No abstract

Teleseismic receiver function analysis of seismograms recorded on a ∼700 km long profile of 17 broadband seismographs traversing the NW Himalaya shows a progressive northward deepening of the Indian Moho from ∼40 km beneath Delhi south of the Himalayan foredeep to ∼75 km beneath Taksha at the Karakoram Fault. Similar studies by Wittlinger et al. (2004) to the north of the Karakoram Fault show that the Moho continues to deepen to ∼90 km beneath western Tibet before shallowing substantially to 50–60 km at the Altyn Tagh Fault. The continuity of the Indian Moho imaged in the receiver functions reported here, along with those of Wittlinger et al. (2004), suggest that in this part of the Himalayan orogen the Indian plate may penetrate as far as the Bangong Suture, and possibly as far north as the Altyn Tagh.

We present crustal thickness and Poisson's ratio determinations from receiver function analyzes at 32 sites on the Archaean and Proterozoic terrains of South India. The crustal thickness in the late Archaean (2.5 Ga) Eastern Dharwar Craton varies from 34–39 km. Similar crustal thickness is observed beneath the Deccan Volcanic Province and the Cuddapah basin. The most unexpected result is the anomalous present‐day crustal thickness of 42–51 km beneath the mid‐Archaean (3.4–3.0 Ga) segment of the Western Dharwar Craton. Since the amphibolite‐grade metamorphic mineral assemblages (5–7 Kbar paleopressures) in this part of Western Dharwar Craton equilibrated at the depths of 15–20 km, our observations suggest the existence of an exceptionally thick (57–70 km) crust 3.0 Ga ago. Beneath the exhumed granulite terrain in southernmost India, the crustal thickness varies between 42–60 km. The Poisson's ratio ranges between 0.24–0.28 beneath the Precambrian terrains, indicating the presence of intermediate rock type in the lower crust. These observations of thickened crust suggest significant crustal shortening in South India during the Archaean.

[1] The south Indian shield is a collage of Precambrian terrains gathered around and in part derived from the Archean-age Dharwar craton. We operated seven broadband seismographs on the shield along a N-S corridor from Nanded (NND) to Bangalore (BGL) and used data from these to determine the seismic characteristics of this part of the shield. Surface wave dispersion and receiver function data from these sites and the Geoscope station at Hyderabad (HYB) give the shear wave velocity structure of the crust along this 600 km long transect. Inversion of Rayleigh wave phase velocity measured along the profile shows that the crust has an average thickness of 35 km and consists of a 3.66 km s À1 , 12 km thick layer overlying a 3.81 km s À1 , 23 km thick lower crust. At all sites, the receiver functions are extremely simple, indicating that the crust beneath each site is also simple with no significant intracrustal discontinuities. Joint inversion of the receiver function and surface wave phase velocity data shows the seismic characteristics of this part of the Dharwar crust to be remarkably uniform throughout and that it varies within fairly narrow bounds: crustal thickness (35 ± 2 km), average shear wave speed (3.79 ± 0.09 km s À1 ), and V p /V s ratio (1.746 ± 0.014). There is no evidence for a high velocity basal layer in the receiver function crustal images of the central Dharwar craton, suggesting that there is no seismically distinct layer of mafic cumulates overlying the Moho and implying that the base of the Dharwar crust has remained fairly refractory since its cratonization.

No abstract