An integrated petrologic, thermochronologic, and numerical modeling
study constrains 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 C. 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 similar
to 550 degrees C. An inverted thermal field gradient of similar to 30
degrees C/km beneath the Vaikrita thrust is calculated. An inverted
thermal field gradient of similar to 20 degrees C/km is also documented
beneath a klippe of the Main Central thrust hanging wall similar to 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 (40)Ar/(39)Ar
thermochronology reveals that the exposed rock in the Kumaun and Garwhal
Lesser Himalaya underwent cooling below the closure temperature (T(c))
for white mica at different times in the late Tertiary. The proximal
footwall of the Munsiari thrust experienced cooling below the white mica
T(c) 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, similar to 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.