Seismic Imaging of South Indian Lithosphere

Basak, Suman (2025) Seismic Imaging of South Indian Lithosphere. PhD thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

The South Indian lithosphere is one of the most diverse geological regions in the world comprising the youngest volcanic province in the north, the Deccan Volcanic Province (DVP), to the granite-greenstone (3.4 Ga) of the Dharwar Craton (DC) in the south. Continuing southward, the craton passes into Pan African granulite and khondalite terrain, as well as 2.5 Ga high-grade granulite and charnokite. Being a portion of the Indian plate, the craton has been shaped by events such as the division of east Gondwanaland around 130Ma and seafloor spreading due to the Marion plume around 90Ma ago, and lastly, the Reunion plume around 65Ma ago. It preserves within it a history of approximately 3.5 billion years. Thereby determining the lithospheric structure and the various crust and upper mantle discontinuities beneath it plays a key role in understanding its formation and evolution. Numerous crustal studies have been conducted throughout the years, which have greatly aided in the highly accurate determination of the shallow depth structures. However, the lithospheric studies largely connected to deeper depths have given drastically variable results utilizing different geophysical methodologies mostly due of the resolution constraint in resolving deeper earth. We employed the surface wave tomography (SWT) method to address the horizontal resolution issue. SWT method was applied to seismic data recorded at 51 broadband stations operated during 2009-2012 with magnitude M ≥ 4.5 and epicentral distances from 5°-30°. This attributed to a total of 4873 source-station ray paths for which the dispersion curves were calculated to obtain Rayleigh-wave group velocity maps from 10-200 secs. The dense ray coverage resulted in achieving a very high horizontal resolution of 0.5°-1° for the first time in the study region. Next to address the vertical resolution we improved the P-wave autocorrelation method (PWA) with the integration of techniques like spectral whitening, phase weighted stacking, and automatic gain control. This was then applied on teleseismic datasets with magnitude M ≥ 5.5. This method was efficient in identifying the major shallow upper mantle discontinuities down to 250 km depth with a very high vertical resolution (<0.6 km). Combining the techniques we were able to overcome the resolution constrain faced by most of the other geophysical methods to resolve deeper discontinuities. There are lots of debate on thickness of Lithosphere Asthenosphere Boundary (LAB) beneath Dharwar craton. Few researchers suggested that LAB is thin beneath Dharwar craton. However, some other researcher showed that the LAB beneath Dharwar craton is thick, and they proposed that the low velocity observed at shallow depth correspond to Mid-Lithospheric Discontinuity (MLD). The comprehensive imaging of the LAB thickness throughout the craton facilitated in addressing of two critical aspects of the craton. Firstly, the influence of plumes and hotspots on the rapid movement of the Indian plate, and secondly, the geological heterogeneity between the northern and southern blocks of the Southern Granulite terrain (SGT), as well as the impact of ancient magmatic events on this region. Here, we have imaged LAB thickness along with other lithospheric discontinuities with greater accuracy, using Rayleigh wave group velocity tomography data and P-wave autocorrelation method in different geological blocks beneath South India. Quantitatively, we observed a thinner crust (∼35 km) in the late Archean (∼2.7 Ga) EDC and relatively higher (∼40-50 km) in adjoining terrains (mid-Archean and Proterozoic). The mid-Archean (∼3.36 Ga) WDC showed a thicker crust (∼50 km). The WDC also has a thicker lithosphere with a lithosphere-asthenosphere boundary (LAB) around an average of 160-190 km. The southern root of the lithosphere of WDC continued till the northern province of SGT which showed thicker LAB compared to the rest of SGT (∼120 km). The LAB in the late Archean EDC along with the Cuddapah province was found to be flatter and less thin (∼140-160 km) compared to WDC. The coastal areas showed a thin LAB (∼110-120 km) except for the WDC boundary. The southernmost tip showed a very thin LAB. We also present a comprehensive study of the energy attenuation properties of the South Indian crust by calculating the regional variations Lg-wave quality factor (Q). We used a two-station method for seismic waves with epicentral distance of less than 15° to calculate individual Q values. The final Qo values were calculated using attenuation tomography incorporating the damped least square iterative method. The high-density ray path facilitates the completely resolving the region of study. The stable cratonic region exhibited a high Q, as anticipated. The SGT region and the coastal areas showed low Q values. Also, the geophysical parameters, including bulk thickness, Vp/Vs ratio, and heat flow, were examined and correlations were established with our attenuation results.

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