Transient Quaternary fault activity in Central Nepal

Central Nepal is a key natural laboratory for investigating crustal kinematics, exhumation, and thermochronometric records within a critically tapered orogenic wedge. In the High Himalayas, thermochronologic data record remarkably young cooling ages and rapid late Cenozoic exhumation rates, yet the kinematics driving these patterns remain actively debated. Most models aiming to explain the physiographic transition across the Himalayan range propose either predominantly in-sequence deformation focussed on the Main Himalayan Thrust (MHT), with underplating and the growth of a Lesser Himalayan duplex, or significant out-of-sequence (OOS) faulting in the Main Central Thrust (MCT) zone. Existing thermochronometric datasets allow for both end-member interpretations, highlighting the non-uniqueness of steady-state kinematic models based on traditional thermochronometers alone.

We address this issue by adding trapped-charge thermochronometers (luminescence and ESR thermochronometry), which, owing to their extremely low effective closure temperatures, are uniquely sensitive to <1–2 Myr transients in near‑surface thermal histories. These data provide unprecedented sensitivity to short-lived Quaternary pulses of exhumation, potentially associated with OOS fault reactivation. We present new luminescence and ESR thermochronometry data from bedrock samples collected across the four major valleys of the Narayani basin in central Nepal (from west to east: Kali Gandaki, Marsyangdi, Buri Gandaki, and Trisuli), spanning the MCT zone and the High Himalayan range.

Using the thermo-kinematic code PECUBE, we invert this multi-system thermochronometer dataset (including both traditional and trapped-charge thermochronometers) through neighbourhood-algorithm exploration of fault slip rates and activity timing. We test three kinematic scenarios: (i) purely steady-state, ramp-focused uplift along the MHT; (ii) stepwise acceleration linked to mid-crustal duplex initiation at ~10 Ma; and (iii) short-lived Quaternary pulses of exhumation associated with transient MCT reactivation superimposed on long-term MHT-driven uplift.

Our inversion results show that trapped-charge thermochronometers require transient Quaternary uplift pulses in the High Himalayas to reproduce the observed cooling patterns. We quantify the timing, magnitude, and spatial distribution of OOS slip, revealing lateral variations in reactivation activity among the four valleys and testing their correlation with variations in MHT coupling and orographic precipitation patterns. Overall, our results provide quantitative bounds on the timing, magnitude, and spatial variability of Quaternary uplift transients in the High Himalayas, and demonstrate that transient MCT reactivation is required to reconcile thermochronologic data with topography and structural constraints, refining the late Cenozoic kinematic evolution of the MHT–MCT system.