Variations of Hales Discontinuity Beneath South India

Kosre, Goukaran Kumar (2016) Variations of Hales Discontinuity Beneath South India. Masters thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

Thermodynamic studies show the spinel-garnet transition in fertile and hot mantle should be relatively narrow and should show up in seismological studies as a discontinuity. The evidence for a shallow lithospheric mantle discontinuity was first proposed by Hales (1969) based on seismological travel time measurement from the Early Rise experiment in Central United States, where a ~4% increase in the S-wave velocity at a depth of 75 km was observed. The recent studies show, in cratonic blocks with colder geotherms, it appears at greater depths and over broader intervals, that is, from the Moho to 150 km depth. Different studies interpreted Hales discontinuity may be due to seismic anisotropy or pervasive partial melts or cation ordering in mantle olivine. In the present study an attempt is made to model the Hales discontinuity in the South Indian shield, by jointly inverting group velocity dispersion and receiver functions, calculated from teleseismic earthquakes recorded at 16 broadband seismograph locations in the study region. South Indian shield is an amalgamation of several crustal blocks, namely, Eastern Dharwar Craton (EDC), Western Dharwar Craton (WDC), Southern Granulite Terrain (SGT) etc. Inversion modeling results show deeper Hales discontinuity (~80-110 km depth) in the south of WDC and SGT, while in the north of Western Dharwar Craton and Eastern Dharwar Craton it varies from ~60 -94 km. It is also seen that Hales Discontinuity is observed at greater depth in the western part of Dharwar Craton, compared to eastern part.The variations of thickness and depth of Hales discontinuity may due to different Iron content ,Chromium number, geotherm(temperature variation) or their combination.

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