kathmandu nappe in central nepal
129605810
Sedimentology of a Mid Late Ordovician carbonate mud mound complex from the Kathmandu nappe in Central Nepal
Abstract This sedimentological study of the Godavari quarry is the first relating to the Palaeozoic Tethyan sedimentary rocks of the Kathmandu nappe (Central Nepal) Sedimentological analyses led to the identification of six microfacies belonging to a large carbonate mud mound complex, which can be divided into mound, flank and off mound main depositional settings. Identification of two dasycladaceans (Dasyporella cf. silurica Stolley, 1893 and Vermiporella sp. in the mound facies gives a Mid Late Ordovician age to this newly discovered Godavari carbonate mud mound, which makes this mound one of the oldest ever described in the Asian continent. The mound microfacies are characterized by a high micritic content, the presence of stromatactis and the prevalence of red coloured sediments (the red pigmentation probably being related to organic precipitation of iron) The flank microfacies are characterized by a higher crinoid and argillaceous content and the presence of bio and lithoclasts concentrated in argillaceous lenses. Finally, the off mound microfacies show very few bioclasts and a high argillaceous content. Palaeoenvionmental interpretation of microfacies, in terms of bathymetry, leads us to infer that the Godavari mud mound started to grow in a deep environment setting below the photic and wave action zones and that it evolved to occupy a location below the fair weather wave base. Cementation of cavities within the mound facies underlines a typical transition from a marine to a burial diagenetic environment characterized by: (1) a radiaxial non luminescent feroan calcite cement (marine) showing a bright orange luminescent band in its middle part; (2) a bright zoned orange fringe of automorphic feroan calcite (meteoric phreatic) (3) a dull orange xenomorphic feroan calcite cement in the centre of cavities (burial) and (4) a saddle dolomite within the centre of larger cavities. The faunal assemblage (diversity and relative proportion) of the Godavari mound facies is dominated by crinoids and ostracods, which makes this carbonate mud mound comparable to the Meiklejohn Peak mounds (Nevada)
Damien Pas Anne-Christine da Silva M R Dhital Frederic Boulvain
55601187
Petrological constraints on the tectonic setting of the Kathmandu Nappe in the Langtang Gosainkund Helambu regions, Central Nepal Himalaya
The Gosainkund Helambu region in central Nepal occupies a key area for the development of Himalayan kinematic models, connecting the well investigated Langtang area to the north with the Kathmandu Nappe (KN) whose interpretation is still debated, to the south. In order to understand the structural and metamorphic architecture of the Greater Himalayan Sequence (GHS) in this region, a detailed petrological study was performed, focusing on selected metapelite samples from both the Gosainkund Helambu and Langtang transects. The structurally lowest sample investigated belongs to the Lesser Himalayan Sequence; its metamorphic evolution is characterized by a narrow hairpin P T path with peak P T conditions of 595 25 degC, 7.5 1 kbar. All of the other samples here investigated belong to the GHS. Along the Langtang section, two tectono metamorphic units have been distinguished within the GHS: the Lower Greater Himalayan Sequence (L GHS) characterized by peak P T conditions at 728 11 degC, 10 0.5 kbar (corresponding to a T/depth ratio of 22 1 degC km 1) and the structurally higher Upper Greater Himalayan Sequence, with peak metamorphic conditions at 780 20 degC, 7.8 0.8 kbar (corresponding to a T/depth ratio of 31 4 degC km 1) This confirms the existence of a main tectono metamorphic discontinuity within the GHS, as previously suggested by other authors. The results of petrological modelling of the metapelites from the Gosainkund Helambu section show that this region is entirely comprised within a sub horizontal and thin L GHS unit: the estimated peak metamorphic conditions of 734 19 degC, 10 0.8 kbar correspond to a uniform T/depth ratio of 23 3 degC km 1. The metamorphic discontinuity identified along the Langtang transect and dividing the GHS in two tectono metamorphic units is located at a structural level too high to be intersected along the Gosainkund Helambu section. Our results have significant implications for the interpretation of the KN and provide a contribution to the more general discussion of the Himalayan kinematic models. We demonstrate that the structurally lower unit of the KN (known as Sheopuri Gneiss) can be correlated with the L GHS unit; this result strongly supports those models that correlate the KN to the Tethyan Sedimentary Sequence and that suggest the merging of the South Tibetan Detachment System and the Main Central Thrust on the northern side of the KN. Moreover we speculate that, in this sector of the Himalayan chain, the most appropriate kinematic model able to explain the observed tectono metamorphic architecture of the GHS is the duplexing model, or hybrid models which combine the duplexing model with another end member model.
Giulia Rapa Chiara Groppo Pietro Mosca Franco Rolfo
131755844
Uplift Mechanism of the Kathmandu Nappe, Central Nepal:
The Himalayan orogen represents a fold and thrust belt characterized by a series of foreland propagating thrust systems with an out of sequence thrust (OST) Geological and geomorphological evidence implies that the Himalayan upheaval started from the north and shifted gradually southward with time. The uplift seems to have been accelerated in the Higher Himalaya since the Pliocene. To investigate the role of OST in recent uplifts of the Higher Himalaya and to estimate the denudation rate of the latter, we carried out fission track (FT) dating on zircon and apatite of high grade pelitic and granitic gneisses from both sides of an inferred OST north of the Kathmandu nappe and along a traverse across Gosainkund Lekh in the northern root zone of the Kathmandu nappe.Nine zircon FT ages of sites in the Gosainkund Lekh area range from 1.5 Ma (at 1525 m) to 2.8 Ma (at 5045 m) Ages of four samples from the northern slope of the area increase linearly with elevation, and yield an average denudation rate of 2.4 mm/y. On the other hand, six samples from the southern slope yield identical ages (2.5 2.8 Ma) regardless of their sample elevations (1150 5045 m) The zircon ages of the area suggest that rocks on the southern slope passed evenly the depth of closure temperature of zircon at around 2.6 Ma. Following this, the northern part of the southern slope (the highest part of Gosainkund Lekh) and the northern slope were uplifted more rapidly than the southern slope.Eight zircon FT ages and seven apatite FT ages from rocks around the Sheopuri zone range from 4.4 Ma to 6.9 Ma and 4.3 Ma to 5.5 Ma, respectively. These ages vary irrespective of their sample elevations (1200 2400 m) Furthermore, zircons and apatites from both sides of the OST (Trisuli Likhu fault) yield almost the same FT ages, suggesting that the OST has been inactive since the latest Miocene. It is noteworthy that zircon FT ages from the sample elevations between 1200 m and 1800 m in the present area together with zircon FT ages (ca. 9 Ma) reported from early Paleozoic granites in the southern part of the Kathmandu nappe suggest a general trend of younging towards the north between 9 Ma and 1.5 Ma. On the basis of such a trend, two possible uplift mechanisms of the Kathmandu nappe and its northern root zone in the Higher Himalaya are presented. Both mechanisms might be related to crustal ramp due to northward wedging of Indian mid crust beneath the Himalaya.
Kazunori Arita Hiroto Ohira
204255127
U Pb ages of the Nardanda Pegmatite of the Kathmandu Crystalline Nappe and augen gneisses of the Gosaikund Crystalline Nappe, Central Nepal Himalaya
Santa Man Rai Bishal Nath Upreti Randy R Parrish P Copeland Patrick Le Fort
129780862
Tectonothermal evolution of the Lesser Himalaya, Nepal: Constraints from 40Ar/39Ar ages from the Kathmandu Nappe
Abstract The Himalaya is a fold and thrust wedge formed along the northern margin of the Indian continent, and consists of three thrust bounded lithotectonic units; the Sub Himalaya, the Lesser Himalaya, and the Higher Himalaya with the overlying Tethys Himalaya from south to north, respectively. The orogen scale, intracrustal thrusts which bound the above lithotectonic units are splays off an underlying subhorizontal dkcollement, and show a southward propagating piggy back sequence with an out of sequence thrust. Among these thrusts, the Main Central Thrust zone (MCT zone) has played a major role in Himalayan tectonics. The MCT zone represents a shear zone which has accommodated southward thrusting of the Higher Himalayan crystalline thrust sheet over the Lesser Himalayan sequence for ~140 km. The Kathmandu Nappe in central Nepal has been transported over the Lesser Himalayan metasediments along the MCT zone, and is locally separated from the Higher Himalayan thrust sheet in the north by an out of sequence thrust. 40Ar/39Ar ages have been determined for one whole rock phyllite and six muscovite concentrates from metasedimenta ry rocks and variably deformed granites in the Kathmandu Nappe. These ages range from 44 Ma to 14 Ma, and suggest a record of both Eo Himalayan (Eocene) and Neo Himalayan (Miocene) tectonothermal events in the Tertiary Himalayan orogeny. The Miocene event was associated with translation along the MCT zone. No tectonothermal event of the Late Miocene to Early Pliocene ages have been reported near the MCT zone in southern Lesser Himalayan crystalline nappe or klippe, although such events have been documented within and around the MCT zone in the northern root zone of the Higher Himalaya. This suggests that out of sequence thrusting may have occurred between 14 Ma and 5 Ma, probably during the period 10 7.5 Ma. Since then the frontal MCT zone below the Kathmandu Nappe has been inactive, but the MCT zone in the northern root zone has remained active. The rapid increase in denudation rates of the Higher Himalaya since the Late Miocene may have been caused by ramping along the out of sequence thrust at depth.
Kazunori Arita R David Dallmeyer Akira Takasu
12631808
Thermal structure and exhumation history of the Lesser Himalaya in central Nepal
The Lesser Himalaya (LH) consists of metasedimentary rocks that have been scrapped off from the underthrusting Indian crust and accreted to the mountain range over the last ~20 Myr. It now forms a significant fraction of the Himalayan collisional orogen. We document the kinematics and thermal metamorphism associated with the deformation and exhumation of the LH, combining thermometric and thermochronological methods with structural geology. Peak metamorphic temperatures estimated from Raman spectroscopy of carbonaceous material decrease gradually from 520deg 550degC below the Main Central Thrust zone down to less than 330degC. These temperatures describe structurally a 20deg 50degC/km inverted apparent gradient. The Ar muscovite ages from LH samples and from the overlying crystalline thrust sheets all indicate the same regular trend; i.e. an increase from about 3 4 Ma near the front of the high range to about 20 Ma near the leading edge of the thrust sheets, about 80 km to the south. This suggests that the LH has been exhumed jointly with the overlying nappes as a result of overthrusting by about 5 mm/yr. For a convergence rate of about 20 mm/yr, this implies underthrusting of the Indian basement below the Himalaya by about 15 mm/yr. The structure, metamorphic grade and exhumation history of the LH supports the view that, since the mid Miocene, the Himalayan orogen has essentially grown by underplating, rather than by frontal accretion. This process has resulted from duplexing at a depth close to the brittle ductile transition zone, by southward migration of a midcrustal ramp along the Main Himalayan Thrust fault, and is estimated to have resulted in a net flux of up to 150 m^2/yr of LH rocks into the Himalayan orogenic wedge. The steep inverse thermal gradient across the LH is interpreted to have resulted from a combination of underplating and post metamorphic shearing of the underplated units.
Laurent Bollinger Jean-Philippe Avouac O Beyssac Elizabeth J Catlos T Mark Harrison M Grove Bruno Goffe Soma Nath Sapkota
53500554
Inverted metamorphic zonation in the hanging and foot walls of the Mahabharat Thrust, Kathmandu Trishuli area, central Nepal
Geological study was carried out along the Pasang Lahmu Highway from Kathmandu to Trishuli Bazaar covering both theLesser Himalayan autochthonous unit and the Kathmandu Nappe. The Lesser Himalayan rocks in the study area belong to theKunchha Formation, Benighat Slate and the Robang Formation of the Nawakot Complex. The Kathmandu Nappe (KathmanduComplex) comprises the Kalitar Formation, Gneiss Zone, Tistung Formation, Sopyang Formation and the Chandragiri Limestone.Petrographic study was carried out in the samples representing all the lithological units along the Pasang Lahmu Highway.The study shows that the Kunchha Formation belongs to the biotite zone. The overlying units (Benighat Slate and RobangFormations) belong to the garnet zone. It is a clear evidence of inverted metamorphic zonation at the foot wall of the KathmanduNappe. The rocks of the Kathmandu Complex above the Mahabharat Thrust north of Kakani also show an inverse metamorphiczonation, i.e. the garnet zone is overlain by the sillimanite zone. However, in the southern part, the metamorphic zonation isnormal with biotite zone overlain by the chlorite zone. Although the inverted metamorphic zonation at the hanging wall of theMahabharat Thrust may be related to the high temperature contact metamorphism by pegmatite injection, the inverted metamorphismat the footwall needs an explanation. DOI: http:/dx.doi.org/10.3126/bdg.v14i0.5439 Bulletin of the Department of Geology, Vol. 14, 2011 pp. 51 58
Sobit Prasad Thapaliya Lalu Paudel
129682373
An overview of the metamorphic evolution in Central Nepal
Abstract This paper summarizes the studies of the metamorphic evolution of Central Nepal carried out by Nepali and international teams in the last 25 years. In Central Nepal, three metamorphic units are recognized. (1) The southernmost zone is the Lesser Himalaya, which is characterised by an inverted mineral zoning towards the Main Central Thrust (MCT) zone; (2) the Kathmandu nappe corresponds to an early
Stephane Guillot
146242984
Finite strain variation across the Mahabharat Thrust in central Nepal Himalaya
This paper deals with the three dimensional strain across the Mahabharat Thrust (MT) in the Malekhu area in central Nepal. The MT served as a glide plane for the Kathmandu Nappe. Its footwall is made up of phyllites, quartzites, and amphibolites, whereas the hanging wall contains garnetiferous schists, biotite schists, and quartzites with a few lenses of augen gneiss. A three dimensional strain analysis reveals that Nadai's amount of strain intensity (e s ranges from 0.396 to 0.575 in the footwall indicating an increasing trend towards the proximity of the MT. In contrast, the hanging wall shows an increase in e s magnitude away from the MT and its value varies between 0.556 (at the basal part) and 0.795 (upper part) Microtextures and structures revealed dynamic recrystallisation of the footwall and static recrystallisation of the hanging wall rocks. The shape of three dimensional strain ellipsoids, types of microstructures, and mechanisms of grainscale deformation indicated that the footwall was dominantly affected by simple shear deformation at lower temperatures while the hanging wall suffered from pure shear with minor sub simple shear deformation at relatively higher temperatures.
Deepak Chamlagain Daigoro Hayashi
129045063
Pressure temperature evolution in the Kathmandu and Gosainkund regions, Central Nepal
Abstract The Kathmandu and Gosainkund regions in central Nepal are occupied by a metamorphic sequence of upper amphibolite to granulite facies metasedimentary rocks. On the basis of structure and lithology, three units are distinguished: the Gosainkund Crystalline Nappe (GCN) corresponding to the southward continuation of the Higher Himalayan Crystallines (HHC) of Langtang; and the Kathmandu Crystalline Nappe (KCN) representing an out of sequence thrust sheet in the Lesser Himalaya. The boundary between the two nappes corresponds to the Main Central Thrust (MCT) The GCN and the KCN are thrust over the Lesser Himalaya along the MCT and the Mahabharat Thrust (MT) respectively. Structural and petrographical data demonstrate that the metamorphic assemblage in the three units is related to movements on the MCT and MT, with a well preserved inverted thermal gradient in the MCT zone. Both conventional and Thermocalc thermobarometric methods are used to calculate pressure and temperature. The upper Lesser Himalayan sample records syn MCT metamorphic conditions of 750+ 150 MPa and 566+ 136degC, the KCN samples record P T conditions from 900+ 140 to 720+ 420 MPa and 700+ 60 to 484+ 194degC, while the GCN samples equilibrated at upper amphibolite to granulite facies conditions from 890+ 260 to 583+ 198 MPa and 754+ 40 to 588+ 100degC (error estimation quoted at 2 s The final equilibration of the upper section of the GCN occurred with a pressure and temperature drop of 170 MPa and 60degC respectively, associated with NNW normal ductile shearing. The good preservation of the metamorphic conditions in the different units, with no field and petrographic evidence of a metamorphic discontinuity across the MCT and MT zones suggests that the final exhumation of the nappe piles occurred along the MBT.
Santa Man Rai Stephane Guillot Patrick Le Fort Bishal Nath Upreti