: the tectonically active western U.S., the largely quiescent Archean and Proterozoic cratons of the central U.S., and the Phanerozoic orogen and rifted margin of the eastern U.S. The transitions between these regions are clearly observed with Sp converted wave images of the uppermost mantle. We use common conversion point stacked Sp waves recorded by EarthScope's Transportable Array and other permanent and temporary broadband stations to image the transition from a strong velocity decrease at the lithosphere‐asthenosphere boundary (or LAB) beneath the western U.S. to deeper, less continuous features moving east that largely lie within the lithosphere. Only sparse, localized, weak phases are seen at LAB depths beneath the cratonic interior. Instead, we observe structures within the cratonic lithosphere that are most prominent within the Archean lithosphere of the Superior Craton. The transition from west to east is clearly revealed by cluster analysis, which also shows eastern U.S. mantle velocity gradients as more similar to the western U.S. than the ancient interior, particularly beneath New England and Virginia. In the western U.S., the observed strong LAB indicates a large enough velocity gradient (an average velocity drop of 10 ± 4.5% distributed over 30 ± 15 km) to imply that melt has ponded beneath the lithosphere.

Structural Evolution of the Rifted Margin off Northern Labrador: The Role of Hyperextension and Magmatism

AbstractHigh‐quality seismic reflection data from the offshore northern Labrador rifted margin allow imaging of the extended and rifted crust both along and across the continental margin and are described in conjunction with available seismic velocity and gravity data. The margin formed within cold, thick cratonic lithosphere. Both Archean continental basement and a discrete, undulating, high‐amplitude, deep reflection about 10 km below basement are observed. The deeper reflection can be correlated with the crust‐mantle boundary as measured on previous wide‐angle seismic data in the region. This reflection, termed here the L‐reflection, appears to be the equivalent to other top‐mantle detachments found elsewhere on magma‐poor rifted margins. However, normal mantle velocities have been observed to lie just below the reflection, suggesting that it may not be related to the formation of weak serpentinized mantle. A high‐velocity and density zone occupies the outer shelf seaward of the L‐reflection where basement is transparent, which may represent highly mafic crust or serpentinized mantle. A crustal reconstruction of this margin and its conjugate shows marked asymmetry, with a wider zone of crustal thinning on the Greenland margin. These crustal thinning profiles are comparable to those on other conjugate margins within cratonic lithosphere. While some of the attributes of this margin are those of a magma‐poor system, at the ocean‐continent transition, thick igneous crust created a magma‐rich zone in Paleocene time when a hot spot was active in the Davis Strait to the north. Thus, this margin exhibits characteristics of both magma‐rich and magma‐poor systems.

The geophysical characteristic of the lower lithosphere and asthenosphere in the marginal zone of the East European Craton

Seismic P- and S-wave velocities of the lower lithosphere and underlying asthenosphere at the SW margin of the East European Craton in northern Poland were obtained with different seismic techniques: seismic refraction, P-residuals of the first arrivals from teleseismic earthquakes, P-wave receiver function, and inversion of the Rayleigh surface wave dispersion curves, the last two using data collected in the passive seismic experiment “13 BB star”. The uniform array consisted of 13 stations deployed in a 120 km in diameter area. Below the depth of 180–220 km a decrease of about 6% of the S-wave velocity is interpreted as a thermal gradient zone corresponding to a lithosphere–asthenosphere transition. The average mantle velocities down to a depth of 300 km beneath the array are relatively high, exceeding values for other Precambrian cratons by 0.1–0.2 km/s, and cannot be modeled by reasonable mantle peridotite compositions in the lithospheric part of the profile. We suggest that significant peridotite anisotropy could explain the misfit between measured and calculated seismic velocities in the lithosphere.

Joint Inversion for Lithospheric Structures: Implications for the Growth and Deformation in Northeastern Tibetan Plateau

AbstractSeveral geodynamic models have been proposed for the deformation mechanism of Tibetan Plateau (TP), but it remains controversial. Here we applied a method of joint inversion of receiver functions and surface wave dispersions with P wave velocity constraint to a dense linear array in the NE Tibet. The results show that the geological blocks, separated by major faults at the surface, are characterized by distinct features in the crust, the Moho, and the uppermost mantle. The main features include crustal low‐velocity zones (LVZs) with variable strengths, anomalous Vp/Vs ratios that are correlated with LVZs, a large Moho jump, and other abrupt changes near major faults, strong mantle lithosphere anomalies, and correlation of crustal and mantle velocities. The results suggest a lithospheric‐scale deformation of continuous shortening as well as localized faulting, which is affected by the strength of the lithosphere blocks. The thickened mantle lithosphere can be removed, which facilitates the formation of middle‐lower crustal LVZ and flow. However, such flow is likely a consequence of the deformation rather than a driving force for the outward growth of the TP. The proposed model of TP deformation and growth can reconcile the continuous deformation within the blocks and major faults at the surface.

Petrological Geodynamics of Mantle Melting II. AlphaMELTS + Multiphase Flow: Dynamic Fractional Melting

In this second installment of a series that aims to investigate the dynamic interaction between the composition and abundance of the solid mantle and its melt products, the classic interpretation of fractional melting is extended to account for the dynamic nature of the process. A multiphase numerical flow model is coupled with the program AlphaMELTS, which provides at the moment possibly the most accurate petrological description of melting based on thermodynamic principles. The conceptual idea of this study is based on a description of the melting process taking place along a 1-D vertical ideal column where chemical equilibrium is assumed to apply in two local sub-systems separately on some spatial and temporal scale. The solid mantle belongs to a local sub-system (ss1) that does not interact chemically with the melt reservoir which forms a second sub-system (ss2). The local melt products are transferred in the melt sub-system ss2 where the melt phase eventually can also crystallize into a different solid assemblage and will evolve dynamically. The main difference with the usual interpretation of fractional melting is that melt is not arbitrarily and instantaneously extracted from the mantle, but instead remains a dynamic component of the model, hence the process is named dynamic fractional melting (DFM). Some of the conditions that may affect the DFM model are investigated in this study, in particular the effect of temperature, mantle velocity at the boundary of the mantle column. A comparison is made with the dynamic equilibrium melting (DEM) model discussed in the first installment. The implications of assuming passive flow or active flow are also considered to some extent. Complete data files of most of the DFM simulations, four animations and two new DEM simulations (passive/active flow) are available following the instructions in the supplementary material.

A North Pole thermal anomaly? Evidence from new and existing heat flow measurements from the central Arctic Ocean

Constraining the thermal evolution of the Arctic Ocean is hampered by notably sparse heat flow measurements and a complex tectonic history. Previous results from the Lomonosov Ridge in the vicinity of the North Pole, and the adjacent central Amundsen Basin reveal varied values, including those higher than expected considering plate cooling or simple uniform stretching models. Furthermore, in the vicinity of the North Pole an anomalously slow velocity perturbation exists in upper mantle seismic tomography models. However, whether these observations are related to a thermal anomaly in the mantle remains unknown. We present new heat flow results gathered from 17 sediment cores acquired during the “Arctic Ocean 2016” and “SWERUS-C3” expeditions on the Swedish icebreaker Oden. Three sites located on oceanic lithosphere in the Amundsen Basin between 7°W-71E° reveal surface thermal conductivity of 1.07–1.26 W/mK and heat flow in the order of 71–95 mW/m2, in line-with or slightly higher (1–21 mW/m2) than expected from oceanic heat flow curves. These results contrast with published results from further east in the Amundsen Basin, which indicated surface heat flow values up to 2 times higher than predicted from oceanic crustal cooling models. Heat flow of 49–61 mW/m2 was recovered from the Amerasia Basin. Sites from the submerged continental fragments of the Lomonosov Ridge and Marvin Spur recovered heat flow in the order of 53–76 and 51–69 mW/m2 respectively. When considering the additional potential surface heat flux from radiogenic heat production in the crust, these variable measurements are broadly in line with predictions from uniform extension models for continental crust. A seismically imaged upper mantle velocity anomaly in the central Arctic Ocean may arise from a combination of compositional and thermal variations but requires additional investigation. Disentangling surface heat flow contributions from crustal, lithospheric and mantle processes, including variable along-ridge rifting rates and timing, density and phase changes, conductive and advective dynamics, and regional tectonics, requires further analysis.

Structure of oceanic crust and serpentinization at subduction trenches

The subducting oceanic lithosphere may carry a large amount of chemically bound water into the deep Earth interior, returning water to the mantle, facilitating melting, and hence keeping the mantle mobile and, in turn, nurturing plate tectonics. Bending-related faulting in the trench–outer rise region prior to subduction has been recognized to be an important process, promoting the return flux of water into the mantle. Extensional faults in the trench–outer rise are opening pathways into the lithosphere, supporting hydration of the lithosphere, including alteration of dry peridotite to water-rich serpentine. In this paper, we review and summarize recent work suggesting that bend faulting is indeed a key process in the global water cycle, albeit not yet well understood. Two features are found in a worldwide compilation of tomographic velocity models derived from wide-angle seismic data, indicating that oceanic lithosphere is strongly modified when approaching a deep-sea trench: (1) seismic velocities in both the lower crust and upper mantle are significantly reduced compared to the structure found in the vicinity of mid-ocean ridges and in mature crust away from subduction zones; and (2) profiles shot perpendicular to the trench show both crustal and upper mantle velocities decreasing systematically approaching the trench axis, highlighting an evolutionary process because velocity reduction is related to deformation, alteration, and hydration. P-wave velocity anomalies suggest that mantle serpentinization at trenches is a global feature of all subducting oceanic plates older than 10–15 Ma. Yet, the degree of serpentinization in the uppermost mantle is not firmly established, but may range from 1.9 in the trench, supporting the development of water-filled cracks where bend faulting occurs. In the mantle, the Vp/Vs ratio increases from ∼1.75 in the outer rise to values of >1.8 at the trench, indicating the increasing intensity of serpentinization.

Structure of the crust and upper mantle beneath the Parnaíba Basin, Brazil, from wide-angle reflection–refraction data

Abstract The Parnaíba Basin is a Phanerozoic intracontinental basin within the South America plate, lying on top of and within Precambrian terranes. The Parnaíba Basin Analysis Program wide-angle reflection–refraction (WARR) lies east–west and is 1150 km long profile crossing the basin and its margins. The WARR results show that the crust and uppermost mantle along the profile consist of the Amazonian Craton and Borborema Province, and the Grajaú and Teresina domains comprising the Parnaíba block hidden below the sedimentary cover of the basin itself. The lithospheric characteristics of the Parnaíba block and their differences from the adjacent Precambrian Amazonian Craton and Borborema Province elucidate some aspects of the present day existence of the sedimentary basin covering it. Important elements include the presence of a high mantle velocity and high-velocity lowermost crustal region, interpreted as linked to the intrusion of mafic material into the crust underlying the Grajaú domain, and indications that the crust in this area has been intruded since its consolidation in the Neoproterozoic. It is tentatively proposed that magmatism is related to the inferred thinning of the lower crust of the Teresina and Borborema segments of the profile, with this, in turn, linked to Cretaceous extensional tectonics and the opening of the South Atlantic Ocean.

Crust and upper mantle velocity structure beneath the Ordos Block and its tectonic significance

The Ordos Block in the western North China Craton (NCC) occupies an important position in the tectonic evolution and deep earthquake environment research in China continent. In previous seismic tomography studies, the sparse distribution of seismic stations resulted in a relatively low lateral resolution of 3D structure of crust and upper mantle in the Ordos Block. In this paper, a dense broadband seismograph network was established, including the permanent seismic stations of the regional networks and the temporary seismic stations funded by the ″Destruction of the North China Craton″ project and the North China Scientific Seismic Array. Based on the waveform data recorded by 338 seismic stations in the Ordos Block and surrounding regions, we used FMTOMO traveltime tomography method to determine the high-resolution P-wave velocity structures of the crust and upper mantle down to 400 km depth, and analyzed the relationship between deep structure and a range of geodynamic issues, such as the volcanic and strong earthquake activities and the destruction of the NCC. The tectonic implications of the lateral and the vertical variations of P-wave velocity in the crust and upper mantle structures are described as follow. The transverse variation and vertical difference of velocity structure of crust and uppermost mantle indicate significant correlations between deep tectonics and surface geological setting. The thick lithosphere with positive velocity anomaly beneath the Ordos block suggests that the Ordos block did not suffer extensive destruction. The velocity anomaly distribution of the Taihang orogenic belt in the central NCC is characterized by discontinuous strip anomalies, which is significantly different from that of the North China Plain and Ordos block. The low-velocity anomalies deeper than 400 km might be related to the lithosphere destruction in the eastern NCC. A low-velocity columnar anomaly down to 400 km depth beneath the Datong volcanic groups indicates that the magmatism of the Datong volcanic group might originate from the mantle transitional zone, due to the upwelling and expulsion of basaltic magma caused by the subduction of the Pacific plate. The low-velocity zone in approximately EW direction in the upper mantle between the Ordos Block and the Sichuan basin suggests the possible existence of a narrow outflow channel of the asthenosphere. In summary, the low-velocity anomaly down to the upper mantle in the Ordos Block and surrounding regions indicates that the north-eastward compressive stress of the Tibetan Plateau and the westward subduction of the Pacific plate may induce the migration and upwelling of mantle materials, causing the overall uplift of the Ordos Block and the formation of extensional fault belt in surrounding regions. The vertical upwelling and lateral movement of the deep materials provide the stress conditions for the preparation and occurrence of strong earthquakes resulting in frequent occurrence of strong earthquakes in the regions around the Ordos Block.

Mantle transition zone beneath the central Tien Shan: Lithospheric delamination and mantle plumes

We investigate structure of the mantle transition zone (MTZ) under the central Tien Shan in central Asia by using recordings of seismograph stations in Kyrgyzstan, Kazakhstan and adjacent northern China. We apply P-wave receiver functions techniques and evaluate the differential time between the arrivals of seismic phases that are formed by P to SV mode conversion at the 410-km and 660-km seismic boundaries. The differential time is sensitive to the thickness of the MTZ and insensitive to volumetric velocity anomalies above the 410-km boundary. Under part of the southern central Tien Shan with the lowest S wave velocity in the uppermost mantle and the largest thickness of the crust, the thickness of the MTZ increases by 15–20km relative to the ambient mantle and the reference model IASP91. The increased thickness is a likely effect of low (about −150K) temperature. This anomaly is indicative of delamination and sinking of the mantle lithosphere. The low temperature in the MTZ might also be a relic of subduction of the oceanic lithosphere in the Paleozoic, but this scenario requires strong coupling and coherence between structures in the MTZ and in the lithosphere during plate motions in the last 300Myr. Our data reveal a reduction of thickness of the MTZ of 10–15km under the Fergana basin, in the neighborhood of the region of small-scale basaltic volcanism at the time near the Cretaceous-Paleogene boundary. The reduced thickness of the MTZ is the effect of a depressed 410-km discontinuity, similar to that found in many hotspots. This depression suggests a positive temperature anomaly of about 100–150K, consistent with the presence of a thermal mantle plume. A similar depression on the 410-km discontinuity is found underneath the Tarim basin.