Tectonics of the southern Annapurna Range, central Nepal Himalaya

Abstract

Over the past two decades, several competing dynamic models have emerged to explain the tectonic evolution of the Himalayan thrust belt. Basic aspects of the geology of the range remain relatively poorly known, however, limiting the accuracy of dynamic and kinematic models. For example, the location of a mega-thrust now exposed in the interior of the range, the Main Central thrust (MCT), is only approximately known across much of the Himalaya. Both because the MCT accommodated at least 150 kilometers of slip during the mid-late Tertiary and because it carries in its hanging wall the highest-grade metamorphic rocks exposed in the range, the MCT is a prominent feature in all models that seek to explain the tectonic evolution of the Himalaya. Whole-rock Nd isotopes and detrital zircon U-Pb ages allow the discrimination of hanging wall and footwall rocks of the MCT. Application of these techniques in the Annapurna Range of central Nepal unambiguously locates the MCT. Microstructural analyses confirm the presence of a thrust-sense ductile shear zone superimposed on this isotopic boundary.With improved locations of the MCT and other faults, it is now possible to understand thermobarometric and kinematic data in more correct structural context. Thermobarometric estimates demonstrate that rocks exposed in fault-bounded blocks in the Modi Khola transect experienced approximately constant peak conditions, and that peak conditions change dramatically across large faults. Pressure-temperature estimates combined with constraints on the extent of metasomatism indicate the presence of a large normal fault one kilometer structurally above the MCT. Preliminary Th-Pb dating of monazite suggests that this normal fault may have been active during slip on the MCT. The presence of such a proximal normal fault slipping synchronously with the MCT challenges some current representations of dynamic models for the evolution of the thrust belt. In situ Th-Pb ages of monazite inclusions in garnet, chemical and age zoning in the inclusions, and textural relationships between monazite, garnet, and nearby minerals demonstrate that interpretation of the tectonic significance of Tertiary Th-Pb ages from matrix monazite is simpler than interpretation of ages from inclusions in garnet.