Common Conversion Point Stacking Research Articles

Imaging the crustal structure at the northeast corner of the indenting Indian Plate in the Eastern Himalayan Syntaxis

SUMMARY Crustal configuration beneath the indenting northeast corner of the Indian Plate in the Eastern Himalayan Syntaxis has been investigated with the help of receiver function (RF) analysis of teleseismic earthquakes recorded by 19 broad-band seismological stations. The common conversion point stacking of RFs and 1-D velocity models obtained through inversion provide new information on the intracrustal structure. The study reveals the signature of the Main Himalayan Thrust (MHT) beneath the Lohit Valley at ∼22–26 km depth. The MHT is not prominent in the Siang window plausibly due to large-scale crustal deformation related to the formation of the window and antiform folding. Unlike in the western and central Himalaya, the MHT does not play a major role in seismogenesis in the Lohit Valley and Siang Window, where seismicity is active up to the crustal depth of ∼40 km. The crustal thickness increases from ∼38 km at Pasighat in the south to ∼50 km at the northernmost station (Gelling) in the Siang window. In Lohit Valley, the crustal thickness increases from ∼40 km at Mahadevpur in the west to ∼54 km in the Tidding–Tuting suture zone, which again shallows to ∼51 km in the eastern Lohit Plutonic Complex (Walong station). The thinner crust beneath the Tidding–Tuting suture compared to the Indus Tsangpo Suture Zone of northwest Himalaya is caused due to the differences in convergence rate, higher exhumation rate and mechanisms to accommodate collision and rotational tectonics.

Crustal structure and geodynamics of the eastern Qilian orogenic belt, NE margin of the Qinghai-Tibet plateau, revealed by teleseismic receiver function

The eastern segment of the Qilian orogenic belt, comprising the Linxia block and Longzhong block, is at the Qinghai–Tibet Plateau’s northeastern margin. The area has experienced multiple tectonic events, including closure of the Qilian Ocean, convergence of the North China block and Qilian terrane, and collision of the Indian and Eurasian plates, forming a complex tectonic framework. To investigate the area’s geological evolution and the suture’s current location between the blocks, we used 3-year data recorded by 33 portable ChinArray II broadband stations (2013–2016). Using three teleseismic P-wave receiver function methods, H-κ stacking and common conversion point stacking (CCP), crustal structure, Poisson’s ratio, and Moho morphology were obtained at 33 stations. The results are described as follows: 1) The Maxianshan fault is an important boundary fault that divides the Linxia block and Longzhong block. The Linxia block’s layered crustal structure is obvious, and there is a low-velocity anomaly in the middle and lower crust, which may contain saline fluid and has Japanese-type island arc characteristics. 2) The layered structure of the Longzhong block’s upper crust is significant, while the middle and lower crust’s layered structure is weak with weak low-velocity characteristics and oceanic-island basaltic crust characteristics. The Longzhong block may have originally been formed by Mariana-type island arcs. 3) The Conrad interface and Moho lateral variation in the Ordos block’s southwestern margin are weak, showing stable craton characteristics. 4) Our results show that the Maxianshan fault cuts through the Earth’s crust and is a continuous west-dipping negative seismic phase in the Common Conversion Point section. The fault zone is the suture line between the Linxia block and Longzhong block. 5) The middle and upper crust of the Liupanshan tectonic belt is thrust upwards on the Ordos block’s southwestern margin, providing deep structural evidence of the Cenozoic uplift of the Liupanshan structural belt.

The mantle transition zone beneath eastern North America: Receiver functions and tomographic velocity models

The eastern continental margin of North America, despite being a passive margin at present, records a comprehensive tectonic history of both mountain building and rifting events. This record is punctuated by several igneous events, including those associated with the Great Meteor and Bermuda hotspots. To gain a better understanding of the state of the mantle beneath this region, we employ the massive quantity of seismic data recorded by the USArray to image the mantle transition zone beneath eastern North America. To construct these images, we first calculate P-to-s receiver functions using an iterative time-domain deconvolution algorithm. These receiver functions are then automatically filtered by their quality, using a set of rigorous criteria, and subsequently summed using common conversion point stacking. We present several cross sections through these stacks, which show remarkable features such as a thinned transition zone beneath the independently observed northern Appalachian and central Appalachian low-wavespeed anomalies, as well as a thickened transition zone beneath western Tennessee associated with the Laramide slab stagnating at depth. In addition to discussing these geologically relevant features, we perform a technical analysis of the effects of using various seismic velocity models for the moveout correction of our receiver functions. We find that the thickness of the mantle transition zone under eastern North America is a robust measurement, while the resolved depths of the 410 and 660 km discontinuities are model dependent.

Seispy: Python Module for Batch Calculation and Postprocessing of Receiver Functions

Abstract Seispy is a graphical interface Python module for receiver function (RF) calculation and postprocessing in seismological research. Automated workflows of RF calculations facilitate processing large volume of different types of seismic data. The graphical user interface enables an intuitive and straightforward evaluation of RF quality. All parameters about the preprocessing for RF estimation can be adjusted based on user preference. Water-level frequency-domain deconvolution and iterative time-domain deconvolution for RF estimation are available in Seispy. The current version of Seispy contains five main modules for the postprocessing of RF, such as H-κ stacking, crustal anisotropic estimation, harmonic decomposition, and 2D and 3D common conversion point (CCP) stacking. The CCP stacking in the different application scenarios can be handled by a rich collection of modules, such as time-to-depth conversion, 2D or 3D CCP stacking, and adaptive station or bin selection for CCP stacking profiles in a dense seismic array or a linear seismic array. As a Python module, functions in the Seispy can be called easily in Python scripts for other purposes. The modular design allows new functionality to be added in a collaborative development environment. Licensed under GPLv3, Seispy allow users and developers to freely use, change, share, and distribute copies of the package.

Imprints of subducted Palaeo-Tethys oceanic lithosphere on upper-mantle discontinuities and the formation of the Emeishan large igneous province

SUMMARYGenerally, the upwelling of a mantle plume is suggested to have led to the formation of the Emeishan large igneous province (ELIP). However, this idea has been challenged by recent geological and seismological studies. In this investigation, three profiles of velocity perturbations crossing the ELIP are drawn following previous tomographic studies, and common conversion point (CCP) stacking of receiver functions in the ELIP is carried out. A slab-like high-velocity structure is revealed, which may be associated with the lithosphere of the Palaeo-Tethys Ocean (LPTO) subducting toward the northeast. CCP stacking of receiver functions shows that the LPTO generated imprints on the 410- and 660-km discontinuities. A comprehensive analysis suggests that the LPTO may have induced large-scale mantle upwelling, which contributed to the formation of the ELIP.

Crustal and Uppermost Mantle Heterogeneities Across the Ailaoshan Red River Shear Zone, SE Tibet: Implications for Cenozoic Magmatic Activity

AbstractThe Ailaoshan Red River shear zone (ARSZ) was formed in the Mesozoic as a suture zone between the Indochina block and the Yangtze craton. Since the Cenozoic, block extrusion due to the Indo‐Asian collision has reactivated the fault zone and caused large‐scale shearing. Affected by the Cenozoic orogeny, a large volume of magmatic and metamorphic rocks developed in the ARSZ, forming many orogenic gold deposits. However, the source and the geodynamic process of these magmatic activities are still unclear. To gain a basic understanding of the subsurface magmatic activity, we deployed a dense array of 24 broadband seismic stations across the Daping and Chang'an gold deposits at the southern end of the ARSZ. Receiver function analysis, common conversion point stacking, and a joint inversion of receiver functions and surface wave dispersions are performed to image the detailed structure of the crust and uppermost mantle. Low‐velocity zones in the mid‐lower crust and thinned lithosphere (∼70 km) are imaged under the ARSZ. The observed subsurface structures are verified by 3D numerical modeling with the SEM‐FK method. We speculate that the mantle upwelling caused by lithospheric delamination has provided the main source of the mantle component in the magmatic rocks since ∼35 Ma; afterward, high temperatures produced partial melting in the lower crust, which was emplaced along active shear zones.

Seismic Evidence for Stepwise Lithospheric Delamination Beneath the Tibetan Plateau

AbstractSeveral models were proposed to account for the tectonic evolution of Tibetan Plateau, among which delamination of the Tibetan lithosphere is suggested to be a leading process. However, direct evidence for the removed Tibetan lithosphere in the deep mantle remains highly lacking. Here we image the mantle transition zone (MTZ) structure beneath western Tibet and Himalaya by common conversion point stacking of 25254 receiver functions (RFs) with unprecedented clarity. One striking feature is the significant depression (15–20 km) of 660‐km discontinuity beneath Himalaya, which indicates the presence of the detached Lhasa lithospheric mantle (∼26 Ma) at the bottom of the MTZ. Our results also show ∼15 km uplift of 410‐km discontinuity (d410) and small d410 amplitude beneath western Tibet, which we ascribe to the more recently delaminated Qiangtang lithosphere (∼15 Ma). Furthermore, the small dependence of d410 amplitude on frequency indicates that the MTZ is anhydrous in the study area.

2‐D Analytical P‐to‐S and S‐to‐P Scattered Wave Finite Frequency Kernels

AbstractScattered wave imaging provides a powerful tool for understanding Earth's structure. The development of finite frequency kernels for scattered waves has the potential for improving the resolution of both the structure and magnitude of discontinuities in S‐wave velocity. Here we present a 2‐D analytical expression for teleseismic P‐to‐S and S‐to‐P scattered wave finite‐frequency kernels for a homogeneous medium. We verify the accuracy of the kernels by comparing to a spectral element method kernel calculated using SPECFEM2D. Finally, we demonstrate the ability of the kernels to recover seismic velocity discontinuities with a variety of shapes including a flat discontinuity, a discontinuity with a sharp step, a discontinuity with a smooth bump, and an undulating discontinuity. We compare the recovery using the kernel approach to expected recovery assuming the classical common conversion point (CCP) stacking approach. We find that the P‐to‐S kernel increases recovery of all discontinuity structures in comparison to CCP stacking especially for the shallowest discontinuity in the model. The S‐to‐P kernel is less successful but can be useful for recovering the curvature of shallow discontinuity undulations. Finally, although we observe some variability in the amplitude of the kernels along the discontinuities, the kernels show some potential for recovering the magnitude of the velocity contrast across a discontinuity.

Receiver Function Imaging of the Crustal Structure Beneath Northern Taiwan Using Dense Linear Arrays

In order to realize the crustal structure in Taiwan, the receiver function method was used to analyze the teleseismic waveforms recorded by two orthogonal broadband linear arrays deployed in northern Taiwan in the east–west and south–north directions by the TAiwan Integrated GEodynamics Research (TAIGER) project from 2007 to 2009. By incorporation with Common Conversion Point (CCP) stacking, the Moho discontinuities beneath northern Taiwan were imaged. Based on the CCP stack of receiver functions in the east–west direction array, a collision boundary between the Philippine Sea Plate and the Eurasian Plate appears at the east of Taiwan. The Moho depth of the Eurasian Plate in west Taiwan is flat and 30 km; the Moho depth of the Philippine Sea Plate below the Central Mountain Range is about 55 km; in the east, the Moho depth of the Ryukyu arc is about 40 km. The south–north profile shows a brittle–ductile transition zone at depths of 15–20 km beneath central Taiwan from south to north. Moreover, the Moho depth of the Eurasian Plate is about 20–25 km in northern Taiwan. The Moho depth appears to deepen from north to south. The deepest Moho is located at the junction of the two profile lines, the Philippine Sea Plate, and has a depth of 60 km. According to these Moho depths, the crustal structure is thin and flat in the west part of northern Taiwan which is similar to the thin-skin model. However, the lithosphere is deformed and forms the mountain root in the east part which is similar to the lithospheric collision model.

Moho Complexity in Southern California Revealed by Local PmP and Teleseismic Ps Waves

AbstractThe Moho discontinuity plays an important role in crustal growth and evolution. In this study, we delineate the Moho geometry in southern California by jointly using local Moho‐reflected waves PmP and teleseismic Moho‐converted waves Ps. To well constrain the Moho geometry, we have developed a two‐stage process to pick PmP waves and have created a reliable PmP travel time data set with a total of 10,192 picks. We have also extracted 38,648 high‐quality P‐wave receiver functions (RFs). The Moho depth is initially estimated via the common conversion point (CCP) stacking of RFs and then refined by inverting the PmP travel time data in a community velocity model (CVM‐H, version 15.1.1). The newly built Moho geometry is generally consistent with the California Moho Model version 1.0 (CMM‐1.0), that is, a shallow Moho beneath the Salton Trough (23 km), a uniformly shallow Moho beneath the Mojave Desert and the Basin and Range (<29 km), and a sliver of deep Moho under the western Peninsular Ranges, the eastern Transverse Ranges, and the western Sierra Nevada (>34 km). However, our Moho model reveals some new features different from the CMM–1.0, such as a deep Moho (∼34 km) beneath the northern end of the central and western Transverse Ranges, consistent with the observation of deep seismicities due to a thick brittle crust there. We also find a gradual transition from the lower crust to the uppermost mantle beneath the western Peninsular Ranges, leading to the rareness of pickable PmP waves as well as weak Moho‐converted signals there.

Weak and vanishing upper mantle discontinuities generated by large-scale lithospheric delamination in the Longmenshan area, China

A large amount of high-quality teleseismic data is used for common conversion point (CCP) stacking of receiver functions in the Longmenshan area and near-by region. The results show that a large-scale high-velocity anomaly (LHVA) or lithospheric delamination can modify the structure of upper mantle discontinuities or weaken the phase boundary of olivine, which is a very important finding and can be used to assess stagnant slabs in the mantle transition zone globally. The deepening region of the 660 km discontinuity beneath the Songpan-Ganzi terrane identified in this study might be generated by the LHVA.

Seismic structure across central Myanmar from joint inversion of receiver functions and Rayleigh wave dispersion

The active tectonics in Myanmar is governed by the ongoing northward indentation and obliquely-eastward subduction of India into Eurasia. So far, detailed seismic structure of the crust and uppermost mantle at the eastern flank of the India-Eurasia collision zone remains highly debated. With seismic waveforms recorded at 79 broadband stations in Myanmar, we build a regional shear velocity model in the depth range of 0–80 km by joint inversion of ambient noise derived Rayleigh wave dispersion and P-wave receiver functions. Common conversion point stacking was performed along two representative profiles. We observe clear variations in the seismic velocity and discontinuity structures beneath this region. 1) A sedimentary layer covers the eastern fore-arc trough of the Central Myanmar Basin, with shear velocity less than 2.5 km/s and thickness increasing from ~8 km at 22°N to ~18 km at 23°N. The fore-arc Chindwin basin is evidently thicker than the back-arc Shwebo basin, an abrupt drop in sediment thickness towards the east appears immediately below the Wuntho-Popa magmatic arc. 2) Crustal low-velocity (LV) anomalies (<3.3 km/s) in the Indo-Burma Ranges probably reflect the Bengal sediments accreted to the toe of the overlying Burma plate, 3) The underlying LV layer with a thickness of over 20 km below the Burma Moho (30–40 km in depth) is indicative of the eastward subduction of the Indian continental crust, the top boundary of which is imaged with a dip angle of ~20°. 4) Upper mantle LV anomalies filling the majority of the back-arc domain at depths greater than 60 km display a connection to a small-scale, subcrustal LV body beneath the Monywa volcano, possibly forming a fluid- or melt-rich mantle channel.

The Structure of the Upper Mantle Transition Zone Beneath Northeast China Associated With Mantle Plume Migration

AbstractThe stagnation and dehydration of the Pacific Plate slab in the mantle transition zone (MTZ) are widely accepted to have resulted in Mesozoic and Cenozoic volcanic activities and the formation of the Songliao Basin in NE China. However, this notion has been challenged by recent seismic studies. Alternatively, a mantle plume may have generated the large‐scale volcanism and may have led to the formation of the Songliao Basin. In this study, a detailed analysis involving common conversion point stacking of receiver functions was carried out. The results reveal a significantly deepened region of the 410 km discontinuity and an elevated region of the 660 km discontinuity in the center of NE China (specifically, the Songliao Basin). The combination of these results with those of a previous study suggests that an upwelling mantle plume was located under the Songliao Basin in the Mesozoic. Furthermore, the distinctive structure of the MTZ beneath the southern part of the Songliao Basin identified in this study corresponds to mantle plume upwelling (a mushroom‐shaped low‐velocity anomaly), which may be related to the Changbaishan volcanic activity in the Cenozoic.

High‐Resolution Ps Receiver Function Imaging of the Crust and Mantle Lithosphere Beneath Southern New England and Tectonic Implications

AbstractSouthern New England exhibits diverse geologic features resulting from past tectonic events. These include Proterozoic and early Paleozoic Laurentian units in the west, several Gondwana‐derived terranes that accreted during the Paleozoic in the east, and the Mesozoic Hartford Basin in the central part of the region. The Seismic Experiment for Imaging Structure beneath Connecticut (SEISConn) project involved the deployment of a dense array of 15 broadband seismometers across northern Connecticut to investigate the architecture of lithospheric structures beneath this region and interpret how they were created and modified by past tectonic events in the context of surface geology. We carried out P‐to‐S receiver function analysis on SEISConn data, including both single‐station analysis and common conversion point (CCP) stacking. Our images show that the westernmost part of Connecticut has a much deeper Moho than central and eastern Connecticut. The lateral transition is a well‐defined, ∼15 km step‐like offset of the Moho over a ∼20 km horizontal distance. The Moho step appears near the surface boundary between the Laurentian margin and the Gondwana‐derived Moretown terrane. Possible models for its formation include Ordovician underthrusting of Laurentia and/or modification by younger tectonic events. Other prominent features include a strong positive velocity gradient (PVG) beneath the Hartford basin corresponding to the bottom of the sedimentary units, several west‐dipping PVGs in the crust and mantle lithosphere that may correspond to relict slabs or shear zones from past subduction episodes, and a negative velocity gradient (NVG) that may correspond to the base of the lithosphere.

Crustal thickness in southeast Tibet based on the SWChinaCVM-1.0 model

Extensive geophysical research has been carried out in southeast Tibet due to the complex geological structure and high seismicity. The study of the Moho interface can provide useful information for the geophysical research. SWChinaCVM-1.0 is the first version of a regional community velocity model for the Sichuan-Yunnan region and aims to provide a reference for seismological research in the region. Based on this model, a high-precision community Moho interface model can be obtained using common conversion point (CCP) stacking, to further expand the applicability of SWChinaCVM-1.0. Before CCP stacking, it is necessary to check the fit of the practical and theoretical travel times of the Ps phase and its multiples under the constraints of the model. Using this method, referred to as H-κ0 stacking, we determined the Moho interface. However, the multiples were poorly fitted with the theoretical travel times at many stations, and the crustal thickness obtained was inconsistent with previous findings. Therefore, we speculate that the SWChinaCVM-1.0 model has some limitations that render it incompatible with the CCP stacking method. To further verify this hypothesis, we performed H-κ stacking using the vP-vS model, vP model, vS model, and the regional average velocity vP0 and compared the results with those of H-κ0 stacking. The results of the comparison indicate that the SWChinaCVM-1.0 model requires improvements. In addition, by analyzing the SWChinaCVM-1.0 inversion process, we found that the vS model was more reliable than the vP model. Finally, we adopted the crustal thickness obtained using H-κ stacking based on the vS model as the initial version of a community Moho interface model for the Sichuan-Yunnan region (SWChinaCVM-MOHO-1.0).

Lithospheric structure beneath southern Iberia and northern Morocco constrained by 3D Kirchhoff-approximate GRT imaging

AbstractThe slab beneath the Alboran Sea is a consequence of the collision between two continents (Europe and Africa), which was initiated along the northeastern Spanish coast, experienced slab rollback and migrated to the area adjacent to the two continents. The tectonic background in this area includes episodes of collisions with adjacent continents as well as extension of those basins in the western Mediterranean. Here, we present three-dimensional (3D) Kirchhoff-approximate generalized Radon transform (GRT) images to further constrain the lithospheric structures previously identified by other researchers. The GRT images were calculated from the same P-to-S (Pds or Ps) teleseismic receiver functions (RFs) as the previous common conversion point (CCP) stacking, but the GRT data provide figures with greater resolution than the Pds RFs CCP results. This study indicates that the Alboran Slab may have completely detached from the crustal base under the western Betics Mountains and that a larger range of lithospheric ‘peeling off’ developed beneath the western part of the Betics Mountains than some previous results have claimed. The observed thin lithosphere under the Middle Atlas (MA) and eastern High Atlas (HA) may have a geodynamic relationship with lithospheric delamination under the eastern part of the Rif Mountains, which has also led to the thin lithosphere under the eastern Rif. According to the thick lithosphere under the western HA, the shallow LAB under the MA and eastern HA may have no heat-flow connection with the Canary mantle plume, as stated in several previous studies.

New Approaches to Multifrequency Sp Stacking Tested in the Anatolian Region

AbstractThis study presents an improved approach to common‐conversion point stacking of converted body waves that incorporates scattering kernels, accurate and efficient measurement of stack uncertainties, and an alternative method for estimating free surface seismic velocities. To better separate waveforms into the P and SV components to calculate receiver functions, we developed an alternative method to measure near‐surface compressional and shear wave velocities from particle motions. To more accurately reflect converted phase scattering kernels in the common‐conversion point stack, we defined new weighting functions to project receiver function amplitudes only to locations where sensitivities to horizontal discontinuities are high. To better quantify stack uncertainties, we derived an expression for the standard deviation of the stack amplitude that is more efficient than bootstrapping and can be used for any problem requiring the standard deviation of a weighted average. We tested these improved methods on Sp phase data from the Anatolian region, using multiple band‐pass filters to image velocity gradients of varying depth extents. Common conversion point stacks of 23,787 Sp receiver functions demonstrate that the new weighting functions produce clearer and more continuous mantle phases, compared to previous approaches. The stacks reveal a positive velocity gradient at 80–150 km depth that is consistent with the base of an asthenospheric low‐velocity layer. This feature is particularly strong in stacks of longer period data, indicating it represents a gradual velocity gradient. At shorter periods, a lithosphere‐asthenosphere boundary phase is observed at 60–90 km depth, marking the top of the low‐velocity layer.

Receiver Function Velocity Analysis Technique and Its Application to Remove Multiples

AbstractResults of P wave receiver function (RF) analysis (e.g., H‐κ stacking, common conversion point (CCP) stacking, and migration) depend on the velocity models employed. Converted phases of crustal and upper mantle discontinuities are often interfered by multiple reflections from shallower structures, adding further complexity to the interpretation of P wave RF images. In this study, we propose a receiver function velocity analysis technique (RFVAT) that can constrain the velocity structure above discontinuities and remove multiple reflections arising from target discontinuities. By scanning the amplitude spectra of the Ps and PpPs phases, we can obtain precise arrival times of these phases for interfaces above 250 km (errors &lt; 0.13 s) and derive the average velocities and thicknesses of the layers between interfaces in the crust and upper mantle. Synthetic experiments are used to demonstrate that crustal and upper mantle discontinuities can be coherently identified and that multiple reflections can be effectively removed in the CCP stacking images of P wave RFs using the RFVAT even with low signal‐to‐noise ratio data. By applying the proposed RFVAT to real P wave RF data from the northeastern part of the North China Craton, we achieve an improvement in CCP stacking image of lithospheric discontinuities, especially for the lithosphere‐asthenosphere boundary (LAB). Our P wave RF images reveal distinct lateral variations of the LAB depth. The lithospheric thickness extends to ~120 km beneath the central orogenic belt and decreases sharply to ~80 km at the west edge of the eastern block, consistent with previous S wave RF studies.

Upwelling mantle plume and lithospheric delamination beneath the North China Craton

Various models have been proposed for the destruction of the Eastern North China Craton (NCC); however, it is still a highly controversial scientific issue. In this study, abundant teleseismic data recorded by permanent and temporary stations in the NCC were collected, and P-wave tomography and common conversion point (CCP) stacking of receiver functions were performed. The results from this study reveal an upwelling mantle plume and a shallow Lehmann discontinuity (or the lower boundary of a low-velocity zone) beneath the Eastern NCC. Additionally, a large-scale, plate-like high-velocity anomaly is defined under the Eastern NCC that might be related to the delamination of a lithospheric keel. Based on the above results and assessment, NCC destruction might include two causes: lithospheric keel removal or delamination and mantle plume upwelling.

Crustal Thickness Variation Across the Sea of Marmara Region, NW Turkey: A Reflection of Modern and Ancient Tectonic Processes

AbstractThe Marmara region in Turkey is an important geological setting, both from a tectonic and a seismic hazard/risk perspective. We present a new map of crustal thickness variation across this complex region to better understand the interplay of past and present tectonic processes that have formed present‐day structure. Maps of crustal thickness are created using Ps converted phases and receiver function (RF) analysis of earthquakes recorded at all publicly available seismic stations and stations in the national monitoring network (run by AFAD Disaster and Emergency Management Authority Turkey). RFs are converted from time to depth using a local 3‐D full‐waveform tomographic model and are combined in multiphase common conversion point stacks. Direct P to S converted arrivals and associated multiples are mapped to produce continuous maps of the Moho discontinuity. Results show Moho depths ranging from 26–41 km with a regional trend of westward thinning reflecting the effects of the extensional regime in western Anatolia and the neighboring Aegean Sea. The thinnest crust is observed beneath the western end of the Sea of Marmara, attributed to transtensional basin opening. A distinct region of increased crustal thickness bounded by the West Black Sea Fault in the west, and the northern strand of the North Anatolian Fault in the south, defines the ancient crustal terrane of the Istanbul Zone. Isostatic arguments indicate that the thickened crust and lower elevation in the Istanbul Zone require it to be underlain by thicker lithosphere, a conclusion that is consistent with its hypothesized origin near the Odessa shelf.