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1.
Kinematic analysis and field mapping of the Homestake shear zone (HSZ) and Slide Lake shear zone (SLSZ) in central Colorado may provide insight into the interaction between subvertical and low-angle shear zones in the middle crust. The northeast-striking, steeply dipping HSZ comprises a ∼10-km-wide set of anastomosing ductile shear zones and pseudotachylyte-bearing faults. Approximately 4 km south of the HSZ, north–northeast-striking, shallowly dipping mylonites of the SLSZ form three 1–10-m-thick splays. Oblique stretching lineations and shear sense in both shear zones record components of dip-slip (top-up-to-the-northwest and top-down-to-the-southeast) and dextral strike-slip movement during mylonite development. Quartz and feldspar deformation mechanisms and quartz [c] axis lattice preferred orientation (LPO) patterns suggest deformation temperatures ranging from ∼280–500 °C in the HSZ to ∼280–600 °C in the SLSZ. Quartz [c] axis LPOs suggest plane strain general shear across the shear system. Based on the relative timing of fabric development, compatible kinematics and similar deformation temperatures in the SLSZ and the HSZ, we propose that both shear zones formed during strain localization and partitioning within a transpressional shear zone system that involved subvertical shuffling in the mid-crust at 1.4 Ga.  相似文献   

2.
Strain distribution patterns are described from several naturally occurring ductile shear zone terminations and a two-fold classification is suggested. Type I termination patterns show a symmetrical decrease in strain area and intensity, and are associated with very low shear strain (ψ) values in the main shear zone. The more common Type II termination patterns show a symmetrical increase in strain area with decrease in strain intensity. Some brittle-ductile examples of Type II terminations contain secondary shears that resemble splay faults. All the available evidence suggests that the deformation near the terminations of these shear zones is not plane strain, nor do the observed strain patterns readily fit the existing theoretical models.  相似文献   

3.
Principal strain orientations (minimum horizontal compression—ex and maximum horizontal compression—ey) were established at three different types of plate tectonic boundary: two transform faults, an oceanic ridge located on the Southeast Indian Ridge and a trench located close to the South Sandwich Archipelago. To establish the strain patterns in each zone, 104 earthquake focal mechanisms (centroid-moment tensor solutions for earthquakes with mb≥4; Harvard seismology data, CMT) were examined by fault population analysis. Despite the existence of only one tectonic process that controlled deformation in these zones (divergence, convergence or passive displacement), and only one main strain tensor, several coeval strain ellipsoids were found. These differed from the main strain tensor in the location of the principal strains. In general, permutations were observed between the principal strains, i.e., interchanges between the location of the principal strain axes maintaining the strain ellipsoids in the same 3D orientation. Only in some cases were changes in the ellipsoid orientation associated with major structures.  相似文献   

4.
西秦岭北缘武山—鸳鸯镇构造带磁组构特征   总被引：2，自引：0，他引：2  
构造与磁组特征揭示出武山-鸳鸯镇构造带为一条复合性断裂带,变形样式表现为花状构造形态。野外及显微构造特征表明,先期韧性变形为右行剪切,发育于中、深构造层次;66个构造岩样品的磁化率椭球形态分析表明,其以平面和压扁应变为主,总体较高的磁化率各向异性度表现了构造带的强变形特征;磁化率椭球主轴方位显示NW和NEE走向两组磁面理的存在,暗示高应变剪切带在平面上可能以共轭或网格状形态出露,锐夹角分线近EW向;高角度磁面理及较为发育的低倾伏角磁线理暗示了沿构造带近EW向的走滑剪切,部分高倾伏角磁线理可能与构造带的挤压和(或)转换挤压相关,而相对集中的磁面理与相对分散的磁线理也表明了构造带的平面及压扁应变体制。强烈的右行转换挤压奠定了西秦岭北缘现今的反"S"型区域构造,表明碰撞造山过程中,西秦岭诸中、小块体一定程度的向西挤逸。中、新生代沿构造带继承性的发育以西秦岭北缘(渭河)断裂为中心的一系列正花状左行走滑构造,构成青藏高原东北边缘物质逃逸及应力释放与调整的重要边界。  相似文献   

5.
走滑断裂带对中国西部压扭性叠合盆地大中型油气田形成与分布具有重要的控制作用,也是研究难点之一.基于高密度三维地震资料,本文采用多种地震构造解析技术,瞄准噶尔盆地腹部侏罗系开展了精细走滑断裂带解释和变形样式分析.在燕山Ⅱ幕构造活动期,侏罗系发育了NWW向左行压扭性和NE向左行张扭性两类走滑断裂带.它们都是由4组剪切断层复合而成,共同遵从左行简单剪切模式,但几何学特征和构造属性差异很大.NWW和NE向走滑断裂带不存在共轭剪切关系,而是在钝夹角区（135°左右）普遍具有弧形联合与归并趋势.在构造变形中,两类同期左行走滑断裂带弧形联合控制了变形区域旋扭形变和剪切破裂,构成了一个大尺度“面”状旋扭构造体系.旋扭构造变形样式对中亚陆内造山带研究具有一定借鉴意义,也为压扭盆地的油气勘探实践提供了新思路.   相似文献   

6.
We report for the first time the occurrence of polygonal faults in sandstone, which is compelling given that layer-bound polygonal fault systems have been observed so far only in fine-grained sediments such as clay and chalk. The polygonal faults are shear deformation bands that developed under shallow burial conditions via strain hardening in dm-wide zones. The edges of the polygons are 1–5 m long. The shear deformation bands are organized as conjugate faults along each edge of the polygon and form characteristic horst-like structures. The individual deformation bands have slip magnitudes ranging from a few mm to 1.5 cm; the cumulative average slip magnitude in a zone is up to 10 cm. The deformation bands heaves, in aggregate form, accommodate a small isotropic horizontal extension (strain <0.005). The individual shear deformation bands show abutting T-junctions, veering, curving, and merging where they mechanically interact. Crosscutting relationships are rare. The interactions of the deformation bands are similar to those of mode I opening fractures. The documented fault networks have important implications for evaluating the geometry of km-scale polygonal fault systems in the subsurface, top seal integrity, as well as constraining paleo-tectonic stress regimes.  相似文献   

7.
Abstract

Neogene deformations have deeply disturbed the initial architecture of the pile of nappes within the Eastern Betic zone. The Arc of Aguilas, displaying a southeast-facing concavity, is a spectacular example of such a post-nappe structuration. Miocene deposits involved in the torsion of the Arc provide a chronology of the deformation. The Arc of Aguilas is one element within a system of rigid-plastic indentation including the following units, from the inner (SE) to the outer (NW) zones : — A rigid block, little deformed, located in the present day abyssal plain, play the part of the indenter.

— A structural pad corresponding to the Aguilas Arc itself. It was severely folded during Miocene times.

— A large peripheral zone mainly subjected to faulting during the Neogene (essentially strike-slip faults). These faults control the evolution of different types of sedimentary basins during the Late Neogene (Tortonian to Pliocene).

Two large shear zones: N020 sinistral (Palomares and Terreros faults), N100 dextral (Las Moreras faults) guided the deformation of the Aguilas Arc within a compressive stressfield of which major tensor axis oscillated between NW-SE and N-S.  相似文献   

8.
The southern Andes plate boundary zone records a protracted history of bulk transpressional deformation during the Cenozoic, which has been causally related to either oblique subduction or ridge collision. However, few structural and chronological studies of regional deformation are available to support one hypothesis or the other. We address along- and across-strike variations in the nature and timing of plate boundary deformation to better understand the Cenozoic tectonics of the southern Andes.Two east–west structural transects were mapped at Puyuhuapi and Aysén, immediately north of the Nazca–South America–Antarctica triple junction. At Puyuhuapi (44°S), north–south striking, high-angle contractional and strike-slip ductile shear zones developed from plutons coexist with moderately dipping dextral-oblique shear zones in the wallrocks. In Aysén (45–46°), top to the southwest, oblique thrusting predominates to the west of the Cenozoic magmatic arc, whereas dextral strike-slip shear zones develop within it.New 40Ar–39Ar data from mylonites and undeformed rocks from the two transects suggest that dextral strike-slip, oblique-slip and contractional deformation occurred at nearly the same time but within different structural domains along and across the orogen. Similar ages were obtained on both high strain pelitic schists with dextral strike-slip kinematics (4.4±0.3 Ma, laser on muscovite–biotite aggregates, Aysén transect, 45°S) and on mylonitic plutonic rocks with contractional deformation (3.8±0.2 to 4.2±0.2 Ma, fine-grained, recrystallized biotite, Puyuhuapi transect). Oblique-slip, dextral reverse kinematics of uncertain age is documented at the Canal Costa shear zone (45°S) and at the Queulat shear zone at 44°S. Published dates for the undeformed protholiths suggest both shear zones are likely Late Miocene or Pliocene, coeval with contractional and strike-slip shear zones farther north. Coeval strike-slip, oblique-slip and contractional deformation on ductile shear zones of the southern Andes suggest different degrees of along- and across-strike deformation partitioning of bulk transpressional deformation.The long-term dextral transpressional regime appears to be driven by oblique subduction. The short-term deformation is in turn controlled by ridge collision from 6 Ma to present day. This is indicated by most deformation ages and by a southward increase in the contractional component of deformation. Oblique-slip to contractional shear zones at both western and eastern margins of the Miocene belt of the Patagonian batholith define a large-scale pop-up structure by which deeper levels of the crust have been differentially exhumed since the Pliocene at a rate in excess of 1.7 mm/year.  相似文献   

9.
The anisotropy of magnetic susceptibility (AMS) of 351 specimens from 51 sites across the Ailao Shan–Red River shear zone (ASRR) was measured to determine its magnetic fabric. Rocks range westward from core schistose gneiss, through low-grade schist, to Triassic sediment. Magnetic ellipticity analysis shows that 41 of 51 sites have an oblate compressional fabric and the other 10 sites have a prolate fabric. PJ value drops by 22.4% in the low-grade schist and by 27.4% in the Triassic sediment on average with respect to the gneiss, suggesting a rapid decrease of deformational intensity. The directions of principal susceptibilities are closely related to the deformation of the Ailao Shan–Red River shear zone. The susceptibility plane always coincides with the schistosity or cleavage plane. Most of the maximum susceptibility axes trend NW–SE. In the shear zone, the maximum susceptibility axes (Kmax) are parallel to the lineation within the foliation plane. With increasing distance from the shear zone, there is a trend that they become parallel to the down-dip of reverse faults or cleavage. This indicates changes in deformation mode, inside and outside the shear zone. Within the shear zone, horizontal movement is dominant. Outside, shortening prevails. The overall minimum magnetic axes align NE–SW with subhorizontal to low dip angles, suggesting that the dominant shortening is NE–SW directed. Caution should be exercised when AMS is used to determine shear sense in strong shear zones because the angle between the minimum susceptibility axis (Kmin) and pole of foliation is small, and also because the attitude of foliation varies from place to place. They result in unreliable or even wrong shear sense. Another important result is the axial ratio of magnetic susceptibility ellipsoid along the study section. With these data, it is possible to establish an axial ratio relationship between the finite strain ellipsoid and magnetic susceptibility ellipsoid for quantitative calculation of offset.  相似文献   

10.
The average seismic strain rate is estimated for the seismotectonic zone of the northern/central parts of the Gulf of Suez. The principal strain rate tensor and velocity tensor were derived from a combination of earthquake focal mechanisms data and seismic moment of small-sized earthquakes covering a time span of 13 years (1992–2004). A total of 17 focal mechanism solutions have been used in the calculation of the moment tensor summation. The local magnitudes (MLs) of these events range from 2.8 to 4.7. The analysis indicates that the dominant mode of deformation in the central and northern parts of the Gulf of Suez is extension at a rate of 0.008 mm/year in N28°E direction and a small crustal thinning of 0.0034 mm/year. This low level of strain means that this zone experienced a little seismic deformation. There is also a right lateral shear motion along the ESE–WNW direction. This strain pattern is consistent with the predominant NW–SE normal faulting and ESE–WNW dextral transtensive faults in this zone. Comparing the results obtained from both stress and strain tensors, we find that the orientations of the principal axes of both tensors have the same direction with a small difference between them. Both tensors show a predominantly extensional domain. The nearly good correspondence between principal stress and strain orientations in the area suggests that the tectonic strength is relatively uniform for this crustal volume.  相似文献   

11.
The seismicity and the associated seismic hazard in the central part of the Pannonian region is moderate, however the vulnerability is high, as three capital cities are located near the most active seismic zones. In our analysis two seismically active areas, the Central Pannonian and Mur-Mürz zones, have been considered in order to assess the style and rate of crustal deformation using Global Positioning System (GPS) and earthquake data.We processed data of continuous and campaign GPS measurements obtained during the years 1991–2007. Velocities relative to the stable Eurasia have been computed at HGRN, CEGRN and EPN GPS sites in and around the Pannonian basin. Uniform strain rates and relative displacements were calculated for the investigated regions. GPS data confirm the mostly left lateral strike slip character of the Mur-Mürz–Vienna basin fault system and suggest a contraction between the eastward moving Alpine-North Pannonian unit and the Carpathians.The computation of the seismic strain rate was based on the Kostrov summation. The averaged unit norm seismic moment tensor, which describes the characteristic style of deformation, has been obtained from the available focal mechanism solutions, whereas the annual seismic moment release showing the rate of the deformation was estimated using the catalogues of historical and recent earthquakes.Our analysis reveals that in the Central Pannonian zone the geodetic strain rate is significantly larger than the seismic strain rate. Based on the weakness of the lithosphere, the stress magnitudes and the regional features of seismicity, we suggest that the low value of the seismic/geodetic strain rate ratio can be attributed to the aseismic release of the prevailing compressive stress and not to an overdue major earthquake. In the Mur-Mürz zone, although the uncertainty of the seismic/geodetic strain rate ratio is high, the seismic part of the deformation seems to be notably larger than in the case of the Central Pannonian zone. These results reflect the different deformation mechanism, rheology and tectonic style of the investigated zones.  相似文献   

12.
The current status of the kinematics and strain geometry of high-strain zone studies is briefly summarized. A general high-strain zone has a triclinic deformation path, and monoclinic shear zones are special end member cases. Fabrics observed in natural shear zones and theoretical considerations based on continuum mechanics are compatible with this conclusion. Non-steady deformation paths remain difficult to deal with, and may ultimately rely on a realistic mechanical treatment of high-strain zones which may be possible when our knowledge of the mechanical behavior of rocks under natural deformation conditions is improved. An examination of the phenomenon of slip partitioning in transpressional plate boundary regions shows that the bulk deformation path in the forearc area (trench-parallel high-strain zone) is generally triclinic. The Alpine Fault in the South Island of New Zealand provides an example of a currently active triclinic shear zone. The Southern Knee Lake shear zone of Manitoba, Canada, provides an Archean example of a triclinic shear zone.  相似文献   

13.
Folds and thrust faults formed by layer-parallel shortening coaxial with extensional structures such as normal dip-slip faults and ductile necking structures with orthorhombic fabric symmetry are usual, but little-recognised structures formed within normal dip-slip shear zones bounding rifts. They are generated because of the shear distribution in a zone of progressive deformation and may be later extended and disrupted depending on which part of the strain ellipsoid they may be located. We here describe folds and thrust faults from the southern margin of the Ala?ehir Rift in western Turkey as an opportunity to discuss the properties of pure extension–related structures formed by layer-parallel shortening. Such structures are more commonly generated during the early stages of rifting, when deformation rates are slow and the shear zones broader than those forming later in the life of a rift when strain rates are usually higher. Such structures have commonly been mistaken for witnesses documenting regional episodes of shortening rather than as integral parts of the extensional structures forming rifts. Not all layer-parallel shortening-related structures therefore indicate regional shortening. We plead that hasty statements concerning the meaning of geological structures at all scales be avoided before a thorough understanding of bulk strains that have affected a region are properly understood.  相似文献   

14.
The Alpi Apuane region of the Northern Apennines appears to have been deformed within a large-scale, low-angle shear zone with an overthrust sense of movement. The presence of mineral stretching lineations, folds progressively rotated into the X strain direction, and schistosities which intersect the nappe boundaries at small angles suggest that a component of shear strain occurred during the deformation. The strain ratios and orientations on two-dimensional sections have been determined from deformed marble breccias, reduction spots, and oncalites. Data from three or more non-perpendicular, non-principal sections have been combined to determine the finite strain ellipsoids at 33 sites within the shear zone.The finite strains have been separated into components of simple shear (γ), longitudinal strain (λ), and volume change (Δ). Algebraic expressions have been derived and graphs constructed which enable components of γ, γ and Δ, and γ and λ to be determined directly from a knowledge of strain ratio (R) within the shear zone and the angle (θ) between the principal strain direction and the shear zone boundary. The Alpi Apuane data indicate that neither simple shear alone, nor simple shear with volume change can satisfactorily explain the observed strains. Consideration of simple shear plus longitudinal strain leads to a general relationship in which the value of shear increases, and the values of longitudinal strain change along a SW-NE profile across the zone. Integration of the resulting shear strain-distance curves gives a minimum displacement of 4 km within the shear zone. Combination of the finite strains with the total time of deformation known from K/Ar studies leads to average strain rates from 1.4 to 9.6 × 10−15 sec−1.A characteristic flat-ramp-flat geometry initially formed the boundaries of what was later to develop into the overthrust shear zone, and deformation of the underlying crystalline basement is believed to have occurred by ductile shearing. Estimates of 21% crustal shortening for the region suggest that the crustal thickness prior to deformation was approximately 20 km in this part of the Northern Apennines.  相似文献   

15.
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16.
青藏高原中部第四纪左旋剪切变形的地表地质证据   总被引：7，自引：5，他引：2  
在青藏铁路的格尔木—拉萨段进行的活动断裂调查发现,在沱沱河—五道梁之间宽约150km的地段内发育了多条由北西西向次级断层左列分布构成的北西西向和北西向左旋张扭性断裂带,在断裂带之间则发育"S"型的北东向裂陷盆地和雁列分布的菱形裂陷盆地,盆地边界断裂也为左旋张扭性质。上述断裂带和裂陷带主要形成于第四纪,它们构成了宽约150km的不均匀的左旋简单剪切变形域,该变形域的整体活动性较弱,属于弱的不均匀剪切变形域。但其中的二道沟断陷盆地是个例外,该盆地边界断裂的垂直活动速率约为0 5mm/a,左旋活动速率介于0 8～1 0mm/a之间。而在整个左旋剪切变形带累计的左旋走滑速率不会超过6mm/a,它们所调节的昆仑山与唐古拉山之间的地壳南北缩短量也可能仅占总缩短量的15%～30%。上述弱剪切变形域与强烈左旋走滑的昆仑断裂系共同构成了高原中部的左旋剪切变形带,它们在印度板块与欧亚板块强烈碰撞的构造动力学背景下,起着调节青藏高原南北向缩短的重要作用。  相似文献   

17.
Geometrical relations between quartz C-axis fabrics, textures, microstructures and macroscopic structural elements (foliation, lineation, folds…) in mylonitic shear zones suggest that the C-axis fabric mostly reflects the late-stage deformation history. Three examples of mylonitic thrust zones are presented: the Eastern Alps, where the direction of shearing inferred from the quartz fabric results from a late deformation oblique to the overall thrusting; the Caledonides nappes and the Himalayan Main Central Thrust zone, where, through a similar reasoning, the fabrics would also reflect late strain increments though the direction of shearing deduced from quartz fabric remains parallel to the overall thrusting direction. Hence, the sense of shear and the shear strain component deduced from the orientation of C-axis girdles relative to the finite strain ellipsoid axes are not simply related nor representative of the entire deformation history.  相似文献   

18.
The strain geometry, microstructure and metamorphism is described from two minor shear zones from the Chatelaudren metagabbro, N. Brittany. A serially slabbed shear zone reveals a strain geometry consistent with simple shear deformation. Strain calculations based on X trajectory angles coincide with those obtained from elliptical mineral clusters at high values of strain only. Strain profiles typically show a broad low-strain region with a narrow high-strain peak at the centre of the zones. Microstructures typically show distinct grain size reduction in both amphibole and feldspar towards the high strain region of shear zones, and this is discussed in terms of deformation mechanisms related to strain softening. A palaeostress estimate based on recrystallized feldspar grain sizes gives a differential stress of 32 MPa for the low strain region and 119 MPa in the shear zone centre. Electron probe analyses reveal chemical and mineralogical changes accompanying metamorphism within the shear zones. This suggests local conditions favourable for ionic diffusion and the activity of fluids is implied.  相似文献   

19.
We conducted a comprehensive 40Ar/39Ar geochronological study of the Jiali and Gaoligong shear zones to obtain a better understanding of crustal deformation and tectonic evolution around the Eastern Himalayan Syntaxis (EHS). The new age data reveal that the main phase of deformation in the Jiali and Gaoligong shear zones occurred from 22 to 11 Ma and from 18 to 13 Ma, respectively. Structural data collected during this study indicate that the Jiali shear zone underwent a change in shear sense from sinistral to dextral during its movement history. Based on a comparison with the deformation histories of other major shear zones in the region, we argue that the initial sinistral motion recorded by the Jiali shear zone was coincident with that of the Ailao Shan–Red River shear zone, which marked the northern boundary of the southeastward extrusion of the Indochina block during the Early Miocene. From the Middle Miocene (～18 Ma), the Jiali shear zone changed to dextral displacement, becoming linked with the dextral Gaoligong shear zone that developed as a consequence of continued northward indentation of the Indian continent into Asia. Since this time, the Jiali and Gaoligong shear zones have been united, defining the southwestern boundary of the EHS during clockwise rotation of the eastward-extruding Tibetan block, as revealed by recent GPS data. The temporal change in regional deformation pattern from southeastward block extrusion to clockwise rotation of crustal fragments may have played an important role in the development of the eastern Himalayan drainage system around the EHS.  相似文献   

20.
The MCT Zone of Bhagirathi valley of Garhwal Himalaya is characterized by numerous mesoscopic ductile shear zones. These shear zones are developed in response to nearly NNE-SSW maximum horizontal compression and provide an opportunity to study the variation in strain and crystallographic fabrics within the ductile shear zones.The grain shape and orientation of quartz under microscope reflect that strain is higher in the center and it progressively decreases towards the shear zone boundary. The preferred orientation of quartz c-axes across the shear zone suggests that the single girdle of the quartz c-axes are probably first developed at the shear zone boundary and become prominent in the center of shear zone with increase in the intensity of deformation. The strong crystallographic preferred orientation normal to foliation suggests that the internal deformation of the quartz might have taken place by dislocation creep mechanism exhibiting a non-coaxial deformation history.  相似文献   

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