Seismic slip recorded in tourmaline fault mirrors from Elba Island (Italy)

This paper reports the first example of fault mirrors developed in an unusual protolith, consisting of tourmaline crystals with interstitial goethite. The deformation mechanisms active in the fault zone have been investigated from the outcrop to the nanoscale, aiming to identify possible traces of frictional heating at seismic slip rate, as observed for other fault mirrors in different protoliths. The investigation revealed the superposition of two main deformational stages. The first was dominated by brittle processes and produced a cataclastic/ultracataclastic principal slip zone, a few mm thick; the second was associated with seismic slip and produced a sharp discontinuity (the principal slip surface) within the cataclastic/ultracataclastic zone. The mirror-like coating, a few microns thick, occurs on the principal slip surface, and is characterized by 1) absence of interstitial goethite; 2) occurrence of truncated tourmaline crystals; 3) highly variable grain size, from 200 μm to 200 nm; 4) tourmaline close packing with interlobate grain boundaries, and 5) tourmaline random crystallographic orientation.Micro and nanostructural investigations indicate the occurrence of thermally-activated processes, involving both interstitial goethite and tourmaline. In particular, close to the principal slip surface, goethite is completely decomposed, and produced an amorphous porous material, with local topotactic recrystallization of hematite. Tourmaline clasts are typically characterized by strongly lobate boundaries, indicative of reaction and partial decomposition at grain boundaries. TEM observations revealed the occurrence of tourmaline nanograins, a few tens of nm in size, characterized by rounded shape and fading amorphous boundaries, that cannot be obtained by brittle processes. Lastly, the peculiar interlobate microstructure of the mirror surface is interpreted as the result of grain boundary recrystallization processes taking place by deformation at high-T conditions. Our results show that tourmaline fault mirrors recorded localized high-T processes triggered by frictional heating and can be therefore considered as reliable traces of ancient earthquakes.     

从地震信息看红河-金沙江断裂带构造特征

红河—金沙江断裂带,根据地质结构的差异可分为北段羌塘向北叠瓦逆冲断裂带,中段三江—哀牢山向东叠瓦逆冲断裂带,南段河内—中建正反转走滑断裂带。北段和中段均由三组基本平行的叠瓦逆冲断层构成,显示以强烈挤压为特征,但后者相对较弱;南段则经历了古近纪由挤压反转为拉张,形成一些断陷构造,新近纪又反转为挤压,断陷转变为坳陷,形成河内、莺歌海及中建等坳陷。该南段的主断层为正反转构造逆冲带的前锋太平断裂带,与原先人们所认为的不相同。红河—金沙江断裂带的挤压特征总体上由北往南逐渐减弱,其演化受特提斯构造活动所控制。断裂带的油气勘探成果表明,在喜马拉雅期形成的断坳结构盆地中油气显示较好。     

Late Pliocene–recent tectonic setting for the Tianchi volcanic zone, Changbai Mountains, northeast China

Mafic volcanic rocks have erupted in the Tianchi volcanic zone, Changbai Mountains, northeast China, since late Pliocene time. The zone formed in an extensional environment during early-middle Cenozoic time, and in a compressional environment during late Cenozoic. Crustal thickness (about 40 km) in the Changbai Mountains is larger than the regional average of 34–36 km to the northwest and southeast. The conduit for magma upwelling was not coincident with the NE-striking regional faults, but seem to be confined to a deep-seated NW–WNW-striking fault zone. Since the late Pliocene, the Tianchi volcanic zone was subjected to crustal uplift within an intracontinental, weakly compressional environment (with minor WNW–ESE shearing) related to the westward subduction of the West Pacific plate. The nature of this volcanism is not typical of active, subduction-related continental margin volcanism. The magmatic evolutionary process evolved from trachybasalt through basaltic trachyandesite, trachyte, and pantellerite.     

The transition from an active to a passive margin (SW end of the South Shetland Trench, Antarctic Peninsula)

The lateral ending of the South Shetland Trench is analysed on the basis of swath bathymetry and multichannel seismic profiles in order to establish the tectonic and stratigraphic features of the transition from an northeastward active to a southwestward passive margin style. This trench is associated with a lithospheric-scale thrust accommodating the internal deformation in the Antarctic Plate and its lateral end represents the tip-line of this thrust. The evolutionary model deduced from the structures and the stratigraphic record includes a first stage with a compressional deformation, predating the end of the subduction in the southwestern part of the study area that produced reverse faults in the oceanic crust during the Tortonian. The second stage occurred during the Messinian and includes distributed compressional deformation around the tip-line of the basal detachment, originating a high at the base of the slope and the collapse of the now inactive accretionary prism of the passive margin. The initial subduction of the high at the base of the slope induced the deformation of the accretionary prism and the formation of another high in the shelf—the Shelf Transition High. The third stage, from the Early Pliocene to the present-day, includes the active compressional deformation of the shelf and the base-of-slope around the tip-line of the basal detachment, while extensional deformations are active in the outer swell of the trench.     

Radon concentrations in soil gas,considering radioactive equilibrium conditions with application to estimating fault-zone geometry

A calculation method for determining the amount of Rn isotopes and daughter products at the start of measurement (CRAS) is proposed as a more accurate means of estimating the initial Rn concentration in soil gas. The CRAS utilizes the decay law between ²²²Rn and ²²⁰Rn isotopes and the daughter products ²¹⁸Po and ²¹⁶Po, and is applicable to α-scintillation counter measurements. As Rn is both inert and chemically stable, it is useful for fault investigation based on the soil gas geochemistry. However, the total number of α particles emitted by the decay of Rn has generally been considered to be proportional to the initial Rn concentration, without considering the gas condition with respect to radioactive equilibrium. The CRAS method is shown to be effective to derive Rn concentration for soil gases under both nonequilibrium conditions, in which the total number of decays increases with time, and equilibrium conditions, which are typical of normal soil under low gas flux. The CRAS method in conjunction with finite difference method simulation is applied to the analysis of two active fault areas in Japan, and it is demonstrated that this combination could detect the sharp rises in ²²²Rn concentrations associated with faults. The method also allows the determination of fault geometry near the surface based on the asymmetry variation of the Rn concentration distribution when coupled with a numerical simulation of ²²²Rn transport. The results for the new method as applied to the two case studies are consistent with the data collected from the geological survey. It implies that the CRAS method is suitable for investigating the fault system and interstitial gas mobility through fractures. The present analyses have also demonstrated that high Rn concentrations require the recent and repeated accumulation of ²²²Rn parents (²³⁰Th and ²²⁶Ra) in fault gouges through deep gas release during fault movement.     

Early Permian strike-slip basin formation and felsic volcanism in the Manning Group,southern New England Orogen,eastern Australia

The Manning Group is characterised by rapidly filled strike-slip basins that developed during the early Permian along the Peel--Manning Fault System in the southern New England Orogen. Typically, the Manning Group has been difficult to date owing to the lack of fossiliferous units or igneous rocks. Thus, the timing of transition from an accretionary convergent margin in the late Carboniferous to dominantly strike-slip tectonic regimes that involved development and emplacement of the Great Serpentinite Belt (Weraerai terrane) is not well constrained. One exception are rhyolites of the Ramleh Volcanics that were erupted into the Echo Hills Formation. These developed along the dextral Monkey Creek Fault splay east of the Peel--Manning Fault System. Zircons extracted from the Ramleh Volcanics yield a U–Pb (SHRIMP) age of 295.6?±?4.6?Ma that constrains the minimum age of deposition in this basin to earliest Permian. Whole-rock geochemistry indicates these are peraluminous felsic melts enriched in LREE and incompatible elements with strong depletions in U, Nb, Sr and Ti. These are similar in age and composition to the nearby S-type Bundarra and Hillgrove plutonic supersuites. We suggest that extensive movement along the east-dipping Peel--Manning Fault System was responsible, not only for strike-slip basin development at the surface (Manning Group), but was also the locus for crustal melting that was responsible for generating S-type felsic melts that utilised hanging-wall fault splays as conduits to the surface or to coalesce in the crust as batholiths exclusively to the east of the Peel--Manning Fault System.     

Introduction

Manisa Fault is a geomorphologically distinct normal dip-slip fault, which oversees the southern edge of the Manisa Graben that is a continuum of the Gediz Graben towards the west. This study aims to determine the neotectonic activity of the Manisa Fault and the most recent time of the change in its stress condition through age-dating data obtained by using ²³⁰Th/²³⁴U dating methodology applied on the calcite coating that develops over hanging-wall of the Manisa Fault and the calcite veins that occur as fracture fillings. The age of the calcite precipitations associated with the Manisa Fault was determined to be between 307?±?203 and 444?±?101?ka by using the ²³⁰Th/²³⁴U dating method. Evaluation of the carbonate precipitations on the Manisa Fault along with the age data and the kinematic indicators ascertained that the Manisa Fault switched to a dip-slip normal faulting character from Middle Pleistocene onwards and that the region was under the effect of a NE–SW directional extensional regime. In addition, the opening rate was attempted to be determined using the roll-over anticline structure that advanced depending upon the movement of the fault on the upper horizontal strata of colluviums, which developed in association with the Manisa Fault. Along with the evaluation of the rise in the horizontal stratification in colluvium and the obtained age data, opening rate of the Manisa Fault was determined as 0.01?mm?y^?1.     

基于形变观测分析中国台湾东部花东纵谷断层运动特征

花东纵谷断层是中国台湾动力作用和地壳运动变形最强烈的断层之一,其断层运动特征和强震危险程度一直备受学者的关注。文中分别以同震地表位移、1992-1999年震间形变数据为约束,反演2003年成功MW 6.8地震同震位错分布和花东纵谷断层震间运动特征。结果表明：花东纵谷断层北段处于强闭锁状态（闭锁率高达0.9）,闭锁深度深（约27 km）;南段闭锁程度较弱（闭锁率约0.5）,闭锁深度较浅（约12 km）;中段闭锁程度与闭锁深度介于南北段之间。另一方面,2003年成功MW 6.8地震微观震中位于震间无震滑移区与闭锁区的过渡带附近。依据同震位错、震间断层运动反演结果,以及历史强震破裂分布特征,分析认为,花东纵谷断层南北段运动方式存在差异性,北段主要以强震形式运动,南段以蠕滑和地震两种形式运动。自1951年花莲-台东ML 7.3地震序列后,花东纵谷断层南段、中段和北段至2016年所累积的矩能量分别等价MW 6.4、MW 7.0、MW 7.4地震;若发生级联破裂,整个断层至2016年所累积的矩能量等价MW 7.5地震。     

宁河地区地震探测结果与桐城断裂浅部结构特征初探

桐城断裂是一条对唐山地震有地壳形变反映的活动断裂,是沧县隆起与黄骅凹陷北部的分界线。本次研究在桐城断裂宁河跨断层场地采用浅层地震探测方法,布设了两条近南北向的浅层人工地震物探剖面,获得了两条反映不同深度的高质量叠加时间、深度剖面图,查明了目标断裂的结构特征。本次工作在研究区探测出7条断裂,发现桐城断裂是由多条主断裂和次级断裂组成的断裂带,但本次工作测线布设相对较短且探测深度有限,因此,通过总结相关部门专家和学者在研究区周边的地震探测成果,初步认为断裂带在浅部呈"花状",并向下呈汇聚趋势。另外,结合前人学者对研究区及周边的钻孔和地层研究工作,通过钻孔地层对比,认为桐城断裂是一条中更新世活动断裂。根据断裂上断点的准确坐标位置以及桐城断裂产状特征,发现宁河跨断层监测场地有效的对F2、F3和F4三条断裂进行了监控,并根据前人对研究区应力应变研究结果,认为宁河跨断层监测场地的巨幅形变反映了F2和F3两条断裂的垂直运动特征。但其形变特征与唐山地震的关系是需要进一步探索的。     

南海北部阳江‒一统暗沙断裂带与新近纪岩浆活动

阳江?一统暗沙断裂带是南海北部珠江口盆地极其重要的中?新生代构造带和转换带.基于钻井资料和大范围、高密度的二维、三维地震资料,本文初步揭示阳江?一统暗沙断裂带走向为NW-NWW向、宽约30 km,沿着断裂走向从陆架至洋陆边界断裂带可分为北?中?南三段,断裂在新生代选择性活化,具有多重走滑断裂叠合和基底岩浆底辟强烈等特点...