Recognition of an episode of post-collisional normal faulting in North West Pakistan: Constraints from fission-track analysis

The Sillai Patti carbonatite has been intruded along a NNE–SSW striking, southward dipping Sillai Patti Fault at the contact of the metasediments with the granitic gneiss. Both, time of emplacement and the sense of movement of the blocks on either side of this fault are under considerable dispute. Fission-track analysis on apatite from five carbonatite samples yielded an emplacement age of 29.3 ± 1.2 Ma. Comparison of the textures and cooling rates indicates that the granitic gneiss on the southern side of the fault has been uplifted at a faster rate than the carbonatite itself located on the northern side. The abrupt jump from lower fission-track ages in the south to higher fission-track ages in the north across the fault has also been noticed. Moreover, a faster total exhumation of 6.67 km during the past 24.4 ± 2.9 Ma for the southern hanging wall of the fault has been noticed based on fission-track analysis of zircon as compared to the slower maximum possible total exhumation of ≤ 1.67 km derived from the fission-track analysis of apatite during the past 29.3 ± 1.2 Ma for the northern footwall. These facts reveal that the Sillai Patti Fault, a southward extension of the Main Mantle Thrust (MMT), showing an apparently thrust sense of movement on local scale at Sillai Patti, has behaved as a normal fault on regional scale further to the north at the contact of the Indian Plate with the Kohistan Island Arc. During this episode of normal faulting the Kohistan Island Arc slid down and to the north along the MMT relative to the Indian Plate. Recognition of an episode of normal faulting during Oligocene on the basis of this study clearly demonstrates that carbonatitic magmatism took place in the region in a post collisional extensional environment along normal fault(s).

► FT age of 29.3 ± 1.2 Ma on apatite from the Sillai Patti carbonatite was obtained.
► This age is concordant with FT and all the high temperatures ages within the region.
► Age of 29.3 ± 1.2 Ma represents emplacement time of the Sillai Patti carbonatite.
► Emplacement of Sillai Patti Carbonatite took place at near surface shallow crustal levels.
► Emplacement of Sillai Patti carbonatite took place during post collisional normal faulting.