P- andSV-velocity structure of the South Portuguese Zone fold-and-thrust belt, SW Iberia, from traveltime tomography
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Imaging the architecture of the shallow crust of the South Portuguese Zone fold-and-thrust belt is essential to extend surface mapped geological information to depth and to help in developing models of the ore-bearing Iberian Pyrite Belt part of the Variscan orogeny. The recently acquired IBERSEIS seismic-reflection data set provides, for the first time, detailed images of the entire crust, but source-generated noise masks the earliest reflections and limits the shallowest observed signals to depths >500 m. We inverted P- and SV first-arrival traveltimes for the smoothest minimum-structure velocity models, imaging the shallowest few hundreds of metres along four in total ∼60-km-long profiles. A comparison of a 2-D and 2.5-D (3-D forward and 2-D inverse problem) crooked-line inversion scheme revealed that the crooked-line geometry has a negligible effect on the final images. Resolution of the final preferred models was assessed on the basis of an extensive series of checkerboard tests, showing a slightly lower resolution capability of the SV-data due to greater data uncertainty, fewer number of picks and more limited source–receiver offsets compared with the P-data. The preferred final models compare favourably with the mapped surface geology, showing relatively high and uniform velocities (>5.25 km s−1) for the flysch group in the southern part of the investigation area. Low velocities (∼4.5 km s−1) are found for the 'La Puebla de Guzman antiform' in the centre of the investigation area, where the phyllite–quartzite group is exposed. Velocities fluctuate the most along the northernmost ∼20 km. Velocity variations reflect more the state of tectonic deformation than being directly correlated with the mapped lithologies. Based on a comparison with coincident seismic-reflection data along the southern half of the area, we suggest that two areas of low to intermediate ratios (∼1.85–1.9) correspond to occurrences of thick and less deformed flysch-group units, whereas high ratios (∼1.95) are interpreted to indicate increased porosity due to intense fracturing.Keywords:
Seismic vibrator
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According to the surface geologic surveys and seismic section interpretations, it is held that Kuche fold—thrust belts have the structural characteristics of zonal assemblage, that is, the southern detachment fold each matches with the northern fault-propagation fold each, for example, Kasangtuokai anticline with Jidike anticline, Tuzimaza anticline with Dawanqi anticline, Yakelike anticline with Misikantage anticline, eastern Qiulitage anticline with Yaken anticline etc. Besides the characteristics of zoning in S—N direction, layering vertically and sectioning in E—W direction, Kuche fold—thrust belts have the characteristic of structures blocking, which can be divided into four blocks of Quele, Kela, Dina and Yangxia from the west to the east. The hydrocarbon accumulation in the Kuche fold—thrust belt mainly relates to the lateral ramp anticlines.
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Echelon formation
Growth fault
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Thrust fault
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Clockwise
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The purpose of this report is to discuss the origin and mechanism of landslides in the vicinity of Higashiyama fold zone from the tectonomechanical standpoint. Northern part of Higashiyama anticline is represented as the ansymmetrically overturned anticline, because, the west limb has steep dips and the east limb has gentle dips.Here the landslides occur at the different areas. One is the eastern limb and the syncline area, where the origin of landslides is considered solely due to physical and chemical properties of rocks.On the other hand, the southern part of Higashiyama anticline is symmetrical, and both limbs have high angles, where landslides occur intermittently along the anticline axis. We concluded that it is based mainly on the structurally disturved anticline.
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Abstract The study area is considered to be a part of Arabian plate, located within High Folded Zone of the Zagros Fold-Thrust Belt in the Iraqi Kurdistan Region. This area consists of a major anticline called Harir anticline. The study of this structure was conducted along five traversal sections through 57 field stations in order to understand the geometry and elucidate structural model for generation and propagation of this fold. Thirteen formations were exposed covering this anticline, ranging in age from Early Cretaceous up to the Pleistocene. Geometrical analysis of Harir fold indicates that this fold is asymmetrical double plunging anticline, gentle to open and non-cylindrical, curvi-planar anticlinal structure. Due to presence of three strike slip-faults cross cutting this anticline, it segmented into four transversal blocks which are namely Batas, Harir, Sheikh-Mamudian and Ashkafta blocks. Each block has its own structural, sedimentlogical, and morphological properties making it different from the other blocks. Detailed balanced and retro-deformable cross sections in the studied area reveal that the shortening increases gradually from northwest toward middle part and decreases towards southeastern part of Harir Anticline. The calculated shortening consecutively is 6.76%, 14.30% and 14.31 % 10.52%, 5.48% from northwestern plunging area to the southeastern plunge. The depth to detachment surface has been calculated, ranging between 10-10.5km below the regional level of Bekhme Formation. This depth coincides with surface of the Upper Paleozoic Ora shale formation, which indicates this anticline is generated from this depth and propagated upwards. Structural model is constructed for this anticline taking into consideration the two mechanisms informing this structure; one is the occurrence of slipping along detachment surface and second is the effect of the strike-slip faults (Tear faults) cross cutting Harir structure. These two mechanisms acted synchronously and beginning at least from Late Cretaceous to the end of the Tertiary.
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Echelon formation
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Regional 3D seismic data of the deep-water area offshore NW Borneo provide a detailed picture of the interaction between sedimentary processes on a continental slope and the growth of major folds over a time period of ca. 3.5–5 Ma. In the deep-water area, the estimated rates of fold propagation of tens of mm/yr−1, shortening rates of mm/yr−1, and fold segment lengths of tens of kilometers indicate the studied folds are similar in scale and deformation rate to folds in orogenic belts such as the Zagros Mountains and Himalayas. Feedbacks between sediment dispersal patterns, anticline growth, and structural style are manifested in many ways, and are enhanced by the presence of weak, poorly lithified, synkinematic sediments at fold crests that undergo mass wasting as the fold grows. As folds tighten they range in geometry through simple folds—folds affected by crestal normal faults, folds with crestal normal faults and rotational slides, and folds with forelimb degradation complexes and pronounced erosional unconformities. The unconformity surfaces are either elongate parallel to the fold axes (related to local mass wasting) or perpendicular (related to flows crossing the anticlines). Mass wasting processes in the study area that are large in scale compared with folds (i.e., giant landslides) have modified anticline shape by erosion, and are little affected by anticline topography. More commonly, gravity flows are relatively small compared with the anticlines, and transport pathways are influenced by anticline surface topography. The factors influencing sediment pathway changes during anticline growth include: proximal to distal propagation of folds, lateral propagation of folds, and changing locations of fold growth with time. Assuming an overall consistency in sediment supply, local relative changes in sediment supply to individual piggyback basins are defined by relative changes in sediment supply (generally increasing with time) with respect to growth in anticline amplitude. Such changes may be due to sediment infilling of depressions farther updip and prograding downslope with time or to changes in rate of deformation with time. Initial sediment pathways were predominantly subparallel to growing faults. As folds matured, canyons and channels exploited low points (e.g., fold linkage or intersection points) or weak points (e.g., mud pipes) on folds to initiate transverse channel systems. At a late stage, transverse channels carved up the once long and linear surface ridges into isolated segments.
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