Brian S. Currie

Palaeo-altimetry of the late Eocene to Miocene Lunpola basin, central Tibet

The elevation history of the Tibetan plateau provides direct insight into the tectonic processes associated with continent–continent collisions. Here we present oxygen-isotope-based estimates of the palaeo-altimetry of late Eocene and younger deposits of the Lunpola basin in the centre of the plateau, which indicate that the surface of Tibet has been at an elevation of more than 4 kilometres for at least the past 35 million years. We conclude that crustal, but not mantle, thickening models, combined with plate-kinematic solutions of India–Asia convergence, are compatible with palaeo-elevation estimates across the Tibetan plateau.

A new approach to stable isotope-based paleoaltimetry: implications for paleoaltimetry and paleohypsometry of the High Himalaya since the Late Miocene

The change in oxygen isotopic composition of precipitation is modeled using equilibrium fractionation during Rayleigh distillation linked to the thermodynamics of atmospheric ascent and water vapor condensation. The primary controllers of the vertical variation in isotopic composition with elevation are the low elevation temperature and relative humidity as these control the vertical distribution of condensation. An empirical fit of precipitation versus model condensation based on Alpine stations is derived. This fit is represented in the model as the weighted mean composition of condensation within a 1000 m thick air parcel 1500 ˛ 500 m above the ground surface and is used for all other regions. Comparison of model versus observed modern precipitation reveals a close fit, particularly of more highly elevated sites. Comparison of modern waters in the Himalayas and southern Tibet with model predictions, particularly as revealed by comparison of observed and predicted hypsometry provides additional support to the validity of the model. Finally, application of this model to estimates of paleo-waters in the Himalayas and southern Tibet reveals that this region had already achieved its present hypsometry by the Late Miocene, about 10 Ma ago. fl 2001 Published by Elsevier Science B.V.

Age of Initiation of the India‐Asia Collision in the East‐Central Himalaya

We document the stratigraphy and provenance of the lower Tertiary terrigenous sections in the Zhepure Shan region of the Tethyan Himalaya, southern Tibet, using petrographic and geochemical whole‐rock and single‐grain techniques. The Cretaceous–early Tertiary shelf deposits of shallow marine carbonates and siliciclastics of the former Indian passive margin near the western end of the Zhepure Shan are conformably overlain by lower Tertiary clastic rocks. Sandstones in the Jidula Formation (Paleocene) mostly contain monocrystalline quartz grains of cratonic origin. In contrast, significant amounts of immature framework grains with a distinct ophiolitic and volcanic arc influence are present in the Youxia (Early Eocene) and Shenkeza (post–Early Eocene) formations. Major, trace, and rare earth element concentrations in both sandstones and shales complement the petrographic data and indicate that the source of the Jidula Formation consisted primarily of quartzose basement rocks, probably of Indian continental origin, whereas the sediments of the Youxia Formation were mainly derived from the uplifted Gangdese arc‐trench system associated with the obduction of the Asian subduction complex. The compositions of Cr‐rich spinels in the Youxia and Shenkeza sandstones resemble those from fore‐arc peridotites and were most likely derived from the arc and ophiolite rocks along the developing Yarlung‐Zangbo suture to the north. No spinels have been observed in the Jidula sandstones. Therefore, the early Tertiary detrital clastics in the Zhepure Shan record a marked change in provenance and sediment character and specifically at the time of deposition of the Youxia Formation, which contains a zone P‐8 foram assemblage. This change indicates that the onset of India‐Asia collision and the first development of the foreland basin immediately south of the India‐Asia suture zone occurred at \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

Neogene climate change and uplift in the Atacama Desert, Chile

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Middle Miocene paleoaltimetry of southern Tibet: Implications for the role of mantle thickening and delamination in the Himalayan orogen

\end{document} Ma in the both the western (Zanskar) and eastern (this study) Tethyan Himalaya.

The Geological, Isotopic, Botanical, Invertebrate, and Lower Vertebrate Surroundings of Ardipithecus ramidus

The origin of Australopithecus, the genus widely interpreted as ancestral to Homo, is a central problem in human evolutionary studies. Australopithecus species differ markedly from extant African apes and candidate ancestral hominids such as Ardipithecus, Orrorin and Sahelanthropus. The earliest described Australopithecus species is Au. anamensis, the probable chronospecies ancestor of Au. afarensis. Here we describe newly discovered fossils from the Middle Awash study area that extend the known Au. anamensis range into northeastern Ethiopia. The new fossils are from chronometrically controlled stratigraphic sequences and date to about 4.1–4.2 million years ago. They include diagnostic craniodental remains, the largest hominid canine yet recovered, and the earliest Australopithecus femur. These new fossils are sampled from a woodland context. Temporal and anatomical intermediacy between Ar. ramidus and Au. afarensis suggest a relatively rapid shift from Ardipithecus to Australopithecus in this region of Africa, involving either replacement or accelerated phyletic evolution.

Geochemical evaluation of fenghuoshan group lacustrine carbonates, north-central Tibet : Implications for the paleoaltimetry of the eocene Tibetan plateau

The relationship between Andean uplift and extreme desiccation of the west coast of South America is important for understanding the interplay between climate and tectonics in the Central Andes, yet it is poorly understood. Here we use soil morphological char- acteristics, salt chemistry, and mass independent fractionation anomalies ( 17 O values) in dated paleosols to reconstruct a middle Miocene climatic transition from semiaridity to extreme hyperaridity in the Atacama Desert. Paleosols along the southeastern margin of the Calama Basin change from calcic Vertisols with root traces, slickensides, and gleyed horizons to an extremely mature salic Gypsisol with pedogenic nitrate. We interpret this transition, which occurred between 19 and 13 Ma, to represent a change in precipitation from 200 mm/yr to 20 mm/yr. This drastic reduction in precipitation likely resulted from uplift of the Central Andes to elevations 2 km; the uplift blocked moisture from the South American summer monsoon from entering the Atacama. The mid-Miocene Gyp- sisol with pedogenic nitrate is located at elevations between 2900 and 3400 m in the Cal- ama Basin, significantly higher than modern nitrate soils, which occur below 2500 m. Modern and Quaternary soils in this elevation zone contain soil carbonate and lack ped- ogenic gypsum and nitrate. We infer that 900 m of local surface uplift over the past 10 m.y. displaced these nitrate paleosols relative to modern nitrate soils and caused a return to wetter conditions in the Calama Basin by decreasing local air temperatures and creating an orographic barrier to Pacific air masses.

Structural configuration of the Early Cretaceous cordilleran foreland-basin system and Sevier thrust belt, Utah and Colorado

Two hundred thirty million years ago, in what is now Argentina, dinosaurs could be found as the dominant carnivores or as small herbivores. Upper Triassic rocks in northwestern Argentina preserve the most complete record of dinosaurs before their rise to dominance in the Early Jurassic. Here, we describe a previously unidentified basal theropod, reassess its contemporary Eoraptor as a basal sauropodomorph, divide the faunal record of the Ischigualasto Formation with biozones, and bracket the formation with 40Ar/39Ar ages. Some 230 million years ago in the Late Triassic (mid Carnian), the earliest dinosaurs were the dominant terrestrial carnivores and small herbivores in southwestern Pangaea. The extinction of nondinosaurian herbivores is sequential and is not linked to an increase in dinosaurian diversity, which weakens the predominant scenario for dinosaurian ascendancy as opportunistic replacement.