[1] The Ramgarh thrust is one of the major fault systems of the Himalayan thrust belt in Nepal and northern India. The Ramgarh thrust sheet is ∼0.2–2.0 km thick and can be traced along strike the entire length of the Himalaya in Nepal. The fault generally places the oldest Paleoproterozoic rocks in the Lesser Himalayan series upon younger Lesser Himalayan rocks or lower Miocene foreland basin deposits. Regional balanced cross sections suggest that the Ramgarh thrust had at least ∼120 km of initial south vergent displacement. Subsequently, the frontal part of the thrust experienced further slip as the roof thrust for a large duplex in underlying Lesser Himalayan rocks. Ramgarh hanging wall strata are greenschist-grade phyllite, quartzite, and augen gneiss, all of which locally exhibit phyllonitic and mylonitic fabrics that indicate a top-to-the-south sense of shear. Structural fabrics in the Ramgarh thrust sheet are generally parallel to the fabrics in rocks above and below the thrust sheet. Regional and local mapping of the Ramgarh thrust in Nepal demonstrates that the fault always places a hanging wall flat upon a footwall flat, except where local lateral ramps complicate its geometry. Similarly, the structurally overlying Main Central thrust always places a hanging wall flat in Greater Himalayan series rocks upon the regionally flat Ramgarh thrust sheet. These geometric relationships preclude kinematic and thermal models that elevate Greater Himalayan and lower Lesser Himalayan rocks along high-angle thrust ramps in the vicinity of the present traces of the Ramgarh and Main Central thrust faults. Instead, the corresponding footwall ramps for these thrusts must be located more than 100 km north of the current trace of the Main Central thrust. The present steep dips of the Ramgarh and Main Central thrust sheets can be attributed to tilting during emplacement of structurally lower thrust sheets within a large antiformal duplex that occupies most of the Lesser Himalayan zone. The Ramgarh thrust sheet overlaps a bed length of at least 100 km in lower Miocene foreland basin deposits, indicating that a significant amount of displacement on the thrust must have occurred after ∼15 Ma. Growth of the Lesser Himalayan duplex and additional slip on the frontal part of the Ramgarh thrust occurred from ∼12 to 5 Ma. The presence of a major greenschist-grade metasedimentary thrust sheet composed of Lesser Himalayan rocks directly below the Main Central thrust suggests that the famous “inverted metamorphism” in this region is a result of structural inversion. Similarly, the concept of a broad zone of intense shear strain related exclusively to emplacement of the Main Central thrust sheet is probably invalid in Nepal.

Structural geology and regional tectonic significance of the Ramgarh thrust, Himalayan fold-thrust belt of Nepal

Geodynamics and metallogeny of the eastern Tethyan metallogenic domain

For the period 1995–2000, the Nepal seismic network recorded 37 ± 8% fewer earthquakes in the summer than in the winter; for local magnitudes ML > 2 to ML > 4 the percentage increases from 31% to 63% respectively. We show the probability of observing this by chance is less than 1%. We find that most surface loading phenomena are either too small, or have the wrong polarity to enhance winter seismicity. We consider enhanced Coulomb failure caused by a pore-pressure increase at seismogenic depths as a possible mechanism. For this to enhance winter seismicity, however, we find that fluid diffusion following surface hydraulic loading would need to be associated with a six-month phase lag, which we consider to be possible, though unlikely. We favor instead the suppression of summer seismicity caused by stress-loading accompanying monsoon rains in the Ganges and northern India, a mechanism that is discussed in a companion article.

https://hal.archives-ouvertes.fr/hal-00310262/document

Seasonal modulation of seismicity in the Himalaya of Nepal

The Higher Himalayan Crystalline (HHC) in the Bhagirathi river section (India) on fieldwork reveals two extensional ductile top-to-N/NE shear sub-zones—the ‘South Tibetan Detachment System’ and the ‘Basal Detachment’—besides a preceding top-to-S/SW ductile shear. A top-to-N/NE brittle shear was identified as backthrusts from the HHC (except its northern portion) that occur repeatedly adjacent to numerous top-to-S/SW brittle shears as fore-thrusts. The northern portion of the HHC—the Gangotri Granite—exhibits infrequent total six extensional and compressional brittle shear senses. The backthrusts could be due to a low friction between the lower boundary of the HHC (i.e. the Main Central Thrust-Zone) and the partially molten hot rock materials of the HHC. Subduction of the Eurasian plate towards S/SW below the Indian plate more extensively in the Garhwal sector could be the second possible reason. Presence of two ductile extensional shear sub-zones may indicate channel flow (or several exhumation mechanisms) of the HHC in a shifting mode (similar to Mukherjee et al. in Int J Earth Sci 101:253–272, 2012). The top-to-S/SW extensional brittle shear exclusively within the upper (northern portion) of the HHC and a top-to-S/SW brittle shear within the remainder of it is a possible indicator of critical taper deformation mechanism. Thus, this work provides the field evidences of possibly both channel flow and critical taper conditions from a Higher Himalayan section, besides that by Larson et al. (Geol Soc Am Bull 122:1116–1134, 2010).

Higher Himalaya in the Bhagirathi section (NW Himalaya, India): its structures, backthrusts and extrusion mechanism by both channel flow and critical taper mechanisms

Meta-sedimentary rocks including marbles and calcsilicates in Central Dronning Maud Land (CDML) in East Antarctica experienced a Pan-African granulite facies metamorphism with peak metamorphic conditions around 830 ± 20 °C at 6.8 ± 0.5 kbar which was accompanied by the post-kinematic intrusion of huge amounts of syenitic (charnockitic) magmas at 4.5 ± 0.7 kbar. The marbles and calcsilicates may represent meta-evaporites as indicated by the occurrence of metamorphic gypsum/anhydrite and Cl-rich scapolite that formed in the presence of saline fluids with X
NaCl in the range 0.15–0.27. The marbles and calcsilicates bear biotite, tremolite and/or hornblende and humite group minerals (clinohumite, chondrodite and humite) which are inferred to have crystallized at about 650 °C and 4.5 kbar. The syenitic intrusives contain late-magmatic biotite and amphibole (formed between 750 and 800 °C) as well as relictic magmatic fayalite, orthopyroxene and clinopyroxene. Two syenite and two calcsilicate samples contain fluorite. Corona textures in the marbles and calcsilicates suggest very low fluid-rock ratios during the formation of the retrograde (650 °C) assemblages. Biotite in all but two syenite samples crystallized at log(f
H
2
O/f
HF) ratios of 2.9 ± 0.4, while in the calcsilicates, both biotite and humite group minerals indicate generally higher log(f
H
2
O/f
HF) values of up to 5.2. A few samples, though, overlap with the syenite values. Log(f
H
2
O/f
HCl) derived from biotite covers the range 0.5–2.6 in all rock types. Within a single sample, the calculated values for both parameters vary typically by 0.1 to 0.8 log units. Water and halogen acid fugacities calculated from biotite-olivine/orthopyroxene-feldspar-quartz equilibria and the above fugacity ratios are 1510–2790 bars for H2O, 1.3–5.3 bars for HF and 7–600 bars for HCl. The results are interpreted to reflect the reaction of relatively homogeneous magmatic fluids [in terms of log(f
H
2

 O
 /f
HF)] derived from the late-magmatic stages of the syenites with both earlier crystallized, still hotter parts of the syenites and with adjacent country rocks during down-temperature fluid flow. Fluorine is successively removed from the fluid and incorporated into F-bearing minerals (close to the syenite into metamorphic fluorite). In the course of this process log(f
H
2

 O
 /f
HF) increases significantly. Chlorine preferably partitions into the fluid and hence log(f
H
2

 O
 /f
HCl) does not change markedly during fluid-rock interaction.

Halogen-bearing minerals in syenites and high-grade marbles of Dronning Maud Land, Antarctica: monitors of fluid compositional changes during late-magmatic fluid-rock interaction processes

Copper mineralization around the Ahar batholith, north of Ahar (NW Iran): Evidence for fluid evolution and the origin of the skarn ore deposit

Petrochemical studies of granitoid rocks from the eastern part of Kumaun region suggest that the leading edge of India represents an active arc during Late Paleoproterozoic times. It has been observed that melt generation for granodiorite rocks from the eastern Almora Nappe and Chhiplakot klippe along with the Askot klippe was caused through a subduction-related process involving hydrous partial melting of a Paleoproterozoic amphibole- and/or garnet-bearing mafic source with the involvement of sediments from the subduction zone. The medium- to high-K basic rocks, common in subduction-related magmatic arcs, can also explain the generation of the high-K granodiorites of the Chhiplakot klippe. The augen gneisses from the eastern Almora nappe and Chhiplakot klippe along with the Askot klippe further show geochemical similarity with the associated granodiorites, suggesting there is a genetic linkage with one another.

Arc magmatism in eastern Kumaun Himalaya, India: A study based on geochemistry of granitoid rocks

Hydrothermal fluids are natural heated water solutions wherein variety of elements, compounds and gases may be dissolved. They are generated by diverse crustal and mantle geological processes including basinal fluid interaction, magmatic differentiation and mantle degassing. Mixing of the fluids of two different origins is often possible, and their solidified product may be economic. As the magmatic hydrothermal fluid is formed during the course of magmatic evolution from low volatiles and CO2-CH4 rich at primary basaltic magmatic phase to the saline water rich during granite/pegmatite formation, large number of constituents including the common S, Cl, F, Na, K, N2, metals and even REE may be found in it. The enrichment of halogen in the magmatic hydrothermal fluids promotes the partitioning of economically useful elements like Cu, Pb, Zn, W, Mn, Li, Rb, Sr and Ba from melt to the fluid phase. Salinity of this hydrothermal fluid, which varies from near 0 to >50 wt.% eq. NaCl, is a function of pressure. A wide range of immiscibility in the magmatic fluids results compositional modification of the gradually developing phases from nearly anhydrous melt to last residual low temperature water solution. Wet magma with higher mol% of water flows easily and exsolves water at low pressure regime. Influx of water in the heated rock suit can lower the liquidus temperature triggering melting at a lower temperature than the anhydrous melting. Fluid inclusions have been widely used to understand behaviour of ore forming fluids and the magmatic immiscibility such as silicate melt, H2O-CO2, hydrosaline melt, dense CH4 and sulphide-metal melt. Ore deposition is generally linked with the late stage of magmatic hydrothermal fluid, and its repeated pulses may lead to the formation of large ore bodies. In addition to saline aqueous fluid, the volatiles like H2S, CO2, SO2, SO4, HCl, B and F, are found as significant ore-depositing agents in magmatic-hydrothermal fluids. A hydrothermal fluid may dissolve economically useful elements or simply act as carrier for them. PVTX conditions obtained from fluid inclusions are vital for defining hydrothermal system and resulting ore mineralization, though such interpretation is largely based on the knowledge of their phase equilibria. The fluid process related to the epithermal Au deposits, porphyry type deposits, Malanjkhand Cu deposit and Balda-Tosham tungsten province, India, have also been discussed briefly.

Hydrothermal Fluids of Magmatic Origin

http://directory.umm.ac.id/Data%20Elmu/jurnal/J-a/Journal%20of%20Asian%20Earth%20Science/Vol19.Issue1-2.Feb2001/420.pdf

Rajesh Sharma

Papers

Copper mineralization around the Ahar batholith, north of Ahar (NW Iran): Evidence for fluid evolution and the origin of the skarn ore deposit

Arc magmatism in eastern Kumaun Himalaya, India: A study based on geochemistry of granitoid rocks

Hydrothermal Fluids of Magmatic Origin

Fluid events and exhumation history of the main central thrust zone Garhwal Himalaya (India)

Features and characterization of graphite in Almora Crystallines and their implication for the graphite formation in Lesser Himalaya, India