Elevated equilibrium geotherm in stable continental shield : evidence from integrated structural, hydrological and electromagnetic studies on non-volcanic hot springs in the Eastern Ghats Belt, India

Field data: These data were collected by us during fieldwork, following which some data was generated after analysis in the laboratory. The study area is in and around Khurda district, Odisha, India. Water samples were collected from the Atri and Tarbalo hot springs, and from tube- and dug-wells in the surrounding area. Structural data was collected from the exposures in and around these two hot springs, with strike and dips of penetrative fabrics at the localities shown in Fig. 2 were measured using a brunton. Mineral chemical data for monazite: Major and trace element analyses of the monazite grains, specifically for the U, Th and Pb content, were carried out on a Cameca SX-100 Electron Probe Micro Analyzer (EPMA), in IIT Kharagpur which is equipped with four wavelength dispersive spectrometers. During monazite spot analysis, LaB6 electron source was used with an acceleration voltage of 20kV, beam current of 150 nA and beam size of 1µm. The following natural samples were used as standards: vanadinite for Pb, ThO2 for Th; UO2 for U, apatite for Ca and P, yttrium aluminum garnet for Y; hematite for Fe; corundum for Al and Th-glass for Si; and synthetic glass containing rare earth elements (REE - 4 wt %) for La, Ce, Pr, Nd, Sm, Gd, Dy and Ho. Water Geochemistry: For the water samples, field parameters like temperature, EC (Electrical Conductivity), alkalinity, pH, TDS (Total Dissolve Solid) and DO (Dissolved Oxygen) of the thermal and non-thermal waters were measured directly in the field. The temperature of the water was measured by using a digital thermometer while the pH, EC, TDS and DO were recorded by using multiparameter kit (HANNA). Alkalinity of the water was determined by titration using HCl acid and 2-3 drops of phenolphthalein indicator. Water samples were collected after filtering into high density polyethylene (HDPE) bottles and divided into five parts. One part was acidified with nitric acid to determine the dissolved cations; drops of chloroform were added to a second part from where dissolved anions were analyzed. Other parts were used to measure tritium, stable isotopes (δ2H and δ18O) and 14C of the water samples. Cation analysis was carried out in ICP-OES and anions are analyzed in Ion Chromatography. Stable isotope analysis was done in the BRNS lab with Isotope Ratio Mass Spectrometer and tritium and C-14 analyses were done in liquid scintillation counter. Electromagnetic (VLF data): The VLF-EM data was acquired and analyzed in three phases. The first phase data was acquired in a grid pattern along parallel profiles. The direction of the profiles was N-S with intervals of 10 m; each profile was 50 m apart. The second phase data (Profiles P5-P15) was acquired around Atri hot spring by following the previously described specification. The regional strike of lithologies is approximately in the E-W direction (Fig. 2 and Fig 7), and therefore, profiles were all taken along a N-S direction. During the field work a transmitter with frequency 19.8 kHz was found to have maximum field strength in the E-W (strike) direction. Therefore, all VLF measurements were performed using this transmitter. A total of eleven profile surveys were carried out around the hot spring. Seven profile surveys were carried out to the west of the hot spring, and the rest were acquired to its east. Along each profile, survey was carried out at intervals of 25 m; between the profiles the distance was around 50 m. In the last phase of VLF measurements, several profiles (Profiles P16-P20) were taken in between the Atri and Tarbalo hot springs. The profiles were regionally and unevenly spaced, but always along a N-S traverse, with data along profile collected at intervals of 10m. A total of five N-S VLF profiles were acquired by using the ABEM WADI instrument.