Contrasts in Tectonic style of the Central and Southern Appalachians, United States: Insights from seismic reflection data

Two reflection seismic transects, one across the central Appalachians in Virginia and the other across the southern Appalachians in Georgia, reveal a significant contrast in mid- and lower crustal reflectivity from east to west. Data from east of the Blue Ridge geologic province in Virginia and to the east of the Inner Piedmont in Georgia show a highly reflective crust extending from the near-surface to the Moho, including zones of east-dipping reflections, a sub-horizontal reflection signature at 7 seconds, and a west-dipping Moho. Reflection seismic data from west of the Blue Ridge in Virginia and Inner Piedmont farther south are characterized by reflector geometries related to deformation above a master decollement, leading to classic ‘thin-skinned’ tectonic structures in the overlying allochthon, and few if any apparent structures in the underlying basement. The location of the Iapetan rifted margin, the preexistence of favorably oriented structures to the east of this point, and sub-horizontal weak zones within the lower Paleozoic shelf strata have played critical roles in the distribution of seismic reflector geometry. Seismic reflection signatures seen in the southeastern United States are a result of multiple episodes of deformation from the early Paleozoic through the middle Mesozoic. Oblique stresses during late Paleozoic time produced transpression that manifested itself as predominantly strike-slip faulting to the east of the Blue Ridge/Inner Piedmont. Onlapping lower Paleozoic shelf strata responded to tectonic stresses through thin-skinned deformation above a master decollement during the late Paleozoic Alleghanian orogeny, aided in part by sub-horizontal zones of weakness in the strata. This partitioning of strain was supported via tectonic buttressing provided by Precambrian continental crust that was little deformed in the Taconic orogeny. During the Alleghanian orogeny, the variations in Valley and Ridge deformational style between the central and southern Appalachians were controlled by the original shape of the continental edge. Further deformation during Mesozoic extension occurred to the east of the Precambrian rift margin in the region where favorably oriented faults were reactivated, leading to the rotation of the fault zones from more steeply dipping initial orientations, the merging of the mid-crustal reflection zone with the Moho, and the formation of Mesozoic basins and antiformal reflections in the seismic sections.     

High-resolution seismic imaging of crooked two-dimensional profiles in greenstone belts of the Canadian shield: results from the Swayze area,Ontario, Canada

In 2017, the Metal Earth multi-disciplinary exploration project acquired a total of 921 km of regional deep seismic reflection profiles and 184 km of high-resolution seismic reflection profiles in the Abitibi and Wabigoon greenstone belts of the Superior province of Canada. The Abitibi belt hosts several world-class mineral deposits, whereas the Wabigoon has sparse economic mineral deposits. Two high-resolution surveys in the Swayze area, a poorly endowed part of the western Abitibi greenstone belt, served as pioneer surveys with which to better understand subsurface geology and design a strategy to process other surveys in the near future. Swayze seismic data were acquired with crooked survey geometries along roads. Designing an effective seismic processing flow to address these geometries and complex geology required straight common midpoint lines along which both two-dimensional prestack dip-moveout correction and poststack migration processing were applied. The resulting seismic sections revealed steeply dipping and subhorizontal reflections; some correlate with folded surface rocks. An interpreted fault/deformation zone imaged in Swayze north would be a target for metal endowment if it extends the Porcupine–Destor structure. Because of the crooked line geometry of the surveys, two-dimensional /three-dimensional prestack time migration and swath three-dimensional processing were tested. The prestack time migration algorithm confirmed reflections at the interpreted base of the Abitibi greenstone belt. The swath three-dimensional images provided additional spatial details about the geometries of some reflections, but also had less resolution and did not detect many reflectors observed in two dimensions. Geological contacts between felsic, mafic and ultramafic greenstone rock layers are thought the main cause of reflectivity in the Swayze area.     

Crustal structure in Dabieshan UHP metamorphic belt and its tectonic implication

IntroductionGeologistsfirstlyfoundcoesite-bearingecologitesattheendof1980'sandthenthemicrodiamond(Xu,elal,1992)inDabieshanarea.Theultra-highpressure(UHP)metamorphismandthegeodynamicprocessesofDabieorogenhaveattractedmanygeoscientists(Wang,etal,1989,O...     

Crustal thickness variation across the northeast Iran continental collision zone from teleseismic converted waves

For the first time, we present the variation of crust–mantle boundary beneath the northeast Iran continental collision zone which is genetically part of the Alpine–Himalayan orogeny and beneath Central Iran which is a less-deformed tectonic block. The boundary was imaged by stacking teleseismic P–S converted phases and shows a strong variation of Moho from 27.5 km under Central Iran to 55.5 km beneath the Binalud foreland basin. The thickest crust is not located beneath the high topography of the Kopeh Dagh and Binalud mountain ranges suggesting that these mountain ranges are not supported by a crustal root. The simple gravity modeling of the Bouguer anomaly supports this idea.     

广西地震活动与我国主要地震区(带)地震活动的相关分析

应用灰色系统理论的关联分析方法研究了广西地震活动与我国主要地震区（带）地震活动的相关性．结果表明广西地震活动与东南沿海地震带地震活动的关联程度最大（ｒ＝０．８６）,根据这一结果,对广西与东南沿海地震带的地震活动的相关性作了进一步探讨,发现本世纪以来广西地震活动与东南沿海地震带的地震活动同步起伏,两区（带）之间发生中强地震的互相对应关系特别明显．     

Mesozoic subduction-accretion zone in northeastern South China Sea inferred from geophysical interpretations

The northern South China Sea (SCS) is an impor- tant offshore exploration area of China, producing over 10 million tons of oil from Cenozoic reservoirs annually since 1996. The increasing energy consump- tion in China demands a rapid expansion of hydrocar- bon exploration to new types, including pre-Cenozoic sources and reservoirs. This has stimulated a new phase of pre-Cenozoic researches in the area. So far more than 100 exploration wells have reached the pre-Cenozoic basement in northe…     

长江中下游成矿带及邻区三维Moho面结构:来自人工源宽角地震资料的约束

为深入理解长江中下游地区在中生代成矿的深部动力学过程,对跨越宁芜矿集区地质廊带内的非纵剖面反射/折射地震数据进行动校正和时深转换处理,获得了非纵方向的Moho面深度;联合纵测线和非纵测线上Moho面深度数据,获得了长江中下游成矿带及邻区的三维Moho面深度结构.结果显示宁芜矿集区下方的Moho面整体较浅,约32~34km,华北块体合肥盆地内Moho面整体较深,约34~35km.Moho面深度和区域布格重力异常变化趋势对应良好.宁芜矿集区下方Moho面呈上隆特征,支持长江中下游地区成矿模式中增厚岩石圈发生拆沉、软流圈的上隆及底侵作用等动力学过程.Moho面平行于成矿带走向的变化趋势,预示长江中下游成矿带地壳和上地幔在板块边界发生了NE-SW向的切向流动变形.郯庐断裂带两侧,Moho面深度变化较大,表明地表近陡立的郯庐断裂为深大断裂,深部可能切穿Moho面并延伸至上地幔.     

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.     

The velocity structure of the upper mantle in the transition zone from the East European Platform to Western Europe from seismic noise data

The surface wave tomography from ambient seismic noise recorded at stations in Western Europe (WE) and on the East European Platform (EEP) revealed the structure of the crust and upper mantle in the transitional zone from the Precambrian platform to the younger geological units in Western Europe. The Tornquist-Teisseyre Line separating these structures is clearly traced as a transition zone from the high velocities beneath EEP to the low velocities beneath WE in the crust and upper mantle, which extends to a depth of 150?C170 km. Below 200 km the relationship between the velocities beneath EEP and WE becomes the opposite. A similar relationship between the velocities in the upper mantle down to a depth of 300 km is observed on the southern boundary, where EEP borders on the northern segment of the Alpine-Himalayan seismic belt.     

Crustal Configuration of the Northwest Himalaya Based on Modeling of Gravity Data

The gravity response and crustal shortening in the Himalayan belt are modeled in detail for the first time in the NW Himalaya. The Bouguer gravity anomaly along a ~450-km-long (projected) transect from the Sub-Himalaya in the south to the Karakoram fault in the north across the Indus-Tsangpo Suture Zone is modeled using spectral analysis, wavelet transform and forward modeling. The spectral analysis suggests three-layer interfaces in the lithosphere at 68-, 34- and 11-km depths corresponding to the Moho, the Conrad discontinuity and the Himalayan decollement thrust, respectively. The coherence, admittance and cross spectra suggest crustal shortening because of convergence compensated by lithospheric folding at 536- and 178-km wavelength at the Moho and the upper-crustal level. An average effective elastic thickness of around 31 km is calculated using the coherence method. The gravity data are modeled to demarcate intracrustal to subcrustal regional thrust/fault zones. The geometrical constraints of these faults are obtained in the space scale domain using the wavelet transform, showing good correlation with the major tectonic boundaries. The crustal configuration along the transect shows how the Moho depth increases from 45 to 80 km towards the north with the locus of flexure of the Indian crust beneath the Higher Himalayan zone. The combination of forward modeling and wavelet analysis gives insight into the subsurface extent and geometry of regional structures across the NW Himalaya.     

西秦岭—东昆仑及邻近地区地壳结构——深地震宽角反射/折射剖面结果

在青藏高原东北缘,穿过阿尼玛卿缝合带东端完成了一条637 km的近南北向深地震宽角反射/折射剖面.获得的地壳结构剖面表明,该地区Moho界面埋深48～51 km,北浅南深,横向变化不大,而地壳内部构造在不同的地质构造块体差异明显.在下地壳内出现的两组能量较强的P₃、P₄波组,反映了研究区下地壳的反射性质和多层结构特征.阿坝弧形断裂以南和阿尼玛卿缝合带附近壳内界面变形强烈,壳内低速异常结构明显,特别是在缝合带下方20 km以下的中下地壳异常的低速结构可以解释为存在延伸至中下地壳的破碎带构造特征.在剖面南段反映西秦岭褶皱带至松甘块体相应的地震记录出现复杂、强烈的中下地壳反射和相对较弱的Moho反射震相是该地区地壳结构的明显特征.     

南北地震带中段深部构造与地震

南北地震带中段位于中国大腹部，是青藏块体、华北块体和扬子块体的中间枢纽部位。根据该区布格重力异常图，采用Ｐａｒｋｅｒ’ｓ三维位场方法进行莫氏面反演并计算该区不同窗口的剩余场推断和解释了条岩石圈断裂，进而研究它们与地震的关系。     

ANALYSIS ON STRUCTURAL FEATURES:INSIGHTS FROM SATELLITE-DERIVED GRAVITY MODEL AND SEISMIC PATTERN

In this paper, based on a large number of cumulative observational data from the seismic monitoring network in China, we grid the research area to calculate the density values at each grid node and convert the qualitative earthquake epicenter distribution to quantitative seismic pattern. Minimum magnitude of completeness(M_C)is determined by magnitude-rank analysis, which provides lower limit earthquake and original time. New satellite-derived gravity model v23.1, which is based on satellites CryoSat-2 and Jason-1 data, is used to determine the Bouguer gravity anomaly derived from free-air gravity anomaly and elevation database sets SRTM30, and ultimately, the complete Bouguer correction is obtained. In this paper, the Xingtai earthquake zone and Tanlu fault zone (Anhui segment) are selected for case study. Bouguer gravity anomaly presents a NE-trending U-shaped narrow strip in the Xingtai earthquake zone, and its location is consistent with Shulu Fault Basin. Grid density value contours are restricted by the U-shaped strip, and the extreme value of seismic activity density lies in the bottom of the U-shaped strip as shown in the cross section. The results of Bouguer gravity anomaly and upward continuations to the different heights show good linearity and gradient in the Tanlu fault zone (Anhui segment); and both long-axis direction of seismic pattern and nodal plane strike of seismogenic fault from focal mechanism solutions trend NNE. In short, the Tanlu fault zone(Anhui segment)is a large deep-seated fault that still has the ability to control seismic activity along it. Based on the measured gravity and magmatic data, using the edge detection TDX method to interpret the concealed boundary of the Anqing M4.8 earthquake near the Tanlu fault, and combining with the results from deep seismic reflection profiles of the study area, we discussed the causative fault of the Anqing earthquake.     

Volcanoes and regional geophysical trends

Active volcanoes occur in at least two fundamentally different tectonic settings. Taphrogenic volcanoes are aligned along the mid-ocean fracture system which is characterized by a broad ridge of rugged relief, «tensional» horizontal stress components perpendicular to the ridge, shallow earthquakes in a zone vertically beneath the ridge crest, thin to normal occanic crust and low to normal seismic velocities in the uppermost mantle, positive free-air gravity anomalies, and high heat flow. Orogenic volcanoes are aligned along the mobile Pacific rim and Indonesian archipelago which are characterized by double or single arcuate ridges with a deep oceanic trench on the convex side, compressional horizontal stress components perpendicular to the arcs, shallow to deep earthquakes in a zone dipping from the trench to beneath the volcanoes and beyond, transitional crustal thicknesses and seismic velocities, parallel belts of negative and positive free-air gravity anomalies from the trench to the volcanic arc, and low heat flow from the trench. The diverse nature of most geophysical lineaments associated with volcanism suggests that magma generation is independent of these phenomena. The remaining correlation of shallow earthquakes with taphrogenic volcanocs and intermediate depth earthquakes with orogenic volcanoes suggests that active fracture systems reaching these depths can tap latent magma sources. Seismic evidence for a low velocity layer beginning 100 km beneath continents and 60 km beneath oceans gives independent support to this hypothesis.     

渭河盆地及邻区重力异常小波多尺度分解与解释

基于EGM2008重力场模型计算获得了渭河盆地及邻区布格重力异常。采用小波多尺度分解方法对布格重力异常进行了4阶小波逼近和小波细节分解,同时基于平均径向对数功率谱方法定量化地计算出1~4阶小波细节和小波逼近所对应的场源平均埋深。结合区域地质和地震资料,对获得的重力场结果进行分析,得到如下结论:①鄂尔多斯地块、渭河盆地、秦岭造山带3个一级构造单元的布格重力异常之间存在明显差异;构造区内部重力异常也存在横向的显著差异。布格重力异常的走向、规模、分布特征与二级构造区及主要的断裂具有一定的对应关系。②渭河盆地及邻区布格重力异常1~4阶细节对应4~23 km不同深度的场源信息,鄂尔多斯地块南缘东、西部的地壳结构存在明显的差异;渭河盆地凹陷、凸起构造区边界清晰,断裂边界与重力异常边界具有较好的一致性;秦岭造山带重力异常连贯性不好,东、西部重力异常变化特征表现出明显的差异。③渭河盆地及邻区布格重力异常分布与莫霍面埋深具有非常明显的镜像关系。渭河盆地及邻区地震主要分布在六盘山—陇县—宝鸡断裂带、渭河断裂与渭南塬前断裂交汇处、韩城断裂与双泉—临猗断裂交汇处。渭河盆地及邻区重力异常主要由中上地壳剩余密度体所影响,这可能是该区地震以浅源地震为主的主要原因。     

南北地震带南段地壳厚度重震联合最优化反演

重力反演方法是研究地壳结构和物性界面起伏的有效地球物理手段之一.本文收集了南北地震带南段67个已有的固定台站接收函数反演的Moho面深度结果,并使用基于EGM2008重力异常模型计算的布格重力异常,验证了本文提出的重震联合密度界面反演方法的有效性.利用接收函数对台站下方Moho面深度估计作为先验约束,定义了一类评价函数,通过对重力反演算法中尺度因子,平移因子和稳定性因子的最优选择,最小化重力反演结果与接收函数模型之间的差异.结果表明,本文提出的方法,可以有效地同化不同地球物理方法获得的反演模型,且通过重震联合反演可以改进由于对空间分布不均匀的接收函数结果插值可能而引起的误差.本文还通过引入Crust1.0的Moho面深度为初值,同时考虑地壳密度的横向不均匀分布,通过模型之间的联合反演有效改善了地球物理反演模型间的不一致性问题.本文反演得到的最优化Moho面深度模型与已知67个台站位置接收函数模型之间的标准差约1.9km,小于Crust1.0与接收函数结果模型之间标准差为3.73km的统计结果.本文研究结果对于同化重震反演结果、精化地壳密度界面模型,都具有十分重要的参考意义.     

Crust and upper mantle structure of the Ailao Shan-Red River fault zone and adjacent regions

Using arrival data of the body waves recorded by seismic stations, we reconstructed the velocity structure of the crust and upper mantle beneath the southeastern edge of the Tibetan Plateau and the northwestern continental margin of the South China Sea through a travel time tomography technique. The result revealed the apparent tectonic variation along the Ailao Shan-Red River fault zone and its adjacent regions. High velocities are observed in the upper and middle crust beneath the Ailao Shan-Red River fault zone and they reflect the character of the fast uplifting and cooling of the metamorphic belt after the ductile shearing of the fault zone, while low velocities in the lower crust and near the Moho imply a relatively active crust-mantle boundary beneath the fault zone. On the west of the fault zone, the large-scale low velocities in the uppermost mantle beneath western Yunnan prove the influence of the mantle heat flow on volcano, hot spring and magma activities, however, the upper mantle on the eas     

THE DEPTH OF MOHO INTERFACE AND CRUSTAL THICKNESS IN SICHUAN-YUNNAN REGION,CHINA

By using moving average method to separate Bouguer gravity anomaly field in Sichuan-Yunnan region, we got the low-frequency Bouguer gravity anomaly field which reflects the undulating of Moho interface. The initial model is obtained after seismic model transformation and elevation correction. Then, we used Parker method to invert the low-frequency Bouguer gravity anomaly field to obtain the depth of Moho interface and crustal thickness in the area. The results show that the Qinghai-Tibet block in the northwest of the study area deepens and thickens from the edge to the interior, with the depth of Moho interface and the crust thickness of about 52~62km and 54~66km, respectively. The depth of Moho interface in Sichuan Basin is about 38~42km. In Sichuan-Yunnan block, the depth of Moho interface is about 42~62km from southeast to northwest. Beneath the West Yunnan block, west of the Red River fault zone, the Moho depth is about 34~52km from south to north. The Longmen Mountains and Red River fault zone are the gradient zone of the Moho depth change. Along the Red River fault zone, the depth difference of Moho interface is increasing gradually from north to south. No obvious uplift is found on the Moho interface of Panzhihua rift valley. The depth of Moho interface distribution in Sichuan and Yunnan is obviously restricted by the collision between the Indian plate and the Eurasian plate and the lateral subduction of the Indo-China peninsula. The mean square error of the depth of Moho interface is less than 1.7km between the result of divisional density interface inversion and artificial seismic exploration. At the same time, we compared the integral with divisional inversion result. It shows that:in areas where there is obvious difference between the crust velocity and density structure in different tectonic blocks, the use of high resolution seismic exploration data as the constraints to the divisional density interface inversion can effectively improve the reliability of inversion results.     

芦山—康定地区布格重力异常及其归一化梯度图像的构造物理涵义

芦山—康定地区是川滇块体、松潘—甘孜块体和华南块体三个块体过渡的"Y"型交汇区,构造变形十分强烈.本文对EGM2008计算的布格重力异常进行1~5阶离散小波变换,得到三方向分量平方和的平方根(HVDM)图像;利用实测剖面布格重力异常数据,得到剖面的布格重力异常归一化总梯度(NFG)图像.结果分析表明:(1)垂直于龙门山断裂带南段剖面的NFG图像显示推覆构造体前端切割较浅、后端逐步变深至中地壳,说明松潘—甘孜块体在深约10~30km之间存在滑脱构造,在青藏高原东向挤出和四川盆地的阻挡作用下,造成深、浅部构造差异性运动,形成逆冲推覆的龙门山构造带;(2)HVDM图像和剖面的NFG图像均显示龙门山断裂带西南段与中段和东北段不同,松潘—甘孜块体对四川盆地的逆冲推覆作用沿北东方向具有分段性;(3)雅江—洪雅剖面NFG图像显示鲜水河断裂带和龙门山断裂之间存在高梯度变化带,在鲜水河断裂带下方强变形带不仅在20km左右东倾至龙门山断裂带前缘,且逐渐近垂直向下伸入至少到下地壳,反映了两大断裂带交汇区域变形作用较强.川滇块体内部和四川盆地内部则显示低值,说明其变形作用较弱.强烈左旋剪切的鲜水河断裂带对芦山—康定地区构造活动具有主要的控制作用.     

青藏高原东南缘多尺度重力场及构造含义

本文基于EGM2008重力场模型研究了青藏高原东南缘均衡重力异常和多尺度的布格重力异常特征,以鲁甸和景谷地震为例,认识其深部构造环境和动力学过程,为该区域的构造运动和地震孕育环境研究提供依据.结果表明,研究区布格重力异常和均衡重力异常与地质构造格局相关性较好,川滇地块剧烈的区域布格重力异常和非均衡状态与其强烈的地壳变形...