Frozen subduction in the Yangtze block:insights from the deep seismic profiling and gravity anomaly in east Sichuan fold belt

The Sichuan basin is the main part of the middle-upper Yangtze block, which has been experienced a long-term tectonic evolution since Archean. The Yangtze block was regarded as a stable block until the collision with the Cathaysia block in late Neoproterozoic. A new deep seismic reflection profile conducted in the eastern Sichuan fold belt（ESFB） discovered a serials of south-dipping reflectors shown from lower crust to the mantle imply a frozen subduction zone within the Yangtze block. In order to prove the speculation, we also obtain the middle-lower crustal gravity anomalies by removing the gravity anomalies induced by the sedimentary rocks and the mantle beneath the Moho, which shows the mid-lower crustal structure of the Sichuan basin can be divided into eastern and western parts. Combined with the geochronology and Aeromagnetic anomalies, we speculated the Yangtze block was amalgamated by the West Sichuan and East Sichuan blocks separated by the Huayin-Chongqing line. The frozen subduction zone subsequently shifted to a shear zone accommodated the lower crustal shortening when the decollement at the base of the Nanhua system functioned in the upper plate.

Crustal velocity, density structure, and seismogenic environment in the southern segment of the North- South Seismic Belt, China

The southern segment of the North-South Seismic Belt in China is a critical region for earthquake preparedness and risk reduction efforts.However,limited by the low density of seismic stations and the use of single-parameter physical structural models,the deep tectonic features and seismogenic environment in this area remain controversial.Thus,a comprehensive analysis based on high-resolution crustal structures and multiple physical parameters is required.In this study,we applied the ambient noise tomography method to obtain the three-dimensional(3D)crustal S-wave velocity structure using continuous waveform data from 112 permanent stations and 350 densely distributed temporary stations in the southern segment of the North-South Seismic Belt.Then,we obtained the high-resolution 3D density structure through wavenumber-domain 3D gravity imaging constrained by the velocity structure.The low-velocity and low-density anomalies in the upper crust of the study area were mainly distributed in the Sichuan Basin and around Dali and Simao,while the high-velocity and high-density anomalies were primarily distributed in the Panxi region,corresponding to the surface geological features.Two prominent low-velocity and low-density anomalies were observed in the middle and lower crust:one to the west of the Songpan-Garzêblock and Sichuan-Yunnan diamond-shaped block,and the other near the Anninghe-Xiaojiang fault.Combined with the spatial distribution of seismic events in the study area,we found that previous earthquakes predominantly occurred in the transition zones between high and low anomaly regions and in the low-velocity and low-density zones in the upper crust.In contrast,moderate-to-strong earthquakes mainly occurred within the transition zones between high and low anomaly regions and close to the high-velocity and high-density regions,often with low-velocity and low-density layers below their hypocenters.Fluids play a critical role in the seismogenic process by reducing fault strength and destabilizing the stress state,which may be a triggering factor for earthquakes in the study area.Additionally,the upwelling of molten materials from the mantle may lead to energy accumulation and stress conce-ntration,providing an important seismogenic background for moderate-to-strong earthquakes in this area.

Observation and research of deep underground multi-physical fields—Huainan–848 m deep experiment

Compared with the surface,the deep environment has the advantages of allowing“super-quiet and ultra-clean”-geophysical field observation with low vibration noise and little electromagnetic interference,which are conducive to the realization of long-term and high-precision observation of multi-physical fields,thus enabling the solution of a series of geoscience problems.In the Panyidong Coal Mine,where there are extensive underground tunnels at the depth of 848 m below sea level,we carried out the first deep-underground geophysical observations,including radioactivity,gravity,magnetic,magnetotelluric,background vibration and six-component seismic observations.We concluded from these measurements that(1)the background of deep subsurface gravity noise in the long-period frequency band less than 2 Hz is nearly two orders of magnitude weaker than that in the surface observation environment;(2)the underground electric field is obviously weaker than the surface electric field,and the relatively high frequency of the underground field,greater than 1 Hz,is more than two orders of magnitude weaker than that of the surface electric field;the east-west magnetic field underground is approximately the same as that at the surface;the relatively high-frequency north-south magnetic field underground,below 10 Hz,is at least one order of magnitude lower than that at the surface,showing that the underground has a clean electromagnetic environment;(3)in addition to the highfrequency and single-frequency noises introduced by underground human activities,the deep underground space has a significantly lower background vibration noise than the surface,which is very beneficial to the detection of weak earthquake and gravity signals;and(4)the underground roadway support system built with ferromagnetic material interferes the geomagnetic field.We also found that for deep observation in the“ultra-quiet and ultra-clean”environment,the existing geophysical equipment and observation technology have problems of poor adaptability and insufficient precision as well as data cleaning problems,such as the effective separation of the signal and noise of deep observation data.It is also urgent to interpret and comprehensively utilize these high-precision multi-physics observation data.

Magnetized magma intrusions being sources of a weak and a strong lunar magnetic anomaly revealed by 3D distribution of magnetization

In this work,we aim to investigate the origin of the magnetic carriers in the lunar crust and the intensity of the ancient dynamo field.The magnetization and depth range of magnetic carriers are studied under a weak and a strong magnetic anomaly in Mare Tranquillitatis and in Oceanus Procellarum,respectively,where the surface ages are 3.6 and 3.3 billion years.A sophisticated three-dimensional amplitude inversion software program from a geophysical survey is used to reconstruct the distributions of magnetization in the lunar crust.Because no globally measured surface magnetic field exists for the Moon,a crustal magnetic anomaly model with a grid resolution of 0.2°is used.The depth range of the magnetic source is fixed by the boundary identified by a relative criterion,which is 20%of the recovered maximum magnetization.The central burial depths of the magnetic carriers are approximately 15 km and 25 km under Reiner Gamma and Mare Tranquillitatis,respectively.The volumes of the two magnetic sources are at scales of 104 and 105 km3,respectively.The aforementioned differences may imply a hotter crust under Reiner Gamma than Mare Tranquillitatis by 3.3 billion years.The results support the view that the magma intrusions magnetized by an ancient magnetic field could be the origin of magnetic anomalies under Reiner Gamma and Mare Tranquillitatis.Compared with previous works,the maximum magnetization of 3 A/m under Reiner Gamma supports the intensity of the field being several microteslas.

壁面微沟槽与表面活性剂耦合对管道中湍流减阻特性的影响

通过矩形管道压降实验研究了壁面微沟槽和表面活性剂的减阻性能及联合减阻的增益效果,用粒子成像测速仪分析了流场特性。实验所用的微沟槽为3种不同结构的顺流向V形沟槽,表面活性剂为十六烷基三甲基氯化胺(CTAC),水杨酸钠(NaSal)作为补偿离子。结果表明,壁面微沟槽和表面活性剂溶液均有减阻效果,二者耦合后减阻率进一步提升,最高减阻率为48.26%。微沟槽的减阻性能主要作用在近壁区,通过影响边界层平均流速、速度脉动强度和涡结构,减少表面活性剂的湍动能损耗。当超过表面活性剂的临界雷诺数后,沟槽尖端的高剪切力会加剧胶束结构分解。表面活性剂能抑制湍流涡的演变,扩大微沟槽有效减阻的雷诺数范围。