Unravelling Deformation Textures in Himalayan Rocks

Nicholas Hunter, Chris Wilson and Roberto Weinberg (Monash University) and Vladimir Luzin (ANSTO)
 
The Himalayas are renowned for their exceptional topography and mountain ranges, spanning laterally over 2200 km. These spectacular exposures have come about due to ongoing collision between the Indian and Eurasian tectonic plates, and it is the deformation processes occurring underneath the surface that drive the uplift of mountains. In the ductile crust, the intense strains associated with collision are localised along lineaments which geologists refer to as shear zones. Under the surface, the Himalaya comprises a network of ductile shear zones, the most significant of which is named the Main Central Thrust (MCT). This shear zone plays the integral role of emplacing hot rocks of the deep crust atop cold rocks on the Indian plate, which facilitates a high degree of crustal shortening and strain accommodation.
 
The Main Central Thrust provides an interesting case study for neutron texture analysis in naturally deformed rocks. The geological sequence across the shear zone contains intercalated layers of pure quartzite, which have been sheared at higher degrees of deformation with increased proximity to the shear zone core. In nature, most rocks comprise a wide suite of rheologically contrasting minerals, so it is often difficult to relate naturally deformed rocks to the more simple geological materials experimentally deformed in the laboratory. The analysis of quartzite rocks from the Main Central Thrust has allowed us to understand the development of deformation textures in nature, and thus we can link our empirical observations with what has been found in the laboratory.
 
Himalayan rocks figure 1
 
Fig. 1 – Geological map of the Alaknanda Valley (NW India). Here, the Main Central Thrust defines a broad zone of deformation (orange), situated between cold rocks of the Lesser Himalayan Sedimentaries (blue), and hotter rocks of the Joshimath Formation (green). Locations of sampling is shown in yellow, only quartz-rich samples are indicated. 
 
We used the KOWARI strain scanner to study rocks from the Alaknanda Valley (NW India), where the Main Central Thrust is well exposed. The samples for the study were collected over two field trips between the monsoon seasons of 2013 and 2015, spanning the total width of the shear zone of approximately 7 km (Fig. 1). Over 20 samples were carefully selected for neutron diffraction, the majority comprising >95% quartz, with minor quantities of mica.
 
Results from our quartzite samples demonstrate that intense shear strain was accommodated during the deformation (Fig. 2). The intensity of strain increases towards the north, where our pole figures reveal a marked shift in their maxima to more clustered distributions. This shift is typically associated with major changes in temperature and shear strain, and likely reflects the region where the hotter hanging wall of the shear zone is situated (Stipp et al., 2002; Heilbronner & Tullis, 2006). 
 
Himalayan rocks figure 2
 
Fig. 2 – Strain textures of quartzites from across the MCT sequence. 
 
Our results bear striking comparison with textures produced in experimental sheared quartzites (Heilbronner and Tullis 2006). Moreover, we have documented clear systematic changes in the texture of Main Central Thrust rocks as a function of location within the shear zone, an observation that has not been recorded in other sections of the Himalaya (Bouchez & Pecher, 1981; Bhattacharya & Weber, 2004). Thus, our results provide one of the first insights into texture development in the Himalaya. We are currently applying further quantitative analysis to the sample set, which will allow us begin unravelling the dynamic factors controlling shear zone evolution.
 
References:
 
Bhattacharya, A. R. & Weber, K. (2004) Fabric development during shear deformation in the Main Central Thrust Zone, NW-Himalaya, India. Tectonophysics, 387, 23-46.
 
Bouchez, J.-L. & Pecher, A. (1981) The Himalayan Main Central Thrust pile and its quartz-rich tectonites in central Nepal. Tectonophysics, 78, 23-50.
 
Heilbronner, R. & Tullis, J. (2006) Evolution of c axis pole figures and grain size during dynamic recrystallization: Results from experimentally sheared quartzite. Journal of Geophysical Research: Solid Earth, 111, B10202.
 
Stipp, M., Stünitz, H., Heilbronner, R. & Schmid, S. M. (2002) The eastern Tonale fault zone: A 'natural laboratory' for crystal plastic deformation of quartz over a temperature range from 250 to 700 °C. Journal of Structural Geology, 24, 1861-1884.