The deep geology of South India inferred from Moho depth and Vp/Vs ratio

Abstract

We present a comprehensive study of thickness and composition of the crust; and the nature of crust–mantle boundary beneath Southern India using P-wave receiver function from 119 seismic stations. Data from distributed network of seismograph location encompass geological domains like mid to late Archean Dharwar craton, Archean and Proterozoic metamorphic terrains, Proterozoic basin, rifted margins and escarpments, and Deccan volcanics. Except for the mid to lower crust exhumed Archean terrains (of West Dharwar and Southern Granulite) all other geological domains have crustal thickness in the range 33–40 km. In the western Dharwar, crustal thickness increases from ∼40 km in the north to over 50 km in the south. The Archean domain of granulite terrain is thicker (40–45 km) and more mafic compared to its counterpart in south deformed at 550 Ma. Most of the crustal blocks have low to moderate Vp/Vs (1.72–1.76) representing a felsic to intermediate composition. Exception to the above include Archean granulite terrain with high Vp/Vs (1.76–1.81) suggestive of more mafic crust beneath them. When accounted for the paleo burial depth of 15–25 km, the study suggests a possible Himalaya–Tibet like scenario beneath the mid-late Archean in southwestern Dharwar and north granulite terrain whose deeper crust has progressively densified. This led to a gradational crust–mantle transition that is otherwise sharp elsewhere. The study suggests a more homogenized and felsic nature of the Precambrian crust beneath the terrains formed after 2.6 Ga, possibly due to delamination of the mafic lower crust. Our study does not suggest any distinction between late Archean and Proterozoic crust. The Deccan volcanism at 65 Ma does not appear to have altered the crustal character beneath it and is similar to the adjoining late Archean east Dharwar craton. The western Ghat escarpment and the coastal plain formed due to separation of India from Madagascar are underlain by mafic lower crust.