Junsheng Nie

Late Cenozoic deformation and uplift of the NE Tibetan Plateau: Evidence from high-resolution magnetostratigraphy of the Guide Basin, Qinghai Province, China

The Cenozoic intramontane Gonghe–Guide Basin in Qinghai Province, China, is tectonically controlled by the sinistral strike-slip framework of the Kunlun and Altyn Tagh–South Qilian faults in the northeastern Tibetan Plateau. The basin is filled 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 five 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 reflect 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 reflects 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 flexure 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.

Loess plateau storage of northeastern Tibetan plateau-derived Yellow River sediment

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 rivers 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.

Linking sedimentation in the northern Andes to basement configuration, Mesozoic extension, and Cenozoic shortening: Evidence from detrital zircon U-Pb ages, Eastern Cordillera, Colombia

Laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) analyses of 29 samples from the Eastern Cordillera 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.

Resolving uplift of the Northern Andes using detrital zircon age signatures

Uplift of the Eastern Cordillera in the northern Andes has been linked to orographic climate change and genesis of South America’s largest river systems. The timing of initial uplift remains poorly constrained, with most estimates ranging from ca. 60 to ca. 5 Ma. New detrital zircon U-Pb ages from proximal fill of the Llanos foreland basin in Colombia reveal a pronounced mid-Cenozoic shift in provenance from an Amazonian craton source to an Andean fold-thrust belt source. This shift corresponds with changes in detrital zircon (U-Th)/He ages, a conglomeratic unroofing sequence, and a sharp increase in foredeep accumulation rates. These nearly simultaneous changes in zircon age spectra, clast compositions, and sediment accumulation are attributable to latest Oligocene uplift of the eastern flank of the Eastern Cordillera. The timing relationships suggest an early activation of the frontal thrust system, implying a long-term (up to 25 m.y.) cessation of orogenic wedge advance, potentially driven by structural inheritance and/or climate change. INTRODUCTION Surface uplift of the Eastern Cordillera in the northern Andes has had a profound effect on orographic climate change (Mora et al., 2008), growth of large continental drainage systems (Fig. 1) (Amazon, Orinoco, and Magdalena rivers; Hoorn et al., 1995; Díaz de Gamero, 1996), and biologic evolution of neotropical rainforests (Hooghiemstra and Van der Hammen, 1998; Jaramillo et al., 2006). Most estimates for the onset of uplift along the eastern flank of the Colombian Andes (Fig. 2) range from Paleocene to Pliocene time (Van der Hammen et al., 1973; Dengo and Covey, 1993; Cooper et al., 1995; Bayona et al., 2008; Parra et al., 2009a). Initial uplift has proven difficult to constrain by conventional methods. First, recent zircon fission track data provide a minimum age but do not uniquely pinpoint the precise onset of earliest uplift-induced exhumation (Parra et al., 2009b). Second, insights from synorogenic growth strata are commonly limited by inadequate exposure, poor seismic resolution, and GSA Today, v. 20, no. 7, doi: 10.1130/GSATG76A.1 minimal variation in stratal dip (e.g., Toro et al., 2004). Third, clastic compositional records of erosional unroofing are hindered by the uniformly high-maturity (quartz-dominated) sand compositions imposed by intense tropical weathering (e.g., Johnsson et al., 1988). In this study, we utilize U-Pb and (U-Th)/He ages of detrital zircon grains from the Colombian Andes to demonstrate that initial uplift-induced exhumation along the eastern flank of the fold-thrust belt had commenced by ca. 26–23 Ma. Timing relationships revealed by geochronological data coincide with shifts in conglomerate clast compositions and sediment accumulation rates and provide new insights into the pace of orogenic wedge advance. GEOLOGIC SETTING The Eastern Cordillera of Colombia forms a 1–4-km-high orographic barrier separating the intermontane Magdalena Valley from the Llanos foreland basin (Fig. 2). The 100–200-kmwide range is bounded by a frontal thrust system consisting of inverted normal faults and newly formed fold-thrust structures (Cooper et al., 1995; Mora et al., 2006). Following Jurassic– Early Cretaceous rifting, Andean orogenesis began with latest Cretaceous–Paleocene shortening in the Central Cordillera and early foreland basin evolution in the present-day Magdalena Figure 1. Map of South America showing main river systems (Magdalena, Orinoco, Amazon, and Parana) and Precambrian crustal provinces of the Amazonian craton (after Cordani et al., 2000; Chew et al., 2007).

Tracking exhumation of Andean ranges bounding the Middle Magdalena Valley Basin, Colombia

The shortening history of the Andes is important for understanding retroarc deformation along convergent margins and forcing mechanisms of Cenozoic climate. However, the timing of uplift in the northern Andes is poorly constrained, with estimates ranging from Cretaceous to Pliocene. Detrital zircon U-Pb ages from the Middle Magdalena Valley Basin in Colombia reveal two provenance shifts during Cenozoic time. The fi rst shift occurs between early and late Paleocene strata, where U-Pb results show a switch from Proterozoic-dominated to Phanerozoic-dominated age spectra. We attribute this change to uplift-related exhumation of the Central Cordillera. The second shift occurs between middle-late Eocene and late Oligocene strata, where increased Grenville ages and diminished Mesozoic ages can be linked to uplift of the Eastern Cordillera. Our results show that signifi cant pre-Neogene deformation affected the northern Andes, underscoring the potential importance of Andean uplift on the dynamics of Paleogene climate.

Pacific freshening drives Pliocene cooling and Asian monsoon intensification

The monsoon is a fundamental component of Earths climate. The Pliocene warm period is characterized by long-term global cooling yet concurrent monsoon dynamics are poorly known. Here we present the first fully quantified and calibrated reconstructions of separate Pliocene air temperature and East Asian summer monsoon precipitation histories on the Chinese Loess Plateau through joint analysis of loess/red clay magnetic parameters with different sensitivities to air temperature and precipitation. East Asian summer monsoon precipitation shows an intensified trend, paradoxically at the same time that climate cooled. We propose a hitherto unrecognized feedback where persistently intensified East Asian summer monsoon during the late Pliocene, triggered by the gradual closure of the Panama Seaway, reinforced late Pliocene Pacific freshening, sea-ice development and ice volume increase, culminating in initiation of the extensive Northern Hemisphere glaciations of the Quaternary Ice Age. This feedback mechanism represents a fundamental reinterpretation of the origin of the Quaternary glaciations and the impact of the monsoon.

Mixing of Source Populations Recorded in Detrital Zircon U-Pb Age Spectra of Modern River Sands

Detrital zircon U-Pb geochronology is widely used in reconstructing sedimentary provenance, sediment dispersal pathways, and tectonic histories. These applications implicitly assume that sample age distributions from sedimentary basins closely match the zircon age populations within the drainage catchments of the eroding source region. However, recent studies question this largely untested assumption. Here we compare detrital zircon data from unconsolidated sand samples from two modern rivers in the Colombian foreland with data from Mesozoic-Cenozoic sedimentary strata exposed in their respective Andean catchments (∼1500 km2). We forward model the expected detrital zircon age distribution from each mountainous catchment by integrating age data from all exposed sedimentary units into two modeled age distributions: one assuming equal contributions from each formation and one in which each formation’s contribution is proportional to its exposure area within the catchment. Multiple statistical methods show that the area-proportional models most closely match the modern river data. We further test the ability to estimate the contributing source areas by iteratively solving for formation exposure areas to minimize the misfit between the model and the modern river data. The results show a robust correlation between modeled and observed source areas when considered at the epoch level and a qualitative correlation when considered at the formation level. We conclude that detrital zircon age populations of the Colombian foreland basin fill accurately reflect mixing of their Andean sources, in proportions roughly equal to their exposed areas within each catchment. We further conclude that the approximate relative exposure areas of contributing sources can be estimated from detrital zircon analyses. These results highlight the need for consideration of variations in the exposure area rather than solely the presence or absence of competing sediment sources in detrital zircon provenance studies.

A rock magnetic study of loess from the West Kunlun Mountains

West Kunlun Mountains have continuous loess deposits. However, very few papers have explored the rock magnetic characteristics of loess deposited there. Thus, although Kunlun Mountains loess is one of the useful paleoclimate archives, its importance has not been emphasized. Here we present detailed rock magnetic and bulk grain-size studies on a 207 m loess core on the northern slope of the West Kunlun Mountains. We find that magnetic susceptibility of loess from the northern slope of the West Kunlun Mountains is not controlled by changes in the concentration of ultrafine pedogenic magnetite and maghemite, but instead controlled by changes in the concentration of aeolian multidomain magnetite and maghemite. This is unsurprising given the low rainfall of the Kunlun region and the lack of postdepositional weathering and soil formation in these aeolian sequences. In addition, we find that magnetic susceptibility values increase dramatically at 111 m corresponding to an age of similar to 0.5 Ma. The magnetic susceptibility increase is accompanied by coarsening of both bulk and magnetic grain size and softening of magnetic mineralogy. We tentatively tie these changes in rock magnetic and grain-size parameter records to an expansion of desert environment.

Tibetan uplift intensified the 400 k.y. signal in paleoclimate records at 4 Ma

Knowledge of when and how a specific climatic threshold is crossed is one key to an understanding of the evolution of Earths climate system. We have observed a simultaneous intensification of the similar to 400 k.y. cycle, in both proxy records for global ice volume and for East Asian monsoon precipitation on the Chinese Loess Plateau at ca. 4 Ma, indicating that important climatic thresholds were crossed. The 400 k.y. climatic cycles can be substantially intensified by clipped responses to insolation. We argue that Tibetan uplift accelerated at ca. 4 Ma and triggered such responses. The weaker orbital-band climatic signal during the interval 4-3.6 Ma indicates that the thresholds were only crossed slightly.

Growth of the Qaidam Basin during Cenozoic exhumation in the northern Tibetan Plateau: Inferences from depositional patterns and multiproxy detrital provenance signatures

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.