Adakites are geochemically distinct intermediate to felsic lavas found exclusively in subduction zones. Here we report the first example of such magmas from southern Tibet in an active continental collision environment. The Tibetan adakites were emplaced from ca. 26 to 10 Ma, and their overall geochemical characteristics suggest an origin by melting of eclogites and/or garnet amphibolites in the lower part (≥50 km) of thickened Tibetan crust. This lower-crustal melting required a significantly elevated geotherm, which we attribute to removal of the tectonically thickened lithospheric mantle in late Oligocene time. The identification of collision-type adakites from southern Tibet lends new constraints to not only the Himalayan-Tibetan orogenesis—how and when the Indian lithosphere started underthrusting Asia can be depicted—but also the growth of the early continental crust on Earth that consists dominantly of the tonalite-trondhjemite-granodiorite suites marked by adakitic geochemical affinities.

http://argonlab.gl.ntu.edu.tw/papers/Chung(2003)Geology-Adak.pdf

Adakites from continental collision zones: Melting of thickened lower crust beneath southern Tibet

Understanding ancient climate changes is hampered by the inability to disentangle trends in ocean temperature from trends in continental ice volume. We used carbonate “clumped” isotope paleothermometry to constrain ocean temperatures, and thereby estimate ice volumes, through the Late Ordovician–Early Silurian glaciation. We find tropical ocean temperatures of 32° to 37°C except for short-lived cooling by ~5°C during the final Ordovician stage. Evidence for ice sheets spans much of the study interval, but the cooling pulse coincided with a glacial maximum during which ice volumes likely equaled or exceeded those of the last (Pleistocene) glacial maximum. This cooling also coincided with a large perturbation of the carbon cycle and the Late Ordovician mass extinction.

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The Magnitude and Duration of Late Ordovician–Early Silurian Glaciation

The history of the southern central Andes, including the world’s second largest plateau and adjacent intermontane basins and ranges of the Eastern Cordillera and the northern Sierras Pampeanas of Argentina and Bolivia, impressively documents the effects of tectonics and topography on atmospheric circulation patterns, the development of orographic barriers, and their influence on erosion and landscape evolution at various timescales. Protracted aridity in the orogen interior has facilitated the creation and maintenance of the Puna-Altiplano plateau. Contraction and range uplift, filling of basins, and possibly wholesale uplift of the plateau increased gravitational stresses in the orogen interior, which caused the eastward migration of deformation into the foreland and successive aridification. The uplift of the Andean orogen has also had a far-reaching influence on atmospheric and moisture-transport patterns in South America. This is documented by the onset of humid climate conditions on the eastern side of the Andes in late Miocene time, which was coupled with the establishment of dramatic precipitation gradients perpendicular to the orogen, and changes in tectonic processes in the Andean orogenic wedge.

/pdf/tectonics-and-climate-of-the-southern-central-andes-3stq8x48ur.pdf

Tectonics and Climate of the Southern Central Andes

The Himalayan-Tibetan orogen culminated during the Cenozoic India – Asia collision, but its geological framework and initial growth were fundamentally the result of multiple, previous ocean closure and intercontinental suturing events. As such, the Himalayan-Tibetan orogen provides an ideal laboratory to investigate geological signatures of the suturing process in general, and how the Earth9s highest and largest orogenic feature formed in specific. This paper synthesizes the Triassic through Cenozoic geology of the central Himalayan-Tibetan orogen and presents our tectonic interpretations in a time series of schematic lithosphere-scale cross-sections and paleogeographic maps. We suggest that north-dipping subducting slabs beneath Asian continental terranes associated with closure of the Paleo-, Meso-, and Neo-Tethys oceans experienced phases of southward trench retreat prior to intercontinental suturing. These trench retreat events created ophiolites in forearc extensional settings and/or a backarc oceanic basins between rifted segments of upper-plate continental margin arcs. This process may have occurred at least three times along the southern Asian margin during northward subduction of Neo-Tethys oceanic lithosphere: from ∼174 to 156 Ma; 132 to 120 Ma; and 90 to 70 Ma. At most other times, the Tibetan terranes underwent Cordilleran-style or collisional contractional deformation. Geological records indicate that most of northern and central Tibet (the Hoh-Xil and Qiangtang terranes, respectively) were uplifted above sea level by Jurassic time, and southern Tibet (the Lhasa terrane) north of its forearc region has been above sea level since ∼100 Ma. Stratigraphic evidence indicates that the northern Himalayan margin of India collided with an Asian-affinity subduction complex – forearc – arc system beginning at ∼60 Ma. Both the Himalaya (composed of Indian crust) and Tibet show continuous geological records of orogenesis since ∼60 Ma. As no evidence exists in the rock record for a younger suture, the simplest interpretation of the geology is that India – Asia collision initiated at ∼60 Ma. Plate circuit, paleomagnetic, and structural reconstructions, however, suggest that the southern margin of Asia was too far north of India to have collided with it at that time. Seismic tomographic images are also suggestive of a second, more southerly Neo-Tethyan oceanic slab in the lower mantle where the northernmost margin of India may have been located at ∼60 Ma. The geology of Tibet and the India – Asia suture zone permits an alternative collision scenario in which the continental margin arc along southern Asia (the Gangdese arc) was split by extension beginning at ∼90 Ma, and along with its forearc to the south (the Xigaze forearc), rifted southward and opened a backarc ocean basin. The rifted arc collided with India at ∼60 Ma whereas the hypothetical backarc ocean basin may not have been consumed until ∼45 Ma. A compilation of igneous age data from Tibet shows that the most recent phase of Gangdese arc magmatism in the southern Lhasa terrane initiated at ∼70 Ma, peaked at ∼51 Ma, and terminated at ∼38 Ma. Cenozoic potassic-adakitic magmatism initiated at ∼45 Ma within a ∼200-km-wide elliptical area within the northern Qiangtang terrane, after which it swept westward and southward with time across central Tibet until ∼26 Ma. At 26 to 23 Ma, potassic-adakitic magmatism swept southward across the Lhasa terrane, a narrow (∼20 km width), orogen-parallel basin developed at low elevation along the axis of the India – Asia suture zone (the Kailas basin), and Greater Himalayan Sequence rocks began extruding southward between the South Tibetan Detachment and Main Central Thrust. The Kailas basin was then uplifted to >4 km elevation by ∼20 Ma, after which parts of the India – Asia suture zone and Gangdese arc experienced >6 km of exhumation (between ∼20 and 16 Ma). Between ∼16 and 12 Ma, slip along the South Tibetan Detachment terminated and east-west extension initiated in the northern Himalaya and Tibet. Potassic-adakitic magmatism in the Lhasa terrane shows a northward younging trend in the age of its termination, beginning at 20 to 18 Ma until volcanism ended at 8 Ma. We interpret the post-45 Ma geological evolution in the context of the subduction dynamics of Indian continental lithosphere and its interplay with delamination of Asian mantle lithosphere.

Mesozoic─Cenozoic geological evolution of the Himalayan-Tibetan orogen and working tectonic hypotheses

The Rossiter-McLaughlin (RM) effect is the distortion of stellar spectral lines that occurs during eclipses or transits, due to stellar rotation. We assess the future prospects for using the RM effect to measure the alignment of planetary orbits with the spin axes of their parent stars, and to confirm exoplanetary transits. We compute the achievable accuracy for the parameters of interest, in general and for the five known cases of transiting exoplanets with bright host stars. We determine the requirements for detecting the effects of differential rotation. For transiting planets with small masses or long periods (as will be detected by forthcoming satellite missions), the velocity anomaly produced by the RM effect can be much larger than the orbital velocity of the star. For a terrestrial planet in the habitable zone of a Sunlike star found by the Kepler mission, it will be difficult to use the RM effect to confirm transits with current instruments, but it still may be easier than measuring the spectroscopic orbit.

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Prospects for the characterization and confirmation of transiting exoplanets via the rossiter-mclaughlin effect

Annual review of earth and planetary sciences v. 7, Editor: F. A. Donath; Associate Editors: F. G. Stehli, and G. W. Wetherill, Annual Reviews, Inc., 4139 El Camino Way, Palo Alto, California, 94036, 1979, 517p., 17 (U.S.), 17.50 elsewhere

D.F. Merriam

Papers

Annual review of earth and planetary sciences v. 7, Editor: F. A. Donath; Associate Editors: F. G. Stehli, and G. W. Wetherill, Annual Reviews, Inc., 4139 El Camino Way, Palo Alto, California, 94036, 1979, 517p., 17 (U.S.), 17.50 elsewhere