Two-step Gravity Inversion Reveals Variable Architecture of African Cratons

The lithospheric build-up of the African continent is still to a large extent unexplored. In this contribution, we present a new Moho depth model to discuss the architecture of the three main African cratonic units, which are: West African Craton, Congo Craton, and Kalahari Craton. Our model is based on a two-step gravity inversion approach that allows variable density contrasts across the Moho depth. In the first step, the density contrasts are varied for all non-cratonic units, in the second step for the three cratons individually. The lateral extension of the tectonic units is defined by a regionalization map, which is calculated from a recent continental seismic tomography model. Our Moho depth is independently constrained by pointwise active seismics and receiver functions. Treating the constraints separately reveals a variable range of density contrasts and different trends in the estimated Moho depth for the three cratons. Some of the estimated density contrasts vary substantially, caused by sparse data coverage of the seismic constraints. With a density contrast of Δ ρ = 200 kg/m3 the Congo Craton features a cool and undisturbed lithosphere with smooth density contrasts across the Moho. The estimated Moho depth shows a bimodal pattern with average Moho depth of 39–40 km for the Kalahari and Congo Cratons and 33–34 km for the West African Craton. We link our estimated Moho depth with the cratonic extensions, imaged by seismic tomography, and with topographic patterns. The results indicate that cratonic lithosphere is not necessarily accompanied by thick crust. For the West African Craton, the estimated thin crust, i.e. shallow Moho, contrasts to thick lithosphere. This discrepancy remains enigmatic and requires further studies.

De-noising Receiver function data using the unsupervised deep learning approach

Summary The converted wave data (P-to-s or S-to-p), traditionally termed as receiver functions, are often contaminated with noise of different origin that may lead to the erroneous identification of phases and thus influence the interpretations. Here we utilize an unsupervised deep learning approach called Patchunet to de-noise the converted wave data. We divide the input data into several patches, which are input to the encoder and decoder network to extract some meaningful features. The method de-noises an image patch-by-patch and utilizes the redundant information on similar patches to obtain the final de-noised results. The method is first tested on a suite of synthetic data contaminated with various amount of Gaussian and realistic noise and then on the observed data from three permanent seismic stations: HYB (Hyderabad, India), LBTB (Lobatse, Botswana, South Africa), COR (Corvallis, Oregon, USA). The method works very well even when the signal-to-noise ratio is poor or with the presence of spike noise and deconvolution artifacts. The field data demonstrate the effectiveness of the method for attenuating the random noise especially for the mantle phases, which show significant improvements over conventional receiver function based images.

The Splitting 660 km Discontinuity Associated with Lithospheric Delamination in the Northern Part of the North-South Seismic Zone, China

The north-south seismic zone (NSSZ) is a destructive zone of large-scale earthquakes in China, and the earthquake mechanism associated with deep structures remains unclear. Previous studies have indicated that lithospheric delamination or absence of lithospheres in the western part of the NSSZ may facilitate the eastern extrusion of the Tibetan Plateau and lead to stress accumulation and release. However, the deep process of lithospheric delamination needs to be further clarified. In this study, I collect abundant high-quality teleseismic data recorded by permanent seismic stations and perform common conversion point (CCP) stacking of receiver functions in the north part of the NSSZ. The results show that lithospheric delamination might result in the splitting 660 km discontinuity and a thickening region of the mantle transition zone (MTZ).

Seismic Structure Beneath the Wellington Region from Receiver Functions

<p>Seismic velocity structures, interpreted as being associated with the Hikurangi subduction system beneath the lower North Island of New Zealand, are imaged using stacked P wave receiver functions computed using teleseismic earthquakes. Receiver functions are a seismological technique that exploits the phenomenon of wave conversion. The upcoming P wave interacts with seismic velocity impedance contrasts below the receiving station to produce polarized P to SV converted phases. The time delay between the first arriving P wave and the SV converted phase is interpreted to infer the depth of interfaces and the velocity structure directly below the receiver, allowing estimates to be made of the physical properties of the interface. Passive seismic data were recorded at eighteen seismic stations deployed across a ~90km transect stretching across the breadth of lower North Island of New Zealand, from Kapiti Island, 5km off the west coast, to the eastern coast. The transect is oriented normal to the strike of the subducting Pacific Plate, as it dives beneath the overriding Australian Plate. Data were recorded at 10 broadband and 2 short period sensors, deployed as part of the Seismic Array Hikurangi Project (SAHKE 1 deployment), 3 Geonet (New Zealand Geonet Project) permanent short period stations, and 3 temporary stations from part of the 1991-1992 POMS project. Seismic data were recorded between November 2009 and March 2010 on the short period sensors and up to 18 months on the broadband sensor. Data recorded between November 2009 and November 2011 were utilised from the Geonet stations. P wave receiver functions are computed using the multi-taper correlation method using 389 > 6.0 Mw teleseismic earthquakes recorded at the individual seismic stations. A total of 1082 individual receiver functions from all the stations are stacked for both the individual stations and as a ‘super-stack’ across the complete transect, using the common conversion point (CCP) method. The CCP stack shows a distinct, thick low velocity layer (LVL), dipping to the west, from ~18km depth in the east to ~30km depth in the west. This is above a higher velocity layer, also dipping west, at depths of between ~22km and ~ 37km. The LVL is interpreted as being subducted sediments overlying the higher velocity plate interface. Structures towards the west indicate the presence of possibly imbricated features associated with the overriding plate. Deeper structures, down to a depth of 140km are evident, but have less clarity than the shallower features. Some of the deeper layers appear to be dipping towards the west, some to the east. The results of the CCP stack agree well with results from active source methods.</p>

Seismic Structure Beneath the Wellington Region from Receiver Functions

<p>Seismic velocity structures, interpreted as being associated with the Hikurangi subduction system beneath the lower North Island of New Zealand, are imaged using stacked P wave receiver functions computed using teleseismic earthquakes. Receiver functions are a seismological technique that exploits the phenomenon of wave conversion. The upcoming P wave interacts with seismic velocity impedance contrasts below the receiving station to produce polarized P to SV converted phases. The time delay between the first arriving P wave and the SV converted phase is interpreted to infer the depth of interfaces and the velocity structure directly below the receiver, allowing estimates to be made of the physical properties of the interface. Passive seismic data were recorded at eighteen seismic stations deployed across a ~90km transect stretching across the breadth of lower North Island of New Zealand, from Kapiti Island, 5km off the west coast, to the eastern coast. The transect is oriented normal to the strike of the subducting Pacific Plate, as it dives beneath the overriding Australian Plate. Data were recorded at 10 broadband and 2 short period sensors, deployed as part of the Seismic Array Hikurangi Project (SAHKE 1 deployment), 3 Geonet (New Zealand Geonet Project) permanent short period stations, and 3 temporary stations from part of the 1991-1992 POMS project. Seismic data were recorded between November 2009 and March 2010 on the short period sensors and up to 18 months on the broadband sensor. Data recorded between November 2009 and November 2011 were utilised from the Geonet stations. P wave receiver functions are computed using the multi-taper correlation method using 389 > 6.0 Mw teleseismic earthquakes recorded at the individual seismic stations. A total of 1082 individual receiver functions from all the stations are stacked for both the individual stations and as a ‘super-stack’ across the complete transect, using the common conversion point (CCP) method. The CCP stack shows a distinct, thick low velocity layer (LVL), dipping to the west, from ~18km depth in the east to ~30km depth in the west. This is above a higher velocity layer, also dipping west, at depths of between ~22km and ~ 37km. The LVL is interpreted as being subducted sediments overlying the higher velocity plate interface. Structures towards the west indicate the presence of possibly imbricated features associated with the overriding plate. Deeper structures, down to a depth of 140km are evident, but have less clarity than the shallower features. Some of the deeper layers appear to be dipping towards the west, some to the east. The results of the CCP stack agree well with results from active source methods.</p>

Deep Structure and Dynamics of the Central Balkan Peninsula from Seismic Data

Abstract—Analysis of P- and S-receiver functions for 19 seismic stations on the Balkan Peninsula has been performed. Half of the stations are in Bulgaria. The crustal thickness varies from 28–30 to 50 km. The ratio of longitudinal and shear wave velocities in the upper crust reaches 2.0 in some places. In the southwest of the study area, the 410-km seismic boundary is uplifted by 10 km relative to nominal depth. The elevation may be caused by hydration and/or cooling of the mantle transition zone under the influence of the Hellenic subduction zone. A low S-wave velocity layer related to the 410-km boundary may be located atop this boundary. In the northwestern part of the study area this layer is present in spite of the absence of the 410-km boundary. A similar paradox has been previously noted in central Anatolia. Indications of a low-velocity layer are present at a depth exceeding 410 km. The simultaneous inversion of the receiver functions of the two types (P and S) and the Rayleigh wave phase velocities reveals a large (7–9%) decrease in the S-wave velocity in the upper mantle of southern Bulgaria and northern Greece. The thickness of the low-velocity layer (asthenosphere) is about 50 km. The lithosphere-asthenosphere boundary (LAB) is at depths of 40 to 60 km. In terms of tectonics, this zone is characterized as the South Balkan extension system. To the north of 43° N, the S-wave velocity in the upper mantle is usually at least 4.4 km/s and the LAB is not detected or is detected at a depth of over 80 km. The SKS analysis of azimuthal anisotropy reveals lateral zoning in the upper mantle that is correlated to velocity zoning. Probably, the mechanically weak low-velocity mantle of the South Balkan system is easily deformed, and the azimuth of the fast direction of anisotropy (20°) indicates the direction of extension. At the northern stations, the fast direction (about –30°) may be a reflection of an older process.

Variations of crustal thickness and average Vp/Vs ratio beneath the Shanxi Rift, North China, from receiver functions

The Shanxi Rift located in the central part of the North China Craton (NCC) as a boundary between the Ordos block and the Huabei basin. The Shanxi graben system is a Cenozoic rift and originated from back-arc spreading related to westward subduction of the western Pacific and far field effects caused by northward subduction of the Indian plate. It has also had strong earthquake activity in China since the Quaternary. To investigate the tectonic evolution and tectonic setting of strong earthquakes in the Shanxi Rift, we apply the receiver function $$H$$ H -$$\kappa$$ κ stacking method to determine the crustal thickness and average Vp/Vs ratio in the area. The results show that the thickness of the crust increases from approximately 30 km in the Huabei basin to approximately 47 km in the Yinshan Mountains with a close correlation between the Moho depth and topography. The Yuncheng, Linfen and Taiyuan grabens have varying degrees of crustal thinning. The crustal average Vp/Vs ratio in the Shanxi Rift has significant heterogeneity; the high Vp/Vs ratio (~ 1.85) are found in the Datong and Yuncheng grabens, and Vp/Vs ratio of the Taiyuan and Linfen grabens is approximately 1.75 which close to the global average value ~ 1.782. Combining the observations in this study with previous research, we suggest that the grabens in the Shanxi Rift experienced extensional deformation from south to north and that the possibility of strong earthquakes in the central part of the Shanxi seismic belt is greater than that on the northern and southern sides.

Crustal Anisotropy Beneath the Trans-North China Orogen and its Adjacent Areas From Receiver Functions

As an important segment of the North China Craton, the Trans-North China Orogen (TNCO) has experienced strong tectonic deformation and magmatic activities since the Cenozoic and is characterized by significant seismicity. To understand the mechanism of the crustal deformation and seismic hazards, we determined the crustal thickness (H), Vp/Vs ratio (κ) and crustal anisotropy (the fast polarization direction φ and splitting time τ) beneath the TNCO and its adjacent areas by analyzing receiver function data recorded by a dense seismic array. The (H, κ) and (φ, τ) at a total of 309 stations were measured, respectively. The Moho depth varies from ∼30 km beneath the western margin of the Bohai bay basin to the maximum value of ∼48 km beneath the northern Lüliang Mountain, which shows the positive and negative correlations with the elevation and the Bouguer anomaly. The average φ is roughly parallel to the strikes of the faults, grabens and Mountains in this study area, whereas a rotating distribution is shown around the Datong-Hannuoba volcanic regions. Based on the φ measured from the Moho Ps and SKS/SKKS phases, we propose that the crustal deformation and seismic hazards beneath the TNCO could be due to the counterclockwise rotation of the Ordos block driven by the far-field effects of the India-Eurasian collision.

Bulk Crustal Properties and Layered Velocity Structure in Fujian, SE China: Constraints From P and S Receiver Functions

Multiple tectonic events since the Neoproterozoic era have framed the present-day lithosphere in the Fujian province affiliated with the eastern part of the South China Block. Comprehensive information of the crustal structure and bulk properties can aid to understand the geological features and tectonic processes of still much debate in this region. An attempt is made in this study to explore crustal thickness and internal velocities across Fujian using the teleseismic receiver functions (RFs). The H-V stacking of joint P and S RFs improves to simultaneously estimate crustal thickness, average Vp and Vs, and derived Vp/Vs ratio and bulk sound speed in three backazimuth sectors for each of 17 stations. Furthermore, a Neighborhood Algorithm nonlinear inversion of P RFs is employed to determine the layered structures of Vs and Vp/Vs beneath all the stations. Results indicate the crustal thickness varies from at most ∼35 km in northwest Fujian to 30–35 km in the inland mountains and 27–30 km in the southeastern coasts. The inferred Moho geometry is nonplanar or inclined across the Zhenghe-Dapu (ZD) and Changle-Zhaoan (CZ) fault zones, especially in the southern ZD fault area. The average Vp/Vs suggests that the crust is predominantly felsic in the Wuyi-Yunkai orogen and intermediate-to-mafic in the Cretaceous magmatic and metamorphic zones. A high-velocity upper crust along the coastline is revealed, which attributes to the Pingtan-Dongshan metamorphic belt. At the sites near the ZD fault zone, the intracrustal negative discontinuity occurs at a shallower depth of ∼15 km marking an abrupt Vs decrease into the low-velocity mid-to-lower crustal layer, probably linked to the closed paleo-rift basin remnants. The lower crust across the Fujian is generally characterized by relatively lower Vs and higher Vp/Vs (1.80–1.84) consistent with those of the mafic-ultramafic rocks, which do not support the proposed extensive magmatic underplating in the Late Mesozoic.