Active normal faulting during the 1997 seismic sequence in Colfiorito,Umbria: Did slip propagate to the surface?

In order to determine whether slip during an earthquake on the 26th September 1997 propagated to the surface, structural data have been collected along a bedrock fault scarp in Umbria, Italy. These collected data are used to investigate the relationship between the throw associated with a debated surface rupture (observed as a pale unweathered stripe at the base of the bedrock fault scarp) and the strike, dip and slip-vector. Previous studies have suggested that the surface rupture was produced either by primary surface slip or secondary compaction of hangingwall sediments. Some authors favour the latter because sparse surface fault dip measurements do not match nodal plane dips at depth. It is demonstrated herein that the strike, dip and height of the surface rupture, represented by a pale unweathered stripe at the base of the bedrock scarp, shows a systematic relationship with respect to the geometry and kinematics of faulting in the bedrock. The strike and dip co-vary and the throw is greatest where the strike is oblique to the slip-vector azimuth where the highest dip values are recorded. This implies that the throw values vary to accommodate spatial variation in the strike and dip of the fault across fault plane corrugations, a feature that is predicted by theory describing conservation of strain along faults, but not by compaction. Furthermore, published earthquake locations and reported fault dips are consistent with the analysed surface scarps when natural variation for surface dips and uncertainty for nodal plane dips at depth are taken into account. This implies that the fresh stripe is indeed a primary coseismic surface rupture whose slip is connected to the seismogenic fault at depth. We discuss how this knowledge of the locations and geometry of the active faults can be used as an input for seismic hazard assessment.     

南冲绳海槽地震反射波特征及其地质解释

对南冲绳海槽进行反射地震调查 ,结果表明 :(1 )海槽盖层主要由反射层组 (时代相当于第四纪 )和反射层组 (时代相当于上新世 )组成 ,推测槽底局部存在中新统。槽底沉积物主要源自中国大陆。轴部目前仍处在裂陷作用阶段 ;(2 )断裂极为发育 ,可分 NE— SW向(西南端转为近 E— W向 )和 NW— SE向两组 ,分别属张性及张扭性断裂 ,后者切割前者 ;(3)岩浆活动十分强烈 ,东南缘岩浆活动尤甚 ,推测其岩性以中、基性岩为主     

多地震活动性参数在断裂带现今活动习性与地震危险性评价中的应用与问题

为了分析将地震活动性参数用于判定断裂带现今活动习性、进而评估长期地震危险性的可行性,文中介绍了沿断裂带进行b值扫描与填图的方法,以及进行断裂带分段的多参数值(—b、E、n和—a/—b)组合分析的资料处理、计算及分析步骤;提出了为进行多参数计算的、断裂带分段的参考判据,进而概括了根据计算的参数值、结合强震历史背景、现今地震分布综合分析断裂带现今活动习性空间差异的方法与思路。以5条地震活动水平和监测能力各异的断裂带为试验对象,基于归纳的方法综合分析了各断裂带现今活动习性的空间差异及潜在的地震危险段。文中还就断裂的震后调整运动与低b值的关系以及精定位的地震资料在参数计算中的合理使用等问题进行了讨论。主要认识为:以b值为主的若干地震活动性参数的空间分布可有效地用于断裂带现今活动习性及潜在强震危险段落的判定。沿断裂带b值扫描与填图以及断裂带分段多参数值组合分析两种方法,可分别应用于地震监测能力强和一般的地区;若在监测能力强的地区将两种方法结合起来使用,可获得更可靠的结果。晚期余震活跃或者大震后调整运动的断裂段也可表现出较低b或—b值的特点     

三角剪切断层传播褶皱作用理论与应用

基底断层在沉积盖层中传播所形成的褶皱形态难以用平行膝折褶皱理论进行解释,这在于两者的流变学性质有很大差异。Erslev提出了三角剪切断层传播褶皱理论,认为下伏断层的脆性强破裂变形为向上变宽的三角形分布式剪切所调节,三角形顶点固定于断层端点。Hardy和Ford拓展了这一理论并成功地建立数字模拟模型,Allmendinger进一步建立与完善了三角剪切的正演模型与反演方法。通过运动学模型预测结果与天然构造观察和相似模拟实验结果的对比分析,以及通过一系列力学模型对运动学模型的检验,三角剪切断层传播褶皱理论被证实并获得了广泛应用。对前陆盆地、克拉通盆地和走滑盆地的基底卷入型构造与走滑或斜向滑动构造,都可以应用三角剪切断层传播褶皱理论来分析变形样式及其分布特征。该理论可以有效地预测隐伏断层的初始破裂点、断层传播量与发育部位,已成功地应用于工程地质与地震灾害预报等方面。     

环太平洋俯冲带内双地震带及其成因机制研究进展

俯冲带作为地球最为庞大的循环系统的重要组成部分,已成为地球科学的研究热点之一.很多俯冲带,特别是环太平洋俯冲带内的中深源地震,在空间上呈明显的分层分布,并且各层地震具有不同的震源机制,即所谓的双地震带现象.本文简要介绍了环太平洋双地震带形态特征与震源机制的空间分布,并回顾了双地震带的几种成因模型.根据形态特征和震源机制的差异,中源深度的双地震带可以分为两类,其中,一类双地震带对应上、下二层分别为压缩和张性的地震分布;另一类双地震带的震源分布较浅,且其浅部地震以横向压缩为主.此外,日本本州东北俯冲带的地震分布可能是由三层地震带组成的,而且汤加、伊豆-小笠原地区还发现深源深度的双地震带.通过对双地震带的形态特征以及其热力学条件的研究,人们从抗弯作用、脱水脆化、相变断层等多方面,尝试建立解释双地震带成因的模型.目前,大多数研究利用数值计算结果,结合蛇纹石脱水脆化、相变断层模型,能够不同程度地分别解释中源和深源双地震带成因.不过,这些模型几乎相互独立,并不能同时解释中源和深源双地震带.有人试图尝试用统一模型解释中深源地震成因,例如,先前存在的断层模型,不过该模型还不很具有说服力.也可能是多种因素的联合作用,共同影响着俯冲板内的温度场、应力场分布.     

像素图数学基础变换方法研究

针对像素图数学基础变换问题 ,提出了一种间接变换思路 ,给出了具体的变换方法 ,包括整块变换和分块变换及拼接等问题 ,并作了相关的实验。     

川滇交界地区地壳结构及现代地壳活动模式

根据地球物理异常及大地测深资料探讨川滇交界地区的地壳深部构造背影及地壳结构特征,并综合多方面的研究资料,以两板块碰撞、青藏高原窿升为构造背景,以川滇菱形断块运动为基本模式,全面系统地揭示川滇交界面区的现代地壳活动性。从地壳运动图象中可以清楚地看到断块差异性活动是现代地壳活动主要形式。     

论我国微细浸染型金矿床与沉积盆地演化的关系——以右江盆地为例

对华南右江盆地中某些微细浸染型金矿床开展了系统的地质学、组构学以及矿床地球化学研究。地质学和组构学研究表明，这类矿床具有大量的沉积阶段－成岩阶段的成因特征，在沉积物沉积－成岩期间就已经成矿。矿化与碳酸盐岩孤台的密切联系和丰富的成岩期软变形组构，表明成矿与控制海底地形的同沉积期断裂活动密切相关。而大量泄水构造和液化层理的出现则表明，在沉积物成岩阶段，沉积柱中曾有大量流体的活动。而且这种盆地流体的活动与成矿有密切关系。丰富的生物－有机成因组构则显示，成矿与沉积有机质的演化有着密切的内在联系。另外，矿床没有显示出与岩浆活动的任何关系。因此，右江盆地的微细浸染型金矿床与美国的卡林型金矿床不同，可能是离散大陆边缘张性盆地演化过程中盆地流体活动的产物之一。这一认识也得到地球化学研究不同程度的支持。     

Late Cenozoic faulting in SW Bulgaria: Fault geometry,kinematics and driving stress regimes. Implications for late orogenic processes in the Hellenic hinterland

We investigate the geometry and kinematics of the faults exposed in basement rocks along the Strouma River in SW Bulgaria as well as the sequence of faulting events in order to place constraints on the Cenozoic kinematic evolution of this structurally complex domain. In order to decipher the successive stress fields that prevailed during the tectonic history, we additionally carried out an analysis of mesoscale striated faults in terms of paleostress with a novel approach. This approach is based on the P–T axes distribution of the fault-slip data, and separates the fault-slip data into different groups which are characterized by kinematic compatibility, i.e., their P and T axes have similar orientations. From these fault groups, stress tensors are resolved and in case these stress tensors define similar stress regimes (i.e., the orientations of the stress axes and the stress shape ratios are similar) then the fault groups are further unified. The merged fault groups after being filled out with those fault-slip data that have not been incorporated into the above described grouping, but which present similar geometric and kinematic features are used for defining the final stress regimes. In addition, the sequence of faulting events was constrained by available tectonostratigraphic data.Five faulting events named D1, D2, D3, D4 and D5 are distinguished since the Late Oligocene. D1 is a pure compression stress regime with σ₁ stress axis trending NNE-SSW that mainly activated the WNW-ESE to ENE-WSW faults as reverse to oblique reverse and the NNW-SSE striking as right-lateral oblique contractional faults during the Latest Oligocene-Earliest Miocene. D2 is a strike-slip − transpression stress regime with σ₁ stress axis trending NNE-SSW that mainly activated the NNW-SSE to N-S striking as right-lateral strike-slip faults and the ENE-WSW striking faults as left-lateral strike-slip ones during the Early-Middle Miocene. D3 extensional event is associated with a NW-SE to WNW-ESE extension causing the activation of mainly low-angle normal faults of NE-SW strike and NNE-SSW to NNW-SSE striking high-angle normal faults. D4 is an extensional event dated from Late Miocene to Late Pliocene. It activated NNW-SSE to NW-SE faults as normal faults and E-W to WNW-ESE faults as right-lateral oblique extensional faults. The latest D5 event is an N-S extensional stress regime that dominates the wider area of SW Bulgaria in Quaternary times. It mainly activated faults that generally strike E-W (ENE-WSW and WNW-ESE) normal faults, along which fault-bounded basins developed. The D1 and D2 events are interpreted as two progressive stages of transpressional tectonics related to the late stages of collision between Apulia and Eurasia plates. These processes gave rise to the lateral extrusion of the Rhodope and Balkan regions toward the SE along the Strouma Lineament. The D3 event is attributed to the latest stage of this collision, and represents the relaxation of the overthickened crust along the direction of the lateral extrusion. The D4 and D5 events are interpreted as post-orogenic extensional events related to the retreat of the Hellenic subduction zone since the Late Miocene and to the widespread back-arc Aegean extension still prevailing today.     

Formation of lode‐style gold mineralisation during Tabberabberan wrench faulting at Lefroy,eastern Tasmania

The Lefroy Goldfield in eastern Tasmania is anomalous in southeastern Australia because mineralised fault reefs (i.e. reefs that are also faults) strike in an easterly direction at a high angle to the predominantly northwest strike of bedding and folds. Gold mineralisation is of Early to Middle Devonian age, with reef formation coinciding with a third regionally compressive deformation event (D₃), and a second phase of Tabberabberan orogenesis. Mineralised reefs are hosted by Mathinna Supergroup turbidites of Cambrian to Ordovician age and extend for up to 2 km across the boundary between the sandstone‐dominated Stony Head Sandstone and the shale‐dominated Turquoise Bluff Slate. Ore shoots in the reefs plunge moderately west and, in the Volunteer Mine, coincide with the intersection of the reef and a D₁/D₂ thrust contact. The subvertical orientation and discordant relationship of the mineralised reefs to bedding, as well as the lack of gold mineralisation along bedding and pre‐D₃ structures, indicate that the reefs formed during a period of wrench faulting. In contrast to lode‐style deposits in Victoria, the far‐field minimum compressive stress at Lefroy during reef formation was not vertical but, rather, occupied a subhorizontal orientation.