Mesozoic and Cenozoic tectonic history of the central Chinese Tian Shan: Reactivated tectonic structures and active deformation
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[1] The present-day topography of the Tian Shan range is considered to result from crustal shortening related to the ongoing India-Asia collision that started in the early Tertiary. In this study we report evidence for several episodes of localized tectonic activity which occurred prior to that major orogenic event. Apatite fission track analysis and (U-Th)/He dating on apatite and zircon indicate that inherited Paleozoic structures were reactivated in the late Paleozoic-early Mesozoic during a Cimmerian orogenic episode and also in the Late Cretaceous-Paleogene (around 65–60 Ma). These reactivations could have resulted from the accretion of the Kohistan-Dras arc or lithospheric extension in the Siberia-Mongolia zone. Activity resumed in the late Mesozoic prior to the major Tertiary orogenic phase. Finally, the ongoing deformation, which again reactivates inherited tectonic structures, tends to propagate inside the endoreic basins that were preserved in the range, leading to their progressive closure. This study demonstrates the importance of inherited structures in localizing the first increments of the deformation before it propagates into yet undeformed areas.Keywords:
Fission track dating
Paleogene
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Closure temperature
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Abstract Apatite fission-track thermochronology has been used to study the post-Caledonian denudation history of northern Scandinavia. Post-orogenic denudation progressively shifted from the interior of the continent towards the North Atlantic margin. The present-day area of maximum elevation in the Northern Scandes mountain range has experienced continuous denudation at least since Jurassic time. In Jurassic-Cretaceous time, the area north and east of this region experienced either no denudation at all or some denudation followed by a transient thermal event with a peak temperature in late Cretaceous time. Final denudation of the area to the east of the Northern Scandes probably started in late Cretaceous-Paleogene time and possibly accelerated in Neogene time. The denudation history of northern Scandinavia can be explained by scarp retreat of an uplifted rift flank. The pattern and timing of denudation of the Northern Scandes is different from that of the Southern Scandes, which experienced domal-style, late-stage postrift uplift in Neogene time. Geomorphological observations, offshore data from the Atlantic and Barents Sea margins, and scarce stratigraphical information from the mainland are in general agreement with the new thermochronological data.
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Fission track dating
Denudation
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Abstract We assess the proposal of Hendriks & Redfield ( Earth and Planetary Science Letters , 236 , 443–458, 2005) that cross-over of the predicted apatite fission track (AFT)>(U–Th–Sm)/He (AHe) age relationship in the southeastern Fennoscandian shield in southern Finland reflects α-radiation-enhanced annealing (REA) of fission tracks at low temperatures and that more robust estimates of the denudation history are recorded through reproducible AHe data. New AHe results from southern Finland showing variable dispersion of single-grain ages may be biased by different factors operating within grains, which tend to give a greater weighting towards older age outliers. AHe ages from mafic rocks show the least dispersion and tend to be consistently lower than their coexisting AFT ages. In general, it is at the younger end of the single-grain variation range from such lithologies where most meaningful AHe ages can be found. AHe data from multigrain aliquots are, therefore, of limited value for evaluating thermal histories in southern Finland, especially when compared against coexisting AFT data as supporting evidence for REA. New, large datasets from the southern Canadian and Western Australian shields show the relationship between AFT age, single-grain age or mean track length as a function of U content (determined by the external detector method). These do not display the moderately strong inverse correlations previously reported from southern Finland in support of REA. Rather, the trends are inconsistent and generally exhibit weak positive or negative correlations. This is also the case for plots from both shields, as well as those from southern Finland, where AFT parameters are plotted against effective U concentration [eU] [based on U and Th content determined by inductively coupled plasma-mass spectroscopy (ICP-MS)], which weights decay of the parents more accurately in terms of their α‐productivity. Further, samples from southern Finland yield values of chi-square χ 2 >5%, indicating that there is no significant effect of the range of uranium content between grains within samples on the AFT ages, and that they are all consistent with a single population. The oldest AFT ages in southern Finland apatites (amongst the oldest recorded from anywhere) are found in gabbros, which also have the highest Cl content of all samples studied. We suggest, that it is Cl content rather than REA that has influenced the annealing history of the apatites, which have experienced a history including reburial into the partial annealing zone by Caledonian Foreland basin sedimentation. The study of apatite from low U and Th rocks, with relatively low levels of α-radiation damage may provide the most practical approach for producing reliable results for AFT and AHe thermochronometry studies in cratonic environments.
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Abstract The Qilian Shan is situated along the northeastern margin of the Tibetan Plateau, and has undergone multiple episodes of tectonic deformation since the Paleozoic. Strikingly, tectonic deformation in the Qilian Shan and adjacent areas encompasses newly formed and inherited lithospheric and crustal structures. This study presents the Mesozoic‐Cenozoic post‐orogenic thermo‐tectonic histories of these active tectonic units utilizing an integrated approach combining apatite U‐Pb and fission track thermochronometers. The apatite U‐Pb dates are dominantly Paleozoic, mainly recording post‐magmatic cooling of the sampled granitoid rocks. The apatite fission track data record Late Triassic to Cretaceous cooling ages (central ages between ∼223 and ∼88 Ma), and most of the measured mean track lengths are between ∼13 and ∼12 μm. The apatite fission track results and thermal history models revealed that the Qilian Shan, Longshou Shan and Beida Shan experienced various moderate to rapid basement cooling episodes during the Late Triassic to Early Cretaceous, which we interpret as a far‐field response to a series of distant tectonic events including the Paleo‐Asian Ocean's closure, Qiangtang collision, Tethys‐deformation and Lhasa collision. During this period, these fault‐bounded tectonic units displayed different exhumation patterns, with the North Qilian Shan exhibiting a late Early Cretaceous rapid denudation while others preserved uplifted apatite partial annealing zones.
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