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In this paper, we will use the knowledge of ultrahigh-pressure (UHP) metamorphic evolution as independent data to constraint the geometry and reconstruct the tectonic evolution of the early stage of the India-Asia collision. The two UHP units recognized in NW Himalaya (Kaghan in Pakistan and Tso Morari in India) belong to distal parts of the continental Indian margin subducted between 55 and 45 Ma at a minimum depth of 100 km (e.g. Guillot et al. 2003 for review), evolving simultaneously during the early Himalayan evolution. They are interpreted as the signature of the early subduction of the Indian continental plate at the Paleocene-Eocene boundary. The metamorphic conditions are synthesized in Table 1 for both units. Even if similar protoliths are involved and both UHP units record the same maximum depth of about 100 km, some differences in P-T-t conditions may be emphasized. Firstly, the temperature of the metamorphism peak is significantly lower than a minimum of 100°C in the Tso Morari unit suggesting that the subduction rate is higher (Peacock 1992) in the Eastern part. Secondly, the western Kaghan unit seems to be involved in the subduction zone 4 Ma (~ 53 Ma) after the Eastern Tso Morari unit (~ 57 Ma). The later implication of the Kaghan unit in the subduction zone is related to its greater internal localization on the Indian margin, which, combined with a lower subduction rate, induce a higher temperature peak. As the UHP units are buried and exhumed along the subduction plane (e.g. Chemenda et al. 2000), the dip angle of the subduction plane can be deduced from the geometry of the subduction and the timing of the processes. The sinus of dip angle is equal to the amount of vertical displacement (D) during an interval of time, divided by the amount of Indian plate subduction during the same time interval (H). Those two data are independently measured, D from the exhumed rocks, H from the motion of the Indian plate.

Himalayan ultrahigh pressure rocks and warped Indian subduction plane