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

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As noted by Law et al. (2004), rigid grain methods may systematically underestimate the
vorticity number if clasts of large aspect ratio are not present. Moreover, rigid grain methods
may under estimate W m if strain has not been sufficient to allow clasts to reach a stable sink
position. Following Xypolias & Kokkalas (2006), we interpret the rather sharp breaks between
clasts in stable sink positions and clasts in freely rotating positions on the RGN (Figures 10b, d,
f) to reflect that strain was indeed sufficient for stable sinks to be reached. The upper bound of
the range of W m values determined using rigid grain techniques, therefore, might more closely
reflect the true value of the kinematic vorticity number (Law et al. 2004).
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A second method used in the study for estimating the vorticity of flow incorporates strain
ratio data measured in the XZ-plane of finite strain (R xz
) and relates it to the angle between the
foliation and the flow plane infered from quartz c-axis fabrics. This method was proposed by
Wallis (1992, 1995) and has recently been applied to both the top and middle of the Greater
Himalayan sequence (Grasemann et al. 1999; Law et al. 2004). Strain ratio data in this study
have been derived from deformed relict quartz grains (as in Bailey et al. 2004; Law et al. 2004;
Xypolias & Kokkalas 2006). In sample 070B the original grain shape aspect ratios have been
preserved by muscovite surrounding the original quartz grain, inhibiting grain boundary
migration (Figure 11a,b). The aspect ratio and orientation with respect to foliation of 38 relict
quartz grains were measured in sample 070B and analyzed using the matrix method of
Shimamoto & Ikeda (1976; using the program MACSTRAIN 2.4 written by Kanagawa 1992)
and the Rf/phi/theta curve method of Lisle (1985). The calculated strain ratios for sample 070B
ranged between 2.36 using the matrix method and 2.41, with a chi-squared value of 2.02 (see
Lisle [1985] for explanation) using the Rf/phi/theta curve method. These strain ratios can be
related to W m , using the angle between the macroscopic foliation and flow plane (Figure 11b),
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