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Palaeomagnetic investigations often encounter multiple magnetization components, where secondary processes have obscured, partially overprinted or completely replaced the original (primary) remanent magnetization. Identification and separation of primary and secondary magnetizations are generally carried out with principal component analysis of stepwise demagnetization data. However, rocks may contain multiple generations of magnetic minerals with overlapping unblocking ranges that complicate the discrimination of components when applying best-fitting line procedures. Developing a method to differentiate and quantify contributions of overlapping magnetic components using directional data is therefore highly desirable. This paper presents a method to unmix stepwise demagnetization data using an inverse modelling approach. We show that the method is capable of accurately resolving two or three magnetic components with overlapping or superimposed unblocking spectra as well as quantifying absolute component contributions. The method depends on accurate identification and selection of end-member components prior to analysis; in doing so, the method can help palaeomagnetists understand how magnetization components combine to explain their data. We show that the dilution of one component by more than ca. 25 per cent from another component can result in linear demagnetization curves that decay to the origin on orthogonal plots, but whose best-fitting direction can significantly deviate from both end-members. The efficacy of the method is demonstrated through examples of demagnetization data from hematite and/or magnetite-bearing sandstones from China. This method can be broadly applied to all multicomponent magnetization problems in palaeomagnetism. [ABSTRACT FROM AUTHOR]

The Daliang Shan is a tectonic unit that connects the active southeastern Tibetan Plateau with the stable South China block. As a newly generated (formed later than the Xianshuihe–Xiaojiang active fault system) seismotectonic zone induced by the Tibetan tectonics, the detailed constraints of the crustal deformation are central to the understanding of the kinematics and dynamics of the Tibetan expansion. This paper establishes and analyses a high-spatial resolution global positioning system (GPS) velocity field from a dense GPS network in this region. Our modelling results indicate that, in contrast to the equivalent sinistral strike-slip rate of approximately 5 mm yr–1 on the Anninghe–Zemuhe and Daliangshan faults, their inferred interseismic locking depth varies within a large range. The southern segment of the Anninghe Fault and the middle segment of the Daliangshan Fault have deep locking depths of ∼13 km, indicating that the seismic risk is high in these areas. In addition, the detectable counter-clockwise rotation rate of 0.35 ± 0.12° Myr–1 of the Mabian block makes a significant contribution of ∼50 per cent to the strike-slip motion on its boundary faults. This counter-clockwise rotation may be induced by a left-lateral shear gradient with southeastward motion relative to the South China-fixed reference frame, indicating the significance of a simple-shear pattern in exploring the kinematics of the encroachment of the Tibetan tectonics upon a stable block (craton). [ABSTRACT FROM AUTHOR]

Receiver function (RF) analysis is an indispensable method that is widely utilized to image the deep structure of the crust and upper mantle. Although the P -wave receiver function (PRF) has the advantages of a higher signal-to-noise ratio and higher resolution than S -wave receiver function, the information about P -to- S converted (Ps) phases from velocity discontinuities of the crust and upper mantle is usually obstructed by multiples from shallower crustal interfaces. Sometimes it is difficult to identify useful information about lithospheric discontinuities. In this paper, we propose a method for suppressing multiples and isolating the Ps phases of RFs. This task is accomplished by exploiting the differences in arrival times resulting from the slowness between the conversion and the multiples that occur at the same interface in the PRFs and by separating them in the Radon domain, which enables removal of crustal and sedimentary multiples. This method can effectively remove multiples and isolate the useful signals of Ps phases contained in the PRFs. We test the method on synthetic PRFs and demonstrate that crustal multiples can be effectively eliminated or suppressed and that the Ps phase of lithospheric discontinuities can be coherently traced. Next, we apply the method to real PRF data collected from the seismic station of the China Array seismic experiment (Phase II and Phase III) in the Ordos block and its adjacent area, and successfully obtain an improved common conversion point stacking image of lithospheric discontinuity structures in the depth domain, particularly for detecting the lithosphere–asthenosphere boundary. [ABSTRACT FROM AUTHOR]

Flat slab subduction has been ascribed to a variety of causes, including subduction of buoyant ridges/plateaus and forced trench retreat. The former, however, has irregular spatial correlations with flat slabs, while the latter has required external forcing in geodynamic subduction models, which might be insufficient or absent in nature. In this paper, we present buoyancy-driven numerical geodynamic models and aim to investigate flat slab subduction in the absence of external forcing as well as test the influence of overriding plate strength, subducting plate thickness, inclusion/exclusion of an oceanic plateau and lower mantle viscosity on flat slab formation and its evolution. Flat slab subduction is reproduced during normal oceanic subduction in the absence of ridge/plateau subduction and without externally forced plate motion. Subduction of a plateau-like feature, in this buoyancy-driven setting, enhances slab steepening. In models that produce flat slab subduction, it only commences after a prolonged period of slab dip angle reduction during lower mantle slab penetration. The flat slab is supported by mantle wedge suction, vertical compressive stresses at the base of the slab and upper mantle slab buckling stresses. Our models demonstrate three modes of flat slab subduction, namely short-lived (transient) flat slab subduction, long-lived flat slab subduction and periodic flat slab subduction, which occur for different model parameter combinations. Most models demonstrate slab folding at the 660 km discontinuity, which produces periodic changes in the upper mantle slab dip angle. With relatively high overriding plate strength or large subducting plate thickness, such folding results in periodic changes in the dip angle of the flat slab segment, which can lead to periodic flat slab subduction, providing a potential explanation for periodic arc migration. Flat slab subduction ends due to the local overriding plate shortening and thickening it produces, which forces mantle wedge opening and a reduction in mantle wedge suction. As overriding plate strength controls the shortening rate, it has a strong control on the duration of flat slab subduction, which increases with increasing strength. For the weakest overriding plate, flat slab subduction is short-lived and lasts only 6 Myr, while for the strongest overriding plate flat slab subduction is long-lived and exceeds 75 Myr. Progressive overriding plate shortening during flat slab subduction might explain why flat slab subduction terminated in the Eocene in western North America and in the Jurassic in South China. [ABSTRACT FROM AUTHOR]

In this paper, convolutional neural networks (CNNs) were used to distinguish between tectonic and non-tectonic seismicity. The proposed CNNs consisted of seven convolutional layers with small kernels and one fully connected layer, which only relied on the acoustic waveform without extracting features manually. For a single station, the accuracy of the model was 0.90, and the event accuracy could reach 0.93. The proposed model was tested using data from January 2019 to August 2019 in China. The event accuracy could reach 0.92, showing that the proposed model could distinguish between tectonic and non-tectonic seismicity. [ABSTRACT FROM AUTHOR]

Magnetic susceptibility and natural remanent magnetization of rocks are useful parameters to study geological structures and geodynamic processes. Traditional widely used algorithms for the inversion of magnetic data can recover the distribution of the apparent susceptibility or total magnetization intensity, but do not provide information on the remanent magnetization. In this paper, we propose a framework to directly invert for the magnetic susceptibility and the natural remanent magnetization vector using surface or airborne magnetic data, assuming that the Köenigsberger ratio of the rock is known or approximately deducible. The susceptibility and remanence are computed using two different approaches: (1) the susceptibility, intensity, and direction of the remanent magnetization are continuously recovered for each discretized cell and (2) the remanence direction is assumed to be uniform in each subzone and is iteratively computed as discrete values. Both processes are implemented using the preconditioned conjugate gradient algorithm. The method is tested on three synthetic models and one field data set from the Zaohuohexi iron-ore deposit, Qinghai Province, northwest China. The results of the continuous inversion show the trend of the remanent magnetization directions, while the discrete inversion yields more specific values. This inversion framework can determine the source bodies' geometry and position, and also provide superposed and comprehensive information on the natural remanent magnetization, which may be useful to investigate geological bodies bearing stable primary remanent magnetization. [ABSTRACT FROM AUTHOR]

The Ground-source Airborne Time-domain Electromagnetic (GATEM) method is an efficient electromagnetic prospecting technique for engineering, geological surveying and mineral exploration. The electromagnetic response and inversion are obtained via the electrical conductivity of the geological medium, which is conventionally characterized by a homogeneous half-space or a piecewise smooth spatial distribution. However, these approximations hardly reflect the conditions of real geological media because of their rough, self-similar characteristics. This paper primarily considers the GATEM response and inversion with a long grounded wire source in a rough geological medium based on the fractional diffusion. To manage the rough geological medium, the fractional Maxwell equation is solved in the Laplace domain, and then the solution is transformed to the time domain using Guptasarmas digital filter method. According to the mapping relationship between the resistivity and the induced voltage theoretically calculated in a rough geological medium, the neural network method is used to invert the resistivity. The characteristics of the response in a rough medium are analysed. Then, the inversion results are verified for a layered rough geological medium model; these results are closer to those of the real model than the results of the method derived using a smooth spatial distribution. Finally, the method is applied to GATEM field data in Jincheng, Shanxi Province, China. The inversions performed using a rough geological medium result in a reliable conductivity distribution. [ABSTRACT FROM AUTHOR]

Trend drift is an annoying background interference in induced polarization (IP) exploration, which has great influence on the final calculation of apparent complex resistivity spectrum at low frequency (<0.1 Hz). This paper proposed a modified empirical mode decomposition (EMD) detrending technique for multiperiod IP data. The method uses local extreme values of the rising edges and the falling edges to form multiple envelopes and then to fit and eliminate the trend term. Through comparing with the traditional EMD methods using IP data with simulated trend drift, we find that the modified method can be used to obtain a more accurate fitting trend and the computational cost is only a fraction of that of the conventional one. Additionally, this detrending is little affected by other strong noise. We also used IP data with and without trend interferences to analyse this method, respectively. The results show that, for data without trend drift, the signals remain almost unchanged; however, for data with strong trend drift, the data quality is greatly improved and the calculation error is reduced. This technique is also applied to a large-scale multiperiod full-waveform IP data acquired in Zhegu Zn-Sb-Ag polymetallic deposit in southern Tibet, China. The apparent complex resistivity and phase of a survey line, a planar contour map and a pseudo-section with and without using the modified EMD were compared, respectively. Overall, before EMD detrending, the apparent phase results are rough and full of outliers. After detrending, the profiles are smooth and reasonable, and the outliers disappear. Both the results demonstrated that our proposed method can be adopted to effectively suppress trend drift interference without additional deviation in distributed full-waveform IP exploration. [ABSTRACT FROM AUTHOR]