Laura Ritchie

The MacEachern - Ponsford Memorial Award - 2004

The Greater Himalayan Sequence (GHS) is the metamorphic core of the Himalaya bounded by the Main Central Thrust (MCT) in the south and the extensional South Tibetan Detachment (STD) in the north. The GHS of the Bhutan Himalaya also contains the out of sequence Kakhtang Thrust and several sedimentary klippen associated with the STD. The dominant metamorphic feature of the GHS is an inverted metamorphic sequence that occurs from the MCT (amphibolite facies) with higher metamorphic grades (up to granulite facies with associated migmatites and leucogranites) occurring towards the north. In Bhutan, this inverted metamorphic sequence is repeated several times within the GHS, with lower grade metamorphic assemblages (garnet staurolite conditions) occurring in several locations B near the MCT, at the base of the klippen, and in footwall of Kakhtang Thrust.

A comparison of the deformational and metamorphic history of garnet staurolite biotite schists from the sole and roof of the GHS provides insight on the tectonic history of the GHS as a whole and in relation to the other structural units of the Bhutan Himalaya.

Geothermobarometry results of the garnet staurolite schists show that just below the MCT peak metamorphic conditions occurred at 611"27 1C and 10.2"1.0 kbar, contemporaneous to dominant flattening combined with top to the south shearing. The Paro metasediments in the Paro region of eastern Bhutan show a peak metamorphic temperature of 702"43 1C and pressure of 9.8"1.6 kbar. This unit, previously described as either part of the LHS or of the GHS, is reinterpreted as a window to the Lesser Himalayan Sequence because it is bounded by opposite dipping normal faults and has metamorphic conditions compatible with that structural level. Pressure conditions decrease above the STD with an increase in structural level, whereas the temperature conditions range from 608"251C to 715"501C. These garnet staurolite schists of the Chekha Formation exhibit a dominant deformation of vertical shortening, overprinted by north to northwest normal fault kinematics as expected in the STD shear zone. A garnet staurolite schist of the footwall of the Kakhtang Thrust is petrographically equivalent to those of the Chekha Formation and exhibits the same dominant pure shear flattening, with P T conditions of 654"261C and 8.2"1.2 kbar similar to those in the klippen. This unit, the Naspe Formation, can be thus correlated as part of the Chekha Formation.

The metamorphic data obtained here combined with published data indicate that the peak temperature increases in the northwards towards higher structural levels, and then above the STD progressively decreases. At the same time, the pressure at peak temperature progressively decreases from ca. 10 kbar at the base of the GHS to approximately 8 kbar in the roof of the GHS. This relatively simple P T pattern has been subsequently overprinted and repeated by later faulting and thrusting. This P T pattern in the GHS provides an important constraint for models of Himalayan tectonics.

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Pages: 230
Supervisor: Djordje Grujic