Detailed analysis of successive pTRMs carried by pyrrhotite in Himalayan metacarbonates: an example from Hidden Valley, Central Nepal

Low-grade metacarbonates from the Tethyan Himalaya were sampled for palaeomagnetic studies in Hidden Valley (Central Nepal). The remanence is carried by pyrrhotite, evidenced by thermomagnetic runs of susceptibility (Hopkinson peak at similar to 300 degreesC), alternating field demagnetization, isothermal remanent magnetization acquisition and subsequent thermal demagnetization. The palaeomagnetic directions reflect a Tertiary overprint after the main folding event, probably synchronous with the metamorphism. Normal and reverse remanence directions were separated and vary with altitude. It is also possible to retrieve several antiparallel components versus temperature during thermal demagnetization of a single sample. At higher altitudes (4920-5500 m), the first component recorded is reverse (R-1). At a lower temperature a normal component can be extracted (N-1). For sites sampled at lower altitudes (4700-4900 m), the high-temperature reverse component disappears but a medium-temperature reverse component (R-2) demagnetized in a narrow temperature range can be identified in between two normal components (N-1 at high temperature and N-2 at low temperature). At the lowest altitudes (4450-4700 m), only a normal component (N-2) appears. The occurrence of successive normal and reverse polarities in one sample is interpreted as the record of successive reversals of the geomagnetic field during the post-metamorphic Tertiary cooling of the studied area. The polarity versus attitude function is a powerful argument for a thermomagnetic origin of the magnetization. No obvious rotations around a vertical axis with respect to the stable Indian plate are evidenced for the Tertiary. However, the inclination is not consistent with the expected inclination. Main Central Thrust ramping can be invoked to explain our observations. R-1, N-1 and N-2 inclinations are slightly different and their tendency is consistent with tilting towards the north during magnetization acquisition. The minimum total amount of such tilting is around 25 degrees. Accurate geochronological data from the Tethyan Himalaya would be of a great help for better resolution.