John Hirschmiller

1st Class Honours
Best Thesis Award

The 2500 km long Himalayan orogen is characterized by continuity of the principal lithotectonic units.  There is evidence for faster convergence rates in the eastern portion the orogen compared to the western portion; in addition, present-day precipitation rates and Late Miocene erosion rates indicate a west-to-east increasing gradient.  Further complications are induced by the Shillong Plateau, which is a basement pop-up structure in front of the eastern Himalaya, and the only active structure in the Himalayan foreland that could accommodate or partition 4-7 mm/yr of plate convergence.  This study aims to test whether these differences are reflected in the rates of tectonic activity and shortening along the range.  To do so I have constructed balance cross-sections for 10 transects across the Siwalik Group.

            The Siwalik Group comprises the deformed part of the Neogence foreland basin along the southern orogen margin.  The group consists of synorogenic sediments, which date back to ca. 18.5 Ma and form the youngest and frontal parts of the Himalayan fold-and-thrust belt.  Thrust faults in the Sub-Himalaya are splays of a major decollement (the Main Himalayan Thrust), which spans the entire Himalaya thrust belt.  Several south-verging thrusts define the deformation and shortening in the Siwalik Group:  (1) the Main Boundary Thrust is the backstop of the Siwalik group against the Lesser Himalaya, (2) a succession of duplexes are present within the Sub-Himalaya, and (3) the Main Frontal Thrust is the frontal deformed toe.

            During the last 11 myr, convergence rates between the Indian plate and the Eurasian plate were at a steady rate, but with a lateral gradient of ca. 34 mm/yr in the northwest to ca. 44 mm/yr in the northeast of India.  Current GPS velocities are consistent with plate convergence velocities, since rates are ca. 10 mm/yr greater in the east than the west.  By constructing internally consistent cross-sections, the shortening rates and strain rates obtained helped determine difference in strain rates and shortening rates along the Himalaya.  There is a distinct west-to-east increase in strain rates across the Himalaya arc, which correlates with two external parameters.

            The exact cause of the west-to-east trend of Pliocene to present shortening can only be determined through quantitative testing of parameters.  However, it is seen that rainfall amounts along the Himalayan arc have a direct correlation to strain rates, as there are higher strain rates and rainfall in the west than the east.

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Pages: 127
Supervisor: Djordje Grujic