Comment on “Miocene to Quaternary tectonostratigraphic evolution of the middle section of the Burdur-Fethiye Shear Zone, south-western Turkey: Implications for the wide inter-plate shear zones. Tectonophysics 690, 336–354”

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.

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

10.5194/egusphere-egu21-3824 ◽

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

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&#176;C) than in the leading edge (350-550&#176;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.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 Rs-values toward leading edge exposures of the same shear zones. Additionally, the &#945;-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.

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The internal structure and composition of a plate boundary-scale serpentinite shear zone: The Livingstone Fault, New Zealand

10.5194/se-2019-62 ◽

2019 ◽

Author(s):

Matthew S. Tarling ◽

Steven A. F. Smith ◽

James M. Scott ◽

Jeremy S. Rooney ◽

Cecilia Viti ◽

...

Keyword(s):

New Zealand ◽

Internal Structure ◽

Shear Zone ◽

Plate Boundary ◽

Shear Zones ◽

East Side ◽

Local Dispersion ◽

Fine Grained ◽

Shear Sense ◽

Tectonic Settings

Abstract. Deciphering the internal structural and composition of large serpentinite-dominated shear zones will lead to an improved understanding of the rheology of the lithosphere in a range of tectonic settings. The Livingstone Fault in New Zealand is a > 1000 km long terrane-bounding structure that separates the basal portions (peridotite; serpentinised peridotite; metagabbros) of the Dun Mountain Ophiolite Belt from quartzofeldspathic schists of the Caples or Aspiring Terranes. Field and microstructural observations from eleven localities along a strike length of c. 140 km show that the Livingstone Fault is a steeply-dipping, serpentinite-dominated shear zone tens to several hundreds of metres wide. The bulk shear zone has a pervasive scaly fabric that wraps around fractured and faulted pods of massive serpentinite, rodingite and partially metasomatised quartzofeldspathic schist up to a few tens of metres long. S-C fabrics and lineations in the shear zone consistently indicate a steep Caples-side-up (i.e. east-side-up) shear sense, with significant local dispersion in kinematics where the shear zone fabrics wrap around pods. The scaly fabric is dominated (> 98 vol %) by fine-grained (&ll; 10 μm) fibrous chrysotile and lizardite/polygonal serpentine, but infrequent (

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

Geosciences ◽

10.3390/geosciences10080288 ◽

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.

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Macro- and microstructural analysis of the Zhujiafang ductile shear zone, Hengshan Complex: Tectonic nature and geodynamic implications of the evolution of Trans−North China orogen

Geological Society of America Bulletin ◽

10.1130/b35672.1 ◽

2020 ◽

Author(s):

Lingchao He ◽

Jian Zhang ◽

Guochun Zhao ◽

Changqing Yin ◽

Jiahui Qian ◽

...

Keyword(s):

Shear Zone ◽

North China ◽

Shear Zones ◽

Crustal Thickening ◽

Slip Systems ◽

High Grade ◽

Ductile Shear Zone ◽

Crustal Rocks ◽

Ductile Shear ◽

Lower Crustal

In worldwide orogenic belts, crustal-scale ductile shear zones are important tectonic channels along which the orogenic root (i.e., high-grade metamorphic lower-crustal rocks) commonly experienced a relatively quick exhumation or uplift process. However, their tectonic nature and geodynamic processes are poorly constrained. In the Trans−North China orogen, the crustal-scale Zhujiafang ductile shear zone represents a major tectonic boundary separating the upper and lower crusts of the orogen. Its tectonic nature, structural features, and timing provide vital information into understanding this issue. Detailed field observations showed that the Zhujiafang ductile shear zone experienced polyphase deformation. Variable macro- and microscopic kinematic indicators are extensively preserved in the highly sheared tonalite-trondhjemite-granodiorite (TTG) and supracrustal rock assemblages and indicate an obvious dextral strike-slip and dip-slip sense of shear. Electron backscattered diffraction (EBSD) was utilized to further determine the crystallographic preferred orientation (CPO) of typical rock-forming minerals, including hornblende, quartz, and feldspar. EBSD results indicate that the hornblendes are characterized by (100) <001> and (110) <001> slip systems, whereas quartz grains are dominated by prism <a> and prism <c> slip systems, suggesting an approximate shear condition of 650−700 °C. This result is consistent with traditional thermobarometry pressure-temperature calculations implemented on the same mineral assemblages. Combined with previously reported metamorphic data in the Trans−North China orogen, we suggest that the Zhujiafang supracrustal rocks were initially buried down to ∼30 km depth, where high differential stress triggered the large-scale ductile shear between the upper and lower crusts. The high-grade lower-crustal rocks were consequently exhumed upwards along the shear zone, synchronous with extensive isothermal decompression metamorphism. The timing of peak collision-related crustal thickening was further constrained by the ca. 1930 Ma metamorphic zircon ages, whereas a subsequent exhumation event was manifested by ca. 1860 Ma syntectonic granitic veins and the available Ar-Ar ages of the region. The Zhujiafang ductile shear zone thus essentially record an integrated geodynamic process of initial collision, crustal thickening, and exhumation involved in formation of the Trans−North China orogen at 1.9−1.8 Ga.

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Eburnean deformation pattern of Burkina Faso and the tectonic significance of shear zones in the West African craton

BSGF - Earth Sciences Bulletin ◽

10.1051/bsgf/2020001 ◽

2020 ◽

Vol 191 ◽

pp. 2 ◽

Cited By ~ 1

Author(s):

Dominique Chardon ◽

Ousmane Bamba ◽

Kalidou Traoré

Keyword(s):

Burkina Faso ◽

Shear Zone ◽

Regional Scale ◽

Kinematic Model ◽

Shear Zones ◽

West African ◽

Deformation Pattern ◽

The West ◽

West African Craton ◽

Regional Flow

Shear zones of the Paleoproterozoic Eburnean accretionary Orogen (West African craton) are investigated by means of large-scale structural mapping. Regional scale (10-100 km) mapping was based on the aeromagnetic survey of Burkina Faso and craton-scale (1000 km) mapping on a compilation of fabric data. At both scales, shear zones are arranged as an anastomosed transpressional network that accommodated distributed shortening and lateral flow of the orogenic lithosphere between the converging Kénéma-Man and Congo Archean provinces. Structural interference patterns at both scales were due to three-dimensional partitioning of progressive transpressional deformation and interactions among shear zones that absorbed heterogeneities in the regional flow patterns while maintaining the connectivity of the shear zone network. Such orogen-scale kinematic patterns call for caution in using the deformation phase approach without considering the “bigger structural picture” and interpreting displacement history of individual shear zones in terms of plate kinematics. The West African shear zone pattern is linked to that of the Guiana shield through a new transatlantic correlation to produce an integrated kinematic model of the Eburnean-Transamazonian orogen.

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The influence of dikes on auriferous shear zone development within granitoid intrusions: the Bourlamaque pluton, Val-d'Or district, Abitibi greenstone belt

Canadian Journal of Earth Sciences ◽

10.1139/e93-170 ◽

1993 ◽

Vol 30 (9) ◽

pp. 1924-1933 ◽

Cited By ~ 9

Author(s):

Abdelhay Belkabir ◽

François Robert ◽

L. Vu ◽

C. Hubert

Keyword(s):

Shear Zone ◽

Shear Zones ◽

Slip Direction ◽

Principal Stresses ◽

Quartz Veins ◽

Profound Influence ◽

Complex Shear ◽

Zone Development ◽

Granitoid Intrusions ◽

Geometry And Kinematics

Shear-zone-related gold–quartz veins in granitoid intrusions are commonly intimately associated with mafic dikes, which may have a profound influence on the localization, orientation, and kinematics of auriferous shear zones. The Bourlamaque pluton of the Val-d'Or district contains several economic auriferous shear zones, most of which follow and overprint diorite dikes. Mineralization in all deposits consists of quartz–tourmaline–pyrite veins in reverse- oblique orientation with a significant range of strike, dip, and slip direction. The geometry and kinematics of shear zone and vein array within the pluton is more complex than the simple conjugate pattern predicted for a deforming homogeneous intrusion. The stress tensor determined from the auriferous shear zones within the pluton indicates the same northerly-directed compression recorded by similar shear zones outside the pluton. This indicates that the complex shear zone and vein pattern within the pluton reflects the influence of diorite dikes, which acted as weak layers that were activated during subsequent deformation, showing the importance of layer anisotropy in auriferous shear zone development.The plunges of orebodies bear simple geometric relationships to the slip direction along a host shear zone: these are generally perpendicular to, or in some cases parallel to, the slip direction. Knowledge of the slip directions along activated dikes would therefore allow prediction of the possible plunge(s) of orebodies at early stages of exploration programs. Slip direction along an activated layer is controlled by the orientation of the layer with respect to the stress field and by the relative magnitudes of the three principal stresses. Using techniques developed for analysis of fault slip data, both parameters can be determined, provided there is a sufficient database, and slip direction can be predicted for activated layers of any orientations.

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

Canadian Journal of Earth Sciences ◽

10.1139/e93-115 ◽

1993 ◽

Vol 30 (7) ◽

pp. 1338-1354 ◽

Cited By ~ 19

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.

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