The Himalayan fold-and-thrust belt preserves one of Earth's most complete and accessible records of continental collision — a natural laboratory for understanding how major tectonic boundaries express themselves in rock. This investigation documented that record across a carefully selected transect from the Lesser Himalayan sequence to the Higher Himalayan Crystallines.
Objectives
- Map the distribution of major lithological units across the transect at 1:10,000 scale
- Collect systematic structural measurements at 48 stations to define fold geometry and thrust kinematics
- Collect representative rock samples from each major unit for petrographic analysis
- Construct balanced geological cross-sections across the Main Central Thrust zone
- Interpret the deformation sequence and relate it to the regional tectonic framework
Geological Setting
The study transect crosses three major lithotectonic units of the Nepal Himalaya. The Lesser Himalayan Sequence (LHS) comprises metasedimentary rocks — phyllite, quartzite, metasandstone, and dolostone — of Proterozoic to Lower Palaeozoic age, deformed by southward-vergent folds and thrust sheets. The Main Central Thrust (MCT) zone separates the LHS from the overlying Higher Himalayan Crystallines (HHC), which consist of high-grade metapelites, orthogneisses, and leucogranites of Precambrian to Eocene age.
The MCT zone itself is a broad ductile shear zone up to several kilometres wide, characterised by mylonite, phyllonite, and augen gneiss with strong top-to-south shear sense indicators.
Methodology
Field Mapping
Systematic mapping along the transect was conducted over 14 field days, covering approximately 120 km of traverses. At each of 48 stations, the following data were collected: bedding orientation (strike and dip), fold axis orientation (plunge and trend), joint sets (orientation, spacing, roughness, aperture), fault orientation and kinematic indicators (slickenlines, fibres, offset markers), and lithological description.
Petrographic Analysis
Twenty-two oriented rock samples were collected from key structural positions — including footwall and hanging wall lithologies on either side of the MCT, mylonite zones within the shear zone, and representative specimens from each major lithostratigraphic unit. Thin sections were prepared and examined under polarised light microscopy at Tribhuvan University.
Key Results
Structural Finding
Stereonet analysis of 48 measurement stations confirms a dominant NW-SE structural grain with southward fold vergence throughout the Lesser Himalayan sequence, transitioning to a diffuse, high-strain fabric with top-to-south kinematic indicators within the MCT shear zone — consistent with progressive southward propagation of the thrust system.
Lesser Himalayan Sequence
Fold analysis reveals two phases of deformation: an early phase (D1) producing recumbent to gently inclined folds with axial planes dipping 40–60° northward, overprinted by a later upright to steeply inclined fold phase (D2) with NW-SE axial trends. Thrust faults of the Main Boundary Thrust system root northward and carry LHS rocks southward over Siwalik molasse deposits.
MCT Shear Zone
The MCT zone extends over a 4.2 km structural thickness in the study transect. Key kinematic indicators include: S-C fabrics in mylonitic phyllonite, rotated porphyroclasts (feldspar and quartz) with asymmetric pressure shadows, and shear band cleavage — all indicating a consistent top-to-south (normal-sense relative to the thrust) shear direction, confirming ongoing southward transport.
Petrographic Observations
Thin-section analysis of MCT zone rocks reveals dynamic recrystallisation of quartz (bulging, subgrain rotation) and syn-kinematic growth of biotite and muscovite along the foliation — indicating deformation at greenschist-facies conditions (approximately 350–450°C), consistent with published pressure-temperature paths for the MCT zone in central Nepal.
Interpretation
The structural data confirm that the study transect records a progressive southward-younging deformation sequence consistent with critical wedge mechanics. Earlier, deeper-level ductile deformation recorded in the HHC and MCT zone is overprinted by shallower, more brittle thrust deformation in the Lesser Himalayan sequence — reflecting the progressive incorporation of new material at the base of the growing orogenic wedge as the Indian plate continues to underthrust Asia at approximately 18–20 mm/year.