Marine accumulations of terrigenous sediment are widely assumed to accurately record climatic- and tectonic-controlled mountain denudation and play an important role in understanding late Cenozoic mountain uplift and global cooling. Underpinning this is the assumption that the majority of sediment eroded from hinterland orogenic belts is transported to and ultimately stored in marine basins with little lag between erosion and deposition. Here we use a detailed and multi-technique sedimentary provenance dataset from the Yellow River to show that substantial amounts of sediment eroded from Northeast Tibet and carried by the river's upper reach are stored in the Chinese Loess Plateau and the western Mu Us desert. This finding revises our understanding of the origin of the Chinese Loess Plateau and provides a potential solution for mismatches between late Cenozoic terrestrial sedimentation and marine geochemistry records, as well as between global CO2 and erosion records.

The Cenozoic intramontane GongheGuide Basin in Qinghai Province, China, is tectonically controlled by the sinistral strikeslip framework of the Kunlun and Altyn Tagh-South Qilian faults in the northeastern Tibetan Plateau. The basin is fi lled with thick Cenozoic clastic sedimentary formations, which provide important evidence of the deformation of this part of the plateau, although they have long lacked good age constraints. Detailed magnetostratigraphic and paleontologic investigations of fi ve sections in the Guide Basin and their lithologic and sedimentary characteristics allow us to divide a formerly undifferentiated unit (the Guide Group) into six formations (where ages are now magnetostratigraphically well established, they are given in parentheses): the Amigang (1.8-2.6 Ma), Ganjia (2.6-3.6 Ma), and Herjia formations (3.6 to ca. 7.0-7.8 Ma), and the older Miocene Ashigong, Garang, and Guidemen formations. These rocks document a generally upward coarsening sequence, characterized by increasing accumulation rates. Increasing gravel content and sizes of its components, changes of bedding dips and source rock types, and marginal growth faults collectively refl ect accelerated deformation and uplift of the NE Tibetan Plateau after 8 Ma, punctuated by a sharp increase in sedimentation rate at ca. 3.2 Ma that refl ects the boulder conglomerates of the Ganjia formation. Interestingly, much of the vergence of the compressional deformation in the basin is to the south, accommodated by a sequence of six thrusts (F1-F6), which become active one by one progressively later toward the south, undoubtedly contributing to the uplift of this part of the plateau. F1 likely initiated the Guide Basin due to crustal fl exure in the Oligocene, F2 was active in the early Miocene, F4 and F5 at ca. 3.6 Ma, and F6 was active in the early Pleistocene. The detailed late Miocene and younger magnetostratigraphy allows us to place much improved time constraints on the deformation and, hence, uplift of northeastern Tibet, which, when compared with ages for events on other parts of the plateau, provides important boundary conditions for the geodynamical evolution of Tibet.

Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analyses of 29 samples from the EasternCordillera of Colombia reveal the origin of northern Andean basement and patterns of sedimentation during Paleozoic subsidence, Jurassic-Early Cretaceous extension, Late Cretaceous postrift subsidence, and Cenozoic shortening and foreland-basin evolution. U-Pb geochronological results indicate that presumed Precambrian basement is mainly a product of early Paleozoic magmatism (520-420 Ma) potentially linked to subduction and possible collision. Inherited zircons provide evidence for Mesoproterozoic tectonomagmatic events at 1200-1000 Ma during Grenville-age orogenesis. Detrital zircon U-Pb ages for Paleozoic strata show derivation from Andean basement, syn depositional magmatic sources (420-380 Ma), and distal sources of chiefl y Mesoproterozoic basement (1650-900 Ma) in the Amazonian craton (Guyana shield) to the east or in possible continental terranes along the western margin of South America. Sedimentation during Jurassic-Early Cretaceous rifting is expressed in detrital zircon age spectra as Andean basement sources, recycled Paleozoic contributions, and igneous sources of Carboniferous-Permian (310-250 Ma) and Late Triassic-Early Jurassic (220-180 Ma) origin. Detrital zircon provenance during continued Cretaceous extension and postrift thermal subsidence recorded the elimination of Andean basement sources and increased infl uence of craton-derived drainage systems providing mainly Paleoproterozoic and Mesoproterozoic (2050-950 Ma) grains. By Eocene time, zircons from the Guyana shield (1850-1350 Ma) dominated the detrital signal in the easternmost Eastern Cordillera. In contrast, coeval Eocene deposits in the axial Eastern Cordillera contain Late Cretaceous-Paleocene (90-55 Ma), Jurassic (190-150 Ma), and limited Permian-Triassic (280-220 Ma) zircons recording initial uplift and exhumation of principally Mesozoic magmatic-arc rocks to the west in the Central Cordillera. Oligocene-Miocene sandstones of the proximal Llanos foreland basin document uplift-induced exhumation of the Eastern Cordillera fold-thrust belt and recycling of the Paleogene cover succession rich in both arc-derived detritus (dominantly 180-40 Ma) and shield-derived sediments (mostly 1850-950 Ma). Late Miocene-Pliocene erosion into the underlying Cretaceous section is evidenced by elimination of Mesozoic-Cenozoic zircons and increased proportions of 1650-900 Ma zircons emblematic of Cretaceous strata.

Sedimentologic and provenance analyses for the Qaidam Basin in the northern Tibetan Plateau help to elucidate the stratigraphic signatures of initial deformation and exhumation in basin-bounding ranges. The basin recorded sedimentary transitions in response to uplift and unroofing of several distinctive source regions. Along the NE basin margin, a detrital record of exhumation and basin isolation is preserved in the 6200-m-thick Cenozoic succession at the Dahonggou anticline. An up-section shift from axial fluvial and marginal lacustrine deposition to transverse fluvial sedimentation suggests progradation and increasingly proximal sediment sources, reflecting activation and advance of crustal deformation. Provenance results from sandstone petrology, U-Pb geochronology, and heavy mineral analyses indicate initial late Paleocene-early Eocene derivation from igneous, metamorphic, and sedimentary sources, consistent with Permian-Triassic arc rocks dominating the southern (Kunlun Shan) or southwestern (Qimen Tagh) basin margins. Up-section variations in sediment composition and detrital zircon U-Pb age distributions are attributed to Eocene-Oligocene derivation from lower Paleozoic and Mesozoic igneous and metamorphic rocks of the central to northern Qilian Shan-Nan Shan. Disappearance of igneous sources and persistence of metamorphic sources are consistent with derivation from the southern Qilian Shan-Nan Shan during early-middle Miocene shortening along the frontal Nan Shan-North Qaidam thrust belt. These results are supported by paleocurrent analyses revealing an Eocene shift from roughly E-directed (axial) to SW-directed (transverse) dispersal of sediment. Variations in lithofacies, composition, U-Pb ages, and paleoflow are consistent with late Paleocene-early Eocene exhumation in the Kunlun Shan followed by middle Eocene-middle Miocene exhumation in the Qilian Shan-Nan Shan. The up-section disappearance and reappearance of diagnostic U-Pb age populations can be associated with progressive unroofing of multiple thrust sheets, successive input of sedimentary and magmatic sources, and southward encroachment of Qilian Shan-Nan Shan shortening into the Qaidam Basin. The sedimentary record presented here indicates that during the Paleogene, the unified Qaidam-Tarim Basin was partitioned and uplifted as it was incorporated into the growing Tibetan Plateau. Comparison with basins on and surrounding the Tibetan Plateau suggests that basement strength and lateral homogeneity, and formation of syndepositional structural dams are among the primary controls on formation of giant sedimentary basins.

No abstract