Widespread anatexis was a regional response to the evolution of the Himalayan‐Tibetan Orogen that occurred some 30 Ma after collision between Asia and India. This paper reviews the nature, timing, ...duration and conditions of anatexis and leucogranite formation in the Greater Himalayan Sequence (GHS), and compares them to contemporaneous granites in the Karakoram mountains. Himalayan leucogranites and associated migmatites generally share a number of features along the length of the mountain front, such as similar timing and duration of magmatism, common source rocks and clockwise P–T paths. Despite commonalities, most papers emphasize deviations from this general pattern, indicating a fine‐tuned local response to the dominant evolution. There are significant differences in P–T–XH2O conditions during anatexis, and timing in relation to regional decompression. Further to that, some regions underwent a second event recording melting at low pressures. Zircon and monazite ages of anatectic rocks range between c. 25 and 15 Ma, suggesting prolonged crustal melting. Typically, a single sample may have ages covering most of this 10 Ma period, suggesting recycling of accessory phases from metamorphic rocks and early‐formed magmas. Recent studies linking monazite and zircon ages with their composition, have determined the timing of prograde melting and retrograde melt crystallization, thus constraining the duration of the anatectic cycle. In some areas, this cycle becomes younger down section, towards the leading front of the Himalayas, whereas the opposite is true in other areas. The relationship between granites and movement on the South Tibetan Detachment (STD) reveals that fault motion took place at different times and over different durations requiring complex internal strain distribution along the Himalayas. The nature and fate of magmas in the GHS contrast with those in the Karakoram mountains. GHS leucogranites have a strong crustal isotopic signature and migration is controlled by low‐angle foliation, leading to diffuse injection complexes concentrated below the STD. In contrast, the steep attitude of the Karakoram shear zone focused magma transfer, feeding the large Karakoram‐Baltoro batholith. Anatexis in the Karakoram involved a Cretaceous calcalkaline batholith that provided leucogranites with more juvenile isotopic signatures. The impact of melting on the evolution of the Himalayas has been widely debated. Melting has been used to explain subsequent decompression, or conversely, decompression has been used to explain melting. Weakening due to melting has also been used to support channel flow models for extrusion of the GHS, or alternatively, to suggest it triggered a change in its critical taper. In view of the variable nature of anatexis and of motion on the STD, it is likely that anatexis had only a second‐order effect in modulating strain distribution, with little effect on the general history of deformation. Thus, despite all kinds of local differences, strain distribution over time was such that it maintained the well‐defined arc that characterizes this orogen. This was likely the result of a self‐organized forward motion of the arc, controlled by the imposed convergence history and energy conservation, balancing accumulation of potential energy and dissipation, independent of the presence or absence of melt.
Mountain regions of the world face unprecedented climate-induced changes and associated sustainable development challenges. Retreating glaciers, degrading permafrost and rapid mass movements on the ...one hand and glacier-related disasters, on the other hand, are the sentinels of these phenomena. In this study, we focus our attention on the Makalu Barun region in the Nepal Himalaya, and characterize four main morphoclimatic zones, building on repeated field surveys and interpretation of remote sensing imagery. We distinguish four distinct zones: (i) extreme glacial zone; (ii) glacial zone; (iii) periglacial zone; and (iv) seasonally cold/warm humid zone. While extreme glacial zone is stagnant in its area, remaining three zones have been experiencing areal/location changes associated with changing climate, glacier extent and permafrost distribution. We describe dominant geomorphic processes and typical landforms of these zones in detail, highlighting the role of mass wasting processes and far-reaching process chains acting across distinct morphoclimatic zones. The study provides evidence of very dynamic landform evolution which indicates extreme geomorphological hazards in the Nepal Himalaya.
This paper describes the course of migration and expansion of small mammals in the Makalu Barun region influenced by the orogenetic uplift of the East Nepal Himalaya and climatically conditioned ...changes in the extent of morphogenetic zones from the Upper Pleistocene up to the present. The results of zoological and parasitological research are compounded with the knowledge of the dynamic development of landforms, which testifies to significant changes in the high-mountain environment during the Quaternary. The migration of Palearctic species of small mammals across the gradually emerging orographical barrier during the orogenesis of the High Himalaya was completely interrupted by the glaciation in the Upper Pleistocene. This extensive glaciation also excluded occurrence and survival of small mammals in the high-mountain valleys of the Makalu Barun region. Migration routes and the extension of the territory of small mammals remained open only in the periglacial zone of the Arun and Barun Khola valleys. Following the interstadial period of warmer and humid climate conditions were changed by the Late Glacial Maximum when small mammals were again pushed away from heavily glaciated valleys to the lower altitude periglacial zone. During the Holocene interglacial, the occurrence of fauna and flora in the high-mountain valleys depended on repeated spatial changes of periglacial and glacial morphoclimatic zones. Current biogeographical hazards are accentuated due to the rapid retreat of glaciers, the expansion of the periglacial morphoclimatic zone and the increased human impact in the High Himalaya.
The Main Central Thrust Zone (MCTZ) is a key tectonic feature in the architecture of the Himalayan chain. In the Arun valley of the eastern Nepal Himalaya, the MCTZ is a strongly deformed package of ...amphibolite- to granulite-facies metapelitic schist and granitic orthogneiss. This package is tectonically interposed between the underlying, low-grade, Lesser Himalaya sequences and the overlying, high-grade and locally anatectic, Higher Himalayan Crystallines (HHC). The MCTZ is characterized by a well documented inverted metamorphism from the Grt–Bt zone, across the Ky-in, St-in and -out, Kfs-in, Ms-out and Sil-in isograds. Partial melting with local occurrence of migmatitic segregations has been rarely reported from the highest structural levels of the MCTZ.
While it is widely accepted that thrusting along the MCT occurred during the Miocene, geochronological data constraining the timing of crustal anatexis in the upper portion of the MCTZ are still lacking. In order to understand the link between partial melting in the MCTZ and the Miocene activation of the MCT, we present the
P–
T–time evolution of a kyanite-bearing anatectic gneiss occurring at the highest structural levels of the MCTZ, along the Arun–Makalu transect (eastern Nepal).
Microstructural observations combined with
P–
T pseudosection analysis show that dehydration partial melting occurred in the kyanite-field. After reaching peak conditions at about 820
°C, 13
kbar, the studied sample experienced decompression accompanied by cooling down to 805
°C, 10
kbar, which caused
in situ melt crystallization. SHRIMP monazite and zircon geochronology provides evidence that the anatexis affecting the upper portion of the MCTZ occurred during Early Oligocene (∼
31
Ma). These results demonstrate that in the upper MCTZ, at least in the eastern Himalaya, crustal anatexis was earlier than, and not a consequence of, decompression linked to exhumation along the MCT.
The Makalu leucogranite in the eastern Nepal Himalaya is a multiphase intrusion forming the structurally highest foliation‐parallel sheets along the top of the Greater Himalayan Sequence. It is part ...of a chain of Miocene granites seen continuously along the length of the Himalaya and is composed of Grt + Tur + Ms ± Bt leucogranites but, unlike most other Himalayan granites, also locally contains coarse‐grained cordierite. The cordierite‐bearing leucogranite intrudes through and overlies lower sheets of “normal” tourmaline granites and represents the most recent phase of magmatism. Cross‐cutting feeder dykes channelled magma up from the source region within the sillimanite grade Barun gneiss to the upper sheet. Petrology shows evidence for muscovite dehydration melting (∼700°C) in the upper part of the Barun gneiss of the Greater Himalayan Sequence, which retains biotite, indicating that melting temperatures did not exceed 800°C. Secondary cordierite around garnet in these gneisses and the presence of cordierite in leucogranites record the last low‐pressure decompression phase of melting. P‐T determinations detail peak sillimanite grade metamorphism at 713°C/5.9 kbar, with a secondary cordierite overprint at 618°C/2.1 kbar; this P‐T transition lies wholly within the modeled melt field. Monazite, zircon, and xenotime geochronology links the metamorphism and the different leucogranites. The main phase of leucogranite production occurred from 24 to 21 Ma, while the most recent melting occurred in the cordierite leucogranite and the migmatitic Barun gneisses at 15.6 ± 0.2 and 16.0 ± 0.6 Ma, respectively. Pseudosections for the migmatitic Barun gneiss and cordierite leucogranite show conditions of final cordierite bearing melt crystallization at approximately 4 kbar and 700°C and two main phases of melting: one associated with muscovite dehydration melting and one associated with formation of cordierite. These data support the channel flow model for the Greater Himalaya where decompression melting was coeval with southward ductile extrusion of a partially molten layer of middle crust during the Early and Middle Miocene.
Identifying the location and nature of the Main Central Thrust Zone (MCTZ) is a major challenge in most of the Himalayan chain. As a contribution to clarifying this conundrum, in eastern Nepal a ...number of metapelite samples were selected for petrological study along a transect on the eastern flank of the Arun Tectonic Window. Both to the west and east of the study area, the Makalu and Kangchendzonga transects show metamorphic units characterized by a well-documented inverted metamorphism, with metamorphic grade increasing northward from lower (Lesser Himalaya) to higher (High Himalayan Crystallines) structural levels across the MCTZ. A detailed petrological study of the selected metapelite sequence allowed us to recognize three superimposed tectonometamorphic units, which are separated by cryptic and transitional metamorphic discontinuities. These units are characterized by different P–T evolutions, peculiar zoning styles of garnets and contrasting T/depth ratios. Specifically: (1) the structurally lowest sample shows a prograde P–T path characterized by an increase in both P and T, up to peak metamorphic conditions of 550°C and 0·65 GPa; (2) two structurally intermediate samples preserve relics of a prograde history characterized by heating and decompression up to peak metamorphic conditions of 600–650°C and 0·85–0·95 GPa; (3) the structurally highest samples consist of mostly unzoned minerals and assemblages, interpreted as chemically equilibrated, that do not preserve relics of their prograde metamorphic history. Peak metamorphic conditions of 655°C, 0·75 GPa (still inside the white mica stability field), and a minimum temperature of 790°C at 1·05 GPa (beyond the stability limit of white mica) have been determined for these samples. These data provide new constraints on the location, tectonic setting and metamorphic evolution of the Himalayan units in a hitherto poorly known area.
New apatite and zircon fission track (FT) data from the summit slopes of Everest and along the Barun, Arun, Dudh Kosi, and Kangshung valleys that drain the Everest and Makalu massifs cover a vertical ...sample transect of almost 8000 m of the Eastern Nepal Greater Himalayan Sequence (GHS). Apatite FT ages range from 0.9 ± 0.3 Ma to 3.1 ± 0.3 Ma in the GHS with ages increasing systematically with elevation. Apatite FT ages in the Everest Series and summit Ordovician limestones are much older, up to 30.5 ± 5.1 Ma. Zircon FT ages from the GHS range from 3.8 ± 0.4 Ma to 16.3 ± 0.8 Ma. The brittle exhumation rates calculated from these data show the GHS was exhumed, since ∼9 Ma, at an average rate of 1.0 ± 0.2 mm/a. Pliocene exhumation rates are higher at 1.7 ± 0.3 mm/a. These values are not significantly different from the estimate of ductile exhumation rates of 1.8 mm/a recorded by metamorphic minerals undergoing decompression between 18.7 and 15.6 Ma but are well below the values (up to 10 mm/a) used by thermomechanical models for ductile channel flow in the GHS. If representative of the GHS these models will therefore require further ‘tuning’. Higher exhumation rates in the Pliocene have also been observed in other parts of the Himalaya and points to a regional cause, likely increased erosion due to the onset of late Pliocene–Pleistocene glaciation of the high Himalaya.
Key Points
Fission track data reveals the rate and timing of exhumation
Reduction in exhumation is synchronous with cessation of channel flow
Tectonics, climate, and erosion all influence exhumation
In recent decades, many of the larger glaciers in the Himalaya and Andes that have experienced increased melting have become glacial lakes. Some of these lakes present a risk of glacial lake outburst ...floods that can unleash stored lake water and eroded debris, often causing enormous devastation downstream. Many of these new glacial lakes have formed in the Mt. Everest and Makalu Barun National Parks of Nepal, nine of which in the remote Hinku and Hongu valleys have been designated as “potentially dangerous” based on remote sensing analyses. Until recently, however, relatively little ground-based information was available for these lakes, including their physical characteristics, danger level, prospective downstream impacts in the event of an outburst, and mitigation methods appropriate and applicable to remote regions within the Nepal Himalaya. This paper describes three separate, interdisciplinary expeditions to the Hinku and Hongu valleys between 2009 and 2012 that were designed to close these information gaps. Each expedition combined remote sensing with field-based analyses, repeat photography, interviews with local people, bathymetric surveys, ground penetrating radar, and flood modeling. Eight of the “potentially dangerous” lakes surveyed were found to be stable, and one that had escaped mention in previous studies (L464) was found to contain a high risk of an outburst flood. In the data-deficient high mountain world, we suggest that the combined use of sophisticated remote sensing and modeling technologies with traditional, field-based methods can provide the most thorough understanding of glacial lakes possible at this time, including the actual risks that they pose as well as the most appropriate and community-based risk reduction strategies.
Like many developing countries, Nepal has adopted a community-based conservation (CBC) approach in recent years to manage its protected areas mainly in response to poor park-people relations. Among ...other things, under this approach the government has created new "people-oriented" conservation areas, formed and devolved legal authority to grassroots-level institutions to manage local resources, fostered infrastructure development, promoted tourism, and provided income-generating trainings to local people. Of interest to policy-makers and resource managers in Nepal and worldwide is whether this approach to conservation leads to improved attitudes on the part of local people. It is also important to know if personal costs and benefits associated with various intervention programs, and socioeconomic and demographic characteristics influence these attitudes. We explore these questions by looking at the experiences in Annapurna and Makalu-Barun Conservation Areas, Nepal, which have largely adopted a CBC approach in policy formulation, planning, and management. The research was conducted during 1996 and 1997; the data collection methods included random household questionnaire surveys, informal interviews, and review of official records and published literature. The results indicated that the majority of local people held favorable attitudes toward these conservation areas. Logistic regression results revealed that participation in training, benefit from tourism, wildlife depredation issue, ethnicity, gender, and education level were the significant predictors of local attitudes in one or the other conservation area. We conclude that the CBC approach has potential to shape favorable local attitudes and that these attitudes will be mediated by some personal attributes.
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