Kinematics of the Greater Himalayan sequence, Dhaulagiri Himal ...

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sequence of the Everest region show significant pure shear with W m values ranging between 0.63
and 0.77 (57 – 44 % pure shear) and an average value of 0.69 or 52% pure shear (Jessup et al.
2006). When the tectonic framework of each study area is reinterpreted to reflect the MCT of
Searle et al. (2008), the results of all vorticity studies can be incorporated to provide insight into
vorticity variation across the Greater Himalayan sequence (Figure 14a). Such a compilation
should be viewed with caution, however, as the pressure-temperature-time conditions specific to
each specimen used for vorticity analysis is not known and the relative location of the samples
within the Greater Himalayan sequence with respect to the hinterland and foreland changes along
the orogen. If the vorticity data are taken at face value simple shear appears to dominate near the
upper boundary of the Greater Himalayan sequence, while almost equal portions of pure shear
and simple shear are measured in the interior and lower portions (Figure 14b). Increasing pure
shear towards the base of the Greater Himalayan sequence may be partially attributable to
lithostatic loading. The recognition of significant pure shear implies: (1) thinning and along-dip
extension of the Himalayan infrastructure (Law et al. 2004; Jessup et al. 2006); and (2) increased
strain rates and extrusion rates relative to that expected in a simple shear system (Law et al.
2004; Jessup et al. 2006).
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6.4 Implications for mid-crustal extrusion processes
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The extrusion of the mid-crustal metamorphic core from beneath Tibet has been
attributed to contemporaneous movement along the MCT and the STDS during the Miocene
(Grujic et al. 1996; Grasemann et al. 1999; Beaumont et al. 2006; Godin et al. 2006a;). During
and subsequent to the southward extrusion of the Greater Himalayan sequence from beneath
Tibet the décollements at the top and bottom of the panel developed into wide shear zones, the
boundaries of which have often been mapped as separate faults with the designations ‘1’ and ‘2’
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