scholarly journals Texture Asymmetries as Shear Sense Indicators in Naturally Deformed Mono- and Polyphase Carbonate Rocks

1999 ◽  
Vol 33 (1-4) ◽  
pp. 61-74 ◽  
Author(s):  
B. Leiss ◽  
S. Siegesmund ◽  
K. Weber
Keyword(s):  
Shear Zone ◽  
Carbonate Rocks ◽  
Shear Plane ◽  
Deformation Texture ◽  
Strike Slip ◽  
Strike Slip Fault ◽  
Deformation Mechanisms ◽  
Shear Sense ◽  
Shear Sense Indicators ◽  
Main Deformation

The microstructural and quantitative texture analyses of a naturally deformed calcite mylonite, a dolomite mylonite and a dolomitic calcite mylonite reveal different texture asymmetries for comparable deformation conditions. Calcite shows a c-axis maximum rotated against the shear sense with regard to the main shear plane. In contrast, the dolomite shows a c-axis maximum rotated with the shear sense. In accordance with the experimental and simulated textures from the literature, this difference proves e-twinning and r-slip for calcite and f-twinning and c-slip for dolomite as the main deformation mechanisms. The dolomitic calcite mylonite shows for both the calcite and the dolomite a c-axis maximum rotated against the shear sense. On account of the microstructure of this sample, the dolomite texture has been passively overtaken from the deformation texture of calcite during a late-deformative dolomitization. The results significantly contribute to the interpretation that the sampled shear zone is a transpressive strike–slip fault.

Microstructures and deformation temperature of carbonate mylonites in Shajigami shear zone at eastern margin of Abukuma Mountains, Northeastern Japan

2021 ◽  
Author(s):  
Hiroaki Yokoyama ◽  
Jun Muto ◽  
Hiroyuki Nagahama
Keyword(s):  
Grain Size ◽  
Shear Zone ◽  
Deformation Temperature ◽  
Microstructural Analysis ◽  
Shear Plane ◽  
Strike Slip ◽  
Eastern Margin ◽  
Northeastern Japan ◽  
Shear Sense ◽  
Shear Component

<p>　　Microstructural analysis is essential for estimating the deformation conditions of plastically deformed rocks. In this study, we analyze the microstructures of carbonate mylonites and deformation conditions in natural shear zone to reconstruct tectonics. Carbonate mylonites originated from late Carboniferous Tateishi Formation and mylonitized in middle Cretaceous by the strike-slip motion of Shajigami shear zone in the eastern margin of the Abukuma Mountain, Northeastern Japan.<br>　　Microstructural analysis was carried out by optical microscope and electron backscattered diffraction (EBSD) mapping to determine grain size, aspect ratio, shape preferred orientation (SPO) and crystallographic preferred orientation (CPO) of calcite aggregates.<br>　　Pervasive deformation twins and dynamically recrystallized grains are observed. Although most porphyroclasts show symmetric structure, some show asymmetric structure that indicates dextral shear sense. Mean dynamically recrystallized grain size is 16-67 µm, and it decreases close to the shear zone. CPOs show that <em>c</em>-axes concentrate normal to the shear plane or slightly rotate to the shear sense. The strong CPOs suggest that the dominant deformation mechanism is dislocation creep. SPOs show the foliation which is slightly oblique or almost parallel to the shear plane. However, we observed the SPOs parallel to the shear plane at the location 150 m away from the shear zone.  The 3D dynamically recrystallized grain shapes are between plane-strain ellipsoid and oblate ellipsoid. The grain shapes tend to be relatively polygonal close to the shear zone, while more elongated further away from the shear zone. The distribution of the carbonate mylonite originated from same Tateishi Formation is known to be about 5 km apart from the Shajigami shear zone (Tateishi location). However, based on many aspects of differences in microstructures among both locations such as SPOs of recrystallized grains, we infer that the deformation of Shajigami shear zone was not related to one at Tateishi location. The pervasive dynamic recrystallization suggests that the deformation temperature was at least 200°C. Observed type Ⅱ and type Ⅲ twin morphologies (Burkhard, 1993) of calcite grains suggest deformation temperature below 300°C. <br>　　These results indicate that the deformation of the Shajigami shear zone was in the range from 200 to 300℃ and deformation was stronger near the shear zone. In addition, the polygonal grain shape close to the shear zone suggests that the deformation temperature is higher close to the shear zone. Furthermore, SPOs show that pure shear component is larger than simple shear component in terms of SPOs that almost parallel to the shear plane away from the shear zone. This study including several additional results will provide the microstructural development of carbonate mylonites in natural strike-slip shear zones deformed near the brittle-ductile condition of the upper crust.</p>

Structural and kinematic analysis of the Shagawa Lake shear zone, Superior Province, northern Minnesota: implications for the role of vertical versus horizontal tectonics in the Archean

2010 ◽  
Vol 47 (12) ◽  
pp. 1463-1479 ◽  
Author(s):  
Emerald J. Erickson
Keyword(s):  
Shear Zone ◽  
Horizontal Displacement ◽  
Northern Region ◽  
Strike Slip ◽  
Superior Province ◽  
Plane Normal ◽  
North Side ◽  
Shear Sense ◽  
Narrow Width ◽  
Shear Sense Indicators

The steeply dipping Shagawa Lake shear zone, which strikes ENE through the Vermilion District of the Superior Province in northeastern Minnesota, is marked by zone-parallel foliation and a mineral elongation lineation (Le). Le includes (i) a broad population with pitch of 90° ± 60°, and (ii) a less abundant population with pitch of 15° ± 15°. Shallowly plunging Le is rare, and where it occurs, it overprints the pervasive steeply plunging Le. Shear sense indicators occur within the L–S tectonite motion plane, normal to foliation and parallel to Le. Microstructures define both south-side-up and north-side-up displacement domains, but no spatial patterns emerge across the shear zone. L–S tectonites with east-plunging Le indicate either south- or north-side-up shear parallel to Le, whereas L–S tectonites with west-plunging Le indicate predominantly north-side-up shear parallel to Le. Strike-slip L–S tectonites are rare, but consistently record sinistral shear. Overprinting relationships and structural-kinematic patterns can be attributed to sinking of the Vermilion District volcanic basin and relative rise of the southern region followed by rise of the northern region, as represented by the Giants Range Batholith and Vermilion Granitic Complex, respectively. Structural and kinematic evidence indicates that a process like sagduction–diapirism could explain the rising granitoids and sinking volcanic basin. The narrow width of the shear zone, the need for structural dates and radiometric dates of the surrounding plutons, and lack of documented strike-slip kinematic data make it difficult to evaluate when and how the shear zone transitioned to horizontal displacement.

Regional setting and kinematic features of the Needle Falls Shear Zone, Trans-Hudson Orogen

1993 ◽  
Vol 30 (7) ◽  
pp. 1338-1354 ◽  
Author(s):  
Mel R. Stauffer ◽  
John F. Lewry
Keyword(s):  
Shear Zone ◽  
Simple Shear ◽  
Country Rock ◽  
Shear Zones ◽  
Small Scale ◽  
Lake Zone ◽  
Shear Sense ◽  
Regional Tectonic ◽  
Ductile Shear ◽  
Shear Sense Indicators

Needle Falls Shear Zone is the southern part of a major northeast-trending ductile shear system within the Paleoproterozoic Trans-Hudson Orogen in Saskatchewan. Throughout its exposed length of ~400 km, the shear zone separates reworked Archean continental crust and infolded Paleoproterozoic supracrustals of the Cree Lake Zone, to the northwest, from mainly juvenile Paleoproterozoic arc terrains and granitoid plutons of the Reindeer Zone, to the southeast. It also defines the northwest margin of the ca. 1855 Ma Wathaman Batholith, which forms the main protolith to shear zone mylonites. Although not precisely dated, available age constraints suggest that the shear zone formed between ca. 1855 and 1800 Ma, toward the end of peak thermotectonism in this part of the orogen.In the Needle Falls study area, shear zone mylonites exhibit varied, sequentially developed, ductile to brittle fabric features, including C–S fabrics, winged porphyroclasts (especially delta type), small-scale compressional and extensional microfaults ranging from thin ductile shear zones to late brittle faults, early isoclinal and sheath folds, later asymmetric folds related to compressional microfaults, and variably rotated and (or) folded quartz veins. All ductile shear-sense indicators suggest dextral displacement, as do most later ductile–brittle transition and brittle features. In conjunction with a gently north–northeast-plunging extension lineation, such data indicate oblique east-side-up dextral movement across the shear zone. However, preexisting structures in country rock protoliths rotate into the shear zone in a sense contrary to that predicted by ideal dextral simple shear, a feature thought to reflect significant flattening across the shear zone. Other ductile to brittle fabric elements in the mylonites are consistent with general noncoaxial strain, rather than ideal simple shear. Amount of displacement cannot be measured but indirect estimates suggest approximately 40 ± 20 km.The Needle Falls Shear Zone is too small and has developed too late in regional tectonic history to be considered a crustal suture. Rather, it is interpreted as either a late-tectonic oblique collisional structure or as the result of counterclockwise oroclinal rotation of the southern part of the orogen.

scholarly journals Structural Overview and Morphotectonic Evolution of a Strike-Slip Fault in the Zone of North Almora Thrust, Central Kumaun Himalaya, India

2016 ◽  
Vol 2016 ◽  
pp. 1-16 ◽  
Author(s):  
Lalit M. Joshi ◽  
Pitamber D. Pant ◽  
Bahadur S. Kotlia ◽  
Girish C. Kothyari ◽  
Khayingshing Luirei ◽  
...  
Keyword(s):  
Shear Zone ◽  
Strike Slip ◽  
Strike Slip Fault ◽  
Asymmetry Factor ◽  
Base Line ◽  
Gradient Index ◽  
River Channels ◽  
Fluvial Terraces ◽  
Dextral Strike Slip Fault ◽  
Dextral Strike Slip

The aim of the present research is to provide the base line details of the NNW-SSE trending Raintoli fault (RF) which is running parallel to the North Almora Thrust (NAT) along the Saryu valley from Seraghat-Naichun to Seri in the central sector of the Uttarakhand Himalaya, India. The RF is characterized as dextral strike slip fault and behaves as a ductile shear zone within the zone of NAT. The dextral sense of shear movement of RF is delineated by the fabric of the shear zone rocks including microscopically observed indicators such as sigma and delta porphyroclasts, quartz c-axis, and the field structural data. Additionally, in the quaternary period the dextral strike slip fault is reactivated with oblique slip component as characterized by various geomorphic indicators, for example, triangular facets, abandoned river channels, unpaired fluvial terraces, and V-shaped valleys with recurrent seismicity. Further, the morphometric parameters including Valley Floor Width to Valley Height (Vf), asymmetry factor (AF), and gradient index (GI) further prove active nature of RF as suggested by low values of hypsometric integration, V-shaped valley, higher gradient index, and tilting of Saryu basin.

scholarly journals Strain Pattern Analysis of Mylonites From Sitampundi-Kanjamalai Shear Zone, Thiruchengode, South India

2019 ◽  
Vol 1 (1) ◽  
pp. 25-34
Author(s):  
Thirukumaran V ◽  
Biswal T.K ◽  
Sundaralingam K ◽  
Sowmya V ◽  
Boopathi S ◽  
...  
Keyword(s):  
Shear Zone ◽  
Simple Shear ◽  
Pure Shear ◽  
Strain Analysis ◽  
Strain Pattern ◽  
Shear Sense ◽  
Shear Component ◽  
Deep Crustal Level ◽  
Dextral Shear ◽  
Shear Sense Indicators

This study aims to investigate the petrography and strain pattern of mylonites from parts of N-S trending Sitampundi-Kanjamalai Shear Zone (SKSZ) around Thiruchengode. The petrographic study indicates the presence of recrystallized quartz, K-feldspar, plagioclase, biotite and some hornblende. The kinematic analysis of Mylonites was done with the help of shear sense indicators such as recrystallized type quartz (quartz ribbon) around the cluster of feldspar, S-C fabric shows dextral shear sense and some sinisterly shear sense in some parts of SASZ which can be considered as a product of partitioning of both strain and vorticity between domains. These all indicates the simple shear extension along E-W direction and the mylonitic foliation shows the pure shear compression along N-S direction. Further the study of bulk strain analysis by Flinn plot method using L and T section of mylonite shows k<1 which lies in the field of flattening zone of finite strain. The kinematic vorticity number is calculated by Rxz/β method which gives the value of 0.36 indicating the general shear. The rigid grain graph shows that the pure shear component is more ­­­­dominant than the simple shear component. The analysis leads to the conclusion that the mylonite has experienced a high temperature shearing of above 700°cat deep crustal level.

Complex kinematics in a major ductile shear zone, NW Shetland: Evidence of ductile extrusion during Caledonian transpression

2021 ◽  
Author(s):  
Timothy Armitage ◽  
Robert Holdsworth ◽  
Robin Strachan ◽  
Thomas Zach ◽  
Diana Alvarez-Ruiz ◽  
...  
Keyword(s):  
Shear Zone ◽  
Hanging Wall ◽  
Regional Scale ◽  
Shear Zones ◽  
White Mica ◽  
Strike Slip ◽  
Amphibolite Facies ◽  
Lateral Extrusion ◽  
Shear Sense ◽  
Ductile Shear

&lt;p&gt;Ductile shear zones are heterogeneous areas of strain localisation which often display variation in strain geometry and combinations of coaxial and non-coaxial deformation. One such heterogeneous shear zone is the c. 2 km thick Uyea Shear Zone (USZ) in northwest Mainland Shetland (UK), which separates variably deformed Neoarchaean orthogneisses in its footwall from Neoproterozoic metasediments in its hanging wall (Fig. a). The USZ is characterised by decimetre-scale layers of dip-slip thrusting and extension, strike-slip sinistral and dextral shear senses and interleaved ultramylonitic coaxially deformed horizons. Within the zones of transition between shear sense layers, mineral lineations swing from foliation down-dip to foliation-parallel in kinematically compatible, anticlockwise/clockwise-rotations on a local and regional scale (Fig. b). Rb-Sr dating of white mica grains via laser ablation indicates a c. 440-425 Ma Caledonian age for dip-slip and strike-slip layers and an 800 Ma Neoproterozoic age for coaxial layers. Quartz opening angles and microstructures suggest an upper-greenschist to lower-amphibolite facies temperature for deformation. We propose that a Neoproterozoic, coaxial event is overprinted by Caledonian sinistral transpression under upper greenschist/lower amphibolite facies conditions. Interleaved kinematics and mineral lineation swings are attributed to result from differential flow rates resulting in vertical and lateral extrusion and indicate regional-scale sinistral transpression during the Caledonian orogeny in NW Shetland. This study highlights the importance of linking geochronology to microstructures in a poly-deformed terrane and is a rare example of a highly heterogeneous shear zone in which both vertical and lateral extrusion occurred during transpression.&lt;/p&gt;&lt;p&gt;&lt;img src=&quot;https://contentmanager.copernicus.org/fileStorageProxy.php?f=gepj.0cf6ef44e5ff57820599061/sdaolpUECMynit/12UGE&amp;app=m&amp;a=0&amp;c=d96bb6db75eed0739f2a6ee90c9ad8fd&amp;ct=x&amp;pn=gepj.elif&amp;d=1&quot; alt=&quot;&quot;&gt;&lt;/p&gt;

scholarly journals Polyphase deformation along the South Bohemian Batholith-Moldanubian nappes boundary – The Freyenstein Fault System (Bohemian Massif/Austria)

2020 ◽  
Vol 113 (1-2) ◽  
pp. 139-153
Author(s):  
Gerit E. U. Griesmeier ◽  
Christoph Iglseder ◽  
Ralf Schuster ◽  
Konstantin Petrakakis
Keyword(s):  
Shear Zone ◽  
Bohemian Massif ◽  
Potassium Feldspar ◽  
Strike Slip ◽  
Greenschist Facies ◽  
Fault System ◽  
Ductile Shear Zone ◽  
The South ◽  
Shear Sense ◽  
Ductile Shear

AbstractThis work describes the Freyenstein Fault System, which extends over 45 km in the southeastern part of the Bohemian Massif (Lower Austria). It represents a ductile shear zone overprinted by a brittle fault located at the eastern edge of the South Bohemian Batholith towards the Moldanubian nappes. It affects Weinsberg- and a more “fine-grained” granite, interlayered aplitic granite and pegmatite dikes as well as paragneiss of the Ostrong Nappe System. The ductile shear zone is represented by approximately 500 m thick greenschist-facies mylonite dipping about 60° to the southeast. Shear-sense criteria like clast geometries, SCC`-type shear band fabrics as well as abundant microstructures show top to the south/ southsouthwest normal shearing with a dextral strike-slip component. Mineral assemblages in mylonitized granitoid consist of pre- to syntectonic muscovite- and biotite-porphyroclasts as well as dynamically recrystallized potassium feldspar, plagioclase and quartz. Dynamic recrystallization of potassium feldspar and the stability of biotite indicate upper green-schist-facies metamorphic conditions during the early phase of deformation. Fluid infiltration at lower greenschist-facies conditions led to local sericitization of feldspar and synmylonitic chloritisation of biotite during a later stage of ductile deformation. Finally, a brittle overprint by a north-south trending, subvertical, sinistral strike-slip fault that shows a normal component is observed. Ductile normal shearing along the Freyenstein Shear Zone is interpreted to have occurred between 320 Ma and c. 300 Ma. This time interval is indicated by literature data on the emplacement of the hostrock and cooling below c. 300°C inferred from two Rb-Sr biotite ages measured on undeformed granites close to the shear zone yielding 309.6 ± 3 Ma and 290.9 ± 2.9 Ma, respectively. Brittle sinistral strike-slip faulting at less than 300°C presumably took place not earlier than 300 Ma. Early ductile shearing along the Freyenstein Fault System may be genetically, but not kinematically linked to the Strudengau Shear Zone, as both acted in an extensional regime during late Variscan orogenic collapse. A relation to other major northeast-southwest trending faults of this part of the Bohemian Massif (e.g. the Vitis-Pribyslav Fault System) is indicated for the phase of brittle sinistral movement.

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