David B. Rowley

Age of initiation of collision between India and Asia : A review of stratigraphic data

Abstract The collision of India with Asia is perhaps the most profound tectonic event to have occurred in past 100 Ma. It is responsible for the uplift of the Himalayas and Tibetan Plateau and has been argued to have been responsible for geological, geochemical, and climatological consequences of global extent. Yet the age of initiation of this collision remains poorly constrained. The literature is replete with estimates that range from the Late Cretaceous (> 65 Ma) to latest Eocene (

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.

Sr isotope evolution of seawater: the role of tectonics

We use a high-resolution seawater Sr isotopic evolution curve for the last 100 m.y. in conjunction with modern riverine Sr flux measurements, and also geologic, tectonic and geochronological data, to make the case for a close relationship between seawater Sr isotopic composition and the India Asia continental collision. Using a simple seawater Sr budget model we begin by showing that the Sr flux associated with alteration of seafloor basalts is too small and does not have the right time evolution to account for much of the seawater Sr isotopic curve of the last 100 m.y. The flux of dissolved Sr carried by rivers originating in the Himalaya-Tibet region on the other hand is presently a significant fraction of the global Sr budget. We calculate how this riverine flux would have had to change with time in order to match the observed seawater Sr isotopic curve and find that the riverine flux remains relatively constant prior to the collision of India with Asia but then increases very significantly after collision. We note that the period of most rapid change in seawater Sr isotopic ratio, from 20 Ma to 15 Ma, is also a period of exceptionally high erosion in parts of the Himalayas and the Tibetan Plateau. As further evidence that Sr derived from the collision of India with Asia plays a major role in the Sr isotopic evolution of seawater we show that the total amount erosion of the Himalaya-Tibetan Plateau since collision, which we calculate separately, represents a total amount of Sr that is very nearly the same as the cumulative amount required by the Sr isotopic change of seawater since collision. The relationship between erosion and riverine Sr flux allows us to use the Sr isotopic evolution of seawater to reconstruct a history of erosion since collision, and we find that the erosion rate accelerates with time since collision, with the present having the largest rate. When we apply the Sr budget model to the entire Phanerozoic using a new compilation of deformed continental area versus time we find that we can account for the large-scale structure of the seawater Sr isotopic curve, but fail to reproduce several local maxima and minima, especially in the period 100-300 Ma. The present high S7Sr/S6Sr of seawater and similar highs in the Devonian and Cambrian do correlate with extensive deformation on the continents.

Ages of ultrahigh pressure metamorphism and protolith orthogneisses from the eastern Dabie Shan: U/Pb zircon geochronology

The Dabie Shan contains two rare geologic features. One, the occurrence of ultra-high pressure metamorphic rocks (peak P > 2.7 GPa = UHP) in the east-central part of this belt, and two, some of these UHP rocks are characterized by anomalously 18O-depleted oxygen isotopic compositions. Geochronological data are presented that constrain the age of the UHP metamorphism, the ages of protoliths, and potentially the age of hydrothermal alteration believed to be responsible for the anomalous oxygen isotopic compositions. The age of UHP metamorphism in the eastern Dabie Shan is dated at 218.5 ± 1.7 Ma using U/Pb in zircon separated from host gneisses. This age is identical within uncertainty with a less precise age using U/Pb in zircon separated from UHP eclogite (225.5 + 3/−6 Ma) and with previously published ages from UHP eclogites of the Dabie Shan. Map relationships strongly support an in-situ, as opposed to exotic, orgin of the UHP eclogites in this region. The identical ages of UHP metamorphism in the gneisses and eclogites provides additional support for this interpretation. The age of the protolith gneisses is dated as 772.5 ± 9.5 Ma based on the upper intercept of the zircons which accords well with other dates of granitic gneisses in the Qinling belt. The association of mafic and felsic magmatism is interpreted to reflect rifting along the northern margin of the Yangtze block at this time. These upper intercept ages may date the hydrothermal circulation of 18O-depleted meteoric water which gave rise to the anomalously 18O-depleted UHP protoliths of the Dabie Shan and Sulu region. The coincidence of these protolith ages with the early Sinian glaciation of South China is consonant with the suggestion that the highly anomalous oxygen isotopic compositions in this region date from this time. The upper intercept age of the eclogite is imprecisely determined at 447 +82/−79 Ma but is younger than the host gneisses and is not characterized by anomalous oxygen isotopic compositions.

A plate-kinematic framework for models of Caribbean evolution

Abstract We define the former relative positions and motions of the plates whose motions have controlled the geological evolution of the Caribbean region. Newly determined poles of rotation defining the approximate spreading histories of the central North and the South Atlantic oceans are given. For the late Jurassic-Early Cretaceous anomaly sequence of the central North Atlantic, we have used previously published ∗ definitions of fracture-zone traces and magnetic anomaly picks, redetermining the pole positions and angular rotations for various isochrons on an Evans and Sutherland interactive graphics system. For magnetic anomalies younger than the Cretaceous Quiet Period in both oceans, we (1) used Seasat altimeter data to help define fracture-zone traces, and (2) identified and used marine magnetic anomalies to determine the positions of spreading isochrons along the flowlines indicated by the fracture zones. By the finite difference method, the relative paleopositions and the relative motion history between North and South America were computed. This analysis defines the size and shape (and the rate at which the size and shape changed) of the interplate region between North and South America since the Middle Jurassic. Thus, a plate-kinematic framework is provided for the larger plates pertaining to the Caribbean region, in which can be derived more detailed scenarios for Gulf of Mexico and Caribbean evolution. North and South America diverged to approximately their present relative positions from Late Triassic? to Early Campanian (about 84 m.y. ago) time. This is the period during which the Gulf of Mexico and a Proto-Caribbean seaway were formed. Since the Campanian, only minor relative motion has occurred; from Early Campanian through to Middle Eocene times. South America diverged only another 200 km, and since the Middle Eocene, minor N-S convergence has occurred. These very minor post-Early Campanian motions have probably been accommodated by imperfect shear and compression along the Atlantic fracture zones to the east of the Lesser Antilles, and along the northern and southern borders of the Caribbean Plate. Accordingly, it is suggested that from Campanian time to the present, the relative motions between the North and South American plates have had only minor effects on the structural development of the Caribbean region. Primarily using the data of Engebretson et al. ∗∗ , the convergence history of Pacific plates with North America was calculated for two points near the western Caribbean. By completing finite difference solutions, the convergence history of the Pacific plates with the Caribbean and South American plates can be approximated. The direction and rate of convergence of the Pacific plates with the Americas may have controlled the style of subduction and possible microplate migration along the North American, South American and western Caribbean boundaries that define the eastern Pacific plate margin.

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

Plate-kinematic reconstructions of the North Atlantic and Arctic: Late Jurassic to Present

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