Flat Moho Research Articles

Seismic structure of the Eastern European crust and upper mantle from probabilistic ambient noise tomography

The Eastern European lithosphere is a natural laboratory to study continental formation and evolution through time, comprising Archean continental remnants, Proterozoic rifts and belts, and younger accreted terranes. We investigate the seismic structure of the East European Craton (EEC) crust and uppermost mantle, and the transition from Precambrian to Phanerozoic Europe across the Trans European Suture Zone (TESZ) using probabilistic transdimensional ambient noise tomography. We cross-correlate noise recorded at broadband seismic stations from Eastern, Northern, and Central Europe, remove earthquake signals using continuous wavelet transform, and extract Rayleigh wave phase velocity dispersion curves. We invert these for the highest resolution shear wave velocity model of the Eastern European lithosphere to date, using Markov chain Monte Carlo Bayesian inversion. Our shear wave velocity model exhibits spatial correlation with major tectonic units and bears similarities with active seismic survey profiles in terms of seismic velocity patterns and main discontinuities. The crust thickens across the TESZ boundary and the mantle is seismically faster than beneath younger terranes, consistent with a less dense Precambrian lithosphere in the EEC. The crust and lithosphere beneath the Pannonian region is hyper-extended but the adjacent Transylvanian basin crust shows significant heterogeneity. The Precambrian building blocks of the EEC exhibit contrasting seismic fabrics. The Baltic orogens of Fennoscandia are underlain by uniform crust with a flat Moho, while Sarmatia shows alternating high and low velocity layers and a regional south-dipping crustal boundary from beneath the Ukrainian Shield towards the Crimean Peninsula. The last observation supports a geodynamic style driven by horizontal rather than vertical tectonics, with fundamental implications for the formation and evolution of early continents.

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

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

Crustal structure and magmatism of the southern Kyushu-Palau ridge

The Kyushu–Palau Ridge (KPR) represents a natural laboratory for understanding tectonic evolution and magmatism from initial subduction to initial back-arc spreading, thus demonstrating along−/across-arc variations in structures. However, how magma emplaces and causes diversities in arc structures remains unclear, and the southern KPR structure has not been well studied. Herein we studied the crustal magmatism at the southern KPR based on the detailed velocity structure imaged by two two-dimensional wide-angle seismic profiles. Results show that the KPR's crust to the west is 6.0–7.6-km-thick with one summit at the basement and a flat Moho boundary at a 12-km depth; to the east, the crust has a thickness of 7.0–11.1 km, and three summits are observed at the basement sharing one depressed Moho boundary (12–15 km depth). These structural diversities indicate the presence of two independent seamounts (west and east), with different periods of magmatism. By calculating the duration of volcanism, we hypothesized that the formation of the western seamount of the KPR requireed a magmatism duration of ∼4 Myr and one stable volcanic source. In contrast, the eastern seamount needed two volcanic sources and a total magma loading time of ∼10 Myr with ∼19-Myr period gap in magmatism. By analyzing the KPR evolution, we conclude that 1) multiple volcanic sources and episodic magmatism caused the diversities in arc structures at the KPR; 2) the initiation of back-arc spreading at ∼26 Ma might have inhibited the magmatism related to the subduction; however, the back-arc-related partial remelting of the lower island arc would have led to a new magma source and reconstructed the preexisting island arc, thus leaving multiple conduits inside it.

Intraplate lithospheric extension revealed by seismic reflection profiling of South China

How lithospheric extension evolves in an intraplate setting remains uncertain due to the lack of reliable constraints on the lithospheric architecture. Here we present seismic reflection data across the Cretaceous extensional system of South China. Our results show that extension in magma-poor conditions was accommodated by localized normal faulting in the upper crust and distributed ductile stretching in the lower crust, followed by localized crustal necking and Moho uplift associated with mantle shear-zone formation. These vertically spaced crustal and mantle features appear to be kinematically linked as follows. First, lower-crustal stretching was accompanied by normal faulting and localized exhumation of extensional domes along low-angle detachments in the upper crust. Second, sub-horizontal lower-crustal stretching tends to compensate for upper-crustal heterogeneous thinning and distribute crustal strain evenly, enabling the crust to maintain an overall smooth and flat Moho. Third, mantle shear zones likely affected lithospheric extension by controlling localized Moho uplift and crustal necking. Our compilation of seismic observations suggests that the extensional modes vary laterally from magma-poor to magma-rich conditions, reflected in increased crustal melting, decreased crust-mantle decoupling, and the replacement of a two-layer (high-strength vs. low-strength) lithospheric mantle by a single-layer, low-strength lithospheric mantle. These findings reveal a first-order configuration of depth-dependent extension over ∼800 km, with vertical and lateral variations as a function of lithospheric strength, rheology, and temperature. This extension mechanism provides a basis for assessing modes of lithospheric extension in other tectonic settings.

The boundary between the North China Craton and the Central Asian Orogenic Belt in NE China: Seismic evidence from receiver function imaging

• 38 broadband seismic stations (profile NCISP-10, 400-km-long) were deployed across the North China Craton (NCC) and the Central Asian Orogenic Belt (CAOB) in Northeast China. • Receiver function CCP images reveal a southward thrust nappe structure at the boundary of the NCC and the CAOB. • Yilan-Yitong fault (YYF) cuts through the Moho and has a vertical offset of 2–3 km indicating that it represents an ultra-crust normal fault. This study imaged the crustal structure and tectonic state of the interacting North China Craton (NCC) and Central Asian Orogenic Belt (CAOB) using receiver function imaging. The seismic profile derived from the NCISP-10 broadband seismic array spanning a northeasterly area of the NCC and a southeasterly area of the CAOB. The profile shows a flat Moho with localized anomalies but generally resting at an average depth of 32 km. The average Vp/Vs ratio was about 1.75 but the crust around the Songliao Basin exhibits a relatively high Vp/Vs ratio and a shallow Moho. An obvious velocity discontinuity appears within the crust beneath the Songliao Basin as evidenced by theoretical model tests which managed effects from multiple waves generated by sedimentary layers in the basin. This discontinuity inclines gently and is interpreted to represent the deep extension of the Chifeng-Kaiyuan fault (CKF), which itself serves as the surface boundary between the NCC and CAOB. The NCC feature appears to the south of this boundary, while a nappe structure formed during subduction-collision and composed primarily of a subduction-accretionary complex lies to the north. The Tanlu Fault consists of two branches in the study area, the Yilan-Yitong fault (YYF) and the Dunhua-Mishan fault (DMF). The YYF cuts through the Moho and has a vertical offset of 2–3 km indicating that it represents a crustal-scale normal fault. The DMF gives no obvious indication of vertical offset in the images but may be mainly characterized by sinistral strike slip movement.

Deep Learning‐BasedH‐κMethod (HkNet) for Estimating Crustal Thickness andVp/VsRatio From Receiver Functions

AbstractThe teleseismic receiver function (RF) is commonly used to determine major interfaces of the Earth. If the crustalVpis known approximately, the Moho convertedPsphase and crustal multiple reverberations can be used to determine the thickness (H) and averageVp/Vsratio (κ) of the crust. A widely used method for this isH‐κstacking (Zhu & Kanamori, 2000,https://doi.org/10.1029/1999JB900322), which uses grid search and superposition to find the maximum coherent energy of the MohoPsand its reverberated multiples phases. However, this method assumes a homogeneous isotropic crust and a flat Moho. Furthermore, it is affected by the reference crustalVp. Improved methods, such as theH‐κ‐cmethod for anisotropic media and inclined interfaces (J. Li et al., 2019,https://doi.org/10.1029/2018JB016356), help alleviate the problem. In this paper, we propose a new method that uses deep learning to estimateHandκ. Our method is divided into two steps. The first step employs a denoise architecture (the DenoiseNet) to reduce the noise level of the RFs and restore missing back‐azimuthal information. In the second step, our new deep learning network (the HkNet) is used to estimateHandκ. Deep learning has the inherent ability to automatically extract complex features from RFs, which allows us to estimate the parameters in complex media with different crustalVp. Synthetic data tests show that the proposed method achieves better accuracy than theH‐κandH‐κ‐cmethods. Applications to real data show that the proposed method is robust and reliable in a wide range of geological settings.

New Constraints for the On‐Shore Makran Subduction Zone Crustal Structure

AbstractThe Makran Subduction Zone is the primary seismic/tsunami hazard of the northwestern Indian Ocean, but little is known of its on‐shore seismic structure. We derived a shear wave velocity model extending to 100 km depth beneath a ∼400 km‐long seismic profile oriented parallel to the convergence vector of the Arabian Sea Plate. Receiver function/surface wave analysis shows that the average structure in the coastal region comprises a ∼22–28 km‐thick low wavespeed sedimentary cover and a 6–8 km‐thick gradient zone overlying 100 km‐thick high wavespeed upper mantle. The ocean‐basement interface dips gently northward, remaining a positive impedance contrast to ∼50 km depth at ∼250 km north of the coast where it disappears as the basaltic/gabbroic oceanic crust has probably transformed to eclogite. Further north, a weak arrival at ∼5 s in the receiver functions appears, grading northward into the Moho arrival of the continental Iranian Plateau. This disruption in the seismic signature of the Moho occurs in the forearc region where the dip of the subducting oceanic plate steepens. The southern Iranian Plateau's continental crust has an average Vs of 3.55 ± 0.05 km s−1, an almost flat Moho 40–45 km deep, and a sub‐Moho mantle Vs of 3.75 ± 0.05 km s−1 in the 50–80 km depth range. Weak Moho conversions probably result from ∼20% serpentinization of peridotite in the mantle wedge. Receiver functions indicate a flat continental Moho – no crustal root beneath the high topography region of the volcanic belt, which therefore must be compensated by low upper mantle densities. The high Vp/Vs ratio observed for the mantle wedge suggests ∼1%–2% partial melt.

Extracting Reliable P‐Wave Reflections From Teleseismic P Wave Coda Autocorrelation

AbstractRecently, many studies have demonstrated the use of teleseismic P wave coda autocorrelation for imaging lithosphere structures. However, the reliability of the extracted reflections remains uncertain and a means of evaluation is lacking. In this study, we propose a velocity analysis method that conveniently solves this problem in place of a synthetic experiment. This method considers the average velocity used for the moveout correction as an unknown quantity and then uses a continuously varying average velocity for the moveout correction. Finally, this method obtains a stacked result that varies with the average velocity and the vertical two‐way travel time to produce a diagram. This method is similar to the velocity analysis method used in exploration seismology. In this diagram, reliable reflections correspond to focused energy clusters, while noise lacks this feature. Therefore, this method helps determine which reflections are reliable, while also finding the appropriate parameters for data processing. Synthetic data tests demonstrate the validity of this method as well as a test of field data for station BOSA in the Kaapvaal craton, South Africa, which illustrates the successful application of the method in the case of a sharp and flat Moho discontinuity. Finally, we applied the method to the NCISP‐6 dense array and observed obvious energy clusters, representing reflections from the Moho discontinuity in the results for most stations. The depth and shape of the Moho discontinuity determined by this test are consistent with receiver function results, which verify the robustness of this method in relatively complex applications.

Crustal structure and Curie point depth in central Mexico inferred from the spectral analysis and forward modeling of potential field data

Different interpretations of crustal thickness variations have been proposed for central Mexico. The studied region encompass the transition between the Mesa Central, an elevated plateau with low relief, and the Sierra Madre Oriental, a late Mesozoic–early Cenozoic fold-thrust belt at lower mean elevation. Crustal models discussed here are based on three different published hypotheses: 1. Homogenous crustal thickness and a flat Moho beneath both provinces; 2. Thicker crust beneath the Mesa Central compared to thinner crust under the Sierra Madre Oriental; and 3. Thinner crust below the plateau and thicker crust beneath the fold-thrust belt. This last configuration implies the presence of lower-density mantle under the plateau to produce the higher mean elevation. We tested these conflicting configurations using gravity and magnetic data to produce forward models of the previously published interpretations and to obtain new crustal thickness and Curie point depth estimates. Our results show significantly higher error for model 1 and similar errors for models 2 and 3, ruling out model 1 but preventing discrimination between models 2 and 3 through forward modeling. However, the geothermal state results along with our depth-to-Moho estimates indicate the presence of a wide region beneath the eastern portion of the Mesa Central with thicker (40–52 km) and colder (−35 to −45 mWm−2) crust as compared to thinner crust (34 to <40 km) with elevated heat flow values (−50 to −85 mWm−2) beneath the portion of the Sierra Madre Oriental in the study area. Broad negative isostatic anomalies in the Mesa Central are evidence for excess mass/density under the plateau. Our findings are more consistent with model 2 and incompatible with model 3, in which the presence of shallow asthenosphere and partial melt under the plateau implies lower density rocks beneath the plateau and a corresponding steeper geothermal gradient.

Variations in crustal and lithospheric structure across the Eastern Indian Shield from passive seismic source imaging: Implications to changes in the tectonic regimes and crustal accretion through the Precambrian

Changes in the dominant tectonic regimes and the modes of formation and maturation of the continental crust during the Precambrian are a subject of active debate. Here, we discuss these issues in terms of new information on the crustal and lithospheric architecture of the different terranes in the Eastern part of the Indian Shield (EIS). We present well-constrained estimates of the crustal and lithospheric thickness characterizing different terranes of the EIS through the joint inversion of P-receiver functions (PRF) and fundamental mode group velocity dispersion data of Rayleigh waves (10–100 s). The lithospheric thicknesses are also computed independently using the S-receiver function (SRF) modeling. The study involved 2167 PRFs and 100 SRFs from 15 broadband stations. Our modeling detects a marked crustal (~4–8 km) and lithospheric (~15–20 km) thinning beneath the Singhbhum-Odisha-Craton (SOC), spatially correlating with the central part of the craton comprising the gneissic terrain contrasted by a crustal thickening (~2–6 km) beneath the horseshoe-shaped Iron Ore Group. We propose that such a crustal configuration beneath the Paleoarchean craton may suggest the dominance of vertical tectonic processes such as thickening of an oceanic mafic plateau and recurrent melting producing the TTGs and the Singbhum Granite suite. Infra-crustal reworking involving Rayleigh-Taylor instabilities and gravitational processes could explain the crustal architecture of the SOC much similar to a few other Paleoarchean cratons such as Pilbara (Australia), Barberton and Kappvaal (South Africa). Our modeling has also revealed a relatively smaller degree of crustal (2–4 km) and lithospheric thinning (4–10 km) beneath the Eastern Ghat Mobile Belt, south of the SOC. We observe a relatively thicker crust (~42 km) characterized by a flat Moho over a large area (~40,000 sq. km) encompassing the Chota Nagpur Granite Gneiss Terrane (CGGT). Such thickening is consistent with the convergence and collision tectonics involving the SOC and CGGT. However, we note a 15–20 km lithospheric thinning associated with the CGGT. Such thinning may mainly relate to Phanerozoic rifting along the Damodar graben and Rajmahal magmatism. Broadly, our model of the crustal and lithospheric architecture of the EIS reflects the imprints of northward subduction of SOC beneath the CGGT.

PRISM3D: a 3-D reference seismic model for Iberia and adjacent areas

SUMMARY We present PRISM3D, a 3-D reference seismic model of P- and S-wave velocities for Iberia and adjacent areas. PRISM3D results from the combination of the most up-to-date earth models available for the region. It extends horizontally from 15°W to 5°E in longitude, 34°N to 46°N in latitude and vertically from 3.5 km above to 200 km below sea level, and is modelled on a regular grid with 10 and 0.5 km of grid node spacing in the horizontal and vertical directions, respectively. It was designed using models inferred from local and teleseismic body-wave tomography, earthquake and ambient noise surface wave tomography, receiver function analysis and active source experiments. It includes two interfaces, namely the topography/bathymetry and the Mohorovičić (Moho) discontinuity. The Moho was modelled from previously published receiver function analysis and deep seismic sounding results. To that end we used a probabilistic surface reconstruction algorithm that allowed to extract the mean of the Moho depth surface along with its associated standard deviation, which provides a depth uncertainty estimate. The Moho depth model is in good agreement with previously published models, although it presents slightly sharper gardients in orogenic areas such as the Pyrenees or the Betic-Rif system. Crustal and mantle P- and S-wave wave speed grids were built separately on each side of the Moho depth surface by weighted average of existing models, thus allowing to realistically render the speed gradients across that interface. The associated weighted standard deviation was also calculated, which provides an uncertainty estimation on the average wave speed values at any point of the grid. At shallow depths (&amp;lt;10 km), low P and S wave speeds and high VP/VS are observed in offshore basins, while the Iberian Massif, which covers a large part of western Iberia, appears characterized by a rather flat Moho, higher than average VP and VS and low VP/VS. Conversely, the Betic-Rif system seems to be associated with low VP and VS, combined with high VP/VS in comparison to the rest of the study area. The most prominent feature of the mantle is the well known high wave speed anomaly related to the Alboran slab imaged in various mantle tomography studies. The consistency of PRISM3D with previous work is verified by comparing it with two recent studies, with which it shows a good general agreement.The impact of the new 3-D model is illustrated through a simple synthetic experiment, which shows that the lateral variations of the wave speed can produce traveltime differences ranging from –1.5 and 1.5 s for P waves and from –2.5 and 2.5 s for S waves at local to regional distances. Such values are far larger than phase picking uncertainties and would likely affect earthquake hypocentral parameter estimations. The new 3-D model thus provides a basis for regional studies including earthquake source studies, Earth structure investigations and geodynamic modelling of Iberia and its surroundings.

Seismic Evidence for Proterozoic Collisional Episodes along Two Geosutures within the Southern Granulite Province of India

Abstract The Southern Granulite Province of India had witnessed episodes of multiple tectonic activities, leading to sparsely preserved surface geological features. The present study is focused on unraveling the geodynamic evolution of this terrain through measurement of Moho depth and Vp/Vs ratio using data from a large number of broadband seismic stations. These results unambiguously establish three domains distinct in Moho depth and crustal composition. An intermediate to felsic crust with a 7–10 km step-in-Moho is delineated across the Moyar–Bhavani region. Anomalously high felsic crust with abrupt jump in Moho (~8–10 km) together with a dipping feature at deeper level characterizes the transition from eastern to southern segments of the Jhavadi–Kambam–Trichur region. By contrast, the central zone hosting the Palghat–Cauvery shear zone records uniform felsic crust and flat Moho. Drawing analogy from similar results in different parts of the globe, juxtaposition of petrologically dissimilar crustal blocks characterized by varied depths to the Moho is argued to point towards unambiguous presence of two distinct geosutures in the study area: one along the Moyar–Bhavani region and the other across the Jhavadi–Kambam–Trichur. This inference is corroborated by the presence of layered meta-anorthosite, related rock suites, and mafic-ultramafic bodies, supporting the view of a suprasubduction setting in the Moyar–Bhavani region. The Jhavadi–Kambam–Trichur area is marked by operation of the Wilson cycle by way of sparsely preserved geological features such as the presence of ophirags (ophiolite fragments), alkali syenites, and carbonatites. Geochronological results suggest that the suturing along Moyar–Bhavani took place during the Paleoproterozoic and that along Jhavadi–Kambam–Trichur was during the late Neoproterozoic.

Lateral growth of NE Tibetan Plateau restricted by the Asian lithosphere: Results from a dense seismic profile

We present high-resolution receiver function images along a 700-km long dense seismic array extending from northern Tibetan Plateau to the Alxa block, crossing the entire Qilian thrust belt (QTB). The dense stations, with less than ~2 km station intervals, allow the receiver functions to unveil unprecedented details of crustal structures across the northern frontier of the growing Tibetan Plateau. The migration image shows a thickened and strongly deformed QTB crust, with an uneven Moho and complex internal structures that are indicative of pure-shear shortening. The Alxa block, in contrast, has a thinner crust, a flat Moho, and little internal crustal deformation. These results suggest that the lateral growth of NE Tibetan Plateau is restricted by the strong Asian lithosphere, which shows no visible subduction beneath the Tibetan Plateau as previously suggested.

Cenozoic crustal-scale duplexing and flat Moho in southern Tibet: Evidence from reflection seismology

Although the Lhasa terrane of southern Tibet is an integral part of the Tibetan plateau, how the area has achieved its current high altitude of ~5000 m remains poorly understood. This is largely due to the fact that little is known about deep (>15 km) crustal structures and the depth and geometry of the Moho below the terrane. In this study, we address this question by acquiring and analyzing high-resolution seismic reflection data gathered from four high-energy dynamite shots along a 137-km north-south traverse spanning the southern Lhasa terrane. Our new seismic data reveal the presence of north-dipping reflectors in the crust, a seismically weak-reflection layer at the lowermost crust, and a sub-horizontal and locally discontinuous Moho at depth of 69–72 km with a crustal velocity of 6.30 km/s. The discordant relationship between the crustal dipping reflectors, the subhorizontal lower crustal layer, and the flat Moho suggests that the ~12-km basal section of the Lhasa terrane crust is a decoupling zone accommodating different styles of lithospheric-scale deformation. Because the north-dipping reflectors can be projected and/or connected with the reflectors in the Cenozoic Himalayan thrust system and the southernmost Lhasa terrane, we propose that the dipping crustal reflectors are parts of a south-directed crustal-scale thrust duplex created by the Cenozoic India-Asia collision. The relatively discontinuous and horizontal geometry at the base of the crust indicates that the Moho was reconstituted through ductile deformation in the lowermost crust caused by abnormally high temperature.

Mantle influx compensates crustal thinning beneath the Cathaysia Block, South China: Evidence from SINOPROBE reflection profiling

The crustal architecture of the Cathaysia Block, South China, is a key element in understanding the late Mesozoic tectono-magmatic history of East Asia, and includes a spectacular (∼260,000 km2) basin and igneous province that forms a natural laboratory for unravelling the interplay of extension, magmatism and crust-mantle interaction. Here we analyze the whole crustal architecture of East Cathaysia through a ∼300-km-long SINOPROBE deep seismic reflection profile. This profile reveals that East Cathaysia is underlain by two seismically distinct terranes (i.e., the Wuyishan and Coastal terranes), separated by a low-angle, northwest-dipping set of mantle-penetrating reflectors interpreted as the Zhenghe-Dapu fault zone, which documents Neoproterozoic terrane amalgamation and Cretaceous extensional reactivation. The seismic image of the Coastal terrane shows large-scale low reflectivity truncated by arrays of high-reflective subhorizontal bands of layered reflectors, corresponding to an extension-related crustal architecture dominated by extensive Cretaceous magmatism. The layered reflectors are interpreted to represent mantle-derived mafic sills, which document significant magmatic underplating associated with asthenospheric upwelling beneath the terrane. This interpretation accounts for the localized distribution of Cretaceous magmatic rocks in the Coastal terrane. We suggest that the magmatic underplating added large volumes of mantle-derived material to the crust, compensating for the crustal thinning. Such a magma-compensating process might have formed an integral part of crustal reworking and growth in creating the magma-dominated crust and the broadly flat Moho of the Coastal terrane during the Cretaceous.

3-D shear wave velocity model of the lithosphere below the Sardinia–Corsica continental block based on Rayleigh-wave phase velocities

SUMMARY Rayleigh-wave dispersion curves from both ambient noise and teleseismic events allow us to provide the first high-resolution 3-D shear wave velocity (VS) model of the crust and upper mantle below the Sardinia–Corsica microplate, an important continental block for understanding the evolution of the central-western Mediterranean. For a wide range of periods (from 3 to ∼30 s), the phase velocities of the study area are systematically higher than those measured within the Italian peninsula, in agreement with a colder geotherm. Relative and absolute variations in the VS allow us to detect a very heterogeneous upper crust down to 8 km, as opposed to a relatively homogeneous middle and lower crust. The isosurface at 4.1 km s−1 is consistent with a rather flat Moho at a depth of 28.0 ± 1.8 km (2σ). The lithospheric mantle is relatively cold, and we constrain the thermal lithosphere–asthenosphere boundary at ∼100 km. We find our estimate consistent with a continental geotherm based on a surface heat flow of 60 mW m−2. Our results suggest that most of the lithosphere endured the complex history of deformation experienced by the study area and imply, in general, that deep tectonic processes do not easily destabilize the deeper portion of the continental lithosphere, despite leaving a clear surface signature.

Imaging with pre-stack migration based on Sp scattering kernels

SUMMARYSp receiver functions have been widely used to detect the lithosphere–asthenosphere boundary (LAB) and other mantle discontinuities. However, traditional common conversion point (CCP) stacking can be biased by the assumption of horizontal layers and this method typically underestimates scattering amplitudes from velocity boundaries with significant dips. A new pre-stack migration method based on recently developed Sp scattering kernels offers an alternative that more accurately captures the timing and amplitude of scattering. When calculating kernels, Sp-S times are estimated with the fast-marching method, and scattering amplitude versus direction, geometrical spreading and phase shifts are accounted for. To minimize imaging artefacts with larger station spacing, Sp receiver functions are interpolated to more closely spaced pseudo-stations using either compressive sampling or spatial averaging algorithms. To test the kernel-based stacking method, synthetic Sp phases were predicted using SPECFEM2D for velocity models with a flat Moho and a negative mantle velocity gradient with a ramp structure. The kernel-based stacking method resolves horizontal interfaces equally well as CCP stacking and outperforms CCP stacking when imaging boundaries with dips of more than 8°, although dip resolution is still limited. Use of more vertically incident phases such as SKSp improves retrieval of dipping discontinuity segments. A second approach is to down-weight the portions of the kernels that have the greatest positive interference among neighbouring stations, thus enhancing scattering from dipping structures where positive interference is lower. With this downweighting, the kernel-based stacking method applied to Sp data is able to continuously resolve LAB discontinuities with dips up to 15° and to partially resolve continuous LAB discontinuities with dips of ∼20°. The intrinsic properties of teleseismic Sp phase kernels limit their ability to resolve LAB structures with dips of ∼20–35°, but still larger dips of ∼40–50° are resolvable with dense and appropriately placed stations. Analysis of Sp scattering kernels also explains the effectiveness of CCP stacking for quasi-horizontal interfaces.

A Generalized H‐κ Method With Harmonic Corrections on Ps and Its Crustal Multiples in Receiver Functions

AbstractThe H‐κ method (Zhu &amp; Kanamori, 2000, https://doi.org/10.1029/1999JB900322) has been widely used to estimate the crustal thickness (H) and the ratio of P to S velocities (VP/VS ratio, κ) with receiver functions. However, in regions where the crustal structure is complicated, the method may produce biased results, arising particularly from dipping Moho and/or crustal anisotropy. H‐κ stacking in case of azimuthal or radial anisotropy with flat Moho has been proposed but not for cases with plunging anisotropy and dipping Moho. Here we propose a generalized H‐κ method called H‐κ‐c, which corrects for these effects first before stacking. We consider rather general cases, including plunging anisotropy and dipping interfaces of multiple layers, and use harmonic functions to correct for arrival time variations of Ps and its crustal multiples with back azimuth (θ). Systematic synthetic tests show that the arrival time variations can be well fitted by cosθ and cos2θ functions even for very complex crustal structures. Correcting for the back azimuthal variations significantly enhances H‐κ stacking. We verify the feasibility of the H‐κ‐c method by applying it to 40 permanent stations in various geological setting across the Mainland China. The results show clear improvement after the harmonic corrections, with clearer multiples and stronger stacking energy, as well as more reliable H‐κ values. Large differences in H (up to 5.0 km) and κ (up to 0.09) between the new and traditional methods occur mostly in mountainous regions, where the crustal structure tends to be more complex. We caution in particular about systematic bias when the traditional method is used in the presence of dipping interfaces. The modified method is simple and applicable anywhere in the world.

The Moho structure beneath the Yarlung Zangbo Suture and its implications: Evidence from large dynamite shots

The Yarlung Zangbo Suture (YZS) is the collisional front between the Indian and Eurasian plates. The deep structure beneath the YZS has long been a focus of research. Its particular tectonic location and formation time provide a natural laboratory for the study of ongoing continental collisional processes. Lateral variations in the Moho discontinuity provide evidence of basic processes governing plate tectonic evolution. The depth and geometry of the Moho provides first-order information for restoration of complex geodynamic systems. This paper presents a studying result of the Moho, both adjacent to and beneath the YZS, obtained using five large dynamite shots along two profiles. These five large shots were combined to construct two single-fold profiles (YZS-A and YZS-B) and obtain high-resolution images of the Moho beneath the YZS. Both profiles show that continental crust thickness beneath the YZS is twice the global average of continental crust, and the depth of the Moho along profile YZS-B is deeper than YZS-A. YZS-A (81°E) indicates a relatively flat Moho without offset related to continent-continent convergence beneath the collision zone. Conversely, a Moho structure with offset and overlapping reflections was imaged beneath profile YZS-B (88°E). Significant lateral variations in the geometry of the Moho discontinuity along the YZS may indicate that the Indian lithosphere is thrusting beneath Tibet at a different angle from west to east.

Precambrian Tectonic Discontinuities in Western Laurentia: Broadband Seismological Perspectives on the Snowbird and Great Falls Tectonic Zones

AbstractThe Snowbird Tectonic Zone (STZ) in southwestern Canada and the Great Falls Tectonic Zone (GFTZ) in northern Montana are two structural lineaments with major implications for the formation and evolution of the western North American craton. In this study, we examine the origins of these two proposed Proterozoic sutures using broadband seismic data from Alberta and USArray stations. We find substantial spatial variations in crustal structure and thickness across both the STZ and GFTZ based on an integrated analysis of P‐to‐S receiver functions, shear velocities, and potential field measurements. The STZ marks the transition from deep (NW, ~44 km) to shallow (SE, ~39 km) Moho in central Alberta. This steep Moho relief, in conjunction with elevated Vp/Vs ratios, provide compelling evidence for Proterozoic subduction and magmatism. In northern Montana, the Moho is depressed by ~6 km within the fold‐and‐thrust belts of the Trans‐Montana Orogen, where high (&gt;1.84) Vp/Vs ratios spread along the collisional suture of the Dillion Shear Zone and terminates sharply along the reworked margin of the Wyoming craton. These observations, coupled with widespread lower crustal conversions, are concordant with a Paleoproterozoic orogeny resulting from collision(s) between Archean cratons. On the other hand, a flat Moho under major northeast striking potential field lineaments in the northernmost Montana favors an origin as a ductile shear zone, rather than suture, in the orogenic hinterland. We find evidence for crustal underplating beneath the GFTZ, though this anomalous layer is smaller than previous estimates and appears to be confined to the Trans‐Montana Orogen.