Two-mica and tourmaline leucogranites from the Everest–Makalu region (Nepal–Tibet). Himalayan leucogranite genesis by isobaric heating?

In the Higher Himalaya of the region from Cho Oyu to the Arun valley northeast of Makalu, the Miocene leucogranites are not hosted only in the upper High Himalayan Crystallines (HHC); a network of dykes also cuts the lower HHC and the Lesser Himalayan Crystallines (LHC).

The plutons and dykes are mainly composed of two-mica (muscovite+biotite±tourmaline±cordierite±andalusite±sillimanite) leucogranite, with tourmaline≤2.6% and biotite>1.5% modal, and tourmaline (muscovite+tourmaline±biotite±sillimanite ±garnet±kyanite±andalusite±spinel±corundum) leucogranite, with tourmaline>2.2% and biotite<1.5% modal.

Both leucogranite types were produced by partial melting in the andalusite–sillimanite facies series, under LP/HT conditions constrained by the occurrence of peritectic andalusite and cordierite. The geochemical features of the leucogranites suggest that tourmaline leucogranite was produced by muscovite dehydration melting in muscovite-rich metapelites at P350 MPa and T≥640°C, whereas two-mica leucogranite was produced by biotite dehydration melting in biotite-rich metapelites at P300 MPa and T≥660–710 °C.

Melting in fertile muscovite-rich metapelites of the top of both the HHC and LHC produced magmas which were emplaced at the same structural level in which they had been generated. Melting in the biotite-rich gneiss of both the HHC and LHC produced hotter magmas which were transported upwards by dyking and eventually coalesced in the plutons of the upper HHC. A similar process also produced a network of two-mica granite at the top of the LHC in the Ama Drime–Nyönno Ri Range northeast of Makalu.

The prograde character of leucogranite melt-producing reactions in the Everest–Makalu area suggests that, here, the generation of Miocene leucogranites took place in a regime of nearly isobaric heating following nearly adiabatic decompression.     

Features of volcanic tremors on Mt. Etna (Sicily) during the March–August 1983 eruption

The seismic analysis of the volcanic tremors preceding and accompanying the Etnean eruption of March–August, 1983 has shown a significant variation in the spectral content before the beginning of the eruption, the tremor peaks at 1.4 and 1.6 Hz — which might be associated with the feeding pipes of the NE crater (Schick et al., 1982a) — being the dominant feature of the spectra.A model of eruption mechanism is proposed where a feeder dyke would connect the NE crater with the effusive fracture.     

High-permeability layers for remediation of ground water; go wide, not deep

A nitrate-reactive porous media layer comprising wood particles with very high hydraulic conductivity (K approximately 1 cm/s) was used to successfully treat nitrate in a shallow sand-and-gravel aquifer in southern Ontario. Nitrate concentrations of 1.3 to 14 mg/L as N in the aquifer were attenuated to <0.5 mg/L as N in the reactive layer. Borehole dilution testing indicated that ground water velocities in the reactive layer, although variable, averaged five times higher than in the surrounding aquifer, suggesting that the layer was capturing ground water flow from deeper in the aquifer. The use of high-K reactive media opens up the possibility of installing permeable reactive barriers as horizontal layers in the shallow water table zone that do not necessarily have to penetrate the full depth of a contaminant plume to be effective. Model simulations show that the depth of capture of a high-K layer increases as the layer width in the direction of flow increases. Shallower emplacement could decrease barrier costs at some sites.     

Geology of the Higher Himalayan Crystallines in Khumbu Himal (Eastern Nepal)

In this paper we present the current geological knowledge and the results of new geological and structural investigations in the Cho Oyu-Sagarmatha-Makalu region (Eastern Nepal and Southern Tibet).The tectonic setting of the middle and upper part of the Higher Himalayan Crystallines (HHC) and Tibetan Sedimentary Sequence is characterized by the presence of pervasive compressive tectonics with south-verging folds and shear zones overprinted by extensional tectonics.In the middle and upper part of the HHC two systems of folds (F2a and F2b) have been recognized, affecting the S1 high-grade schistosity causing kilometer-scale upright antiforms and synforms. The limbs of these upright folds are affected by F3 collapse folds, top-to-SE extensional shear zones and extensional crenulation cleavages linked to extensional tectonics.The uppermost portion of the HHC and the lower part of the Tibetan Sedimentary Sequence is affected by two major extensional fault zones with a top-to NE direction of movement. The lower ductile extensional shear zone brings into contact the North Col Formation with the high grade gneisses and micaschists of the HHC. It is regarded as the main feature of the South Tibetan Detachment System. The upper low-angle fault zone is characterized by ductile/brittle deformation and thin levels of cataclasites and brings the slightly metamorphosed Ordovician limestones into contact with the North Col Formation. Extensional tectonics continued with the formation of E–W trending high angle normal faults.Three metamorphic stages of Himalayan age are recognized in the HHC of the Sagarmatha-Makalu region. The first stage (M1) is eclogitic as documented by granulitized eclogites collected at the top of the Main Central Thrust Zone in the Kharta region of Southern Tibet. The second event recorded in the Kharta eclogites (M2) was granulitic, with medium P (0.55–0.65 GPa) and high T (750–770°C), and was followed by recrystallization in the amphibolite facies of low pressure and high T (M3). The first event has also been recorded in the overlying Barun Gneiss, where M1 was followed by decompression under increasing T, the M2 event, producing the dominant mineral assemblage (garnet-sillimanite-biotite), and then by strong decompression under high T, with growth of andalusite, cordierite and green spinel. Also, changes in phase compatibilities suggest an increase in metamorphic temperature (T) coupled with a decrease in metamorphic pressure (P) in some of the thrust sheets of the MCT Zone.A telescoped metamorphic zonation ranging from the sillimanite to the staurolite and biotite zones is characteristic of the ductile extensional shear zone which is the lower part of the STDS in the Sagarmatha region. Evidence for decompression under increasing temperature, anatexis and leucogranite emplacement accompanying extension in the HHC was found throughout the whole ductile shear zone, particularly in metapelites both below and above the Makalu leucogranite and in micaschists of the staurolite zone.     

Geology and metamorphism of the Ladakh Terrane and Shyok Suture Zone in the Chogo Lungma – Turmik area (northern Pakistan)

Abstract

Metamorphism of the Askore Amphibolite, metabasaltic and metasedimentary medium-grade hornblendebearing schists at the northernmost portion of the Ladakh Terrane and of the Shyok Suture Zone, mainly a low-grade volcano-sedimentary series, has been studied in the area between the Chogo Lungma glacier and the Indus river halfway between Skardu and Rondu.

In the Askore Amphibolite the peak assemblage in the amphibolite facies defines the regional metamorphic foliation, and is overprinted by a later static recrystallization at comparable P-T conditions. In spite of similar peak temperatures (630–650°C), geobarometry based on amphibole composition reveals a marked difference between garnet – epidote – andesine amphibolites exposed just above the Main Mantle Thrust at the head of Turmik valley, which equilibrated at high pressures (about 10 kbar) in late Miocene (Tortonian), and biotite – epidote – oligoclase amphibolites outcropping at the mouth of Turmik valley, which equilibrated at pressures of c. 6 kbar before late Eocene (Priabonian).

The Dasu Ultramafite and other smaller lens-shaped bodies of low- to medium-grade metaperidotite separate the Ladakh Terrane from the Shyok Suture Zone. They are antigorite serpentinites, often with talc and magnesite, in which relict cumulitic structures are locally recognisable. The ultramafites may represent remnants of oceanic lithosphere separating the Ladakh-Kohistan island arc from the Asian plate, or they may be deep crustal rocks stripped from the basement of the arc.

The mostly greenschist-facies Shyok Suture Zone shows the lithology of a calc-alkaline volcano-sedimentary series. It is supposed to be a remnant of a back arc basin of early Cretaceous age, separating the arc from the southern margin of Asia. Chloritoid, kyanite and biotite have been found in individual thrust sheets occurring at different structural levels and totally subordinate in volume to very low- and low-grade rocks. Such sharp differences in mineral paragenesis, together with field evidence of local shear, suggest a complex internal structure for the Shyok Suture Zone. From the head of Chogo Lungma glacier to the Basha valley, close to the contact with the Karakorum Metamorphic Complex, the rocks of the Shyok Suture Zone record a late Miocene metamorphic event at medium pressures and temperatures. Thermobarometric and geochronological evidence suggests that this event can be related to the exhumation and thrusting of the Karakorum metamorphic core over the Shyok Suture Zone.     

Water level as the main driver of the alternation between a free-floating plant and a phytoplankton dominated state: a long-term study in a floodplain lake

This 10-year field data study explores the relevance of water level fluctuations in driving the shift from a free-floating plant (FFP) to a phytoplankton dominated state in a shallow floodplain lake from the Lower Paraná River. The multi-year natural flood pulse pattern in the Lower Paraná River drove the ecosystem regime from a FFP-dominant state during very high waters (1998–1999) to absolute phytoplankton prevalence with blooms of nitrogen fixing Cyanobacteria during extreme low waters (2008–2009). Satellite images support the observed changes over the decade and show the decrease of the surface lake area covered by FFP as well as the modification of the spectral firm in open waters, which documents the significant increases in phytoplankton chlorophyll a concentrations. We discuss the possibility that, despite a slow eutrophication in these highly vegetated systems, water level changes and not nutrients account for the shift from a floating macrophyte community to phytoplankton dominance. Cyclic shifts may occur in response to the seasonal floodpulse, but more strongly, as indicated by our results, in association to the extreme drought and flood events related to the El Niño Southern Oscillation, which is linked to discharge anomalies in the Paraná River.     

A test of transferability for landslides susceptibility models under extreme climatic events: application to the Messina 2009 disaster

A model building strategy is tested to assess the susceptibility for extreme climatic events driven shallow landslides. In fact, extreme climatic inputs such as storms typically are very local phenomena in the Mediterranean areas, so that with the exception of recently stricken areas, the landslide inventories which are required to train any stochastic model are actually unavailable. A solution is here proposed, consisting in training a susceptibility model in a source catchment, which was implemented by applying the binary logistic regression technique, and exporting its predicting function (selected predictors regressed coefficients) in a target catchment to predict its landslide distribution. To test the method, we exploit the disaster that occurred in the Messina area (southern Italy) on 1 October 2009 where, following a 250-mm/8-h storm, approximately two thousand debris flow/debris avalanches landslides in an area of 21 km² triggered, killing 37 people and injuring more than 100, and causing 0.5 M € worth of structural damage. The debris flows and debris avalanches phenomena involved the thin weathered mantle of the Varisican low to high-grade metamorphic rocks that outcrop in the eastern slopes of the Peloritani Mounts. Two 10-km²-wide stream catchments, which are located inside the storm core area, were exploited: susceptibility models trained in the Briga catchment were tested when exported to predict the landslides distribution in the Giampilieri catchment. The prediction performance (based on goodness of fit, prediction skill, accuracy and precision assessment) of the exported model was then compared with that of a model prepared in the Giampilieri catchment exploiting its landslide inventory. The results demonstrate that the landslide scenario observed in the Giampilieri catchment can be predicted with the same high performance without knowing its landslide distribution: we obtained, in fact, a very poor decrease in predictive performance when comparing the exported model to the native random partition-based model.     

Rainfall stochastic modeling for runoff forecasting

Rainfall fields estimation over a catchment area is an important stage in many hydrological applications. In this context, weather radars have several advantages because a single-site can scan a vast area with very high temporal and spatial resolution. The construction of weather radar systems with dual polarization capability allowed progress on radar rainfall estimation and its hydro-meteorological applications. For these applications of radar data it is necessary to remove the ground clutter contamination with an algorithm based on the backscattering signal variance of the differential reflectivity. The calibration of the GDSTM model (Gaussian Displacements Spatial-Temporal Model), a cluster stochastic generation model in continuous space and time, is herewith presented. In this model, storms arrive in a Poisson process in time with cells occurring in each storm that cluster in space and time. The model is calibrated, using data collected by the weather radar Polar 55C located in Rome, inside a square area of 132 × 132 km², with the radar at the centre. The GDSTM is fitted to sequences of radar images with a time interval between the PPIs scans of 5 min. A generalized method of moment procedure is used for parameter estimation. For the validation of the ability of the model to reproduce internal structure of rain event, a geo-morphological rainfall-runoff model, based on width function (WFIUH), was calibrated using simulated and observed data. Several rainfall fields are generated with the stochastic model and later they are used as input of the WFIUH model so that the forecast discharges can be compared to the observed ones.     

Rb/Sr geochronology of granites and gneisses from the Mount Everest region,Nepal Himalaya

Nineteen samples of granites, orthogneisses and paragneisses from the High Himalaya basement nappe(s) of the Mount Everest region have been dated by the Rb/Sr method. The post-metamorphic tourmaline leucogranites of the upper Imja Drangka (Nuptse, Lhotse Glacier) have high initial Sr⁸⁷/Sr⁸⁶ characteristic of an anatectic origin from crustal material. A whole-rock isochron age of 52 m. y. (Early Eocene) has been obtained for the samples from the granite body of Lhotse Glacier; apparently Sr isotopic homogenization was not reached throughout the much larger Nuptse granite. The granite precursor of the migmatitic orthogneisses from the upper Dudh Kosi valley has an age of 550 ± 16 m. y. (whole rock isochron) and a high initial Sr⁸⁷/Sr⁸⁶ ratio indicating its origin from an older basement complex. The Rb/Sr data on paragneisses from the south face of Lhotse do not define an isochron, possibly reflecting isotopic hetero-geneity in the sedimentary protoliths and incomplete homogenization during a late Precambrian metamorphism. All the mineral ages fall in the time span from 15 to 17 m. y. They represent cooling ages reflecting a regional phase of major uplift in the Middle Miocene and post-dating the peak of the Himalayan metamorphism which the data from the Mt. Everest region place in pre-Eocene times.