Tectonic control on exhumation and seismicity in the Garhwal‐Kumaun Himalaya,NW‐India

Despite similar geological and tectonic setting along the Himalayan orogen, distinct thermochronological/exhumational and seismicity variability exists between the Kumaun and the Garhwal regions of the NW‐ Himalaya. The processes responsible for such variability are still debated. To understand this, published thermochronological ages from several traverses across the Higher Himalayan Crystalline (HHC) and Lesser Himalayan Crystalline (LHC) have been correlated with the seismicity pattern in both Garhwal and Kumaun segments. The seismicity pattern coincides with the zone of rapid uplift and exhumation. The profiles of seismicity across the Kumaun and the Garhwal regions agree with the existence of the Main Himalayan Thrust (MHT) underlying the regions and reflect its geometry and architecture. Slip along the MHT is responsible for occurrence of seismicity on decade time‐scale and exhumation pattern on Myr time‐scale of the Himalaya.     

Is an onset vortex important for monsoon onset over Kerala?

Inter-annual variability in the formation of the mini warm pool [sea-surface temperature (SST)>30°C] over the south eastern Arabian Sea (SEAS) and its role in the formation of the monsoon onset vortex (MOV) has been examined using two independent SST data sets. The role of SST, convection, integrated columnar water vapour and the low-level jet in the setting up of the monsoon onset over Kerala (MOK) is examined. It is found that the MOV which forms over the SEAS region upsets the delicate balance between convection, buildup of moisture and strengthening and deepening of the westerlies over the SEAS that is needed for the setting up of the MOK. Thus, the formation over the SEAS of an MOV is not necessarily conducive for MOK. Furthermore, it is shown that a mini warm pool over the southeastern Arabian Sea is not a sufficient condition for the formation of an MOV because such a warm pool is present over this region during most of the years, but an MOV does not necessarily form over there.     

Radioactive quartz-pebble conglomerates from western margin of Bonai granite pluton,Sundargarh district,Orissa — A new find

Several radioactive quartz-pebble conglomerate (QPC) occurrences at the western margin of Archaean Bonai granite and overlying Iron Ore Group (IOG) rocks have recently been located over a total strike length of 8–10 km intermittently in a NE-SW to E-W trend with steep dips due north-west to north in parts of Sundargarh district of Orissa. The QPC samples have analysed up to 0.039% U₃O₈ and 0.035% ThO₂ with high concentration of Y (74 to 518 ppm), La(<100 to 880 ppm), Cr ( 126 to 633 ppm), Zr (137 to 1250 ppm) and Pb (31 to 581 ppm). Cellulose Nitrate (CN) film studies of few QPC samples indicated adsorbed uranium over goethite and infiltrated ferruginous material (limonite), secondary uranium as encrustation and fracture filling and discrete sub-rounded grains of monazite, zircon, allanite and rare xenotime in the matrix of QPC as radioactive phases. Higher content of Th over U, elevated concentration of Y and La in QPC eliminates the possibility of its low temperature product by epigenetic processes. Poor correlation of U with elements like Pb, Y, Zr, La and Cr can be explained due to surficial leaching of uranium from QPC after its deposition as reflected by adsorbed U over iron-oxides and low U/Th ratio in QPC in the area.     

Satellite detection of carbon monoxide emission prior to the Gujarat earthquake of 26 January 2001

NOAA AVHRR images have clearly shown anomalous changes in land surface temperature associated with earthquakes in the past two decades. Soon after the Gujarat earthquake of January 26, 2001, an anomalous increase in land surface temperature was inferred from MODIS satellite data a few days prior to the main earthquake event. The cause of such an anomalous change in surface temperature prior to the earthquake is attributed to many probable phenomena, but no definite cause has been identified. In the present study, changes of a complementary nature were found of land surface temperature associated with the emission of CO from the epicentral region. The observed changes on land and atmosphere associated with the Gujarat earthquake of 26 January, 2001, show the existence of strong coupling between land, atmosphere and ionosphere.     

Stable isotope (δ<Superscript>13</Superscript>C and δ<Superscript>15</Superscript>N) values of sediment organic matter in subtropical lakes of different trophic status

Lake sediments contain archives of past environmental conditions in and around water bodies and stable isotope analyses (δ¹³C and δ¹⁵N) of sediment cores have been used to infer past environmental changes in aquatic ecosystems. In this study, we analyzed organic matter (OM), carbon (C), nitrogen (N), phosphorus (P), and δ¹³C and δ¹⁵N values in sediment cores from three subtropical lakes that span a broad range of trophic state. Our principal objectives were to: (1) evaluate whether nutrient concentrations and stable isotope values in surface deposits reflect modern trophic state conditions in the lakes, and (2) assess whether stratigraphic changes in the measured variables yield information about shifts in trophic status through time, or alternatively, diagenetic changes in sediment OM. Three Florida (USA) lakes of very different trophic status were selected for this study. Results showed that both δ¹³C and δ¹⁵N values in surface sediments of the oligo-mesotrophic lake were relatively low compared to values in surface sediments of the other lakes, and were progressively lower with depth in the sediment core. Sediments of the eutrophic lake had δ¹³C values that declined upcore, whereas δ¹⁵N values increased toward the sediment surface. The eutrophic lake displayed δ¹³C values intermediate between those in the oligo-mesotrophic and hypereutrophic lakes. Sediments of the hypereutrophic lake had relatively higher δ¹³C and δ¹⁵N values. In general, we found greater δ¹³C and δ¹⁵N values with increasing lake trophic state.     

Statistical corrections of spatially interpolated missing precipitation data estimates

Spatial interpolation methods used for estimation of missing precipitation data generally under and overestimate the high and low extremes, respectively. This is a major limitation that plagues all spatial interpolation methods as observations from different sites are used in local or global variants of these methods for estimation of missing data. This study proposes bias‐correction methods similar to those used in climate change studies for correcting missing precipitation estimates provided by an optimal spatial interpolation method. The methods are applied to post‐interpolation estimates using quantile mapping, a variant of equi‐distant quantile matching and a new optimal single best estimator (SBE) scheme. The SBE is developed using a mixed‐integer nonlinear programming formulation. K‐fold cross validation of estimation and correction methods is carried out using 15 rain gauges in a temperate climatic region of the U.S. Exhaustive evaluation of bias‐corrected estimates is carried out using several statistical, error, performance and skill score measures. The differences among the bias‐correction methods, the effectiveness of the methods and their limitations are examined. The bias‐correction method based on a variant of equi‐distant quantile matching is recommended. Post‐interpolation bias corrections have preserved the site‐specific summary statistics with minor changes in the magnitudes of error and performance measures. The changes were found to be statistically insignificant based on parametric and nonparametric hypothesis tests. The correction methods provided improved skill scores with minimal changes in magnitudes of several extreme precipitation indices. The bias corrections of estimated data also brought site‐specific serial autocorrelations at different lags and transition states (dry‐to‐dry, dry‐to‐wet, wet‐to‐wet and wet‐to‐dry) close to those from the observed series. Bias corrections of missing data estimates provide better serially complete precipitation time series useful for climate change and variability studies in comparison to uncorrected filled data series. Copyright © 2013 John Wiley & Sons, Ltd.     

Geochemistry and geochronology of A-type Barabazar granite: Implications on the geodynamics of South Purulia Shear Zone,Singhbhum craton,Eastern India

The Barabazar granite, exposed at the northern margin of Singhbhum craton, Eastern India, occurs along the South Purulia Shear Zone (SPSZ) and is emplaced into the Palaeoproterozoic metapelites and felsic volcanics of Singhbhum Group. Geochemical, petrographical and geochronological studies on the Barabazar granite addressed in the work have wide implications on understanding the geodynamics of SPSZ during Palaeoproterozoic to Mesoproterozoic. Geochemically, Barabazar granite displays limited range of major oxides, alkali enrichment and highly fractionated features (SiO₂ > 75%; Eu/Eu* = 0.16–0.33; enrichment of K, Rb, Th, U and Nb; depletion of Ba, Sr, P and Ti). It is predominantly peraluminous (molar Al₂O₃/CaO+Na₂O+K₂O (A/CNK) =1.14–144) and contains abundant alkali feldspar, perthite, and minor plagioclase, biotite and accessory minerals. Geochemical and petrological data indicates that it is A-type granite, which formed in ‘Within plate granite’ tectonic set up. The Barabazar granite was emplaced at ca. 1771 Ma (Pb-Pb) in rift related environs and evolved by partial melting of stabilized lower/middle crust (initial ⁸⁷Sr/⁸⁶Sr = 0.7302 ± 0.0066 and μ₁ = 8.5 ± 0.5). Subsequently, the shear zone (SPSZ) developed during the closure of the riftogenic basin and was reactivated during the Grenvillian orogeny (Ca. 900–1300 Ma), resulting in rehomogenisation of the strontium isotopes and thereby yielding younger whole-rock Rb-Sr isotope age of c. 971 Ma for the Barabazar granite. Probably during this tectonic event, the Singhbhum craton (Southern India Shield) would have finally juxtaposed with Northern Indian Shield along Central Indian Tectonic Zone (CITZ) during the global Grenvillian orogeny.