New Constraints on the Structural and Exhumation History of the Kirthar Fold-and-Thrust Belt, Western Pakistan

The western fold-and-thrust belt (FTB) of Pakistan is structurally divided into the Sulaiman FTB in the north and the Kirthar FTB in the south, separated by the Quetta syntaxis. We investigate the spatiotemporal structural evolution of the Kirthar FTB with low-temperature thermochronology (Apatite U-Th-Sm/ He (AHe), Fission track (AFT), and Zircon U-Th/He (ZHe)) age data. Our dataset includes 14 AHe, 9 AFT, and 4 ZHe surface ages, supplemented by 18 AFT, 56 vitrinite reflectance (VR), and 9 Rock-Eval (Ro) samples from foreland wells.

In the Northern Kirthar, partially reset Eocene AHe ages and AFT age of 135 ± 34 Ma, combined with unreset ZHe ages (187.7-424.6 Ma) from Cretaceous samples, indicate shallow burial and preserved signatures of pre-Oligocene uplift. In contrast, Central Kirthar records resetting of Paleocene to Oligocene samples with AHe ages (~6–8 Ma), however; the AFT (~119 ± 43 Ma) and ZHe ages ages (63-433 Ma) show partial resetting, suggesting Late Miocene to Pliocene rock uplift from ~3 km depth. In Southern Kirthar, the westernmost samples yield Oligocene AHe ages of 5.3 ± 1.7 Ma, AFT ages of 23.3 ± 8.3 Ma, and un-reset ZHe ages (202.0 to 390.7 Ma), suggesting burial of ~3 km before exhumation. However, range-front samples show different patterns with a partial reset age (21.2–6.5 Ma) for, hanging-wall Oligocene and footwall Miocene samples (18 Ma). In the Katawaz Basin, deeper burial (~3-5km) is evident in the northern sector, while southern sectors experienced only limited burial, insufficient to reset AHe and AFT ages.