Structure of the deep crust beneath the central Indian tectonic zone: an integration of geophysical and xenolith data

Xenoliths of garnet granulite and “crustal” eclogites in alkaline dykes from the northern margin of the Dharwar craton suggest that the deep crust beneath this region is dominated by mafic granulite. The granulites consist primarily of clinopyroxene, orthopyroxene, garnet and plagioclase. Clinopyroxene from most garnet-bearing lithologies is eclogitic with Jd/Ts > 0.5; the Jd component varies from > 5 to 8%. Garnets from the granulites are similar to those in crustal eclogites whereas the ones from pyroxenites are akin to those in kimberlites. Composite xenoliths are indicative of interstratification of the lower crust by ultramafic rocks represented by pyroxenites and websterites that display prominent, high density reflectors in deep seismic sounding profiles. They represent cumulates in relict magma chambers and are expressed as conductors in magnetotelluric transects. Thermobarometric estimates for granulite xenoliths are 740–1094 °C, 0.71–1.3 GPa equivalent to depths of 20–40 km, whereas those for pyroxenite xenoliths are 726–1042 °C, 0.57–1.05 GPa. These provide for the first time a spatial context to these rock types, suggesting derivation from lower crustal depths. The lower crustal rocks beneath the Central Indian Tectonic Zone are near to their melting point as is evident from the anomalously high geothermal gradient due to which they show lower Vps than those characteristic of typical garnet granulites. The deep crust has undergone thickening both due to crustal inflation and under- and intra-plating beyond its original pre-rift thickness, overcompensating for the crustal thinning due to extension. Lateral flow of the lower crust has also contributed to crustal thickening in addition to under- and intra-plating. Transport of midcrustal rocks to the surface along ductile shear zones has caused additional crustal modification

Electrical structure beneath Schirmacher Oasis, East Antarctica: a magnetotelluric study

Maitri Station (70.76°S; 11.73°E) is located in Schirmacher Oasis, a coastal nunatak in north-central Dronning Maud Land covering an area of 35 km2. Here, we report results from the first magnetotelluric experiments and delineate the deep electrical conductivity structure under Schirmacher Oasis using the data acquired during the 24th Indian Antarctic Scientific Expedition. The magnetotelluric method has the advantage of shallow to deeper level coverage as the data acquisition covers a wide frequency band of 10−3–103 Hz, permitting different penetration depths depending on the frequency and conductivity of the layer under investigation. The modelling results indicate the presence of a highly resistive (8000–10 000 ohm m) upper crust, which shows a lateral variation in thickness from 20 km (below site 6) in the east to 10 km (between sites 1 and 2) in the west. It is underlain by a less resistive (500–600 ohm m) lower crust. The highly resistive upper crustal structure supports the existing notion that western Dronning Maud Land is a stable, cratonic platform. Results of free-air gravity, seismic, geomagnetic and surface wave dispersion investigations in East Antarctica also indicate a cratonic-type crust. The results of our study allow us to identify a westward thinning of the upper crust with a marked boundary between sites 1 and 2. We also find evidence for the continuity of the Mozambique mobile belt in East Antarctica on the western side of Schirmacher Oasis