Rapid cooling of crustal rocks is commonly interpreted as evidence for rapid exhumation or erosion during orogenesis. In contractional settings such as the Himalaya, however, thrust cessation can itself generate transient thermal conditions capable of producing ultra-rapid cooling (>100 °C/Myr) without intensified surface denudation. Here, we test this mechanism using new thermochronological data from the Bhutan Himalaya together with forward thermo-kinematic modelling. We present 11 new bedrock zircon fission-track (ZFT) ages (7.4–14.4 Ma) that complement published muscovite 40 Ar/ 39 Ar and rutile U–Pb data from the same region. The close agreement in age among systems with markedly different closure temperatures implies at least 300 °C of cooling within 1–2 Myr, indicating an episode of ultra-rapid cooling in the Middle Miocene. We evaluate this episode using one-dimensional analytical models and three-dimensional numerical models parameterised for thrust cessation along the Main Himalayan Thrust. Our models show that termination of thrusting initiates conductive thermal relaxation in the upper crust, producing cooling rates of up to 380 °C/Myr during the first ∼0.2 Myr above the basal-detachment ramp, declining over the following ∼2 Myr. These transient high cooling rates compress effective closure temperatures and can yield tightly clustered thermochronological ages across systems with widely different nominal closure temperatures, a pattern commonly attributed to rapid exhumation. Under sufficiently rapid cooling, effective closure temperatures of nominally higher- and lower-temperature systems may even overlap. We conclude that conductive relaxation of a previously advected, high-gradient thermal field can match or exceed cooling rates expected from erosion-driven exhumation. The numerical experiments are intended as conceptual end-member tests rather than a new inversion of the full Bhutan age field, but they show that post-thrust thermal relaxation must be considered when interpreting thermochronological records in active orogens. • Thrust cessation or slowdown causes conductive relaxation of previously advected isotherms. • Bhutan Himalaya thermochronological ages record Middle Miocene ultra-rapid cooling. • Overlapping thermochronometer ages require at least 300 °C of cooling in 1–2 Myr. • Cooling rate peaks above the basal-detachment ramp at up to 380 °C/Myr. • Age clustering and closure-temperature compression need not imply rapid exhumation. • The depth at which closure temperatures are attained increases with time.
Grujic et al. (Wed,) studied this question.