Ductile Shear As A Cause of Inverted Metamorphism: Example From the Nepal Himalaya: A Discussion

Abstract

Hubbard (1996) postulated the cause of well-known development of kyanite as well as other index minerals representing dominant metamorphism was metamorphic inversion in a section of the eastern Nepal Himalaya due to the intense ductile shearing either preor synchronous with, but outlasted the deformation because of paucity of growth of metawithin the Main Central Thrust (MCT) zone and the overlying Tibetan Slab on the basis of geologimorphic minerals across the main tectonic fabric (Hubbard 1996, p. 496). cal and textural data (cf. Hubbard 1989). The main thesis of her recent paper revolves around the fact The metamorphic belts of the NW Himalaya and the eastern Nepal are characterized by (i) ductile that the distribution of inverted metamorphic isograds is confined to intensely sheared, ,5 km thick shear zones of high strain revealing S-C fabric and other shear structures having a top-to-south/southzone of the MCT, which either post-dated or outlasted predominant metamorphic assemblages (p. west vergence, not only in the basal parts but also in the Tibetan Slab at higher structural levels; 495). Conceptually, right-way-up metamorphic iso(ii) growth of metamorphic minerals is dominantly preto synchronous, but occasionally outlasting grads in the young orogenic belts like the Himalaya can be very efficiently inverted due to intense the main deformation; (iii) lack of large-scale overturned or recumbent folds in many sections, except shearing in a broad ductile shear zone. We wish to point out that the model, proposed by Hubbard Zanskar (Searle et al. 1992), and the Beas Valley, Himachal Pradesh (Frank et al. 1977); and (iv) pres(1996) for the eastern Nepal, is the same in principle as ours for the northwestern Himalayas pubence of inverted metamorphism in the lower and middle sequences and a normal sequence in the lished in Geology in 1993. Earlier, Hubbard (1989) fixed the upper boundary upper parts near the Tethyan sedimentary cover. Numerous structures having predominantly topof the 3–5 km thick MCT zone from this area arbitrarily, as she has documented that ‘‘The upper and to-southwest sense of ductile shearing have been frequently documented across many tectonic units lower boundaries of the MCT zone are not distinct, so for the sake of discussions the author defines of the Lesser Himalaya, HHC, and Tethyan Sedimentary Zone (Brunel 1986; Treloar and Rex 1990; boundaries that include the majority of high-strain zone’’ (p. 20). Ductile shear zones were also obPatel et al. 1993) and can, therefore, be correlated with the intense ductile shearing irrespective of the served by her in the overlying Tibetan Slab, where ‘‘Gneisses of the Tibetan slab are foliated though presence of the discrete thrusts or wide thrust zones (Jain and Anand 1988). foliation is somewhat less penetrative than probable coeval mylonitic fabric within the MCT zone’’ The ductile shear model, postulated by Hubbard (1996), bears strong structural similarities to the (p. 21), indicating synchrony of two dominant planar structures in this region. Within the MCT zone, one given earlier by Jain and Manickavasagam (1993), as it is evident from the simplified version she recorded that ‘‘The apparent metamorphic grade across the MCT zone gradually increases toof our model, where effects of post-metamorphic/ thrusting, folding, and other zones of high strain wards structurally higher levels’’ (p. 20), where fibrolite and sillimanite are developed along the folihave not been considered (figure 1). Keeping in mind the regional geological and tectonic variaation as well as in the Tibetan Slab (Hubbard 1989); thus an apparent structural and metamorphic grations that may be expected over the distance of about 1000 km from the NW Himalaya, it is likely dation persists near the upper contact of the MCT zone with the Tibetan Slab. Within the MCT zone, that the growth of metamorphic minerals may be diachronous within the Higher Himalayan shear zone. One of the very basic fact of the NW Hima1 Manuscript received February 1, 1997.