Two of the most popular mechanisms for thickening the crust beneath the Tibetan Plateau are (1) pure shear with faulting and folding in the upper crust and horizontal shortening below and (2) flow of lower or middle crust without significant shortening of the upper crust. To help discriminate between the relative contributions of these two mechanisms, well-constrained estimates of upper crustal shortening are needed. Here we document the Cenozoic shortening budget across the northeastern Tibetan Plateau margin with several balanced cross sections that exploit thermochronological and magnetostratigraphic constraints. These sections indicate 11 ⁺²/-1% east-west shortening since middle Miocene time and ∼9 ⁺²/-3% NNE-SSW shortening between middle Eocene and middle Miocene times with little subsequent shortening of this orientation. Shortening rates accelerate fivefold after middle Miocene time. Given the present-day crustal thickness of 56 4 km in northeastern Tibet, crustal restorations that remove Cenozoic shortening suggest that the northeastern Tibetan crust was 45±5 km thick prior to the India-Asia continental collision. This precollision thickness estimate is equivalent to average continental crustal thicknesses both adjacent to the Tibetan Plateau (44±4 km) and globally (41±6 km) and suggests that pure shear alone may account for Cenozoic crustal thickening in northeastern Tibet. In contrast to eastern Tibet where, in the absence of significant shortening structures, crustal flow has been invoked to explain the addition of crustal material since middle Miocene time, our results may obviate lower crustal flow as a necessary crustal thickening agent in northeastern Tibet. Copyright © 2012 by the American Geophysical Union.

Sedimentary rocks in Tibetan Plateau basins archive the spatiotemporal patterns of deformation, erosion, and associated climate change that resulted from Cenozoic continental collision. Despite growing understanding of basin development in northeastern Tibet during initial India-Asia collision, as well as in the late Miocene-Holocene, surprisingly little is known about the intervening period: a time when the plateau may have undergone fundamental tectonic changes. To fill this gap, we present new magnetostratigraphy from a >2300-m-thick fluviolacustrine succession that spans ca. 30-9.3 Ma. An integrated analysis of sedimentology, subsidence, and provenance from this section reveals the sequential, pulsed erosion of multiple ranges bordering the Xunhua Basin. Emergence of the WNW-trending Laji Shan is highlighted by a doubling of sediment accumulation rates between 24 and 21 Ma and a transition to coarse allu vial facies at 20.3 Ma. Detrital zircon U/Pb age spectra show that these coarse sediments came from basement terranes within the Laji Shan. Together these observations suggest accelerated growth of the Laji Shan and its coupled foreland basin at ca. 22 Ma. The most rapid accumulation rates in Xunhua Basin occur within the finest-grained strata and suggest an underfilled basin during the fastest interval of Laji Shan deformation. Growth of the Laji Shan occurred northward of the contemporaneous plateau margin, which had been defined since ca. 45-50 Ma by the West Qinling, lying ~60 km farther south. Hence, following ~20-25 m.y. of apparent stability, the deformation front in this region jumped ~60 km to the north at ca. 22 Ma. Subsequently, growth of the north-trending Jishi Shan occurred at ca. 13 Ma and is highlighted by an acceleration in Xunhua Basin accumulation rates between 12 and 9 Ma, as well as by a significant change in detrital zircon provenance of nearby Linxia Basin deposits by 11.5 Ma. Initial growth of the WNW-trending Laji Shan in the early Miocene and subsequent growth of the north-trending Jishi Shan ~10 m.y. later support interpretations of a middle Mio cene kinematic reorganization in northeastern Tibet. © 2012 Geological Society of America.

Recent studies of the northeastern part of the Tibetan Plateau have called attention to two emerging views of how the Tibetan Plateau has grown. First, deformation in northern Tibet began essentially at the time of collision with India, not 10-20 Myr later as might be expected if the locus of activity migrated northward as India penetrated the rest of Eurasia. Thus, the north-south dimensions of the Tibetan Plateau were set mainly by differences in lithospheric strength, with strong lithosphere beneath India and the Tarim and Qaidam basins steadily encroaching on one another as the region between them, the present-day Tibetan Plateau, deformed, and its north-south dimension became narrower. Second, abundant evidence calls for acceleration of deformation, including the formation of new faults, in northeastern Tibet since ~15 Ma and a less precisely dated change in orientation of crustal shortening since ~20 Ma. This reorientation of crustal shortening and roughly concurrent outward growth of high terrain, which swings from NNE-SSW in northern Tibet to more NE-SW and even ENE-WSW in the easternmost part of northeastern Tibet, are likely to be, in part, a consequence of crustal thickening within the high Tibetan Plateau reaching a limit, and the locus of continued shortening then migrating to the northeastern and eastern flanks. These changes in rates and orientation also could result from removal of some or all mantle lithosphere and increased gravitational potential energy per unit area and from a weakening of crustal material so that it could flow in response to pressure gradients set by evolving differences in elevation. Key Points The north-south limits of Tibet were set by lateral variations in strength Roughly 15 million years ago, deformation of NE Tibet accelerated Since 20-15 million years ago, the orientation of shortening rotated eastward ©2013. American Geophysical Union. All Rights Reserved.