Tectonic processes that built the foothills of the Himalayas

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Washington, August 1 (ANI): In a novel study, scientists have used new observations and quantitative data-sets to unravel the tectonic processes responsible for building the foothills of the Himalayas - the Lesser Himalayas in northwest India.

The immense and spectacular Himalayas are Earth's most impressive manifestation of plate tectonics.

Despite their importance in our understanding of mountain building processes, large swathes of the Himalayas remain poorly understood in terms of their geology and the tectonic processes which built them.

Now, Julien Celerier and colleagues from Research School of Earth Sciences, Australian National University, Canberra, have integrated petrologic, thermochronologic, and numerical modeling to study the thermal evolution of the Lesser Himalayan Sequence in the Kumaun and Garwhal regions of India.

South of the Tons thrust, peak metamorphic temperatures do not exceed 330 degrees Celsius.

On the northern side, a transition toward higher temperatures adjacent to the Main Central thrust hanging wall is documented.

In the immediate footwall of the Main Central thrust, peak temperatures average 550 degrees C.

An inverted thermal field gradient of 30 degrees C/km beneath the Vaikrita thrust is calculated.

An inverted thermal field gradient of - 20 degrees C/km is also documented beneath a klippe of the Main Central thrust hanging wall - 25 km south of the Vaikrita thrust, and a peak footwall temperature of 530 degrees C is recorded close to the shear zone at the base of the klippe.

The mechanism for thermal metamorphism in the Lesser Himalayan Sequence is interpreted to be conduction of heat from the Main Central thrust fault and/or hanging wall to the footwall.

A localized thermal field gradient is also associated with the Tons thrust.

The 40Ar/39Ar thermochronology reveals that the exposed rock in the Kumaun and Garwhal Lesser Himalaya underwent cooling below the closure temperature (Tc) for white mica at different times in the late Tertiary.

The proximal footwall of the Munsiari thrust experienced cooling below the white mica Tc at ca. 8.5 Ma.

South of the Munsiari thrust, in the proximal footwalls of the Chaukori and Askot klippen, muscovites ages are ca. 11.5 Ma, - 3 Ma older than samples to the north.

The age differences in samples from northern and southern exposures are interpreted to document cooling related to thermal relaxation following passage of the Main Central thrust hanging wall atop the Lesser Himalayan Sequence.

Results of thermal modeling achieve good fits to the data when scenarios involve an early Miocene phase of overthrusting of a hot hanging wall over a downgoing footwall, followed by the initiation of a duplex within Lesser Himalayan Sequence rocks. (ANI)

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