87,126 research outputs found
Timing of the Arabia-Eurasia continental collision—Evidence from detrital zircon U-Pb geochronology of the Red Bed Series strata of the northwest Zagros hinterland, Kurdistan region of Iraq
No full textOne of the major debated aspects of the Zagros orogenic system in the Middle East is the timing of onset of continental collision between Arabia and Eurasia. The Zagros hinterland in the Kurdistan region of Iraq contains an ~2-km-thick clastic depositional sequence of the Red Bed Series (RBS) that rests unconformably on the Arabian foreland and structurally below the Main Zagros fault, which carries the allochthonous volcaniclastic rocks of the Walash and Naopurdan groups in its hanging wall. Detrital zircon (DZ) U-Pb geochronol- ogy constrains both the depositional age and the provenance of the RBS and pinpoints the timing of initial arrival of Eurasian sediment on the Arabian plate. The youngest DZ U-Pb ages for the laterally extensive (~150 km) basal RBS (Suwais unit) imply a middle Oligocene (ca. 26 Ma) maximum depositional age. The provenance data reveal dominant DZ U-Pb age modes of late Paleocene (ca. 55–60 Ma) and middle Eocene (ca. 37–44 Ma) and, importantly, the presence of ~10%–15% DZ grains that are unequivocally derived from Eurasia, includ- ing of Jurassic (150–200 Ma) and late Paleozoic (270–380 Ma) DZ age modes. These data suggest that the RBS deposits were mainly sourced from forearc- and/or arc-related terranes along the southwest margin and hinterland of Eurasia. We advocate that by ca. 26 Ma, Neo- Tethys oceanic crust had been consumed and that Arabia-Eurasia continental collision well was underway as indicated by deposition of strata with Eurasian provenance on the Arabian margin. These DZ U-Pb data from the RBS highlight the significance of provenance data from synorogenic deposits in revealing the timing of initial continent collision by document- ing the earliest arrival of upper-plate sediment on the lower plate
Detrital zircon provenance record of the Zagros mountain building from the Neotethys obduction to the Arabia–Eurasia collision, NW Zagros fold–thrust belt, Kurdistan region of Iraq
No full textRecognition of a new angular unconformity and the synthesis of new detrital zircon U–Pb and (U–Th) / He provenance records, including zircon (U–Th) / He double dating, from the NW Zagros region elucidate the basin dy- namics of the foreland wedge-top and intermontane units, as well as the tectonic processes in the source terranes in response to the different geodynamic phases. In this con- tribution, we present field observations and detrital zircon provenance data from hinterland basins to reconstruct the basin dynamics and the underlying tectonic controls in the NW Zagros area in the Kurdistan region of Iraq. Results reveal that the deposition of the suture zone units of the Red Beds Series (RBS; Suwais Group, Govanda Formation, Merga Group) occurred in an intermontane basin on top of folded Upper Cretaceous units and that the RBS deposits rest with an angular unconformity on the underlying older strata. The RBS provenance data point to the Paleogene Walash– Naopurdan–Kamyaran (WNK) complex as a source area and imply a substantial decrease in magmatism by ∼ 36 Ma, as reflected by the youngest age peaks. New detrital zircon provenance data from the hinterland wedge-top units of the proto-Zagros foreland basin (the Tanjero, Kolosh, and Ger- cus formations) exhibit exclusive derivation from the Upper Cretaceous Neotethys ophiolitic terranes, which differs from the provenance of the older Lower Cretaceous and Paleozoic units that are dominated by the Paleozoic and Neoprotero- zoic age spectra. These shifts in provenance between differ-
ent tectonostratigraphic units argue for a sediment source re- versal from the west to the east in response to ophiolite ob- duction, arrival of the WNK complex, and commencement of the Arabia–Eurasia continental collision during the latest Eocene (< 36 Ma). According to the provenance data, the in- cipient collision was followed by the deposition of the RBS in the hinterland of the proto-Zagros fold–thrust belt as well as the connection of drainages with the collision-related Neo- gene foreland basin
Thermo-tectonic history of the Issyk-Kul basement (Kyrgyz Northern Tien Shan, Central Asia)
No full textAbstract not availableJohan De Grave, Stijn Glorie, Mikhail M. Buslov, Daniel F. Stockli, Michael O. McWilliams, Vladislav Yu. Batalev, Peter Van den haut
Timing of the Arabia-Eurasia continental collision—Evidence from detrital zircon U-Pb geochronology of the Red Bed Series strata of the northwest Zagros hinterland, Kurdistan region of Iraq: REPLY
No full text(U-Th)/He Dating of Phosphates: Apatite, Monazite, and Xenotime
No full textThe common phosphate minerals, apatite Ca_5(PO_4)_3(F,OH,Cl), monazite
(Ce,La,Th)PO_4, and xenotime YPO_4, have found widespread use in geochronology
because they incorporate U and Th into their structures. For example, apatite usually has
a few tens of ppm of both U and Th, while monazite and xenotime usually have hundreds
of ppm to weight percent levels of these elements. As a consequence, these phosphates
can be dated using several fundamentally different isotopic techniques. Elsewhere in this
volume Harrison et al. describe phosphate dating using ingrowth of radiogenic Pb, the
final daughter of U and Th series decay, and Gleadow et al. describe dating based on
damage tracks from the spontaneous fission of ^(238)U. The most recently developed dating
technique applied to phosphates, described in this chapter, uses the accumulation of
α particles from U and Th series decay, (U-Th)/He dating. While phosphate U-Th-Pb
dating is usually used to date high temperature events such as crystallization of igneous
rocks and the timing of prograde metamorphism, fission track and (U-Th)/He dating are
more commonly used to establish cooling histories through low temperatures, for
example, in the range ~110-40°C in the case of apatite.
Dating of minerals using radiogenic He was first explored shortly after the discovery
of radioactivity (Strutt 1908) and was investigated extensively in the 1950s and 1960s,
mostly on very U- and Th-rich minerals such as zircon and titanite (Hurley 1952, 1954;
Damon and Kulp 1957, Damon and Green 1963). Apatite He dating was first investigated
by Zeitler et al. (1987), who studied the diffusion rate of He from apatite and proposed
that apatite He dating might provide a useful thermochronometer, recording cooling
through about 100°C. Further studies both in the laboratory (Lippolt et al. 1994, Wolf et
al. 1996b, Warnock et al. 1997, Farley 2000) and in the natural setting (House et al. 1999,
Stockli et al. 2000) have confirmed this idea, and the technique has now been applied to a
range of tectonic, geologic and geomorphologic problems (e.g., House et al. 1997, 1998;
Spotila et al. 1997, Farley et al. 2001, Stockli et al. 2000). Monazite and xenotime have
only recently come under scrutiny for He geochronology (e.g., Pik and Marty 1999), and
little is yet known of their potential for routine geo- or thermochronometry
Variable helium diffusion characteristics in fluorite
No full textPrecise analysis of the diffusion characteristics of helium in fluorite is crucial for establishing the new fluorite (U-Th-Sm)/He thermochronometer (FHe), which potentially provides a powerful tool for dating ore deposits unsuitable for the application of conventional geochronometers. Incremental helium outgassing experiments performed on fluorites derived from a spectrum of geological environments suggest a thermally activated volume diffusion mechanism. The diffusion behaviour is highly variable and the parameters range between log D-0/a(2) = 0.30 +/- 0.27-7.27 +/- 0.46 s(-1) and E-a = 96 +/- 3.5-182 +/- 3.8 kJ/mol. Despite the fact that the CaF2 content of natural fluorites in most cases exceeds 99 weight percent, the closure temperature (T-c) of the fluorite (U-Th-Sm)/He thermochronometer as calculated from these diffusion parameters varies between 46 +/- 14 degrees C and 169 +/- 9 degrees C, considering a 125 mu m fragment size. Here we establish that minor substitutions of calcium by rare earth elements and yttrium (REE + Y) and related charge compensation by sodium, fluorine, oxygen and/or vacancies in the fluorite crystal lattice have a significant impact on the diffusivity of helium in the mineral. With increasing REE + Y concentrations F vacancies are reduced and key diffusion pathways are narrowed. Consequently, a higher closure temperature is to be expected. An empirical case study confirms this variability: two fluorite samples from the same deposit (Horni Krupka, Czech Republic) with ca. 170 degrees C and ca. 43 degrees C T-c yield highly different (U-Th-Sm)/He ages of 290 +/- 10 Ma and 79 +/- 10 Ma, respectively. Accordingly, the fluorite sample with the high T-c could have quantitatively retained helium since the formation of the fluorite-bearing ores in the Permian, despite subsequent Mesozoic burial and associated regional hydrothermal heating. In contrast, the fluorite with the low T-c yields a Late Cretaceous age close to the apatite fission track (AFT) and apatite (U-Th)/He ages (AHe) from the same locality. Remarkably, thermal modelling of FHe yields comparable results to the well-established modelling based on AFT and AHe. (C) 2016 Elsevier Ltd. All rights reserved.German Research Foundation (DFG) [DU373/6
Variations on the Author
No full text“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Cenozoic Exhumation and Foreland Basin Evolution of the Zagros Orogen During the Arabia‐Eurasia Collision, Western Iran
No full text[Newspaper Clipping: Author Claims Evidence of Second JFK Assassin #1]
No full textNewspaper article titled "Author Claims Evidence of Second JFK Assassin." The article states that author Richard J. Whalen concluded "that there is circumstantial evidence to support the theory of a second assassin in the shooting of President John F. Kennedy.
Multi-stage magmatic activity in the Neotethyan ophiolite of Penjween: Reply on comments on “Generation and exhumation of granitoid intrusions in the Penjween ophiolite complex, NW Zagros of the Kurdistan region of Iraq: Implications for the geodynamic evolution of the Arabia-Eurasia collision zone” by Mohammad et al.
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