Crust Characterization below North-East India

Abstract

Highly seismic and complex tectonic regions of the world, North-East India is a triple junction formed by the collision of the Indian plate with the Eurasian plate, depicting the collision tectonics and collision with the Burmese plate depicting the subduction tectonics. It also includes the Eastern Himalayan Syntaxis (EHS), which is present at the juxtaposition of the 3 plates. There are 19 broadband seismological stations installed by the National Centre of Seismology (NCS), and for each station, receiver function (RF) analysis has been done for the teleseismic earthquakes recorded by these stations. The RFs are stacked to compute the 1-D S-wave velocity model, computed using the Nearest Neighbourhood Algorithm (NNA). While implementing, we consider the sub-surface to be composed of 6 layers and 4 parameters are assigned to each layer producing a 24-dimensional parameter space for the purpose of modeling. As we implement this technique, we obtain 1-D velocity models for all the stations. The stations are widely distributed all over the region of North-east India, such as the Eastern Himalayas, Shillong-Mikir plateau, Brahmaputra valley, Tripura fold belt, and the Indo-Burma ranges. As we move from North to South, the stations in the Eastern Himalaya (GTK, TAWA, ITAN, and ZIRO) and the stations in the Indo-Burma ranges (IMP, MOKO, KOHI, and LKP) show a greater depth of the Moho discontinuity than the stations in the Shillong-Mikir Plateau (DHUB, TURA, and SILR), the Brahmaputra valley (GUWA, TEZP, and JORH) and the Tripura-fold Belt (AGT, AZL, BEL, and SAIH). At Moho depth, we observe a gradational increase in velocity, in most cases. The velocity model for station ZIRO shows the presence of an Intra-crustal low-velocity zone (IC-LVZ) from 10 km to 24 km, which might indicate the Main Himalayan Thrust (MHT) present below this station. Similar is the case for the station GTK, which shows the presence of 18 km thick middle crust. In IBR, station MOKO shows a 16-km thick IC-LVZ present in the lower crust. The average crustal velocity for the station LKP till the depth of 12 km shows a crustal S-wave velocity of 3.4 km/s resembling the ophiolitic crust present below the station. Other stations in IBR such as MOKO, KOHI, and IMP show a gradational decrease in velocity till 14 km, which might be due to the presence of Disang shales and flysch sediments. The velocity model obtained for the stations, DHUB and TURA, in Shillong-Mikir Plateau might indicate the presence of a felsic crust. Stations in the Tripura-Fold Belt such as AGT and BEL and station SILR show the presence of alternate low and high-velocity layers at various depth levels. The stations AZL and SAIH show a similar pattern of velocity variation till the depth of 10 km ranging from 3.0 km/s to 3.2 km/s indicating the presence of shale and phyllite sediments. Below the upper crust of AZL and SAIH, these stations show a gradational increase in velocity.