Palaeomagnetic study of Cretaceous red beds from the eastern Hindukush ranges, northern Pakistan: palaeoreconstruction of the Kohistan-Karakoram composite unit before the India-Asia collision

Abstract

We report palaeomagnetic results obtained from Cretaceous red bed samples, collected from the eastern Hindukush mountains, northern Pakistan. Rock magnetic studies revealed specular haematite as the dominant remanence carrier, while pigmentary haematite, magnetite and goethite are responsible for the acquisition of secondary magnetizations. Thermal treatment generally revealed three components of magnetization. The most unstable component (A) was removed between 200 °C and 500 °C, which corresponds to the local present field direction. The intermediate component (B) was generally separated between 200 °C and 660 °C. The omnipresence of this complex secondary component across the Northern Suture Zone suggests that movement of chemically active orogenic fluids at, or slightly before, the time of the India–Asia collision was probably responsible for the authigenic remagnetization processes. The characteristic component (C) was generally unblocked between 600 °C and 685 °C. The characteristic directions from 12 sites fall into two groups. The sites from the Purit Formation give a tilt-corrected mean direction of D = 311.2°, I = –4.4°, k = 12.4, α95 = 17.8°, indicating a counter-clockwise rotation of about 66° and 15° relative to Eurasia and India, respectively. The tilt-corrected mean direction for the Reshun and Drosh formations is D = 263.0°, I = 2.2°, k = 52.7, α95 = 10.6°, which indicates that the sampling area experienced a counter-clockwise rotation of about 114° and 63° with respect to Eurasia and India, respectively. In both cases the palaeomagnetic rotation is consistent with the regional superstructure of the Hindukush–Pamir–Karakoram syntaxial bend. The difference in rotation between these two groups is probably due to local tectonic movement caused by the net rotation of thrust sheets along the Northern Suture Zone. The palaeolatitudes calculated for Purit and Reshun + Drosh formations are 2.2°S and 1.1°N, respectively. This indicates that about 4000 km of convergence has taken place since the mid-Cretaceous, of which about 90 per cent post-dates the India–Asia collision. The Late Cretaceous inclination values across the Northern Suture Zone are almost identical, which indicates that suturing had been completed at the time of ChRM acquisition.