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1.
《Journal of Structural Geology》1999,21(8-9):1049-1055
One model for the development of hybrid shear fractures is transitional–tensile fracture propagation, a process described as the in-plane propagation of a crack subject to a shear traction while held open by a tensile normal stress. Presumably, such propagation leads to a brittle structure that is the hybrid of a joint and a shear fracture. Crack–seal veins with oblique fibers are possible candidates. While these veins clearly show shear offset, this is not conclusive evidence that a shear traction was present at the time of initial crack propagation. Many recent structural geology textbooks use a parabolic Coulomb–Mohr failure envelope to explain the mechanics of transitional–tensile fracturing. However, the laboratory experiments cited as demonstrating transitional–tensile behavior fail to produce the fracture orientation predicted by a parabolic failure envelope. Additional attempts at verification include field examples of conjugate joint sets with small acute angles, but these conjugate joints form neither simultaneously nor in the stress field required by the transitional–tensile model. Finally, linear elastic fracture mechanics provides strong theoretical grounds for rejecting the notion that individual cracks propagate in their own plane when subject to a shear traction. These observations suggest that transitional–tensile fracture propagation is unlikely to occur in homogeneous, isotropic rock, and that it is not explained by a parabolic Coulomb–Mohr failure envelope as several recent structural geology textbooks have suggested. 相似文献
2.
An inventory of artesian springs emerging from fractures (fracture springs) was conducted in the Pinh?o River Basin and Morais
Massif, northern Portugal, comprising an area of approximately 650 km2. Over 1,500 springs were identified and associated with geological domains and fracture sets. Using cross-tabulation analysis,
spring distributions by fracture sets were compared among geological environments, and the deviations related to differences
in rock structure and, presumably, to differences in deformational histories. The relation between spring frequencies and
rock structures was further investigated by spectral determination, the model introduced in this study. Input data are the
spring frequencies and fracture lengths in each geological domain, in addition to the angles between fracture strikes and
present-day stress-field orientation (θ). The model's output includes the so-called intrinsic densities, a parameter indexing spring occurrence to factors such as
fracture type and associated deformational regime and age. The highest densities (12.2 springs/km of lineament) were associated
with young shear fractures produced by brittle deformation, and the lowest (0.1) with old tensional and ductile fractures.
Spectral determination also relates each orientation class to a dominant structural parameter: where spring occurrence is
controlled by θ, the class is parallel to the present-day stress-field orientation; where the control is attributed to the length of fractures,
the spring occurrence follows the strike of large-scale normal faults crossing the region.
Electronic Publication 相似文献
3.
Systematic field mapping of fracture lineaments observed on aerial photographs shows that almost all of these structures are positively correlated with zones of high macroscopic and mesoscopic fracture frequencies compared with the surroundings. The lineaments are subdivided into zones with different characteristics: (1) a central zone with fault rocks, high fracture frequency and connectivity but commonly with mineral sealed fractures, and (2) a damage zone divided into a proximal zone with a high fracture frequency of lineament parallel, non-mineralized and interconnected fractures, grading into a distal zone with lower fracture frequencies and which is transitional to the surrounding areas with general background fracturing. To examine the possible relations between lineament architecture and in-situ rock stress on groundwater flow, the geological fieldwork was followed up by in-situ stress measurements and test boreholes at selected sites. Geophysical well logging added valuable information about fracture distribution and fracture flow at depths. Based on the studies of in-situ stresses as well as the lineaments and associated fracture systems presented above, two working hypotheses for groundwater flow were formulated: (i) In areas with a general background fracturing and in the distal zone of lineaments, groundwater flow will mainly occur along fractures parallel with the largest in-situ rock stress, unless fractures are critically loaded or reactivated as shear fractures at angles around 30° to σH; (ii) In the influence area of lineaments, the largest potential for groundwater abstraction is in the proximal zone, where there is a high fracture frequency and connectivity with negligible fracture fillings. The testing of the two hypotheses does not give a clear and unequivocal answer in support of the two assumptions about groundwater flow in the study area. But most of the observed data are in agreement with the predictions from the models, and can be explained by the action of the present stress field on pre-existing fractures. 相似文献
4.
Tapas Acharya Sisir Kanti Nag Sukumar Basumallik 《Journal of the Geological Society of India》2012,80(5):723-730
Filter analysis of lineaments in Precambrian metamorphic rocks was used to delineate fracture-correlated lineaments and hydraulically significant fractures. The unfiltered analysis technique fails to show correlation between major lineaments and fractures. Domain-based and discrete filtering techniques successfully identify fracture-correlated lineaments within the brittle-ductile shear zone in conjunction with fractures characterized by high fracture frequencies (>10/m). The locales of hydraulically significant fractures can thus be assessed if the geological controls governing the spatial distribution of fracture frequencies are computed using structural domain approach. The concurrence of fracture-correlated lineaments and hydraulically significant fractures within the brittle-ductile shear zone is evident. 相似文献
5.
Liu Enhuai 《地学学报》1990,2(3):257-263
Based on research into the macro- and micro-structures within the Eastern Shandong gold metallogenetic province, fractures are the main ore-controlling and ore-bearing structures, revealing a process of ductile deformation in the early period and brittle deformation in the later period, when gold mineralization mainly occurred. The ore-bearing structures can be classified into five types: (a) fracture structures; (b) fissure structures; (c) fracture and fissure structures; (d) fracture, fissure and concealed explosion breccia pipe structures; and (e) fracture and explosive breccia structures. NE-trending fractures are the most favourable to the formation of gold orebodies, next in importance are the NNE-trending fractures. NW-trending fractures controlled the distribution and the maximum depth of gold ore shoots during mineralization and intercept the veins with dextral offsets subsequent to mineralization. The different types of orebodies each have their own specific structural controls. Furthermore, the vertical zonation between the auriferous silicified rock and sericite–quartz rock is interpreted as resulting, from convective circulation. The author analyses the areal structural history, sets up a structural model and suggests that the difference between the orebodies in the Zhaoyuan–Yexian and Mupin-Rushan metallogenetic areas results from the distinct structural controls on the ore-bearing fractures in the two areas. 相似文献
6.
7.
Cristiano Chimpliganond Marcelo Assumpção Mônica Von Huelsen George Sand França 《Tectonophysics》2010,480(1-4):48-56
On December 9, 2007, a 4.9 mb earthquake occurred in the middle of the São Francisco Craton, in a region with no known previous activity larger than 4 mb. This event reached intensity VII MM (Modified Mercalli) causing the first fatal victim in Brazil. The activity had started in May 25, 2007 with a 3.5 magnitude event and continued for several months, motivating the deployment of a local 6-station network. A three week seismic quiescence was observed before the mainshock. Initial absolute hypocenters were calculated with best fitting velocity models and then relative locations were determined with hypoDD. The aftershock distribution indicates a 3 km long rupture for the mainshock. The fault plane solution, based on P-wave polarities and hypocentral trend, indicates a reverse faulting mechanism on a N30°Ε striking plane dipping about 40° to the SE. The rupture depth extends from about 0.3 to 1.2 km only. Despite the shallow depth of the mainshock, no surface feature could be correlated with the fault plane. Aeromagnetic data in the epicentral area show short-wavelength lineaments trending NNE–SSW to NE–SW which we interpret as faults and fractures in the craton basement beneath the surface limestone layer. We propose that the Caraíbas–Itacarambi seismicity is probably associated with reactivation of these basement fractures and faults under the present E–W compressional stress field in this region of the South American Plate. 相似文献
8.
Hydraulic conductivity distribution in crystalline rocks, derived from inflows to tunnels and galleries in the Central Alps, Switzerland 总被引：4，自引：3，他引：1
Inflow data from 23 tunnels and galleries, 136 km in length and located in the Aar and Gotthard massifs of the Swiss Alps, have been analyzed with the objective (1) to understand the 3-dimensional spatial distribution of groundwater flow in crystalline basement rocks, (2) to assess the dependency of tunnel inflow rate on depth, tectonic overprint, and lithology, and (3) to derive the distribution of fracture transmissivity and effective hydraulic conductivity at the 100-m scale. Brittle tectonic overprint is shown to be the principal parameter regulating inflow rate and dominates over depth and lithology. The highest early time inflow rate is 1,300 l/s and has been reported from a shallow hydropower gallery intersecting a 200-m wide cataclastic fault zone. The derived lognormal transmissivity distribution is based on 1,361 tunnel intervals with a length of 100 m. Such interval transmissivities range between 10?9 and 10?1 m2/s within the first 200–400 m of depth and between 10?9 and 10?4 m2/s in the depth interval of 400–1,500 m below ground surface. Outside brittle fault zones, a trend of decreasing transmissivity/hydraulic conductivity with increasing depth is observed for some schistous and gneissic geological units, whereas no trend is identified for the granitic units. 相似文献
9.
The aim of the present study is to investigate the lineaments of Kolli hills of Tamil Nadu State for which CARTOSAT-1 satellite’s DEM output has been made use of. The extracted lineaments were analysed using ArcGIS and Rockworks software. The total number and length of lineaments are 523 and 943.81 km, respectively. Shorter lineaments constitute about 3/4th of the total number of lineaments. The density of the lineaments varies from 0 to 7.41 km/km2, and areas of very high to high density are restricted to the south central, central and north eastern parts, and these areas reflect the high degree of rock fracturing and shearing which makes these areas unsuitable for the construction of dams and reservoirs. However, these areas could be targeted for groundwater exploitation as they possess higher groundwater potential. The lineaments are oriented in diverse directions. However, those orienting in ENEWSW, NE-SW and NW-SE are predominating followed by those oriented in sub E-W and sub N-S directions. These orientations corroborate with results of previous regional studies and with orientations of prominent geological structures and features of the study area. Distinct variation in the predominant orientations of lineaments of varied sizes is observed, while the shorter ones are oriented in either NW-SE or NNW-SSE directions, the longer ones are oriented in either NE-SW or ENE-WSW. A comparative analysis of lineament datasets of the eight azimuth angles and the final lineament map underlines the need to extract lineaments from various azimuth angles to get a reliable picture about the lineaments. 相似文献
10.
Estimating regional-scale fractured bedrock hydraulic conductivity using discrete fracture network (DFN) modeling 总被引：1，自引：1，他引：0
Estimating bedrock hydraulic conductivity of regional fractured aquifers is challenging due to a lack of aquifer testing data and the presence of small and large-scale heterogeneity. This study provides a novel approach for estimating the bedrock hydraulic conductivity of a regional-scale fractured bedrock aquifer using discrete fracture network (DFN) modeling. The methodology is tested in the mountainous Okanagan Basin, British Columbia, Canada. Discrete fractures were mapped in outcrops, and larger-scale fracture zones (corresponding to lineaments) were mapped from orthophotos and LANDSAT imagery. Outcrop fracture data were used to generate DFN models for estimating hydraulic conductivity for the fractured matrix (K m). The mountain block hydraulic conductivity (K mb) was estimated using larger-scale DFN models. Lineament properties were estimated by best fit parameters for a simulated pumping test influenced by a fracture zone. Unknown dip angles and directions for lineaments were estimated from the small-scale fracture sets. Simulated K m and K mb values range from 10–8 to 10–7?m/s and are greatest in a N–S direction, coinciding with the main strike direction of Okanagan Valley Fault Zone. K mb values also decrease away from the fault, consistent with the decrease in lineament density. Simulated hydraulic conductivity values compare well with those estimated from pumping tests. 相似文献
11.
Agust Gudmundsson 《地学学报》1992,4(4):464-471
The divergent plate boundary in Iceland is characterized by 40–80 km long and 5–10 km wide swarms of tension fractures (∼102 m long) and normal faults (∼103 m long). The upper part of the crust is mainly composed of lava flows, with abundant columnar joints that are mostly perpendicular to the lava contacts. The lava flows are horizontal at the surface of the rift zone but become tilted at the rate of 1° for every 150 m depth in the crust. At the surface of the rift zone the joints are vertical and parallel to the vertical principal stress. Because of tilting of the lava pile, the columnar joints become oblique to this stress, hence becoming potential shear fractures, and form echelon sets at greater depths in the crust. Theoretical considerations suggest that normal faults start to nucleate on sets of en echelon columnar joints and/or large-scale tension fractures at crustal depths of 0.5–1.5 km. The width (depth) must be the smallest (controlling) dimension of many faults. Nevertheless, there is a positive linear relation (r = 0.91) between maximum throw and length of the Holocene faults. If the faults grow as self-similar structures, the throw-length relationship can be explained by a similar relation between fault length and width. 相似文献
12.
Present-day stress orientations in the Northern Perth Basin have been inferred from borehole breakouts and drilling-induced tensile fractures observed on image logs from eight wells. Stress indicators from these wells give an east – west maximum horizontal stress orientation, consistent with stress-field modelling of the Indo-Australian Plate. Previous interpretations using dipmeter logs indicated anomalous north-directed maximum horizontal stress orientations. However, higher-quality image logs indicate a consistent maximum horizontal stress orientation, perpendicular to dominant north – south and northwest – southeast fault trends in the basin. Vertical stress was calculated from density logs at 21.5 MPa at 1 km depth. Minimum horizontal stress values, estimated from leak-off tests, range from 7.4 MPa at 0.4 km to 21.0 MPa at 0.8 km depth: the greatest values are in excess of the vertical stress. The maximum horizontal stress magnitude was constrained using the relationship between the minimum and maximum horizontal stresses; it ranges from 8.7 MPa at 0.4 km to 21.3 MPa at 1 km depth. These stress magnitudes and evidence of neotectonic reverse faulting indicate a transitional reverse fault to strike-slip fault-stress regime. Two natural fracture sets were interpreted from image logs: (i) a north- to northwest-striking set; and (ii) an east-striking set. The first set is parallel to adjacent north- to northwest-striking faults in the Northern Perth Basin. Several east-striking faults are evident in seismic data, and wells adjacent to east-striking faults exhibit the second east-striking set. Hence, natural fractures are subparallel to seismically resolved faults. Fractures optimally oriented to be critically stressed in the present-day stress regime were probably the cause of fluid losses during drilling. Pre-existing north- to northwest -striking faults that dip moderately have potential for reactivation within the present-day stress regime. Faults that strike north to northwest and have subvertical dips will not reactivate. The east-striking faults and fractures are not critically stressed for reactivation in the Northern Perth Basin. 相似文献
13.
The relationship between major structural lineaments and locations of ore deposits in Iran has been investigated using geospatial data. In the course of lineament extraction, satellite images, aeromagnetic data, digital elevation model (DEM) and structural maps were processed and the lineaments and large-scale faults were identified. The extracted lineaments, based on subjective assessment, from each dataset were imported into GIS software and the “lineament map of Iran” was prepared by data integration. The analysis for selecting significant lineament was mainly based on fault correlated lineament and lineament with field map fractures, which was sets as benchmarks for compiling a final output map. Four major regional lineament trends of N–S, E–W, NW–SE and NE–SW were identified in the data of all images, which are corresponded to the structural zones and the major fault systems of Iran. The mineral deposits (active and abandoned) and mineral indications database compiled are based on the published maps, papers, reports and the ore deposits data files of Geological Survey of Iran. Integrating the output of these two datasets by GIS software resulted in the “Combined Map of Lineaments and Gold, Copper, Lead, Zinc and Iron Deposits of Iran”. The number and distance of ore deposits toward the lineaments were processed by the counting and cumulative methods in the GIS software's. Approximately, over 90% of the ore deposits of Iran are located in the central part of the lineaments (15 km on each side) which are concordant with a definition of large lineament. About 50% of these mineral deposits are closer than 5 km to the lineaments. There are significant correlations between lineament density and intersections with ore deposits occurrences. The observed associations at this scale are informative in establishing exploration strategy and decreasing exploration risks for detailed work on ore deposit scale. 相似文献
14.
The Schöllenen Gorge in the Reuss Valley of the Central Swiss Alps (Figs. 1 and 2) is a famous tourist attraction and ideal location for the study of the properties and formation mechanisms of uplift and post-uplift unloading joints. The gorge is situated in the southern part of the Central Aar Granite, a granitic batholith which intruded about 300 million years ago. The magmatic fabric of this batholith (Fig. 4) has only been locally modified during Alpine tectonic and metamorphic overprinting, mainly in the vicinity of ductile-brittle shear zones. The up to 600 m deep gorge provides an ideal opportunity to study the complex fracture systems of the batholith, and tunnels of the Göschenen hydropower system allow the study of the fracture patterns below ground surface. Outcrop, tunnel and remote mapping of fractures in the study area lead to the recognition of two probably syntectonic (Oligocene-Miocene) joint sets (S and Q joints) and three generations of uplift and post-uplift joints (unloading joints). The frequent S joints run nearly parallel to the Alpine schistosity, i.e. striking approximately E–W and dipping steeply to the south (Figs. 5 and 7). The less frequent Q joints dip steeply to SW; the angle between the two joint sets ranges between 60 and 80 degrees. The first generation of uplift joints (called L- joints) is subhorizontal and probably related to Alpine extensional veins filled with fissure quartz (Zerrklüfte). These veins formed during the late Alpine (Miocene) uplift of the Aar Granite (Mullis 1996). A first generation of post-uplift joints (T1 joints) strikes parallel to the valley axes and dip with 30–45 degrees towards the valley bottom. This set probably formed during an earlier stage of glacial valley erosion in the Pleistocene (Figs. 9–11). The youngest generation of post-uplift joints (T2 joints) is orientated parallel to the present ground surface of the Schöllenen Gorge and to erosional surfaces with glacial striations (Figs. 9–11 and 21). The frequency and size of these joints seems to decrease with depth below the ground surface. In one tunnel, post-uplift joints could be observed within a horizontal and vertical distance from the ground surface of 150 and 80 meters. Post-uplift joints only form in granites with a primary fabric that has not been intensively overprinted by brittle or ductile Alpine tectonic deformations. Fractographic investigations, i.e. investigations of crack propagation markers on joint surfaces, confirm this relative age of the fracture sets and give valuable insights into the formation mechanisms of post-uplift joints. Post-uplift joints show intense and 5–10 meter long plumose markings and only rarely arrest lines (Figs. 18a and 20). It can be shown that sets of post-uplift joints join at pre-existing (uplift and syntectonic) fractures to form large (50–100 m sized) curved exfoliation structures (Fig. 19). The growth direction of the post-uplift joints is mainly in subhorizontal directions (Figs. 19 and 20). Fractographic markings, spatial and depth distributions as well as the relative size of post-uplift fractures are explained within the mechanical framework of uniaxial and biaxial compression tests on intact granite samples and samples with artificial flaws. Most of these experiments have been carried out in the framework of studies related to brittle failure (spalling and rockbursting) around deep mining drifts and tunnels in hard rock’s (e.g. Hoek & Bieniawski 1965, Read et al. 1998, Eberhardt et al. 1999). As suggested already by Holzhausen & Johnson (1979), post-uplift fractures form as extension fractures in a compressive stress field with small confining stress. Laboratory tests carried out on artificial Griffith cracks suggest that the macroscopic fracture size is mainly controlled by the ratio of the smallest to the largest principal stress (σ3/σ1), the so-called spalling limit. In steep slopes this ratio should increase with depth below ground surface (Fig. 24c), leading to smaller exfoliation fractures with increasing depth. The spatial occurrence of post-uplift fractures along the surface topography is a function of the deviatoric stress level (Fig. 24a) and/or the development of local tensile stresses (Fig. 24d). Preliminary numerical simulations of these failure criteria in a multistage glacial erosion model (Fig. 23) allow some of the observed patterns of post-uplift fracture distributions to be reproduced. post-uplift joints in steep glacial valleys play an important role in valley erosion and in connection with the risk of rock falls, the safety of traffic corridors, and the inflow of water to near-surface tunnels and hydropower caverns. The depth dependant sizes, frequencies and hydraulic conductivities of these fractures can be directly related to the occurrence and magnitudes of the corresponding hazards. 相似文献
15.
Footwall rocks of the northern Snake Range detachment fault (Hampton and Hendry's Creeks) offer exposures of quartzite mylonites (sub-horizontal foliation) that were permeated by surface fluids. An S–C–C′ mylonitic fabric is defined by dynamically recrystallized quartz and mica. Electron backscatter diffraction analyses indicate a strong preferred orientation of quartz that is overprinted by two sets of sub-vertical, ESE and NNE striking fractures. Analyses of sets of three perpendicular thin sections indicate that fluid inclusions (FIs) are arranged according to macroscopic fracture patterns. FIs associated with NNE and ESE-striking fractures coevally trapped unmixed CO2 and H2O-rich fluids at conditions near the critical CO2–H2O solvus, giving minimum trapping conditions of T = 175–200 °C and ∼100 MPa H2O-rich FIs trapped along ESE-trending microcracks in single crystals of quartz may have been trapped at conditions as low as 150 °C and 50 MPa indicating the latest microfracturing and annealing of quartz in an overall extensional system. Results suggest that the upper crust was thin (4–8 km) during FI trapping and had an elevated geotherm (>50 °C/km). Footwall rocks that have been exhumed through the brittle-ductile transition in such extensional systems experience both brittle and crystal-plastic deformation that may allow for circulation of meteoric fluids and grain-scale fluid–rock interactions. 相似文献
16.
M.D. Wood 《Tectonophysics》1979,52(1-4)
Tilt due to massive hydraulic fractures induced in sedimentary rocks at depths of up to 2.2 km have been recorded by surface tiltmeters. Injection of fluid volumes up to 4 · 105 liters and masses of propping agent up to 5 · 105 kg is designed to produce fractures approximately 1 km long, 50–100 m high and about 1 cm wide. The surface tilt data adequately fit a dislocation model of a tensional fault in a half-space. Theoretical and observational results indicate that maximum tilt occurs at a distance off the strike of the fracture equivalent to 0.4 of the depth to the fracture. Azimuth and extent of the fracture deduced from the geometry of the tilt field agree with other kinds of geophysical measurements. Detailed correlation of the tilt signatures with pumping parameters (pressure, rate, volume, mass) have provided details on asymmetry in geometry and growth rate. Whereas amplitude variations in tilt vary inversely with the square of the depth, changes in flow rate or pressure gradient can produce a cubic change in width. These studies offer a large-scale experimental approach to the study of problems involving fracturing, mass transport, and dilatancy processes. 相似文献
17.
月球雨海北部陆地区域构造及其含义 总被引：1，自引：0，他引：1
月球雨海北部陆地是雨海多环盆地的第二层,平均高程约-1 km。DEM图像显示,大量来自虹湾与柏拉图月坑的掘积物使本地区高程变得非常不均一。统计了研究区内的月坑,并根据其深度与宽度之比(深宽比)将它们划分为4组。深宽比较小而扁率较大的月坑被认为是较古老的月坑。这些古老月坑分布于比较接近月海的位置。对研究区内线性构造的制图研究揭示了3个优选方位,分别是E—W、NEE—SWW和NW—SE向。这种分布样式与月球格子构造系统大致匹配,因而它们很可能形成于雨海事件之前。这些线性构造,包括断裂与月溪,在月海玄武岩泛滥时期为玄武质岩浆的侵入提供了大量通道。在研究区内一些地形较低的地点,玄武岩上侵并出露在月表,它们的FeO平均含量接近但是略低于月海玄武岩。总结了本地区的地质构造演化历史,并且推论月球上的确存在类月海的陆地。 相似文献
18.
川东南丁山地区是近年来四川盆地页岩气勘探开发的热点区域,裂缝的发育对页岩含气性及保存条件有重要的影响。综合运用野外露头、岩心、测井资料,结合岩石脆性矿物含量、岩石力学参数等数据,深入分析龙马溪组页岩裂缝发育特征和控制因素,并探讨了裂缝发育对含气性的影响。结果表明,丁山地区龙马溪组页岩裂缝主要以构造成因的剪切缝为主,裂缝优势方位共6组,主要包括4组平面剪切缝和2组剖面剪切缝,其发育主要受2个方向、3个阶段的构造应力场影响而成;裂缝延伸稳定,平均密度小,宽度小,充填程度高,主要被方解石和黄铁矿等充填。裂缝受控因素主要包括古构造应力场、构造部位、脆性矿物组分、岩石力学性质等;断层对裂缝发育具有明显的控制作用,其中断层两盘均存在裂缝发育程度急剧下降的临界范围,临界范围内裂缝发育程度高,超过此临界范围,裂缝发育程度变差且变化趋于平缓;不同期次的裂缝中,形成时间晚、规模过大、充填程度不高、与现今地应力方向一致或呈低角度相交的裂缝易造成页岩气的散失,对提高页岩含气性不利;龙马溪组岩石脆性矿物含量高,脆性指数属中等偏上程度,有利于构造缝发育且可压性较好。随着距齐岳山断裂距离的适当增加,龙马溪组页岩埋藏深度适中,地层压力增大,抗压强度增强高,脆性指数适中,构造保存条件变好,有利于不同方位的裂缝发育和页岩含气量的增加,位于该区域的DY2井与DY4井均位于该有利区域,含气性良好。研究结果对下一步深化页岩气勘探开发具有重要指导作用。 相似文献
19.
《Journal of Structural Geology》2001,23(2-3):165-182
An almost horizontal pahoehoe surface in a Holocene plagioclase basanite lava on Tenerife displays three scales of fracture within the surface crust. An early-formed set of large-scale fractures divides up the surface into an orthogonal set of rectangular slabs with dimensions of several metres and depths of 10–12 cm. The shortest slab dimension is parallel to the flow emplacement direction, inferred from a strong surface lineation. The slabs are domed with the centre an average of 9.6 cm (with range 4–19.6 cm) above the edges of the slabs. Profiles of the slabs normal and perpendicular to the margins and through the crest indicate that they can be described by a power law in which the deflection of the slab, h, is related to the distance from the crest, x, with an exponent between 2 and 3. Analysis of joints within the slabs indicates two smaller scale networks. An intermediate scale joint network bounds blocks with rectilinear to polygonal shapes in plan-view and has a characteristic mean spacing of 24.2 cm (range 10.5–48 cm). The major fractures in this set are normal and parallel to the slab margins. A smaller-scale joint network bounds polygonal equant blocks in plan-view and has characteristic spacing of 6.4 cm (range 3.7–10.5 cm). A model of cooling from the pahoehoe surface is used to constrain the growth of the crust and timing of fracture development. The large-scale slabs are attributed to localised accumulation of gas beneath the growing crust causing buoyant forces. The tensile stresses caused by uplift are sufficient to form the large-scale fractures after 2 or 3 h of cooling. The intermediate scale fracture network is attributed to the flexure of the slab crust. The smaller scale polygonal joint network is related to the build up of isotropic tensile stresses in the cooling slab crust due to thermal contraction with fracture development being promoted by the flexure of the slabs. An analysis of the slab deformation indicates that lava crust is weak. The weakness is explained by division of the crust into three zones: an outer zone with small scale joints that cause negligible strength, a middle zone of elastic behaviour in which stress can build up, and a lower zone of plastic deformation. The crustal slabs display profiles similar to that expected in a bending elastic plate. The deformation of the 10-cm-thick crust can be explained if the elastic zone was about 2-cm thick. This result agrees with an independent calculation of elastic zone thickness based on the position of the brittle–ductile transition being located at the 600°C isotherm at a depth of about 2 cm when the crustal slabs were rifted apart. 相似文献
20.
Saumen Maiti Gautam Gupta Vinit C. Erram Ram Krishna Tiwari 《Environmental Earth Sciences》2013,68(3):779-794
Modeling resistivity profiles, especially from hard rock areas, is of specific relevance for groundwater exploration. A method based on Bayesian neural network (BNN) theory using a Hybrid Monte Carlo (HMC) simulation scheme is applied to model and interpret direct current vertical electrical sounding measurements from 28 locations around the Malvan region, in the Sindhudurg district, southwest India. The modeling procedure revolves around optimizing the objective function using the HMC based sampling technique which is followed by updating each trajectory by integrating the Hamiltonian differential equations via a second order leapfrog discretization scheme. The inversion results suggest a high resistivity structure in the north-western part of the area, which correlates well with the presence of laterites. In the south-western part, a very high conductive zone is observed near the coast indicating an extensive influence of saltwater intrusion. Our results also show that the effect of intrusion of saline water diminishes from the south-western part to the north-eastern part of the region. Two dimensional modeling of four resistivity profiles shows that the groundwater flow is partly controlled by existing lineaments, fractures, and major joints. Groundwater occurs at a weathered/semi-weathered layer of laterite/clayey sand and the interface of overburden and crystalline basement. The presence of conduits is identified at a depth between 10 and 15 m along the Dhamapur–Kudal and Parule–Oros profiles, which seems to be potential zone for groundwater exploration. The NW–SE trending major lineaments and its criss-cross sections are indentified from the apparent and true resistivity surface map. The pseudo-section at different depths in the western part of the area, near Parule, shows extensive influence of saltwater intrusion and its impact reaching up to a depth of 50 m from the surface along the coastal area. Further, the deduced true electrical resistivity section against depth correlates well with available borehole lithology in the area. Present analyses suggest that HMC-based BNN method is robust for modeling resistivity data especially in hard rock terrains. These results are useful for interpreting fractures, major joints, and lineaments and crystalline basement rock and also for constraining the higher dimensional models. 相似文献