Hales Discontinuity in the Continental Lithosphere: Seismological and Petrological Models

Chaudhury, Jashodhara (2021) Hales Discontinuity in the Continental Lithosphere: Seismological and Petrological Models. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Jashodhara Chaudhury (14RS037)) 14RS037_DES.pdf - Submitted Version Restricted to Repository staff only Download (15MB)

Abstract

We model the depth and Vs structure of the Hales discontinuity in the sub-continental lithospheric mantle (SCLM) beneath southern India, northeast India and five shields/cratons across the globe. In southern India we study the Eastern Dharwar Craton (EDC) and the Southern Granulite Terrain (SGT). We use P-wave receiver function (P-RF) analysis and joint inversion with Rayleigh wave phase velocity dispersion to model the SCLM structure. We calculate P-RFs at higher frequency (fmax=0.46 Hz), compared to previous studies, to show that the Hales discontinuity P-to-S converted phase (Phs) is distinct from crustal reverberations. The Phs at stations HYB and GBA, in the EDC, arrive at ~11.5 s and ~12 s, respectively. From joint inversion the Hales discontinuity beneath HYB and GBA are modeled at a depth of 107±5 km and 113±4 km, with Vs increase of ~3% and ~4%, respectively. For station KOD, in SGT, the Phs destructively interferes with the negative mid-crustal reverberation at many ray-parameters, which explains its apparent absence in previous studies. We isolated those P-RFs where Phs is distinct at ~11 s and modeled it at a depth of 102±3 km. Common conversion point stack profiles constructed by depth migrating P-RFs through the Vs model, show an undulatory nature of the Hales discontinuity. From data of mantle xenoliths (Wajrakarur kimberlite field) we calculate Vs of mantle peridotite and kyanite eclogite, using published bulk-rock compositions. At the Hales discontinuity depth and temperature derived from Indian Shield geotherm, we observed a good match to the Vs structure of the Hales discontinuity. Our results support the geodynamic model of the Hales discontinuity being an interface of paleo-subducted eclogitic oceanic crust embedded within the upper mantle peridotite. Global observations of mantle reflectors within the continental lithosphere, at depths similar to Hales discontinuity, has been related to relict-subduction and independently support our model. We then model the depth and Vs structure of the Hales discontinuity beneath northeast (NE) Indian lithosphere using P-RF analysis and joint inversion with Rayleigh wave group velocity dispersion. Finally, we model the Hales discontinuity at chosen stations in the Canadian Shield, Siberian Shield, Arabian Shield, Kaapvaal Craton and Baltic Shield. Forward modeling of P-RFs have been used to estimate the depth and Vs change at the Hales discontinuity beneath these cratons and shields. Our modeled Hales discontinuity depths beneath these regions are in accord with the previously modeled depths. However, beneath the Kaapvaal Craton station BOSA, we could not identify the Phs phase in the observed P-RFs. To test our geodynamic model for these shields and cratons, further studies of petrological modeling of Vs structure using mantle xenolith data will be attempted.

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