scholarly journals Kinematics and Timing Constraints in a Transpressive Tectonic Regime: The Example of the Posada-Asinara Shear Zone (NE Sardinia, Italy)

Geosciences ◽  
2020 ◽  
Vol 10 (8) ◽  
pp. 288
Author(s):  
Rodolfo Carosi ◽  
Alessandro Petroccia ◽  
Salvatore Iaccarino ◽  
Matteo Simonetti ◽  
Antonio Langone ◽  
...  
Keyword(s):  
Shear Zone ◽  
Finite Strain ◽  
Pure Shear ◽  
Deformation Gradient ◽  
Shear Zones ◽  
Temperature Deformation ◽  
Timing Constraints ◽  
Important Variation ◽  
Deformation Phase ◽  
First Time

Detailed geological field mapping, integrated with meso- and microstructural investigations, kinematic of the flow and finite strain analyses, combined with geochronology, are fundamental tools to obtain information on the temperature–deformation–timing path of crystalline rocks and shear zone. The Posada-Asinara shear zone (PASZ) in northern Sardinia (Italy) is a steeply dipping km-thick transpressive shear zone. In the study area, located in the Baronie region (NE Sardinia), the presence of mylonites within the PASZ, affecting high- and medium-grade metamorphic rocks, provides an opportunity to quantify finite strain and kinematic vorticity. The main structures of the study area are controlled by a D2 deformation phase, linked to the PASZ activity, in which the strain is partitioned into folds and shear zone domains. Applying two independent vorticity methods, we detected an important variation in the percentage of pure shear and simple shear along the deformation gradient, that increases from south to north. We constrained, for the first time in this sector, the timing of the transpressive deformation by U–(Th)–Pb analysis on monazite. Results indicate that the shear zone has been active at ~325–300 Ma in a transpressive setting, in agreement with the ages of the other dextral transpressive shear zones in the southern Variscan belt.

The kinematic vorticity analysis of ductile shear zones of Ambaji Granulite, NW India and its tectonic implications

2020 ◽  
Author(s):  
Sudheer Kumar Tiwari ◽  
Anouk Beniest ◽  
Tapas Kumar Biswal
Keyword(s):  
High Temperature ◽  
Low Temperature ◽  
Lateral Displacement ◽  
Pure Shear ◽  
Tectonic Setting ◽  
Shear Zones ◽  
Normal Faults ◽  
Temperature Phase ◽  
Brittle Ductile Transition ◽  
Deformation Phase

<p>The Neoproterozoic (834 – 778 Ma) Ambaji granulite witnessed four deformation phases (D<sub>1</sub>- D<sub>4</sub>), of which the D<sub>2</sub> deformation phase was most significant for the exhumation of granulites in the ductile regime. We performed a field study to investigate the tectonic evolution of the D<sub>2</sub> deformation phase and investigated the deformation evolution of the ductile extrusion of the Ambaji granulite by estimating the vorticity of flow (Wm) with the Rigid Grain Net and strain ratio/orientation techniques.</p><p>During the D<sub>2</sub> deformation phase, the S<sub>1</sub> fabric was folded by F<sub>2</sub> folds that are coaxial with the F<sub>1</sub> folds. The F<sub>2</sub> folds were produced in response to NW-SE compression. Because the large shear zones are oriented parallel to the axial plane of the F<sub>2</sub> folds, they likely formed simultaneously during the D<sub>2</sub> deformation phase. Compression during the D<sub>2</sub> deformation phase accommodated most of the exhumation of the granulite along the shear zones. D<sub>2</sub> shearing was constrained between 834 ± 7 to 778 ± 8 Ma (Monazite ages).</p><p>The shear zones evolved from a high temperature (>700 °C) thrust-slip shearing event in the lower-middle crust to a low temperature (450 °C) retrograde sinistral shearing event at the brittle-ductile-transition (BDT). The Wm estimates of 0.32–0.40 and 0.60 coincide with the high temperature event and suggests pure shear dominated deformation. The low temperature phase coincides with Wm estimates of 0.64–0.87 and ~1.0, implying two flow regimes. The shear zone was first affected by general non-coaxial deformation and gradually became dominated by simple shearing.</p><p>We interpreted that the high temperature event happened in a compressive tectonic regime, which led to horizontal shortening and vertical displacement of the granulite to the BDT. The low temperature event occurred in a transpressive tectonic setting that caused the lateral displacement of the granulite body at BDT depth. The Wm values indicate a non-steady strain during the exhumation of granulite. From the BDT to surface, the Ambaji granulite exhumed through the NW-SE directed extension for normal faults via brittle exhumation through crustal extension and thinning.</p>

Metagabbronorite from DSDP hole 334: an example of high-temperature deformation and recrystallization near the Mid-Atlantic Ridge

1977 ◽  
Vol 14 (4) ◽  
pp. 886-898 ◽  
Author(s):  
Herwart Helmstaedt ◽  
John M. Allen
Keyword(s):  
Shear Zone ◽  
Regional Metamorphism ◽  
Shear Zones ◽  
Granulite Facies ◽  
High Temperature Deformation ◽  
Temperature Deformation ◽  
Mid Atlantic Ridge ◽  
Magmatic History ◽  
Highly Strained ◽  
Zoned Plagioclase

Five gabbros and three peridotites from hole 334 were examined for postmagmatic deformational changes and metamorphic recrystallization. The condition of the gabbros ranges from unaltered, to slightly altered and deformed, to highly deformed and metamorphosed, the latter type being confined to narrow shear zones. Petrofabric studies and electron microprobe analyses of mineral phases of a foliated metagabbronorite from such a shear zone suggest the following post-magmatic history: (1) strong plastic deformation and anhydrous synkinematic recrystallization of igneous plagioclase (An90−85), clinopyroxene, and orthopyroxene to the granulite facies mineral assemblage of strongly zoned plagioclase (An80−65), diopside, and orthopyroxene; relics of the igneous minerals survived as highly strained porphyroclasts; (2) synkinematic growth of amphibole consisting of a patchy intergrowth of tremolite and actinolitic hornblende; the amphibole fabric is compatible with strain estimates by the centre-point method; (3) mainly static hydrous alteration — growth of (?) vermiculite and sericitization.Penetrative deformation probably initiated shortly after intrusion and took place during cooling of the rocks from approximately 800 °C to 300 °C. The bulk of the strain in the metagabbronorite of the shear zone predates the serpentinization of the peridotites. Many textural and fabric features in alpine-type gabbro–peridotite complexes that are ascribed to regional metamorphism and deformation during orogenic emplacement may originate shortly after intrusion near or at an accreting plate margin.

Strain variation in shear belts

1970 ◽  
Vol 7 (3) ◽  
pp. 786-813 ◽  
Author(s):  
J. G. Ramsay ◽  
R. H. Graham
Keyword(s):  
Shear Zone ◽  
Strain State ◽  
Finite Strain ◽  
High Strain ◽  
Shear Zones ◽  
Finite Strains ◽  
Strain Variation ◽  
Strain Ellipsoid ◽  
Intensity Of Development ◽  
Variable Displacement

In rocks deformed by natural orogenic processes it is usual to find that the finite strain state varies from locality to locality. In some deformed rocks high strain states are localized within approximately planar zones commonly known as "shear belts".The general relationships that exist between variable displacement and variable strain state are established, and these general equations are solved for particular types of strain within shear zones. Only a limited number of types of solution are possible. Using these solutions the geometric forms of the structures found in shear zones in several regions are analyzed. Methods for computing the finite strain through these zones are described, and these finite strains are integrated to determine the total displacements across these zones. Schistosity is developed in some of the shear zones described. It is not parallel to the walls of the shear zone and is therefore not parallel to the dominant displacement (shear) directions. The schistosity appears to be formed perpendicular to the principal finite shortening (i.e. perpendicular to the shortest axis of the finite strain ellipsoid). Variations of the schistosity planes represent variations in the finite strain trajectories of XY planes in the strain states ([Formula: see text] ellipsoid axes). The intensity of development of the schistosity is correlated with the values of the principal finite strains.

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

<p>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 –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/θ 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.</p><p>Keywords: Shear zone, Deformation, Vorticity, 3D strain analysis, Monazite dating</p>

scholarly journals Superposition of structures in the interference zone between the southern Brasília belt and the central Ribeira belt in the region SW of Itajubá (MG), SE Brazil

2016 ◽  
Vol 46 (4) ◽  
pp. 547-566 ◽  
Author(s):  
Rodrigo Vinagre ◽  
◽  
Rudolph Allard Johannes Trouw ◽  
Hugo Kussama ◽  
Rodrigo Peternel ◽  
...  
Keyword(s):  
Shear Zone ◽  
Shear Zones ◽  
The Other ◽  
Northwestern Part ◽  
Deformation Phase ◽  
Se Brazil ◽  
Two Samples ◽  
Minor Part ◽  
A Minor ◽  
Three Phases

ABSTRACT: The study area is localized in the Socorro nappe, part of the southern Brasília belt, with a minor part in the Embu terrane, part of the central Ribeira belt. Three phases of deformation were detected, Dn-1, Dn and Dn+1. Sn-1 seems to be generally transposed into Sn, but in the northwestern part it is well preserved, dipping about 60º to W and SW, with a stretching and/or mineral lineation plunging down dip. The main foliation in most of the area is Sn, dipping about 70º to SSE. Dn folds are tight to isoclinal, with axes that plunge about 40º to SW. Quartz-feldspathic segregation veins are folded by Dn. The structures related to Dn-1 and Dn are cut and modified by four important shear zones ascribed to deformation phase Dn+1. Two samples of a granite that is elongated along the Caxambu shear zone, and also cut by it, were dated. One yielded a crystallization age of 575 ± 5 Ma, and the other one, from the shear zone, an age of 567 ± 8 Ma, interpreted either as representing the age of movement along the Caxambu shear zone, or as metamorphic growth.

scholarly journals The Dabie Extensional Tectonic System: Structural Framework

2012 ◽  
Vol 2012 ◽  
pp. 1-8 ◽  
Author(s):  
Quanlin Hou ◽  
Hongyuan Zhang ◽  
Qing Liu ◽  
Jun Li ◽  
Yudong Wu
Keyword(s):  
Shear Zone ◽  
Shear Zones ◽  
Ultrahigh Pressure ◽  
Metamorphic Complex ◽  
The South ◽  
Magma Intrusion ◽  
The North ◽  
Extensional Tectonic ◽  
Mechanism Transition ◽  
Tectonic System

A previous study of the Dabie area has been supposed that a strong extensional event happened between the Yangtze and North China blocks. The entire extensional system is divided into the Northern Dabie metamorphic complex belt and the south extensional tectonic System according to geological and geochemical characteristics in our study. The Xiaotian-Mozitan shear zone in the north boundary of the north system is a thrust detachment, showing upper block sliding to the NNE, with a displacement of more than 56 km. However, in the south system, the shearing direction along the Shuihou-Wuhe and Taihu-Mamiao shear zones is tending towards SSE, whereas that along the Susong-Qingshuihe shear zone tending towards SW, with a displacement of about 12 km. Flinn index results of both the north and south extensional systems indicate that there is a shear mechanism transition from pure to simple, implying that the extensional event in the south tectonic system could be related to a magma intrusion in the Northern Dabie metamorphic complex belt. Two 40Ar-39Ar ages of mylonite rocks in the above mentioned shear zones yielded, separately, ~190 Ma and ~124 Ma, referring to a cooling age of ultrahigh-pressure rocks and an extensional era later.

Effect of kinematics of orogenic wedge on kinematic evolutionary paths and deformation profiles of major shear zones: An example from the eastern Himalaya 

2021 ◽  
Author(s):  
Pritam Ghosh ◽  
Kathakali Bhattacharyya
Keyword(s):  
Shear Zone ◽  
Strain Softening ◽  
Leading Edge ◽  
Shear Zones ◽  
Deformation History ◽  
Progressive Deformation ◽  
Trailing Edge ◽  
Orogenic Wedge ◽  
Transport Direction ◽  
Evolutionary Paths

<p>We examine how the deformation profile and kinematic evolutionary paths of two major shear zones with prolonged deformation history and large translations differ with varying structural positions along its transport direction in an orogenic wedge. We conduct this analysis on multiple exposures of the internal thrusts from the Sikkim Himalayan fold thrust belt, the Pelling-Munsiari thrust (PT), the roof thrust of the Lesser Himalayan duplex (LHD), and the overlying Main Central thrust (MCT). These two thrusts are regionally folded due to growth of the LHD and are exposed at different structural positions. The hinterlandmost exposures of the MCT and PT zones lie in the trailing parts of the duplex, while the foreland-most exposures of the same studied shear zones lie in the leading part of the duplex, and thus have recorded a greater connectivity with the duplex. The thicknesses of the shear zones progressively decrease toward the leading edge indicating variation in deformation conditions. Thickness-displacement plot reveals strain-softening from all the five studied MCT and the PT mylonite zones. However, the strain-softening mechanisms varied along its transport direction with the hinterland exposures recording dominantly dislocation-creep, while dissolution-creep and reaction-softening are dominant in the forelandmost exposures. Based on overburden estimation, the loss of overburden on the MCT and the PT zones is more in the leading edge (~26km and ~15km, respectively) than in the trailing edge (~10km and ~17km, respectively), during progressive deformation. Based on recalibrated recrystallized quartz grain thermometer (Law, 2014), the estimated deformation temperatures in the trailing edge are higher (~450-650°C) than in the leading edge (350-550°C) of the shear zones. This variation in the deformation conditions is also reflected in the shallow-crustal deformation structures with higher fracture intensity and lower spacing in the leading edge exposures of the shear zones as compared to the trailing edge exposures.</p><p>The proportion of mylonitic domains and micaceous minerals within the exposed shear zones increase and grain-size of the constituent minerals decreases progressively along the transport direction. This is also consistent with progressive increase in mean R<sub>s</sub>-values toward leading edge exposures of the same shear zones. Additionally, the α-value (stretch ratio) gradually increases toward the foreland-most exposures along with increasing angular shear strain. Vorticity estimates from multiple incremental strain markers indicate that the MCT and PT zones generally record a decelerating strain path. Therefore, the results from this study are counterintuitive to the general observation of a direct relationship between higher Rs-value and higher pure-shear component. We explain this observation in the context of the larger kinematics of the orogen, where the leading edge exposures have passed through the duplex structure, recording the greatest connectivity and most complete deformation history, resulting in the weakest shear zone that is also reflected in the deformation profiles and strain attributes. This study demonstrates that the same shear zone records varying deformation profile, strain and kinematic evolutionary paths due to varying deformation conditions and varying connectivity to the underlying footwall structures during progressive deformation of an orogenic wedge.</p>

Realistic hardening-to-softening transition effects of metals over the finite strain range up to failure

2021 ◽  
Vol ahead-of-print (ahead-of-print) ◽  
Author(s):  
Zi-Han Xu ◽  
Lin Zhan ◽  
Si-Yu Wang ◽  
Hui-Feng Xi ◽  
Heng Xiao
Keyword(s):  
Finite Strain ◽  
Plastic Work ◽  
Uniaxial Tensile ◽  
Uniaxial Tensile Test ◽  
Trial And Error ◽  
Content Type ◽  
Transition Effects ◽  
Iterative Procedures ◽  
First Time ◽  
Hardening And Softening

PurposeA new approach is proposed toward accurately matching any given realistic hardening and softening data from uniaxial tensile test up to failure and moreover, toward bypassing usual tedious implicit trial-and-error iterative procedures in identifying numerous unknown parameters.Design/methodology/approachFinite strain response features of metals with realistic hardening-to-softening transition effects up to eventual failure are studied for the first time based on the self-consistent elastoplastic J2-flow model with the logarithmic stress rate. As contrasted with usual approximate and incomplete treatments merely considering certain particular types of hardening effects such as power type hardening, here a novel and explicit approach is proposed to obtain a complete form of the plastic-work-dependent yield strength over the whole hardening and softening range.FindingsA new multi-axial evolution equation for both hardening and softening effects is established in an explicit form. Complete results for the purpose of model validation and prediction are presented for the finite strain responses of monotonic uniaxial stretching up to failure.Originality/valueNew finite strain elastoplastic equations are established with a new history-dependent variable equivalently in place of the usual plastic work. With these equations, a unified and accurate simulation of both gardening and softening effects up to failure is achieved for the first time in an explicit sense without involving usual tedious implicit trial-and-error iterative procedures.

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