Depth Of Lithosphere-asthenosphere Boundary Research Articles

Lithospheric layering beneath northern Central Africa by S-to-P receiver functions

Using a total of 4520 S-to-P receiver functions (SRFs) recorded by 19 broadband seismic stations belonging to the Libyan Center for Remote Sensing and Space Science and the Incorporated Research Institutions for Seismology Data Management Center, we imaged the depth of the lithosphere–asthenosphere boundary (LAB) beneath northern Central Africa. This boundary occurs over the depth range of 57–124 km which we imaged in consecutive circular bins (radius of 2°) using a high number of receiver functions. The mean depth of the discontinuity is 80 ± 17 km, which is significantly shallower than the global average of ~250 km, commonly found beneath ancient cratons. The SRFs in the study area produced 156 bins with observable arrivals from the LAB. All the stacked traces were plotted along eight latitudinal profiles from 20°N to 35°N. The observed depth of the LAB increases systematically toward the northern central part of the study area from approximately 67 km–120 km. The apparent depth of the LAB increases from 70 to 90 km from 21°N to 28°N and then further increases to 120 km from 28°N to 34°N. These depth variations are extreme beneath the northern central part of Libya. The LAB depth beneath the Hoggar volcanic province area is relatively low (~57 km) compared to other areas. This study provides new constraints on regional-scale tectonic processes such as lithospheric stretching or thinning, due to partial melting beneath the study region that may be an effect of the LAB topography in this part of northern Central Africa.

The relationship between crust-lithosphere structures and seismicity on the southeastern edge of the Tibetan Plateau

The crust-lithosphere structure, especially the discontinuities such as Moho and lithosphere-asthenosphere boundary (LAB), is important in the investigation of geodynamic process implications and tectonic evolution of the lithosphere and regional seismic activity. We stacked the S receiver functions from 51 permanent broad-band stations to investigate the crust-lithosphere structures beneath the southeastern edge of the Tibetan Plateau and further discussed the mechanism of lower-crust earthquakes, the regional tectonics deformation characteristics, and the relationship between the two discontinuities and seismicity in this region. The results show that the Moho depth increases from 40 to 52km beneath the Sichuan Basin to 56–74km on the both western side of the Longmenshan (LMS) fault and Anninhe-Zemuhe fault. The LAB depth ranges from 130 to 170km beneath the Sichuan Basin, and a shallow belt ranges from 100km to 130km around the Sichuan Basin. On the Moho depth contour map, 374 moderate earthquakes that accounted for 72.2%, corresponding to the depth range of 44–52km and 60–68km on the both sides of a narrow strip (longitude 102°~104°). On the LAB depth contour map, 321 earthquakes accounted for 62.15% in the depth range of 130–150km. The variation of LAB corresponds to the increasing crustal thickness, intensive lower crustal earthquakes (hypocenters about 60km and magnitude 4.0≤Ms.≤4.9) and surface abrupt elevations. It implied that Normal strong earthquakes generate the lower crustal earthquakes or aftershocks that resulted in the shear zones where fluid-induced metamorphic transformations, the eclogitization of dry granulite, and further affect the crustal thickness and deformation to control the spatial distribution of deep faults and seismicity.

Complex structure of upper mantle beneath the Yadong-Gulu rift in Tibet revealed by S-to-P converted waves

The convergence between the Indian and Eurasian plates has produced the thick crust and uplifted the Tibetan plateau since about 50 Ma. However, the deformation of the mantle lithosphere is still a puzzle. The geometry of the subducting Indian mantle lithosphere beneath the plateau and the thickening or/and delamination of the Tibetan mantle lithosphere are the keys for understanding the continental collision process and the evolution of the plateau. However, knowledge has been restricted due to sparse data coverage in Tibet. In this study, S-wave receiver functions are calculated using tele-seismic waveforms recorded by two broadband arrays in central Tibet to image the lithospheric structure, mainly the depth variation of the lithosphere-asthenosphere boundary (LAB). Our results show that the depth of the Tibetan LAB decreases from ∼150 km in the west to ∼120 km in the east across the north-south trending Yadong-Gulu rift. Similarly, the LAB depth of the subducting Indian slab decreases from ∼270 km in the west to ∼200 km in the east, and the northernmost subducting Indian slab can be observed beneath the Bangong-Nujiang suture. These observations suggest that the thickness of the Tibetan lithosphere and the depth of the underlying Indian slab are segmented across the Yadong-Gulu rift in different degrees. The abrupt changes imply that the subducting Indian slab has been torn, which provided an upwelling channel for the asthenosphere contributing to the development of the rift.

The application of a gravimetric forward modelling of the lithospheric structure for an estimate of the average density of the upper asthenosphere

The average density of 3300 kg m−3 is often attributed for the asthenosphere. In this study, we inspect this value by estimating the average value of the (upper) asthenosphere based on applying the gravimetric forward modelling of major known lithospheric density structures. The LITHO1.0 global seismic model of the lithospheric density structure is used for this purpose, while considering that the lithosphere-asthenosphere boundary (LAB) is rheological, conventionally taken at the 1300 °C isotherm, above which the mantle behaves in a rigid fashion and below which it behaves in a ductile fashion. According to our result, the average density of the upper asthenosphere is roughly 3400 kg m−3. This density value closely agrees with the corresponding average value 3371 kg m−3 computed based on an empirical density model provided in the Preliminary Reference Earth Model (PREM), while using the LITHO1.0 LAB depth data. We also demonstrate that the sub-lithospheric mantle gravity map exhibits mainly a thermal signature. The most prominent features in this gravity map are mid-oceanic spearing ridges marked by gravity lows, while oceanic subductions in the West Pacific are characterized by the most pronounced gravity highs.

Rheological stratification and spatial variations in the effective elastic thickness of the lithosphere underneath the central Anatolian region, Turkey

In this study, the regional components of global model EGM08 Bouguer anomalies obtained by low pass filtering were inverted to map the geometries of Moho and Lithosphere-Asthenosphere Boundary (LAB) of the central Anatolian region. It was determined the Moho and LAB depths in the region to be 35.8–41.2 km and 67–91 km, respectively. The results from rheological modeling indicate mechanical decoupling of the crust and uppermost lithospheric mantle in eastern part and coupling in the western part of the study area. We also compare the rheological stratification with the focal depth distribution of earthquakes to examine the possible discrepancies between the brittle-ductile transition zone and the maximum depths of earthquakes along the selected profiles. The spatial variations of effective elastic thicknesses (EET) of the lithosphere have been estimated from the strength of the crust and lithospheric mantle by implying deformation gradient at Moho and LAB. The EET values vary in the range of 19–24.3 km. Although the EET values are relatively high in the eastern part of the region, lower EET values are directly underlain by thinned lithosphere of northwestern and southwestern part of central Anatolian region. We also analyze the crustal rheologies obtained from the lithospheric strength by delineating the pattern of crustal seismic activities.

Seismic Evidence for Lateral Asthenospheric Flow Beneath the Northeastern Tibetan Plateau Derived From S Receiver Functions

AbstractWe present detailed lithospheric images of the NE Tibetan Plateau by applying the depth migration technique to S receiver functions derived from 113 broadband stations. Our migrated images indicate that the lithosphere‐asthenosphere boundary (LAB) lies at depths of 105–120 km beneath the Qilian terrane and reaches depths of 126–140 km below the Alxa and Ordos blocks. The most prominent variation in the LAB depth is the presence of LAB steps of no less than 20 km in the transition zone between the active NE Tibetan Plateau and the surrounding cratonic Alxa and Ordos blocks, which conflicts with the model of southward subduction of the Alxa and Ordos blocks. Furthermore, the marked LAB steps occur at 130 ± 10 km away from the southern surficial boundary faults between the NE Tibetan Plateau and the surrounding tectonic provinces, corresponding to the North Qilian fault and the Liupanshan fault, respectively. Therefore, we propose that these scenarios of LAB can be attributed to the delamination of fragmented mantle lithosphere in the transition zone between the NE Tibetan Plateau and the surrounding Alxa and Ordos blocks, triggered by lateral asthenospheric flow. In addition, our observations of a thin lithosphere with thickness of 107–115 km beneath the Songpan‐Ganzi terrane and the west Qinlin orogen greatly facilitate the process of underlying lateral asthenospheric flow. The isostatic uplift of the plateau caused by the delamination of fragmented mantle lithosphere, together with increased horizontal compressive stress, may have led to the outward growth of the NE Tibetan Plateau.

Lithospheric layering beneath the contiguous United States constrained by S-to-P receiver functions

To image upper mantle seismic discontinuities beneath the contiguous United States, a total of 284,121 S-to-P receiver functions (SRFs) recorded by 3,594 broadband seismic stations in the EarthScope Transportable Array and other permanent and temporary deployments are stacked in circular bins of 2° in radius. A robust negative arrival, representing a sharp discontinuity of wave speed reduction with depth, is visible in virtually all the stacked traces in the depth range of 30–110 km. Beneath the western U.S., the mean depth of this discontinuity is 69±17 km, and beneath the eastern U.S., it is 76±5 km, both are comparable to the depth of the tomographically-determined lithosphere–asthenosphere boundary (LAB). In contrast, the depth of the discontinuity beneath the stable cratonic region of the central U.S. is 87±6 km, which is significantly shallower than the ∼250 km LAB depth determined by seismic tomography. Based on the amplitude of the corresponding arrival in the SRFs and findings from previous seismic tomography and mantle xenolith studies, this discontinuity beneath the central U.S. is interpreted as the top of an intra-lithospheric low wave speed, probably phlogopite-rich layer. The observations provide new constraints on a number of regional scale tectonic processes, such as lithospheric stretching in the Texas–Louisiana Gulf Coastal Plain and the Basin and Range Province, and possible lithospheric basal erosion beneath the northeastern U.S.

Lithospheric structure of the South China Sea and adjacent regions: Results from potential field modelling

This work aims to investigate the crustal and lithospheric mantle thickness of the South China Sea (SCS) and adjacent regions. The crust-mantle interface, average crustal density, and lithospheric mantle base are calculated from free-air gravity anomaly and topographic data using an iterative inversion method. We construct a three-dimensional lithospheric model with different hierarchical layers. The satellite-derived gravity is used to invert the average crustal density and Moho (crust-mantle interface) undulations. The average crustal density and LAB (lithosphere–asthenosphere boundary) depths are further adjusted by topographic data under the assumption of local isostasy. The average difference in Moho depths between this study and the seismic measurement results is <1.5 km. The results show that in oceanic regions, the Moho depths are 7.5–30 km and the LAB depths are 65–120 km. The lithospheric thickness of the SCS basin and the adjacent regions increases from the sea basin to the continental margin with a large gradient in the ocean-continent transition zones. The Moho depths of conjugate plots during the opening of SCS, Zhongsha Islands and Reed Bank, reveal the asymmetric spreading pattern of SCS seafloor spreading. The lithospheric thinning pattern indicate two different spreading directions during seafloor spreading, which changed from N-S to NW-SE after the southward transition of the spreading axis. The lithosphere of the SCS basin and adjacent regions indicate that the SCS basin is a young basin with a stable interior lithosphere.

Seismic Constraints on the Lithosphere–Asthenosphere Boundary Beneath the Izu-Bonin Area: Implications for the Oceanic Lithospheric Thinning

The lithosphere–asthenosphere boundary (LAB) is the seismic discontinuity with negative velocity contrasts in the upper mantle. Seismic detections on the LAB are of great significance in understanding the plate tectonics, mantle convection and lithospheric evolution. In this paper, we study the LAB in the Izu-Bonin subduction zone using four deep earthquakes recorded by the permanent and temporary seismic networks of the USArray. The LAB is clearly revealed with sP precursors (sdP) through the linear slant stacking. As illustrated by reflected points of the identified sdP phases, the depth of LAB beneath the Izu-Bonin Arc (IBA) is about 65 km with a range of 60–68 km. The identified sdP phases with opposite polarities relative to sP phases have the average relative amplitude of 0.21, which means a 3.7% velocity drop and implies partial melting in the asthenosphere. On the basis of the crustal age data, the lithosphere beneath the IBA is located at the ~ 1100 °C isotherm calculated with the GDH1 model. Compared to tectonically stable areas, such as the West Philippine Basin (WPB) and Parece Vela Basin (PVB) in the Philippine Sea, the lithosphere beneath the Izu-Bonin area shows the obvious lithospheric thinning. According to the geodynamic and petrological studies, the oceanic lithospheric thinning phenomenon can be attributed to the strong erosion of the small-scale convection in the mantle wedge enriched in volatiles and melts.

Lithospheric thickness under the Dinarides

The nature of the interaction between Adriatic and Euroasian lithospheric plates in the Dinarides is important for understanding the complex tectonic history of the central Mediterranean. Using the data from permanent and temporary seismic stations in the wider Dinaric region, we imaged the lithospheric and upper mantle structure under this area. Specifically, we focused on mapping the lithosphere asthenosphere boundary (LAB) using the S receiver functions in order to establish boundaries between different tectonic domains present in this region. The lithospheric thickness in the investigated area varies between ∼50 and ∼160 km with high degree of variability between adjacent tectonic realms. Below northwestern Dinarides the LAB depth varies between 100 and 120 km thinning towards Adriatic sea and Pannonian basin, to 90 and 70 km respectively. In the central Dinaric region (Lika region) we find anomalously thin lithosphere with thickness varying between 50 and 70 km and weak velocity gradient defining the LAB. Further south the signal from the LAB is more pronounced with lithosphere getting thicker again with average depths around 90 km. The intriguing observation of thinned lithosphere under central part of the Dinarides coincides with the zone of lower seismicity and with the tomographic images showing the slab gap in this area.

The lithosphere–asthenosphere boundary beneath the South Island of New Zealand

Lithosphere–asthenosphere boundary (LAB) properties beneath the South Island of New Zealand have been imaged by Sp receiver function common-conversion point stacking. In this transpressional boundary between the Australian and Pacific plates, dextral offset on the Alpine fault and convergence have occurred for the past 20 My, with the Alpine fault now bounded by Australian plate subduction to the south and Pacific plate subduction to the north. Using data from onland seismometers, especially the 29 broadband stations of the New Zealand permanent seismic network (GeoNet), we obtained 24,971 individual receiver functions by extended-time multi-taper deconvolution, and mapped them to three-dimensional space using a Fresnel zone approximation. Pervasive strong positive Sp phases are observed in the LAB depth range indicated by surface wave tomography. These phases are interpreted as conversions from a velocity decrease across the LAB. In the central South Island, the LAB is observed to be deeper and broader to the northwest of the Alpine fault. The deeper LAB to the northwest of the Alpine fault is consistent with models in which oceanic lithosphere attached to the Australian plate was partially subducted, or models in which the Pacific lithosphere has been underthrust northwest past the Alpine fault. Further north, a zone of thin lithosphere with a strong and vertically localized LAB velocity gradient occurs to the northwest of the fault, juxtaposed against a region of anomalously weak LAB conversions to the southeast of the fault. This structure could be explained by lithospheric blocks with contrasting LAB properties that meet beneath the Alpine fault, or by the effects of Pacific plate subduction. The observed variations in LAB properties indicate strong modification of the LAB by the interplay of convergence and strike-slip deformation along and across this transpressional plate boundary.

Structure of the oceanic lithosphere and upper mantle north of the Gloria Fault in the eastern mid‐Atlantic by receiver function analysis

AbstractReceiver functions (RF) have been used for several decades to study structures beneath seismic stations. Although most available stations are deployed on shore, the number of ocean bottom station (OBS) experiments has increased in recent years. Almost all OBSs have to deal with higher noise levels and a limited deployment time (∼1 year), resulting in a small number of usable records of teleseismic earthquakes. Here we use OBSs deployed as midaperture array in the deep ocean (4.5–5.5 km water depth) of the eastern mid‐Atlantic. We use evaluation criteria for OBS data and beamforming to enhance the quality of the RFs. Although some stations show reverberations caused by sedimentary cover, we are able to identify the Moho signal, indicating a normal thickness (5–8 km) of oceanic crust. Observations at single stations with thin sediments (300–400 m) indicate that a probable sharp lithosphere‐asthenosphere boundary (LAB) might exist at a depth of ∼70–80 km which is in line with LAB depth estimates for similar lithospheric ages in the Pacific. The mantle discontinuities at ∼410 km and ∼660 km are clearly identifiable. Their delay times are in agreement with PREM. Overall the usage of beam‐formed earthquake recordings for OBS RF analysis is an excellent way to increase the signal quality and the number of usable events.

THE NATURE OF THE LITHOSPHERE‐ASTHENOSPHERE BOUNDARY BENEATH THE CENTRAL SOUTH AMERICA AREA FROM THE STACKING OF sP PRECURSORS

AbstractThe lithosphere‐asthenosphere boundary (LAB) is a seismic discontinuity with negative velocity contrast in the upper mantle. Seismic detections of the LAB in subduction zone region are helpful to understand the interaction between the lithosphere and asthenosphere and the geodynamic process associated with the slab subduction. In this paper, the vertical broadband waveforms are collected from four deep earthquakes occurred from 2006 to 2012 beneath the central South America area. The seismic waveforms are processed with the linear slant stack method to get the vespagrams in the relative travel‐time vs. slowness domain, and the sP precursors reflected at the bottom of the LAB (sLABP) are successfully extracted. Based on the one‐dimensional modified velocity model (IASP91‐SA), the horizontal distribution for the six sLABP reflecting points is obtained and divided into the western part (I) and the eastern part (II). In the part I, the LAB depth ranges between 60 km and 63 km, with the average depth of 61 km and the topography of 3 km; in the part II, the LAB depth ranges between 78 km and 82 km, with the average depth of 80 km and the topography of 4 km. Our results reveal the increasing LAB depths from west to east in the central South America area, and the trend may possibly represent the reformation differences of the continental lithosphere. We infer that near the trench, the continental lithosphere may be subjected to the stronger erosion for the higher degree of partial melting and more fertile melts in the asthenosphere; while far from the trench, the continental lithosphere may be subjected to the weaker erosion for the lower degree of partial melting and less fertile melts in the asthenosphere.

Tearing of the mantle lithosphere along the intermediate‐depth seismicity zone beneath the Gibraltar Arc: The onset of lithospheric delamination

AbstractThe intermediate‐depth seismicity (IDS) beneath the Gibraltar Arc is enigmatic. So far, there is no general consensus on its relationship with the ongoing tectonic processes. We analyzed S wave receiver functions (SRFs) with data recorded by a dense N‐S seismic profile deployed across the Sierra Nevada in southern Spain. SRF piercing points at depths of the lithosphere‐asthenosphere boundary (LAB) sample an area of the IDS zone, providing an ideal opportunity to study the lithospheric structure at the IDS zone. We observe an abrupt change in the LAB depth along a profile from north to south across the northern branch of the IDS. The LAB depth changes from 90 to 100 km north of the IDS to ~130 km south of it. We propose that the IDS marks a tear in the Iberian mantle lithosphere along its entire length, implying an ongoing lithospheric delamination process that produces the seismicity at its onset.

Lithospheric thickness estimation beneathNorthwesternSouthAmerica from anS‐wave receiver function analysis

AbstractWe make use of theS‐to‐Preceiver function technique beneath Colombia and neighboring regions to make a first‐order approximation of the depth of the Lithosphere‐Asthenosphere Boundary (LAB) and, therefore, of lithospheric thickness. A deconvolution technique was used to calculate the receiver functions, and after a moveout correction and a time‐depth conversion, LAB depths for different tectonic regions of northwestern South America were obtained. Results are typically between 65 and 110 km, consistent with previous estimates around the world and other regions in South America. Lithospheric thickness beneath an oceanic island in the Caribbean is ∼80 km, whereas for the Ecuador‐Colombia Trench it is ∼65 km, and around 100 km for the Panama Arc. The transition to the continent is associated with an increase in LAB depth, where it can reach ∼110 km, with no significant differences among terranes and/or tectonic blocks.

Seismic evidence of the lithosphere-asthenosphere boundary beneath Izu-Bonin area

The lithosphere-asthenosphere boundary (LAB) is the seismic discontinuity with the negative velocity contrast in the upper mantle. The plate tectonic theory describes that the rigid lithosphere translate coherently over the ductile asthenosphere, and the LAB implies the decoupling between the two layers. The seismic velocity, viscosity, electrical resistivity and other physical parameters change with the depths across the boundary. The LAB has become a focus of seismology studies for its significance in understanding the plate motions, the mantle convection and the lithospheric evolution processes. Seismic detections on the LAB in subduction zone areas are helpful to understand the interactions between the lithosphere and asthenosphere layers and the geodynamic processes related with the slab subductions. For the dense permanent networks and transportable arrays of the USArray, the abundant waveforms of the deep earthquake events beneath the Izu-Bonin area can be recorded and retrieved. In this study, the vertical broadband waveforms are collected from three earthquake events occurring from 2002 to 2014 with the focal depths of 400–600 km. The source parameters are taken from the Preliminary Determinations of Epicenters catalog of US Geological Survey, and the focal fault solutions are taken from the global Centroid-Moment-Tensor project. The seismic waveforms are manually selected with two criteria: (1) simple sP arrivals with clear peaks, and (2) high signal-to-noise ratios and without abnormal spikes. In order to enhance the weak unknown precursors, the linear slant stack method, which is the special form of the N -th root slant stack method and phase-weighted stack method, is applied to scan the differential slowness and stack the seismograms in the domain of differential travel-time and differential slowness. The selected waveform data is processed to obtain the vespagrams and the stacked waveforms. The sP precursors reflected on the LAB ( s LAB P ), which have the opposite polarities with the amplitude ratios of 0.17–0.21 relative to the sP phases, are successfully extracted in our results. To reduce the possible effects of the crustal structures on the propagations of sP phases, the one-dimensional modified velocity model (IASP91-IB) is established by replacing the crust part of the IASP91 model with the CRUST1.0 model. On the basis of the modified model, we obtain the distributions for six reflected points of the s LAB P phases near the source area. Our results reveal that the LAB depths range between 58 and 65 km beneath the Izu-Bonin Arc, with the average depth of 62 km and the small topography of 7 km. Based on the half-space cooling model for the oceanic lithosphere, the lithosphere beneath the Izu-Bonin Arc is about located at the isotherm of 950°C, and the lithosphere beneath the West Philippine Basin (WPB) and Parece Vela Basin (PVB) is about located at the isotherm of 1100°C. Compared with the results of the tectonic stable areas (WPB and PVB) in the Philippine Sea, the lithosphere beneath the Izu-Bonin Arc shows the obvious thinning phenomenon. The numerical modeling experiments have revealed the strong erosions of the convecting asthenosphere in the mantle wedge, resulting in the destruction of the craton. We infer that the lithospheric thinning beneath the Izu-Bonin Arc is closely related with the partial melting, which is caused by the volatiles continuously released from the subducted western Pacific slab, and the strong erosions of the small-scale mantle convections in the back-arc mantle wedge.

Seismic evidence of the lithosphere-asthenosphere boundary beneath the Tonga area, southwestern Pacific

We study the lithosphere-asthenosphere boundary (LAB) in the Tonga subduction zone using four deep focal events recorded by the Capital Seismic Network (CSN) of China and the Northeast China Extended Seismic Array (NECESSArray). The existence and depths of the LAB beneath the Lau Ridge are clearly revealed by the linear slant stacking of observed sP precursors (sdP). As illustrated by the reflected points of robust sdP phases from LAB, the depth of LAB in the northern Lau Ridge is about 63km with a range of 63–64km, in the northwest is about 77km with a range of 76–78km and in the south is about 72km. The uncertainty of determined depths is no more than 4km given the effects of the picking errors of arrival times and velocity heterogeneities. Our results suggest that the oceanic lithosphere of the Lau Ridge is thinner in the northern part than in the northwestern and the southern parts. According to the previous geodynamic and petrological studies, the oceanic lithospheric thinning in the northern Lau Ridge should be related to the strong erosion of the active mantle convection in the back-arc mantle wedge with the enrichment of the volatiles and melts.

Age-independent seismic anisotropy under oceanic plates explained by strain history in the asthenosphere

The depth of the oceanic lithosphere–asthenosphere boundary (LAB), as inferred from shear wave velocities, increases with lithospheric age, in agreement with models of cooling oceanic lithosphere. On the other hand, the distribution of radial anisotropy under oceanic plates is almost age-independent. In particular, radial anisotropy shows a maximum positive gradient at a depth of ∼70 km, which, if used as a proxy, indicates an age-independent LAB depth. These contrasting observations have fueled a controversy on the seismological signature of the LAB. To better understand the discrepancy between these observations, we model the development of lattice preferred orientation (LPO) in upper mantle crystal aggregates and predict the seismic anisotropy produced by plate-driven mid-ocean ridge flows. The model accounts for the progressive cooling of the lithosphere with age and can incorporate both diffusion and dislocation creep deformation mechanisms. We find that an age-independent distribution of radial anisotropy is the natural consequence of these simple flows. The depth and strength of anisotropy is further controlled by the deformation regime – dislocation or diffusion creep – experienced by crystals during their ascent towards, and subsequent motion away from, the ridge axis. Comparison to surface wave tomography models yield constraints on rheological parameters such as the activation volume. Although not excluded, additional mechanisms proposed to explain some geophysical signatures of the LAB, such as the presence of partial melt or changes in water content, are not required to explain the radial anisotropy proxy. Our prediction, that the age-independent radial anisotropy proxy marks the transition to flow-induced asthenospheric anisotropy, provides a way to reconcile thermal, mechanical and seismological views of the LAB.

Upper mantle structure across the Trans-European Suture Zone imaged by S-receiver functions

We present a high-resolution study of the upper mantle structure of Central Europe, including the western part of the East European Platform, based on S-receiver functions of 345 stations. A distinct contrast is found between Phanerozoic Europe and the East European Craton across the Trans-European Suture Zone. To the west, a pronounced velocity reduction with depth interpreted as lithosphere–asthenosphere boundary (LAB) is found at an average depth of 90 km. Beneath the craton, no strong and continuous LAB conversion is observed. Instead we find a distinct velocity reduction within the lithosphere, at 80–120 km depth. This mid-lithospheric discontinuity (MLD) is attributed to a compositional boundary between depleted and more fertile lithosphere created by late Proterozoic metasomatism. A potential LAB phase beneath the craton is very weak and varies in depth between 180 and 250 km, consistent with a reduced velocity contrast between the lower lithosphere and the asthenosphere. Within the Trans-European Suture Zone, lithospheric structure is characterized by strong heterogeneity. A dipping or step-wise increase to LAB depth of 150 km is imaged from Phanerozoic Europe to 20–22° E, whereas no direct connection to the cratonic LAB or MLD to the east is apparent. At larger depths, a positive conversion associated with the lower boundary of the asthenosphere is imaged at 210–250 km depth beneath Phanerozoic Europe, continuing down to 300 km depth beneath the craton. Conversions from both 410 km and 660 km discontinuities are found at their nominal depth beneath Phanerozoic Europe, and the discontinuity at 410 km depth can also be traced into the craton. A potential negative conversion on top of the 410 km discontinuity found in migrated images is analyzed by modeling and attributed to interference with other converted phases.

Lithospheric Structure of the Northeastern North China Craton Imaged by S Receiver Functions

Lithosphere thickness variation is important for understanding the significant tectonic reactivation of the North China Craton (NCC) in the Mesozoic and Cenozoic time. Here, we determined the lithospheric structure in the northeastern NCC using S receiver functions from 305 teleseismic events recorded by 223 seismic stations. The Moho and lithosphere–asthenosphere boundary (LAB) are imaged clearly beneath the region. The Moho depth decreases from ~45 km beneath the western NCC to ~25 km beneath the eastern NCC. We found that the lithospheric thickness varies from 60 to 80 km beneath the Trans-North China Orogen (TNCO) and eastern NCC with no significant change of the LAB depth. The lithosphere thickness beneath the northwestern Ordos plateau is 100–130 km. In addition, there is a mid-lithosphere discontinuity at a depth of 80 km beneath the plateau that is connected to the base of thinned lithosphere in TNCO and eastern NCC. We suggest that the mid-lithosphere discontinuity represents a mechanically weak zone in the original cratonic lithosphere of the NCC. The material in the lower lithosphere of the craton, when warmed and hydrated by water released from the subducting slab of Western Pacific, became weak due to decrease in viscosity and/or partial melting and was subsequently removed through small-scale mantle convections.