Garhwal Lesser Himalaya Research Articles

Provenance of the Proterozoic Lesser Himalayan siliciclastics, northwest Himalaya, India: Implications to terrain accretion and crustal evolution

We present petrographical and geochemical data for the Lesser Himalayan sedimentary succession from the Tons Valley section of the Garhwal Lesser Himalaya. The succession consists of the least metamorphosed shale-dominant Chandpur Formation overlain by coarser-grained sandstones of Nagthat Formation. Petrographic studies of the Jaunsar Group siliciclastics reveal that apart from quartz 84 ± 9%, feldspar forms 5 ± 4% (with PF > KF) and rock fragments (micro- to mega-crystalline chert, orthoquartzite, gneisses, phyllites, and schists) constitute the remaining 11 ± 8%. The occurrence and existence of the labile components like feldspars, and rock fragments such as phyllites and schists, indicate less weathering and a relatively short distance of transport and provenance from a clastic wedge with a recycled nature of sediments. The major element discriminant function analysis indicates that the Nagthat Formation samples plot in the quartzose sedimentary-felsic igneous field and the Lower Chandpur Formation plots in the intermediate igneous provenance, supporting derivation from an Upper Continental Crust-like source. The TiO2 vs Ni diagram indicates acidic source rocks. Trace and REE combined discriminant diagrams support significant mafic input and confirm mafic and ultramafic rocks in the source area. In the Eu/Eu* vs GdN/YbN plot, lower values of Eu/Eu* suggest that the igneous/metamorphic provenance was affected by intra-crustal differentiation. The GdN/YbN > 2 value indicates a contribution from older sources like the Archean, whereas Phanerozoic granites have a ratio <2. It gives a clear indication that the detrital derivation for the Jaunsar Group is a product of a mixed source, consisting of a major portion of the thick sediments and meta-sediments of the Aravalli-Delhi Supergroup, along with the basement granite and granite gneiss from the Banded Gneissic Complex (BGC) and Bundelkhand Granite (BG). During the deposition of the Jaunsar Group of sediments, there was no substantial change in the provenance; however, as evidence indicates, it can be inferred that as time passed and the erosion deepened and widened, the primary sources such as BG and BGC were exposed more and more gradually. Sedimentation continued in the NW sector all through the ages of the Columbian until the gradual breakup of the Rodinia supercontinent.

Geomorphological and Statistical Assessment of Tilt-Block Tectonics in the Garhwal Synform: Implications for the Active Tectonics, Garhwal Lesser Himalaya, India

Active tectonics is manifested in geomorphological features such as drainage basins and drainage patterns. Geomorphic parameters asymmetry factor (AF) and transverse topography symmetry factor (T) is calculated for 94 third order basins of the Garhwal synform to decipher the tilt-block tectonics based on remote sensing and geographical information system (GIS) techniques. The quantitative analysis of the AF suggests that all the 94 basins are asymmetric and gentle to steeply tilted, indicating active tectonics and early and late stage of development, respectively. The mean vector magnitude (θv) of T suggests the migration of the basin stream towards the south in most basins (60%), suggesting a unidirectional tilting of the tectonic block. The χ2 test for statistical significance indicates that the θv is significant for southern and northern limb basins. The χ2 test affirms that the third order basin position on either side of the main channel of the river basin influences the tilt direction. The regional tectonics suggests migration of the Lansdowne klippe towards the south, as the majority of third order basins show southward tilt. The study provides a quick appraisal of tilting in the tectonic blocks of active margins, such as in the Himalayas.

Seismically-Induced Soft Sediment Deformation Structures In and Around Chamoli, Garhwal Himalaya, India

Soft-sediment deformation structures were identified along Nandakini River valley near Chamoli village of Garhwal Lesser Himalaya. The deformation, in each outcrop, is restricted to a single stratigraphic layer bounded by undeformed flat-lying layers, eliminating the influence of slope failure. The structures are multiple liquefaction features like sand dikes, flame structures, pear-drop disturbance and downward warping of beds. The preferred interpretation is that the deformations resulted from earthquake-induced liquefaction and their existence adjacent to the Nandaprayag Fault can be related to episodic seismicity that occurred along this fault. However, more data and dating technique is needed to constrain the timing and distance to the causative seismic source. It is possible that more than one earthquake has effected the deformations.

Asymmetric development of the Himalaya: quantitative evidence from strain analysis in Kimin-Ziro-Tamen area, Arunachal Lesser Himalaya, India

The eastern segment of the Himalaya makes a quarter of the mountain belt. While its plate tectonic setting is well understood, the intensity of deformation along the thrusts is not well constrained. This knowledge gap has restricted a quantitative comparison of deformation and crustal shortening in the Eastern Lesser Himalaya with the Central and Western Lesser Himalaya. In this study field evidence, together with microstructural investigations, and two-dimensional (2D) finite strain analysis are used to decipher the deformation pattern in the Kimin-Ziro-Tamen area of the Eastern Lesser Himalaya. Present investigations reveal that micro-to-outcrop-scale structures such as plastically deformed quartz grains, crenulation cleavages, S-C fabric, pull-apart grains, pinch-n-swell structures, boudins and faults confirm that the rocks have been subjected to ductile and brittle-ductile phases of deformation with a regional top-to-south shear sense. The 2D finite strain estimated here is comparable to that in other parts of Eastern Lesser Himalaya, such as the Sikkim and Bhutan Himalaya. However, the strain estimated here is higher than in the Central and Western Lesser Himalaya, such as in the Central Nepal Himalaya, Almora Crystalline Zone, the Garhwal Lesser Himalaya and the Hazara-Kashmir Syntaxis in Pakistan. This provides quantitative proofs of greater deformation and crustal shortening in the Eastern Lesser Himalaya than the Central and Western Himalaya, and of the asymmetric development of Himalaya.

Landslide Studies Between Devprayag and Pali along National Highway-7, Tehri District, Garhwal Lesser Himalaya

Landslide Studies Between Devprayag and Pali along National Highway-7, Tehri District, Garhwal Lesser Himalaya

Refraction of micro-fractures due to shear-induced mechanical stratigraphy in a low-grade meta-sedimentary rock

Refracted markers, viz., cleavages and faults, across rock layers are well-documented structural features that develop as the structures propagate through layers of varying competency. We study the Palaeoproterozoic low grade meta-greywacke of the Rautgara Formation, Garhwal Lesser Himalaya, Uttarakhand, India. The focus is on the micro-fractures that cut the flaky-mineral rich cleavage (c-) and the porphyroclast-rich microlithon (m-) domains of the disjunctive foliation planes. Although the rock appears unsheared mesoscopically, in the micro-scale S–C fabric, shadow zones and tails of few quartz porphyroclasts exhibit a top-to-SW ductile shear. A mean kinematic vorticity number (Wm) of ~0.73 has already been determined from this rock. Our renewed study of thin-sections reveal fracture refraction patterns that match with the findings of various known analogue- and analytical models, viz., (i) higher competency contrast between c- and m-domains favours extension fractures over shear fractures, which develop more in the m-domains. Shear fractures dominate in the c-domains, (ii) the angle (ϴ) between fracture and the ‘layer normal’ is higher (>70°) inside the less-competent layers, (iii) a dominant simple shear in the brittle regime produces the P-planes at an angle to the primary shear Y-plane. In one such case, ϴ measured from thin-section for 15 successive sub-parallel c-and m-domains show that the most viscous m-domain is ~24 times more viscous than the lowest viscous c-domain. Additionally, out of the eight c-layers, the most viscous c-domain is 3.4 times more viscous than the least viscous c-domain. Similarly, out of the seven m-domains, the most viscous m-domain has a viscosity four times more than the least viscous m-domain. Knowing viscosity ratio of different layers in rocks will enable better analogue and analytical tectonic models. Our numerical models of general shear on linear elastic materials similar to the studied rock type, however, show that the rheological contrast does not influence the curvature of the shear-induced fractures at the boundaries between the quartz-rich sandstone and the mica-rich domains. Close-spaced impurities/notches may curve fracture domains across the layer boundaries producing a ‘false’ impression of fracture refraction. Moreover, the first principal strain axis (ε1) does not reorient across the layers except close to the notches. Nevertheless, the current study shows micro-scale development of mechanical stratigraphy under the influence of the ongoing tectonic deformation and quantifies the domain-wise competence contrasts with the help of refracted fractures.

Crustal anisotropy from shear-wave splitting of local earthquakes in the Garhwal Lesser Himalaya

SUMMARYCrustal anisotropy of the Garhwal Lesser Himalaya has been studied using local earthquake data from the Tehri seismic network. Earthquakes with magnitude (mL) up to 3, which occurred between January 2008 to December 2010, have been used for the shear wave splitting (SWS) analysis. SWS measurements have been done for steeply incident ray paths (ic ≤ 45°) to estimate the anisotropy fast axis orientation (ϕ) and the delay time (∂t). A total of 241 waveforms have been analysed, which yielded 209 splitting measurements, and 32 null results. The analysis reveals spatial and depth variation of ϕ and ∂t, suggesting complex anisotropic structure beneath the Garhwal Lesser Himalaya. The mean ∂t is estimated to be 0.07 ± 0.065 s with a mean depth normalized ∂t of 0.005 s km–1. We present the ϕ and Vs per cent anisotropy results by segregating these as a function of depth, for earthquakes originating above and below the Main Himalayan Thrust (MHT); and spatially, for stations located in the Outer Lesser Himalaya (OLH) and the Inner Lesser Himalaya (ILH). Earthquakes above the MHT sample only the Himalayan wedge, while those below the MHT sample both the underthrust Indian crust and the Himalayan wedge. Within the Himalayan wedge, for both OLH and ILH, the mean ϕ is oriented NE–SW, in the direction of maximum horizontal compressive stress axis (SHmax). This anisotropy is possibly due to stress-aligned microcracks controlled by the local stress pattern within the Himalayan wedge. The mean of normalized ∂t for all events originating within the Himalaya is 0.006 s km–1, which yields a Vs per cent anisotropy of ∼2.28 per cent. Assuming a homogeneous distribution of stress-aligned microcracks we compute a crack density of ∼0.0228 for the Garhwal Lesser Himalaya. At stations close to the regional fault systems, the mean ϕ is subparallel to the strike of the faults, and the anisotropy, locally, appears to be structure-related. For earthquakes originating below the MHT, in OLH, the mean ϕ orientation matches those from the Himalayan wedge and the normalized ∂t decreases with depth. This suggests depth localization of the anisotropy, primarily present within the Himalayan wedge. In the ILH, we observe large variations in the mean ϕ orientation and larger values of ∂t close to the regional fault/thrust systems. This is possibly a composite effect of the structure-related shallow crustal anisotropy and the frozen anisotropy of the underthrusting Indian crust. However, these cannot be segregated in this study.

Field documentation and genesis of the back-structures from the Garhwal Lesser Himalaya, Uttarakhand, India

Abstract Collisional fold-and-thrust belts are characterized by foreland-verging thrusts. Conversely, structures with hinterland-ward vergence, known as the back-thrusts, also exist. Strain intensification, critical taper deformation and the presence of thrust ramps generate back-thrusts. This study focuses on the exposure-scale brittle and ductile structures showing hinterland-ward vergence (back-structures) from a part of the Garhwal Lesser Himalaya, NW India, mainly along the Bhagirathi river section. In our field-traverse, back-structures were found at 31 locations. Towards the north, in the Outer Lesser Himalaya, the back-structures are located on the inverted limb of the Mussoorie Syncline (Group 1). The Tons Thrust is a south-dipping thrust (i.e. back-thrust). Hence, the Tons Thrust and nearby areas show intense back-structures (Group 2). In the Inner Lesser Himalaya, back-structures have been generated by shearing related to the folded Berinag Thrust (Group 3). The back-structures at and near the Main Central Thrust Zone (MCTZ) (Group 4) can be correlated with the presence of the Delhi–Haridwar Ridge. In this way, this study establishes the back-structures to be an integral part of the Garhwal Lesser Himalaya and provides the genesis of those structures by correlating them with the (local) tectonic settings. Supplementary material : Tables listing seismic events and the GPS coordinates of the field locations, and figures showing structures at these field locations are available at https://doi.org/10.6084/m9.figshare.c.4339784

Age and geochemistry of the Paleoproterozoic Bhatwari Gneiss of Garhwal Lesser Himalaya, NW India: implications for the pre-Himalayan magmatic history of the Lesser Himalayan basement rocks

Abstract The Bhatwari Gneiss of Bhagirathi Valley in the Garhwal Himalaya is a Paleoproterozoic crystalline rock from the Inner Lesser Himalayan Sequence. On the basis of field and petrographic analyses, we have classified the Bhatwari Gneiss into two parts: the Lower Bhatwari Gneiss (LBG) and the Upper Bhatwari Gneiss (UBG). The geochemical signatures of these rocks suggest a monzonitic protolith for the LBG and a granitic protolith for the UBG. The UBG has a calc-alkaline S-type granitoid protolith, whereas the LBG has an alkaline I-type granitoid protolith; the UBG is more fractionated. The trace element concentrations suggest a volcanic arc setting for the LBG and a within-plate setting for the UBG. The U–Pb geochronology of one sample from the LBG gives an upper intercept age of 1988 ± 12 Ma ( n = 10, MSWD = 2.5). One sample from the UBG gives an upper intercept age of 1895 ± 22 Ma ( n = 15, MSWD = 0.82), whereas another sample does not give any upper intercept age, but indicates magmatism from c. 1940 to 1840 Ma. Based on these ages, we infer that the Bhatwari Gneiss has evolved due to arc magmatism and related back-arc rifting over a time period of c. 100 Ma during the Proterozoic. This arc magmatism is related to the formation of the Columbia supercontinent. Supplementary material: LA–ICP-MS calibration and adjusted major element data used for computing the isometric log-ratio transformations are available at https://doi.org/10.6084/m9.figshare.c.4272158

Role of grain-size in phyllonitisation: Insights from mineralogy, microstructures, strain analyses and numerical modeling

Brittle Y- and P-planes exist in an exposure of greywacke in the Garhwal Lesser Himalaya, India. Although, Y-planes are well developed throughout, the P-planes are prominent only in some parts (domain-A), and not elsewhere (domain-B). To investigate why the P-planes developed selectively, the following studies were undertaken: 1. Clay-separated XRD analyses: clinochlore and illite are present in both the domains. 2. Strain analyses by Rf-φ method: it deduces strain magnitudes of ∼1.8 for the ductile deformed quartz grains from both the domains A and B. 3. Grain size analyses of quartz clasts: domain-A is mostly composed of finer grains (area up to 40,000 μm2), whereas domain-B consists of a population of coarser grains (area >45,000 μm2). A 2D finite element modeling of linear elastic material was performed using COMSOL software to investigate the control of grain-size variation on the generation brittle shear planes. The results of numerical modeling corroborate the known fact that an increase in grain-size reduces the elastic strain energy density. A broader grain-size distribution increases the effects of diffusion creep and resists the onset of dislocation creep. Thus, rocks with coarser grain population (domain B) tend to resist the generation of shear fractures, unlike their fine-grained counterpart (domain A).

Three-dimensional velocity structure around Tehri region of the Garhwal Lesser Himalaya: constraints on geometry of the underthrusting Indian plate

Three-dimensional velocity structure around Tehri region of the Garhwal Lesser Himalaya: constraints on geometry of the underthrusting Indian plate

Q Anisotropy in the region of Garhwal Himalaya

Empirical relations have been found out to describe the decay in amplitude with distance of s-waves in the region of Garhwal Himalaya. These functions results from the analysis of spectral amplitudes of three component digital records from 19 local earthquakes obtained from 12-station seismological network deployed in the Garhwal lesser Himalaya. The events lie in a magnitude range from ML 2 to ML 4.5, epi-central distance 5 km to 120 km and focal depth from 5 km to 50 km. To determine the variations of the quality factor Qin the direction of wave propagation (Q anisotropy), independent estimates have been made of the attenuation of SV- and SH-waves using vertically polarized and transversely polarized components, respectively. Frequency analyzed (1-12 Hz) and in the whole distance range, the frequency dependence of Q can be approximated as Qsh = 74.3 f 1.0 for the SH-wave and Qsv = 81.5 f 0.95 for the SV-waves. The small difference between SH and SV spectral decay, suggests that Q anisotropy is negligible. However, In the distance range of 5-50 km, Qsv tends to be significantly greater than Qsh at frequency &gt; 8 Hz. So in this distance range we observed Q anisotropy in the studied region. The dependence of Q value with frequency in this range is Qsh = 110 f 0.94 for SH- wave and Qsv = 134 f1.0 for SV-waves.

Water–rock interaction on the development of granite gneissic weathered profiles in Garhwal Lesser Himalaya, India

The development and sustainability of weathered profiles are very difficult in the Himalaya due to its complex lithology, tectonic history and fast erosion. Despite this, two weathered profiles namely WPa (weathered profile a) and WPb (weathered profile b) which have sustained erosion are developed on porphyry granite gneiss and granite gneissic lithology in Alaknanda valley of the Garhwal Lesser Himalaya. Systematic sampling of these two weathered profiles was done from bottom to top and they were chemically analysed to understand the elemental mobility in each profile. Major, trace and rare earth element studies show dissimilar behaviour with the advancement of weathering. In WPa profile, the CIA value of LAR (LAR) is 50 which reveals that the rock has not suffered any alteration but in WPb profile, the CIA value of LAR is 64 which indicates significant amount of chemical alteration. A–CN–K projection also exhibits similar behaviour. Further, the relative mobility of all the major and trace elements show variable elemental distribution in both the profiles. Enrichment of Mg, Fe, Ti, Al, Co, Ni, Zr, LREE and depletion of Na, K, P, Ca, Si, LILE and HFSE are observed in WPa profile; while the depletion of Na, K, Ca, P, Si, HREE and enhancement of Fe, Mn, Ti, Sc, Co, Zr, LREE are noticed in WPb profile. The rare earth elements also show a dissimilar mobilization pattern in both the profiles due to their strong dependency on lithology, and corresponding climate and tectonic interaction. Contrasting elemental mobility in both the profiles depict the major role in disparity of lithological characters and subsequent development of fractures produced by the major thrust system (Ramgarh thrust) which made an easy passage for rain water, thus causing the development of a chemically altered profile in the Lesser Himalayan region. Further, the present study infers the climate and tectonic milieu which is responsible for the development of such weathered profiles in Himalayan sector.

Attenuation of coda waves in the Garhwal Lesser Himalaya, India

Qc estimates for the Uttarkashi and the Chamoli regions of the Garhwal Lesser Himalaya have been obtained by analyzing the coda waves of 159 local earthquakes recorded during 2008 and 2009 employing a 12-station seismological network. Earthquakes around the Uttarkashi region are located in the epicentral distance range of 5.0 to 93.9 km, focal depth range of 1.63 to 42.13 km, and coda magnitude range of 0.2 to 2.9, whereas earthquakes around Chamoli region are located in the epicentral distance range of 19.8–109.2 km, focal depth range of 1.36 to 40.72 km, and coda magnitude range of 1.0 to 3.0. The coda waves of 30 s duration, recorded on 982 seismograms, have been analyzed in seven frequencies range centered at 1.5, 3.0, 6.0, 9.0, 12.0, 18.0, and 24.0 Hz for four to five lapse time windows (LTW) using the single backscattering model given by Aki and Chouet (J Geophys Res 80:3322–3342, 1975). Mean value of Qc estimates vary from 76 at 1.5 Hz to 2201 at 24.0 Hz for LTW range of 10–40 s and from 216 at 1.5 Hz to 3243 at 24.0 Hz for LTW range of 50–80 s (for the Uttarkashi region) and from 147 at 1.5 Hz to 2273 at 24.0 Hz for LTW range of 20–50 s and from 188 at 1.5 Hz to 2826 at 24.0 Hz for LTW range of 50–80 s (for Chamoli region). The Qc values thus obtained showed a clear dependence on frequency and LTW and frequency dependence Qc relationships, Qc = Q0fη, for LTWs that have been obtained as Qc = 57f1.20 (10–40 s), Qc = 97f1.07 (20–50 s), Qc = 116f1.03 (30–60 s), Qc = 130f1.03 (40–70 s), and Qc = 162f0.95 (50–80 s) for Uttarkashi region and Qc = 107f0.95 (20–50 s), Qc = 115f0.96 (30–60 s), Qc = 128f0.95 (40–70 s), and Qc = 145f0.95 (50–80 s) for Chamoli region.

Seasonal variation in the deformation rate in NW Himalayan region

We report GPS measurements of continuous observations from the multi- parametric geophysical observatory (MPGO) at Ghuttu, Garhwal Lesser Himalaya. Other than the evidence of secular motion depicting strain accumulation due to locking of the underneath seismically active detachment, measurements at Ghuttu show annual variation of ±4 mm on horizontal component. Such variations are more prominent in the north coordinate and do not directly correlate with the meteorological parameters such as variations in rainfall, water table, and atmospheric pressure measured at the MPGO observatory. These variations are also not the artefact of data processing and network. They correlate with the water load storage in the Ganga plains, with minimum in dis- placement coinciding with the maximum storage of water in Ganga plains immediately after the monsoon and vice versa. Such variations also appear to cause annual variation in the low-magnitude earthquake frequency in the Himalayan region, being relatively more in the winter period.

Soft-sediment deformation structures from the Paleoproterozoic Damtha Group of Garhwal Lesser Himalaya, India

Soft-sediment deformation (SSD) structures are observed in the Paleoproterozoic Damtha Group in the Garhwal Lesser Himalaya for a lateral extent of over 110km in comparable litho-sections. The SSD structures are primarily observed in medium- to fine-grained sandstones, siltstones and shale at sand–mud interfaces that were deposited in an inter-tidal to supra-tidal setting of the shallow marine platform. The SSD structures are conspicuously absent in horizons within facies dominated by a single grain size. The SSD structures include load structures, namely drops, pseudonodules and ball-and-pillow structures, along with contorted beds, slumps, cusps and water-escape structures of varying geometry and dimensions. The SSD structures are invariably bounded by undeformed beds and their dimensions depend on the thickness of bed affected by liquefaction. The observed SSD structures show distinct similarities with liquefaction-induced deformation structures in geological records and those produced in the laboratory under reverse and normal density gradient conditions. The recurring nature of SSD structures in the stratigraphic column over large spatial distances points toward a trigger mechanism with recurrent activity that affected the basin on a regional scale such as prolonged rifting of the Damtha basin during the Paleoproterozoic. This would have caused frequent earthquakes and shakes of the basin floor, producing recurrent SSD structures. In view of the similarity with seismites and absence of a viable alternative, rift-related seismicity is preferred as the trigger mechanism for the observed SSD structures in the Damtha Group.

Petrography, geochemistry and regional significance of crystalline klippen in the Garhwal Lesser Himalaya, India

Uphalda gneisses (UG) is a crystalline klippe located near Srinagar in Garhwal Himalaya. These gneisses are compared with Debguru porphyroids (DP) (≈Ramgarh group) of Garhwal–Kumaun Himalaya and Baragaon mylonitic gneisses (BMG) of Himachal Himalaya. Petrographic study reveals that the deformation of UG was initiated at higher temperature (above 350°C) and continued till lowering of temperature and deformation led to the mylonitization. Geochemically, these granitic gneisses (UG, DP and BMG) exhibit similar composition. Features such as high molecular A/CNK value (>1), presence of normative corundum and absence of normative diopside, enhanced Rb/Sr, Rb/Zr ratios, enrichment of Th and containing rounded zircons support their crustally-derived S-type granitic nature. The linear plot in major oxides is interpreted in terms of fractional crystallization processes. Mantle normalized multi-element spider diagram of UG illustrates depletion of Ba, Nb, Sr, P and Ti and enrichment of Th and show similarities with DP and BMG. Similarities were observed in lithology, petrographic characters and chemical composition of UG, DP, BMG and Ulleri augen gneisses (Nepal). Comparison with the rocks of Higher Himalayan crystallines (≈Vaikrita), suggests that these rocks (UG) are not transported from Higher Himalaya as understood earlier. This study however proposes that, these gneissic bodies represent an older basement occurring as a tectonic sliver which emplaced within the cover sequence as wedges at different structural levels. This is a regional phenomena observed throughout the Lesser Himalayan region.

Seasonal and daily variation of radon at 10m depth in borehole, Garhwal Lesser Himalaya, India

Mostly accepted and widely reported radon (Rn(222)) measurements, a tool for earthquake precursor research, is a part of multi-parametric geophysical observation in the Garhwal Lesser Himalaya for earthquake related studies. Radon is being recorded continuously at an interval of 15 min at 10 m depth in a 68 m deep borehole. Three years high resolution 15 min data at 10 m depth shows a complex trend and has a strong seasonal effect along with some diurnal, semi-diurnal and multi-day recurring trends. A well-defined seasonal pattern is prominent with a high emanation in summer and low values in winter accounting for about a 30% decrease in count values in winter when the atmospheric temperature is very low at this station located 1.90 km above mean sea level. Diurnal, semi-diurnal and multi-day trends in this time-series are mainly observed during April-May and October-November. This is the period of spring and autumn when there is a high contrast in day-night atmospheric temperature. Hence the high fluctuation in Rn concentration is mainly caused by the temperature contrast between the air-column inside the borehole and the atmosphere above the earth's surface.

Landslide susceptibility zonation mapping and its validation in part of Garhwal Lesser Himalaya, India, using binary logistic regression analysis and receiver operating characteristic curve method

A landslide susceptibility zonation (LSZ) map helps to understand the spatial distribution of slope failure probability in an area and hence it is useful for effective landslide hazard mitigation measures. Such maps can be generated using qualitative or quantitative approaches. The present study is an attempt to utilise a multivariate statistical method called binary logistic regression (BLR) analysis for LSZ mapping in part of the Garhwal Lesser Himalaya, India, lying close to the Main Boundary Thrust (MBT). This method gives the freedom to use categorical and continuous predictor variables together in a regression analysis. Geographic Information System has been used for preparing the database on causal factors of slope instability and landslide locations as well as for carrying out the spatial modelling of landslide susceptibility. A forward stepwise logistic regression analysis using maximum likelihood estimation method has been used in the regression. The constant and the coefficients of the predictor variables retained by the regression model have been used to calculate the probability of slope failure for the entire study area. The predictive logistic regression model has been validated by receiver operating characteristic curve analysis, which has given 91.7% accuracy for the developed BLR model.

Frequency and polarization characteristics of geomagnetic induction response of Garhwal Lesser Himalaya conductivity anomaly

A network of fluxgate magnetometers was operated over Garhwal Lesser Himalaya Seismic Belt (GLHSB) in Uttarkashi area during 1987–1990. This study delineated a narrow conductive zone south of MCT and named as Garhwal Lesser Himalaya Conductivity Anomaly (GLHCA). Pseudo-sections of |Z/H| for a traverse across GLHCA implicate that the top of the anomaly is within 10 km depth and mostly of 2D nature. The tectonic formation associated with the anomaly is found to be polarization sensitive and the polarization characteristics show that the anomaly is not parallel to Main Central Thrust (MCT) but inclined by about 10°–30° in anti clockwise direction. The frequency and polarization characteristics of the induction response of GLHCA are discussed in this paper.