Noriko Hasebe

Towards zircon fission-track thermochronology: Reference framework for confined track length measurements

Abstract Horizontal confined track (HCT) lengths and angles to the crystallographic c -axis were measured for both unannealed spontaneous and induced fission tracks (FT) in zircons to establish a reference framework of HCT length measurement. Induced track lengths were analysed on eleven zircon separates from various rock types, including four age standards, and spontaneous lengths on three zircon standards were analysed. In general, the observed HCT lengths in each sample are slightly anisotropic, with shorter tracks having greater angles to the c -axis. In addition, all samples show large anisotropy in the number of observed HCTs, particularly for induced tracks. As an observed mean HCT length is controlled by the combined influence of these two anisotropies, selective measurement of tracks that have specified azimuth angles to the c -axis (e.g., 60–90°) is recommended in annealing studies or thermochronological applications. Mean spontaneous and induced HCT lengths of 60–90° are approximately concordant for individual age standards, although slightly controlled by the difference in overetching effects due to the different etching criteria adopted. This indicates that natural track annealing at an ambient surface temperature at geological timescales is negligible in zircon, in contrast to the case in apatite. Mean lengths of induced HCTs do not differ significantly between the zircons analysed. Therefore the overall mean and standard deviation values of HCT lengths for 60–90° azimuth angles, i.e. 10.50 ± 0.04 (1 standard error) μm and 0.76 ± 0.03 μ m for spontaneous tracks and 10.73 ± 0.04 and 0.63 ± 0.03 μ m for induced tracks, respectively, offer reliable references for HCT length measurement in zircon FT thermochronology.

The evidence of along-arc differential uplift of the Shimanto accretionary complex: Fission track thermochronology of the Kumano Acidic Rocks, Southwest Japan

Abstract The Kumano Acidic Rocks (KARs) are the most extensive of the Tertiary granitic bodies in the southwest Japanese fore-arc. They intrude the Cretaceous-lower Miocene Shimanto accretionary complex, perturbing its initial thermal records related to accretion. To assess the timing of igneous activity and subsequent cooling history of the KARs, fission-track (FT) ages were measured on seventeen zircon and three apatite samples, along with four zircon confined track-lengths. Samples were collected mainly from the Granite Porphyry of the KARs along two traverses intersecting expected isotherms. Zircon and apatite ages show good agreement at approximately 15 Ma and, in conjunction with the observed track-lengths, suggest rapid uniform cooling of the KARs in the temperature range ~ 100− ~ 300°C. In addition, based on the petrographie features of the KARs, the inferred thermal history is best explained by rapid cooling subsequent to the eruption of felsic magma. The rapid cooling of the KARs at approximately 15 Ma indicates that the basement Shimanto accretionary complex in the area had already cooled at that time. FT apatite ages of ~ 10 Ma were reported in the Muroto Peninsula, which is about 100 km southwest of the Kii Peninsula. These contrasting results suggest differential uplift of the Shimanto accretionary complex along its length, presumably caused by some features characterizing the subducting Shikoku Basin. Furthermore, the cooling age of the KARs places constraints on the ages of the Orbulina Datum and the clockwise rotation of Southwest Japan.

Geological partial annealing zone of zircon fission-track system: additional constrains from the deep drilling MITI-Nishikubiki and MITI-Mishima

Abstract Fission track (FT) analysis of zircon from rhyolite and sandstone samples of Ministry of International Trade and Industry (MITI)-Nishikubiki and MITI-Mishima boreholes has given additional constraints on the temperature range of the zircon partial annealing zone (ZPAZ) over a geological time scale. The advantages of these samples are the following: (1) they have been exposed to a stable temperature for ∼1 million years, and (2) it is possible to obtain data at successive temperatures. Ages and confined track lengths were measured in samples from depths of ∼3.5–5.6 km with a current environmental temperature of ∼223 °C at the deepest. Track lengths in samples below 200 °C are still long and FT ages are compatible with their sedimentary ages, showing no evidence of annealing for a heating duration of 1 million years. Samples at 205 °C seem to yield shortened tracks as a result of annealing. Because of a low number of zircon crystals, only four track lengths were measured for the deepest sample whose mean length was shortest among analysed samples. These data are consistent with previous results from ultra-deep boreholes.

The effect of chemical etching on LA-ICP-MS analysis in determining uranium concentration for fission-track chronometry

Abstract LA–ICP-MS (laser ablation–inductively coupled plasma-mass spectrometry) has the potential to measure uranium concentration for fission-track (FT) chronometry as an alternative to thermal neutron-induced fission of 235U. This study examines the effect that chemical etching, required to reveal spontaneous fission tracks of 238U, has upon LA–ICP-MS analyses. Uranium concentrations were measured before and after etching for six large gem-quality apatite crystals and six zircon samples – three large crystals and three FT age standards. Comparison of the results shows no significant difference in 238U concentrations measured on the etched and unetched mineral surfaces. The 238U concentrations determined by the LA–ICP-MS provide reasonable FT ages for the zircon age standards, which, with the previously reported LA–ICP-MS apatite FT results, promotes the use of the LA–ICP-MS for FT chronometry.

Melange-forming processes in the development of an accretionary prism: Evidence from fission track thermochronology

To give thermotectonic constraints on melange-forming processes and to understand the significance of melange in the evolution of accretionary complexes, fission track analysis was carried out on samples collected from sandstone blocks in melange matrix and coherent sandstone sequences adjacent to melange units in the Cretaceous to Miocene Shimanto accretionary complex, southwest Japan, which is one of the typical accretionary complexes with an accumulation of geological information. Nineteen zircon ages and 15 track length distributions from melanges indicate with some local variation that materials with higher maximum paleotemperatures are exposed at the rear part of the prism, as is the case of the coherent units. Two apatite ages are contemporaneous with previously reported apatite ages of ∼10 Ma obtained from coherent units in the region, emphasizing the regional cooling through the apatite closure temperature of ∼100°C at that time. There is no evidence for substantial relative motion of the melanges with respect to the surrounding coherent units; melange formation is explained by processes that exclude large vertical transport (i.e., 5 km) of melange forming materials. These thermotectonic characteristics observed in the Shimanto accretionary complex are maintained during subsequent local internal deformation caused by the successive growth of the wedge.

Confirmation of a Middle Jurassic age for the Eedemt Formation in Dundgobi Province, southeast Mongolia: constraints from the discovery of new spinicaudatans (clam shrimps)

Two spinicaudatan species, Triglypta eedemtensis Li sp. nov. and Dundgobiestheria mandalgobiensis Li gen. et sp. nov., are described on the basis of scanning electron microscopy (SEM) imaging of newly collected specimens from paper-thin laminated black shale of the Eedemt Formation exposed at the Eedemt locality in the Khootiin Khotgor coal mine region of Dundgobi Province in southeast Mongolia. Triglypta eedemtensis is ornamented mainly with puncta and a punctate fine reticulum; radial lirae occur only on two or three growth bands near the venter. The small spinicaudatan Triglypta is a common component of the Middle Jurassic Euestheria ziliujingensis fauna and Sinokontikia fauna, and is a typical taxon in the Middle Jurassic lacustrine sequences of northern Hebei and the Junggar and Turpan basins of the Xinjiang Autonomous District of China; however, it does not occur in stratigraphically higher units elsewhere. Therefore, the age of the Eedemt Formation should be considered Middle Jurassic rather than Early Cretaceous. The Eedemt Formation is much older than the Early Cretaceous Shinekhudag Formation in the Shine Khudag area of southeast Mongolia.

Possible Age Models for Lake Onuma Lacustrine Sediments Based on Tuffs Recovered in Three Cores

Lake Onuma, Hokkaido, Japan, is located south of Hokkaido-Komagatake volcano. The present Lake Onuma was formed by an intensive eruption and partial collapse of the volcanic cone in 1640 AD, which caused a debris flow that dammed the drainage. Three cores (ON12A, C, and D) were recovered from Lake Onuma to examine the environmental change in the region. These three cores are correlated based on lithological descriptions and water content fluctuation. Volcanic deposits Ko-a (1929 AD), Ko-c1 (1856 AD), Ko-d (1640 AD), and possibly Ko-c2 (1694 AD) were successfully identified with the help of radiocarbon age dating, 137Cs radioactivity measurement, and chemical analyses, to construct an age model of these cores.

Comparison of Luminescence Dating Methods on Lake Sediments from a Small Catchment: Example from Lake Yogo, Japan

When applying luminescence dating to sediment deposited in aquatic environments, a key issue for accurate age determination is resetting of acquired luminescence in sediment by surface exposure (bleaching) before burial. The time needed for bleaching is known to vary among the signals used in three methods: optically stimulated luminescence (OSL), infrared stimulated luminescence (IRSL), and post-infrared IRSL (pIRIR). A comparison of luminescence ages from these different signals is therefore useful to assess whether a sample was fully bleached before burial. In a comparison of OSL, IRSL50/225 and pIRIR225 ages of eight samples of fine-grained sediment from a 294-cm-long sediment core from Lake Yogo, a small-catchment lake in central Japan, the IRSL50/225 and pIRIR225 ages were much older than the OSL ages. The IRSL50/225 residual signals were close to zero, and the difference between pIRIR225 and OSL signals was much larger than the pIRIR225 residual signals. Thus, IRSL50/225 and pIRIR225 signals were not completely bleached, which we attribute to the short sediment transport distance in this small catchment. Five corrected bulk radiocarbon (14C) ages agreed with the OSL ages, except for two intervals in which OSL ages were about 500 and 1,900 years older than the corrected 14C ages. These discrepancies are attributable to incomplete bleaching related to sediment transport, whereas the rest of the OSL ages show no evidence of incomplete bleaching. This study shows that even in samples in which the pIRIR225 and IRSL50/225 signals are not well-bleached, OSL dating yields accurate age estimates because of the faster bleaching rate.

Palaeohydrological and Palaeoenvironmental Fluctuations of the Historic Eurimji Lake

Lake Eurimji is located in the northern part of Jaecheon, Chungbug Province, in South Korea. It is well known for its scenic beauty as a nationally registered landscape site (Yang GS, Jungwon Munhwa 14:1–38, 2010). Since the late Bronze Age, its water has been used for agricultural irrigation. The lake was initially formed at the mouth of a fanglomerate where Jurassic granites blocked the main course of a stream channel at about 305 m above sea level (m.a.s.l.). Natural levees grew laterally near the main outlets of the fan-valley mouth up to about 307 m.a.s.l. The lake accumulated organic sandy mud on the sand-and-gravel streambeds extending toward the southern valley mouth near Yongdu Mt., the ages of which were dated as early as ca. 3,000 calibration years Before Present (cal-yr BP). The early evidence of lake deposits is supported by radiocarbon ages for organic muds below the lowermost artificial layers in Lake Eurimji, which vertically constitute the bottom of Eurimji bank or dyke at about 306.5 m.a.s.l. The height is about 15 m. The present study reconstructs the paleo-precipitation of Lake Eurimji by applying an age mode to core sediments in the ER3-1 borehole using the mean grain size of sediments trapped at the bottom of the lake. The results show that lake sediments accumulated at an extremely high rate, ca. 3.49 mm/year during the Early Three Kingdoms Period, around 2,120–1,340 cal-yr BP. The average grain size of the trapped sediments is proportional to the total amount of precipitation during the elapsed time of each monitoring session. Both the paleoclimatic cyclicity and historical documents indicate that droughts in the lake catchment areas prevailed at 80 AD, 120 AD, 160 AD, 310 AD, 350 AD, 460–500 AD, and 620 AD, while relatively wet conditions occurred at approximately 90 AD, 150 AD, 200 AD, 240 AD, 320 AD, 360 AD, 435 AD, and 530 AD. These drought records suggest a multi-decadal cyclicity, particularly during the Early Three Kingdoms Period. This may imply that multi-decadal to century scale fluctuations of winter and summer monsoons controlled the sedimentary regime of Lake Eurimji catchment areas during this period in South Korea.

Uplift and denudation history of the Akaishi Range, a thrust block formed by arc‐arc collision in central Japan: Insights from low‐temperature thermochronometry and thermokinematic modeling

Fission-track (FT), (U–Th)/He (He), and U–Pb data were used to identify the denudation history of the Akaishi Range, central Japan. The northern Akaishi Range is bounded on the east by the Itoigawa-Shizuoka Tectonic Line Fault Zone (ISTL-FZ). The thermochronometric ages progressively decreases with the decreasing distance to the ISTL-FZ. Thermo-kinematic calculations suggest that the age pattern observed can be explained by a 5–7.5 mm/yr reverse slip on the ISTL-FZ that dips 34–45° west and soles onto detachment at a20–22.5 km depth. By assuming the same geometry and slip rate of the fault, thebedrock uplift rates and denudation rates are estimated at ~4 mm/yr. Thus, the uplift and denudation style of the northern Akaishi Range is well explained asa simple tilted thrust block that has been exhumed along the listric ISTL-FZ. On the other hand, considering both the difference in apatite FT age and the active fault distribution, the southern Akaishi Range might be different in uplift origin and timing, although the difference in apatite FT ages between them may be attributable to the chlorine content variation in apatite. The inferred total denudation is larger than several kilometers and likely exceeds 10 km since the beginning of the northern Akaishi Range uplift, suggesting that the low-relief surfaces on the ridges and the relatively constant elevations of the summits reflect post-uplift denudation rather than pre-existing low-relief landforms.