Preserved and modified mid-Archean crustal blocks in Dharwar craton: Seismological evidence

Abstract

We report significant lateral variability in shear wave velocity and Moho depth in the Archean crust, beneath the Dharwar craton, using earthquake waveform data recorded over 50 broadband seismographs. The craton is a continuously exposed Archean continental fragment divided into the west Dharwar craton (WDC) of age 2.7–3.36Ga, and the east Dharwar craton (EDC) of age, dominantly, 2.5Ga. The craton progressively transitions into the Southern Granulite Terrain (SGT) with age of metamorphism around 2.6Ga.The inversion and modeling of receiver function data reveal significant variation of Moho depth, viz., 38–54km in the WDC, and 40–46km in the SGT and 32–38km in the EDC. The average shear wave velocity (Vs) of crust beneath the WDC is ∼3.85km/s as compared to ∼3.6km/s in the EDC. We infer highly variable thickness (16–30km) of mafic cumulate (Vs≥4.0km/s and Vp≥7.0km/s) beneath the WDC, in contrast with a thin one (<5km) beneath the late Archean EDC. The 3.36Ga greenstone belt in the WDC has maximum basal layer thickness of ∼30km. These results suggest the intermediate-mafic composition and exceptional thickness of crust beneath the WDC (>50km) as compared to felsic to intermediate composition for the EDC crust with almost flat Moho (down to ∼38km). These results suggest preserved mafic crustal root beneath the middle Archean terrain in the WDC that has remained inert since then. On the other hand, felsic to intermediate composition of crust with a nearly flat Moho beneath the late Archean EDC could be a consequence of regional delamination of lower crust. The preserved distinct Moho topography across the Archean terrains suggests it as compositional boundary. Considering the surface exposure of 15–20km crust, based on P–T condition, in the granulite segment of the WDC, we speculate a Himalaya-like crustal thickness (50–70km) beneath the middle Archean crust pointing toward a plate tectonic-like scenario at ∼3.0Ga.