犁式正断层(1istric normal fault)

犁式正断层也有称剥离断层(Denudational fault),它是一种倾角随深度变缓的低角度正断层,形态似犁状,故名为犁式正断层。其特点,上盘地层沿倾向下滑,较浅层位的年青岩层直接盖于较深层位的岩层之上,断层上陡,向下变平而联合成一个基底滑脱面,断层面     

Are feldspar-to-mica reactions necessarily reaction-softening processes in fault zones?

Reaction-softening by mineralogical changes from feldspars to sericite has been documented from many fault zones. During external crystalline basement deformation in the Alpine orogeny, the Ser Barbier thrust and splay faults in the Pelvoux Massif experienced ultracataclasis and sericitisation. Microstructural information and geochemical data from the fault rocks suggest that different muscovitisation reactions occurred at different times within the evolution of the fault zone, and each reaction had its own impact on fault rheology. Early cataclasis aided chemical breakdown of orthoclase feldspars to muscovite, yet quartz release accompanying this process resulted in local cementation and consequent hardening of the ultracataclasite. Continued deformation was accompanied by muscovitisation of the albite feldspar, and resulted in the formation of mica-rich fault rocks which experienced progressive silica removal by the fluid with increasing deformation. At this stage, reaction-enhanced ductility dominated. Much of the early cemented ultracataclasites escaped later deformation, and their low permeability allowed preservation of their early geochemical characteristics by preventing later fluid access. Such findings demonstrate how the complex interplay between deformation processes and geochemical reactions may result in a changing rheology during fault zone evolution.     

Rock pulverization and localization of a strike-slip fault zone in dolomite rocks (Salzach–Ennstal–Mariazell–Puchberg fault,Austria)

Detailed investigations of dolomite fault rocks, formed at shallow crustal depths along the Salzach–Ennstal–Mariazell–Puchberg (SEMP) fault system in the Northern Calcareous Alps, revealed new insights into cataclasite formation. The examined Miocene, sinistral strike-slip faults reveal grain size reduction of dolomite host rocks by tensile microfracturing at a large range of scales, producing rock fragments of centimetre to micrometre sizes. In situ fracturing leads to grain size reduction down to grain sizes <25 μm, producing mosaic breccias and fault rocks which have previously been described as “initial/embryonic” and “intermediate” cataclasites. At all scales, grain fragments display little to no rotation and no or minor evidence of shear deformation. The observed microstructures are similar to those previously described in studies on pulverized rocks. Microstructural investigations of cataclasites and mosaic breccias revealed aggregations of small dolomite grains (<50 μm) that accumulated on top of large fragments or as infillings of V-shaped voids between larger grains and show constant polarity throughout the investigated samples. Fabrics indicate deposition in formerly open pore space and subsequent polyphase cementation. The newly described tectonic geopetal fabrics (geopetal-particle-aggregates, GPA) prove that these faults temporarily passed through a stage of extremely high porosity/permeability prior to partial cementation.     

Seismic reflection imaging of active faults and their tectonic behavior in the Northern Moroccan margin. Is the Nekor fault a pure strike-slip fault?

The study of 1000-km seismic reflection profiles, along the Northern Moroccan margin, allowed browsing new imaging in detail about the regional geological structures and their functioning. To achieve this goal, we elaborated a high-resolution depth model and a global tectonic sketch. The influence of recent tectonic activity is manifested by normal and strike-slip faults, trending mainly 70° N and 125° N. In this segment, the Nekor strike-slip fault seems to be connected to a secondary major fault system that changes direction from 30° N to 70° N, and changing behavior to left-lateral strike-slip fault with normal component. Analysis of local seismic activity recorded from 1990 to 2014 with moderate magnitudes activity shows alignments in clear superposition with the detected active faults in seismic reflection lines.     

Deformation band populations in fault damage zone—impact on fluid flow

Fault damage zones in highly porous reservoirs are dominated by deformation bands that generally have permeability-reducing properties. Due to an absence of sufficiently detailed measurements and the irregular distribution of deformation bands, a statistical approach is applied to study their influence on flow. A stochastic model of their distribution is constructed, and band density, distribution, orientation, and flow properties are chosen based on available field observations. The sensitivity of these different parameters on the upscaled flow is analyzed. The influence of a heterogeneous permeability distribution was also studied by assuming the presence of high permeability holes within bands. The fragmentation and position of these holes affect significantly the block-effective permeability. Results of local upscaling with a diagonal and full upscaled permeability tensor are compared, and qualitatively similar results for the flow characteristics are obtained. Further, the procedure of iterative local–global upscaling is applied to the problem.     

`Crack–seal', slip: a new fault valve mechanism?

This paper presents a new model of fault development in carbonate rocks involving a crack–seal–slip sequence. The structures of sheared calcite veins from the Les Matelles outcrop, Languedoc (S. France), and the observations used to construct this new model which integrates aspects of `crack–seal' evolution of calcite-filled veins with concepts of fault valve behaviour are described. In our model, hydraulic mode I reopening of an oblique pre-existing vein in an overall strike-slip stress regime is accompanied by precipitation of calcite, but significant fault slip cannot occur initially despite this obliquity because the ends of the pre-existing structure limit further reopening propagation beyond the tips. The rate of aligned calcite precipitation keeps pace with the rate of dilation of the structure, so that calcite cement essentially seals the system. Stress concentrations at the tips are allowed to rise with reopening until failure of the tip zone results in branch crack formation, triggering both slip along the vein and hydraulic pressure drop. This is followed by sealing within the branch cracks. Such a crack–seal–slip cycle may be repeated several times, as evidenced by fault-perpendicular calcite vein growth interlayered with calc-mylonite lamellae within these structures. Later cycles will become less pronounced because strength recovery of the sealed branch cracks does not regain the initial strength of the intact rock. This model could apply at various scales, and could be a mechanism for triggering earthquakes.     

Formation of chlorite during thrust fault reactivation. Record of fluid origin and P–T conditions in the Monte Perdido thrust fault (southern Pyrenees)

The chemical and isotopic compositions of clay minerals such as illite and chlorite are commonly used to quantify diagenetic and low-grade metamorphic conditions, an approach that is also used in the present study of the Monte Perdido thrust fault from the South Pyrenean fold-and-thrust belt. The Monte Perdido thrust fault is a shallow thrust juxtaposing upper Cretaceous–Paleocene platform carbonates and Lower Eocene marls and turbidites from the Jaca basin. The core zone of the fault, about 6 m thick, consists of intensely deformed clay-bearing rocks bounded by major shear surfaces. Illite and chlorite are the main hydrous minerals in the fault zone. Illite is oriented along cleavage planes while chlorite formed along shear veins (<50 μm in thickness). Authigenic chlorite provides essential information about the origin of fluids and their temperature. δ¹⁸O and δD values of newly formed chlorite support equilibration with sedimentary interstitial water, directly derived from the local hanging wall and footwall during deformation. Given the absence of large-scale fluid flow, the mineralization observed in the thrust faults records the P–T conditions of thrust activity. Temperatures of chlorite formation of about 240°C are obtained via two independent methods: chlorite compositional thermometers and oxygen isotope fractionation between cogenetic chlorite and quartz. Burial depth conditions of 7 km are determined for the Monte Perdido thrust reactivation, coupling calculated temperature and fluid inclusion isochores. The present study demonstrates that both isotopic and thermodynamic methods applied to clay minerals formed in thrust fault are useful to help constrain diagenetic and low-grade metamorphic conditions.     

Discussion on initial mechanism of the Tan-Lu fault zone北大核心CSCD

The Dabie and Sulu orogens between the North China and the Yangtze cratons were left-laterally offset about 4(H) km along the NE-striking Tan-Lu Fault Zone. The fault zone terminates abruptly at the southeastern corner of the Dabie Orogen, suggesting unique origin of the fault zone which remains controversial. Structures in the Zhangbaling Croup and Feidong Complex in the Zhangbaling Uplift formed in a flat-lying ductile detachment zone with a shear sense of top to the SSW. Whereas, the Tan-Lu shear zone in the l.ujiang area exhibits as a sinistral ductile shear zone. Thus, the Tan-Lu Fault Zone in the east of the Dabie Orogen experienced two phases of deformation. The first phase deformation exhibits as sinistral ductile shear belts, the sinistral ductile shear zone was then involved in the NK-SW trending tightly folds and thrusts deformation. The Susong Complex and Zhangbaling Group in the Dabie Orogens exhibit as exhumation structures. According previous muscovite 4'Ar/,>Ar ages and deformation of syn-collisional folds and thrusts, we propose an indentation-induced continent-Tearing model for the initialization the Tan-Lu Fault Zone.     

The Palmerville fault — a “fundamental” structure in North Queensland∗

The Palmerville Fault in North Queensland is an important structure with a probable length of at least 600 miles, extending from Princess Charlotte Bay in the north to Townsville in the south, and controlling the configuration of the coast line of eastern Cape York and at Halifax Bay. The fault forms the western boundary of both the Palaeozoic Hodgkinson Basin (Tasman Geosyncline) and of the Mesozoic Laura Basin, and has been active at least four times since the Silurian. The latest movements took place during the late Cainozoic.     

U–Pb–Hf zircon study of two mylonitic granite complexes in the Talas-Fergana fault zone,Kyrgyzstan, and Ar–Ar age of deformations along the fault

A 2000 km long dextral Talas-Fergana strike–slip fault separates eastern terranes in the Kyrgyz Tien Shan from western terranes. The aim of this study was to constrain an age of dextral shearing in the central part of the fault utilizing Ar–Ar dating of micas. We also carried out a U–Pb–Hf zircon study of two different deformed granitoid complexes in the fault zone from which the micas for Ar dating were separated. Two samples of the oldest deformed Neoproterozoic granitoids in the area of study yielded U–Pb zircon SHRIMP ages 728 ± 11 Ma and 778 ± 11 Ma, characteristic for the Cryogenian Bolshoi Naryn Formation, and zircon grains analyzed for their Lu–Hf isotopic compositions yielded εHf(t) values from −11.43 to −16.73, and their calculated tHfc ages varied from 2.42 to 2.71 Ga. Thus varying Cryogenian ages and noticeable heterogeneity of Meso- to Paleoproterozoic crustal sources was established for mylonitic granites of the Bolshoi Naryn Formation. Two samples of mylonitized pegmatoidal granites of the Kyrgysh Complex yielded identical ²⁰⁶Pb/²³⁸U ages of 279 ± 5 Ma corresponding to the main peak of Late-Paleozoic post-collisional magmatism in the Tien Shan (Seltmann et al., 2011), and zircon grains analyzed for their Lu–Hf isotopic compositions yielded εHf(t) values from −11.43 to −16.73, and calculated tHfc ages from 2.42 to 2.71 Ga indicating derivation from a Paleoproterozoic crustal source. Microstructural studies showed that ductile/brittle deformation of pegmatoidal granites of the Kyrgysh Complex occurred at temperatures of 300–400 °C and caused resetting of the K–Ar isotope system of primary muscovite. Deformation of mylonitized granites of the Bolshoi Naryn Formation occurred under high temperature conditions and resulted in protracted growth and recrystallization of micas. The oldest Ar–Ar muscovite age of 241 Ma with a well defined plateau from a pegmatoidal granite of the Kyrgysh Complex is considered as a “minimum” age of dextral motions along this section of the fault in the Triassic while younger ages varying from 227 Ma to 199 Ma with typical staircase patterns indicate protracted growth and recrystallization of micas during ductile deformations which continued until the end of the Triassic.     

Clay-smear continuity and normal fault zone geometry – First results from excavated sandbox models

The continuity of clay-rich fault gouge has a large effect on fluid transmissibility of faults in sand–clay sequences, but clay gouge continuity and composition in 3D are not well known. We report observations of 3D clay smear continuity in water-saturated sandbox experiments where the sheared clay layers were excavated after deformation. The experiments build on existing work on the evolution of clay gouge in similar 2D experiments where interpretations were made in profile view.We used well-known model materials (“Benchmark” sand and uncemented kaolinite–sand mixtures) that were further characterized using standardized geotechnical tests and triaxial compression experiments at effective pressures corresponding to the sandbox experiments. Results show a nonlinear failure envelope of the sand, in agreement with existing models. Unconfined compression experiments with the clay show cohesion around 50 Pa and brittle behavior.A sheared, ductile clay layer embedded in sand above a 70° dipping basement fault reveals a complex, natural-looking clay gouge architecture with relay ramps, breached relays and fault lenses. The clay gouge shows clear variations in composition and thickness and becomes locally discontinuous at throw-thickness ratios above 7, in contrast to our earlier 2D observations where discontinuous clay-gouge only formed in cemented clay layers. In addition to tectonic telescoping in the relays, the thin, continuous parts of the clay gouge were transformed from an initial pure clay by mechanical mixing of sand and clay.We also discuss the applicability of these results to the evolution of normal fault zones and deformation bands in sand–clay sequences at effective pressures below the onset of cataclasis and conclude that in fault zones a higher degree of internal segmentation reduces the probability of the formation of discontinuities.     

First tectonic-geomorphology study along the Longmu–Gozha Co fault system,Western Tibet

The Longmu–Gozha Co left-lateral strike-slip fault system (LGCF) is located in remote western Tibet, forming a triple junction with both the Altyn Tagh fault (ATF) and the Karakorum fault (KF), the two major strike-slip faults in the region. The Ashikule, Gozha Co and Longmu Co faults are clear and distinct left-stepping en-echelon faults, together forming the LGCF system. Although poorly documented, quantifying its activity remains a key problem to understand the kinematics and the tectonic history of the westernmost Tibetan Plateau. Indeed, the Karakax fault (NW segment of the ATF), LGCF and KF together control the tectonics of western Tibet which itself controls the extrusion of Tibet towards the east, with the LGCF acting as a natural boundary for eastward motion of the Tibetan Plateau due to India's northward impingement. The LGCF system shows clear and impressive morphological indications of left-lateral active shear, that we quantify using field measurements (terrestrial LIDAR) along with ¹⁰Be surface-exposure dating. Our data suggest a slip-rate < 3 mm/yr, consistent with geodetic and block model studies. While it is on the order of the Karakax fault slip-rate (~ 2 mm/yr), it is smaller than those along the ATF and KF (> 9 and > 8 mm/yr, respectively), yielding a few mm/yr of extension accommodated most likely in the Ashikule graben and surroundings, located between the ATF and Karakax faults. Numerous evidences of recent tectonic-related events are present in the vicinity, such as the 1951 volcanic eruption as well as the 2008 and 2014 Ms 7.3 Yutian earthquakes, attesting of its high activity. In addition, the LGCF's en-echelon geometry and identical direction with the ATF, as well as smaller geological offsets and lower slip-rate compared to those on the surrounding faults, suggest that this segment of the ATF may be the most recent.     

Paleostress reconstruction from calcite twin and fault–slip data using the multiple inverse method in the East Walanae fault zone: Implications for the Neogene contraction in South Sulawesi,Indonesia

A new approach for paleostress analysis using the multiple inverse method with calcite twin data including untwinned e-plane was performed in the East Walanae fault (EWF) zone in South Sulawesi, Indonesia. Application of untwinned e-plane data of calcite grain to constrain paleostress determination is the first attempt for this method. Stress states caused by the collision of the south-east margin of Sundaland with the Australian microcontinents during the Pliocene were successfully detected from a combination of calcite-twin data and fault–slip data. This Pliocene NE–SW-to-E–W-directed maximum compression activated the EWF as a reverse fault with a dextral component of slip with pervasive development of secondary structures in the narrow zone between Bone Mountain and Walanae Depression.     

Cenozoic fault control on ‘deep lead’ palaeoriver systems,Central Highlands,Victoria

Basaltic eruptions across the Central Highlands of Victoria have sealed in-place Early to middle Cenozoic palaeodrainage systems (also known as deep leads). The basal gravels of the deep leads have been mined extensively in the past for their rich placer-gold deposits. Detailed mapping of the distribution of all palaeorivers has been carried out using drilling results and modern aeromagnetic/radiometric surveys. The palaeochannel isopachs (including basalt and sediment) do not thicken in a modern downvalley direction. Instead, deeper depressions alternate with shallower areas. The variations in thickness, and parts of the palaeochannel courses, are controlled by a series of east-northeast-trending basement highs. The basement highs are caused by a set of east-northeast-trending (Otway Basin-style) faults visible on radar shuttle imagery in the Central Highlands. They have not previously been recognised in regional geological mapping. Most published fault trends are north – south oriented, parallel to the strike of the Palaeozoic basement rocks. Exceptions occur at Ballarat where there is an orthogonal east-northeast set mapped in underground quartz reef workings that show right-lateral strike-slip movements. The east-northeast faults show half-graben block-style rotational movement on basement, creating north- and south-facing fault scarps along the horst ridges. Where palaeochannels overlie the grabens, valleys broadened, infill thickens, and locally drainage directions may change. When the drainage cuts through the horsts, steeper incised valleys result, and this is where, in the historical past, some gold leads were ‘lost’. The initial timing of the block movement pre-dates at least the Early Oligocene to Late Miocene ages of the basal palaeovalley sediments, as shown by revised palynological dating. In places, the modern drainage divide coincides with east-northeast-trending faults. In the Ballarat area, an earlier divide accentuated by the aeromagnetic palaeodrainage mapping occurs up to 25 km south and appears to pre-date the earliest basalt flows at around 7.0 Ma. This evidence suggests the divide can change position through time by differential movements along east-northeast faults and transferral of maximum uplift to adjacent blocks.     

Fluid behaviors from the seismogenic region of Longmenshan fault zone,eastern Tibetan PlateauSCIEI北大核心CSCD

震间期、同震期和震后期流体对断裂带物质的强度和运动性质起到重要作用。前人已识别出断裂带浅部区域流体对断层的弱化以及矿物沉淀导致的断层愈合,然而对于断裂带深部流体的研究鲜有报道。为深入了解孕震区流体行为以及地震成核过程中流体对断层的影响,本文以龙门山断裂带的映秀-北川断裂南段虹口乡八角庙村附近碎裂岩滑动带中石英和方解石脉为研究对象,通过对断裂带脉体的显微构造、碳氧同位素和主量元素含量等分析,开展地震相关脉体的特征结构、流体来源和矿物沉淀环境的研究。结果表明,碎裂岩主滑移带由颜色结构不同的三层断层泥和细小的方解石条带组成,在主滑动带边部和上盘碎裂岩中则分别发育了指示震间期、同震期和震后期三个阶段断层活动的脉体:(1)沿阶步生长的纤维状方解石脉和拉伸型柱状颗粒方解石脉;(2)断层泥楔入脉;(3)近等粒状方解石脉以及具有横向竞争生长结构的非等粒状方解石脉和石英脉。它们分别代表了震间期封闭的还原环境下的微滑动、同震外源高压流体注入以及震后开放的氧化环境至还原环境下的矿物沉淀。碳氧同位素结果表明主滑动带和碎裂岩方解石脉δ^(18)O V-PDB值为-20.5‰~-20.3‰,低于围岩碳酸钙胶结物,表明方解石脉具有大气水来源特征。方解石沉淀温度结合地温梯度表明方解石脉的形成深度大于4km,与碎裂岩形成深度及龙门山断裂带震源深度一致。该地区方解石脉和石英脉的研究深化了关于龙门山断裂带孕震区流体行为的理解,并且对进一步认识震间期、同震期和震后期断层的强度变化机制具有重要意义。     

Late Quaternary paleoseismic evidence on the Munébrega half-graben fault (Iberian Range,Spain)

The Munébrega Plio-Quaternary half-graben is a NW-SE trending neotectonic depression located in the central sector of the intraplate Iberian Range (NE Spain). The master fault of the half-graben offsets an Upper Pleistocene pediment deposit, forming an upslope-facing scarp. A trench dug across the fault scarp exposed a 25-m wide deformation zone consisting of graben and horst fault blocks with fissures in the upper part of the scarp, and a monoclinal flexure affected by normal and reverse faults in the lower part of the scarp. We infer a minimum of three faulting events over the past 72 ka, yielding an average (maximum) recurrence interval of 24 ka. The oldest event (72–41 ka) produced an antislope scarp on the relict pediment surface, confining deposition to the downthrown block. Cross-cutting faults affecting sedimentary units deposited in the sediment trap produced by the first event provide evidence for at least two younger events (33–19? ka). The measured cumulative vertical displacement (7.4 m) yield a minimum vertical slip rate of 0.10 ± 0.01 mm/year (2σ error) for the past 72 ka. If the paleoearthquakes ruptured the whole mappable length of the fault (ca. 20 km), they probably had moment magnitudes ca. 6.9 (Stirling et al. Bull Seismol Soc Am, 2002). Such earthquakes would have been more than a magnitude unit larger than the largest ones recorded historically in the Iberian Range. These results suggest that the official seismic hazard assessments, based solely on the historic and instrumental record, may underestimate the seismic hazard in the area.     

Crimea–Kopet Dagh zone of concentrated orogenic deformations as a transregional late collisional right-lateral strike-slip fault

It is shown that the Crimea, Caucasus, and Kopet Dagh fold systems make up a single whole unified by a lithospheric strike-slip fault zone of concentrated dislocations. The strike-slip fault that dissects the sedimentary cover and consolidated crust is rooted in subcrustal layers of the mantle. The notions about strike-slip dislocations in the structure of the Crimea–Kopet Dagh System are considered. Comparative analysis of structure, age, and amplitude of strike-slip fault segments is carried out. The effect of strike-slip faulting on the deep-seated and near-surface structure of the Earth’s crust is considered. Based on estimation of strike-slip offsets, the paleogeography of Paleogene basins is refined; their initial contours, which have been disturbed and fragmented by slipping motion strike-slip displacement, have been reconstructed.