METAMORPHISM AND SUBDUCTION IN THE SWAT REGION, PAKISTAN HIMALAYA

Three domal structures, the Indus Syntaxis in the east and the composite Loe Sar-Kotah dome in the west, characterize the northern continental margin of the Indian plate in Swat, Pakistan. This area was autochthonous prior to Miocene foreland thrust faulting and is structurally below eclogite-bearing thrust sheets of the adjacent Kaghan area to the east. The Indus syntaxis has a wave height greater than 10 km and has undergone multiple periods of erosional exhumation prior to, during, and following Paleogene orogeny. Exhumation during orogeny is suggested because Indus mélange units are present at the flanks of the dome but are truncated at the crest. ⁴⁰Ar-³⁹Ar hornblende dates from the core of the structure are Precambrian whereas those along the margin are Himalayan including circa 67 and 50 Ma. This suggests that Himalayan-age metamorphism in the core of the structure was shut-off early during the metamorphic cycle possibly due to erosional exhumation before reaching temperatures high enough to reset hornblende. The 67 and 50 Ma ages suggest that it may be possible to delineate a Precambrian-Himalayan argon-closure age boundary along the margins of the structure. These ages also likely represent the earliest stages of metamorphism. The composite Loe Sar-Kotah dome is smaller than the Indus syntaxis with wave heights less than 6 and 4 km respectively. ⁴⁰Ar-³⁹Ar hornblende dates are mostly between 30 and 40 Ma and metamorphic temperature estimates exceed 600 °C in the core of the structure. This implies that the Loe Sar-Kotah dome achieved higher peak metamorphic temperatures than the Indus syntaxis and that cooling was delayed by at least 10 million years. Both the 67 and 50 Ma dates are older than eclogite facies metamorphism in Kaghan and the 67 Ma date is older than inferred circa 54 Ma India-Kohistan collision. The Kohistan arc cuts across both Indian plate and Indus mélange metamorphic fabric along the Kohistan fault and U-Pb dates on truncated granitic rock indicate fault activity after 45.8 ±0.7 Ma. Although still permissive as an assumption, there is no direct evidence to tie Indian plate metamorphism here or in Kaghan with India-Kohistan collision. Metamorphism is not associated with underthrusting beneath the Kohistan arc but, instead, with underthrusting beneath Indus mélange prior to final emplacement of Kohistan.