ELECTRICAL CONDUCTIVITY IMAGING OF UTTARAKHAND HIMALAYA USING MT METHOD

Abstract

The present thesis reports the results of magnetotelluric (MT) investigations in the Uttarakhand (Garhwal-Kumaon) Himalaya where very limited geophysical imaging of deep structures exists and it is a major knowledge gap in tracking the geodynamic evolution of the Himalaya, particularly, in our understanding of the seismogenesis of earthquakes. With this background and rationale, MT measurements on Bijnaur-Mallari profile, starting from the Indo-Gangetic Plains, passing over the Sub-, Lesser- and Higher- Himalaya and terminating in Tethyan Himalaya were carried out. The choice of profile was justified as this segment has witnessed high frequency of large and moderate earthquakes and the accumulated strains may drive the next Great earthquake. The choice of the method is justified because electrical resistivity, by virtue of its sensitivity to interconnected fluids and temperature, has proved a potential marker of active dynamics and stress build up. During most of the periods of field-surveys, when recording of components of MT fields for frequency band of 1000 Hz to 4096s was carried out, there happened to occur unprecedented solar minimum. The extremely low amplitude of natural EM source signals made the acquisition and processing for MT response functions an ultimate challenge. The problem was further compounded by power leakage. Despite longer occupancy at each site, careful editing and robust processing, statistically stable MT impedances could be recovered only up to periods of 100s that too only at 18 sites out of total 53 sites occupied. At the 17 sites out of remaining 35, the noise free magnetic data were used to compute tippers up to 1000s for joint interpretation of TE, TM and vertical magnetic field transfer functions. Combination of Swift and Bahr's Phase sensitive skew parameters helped in establishing that the observed impedances are compatible with 2D resistivity model distribution. The phase tensor analysis in conjunction with induction arrow behavior and the correlation with regional tectonic trend, helped to adopt N45°W direction as the regional geoelectric strike. Finally, we utilized GB decomposition at each station to recover MT response in regional co-ordinate by fixing the strike with N45°W. To obtain the most persistent features, geoelectrical model was obtained by inverting simultaneously the MT responses corresponding to TM and TE modes and the tippers. This strategy helped to recover geoelectrical model to a depth of 40 km. Most salient structural features and their possible tectonic and geodynamic implications can be summarized as follows: Near surface resistivity distribution supports the geological hypothesis that high resistive sheets occupying the core regions of Garhwal and Almora Klippen are basically the Higher Himalayan Crystallines, which have transgressed southward by tectonic movement along the MCT thrust. The roughly 10 km wide MCT zone is seen as a low resistivity steeply dipping structure, which flattens out further north to be traced as a moderate resistive layer. The inter-crustal low resistivity layer (IC-LRL) is most dominant feature whose upper surface excellently traces the geometry of the seismic active detachment. The low resistivity of the layer is attributed to the trapping of low fractions of fluids released by metamorphic dehydration reactions and/or tectonically induced neutral buoyancy. The role of fluid concentration and movement beneath the seismogenic depths are invoked to explain: > Such fluid-filled porosities influence the rheological properties of the crust and, dramatically reduce the shear strength causing focusing of tectonic forces on a single detachment plane. The alignment ofhypocenters of 1991-Uttarkashi, 1999- Chamoli earthquakes and ofa number ofother large earthquakes helps to view this plane as a brittle-ductile transition. > Presence of fluids in interconnected porosities marks the onset of brittle-ductile transition and thus defines the sharp cut-off depth ofthe crustal seismicity. This possibly explains the confinement of the earthquakes to a crustal section of 15-20 km. > In compressional regime, the fluid propagation above the stagnation zone through over pressured zones like thrust and shear zones is seen as a locale of concentrated seismicity. > The confinement of aftershocks of 1999-Chamoli earthquake to a narrow pocket centered on the Alaknanda Fault seems to be an ideal case where aftershock sequence might have been triggered by upward propagation ofthe fluids by either hydro-fracturing or fluid-pressure pulses following the release of accumulated strains in the rupture zone of main shock.

> The mapped discontinuous jump in the IC-LRL beneath the northern limit of the Lesser Himalaya is structurally visualized as a ramp structure, causing concentration of low magnitude earthquakes to a narrow belt straddling the MCT. The high conductance of the low resistivity block suggests presence of partial melt, may be associated with granitic intrusion during the Tertiary period. > Integration of the present resistivity section with that from Keylong-Panamik permits continuation of the IC-LRL all throughout the collision zone. The evidence of partial melt related with extremely high conductance zone under the Trans- Himalaya favours extrusion of ductile flow southward. The integrated picture of electrical resistivity section across Himalaya permits us to conclude that while thrust-wedge tectonic evolution model dominates the Outer and Lesser Himalaya and is able to explain the space-depth distribution of seismicity in the Himalaya, the Channel flow model is the dominant player and is able to solve the puzzle of emplacement of leucogranites belt in the Tethys Himalaya and accounts for inverted metamorphism in Higher and Tethys Himalaya. To sum up, incorporation of the role of fluids on the rheology of crustal rocks, propagation of fluids to over pressurized shear zone can be a useful guide in constraining the seismotectonic model of the Himalayan seismicity.