Along‐Strike Variation in the Initiation Timing of the North‐Trending Rifts in Southern Tibet as Revealed From the Yadong‐Gulu Rift

A key issue in the Cenozoic evolution of the Tibetan plateau is the geodynamic drivers for north-trending rifting in southern Tibet. Recent studies have demonstrated an eastward propagation pattern for rift initiation, but the along-strike variations remain poorly resolved. Two models that predict different north-south rift kinematics include northward underthrusting or southward tearing of the Indian lithospheric slab, predicting a northward or southward propagation trend of individual rifts along strike, respectively. The Yadong-Gulu rift (YGR) is an ideal case to investigate this issue due to its long strike length (similar to 500 km) and location above proposed slab-tear structures. Here, we compile constraints on both rift initiation and acceleration, and report new apatite fission track and (U-Th)/He thermochronological data along the southern segment of YGR. Our main findings are as follows. First, the rifts west of the YGR initiated simultaneously along strike, which we suggest is at odds with predictions of either slab-tear or slab-underthrusting models. However, most of these rifts show a northward younging pattern in rift acceleration, which may be governed by low-angle Indian slab underthrusting released by slab tearing. Second, the initiation timing of the Yadong rift is constrained at similar to 13-11 Ma. Combined with published constraints along strike, we demonstrate a clear northward propagation in rift initiation along the YGR. This kinematic pattern may be affected by its orientation of the most oblique northeast-trending among all rift systems or the outward expansion of the Himalayan arc.