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 global navigation satellite system interferometric reflectometry (GNSS-IR) technique based on signal-to-noise ratio (SNR) data is widely used for snow depth retrieval. Since snow depth retrieval in a snow-free state is very important for meteorological monitoring and many corrections are postprocessed to improve the retrieval accuracy, we propose a GNSS-IR snow depth retrieval model based on a backpropagation (BP) neural network optimized by a genetic algorithm to detect the snow state and predict snow depth using the frequency, amplitude, and phase of the multipath oscillation term as input features. Global positioning system (GPS) data collected from the P351 station of the Plate Boundary Observation (PBO) network and measured snow depth from the snow telemetry (SNOTEL) network were used to conduct the experiments. The accuracy of daily snow state detection for the experimental station exceeded 96%. Combined with the snow state detection results for snow depth regression prediction, the experimental results show that the root-mean-square error (RMSE) of the snow depth retrieval results for the P351 station is 12.09 cm. Compared with the traditional model, the retrieval accuracy is improved by 29.1%, and the correlation coefficient also reaches 0.97, indicating that the proposed snow depth retrieval model not only has high accuracy but also has strong stability. In this study, snow state detection is proposed to improve the retrieval accuracy in snow-free conditions, and the possibility of snow depth retrieval without antenna height is provided.

Description: Global Navigation Satellite System-Reflectometry (GNSS-R) is one of the main technologies used for sea ice remote sensing detection and is based on the multipath interference effect of satellite signals. To improve the GNSS-R sea ice detection performance in terms of accuracy, robustness to noise, and data utilization, a linear discriminant analysis (LDA)-based method was proposed in this article. Delay-Doppler maps (DDMs) collected from TechDemoSat-1 (TDS-1) were employed as input and classified into different types based on the signal-to-noise ratio (SNR) related to the noise effect. For low-effect-noise DDMs, the LDA-based sea-ice detection method presented an accuracy of 95.03%, verifying the feasibility of LDA-based GNSS-R sea-ice detection. For the middle noise effect and high noise effect DDMs, the LDA-based method is more robust to noise effects than the convolutional neural network (CNN) method. Although the detection accuracy decreased when the SNR decreased or the integral delay waveform average (IDWA) increased, the LDA-based method was more robust than the CNN-based one. The data utilization and melting period were also analyzed to account for variations in detection accuracy. The LDA-based method used 67.82% more data than previous experiments with threshold IDWA ≤58 210.32 and SNR 〉−17.48 dB. The melting periods were analyzed based on the noise, SNR, surface reflectivity, and permittivity. When the status of sea ice changes, outliers of surface reflectivity appear, the average permittivity varies in [10, 60], and the detection accuracy decreases during the melting period of sea ice. The results show that the correlation coefficient with the National Oceanic and Atmospheric Administration (NOAA) data is up to 0.93, with different thresholds IDWA or IDWA. The LDA-based method predicted results that greatly matched the sea ice distribution from the NOAA data.

Description: How the Tibetan Plateau grew farther north from the India-Asia collision boundary has profound implications on the mechanics of continental deformation and accretion. The Hoh-Xil Basin in the northern Tibetan Plateau was once a foreland basin adjacent to the high-elevation proto-plateau until the Early Miocene and was rapidly uplifted since then. Here we collected 363 broadband seismic stations operated from 2007 to 2020, including 226 stations from five linear arrays mainly deployed in two north-south profiles across the core of the Hoh-Xil Basin with an average interstation distance of ~15 km. Based on Rayleigh wave signals extracted from ambient noise cross-correlations, we obtained more than 13000 dispersion curves and constructed a 3-D S-wave velocity (V〈sub〉S〈/sub〉) model using measurements in 6–65 s periods by the direct inversion method. Our model shows significant lateral variations of V〈sub〉S〈/sub〉 in the crust and uppermost mantle from the southern to the northern Tibetan Plateau, which should reflect different melt fractions according to the theoretical seismic velocity-melt fraction relationship. We observe widespread partially molten crust in the northern Tibetan Plateau but only isolated pockets in the south manifested as low-V〈sub〉S〈/sub〉 anomalies in the middle-lower crust. The spatial correlation of these low-V〈sub〉S〈/sub〉 anomalies with strong uppermost mantle low-V〈sub〉S〈/sub〉 anomalies and young crust-/mantle-derived magmatism in the Hoh-Xil Basin suggests that the plateau grew through magmatic intrusion and accretion induced by the asthenospheric upwelling due to lithospheric mantle removal. These findings lead to implications for the role of delamination-driven magmatism in the continental crust growth in collision orogens.

Description: Fold-and-thrust belts (FTBs) develop widely in and around active orogenic belts on Earth. With the accumulation of structural shortening, thrust-related folds can grow by increasing their amplitudes and lengths, providing insights into the structural evolution of FTBs. Investigations of fold growth patterns and landscape responses through time and space have important implications for hydrocarbon exploration and geo-hazard assessment. Evidence for lateral fold propagation (i.e., increase in length) usually comes from geochronological constraints based on differences in onset timings along strike. However, these traditional methods are time consuming and economically costly. Alternatively, previous studies have proposed the use of geomorphic features to investigate the lateral fold propagation, including spatial variations in the patterns and densities of river channels, the configuration of the topographic profile along the fold crest, and the occurrence of wind gaps. However, such analyses are based on morphological features of present-day landscapes, which still need process-based validations. Here, we conduct a series of landscape evolution models to reproduce fold growth and their associated geomorphic adjustments. Our results show that deflected rivers and inflected fold-crest elevation profile with decreasing gradient are reliable evidence of lateral fold propagation. The location of propagation is in accordance with the distribution of crest-profile slope breaks and the position where river deflection began. Then, we apply these findings to two natural examples located at the deformation fronts of the Tian Shan (NW China). Results suggest that 1) the Kashi anticline has experienced lateral propagation both westwards and eastwards; and 2) that the Yaken anticline has experienced accelerated tectonic uplift as well as eastwards propagation. These findings are supported by the constraints from geochronological dating and quantitative structural analysis. Future work that integrates folding mechanisms may provide novel insights into landscape response to complex deformation in active FTBs.

Description: Although it has long been regarded as creeping climate anomalies that take months or years to fully develop, droughts may occur frequently at subseasonal timescales, and can develop into severe droughts within a few weeks. These rapid-onset droughts are termed “flash droughts”. In recent years, investigating the evolution and changing characteristics of flash droughts at regional to global scales has been a hot topic. However, no consensus has been reached on whether there has been a transition from slow to flash droughts at global scale, because the frequency of slower developing droughts at subseasonal time scales may also increase. Here, we investigate changes in global drought onset speed and the partitioning between flash and slow droughts, and explore the role of anthropogenic climate change in the drought transition. We show that drought intensification rates have sped up over subseasonal timescales, and there has been a transition toward more flash droughts over 3/4 of the global IPCC SREX regions during the past. Detection and attribution by using CMIP6 models show that the transition is associated with amplified anomalies of evapotranspiration caused by anthropogenic climate change. In the future, the transition is projected to expand to most land areas, with larger increases under higher emission scenarios. These findings suggest a global transition to flash droughts under climate change.

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.