Shear Sense Indicators Research Articles

Mineral fish: their morphological classification, usefulness as shear sense indicators and genesis

Mineral fish are sheared and commonly asymmetric mineral grains or clusters of grains. This work reports 11 sub-types of mineral fish showing a top-to-SE sense of ductile shearing in the Karakoram Metamorphic Complex (KMC). The mineral fish are of three broad geometries: sigmoid, lenticular and parallelogram. Reliable senses of shear are indicated by the overall asymmetry and inclination of mineral fish. On the other hand, the true shear sense is not always indicated by either the orientations of their cleavage planes or those of the individual grains in composite mineral fish. The ranges of local orientations of single sigmoid mineral fish that include the lower values (<23°) in the KMC indicate their extensive ductile shearing. The studied mineral fish were products of a range of deformation mechanisms including homogeneous deformation, simple shear, intra-granular slip, crystal-plastic deformation, fracturing and synthetic shearing. Additionally, some examples might have undergone duplex slips and a few nucleated and grew either prior to or during the top-to-SE shearing.

Mesoscopic and magnetic fabrics in arcuate igneous bodies: an example from the Mandi-Karsog pluton, Himachal Lesser Himalaya

AbstractField, microstructural and anisotropy of magnetic susceptibility (AMS) data from the Palaeozoic Mandi-Karsog pluton in the Lesser Himalayan region reveal a concordant relationship between fabric of the Proterozoic host rock and the granite. The pluton displays a prominent arcuate shape on the geological map. The margin-parallel mesoscopic and magnetic fabrics of the granite and warping of the host rock fabric around the pluton indicate that this regional curvature is either synchronous or pre-dates the emplacement of the granite body. Mesoscopic fabric, magnetic fabric and microstructures indicate that the northern part of the pluton preserves its pre-Himalayan magmatic fabric while the central and southern part shows tectonic fabric related to the Tertiary Himalayan orogeny. The presence of NW–SE-trending aplitic veins within the granite indicates a post-emplacement stretching in the NE–SW direction. Shear-sense indicators in the mylonites along the margin of the pluton suggest top-to-the-SW shearing related to the Himalayan orogeny. Based on these observations, it is envisaged that the extension that gave rise to this rift-related magmatism had a NE–SW trend, that is, normal to the trend of the aplite veins. Subsequently, during the Himalayan orogeny, compression occurred along this same NE–SW orientation. These findings imply that the regional curvature present in the Himachal Lesser Himalaya is in fact a pre-Himalayan feature and the pluton has formed by filling a major pre-Himalayan arcuate extension fracture.

Application of SEM-EBSD to regional scale shear zone analysis: A case study of the Bhavani Shear Zone, South India

The Bhavani Shear Zone (BSZ), part of the Palghat-Cauvery shear system of southern India, comprises a number of subparallel shear zones ranging in width from a few centimetres to tens of metres. These shear zones are characterised by progressive mylonitisation and shear fabrics corresponding to increasing strain, as documented by grain size reduction, flattening, elongation and dimensional preferred orientation of constituent minerals (quartz, hornblende, biotite, plagioclase and orthoclase). However, SEM-EBSD measured crystal preferred orientations (CPO) of these minerals are more complex and indicate interchange of maximum (X) and intermediate (Y) tectonic axes during different phases of deformation, with the minimum axis (Z) remaining approximately constant. The tectonic evolution of the BSZ and the disposition of the surrounding Archaean and Pan-African granulites are interpreted therefore in terms of polyphase deformation involving both dip-slip and strike-slip movement events in a multiply reactivated setting. SEM-EBSD CPO analysis provides clearer indication of potentially conflicting movement directions that are otherwise poorly constrained by shear markers and shear sense indicators.

Mylonites: lessons from Eriboll

Abstract Eriboll has been an important area in understanding the geology and structure of the NW Highlands of Scotland. It came to prominence during the Highland Controversy between Nicol and Murchison in the 1850s. Nicol recognized a major regional, or grand, dislocation (the Moine Thrust Zone) at Eriboll whereas Murchison denied its existence. An important element in the resolution of the controversy was the repeated observation of a distinctive schistose rock that often marked Nicol's dislocation. Lapworth named it a ‘mylonite’ and related its development to mechanical metamorphism resulting from tectonic (milling) movements along the dislocation. Peach, Horne and co-workers, whose contributions to early mylonite studies are often overlooked, described the effects of increasing strain on mylonite development, recorded the prominent lineation within mylonites and identified its kinematic significance via thrust geometries and a first use of shear sense indicators. In this contribution those elements of the Highland Controversy that led to the identification of mylonites will be reviewed, concentrating on the contributions of Lapworth and of Peach, Horne and co-workers. It will include an examination and discussion of the mylonites from Lapworth's locality and of the mylonitized Moine schists at Eriboll. It will conclude with comments on mylonite classification and nomenclature.

Rotating micro-structures in Antarctic cold basal ice: implications for glacier flow and its interpretation

Structural analyses were conducted in the basal zone of an Antarctic glacier. The studied basal ice sequence was retrieved from a 20-m-long subglacial tunnel dug at the margin of the glacier and is at the temperature of −17°C. For the first time, rotating clast systems embedded within debris-rich ice were thin-sectioned using specially designed cutting techniques. The observed structures reflect the occurrence of pervasive shearing at the base of the glacier, and can be used as shear sense indicators. In addition, some of these structures provide evidence for the presence of thin liquid films at the time of formation despite the marked freezing temperature of the ice. It is showed here that cautious analysis of deformation structures present in debris-bearing ice may bring insights not only into the flow dynamics of the embedding matrix, but also into the behaviour of the interstitial fluid network at the base of cold glaciers and ice sheets.

Synoblique convergent and extensional deformation and metamorphism in the Neoproterozoic rocks along Wadi Fatira shear zone, Northern Eastern Desert, Egypt

The Wadi Fatira area occurs at the southern margin of the Northern Eastern Desert (NED) of Egypt and is occupied by highly sheared metavolcanics tectonically alternated with banded iron formations and intruded by Barud tonalite–granodiorite, post-tectonic gabbroic and granitic intrusions. Detailed structural investigation showed that the schists and migmatitic amphibolites are formed by shearing in metavolcanics and syntectonic Barud tonalite–granodiorite due to movement along the Wadi Fatira shear zone (WFSZ). This shear zone starts as a NW–SE striking fault along Wadi Barud Al Azraq and the Eastern part of Wadi Fatira and turns to a E–W trending fault to the north of Wadi Fatira. Microstructural shear sense indicators such as asymmetric geometry of porphyroclasts such as σ-type and asymmetric folds deforming fine-grained bands which are frequently found around porphyroclasts indicate sinistral sense of shearing along the WFSZ. This shear zone is characterized by transitions from local convergence to local extension along their E–W and NW–SE trending parts, respectively. The NW–SE part of the WFSZ is of about 200 m in width and characterized by synmagmatic extensional features such as intrusion of synkinematic tonalite, creation of NE–SE trending normal faults, and formation of migmatitic amphibolites and schlieric tonalites. This part of the shear zone is metamorphosed under synthermal peak metamorphic conditions (725°C at 2–4 kbar). The E–W compressional part of the WFSZ is up to 3 km in width and composed of hornblende, chlorite, actinolite, and biotite schists together with sheared intermediate and acidic metatuffs. Contractional and transpressional structures in this part of the WFSZ include E–W trending major asymmetrical anticline and syncline, nearly vertical foliation and steeply pitching stretching lineations, NNE dipping minor thrusts, and minor intrafolial folds with their hinges parallel to the stretching lineation. P–T estimates using mineral analyses of plagioclase and hornblende from schists and foliated metavolcanics indicate prograde metamorphism under medium-grade amphibolite facies (500–600°C at 3–7 kbar) retrogressed to low-grade greenschist facies (227–317°C). The foliation in Barud tonalite–granodiorite close to the E–W part of the WFSZ runs parallel to the plane of shearing and the tonalite show numerous magmatic flow structures overprinted by folding and ductile shearing. The WFSZ is similar to structures resulted from combined simple shear and orthogonal shortening of oblique transpressive shear zones and their sense of movement is comparable with the characteristics of the Najd Fault System.

Thermochronometric constraints on the tectonic evolution of the Serifos detachment, Aegean Sea, Greece

We constrain the timing and kinematics of the Serifos detachment in the southwestern Cyclades, Greece, using low-temperature thermochronometry. Fission-track dating shows that the Serifos detachment was active between ~13 and 6 Ma and that the Serifos granodiorite in its footwall intruded at or before ~12–11 Ma into the extensional shear zone and initially cooled very rapidly at rates >180°C per million year. The mylonite zone at the top of the granodiorite and mylonitic structures in its country rocks record a consistent top-SSW shear sense in the ductile crust. In the brittle regime top-NNE shear-sense indicators occur as well. Conjugate top-SSW and top-NNE high-angle normal faults are the youngest deformational features and cut across the detachment. Age–distance relationships for the fission-track data display a relatively flat pattern. We discuss a model advocating initial top-SSW movement on the Serifos detachment before and during emplacement of the granodiorite. Updoming of the detachment during exhumation and cooling caused subsequent bivergent extension in the brittle crust.

Pressure-temperature paths and exhumation of Late Ordovician-Early Silurian blueschists and associated metamorphic nappes of the Salinic Brunswick subduction complex, northern Appalachians

The Late Ordovician–Early Silurian Brunswick subduction complex in northern New Brunswick preserves a nearly intact southeast-facing forearc terrane with clear links among Silurian subduction, under-plating, exhumation, and forearc sedimentation. The forearc terrane grew over time due to successive accretion and underplating of seamounts and isolated ribbons made up of continental and transitional crust concomitant with foreland migration of the trench. A sliver of strongly deformed epidote-blueschists derived from an accreted seamount was stacked together with slightly lower-pressure winchite-bearing tectonites and medium- to high-pressure greenschists of the Fournier, California Lake, and Tetagouche blocks into a series of D 1 metamorphic nappes during Salinic closure of the Tetagouche-Exploits backarc basin. The blueschists (~375 °C, 7.2 kbar) locally preserve mineralogical (zoning from actinolite or barroisite core to a rim of glaucophane) and microstructural evidence for a counterclockwise pressure-temperature ( P-T ) path ascribed to underthrusting beneath the young oceanic lithosphere of the Fournier block shortly after the inception of subduction at ca. 450 Ma. The next phase of accretion is represented by underplating of the California Lake block at ca. 442 Ma. This may have caused emplacement of the blueschists above the slightly lower metamorphic grade Canoe Landing Lake winchite-bearing nappe (350 °C, 6.5 kbar). Underplating of the Tetagouche block at ca. 435 Ma extruded the blueschist sliver and underlying nappes of the California Lake block immediately above greenschist-facies nappes (~5.8 kbar) and beneath lower-pressure (4–5 kbar) greenschist-facies basalt and gabbro of the ophiolitic Fournier block, which was deformed and imbricated earlier, probably during accretion of the California Lake block. Shear sense indicators confirm that the hanging-wall shear zone of the blue-schist sliver accommodated Early Silurian normal motion. D 1 is generally time-trans-gressive due to down-stepping of the under-plating-related deformation. Metamorphic relicts, as well as zoning in amphibole and epidote, indicate that the Canoe Landing Lake and other nappes underwent clockwise P-T paths related to subduction of the cool oceanic lithosphere and/or accretion of the California Lake and Tetagouche blocks.

Transpressive dextral shear in the Italva-Itaperuna section, Northern State of Rio de Janeiro, Brazil

Structural analysis carried out on a segment of the Neoproterozoic Ribeira Belt, southeastern Brazil, show that it represents part of the transpressive dextral orogen related to the Central Mantiqueira Province. NNE-trending and steeply dipping regional mylonitic belts form anastomosed geometry, and describe a map-scale, S-C-like structure that is characterized by their deflection towards NE near the Além Paraíba Lineament. Lithological and structural control related to deformation partition were responsible for the formation of felsic mylonitic granulites with S-type granites lenses developed in ductile shear zones, alternated with less deformed intermediate to basic granulites associated with charnockites. The dextral shear sense indicators are consistent with transpressive deformation in the region and are common especially at the border of the main shear zones. The presence of S-type leucogranite may lead to variations of linear and planar relationships, which result in local extension zones. These elements are consistent with oblique continental collision considering the São Francisco Craton as a stable block.

The Sha’it–Nugrus shear zone separating Central and South Eastern Deserts, Egypt: A post-arc collision low-angle normal ductile shear zone

The northerly dipping Sha’it–Nugrus shear zone (SNSZ) is the boundary separating the Central Eastern Desert from the South Eastern Desert of Egypt. The hangingwall of this shear zone is composed of low-grade metavolcanics and ophiolitic nappes of the Central Eastern Desert, while the footwall consists of South Eastern Desert high-grade metapsammitic gneisses (Migif-Hafafit gneissic complex). The SNSZ is about 700 m thick and represents the shear foliated lower parts of the hangingwall and upper parts of the footwall. A significant part of the SNSZ has been truncated by a later normal fault along Wadi Sha’it, however the SNSZ is well-preserved along Wadi Nugrus. Features of the SNSZ include shear-related schistosity (termed Ss), mylonite zones, sheared syn-kinematic granitoid intrusions, diverse metasomatism and metamorphic effects (higher T overprinting of hangingwall lithologies and retrogression of footwall lithologies). Shear-sense indicators clearly show top-to-N or NW displacement sense. SNSZ structures overprint arc collision related nappe structures (∼680 Ma) and are therefore post-arc collision. SNSZ syn-kinematic intrusives have been dated at ∼600 Ma. The SNSZ is deformed (regionally and locally folded and thrust dissected) during later NE–SW compressive tectonism. The SNSZ had an originally approximately E–W strike, low-angle N-dip and a normal shear sense, making this an example of a low-angle normal ductile shear (LANF) or detachment fault. The steep NE dip of Ss foliations and low-pitching slip lineations along Wadi Nugrus are due to NW–SE folding of the SNSZ, and do not indicate a sinistral strike-slip shear zone. The normal shear sense activity is responsible for juxtaposing the low-grade Central Eastern Desert lithologies against South Eastern Desert gneisses. A displacement of 15–30 km is estimated on the SNSZ, which is comparable to LANF displacements in the Basin and Range province of the western USA. Frictional resistance along this shear was probably reduced by high magmatic fluid pressure and hydrothermal fluid pressure. The vastness and diversity of the hydrothermal activity along this shear zone is a characteristic of other LANFs in the Eastern Desert, e.g. at Gabal El-Sibai, and may be Gabal Meatiq. The SNSZ formed during the Neoproterozoic extensional tectonic phase of Eastern Desert that began ∼600 Ma, and followed arc collision and NW-ward ejection of nappes.

Cenozoic extensional tectonics of the Western Anatolia Extended Terrane, Turkey

The Western Anatolia Extended Terrane in Turkey is located on the eastern side of the Aegean Extended Terrane and contains one of the largest metamorphic core complexes in the world, the Menderes massif. It has experienced a series of continental collisions from the Late Cretaceous to the Eocene during the formation of the Izmir-Ankara-Erzincan suture zone. Based our field work and monazite ages, we suggest that the north-directed postcollisional Cenozoic extension in the region is the product of three consecutive stages, triggered by three different mechanisms. The first stage was initiated about 30 Ma ago, in the Oligocene by the Orogenic Collapse the thermally weakened continental crust along the north-dipping Southwest Anatolian shear zone. The shear zone was formed as an extensional simple-shear zone with listric geometry at depth and exhibits predominantly normal-slip along its southwestern end. But, it becomes a high-angle oblique-slip shear zone along its northeastern termination. Evidence for the presence of the shear zone includes (1) the dominant top to the north-northeast shear sense indicators throughout the Menderes massif, such as stretching lineations trending N10E to N30E; and (2) a series of Oligocene extensional basins located adjacent to the shear zone that contain only carbonate and ophiolitic rock fragments, but no high grade metamorphic rock fragments. During this stage, erosion and extensional unroofing brought high-grade metamorphic rocks of the Central Menderes massif to the surface by the early Miocene. The second stage of the extension was triggered by subduction roll-back and associated back-arc extension in the early Miocene and produced the north-dipping Alaşehir and the south-dipping Büyük Menderes detachments of the central Menderes massif and the north-dipping Simav detachment of the northern Menderes massif. The detachments control the Miocene sedimentation in the Alaşehir, Büyük Menderes, and Simav grabens, containing high-grade metamorphic rock fragments. The third stage of the extension was triggered by the lateral extrusion (tectonic escape) of the Anatolian plate when the North Anatolian fault was initiated at about 5 Ma. This extensional phase produced the high-angle faults in the Alaşehir, Büyük Menderes and Simav grabens and the high-angle faults controlling the Küçük Menderes graben.

Kinematics of Franciscan Complex exhumation: New insights from the geology of Mount Diablo, California

Three major lithotectonic units of an ancient arc-trench system are present in western California. In ascending structural order, they include: the Franciscan Complex, a remnant accretionary prism partly including high-pressure (blueschist-facies) metamorphic rocks in a subduction melange; the relatively unmetamorphosed Coast Range ophiolite; and the Great Valley Group forearc sediments. In the Mount Diablo study area east of San Francisco, the subduction complex and ophiolite are juxtaposed across the Coast Range fault, and together they represent a structurally attenuated section through the ancestral forearc crust exposed by uplift and folding of the late Cenozoic Mount Diablo anticline. The Coast Range fault is locally associated with a sheared zone in the lower part of the attenuated ophiolite. Shear-sense indicators in the serpentinite, such as S-C fabrics, rotated clasts, and slick-enfibers, record the deformation kinematics and consistently indicate that the ophiolite moved down in a normal sense relative to the Franciscan rocks in the modern reference frame. Based on these kinematic relations and metamorphic facies contrasts between the Franciscan and the Coast Range Ophiolite, as much as 6–18 km structural attenuation has occurred along the Coast Range fault. After restoration of vertical axis rotation and late Cenozoic fold deformation, the Coast Range fault dips at a low angle toward the northeast, and shear-sense data restore to top-to-the-northeast motion on the Coast Range fault, implying normal displacement of the ophiolite with respect to the structurally underlying Franciscan Complex. These relations are consistent with the hypothesis that the Franciscan rocks were exhumed by crustal attenuation along a low-angle fault system. These data suggest that the Coast Range fault dominantly accommodated normal displacement and ductile thinning, and that these processes were primarily responsible for the structural exhumation of high-pressure assemblages in the Franciscan Complex relative to the structurally overlying forearc crust.

Evolution of the Xiaotian-Mozitan fault and its implications for exhumation of Dabie HP–UHP rocks

The Xiaotian-Mozitan fault (XMF) located north of the Dabie orogenic belt separates the North Dabie complex to the south from the Beihuaiyang low-grade metamorphic rocks to the north. It comprises several NW-striking ductile shear zones and brittle faults. The brittle faults obviously overprinted on the ductile shear zones and promoted the development of the volcanic basins in early Cretaceous to the north, which suggests that the brittle faults were normal faults formed in early Cretaceous during doming of the Dabie orogenic belt. The ductile shear zone superposed on the north Dabie gray gneiss, and it is an important channel where the Dabie HP–UHP rocks exhumed. For obtaining new structural constraint on exhumation of the HP–UHP rocks, we present here experimental results on the microstructure, quartz C-axis fabrics and the microprobe analyses of phengite. The ductile shear zone was determined to be formed at a temperature of 600–650 °C and pressure of 1.1 GPa by the mineral deformation, microprobe analyses and geobarometry of Si-in-phengite of the mylonite, the results suggest that the mylonite now exposed on the surface experienced an upper amphibolite-facies metamorphism in the lower crust. The mineral stretching lineation varies from horizontal in the east segment to sub-dip in the west. Shear sense indicators from outcrop and thin sections of orientated specimen and quartz C-axis fabrics suggest that the XMF is a sinistral normal fault. The kinematics analysis of the ductile shear zone indicates that the exhumation of Dabie HP–UHP rocks is the results of a SE-directed extrusion and an anticlockwise rotation around its eastern pivot simultaneously.

Quartz fabrics in the Alpine Fault mylonites: Influence of pre-existing preferred orientations on fabric development during progressive uplift

Strong quartz crystallographic preferred orientations (CPOs) were developed during dislocation creep in the mylonite zone exposed in the hanging wall of the dextral reverse Alpine Fault Zone, New Zealand. The CPOs have a consistent asymmetry indicating a high ratio of simple to pure shear strain, with a shear sense of dextral-up to the NW, consistent with the mesoscopic shear sense indicators and with slip on the active fault. There is a transition from Y-maxima and asymmetric single girdles in mylonites and ultramylonites within 300 m of the present fault trace, to cross-girdle fabrics in the protomylonites further from the fault. The strong Y-maxima or single girdle CPOs are ascribed to high ductile shear strains under amphibolite facies conditions while the cross-girdle patterns found in the protomylonites are interpreted to represent deformation under lower temperature conditions. However, the observed fabric transition cannot logically be attributed to variations in temperature during the last increment of deformation. The highly oriented Y-maximum fabrics formed at high temperature contain very few grains suitably oriented for basal <a> slip, so that the slip systems activated during subsequent shear at lower temperatures during exhumation were prism <a> or rhomb <a>. Further from the fault, where shear strains under high temperature conditions were lower, weaker fabrics developed under these conditions were modified at higher levels in the crust into crossed girdle patterns. This interpretation implies that intense localisation of shear strain along the fault zone within the lower crust must have occurred in order for the high-temperature fabrics to become sufficiently intense to be preserved. This in turn implies that deep-seated localised shear was taking place early during the evolution of the current oblique-slip plate boundary fault. The results of this study also indicate that the current model that operation of certain slip systems in quartz is mostly a function of temperature is incorrect. Future work should consider the effects of deformation history, total strain and the presence of other mineral phases.

Age and evolution of late Mesozoic metamorphic core complexes in southern Siberia and northern Mongolia

Numerous Cretaceous metamorphic core complexes (MCCs) extend from Transbaikalia in Russia to northern Mongolia within the Central Asian Orogenic Belt. We investigated the Buteel and Zagan MCCs in detail. Shear sense indicators in mylonitized rocks show footwall-to-the-NW tectonic transport. Single zircon dating of footwall rocks in the Buteel MCC establishes the emplacement of granitoid orthogneiss precursors at 240–211 Ma, a felsic metavolcanic rock at 265.0 ± 1.2 Ma, a syenite at 265.5 ± 1.2 Ma and a metarhyolite of the pre-granitoid basement at 553.6 ± 2.9 Ma. A peralkaline granite intruding orthogneisses of the Zagan MCC has a new U–Pb zircon age of 151.6 ± 0.7 Ma. 40 Ar/ 39 Ar ages of 133.5 ± 1.8 Ma of hornblende from amphibolite and 122.6 ± 1.8 Ma of biotite from mylonitized gabbro–dolerite of the Buteel MCC are interpreted as cooling ages representing the time of deformation in the footwall. Geological data suggest that the MCCs in Transbaikalia and northern Mongolia formed as a result of extension in a crust that had previously been thickened by abundant calc-alkaline magmatism in an Andean-type setting on the border of the closing Mongol–Okhotsk ocean, by widespread collisional to post-collisional thrusting, and by extensive alkaline–peralkaline magmatism.

The initiation and evolution of the transpressional Straight River shear zone, central Fiordland, New Zealand

Structural data and U/Pb geochronology on zircon from central Fiordland, New Zealand show the role of pre-existing structural heterogeneities in the kinematic evolution of a newly discovered zone of transpression. The Straight River shear zone consists of steep zones of high strain that are superimposed onto older fabrics across a 10 × 80 km region. The older foliation formed during two periods of tectonism: contraction and magmatism of mostly Carboniferous (∼312–306 Ma) age and Early Cretaceous batholith emplacement ending by 113.4 ± 1.7 Ma followed by extension that ceased by 88.4 ± 1.2 Ma. The primary mechanism for the formation of steep shear zone foliations was the folding of these older fabrics. Conjugate crenulation cleavages associated with the folding record shortening at high angles to the shear zone boundaries. Fold axial surfaces and axial planar cleavages strike parallel to the shear zone with increasing strain as they progressively steepened to subvertical. In most areas, shear sense flips from oblique-sinistral (east-side-down component) to oblique-dextral (west-side-down) across zones of intermediate and high strain. High strain zones display subvertical mineral lineations, steep strike-slip faults and shear sense indicators that record strike-slip motion across the steep lineations. These patterns reflect triclinic transpression characterized by narrow zones of mostly strike-slip deformation and wide zones of mostly contraction. Zones of high strain align with offshore traces of late Tertiary strike-slip faults, suggesting that a previously undocumented component of late Tertiary shortening and strike-slip motion is accommodated within Fiordland.

Evolução tectônica da Zona de Cisalhamento Caxambu, MG

The NE-SW trending Caxambu Shear Zone (CSZ) is an example of a group of similar shear zones that crop out in southern Minas Gerais and adjacent Rio de Janeiro states. They represent the final stage of the Neoproterozoic tectonic activity related to the Brasiliano Orogeny in this region. It is a dextral shear zone with predominantly ductile behavior. The CSZ affects metaigneous and metasedimentary rocks of a Paleoproterozoic basement with its Neoproterozoic cover denominated Andrelândia Megasequence (AMS). Six mapping units were recognized: orthogneiss (basement), banded biotite gneiss (AMS), quartzite (AMS), biotite schist (AMS), garnet muscovite schist (AMS) and a plug of alkaline intrusive rocks of probable Cretaceous age. Two groups of structures were recognized in the area. The first group, attributed to deformation phase D B , related to the Brasilia Belt, includes the main foliation and associated lineation and extensive nappe structures with tectonic transport top to the east. The second group, related to the Ribeira Belt and labeled D R , includes the Caxambu Shear Zone. The CSZ is more then 100 km long and about 2.5 km wide; its dextral horizontal displacement in the studied area is estimated at about 14.5 km. Including published estimates from adjacent areas an average value of 17.6 km is obtained, resulting in na approximate average value for γ of simple shear of seven. Associated to the CSZ mylonitic rocks were formed containing the following shear sense indicators: mica fish and other minerals with similar shape and asymmetry, oblique foliation, S-C structures, mantled porphyroclasts and asymmetric microfolds. The metamorphic conditions during mylonitization are estimated at lower amphibolite facies.

Slip partitioning and inclined dextral transpression along the Zagros Thrust System, Iran

Flexural duplex structures and shear zones are well developed in the Heneshk area, in southwestern Iran. The Zagros Thrust System in this area consists of eight sheets of NW-striking, NE-dipping dextral strike-slip duplexes that are linked with imbricate fans and oblique-slip thrusts. The Zagros Thrust System is a portion of the internal zone of the Zagros orogenic belt. This internal zone is characterized by penetrative plastic deformation and metamorphism. Based on the kinematic vorticity number ( W k = 0.73 ± 0.02) calculated from quartz porphyroclasts of the Goshti shear zones along the Zagros Thrust System, the estimated θ angle between the maximum instantaneous strain axis (ISA) and the transpressional zone boundary is 33°. The estimated α angle of the plate convergence or the flow apophysis of the displacement field is 25°.The presence of dominantly dextral shear sense indicators in the area is consistent with dextral inclined transpressional convergence. The mean estimated finite deformation ( W m ) value indicates relative contributions of 47% pure shear and 53% simple shear for the deformation which involved both strike-slip and oblique slip displacements. In this inclined transpression, about 40% strike-slip partitioning was required to accommodate the finite strain and re-orientation of instantaneous strain.

Extrusão tectônica e transporte lateral de massa na porção central do cinturão Paraíba do Sul, seção Três Rios - Matias Barbosa (RJ/MG)

This paper deals with the geometry and kinematics of structures, described along a section cross-cutting the northern limb of a NE-trending regional fan-like geometry. The structure occurs on the core of the Ribeira orogen in SE Brazil. Three structural domains were described, from south to north. In the domain I, the mylonitic foliation runs towards NE with sub-vertical to vertical dip. Stretching lineation is horizontal, and assymmetric structures suggest dextral transpressive motion in the core of the divergence (domain I). Tablet-of-chocolate boudinage suggests secondary vertical stretching and shortening orthogonal to the flow plane occurs. Northwards occurs a crustal slice composed mainly by orthogneiss, and minor metassediments and deformed granitoids, called Paraíba do Sul domain – or domain II. This slice tectonically overlies the granulitic rocks of the Juiz de Fora domain (domain III). Foliation in domain II delineates a guirdle on the stereoplot, with slowly ENE- plunging axes. Stretching lineation plunges slowly to NE, parallel to the girdle axes. Tight to isoclinal folds with axis parallel to the stretching lineation occur within the mylonitic foliation. Slowly southward dipping mylonitic foliation (and associated folds) are deformed by open to tight, nearly upright extension-parallel folds with axial surfaces sub-parallel to the regional transpression zones. L-tectonites in deformed granitoids, and mullion structures parallel to the axes of both families of folds also were described. Shear-sense indicators suggest top-to-SWW shearing, sub-parallel and oblique to the orogenic trend, coeval with a NW-SE contractional component. Later structures are ductile-ruptile top-toNW thrusts and sinistral transtrative shear zones. The structural evolution of the area is interpreted in terms of regional dextral unconfined transpression. Our observations suggest that strain path partitioning during oblique extrusion and convergence may explain coeval parallel orogenic motion either in flat lying or steeply dipping shear zones, and WNW-directed dextral thrusts described in the deeper portions of the belt. Tectonic extrusion may also explain the presence of L-tectonites and the absence of vertical stretching lineation in the highly strained zones in the Paraíba do Sul Belt.

Extrusão tectônica e transporte lateral de massa na porção central do cinturão Paraíba do Sul, seção Três Rios - Matias Barbosa (RJ/MG)

This paper deals with the geometry and kinematics of structures, described along a section cross-cutting the northern limb of a NE-trending regional fan-likegeometry. The structure occurs on the core of the Ribeira orogen in SE Brazil. Three structural domains were described, from south to north. In the domain I, the mylonitic foliation runs towards NE with sub-vertical to vertical dip. Stretching lineation is horizontal, and assymmetric structures suggest dextral transpressive motion in the core of the divergence (domain I). Tablet-of-chocolate boudinage suggests secondary vertical stretching and shortening orthogonal to the flow plane occurs. Northwards occurs a crustal slice composed mainly by orthogneiss, and minor metassediments and deformed granitoids, called Paraiba do Sul domain – or domain II. This slice tectonically overlies the granulitic rocks of the Juiz de Fora domain (domain III). Foliation in domainII delineates a guirdle on the stereoplot, with slowly ENE- plunging axes. Stretching lineation plunges slowly to NE, parallel to the girdle axes. Tight to isoclinal folds with axis parallel to the stretching lineation occur within the mylonitic foliation. Slowly southward dipping mylonitic foliation (and associated folds) are deformed by open to tight, nearly upright extension-parallel folds with axial surfaces sub-parallel to the regional transpression zones. L-tectonites in deformed granitoids, and mullion structures parallel to the axes of both families of folds also were described. Shear-sense indicators suggest top-to-SWW shearing, sub-parallel and oblique to the orogenic trend, coeval with a NW-SE contractional component. Later structures are ductile-ruptile top-to- NW thrusts and sinistral transtrative shear zones. The structural evolution of the area is interpreted in terms of regional dextral unconfined transpression. Our observations suggest that strain path partitioning during oblique extrusion and convergence may explain coeval parallel orogenic motion either in flat lying or steeply dipping shear zones, and WNW-directed dextral thrusts described in the deeper portions of the belt. Tectonic extrusion may also explain the presence of L-tectonites and the absence of vertical stretching lineation in the highly strained zones in the Paraiba do Sul Belt.