Signatures of mountain-building: Middle Miocene reorganization of deformation, erosion, and deposition on the northeastern Tibetan plateau

The timing and nature of lateral expansion and upward growth of the Tibetan Plateau remains enigmatic, despite its relevance for exploring both the competing geodynamic processes responsible for the world's largest topographic perturbation and its climatic consequences. Variations in the direction and timing of range growth demonstrate how India-Asia convergence was manifest in northeastern Tibet (near 36°N, 103°E) throughout the Cenozoic. I detect a >45° change in shortening direction and three-fold acceleration in shortening rate in middle Miocene time by examining (i) thermochronological relief transects from contractional ranges in combination with (ii) stratigraphic successions from adjacent foreland basins and (iii) structural cross-sections. Apatite (U-Th)/He and fission-track age-elevation relationships suggest an onset of rapid exhumation in the east-west trending Laji Shan at ∼22 Ma. Rapid exhumation of the adjacent north-trending Jishi Shan, however, did not commence until ∼13 Ma. Further support for this diachronous Miocene range growth emerges from records of subsidence and provenance from magnetostratigraphically-dated sections in the adjacent Xunhua and Linxia basins. Emergence of the east-west trending Laji Shan is highlighted in the Xunhua basin by a doubling of sediment-accumulation rates between 24 and 21 Ma and by the introduction at 20 Ma of fluvial facies for which detrital zircon U/Pb age spectra show were sourced from basement terranes within the Laji Shan. In contrast, growth of the north-trending Jishi Shan is expressed by a 50% acceleration in Xunhua accumulation rates between 12 and 9 Ma and a doubling of detrital zircons derived from Jishi Shan sources by 11.5 Ma in the Linxia basin. Finally, balanced cross-sections indicate (i) 11% east-west shortening since middle Miocene time, and (ii) ∼9% NNE-SSW shortening since middle Eocene time, obviating lower crustal flow as a thickening agent. Together these diverse observations document a middle Miocene change in the kinematic style of plateau deformation from initial north-south contraction along a trajectory that mimicked the India-Asia collision to the later onset of east-west contraction and intensified shortening. This kinematic shift in northeastern Tibet coincides with significant changes in style and rate of deformation across the plateau and may herald a plateau-wide reorganization of deformation.