Main Central Thrust Zone Research Articles

Plio-Pleistocene rapid uplift process of the Nepal Himalaya revealed from fission-track ages

Himalayan Journal of Sciences Vol.2(4) Special Issue 2004 pp. 98-99

Orogen-parallel extension and exhumation enhanced by denudation in the trans-Himalayan Arun River gorge, Ama Drime Massif, Tibet-Nepal

Focused denudation and mid-crustal flow are coupled in many active tectonic settings, including the Himalaya, where exhumation of mid-crustal rocks accommodated by thrust faults and low-angle detachment systems during crustal shortening is well documented. New structural and (U-Th)/He apatite data from the Mount Everest region demonstrate that the trans-Himalayan Ama Drime Massif has been exhumed at a minimum rate of ~1 mm/yr between 1.5 and 3.0 Ma during orogen-parallel extension. The Ama Drime Massif offsets the South Tibetan detachment system, and therefore the South Tibetan detachment system is no longer capable of accommodating south-directed mid-crustal flow or coupling it with focused denudation. Previous investigations interpreted the NNE-SSW–striking shear zone on the west side of the Ama Drime Massif as the Main Central thrust zone; however, our data show that the Ama Drime Massif is bounded on either side by 100–300-m-thick normal-sense shear zone and detachment systems that are kinematically linked to young brittle faults that offset Quaternary deposits and record active orogen-parallel extension. When combined with existing data, these results suggest that the Ama Drime Massif was exhumed during orogen-parallel extension that was enhanced by, or potentially coupled with, denudation in the trans-Himalayan Arun River gorge. This model provides important insights into the mechanisms that exhumed trans-Himalayan antiformal structures during orogen-parallel extension along the southern margin of the Tibetan Plateau.

Nd isotopic data reveal the material and tectonic nature of the Main Central Thrust zone in Nepal Himalaya

The Main Central Thrust (MCT) is a tectono-metamorphic boundary between the Higher Himalayan crystallines (HHC) and Lesser Himalayan metasediments (LHS), reactivated in the Tertiary, but which had already formed as a collisional boundary in the Early Paleozoic. To investigate the nature of the MCT, we analyzed whole-rock Nd isotopic ratios of rocks from the MCT and surrounding zones in the Taplejung–Ilam area of far-eastern Nepal, Annapurna–Galyang area of central Nepal, and Maikot–Barekot area of western Nepal. We define the MCT zone as a ductile–brittle shear zone between the upper MCT (UMCT) and lower MCT (LMCT). The protoliths of the MCT zone may provide critical constraints on the tectonic evolution of the Himalaya. The LHS is lithostratigraphically divided into the upper and lower units. In the Taplejung–Ilam area, different lithologic units and their ε Nd (0) values are as follows; HHC (− 10.0 to − 18.1), MCT zone (− 18.5 to − 26.2), upper LHS unit (− 17.2), and lower LHS unit (− 22.0 to − 26.9). There is a distinct gap in the ε Nd (0) values across the UMCT except for the southern frontal edge of the Ilam nappe. In the Annapurna–Galyang and Maikot–Barekot areas, different lithologic units and their ε Nd (0) values are as follows; HHC (− 13.9 to − 17.7), MCT zone (− 23.8 to − 26.2 except for an outlier of − 12.4), upper LHS unit (− 15.6 to − 26.8), and lower LHS unit (− 24.9 to − 26.8). These isotopic data clearly distinguish the lower LHS unit from the HHC. Combining these data with the previously published data, the lowest ε Nd (0) value in the HHC is − 19.9. We regard rocks with ε Nd (0) values below − 20.0 as the LHS. In contrast, rocks with those above − 19.9 are not always the HHC, and some parts of them may belong to the LHS due to the overlapping Nd isotopic ratio between the HHC and LHS. Most rocks of the MCT zone have Nd isotopic ratios similar to those of the LHS, but very different from those of the HHC. The spatial patterns in the distribution of ε Nd (0) value around the UMCT suggest no substantial structural mixing of the HHC and LHS during the UMCT activities in the Tertiary. A discontinuity in the spatial distribution of ε Nd (0) values is laterally continuous along the UMCT throughout the Himalayas. These facts support the theory that the UMCT was originally a material boundary between the HHC and LHS, suggesting the MCT zone was mainly developed with undertaking a role of sliding planes during overthrusting of the HHC in the Tertiary.

A stream sediment geochemical survey of the Ganga River headwaters in the Garhwal Himalaya

This study models geochemical and adjunct geologic data to define provinces that are favorable for radioactive-mineral exploration. A multi-element bed-sediment geochemical survey of streams was carried out in the headwaters region of the Ganga River in northern India. Overall median values for uranium and thorium (3.6 and 13.8 ppm; maxima of 4.8 and 19.0 ppm and minima of 3.1 and 12.3 ppm respectively) exceed average upper crustal abundances (2.8 and 10.7 ppm) for these radioactive elements. Anomalously high values reach up to 8.3 and 30.1 ppm in thrust zone rocks, and 11.4 and 22.5 ppm in porphyroids. At their maxima, these abundances are nearly four- and three-fold (respectively) enriched in comparison to average crustal abundances for these rock types. Deformed, metamorphosed and sheared rocks are characteristic of the main central thrust zone (MCTZ). These intensively mylonitized rocks override and juxtapose porphyritic (PH) and proterozoic metasedimentary rock sequences (PMS) to the south. Granitoid rocks, the major protoliths for mylonites, as well as metamorphosed rocks in the MCT zone are naturally enriched in radioelements; high values associated with sheared and mylonitized zones are coincident with reports of radioelement mineralization and with anomalous radon concentrations in soils. The radioelement abundance as well as REE abundance shows a northward enrichment trend consistent with increasing grade of metamorphism indicating deformation-induced remobilization of these elements. U and Th illustrate good correlation with REEs but not with Zr. This implies that zircon is not a principal carrier of U and Th within the granitoid-dominant thrust zone and that other radioelement-rich secondary minerals are present in considerable amounts. Thus, the relatively flat, less fractionated, HREE trend is also not entirely controlled by zircon. The spatial correlation of geologic boundary zones (faults, sheared zones) with geochemical and with geophysical (Rn) anomalies infers ore mineralization by hydrothermal processes generated during multiple episodes of deformation and thrusting. The geologic setting of the anomalies also suggests that crystalline rocks (MCT Zone) along the nearly 2500 km length of the LesserHimalayan belt, where in the vicinity of thrust and fault zones, have potential for radioelement mineralization. Zones of higher concentrations of radioelements delineated by this study and locations of anomalous radon discharge determined by other investigations may indicate a potential health hazard over the long term. However, the low human population density precludes direct manifestation of health effects attributable to chronic exposure to these radioelements; however, the magnitude of natural concentrations suggests the need for more detailed studies and monitoring.

Structural evolution and vorticity of flow during extrusion and exhumation of the Greater Himalayan Slab, Mount Everest Massif, Tibet/Nepal: implications for orogen-scale flow partitioning

Abstract The Greater Himalayan Slab (GHS) is composed of a north-dipping anatectic core, bounded above by the South Tibetan detachment system (STDS) and below by the Main Central thrust zone (MCTZ). Assuming simultaneous movement on the MCTZ and STDS, the GHS can be modelled as a southward-extruding wedge or channel. New insights into extrusion-related flow within the GHS emerge from detailed kinematic and vorticity analyses in the Everest region. At the highest structural levels, mean kinematic vorticity number ( Wm ) estimates of 0.74–0.91 ( c. 45–287fb3e69cure shear) were obtained from sheared Tethyan limestone and marble from the Yellow Band on Mount Everest. Underlying amphibolite-facies schists and gneisses, exposed in Rongbuk valley, yield Wm estimates of 0.57–0.85 ( c. 62–357fb3e69cure shear) and associated microstructures indicate that flow occurred at close to peak metamorphic conditions. Vorticity analysis becomes progressively more problematic as deformation temperatures increase towards the anatectic core. Within the MCTZ, rigid elongate garnet grains yield Wm estimates of 0.63–0.77 ( c. 58–447fb3e69cure shear). We attribute flow partitioning in the GHS to spatial and temporal variations that resulted in the juxtaposition of amphibolite-facies rocks, which record early stages of extrusion, with greenschist to unmetamorphosed samples that record later stages of exhumation.

Pulsed channel flow in Bhutan

Abstract We summarize our results from Bhutan and interpret the Greater Himalaya Sequence (GHS) of Bhutan, together with a portion of the underlying Lesser Himalaya Sequence, in the context of recently published channel flow models. For the GHS rocks now exposed in Bhutan the depth for beginning of muscovite dehydration melting (approximately 750°C at 11 kbar) and associated weakening of these rocks is constrained by geobarometry to be at about 35–45 km. The location of initial melting was down-dip and over 200 km to the north of Bhutan. Melt was produced and injected into ductilely deforming metamorphic rocks as they were extruded towards the south between the Main Central Thrust (MCT) and the South Tibetan Detachment zones. The lateral flow of low viscosity rocks at these depths occurred under southern Tibet between 22 Ma and 16 Ma. Subsequently, the channel rocks decompressed from 11 to 5 kbar (from 35 km to a depth of 15 km), but maintained high temperatures, between about 16 Ma and 13 Ma. The data from Bhutan are consistent with channel flow models if there were several pulses of channel flow. The first, between 22 and 16 Ma, produced the rock seen in the lower half of the GHS of Bhutan. A second pulse, which is cryptic, is inferred to have led to the uplift and exhumation of the MCT zone. A third, in central Bhutan, is exposed now as the hanging wall of the Kakhtang thrust, an out-of-sequence thrust that was active at 12–10 Ma. The latter two pulses likely broke around a plug at the head of the first pulse that was formed where the melt in the channel had solidified.

Transtensional deformation in the central Himalaya and its role in accommodating growth of the Himalayan orogen

Field mapping, structural analysis, and geochronologic data from northwestern Nepal reveal major normal right‐slip motion along a previously unrecognized west‐northwest striking system of shear zones that we term the Gurla Mandhata–Humla fault system (GMH). The GMH obliquely cuts across the Greater Himalayan Crystalline sequence and into the Lesser Himalayan imbricate thrust belt via two right‐step‐over structures. The average slip direction on the GMH parallels the strike of the Himalayan orogen. Motion along this fault system has resulted in an apparent left separation of the South Tibet Detachment, Main Central thrust zone, and Lesser Himalayan imbricate thrust belt along a north striking segment of the fault system. We estimate a minimum of 21 km of net slip on the southern branch of the GMH by restoring the trace of the Main Central thrust zone parallel to the average slip direction on the fault. Taking into account slip estimates from the northern branch of the GMH yields a minimum net slip estimate of 24.4 to 32.4 km for the GMH. The 232Th/208Pb ion microprobe monazite ages from leucogranite bodies indicate that motion on the GMH occurred after 15 Ma. Its initiation immediately followed crustal thickening between the Main Central thrust zone and Indus‐Yalu suture zone. Motion on the GMH is contemporaneous with arc‐normal contraction in the southernmost Himalayan orogen. These observations can be explained by a model that involves foreland propagating structural systems facilitating arc‐normal contraction in the foreland and arc‐parallel extension in the hinterland that work together to maintain the arcuate shape of the Himalayan orogen.

U–Pb zircon ages as a sediment mixing tracer in the Nepal Himalaya

This paper presents a new approach to quantify sediment mixing based on the mixing of U–Pb zircon age distributions within sediment. Two statistical techniques are presented to determine the proportion in which two known age distributions combine to create a known mixed age distribution. These techniques are then used to determine relative erosion rates between adjacent drainage basins above and below the Main Central Thrust (MCT) in the central Nepal Himalaya. The MCT region is coincident with an abrupt north–south change in geomorphic character and mineral cooling ages that are thought to represent an erosional response to higher rock uplift rates north of the MCT zone. However, it is unclear whether the ongoing deformation responsible for the differential uplift rates is: (1) focused on the MCT; (2) at depth along a crustal scale ramp; or (3) along newly mapped thrust faults south of the MCT. Our study explores this issue by comparing modern erosion rates with longer-term erosion rates determined from mineral cooling ages. Zircons were separated from modern river sand and dated by LA-MC-ICPMS before the measured isotopic ratios and ages were used in 1-d and 2-d mixing calculations. The 1-d technique creates probability density functions of zircon ages for each sample and then uses both an iterative and inverse approach to estimate mixing between samples. In contrast, the 2-d technique estimates mixing between probability “fields” defined by the measured 238U/ 206Pb and 207Pb/ 206Pb ratios. Given a finite mixture with perfect sample representation, both techniques produce perfect mixing estimates across a range of mixing proportions. Modeling results demonstrate that given imperfect subsample representation of the complex parent age distribution, differing degrees of subsample smoothing may be required to achieve an accurate mixing estimate. Using mixing of zircon ages as a quantitative proxy for sediment mixing requires a correction for the concentration of zircon in the river sediment. Two new methods for establishing zircon concentration in river sediment are presented demonstrating the existence of 2- to 5-fold differences in zircon concentration between adjacent drainages. Relative erosion rates are estimated by determining the zircon mixing ratio between adjacent drainages which are then normalized by the ratio of zircon concentrations and the ratio of drainage areas. Results show ∼ 3 times higher modern erosion rates south of the MCT in the northernmost Lesser Himalaya. Future applications of this new technique may include reach-scale sediment transport dynamics, improved sedimentary basin analysis, and better interpretation of foreland mineral cooling ages.

Channel flow and ductile extrusion of the high Himalayan slab-the Kangchenjunga–Darjeeling profile, Sikkim Himalaya☆

The geology of the Kangchenjunga–Darjeeling profile in west Sikkim and north Bengal is consistent with the interpretation of the High Himalaya metamorphic sequence as a ductile channel approximately 15–20 km thick, extruding southwards, bounded by major ductile shear zones above (South Tibetan Detachment) and below (Main Central Thrust zone). In the Yoksam to Kangchenjunga section the entire slab is composed of migmatites or leucogranite sheets. Massive sills, or sub-horizontal to gently north-dipping sheets, of leucogranite make up the entire Kangchenjunga massif and extend west at least as far as Jannu. Shear sense indicators show both pure shear flattening fabrics and non-coaxial south-directed simple shear fabrics. The carapace of the melt-filled channel is seen around the southern margin of the Singallila ridge and the Darjeeling klippe, where inverted metamorphic isograds from sillimanite+K-feldspar grade down through kyanite and staurolite to garnet–biotite grade have been well documented. We present an interpretation of the crustal structure of the Sikkim Himalaya, based on field structural observations and utilizing the INDEPTH seismic profile to constrain sub-surface structure to the north. We suggest that widespread melting within sillimanite grade gneisses, triggered sudden and rapid ductile extrusion during the Miocene, by critically affecting the rheology of the middle crust under south Tibet. Leucogranite melts lubricated shear zones, facilitating rapid transport of heat towards the surface. When the leucogranite melts began to cool, ductile extrusion ended, as thrusting propagated down-section into the Lesser Himalaya. Since leucogranite expulsion was largely horizontal, rather than vertical, and since the STD normal sense shear zone is very low-angle, dipping to the north, decompression melting cannot be invoked as an origin for the leucogranites. The heat source for such widespread melting at such shallow levels in the crust can only be explained by having a highly radioactive protolith of Indian plate Proterozoic sediments. The overall map distribution and geometry of leucogranites, migmatites and metamorphic isograds is compatible with both the Everest-Makalu section to the west, and the Bhutan section, east of the Yadong-Gulu rift. We conclude that the channel flow model is a viable explanation for this section of the Himalaya.

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 520°–550°C below the Main Central Thrust zone down to less than 330°C. These temperatures describe structurally a 20°–50°C/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 m2/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.

Fabric development during shear deformation in the Main Central Thrust Zone, NW-Himalaya, India

Quartz microfabrics and associated microstructures have been studied on a crustal shear zone—the Main Central Thrust (MCT) of the Himalaya. Sampling has been done along six traverses across the MCT zone in the Kumaun and Garhwal sectors of the Indian Himalaya. The MCT is a moderately north-dipping shear zone formed as a result of the southward emplacement of a part of the deeply rooted crust (that now constitutes the Central Crystalline Zone of the Higher Himalaya) over the less metamorphosed sedimentary belt of the Lesser Himalaya. On the basis of quartz c- and a-axis fabric patterns, supported by the relevant microstructures within the MCT zone, two major kinematic domains have been distinguished. A noncoaxial deformation domain is indicated by the intensely deformed rocks in the vicinity of the MCT plane. This domain includes ductilely deformed and fine-grained mylonitic rocks which contain a strong stretching lineation and are composed of low-grade mineral assemblages (muscovite, chlorite and quartz). These rocks are characterized by highly asymmetric structures/microstructures and quartz c- and a-axis fabrics that indicate a top-to-the-south sense that is compatible with south-directed thrusting for the MCT zone. An apparently coaxial deformation domain, on the other hand, is indicated by the rocks occurring in a rather narrow belt fringing, and structurally above, the noncoaxial deformation domain. The rocks are highly feldspathic and coarse-grained gneisses and do not possess any common lineation trend and the effects of simple shear deformation are weak. The quartz c-axis fabrics are symmetrical with respect to foliation and lineation. Moreover, these rocks contain conjugate and mutually interfering shear bands, feldspar/quartz porphyroclasts with long axes parallel to the macrosopic foliation and the related structures/microstructures, suggesting deformation under an approximate coaxial strain path. On moving towards the MCT, the quartz c- and a-axis fabrics become progressively stronger. The c-axis fabric gradually changes from random to orthorhombic and then to monoclinic. In addition, the coaxial strain path gradually changes to the noncoaxial strain path. All this progressive evolution of quartz fabrics suggests more activation of the basal, rhomb and 〈 a〉 slip systems at all structural levels across the MCT.

Crustal channel flows: 2. Numerical models with implications for metamorphism in the Himalayan‐Tibetan orogen

Results from a thermal‐mechanical model (HT1) that includes midcrustal channel flow are compatible with many features of the Himalayan‐Tibetan system. Radioactive self‐heating and rheological weakening of thickened model orogenic crust lead to the formation of a hot, low‐viscosity midcrustal channel and a broad plateau. Channel material, corresponding to the Greater Himalayan Sequence (GHS), flows outward from beneath the plateau in response to topographically induced differential pressure. At the plateau flank it is exhumed by focused surface denudation and juxtaposed with cooler, newly accreted material corresponding to the Lesser Himalayan Sequence (LHS). The model channel is bounded by coeval thrust and normal sense ductile shear zones, interpreted to represent the Main Central Thrust (MCT) zone and South Tibetan Detachment system, respectively. Inverted metamorphism associated with the model MCT zone results from distributed ductile shear along the MCT and extrusion of the hot channel. A variety of model P‐T‐t path styles, resembling those observed in the GHS and LHS, are produced for points traveling through contrasting tectonic regimes that coexist in different parts of the model. Predicted times of peak metamorphism, cooling, and erosion of metamorphic facies are generally compatible with observations, although model GHS cooling ages are too young. The times of M1 and M2 metamorphic “events” observed in the GHS correspond to model times of maximum burial and maximum heating, respectively. The results highlight the need to integrate tectonics and metamorphism in continental collision models and demonstrate the importance of lateral transport of both heat and material in large hot orogens.

The South Tibetan Detachment and the Manaslu Leucogranite: A Structural Reinterpretation and Restoration of the Annapurna‐Manaslu Himalaya, Nepal

The South Tibetan Detachment (STD) System comprises both ductile shear zones and brittle low‐angle extensional faults bounding the upper (northern) margin of the high‐grade metamorphic and anatectic rocks of the Greater Himalayan Sequence (GHS). Along the Himalayan chain from Zanskar in the west to Bhutan in the east, leucogranites are restricted to the footwall of the STD and rarely, if ever, intrude across the fault into unmetamorphosed sedimentary rocks of the Tethyan zone. The Manaslu leucogranite (24–19 Ma) was previously thought to be an exception, intruding up as far as the Triassic sediments. We have mapped a newly discovered 350–400‐m‐thick shear zone of high‐strain mylonites along the upper Nar Valley and Pangre glacier (Phu $$\mathrm{Detachment}\,=\mathrm{STD}\,$$ ), west of the Manaslu‐Himlung massif, which wraps around the northern (upper) margin of the Manaslu leucogranite. All rocks beneath the Manaslu leucogranite are metamorphosed, and no leucogranites intrude across this shear zone, in common with observations elsewhere along the Himalaya. The age of motion along the STD in the Manaslu region is constrained as being younger than 18 Ma, not older than 24 Ma as previously thought. The Chame Detachment (CD) is a ductile shear zone wholly within the GHS. Pressure‐temperature conditions of $$\mathrm{diopside}\,+\mathrm{K}\,$$ ‐$$\mathrm{feldspar}\,+\mathrm{tremolite}\,$$ marbles and calc‐silicates, and $$\mathrm{sillimanite}\,\pm \mathrm{kyanite}\,$$ meta‐pelites above the CD are similar to conditions for calc‐silicates, pelites, and augen gneisses below. We have used structural data and PT constraints in conjunction with stratigraphy to restore the entire GHS in the Annapurna‐Manaslu region, prior to the Early Miocene formation of the Manaslu leucogranite and the STD System of normal faults. These Miocene faults and shear zones cut across the earlier Oligocene north‐verging folds exposed in the Annapurna range. Ductile shearing along upper levels of the Main Central Thrust zone and along the CD occurred before 21 Ma, and brittle faulting along the STD occurred after 19–18 Ma. Rapid exhumation and cooling of the GHS and Manaslu granite between 18 and 15 Ma was followed by initiation of east‐west extension in the Thakkhola Graben north of the STD and propagation of thrusting southward into the Lesser Himalaya.

Kinematic history of the Main Central Thrust zone in the Langtang area, Nepal

This paper describes for the first time evidence for a northeastward brittle extensional movement of the Main Central Thrust (MCT) zone overprinting the southward ductile thrust movement. The MCT zone is the major tectonic boundary between the Lesser Himalayan metasedimentary sequence (LHS) and Higher Himalayan crystalline sequence (HHS) and contributes to the kinematic and tectonic evidence of continent–continent collision in the Neogene. The Langtang area, 50 km north of Kathmandu, is a matter of interest in considering the movement of the Kathmandu Nappe because the general trend of foliation of the HHS and the underlying MCT zone is turned from WNW–ESE to N–S or NNE–SSW. The major tectono-lithostratigraphic units of the MCT zone in the study area are divided into three: the lower and middle units included in the LHS and the upper unit even included in the HHS. Characteristic mesoscopic quartz lenses are developed in the schists of the Lower unit forming asymmetric boudins. This asymmetry indicates an extensional (top-to-the-NE) shear sense. However, a thrust sense of top-to-the-SW is also preserved in microscopic ductile shear bands and mica fish. The extensional shear movement in the Lower unit took place after the major thrust movement in the MCT zone as a negative inversion. The Middle unit contains Ulleri-type augen gneiss (Syabru Bensi augen gneiss) which is widely distributed in the MCT zone in the other parts of the Nepalese Himalaya. Asymmetric clast-tails (or clast-pressure shadows) and asymmetric boudins observed in the Syabru Bensi augen gneiss give a dextral-thrust oblique sense of shear. This dextral component is regarded as movement of a N–S-trending lateral ramp during the overthrusting of the Kathmandu Nappe. The Upper unit above the MCT is commonly made up of kyanite-bearing medium pressure type amphibolite-facies pelitic gneiss. Asymmetric deformational features such as mica fish and shear bands in the pelitic gneisses commonly show a thrust (top-to-the-W) sense of movement. After the emplacement of the Kathmandu Nappe (∼9–6 Ma), brittle extensional shear occurred only in the crystalline schists of the Lower unit presumably associated with layer-parallel slip. This extensional movement in the Lower unit is probably caused by the change in strike of foliations, e.g., from a general trend of E–W in the other areas to N–S as seen in the study area. This is because N–S contraction during continued continent–continent collision produced E–W extension normal to the contraction axis (N–S), thus the relative east-directed movement on normal faults took place only in the N–S trending domain of the MCT zone. Another possibility for the extensional movement is squeezing of the upper part of the LHS between the MCT above and an out-of-sequence thrust or the Main Detachment Fault below.

Exhumation of the Main Central Thrust from Lower Crustal Depths, Eastern Bhutan Himalaya

AbstractGeothermometry and mineral assemblages show an increase of temperature structurally upwards across the Main Central Thrust (MCT); however, peak metamorphic pressures are similar across the boundary, and correspond to depths of 35–45 km. Garnet‐bearing samples from the uppermost Lesser Himalayan sequence (LHS) yield metamorphic conditions of 650–675 °C and 9–13 kbar. Staurolite‐kyanite schists, about 30 m above the MCT, yield P‐T conditions near 650 °C, 8–10 kbar. Kyanite‐bearing migmatites from the Greater Himalayan sequence (GHS) yield pressures of 10–14 kbar at 750–800 °C. Top‐to‐the‐south shearing is synchronous with, and postdates peak metamorphic mineral growth.Metamorphic monazite from a deformed and metamorphosed Proterozoic gneiss within the upper LHS yield U/Pb ages of 20–18 Ma. Staurolite‐kyanite schists within the GHS, a few metres above the MCT, yield monazite ages of c. 22 ± 1 Ma. We interpret these ages to reflect that prograde metamorphism and deformation within the Main Central Thrust Zone (MCTZ) was underway by c. 23 Ma. U/Pb crystallization ages of monazite and xenotime in a deformed kyanite‐bearing leucogranite and kyanite‐garnet migmatites about 2 km above the MCT suggest crystallization of partial melts at 18–16 Ma. Higher in the hanging wall, south‐verging shear bands filled with leucogranite and pegmatite yield U/Pb crystallization ages for monazite and xenotime of 14–15 Ma, and a 1–2 km thick leucogranite sill is 13.4 ± 0.2 Ma. Thus, metamorphism, plutonism and deformation within the GHS continued until at least 13 Ma. P‐T conditions at this time are estimated to be 500–600 °C and near 5 kbar. From these data we infer that the exhumation of the MCT zone from 35 to 45 km to around 18 km, occurred from 18 to 16 to c. 13 Ma, yielding an average exhumation rate of 3–9 mm year−1. This process of exhumation may reflect the ductile extrusion (by channel flow) of the MCTZ from between the overlying Tibetan Plateau and the underthrusting Indian plate, coupled with rapid erosion.

Kinematic model for the Main Central thrust in Nepal

We present a kinematic model for the Himalayan thrust belt that satisfies structural and metamorphic data and explains recently reported late Miocene‐Pliocene geochronologic and thermochronologic ages from rocks in the Main Central thrust zone in central Nepal. At its current exposure level, the Main Central thrust juxtaposes a hanging-wall flat in Greater Himalayan rocks with a footwall flat in Lesser Himalayan rocks of the Ramgarh thrust sheet, which is the roof thrust of a large Lesser Himalayan duplex. Sequential emplacement of the Main Central (early Miocene) and Ramgarh (middle Miocene) thrust sheets was followed by insertion of thrust sheets within the Lesser Himalayan duplex and folding of the Main Central and Ramgarh thrusts during late Miocene‐ Pliocene time. Thorium-lead (Th-Pb) ages of monazite inclusions in garnets from central Nepal record the timing of coeval, progressive metamorphism of Lesser Himalayan rocks in the footwall of the Main Central thrust. Although this model does not rule out minor, late-stage reactivation of the Main Central thrust, major late Miocene reactivation is not required.

Kinematic model for the Main Central thrust in Nepal: Comment and Reply

In the Main Central thrust zone of the central Nepalese Himalaya, monazites decrease in age downsection between the Main Central thrust and Main Central thrust 1 (MCT-1 in [Fig. 1][1]) from ca. 22 to 3.3 Ma ([Catlos et al., 2001][2]). This astonishingly systematic result ([Kohn et al., 2001][3]) has

Estimating streaming potentials associated with geothermal circulation at the Main Central Thrust: an example from Tatopani-Kodari hot spring in central Nepal

Streaming potential coefficient and electrical conductivity have been measured in the laboratory as a function of KCI electrolyte conductivity for six crushed rock samples collected at the Main Central Thrust (MCT) zone, near the Tatopani­Kodari hot spring in Central Nepal. Surface conductivity values range from 0.11±0.07 to 1.19±0.13 mS/m and values of the inferred ζ potential vary from -16.3±0.2 mV to -41.2±1.0 mV. These experimental values are used to model the streaming potential coefficient and the rock resistivity as a function of permeability. The electric potential generated on sur face by the geothermal circulation at the MCT zone is then derived using a simple two-dimensional analytical calculation. The maximum expected anomaly depends on the values of poorly known parameters such as the permeability of the MCT but is estimated to be of the order of 20 mV, and in general tends to remain below 100 mV. Such anomalies, although they could reflect variations of crustal parameters associated with stress accumulation, are difficult to detect and do not appear as a promising possibility in the search for earthquake precursors.

Two-dimensional thermal modelling of the early tectonometamorphic evolution in central Himalaya

The Higher Himalayan Crystallines is a major structural feature of the Himalayan belt characterized by a typical collisional and metamorphic evolution from Eocene to Miocene. We use two-dimensional thermal modelling to examine the velocity and the duration of the two main phases of metamorphism in the Higher Himalayan Crystallines. Our calculations suggest that the Eohimalayan metamorphism is not a simple consequence of the burial of the Indian plate below the upper Himalayan wedge. The M1 metamorphic conditions (700 °C, 1 GPa) recorded in the central Himalaya between 37 and 32 Ma corresponds to a period of thermal relaxation of the Higher Himalayan Crystallines. During this period, part of the Himalayan convergence had taken place in the Himalayan wedge along south-vergent thrusting structures, located north and south of the future Main Central Thrust (MCT) zone. Relatively high temperature (∼600 °C) recorded in the MCT between 23 and 18 Ma (Neohimalayan metamorphism) may be explained by high heat production rates, ∼3 μW/m 3, in the upper Himalayan crust and high exhumation rates, greater than 3 mm/year, of the Higher Himalayan Crystalline rocks. By analogy with modern accretionary wedges, we propose that the Eohimalayan phase corresponds to the period of the formation of the Himalayan wedge through several internal and external thrusting. During the subsequent Neohimalayan phase, the Himalayan wedge reached a thermal equilibrium, leading to the localisation of the Himalayan convergence only along the MCT zone.

Pressure-temperature-time path discontinuity in the Main Central thrust zone, central Nepal: Comment and Reply

[Kohn et al. (2001)][1] presented P-T-t paths and ion microprobe Th- Pb monazite age data across the Main Central thrust zone along the Darondi River valley in central Nepal. Here, the structurally lowest garnets within the Main Central thrust zone grew with increasing P and T (loading), whereas