Foliation intersection axes preserved in garnet porphyroblasts from the Swat area, NW Himalaya: A record of successive crustal shortening directions between the Indian plate and Kohistan–Ladakh Island Arc

Abstract

We have investigated the geometry and tectonic significance of porphyroblast inclusion trails in 61 oriented samples from the Swat region of north Pakistan (NW Himalaya). The studied samples represent the Indian-plate cover sequence, deformed and metamorphosed to medium- and high-grade during the Himalayan orogeny. Two complementary techniques were applied to measure the orientation of Foliation Intersection/Inflection Axes (FIA) in porphyroblasts in the studied samples and reveal the existence of three principal FIA sets in the study area (measuring approximately 60 × 30 km). Each FIA set is characterized by a distinctive, regionally consistent trend and relative timing, from old to young: ESE–WNW (FIA set1), E–W (FIA set 2), and broadly NNE–SSW (FIA set 3). This sequence is inferred to record an anticlockwise rotation of the direction of tectonic transport or bulk crustal shortening (perpendicular to FIA) through time, either progressively or in a stepwise fashion, consistent with previous kinematic- and geochronological data regarding collisions between the Indian plate, Kohistan–Ladakh Island Arc (KLIA) and Eurasia between ca. 55 and 40 Ma. FIA set 1 and 2 correlate well with early-formed ESE–WNW and E–W trending thrust-related structures in the study area, respectively, whereas the trend of FIA 3 matches that of relatively young regional-scale folds and shear zones with a conspicuous N–S trend. Hence, the FIA succession has successfully resolved the relative timing of formation between the E–W versus N–S trending structures in the region, which was a longstanding question in the tectonics of the NW Himalaya. In addition, a comparison of orientation data for (symmetrical) ‘millipede’ inclusion trails versus (asymmetrical) ‘curved’ trails indicates that porphyroblasts experienced very limited differential rotations in the region, despite the complex, polyphase deformation history recorded. Perhaps the most fascinating aspect of our findings is the direct relationship that is suggested between FIA trends and successive vectors of relative Indian plate-motion since collision with the KLIA.