Terrain changes, caused by the 15-17 June 2013 heavy rainfall in the Garhwal Himalaya, India: A case study of Alaknanda and Mandakini basins.

Exceptional early high monsoon rains between 15 and 17 June 2013 combined
with discharge from snowmelt water caused widespread floods in every major
river of the Garhwal Himalaya. This catastrophic event triggered widespread
landslides and devastation in the region, affecting the movement of the
people that led to stranding of pilgrims in various pilgrimage routes. This
event caused many casualties and irreparable damage to the infrastructures
and property in the Garhwal Himalaya. A large volume of debris was deposited
in Kedarnath town (3.9 x 106 m3), and a huge amount of debris was removed
from Rambara and surrounding areas (2.6 x 108 m3). The study also found that
villages like Lambaghar, Bhyundar (Alaknanda River Valley), and Rambara
(Mandakini River Valley) were completely washed away, leaving no trace of
earlier settlement. Govindghat and Pulna villages in the Alaknanda River
Valley were also badly damaged. Approximately 0.3 x 106 and 0.72 x 106 m3 of
debris was deposited, respectively. Similarly in the Mandakini Valley,
Kedarnath and Sonprayag towns were also badly damaged and ~3.9 x 106 and
~1.4 x 106 m3 of debris was deposited in the area, respectively.
Simultaneously, the moraine-dammed Chorabari Lake breached releasing ~6.1 x
105 m3 of water with an average rate of ~1429 m3/s (discharge of lake). The
towns of Pandukeshwar in the Alaknanda Valley and Gaurikund in the Mandakini
Valley were partially damaged. However, no evidence of such magnitude of
destruction was reported from the Yamuna River Valley during the same
period. This catastrophic event changed the landscape in many parts of
Uttarakhand, making the whole region more fragile and vulnerable. A disaster
of such magnitude was perhaps not witnessed by the region for at least the
last 100 years.

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First evidence of denitrification vis-à-vis monsoon in the Arabian Sea since Late Miocene

Oxygen minimum zones (OMZ) is the regions of oxygen deficient (O2 < 20 μM) water located in the tropical oceans, which have been proposed to expand in the present scenario of global warming. OMZs play a significant role in producing N2O (a powerful greenhouse gas through the process of denitrification), when the dissolved O2 levels fall below 1 µΜ. A perennial OMZ develops between 150 and 1000 m water depth in the Arabian Sea due to various natural factors such as high surface water productivity and reduced ventilation. The anoxic centers of these OMZs occupy only ~0.8% of the world ocean but are responsible for the highest production of N2 through denitrification (~35% of the global production) out of which the Arabian Sea contributes the largest proportion (~17% of global N2 production).

This study give a synoptic view of long-term evolution of OMZ spanning the past several million years, from the Eastern Arabian Sea. The squeeze cake samples from Site U1456 (sedimentation rate ~10 cm kyr -1) in the Eastern Arabian Sea, were analyzed during the IODP Expedition 355.

The long-term OMZ variability and its coupling with surface water productivity were established by analyzing multiple isotopic and geochemical proxies viz. δ 15N, δ 13C, TOC, TN, and C/N ratio of sedimentary organic matter (SOM).

Our record from Site U1456 spans ~10.2 to 0.03 Ma, but includes several hiatuses dated to ~8.2–9.2 Ma, ~3.7–5.4 Ma, and ~1.6–2.2 Ma. Nevertheless, we interpret that surface water productivity in the Eastern Arabian Sea was low from 10.15 Ma to 3.2 Ma as evident from uniformly low values of TOC and TN.

During this period, the δ 15N did not reach the threshold value (~6‰) indicative of denitrification. This implies that neither the surface water productivity (TOC, TN) nor the OMZ intensity supports any major intensification in SAM strength from ~10 to ~3.2 Ma, which is also documented in the different regions (the South China Sea, the Northern Arabian Sea and the Bay of Bengal). We find that the SAM was weak at ~10 Ma indicating that East Asian Monsoon (EAM) and SAM varied in consonance, without any apparent time lag, on tectonic timescale. At around 8 Ma, δ 15N values vary between 3.7‰ to 5.8‰, i.e., the OMZ was not intense enough to cause denitrification and the surface water productivity was diminished, which implies that SAM did not intensify at ~8 Ma. During the study period, for the first time, the OMZ intensified to the level that denitrification takes place was at ~3.2–2.8 Ma. Recently investigated that the role of the Tibet Plateau in affecting SAM, and found that it simply acts as a physical barrier for northerly cool, dry winds. Its role as an elevated heat source is of secondary importance in affecting the SAM. EAM dynamics is also affected by the Tibet Plateau, which is located in the path of subtropical jet streams. The increase in both the EAM and SAM during ~3.6–2.6 Ma could have resulted in the increased weathering and organic carbon burial, as evident by higher TOC leading to atmospheric CO2 drawdown that would have possibly contributed to Northern Hemisphere Glaciation (NHG) at 2.7 Ma. Thereafter, from 2.8 Ma to ~1.0 Ma, δ 15N values as well as the surface water productivity declined in parallel, indicating relatively weaker SAM. Previous studies also reported the weakened EAM and SAM after ~2.6 Ma, confirming our results, which coincides with the onset of NHG. Finally, the OMZ reached its modern strength, i.e., denitrification became a permanent feature, at about ~1.0 Ma closely following the enhanced surface water productivity. It implies that SAM intensified from ~1.0 Ma as reported in earlier studies viz. the enhanced sedimentation rate in the Indus Fan, the increased chemical weathering from the Bengal Fan and the South China Sea, the rise of magnetic susceptibility and mean sediment flux from the Indian Ocean.

Scientists of the Institute analyzed the strong motion records of recent
Gorkha Nepal earthquake (Mw 7.8), its strong aftershocks and seismic events
of Hindukush region for the estimation of source parameters. The Mw 7.8
Gorkha Nepal earthquake of April 25, 2015 and its six aftershocks of
magnitude range 5.3-7.3 have been recorded at Multi-Parametric Geophysical
observatory (MPGO), Ghuttu, Garhwal Himalaya (India) > 600 km west from the
epicenter of main shock of Gorkha earthquake. The acceleration data of eight
earthquakes occurred in the Hindukush region also recorded at this
observatory which is located > 1000 km east from the epicenter of Mw 7.5
Hindukush earthquake on October 26, 2015. The shear wave spectra of
acceleration record are corrected for the possible effects of anelastic
attenuation at both source and recording site as well as for site
amplification. The strong motion data of six local earthquakes is used to
estimate the site amplification and the shear wave quality factor (Qβ) at
recording site. The frequency dependent Qβ(f) = 124 f 0.98 is computed at
Ghuttu station by using inversion technique. The corrected spectrum is
compared with theoretical spectrum obtained from Brune's circular model for
the horizontal components using grid search algorithm. Computed seismic
moment, stress drop and source radius of the earthquakes used in this work
range 8.20x1016 to 5.72x1020 Nm, 7.1 to 50.6 bars and 3.55 to 36.70 km,
respectively. The results match with the available values obtained by other
agencies.

Geochemistry of the mafic xenoliths from the Kinnaur Kailash Granite, Baspa valley, Himachal Pradesh, India

Mafic enclaves occur in a variety of geologic and tectonic settings and play an important role in the crustal evolutionary processes.  The geochemistry of the mafic xenoliths from Baspa valley of Himachal Pradesh, India has been investigated to characterize their protoliths on the basis of immobile elements, especially trace elements including REE.  The mafic xenoliths occur within the Kinnaur Kailash Granite (KKG). They are elongated in shape with commonly 5-10 cm length, although a few of them attain surface dimensions of even 20-100 cm. These xenoliths are fine-grained and massive to foliated, sometimes with thin felsic stringers parallel to layering. Contact between host granite and mafic xenolith is sharp. They are comprised of clinopyroxene, hornblende, plagioclase, quartz, ilmenite, sphene, ±orthopyroxene, ±garnet.  Hornblende constitutes the dominant mineral of the mafic xenoliths, it occurs as a well-developed matrix mineral and as a cleavage-less massive retrograde mineral around clinopyroxene. Mineral chemistry of the hornblende shows that they are of magnesio-hornblende, ferro-hornblende to ferro-tschermakite variety. Orthopyroxene when present occurs as both discrete grains and microscopic needles exsolved parallel to prismatic cleavage of the clinopyroxene host. Garnetiferrous mafic xenoliths display coronae of garnet around plagioclase and clinopyroxene, and of sphene around ilmenite. The most common retrograde mineral is hornblende, retrogression is distinctly observed as corona textures such as, garnet around clinopyroxene and/or plagioclase, sphene around ilmenite, and hornblende around clinopyroxene.

Their geochemistry show that they have tholeiitic nature with basaltic composition. Compositionally, they range from ‘depleted’ to ‘enriched’ MORB as observed on the binary diagrams of Ti vs V and Zr vs Ti and on ternary diagrams of Zr-Ti-Y and Th-Zr-N.  Likewise, they match with various enriched or ‘transitional’ MORB types as evident from their Zr vs Nb binary plot. Their enriched character when compared with N-MORB, E-MORB and OIB rocks on chondrite and primordial mantle normalized plots reveals that it is intermediate to that of E-MORB and OIB. The geochemistry of the rocks suggest that the enriched components are probably derived by melting of a mantle source with E-MORB or OIB rather than due to the crustal contamination. The study carried out emphasize that the mafic xenoliths have developed in rift environment, and that they are not volcanic rocks of island arc related to subduction tectonics. It is visualized that the mafic xenoliths were formed as cumulate rocks from the tholeiitic magmas that were rising to lower crust levels in a rift environment, which at a later stage got entrapped as restitic material in the host Kinnaur Kailash Granite formed in a collision environment, and propose a change of regime from rift related to collision environment prior to Palaeozoic period.

Summarizing the overall implications of the studies carried out on the mafic xenoliths of Baspa valley of Himachal Pradesh, it can be suggested that, the region under study had experienced change of regime from rift environment to collision environment prior to Paleozoic period. The mafic xenolith were formed as cumulate rocks from the tholeiitic magmas that were rising to lower crust levels in a rift environment, which at latter stages occurred as restitic material within the host Kinnaur Kailash Granite (KKG) that were formed in a collision environment during Paleozoic times. However, the enigma of age of mafic xenolith from Baspa valley can be resolved only by dating them.  The geochemical comparison of mafic xenolith under study with metabasites from other parts of Himalaya further strengthen our view of mafic xenolith formation in a rift environment.

The summary and the highlights of the work presented in a paper by Dr Paramajeet Singh (WIHG) and Dr R.C. Patel (Kurukshetra University, Kurukshetra) are as follows:

Title of the paper:    Post-Emplacement kinematics & exhumation history of the Almoraklippe of the Kumaun-Garhwal Himalaya, NW-India: revealed by fission track thermochronology

Journal Name:International Journal of Earth Sciences

Current Status of the publication: In press

Summary:Tectonically transported crystalline thrust sheet over the Lesser Himalayan meta-sedimentary (LHMS) zone along the Main Central Thrust (MCT) are represented by Almora, Baijnath, Askot and Chiplakot crystalline klippen. The Almora-Dadeldhuraklippe in the Kumaun-Garhwal and western Nepal Himalaya is the witness and largest representative of these crystalline klippen, south of MCT. Here, we investigate the post-emplacement kinematics and exhumation history of the Almoraklippe.  The newly derived Zircon Fission Track (ZFT) ages combined with published Apatite Fission Track (AFT), 40Ar-39Ar ages from the Almora-Dadeldhuraklippe and Ramgarh thrust sheet to quantify the temporal variation in cooling ages and exhumation rates. New ZFT cooling ages from Almoraklippe range between 13.4 ± 0.6 and 21.4 ± 0.9 Ma. The linearly decreasing age trend from SAT to NAT with in Almoraklippe with youngest ZFT age (~ 14 Ma) close to the North Almora Thrust (NAT) in its hanging wall suggests rapid uplift close to the NAT due to its reactivation as back thrust.  The transient exhumation rates  calculated using 1-D thermal modelling of Almoraklippe agrees that the erosion rate was rapid (0.58 mma-1) close to the NAT in its hanging wall and relatively slow (0.31 mma-1) close to the SAT in its hanging wall during 15-11 Ma. It is interpreted that the fission track ages and transient exhumation rate pattern of crystalline klippen show a dynamic coupling between tectonic and erosion processes in the Kumaun-Garhwal Himalaya. However, the tectonic processes play the major role in controlling the exhumation pattern.

Research Highlights:

  • FT data from the Almoraklippe of Kumaun region allows us to constrain the post-emplacement kinematics and exhumation history from early Miocene to late Pliocene.
  • The obtained Zircon Fission Track ages data clearly indicate timing of the development of the Lesser Himalayan Duplexes (LHD) in the Lesser Himalayan Sequences (LHS) and Kasun Thrust (KT) in the Alomraklippe.
  • It has been concluded that the significant jumps in the Apatite Fission Track ages across the Ramgarh Thrust (RT), KT and North Almora Thrust (NAT) can be linked with the reactivation of these major thrust.
  • The 1-D thermal modelling exhumation rates also indicates the spatial-temporal variation in exhumation rates of KT-NAT block and SAT-KT block.
  • Observation shows interaction between spatial and temporal variations in tectonic and exhumation rates within the Almoraklippen of Kumaun Himalaya, NW-India and support that the geometry of crustal scale thrust/faults and their associated kinematics control the topography through uplift and exhumation pathways of rocks.
  • Based on the obtained Fission Track age results and thermal modelling, it has been concluded that tectonics have control over the post-emplacement kinematics and exhumation with the Almoraklippen since middle Miocene.