1,721,169 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
Full text linkOne 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
Full text linkRecognition 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
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 textMulti-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.
No full textGeneration 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 Arabian-Eurasian collision zone
No full textTectonic significance of Cenozoic exhumation and foreland basin evolution in the Western Alps
No full textThe Alps are the archetypical collisional orogenic system on Earth and yet our understanding of processes controlling topographic growth in the Cenozoic remains incomplete. Whereas ideas and models on the Alps are abundant, data from the foreland basin record able to constrain the timing of erosion and sedimentation, mechanisms of basin accommodation and basin deformation are sparse. We combine seismic stratigraphy, micropaleontology, white mica 40Ar/39Ar, detrital zircon (U-Th)/He and apatite fission track thermochronology to Miocene-Pliocene samples from the retro-wedge foreland basin (Saluzzo Basin in Italy) and to Oligocene-Miocene sedimentary rocks from the pro-wedge foreland basin (Bârreme Basin in France) of the Western Alps. Our new data show that exhumation in the Oligocene-Miocene was non uniform across the Western Alps. Topographic growth was underway since the Oligocene and exhumation was concentrated on the pro-side of the orogenic system. Rapid and episodic early Miocene exhumation of the Western Alps was concentrated instead on the retro-side of the orogen and correlates with a major unconformity in the proximal retro-foreland basin. A phase of orogenic construction is recorded by exhumation of the proximal pro-foreland in both the Central and Western Alps at ca. 16 Ma. This is associated with high sedimentation rates, and by inference erosion rates, and suggests that an increase in accretionary flux associated with the dynamics of subduction of Europe under Adria controlled orogenic expansion in the Miocene
Application of combined U-Th-disequilibrium/U-Pb and (U-Th)/He zircon dating to tephrochronology
No full textThe combination of U-Th disequilibrium/U-Pb and (U-Th)/He dating of zircon (ZDD) has provided a relatively new radiometric approach suitable for dating Quaternary zircon -bearing volcanic and pyroclasticdeposits. This approach permits the dating of zircon as young as ca. 2.5 ka and has huge potential for tephrochronology and other fields of Quaternary science. Its applicability range covers the critical time-window in Quaternary geochronology beyond the range of C-14 and below the limit of routine Ar/Ar dating (i.e. between ca. 50 ka and 1.5 Ma). The combined U-Th disequilibrium/U-Pb and (U-Th)/He dating approach can also be applied to other actinide-rich minerals such as allanite or monazite. In <1 Ma young volcanic samples, the integration of the low-temperature (U-Th)/He geochronometer with the high temperature U-Th disequilibrium/U-Pb geochronometers is essential in order to correct for disequilibrium in the U-decay chains which, when ignored, may result in erroneous underestimation of true eruption ages. In this review paper we summarize past developments and the current state of the ZDD method, explain the theoretical principles, and document the necessity of combining (U-Th)/He with U-Th disequilibrium and/or U-Pb methods when dating young volcanic material. We then describe analytical procedures and highlight the advantages of the ZDD method. Finally, we present some examples that illustrate the efficacy of this method to derive reliable eruption ages for young tephras, and outline future directions in methodological development and application for this geochronological tool. (C) 2016 Published by Elsevier B.V
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
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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