Three‐dimensional vorticity and time‐constrained evolution of the Main Central Thrust zone,Garhwal Himalaya (NW India)

Vorticity estimates based on porphyroclasts analysis are limited by the extrapolation to three dimensions of two‐dimensional data. We describe a 3D approach based on the use of X‐ray micro‐computed tomography that better reflects the real 3D geometry of the porphyroclasts population. This new approach for kinematic vorticity analysis in the Munsiari Thrust mylonites, the lower boundary of the Main Central Thrust zone (MCTz) in Indian Himalaya, indicates a large pure shear component during non‐coaxial shearing. ⁴⁰Ar/³⁹Ar ages of micas along the mylonitic foliation of the Munsiari and Vaikrita thrusts (the upper boundary of the MCTz) constrain thrust activity to 5–4 and 8–9 Ma, respectively. Available kinematic vorticity analyses of the Vaikrita mylonites suggest the dominance of a simple shear component. Combining these data, we suggest that the southward and structurally downward shift of deformation along the MCTz was accompanied by a progressive increase in the pure shear component in a general shear flow.     

Structural and metamorphic evolution of the rocks of the Jutogh Group, Chur half-klippe, Himachal Himalayas: A summary and comparison with the Simla area

The rocks of the Jutogh Group in the Himachal Himalayas and their equivalents elsewhere are now considered to represent a several km thick crustal scale ductile shear zone, the so called Main Central Thrust Zone. In this article we present a summary of structural and metamorphic evolution of the Jutogh Group of rocks in the Chur half-klippe and compare our results with those of Naha and Ray (1972) who worked in the adjacent Simla klippe. The deformational history of the Jutogh Group of rocks in the area around the Chur-peak, as deduced from small-scale structures, can be segmented into: (1) an early event giving rise to two sets of very tight to isoclinal and coaxial folds with gentle dip of axial planes and easterly or westerly trend of axes, (2) an event of superimposed progressive ductile shearing during which a plethora of small-scale structures have developed which includes successive generations of strongly non-cylindrical folds, several generations of mylonitic foliation, extensional structures and late-stage small-scale thrusts, and (3) a last stage deformation during which a set of open and upright folds developed, but these are regionally unimportant. The structure in the largest scale (tens of km) can be best described in terms of stacked up thin thrust sheets. Km-scale asymmetric recumbent folds with strongly non-cylindrical hinge lines, developed as a consequence of ductile shearing, are present in one of these thrust sheets. The ductile shearing, large-scale folding and thrusting can be related to the development of the Main Central Thrust Zone. The microstructural relations show that the main phase of regional low-to medium-grade metamorphism (T ≈ 430–600°C andP ≈ 4.5–8.5 kbar) is pre-kinematic with respect to the formation of the Main Central Thrust Zone. Growth zoned garnets with typical bell-shaped Mn profiles and compensating bowl-shaped Fe profiles are compatible with this phase of metamorphism. Some of the larger garnet grains, however, show flat compositional profiles; if they represent homogenization of growth zoning, it would be a possible evidence of a relict high-grade metamorphism. The ductile shearing was accompanied by a low-greenschist facies metamorphism during which mainly chlorite and occasionally biotite porphyroblasts crystallized.     

Geometry and impact of transpressional faulting in polyphasic metamorphic orogenic belts: the Viù Deformation Zone (inner Western Alps)

This article describes the geometry and structural architecture of the Viù Deformation Zone (VDZ), a brittle-ductile to brittle structure affecting the metamorphic units of the inner Western Alps, and its role in modifying the pre-existing syn-metamorphic structural setting. The VDZ reactivates and displaces the contact between two different oceanic units, the Lanzo Ultramafic Complex and the Lower Susa–Lanzo Valleys Unit, characterized by polyphase syn-metamorphic deformation. It shows a strike-slip duplex geometry, constituted by N–S reverse-dextral faults linked by NW–SE antithetical sinistral-reverse faults, and represents a contractional step-over zone along a N–S regional dextral-reverse structure, the Col del Lis-Trana Deformation Zone. Formation of these transpressional structures steepened the Lanzo Ultramafic Complex during the last stages of its exhumation. The 3D geometry of the VDZ was strongly controlled by the reactivation of pre-existing structures, such as the buried western edge of the Ivrea body and metamorphic foliations. Brittle reactivation also induced block rotation along the VDZ, causing anomalous kinematic relations between the VDZ-associated faults. This study, hence, shows that in metamorphic orogens the mechanisms generating strike-slip duplexes may be different from those classically provided by the literature, with brittle reactivation and block rotation strongly prevailing on newly formed faults. In such orogens, moreover, rotations induced by transpressional faulting may sometimes be mistaken for steep syn-metamorphic shear zones. Underestimating the effects of later brittle deformation and associated rotations may cause erroneous interpretations of the tectonic evolution of orogens.     

Integrated 3D geophysical and geological modelling of the Hercynian Suture Zone in the Champtoceaux area (south Brittany, France)

This paper combines geological knowledge and geophysical imagery at the crustal scale to model the 3D geometry of a segment of the Hercynian suture zone of western Europe in the Champtoceaux area (Brittany, France). The Champtoceaux complex consists of a stack of metamorphic nappes of gneisses and micaschists, with eclogite-bearing units. The exhumation of the complex, during early Carboniferous times, was accompanied by deformation during regional dextral strike–slip associated with a major Hercynian shear zone (the South Armorican Shear Zone, SASZ). Dextral shearing produced a km-scale antiformal structure with a steeply dipping axial plane and a steeply eastward plunging axis. Armor 2 deep seismic profile shows that the regional structure was cut by a set of faults with northward thrusting components. Based on the seismic constraint, direct 2D crustal-scale modelling was performed throughout the Champtoceaux fold on seven radial gravity profiles, also using geological data, and density measurements from field and drill-hole samples. The 3D integration of the cross-sections, the digitised geological map, and the structural information (foliation dips) insure the geometrical and topological consistency of all sources of data. The 2D information is interpolated to the whole 3D space using a geostatistical analysis. Finally, the 3D gravity contribution of the resulting model is computed taking into account densities for each modelled geological body and compared to the Bouguer anomaly. The final 3D model is thus compatible with the seismic and gravity data, as well as with geological data. Main geological results derived from the modelling are (i) the overall 3D geometry of the south dipping thrust system interpreted on the seismic profile emphasises northward thrusting and folding of the Champtoceaux complex which was coeval with strike–slip along the South Armorican Shear Zone; (ii) the gravity modelling suggests the presence of a relatively dense body below the Champtoceaux complex that could be interpreted as a result of relative uplift of midcrustal material during thrusting along the E–W trending wrench–thrust system; (iii) the northern limb of the Champtoceaux anticline is a relatively shallow feature; and (iv) Vigneux synkinematic granitic body is a laccolith sheared and rooted along the southern branch of the SASZ and spreads away from the strike–slip zone within weak country-rocks.     

Morphostructural provinces and neotectonics in the Amazonian lowlands of Peru

The Amazonian lowlands of Peru are composed of the Subandean Zone (foreland), the Marañón Basin (foredeep) and the Iquitos Geanticline (Brazilian Craton). The Subandean Zone includes the Subandean Thrust and Fold Belt (STFB) to the west, which crops out mostly in the foothills, and the Subandean Tilted Block Zone (STBZ) to the east, which is principally exposed in the Amazonian lowlands of central Peru. The main trends of river belts are related to structural style. Main rivers in the STFB are antecedent, and secondary drainage is subsequent. In the STBZ, river basins are channelized, parallel to structural grain. The special asymmetric pattern of the Subandean drainage network depends on these two juxtaposed structural regions. As a result, fluvial migration in Peruvian lowlands is more controlled and limited by neotectonics than previously supposed.     

Non-cylindrical, flexural slip folding in the Ardwell flags—a statistical approach

Non-cylindrical, flexural slip folding is described from the well-known coastal section between Ardwell Bay and Kennedy's Pass, near Girvan, southwest Scotland. Bedding plane slip is recognised by the ubiquitous slickenside striations on bedding surfaces, and these linear elements define the ac kinematic plane of fold formation. This intersects the axial surfaces of folds (ab) in the common direction a which is the movement direction during fold propagation. Statistical treatment of the orientation of structural and kinematic elements yields important conclusions concerning fold formation: the Ardwell folds are markedly non-cylindrical and this is a primary feature amplified during the Ardwell Fold Phase.     

Ductile Shearing in Attur Shear Zone and its Relation with Moyar Shear Zone, South India

In the eastern part of southern Peninsular India, the charnockitic hills of the Madras block are cut across by the E-W trending Attur shear zone (ASZ) which is characterised by a thick (1 to 1.5 km) phyllonite zone, showing intense mylonitisation due to ductile shearing. Steeply plunging (70°–80°) stretching lineation on steeply dipping mylonitic foliation within this zone indicates a relative vertical upliftment of the adjacent blocks. A dextral shearing event from west to east is envisaged from the kinematic analysis of shear sense indicators such as S-C fabric, asymmetric folds, asymmetric augens and asymmetric porphyroclasts. Simultaneous development of these features, related to vertical and horizontal movements may be explained by the mechanism of transpressional deformation. The Attur shear zone may be correlated with the Moyar shear zone based on distinct lithological and structural similarities.     

辽宁歪头山铁矿构造研究及构造控矿模式初探

歪头山铁矿屑鞍山式沉积变质铁矿，褶皱、韧性剪切带、断裂是矿区的主要构造形迹。通过构造解析，将矿区鞍山运动中的构造变形划分为3幕。利用共轭节理和断层擦痕反演，对矿区构造应力场进行了定向、定时研究，确定了各期构造应力场。最后，提出了歪头山铁矿“褶皱 韧性剪切带”控矿模式，新模式对矿区的采矿平面设计及寻找可能的漏矿具有指导作用。     

Singhbhum Shear Zone: Structural transition and a kinematic model

The northern fold belt away from the Singhbhum Shear Zone displays a set of folds on bedding. The folds are sub-horizontal with E-W to ESE striking steep axial surfaces. In contrast, the folds in the Singhbhum Shear Zone developed on a mylonitic foliation and have a reclined geometry with northerly trending axes. There is a transitional zone between the two, where the bedding and the cleavage have become parallel by isoclinal folding and two sets of reclined folds have developed by deforming the bedding—parallel cleavage. Southward from the northern fold belt the intensity of deformation increases, the folds become tightened and overturned towards the south while the fold hinges are rotated from the sub-horizontal position to a down-dip attitude. Recognition of the transitional zone and the identification of the overlapping character of deformation in the shear zone and the northern belt enable the formulation of a bulk kinematic model for the area as a whole.     

Emplacement and deformation of the Wyangala Batholith,New South Wales

The Wyangala Batholith, in the Lachlan Fold Belt of New South Wales, is pre‐tectonic with respect to the deformation that caused the foliation in the granite, and was emplaced during a major thermal event, perhaps associated with dextral shearing, during the Late Silurian to Early Devonian Bowning Orogeny. This followed the first episode of folding in the enclosing Ordovician country rocks. Intrusion was facilitated by upward displacement of fault blocks, with local stoping. Weak magmatic flow fabrics are present. After crystallization of the granite, a swarm of mafic dykes intruded both the granite and country rock, possibly being derived from the same tectonic regime responsible for emplacement of the Wyangala Batholith. A contact aureole surrounding the granite contains cordierite‐biotite and cordierite‐andalusite assemblages. Slaty cleavage produced in the first deformation was largely obliterated by recrystallization in the contact aureole.

Postdating granite emplacement and basic dyke intrusion, a second regional deformation was accompanied by regional metamorphism ranging from lower greenschist to albite‐epidote‐amphibolite facies, and produced tectonic foliations, termed S and C, in the granite, and a foliation, S₂, in the country rocks. Contact metamorphic rocks underwent retrogressive regional metamorphism at this time. S formed under east‐west shortening and vertical extension, concurrently with S₂. C surfaces probably formed concurrently with S and indicate reverse fault motion on west‐dipping ductile shear surfaces. The second deformation may be related to Devonian or Early Carboniferous movement on the Copperhannia Thrust east of the Wyangala Batholith.     

银洞沟银金矿矿床地质特征及成因探讨

就构造对该矿的控岩控矿作用和围岩蚀变等进行了探讨,揭示了早期 层次滑脱构造控制了初放源层的形成、韧－脆性推覆型剪切作用控制了含矿石英脉的形成,     

Multistage metamorphism and deformation in high‐pressure metabasites of the northern Adula Nappe Complex (Central Alps,Switzerland)

Metabasites from the northern Adula Nappe Complex (ANC) display a complex microstructural evolution recording episodes of deformation and metamorphic re‐equilibration that were obliterated in the surrounding phengite‐bearing schists. Pre‐D1 and D1 deformation episodes are preserved as mineral inclusions within garnet cores of some amphibole‐bearing eclogites and record high‐temperature greenschist‐/amphibolite‐facies conditions. D2 produced an eclogite‐facies foliation which developed at 580 ± 70°C and 19 ± 3 kbar. D3 was a composite deformation episode which can be divided into three sub‐episodes D3m, D3a and D3b which occurred as the metamorphism evolved from post‐eclogitic high‐pressure and low‐temperature conditions through to amphibolite‐facies conditions at 590 ± 30°C and 11.7 ± 1.3 kbar. The D3 deformation episode was responsible for the development of the S3 regional‐scale foliation in the surrounding schists, whilst D4 caused the development of an S4 greenschist foliation. The composite nature of the D3 episode indicates that rocks of the northern ANC experienced a protracted post‐eclogitic structural reworking and that the current structure of this part of the Alps is a late‐Alpine feature. Copyright © 2010 John Wiley & Sons, Ltd.     

Structural and metamorphic evolution of the Moornambool Metamorphic Complex,western Lachlan Fold Belt,southeastern Australia

The wedge‐shaped Moornambool Metamorphic Complex is bounded by the Coongee Fault to the east and the Moyston Fault to the west. This complex was juxtaposed between stable Delamerian crust to the west and the eastward migrating deformation that occurred in the western Lachlan Fold Belt during the Ordovician and Silurian. The complex comprises Cambrian turbidites and mafic volcanics and is subdivided into a lower greenschist eastern zone and a higher grade amphibolite facies western zone, with sub‐greenschist rocks occurring on either side of the complex. The boundary between the two zones is defined by steeply dipping L‐S tectonites of the Mt Ararat ductile high‐strain zone. Deformation reflects marked structural thickening that produced garnet‐bearing amphibolites followed by exhumation via ductile shearing and brittle faulting. Pressure‐temperature estimates on garnet‐bearing amphibolites in the western zone suggest metamorphic pressures of ～0.7–0.8 GPa and temperatures of ～540–590°C. Metamorphic grade variations suggest that between 15 and 20 km of vertical offset occurs across the east‐dipping Moyston Fault. Bounding fault structures show evidence for early ductile deformation followed by later brittle deformation/reactivation. Ductile deformation within the complex is initially marked by early bedding‐parallel cleavages. Later deformation produced tight to isoclinal D₂ folds and steeply dipping ductile high‐strain zones. The S₂ foliation is the dominant fabric in the complex and is shallowly west‐dipping to flat‐lying in the western zone and steeply west‐dipping in the eastern zone. Peak metamorphism is pre‐ to syn‐D₂. Later ductile deformation reoriented the S₂ foliation, produced S₃ crenulation cleavages across both zones and localised S₄ fabrics. The transition to brittle deformation is defined by the development of east‐ and west‐dipping reverse faults that produce a neutral vergence and not the predominant east‐vergent transport observed throughout the rest of the western Lachlan Fold Belt. Later north‐dipping thrusts overprint these fault structures. The majority of fault transport along ductile and brittle structures occurred prior to the intrusion of the Early Devonian Ararat Granodiorite. Late west‐ and east‐dipping faults represent the final stages of major brittle deformation: these are post plutonism.     

Brittle–ductile deformation in the Glarus thrust Lochseiten (LK) calc-mylonite

The Glarus thrust accommodated at least 30 km of northward displacement strongly localized within a 1-m layer of 'Lochseiten' (LK) calc-mylonite. This layer displays veins in various states of plastic deformation and a wildly refolded foliated gouge texture. Lattice- and shape-preferred orientations are observed within the fine-grained, recrystallized matrix. These features indicate the alternate activity of brittle and ductile deformation mechanisms. In contrast to the classical view that grain boundary sliding (superplasticity) is the dominant deformation mechanism, it is advocated that fluids, derived from the footwall and expelled along the thrust, are responsible for hydrofracturing and cataclastic deformation. In periods between fracture events, deformation was ductile. In this new interpretation, a substantial amount of the total thrust displacement was accommodated by numerous short-lived and strongly localized fracture events at the base of the Verrucano thrust sheet, rather than a permanently weak décollement lithology.     

Exhumation of the lower crust during crustal shortening: an Alice Springs (380 Ma) age for a prograde amphibolite facies shear zone in the Strangways Metamorphic Complex (central Australia)

Foliated garnet-bearing amphibolites occur within the West Bore Shear Zone, cutting through granulite facies gneisses of the Strangways Metamorphic Complex. In the amphibolites, large euhedral garnet (up to 3 cm) occurs within fine-grained recrystallized leucocratic diffusion haloes of plagioclase–quartz. The garnet and their haloes include a well-developed vertical foliation, also present in the matrix. This foliation is the same as that cutting through the unconformably overlying Neoproterozoic Heavitree Quartzite. The textures indicate syn- to late kinematic growth of the amphibolite facies mineral assemblages.
All mineral assemblages record an arrested prograde reaction history. Noteworthy is the growth of garnet at the expense of hornblende and plagioclase, and the breakdown of staurolite–hornblende to give plagioclase–gedrite. These dehydration reactions indicate increasing P – T  conditions during metamorphism, and suggest heating towards the end of a period of intense deformation. Temperature estimates for the garnet–amphibolite and related staurolite–hornblende assemblages from the shear zone are about 600 °C. Pressure is estimated at about 5 kbar.
An Sm–Nd isochron gives an age of 381±7 Ma for the peak metamorphism and associated deformation. This age determination confirms that amphibolite facies conditions prevailed during shear zone development within the Strangways Metamorphic Complex during the Alice Springs Orogeny. These temperature conditions are significantly higher than those expected at this depth assuming a normal geothermal gradient. The Alice Springs Orogeny was associated with significant crustal thickening, allowing exhumation of the granulite facies, Palaeoproterozoic, lower crust. Along-strike variations of the tectonic style suggest a larger amount of crustal shortening in the eastern part of the Alice Springs Orogeny.     

Strain memory of 2D and 3D rigid inclusion populations in viscous flows — What is clast SPO telling us?

We model the development of shape preferred orientation (SPO) of a large population of two- and three-dimensional (2D and 3D) rigid clasts suspended in a linear viscous matrix deformed by superposed steady and continuously non-steady plane strain flows to investigate the sensitivity of clasts to changing boundary conditions during a single or superposed deformation events. Resultant clast SPOs are compared to one developed by an identical initial population that experienced a steady flow history of constant kinematic vorticity and reached an identical finite strain state, allowing examination of SPO sensitivity to deformation path. Rotation paths of individual triaxial inclusions are complex, even for steady plane strain flow histories. It has been suggested that the 3D nature of the system renders predictions based on 2D models inadequate for applied clast-based kinematic vorticity gauges. We demonstrate that for a large population of clasts, simplification to a 2D model does provide a good approximation to the SPO predicted by full 3D analysis for steady and non-steady plane strain deformation paths. Predictions of shape fabric development from 2D models are not only qualitatively similar to the more complex 3D analysis, but they display the same limitations of techniques based on clast SPO commonly used as a quantitative kinematic vorticity gauge. Our model results from steady, superposed, and non-steady flow histories with a significant pure shearing component at a wide range of finite strain resemble predictions for an identical initial population that experienced a single steady simple shearing deformation. We conclude that individual 2D and 3D clasts respond instantaneously to changes in boundary conditions, however, in aggregate, the SPO of a population of rigid inclusions does not reflect the late-stage kinematics of deformation, nor is it an indicator of the unique ‘mean’ kinematic vorticity experienced by a deformed rock volume.     

Strain path partitioning within thrust sheets: microstructural and petrofabric evidence from the Moine Thrust zone at Loch Eriboll,northwest Scotland

Quartz c axis fabrics and microstructures have been investigated within a suite of quartzites collected from the Loch Eriboll area of the Moine Thrust zone and are used to interpret the detailed processes involved in fabric evolution. The intensity of quartz c axis fabrics is directly proportional to the calculated strain magnitude. A correlation is also established between the pattern of c axis fabrics and the calculated strain symmetry.Two kinematic domains are recognized within one of the studied thrust sheets which outcrops immediately beneath the Moine Thrust. Within the upper and central levels of the thrust sheet coaxial deformation is indicated by conjugate, mutually interfering shear bands, globular low strain detrital quartz grains whose c axes are aligned sub-parallel to the principal finite shortening direction (Z) and quartz c axis fabrics which are symmetric (both in terms of skeletal outline and intensity distribution) with respect to mylonitic foliation and lineation. Non-coaxial deformation is indicated within the more intensely deformed and recrystallized quartzites located near the base of the thrust sheet by single sets of shear bands and c axis fabrics which are asymmetric with respect to foliation and lineation.Tectonic models offering possible explanations for the presence of kinematic (strain path) domains within thrust sheets are considered.     

CCSD主孔3000 m岩心构造产状特征及其地质意义

对CCSD主孔3 000 m岩心叶理、具断层擦痕的微断层及断层擦痕产状特征进行了统计分析, 并分析了脆、韧性剪切带的运动学特征, 研究表明(1) 榴辉岩类叶理产状明显较片麻岩类陡, 可能与其岩石能干性较强有关, 因而在挤压褶皱变形过程中表现出不同的变形行为.韧性变形主体表现为顺层剪切, 含断层擦痕的微断层最主要的一组产状与叶理面一致或接近一致, 部分伴随与之呈共轭关系的另一组微断层. (2) 脆、韧性变形以SEE-NWW向剪切为主, 部分为近SN向, 脆性、韧性变形域断层运动方向基本一致.但现在所保留下来的构造变形中韧性变形以SEE向NWW的逆冲型剪切为主, 部分为近SN向韧性剪切作用; 而脆性变形以NWW向SEE的正滑作用为主. (3) 主孔构造应力场初步可划分为4期, 现在所保留的主期构造为SEE-NWW向挤压构造应力场所致.