Numerical analysis of overburden soil subjected to strike-slip fault: Distinct element analysis of Nojima fault

A distinct element method analysis is carried out to examine the development of shear bands in overburden soil subjected to a strike-slip fault. About 2.3 million spherical particles are used in the analysis and the results are compared with the shears observed at the Nojima earthquake fault during the 1995 Hyogoken Nanbu earthquake. En echelon shears and secondary shears which strike at lower angles to the basement fault – typical in strike-slip faults – are observed in the numerical analysis. Simple shear in the horizontal plane and drag due to the dependence of velocity on depth are confirmed to control the helicoidal shape of Riedel shears. Rotation of the compressional direction toward the fault strike as a result of slip along Riedel shears is also verified. It is found that the compressional direction is more horizontal within the area enclosed by Riedel shears than in outside areas and that these compressional directions produce secondary lower-angle shears that are less helicoidal. It is shown that the formation of column-like structures of particles and their subsequent buckling play significant micromechanical roles in three-dimensionally wrenched shears. The results of the numerical analysis, such as shear intervals and striking angles, show a resemblance to observational results at sites where sediment contains coarse grains and is subjected to strike slip with a small dip component, although they are not exactly the same as those observed at locations with similar overburden thicknesses.     

The initiation and linkage of surface fractures above a buried strike-slip fault: An experimental approach

Surface fractures in the overburdened sedimentary rocks, formed above a deep-seated basement fault, often provide important information about the kinematics of the underlying master fault. It has already been established that these surface fractures dynamically evolve and link one another with progressive displacement on the master fault below. In the present study, two different series of riedel-type experiments were carried out with clay analogue models under different boundary conditions viz., (i) heterogeneous simple shear of the cover rocks above a buried strike slip fault (wrench system) and (ii) heterogeneous simple shear with a component of shear-normal compression of the overburden package above a basement fault (transpressional system), to observe the initiation and linkage of surface fractures with varying T′ (where T′ = thickness of the overburden normalized with respect to the width of the master fault). In the wrench system, Riedel (R) shears were linked by principal displacement (Y) shears at early stages (shear strain of 0.8 to 1) in thin (2 < T′ < 18) models and finally (at a minimum shear strain of 1.4) gave rise to a through-going fault parallel to the basement fault without development of any other fracture. Conjugate Riedel (R′) shears develop only within the thicker (T′ > 18) clay models at a minimum shear strain of 0.7. With increasing deformation (at a minimum shear strain of 1.2) two R′ shears were joined by an R shear and finally opened up to make a sigmoidal vein with an asymmetry antithetic to the major fault-movement sense. Under transpression, the results were similar to that of heterogeneous simple shear for layers 2 < T′ < 15. In layers of intermediate thickness (15 < T′ < 25) early formed high angle R shears were cross cut by low angle R shears (at a minimum shear strain of 0.5 and shortening of 0.028) and “Riedel-within-Riedel” shears were formed within thick (T′ > 25) models (at minimum shear strain of 0.7 and shortening of 0.1), with marked angularity of secondary fault zone with the master fault at depth.     

Seafloor spreading, obduction and triple junction tectonics of the Mineoka Ophiolite, Central Japan

The Tertiary Mineoka ophiolite occurs in a fault zone at the intersection of the Honshu and Izu forearcs in central Japan and displays structural evidence for three major phases of deformation: normal and oblique-slip faults and hydrothermal veins formed during the seafloor spreading evolution of the ophiolite at a ridge-transform fault intersection. These structures may represent repeated changes in differential stress and pore-fluid pressures during their formation. The second series of deformation is characterized by oblique thrust faults with Riedel shears and no significant mineral veining, and is interpreted to have resulted from transpressional dextral faulting during the obduction of the ophiolite through oblique convergence and tectonic accretion. This deformation occurred at the NW corner of a TTT-type (trench–trench–trench) triple junction in the NW Pacific rim before the middle Miocene. The third series of deformation of the ophiolite is marked by contractional and oblique shear zones, Riedel shears, and thrust faults that crosscut and offset earlier structures, and that give the Mineoka fault zone its lenticular (phacoidal) fabric at all scales. This deformation phase was associated with the establishment and the southward migration of the TTT Boso triple junction and with the kinematics of oblique subduction and forearc sliver fault development. The composite Mineoka ophiolite hence displays rocks and structures that evolved during its complex geodynamic history involving seafloor spreading, tectonic accretion, and triple junction evolution in the NW Pacific Rim.     

Initiation and development of pull-apart basins with Riedel shear mechanism: insights from scaled clay experiments

Typical pull-apart structures were created in scaled clay experiments with a pure strike-slip geometry (Riedel type experiments). A clay slab represents the sedimentary cover above a strike-slip fault in the rigid basement. At an early stage of the development of the deformation zone, synthetic shear fractures (Riedel shears) within the clay slab display dilatational behaviour. With increasing basal displacement the Riedel shears rotate and open further, developing into long, narrow and deep troughs. The shear displacement and the low angle with the prescribed principal basal fault set them apart from tension gashes. At a more evolved stage, synthetic segments (Y-shears) parallel to the basal principal fault develop and accommodate progressive strike-slip deformation. The Y-shears connect the tips of adjacent troughs developed from the earlier Riedel shears, resulting in the typical rhomb-shaped structures characteristic for pull-apart basins. The Strait of Sicily rift zone, with major strike-slip systems being active from the Miocene to the Present, comprises pull-apart basins at different length scales, for which the structural record suggests development by a mechanism similar to that observed in our experiments.     

Faults in Quaternary cover as a relfection of basement tectonics: Kolguev island,barents sea

The paper is concerned with the results of the detailed study of dislocations in the Pliocene-Quaternary loose sediments exposed as cliffs extending for ～30 km along the rectilinear shore of Kolguev Island. According to seismic data, this lineament is related to the Coastal Fault in the lower part of the sedimentary cover. A system of faults longitudinal, diagonal, and transverse relative to the shoreline is established from observations at the cliffs. Their arrangement in plan view corresponds to the geometry of the right-lateral shear zone, the axis of which almost coincides with the shoreline. This has allowed us to identify the faults as secondary disturbances in the region of dynamic effect of the Coastal Fault in the basement. The kinematics of the secondary faults and their dip azimuth are consistent with echeloned geometry in plan view. The low-angle dip of the reverse-strike-slip faults observed at the outcrops is caused by their near-surface flattening toward the subsided block. The shallow-seated dislocations are related to ductile lateral shear in the vertical plane. The lower layers of the sedimentary cover mimic the horizontal movements in the basement more closely than the upper layers. The data obtained indicate high neotectonic activity of the Barents Sea shelf and specify the geodynamic setting of the region in the Pliocene and Quaternary.     

The current tectonic motion of the Northern Andes along the Algeciras Fault System in SW Colombia

Riedel, synthetic and antithetic type faults, principal displacement zones (PDZ), pull-apart basins (such as lazy-S shaped releasing bend, extensive and rhomboidal shaped and releasing sidestep basins) and minor folds located oblique to the main trace of the Algeciras Fault System (AFS) are interpreted from Landsat TM 5 images and geological mapping. These tectonic features are affecting Quaternary deposits and are related to major historical earthquakes and recent registered seismic events, indicating neotectonic activity of the structure.The AFS is classified as a right lateral wrench complex structure, with an important vertical component in which sedimentary cover and basement rocks are involved. In addition, the system represents a simple shear caused by the oblique convergence between the Nazca Plate and the northern Andes. The transpressive boundary in SW Colombia was previously located along the Eastern Frontal Fault System. However, this paper shows that the AFS constitutes the actual boundary of the current transpressive regime along the Northern Andes, which begins at the Gulf of Guayaquil in Ecuador and continues into Colombia and Venezuela.     

Exhumation of the Tauern window,Eastern Alps

The exhumation of metamorphic domes within orogenic belts is exemplified by the Tauern window in the Eastern Alps. There, the exhumation is related to partitioning of final orogenic shortening into deep-seated thrusts, near-surface antiformal bending forming brachyanticlines, and almost orogen-parallel strike-slip faults due to oblique continental plate collision. Crustal thickening by formation of an antiformal stack within upper to middle crustal portions of the lower lithosphere is a prerequisite of late-stage orogenic window formation. Low-angle normal faults at releasing steps of crustal-scale strike-slip faults accomodate tectonic unloading of synchronously thickened crust and extension along strike of the orogen, forming pull-apart metamorphic domes. Initiation of low-angle normal faults is largely controlled by rock rheology, especially at the brittle-ductile transitional level within the lithosphere. Several mechanisms may contribute to uplift and exhumation of previously buried crust within such a setting: (1) Shortening along deep-seated blind thrusts results in the formation of brachyanticlines and bending of metamorphic isograds; (2) oversteps of strike-slip faults within the wrench zone control the final geometry of the window; (3) unloading by tectonic unroofing and erosional denudation; and (4) vertical extrusion of crustal scale wedges. Rapid decompression of previously buried crust results in nearly isothermal exhumation paths, and enhanced fluid circulation along subvertical tensile fractures (hydrothermal ore and silicate veins) that formed due to overall coaxial stretching of lower plate crust.     

Recent crustal movements in The Central Sierra Nevada—Walker lane region of California—Nevada: Part iii, The Olinghouse fault zone

The Olinghouse fault zone is one of several NE—ENE-trending fault zones and lineaments, including the Midas Trench and the Carson—Carson Sink Lineament, which exhibit left-lateral transcurrent movement conjugate to the Walker Lane in western Nevada. The active portion of this fault zone extends for approximately 23 km, from 16 km east of Reno, Nevada, to the southern extent of Pyramid Lake. The fault can be traced for most of its length from its geomorphic expression in the hilly terrain, and it is hidden only where overlain by recent alluvial sediments. Numerous features characteristic of strike-slip faulting can be observed along the fault, including: scarps, vegetation lines, sidehill and shutter ridges, sag ponds, offset stream channels and stone stripes, enclosed rhombohedral and wedge-shaped depressions, and en-echelon fractures.A shear zone having a maximum observable width of 1.3 km is defined principally by Riedel shears and their symmetrical P-shears, with secondary definition by deformed conjugate Riedel shears. Several continuous horizontal shears, or principal displacement shears, occupy the axial portion of the shear zone. The existence of P-shears and principal displacement shears suggests evolution of movement along the fault zone analogous to the “Post-Peak” or “Pre-Residual Structure” stage.Historic activity (1869) has established the seismic potential of this zone. Maximum intensities and plots of the isoseismals indicate the 1869 Olinghouse earthquake had a magnitude of 6.7. Field study indicates the active length of the fault zone is at least 23 km and the maximum 1869 displacement was 3.65 m of left-slip. From maximum fault length and maximum fault displacement to earthquake magnitude relations, this corresponds to an earthquake of about magnitude 7.     

Recent crustal movements in the Central Sierra Nevada—Walker lane region of California—Nevada: Part ii, The pyramid lake right-slip fault zone segment of the Walker lane

The Pyramid Lake fault zone is within the Honey Lake—Walker Lake segment of the Walker Lane, a NW-trending zone of right-slip transcurrent faulting, which extends for more than 600 km from Las Vegas, Nevada, to beyond Honey Lake, California. Multiscale, multiformat analysis of Landsat imagery and large-scale (1: 12,000) lowsun angle aerial photography, delineated both regional and site-specific evidence for faults in Late Cenozoic sedimentary deposits southwest of Pyramid Lake. The fault zone is coincident with a portion of a distinct NW-trending topographic discontinuity on the Landsat mosaic of Nevada. The zone exhibits numerous geomorphic features characteristic of strike-slip fault zones, including: recent scarps, offset stream channels, linear gullies, elongate troughs and depressions, sag ponds, vegetation alignments, transcurrent buckles, and rhombohedral and wedge-shaped enclosed depressions. These features are conspicuously developed in Late Pleistocene and Holocene sedimentary deposits and landforms.The Pyramid Lake shear zone has a maximum observable width of 5 km, defined by Riedel and conjugate Riedel shears with maximum observable lenghts of 10 and 3 km, respectively. P-shears have formed symmetrical to the Riedel shears and the principal displacement shears, or continuous horizontal shears, isolate elongate lenses of essentially passive material; most of the shears are inclined at an angle of approximately 4° to the principal direction of displacement. This suggests that the shear zone is in an early “PreResidual Structure” stage of evolution, with the principal deformation mechanism of direct shear replacing the kinematic restraints inherent in the strain field.Historic seismic activity includes microseismic events and may include the earthquake of about 1850 reported for the Pyramid Lake area with an estimated Richter magnitude of 7.0. Based on worldwide relations of earthquake magnitude to length of the zone of surface rupture, the Pyramid Lake fault zone is inferred to be capable of generating a 7.0–7.5-magnitude event for a maximum observable length of approximately 6 km and a 6.75–7.25-magnitude event for a half length of approximately 30 km.     

江南造山带北部早中生代岳阳—赤壁断褶带构造特征及变形机制研究

早中生代(晚印支-早燕山期)岳阳-赤壁断褶带位于江南造山带与中扬子前陆盆地交界地带.作者对该构造带进行了地表地质调查,以此为基础探讨了构造剖面结构及构造变形动力机制.岳阳-赤壁断褶带自南而北可分为岳阳-临湘基底滑脱-逆冲带,桃花泉-肖家湾盖层滑脱褶皱带,以及赤壁-嘉鱼前陆盆地断-褶-盆构造带.岳阳-临湘基底滑脱-逆冲带自南而北依次有郭镇向斜、官山背斜、临湘倒转向斜和聂市背斜,组成隔槽式褶皱组合.褶皱轴面多向南倾,褶皱变形面为南华系盖层与冷家溪群褶皱基底间的角度不整合面和顺界面的滑脱断裂面.桃花泉-肖家湾盖层滑脱褶皱带主要发育轴面南倾倒转褶皱,褶皱波长较小,卷入地层为南华系-志留系以及上石炭统-中三叠统沉积盖层.赤壁-嘉鱼前陆盆地断-褶-盆构造带以南倾蒲圻断裂(江南断裂)为南部边界,发育T3-J2前陆盆地沉积,带内褶皱与断裂卷入地层包括沉积盖层以及T3-J2地层:南部断裂与褶皱轴面南倾.北部轴面近直立.自南西至北东,研究区内构造线走向由EW向渐变为NEE-NE向.上述构造分带及变形特征反映出自南向北的运动指向,表明岳阳-赤壁断褶带具前陆冲断带构造性质.从断裂相关褶皱理论出发,以地表构造特征为依据,厘定了岳阳-赤壁地质剖面结构并进行了变形动力机制分析,认识如下:①自南而北、自下而上的多个滑脱层及其间的南倾逆断裂或断坡(主要为江南断裂)组成近似台阶状的逆冲断裂系统,从总体上控制了构造块体的滑移、逆冲以及相应的构造格架或变形分区.②郭镇向斜为基底滑脱褶皱,官山背斜具滑脱褶皱和断裂传播褶皱双重成因,聂市背斜为断裂转折褶皱;临湘向斜为受两侧背斜控制的被动向斜,由于弯滑褶皱作用在其两翼沿不整合界面形成滑脱断裂.③岳阳-临湘基底滑脱-逆冲带隔槽式褶皱的形成主要受控于褶皱基底的滑脱和基底整体的水平压缩,其形成机制类似于肿缩式褶皱.最后讨论认为湘东北-鄂东南地区不存在大规模、长距离的逆冲推覆构造.     

Structural analysis of the Gachsar sub-zone in central Alborz range; constrain for inversion tectonics followed by the range transverse faulting

Analysis of the Gachsar structural sub-zone has been carried out to constrain structural evolution of the central Alborz range situated in the central Alpine Himalayan orogenic system. The sub-zone bounded by the northward-dipping Kandovan Fault to the north and the southward-dipping Taleghan Fault to the south is transversely cut by several sinistral faults. The Kandovan Fault that controls development of the Eocene rocks in its footwall from the Paleozoic–Mesozoic units in the fault hanging wall is interpreted as an inverted basin-bounding fault. Structural evidences include the presence of a thin-skinned imbricate thrust system propagated from a detachment zone that acts as a footwall shortcut thrust, development of large synclines in the fault footwall as well as back thrusts and pop-up structures on the fault hanging wall. Kinematics of the inverted Kandovan Fault and its accompanying structures constrain the N–S shortening direction proposed for the Alborz range until Late Miocene. The transverse sinistral faults that are in acute angle of 15° to a major magnetic lineament, which represents a basement fault, are interpreted to develop as synthetic Riedel shears on the cover sequences during reactivation of the basement fault. This overprinting of the transverse faults on the earlier inverted extensional fault occurs since the Late Miocene when the south Caspian basin block attained a SSW movement relative to the central Iran. Therefore, recent deformation in the range is a result of the basement transverse-fault reactivation.     

Displacement features associated with fault zones: a comparison between observed examples and experimental models

Fault zones commonly consist of discontinuous surfaces or are composed of different kinds of en echelon fractures (tension cracks, R or P-type fractures) which give the brittle shear zone a width. Translation along these faults and related features can occur by sliding on the elementary planes and/or by solution/deposition processes with opening of transtension zones and possible dilatation of the shear zone.In order to understand how fault planes develop and to investigate the mechanical conditions corresponding to natural configurations, displacement along an analogue fault model was examined under conditions of direct shear. The experimental fracture patterns were then compared with the natural features. The structural elements of certain faults created by testing, and showing transpression and transtension zones, were found to be similar to natural domino structures bounded by elementary shears, and could be compared with computed situations.It can be inferred that stresses are reoriented inside the shear zone; the angle between elementary fractures depends on their order of development; transpression and transtension zones occur systematically; and the shear zone undergoes dilatancy under low normal stresses.     

邢北深部勘查区低角度正断层成因浅析

通过邢北深部勘查区钻孔中所见的断层构造迹象分析,认为原来部分定为高角度的正断层是受太行山隆起赞皇变质核杂岩东侧拆离构造带影响而派生的中、小型低角度正断层。由于变质基底与上覆沉积盖层之间为软硬差距较大的主拆离构造面,在其影响下,上覆地层发育了若干个次级拆离构造面,邢北深部低角度正断层正是受基底拆离构造影响而形成。     

Tectonic and lithological constraints on the evolution of the Karoo graben of northern Malawi (East Africa)

The results of a lithostratigraphic, tectonic and kinematic study of the Karoo deposits of northern Malawi are reported. The objective of the lithostratigraphic study is to correlate the deposits of the Karoo basins of northern Malawi with the well-known deposits of southern Tanzania, thus establishing a stratigraphic framework through which the timing of faulting can be constrained. The kinematic analysis of faulting constrains the opening direction for the Karoo graben in this area and provides basic data to discuss the Karoo graben development within the regional tectonic framework of south-eastern Africa. The studied adults are defined by moderately to steeply dipping cataclastic zones with a width of up to 15 m and are characterized by an array of slickensided fault surfaces with different orientations and slip directions. In this study, small faults (offset < 10 m) and meso-scale faults (offset > 10 m, but generally not exceeding 30–40 m) have been distinguished. Methods used to analyse the kinematic data include the ‘pressure tension’ (PT) method, which estimates the principal axes for the bulk brittle strain, and the internal rotation axis (IRA) method, which estimates the axis of bulk internal rotation and the overall sense of slip at the faults. A mass balance calculation reveals a volume increase of up to 16% during cataclastic deformation in the fault zones. The PT method shows an approximately east trending extension direction for faults that occur only in the latest Carboniferous (?) and Early Permian strata, whereas the fault kinematics from faults that cut middle Permian to Early Triassic rocks is characterized by a ESE to SE trending extension direction. The small faults yield essentially the same kinematic results as the meso-scale faults. In a transport-parallel cross-sectional view, the principal extension axes are at an acute angle of approximately 60° to the major fault planes. Given the moderate fault density, the relatively high angle between the orientation of the principal extension axis and the fault planes suggest only a moderate amount of horizontal extension across the Karoo graben of northern Malawi. Riedel structures in the fault zones formed within two conjugate sets of localized shear zones; slip on one set was top to the W/NW and, on the other, top to the E/SE. The two conjugate sets of Riedel structures have an acute angle about the regional shortening axes, implying that no pronounced rotation of the strain axes occurred. The internal rotation axes for the Riedel structures reveal a largely bimodal distribution and inferred weakly monoclinic to orthorhombic symmetry. Therefore the overall deformation during Karoo rifting in northern Malawi is interpreted to be close to a coaxial deformation with a limited amount of horizontal extension.[/p]     

里德尔剪切的组合型式与走滑盆地组合型式的相似性

走滑断裂体系中经常发育里德尔剪切的断裂组合,世界上不同构造背景下与走滑断裂相关的盆地(走滑盆地)也很多见。因此里德尔剪切是地质构造研究中的重要方面。我们从三个方面对里德尔剪切的节理构造组合及盆地组合进行了对比:(1)里德尔剪切构造组合与比例尺无关。里德尔剪切带的(转换)拉张区与沉积盆地的分布区是相似的;(2)断裂的最大位移区与断陷盆地的沉积中心是一致的,断陷盆地长轴平行于断裂走向;(3)物理模拟试验及数学模拟试验都证实了走滑盆地的演化。基于上述认识,我们通过厘米级岩芯标本的观察,结合已发表的盆地资料,提出了6类与里德尔剪切有关的构造组合及断陷盆地组合。(1)雁列状构造及盆地组合:许多盆地发育雁列状构造。同时,与里德尔剪切相关的雁列状盆地的宽度与主剪切断裂的剪切位移呈正相关。(2)帚状或马尾状构造及盆地组合:二者在形态上相似,所以归为一类。成因上,马尾状构造及盆地主要发育在走滑断裂的拉张端部,而帚状构造或盆地反映走滑断裂的旋扭作用,可以在走滑断裂影响区域的任何部位。(3)串珠状构造及盆地组合:该类型的盆地主要是指释压盆地的组合,拉分盆地也可以形成串珠状盆地。(4)S状或Z状构造或盆地组合:左行走滑形成Z状构造或盆地,而右行走划形成S状构造或盆地。(5)多字型构造及盆地组合:是拉分盆地的典型组合,可以过渡到串珠状盆地。(6)复杂的网状构造及盆地组合:通常是由于分布型简单剪切的作用结果。以上盆地组合类型包括大型盆地内次级单元(次级盆地或更次级盆地)的组合,但不包括多成因、多期活动的构造及盆地。     

Shear zones in clay-rich fault gouge: A laboratory study of fabric development and evolution

Clay-rich fault rocks have long been recognized to host distinctive fabric elements, and fault rock fabric is increasingly thought to play a fundamental role in fault mechanical behaviour in the brittle regime. Although the geometries of fabric elements in fault gouges have been described for almost a century, the genesis and evolution of these elements during shear, and their links to bulk mechanical properties, remain poorly understood. We characterize the development and evolution of fabric elements with increasing shear in a variety of clay-rich experimental gouges over shear strains of <1 to >20 and at normal stresses of 2–150 MPa in the double-direct shear configuration. In addition to SEM observations of experiment products at a variety of shear strains, we quantified clay fabric intensity and the degree of grain size reduction using X-ray Texture Goniometry (XTG) and particle size distribution (PSD) measurements. We also measured P- and S-wave velocities during shear to further probe the evolution of shear fabric and gouge properties. We find that clay fabric elements develop in a systematic manner regardless of the gouge material. Riedel shears in the R₁ orientation and boundary-parallel shears are the dominant fabric elements. Riedel shears nucleate at layer margins and propagate into the layer shortly after reaching yield stress. Clay particles rotate into the P-orientation shortly after Riedels propagate through the layer. The Riedel shears are through-going, but are >10× thinner than similar zones observed in coarser granular materials. Our results suggest that the weakness of clay-rich fault gouge may be less a function of anisotropic crystal structure, as has been suggested previously, and more a consequence of very thin shear surfaces permitting deformation in clay-rich materials with minimal dilation or cataclasis. The very thin shear surfaces are a function of the fine grain size of the materials and possibly polymodal PSD's.     

Analogue models of folds above a wrench fault

This paper describes the experimental deformation of models made with sheets of paraffin wax, simulating a bedded cover resting on a basement wrench fault. During the experiments, “en échelon” folds appear in the cover. As a result of early fault motion, folds first appear at heterogeneities in the bedding and with axes at about 45° to the trace of the wrench fault. Further fault displacement causes a bulk rotation of fold axes towards parallelism with the basement wrench fault, and a resulting curvature of fold axes at larger fault displacement.Folding affects an area which tends to quickly stabilize in width, since folding weakens the sheared cover and subsequent deformation is concentrated in it. Axial surfaces of folds are initially upright, then tend to become inclined with an external vergence, forming a fan centered on the basement wrench fault. Deeper layer-deformation, close to the basement, involves fold reorientations that are greater than in the upper layers. Therefore, down a given vertical line, there is no continuity between surface and deep structures. The geometry and orientation of folds appearing at later stages of wrenching is controlled by the geometry and orientation of already extant folds.     

Faulting history at the eastern termination of the High Atlas Fault (Western High Atlas,Morocco)

At its eastern termination, the High Atlas Fault in the Western High Atlas in Morocco, consists of a splay of three faults. In the interjacent fault blocks, Neo- and Paleoproterozoic basement, forming the northernmost extremity of the NW-African Craton, is cropping out. The Precambrian basement witnesses a long history of brittle deformation starting at the end of the Pan-African Orogeny. A subsequent episode of normal faulting can be related to the development of a Hercynian basin along the northern passive margin of the cratonic promontory. With regard to the main tectonic activity in the Western High Atlas, basically two models exist: one emphasising block tectonics reflecting Mesozoic rifting followed by Alpine uplift and inversion, the other emphasising Late Paleozoic dextral wrench tectonics. The analysis of the fault activity along the splay faults reveals a predominantly Alpine history, consisting of the Triassic development of the Atlas Rift along the axial zone of the orogen, followed by uplift and inversion. The Late Jurassic to Cenozoic fault activity took place in a sinistral transpressive regime and was partitioned over the three splay faults. Dextral strike-slip fault activity could not be demonstrated in the fault blocks nor along the splay faults. Therefore the faults were probably not involved in Late Paleozoic dextral wrench tectonics.     

Fault kinematics of the Magallanes-Fagnano fault system,southern Chile; an example of diffuse strain and sinistral transtension along a continental transform margin

A system of left-lateral faults that separates the South American and Scotia plates, known as the Magallanes-Fagnano fault system, defines the modern tectonic setting of the southernmost Andes and is superimposed on the Late Cretaceous – Paleogene Patagonian fold-thrust belt. Fault kinematic data and crosscutting relationships from populations of thrust, strike-slip and normal faults from Peninsula Brunswick adjacent to the Magallanes-Fagnano fault system, presented herein, show kinematic and temporal relationships between thrust faults and sets of younger strike-slip and normal faults. Thrust fault kinematics are homogeneous in the study area and record subhorizontal northeast-directed shortening. Strike-slip faults record east—northeast-directed horizontal shortening, west—northwest-directed horizontal extension and form Riedel and P-shear geometries compatible with left-lateral slip on the main splay of the Magallanes-Fagnano fault system. Normal faults record north-south trending extension that is compatible with the strike-slip faults. The study area occurs in a releasing step-over between overlapping segments of the Magallanes-Fagnano fault system, which localized on antecedent sutures between basement terranes with differing geological origin. Results are consistent with regional tectonic models that suggest sinistral shearing and transtension in the southernmost Andes was contemporaneous with the onset of seafloor spreading in the Western Scotia Sea during the Early Miocene.     

基于地应力实测数据分析郯庐断裂带中段滑动趋势

郯庐断裂带潜在滑动趋势对于区域地壳稳定性评价具有重要的意义。对郯庐断裂带中段(新沂至渤海段)附近12个钻孔共65条实测地应力数据分析,得出该区段最大和最小水平主应力与垂直应力之比Kmax和Kmin的范围,结果表明郯庐断裂带中段主要表现为逆断层的应力状态,局部地区呈现出有利于走滑断层的特征。研究区内应力比K值由西向东逐渐增大,并在东北部达到最大。通过计算得出断层面上的剪应力与有效正应力比值小于Byerlee准则所定义的摩擦系数,说明目前郯庐断裂带中段处于相对稳定状态;同时发现北西向断裂面上的剪应力与有效正应力比值比北东向断裂的大。区内应力积累指标μm的计算结果和地震的分布特征表明,郯庐断裂带中段北西向断裂上地震活动频繁,应力积累程度低;而北东向断层上应力积累程度高,特别是处于郯庐断裂带中段东北部的北东向断层,在应力集中的区域出现地震空白区,推测为断层的锁固段。最终认为在现今应力场下郯庐断裂带中段更易沿北西向的断裂滑动。