INSIGHTS INTO THE EMPLACEMENT OF THE KATHMANDU KLIPPE FROM QUARTZ MICROSTRUCTURES AND TITANITE PETROCHRONOLOGY

The Himalayan Orogen is one of the largest continent-continent collision zones in the world and serves as the preeminent example of an actively evolving mountain belt. Several fundamental questions persist, however, as to how the Himalaya has tectonically evolved through time, including the origin of the Lesser Himalayan Crystallines—a series of fault bound outliers of metamorphic rocks that occur to the south of the main Himalayan structural sequence. Several tectonic models have been proposed to explain the emplacement of these thrust sheets and klippen, wherein different configurations of major faults are invoked to have resulted in the southward transport of these metamorphic rocks.

As an initial attempt to answer some of these questions this study investigated the geometry and timing of deformation in one of these tectonic outliers, the Kathmandu Klippe in central Nepal. Microstructural analysis of rocks collected along four transects across the klippe allows for identification of major structures and quantification of internal strain. Microstructural data collected from several quartzites across the klippe using the Rf-φ and Fry methods yields strain ellipse ratios (Rs values) that range from 1.10 to 2.28 (YZ) and 1.67 to 2.89 (XZ). Strain magnitudes increase systematically with proximity to several major mapped and unmapped structures consistent with their occurrence as broad shear zones and not discrete faults. Among the samples displaying the largest Rs values, are those collected from the Mahabharat Thrust—a large top-south shear zone and possible analogue to the Main Central Thrust. U-Pb dating of strained titanite in these samples yields a date of ~16-18 Ma, overlapping with prior estimates for the timing of slip along the southern MCT. Petrographic study of the titanites indicates these young dates primarily reflect subgrain formation at grain tips and may thus represent the first direct determination of the timing of deformation within the klippe. When combined with information provided by the bulk strain ellipses and shear sense indicators, these results provide important insights into the various models on the development of the Himalaya and, more broadly, how high-grade metamorphic rocks are juxtaposed over low-grade metamorphic rocks in convergent orogens.