Constraining Early Miocene Reactivation of the Main Mantle Thrust (Swat Valley, Pakistan) through Integrated Structural and Thermochronologic Analysis

Unravelling the tectonic evolution of collisional orogens and the forces driving the exhumation of high-grade metamorphic rocks requires constraining the kinematics and timing of major fault systems. The Main Mantle Thrust (MMT) in the Swat Valley (northern Pakistan) marks the Eocene suture between the Indian Plate and the Kohistan–Ladakh Island Arc. Its later reactivation is critical yet debated, with models ranging from dominantly dextral strike-slip faulting with minor normal offset (typically less than a few meters) to significant normal faulting facilitating regional exhumation of high-grade metamorphic rocks. This research integrates structural geology with a new, multi-method low-temperature thermochronology dataset—including zircon and apatite (U–Th)/He and apatite fission-track ages—to contribute to this debate. Structural analysis of over 150 kinematic indicators collected along the MMT and its footwall shows that the MMT experienced semi-ductile to brittle reactivation dominated by top-to-the-north normal faulting. Structures formed under greenschist- to sub-greenschist facies conditions (e.g., C′ shear bands) in the footwall record progressive exhumation, with purely brittle cataclastic deformation marking the final stages. Our new thermochronometer dateset includes zircon (U–Th)/He (ZHe) single-grain ages (14.9 ± 1.2 to 24.4 ± 2.0 Ma), apatite (U–Th)/He (AHe) single-grain ages (7.6 ± 0.4 to 15.9 ± 1.0 Ma), and apatite fission-track (AFT) central ages (15.3 ± 2.4 to 16.4 ± 3.2 Ma). This dataset, alongside its thermal history modelling, reveals a consistent cooling signal across the Swat Valley. Following the Eocene collision and peak metamorphism, a major tectonic shift occurred in the Early Miocene. Our data indicate the onset of rapid cooling at ~20 Ma, with a slightly later initiation at ~18 Ma in the eastern Loe Sar Dome. This distinct phase of rapid cooling records the top-to-the-north normal reactivation of the MMT, which lasted until ~15–14 Ma. Collectively, our results provide structural and timing constraints supporting a model of protracted, normal-sense reactivation of the MMT between ~20 and 15 Ma. This event facilitated the final unroofing of high-grade metamorphic rocks in the Swat Valley.