Crustal structure and shallow-focus seismic activities in the Sumatra subduction zone:Constrains from receiver function inversion

The Sumatra subduction zone is located in the southwest of the Suna plate,between the Euro-Asia Plate and Indo-Austrilian Plates.With the obliquely subducting of the Indo-Austrilian Plate toward the the Euro-Asia Plate,complex tectonics,strong earthquakes and volcanoes have been observed in this area which has become a well experimental field used to study the subduction zone.In this work,we employed receiver function method to evaluate the S-wave velocity structure beneath 5 broadband seismic stations along the Sumatra subduction zone.We selected 332 receiver function waveforms with intelligent software and manual picking methods,including 130,34,42,29 and 97 receiver function waveforms corresponding to BKNI,GSI,LHMI,MNAI,and PMBI sta-tions,respectively.These stacked receiver function waveforms were applied to inversion to estimate Swave structure beneath each station based on a Neighborhood Algorithm(NA).Our results indicate that the sediment layers for GSI,LHMI and MNAI stations are more than 3 km thick,two stations of which are thicker than 6 km(e.g.GSI and LHMI).The difference of receiver function waveforms for NE,SW and W orientation at station GSI where is accompanied with strong thrust earthquakes suggests that there is a complicated structure beneath this station.Station BKNI and PMBI are located on the eastern side of the Sumatra fault and the thickness of their sediment layers is only~1 km.The crustal thickness for back-arc basin is within 30-36 km.However,the crustal thickness of forearc area varies from~26 km of the forearc ridge to 26-30 km of the forearc basin toward continent and its,which suggests that the down dip limit(slab-Moho intersection)of seismogenic zone is within 29-36 km in forearc and explains why the shallow-focus earthquakes play a dominant role in this area.The stable state for the inner wedge of forearc within a seismogenic circle provides a favorable environment for storing stress.Meanwhile,these faults caused by the subducting of Indo-Austrilian Plate constructed a condition(e.g.cracking of intact rocks and frictional sliding)in which it would trigger shallow-focus seismic activities(releasing stress).

Velocity structure in the South Yellow Sea basin based on first-arrival tomography of wide-angle seismic data and its geological implications

The South Yellow Sea basin is filled with Mesozoic-Cenozoic continental sediments overlying pre-Palaeozoic and Mesozoic-Palaeozoic marine sediments.Conventional multi-channel seismic data cannot describe the velocity structure of the marine residual basin in detail,leading to the lack of a deeper understanding of the distribution and lithology owing to strong energy shielding on the top interface of marine sediments.In this study,we present seismic tomography data from ocean bottom seismographs that describe the NEE-trending velocity distributions of the basin.The results indicate that strong velocity variations occur at shallow crustal levels.Horizontal velocity bodies show good correlation with surface geological features,and multi-layer features exist in the vertical velocity framework(depth:0–10 km).The analyses of the velocity model,gravity data,magnetic data,multichannel seismic profiles,and drilling data showed that high-velocity anomalies(>6.5 km/s)of small(thickness:1–2 km)and large(thickness:>5 km)scales were caused by igneous complexes in the multi-layer structure,which were active during the Palaeogene.Possible locations of good Mesozoic and Palaeozoic marine strata are limited to the Central Uplift and the western part of the Northern Depression along the wide-angle ocean bottom seismograph array.Following the Indosinian movement,a strong compression existed in the Northern Depression during the extensional phase that caused the formation of folds in the middle of the survey line.This study is useful for reconstructing the regional tectonic evolution and delineating the distribution of the marine residual basin in the South Yellow Sea basin.

华南地壳结构与构造边界特征:来自地震背景噪声和重力联合成像模型的约束

查明华南板块的地壳精细结构,厘清不同块体的地壳物质属性和构造边界特征,对研究华南成矿带的深部动力学成因以及华南板块的形成演化具有重要意义.随着新方法的发展和对多类型数据的深入挖掘,对壳内精细结构的分辨率逐渐提高,有鉴于此,本研究通过收集华南地区地震台站的波形数据,利用背景噪声面波和布格重力异常联合反演华南板块的地壳S波速度结构.结果表明:联合反演模型获取的布格重力异常与观测重力值拟合较好,印证了新方法的可靠性;上地壳内主要沉积盆地呈现明显的低速特征,地壳厚度由西到东呈现减薄的趋势,且在中地壳内,江南造山带南段的地壳结构呈现北东向的高低速带状穿插分布,在南北重力梯度带和江山-绍兴-萍乡-永州一线存在“蘑菇云”形状的中地壳隆升;随着深度的增加,在21~30 km下地壳,速度的横向变化不再与地壳浅层主要构造相对应,整个区域的速度分布呈现东高西低的趋势,且随着深度的增加这一特征越发明显;在30~40 km深度上,速度变化的东西分界与南北的布格重力异常梯度带近似重合.据此推测,江南造山带与扬子和华夏地块的构造边界分别沿着九江-常德-吉首-铜仁-百色一线和江山-绍兴-萍乡-永州-柳州-钦州一线;区域构造演化的动力学模型显示,扬子地块不断地为江南造山带西翼的北西向逆冲推覆提供驱动力,联合反演获取的地壳结构模型为探讨印支、扬子和华夏地块碰撞拼合的深部动力学过程以及区域成矿作用提供了重要约束.

Investigation of geothermal structure of the Sulawesi,using gravity and magnetic method

The Sulawesi Sea and Sulawesi Island are located in the western Pacific area where volcanic activity,plate subduction,and seismic activity are very active.The Sulawesi basin formed during the Middle Eocene-Late Eocene and nearly half of the Eocene oceanic crust has subducted below the North Sulawesi Trench.The Sulawesi Island was spliced and finalized in the Early Pliocene-Pleistocene during volcanic activity and is recently very active.This area is an optimal location to study volcanic geothermal conditions and subduction initiation mechanisms in the southern part of the western Pacific plate margin,which are important in geothermal and geodynamic research.In this study,we combined 133 heat flow data with gravity and magnetic data to calculate the Moho structure and Curie point depth of the Sulawesi Sea and periphery of the Sulawesi Island,and analyze the distribution characteristics of the geothermal gradient and thermal conductivity.The results show that the average depths of the Moho and Curie surfaces in this area are 18.4 and 14.3 km,respectively,which is consistent with the crustal velocity layer structure in the Sulawesi Basin previously determined by seismic refraction.The average geothermal gradient is 4.96°C(100 m)-1.The oceanic area shows a high geothermal gradient and low thermal conductivity,whereas the land area shows a low geothermal gradient and high thermal conductivity,both of which are consistent with statistical results of the geothermal gradient at the measured heat flow points.The highest geothermal gradient zone occurs in the transition zone from the Sulawesi Sea to Sulawesi Island,corresponding to the spreading ridge of the southward-moving Sulawesi Basin.Comprehensive gravity,magnetic,and geothermal studies have shown a high crustal geothermal gradient in the study area,which is conducive to the subduction initiation.The northern part of the Palu-koro fault on the western side of Sulawesi is likely the location where subduction initiation is occurring.During the process of moving northwest,the northern and eastern branches of Sulawesi Island have different speeds;the former is slow and the latter is fast.These branches also show different deep tectonic dynamic directions;the northern branch tilts north-up and the eastern branch tilts north-down.

Fault-controlled regional magmatism and mineral deposition in central Cathaysia——Evidence from ambient noise tomography

Guangdong Province in the central Cathaysian Block has two world-class metallogenic belts, namely, the Nanling and Southeastern Coastal Metallogenic Belts(NLMB and SCMB), which are spatially coincide with the major regional Ganjiang and Zhenghe-Dapu Fault Zones(GJFZ and ZDFZ). However, what roles the faults played in mineral deposition and magmatism is unclear. Using ambient noise tomography, we obtain a 3-D whole-crust shear wave velocity model. By combining available regional geophysical models, we characterize the architecture of the regional shallow lithosphere and infer its possible tectonic connection to magmatic sources, pathways and surface deposition. The results show that the study area is loosely divided by the two major faults, the GJFZ and ZDFZ, into distinct velocity domains. In the north high Vand low V/Vcrust in the NLMB imply crustal remelting, which leads to the general felsic composition. In the coastal area, the lower crustal high Vanomaly is attributed to upwelling melts associated with Cretaceous magmatic activity. Between mineral belts, a swath of crustal lowvelocity zones extend into the uppermost mantle, manifesting partial melting related to upwelling magmas that may hint at a deep origin of magma from subcrustal lithosphere and likely feed surface mineral deposits through major faults. Secondary NW-trending faults coincide with low velocities and facilitated magmatic migration. A correlation between coastward extension of low velocities and younging of the Jurassic and Cretaceous magmatism is suggestive of a combined effect of slab rollback and a change in the direction of the Paleo-Pacific subduction system. We speculate a regional fault-control model in the central Cathaysian Block for the spatial-temporal evolution of regional deformation and magmatism during the middle Mesozoic.

Neogene subduction initiation models in the western Pacific and analysis of subduction zone parameters

The Neogene is an important period for studying the onset of subduction,with numerous subduction zones forming in the western Pacific,including the Ryukyu,Manila,Philippine,North Sulawesi,Halmahera,New Britain,Solomon,and New Hebrides subduction zones.However,studies on these subduction zones are relatively independent,so it is important to conduct systematic comparative studies.In this paper,we review the initiation models of Neogene subduction in the western Pacific,with the three typical types of subduction initiation models including polarity-reversal,induced subduction re-initiation,and noninherited subduction initiation to form new ruptures.In addition,the parameters of different subduction zones are collated to form five categories:basic features,subducting plate features,upper plate features,kinematic features,and subsequent activity.The regularity of the subduction processes,the specificity of the different subduction cases,and the possible constraints between the subduction initiation types and the characteristics of the subduction zone parameters are discussed and analyzed.The compiled dataset of the subduction zone parameters can provide data support for related studies.