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Fission-track Analyses (fission-track + analysis)
Selected AbstractsCenozoic Exhumation and Thrusting in the Northern Qilian Shan, Northeastern Margin of the Tibetan Plateau: Constraints from Sedimentological and Apatite Fission-Track DataACTA GEOLOGICA SINICA (ENGLISH EDITION), Issue 3 2009Zhaojie GUO Abstract: The Qilian Shan lies along the northeastern edge of the Tibetan Plateau. To constrain its deformation history, we conducted integrated research on Mesozoic,Cenozoic stratigraphic sections in the Jiuxi Basin immediately north of the mountain range. Paleocurrent measurements, sandstone compositional data, and facies analysis of Cenozoic stratigraphic sections suggest that the Jiuxi Basin received sediments from the Altyn Tagh Range in the northwest, initially in the Oligocene (,33 Ma), depositing the Huoshaogou Formation in the northern part of the basin. Later, the source area of the Jiuxi Basin changed to the Qilian Shan in the south during Late Oligocene (,27 Ma), which led to the deposition of the Baiyanghe Formation. We suggest that uplift of the northern Qilian Shan induced by thrusting began no later than the Late Oligocene. Fission-track analysis of apatite from the Qilian Shan yields further information about the deformation history of the northern Qilain Shan and the Jiuxi Basin. It shows that a period of rapid cooling, interpreted as exhumation, initiated in the Oligocene. We suggest that this exhumation marked the initial uplift of the Qilian Shan resulting from the India,Asia collision. [source] The Romagna Apennines, Italy: an eroded duplexGEOLOGICAL JOURNAL, Issue 1 2001A. Cerrina Feroni Abstract The study of clast composition carried out on the alluvial gravels of the Romagna Apennines of northern Italy has provided evidence for an extensive covering of allochthonous units (Ligurian nappe and Epiligurian succession) above the Miocene foredeep deposits (Marnoso-Arenacea Formation), which has been subsequently eroded during the Late Miocene,Pleistocene uplift. This result is confirmed by the burial history outlined in the Marnoso-Arenacea Formation through vitrinite reflectance and apatite fission-track analyses. The Romagna Apennines represent, therefore, a regional tectonic window where the thrust system that displaced the Marnoso-Arenacea Formation crops out. The geometric relations between this thrust system and the basal thrust of the Ligurian nappe, exposed at the boundaries of the Romagna Apennines (Sillaro Zone and Val Marecchia klippe), are consistent with a duplex structure. Thus, the Romagna Apennines thrust system is an eroded duplex. The duplex roof-thrust corresponds to the surface of the synsedimentary overthrust of the Ligurian nappe on the Marnoso-Arenacea Formation; the floor-thrust is located in the pelagic pre-foredeep deposits (Schlier Formation). Copyright © 2001 John Wiley & Sons, Ltd. [source] Electrodynamic Disaggregation: Does it Affect Apatite Fission-Track and (U-Th)/He Analyses?GEOSTANDARDS & GEOANALYTICAL RESEARCH, Issue 1 2010Jörg Giese désagrégation électrodynamique; analyse des traces de fission sur apatite; recuit; analyse (U-Th)/He; diffusion de l'He Apatite fission-track and (U-Th)/He analyses require the liberation of intact idiomorphic apatite grains from rock samples. While routinely being carried out by mechanical methods, electrodynamic disaggregation (ED) offers an alternative approach. The high-voltage discharges produced during the ED process create localised temperature peaks (10000 K) along a narrow plasma channel. In apatite, such high temperatures could potentially reduce the length of fission tracks, which start to anneal at temperatures > 60 °C, and could also enhance He diffusion, which becomes significant at 30,40 °C over geological time scales. A comparison of fission-track analyses and (U-Th)/He ages of apatites prepared both by mechanical (jaw crusher, disk mill) and ED processing provides a way of determining whether heating during the latter method has any significant effect. Apatites from three samples of different geological settings (an orthogneiss from Madagascar, the Fish Canyon Tuff, and a muscovite-gneiss from Greece) yielded statistically identical track length distributions compared to samples prepared mechanically. Additionally, (U-Th)/He ages of apatites from a leucogranite from Morocco prepared by both methods were indistinguishable. These first results indicated that during electrodynamic disaggregation apatite crystals were not heated enough to partially anneal the fission tracks or induce significant diffusive loss of He. Les analyses des traces de fission et des rapports isotopiques (U-Th)/He sur apatite nécessitent la séparation de grains intacts et automorphes d'apatite à partir des échantillons de roche. La désagrégation électrodynamique (DE) offre une approche alternative aux méthodes mécaniques utilisées actuellement en routine. Les décharges de haute tension produite pendant le processus de DE entrainent la formation de pics de température (10000 K) localisés le long d'un étroit canal de plasma. Dans l'apatite, de telles températures peuvent potentiellement réduire la longueur des traces de fission, qui commencent à recuire à des températures > 60 °C, et peuvent aussi favoriser la diffusion de l'Hélium, qui devient significative à des températures de 30,40 °C sur des échelles de temps géologiques. Une comparaison des résultats obtenus à partir des analyses des traces de fission et des âges (U-Th)/He sur des apatites séparées par des moyens mécaniques (concasseur à mâchoires, broyeur à disque) et sur d'autres séparées par la méthode de désagrégation électrodynamique offre un moyen de déterminer si le chauffage lié,à la seconde méthode a un effet significatif. Les apatites séparées par la méthode DE à partir de trois échantillons provenant de différents contextes géologiques (un orthogneiss de Madagascar, le tuf de Fish Canyon et un gneiss à muscovite de Grèce) fournissent des distributions de longueurs de trace de fission statistiquement identiques par rapport à des échantillons préparés mécaniquement. En outre, des âges (U-Th)/He obtenus à partir d'apatites provenant d'un leucogranite du Maroc et préparées par les deux méthodes (DE et mécaniques) sont indiscernables. Ces premiers résultats indiquent que, pendant la désagrégation électrodynamique, les cristaux d'apatite n'ont pas été suffisamment chauffés pour soit recuire partiellement les traces de fission soit provoquer une perte significative par diffusion de l'He. [source] Late Cretaceous-Cenozoic exhumation of the Yanji area, northeast China: Constraints from fission-track thermochronologyISLAND ARC, Issue 1 2010Xiaoming Li Abstract The Yanji area, located at the border of China, Russia, and Korea, where the Phanerozoic granitoids have been widely exposed, was considered part of the orogenic collage between the North China Block in the south and the Jiamusi,Khanka Massifs in the northeast. In this study, the cooling and inferred uplift and denudation history since the late Mesozoic are intensively studied by carrying out apatite and zircon fission-track analyses, together with electron microprobe analyses (EMPA) of chemical compositions of apatite from the granitoid samples in the Yanji area. The results show that: (i) zircon and apatite fission-track ages range 91.7,99.6 Ma and 76.5,85.4 Ma, respectively; (ii) all apatite fission-track length distributions are unimodal and yield mean lengths of 12,13.2 µm, and the apatites are attributed to chlorine-bearing fluorapatite as revealed by EMPA results; and (iii) the thermal history modeling results based on apatite fission-track grain ages and length distributions indicate that the time,temperature paths display similar patterns and the cooling has been accelerated for each sample since ca 15 Ma. Thus, we conclude that sequential cooling, involving two rapid (95,80 Ma and ca 15,0 Ma) and one slow (80,15 Ma) cooling, has taken place through the exhumation of the Yanji area since the late Cretaceous. The maximum exhumation is more than 5 km under a steady-state geothermal gradient of 35°C/km. Combined with the tectonic setting, this exhumation is possibly related to the subduction of the Pacific Plate beneath the Eurasian Plate since the late Cretaceous. [source] Origin of the Blue Ridge escarpment along the passive margin of Eastern North AmericaBASIN RESEARCH, Issue 1 2004James A. Spotila The Blue Ridge escarpment is a rugged landform situated within the ancient Appalachian orogen. While similar in some respects to the great escarpments along other passive margins, which have evolved by erosion following rifting, its youthful topographic expression has inspired proposals of Cenozoic tectonic rejuvenation in eastern North America. To better understand the post-orogenic and post-rift geomorphic evolution of passive margins, we have examined the origin of this landform using low-temperature thermochronometry and manipulation of topographic indices. Apatite (U,Th)/He and fission-track analyses along transects across the escarpment reveal a younging trend towards the coast. This pattern is consistent with other great escarpments and fits with an interpretation of having evolved by prolonged erosion, without the requirement of tectonic rejuvenation. Measured ages are also comparable specifically to those measured along other great escarpments that are as much as 100 Myr younger. This suggests that erosional mechanisms that maintain rugged escarpments in the early post-rift stages may remain active on ancient passive margins for prolonged periods. The precise erosional evolution of the escarpment is less clear, however, and several end-member models can explain the data. Our preferred model, which fits with all data, involves a significant degree of erosional escarpment retreat in the Cenozoic. Although this suggests that early onset of topographic stability is not required of passive margin evolution, more data are required to better constrain the details of the escarpment's development. [source] Thermal anomaly around the Nojima Fault as detected by fission-track analysis of Ogura 500 m borehole samplesISLAND ARC, Issue 3-4 2001Takahiro Tagami Abstract To better understand heat generation and transfer along earthquake faults, this paper presents preliminary zircon fission-track (FT) length data from the Nojima Fault, Awaji Island, Japan, which was activated during the 1995 Kobe earthquake (Hyogo-ken Nanbu earthquake). Samples were collected of Cretaceous granitic rocks from the Ogura 500 m borehole as well as at outcrops adjacent to the borehole site. The Nojima Fault plane was drilled at a depth of 389.4 m (borehole apparent depth). Fission-track lengths in zircons from localities > 60 m distance from the fault plane, as well as those from outcrops, are characterized by the mean values of ,10,11 ,m and unimodal distributions with positive skewness, which show no signs of an appreciable reduction in FT length. In contrast, those from nearby the fault at depths show significantly reduced mean track lengths of ,6,8 ,m and distributions having a peak around 6,7 ,m with rather negative skewness. In conjunction with other geological constraints, these results are best interpreted by a recent thermal anomaly around the fault, which is attributable to heat transfer via focused fluids from the deep interior of the crust and/or heat dispersion via fluids associated with frictional heating by fault motion. [source] Insights in the exhumation history of the NW Zagros from bedrock and detrital apatite fission-track analysis: evidence for a long-lived orogenyBASIN RESEARCH, Issue 5 2010Stéphane Homke ABSTRACT We present the first fission-track (FT) thermochronology results for the NW Zagros Belt (SW Iran) in order to identify denudation episodes that occurred during the protracted Zagros orogeny. Samples were collected from the two main detrital successions of the NW Zagros foreland basin: the Palaeocene,early Eocene Amiran,Kashkan succession and the Miocene Agha Jari and Bakhtyari Formations. In situ bedrock samples were furthermore collected in the Sanandaj-Sirjan Zone. Only apatite fission-track (AFT) data have been successfully obtained, including 26 ages and 11 track-length distributions. Five families of AFT ages have been documented from analyses of in situ bedrock and detrital samples: pre-middle Jurassic at ,171 and ,225 Ma, early,late Cretaceous at ,91 Ma, Maastrichtian at ,66 Ma, middle,late Eocene at ,38 Ma and Oligocene,early Miocene at ,22 Ma. The most widespread middle,late Eocene cooling phase, around ,38 Ma, is documented by a predominant grain-age population in Agha Jari sediments and by cooling ages of a granitic boulder sample. AFT ages document at least three cooling/denudation periods linked to major geodynamic events related to the Zagros orogeny, during the late Cretaceous oceanic obduction event, during the middle and late Eocene and during the early Miocene. Both late Cretaceous and early Miocene orogenic processes produced bending of the Arabian plate and concomitant foreland deposition. Between the two major flexural foreland episodes, the middle,late Eocene phase mostly produced a long-lasting slow- or nondepositional episode in the inner part of the foreland basin, whereas deposition and tectonics migrated to the NE along the Sanandaj-Sirjan domain and its Gaveh Rud fore-arc basin. As evidenced in this study, the Zagros orogeny was long-lived and multi-episodic, implying that the timing of accretion of the different tectonic domains that form the Zagros Mountains requires cautious interpretation. [source] | ||||||||||