Crustal Structure Along the Lawrencepur-Astor Profile in the Northwest Himalayas SUBRATA K. BHUKTA 1 , K. SAIN 1 , and H. C. TEWARI 1 Abstract—During the Pamir Himalayan project in the year 1975 seismic refraction and wide-angle reflection data were recorded along a 270 km long Lawrencepur-Astor (Sango Sar) profile in the northwest Himalayas. The profile starts in the Indus plains and crosses the Main Central Thrust (MCT), the Hazara Syntaxis, the Main Mantle Thrust (MMT) and ends to the east of Nanga Parbat. The seismic data, as published by GUERRA et al. (1983), are reinterpreted using the travel-time ray inversion method of ZELT and SMITH (1992) and the results of inversion are constrained in terms of parameter resolution and uncertainty estimation. The present model shows that the High Himalayan Crystallines (HHC, velocity 5.4 km s )1 ) overlie the Indian basement (velocity 5.8–6.0 km s )1 ). The crust consists of four layers of velocity 5.8–6.0, 6.2, 6.4 and 6.8 km s )1 followed by the upper mantle velocity of 8.2 km s )1 at a depth of about 60 km. Key words: Travel-time, wide-angle, inversion, parameter, velocity. 1. Introduction The northwest Himalayan region has long been of great interest to the earth scientists. Due to this, several international projects (BELOUSSOV et al., 1980; KAILA et al., 1983; CHAMBERLAIN et al., 2002; MELTZER et al., 2001; ZEITLER et al., 2001) have been launched to study the Pamir, Nanga Parbat and Hindukush regions. Ni et al. (1991) computed a 3-D velocity structure from the earthquake data, in the region of Hazara Syntaxis and Main Mantle Thrust. JOHNSON and VINCENT (2002) have developed a gross 3-D velocity model, based on earthquake tomographic studies for a large part of India and Pakistan region. This model includes the velocity structure of the western Himalayan region also but it is very difficult to analyze the crustal velocity model from this picture. A 270 km long seismic refraction and wide angle reflection profile was recorded between Lawrencepur (Indus Plains) and Astor (Sango Sar, to the east of Nanga Parbat-Haramosh Massif [NPHM]), as a part of the Pamir Himalayan Project during the year 1975 (Fig. 1). The area around Lawrencepur is covered by alluvium of 1 National Geophysical Research Institute, Hyderabad 500007, India. E-mail: subrata_ngri@rediffmail.com Pure appl. geophys. 163 (2006) 1257–1277 0033–4553/06/071257–21 DOI 10.1007/s00024-006-0070-x Ó Birkha ¨ user Verlag, Basel, 2006 Pure and Applied Geophysics