EVENT-WISE COLLISIONAL OROGENIC SEQUENCE OF THE INDIAN PLATE WITH EURASIA DEDUCED FROM PORPHYROBLAST INCLUSION TRAIL MICROSTRUCTURES IN THE NW HIMALAYA

‘Millipede’ and ‘rotational’ microstructures preserved in porphyroblasts have extensively been used for the reconstruction of deformation regimes and associated metamorphic paths in many orogenic belts. In this regard, two, rather three, conflicting kinematic models have been postulated involving porphyroblast rotation (e.g. Jiang and Williams, 2004; Jessell et al., 2009), non-rotation (e.g. Aerden and Sayab, 2008; Bell and Hobbs, 2010) or little relative rotation (e.g. Johnson, 2009). These different interpretations on porphyroblast microstructures have important implications for the tectono-metamorphic growth of orogenic system. For this study, we applied advanced 3D microstructural techniques to examine the orientation of inclusion trail patterns preserved in porphyroblasts in medium to high-grade rocks of the Swat region of NW Himalaya in Pakistan. This area represents Indian-plate rocks that were deformed and metamorphosed during the Himalayan orogeny.

Previous workers in the Swat area and adjacent regions assumed ‘rotational’ porphyroblast models to calculate shear strain and kinematics of thrust sheets. However, the timing of porphyroblasts preserving different types of inclusion trail patterns with respect to successive deformation regimes that affected the study area have remained poorly constrained. In order to investigate the growth sequence of porphyroblasts and their relationship with the orogenic evolution of the NW Himalayas, we applied two independent techniques namely ‘asymmetry switch’ and ‘FitPitch’ for determining the orientation of inclusion trails in 3D developed by Hayward (1990) and Aerden, (2003), respectively. Both techniques are based on the measurements of porphyroblast inclusion trail microstructures from multiple, differently oriented thin sections of single samples. Axes of relative porphyroblast–matrix rotation (FIA: Foliation Intersection/Inflection Axes) determined via these methods previously in other orogens have systematically revealed regionally consistent orientations of these microstructures that appear to relate to (successive) crustal shortening directions that can, commonly, no longer be deduced from the study of matrix microstructures alone.

Fifty-five samples analyzed using both techniques yield coherent single- and multi-FIA results that demonstrate the existence of four principle sets of FIA in the study area. These FIA sets can be distinguished based on their specific relative timing, and geographic trends. The FIA trend succession, from old to young is SE-NW (FIA set 1), E-W (FIA set 2), NE-SW (FIA set 3) and NNE-SSW (FIA set 4). FIA set 2 can be further subdivided into set 2a (ESE-WNW) and set 2b (ENE-WSW). The FIA succession suggests an anticlockwise rotation of the bulk shortening directions through time, consistent with the collision history of the Indian plate, Kohistan-Ladakh Island Arc (KLIA) and the Eurasian plate since 55 Ma. The orientation of FIA set 2 can be correlated with early-formed E-W trending thrust-related structures, whereas the trend of FIA set 4 is consistent with the younger regional-scale NNE-SSW trending folds and shear zones. FIA set 1 reflects the orientation of primitive structures that marks the trend of initial collision of the Indian plate with the KLIA. Relics of these primitive structures are scarcely preserved regionally due to transposition by later deformations responsible for FIA sets 2, 3 and 4. In addition, each FIA set is marked by distinct metamorphic cycle associated with unique shortening direction. Our combined data for both ‘millipede’ and ‘rotational’ types of inclusion trails indicate that all developed during a polyphase deformation history without having experienced significant relative rotations between them.

References

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