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1.
Fault affecting silicoclastic sediments are commonly enriched in clay minerals. Clays are sensitive to fluid–rock interactions and deformation mechanisms; in this paper, they are used as proxy for fault activity and behavior. The present study focuses on clay mineral assemblages from the Point Vert normal fault zone located in the Annot sandstones, a Priabonian-Rupelian turbidite succession of the Alpine foredeep in SE France. In this area, the Annot sandstones were buried around 6–8 km below the front of Alpine nappes soon after their deposition and exhumed during the middle-late Miocene. The fault affects arkosic sandstone beds alternating with pelitic layers, and displays throw of about thirty meters. The fault core zone comprises intensely foliated sandstones bounding a corridor of gouge about 20 cm thick. The foliated sandstones display clay concentration along S–C structures characterized by dissolution of K-feldspar and their replacement by mica, associated with quartz pressure solution, intense microfracturation and quartz vein precipitation. The gouge is formed by a clayey matrix containing fragments of foliated sandstones and pelites. However, a detailed petrographical investigation suggests complex polyphase deformation processes. Optical and SEM observations show that the clay minerals fraction of all studied rocks (pelites and sandstones from the damage and core zones of the fault) is dominated by white micas and chlorite. These minerals have two different origins: detrital and newly-formed. Detrital micas are identified by their larger shape and their chemical composition with a lower Fe–Mg content than the newly-formed white micas. In the foliated sandstones, newly-formed white micas are concentrated along S–C structures or replace K-feldspar. Both types of newly formed micas display the same chemical composition confirmed microstructural observations suggesting that they formed in the same conditions. They have the following structural formulas: Na0.05 K0.86 (Al 1.77 Fe0.08 Mg0.15) (Si3.22 Al0.78) O10 (OH)2. They are enriched in Fe and Mg compared to the detrital micas. Newly-formed chlorites are associated with micas along the shear planes. According to microprobe analyses, they present the following structural formula: (Al1,48 Fe2,50 Mg1,84) (Si2,82 Al1,18) O10 (OH)8. All these data suggest that these clay minerals are synkinematic and registered the fault activity. In the gouge samples, illite and chlorite are the major clay minerals; smectite is locally present in some samples.In the foliated sandstones, Kubler Index (KI) ((001) XRD peak width at half height) data and thermodynamic calculations from synkinematic chlorite chemistry suggest that the main fault deformation occurred under temperatures around 220 °C (diagenesis to anchizone boundary). KI measured on pelites and sandstones from the hanging and footwall, display similar values coherent with the maximal burial temperature of the Annot sandstones in this area. The gouge samples have a higher KI index, which could be explained by a reactivation of the fault at lower temperatures during the exhumation of the Annot sandstones formation. 相似文献
2.
面理弯切轴测量技术在造山带研究中的应用——以美国阿肯色河地区为例 总被引：1，自引：1，他引：0
基于变形分解理论提出的面理弯切轴测量技术,通过对变斑晶中多期面理的测定分析,为厘定造山运动中的变形过程提供了新的精确定量研究手段。面理弯切轴数据已经被用于解决地质领域多种问题,例如:研究造山运动过程中变形变质历史以及在区域和造山带尺度对比多期变质作用;论证变斑晶生长过程是否发生旋转;变斑晶成核生长与区域变形过程之间的关系;重建板块运动历史过程;约束不同地质事件的发生时限;划分复杂变形分解类型以及岩浆侵位机制及时限研究。本文主要介绍面理弯切轴测量方法的原理、具体测定方法、研究意义及应用范围,并以美国阿肯色地区为例,详细介绍了面理弯切轴测量技术在造山运动过程区域变形历史重建中的应用。 相似文献
3.
We report the ages of cleavage development in a normally intractable lower greenschist facies slate belt, the Central Maine-Aroostook-Matapedia belt in east-central Maine. We have attacked this problem by identifying the minimum ages of muscovite in a regional Acadian cleavage (S1) and in a local ductile fault zone cleavage (S2) using 40Ar/39Ar geochronology and the ages of crosscutting plutons. Our success stems from the regional low-grade metamorphism of the rocks in which each crystallization event preserves a40Ar/39Ar crystallization age and not a cooling age. Evidence for recrystallization via a pressure solution mechanism comes from truncations of detrital, authigenic, and in some rocks S1 muscovite and chlorite grains by new cleavage-forming muscovite and chlorite grains. Low-blank furnace age spectra from meta-arkosic and slaty rocks climb from moderate temperature Devonian age-steps dominated by cleavage-forming muscovite to Ordovician age-steps dominated by a detrital muscovite component. S1- and S2-cleaved rocks were hornfelsed by granitoids of ∼407 and 377 Ma, respectively. The combination of these minimum ages with the maximum metamorphic crystallization ages establishes narrow constraints on the timing of these two cleavage-forming events, ∼410 Ma (S1) and ∼380 Ma (S2). These two events coincide in time with a change in the plate convergence kinematics from the arrival of the Avalon terrane (Acadian orogeny), to a right-lateral transpression arrival of the Meguma terrane in the Neoacadian orogeny. 相似文献
4.
A. S. A. A. ABU SHARIB 《《地质学报》英文版》2016,90(1):88-105
Foliation inflexion/intersection axes(FIAs)preserved within porphyroblasts that grew throughout Isan orogenesis reveal significant anticlockwise changes in the direction of bulk horizontal shortening between 1670 and 1500 Ma from NE-SW,N-S,E-W to NW-SE.This implies an anticlockwise shift in relative plate motion with time during the Isan orogeny.Dating monazite grains amongst the axial planar foliations defining three of the four FIAs enabled an age for the periods of relative plate motion that produced these structures to be determined.Averaging the ages from monazite grains defining each FIA set revealed 1649±12 Ma for NE-SW shortening,1645±7 Ma for N-S shortening,and 1591±10 Ma for that directed E-W.Inclusion trail asymmetries indicate shear senses of top to the SW for NW-SE FIAs and dominantly top to the N for E-W FIAs,reflecting thrusting towards the SW and N.No evidence for tectonism related to early NE-SW bulk horizontal shortening has previously been detected in the Mount Isa Inlier.Amalgamation of the Broken Hill and possibly the Gawler provinces with the Mount Isa province may have taken place during these periods of NE-SW and N-S-directed thrusting as the ages of tectonism are similar.Overlapping dates,tectonic,metamorphic,and metallogenic similarities between eastern Australia(Mount Isa and Broken Hill terranes)and the southwest part of Laurentia imply a most probable connection between both continental masses.Putting Australia in such position with respect to North America during the Late-Paleo-to-Mesoproterozoic time is consistent with the AUSWUS model of the Rodinia supercontinent. 相似文献
5.
陕西旬北浅变质岩区的大型层滑构造 总被引：2，自引：0，他引：2
笔者着重探讨陕西西旬北地区志留系和泥盆系浅变质岩的构造特征，发现这两套地层在构造式要上有明显的差异，它们存在着大型的顺层滑动构造－层滑构造，层滑构造面上下的两套地层是相对独立的构造变形单元。 相似文献
6.
This paper determines, using simple mathematical models, maps of strain fields and foliation trajectories in the neighbourhood of plutonic bodies intruded into a region before, during, and after a lateral compression. The analyses reveal contrasting features for the three cases, which may be useful in distinguishing pre-, syn-, and post-tectonic plutons in coaxially deformed terranes. The theoretically calibrated strain variation curve, along the regional shortening direction, around a pretectonic, rigid plutonic body shows a zone of strain increase (relative to the far-field strain), whereas that around a syntectonic pluton shows a zone of strain decrease along with a zone of strain increase. The foliations deflect outward from a pretectonic pluton, but they bend towards a syntectonic pluton. The analysis also reveals that syntectonic plutons are likely to be elliptical in plan with axial ratio directly dependent on the ratio of the pluton's stress to the tectonic stress. Tectonic foliations deflect away from post-tectonic plutons. However, the foliated area around a post-tectonic pluton is systematically divided into two sets of zones one characterized by foliations showing compressional features, the other showing extensional features. 相似文献
7.
A similar succession of Foliation Inflection/Intersection Axis (FIAs) trends preserved within porphyroblasts is present in two areas separated by 200 km along the Rocky Mountains. The Precambrian rocks in Central Colorado and Northern New Mexico were affected by deformation and metamorphism from ～1506 to 1366 Ma. A succession of five FIAs trending W–E, SSW–NNE, NNW–SSE, NW–SE and WSW–ENE is distinguished in Central Colorado and dated at 1506 ± 15 Ma, 1467 ± 23 Ma, 1425 ± 18 Ma, not dated and 1366 ± 20 Ma respectively. To the south in Northern New Mexico, a succession of five FIAs trending SSW–NNE, WNW–ESE, NNW–SSE, NW–SE and WSW–ENE is distinguished and dated at 1482 ± 48 Ma, 1448 ± 12 Ma, 1422 ± 35 Ma, not dated and 1394 ± 22 Ma. The excellent correlation of the sequence of FIA trends and their ages between regions reveals a sixfold‐FIA succession across the region with the first developed FIA set in Central Colorado not present in Northern New Mexico and the third FIA set in the region not present in Central Colorado. Preferential partitioning of W–E trending deformation into the Central Colorado region ～1506 ± 15 Ma was followed by SSW–NNE trending deformation that affected both regions at 1470 ± 20 Ma. However, preferential partitioning of WNW–ESE trending deformation into Northern New Mexico at 1448 ± 12 Ma left Central Colorado unaffected. Both regions were then affected by the three remaining periods of orogenesis, the first trending NNW–SSE at 1424 ± 15 Ma followed by one trending NW–SE that has not yet been dated, and then one trending WSW–ENE at 1390 ± 19 Ma. This suggests that the Yavapai terrane was tectonized at ～1506 Ma, prior to amalgamation with the Mazatzal terrane ～1470 Ma. Subsequent orogenesis was initially partitioned preferentially into the Mazatzal terrane, but the following three periods of tectonism affected both terranes in a similar manner. 相似文献
8.
Most of the rocks in Precambrian shield areas have experienced a complex structural and metamorphic evolution, processes which have a strong influence on bedrock quality. The properties vary on both a local and a regional scale. It is highly beneficial to know the variations in detail when exploiting the rocks for industrial purposes. The main objective of the investigation was to study the variation of rock mechanical properties in an originally more or less isotropic rock at various stages of ductile deformation. The rocks investigated were Paleoproterozoic and with ages of ca. 1.80-1.88 Ga, and the areas chosen for sampling were situated north-east of Lake Vänern (Kristinehamn; 10 samples), south of the city of Eskilstuna, South Central Sweden (5 samples) and south of Ödeshög, near Lake Vättern in southern Sweden (7 samples). The 12 samples from the latter two areas are described in this investigation, while the 10 samples from Kristinehamn have been published earlier (Göransson et al., 2004). A comprehensive study of various parameters of importance for bedrock quality has been performed on all samples, e.g. studded tyre test (STT) and Los Angeles test (LA), uniaxial compressive strength (UCS), ultrasonic velocity, perimeter measurements of mineral phases, and petrographic and chemical analyses. The weakly deformed and massive (more or less isotropic) rocks show a tendency towards better properties of abrasion (STT) than the strongly deformed rocks and this can also be said for UCS, reflecting the greater ability of rocks to split along foliation planes. This is not entirely unambiguous, as the more deformed rocks, such as the mylonites, may have varying properties. This depends on the combined effects of, e.g. grain size, recrystallisation and foliation. However, the brittleness (LA) shows somewhat better values with increased deformation. This may depend on higher amounts of dark minerals, as their existence does not affect this test as much as in the case of abrasion tests. The perimeter values of the mineral phases display generally higher values, i.e. grain boundaries for the more strongly deformed rocks are more complicated. However, the values for the investigated mylonites may vary between low and high. The lower value may be due to dynamic recrystallisation and the creation of triple points (static recrystallisation) making the rock weaker. Besides, the development of a strong foliation may decrease rock strength despite the usually finer grain size. The results show that it is extremely important to consider all possible variations of bedrock before classification and exploitation, as the bedrock material in fact is highly heterogeneous. 相似文献
9.
Kiyoshi Fuji-ta Tomoo Katsura Takuya Matsuzaki Masahiro Ichiki Tomoyuki Kobayashi 《Tectonophysics》2007,434(1-4):93-101
We conducted electrical conductivity measurements perpendicular and parallel to mineral foliation in dry gneiss at up to 1000 K and a constant pressure of 1 GPa. The analyzed gneisses were obtained from the Higo metamorphic belt, Kyushu, Japan. As the metamorphic conditions of these rocks have been well determined by previous studies, we were able to select samples that were representative of the middle to lower crust. Prior to the conductivity measurements, the samples were maintained at the maximum temperature for a long period, until the electrical conductivity had stabilized. Our experiment results reveal linear and reproducible conductivity data between temperatures of 600 and 1000 K. Conductivity measured perpendicular and parallel to foliation differ by an order of magnitude over the same temperature window. A plausible explanation for this discrepancy in conductivity is the contrasting configuration of minerals in the two sample orientations, as observed by backscattered electron image (BEI) and electron probe microanalysis (EPMA). We evaluated the conductivity and computed activation energy for each of the samples and compared the results with those of previous studies; our results are consistent with the conductivity values reported for other types of rocks. We also compared the experiment results with data derived from electromagnetic (EM) soundings. Electrical conductivity measurements undertaken perpendicular to foliation can account for the subsurface conductivity structure beneath central Kyushu, Japan. 相似文献
10.
Enrique Gomez-Rivas Paul D. Bons Albert Griera Jordi Carreras Elena Druguet Lynn Evans 《Journal of Structural Geology》2007,29(12):1882-1899
Small-scale faults with associated drag folds in brittle-ductile rocks can retain detailed information on the kinematics and amount of deformation the host rock experienced. Measured fault orientation (α), drag angle (β) and the ratio of the thickness of deflected layers at the fault (L) and further away (T) can be compared with α, β and L/T values that are calculated with a simple analytical model. Using graphs or a numerical best-fit routine, one can then determine the kinematic vorticity number and initial fault orientation that best fits the data. The proposed method was successfully tested on both analogue experiments and numerical simulations with BASIL. Using this method, a kinematic vorticity number of one (dextral simple shear) and a minimum finite strain of 2.5–3.8 was obtained for a population of antithetic faults with associated drag folds in a case study area at Mas Rabassers de Dalt on Cap de Creus in the Variscan of the easternmost Pyrenees, Spain. 相似文献