Record of deformation by secondary magnetic remanences and magnetic anisotropy in the Nar/Phu valley (central Himalaya)

Abstract

Secondary magnetic remanences residing in pyrrhotite and anisotropy of magnetic susceptibility (AMS) were studied in low-grade metamorphic carbonates of the Tethyan Himalaya in Nar/Phu valley (central Nepal) and used for interpretation of tectonic deformations. The characteristic remanence (ChRM) is likely of thermomagnetic origin related to post-peak metamorphic cooling occurring after the Eohimalayan phase (35–32 Ma). The ChRM postdates small-scale folding (main Himalayan folding F1 and F2) as shown by a negative fold test of site mean directions at 99% confidence level, and has been probably acquired between 32 and 25 Ma. Late-orogenic long-wavelength folding associated with the Chako antiform (CA) is recorded by the spatial dispersion of ChRM directions and the distribution of the main axes of the AMS tensor. The mean tilting of the ChRM direction since remanence acquisition (≈20–30°) approximately coincides with the tilting of the CA (31°) at the study area indicating that the pyrrhotite remanence predates the CA (CA formed at <18 Ma according to preliminary U/Pb dating). However, comparison of tilt angles of remanence directions and AMS tensor axes suggests that remanence acquisition was not completed before the onset of the CA formation. This could imply a younger age (Early Miocene or even younger) of the ChRM. Using the distribution of remanence directions along a small-circle as well as the distribution of AMS tensor axes, a clockwise mean rotation of 16° is obtained for a remanence age of ≈30 Ma. An Early Miocene remanence age would not change this result substantially. Compilation of rotations in the Tethyan Himalaya deduced from secondary pyrrhotite remanences reveals an increasing clockwise rotation from the Hidden valley in the W to the Shiar valley in the E (≈150 km distance), incompatible with an oroclinal bending model.