FT Ages Research Articles

東北南部，会津地域周辺における中期更新世テフラの層序と編年

This study shows the revised stratigraphy and correlations of the middle Pleistocene tephras in and around the Aizu area, Northeast Japan. Significant marker tephras in this area are as follows, in descending order of stratigraphy : Nm-SB, TG, Hu-TK, and Kn-KD from volcanoes adjacent to Oze, So-OT, and APm. Stratigraphic positions of Sn-MT and Sn-SK are not clear, but they seem to be positioned near Kn-KD.Nm-SB (110 ka) from Numazawa caldera is mainly a plinian pumice fall deposit distributed in the central to western part of Fukushima Prefecture and northern part of Tochigi Prefecture. Eruption producing Nm-SB associated with ash fall, plinian eruption, and pyroclastic flow or pyroclastic surge. TG (125-135 ka), which is characterized by volcanic glass shards with a low index, was probably derived from the Sunagohara caldera. The distribution of fall-out tephra of TG is similar to that of Nm-SB, and pyroclastic flow deposit of TG is recognized southwest to west of the Numazawa caldera and along the western margin of the Aizu basin. The eruption process of TG comprises plinian eruption, pyroclastic flow, and plinian eruption.So-OT (300-330 ka) is composed of an ignimbrite and a fall-out tephra derived from the Shiobara caldera. This ignimbrite is known as Otahara pyroclastic flow deposit. On the other hand, fall-out tephra of So-OT has been newly identified. APm tephra beds are significant widespread tephras derived from volcano in the Hida mountains at 330-400 ka. This study corrects a correlation of APm in this area, which was shown by Suzuki (1993). Tephras identified as APm in this study are Nm-13, -14, -16 tephras below So-OT.Sn-MT (180-260 ka : FT ages) is composed of an ignimbrite and a fall-out tephra derived from the Sunagohara caldera. The former is part of the Sunagohara-Kubota tephra reported by Yamamoto and Sudo (1996) and the Pyroclastic Flow Deposit I reported by Mizugaki (1993). The latter is the Sunagohara-Kachikata tephra along the western margin of Aizu basin reported by Yamamoto and Sudo (1996), and the Okayaji Volcanic Ash Layer at the eastern foot of Adatara volcano reported by Soda and Saijo (1987). Sn-SK (220 ± 50 ka : FT age), originating from the Sunagohara caldera, was defined by Yamamoto and Sudo (1996). Sn-SK is composed of an ignimbrite and a fall-out tephra characterized by abundant accretionary lapilli. The latter is correlative to the Minowa Volcanic Ash Layer (Soda and Saijo, 1987) distributed at the eastern foot of Adatara volcano.All pyroclastic deposits derived from the Numazawa caldera are Nm-NM (5 ka), Nm-KN (50-55 ka), and Nm-SB (110 ka), and those from the Sunagohara caldera are TG (125-135 ka), Sn-MT (180-260 ka), and Sn-SK (220 ka). This means that, at both caldera, explosive eruptions occurred three times over 260, 000 years, and it appears that the active period of explosive eruptions moved from the Sunagohara caldera to the Numazawa caldera. This resulted in the preservation of volcanic landforms with more dissected caldera landforms at the Sunagohara caldera. Intervals between eruptions at the Numazawa caldera range from 50, 000 to 60, 000 years, and the volumes of the three products are similar, indicating periodic and regular activities with a discharge rate of 0.02-0.06 DRE km3/1, 000 years. On the other hand, the mean interval of eruptions at the Sunagohara caldera is 70, 000 to 40, 000 years, and the discharge rate is estimated to be 0.05-0.08 DRE km3/1, 000 years : the latter is equivalent to or a little larger than that of the Numazawa caldera.

O-26 紀伊半島に分布する中期中新世火成岩のジルコンFT年代(4. 紀伊半島とその周辺における中新世火成作用とテクトニクス(液晶有),口頭発表,一般講演)

O-26 紀伊半島に分布する中期中新世火成岩のジルコンFT年代(4. 紀伊半島とその周辺における中新世火成作用とテクトニクス(液晶有),口頭発表,一般講演)

Late Cenozoic geomorphic and tectonic evolution of the Patagonian Andes between latitudes 42°S and 46°S: An appraisal based on fission-track results from the transpressional intra-arc Liquiñe-Ofqui fault zone

Abstract Fission-track (FT) thermochronology has been applied to investigate the low-temperature cooling and denudation history of the Patagonian Andes along the southern part of the intra-arc transpressional Liquiñe-Ofqui fault zone between 42° and 46°S. The Liquiñe-Ofqui fault is shown to have been the focus of enhanced cooling and denudation initiated between ca. 16 and 10 Ma. Several fault blocks with different cooling histories are identified; these are separated by major oblique- or reverse-slip faults proposed to form the eastern part of a major (crustal-scale) dextral transpression zone. Local very fast rates of cooling and denudation between ca. 7 and 2 Ma were coeval with collision of the Chile Rise (an active mid-oceanic ridge) with the Peru-Chile Trench between ∼47° and 48°S. This location is close to the southern termination of the Liquiñe-Ofqui fault, implying that the collision of the ridge was a major force driving late Cenozoic transpression. The lack of significant cooling and denudation before ca. 16 Ma is indicative of pure strike-slip or transtensional movement along the Liquiñe-Ofqui fault before the collision of the ridge. Digital landscape analysis supports glacial and periglacial erosion as the main contributor to denudation since ca. 7 Ma, leading to restriction of topographic development. The combination of transpression-induced rock uplift and glacial erosion is shown to be very effective at causing localized denudation. Anomalously young FT ages along the Liquiñe-Ofqui fault are attributed to the existence of a late Cenozoic localized heat-flow anomaly along the fault.

Integration of zircon color and zircon fission-track zonation patterns in orogenic belts: application to the Southern Alps, New Zealand

An exhumed crustal section of the Mesozoic Torlesse terrane underlies the Southern Alps collision zone in New Zealand. Since the Late Miocene, oblique horizontal shortening has formed the northeastern–southwestern trending orogen and exhumed the crustal section within it. On the eastern side, rocks are zeolite- to prehnite–pumpellyite-grade greywacke; on the western side rocks, they have the same protolith, but are greenschist to amphibolite facies of the Alpine Schist. Zircon crystals from sediments in east-flowing rivers (hinterland) have pre-orogenic fission-track ages (>80 Ma) and are dominated by pink, radiation-damaged grains (up to 60%). These zircons are derived from the upper ∼10 km crustal section ( unreset FT color zone) that includes the Late Cenozoic zircon partial annealing zone; both fission tracks and color remain intact and unaffected by orogenesis. Many zircon crystals from sediments in west-flowing rivers (foreland) have synorogenic FT ages, and about 80% are colorless due to thermal annealing. They have been derived from rocks that originally lay in the reset FT color zone and the underlying reset FT colorless zone. The reset FT color zone occurs between ∼250 and 400 °C. In this zone, zircon crystals have color but reset FT ages that reflect the timing of orogenesis.

Thermochronologic constraints on the Late Cenozoic exhumation along the Alpine and West Carpathian margins of the Pannonian basin

The uplifted and partly re-exhumed basement of the Neogene Pannonian basin was investigated by the ap- atite fission track thermochronometer along the marginal belt to the Eastern Alps and the Western Carpathians. In some zones the reset Late Miocene apatite FT ages and the high organic maturation of the Neogene sediments give evidences on the high magnitude of the post-Middle Miocene sediment removal. Thermal modeling of the fission track data allows to quantify this removal and to conclude that its magnitude in the investigated sites was in the range of 1-1.5 km. In this pa- per we review the thermochronologic data and integrate them with the seismic, structural and sedimentological evidences on the Late Miocene-Pliocene erosional phases.

Dénudations cénozoïques en Corse : une analyse thermochronologique par traces de fission sur apatites

Abstract An apatite fission track thermochronological survey of Corsica was undertaken in order to decipher its Cenozoic thermo-tectonic evolution. A remarkable geographic zoning of FT ages with a gradient from southwest (older ages) to northwest (younger ones) is interpreted as the result of two major cooling steps, respectively dated at 26–20 Ma and 18–10 Ma. These coolings are attributed to exhumation phases associated with the Oligo-Miocene Liguro-Provencal rifting and the Miocene Tyrrhenian extension, respectively.

The erosion history of the Central Alps: evidence from zircon fission track data of the foreland basin sediments

Fission track dating on detrital zircons of Alpine debris in the Swiss molasse basin provides information about the erosion history of the Central Alps and the thermal evolution of source terrains. During Oligocene times, only sedimentary cover nappes, and Austroalpine basement units were eroded. Incision into Austroalpine basement units is indicated by increasing importance of Cretaceous cooling ages in granite pebbles upsection. Erosion of Penninic basement units started between 25 and 20 Ma. Early Oligocene zircon FT ages show that Penninic basement units were exposed at ∼20 Ma. Deeper Penninic units of the Lepontine Dome became exposed first at ∼14 Ma, contemporaneously with the opening of the Tauern window in the Eastern Alps. A middle Miocene cooling rate of 40 °C Myr−1 is deduced for the Lower Penninic units of the Lepontine Dome.

関東山地北東縁部寄居溶結凝灰岩類の熱水変質Ｋ‐Ａｒ年代

Yorii welded tuffs in the northeastern Kanto Mountains are penetratively crushed and strongly hydrothermally altered. They are younger constituents of the Atokura Nappe and their ages indicate the lower limit of the time when the Atokura Nappe thrusted up on the Sanbagawa belt.The whole rock K-Ar ages of the altered Yorii welded tuffs give 58.0±2.9 Ma which is consistent with zircon FT age (59.6 Ma). This suggests that the hydrothermal alteration followed the formation of welded tuffs at the last stage of cooling process. After this hydrothermal alteration, the Yorii welded tuffs thrusted up on the Sanbagawa belt.

Exhumation history of orogenic highlands determined by detrital fission-track thermochronology

Abstract A relatively new field in provenance analysis is detrital fission-track thermochronology which utilizes grain ages from sediment shed off an orogen to elucidate its exhumational history. Four examples highlight the approach and usefulness of the technique. (1) Fission-track grain age (FTGA) distribution of apatite from modern sediment of the Bergell region of the Italian Alps corresponds to ages obtained from bedrock studies. Two distinct peak-age populations at 14.8 Ma and 19.8 Ma give calculated erosion rates identical to in situ bedrock. (2) Zircon FTGA distribution from the modern Indus River in Pakistan is used to estimate the mean erosion rate for the Indus River drainage basin to be about 560 m Ma −1 , but locally it is in excess of 1000 m Ma −1 . (3) FTGA distribution of detrital apatite and zircon from the Tofino basin records exhumation of the Coast Mountains in the Canadian Cordillera. Comparison of detrital zircon and apatite FT ages gives exhumation rates of c. 200 m Ma −1 during the interval between c. 34 and 54 Ma, but higher rates ( c. 1500 m Ma −1 ) at c. 56 Ma. (4) FTGA analysis of apatite grain ages from a young basin flanking Fiordland in New Zealand indicates that removal of cover strata was followed by profound exhumation at c. 30 Ma, which corresponds to plate reorganization at this time. Exhumation rates at the onset of exhumation were c. 2000–5000 m Ma −1 . These studies outline the technique of detrital FTGA applied to exhumation studies and highlight practical considerations: (1) well-dated, stratigraphically coordinated suites of samples that span the exhumation event provide the best long-term record; (2) strata from the basin perimeter are the most likely to retain unreset detrital ages; (3) the removal of ‘cover rocks’ precedes exhumation of deeply buried rocks, which retain a thermal signal of the exhumation event; (4) steady-state exhumation produces peak ages that progressively young with time and have a constant lag time; (5) same-sample comparison of zircon and apatite peak ages is best in sequences with high-uranium apatite grains (>50 ppm), and peak-ages statistics can be improved by counting numerous apatite grains (>100).

Miocene thermal history of the southwestern margin of the Styrian Basin: vitrinite reflectance and fission-track data from the Pohorje/Kozjak area (Slovenia)

The Kozjak (Possruck) and Pohorje mountains form the southwestern basement rim of the Neogene Styrian Basin. This region was affected by two Tertiary magmatic events: the emplacement of the Oligocene Pohorje tonalite, and of Early/Middle Miocene dacites in the western Pohorje. Vitrinite reflectance and fission-track data are used to reconstruct the thermal history and to constrain the exhumation of the Pohorje/Kozjak area. Early Miocene sediments lacking a thermal overprint contain apatite grains of Eggenburgian (∼19 Ma) cooling age, only 1–2 Ma older than the time of deposition. The cooling rate of the mainly Austroalpine source units was very fast, denoting tectonic denudation. It means that in the Eastern Alps during Early/Middle Miocene time the Pohorje/Kozjak region in addition to the Tauern and Rechnitz windows was supplying sediment into the surrounding basins with nearly syn-sedimentary apatite cooling ages. Vitrinite reflectance anomalies in Early Miocene sediments in the Ribnica–Selnica trough, located between the Kozjak and Pohorje mountains, and at the eastern margin of the Kozjak mountains give evidence for a strong post-depositional thermal overprint. Thermal models based on nearby wells suggest that Miocene temperatures were as high as 220°C. Apatite fission-track ages indicate that the thermal overprint had terminated by middle Badenian (14.4±2.3 Ma) time. Vitrinite reflectance anomalies in the Ribnica–Selnica trough are a result of Early/Middle Miocene volcanic activity. Another vitrinite reflectance anomaly, situated at the eastern margin of the Kozjak mountains, extends eastward into the Somat–Radkersburg area. The heat source in this region is not obvious. Possible heat sources are: (1) a shallow pluton beneath this area, which easily can explain the reconstructed Early/Middle Miocene heat flow of more than 250 mW/m 2; and (2) advective heat transport due to rapid exhumation of hot metamorphic rocks in the Pohorje/Kozjak region. Rapid exhumation is proven by fission-track dating. It would explain the appearance of the nearly syn-sedimentary detrital apatite FT ages in the sediments and the post-depositional heating.

Exhumation of the Rechnitz Window at the border of the Eastern Alps and Pannonian Basin during Neogene extension

The Rechnitz Window is the easternmost Penninic window of the Alps and the only one that is partly covered by Neogene sediments. Zircon and apatite fission-track ages have been measured on the Penninic metasediments of the Rechnitz series to better understand the exhumation of the window at the Alpine-Pannonian border. The zircon FT ages range from 21.9 to 13.4 Ma, similar to the white mica K/Ar ages (19 and 23 Ma). The exhumation rate during the Early Miocene extension was high (∼40°C/Ma). Apatite samples display FT ages of 7.3 and 9.7 Ma, thus the cooling rate was significantly less (7–11°C/Ma) during the post-rift uplift uplift in Late Miocene-Pliocene than in the Early Miocene escape period. Zircon FT ages decrease eastward due to the gradual southeastward slide of the Austroalpine cover along low-angle normal faults. The exhumation of the Rechnitz metamorphic core complex is younger than the unroofing of the eastern Tauern Window. Morphology of detrital zircon crystals of the Penninic metasediments was used for the differentiation of tectonic subunits and to trace the internal structure of the window. One population was probably derived from undifferentiated calc-alkaline granitoids and tholeiitic granitoid sources, while the other derived from evolved calc-alkaline granitoids and alkaline granitoids of anorogenic complexes. The areal distribution of zircon populations is in harmony with the eastward shift of zircon cooling ages. These data indicate the exhumation of deep levels of Penninic metasediments in the centre of the window.

Fission-track constraints on timing of Alpine nappe emplacement and rates of cooling and exhumation, Torrox area, Betic Cordilleras, S. Spain

Fission-track analyses were performed on apatite and zircon from 8 samples of an exhumed gneiss body belonging to the Alpine Alpujárride nappe complex of the Internal Zone of the Betics. The eight FT apatite ages range from 13.0 to 20.4 Ma, with a central age for six of them of 16.6 ± 0.8 Ma (1σ); a single zircon FT age determination yielded a central age of 17.2 ± 2.5 Ma. Apatite fission-track length distributions are narrow with mean lengths between 14 and 14.7 μm. Such distributions are indicative for very high cooling rates, confirming the results of a previous thermochronological study using RbSr, KAr and ArAr techniques; cooling rates of 250–350°C/m.y. from above 600°C to below ∼350°C at 20-18 Ma were indicated by the tight clustering pattern of the mineral ages. This age bracket may represent the best current estimate for the termination of the Alpujárride nappe emplacement in this region and the very fast large-scale tectonic unroofing. Subsequent cooling may be characterised by lower rates of ∼150°C/m.y. in the period 19-17 Ma and down to ∼5°C/m.y. in the period 17-14 Ma as indicated by best-fit thermal history modelling based on spontaneous apatite fission-track length distributions. This late-stage cooling process may (in part) be related to restoring the isotherm pattern disturbed by the rapid uplift. Further cooling at rates of ∼2°C/m.y. is suggested for the period 14-5 Ma and this may be related to late-stage exhumation of the metamorphic rocks. The data indicate that the period of very fast cooling and coeval extensional tectonics in this area ended at ∼18–19 Ma.

Insights on the thermal history of the Valles caldera, New Mexico: evidence from zircon fission-track analysis

The zircon fission-track dating method was applied to the VC-2B core obtained from the active hydrothermal system at Sulphur Springs, Valles caldera, New Mexico. Four samples were analyzed to obtain both zircon ages and track length data from Permian strata to Precambrian quartz monzonite, the present temperatures of which are from 222 to 294 °C. Zircon ages obtained from the deeper three samples indicate partially annealed ages of about 450–600 Ma. Thus, zircons from Permian strata probably originated from Precambrian rocks. Contrary to the normal relationship between FT ages and temperatures of the sampling points, non-correlation between them is probably due to hot fluid flow and/or hot vein emplacement associated with recent volcanic activity. Zircon ages of the shallowest sample were much younger than the Permian, the reason for which is unknown. The closure temperature of the zircon fission-track dating method seems to be higher than that previously determined from geological data.

Age and tectonic evolution of the olympic subduction complex as inferred from FT ages for detrital zircons

Age and tectonic evolution of the olympic subduction complex as inferred from FT ages for detrital zircons

Sedimentary basin analysis by FT ages of detrital zircons

Sedimentary basin analysis by FT ages of detrital zircons

L'apatite de Durango (Mexique): Analyse d'un minéral standard pour la datation par traces de fission

Durango apatite is analyzed by four analysts using three different dating techniques. Both the annealing technique and the external detector technique are applied to basal and prismatic sections cut from single crystals. The population technique is applied to mounts of randomly oriented grains. Provided an individual determination of the geometry factor is performed, all ages are indistinguishable from the reference age of 31.4 ± 0.5 Ma. There is no noticeable influence of either the dating technique or the crystallographic orientation of the dated section on the FT age. Based on the projected lengths of surface tracks, spontaneous track length reduction varies from ∼4% to ∼10% (mean ∼6%), depending on the analyst. Two simple corrections are applied to eliminate the net effect of the missing short tracks on the mean projected length. After correction the amount of shortening of the spontaneous tracks is found to be slightly larger (mean ∼8%) but is still dependent on the analyst (∼4% to ∼12%). The mean is in fairly good agreement with the value obtained from confined track length measurements (∼9%). Finally, an isochronal (1 h) plateau annealing procedure was applied to a separate set of basal and prismatic sections cut from the same single crystals. Annealing temperatures up to 300°C do not affect the FT age. Erratic ages at 350°C are ascribed to the occurrence of unetchable gaps. It is argued that the absence of a distinct plateau up to temperatures where it could be expected from theoretical considerations indicates that the FT age of Durango apatite needs no correction.

丹沢山地東部の中新世凝灰岩のＫ‐ＡｒおよびＦＴ年代

丹沢山地東部の中新世凝灰岩のＫ‐ＡｒおよびＦＴ年代

A reactor intercalibration for fission track-dating and uranium micromapping

Selected positions of three reactors in France, Italy and Brazil were studied for Fission-Track dating and Uranium distribution micromapping. Spatial gradients of neutron fluxes within the chosen irradiation facilities resulted negligible. Comparison of FT ages calculated by different neutron dosimetries (SRM 962a and SRM 963a standard glass; Fish Canyon Tuff apatite used as age standard) shows that equivalent results are obtained, despite widely differing irradiation conditions in three different reactors. Some observed systematics trends are very probably due to the individual fluence determination.

福知山盆地の中部更新統

The Fukuchiyama Basin is one of the small intermontane basins distributed in the Meso-Paleozoic terrain of the northwestern part of the Kinki District. The basin is filled with the Quaternary deposits having wide sedimentary surfaces which are regarded as the extensions of the“Higher Terrace”widely spreading in the central part of the Kinki. The followings have been clarified by the present studies.1) The Quaternary deposits in this basin reaches more than 50m in thickness consisting of five sedimentary units succeeded conformably, which are collectively called the Fukuchiyama Formation. Each unit was formed as a small fan developed along the marginal part of the basin or as fluvial deposit along the main river course flowing into the basin.2) Fig. 13 is the summary of the results of the studies of the Fukuchiyama Formation from viewpoints of paint fossil and paleomagnetism. No evidence of the Metasequoia flora was found, which shows that this formation is correlative to the Upper Part of the Osaka Group. The Sabia bed, intercalated in the upper horizon of the formation, contains the plant remains of warmer or subtropical types and is thought to correspond to the Ma 8 horizon of the Osaka Group.3) The paleomagnetic polarity of the Fukuchiyama Formation is normal except one narrow horizon showing reverse event. This is the character of the Brunhes Normal Epoch.4) The Fukuchiyama Formation closely resembles to the Meimi Formation in the Harima Basin in various points, which has a FT age of 0.49±0.09 Ma in the lowest horizon.5) The age of the Fukuchiyama Formation is inferred to be the Middle Pleistocene, about 0.6 or 0.7 Ma to 0.2 or 0.15 Ma, considering that the Brunhes Normal is generally dated 0.7 Ma to present with brief revesals at 0.11 and 0.02 Ma and Ma 8 horizon of the Osaka Group is intercalated with a volcanic ash layer having a FT ale of 0.38±0.04Ma.

九州中北部に分布する火山岩のＫ‐Ａｒ年代とフィッション・トラック年代の比較検討 火山構造性陥没地の形成開始年代

In central-north Kyushu, a large volume of Pliocene-Quaternary volcanic rocks are distributed in and around the east-west elongated graben (40×30 km), which is considered to be a volcanotectonic depression (VTD). The radiometric ages of the volcanic rocks are isochronally zonated in concordant with the outline of the VTD, so that the ages increase from the center to the rim of the VTD. In this study, K-Ar ages were determined upon four volcanic rocks from the northern rim of the VTD. K-Ar ages of the volcanic rocks from the Hikosan area (northwest part of the VTD) were determined as 5.0±0.4, 4.0±0.2 and 4.7±0.3 Ma, which show close agreement with the respective FT dates (Watanabe et al., in press). In the Yamaga area (northeast part of the VTD), the andesite lava from the Yabakei Formation, whose FT age is reported as 12.1 Ma (Hayashi et al., 1983), was dated by K-Ar method as 5.7±0.3 Ma. The K-Ar ages of the Usa Group which is overlain by the Yabakei Formation near the Yamaga area show 4.8±0.4, 5.3±0.3 and 4.9±1.1 Ma (Kamata, 1985b). Accordingly the previous FT age (12.1 Ma) for the Yabakei Formation is apparently too old, which may account for corrections made for thermal alteration of the specimen and/or contamination of accidental particles. On the other hand, the K-Ar age (5.7 Ma) is considered to be close to the eruption age of the Yabakei Formation in this area. These results indicate that the oldest volcanic rocks, which are distributed on the rim of the VTD, were erupted 5-6 Ma ago. Therefore it is suggested that the VTD has been formed since 5-6 Ma ago in centralnorth Kyushu.