ALBERT

Description: The Walvis Ridge (WR) is the most prominent hotspot track related to the opening in the South Atlantic Ocean. Several hypotheses have been developed to explain its origin and evolution. The presence of a massive magmatic structure at the landfall of the WR in Northwest Namibia raised speculation about the role of a hotspot during the opening of the South Atlantic ocean. To investigate its deeper velocity structure at the junction of the WR with the African continent was the focus of the amphibious seismological WALPASS experiment. In total 12 ocean-bottom seismometers and 28 broad-band land stations were installed between 2010 and 2012 to acquire seismological data. Here, we present the results of seismic ambient noise tomography to investigate to which extent the Tristan hotspot modified the crustal structure in the landward prolongation of the ridge and in the adjacent oceanic basins. For the tomography, vertical and hydrophone component cross correlations for 〉300 d for OBS stations and between 1 and 2 yr for land stations data were analysed. More than 49 000 velocity measurements (742 dispersion curves) were inverted for group velocity maps at 75 individual signal periods, which then had been inverted for a regional 3-D shear wave velocity model. The resulting 3-D model reveals structural features of the crust related to the continent–ocean transition and its disturbance caused by the initial formation of the WR ∼130 Ma. We found relatively thick continental crust below Northwest Namibia and below the near-shore part of the WR, a strong asymmetry offshore with typical, thin oceanic crust in the Namibe Basin (crossing over into the Angola Basin further offshore) to the North and a wide zone of transitional crust towards the Walvis Basin south of the WR.

Description: A sequence of three strong (M W 7.2–6.4) and several moderate (M W 4.4–5.7) earthquakes struck the Pamir Plateau and surrounding mountain ranges of Tajikistan, China, and Kyrgyzstan in 2015–2017. With a local seismic network in operation in the Xinjiang province since August 2015, an aftershock network on the Pamir Plateau of Tajikistan since February 2016, and additional permanent regional seismic stations, we were able to record the succession of the fore-, main-, and aftershock sequences at local distances with good azimuthal coverage. We located 11,784 seismic events and determined the moment tensor for 33 earthquakes. The seismicity delineates the major tectonic structures of the Pamir, i.e., the thrusts that absorb shortening along the plateau thrust front, and the strike-slip and normal faults that dissect the Plateau into a westward extruding and a northward advancing block. Fault ruptures were activated subsequently at increasing distances from the initial M W 7.2 Sarez. All mainshock areas but the initial one exhibited foreshock seismicity which was not modulated by the occurrence of the earlier earthquakes. The tabular ASCII data of the seismic event catalog consist of origin date, time, location, depth and magnitude of the events, along with the quality measures: number of P- and S-wave arrival time picks, location root-mean-square misfit and localization method. The tabular ASCII data of the moment tensor catalog consist of origin date, time, location, the six independent components of the moment tensor, the moment magnitude, and the orientation of the preferred fault plane parameterized as fault strike, dip and rake.

Description: It is widely recognized that collisional mountain belt topography is generated by crustal thickening and lowered by river bedrock erosion, linking climate and tectonics. However, whether surface processes or lithospheric strength control mountain belt height, shape and longevity remains uncertain. Additionally, how to reconcile high erosion rates in some active orogens with long-term survival of mountain belts for hundreds of millions of years remains enigmatic. Here we investigate mountain belt growth and decay using a new coupled surface process and mantle-scale tectonic model. End-member models and the new non-dimensional Beaumont number, Bm, quantify how surface processes and tectonics control the topographic evolution of mountain belts, and enable the definition of three end-member types of growing orogens: type 1, non-steady state, strength controlled (Bm 〉 0.5); type 2, flux steady state, strength controlled (Bm ≈ 0.4−0.5); and type 3, flux steady state, erosion controlled (Bm 〈 0.4). Our results indicate that tectonics dominate in Himalaya–Tibet and the Central Andes (both type 1), efficient surface processes balance high convergence rates in Taiwan (probably type 2) and surface processes dominate in the Southern Alps of New Zealand (type 3). Orogenic decay is determined by erosional efficiency and can be subdivided into two phases with variable isostatic rebound characteristics and associated timescales. The results presented here provide a unified framework explaining how surface processes and lithospheric strength control the height, shape, and longevity of mountain belts.

Description: The Tien Shan provides an ideal site to study mechanism of intracontinental orogeny due to distant effect of Indo-Asian collision. We investigate lithospheric structures, in particular the lithosphere-asthenosphere boundary (LAB), of Central Tien Shan (CTS) using S wave receiver functions. The results show distinct structures across the orogen. Under the southern CTS, the LAB is shallower than that of the Tarim Basin; a 50 km vertical offset implies that part of the lithosphere has been delaminated. Under the middle CTS, two phases of negative velocity gradient are obtained, which may indicate a new LAB and an ongoing delamination underneath. Under the northern CTS and Kazakh Shield northward, the lithosphere is stable although the LAB inclines southward slightly. The two periods of lithospheric delamination under the southern and middle CTS account well for pulsed uplifts of the Tien Shan at ~11-8 Ma and ~5-0 Ma, respectively.

Description: How the crust in the core of the Eastern Himalayan Syntaxis (EHS) deforms responding to the India-Asia collision remains ambiguous. Here we present the first high-resolution receiver functions image of crustal structure along a new NW-SE trending dense nodal array crossing the core of the EHS. Two sets of low velocity zones (LVZs) are clearly observed: one with a flat style beneath the western Lhasa terrane and Higher Himalaya at 18–20 km depth and the other with two west-dipping shapes below the western Yarlung-Zangbo suture within 10–30 km depth. These LVZs caused by partial melting and aqueous fluids are disconnected, impeding the formation of crustal flow. A discontinuous east-dipping intra-crustal discontinuity and a sharp Moho offset of 7 km under the Aniqiao-Motuo shear zone are identified, suggesting that the underthrusting of the Indian lower crust and pure shear mechanisms jointly dominate crustal deformation in the core of the EHS.

Description: Snow depth monitoring is meaningful for climate analysis, hydrological research, and snow disaster prevention. Global navigation satellite system-reflectometry (GNSS-R) technology uses the relationship between the modulation frequency of the signal-to-noise ratio (SNR) and reflector height to monitor snow depth. Existing research on single constellation has made good progress and is gradually developing toward multiconstellation combined inversion. Aiming at the accuracy of snow depth inversion, this article introduces the variational mode decomposition (VMD) algorithm with the characteristics of an adaptive high-pass filter to detrend the SNR data. The experimental results of KIRU station and P351 station show that the VMD algorithm is suitable for different constellations and has better signal separation effect. The snow depth inversion results for both stations are in high agreement with the in situ snow depths provided by the Swedish Meteorological and Hydrological Institute (SMHI) and the SNOTEL network. The root-mean-square error (RMSE) of the inversion results is reduced by 20%–40% compared to the least-squares fitting (LSF) algorithm, and the correlation coefficients are also greatly improved. Moreover, considering that there is no overlap between the climate station and the inversion area, this article introduces the maximum spectral amplitude as another reference data source and obtains basically consistent experimental conclusions. On this basis, the maximum spectral amplitude is used as the input variable of the entropy method, and the feasibility of the combination strategy is studied. The results show that the combined strategy reduces a little inversion error and improves the temporal resolution of snow depth monitoring. It is of great significance for more accurate and rapid monitoring of snow depth changes and disaster warnings and provides an important reference for further research on the GNSS-R technology.

Description: The signal-to-noise ratio (SNR) is important observa- tions in global navigation satellite system-reflectometry (GNSS-R) technology. The oscillation frequency in the SNR arc is sensitive to different reflecting surfaces and can be used to build height model to track the variation of snow depth. However, it is difficult to obtain retrieval results with snow depth of zero in the actual snow depth re- trieval experiments based on GNSS-R technology, which indicates that the classical model has nonnegligible retrieval errors in the snow-free state. This study aims to realize the detection of ground truth information before snow depth retrieval, i.e., classification of snow-free state and snow-covered state. Machine learning was introduced to achieve the aforementioned purpose and the SNR arc was used as the input data. Compared with the current mainstream topography correction algorithms, the algorithm proposed in this study does not rely on any priori ground measured data and has theoretical universality. The detection results can constrain the retrieval snow depth in the snow-free state and, thus, improve the retrieval accuracy. The experimental results for the 2014 seasonal snowpack at P351 station in Idaho, USA, show that the detection results obtained based on support vector machines agree well with the measured snow depth provided by the SNOTEL network, and the overall detection accuracy can reach about 96%. The daily snowpack state is determined by the majority of SNR arcs detected during the day and is only considered reliable if the percentage exceeds 75%. Only one day of the detection results was inaccurate and only 8 days (8/365) did not reach the set threshold of 75%. With the help of the detection results, the root-mean-square error of snow depth retrieval can be reduced from 20 cm in the classical algorithm to 15 cm, which results in a 25% improvement in retrieval accuracy. Moreover, this study broadens the application value of GNSS signals and provides a reference for the application of SNR in the detection field.

Description: The Hoh Xil Basin (HXB) was located in the foreland of the proto-Tibetan Plateau and has obtained a high elevation and thick crust since the Late Oligocene. However, crust thickening cannot be explained by the limited amount of upper crustal shortening and requires a further mechanism. Based on a linear dense nodal array, we use the receiver function method to study the detailed crustal structure beneath the HXB. Our images reveal that the HXB crust has been thickened to 〉70 km and is thicker than the protoplateau's crust. A series of north-dipping interfaces imaged in the lower crust implies widespread imbrication structures. Considering the uplift history of the plateau, we propose that isostatic-driven adjustments could equalize crustal thickness variations across the protoplateau margin in the Paleogene, and the HXB was elevated by extensive imbrications accommodated to a northward injection of the protoplateau lower crust in the Late Paleogene.

Description: We obtain P-wave receiver functions from teleseismic earthquake recordings at a dense seismic broadband transect, deployed along 170 km across the Betic mountain range in southeastern Spain. Migrated images show the crustal structure of the orogen in detail. In particular, they reveal the situation of the subducted Iberian paleomargin, with full preservation of the proximal domain and the ∼50 km wide necking domain. Crustal thinning across the necking domain affects mainly the lower continental crust. The Variscan crust of the Tethys margin is bending downward beneath the Betics, reaching ∼45 km depth, and terminates abruptly at a major slab tear. The distal domain of the paleomargin cannot be reconstructed, but the migrated section suggests that material has been exhumed through the subduction channel and integrated into the Betic orogen. This supports an origin of the HP-LT Nevado-Filabride units from subducted, hyperextended Variscan crust. According to our profile, the present-day eastern Betics appear to have a much more significant contribution from metamorphic Iberian crust than previously thought.

Description: A stationary mantle plume heats the overlying lithosphere from below, generating melts and causing lithospheric erosion. When the lithosphere moves away from the plume, it can be re-established by melt residues that have been since attached, partly due to cooling, at the base of the lithosphere. The Etendeka flood basalt region in northwest Namibia used to overlie the Tristan da Cunha mantle plume in the Early Cretaceous and thus provides an ideal place to examine plume-lithosphere interaction. Here we determine the upper mantle shear wave velocity structure of southern Africa down to 400 km depth by waveform inversion of multi-mode Rayleigh waves in order to find the imprints left by the Tristan da Cunha mantle plume. Thick lithosphere with high shear wave velocities is observed beneath the Congo and Kalahari Cratons, extending down to 200 km depth with the largest thickness beneath the Limpopo Belt. Along the landfall of the Walvis Ridge and Damara Belt, our model reveals a thick lithosphere down to 100–200 km depths. The thick lithosphere seems to constitute the Congo Craton, but the thick Congo cratonic lithosphere is extended farther south than observed in most of the previous models. We propose that the lithosphere along the landward extension of the Walvis Ridge was affected by magmatic processes related to the plume, but has been partially reconstructed since then by melt depletion and successive cooling. A high velocity anomaly beneath the northwest coast of Namibia down to a depth of ∼80 km coincides with distribution of the Etendeka basalt at the surface and the seaward-dipping reflectors at the continental margin. At sub-lithospheric depths, a low-velocity anomaly (at 100–200 km depth) and a high-velocity anomaly (at 250–350 km depth) can be detected. The origin of these anomalies could be related to an ongoing edge-driven convection process.