Receiver Function Imaging Research Articles

Receiver Function Imaging with Reconstructed Wavefields from Sparsely Scattered Stations

Receiver function imaging is an important tool for imaging the local and regional lithospheric structure beneath seismic arrays. The spatial density of stations and uniformity of their spacing in a given study area can strongly influence image quality. Sparsely distributed stations may produce distorted images and artifacts due to uneven spatial sampling. Reconstructing and resampling seismic wavefields to produce virtual stations within a regular, fine‐scale observation system can provide a more accurate image with fewer artifacts. We developed a wavefield reconstruction method to accurately map teleseismic data from a sparsely spaced seismic network into a regular and fine‐scale observation system. The method reconstructs wavefields in each time slice using a 2D cubic‐spline interpolation. As a preprocess step, wavefield reconstruction can significantly improve the receiver function by greatly reducing stair‐step artifacts for dipping structures. We applied the method to data from the Ordos regional network, which has a station interval of ∼30–80 km. The virtual stations rendered image results that showed almost perfect consistency with images produced by a fine‐scale temporary array having uniform station interval of 10 km. Numerical experiments show the wavefield reconstruction method improved lithospheric images derived from traditional receiver function imaging techniques. Receiver function imaging is routinely used to identify velocity discontinuity structures in the crust and upper mantle. Receiver functions are obtained by deconvolving vertical components from horizontal components of teleseismic records (Langston, 1979). Irregularities in distributions of both seismic stations and earthquakes pose challenges for receiver function imaging techniques. Based on theoretical seismograms from 2D models, Ryberg and Weber (2000) deduced that resolving structures of similar scale requires a station spacing of a few kilometers. This result demonstrates the best way to improve receiver function image quality is to increase station density. Station spacings of a few kilometers are rare, due to the high costs of their …

Using a Three-Component Distributed Array of Mixed, Horizontal, and Vertical Single-Component Seismometers to Produce a Receiver Function from a Short Deployment

Receiver function (RF) imaging is an important tool in the investigation of the crust (e.g., Burdick and Langston, 1977; Langston, 1979; Owens et al. , 1984; Owens and Crosson, 1988; Zhang and Langston, 1995) and upper mantle (Vinnik, 1977; Gurrola et al. , 1994). The P ‐to‐ S conversion ( Ps ) from the Moho is typically one of the most prominent phases on P ‐wave RFs. Estimates of the depth and thickness of the Moho (e.g., Grad and Tiira, 2012) and V P / V S ratio of the crust are important goals of many RF investigations (Zhu and Kanamori, 2000). In addition, S ‐to‐ P conversions ( Sp ) on S ‐wave RFs have been used to image the lithosphere–asthenosphere boundary (e.g., Rychert et al. , 2005, 2007; Abt et al. , 2010; Kumar et al. , 2012). Although 2D and 3D imaging of the upper mantle and crust using common conversion point (CCP) stacking of RFs produced from data recorded by dense seismic arrays (e.g., Dueker and Sheehan, 1997; Simmons and Gurrola, 2000; Hansen et al. , 2013) are more powerful tools than 1D imaging, few areas exist with dense enough arrays for such processing. Single‐station RF interpretation is, therefore, still very important in most regions to image the crustal and upper‐mantle discontinuities. Array processing of both single‐ and three‐component seismic data is known to improve the quality of RFs (Langston and Hammer, 2001). Here, we present the results of using array processing applied to a small, mixed network of vertical and horizontal single‐component seismic stations with a relatively short deployment to produce a single RF. We will refer to an array of mixed vertical and horizontal …

The crustal thickness of NE Tibet and its implication for crustal shortening

The crustal deformation model for NE Tibet is key to understanding the outward growth of the plateau, especially along its northern front. This study describes receiver function images of the Moho beneath NE Tibet, as calculated from teleseismic data recorded by regional seismic networks. Moho depth from these images, coupled with results from previous wide-angle reflection/refraction studies (WARR), allowed crustal thickness estimates along several profiles. Crustal shortening was estimated by restoring the present day thickened crust to its assumed initial crustal thickness. Our results show a relatively large amount of crustal shortening (250–350km) at an orientation of N30°E in the central part of NE Tibet. This indicates that prior to shortening, the southern edge of Asia lay 300–400km north of the location indicated by previous S-wave receiver function studies. This discrepancy may result from southward subduction of a segment of Asia's lower-crust beneath central Tibet. It could also arise from crustal thickening predominantly caused by upper-crustal shortening, provided that delamination of eclogitized mafic lower-crust and erosion have not significantly reduced crustal volume.

Sp receiver function imaging of a passive margin: Transect across Texas's Gulf Coastal Plain

The Gulf Coast of Texas has been the subject of intensive geological and geophysical investigation in pursuit of hydrocarbons but studies that penetrate beyond the upper crust are limited to a few refraction profiles and regional surface wave investigations. The passing of EarthScope's Transportable Array has facilitated regional investigations of the lithosphere but its 70-km station spacing does not allow many important tectonic features to be imaged. A broadband seismic transect across the Texas Gulf Coastal Plain was therefore performed in order to image deep structure beneath this passive margin and the transition to the neighboring craton. A 2D Sp receiver function common conversion point (CCP) stacked image produced for this transect reveals several discontinuities in the sub-crustal lithosphere.The region nearest the shoreline is underlain by an anomalous ∼18 km thick low velocity layer that produces a strong negative pulse in the Sp receiver functions. The drop in velocity is too large to be due to any reasonable change in Fe or Mg content but could be produced by partial melt or mantle hydration. It is unlikely that partial melt would still be found in a 160–180-year-old passive margin, such as the Gulf Coast, but hydration, possibly introduced by a through-going Balcones fault system, and resulting serpentinization could produce the observed anomaly.An event with negative polarity appears at a depth of ∼110 km, which we interpret to be the lithosphere–asthenosphere boundary (LAB). Thermal variations alone would not produce a sufficiently sharp discontinuity to be imaged by Sp converted phases. Recent shear-wave splitting studies revealed unusually large delay times in this region, along with fast polarization directions that differ from measurements on the Laurentian craton. Large delay times may imply significant flow, which could also produce frictional heating, due to shearing, and partial melt, which would steepen the velocity gradients.An additional layer is bounded by discontinuities at 145–165 km depth (with negative polarity) and 185–205 km depth (with positive polarity). The appearance of these discontinuities suggests that the mantle beneath the LAB is more complicated here than elsewhere and that the transition from lithosphere to asthenosphere occurs over a broad (85–100-km-thick) depth interval.

Mapping crustal structure beneath southern Tibet: Seismic evidence for continental crustal underthrusting

Receiver function imaging along a temporary seismic array (ANTILOPE-2) reveals detailed information of the underthrusting of the Indian crust in southern Tibet. The Moho dips northward from ~50km to 80km beneath the Himalaya terrane, and locally reaches ~85km beneath the Indus–Yalung suture. It remains at ~80km depth across the Lhasa terrane, and shallows to ~70km depth under the Qiangtang terrane. An intra-crustal interface at ~60km beneath the Lhasa terrane can be clearly followed southward through the Main Himalaya Thrust and connects the Main Boundary Thrust at the surface, which represents the border of the Indian crust that is underthrusting until south of the Bangong–Nujiang Suture. A mid-crustal low velocity zone is observed at depths of 14–30km beneath the Lhasa and Himalaya terranes probably formed by partial melt and/or aqueous fluids.

Moho structure across the San Jacinto fault zone: Insights into strain localization at depth

Ten years of teleseismic earthquakes recorded by broadband seismic instruments from the Anza network–USArray stations around the San Jacinto fault were used to create P receiver function images of the lithospheric structure beneath this major strike-slip fault. Analyses of back-azimuthal variation and location of the conversion points near the fault suggest an ∼8 km vertical offset of the Moho directly beneath the San Jacinto fault. This implies that the fault extends through the entire crust and into the mantle lithosphere, supporting the idea that the strain in the lower crust is localized within a narrow zone. The Moho offset and surface trace of the San Jacinto fault zone are coincident with a compositional boundary in the Peninsular Ranges batholith previously identified in potential field geophysical data and Sr isotope analyses. The position of the offset with respect to this relict geologic feature, which predates the pluton emplacement that formed the batholith, may be a controlling factor in strain location and plate-boundary fault initiation.

Receiver function imaging of lithospheric structure and the onset of melting beneath the Galápagos Archipelago

The Galápagos Archipelago represents an opportunity to investigate the properties of young oceanic lithosphere, the effects of a hotspot anomaly on lithospheric thickness, and melting dynamics in a hotspot-ridge interaction. Here we use data recorded by the SIGNET array and permanent station PAYG on the Islands Santa Cruz and Isabela, respectively. We used P-to-S (Ps) and S-to-P (Sp) receiver functions to constrain crust and mantle structure. A simultaneous deconvolution method was used to constrain 1-D structure and also for the modeling of robust features. A migrated extended multitaper method was used to investigate 3-D structural variations. Ps images a velocity increase with depth at 11±7 km, probably the base of the pre-plume crust, or old Moho. Sp imaging and modeling images a second, deeper velocity increase at 37±7 km depth. A velocity decrease with depth is imaged on average at 75±12 km likely associated with the lithosphere–asthenosphere boundary. This discontinuity is imaged deeper, 82 km, in the southwest and shallower, 66 km, in the northeast near the spreading ridge. Although the trend is consistent with lithospheric thickening with age, the thickness is much larger than predicted by conductive cooling models of 0–10 My oceanic lithosphere. We infer a compositional contribution to velocity variations. Finally, a velocity increase with depth is imaged at ∼125 to 145±15 km depth that is likely associated with the onset of melting. The discontinuity is imaged deeper in 3 sectors of the Galápagos platform-ridge region, all coincident with the slowest surface wave shear velocity anomalies in the upper 100 km. One is located in the southwest in a hypothesized plume location. The other two are to the northwest and northeast, possibly illuminating multiple plume diversions related to complex plume–ridge interactions.

Structure of the crust and the lithosphere beneath the southern Puna plateau from teleseismic receiver functions

We present a teleseismic P and S receiver function study using data from a temporary passive-source seismic array in the southernmost Puna plateau and adjacent regions. The P receiver function images show the distribution of crustal thickness and Vp/Vs ratio for this area. Over much of the southern Puna plateau, the crustal thickness is 50–55 km, whereas to the west a thicker crust (∼60 to 75 km) is observed beneath much of the Andean volcanic arc region. From the Puna southward, there is little obvious change in the crustal thickness across the border of the plateau (south of 28°S). The crust is seen to progressively thin towards the east in the Pampean Ranges where it is 35–40 km thick. The southern Puna plateau is characterized overall by a low crustal Vp/Vs ratio (less than 1.70), implying a felsic crustal composition. An anomalously high Vp/Vs ratio of 1.87 is observed beneath the Cerro Galan volcanic center, in the region where a prominent crustal low-velocity zone identified below ∼10 km depth probably extends into the lower crust. The crustal thickness determined under the Cerro Galan area (59 km) is close to that of the rest of the southern Puna. The prominent high Vp/Vs ratio and low-velocity zone beneath the Galan region implies the presence of a zone of partial melt or a magma chamber is consistent with hypothesis calling for lithospheric delamination beneath the Galan caldera. A widespread crustal low-velocity layer observed beneath much of the southern Puna, correlates well with crustal low-velocity anomalies observed by teleseismic tomography. The lithosphere–asthenosphere boundary beneath the array can be clearly observed by both P and S receiver functions at depths of 70–90 km in agreement with previous studies suggesting a thin lithosphere beneath the high elevated plateau. The mantle transition zone discontinuities appear at expected depths.

A detailed receiver function image of the lithosphere beneath the Kopeh-Dagh (Northeast Iran)

The Kopeh-Dagh fold belts are among the most seismically active areas in Northeastern Iran, which build the northern part of the Alpine–Himalayan orogen in western Asia. They act as the abrupt northeastern limit to active deformation in Iran. We perform a combined P and S receiver function analysis to detect the major discontinuities within the lithosphere beneath Northeast Iran. Our results obtained from 12 short period and broadband seismological stations significantly map the lateral variations of the Moho boundary. Based on P receiver functions, we show that the Moho depth varies from ∼43 km beneath the southern Kopeh-Dagh foreland basin to ∼49 km below the northern part of the basin. S receiver functions reliably reveal an average Moho depth of ∼50–55 km beneath the Kopeh-Dagh mountain range showing the regional shortening in response to the collision of Arabia with Eurasia. Furthermore, we observe clear conversions with negative polarity at ∼8.5–9.5 s in S receiver functions, which could be related to the conversion at the lithosphere–asthenosphere boundary. This may show a relatively thin continental lithosphere of about 85–95 km beneath the Kopeh-Dagh implying that the lithosphere was influenced by geodynamical reworking processes in the past.

3-D crust and mantle structure in southern Ontario, Canada via receiver function imaging

A teleseismic data set from the POLARIS project is used to obtain 3-D images of southern Ontario using two imaging techniques: scattering tomography and common-conversion-point stacking. The resulting images reveal a layered crust, the layering being interrupted by discontinuities associated with major crustal-scale faulting. Breaks in crustal continuity and Moho deflections associated with the Ottawa-Bonnechère Graben indicate that the graben is associated with faulting on a whole crust scale. We also detect similar discontinuities across the Mississauga Domain, supporting the previous interpretation that the domain is bounded by crustal-scale faults. We locate discontinuous sub-lithospheric negative-polarity arrivals which indicate complex three-dimensional structures within the lithosphere and may be associated with subduction remnants or a mid-lithosphere discontinuity.

A rootless rockies—Support and lithospheric structure of the Colorado Rocky Mountains inferred from CREST and TA seismic data

Support for the Colorado high topography is resolved using seismic data from the Colorado Rocky Mountain (CRM) Experiment and Seismic Transects. The average crustal thickness, derived from P wave receiver function imaging, is 48 km. However, a negative correlation between Moho depth and elevation is observed, which negates Airy‐Heiskanen isostasy. Shallow Moho (<45 km depth) is found beneath some of the highest elevations, and therefore, the CRM are rootless. Deep Moho (45–51 km) regions indicate structure inherited from the Proterozoic assembly of the continent. Shear wave velocities from surface wave tomography are mapped to density employing empirical velocity‐to‐density relations in the crust and mantle temperature modeling. Predicted elastic plate flexure and gravity fields derived from the density model agree with observed long‐wavelength topography and Bouguer gravity. Therefore, low‐density crust and mantle are sufficient to support much of the CRM topography. Centers of Oligocene volcanism, e.g., the San Juan Mountains, display reduced crustal thicknesses and lowest average crustal velocities, suggesting that magmatic modification strongly influenced modern lithospheric structure and topography. Mantle velocities span 4.02–4.64 km/s at 73–123 km depth, and peak lateral temperature variations of 600°C are inferred. S wave receiver function imaging suggests that the lithosphere‐asthenosphere boundary is 100–150 km deep beneath the Colorado Plateau, and 150–200 km deep beneath the High Plains. A region of negative arrivals beneath the CRM above 100 km depth correlates with low mantle velocities and is interpreted as thermally modified/metasomatized lithosphere resulting from Cenozoic volcanism.

Seismic imaging of subducting continental lower crust beneath the Pamir

Exhumation of ultra-high pressure metamorphic rocks testifies that the continental crust can subduct to significant depth into the mantle despite its buoyancy. However, direct observation of ongoing subduction of continental crust is rare. The Pamir is regarded as a possible place of active continental subduction because of the intermediate-depth seismicity, crustal xenoliths and estimates of crustal shortening versus convergence rates. Here we present for the first time receiver function images from a passive-source seismic array traversing the Tien Shan and the Pamir plateau showing southward subduction of Eurasian continental crust. In the eastern Pamir, we observe a southerly dipping 10–15km thick low-velocity zone (LVZ) that extends from 50km depth near the base of the crust to more than 150km depth with a dip angle increasing to subvertical. While the upper- and mid-crustal material seems to be shortened and incorporated into the Pamir, the lower Eurasian crust detaches and subducts. In its deeper part (>80km) the LVZ envelopes the intermediate-depth earthquakes. Our observations imply that the complete arcuate intermediate depth seismic zone beneath the Pamir traces a slab of subducting Eurasian continental lower crust.

Normal faulting from simple shear rifting in South Tibet, using evidence from passive seismic profiling across the Yadong-Gulu Rift

Abstract The Tibetan Plateau is undergoing north–south shortening accompanied by west–east extension, as evidenced by the widespread development of north–south trending normal faults, grabens and rifts. While the mode of the north–south shortening has been the main focus of most international studies, knowledge of the deep structure beneath South Tibet is required for understanding the mechanism of the west–east extension. The onset of the north–south trending normal faulting is commonly taken as an indicator that the Tibetan Plateau was uplifted to a near-maximum elevation before entering a collapsing stage. Here we report on the receiver functions of a seismological experiment across the northern segment of the Yadong-Gulu Rift (YGR), one of the youngest rifts in South Tibet. The migrated receiver function images reveal that the YGR is a high-angle normal fault characterized by a 5-km Moho rise from its western to eastern flank, together with distinct differences in the crustal structure and intracrustal seismic conversion patterns between the two flanks. This highly asymmetric lithospheric structure suggests whole-crustal extension controlled by a simple/general shear rifting mechanism. This simple/general shear rifting in the YGR is attributed to an eastward (horizontal) shear at the base of the upper crust, as evidenced by the observed Tibetan GPS velocity field and our observation of shear wave splitting discrepancy among the upper crust, lower crust and lithospheric mantle. We propose that in the YGR, simple shear rifting accommodates the northward injection of the Indian lithosphere, which may suggest that the onset of the north–south normal faulting does not indicate gravitational collapse of the Tibetan lithosphere.

Delamination in the Betic Range: Deep structure, seismicity, and GPS motion

Subduction comes to an end when all oceanic lithosphere is consumed and continental or transitional crust, with positive buoyancy, becomes involved. During continental collision, the final stage of subduction may be characterized by slab breakoff, limited continental subduction, and delamination below the fold-and-thrust belt. We report receiver function images, constructed from a dense array of seismic broadband stations, that reveal the underthrusting of Iberia beneath the Alboran plate. The underthrusted crust is delaminating beneath part of the Betic mountain range, still connected to the foreland lithosphere on one side, and interrupted discontinuously along the other side. Intermediate deep earthquakes associated with the delamination process do not respond to downdip stresses like subduction earthquakes. The GPS velocity field and source mechanisms for shallow earthquakes demonstrate ongoing active shortening on top of the hinge of delamination, and localized perpendicular extension on top of the edge of delamination, introducing independent motion of the overriding crustal block. Delamination beneath the Betic Range drives seismicity by inducing fundamental changes in deformation patterns within the collision orogen.

Evolutionary aspects of lithosphere discontinuity structure in the western U.S.

We have produced common conversion point (CCP) stacked Ps and Sp receiver function image volumes of the Moho and lithosphere‐asthenosphere boundary (LAB) beneath the western United States using Transportable Array data. The large image volumes and the diversity of tectonic environments they encompass allow us to investigate evolution of these structural discontinuities. The Moho is a nearly continuous topographic surface, whereas the LAB is not and the seismic images show a more complex expression. The first order change in LAB depth in the western U.S. occurs along the Cordilleran hingeline, the former Laurasian passive margin along the southwestern Precambrian North American terranes. The LAB is about 50% deeper to the east of the hingeline than to the west, with most of the increase in LAB thickness being in the mantle lithosphere. We infer that the Moho and the LAB are Late Mesozoic or Cenozoic everywhere west of the hingeline, modified during Farallon subduction and its aftermath. Between the hingeline and the Rocky Mountain Front, the LAB, and to a lesser extent the Moho, have been partially reset during the Cenozoic by processes that continue today. Seismicity and recent volcanism in the interior of the western U.S. are concentrated along gradients in crustal and/or lithospheric thickness, for example the hingeline, and the eastern edge of the coastal volcanic‐magmatic terranes. To us this suggests that lateral gradients in integrated lithospheric strength focus deformation. Similarly, areas conjectured to be the sites of convective downwellings and associated volcanism are located along gradients in regional lithosphere thickness.

The lithosphere-asthenosphere boundary observed with USArray receiver functions

Abstract. The dense deployment of seismic stations so far in the western half of the United States within the USArray project provides the opportunity to study in greater detail the structure of the lithosphere-asthenosphere system. We use the S receiver function technique for this purpose, which has higher resolution than surface wave tomography, is sensitive to seismic discontinuities, and is free from multiples, unlike P receiver functions. Only two major discontinuities are observed in the entire area down to about 300 km depth. These are the crust-mantle boundary (Moho) and a negative boundary, which we correlate with the lithosphere-asthenosphere boundary (LAB), since a low velocity zone is the classical definition of the seismic observation of the asthenosphere by Gutenberg (1926). Our S receiver function LAB is at a depth of 70–80 km in large parts of westernmost North America. East of the Rocky Mountains, its depth is generally between 90 and 110 km. Regions with LAB depths down to about 140 km occur in a stretch from northern Texas, over the Colorado Plateau to the Columbia basalts. These observations agree well with tomography results in the westernmost USA and on the east coast. However, in the central cratonic part of the USA, the tomography LAB is near 200 km depth. At this depth no discontinuity is seen in the S receiver functions. The negative signal near 100 km depth in the central part of the USA is interpreted by Yuan and Romanowicz (2010) and Lekic and Romanowicz (2011) as a recently discovered mid-lithospheric discontinuity (MLD). A solution for the discrepancy between receiver function imaging and surface wave tomography is not yet obvious and requires more high resolution studies at other cratons before a general solution may be found. Our results agree well with petrophysical models of increased water content in the asthenosphere, which predict a sharp and shallow LAB also in continents (Mierdel et al., 2007).

USArray Receiver Function Images of the Lithosphere-Asthenosphere Boundary

As early as the 1920s, Gutenberg (1926) equated the low-viscosity asthenosphere with a seismic low-velocity zone. Regions of high and low velocity in the mantle are today usually determined with tomographic methods. These methods are, however, not very sensitive to sharp boundaries. Converted waves have been used for many years to study boundaries in the mantle. A velocity reduction with depth (possibly the lithosphere-asthenosphere boundary, LAB) was found with P -to- S converted waves ( P -receiver functions) beneath North America at ∼100 km depth with a sharpness of about 10 km (Rychert and Shearer 2009). A comparison of the LAB depth from S -receiver functions (see, e.g. , Yuan et al. 2006 and Kumar et al. 2006 for a description of the method and data examples) with that from surface-wave tomography found good agreement in the western United States (Li et al. 2007) and along the East Coast (Rychert et al. 2007). Our results from the California coast are also consistent with the P -to- S and S -to- P images earlier analyzed by Kumar and Kawakatsu (2011), along a profile spanning ocean to continent. A discrepancy exists in the central part of the craton, where the sharp velocity decrease at about 100 km in P -receiver functions (PRF) was confirmed with S -receiver functions (SRF) in contrast to the smooth velocity decrease around 200 km depth known from surface-wave data (Yuan and Romanowicz 2010; Abt et al. 2010; Yuan et al. 2011). The petrophysical nature of the LAB is still very much a subject of discussion ( e.g. , Eaton et al. 2009; Fischer et al. 2010). We think our results from USArray data significantly contribute to this discussion. The earlier results were obtained with relatively few, sparsely distributed seismic stations (Abt et al. 2010). We apply …

Modelling converted seismic waveforms in isotropic and anisotropic 1-D gradients: discontinuous versus continuous gradient representations

Over the past decade, there have been numerous receiver function studies directed at imaging the lithosphere-asthenosphere boundary (LAB). Although it is generally accepted that receiver function phases observed in these studies are derived from physical mode conversions at depth within the lithosphere-asthenosphere transition, it is still debatable as to whether these phases are directly indicative of the LAB. This is because interpretation of receiver function LAB signals relies on understanding the elastic characteristics of the Earth’s outer thermal boundary layer. The main issues for receiver function imaging are the sharpness of the elastic material property transition and, more importantly, what specifically are the material gradients. To test the various transition models, a forward modelling approach is required that allows accurate waveform synthetics for a range of discontinuous and continuous gradients in anisotropic, elastic media. We present a derivation of the reflection and transmission response for continuous one-dimensional (1-D) gradients in generally anisotropic elastic media. We evaluate the influence of 1-D isotropic and anisotropic elastic gradients on the seismic waveform by comparing numerical results of models for discontinuous and continuous transitions. The results indicate that discontinuous representations using layers each with uniform parameters and with thicknesses on the order of approximately 1/3 to 1/8 of the dominant seismic wavelength can be used to accurately model P-to-S and S-to-P mode conversions due to continuous transitions of both isotropic and anisotropic elastic properties. From a practical point of view, when comparing synthetic modelling with observation, this constraint can be relaxed further. The presence of signal noise and/or the result of receiver function stacking techniques will likely obscure these subtle waveform e ff ects. Hence this study suggests that accurate synthetic waveforms for LAB transitions can be modelled with discontinuous gradient representations using a reasonable number of discrete transition layers with layer thicknesses no greater than 1/2 to 1/3 the dominant seismic wavelength.

Intralithospheric mantle structures recorded continental subduction

We investigated the intralithospheric mantle (ILM) structures beneath the North China Craton (NCC) by the seismic imaging with over 16,000 high‐quality receiver functions from 255 broadband stations. These images were created for six profiles spanning different tectonic units in the NCC. Initially, we inferred the crustal velocity model using an integrated receiver function imaging technique. Subsequently, we performed a series of synthetic tests of common conversion point (CCP) stacking images to distinguish between the multiple waves generated by the crustal structure and the velocity discontinuities in the ILM. The ILM structures were constructed by waveform inversion on the basis of these analyses. Our imaging results reveal (1) a homogenous lithospheric mantle in the western NCC, (2) intermittent and juxtaposed velocity interfaces in the ILM beneath the southern NCC, and (3) local high‐velocity and lower‐velocity volumes in the ILM beneath the eastern NCC. In addition, the lower crust beneath the southern NCC is thin, and the Moho dips northward, which present evidence of the Yangtze craton subduction beneath the NCC. The imaged high‐velocity volumes in the ILM beneath the southern NCC were interpreted as a subduction remnant in the uppermost mantle, which reveal a flat subduction channel resulting from the continent‐continent collision between the NCC and the Yangtze craton. The homogeneous ILM beneath a Paleoproterozoic amalgamation zone in the NCC implies that the remnants of the lithosphere that were subducted before 1.8 Ga might have sunk into the deeper mantle.

Evidence for two upper mantle sources driving volcanism in Central Kamchatka

Volcanoes of the Central Kamchatka Depression (CKD) form the most active arc volcano system in the world. Volcanoes of the CKD are positioned ~170km above the subducting Pacific Plate, in excess of the typical global value of ~100km for arc volcanism. We present results of a combined geophysical and petrological study of the main volcanic center in the CKD, and argue for the presence of a second contributing melt source within the mantle wedge. This region of melt generation is separate from the fluid fluxed region above the subducting Pacific Plate; it may explain the presence of the active CKD arc in its current location.Results of receiver function imaging of the upper mantle beneath CKD reveal a distinct area of low velocities at approximately 110km depth that is clearly distinct from the crust of the subducting Pacific Plate. Results of petrological modeling suggest presence of pyroxenite source melt contribution to CKD lavas, alongside previously described peridotite source melts. We contend that our results advance the notion that melting at two separate sources, rather than the simple flux-induced melting within the mantle wedge, drives volcanoes of the CKD.