The structure of the Tuapse shear zone according to the field tectonophysical data

The results of field structural studies of the Tuapse shear zone in the Northwest Caucasus are presented. This zone is characterized by shear displacements of various scales with a dominant horizontal shear, viz., a geodynamic type of the stress state, which leads to the formation of faults with mostly lateral displacement of wings, i.e., along the strike of the fault surface. The quantitative characteristics of the local stress conditions in the shear zone (the positions of principal axes and the Lode–Nadai coefficient) are determined on the basis of cataclastic analysis and geological indicators of the paleostresses. The differences between these characteristics are considered for the large tectonic zones. Significant spatial (areal) variations in orientations of the axes of major normal stresses in the shear zone and their local weak gentle variations are evidence of a consistent general stress direction during the formation of faults during the Late Eocene–Miocene deformation epoch.     

Plate tearing in the northwestern corner of the subducting Philippine Sea Plate

The Philippine Sea Plate (PHS) simultaneously subducts northwestward and collides eastward with the Eurasian Plate (EU) in northeast Taiwan. These two tectonic events induce high seismic activity, which makes northeastern Taiwan one of the most seismically active zones in the world. To understand the mechanical processes at work, we used existing geophysical data and the aftershocks recorded following a recent large strike-slip event occurring within the PHS oceanic crust. During this event, a NW–SE trending left-lateral sub-parallel to the PHS/EU convergence vector was active. As a consequence of the collision/subduction plate geometry, we show that the lithosphere of the northwestern corner of the PHS has been torn in a NW–SE orientation. This tectonic feature is associated with an abrupt tectonic stress boundary and could generate large intra-plate earthquakes.     

Neogene-Eopleistocene shelly limestones in the North Caucasus and Ciscaucasia regions

In the North Caucasus and Ciscaucasia, seven distribution zones of Middle-Upper Miocene, Pliocene, and Eopleistocene shell-detrital limestones are defined: Lower Don, Tsimlyansk-Manych, Western Ciscaucasia, Western Caucasus, Central Ciscaucasia, Kabarda-Chechen, and Eastern Dagestan. The shell-detrital limestones are present among sandy-clayey sediments in the form of intercalations of variable thickness traceable over different distances (up to a few kilometers). The maximum thicknesses of such limestones are characteristic of the Neogene sections in the northwestern pericline of the Greater Caucasus: Taman-Adagum area, where their distribution comprises the stratigraphic interval from the Chokrakian to early Cimmerian regional stages (approximately 10 m.y. long). Their most spacious distribution area is confined to the Central Ciscaucasia zone. The shell-detrital limestones in the Neogene sections mark shallowwater shelf areas (ancient coastal bars, banks, and shoals) with intermittently forming environments favorable for accumulation of significant volumes of shelly detritus. In such paleogeographic settings, the sediments were subjected to repeated erosion with subsequent formation of limestones. Now, they represent building material widely used in the region for different purposes.     

Active tectonism in the central Alps: contrasting stress regimes north and south of the Rhone Valley

An integrated interpretation of seismicity, fault plane solutions and deep seismic reflection data suggests that the NE–SW to NW–SE trending Rhone–Simplon fault zone and the gently S-dipping basal Penninic thrust separate fundamentally different stress regimes in the western Swiss Alps. North of the Rhone-Simplon fault zone, strike-slip earthquakes on steep-dipping faults within the Helvetic nappes are a consequence of regional NW–SE compression and NE–SW extension. To the south, vertical maximum stress and N–S extension are responsible for normal mechanism earthquakes that occur entirely within the Penninic nappes above the basal Penninic thrust. Such normal faulting likely results from extension associated with southward movements (collapse) of the Penninic nappes and/or continued uplift and relative northward displacements of the underlying Alpine massifs. Geological mapping and fission-track dating suggest that the two distinct stress regimes have controlled tectonism in the western Swiss Alps since at least the Neogene.     

Analysis of the stress regime and tectonic evolution of the Azerbaijan Plateau,Northwestern Iran

The increasing number of earthquakes in recent decades in Northwestern Iran and the determination of the epicenters of these events makes possible to estimate accurately the changing tectonic regime using the Win-Tensor inversion focal mechanism program. For this purpose focal mechanism data were collected from various sources, including the Centroid Moment Tensor catalog (CMT). The focal mechanism and fault slip data were analyzed to determine change in the stress field up to the present day. The results showed that two stages of brittle deformation occurred in the region. The first stage was related to Eocene compression in NE–SW direction, which created compressional structures with NW–SE strike, including the North and South Bozgush, south Ahar and Gushedagh thrust belts. The second brittle stage began in the Miocene with NW–SE compression and caused developing thrusts with N–S trends that were active presently. These stress regimes were created by the counter-clockwise rotation of the Azerbaijan plateau caused by movement on strike slip faults and continuous compression between the Arabian plate, the south Caspian basin and the Caucasus region. Pliocene-Quaternary activity of the Sabalan and Sahand volcanoes as well as recent earthquakes occurred as a result of this displacement and rotational movement. The abundance of hot springs in the Ardebil, Hero Abad and Bostanabad areas also bore witness to this activity.     

Geomorphic observations from southwestern terminus of Palghat Gap,south India and their tectonic implications

The region around Wadakkancheri, Trichur District, Kerala is known for microseismic activity, since 1989. Studies, subsequent to 2nd December 1994 (M =4.3) earthquake, identified a south dipping active fault (Desamangalam Fault) that may have influenced the course of Bharathapuzha River. The ongoing seismicity is concentrated on southeast of Wadakkancheri and the present study concentrated further south of Desamangalam Fault. The present study identifies the northwestern continuity of NW–SE trending Periyar lineament, which appears to have been segmented in the area. To identify the subtle landform modifications induced by ongoing tectonic adjustments, we focused on morphometric analysis. The NW–SE trending lineaments appear to be controlling the sinuosity of smaller rivers in the area, and most of the elongated drainage basins follow the same trend. The anomalies shown in conventional morphometric parameters, used for defining basins, are also closely associated with the NW–SE trending Periyar lineament/s. A number of brittle faults that appear to have been moved are consistent with the present stress regime and these are identified along the NW–SE trending lineaments. The current seismic activities also coincide with the zone of these lineaments as well as at the southeastern end of Periyar lineament. These observations suggest that the NW–SE trending Periyar lineaments/faults may be responding to the present N–S trending compressional stress regime and reflected as the subtle readjustments of the drainage configuration in the area.     

Structures and microstructures related to steady-state mantle flow in the Neyriz ophiolite,Iran

Structural analysis in the well-exposed and well-preserved Neyriz ophiolite suggests that it is a relatively undisturbed piece of oceanic lithosphere. Detailed structural mapping of high-T deformation mantle flow revealed the presence of three elliptical shaped diapirs trending NW–SE. These diapirs are characterized by vertical mantle foliations associated with vertical plunging stretching lineations, which progressively incline toward parallelism with the gently NE-dipping Moho. The NW–SE direction of asthenospheric upwelling of diapirs is parallel with the orientations of the vertical sheeted dike complex. This suggests that the Neyriz ophiolite was created by two NW–SE palaeo-ridge axes. These palaeo-ridges are classified as fast-spreading ridges. These ridges are segmented by a dextral palaeo-transform fracture zone. This is consistent with fast-spreading ridges. Comparison between the Neyriz and Oman ophiolites reveals that they show similar characteristics. Most of the Oman palaeo-ridge systems are oriented NW–SE and NNW–SE. They also display similar sheeted dike complex orientations and crustal thickness variations. These two ophiolites originally were part of the Neo-Tethyan oceanic lithosphere and afterwards were separated by the Oman line during continental collision of the Iranian micro-continent and Afro-Arabian continent.     

The present-day active deformation in the central and northern parts of the Gulf of Suez area,Egypt, from earthquake focal mechanism data

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.     

Analysis of the Tectonic Stress Field of SE Sichuan and its Impact on the Preservation of Shale Gas in Lower Silurian Longmaxi Formation of the Dingshan Region,China

Tectonic stress is one of the most active deformation factors in the Earth’s crust. Therefore, determinations of its mode of action and form of conversion are critical for guiding research that evaluates preservation conditions in shale gas. Through a survey that targeted many structural features, such as vertical folds and conjugated joints, we investigated the revolution characteristics of the tectonic stress field, the mode of action and the sequence of tectonic stress in the Dingshan region of SE Sichuan by structural analysis methods. Meanwhile, based on a geologic analysis of tectonic stress mechanisms, the structural stress in the region using finite element numerical simulations is quantitatively analyzed. The results show that the region has experienced tectonic stress fields over four different periods and in four different orientations: NW–SE, nearly N–S, nearly E–W and NE-SW. The NW-SE force occurred earlier than that of the nearly N–S force, followed by nearly E–W compression, and finally N–E force. It is concluded that the structural framework of the study area was formed by the effects of the NE-SW compressional stress, which is still active today. The simulation demonstrated that overlapping parts of the intensive strain zone had a higher maximum principal stress, particularly in areas of energy concentration. As for mud and shale, which show excellent sealing capacity, the key to preserving shale gas is the accumulation of energy and a low degree of breakage. Considering the nip-out line of the Longmaxi Formation in the Dingshan region, the buried depth limit of shale-gas production (4500 m), and fault length, the preservation conditions of shale gas and identified favorable targets within the Longmaxi Formation is evaluated.     

Evolution of the stress and strain fields in the Eastern Cordillera,Colombia

This work integrates stress data from Global Positioning System measurements and earthquake focal mechanism solutions, with new borehole breakout and natural fracture system data to better understand the complex interactions between the major tectonic plates in northwestern South America and to examine how the stress regime in the Eastern Cordillera and the Llanos foothills in Colombia has evolved through time. The dataset was used to generate an integrated stress map of the northern Andes and to propose a model for stress evolution in the Eastern Cordillera. In the Cordillera, the primary present-day maximum principal stress direction is WNW–ESE to NW–SE, and is in the direction of maximum shortening in the mountain range. There is also a secondary maximum principal stress direction that is E–W to ENE–WSW, which is associated with the northeastward “escape” of the North Andean block, relative to stable South America. In the Cupiagua hydrocarbon field, located in the Llanos foothills, the dominant NNE–SSW fractures are produced by the Panama arc–North Andes collision and range-normal compression. However, less well developed asymmetrical fractures oriented E–W to WSW–ENE and NNW–SSE are also present, and may be related to pre-folding stresses in the foreland basin of the Central Cordillera or to present-day shear associated with the northeastward “escape” of the north Andean block. Our study results suggest that an important driver for orogenic deformation and changes in the stress field at obliquely convergent subduction zone boundaries is the arrival of thickened crust, such as island arcs and aseismic ridges, at the trench.     

Stress and faulting in southeast Australia as derived from the strongest earthquakes in the region

In an attempt to clarify the state of stress in Southeast Australia, the spatial and dynamic behaviour of the strongest events in the region (with magnitude greater than 5.0) has been studied in the light of the most recent numerical modelling of the crust. Although the local seismicity has little obvious spatial correlation with the mapped structures, the strongest events line up in a very narrow strip with a NW–SE direction. Composite fault-plane solutions, based on data from all of these earthquakes, allows the general characteristics of the local stress field to be determined, and shows that the orientation of the main compressive stress lies in an E–W direction. Taking into account the number of inconsistent data (40%) for this composite solution and the fault-plane solutions for some individual events, an improved result can be obtained by domain analysis, which shows that northern and southern zones have different orientation of the compressional axes. Although the composite solution for the events in the northern domain shows the same E–W orientation of the main compression as for the whole region, the composite solution for the southern zone reveals a rotation of the compression axis to an azimuth of 120–130°. From the relation between the energy and seismic moment the maximum value of the stress-drop for the whole of SE Australia is estimated as ∼2.0 MPa.     

Changes of Late Mesozoic Tectonic Regimes around the Ordos Basin (North China) and their Geodynamic Implications

A synthesis is given in this paper on late Mesozoic deformation pattern in the zones around the Ordos Basin based on lithostratigraphic and structural analyses. A relative chronology of the late Mesozoic tectonic stress evolution was established from the field analyses of fault kinematics and constrained by stratigraphic contact relationships. The results show alternation of tectonic compressional and extensional regimes. The Ordos Basin and its surroundings were in weak N-S to NNE-SSW extension during the Early to Middle Jurassic, which reactivated E-W-trending basement fractures. The tectonic regime changed to a multi-directional compressional one during the Late Jurassic, which resulted in crustal shortening deformation along the marginal zones of the Ordos Basin. Then it changed to an extensional one during the Early Cretaceous, which rifted the western, northwestern and southeastern margins of the Ordos Basin. A NW-SE compression occurred during the Late Cretaceous and caused the termination of sedimentation and uplift of the Ordos Basin. This phased evolution of the late Mesozoic tectonic stress regimes and associated deformation pattern around the Ordos Basin best records the changes in regional geodynamic settings in East Asia, from the Early to Middle Jurassic post-orogenic extension following the Triassic collision between the North and South China Blocks, to the Late Jurassic multi-directional compressions produced by synchronous convergence of the three plates (the Siberian Plate to the north, Paleo-Pacific Plate to the east and Lhasa Block to the west) towards the East Asian continent. Early Cretaceous extension might be the response to collapse and lithospheric thinning of the North China Craton.     

Tectonics of the Northern Bresse region (France) during the Alpine cycle

Combining fieldwork and surface data, we have reconstructed the Cenozoic structural and tectonic evolution of the Northern Bresse. Analysis of drainage network geometry allowed to detect three major fault zones trending NE–SW, E–W and NW–SE, and smooth folds with NNE trending axes, all corroborated with shallow well data in the graben and fieldwork on edges. Cenozoic paleostress succession was determined through fault slip and calcite twin inversions, taking into account data of relative chronology. A N–S major compression, attributed to the Pyrenean orogenesis, has activated strike-slip faults trending NNE along the western edge and NE–SW in the graben. After a transitional minor E–W trending extension, the Oligocene WNW extension has structured the graben by a collapse along NNE to NE–SW normal faults. A local NNW extension closes this phase. The Alpine collision has led to an ENE compression at Early Miocene. The following WNW trending major compression has generated shallow deformation in Bresse, but no deformation along the western edge. The calculation of potential reactivation of pre-existing faults enables to propose a structural sketch map for this event, with a NE–SW trending transfer fault zone, inactivity of the NNE edge faults, and possibly large wavelength folding, which could explain the deposit agency and repartition of Miocene to Quaternary deformation.     

甘肃金川矿区古构造应力场恢复及演化研究

金川矿区经历了复杂的构造演化历史,目前其成矿期后的构造变形特征和应力场演化阶段仍缺乏精细剖析。文章运用构造解析方法对金川矿区地表基岩中的断层进行了分期和配套,确定了构造变形序列,认为矿区存在4组重要的断层组合,包括北东向逆冲断层和北西向走滑断层、北东向走滑断层和北西向逆冲断层、北西向正断层以及北东东向走滑断层。通过研究断层破裂面及擦痕构造,利用赤平投影法恢复了断层的古构造应力场,结合区域大地构造演化历史,准确限定了金川矿区成矿期后的应力场演化阶段,对认识区域构造演化和开发新远景区具有重要意义。结果显示,金川矿区在成矿期后经历了4期古构造应力场作用,表现为多阶段不同方向的挤压和伸展过程,分别响应了区域中生代以来的一系列构造热事件,Ⅰ期表现为早—中侏罗世的北西—南东向挤压应力场,Ⅱ期为晚侏罗世的北东—南西向挤压应力场,Ⅲ期为早白垩世的北东—南西向伸展应力场,Ⅳ期为晚白垩世以来的北东—南西向挤压应力场。     

Serpentinization-driven extension in the Ronda mantle slab (Betic Cordillera,S. Spain)

We investigate the stress regimes acting during serpentinization and faulting of the largest known subcontinental lithospheric peridotite body, namely the Ronda peridotites (Betic Cordillera, S. Spain). Petrological and structural analyses on serpentinites grown along fault planes crosscutting the peridotite slab, reveal that they were developed during three superposed stress tensors: the oldest one (E1) is characterized by NW–SE sub-horizontal compression; the intermediate one consists in NE–SW to ENE–WSW extension with orthogonal compression (E2); and the youngest one (E3) shows a sub-vertical maximum stress axis and NW–SE sub-horizontal extension. During serpentinization, maximum and minimum stress axes flip between a NW–SE horizontal position and a vertical one in the whole peridotite body (E1 and E3), while E2 represents an intermediate stress stage. Field relationships and previous petrological and geochronological data indicate that serpentinization and associated stress tensors are coeval with intrusive leucogranite dikes crosscutting the peridotites, thus constraining these processes to 19–22 Ma and occurring at upper continental crust depths (P < 4 kbar). Gravity data reveal that the average density of the Ronda mantle slab (~ 2.7–2.8 g/cm³) shows a negligible contrast with the surrounding crustal rocks, thus suggesting that the peridotite body is serpentinized in a great proportion. Our preferred tectonic model to account for the evolution of the Ronda peridotites in the upper crust considers that E1 compression was linked to the collision of the Alborán continental domain with the Iberian passive margin during the Gibraltar Arc formation. Subsequently, the sudden onset of extension recorded within the peridotite slab (E2 and E3) was favored by serpentinization-driven buoyancy.     

山西太古代——中生代构造应力场

本文采用历史分析与力学分析相结合的原则,研究山西太古代至中生代五次主要构造运动的形变特征、主应力方向及分布规律。中太古代晚期（阜平运动）以SSW向挤压为主;晚太古代末期（五台运动）、早元古代末期（吕粱运动）和侏罗纪（燕山运动）受SE—SEE向挤压;白垩纪（四川运动）以SSW向挤压为特征。五次构造运动的最大主压应力均呈近水平方向。     

Structural geochemistry of gold mineralization in the Linglong-Jiaojia district, Shandong Province, China

The Linglong-Jiaojia district is one of the most important regions containing gold deposits in China. These gold deposits can be divided into: a) the pyrite-gold-quartz vein type (Linglong type), which is controlled by brittle-ductile to ductile deformation structures, and b) the alteration-zone type (Jiaojia type), characterized by small veinlets, or the disseminated type recognized in brittle shear zones. Lode gold deposits in the Jiaojia area occur in NE brittle fracture zones, formed in a dominantly simple shear deformation regime, mainly in thrust attitude with a minor sinistral strike slip component. In the Linglong area, the lode gold deposits are located at the intersection of three types of structures: NNE and NE brittle-ductile fault zones and the ENE ductile reverse shear zone in the south of the area. The structural characteristics of these brittle shear zones are consistent with a tectonic NNW-SSE principal stress field orientation. Similar stresses explain the ENE Qixia fold axes, the Potouqing and several other ENE reverse ductile shear zones elsewhere in the region, the Tancheng-Lujiang fault zone and its subsidiaries in the vicinity of the Linglong-Jiaojia district, as well as the southern ENE suture zone north of Qingdao. Therefore these structural systems occurred as part of different major tectonic events under NNW-SSE compression principal stress fields in the area. Gold deposits are hosted in smaller-scale structures within the brittle fault zones and brittle-ductile shear zones. Although ore bodies and, on a smaller scale, quartz ore veins often seem to be randomly oriented, it is possible to explain their distribution and orientation in terms of the simple shear deformation process under which they were developed. The progressive simple shear failure is characterized by various fracture modes (tension and shear) that intervene in sequence. The tension and shear fractures are influenced by the stress level (depth of burial beneath the paleosurface) in their structural behavior, show variable dilatancy (void openings) and extend on all scales. By making use of these characteristics, a progressive failure analysis can be applied to predicting the shape and extent of ore bodies as well as the styles of mineralization at any given location.     

Upper mantle anisotropy of southeast Arabia passive margin [Gulf of Aden northern conjugate margin], Oman

In this study, we used data recorded by two consecutive passive broadband deployments on the Gulf of Aden northern margin, Dhofar region, Sultanate of Oman. The objective of these deployments is to map the young eastern Gulf of Aden passive continental margin crust and upper mantle structure and rheology. In this study, we use shear-wave splitting analysis to map lateral variations of upper mantle anisotropy beneath the study area. In this study, we found splitting magnitudes to vary between 0.33 and 1.0 s delay times, averaging about 0.6 s for a total of 17 stations from both deployment periods. Results show distinct abrupt lateral anisotropy variation along the study area. Three anisotropy zones are identified: a western zone dominated by NW–SE anisotropy orientations, an eastern zone dominated with NE–SW anisotropy orientations, and central zone with mixed anisotropy orientations similar to the east and west zones. We interpret these shorter wavelength anisotropy zones to possibly represent fossil lithospheric mantle anisotropy. We postulate that the central anisotropy zone may be representing a Proterozoic suture zone that separates two terranes to the east and west of it. The anisotropy zones west and east were being used indicative of different terranes with different upper mantle anisotropy signatures.     

Neotectonic transformation of Cenozoic fold structures in the northwestern Caucasus

The performed morphotectonic regionalization of the northwestern Caucasus shows that the fold structures directly expressed in the topography of the territory and continuing to evolve under the settings of contemporary lateral shortening predominate in the northwestern Caucasus. A map of fold structures expressed in the topography of the northwestern Caucasus is presented. The districts distinguished therein correspond to the largest regional tectonic units, the fold topography of which occurs at various stages of tectonic evolution from primary brachyanticlinal ridges of the Taman and Sochi districts to the complex fold–thrust and inversion fold ridges of the axial zone. Data on active newly formed fold and inversion structures are given. These inherited structures develop under the combined action of selective denudation, beddingplane upthrow faulting, and thrusting.