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In the Bhagirathi River Transect of the Garhwal Himalaya, India, the existence of the Jhala Normal Fault (JNF) and its movement sense are disputed. The JNF has been considered either as part of the South Tibetan Detachment System (STDS) or as a distinct, more southerly discontinuity within the Higher Himalayan Crystalline Sequence (HHCS). Field studies reveal that the JNF lies entirely within the HHCS, with both the JNF footwall and hanging-wall preserving thrust-related shear markers within amphibolite facies HHCS rocks. Rare extensional shear markers are, however, observed at the base of the JNF hanging-wall. New U-Pb zircon rim and monazite SHRIMP ages of 33.8 ± 0.8 Ma and 30.7 ± 0.5 Ma obtained in this study represent the timing of metamorphism in the JNF hanging-wall and footwall, respectively. Together, the field and geochronological evidence suggest that during Eocene-Oligocene channel flow in the HHCS, the slow-moving marginal part of the channel representing the JNF hanging-wall was trailing its more rapidly extruding footwall, resulting in apparent normal-sense movement across the JNF. The intrusion of 21.4 ± 2.3 Ma (monazite U-Pb age) tourmaline-bearing leucogranites within the JNF hanging-wall testifies to its ongoing uplift as part of the exhuming Miocene HHCS channel. The absence of any metamorphic break or distinct extensional shear zone at the JNF indicates that it originated as an intra-channel discontinuity rather than a major lithotectonic boundary. |
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