SECONDARY PALAEOREMANENCE OF TETHYAN SEDIMENTS FROM THE ZANSKAR RANGE(NW HIMALAYA)

Palaeomagnetic investigations have been carried out on Tethyan sedimen ts from the NW Zanskar Range (33.9 degrees N, 76.5 degrees E). A total of 455 oriented samples from 41 sites were drilled from four stratigr aphic units of Middle Cretaceous to Early Eocene age (Shillakong Forma tion, Marpo Limestone/Spanboth Formation, Stumpata Quartzarenite/Spanb oth Formation, Kong Formation). A characteristic remanence (ChRM) coul d be isolated for 27 sites (254 specimens) through detailed thermal an d alternating field demagnetization and multicomponent analysis. All t he ChRM directions identified are similar, irrespective of geological age. They fail the fold test and yield an overall in situ mean directi on of D = 336.6 degrees/I = +39.6 degrees with alpha(95) = 5.9 degrees and k = 22.9. This corresponds to a palaeopole position at 66.5 degre es N, 323.5 degrees E (N-pole). Isothermal remanence (IRM) acquisition and thermal demagnetization of a saturation IRM identify pyrrhotite a s the dominant ferrimagnetic mineral and carrier of the ChRM. The pyrr hotite remanence is probably a thermoremanent magnetization acquired w hen the temperature of low-grade metamorphism decreased below about 30 0 degrees C. The ChRM directions are thus clearly secondary in origin, and a negative conglomerate test carried out on the Kong Formation de monstrates that remanence acquisition post-dates the Early Eocene. The mean ChRM inclination suggests that the remanence was acquired at abo ut 20 degrees N. Cooling events in the higher Himalayan crystallines i ndicate an Oligocene age for this remanence acquisition. The mean ChRM declination shows a counterclockwise rotation of 23.4 degrees since r emanence acquisition. Assuming remanence acquisition at about 30 Myr a go, a slight counterclockwise rotation of about 10 degrees relative to the stable Indian Plate can be concluded. This does not fit with the general pattern of palaeomagnetic results from neighbouring areas with in the western syntaxis of the Himalaya, from which a clockwise rotati on relative to the stable Indian Plate is expected for the Zanskar Ran ge.