Age and significance of core complex formation in a very curved orogen: Evidence from fission track studies in the South Carpathians (Romania)

Twenty-four new zircon and apatite fission track ages from the Getic and Danubian nappes in the South Carpathians are discussed in the light of a compilation of published fission track data. A total of 101 fission track ages indicates that the Getic nappes are generally characterized by Cretaceous zircon and apatite fission track ages, indicating cooling to near-surface temperatures of these units immediately following Late Cretaceous orogeny.The age distribution of the Danubian nappes, presently outcropping in the Danubian window below the Getic nappes, depends on the position with respect to the Cerna-Jiu fault. Eocene and Oligocene zircon and apatite central ages from the part of the Danubian core complex situated southeast of this fault monitor mid-Tertiary tectonic exhumation in the footwall of the Getic detachment, while zircon fission track data from northwest of this fault indicate that slow cooling started during the Latest Cretaceous. The change from extension (Getic detachment) to strike-slip dominated tectonics along the curved Cerna-Jiu fault allowed for further exhumation on the concave side of this strike-slip fault, while exhumation ceased on the convex side. The available fission track data consistently indicate that the change to fast cooling associated with tectonic denudation by core complex formation did not occur before Late Eocene times, i.e. long after the cessation of Late Cretaceous thrusting.Core complex formation in the Danubian window is related to a larger-scale scenario that is characterized by the NNW-directed translation, followed by a 90° clockwise rotation of the Tisza-Dacia “block” due to roll-back of the Carpathian embayment. This led to a complex pattern of strain partitioning within the Tisza-Dacia “block” adjacent to the western tip of the rigid Moesian platform. Our results suggest that the invasion of these southernmost parts of Tisza-Dacia started before the Late Eocene, i.e. significantly before the onset of Miocene-age rollback and associated extension in the Pannonian basin.     

新疆阿尔泰造山带构造作用的锆石裂变径迹分析

在新疆阿尔泰造山带所获得的19个锆石裂变径迹年龄变化于155-243Ma之间，明显地分为2组，分别对应于2个构造活动期，早期为155-189Ma，晚期为189-243Ma。这与磷灰石裂变径迹年龄反映的62-100Ma和100-160Ma两个构造期完全一致。早期和晚期构造活动期持续的时间分别为54-60Ma和34-38Ma，而这两个构造期之间的间隔时间，则从早到晚由83-89Ma变为89-93Ma。同时，锆石裂变径迹年龄与距特斯巴汗断裂和巴寨断裂的距离有关，反映这两条断裂带对区域构造演化的控制作用。     

Rainfall–runoff responses on Arctic hillslopes underlain by continuous permafrost,North Slope,Alaska, USA

The Arctic hydrologic cycle is intensifying, as evidenced by increased rates of precipitation, evapotranspiration, and riverine discharge. However, the controls on water fluxes from terrestrial to aquatic systems in upland Arctic landscapes are poorly understood. Upland landscapes account for one third of the Arctic land surface and are often drained by zero‐order geomorphic flowpath features called water tracks. Previous work in the region attributed rapid runoff response at larger stream orders to water tracks, but models suggest water tracks are hydrologically disconnected from the surrounding hillslope. To better understand the role of water tracks in upland landscapes, we investigated the surface and subsurface hydrologic responses of 6 water tracks and their hillslope watersheds to natural patterns of rainfall, soil thaw, and drainage. Between storms, both water track discharge and the water table in the hillslope watersheds exhibited diel fluctuations that, when lagged by 5 hr, were temporally correlated with peak evapotranspiration rate. Water track soils remained saturated for more of the summer season than soils in their surrounding hillslope watersheds. When rainfall occurred, the subsurface response was nearly instantaneous, but the water tracks took significantly longer than the hillslopes to respond to rainfall, and longer than the responses previously observed in nearby larger order Arctic streams. There was also evidence for antecedent soil water storage conditions controlling the magnitude of runoff response. Based on these observations, we used a broken stick model to test the hypothesis that runoff production in response to individual storms was primarily controlled by rainfall amount and antecedent water storage conditions near the water track outlet. We found that the relative importance of the two factors varied by site, and that water tracks with similar watershed geometries and at similar landscape positions had similar rainfall–runoff model relationships. Thus, the response of terrestrial water fluxes in the upland Arctic to climate change depends on the non‐linear interactions between rainfall patterns and subsurface water storage capacity on hillslopes. Predicting these interactions across the landscape remains an important challenge.     

Low-temperature constraints on the Alpine thermal evolution of the Western Carpathian basement rock complexes

The Cretaceous to Palaeogene Alpine exhumation of previously buried Variscan basements is recorded in the southern portion of the Western Carpathians in the Gemeric and Veporic units. The Meso-Cenozoic collisional processes resulted in basement exhumation of the Tatric Unit from Palaeogene to Neogene times. According to zircon and apatite fission track data, the Gemeric Unit, an uppermost thick-skinned thrust sheet, cooled from depth levels of ∼10 up to 2.5 km (temperature interval of ∼250–60 °C) about 88–64 Ma ago, after the collapse of overlying Meliata-Turňa-Silica Mesozoic accretionary prism. The middle and lower thick-skinned thrust sheets, Veporic and Tatric units, cooled from the depths of ∼10 up to 2.5 km ∼110–40 Ma ago. The process was controlled by unroofing of footwall from beneath the Gemeric Unit. About 50–20 Ma ago, the internal zone of Tatric Unit gradually exhumed to depth of <2 km and some parts of the unit appeared at the surface level. However, the external zone of Tatric Unit was buried beneath the Eocene to Lower Miocene sedimentary successions and exhumed to the subsurface level at ∼21–8 Ma ago, as a result of oblique collision of the Western Carpathians with the European Platform.     

Exhumation history of the Peake and Denison Inliers: insights from low-temperature thermochronology

Multi-method thermochronology applied to the Peake and Denison Inliers (northern South Australia) reveals multiple low-temperature thermal events. Apatite fission track (AFT) data suggest two main time periods of basement cooling and/or reheating into AFT closure temperatures (～60–120°C); at ca 470–440 Ma and ca 340–300 Ma. We interpret the Ordovician pulse of rapid basement cooling as a result of post-orogenic cooling after the Delamerian Orogeny, followed by deformation related to the start of the Alice Springs Orogeny and orocline formation relating to the Benambran Orogeny. This is supported by a titanite U/Pb age of 479 ± 7 Ma. Our thermal history models indicate that subsequent denudation and sedimentary burial during the Devonian brought the basement rocks back to zircon U–Th–Sm/He (ZHe) closure temperatures (～200–150°C). This period was followed by a renewal of rapid cooling during the Carboniferous, likely as the result of the final pulses of the Alice Springs Orogeny, which exhumed the inlier to ambient surface temperatures. This thermal event is supported by the presence of the Mount Margaret erosion surface, which indicates that the inlier was exposed at the surface during the early Permian. During the Late Triassic–Early Jurassic, the inlier was subjected to minor reheating to AFT closure temperatures; however, the exact timing cannot be deduced from our dataset. Cretaceous apatite U–Th–Sm/He (AHe) ages coupled with the presence of contemporaneous coarse-grained terrigenous rocks suggest a temporally thermal perturbation related with shallow burial during this time, before late Cretaceous exhumation cooled the inliers back to ambient surface temperatures.     

Possible sources of forecast errors generated by the global/regional assimilation and prediction system for landfalling tropical cyclones. Part I: Initial uncertainties

This paper investigates the possible sources of errors associated with tropical cyclone (TC) tracks forecasted using the Global/Regional Assimilation and Prediction System (GRAPES). The GRAPES forecasts were made for 16 landfalling TCs in the western North Pacific basin during the 2008 and 2009 seasons, with a forecast length of 72 hours, and using the default initial conditions (“initials”, hereafter), which are from the NCEP-FNL dataset, as well as ECMWF initials. The forecasts are compared with ECMWF forecasts. The results show that in most TCs, the GRAPES forecasts are improved when using the ECMWF initials compared with the default initials. Compared with the ECMWF initials, the default initials produce lower intensity TCs and a lower intensity subtropical high, but a higher intensity South Asia high and monsoon trough, as well as a higher temperature but lower specific humidity at the TC center. Replacement of the geopotential height and wind fields with the ECMWF initials in and around the TC center at the initial time was found to be the most efficient way to improve the forecasts. In addition, TCs that showed the greatest improvement in forecast accuracy usually had the largest initial uncertainties in TC intensity and were usually in the intensifying phase. The results demonstrate the importance of the initial intensity for TC track forecasts made using GRAPES, and indicate the model is better in describing the intensifying phase than the decaying phase of TCs. Finally, the limit of the improvement indicates that the model error associated with GRAPES forecasts may be the main cause of poor forecasts of landfalling TCs. Thus, further examinations of the model errors are required.     

The Exhumation History of North Qaidam Thrust Belt Constrained by Apatite Fission Track Thermochronology: Implication for the Evolution of the Tibetan Plateau

Determining the spatio-temporal distribution of the deformation tied to the India-Eurasian convergence and the impact of pre-existing weaknesses on the Cenozoic crustal deformation is significant for understanding how the convergence between India and Eurasia contributed to the development of the Tibetan Plateau. The exhumation history of the northeastern Tibetan Plateau was addressed in this research using a new apatite fission track (AFT) study in the North Qaidam thrust belt (NQTB). Three granite samples collected from the Qaidam Shan pluton in the north tied to the Qaidam Shan thrust, with AFT ages clustering in the Eocene to Miocene. The other thirteen samples obtained from the Luliang Shan and Yuka plutons in the south related to the Luliang Shan thrust and they have showed predominantly the Cretaceous AFT ages. Related thermal history modeling based on grain ages and track lengths indicates rapid cooling events during the Eocene-early Oligocene and since late Miocene within the Qaidam Shan, in contrast to those in the Cretaceous and since the Oligocene-Miocene in the Luliang Shan and Yuka region. The results, combined with published the Cretaceous thermochronological ages in the Qaidam Shan region, suggest that the NQTB had undergo rapid exhumation during the accretions along the southern Asian Andean-type margin prior to the India-Eurasian collision. The Cenozoic deformation initially took place in the North Qaidam thrust belt by the Eocene, which is consistent with the recent claim that the deformation of the northeastern Tibetan Plateau initiated in the Eocene as a response to continental collision between India and Eurasia. The immediate deformation responding to the collision is tentatively attributed to the pre-existing weaknesses of the lithosphere, and therefore the deformation of the northeastern Tibetan Plateau should be regarded as a boundary-condition-dependent process.     

金川铜镍矿床中新生代抬升冷却过程：来自裂变径迹的证据

本文基于热年代学理论,利用锆石和磷灰石裂变径迹测年数据及对磷灰石热史的模拟,分析探讨金川铜镍矿床在中新生代的抬升冷却过程,认为金川超基性岩体自中生代以来一直处于持续的冷却抬升阶段,且在中侏罗世和晚白垩世早期经历了两期快速的抬升冷却事件,推断此事件与青藏高原后期活动时间相对应,证明金川岩体受到该事件的远程效应波及,致使该矿床遭受了一定的改造。     

矿床保存变化研究的热年代学技术方法

成矿后矿床的保存与变化是矿床地质研究的重要组成部分,但研究程度不高,至今仍属于薄弱环节。矿床的保存与其隆升剥露密切相关,深入细致研究恢复矿区矿床的隆升和剥露历史,是揭示矿床保存变化的一种重要途径。本文重点论述研究矿床保存变化的技术方法,通过综合应用裂变径迹、(U-Th)/He、~(40)Ar-~(39)Ar年代学以及La-ICP-MS、锆石U-Pb定年等多种技术手段,研究成矿期次、构造活动期次以及二者间的联系,定量计算不同矿区、不同矿体、不同部位、不同时间的冷却隆升速率、隆升幅度、剥蚀速率和剥蚀量,探讨矿床保存深度与剥蚀量间的关系,总结不同时代、不同矿区矿床保存-变化过程,建立新的矿床地质-保存环境模型,完善矿床预测的综合示踪标志,最终预测不同矿区、不同地段隐伏矿床可能的产出深度,给出矿床可能已经被剥蚀殆尽的区段或地段,为深部找矿和区域成矿潜力评价提供依据。本文可为地质工作者的相关应用提供借鉴和手段。     

Exhumation of the Mesozoic Guojialing Granodiorite: Implication for the Preservation of Gold Deposits in the Jiaobei Terrane,China

Preservation conditions are very important for mineral systems and a suitable exhumation process is critical for endogenetic deposits, especially for those deposits formed in orogenic settings, where deposits are inclined to erode away due to strong uplift. The G uojialing batholith, intruding into the L inglong granites and the J iaodong G roup right before regional gold mineralization, is one of the most important gold ore‐hosting M esozoic intrusions in the J iaobei terrane. Gold deposits and the intrusion together underwent similar tectonothermal evolutionary processes. Exhumation and denudation process of the G uojialing granodiorite was constrained by biotite geobarometry and apatite fission track (FT ) analysis. Biotite geobarometric data yields an emplacement depth of 3.0 km, while denudation since 110 M a was calculated from the FT data at about 2.7 km. FT inverse modeling revealed a rapid uplift since ca 100 Ma. Compared with the gold ore‐forming depth which is confined between 2.5 and 9.5 km by fluid inclusion studies, great gold potential in the depths is inferred in the J iaobei terrane. Our result is consistent, to some extent, with the hypothesis of a M esozoic paleoplateau in E ast C hina.