scholarly journals Quantitative finite strain analysis of the quartz mylonites within the Three Pagodas shear zone, western Thailand

2018 ◽  
Vol 111 (2) ◽  
pp. 171-179
Author(s):  
Pitsanupong Kanjanapayont ◽  
Peekamon Ponmanee ◽  
Bernhard Grasemann ◽  
Urs Klötzli ◽  
Prayath Nantasin
Keyword(s):  
Shear Zone ◽  
Pure Shear ◽  
Strain Analysis ◽  
Strain Ratio ◽  
Kinematic Vorticity ◽  
The North ◽  
Shear Sense ◽  
Shear Component ◽  
The Mean ◽  
Vorticity Analysis

AbstractThe NW–trending Three Pagodas shear zone exposes a high–grade metamorphic complex named Thabsila gneiss in the Kanchanaburi region, western Thailand. The quartz mylonites within this strike–slip zone were selected for strain analysis. 2–dimensional strain analysis indicates that the averaged strain ratio (Rs) for the lower greenschist facies increment of XZ– plane is Rs = 1.60–1.97 by using the Fry’s method. Kinematic vorticity analysis of the quartz mylonites in the shear zone showed that the mean kinematic vorticity number of this increment is Wk = 0.75–0.99 with an average at 0.90 ±0.07. The results implied that the quartz mylonites within the Three Pagodas shear zone have a dominant simple shear component of about 72% with a small pure shear component. A sinistral shear sense is indicated by kinematic indicators from macro– to micro–scale. We conclude that the Three Pagodas shear zone deformed in the process of sinstral shear–dominated transpression, which is similar to the Mae Ping shear zone in the north.

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.

scholarly journals Contrasting mineralogy and strain partitioning across N-S oriented Sitampundi - Kanjamalai Shear in Sitampundi Anorthosite Layered Complex

2021 ◽  
pp. 67-80
Author(s):  
Harish M.K ◽  
Rahul J ◽  
Thirukumaran V
Keyword(s):  
Shear Zone ◽  
Strain Analysis ◽  
Aluminium Oxide ◽  
Strain Ratio ◽  
Metamorphic Grade ◽  
Strain Partitioning ◽  
The North ◽  
North Eastern ◽  
Layered Complex ◽  
North Eastern Part

Sitampundi Anorthosite Layered Complex (SALC) is a complexly folded and metamorphosed terrain that shows different metamorphic grade separated by a regional linear divide. In the north-eastern part of the complex, the anorthosites contain green-colored clinozoisites that are strikingly absent in the western part of the limb. Based on the presence of the clinozoisites, the entire SALC can be divided into two zones. The Sitampundi-Kanjamalai shear zone (SKSZ) separates mega crystals of clinozoite bearing anorthosites from clinozoisite free anorthosites. To add furthermore, strain analysis of different samples of anorthosite on either side of the zones was conducted by employing Flinn method. In general, anorthosites fall into the flattening field. The clinozoisite free anorthosites are more flattening and clinozoisite bearing anorthosites exhibit a slight difference in their strain ratio, ie., it is comparatively less flattening.  Geochemistry of clinozoisites was studied using EPMA & XRD methods. The percentage of oxides obtained from EPMA coincides with that of epidote. But, XRD confirms the mineral to be clinozoisite indicating the transition phase of epidote to clinozoisite. Zoning has had occurred in clinozoisites with aluminium oxide rich core and FeO rich rim. This could be related to a retrogression corresponding to a shearing event.

Non-steady strain in the terrane boundary shear zone of the Ambaji Granulite, NW India: Implications for understanding towards the dynamics of emplacement of the lower-middle crustal rocks.

2020 ◽  
Author(s):  
Ragini Saraswati ◽  
Tapas Kumar Biswal
Keyword(s):  
Shear Zone ◽  
Pure Shear ◽  
Strain Analysis ◽  
Shear Zones ◽  
Strike Slip ◽  
Ductile Deformation ◽  
Mobile Belt ◽  
High Grade ◽  
Crustal Rocks ◽  
Shear Component

&lt;p&gt;Shear zones in the high-grade terranes represent the tectonic- fossils of strain history. One such shear zones, namely Balaram-Jogdadi shear zones defining the terrane boundary of the Ambaji granulites of the South Delhi terrane Aravalli &amp;#8211;Delhi Mobile belt, NW India, provide evidence for strain variation during exhumation of lower-middle crustal rocks. Compilation of field and microscopic analysis of various samples of mylonite from shear zones suggest that the part of shear zone contains high-grade mineral assemblages such as cordierite, sillimanite, spinel, garnet in quartzo-feldspathic mylonite rock and exhibit signature of thrusting in which garnet behaved as brittle phase and quartz and feldspar grain show ductile deformation. 2D and 3D strain analysis estimate a plane to flattening type of strain pattern. Principal strain planes are used to calculate the strain ratios for estimation of variation of strain along the shear zone. This study indicates high-grade mylonite accommodates high strain. The flow of rigid porphyroclasts estimates mean kinematic vorticity number varies from 0.47 to 0.68, which indicates the dominance of pure shear during shearing. Vorticity by the Rs/&amp;#952; method in quartz grain estimates ranges from 0.7 to 0.95, suggesting a non-steady strain towards the end of deformation. High-grade mylonites were overprinted by low-temperature mylonitisation marked by minerals like quartz, feldspar, biotite in which feldspar porphyroclast shows brittle deformation and quartz, biotite show ductile deformation. Several shear kinematics indicate top-to-NW sinistral strike-slip shearing. Thus it has been interpreted that the shear zone had undergone non-steady strain. The initial thrusting phase was dominated by more pure shear component. The strike-slip shearing part was dominated by more simple shear component. Monazite geochronology sets the age of shearing at 834-778 Ma suggesting the exhumation was a transition event between Grenville to Pan-African orogeny.&lt;/p&gt;&lt;p&gt;Keywords: Shear zone, Deformation, Vorticity, 3D strain analysis, Monazite dating&lt;/p&gt;

scholarly journals THE NATURE OF DUCTILE DEFORMATION IN THE PHYLLITE-QUARTZITE UNIT (EXTERNAL HELLENIDES)

2017 ◽  
Vol 43 (1) ◽  
pp. 387
Author(s):  
P. Xypolias ◽  
V. Chatzaras
Keyword(s):  
Spatial Variation ◽  
Pure Shear ◽  
Structural Evolution ◽  
Strain Ratio ◽  
Linear Increase ◽  
Ductile Deformation ◽  
Shear Component ◽  
Geodynamic Models ◽  
Structural Levels ◽  
Penetrative Foliation

This work describes the nature of ductile deformation in the Phyllite-Quartzite (PQ) unit in terms of structural evolution and spatial variation of finite strain and vorticity of flow. The PQ unit is affected by at least three ductile deformation (D1, 2, 3) phases. However, the D2 is the dominant phase resulting in the formation of a penetrative foliation (S2) which is by far the most common structural feature in all scales of observation. A stretching lineation (L2), which trends perpendicular to the structural grain of the belt, is well-developed within the S2 plane. Numerous kinematic criteria clearly indicate west (or south)-directed transport of the PQ unit during D2. This phase is also characterized by a systematic non-linear increase of strain ratio (Rxz) with proximity to the Basal thrust. Spatial variation of kinematic vorticity number reveals an increase of pure shear component of D2 deformation towards the middle structural levels of the unit. These results are used to discuss the validity of various geodynamic models related to the exhumation of the PQ unit.

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

&lt;p&gt;&amp;#12288;&amp;#12288;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.&lt;br&gt;&amp;#12288;&amp;#12288;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.&lt;br&gt;&amp;#12288;&amp;#12288;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 &amp;#181;m, and it decreases close to the shear zone. CPOs show that &lt;em&gt;c&lt;/em&gt;-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. &amp;#160;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&amp;#176;C. Observed type &amp;#8545; and type &amp;#8546; twin morphologies (Burkhard, 1993) of calcite grains suggest deformation temperature below 300&amp;#176;C.&amp;#160;&lt;br&gt;&amp;#12288;&amp;#12288;These results indicate that the deformation of the Shajigami shear zone was in the range from 200 to 300&amp;#8451; 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.&lt;/p&gt;

Buckle folding and deep-crustal shearing of high-grade gneisses at the junction of two major high-strain zones, Central Gneiss Belt, Grenville Province, Ontario

2005 ◽  
Vol 42 (10) ◽  
pp. 1907-1925 ◽  
Author(s):  
N Culshaw
Keyword(s):  
Shear Zone ◽  
High Strain ◽  
Pure Shear ◽  
Crustal Thickening ◽  
Shear Direction ◽  
High Angle ◽  
Grenville Province ◽  
Shear Component ◽  
Central Gneiss Belt ◽  
Upper Boundary

Low-plunging, transport-parallel F3 folds are common at all scales in the Central Gneiss Belt of the Grenville Province, but few of these folds are sheath folds. Where the D1–D2 Parry Sound shear zone intersects the D3 Shawanaga shear zone (SSZ) at a high angle, F3 folds formed at several scales (centimetre to greater than outcrop scale) in layered D1–D2 "straight" gneisses. At the start of their evolution, the F3 folds formed just beyond the SSZ with hinges near orthogonal to the D3 shear direction and with typical buckle features, e.g., wavelengths vary with layer thickness, and hinges are discontinuous and bifurcate. The buckle folds evolved within the SSZ by rotation of hinges towards the shear direction. Even though hinges initiated at a high angle to the shear direction, sheath folds were not produced. In addition to tightening the buckles, the ductile reorientation produced thin–thick (extended–shortened) limb pairs and very straight, ridge-like fold hinges and removed small folds from the extended limbs of larger folds. Such features may serve as criteria to distinguish transport-parallel folds that initiated in layering at high angles to the shear direction from those formed in layers containing the shear direction. A general shear parallel to the SSZ can reproduce several features inferred to mark stages in the progressive reorientation of the folds; the pure shear component of the general shear is inferred to have had a positive stretch direction down the dip of the shear zone, at a high angle to the transport (simple shear) direction. The interplay of buckling and shearing in the study area is, plausibly, the expression of deformation at the upper boundary of a channel-like flow that succeeded initial crustal thickening.

Deformation of the Archean Quetico–Shebandowan subprovince boundary in the Canadian Shield near Kashabowie, northern Ontario

1991 ◽  
Vol 28 (1) ◽  
pp. 116-125 ◽  
Author(s):  
Graham Borradaile ◽  
Robert Spark
Keyword(s):  
Strain Analysis ◽  
Canadian Shield ◽  
Vertical Position ◽  
Low Grade ◽  
Northern Ontario ◽  
Southern Boundary ◽  
The North ◽  
Shear Component ◽  
East West ◽  
Compressional Component

The southern boundary of the Quetico metasedimentary subprovince of the Superior Province of the Canadian Shield near Kashabowie, Ontario, is a vertical, east–west feature affected by dextral transpression that had a north-northwest – south-southeast compressional component. A synmetamorphic, locally D1 microfabric and magnetic-susceptibility fabric with an east-directed extension lineation is kinematically compatible with this pattern. It shows the same bedding–cleavage relationship and the same direction of structural facing on D1 in both the Quetico metasediments and the Shebandowan greenstone subprovince on the south side of the Quetico subprovince. In the low-grade rocks of the study area, there is a single phase of penetrative deformation, giving a nearly vertical schistosity at a consistent angle, anticlockwise with respect to the now nearly vertical east–west-striking strata. The absence of penetrative polyphase deformations may be due to the near parallelism of the subprovince boundary with the shear component of dextral transpression. Strain analysis indicates that the minimum shortening of the greywackes is 40% in a north–south direction. It is tentatively suggested that the shortening, the steepening of strata into a vertical position, and some of the S1 fabric development may have occurred prior to the climax of metamorphism and transpression. If this sequence is correct, the strata would have dipped gently to the north prior to the steepening event, with the embryonic schistosity dipping to the west.

Neoproterozoic extensional detachment in central Madagascar: implications for the collapse of the East African Orogen

2000 ◽  
Vol 137 (1) ◽  
pp. 39-51 ◽  
Author(s):  
ALAN S. COLLINS ◽  
THEODORE RAZAKAMANANA ◽  
BRIAN F. WINDLEY
Keyword(s):  
Shear Zone ◽  
Tectonic Evolution ◽  
Structural Phase ◽  
East African ◽  
Metasedimentary Rocks ◽  
The North ◽  
Basement Rocks ◽  
Granitic Magmatism ◽  
Shear Sense ◽  
East African Orogen

A laterally extensive, Neoproterozoic extensional detachment (the Betsileo shear zone) is recognized in central Madagascar separating the Itremo sheet (consisting of Palaeoproterozoic to Mesoproterozoic sediments and underlying basement rocks) from the Antananarivo block (Archaean/Palaeoproterozoic crust re-metamorphosed in the Neoproterozoic). Non-coaxial deformation gradually increases to a maximum at a lithological contrast between the granitoids and gneisses of the footwall and the metasedimentary rocks of the hangingwall. Ultramylonites at this highest-strained zone show mineral-elongation lineations that plunge to the southwest.σ-, δ- and C/S-type fabrics imply top-to-the-southwest extensional shear sense. Contrasting metamorphic grades are found either side of the shear zone. In the north, where this contrast is greatest, amphibolite-grade footwall rocks are juxtaposed with lower-greenschist-grade hangingwall rocks. The metamorphic grade in the hangingwall increases to the south, suggesting that a crustal section is preserved.The Betsileo shear zone facilitated crustal-scale extensional collapse of the East African Orogeny, and thus represents a previously poorly recognized structural phase in the story of Gondwanan amalgamation. Granitic magmatism and granulite/amphibolite-grade metamorphism in the footwall are all associated with formation of the Betsileo shear zone, making recognition of this detachment important in any attempt to understand the tectonic evolution of central Gondwana.

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