The Transition From North‐South to East‐West Extensional Structures in the Himalayan‐Tibetan Orogenic System

Abstract In southern Tibet, both the South Tibetan detachment system (STDS) and north‐south trending rifts (NSTRs) are prominent extensional structures. There is debate about whether they overlap temporally or not, which would correspond, respectively, to the tectonic transition being an active process driven by internal tectonic forces or a passive process where one structure triggers the other in response to external gravitational or stress relaxation mechanisms. Resolving this controversy is essential for reconstructing the dynamic transition from syn‐collisional crustal thickening to post‐collisional extension. The Yadong rift, located in the central Himalaya and Lhasa terrane, truncates the STDS and thus provides a key structure to unravel this problem. Combining structural analysis with newly determined dates of zircon U‐Pb (∼22 Ma, ∼18 Ma and ∼14 Ma), mica 40 Ar/ 39 Ar (∼14.2–13.5 Ma), and apatite fission track (∼8.4–7.3 Ma), our study indicates that ductile deformation associated with this rift ceased by ∼14.2–13.5 Ma followed by brittle deformation that overlapped with STDS activity (∼14.2–11 Ma). Furthermore, compiled data on the activation timings of both structures along the Himalayas reveal that the timings of brittle motion on the STDS and ductile motion on the NSTRs overlap spatiotemporally along the Himalayas, with an eastward younging pattern, which challenges the popular idea that the transition from N‐S to E‐W extension represents an abrupt geodynamic shift. Instead, our findings support a refined interpretation of the mechanism involving eastward lithospheric flow driven by lateral detachment of the Indian slab, coupled with asynchronous gravitational potential energy gradients.