Kinematic history of the Main Central Thrust zone in the Langtang area, Nepal

Abstract

This paper describes for the first time evidence for a northeastward brittle extensional movement of the Main Central Thrust (MCT) zone overprinting the southward ductile thrust movement. The MCT zone is the major tectonic boundary between the Lesser Himalayan metasedimentary sequence (LHS) and Higher Himalayan crystalline sequence (HHS) and contributes to the kinematic and tectonic evidence of continent–continent collision in the Neogene. The Langtang area, 50 km north of Kathmandu, is a matter of interest in considering the movement of the Kathmandu Nappe because the general trend of foliation of the HHS and the underlying MCT zone is turned from WNW–ESE to N–S or NNE–SSW. The major tectono-lithostratigraphic units of the MCT zone in the study area are divided into three: the lower and middle units included in the LHS and the upper unit even included in the HHS. Characteristic mesoscopic quartz lenses are developed in the schists of the Lower unit forming asymmetric boudins. This asymmetry indicates an extensional (top-to-the-NE) shear sense. However, a thrust sense of top-to-the-SW is also preserved in microscopic ductile shear bands and mica fish. The extensional shear movement in the Lower unit took place after the major thrust movement in the MCT zone as a negative inversion. The Middle unit contains Ulleri-type augen gneiss (Syabru Bensi augen gneiss) which is widely distributed in the MCT zone in the other parts of the Nepalese Himalaya. Asymmetric clast-tails (or clast-pressure shadows) and asymmetric boudins observed in the Syabru Bensi augen gneiss give a dextral-thrust oblique sense of shear. This dextral component is regarded as movement of a N–S-trending lateral ramp during the overthrusting of the Kathmandu Nappe. The Upper unit above the MCT is commonly made up of kyanite-bearing medium pressure type amphibolite-facies pelitic gneiss. Asymmetric deformational features such as mica fish and shear bands in the pelitic gneisses commonly show a thrust (top-to-the-W) sense of movement. After the emplacement of the Kathmandu Nappe (∼9–6 Ma), brittle extensional shear occurred only in the crystalline schists of the Lower unit presumably associated with layer-parallel slip. This extensional movement in the Lower unit is probably caused by the change in strike of foliations, e.g., from a general trend of E–W in the other areas to N–S as seen in the study area. This is because N–S contraction during continued continent–continent collision produced E–W extension normal to the contraction axis (N–S), thus the relative east-directed movement on normal faults took place only in the N–S trending domain of the MCT zone. Another possibility for the extensional movement is squeezing of the upper part of the LHS between the MCT above and an out-of-sequence thrust or the Main Detachment Fault below.