The influence of faults in basin-fill deposits on land subsidence,Las Vegas Valley,Nevada, USA

The role of horizontal deformation caused by pumping of confined-aquifer systems is recognized as contributing to the development of earth fissures in semiarid regions, including Las Vegas Valley, Nevada. In spite of stabilizing water levels, new earth fissures continue to develop while existing ones continue to lengthen and widen near basin-fill faults. A three-dimensional granular displacement model based on Biot's consolidation theory (Biot, MA, 1941, General theory of three-dimensional consolidation. Jour. Applied Physics 12:155–164) has been used to evaluate the nature of displacement in the vicinity of two vertical faults. The fault was simulated as (1) a low-permeability barrier to horizontal flow, (2) a gap or structural break in the medium, but where groundwater flow is not obstructed, and (3) a combination of conditions (1) and (2). Results indicate that the low-permeability barrier greatly enhances horizontal displacement. The fault plane also represents a location of significant differential vertical subsidence. Large computed strains in the vicinity of the fault may suggest high potential for failure and the development of earth fissures when the fault is assumed to have low permeability. Results using a combination of the two boundaries suggest that potential fissure development may be great at or near the fault plane and that horizontal deformation is likely to play a key role in this development. Electronic Publication     

迂里矿段铜铅锌银多金属矿体地质特征及其矿床成因

苏州迂里矿区位于苏州西部太湖之滨，其中迂里矿段铜、铅、锌、银多金属矿体具有埋藏较深，规模较小，品位较高及可综合利用元素较多的特点。矿体受地层岩性、构造、岩体等因素控制，确定了成矿的5个规律。     

Fault zone development and strain partitioning in an extensional strike-slip duplex: A case study from the Mesozoic Atacama fault system, Northern Chile

Upper crustal strike-slip duplexes provide an excellent opportunity to address the fundamental question of fault zone development and strain partitioning in an evolving system. Detailed field mapping of the Mesozoic Atacama fault system in the Coastal Cordillera of Northern Chile documents the progressive development of second- and third-order faults forming a duplex at a dilational jog between two overstepping master faults: the sinistral strike-slip, NNW-striking, Jorgillo and Bolfin faults. These are constituted by a meter-wide core of foliated S-C ultracataclasite and cataclasite, flanked by a damage zone of protocataclasite, splay faults and veins. Lateral separation of markers along master faults is on the order of a few kilometers. Second-order, NW-striking, oblique-slip subsidiary fault zones do not show foliated ultracataclasite; lateral sinistral separations are in the range of  10 to 200 m with a relatively minor normal dip-slip component. In turn, third-order, east–west striking normal faults exhibit centimetric displacement. Oblique-slip (sinistral–normal) fault zones located at the southern termination of the Bolfin fault form a well-developed imbricate fan structure. They exhibit a relatively simple architecture of extensional and extensional-shear fractures bound by low displacement shear fractures. Kinematic analysis of fault slip data from mesoscopic faults within the duplex area, document that the NW-striking and the EW-striking faults accommodate transtension and extension, respectively. Examination of master and subsidiary faults of the duplex indicates a strong correlation between total displacement and internal fault structure. Faults started from arrays of en echelon extensional/extensional-shear fractures that then coalesced into throughgoing strike-slip faults. Further displacement leads to the formation of discrete bands of cataclasite and ultracataclasite that take up a significant part of the total displacement. We interpret that the duplex formed by progressive linkage of horsetail-like structures at the southern tip of the Bolfin fault that joined splay faults coming from the Jorgillo and Coloso faults. The geometry and kinematics of faults is compared with that observed in analog models to gain an insight into the kinematic processes leading to complex strike-slip fault zones in the upper crust.     

Insights from numerical modeling on the hydrodynamics of non-radial flow in faulted media

The objective of this work is to explore the use of flow dimensions as a tool for characterizing hydraulic conditions in faulted media. Transient flow is numerically simulated in synthetic vertically-faulted reservoirs. Analysis of the obtained time series following the Generalized Radial Flow (GRF) model displays combined radial and fractional signals with a flow dimension n=1.5$n=1.5$. Investigating the transient geometry of the frontal equipotential surface shows that fractional flow occurrence is due to abnormal fault diffusion as a consequence of water supply from the matrix under specific conditions. An original hydrodynamical explanation for fractional flow in vertically faulted media is suggested, along with a reinterpretation of the bilinear regime. It is shown that the GRF theory remains valid in such discontinuum as the fundamental relationship between n and the cross-flow area is satisfied. These results provide insights in the use of the flow dimension as a hydraulic diagnostic tool in faulted media.     

Distribution of crustal extension and regional basin architecture of the Albertine rift system, East Africa

By applying a kinematic and flexural model for the extensional deformation of the lithosphere, and using a recently available EROS Data Center topography DEM of Africa in conjunction with new and previous gravity data from Lakes Albert, Edward and George, we have determined the distribution, amplitude, and style of deformation responsible for the formation of the Albertine rift system, East Africa. Further, we have been able to approximate the three-dimensional architecture of the Albertine rift basin by analyzing a series of profiles across and along the rift system for which we also estimate the flexural strength of the rifted continental lithosphere and its along-strike variation. Previous modeling studies of the Lake Albert basin either overestimated the flexural strength of the extended lithosphere and/or underestimated the crustal extension. The single most important factor that compromised the success of these modeling efforts was the assumption that crustal extension was limited to the present-day distribution of the rift lakes. The style of deformation appears to have changed with time, beginning with a regionally distributed brittle deformation across the region that lead progressively to the preferential growth and development of the major border faults and antithetic/synthetic faults within the collapsed hangingwall block. Minor fault reactivation within the footwall block appears to be related to the release of bending stresses associated by the flexural uplift of the rift flank topography. By simultaneously matching the observed and modeled topography and free-air gravity across the Albertine rift system, we have determined a cumulative extension ranging from 6 to 16 km with the maximum extension occurring in the central and northern segments of the basin. Crustal extension is not constrained to the lake proper, but extends significantly to the east within the hangingwall block. Effective elastic thickness, T_e, varies between 24 and 30 km and is unrelated to either the amount of extension or the maximum sediment thickness. The variation of T_e relates possibly to small changes in crustal thickness, heterogeneities in crustal composition, and/or variations in radiogenic crustal heat production. Maximum sediment thickness is predicted to be 4.6 km and occurs within the central region of Lake Albert. Low bulk sediment densities, correlating with the location of major lake deltas, may be indicative of present-day sediment overpressures. Our results show that basin geometry is strongly dependent on the cumulative (and distribution) of lithospheric extension and the flexural rigidity of the lithosphere. Thus, in order to determine the total amount of extension responsible for the formation of a basin system, it is necessary to independently constrain the flexural strength of the lithosphere both during and after extension. Conversely, in order to determine the rigidity of extended lithosphere using the stratigraphy and/or geometry of rift basins and passive margins, it is necessary to independently constrain the cumulative extension of the lithosphere.     

Fracture and in-situ stress patterns and impact on performance in the Khuff structural prospects,eastern offshore Saudi Arabia

We characterized natural fractures and in-situ stresses for exploration and prospect evaluation in nine periclinal structural traps, in the Eastern Province of Saudi Arabia, where several major gas discoveries were made in the deeply buried, Permian–Triassic Khuff Formation. Borehole image logs, oriented cores, seismic, gravity-magnetic data, and dynamic observations were used in the study. Two fracture systems were identified: a younger, major system, which enhances reservoir permeability and an older, minor, fully mineralized system. The older system consists of subordinate northerly striking extensional mesofractures, including joints and faults, which are fully mineralized by anhydrite and calcite. This mineralization occurred during an early diagenetic-phase. This system acted as paleo-fluid conduits, facilitating the occlusion of matrix porosity and deteriorating the reservoir quality in the immediate vicinity of the fractures. The younger system is regionally dominant, and includes mesofractures with persistent strike ranging from NE–SW to ESE–WNW irrespective of local structure. These younger fractures are nearly parallel to the present day maximum horizontal in-situ stress and perpendicular to the minimum horizontal in-situ stress, which are dominated by the Zagros plate tectonics. The development of this system commenced during the convergence of the Arabian and Eurasian plates (Late Cretaceous to Cenozoic) and culminated during the continental collision. The fractures are predominantly extension joints and hybrid (extensional-shear) fractures, and were facilitated by increases in pore pressure due to the oil placement and the subsequent cracking of this oil into gas. Hydrocarbon migration into the Khuff reservoirs was crucial in slowing down diagenesis and preserving both fracture apertures and matrix porosity. Therefore, most of the fractures in this system tend to be partly mineralized, mainly by carbonates, and/or coated with hydrocarbons. These fractures show channel-type apertures that enhance permeability and productivity of the Khuff by up to two fold. The channel apertures can endure operational changes in reservoir pressure with little or no reduction of their permeability. Critically stressed open fractures have no major role. Geomechanical analyses show that they are estimated to occur under the upper limits of differential stresses, within the margin of error of stress estimates. The static and dynamic observations show the permeability and productivity enhancement follows mechanical layering patterns. Production and pressure profiles in individual wells indicate lack of vertical communication (seal breach) across the different reservoir units. Similar pre-production pressure and hydrocarbons across the Khuff reservoir units is the result of normalization over geological time. Hydrocarbon migration across the anhydrite seals happened via episodic paleoseismic pumping along faults with no sufficient vertical offset to permanently breach the reservoir seals.     

十三陵抽水蓄能电站上池西外坡岩体稳定分析

上池西外坡岩体稳定是十三陵抽水蓄能电站主要的地质工程问题之一,由于受到倾向池外的数条缓倾角断层以及不良的岩体结构控制,存在着滑动位移、变形失稳的可能性,电站的安全运行受到潜在的威胁。因此对西外坡岩体进行稳定分析,并进行补强处理是必要的。     

Propagation of normal faults to the surface in basalt, Koae fault system, Hawaii

Normal fault scarps of the Koae fault system on Kilauea volcano consistently display locally breached monoclines underlain by prominent cavities, deep gaping fissures on the footwall, finer fissures on the hanging wall, and buckles at the scarp base. Elastic analyses reveal that this assemblage forms as a fault propagates up towards the surface rather than down from it. Models of a planar blind normal fault with a dip exceeding 60° yield a monocline with a tensile stress concentration at the surface where gaping fissures occur, a stronger subsurface tensile stress concentration near the blind fault tip, where cavities occur, and a compressive stress concentration at the surface where buckles occur. The footwall fissures grow down from the tensile stress concentration at the surface and link with a fault as its scarp grows. In contrast, the cavities initiate at depth near the fault tip and propagate with it up towards the surface. The hanging wall fissures apparently open in response to slip on late-forming blind antithetic faults near the surface. Stoped blocks derived from footwall fissure walls help prop the footwall fissures open as a normal fault breaches the surface. The fissures, cavities, and scarp rubble provide highly conductive hydraulic pathways.     

Quaternary neotectonics of the Somogy Hills, Hungary (part I): Evidence from field observations

Quaternary and directly underlying Late Miocene (Pannonian) outcrops were analysed by structural, tectono-morphologic and sedimentologic methods to describe the main fault directions, to separate mass movements from faulting and folding and to separate earthquake-induced sediment deformations from other (e.g. periglacial) effects in the Somogy Hills. This is a gentle hilly area elevated at 200–300 m above sea level, located immediately south of Lake Balaton, Hungary.

Quaternary outcrops showed several consistent directions of faulting, and co-depositional seismic activity. Three different Mohr-sets of faults/joints could be differentiated in Quaternary sediments. The three sets are considered Late Quaternary since all cut young loess sections and have morphological expressions.

On the basis of the microtectonic measurements and morphotectonic investigations, the following sequence of Quaternary events can be proposed:

1. A (W)NW–(E)SE compression and perpendicular extension would create E–W to WNW–ESE oriented right lateral, NNW–SSE to N–S oriented left lateral shear zones, and NW–SE striking normal faults. Some of these can be evidenced in morphology and among the individual fault measurements. Some reactivated faults might suggest that this field is a relatively older one, but fresh topographic elements suggest that this stress field might be operational sub-recently.

2. A second stress field with NNW–SSE extensional and ENE–WSW oriented compressional directions could be separated. This stress field could create NNE–SSW and NW–SE oriented shear fractures and ENE–WSW oriented conjugate normal faults. Flat thrusts giving ENE directed shear may also be active under this field.

3. A third stress field might be proposed with N–S compression and perpendicular extension directions. This would create NE–SW and NW–SE oriented shear fractures, which are observed in the measured fault data. It is remarkable that the NE–SW faults are all steep, subvertical, and give a very well defined fault set. Based on the fresh topographic expression, this stress field is also sub-recent.

The different sub-recent stress fields and related fault patterns might succeed each other or might alternate through time. The first and third deformations have fresh topographic expressions and cannot play synchronously. The observed features suggest a compressionally active neotectonics of the study area.     

承德地区煤矿分布特征及其受断裂控制的分形研究

运用Fry方法对承德地区已知煤矿点的空间分布形式做了分析,认为矿点整体走向和主要断裂带的走向一致。选择出露矿点较多的A地区,计算其矿点和断裂的分维值,将其与全区相比较,认为承德地区断裂带主要控制了煤层的走向,而煤的形成则受构造、古地理、古气候等因素的综合影响。