scholarly journals Precise Location and Mapping of the Main Central Thrust Zone in Reference to Micro-Structures and Deformation along Khudi-Tal Area of Marsyangdi Valley

2020 ◽  
Vol 22 ◽  
pp. 33-40
Author(s):  
Lokendra Pandeya ◽  
Kabi Raj Paudyal
Keyword(s):  
Regional Metamorphism ◽  
Static Condition ◽  
Shear Zones ◽  
Geological Mapping ◽  
Lesser Himalaya ◽  
Main Central Thrust ◽  
Thin Sections ◽  
Ductile Shearing ◽  
Shear Sense ◽  
Micro Structures

Geological mapping was carried out along Marsyangdi valley in the Khudi - Dahare -Tal area on a scale of 1: 50,000 covering about 142 square kilometers. Recent study aims to locate the Main Central Thrust (MCT) precisely based on lithostratigraphy, micro-structures, deformation, and metamorphism. Several thin sections were observed to study the metamorphism, deformation, and micro-structures developed in the rocks. The rocks sequences in both the Higher Himalaya and the Lesser Himalaya have undergone polyphase metamorphism and deformation. The Lesser Himalaya experienced first burial metamorphism (M1) followed by garnet grade inverted metamorphism related to the MCT activity (M2) followed by retrograde metamorphism (M3) whereas the Higher Himalaya has undergone regional high-pressure/ high-temperature kyanite/ sillimanite- grade prograde regional metamorphism (M1) followed by the (M2) related to ductile sharing which in turn is overprinted by the later post-tectonic retrograde garnet to chlorite grade metamorphism during exhumation. The polyphase deformation is indicated by the cross-cutting foliation and many other features. The deformation phase D1 is associated with the development of the bedding parallel foliation due to burial in both the Higher Himalaya and the Lesser Himalaya. Isoclinal folds and crenulation cleavage were developed before the collision is categorized as D2. Development of nearly N- S trending mineral and stretching lineation, south vergent drag folds, folded S2 cleavage and microscopic shear sense indicators, rotated syn- tectonic garnet grains, etc. were developed during the deformation D3 related to the ductile shearing through the MCT. Various brittle faults and shear zones cross-cutting all earlier features were developed during D4 during the upheaval. The rocks in the MCT zone are affected by intense sharing and mylonitization as indicated by the presence of many mylonitic structures in the thin sections throughout the Lesser Himalaya in the area. Features like polygonization and ribbon quartz with evidence of sub-grain rotation, mica fish, syn-tectonic rotated garnet grains indicate the ductile shearing in the MCT area suggesting the dynamic recrystallization in the MCT zone whereas rocks of the Higher Himalaya show the evidence of recrystallization under static condition. The MCT zone was mapped precisely based on the microstructures and deformation.

Ductile shearing of Apennine allochtonous units and Messinian terrigenous deposits on top of the Inner Apulian Platform (Monte Alpi, Southern Italy)

2020 ◽  
Author(s):  
Giacomo Prosser ◽  
Fabrizio Agosta ◽  
Alessandro Giuffrida ◽  
Claudia Belviso ◽  
Francesco Cavalcante
Keyword(s):  
Shear Zone ◽  
Shear Zones ◽  
Geological Mapping ◽  
Middle Pleistocene ◽  
Low Grade ◽  
Late Pliocene ◽  
X Ray ◽  
Time Space ◽  
Ductile Shearing ◽  
Shear Sense

<p>Mylonites are common structural elements in basement complexes. There, strain localization within shear zones occurs at amphibolite to greenschist facieses. More rarely, it also takes place at low-grade to anchizonal conditions in the external portions of orogenic belts. In the present contribution, we document the large-scale architecture, micro-structure, and mineralogy of a prominent shear zone exposed along the southern flank of the Monte Alpi Unit, southern Apennines, Italy. Deformation localized within the Messinian sedimentary protolith topping the carbonates of the Apulian Platform, and in the lowermost tectonic units of the Apennine allochton. Integration of results achieved after field geological mapping, outcrop structural analyses, optical and SEM micropscopy, and X-Ray diffrattometry permits to assess the time-space evolution of the main deformation mechanisms in the aforementioned shear zone. The shear zone involved Messinian shale, sandstones and conglomerates originally deposited in a foreland basin system, and Mesozoic claystones, limestones, and marls that formed in deep basinal environments. Now days, the mylonitic foliation is sub-parallel to the tectonic contact between the Messinian sedimentary cover of the Apulian carbonates and the overlying allochton. Shear-related deformation produced a foliated mylonitic fabric dipping ca. 20° S, and a well-developed, east-trending stretching lineation defined by aligned quartz and/or calcite grains. The conglomeratic levels were boudinaged, and the individual elongated pebbles re-oriented along slip direction. The microstructure of mylonites is characterized by a fine-grained calcite matrix, which shows an intense foliation due to dark bands made up of oxides, organic matter, and minor phyllosilicates. X-ray diffraction data performed on the Messinian shales and Mesozoic claystones, indicate the presence of mixed layer illite/smectite with 80-90% of illite and R1/R3 ordering thus suggesting an high digenetic grade (temperature: 120-140 °C). The two analyzed lithologies mainly differ in the presence of kaolinite, which occurs in the more proximal Messinian facies. Altogether, outcrop-scale kinematic markers such as shear bands, rootles folds and asymmetric porphyroclasts show a consistent top-to-the-east shear sense. Mineralogical and microstructural data indicate that shearing took place at a depth of 6-7 km during the Early Pliocene emplacement of the Apennine allochton on the Apulian Platform, and then exhumed by Late Pliocene low-angle normal faulting, Lower Pleistocene transpression, and Middle-Pleistocene-Holocene high-angle extensional faulting. In summary, the eastward motion of the allochton produced intense and localized low-temperature shearing in sediments on top of the Apulian Platform and in the overlying allochton. A subsequent reactivation of this shear zones as low-angle normal fault during late Pliocene exhumation is envisioned.</p>

Age of metamorphism in the Lesser Himalaya and the Main Central Thrust zone, Garhwal India: results of illite crystallinity,40Ar–39Ar fusion and K–Ar studies

1995 ◽  
Vol 132 (2) ◽  
pp. 139-149 ◽  
Author(s):  
G. J. H. Oliver ◽  
M. R. W. Johnson ◽  
A. E. Fallick
Keyword(s):  
Regional Metamorphism ◽  
Low Grade ◽  
Supporting Evidence ◽  
Lesser Himalaya ◽  
Main Central Thrust ◽  
Illite Crystallinity ◽  
Lower Eocene ◽  
Thrust Zone ◽  
Lower Palaeozoic ◽  
Upper Paleocene

AbstractIllite crystallinity data from the Lesser Himalaya of Garhwal show that the upper Paleocene-lower Eocene Subathu Formation, deposited immediately prior to or early in the Himalayan collision, has not suffered significant regional metamorphism. The regional metamorphism in the upper Precambrian–lower Palaeozoic Lesser Himalaya must therefore be precollisional. Illite crystallinity results from Lesser Himalayan fossiliferous Permian strata show grades of metamorphism intermediate between upper Paleocene–lower Eocene and Proterozoic–lower Palaeozoic strata indicating a pre-Permian regional metamorphism for the latter.K–Ar whole rock cooling ages provide supporting evidence for pre-collisional regional metamorphism in the Lesser Himalaya. Slates and phyllites below the Main Central Thrust (MCT) show pre-Cenozoic whole rock ages, as old as Ordovician (486 Ma). Whilst resetting of K–Ar whole rock ages has occurred locally in pervasively cleaved Palaeozoic strata (near thrusts?), fracture cleaved Permian and upper Paleocene–lower Eocene sediments give whole rock ages compatible with diagenesis. The illite crystallinity results confirm that these sediments have not been heated above mica blocking temperatures.Muscovite40Ar–39Ar and K–Ar mineral ages within the 5 km thick MCT zone are as young as 8 Ma indicating that temperatures of above ~ 350°C were maintained in the MCT zone for over 10 Ma after high temperature (~ 550°C) shearing on the MCT. This heating did not affect the MCT footwall Lesser Himalaya to any regional extent, where pre-Permian low grade regional metamorphism has not been overprinted.

Exhumation of HP/UHP rocks by normal ductile shearing on top of the Eocene extruding wedge

2020 ◽  
Author(s):  
Michel Ballèvre ◽  
Paola Manzotti
Keyword(s):  
Shear Zone ◽  
Kinematic Model ◽  
Geological Mapping ◽  
Greenschist Facies ◽  
Western Boundary ◽  
Frontal Part ◽  
Ductile Shearing ◽  
Shear Sense ◽  
Eclogite Facies ◽  
Ductile Shear

<p>A popular model for the exhumation of HP-UHP rocks is the ‘extruding wedge’ model, where a crustal slice is bounded at its base by a ‘thrust shear-sense’ fault and to the top by a ‘normal shear-sense’ fault. In the Western Alps, the late Eocene Combin Shear Zone (CSZ) allowed extrusion of a wedge made by the Briançonnais-Piemonte-Liguria (‘Penninic’) stack.</p><p>Geological mapping has established the geometry and continuity of the CSZ from the frontal part of the Dent Blanche Tectonic System to the western boundary of the Sesia Zone. The CSZ has been cut during the Miocene by the brittle Aosta-Ranzola Fault, with an estimated downthrow of the northern block of c. 2.5 km with respect to the southern block. Consequently, the sections observed north (Monte Rosa) and South (Gran Paradiso) of the Aosta Fault display different structural levels in the Alpine nappe stack. The CSZ has been folded (Vanzone phase) during the final part of its history (i.e. when displacement along the CSZ was no more taking place), due to the indentation of the Adriatic mantle. This offers us the unique opportunity to study the change in deformation mechanisms along the shear zone (for a distance parallel to its displacement of about 50 km).</p><p>Salient characteristics of the CSZ are the following. (i) The thickness of the ductile shear zone increases from NW (frontal part of the Dent Blanche) to SE (frontal part of the Sesia Zone), from a few hundred metres to several kilometres. The type of lithologies pervasively reworked by the ductile shear changes along strike (dominantly calcschists from the topmost oceanic units in the Combin Zone, possibly up to the whole of the ‘Gneiss Minuti’ in the frontal Sesia Zone). (ii) The main ductile deformation along the CSZ was taking place at greenschist-facies conditions, overprinting eclogite-facies to greenschist-facies deformations of Cretaceous to Middle Eocene age. The CSZ is cutting and reworking eclogite-facies structures developed in its hangingwall (Sesia) as well as in its footwall (Zermatt). (iii) Ductile displacement along the CSZ is associated with the development in its footwall of south-east-verging, kilometre-scale, folds (Mischabel phase). The sedimentary sequences of the Pancherot-Cime Bianche-Bettaforca Unit may be used to estimate the minimum amount of ‘normal shear sense’ displacement of the order of 15-20 km.</p><p>A kinematic model integrating slab roll-back, ‘thrust shear-sense’ at the base and ‘normal shear-sense’ displacement on top of the Eocene eclogite-facies stack will be presented.</p>

scholarly journals Evolution of structures and hydrothermal alteration in a Palaeoproterozoic supracrustal belt: Constraining paired deformation–fluid flow events in an Fe and Cu–Au prospective terrain in northern Sweden

Solid Earth ◽  
2020 ◽  
Vol 11 (2) ◽  
pp. 547-578 ◽  
Author(s):  
Joel B. H. Andersson ◽  
Tobias E. Bauer ◽  
Edward P. Lynch
Keyword(s):  
Fluid Flow ◽  
Iron Oxide ◽  
Structural Geology ◽  
Hydrothermal Alteration ◽  
Structural Model ◽  
Regional Metamorphism ◽  
Spatial Relationship ◽  
Shear Zones ◽  
Geological Mapping ◽  
Supracrustal Belt

Abstract. An approximately 90 km long Palaeoproterozoic supracrustal belt in the northwestern Norrbotten ore province (northernmost Sweden) was investigated to characterize its structural components, assess hydrothermal alteration–structural geology correlations, and constrain a paired deformation–fluid flow evolution for the belt. New geological mapping of five key areas (Eustiljåkk, Ekströmsberg, Tjårrojåkka, Kaitum West, and Fjällåsen–Allavaara) indicates two major compressional events (D1 and D2) have affected the belt, with each associated with hydrothermal alteration types typical for iron oxide–apatite and iron oxide Cu–Au systems in the region. Early D1 generated a regionally distributed, penetrative S1 foliation and oblique reverse shear zones that show a southwest-block-up sense of shear that formed in response to NE–SW crustal shortening. Peak regional metamorphism at epidote–amphibolite facies broadly overlaps with this D1 event. Based on overprinting relationships, D1 is associated with regional scapolite ± albite, magnetite + amphibole, and late calcite alteration of mafic rock types. These hydrothermal mineral associations linked to D1 structures may form part of a regionally pervasive evolving fluid flow event but are separated in this study by crosscutting relationships. During D2 deformation, folding of S0–S1 structures generated F2 folds with steeply plunging fold axes in low-strain areas. NNW-trending D1 shear zones experienced reverse dip-slip reactivation and strike-slip-dominated movements along steep, E–W-trending D2 shear zones, producing brittle-plastic structures. Hydrothermal alteration linked to D2 structures is a predominantly potassic–ferroan association comprising K-feldspar ± epidote ± quartz ± biotite ± magnetite ± sericite ± sulfides. Locally, syn- or post-tectonic calcite is the main alteration mineral in D2 shear zones that intersect mafic rocks. Our results highlight the importance of combining structural geology with the study of hydrothermal alterations at regional to belt scales to understand the temporal–spatial relationship between mineralized systems. Based on the mapping results and microstructural investigations as well as a review of earlier tectonic models presented for adjacent areas, we suggest a new structural model for this part of the northern Fennoscandian Shield. The new model emphasizes the importance of reactivation of early structures, and the model harmonizes with tectonic models presented by earlier workers based mainly on petrology of the northern Norrbotten area.

Structural evolution, metamorphism and melting in the Greater Himalayan Sequence in central-western Nepal

2018 ◽  
Vol 483 (1) ◽  
pp. 305-323 ◽  
Author(s):  
Rodolfo Carosi ◽  
Chiara Montomoli ◽  
Salvatore Iaccarino ◽  
Dario Visonà
Keyword(s):  
Hanging Wall ◽  
Regional Scale ◽  
Structural Evolution ◽  
Shear Zones ◽  
Geological Mapping ◽  
Eastern Himalaya ◽  
Main Central Thrust ◽  
Tectonic Model ◽  
Time Deformation ◽  
Greater Himalayan Sequence

AbstractJoining geological mapping, structural analysis, petrology and geochronology allowed the internal architecture of the Greater Himalayan Sequence (GHS) to be unraveled. Several top-to-the-south/SW tectonic–metamorphic discontinuities developed at the regional scale, dividing it into three main units exhumed progressively from the upper to the lower one, starting from c. 40 Ma and lasting for several million years. The activity of shear zones has been constrained and linked to the pressure–temperature–time–deformation (P–T–t–D) evolution of the deformed rocks by the use of petrochronology. Hanging wall and footwall rocks of the shear zones recorded maximum P–T conditions at different times. Above the Main Central Thrust, a cryptic tectonometamorphic discontinuity (the High Himalayan Discontinuity (HHD)) has been recognized in Central-Eastern Himalaya.The older shear zone, that was active at c. 41–28 Ma, triggered the earlier exhumation of the uppermost GHS and allowed the migration of melt, which was produced at peak metamorphic conditions and subsequently produced in abundance at the time of the activation of the HHD. Production of melt continued at low pressure, with nearly isobaric heating leading to the genesis and emplacement of andalusite- and cordierite-bearing granites.The timing of the activation of the shear zones from deeper to upper structural levels fits with an in-sequence shearing tectonic model for the exhumation of the GHS, further affected by out-of-sequence thrusts.

scholarly journals Characteristics and field relation of Ulleri Augen Gneiss to country rocks in the Lesser Himalaya: A case study from Syaprubesi-Chhyamthali area, central Nepal

2019 ◽  
Vol 58 ◽  
pp. 89-96
Author(s):  
Jharendra K.C. ◽  
Kabi Raj Paudyal
Keyword(s):  
Geological Mapping ◽  
Lesser Himalaya ◽  
Main Central Thrust ◽  
Nepal Himalaya ◽  
Field Relation ◽  
Central Nepal ◽  
Thrust Zone ◽  
Augen Gneiss ◽  
Host Rocks

The distribution of Ulleri Augen Gneiss and its origin in the Lesser Nepal Himalaya adjacent to the Main Central Thrust zone is stilla debate among the geo-scientists. Geological mapping was carried out along the Syaprubesi-Chhyamthali area of central Nepal with the aim to study the field relation, distribution, deformation and metamorphism of the Ulleri Augen Gneiss. During mapping, close traverses were set to observe the field relation and a number of systematic samples were collected for analysis of composition and texture. Some preliminary findings were obtained related to its geological position and distribution. This gneiss is hosted within the Kuncha Formation, the oldest unit of the Nawakot Group in the Lesser Himalaya. It has been evolved within this unit as a tabular form in some places and lenses in other places. It shows both concordant (i.e., sill type) and discordant (i.e., dike type) relationship with the host rock. It is characterized by augen-shaped porphyroblasts of K-feldspar and S-C mylonitic texture showing top to the SW sense of shear. The S-C structures and lineated textures shown by the minerals are associated with the shearing caused by the movement along the MCT during the syn-MCT metamorphic deformation. It is characterized in different types of lithologies such as augen gneiss, banded gneiss and two-mica gneiss. An attempt is made to explain the petrological characteristics and field relation of the Ulleri Augen Gneiss with the host rocks along with structural aspects. Based on the field relation and texture analysis, the evolution of the protolith of this Ulleri Augun Gneiss can be interpreted as a multi-story emplacement within the host rocks during and immediately after the sedimentation.

scholarly journals Quantifying displacement on the South Tibetan Detachment normal fault, Everest massif, and the timing of crustal thickening and uplift in the Himalaya and South Tibet

2002 ◽  
Vol 26 ◽  
Author(s):  
M. P. Searle ◽  
R. L. Simpson ◽  
R. D. Law ◽  
D. J. Waters ◽  
R. R. Parrish
Keyword(s):  
Large Scale ◽  
Regional Metamorphism ◽  
Normal Fault ◽  
Crustal Thickening ◽  
The Tibetan Plateau ◽  
The South ◽  
Main Central Thrust ◽  
Ductile Shearing ◽  
The Himalaya ◽  
South Tibet

ABSTRACT Lithospheric convergence of India and Asia since collision has resulted in horizontal shortening, crustal thickening and regional metamorphism in the Himalaya and beneath southern Tibet. The boundary between the High Himalaya and the Tibetan plateau is a large scale, north-dipping, low-angle normal fault termed the South Tibetan Detachment (STD) which was active contemporaneously with the Main Central Thrust (MCT) bounding the southern margin of the High Himalaya. Previous studies have estimated minimum northward displacement along the STD of 35 km along the Everest profile. Here, we demonstrate approximately 200 km of southward displacement of footwall sillimanite + cordierite gneisses (minimum 90-108 km), formed at 600-630°C and pressures of 4.0-4.9 kbar ( 14-18 km depth), beneath the STD which acted as a passive roof fault during southward flow of the hot, viscous, ductile middle crust. U-Th-Pb dating of gneisses, sheared and cross-cutting leucogranites indicates that ductile shearing was active at 17-16 Ma, and later brittle motion at <16 Ma cuts all rocks in the footwall. High temperatures (>620°C) were maintained for -14 Ma along the top of the High Himalayan slab from 32-18 Ma, implying active crustal thickening and high topography in south Tibet during this time. The ending of metamorphism and melting in the Himalaya and ductile shearing along the STD coincides with the initiation of strike-slip faulting in SW Tibet and E-W extension in south Tibet.

40Ar/39Ar Dating of Paleoproterozoic Shear Zones in the Ellesmere-Devon Crystalline Terrane, Nunavut, Canadian Arctic

2021 ◽  
Author(s):  
Brandon Caswell ◽  
J.A. Gilotti ◽  
Laura E. Webb ◽  
William C. McClelland ◽  
Karolina Kośmińska ◽  
...  
Keyword(s):  
Regional Metamorphism ◽  
Shear Zones ◽  
Granulite Facies ◽  
Ellesmere Island ◽  
Ductile Deformation ◽  
Mobile Belt ◽  
Tectonic Zone ◽  
Granulite Facies Metamorphism ◽  
North West ◽  
Ductile Shearing

Paleoproterozoic gneisses of the Ellesmere–Devon crystalline terrane on southeast Ellesmere Island are deformed by m-scale, E-striking mylonite zones. The shear zones commonly offset pegmatitic dikes and represent the last episode of ductile deformation. Samples were dated by the <sup>40</sup>Ar/<sup>39</sup>Ar step-heating method to put an upper limit on the time of deformation. Biotite from one tonalitic protolith and five shear zones give geologically meaningful results. Clusters of unoriented biotite grains pseudomorph granulite-facies orthopyroxene in some of the weakly deformed gneisses, whereas the shape preferred orientation of biotite defines the mylonitic fabric. The intrusive age of the tonalitic protolith is 1958 ± 12 Ma, based on previous U-Pb dating of zircon. 40Ar/39Ar analysis of biotite from the same sample gave a plateau age of 1929 ± 23 Ma, which is interpreted as cooling from regional granulite facies metamorphism. Three nearby samples of mylonitic tonalite have <sup>40</sup>Ar/<sup>39</sup>Ar ages that range from ≈1870–1840 Ma. Biotite from two granitic mylonites over 80 km away return high-resolution Ar spectra in the same range, implying that widespread ductile shearing occurred between ≈1870–1840 Ma, or ≈90 m.y. after cooling from regional metamorphism. Although the 2.0–1.9 Ga gneisses of southeast Ellesmere Island correlate with the Inglefield Mobile Belt in North-West Greenland and the Thelon Tectonic Zone, the late shear zones are superimposed on that juvenile arc long after the 1.97 Ga Thelon orogeny.

Structure and shear-sense indicators of the Mesoproterozoic basement of the North Tianshan microcontinent: Example of granitoid gneisses of the Karadjilga pluton, NW Kyrgyzstan

2021 ◽  
Author(s):  
Anastasia Kushnareva ◽  
Andrey Khudoley ◽  
Dmitriy Alexeiev ◽  
Eugeny Petrov
Keyword(s):  
Granite Gneiss ◽  
Shear Zones ◽  
Magmatic Complex ◽  
Thin Sections ◽  
Metasedimentary Rocks ◽  
The North ◽  
Rock Types ◽  
Shear Sense ◽  
Granitoid Gneisses ◽  
Shear Sense Indicators

&lt;p&gt;The Mesoproterozoic Karadjilga pluton is a poorly studied fragment of the North Tianshan microcontinent located in the western Central Asian Orogenic Belt. Metasedimentary rocks surrounding the pluton consist of marbles and mica schists of the Mesoproterozoic Ortotau Group. These rocks constitute a major west-northwest trending syncline with steep to subvertical limbs. The hinge of the fold is well expressed in the west part of the syncline and plunges east with 30-40&amp;#176; angle of plunge. Eastern termination of the syncline is cut by faults. Granitoid gneisses and granites of the Karadjilga pluton crop out in the core of the syncline. The contacts of the pluton are sub-parallel to bedding and schistosity in surrounding rocks. Primary magmatic contacts are locally reworked by reverse faults and thrusts. Our detailed mapping and structural study revealed inhomogeneous deformation of rocks of the Karadjilga pluton. The following rock types are identified: 1) undeformed granite 2) foliated granite 3) granite-gneiss and 4) mylonite. Undeformed granites form &lt;25-30% of total volume of the pluton and are most widespread in the northeast part of the pluton. On some geological maps they are shown as Ordovician or Devonian. However, U-Pb dating of 9 zircon grains by SHRIMP-II (VSEGEI, St. Petersburg, Russia) yielded a 1125&amp;#177;5 Ma concordant age. It agrees with previously reported U-Pb SHRIMP ages for deformed granites and gneisses (Degtyarev et al., 2011; Kr&amp;#246;ner et al., 2013) and indicates that undeformed granites belongs to the same Mesoproterozoic magmatic complex. Foliated granites and gneisses prevail and constitute up to 60-70% of total volume. They form west-northwest trending zones alternating with mylonites or undeformed granite. Mylonites are subordinate and occur mainly along the contacts of the pluton. Shear zones seem to be approximately parallel to the schistosity of deformed granites, but their geometry needs more study and mapping. Shear-sense indicators were studied in the oriented thin sections and are represented mainly by sigma and delta structures and oblique foliation with rare folds and other indicators. In all but one sample only strike-slip displacement has been identified. In the northern part of the pluton sinistral displacement predominates, whereas dextral displacement prevails in the southern part of the pluton. Shear zones are most widespread on the margins of the Karadjilga pluton, but locally also occur in the central part of the pluton, where they form narrow west-northwest trending zones. According to shear-sense indicators, displacement within the Karadjilga pluton occurred mainly in the approximately west-east direction that strongly differs from the north-south sense of displacement in the Paleozoic thrust and fold belts of Tianshan.&lt;/p&gt;&lt;p&gt;The study was supported by the RFBR project 20-05-00252.&lt;/p&gt;

scholarly journals Geology and micro-structure analysis of the MCT zone along Khudi- Bahundanda area of Lamjung District, west-central Nepal

2019 ◽  
Vol 58 ◽  
pp. 105-110
Author(s):  
Sandeep Thapa ◽  
Shashi Tamang ◽  
Kabi Raj Paudyal ◽  
Frédéric Girault ◽  
Frédéric Perrier
Keyword(s):  
Geological Mapping ◽  
Small Scale ◽  
Main Central Thrust ◽  
Geological Structures ◽  
Quartz Crystals ◽  
Central Nepal ◽  
West Central ◽  
Ductile Shearing ◽  
Microscopic Features ◽  
Garnet Zone

Geological mapping was carried out along the Marsyangdi Valley in the Khudi-Bahundanda area of west-central Nepal covering the Main Central Thrust (MCT) zone. The main objectives of the study were to draw a clear picture of lithology, geological structures and micro-tectonics in the rocks. A detail survey on stratigraphy and correlation with central Nepal reveals geological rock units such as the Benighat Slate, the Malekhu Formation and the Robang Formation of the Lesser Himalaya and the Formation I of the Higher Himalaya. Both regional and small-scale geological structures have been studied. The MCT zone has been mapped as a major regional structure in the region. The Bahundanda Thrust (BT), which has brought the older Malekhu Formation over the younger Robang Formation, is an another significant structure mapped. The BT is marked on the basis of fault breccia, slickensides as well as large deposits of debris mass at the fault zone. The study area has undergone poly-metamorphism and dynamic crystallization of minerals. The Lesser Himalayan rocks resemble the garnet zone while the Higher Himalaya rocks resemble to the kyanite grade of metamorphism. The present section clearly shows the inverted metamorphism in the MCT zone as in the other sections of the Himalaya. Microscopic features like ribbon-quartz, polygonization of quartz crystals, grain boundary reduction, mica-fish and rotated garnet grains indicates the ductile shearing in the MCT zone suggesting the dynamic recrystallization during thrust propagation. Numerous outcrop-scale structures like meso-scalefolds, quartz veins, boudinage and ptygmatic folds are abundant folds in the MCT zone and these are mostly E-W trending.

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