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

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M. P. SEARLE ET AL.
stratigraphy, but not structure. Because thrust trajectories cut up
and across stratigraphic section in the transport direction, these
methods are clearly not useful in defining the position of thrust
faults such as the Main Central Thrust. Isograds are metamorphic
reactions that can be mapped (with difficulty) in the field by the
first appearance of key index minerals (sillimanite, kyanite,
staurolite, garnet). Young monazite ages reveal specific information
on growth of the garnet that armours them, or the matrix
that contains them, and fluid infiltration, neither of which are
definitively associated with motion along a thrust fault. Only
structural mapping and strain indicators can define the position
of the Main Central Thrust. Following Hanmer & Passchier
(1991) and Passchier & Trouw (2005), the essential criteria to
define a shear zone are the identification of a strain gradient and
the clear localization of strain.
As the metamorphic isograds are always telescoped along the
base of the Greater Himalayan Sequence with up to 50% pure
shear flattening superimposed on the already ‘frozen-in’ isograds
(Jessup et al. 2006), the position of the inverted metamorphism
often correlates closely, or precisely, with the position of the
Main Central Thrust ductile shear zone. In the western Himalaya,
the Main Central Thrust zone inverted metamorphic isograd
sequence along the base of the Greater Himalayan Sequence has
been mapped around a NW-plunging recumbent anticline, and
has been shown to join up with right-way-up isograds along the
footwall of the South Tibetan Detachment low-angle normal fault
at the top of the Greater Himalayan Sequence (Searle & Rex
1989). The map relationship and timing constraints (Hodges et
al. 1996) show that movement along the Main Central Thrust
and South Tibetan Detachment were synchronous, and that the
Greater Himalayan Sequence moved south, bounded by these
shear zones above and below, during southward extrusion of the
ductile partially molten core of the Greater Himalayan Sequence
(Fig. 9; channel flow model).
We suggest that a common unifying definition for the Main
Central Thrust should be ‘the base of the large-scale zone of
high strain and ductile deformation, commonly coinciding with
the base of the zone of inverted metamorphic isograds, which
places Tertiary metamorphic rocks of the Greater Himalayan
Sequence over unmetamorphosed or low-grade rocks of the
Lesser Himalaya’, similar to that suggested for the Kishtwar
Main Central Thrust section by Stephenson et al. (2000, 2001).
Whereas the Kishtwar section shows an exhumed, deeper, more
internal section across the Main Central Thrust zone, the
Kathmandu nappe and Ramgarh thrust sheets show a shallower,
more external section across the Main Central Thrust (Fig. 8).
The Main Central Thrust ductile shear zone is commonly
bounded along the south (base) by a brittle thrust fault, so a
distinction could be made between the Main Central Thrust
ductile shear zone (up to 2 km or more thick) and the brittle
Main Central Thrust fault (sensu stricto) along its base.
We acknowledge NERC, NSF and NSERC grants respectively to M.P.S.,
R.D.L. and L.G., and UK (M.J.S.), New Zealand (J.M.C.), Canadian
(K.P.L.) and US (M.J.J.) PhD studentships grants. We thank the late
Pasang Tamang, and Pradap Tamang and team for excellent trekking
logistics in the Annapurnas, Tashi Sherpa and Sonam Wangdu in the
Everest region, and Suka Ghale and team in the Manaslu region. The
paper benefited greatly from reviews by Paul Myrow and Richard Brown,
and discussions with Mike Johnson, Randy Parrish and Laurent Bollinger.
Fig. 9. Generalized model for the Main
Central Thrust ductile shear zone and thrust
fault, and Greater Himalayan Sequence
channel flow along the Himalaya. The
South Tibetan Detachment and Main
Central Thrust were active simultaneously
during the early to middle Miocene, and the
deeper ductile shear zones pass upward and
outward into brittle faults with time. The
mid-crustal channel of partially molten
crust separates the brittle deforming
seismogenic upper crust from the rigid,
high-pressure granulite lower crust of the
subducted Indian Shield.