Defining the Himalayan Main Central Thrust in Nepal - Queen's ...

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M. P. SEARLE ET AL.
Fig. 1. Geological map of the Himalaya.
Zones of high strain have been documented across the Main
Central Thrust zone in the Arun Valley by Brunel (1986) and
Brunel & Kienast (1986) using kinematic criteria. Abundant
shear criteria (S–C fabrics, rolled garnets, etc.) and stretching
lineations show southward transport of the Greater Himalayan
Sequence along the Main Central Thrust zone. Quantitative
vorticity studies were first reported from the Sutlej Valley, India
by Grasemann et al. (1999). Law et al. (2004) and Jessup et al.
(2006) showed that mean kinematic vorticity numbers from the
Main Central Thrust zone along the Everest profile in Nepal
yielded 58–44% pure shear component in addition to the
dominant top-to-south simple shear. The fact that metamorphic
isograds across the Main Central Thrust ductile shear zone have
been compressed or telescoped into a 1–2 km thick section
(Searle & Rex 1989; Hubbard 1996) shows that shearing postdates
peak metamorphism.
Here we discuss the various previous methods used to map or
define the Main Central Thrust and then we propose a unifying
definition and map location, in the hope that future studies
relating to the Main Central Thrust will refer to one single Main
Central Thrust ductile shear zone and brittle thrust fault.
Metamorphic isograds and the Main Central Thrust
The earliest attempt to map the Main Central Thrust was carried
out by Bordet (1961), who mapped it along a prominent kyanitebearing
pelite band within sillimanite gneisses in the Arun valley,
but did not describe any structural criteria to support this
placement (Figs 2 and 3). The same location for the Main
Central Thrust was subsequently used by Lombardo et al. (1993)
and Pognante & Benna (1993), who mapped the Main Central
Thrust further north as far as Kharta in southern Tibet. Above
this kyanite-bearing pelite horizon are some 5–8 km thickness of
sillimanite + garnet + biotite + cordierite orthogneiss (variously
called the Barun gneiss, Black gneiss or Jannu–Kangchenjunga
gneiss) with evidence of abundant partial melting (Brunel &
Kienast 1986; Lombardo et al. 1993; Pognante & Benna 1993;
Searle & Szulc 2005). High-grade calc-silicate gneisses and