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ELCTROMAGNETIC INDUCTION STUDIES

 

MAGNETOTELLURIC (MT) STUDIES

MT studies were commenced in this institute in the mid 80s and since then MT surveys have been successfully used in different geophysical settings in India, such as, the Himalayan mountain belt, Deccan Volcanic province (DVP), Narmada Son lineament region, Achaean shields of Rajasthan, Dharwar and Bundelkhand regions. At present the Eastern Syntaxial bend of the Himalaya and the Deccan Volcanics are the main focus of the group. The group is currently going through its inception stage and is also involved with different aspects of its activities, such as, the instrumentation, software/firmware development, field surveys as well as the analysis and interpretation of the data thus obtained.

The major research contributions are as follows:

 
  • 1.  Detection of the high conductivity at the deep crustal levels of the Indus-Tsangpo suture in the NW Himalaya. The Ladakh and Karakoram batholiths are delineated as high resistivity bodies with no obvious roots beneath. Several similarities between these observations and those in the Lhasa block, about 1500 km east of the NW Himalaya, are indicative of a predominantly two dimensional nature of the Himalayan collision belt. Studies are now directed in the NE Himalayan Syntaxial region to decipher the deep configuration of the crust.
  • 2.  Delineation of a Late Cretaceous thrust about 60 km north of Nasik, which provides evidence of the block movements in the DVP area in the recent times. Earlier, the entire Deccan Volcanic province, was assumed to be devoid of any tectonic activity post dating the Precambrian.
  • 3. Identification of an Achaean suture zone in the Dharwar craton along the Chitradurga schist belts along which the west Dharwar craton is subducting beneath its eastern counterpart. The age of this thrust is dated at 3.4 Ga (deposition of the older schists such as the Gadval, Hatti-Maski) to 2.6 Ga (deposition of the younger schist such as Chitradurga-Gadag). Some more transect corridors have been identified across the SGT for MT studies.
  • 4. Studies in the Bundelkhand craton have proposed an EW trending suture in the Bundelkhand region passing through Jhansi, along which two Achaean blocks may have sutured together to form the present day craton.
  • 5. MT studies were perhaps the first ever systematic geophysical studies over the Arakan Yoma folds in the NE Himalaya (Mizoram). These studies indicate that this region may consist of several discrete crustal blocks, which are not participating the the intercontinental subduction along the Himalaya, but are moving vertically. Perhaps the Mizoram region corresponds to one such block, with NS trending faults on its either sides.
  • 6. Studies have already been conducted over the Rajivnagar-Champhai region and Mangan-Gangtok-Siliguri-Dalkhola region. Further studies in Agartala-Silchar and Silchar-Imphal corridor, Sikkim-Assam region and Darranga-Dauki region are planned for the summer of 2006.
  • 7. Studies in the Sikkim region have shown that the main boundary thrust and the Himalayan Foothill thrust merge together at about 10 km depth and extend up to a depth of about 40 km. Further studies in the Shillong plateau are being planned for the March-April survey season.
  • 8. Crustal studies in the Purana basins (Bhima & Kaladgi) and adjoining regions is in progress.

LONG PERIOD MAGNETOTELLURIC (LMT) STUDIES

Knowledge of the internal structure of the lithosphere, particularly the geometry of lithosphere-asthenosphere boundary is important as this boundary controls the geodynamic processes that create (oceanic ridges), modify (mobile belts) and destroy (collision and subduction zones) the lithosphere. Recognising these needs LMT (long period magnetotelluric) measurements was introduced in Indian shield region to probe (i) lithospheric structures (ii) electrical asthenosphere (iii) mantle electrical anisotropy under craton, mobile belts and collision zone and (iv) to explore the possibility of mapping 660Km discontinuity and beyond.

Keeping with the above objectives, LMT surveys are successfully conducted along

 
  • 1.  The Cambay and Kutch Rift Basins in Gujarat, to understand the evolution history and active geodynamic processes in the intraplate MBs, craton-MB transition zones along the thermally active Cambay and Kutch Rift Basin.
  • 2.  Dharwar craton and Eastern Ghats mobile belts for understanding the geodynamic evolution of the craton (WDC and EDC) and mobile belts (EGMB).
  • 3. Laccadive Ridge and Barren Island to establish the evolution and origin of the Laccadive Ridge in SW continental margin, particularly continental vis-à-vis oceanic origin; and trace the imprints of plume lithosphere interaction is recently initiated

Major findings are:

 
  • 1.  Presence of the mid-crustal conductive layer in the depth range of 8-17 km in Kutch simulates fluid-filled ductile layer and incorporation of this in the crustal section of the Kutch Rift can explain the space-depth distribution of the hypocenters in this region. In the tectonic model of the Kutch Rift, the mid-crustal conductive layer is seen as an aseismic zone with hypocenters of most aftershocks of the recent earthquakes seated in above or below this ductile zone.
  • 2.  Analysis of LMT data from Dharwar has revealed lithosphere-asthenosphere boundary at a depth of 220 km confirming well with the heat flow and seismic data. Presence of electrical anisotropy at the upper mantle depth is also indicated. Two distinct strike directions approx. 45-55° at the lower crustal depth and N-S strike at the base of lithosphere are inferred. The former conform to the Dharwar trend while the N-S strike may be related to the alignment of olivine-elongated axis due to movement of the Indian plate prior to the collision. New results along the Karwar-Bellary profile corroborate the above findings
  • 3. The first results of the conductivity imaging as deduced from GDS and LMT studies carried out along the northern part of the Chagos –Laccadive Ridge at several Lakshadweep islands indicates the increase in longitudinal conductance from north to south attributed to the degree of partial melting and relatively younger age of volcanic intrusion to the south. Severe cyclonic storm and sea ingression during the survey period resulted in some data loss and damage to the equipment and hampered progress, Further LMT and GDS investigations in the Lakshadweep are planned, post monsoon 2006.

GEOMAGNETIC DEPTH SOUNDING (GDS) STUDIES:

Map showing the different regional conductivity structures determined from geomagnetic depth sounding experiments

GDS is used for regional crustal and upper mantle studies by deploying array of magnetometers to identify the anomalies in the time varying magnetic fields of the earth, caused by deep conductivity contrast.

The major research contributions are as follows

 
  • 1.  The most significant result is the delineation of a ‘Trans Himalayan conductor (THC)’, which runs from the northeast Indian shield into the foothills of the Himalaya. The position of this conductor coincides with a localized high seismicity zone
  • 2.  Garhwal Lesser Himalaya Conductivity Anomaly’ was established along the Uttarkashi region
  • 3. A plutonic body was delineated near Valsad in south Gujarat, where it is embedded in the up warped asthenosphere along the western continental margin
  • 4.  GDS studies carried out in seismically active Bhuj region point to the concentration of induced currents in the thick sedimentary columns. The high conductance prevalent in the western part of Kutch region, northwest of Lodai, is related to the half Graben formed due to the uplifted Wagad upland
  • 5.  An arcuate-shaped conductor at mid-crustal depth is also found beneath the Satpura ranges. This conductor coincides with the Mandla gravity high and also a geothermal anomaly is observed in this area. It is also seen that the Godavari Graben can be mapped up to the southern limit of Narmada-Tapti lineament shrouded under the thick cover of Deccan traps
  • 6. In the Singhbhum region a linear trend of conductivity anomaly extending in E-W direction is located to the north of Ranchi and Bokaro lying at greater depths. Its source may be the Gondwana Graben of Damodar valley. This technique has also brought out remarkably well the electrical characteristics of Bengal basin marginal fault as also the east-west trending conductor bordering the Singhbhum craton
  • 7. It is also found that South Indian Offshore Conductivity Anomaly (SIOCA) is associated with the relics of the Marion plume outburst. The conductivity anomaly observed beneath the Palk-Strait appears to be associated with the thermal interaction of the Marion plume with the Indian lithosphere when it has passed over the plume. However, the conductivity anomaly observed beneath the Comorin ridge and a rift structure encountered along the west coast margin of India appears to be related to the Reunion hotspot activity

OCEAN BOTTOM MAGNETOMETER (OBM) STUDIES

OBM studies are similar to GDS method and are used for determining electrical conductivity profile below the seafloor. Since India is surrounded by sea on three sides, the Institute felt that this emerging technology should be adopted and utilized to the maximum. A three-component OBM data can aid in resource mapping of the vast oceanic domain in terms of geoelectrical structure. In India, the OBM array studies have been carried out across Barren Island in Andaman Sea, 850 E and 900 E ridges in the Bay of Bengal and also off the coast of Cochin in the Arabian Sea.

COSEISMIC STUDIES

Map showing the subduction of Indian plate beneath the Burmese plate along with the Resistivity values obtained from OBM array studies. Two different conductivity zones have been identified beneath Barren Island, one at a depth of about 80-100 km and another at a shallow depth of about 17-27 km. We need to carry out east-west profile (using LMT & GDS) across Andaman Islands and Barren Island in order to bring out the geoelectrical structure associated with the subducting Indian plate beneath the Burmese plate.
 

The method of electromagnetic induction has also been used to study the Seismicity and Seismotectonics of the Himalayan region, mostly using Electromagnetic Induction by naturally varying geomagnetic fields. Delineation of (i) Integral Sign Conductor across the Frontal Himalaya region and (ii) a resistive body embedded in a north-south oriented conductive belt in Deccan trap region are most important findings

C. K. Rao, A. K. Singh, Gautam Gupta, P. B. V. Subbbarao.



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