Transpressive duplex and flower structure: Dent Fault System, NW England

Revised mapping along the Dent Fault (northwest England) has improved the resolution of folds and faults formed during Variscan (late Carboniferous) sinistral transpression. A NNE-trending east-down monocline, comprising the Fell End Syncline and Taythes Anticline, was forced in Carboniferous cover above a reactivated precursor to the Dent Fault within the Lower Palaeozoic basement. The Taythes Anticline is periclinal due to interference with earlier Acadian folds. The steep limb of the monocline was eventually cut by the west-dipping Dent Fault. The hangingwall of the Dent Fault was dissected by sub-vertical or east dipping faults, together forming a positive flower structure in cross-section and a contractional duplex in plan view. The footwall to the Dent Fault preserves evidence of mostly dip-slip displacements, whereas strike-slip was preferentially partitioned into the hangingwall faults. This pattern of displacement partitioning may be typical of transpressive structures in general. The faults of the Taythes duplex formed in a restraining overlap zone between the Dent Fault and the Rawthey Fault to the west. The orientations of the duplex faults were a response to kinematic boundary conditions rather than to the regional stress field directly. Kinematic constraints provided by the Dent and neighbouring Variscan faults yield a NNW–SSE regional shortening direction in this part of the Variscan foreland.     

寺口子流动水准趋势分析

本文利用带线性项的富氏多项式最小二秉拟合分析法,处理了寺口子水库区22期跨新层流资料。结果表明,在寺口子水库蓄水后,穿过库区的原压性逆断层,具有了似正断层的活动特征。进一步分析认为,这是由于压力水的浮力、区域挤压构造应力和水体载荷静压力共同作用的结果,而压力水的浮力在库区断层活动中起着主导作用。     

Refining the image of the San Andreas Fault near Parkfield, California using a finite difference travel time computation technique

The Parkfield Area Seismic Observatory (PASO) was a dense, telemetered seismic array that operated for nearly 2 years in a 15 km aperture centered on the San Andreas Fault Observatory at Depth (SAFOD) drill site. The main objective of this deployment was to refine the locations of earthquakes that will serve as potential targets for SAFOD drilling and in the process develop a high (for passive seismological techniques) resolution image of the fault zone structure. A challenging aspect of the analysis of this data set was the known existence of large (20–25%) contrasts in seismic wavespeed across the San Andreas Fault. The resultant distortion of raypaths could challenge the applicability of approximate ray tracing techniques. In order to test the sensitivity of our hypocenter locations and tomographic image to the particular ray tracing and inversion technique employed, we compare an initial determination of locations and structure developed using a coarse grid and an approximate ray tracer [Thurber, C., Roecker, S., Roberts, K., Gold, M., Powell, M.L. , and Rittger, K., 2003. Earthquake locations and three-dimensional fault zone structure along the creeping section of the San Andreas fault near Parkfield, CA: Preparing for SAFOD, Geophys. Res. Lett., 30 3, 10.1029/2002GL016004.] with one derived from a relatively fine grid and an application of a finite difference algorithm [Hole, J.A., and Zelt, B.C., 1995. 3-D finite-difference reflection traveltimes, Geophys. J. Int., 121, 2, 427–434.]. In both cases, we inverted arrival-time data from about 686 local earthquakes and 23 shots simultaneously for earthquake locations and three-dimensional Vp and Vp/Vs structure. Included are data from an active source seismic experiment around the SAFOD site as well as from a vertical array of geophones installed in the 2-km-deep SAFOD pilot hole, drilled in summer 2002. Our results show that the main features of the original analysis are robust: hypocenters are located beneath the trace of the fault in the vicinity of the drill site and the positions of major contrasts in wavespeed are largely the same. At the same time, we determine that shear wave speeds in the upper 2 km of the fault zone are significantly lower than previously estimated, and our estimate of the depth of the main part of the seismogenic zone decreases in places by  100 m. Tests using “virtual earthquakes” (borehole receiver gathers of picks for surface shots) indicate that our event locations near the borehole currently are accurate to about a few tens of meters horizontally and vertically.     

A mechanical model for deformation and earthquakes on strike-slip faults

A two-dimensional model for stress accumulation and earthquake instability associated with strike-slip faults is considered. The model consists of an elastic lithosphere overlying a viscous asthenosphere, and a fault of finite width with an upper brittle zone having an elastoplastic response and a lower ductile zone having an elastoviscoplastic response. For the brittle, or seismic, zone the behavior of the fault material is assumed to be governed by a relation which involves strain hardening followed by a softening regime, with strength increasing with depth. For the fault material in the ductile, or aseismic, section, the viscous effect is included through use of a nonlinear creep law, and the strength is assumed to decrease with depth. Hence, because of the lesser strength and the viscous effect, continuous flow occurs at great depths, causing stress accumulation at the upper portion of the fault and leading to failure at the bottom of the brittle zone. The failure is initially due to localized strain softening but, with further flow, the material above the softened zone reaches its maximum strength and begins to soften. This process accelerates and may result in an unstable upward rupture propagation.Relations are developed for the history of deformation within the lithosphere, specifically for the velocity of particles within the fault and at the ground surface. The boundary-element method is used for a quantitative study, and numerical results are obtained and compared with the recorded surface deformation of the San Andreas fault. The effects of geometry and material properties on instability, on the history of the surface deformation, and on the earthquake recurrence time are studied. The results are presented in terms of variations of ground-surface shear strain and shear strain rate, and velocity of points within the fault at various times during the earthquake cycle.It is found that the location of rupture initiation, the possibility of a sudden rupture as opposed to stable creep, and also the ground deformation pattern and its history, all critically depend on the mechanical response of the material within the fault zone, especially that of the brittle section. Shorter earthquake recurrence times are obtained for shallower brittle zones and for a stiffer lithosphere. Lower viscosities of the aseismic zone and the absence of asthenospheric coupling tend to suppress instability and promote stable creep. The model results thus suggest that the overall viscosity of the ductile creeping zone must exceed a minimum value for a sudden upward propagating rupture to take place within the seismic section.     

郯庐断裂带肥东段早白垩世中期走滑运动的年代学证据

郯庐断裂带肥东段西韦地区和桃花源地区出露了大规模的北北东向韧性剪切带。桃花源地区韧性剪切带显示出2期构造变形的叠加。野外构造和显微构造分析皆指示为左行走滑韧性剪切带。糜棱岩中石英与长石的变形行为指示其变形温度分别为400~450℃和500℃。通过对这两处走滑剪切带内糜棱岩化花岗岩脉的锆石LA-ICP-MS定年,获得了3个样品的侵位年龄分别为(133.2±1.9)Ma,(131.3±2.0)Ma,(130.3±2.0)Ma。再结合已有的研究成果,认为在128~124Ma(早白垩世中期)郯庐断裂带发生过左行走滑活动。综合分析表明,郯庐断裂带在晚侏罗世和早白垩世中期分别经历了2期左行走滑活动,而期间和之后的早白垩世则处于伸展活动之中。伸展活动持续较长,控制发育了西侧的合肥盆地及断裂带内一系列岩浆活动;而区域挤压背景下出现的走滑活动则相对短暂。这些演化规律显示该断裂带在晚侏罗世—早白垩世呈现为交替式的走滑和伸展活动。新发现的早白垩世中期走滑活动,与太平洋区伊泽纳崎板块板块运动方向的调整相对应,是大洋板块运动方向短暂调整的构造响应。     

Newly found Tunglo Active Fault System in the fold and thrust belt in northwestern Taiwan deduced from deformed terraces and its tectonic significance

We found active faults in the fold and thrust belt between Tunglo town and the Tachia River in northwestern Taiwan. The surface rupture occurred in 1999 and 1935 nearby the study area, but no historical surface rupture is recorded in this area, suggesting that the seismic energy has been accumulated during the recent time. Deformed fluvial terraces aid in understanding late Quaternary tectonics in this tectonically active area. This area contains newly identified faults that we group as the Tunglo Fault System, which formed after the area's oldest fluvial terrace and appears at least 16 km long in roughly N–S orientation. Its progressive deformations are all recorded in associated terraces developed during the middle to late Quaternary. In the north, the system consists of two subparallel active faults, the Tunglo Fault and Tunglo East Fault, striking N–S and facing each other from opposite sides of the northward flowing Hsihu River, whose course may be controlled by interactions of above-mentioned two active faults. The northern part of the Tunglo Fault, to the west of the river, is a reverse fault with upthrown side on the west; conversely the Tunglo East Fault, to the east, is also a reverse fault, but with upthrown side on the east. Both faults are marked by a flexural scarp or eastward tilting of fluvial terraces. Considering a Quaternary syncline lies subparallel to the east of this fault system, the Tunglo Fault might be originated as a bending moment fault and the Tunglo East Fault as a flexural slip fault. However, they have developed as obvious reverse faults, which have progressive deformation under E–W compressive stress field of Taiwan. Farther south, a west-facing high scarp, the Tunglo South Fault, strikes NNE–SSW, oblique to the region's E–W direction of compression. Probably due to the strain partitioning, the Tunglo South Fault generates en echelon, elongated ridges and swales to accommodate right-lateral strike–slip displacement. Other structures in the area include eastward-striking portion of the Sanyi Fault, which has no evidence for late Quaternary surface rupture on this fault; perhaps slip on this part of Sanyi Fault ceased when the Tunglo Fault System became active.     

青藏高原东北隅强震构造模型

海原断裂带和中卫-同心断裂带是青藏高原东北隅二条主要断裂带。通过区域地质、地貌分析和二条断裂带的结构、活动历史的对比研究,建立了该区走滑和挤压活动的应变分配模型。通过对主要断裂带破裂分段的研究,确定了该区的强震破裂单元。应变分配和破裂分段是建立地震构造模型的二个方面。     

断层与幔源二氧化碳气藏的形成和分布——以渤海湾盆地济阳坳陷为例

幔源CO_2气的形成和分布与不同级别断层早白垩世以来的活动密切相关。郯庐断裂带是研究区最主要的成气断层,拆离断层和变换断层这些地壳断层是次要的成气断层,二者于早白垩世143Ma、124Ma、新生代~43Ma、~24Ma和~8Ma的走滑或伸展活动,以及与之准同时的新生代碱性玄武岩浆活动,控制了幔源CO_2气的分散和聚集。它们与基底断裂、盖层断裂共同组成运移通道,其中拆离滑脱处的低速带和盖层断裂中的顺层断层是重要的水平运移通道。早白垩世古太平洋板块俯冲脱水脱气,产生的幔源CO_2气沿着郯庐断裂带向上分散聚集;新生代以来受控于太平洋板块俯冲方向和速度的改变以及印欧板块碰撞的远程效应,形成幔源CO_2气。与此同时郯庐断裂带切割深度亦逐渐加大,~43Ma碱性岩浆活动亦开始形成幔源CO_2气并主要位于断裂带,24Ma和8Ma(5Ma)为新近纪碱性岩浆活动脱气两个主要形成时期。郯庐断裂带的活动使地幔脱气形成的CO_2沿断层走向向上运移,并在作为重要横向运移通道的拆离断层拆离滑脱处,与因岩浆脱气形成的CO_2汇合,再通过陡倾斜、缓倾斜基底断层、盖层断层的接力传递在浅部聚集成藏。预测郯庐断裂带附近是无机成因油气重要的聚集分布区带。     

南湾水库坝基渗透监测分析与稳定性研究

南湾水库坝基存在顺河断层,查明断层的结构、颗粒组成、透水性及允许渗透比降,判断其可能发生的渗流破坏形式是南湾水库除险加固工程的一个重点和难点。由于现场钻孔管涌试验所得的坝基岩体试验破坏坡降较大,不足以反映断层破碎带最差部位的岩体破坏坡降。在详细分析了南湾水库坝基顺河断层的工程地质条件和水文地质条件后,对坝体和坝基的长期渗透观测资料进行分析,并对坝址区顺河断层带进行渗透稳定性研究,结果表明坝体和坝基的渗透稳定,发生渗透破坏可能性较小。     

从水系的分布形态探讨沂沭断裂带的运动特征

郯城-庐江断裂带是纵贯中国大陆东部,长达2 000 km以上的大断裂带。该研究利用DEM数据和ETM,以及谷歌卫星影像来分析郯城-庐江断裂带中段的沂沭断裂带周边的水系的几何分布形态特征。三维卫星图像解译和野外调查的结果显示：(1)沿断裂带周边发育的水系的几何分布形态主要受由F1-F4 4条主要断层所组成的沂沭断裂带构造所制约;(2)沿断裂带发育的水系不具有典型的走滑断层的系统化的同步转折和位错的变形特征;(3)沂沭断裂带东侧的F1和F2断裂是现今的活动断裂,而F3和F4断裂在第四纪基本没有明显的活动迹象;(4)F1和F2断裂以垂向运动为主,具有高角度的逆断层或正断层的运动特征。该研究结果显示沿断裂发育的水系地貌和几何分布特征可作为评价断裂的活动性和运动特征的地貌标志。