Abstract The timing of Cenozoic deformation in the East Kunlun Mountains, northern Tibet, is crucial for understanding plateau growth but remains disputed due to the scarcity of direct chronological constraints on specific tectonic structures. Here, we present a high‐density‐sampling magnetostratigraphic record from the Xiaohongshan section, interior of the East Kunlun Mountains, where growth strata directly constrain syndepositional thrust activity. Correlation with the geomagnetic polarity timescale, supported by ostracod biostratigraphy, dates the section to 25.7–17.5 Ma and places the onset of growth strata at ca. 23 Ma, marking the initiation of major thrust faulting. We propose that this thrust deformation was triggered by the northward flow of weakened lower crustal material from the Hoh Xil Basin, which was obstructed by the rigid lithospheric keel of the Qaidam Basin, leading to upper crustal shortening and surface uplift. Our results provide a direct temporal link between deep lithospheric processes and surface tectonics in northern Tibet.