Island Arc最新文献

The Middle Miocene was a period of tectonic transition in the southwest Japan arc. The cessation of backarc spreading at 15 Ma has been thought to have caused a pronounced change in the overall stress state from tension to compression in the arc-perpendicular direction. However, the spatial variation in stress throughout the arc, especially in the forearc, which reflects the characteristics of plate subduction, has yet to be delineated. This study investigates the stress field of the forearc region and its temporal change by applying a stress tensor inversion technique to outcrop-scale dilatant fractures. We measured the attitudes of clastic dikes and mineral veins in the Lower–Middle Miocene Tanabe Group, the paleo-forearc basin deposits in the southwestern Kii Peninsula. Results of paleostress analyses show that the Tanabe Group was influenced by (i) stress with an E–W minimum compressional axis before 15 Ma and (ii) stress with a NW–SE maximum horizontal compressional axis after 15 Ma. In contrast to conventional interpretations, we find that the former stress differed from arc-perpendicular tension in the backarc region before 16 Ma. The latter stress differed from trench-parallel tension on the southernmost Kii Peninsula. Such spatial non-uniformity of the forearc stress can be explained by the occurrence of a dynamic backstop, such as a landward area of the out-of-sequence thrust, beneath the Tanabe Group.

Zircon U–Pb dates for detrital grains in the Shakubetsu Formation of the uppermost Eocene Urahoro Group and the Upper Miocene Atsunai Formation distributed in the eastern part of Hokkaido have been determined for the first time. Both weighted mean 48.8 ± 1.4 Ma (n = 80) for < 70 Ma zircons and youngest grain 36.5 + 1.8, −2.5 Ma (Middle to Late Eocene) ages of the Urahoro Group are consistent with the sedimentary ages and Paleogene magmatic arc origin of the detritus clarified in the previous studies. Although the Zircon U–Pb ages (n = 24) of the Atsunai Formation range between 5 and ~1700 Ma, a weighted mean age of 7.05 ± 0.57 Ma (n = 10) for < 10 Ma zircons (youngest cluster) could constrain the maximum sedimentary age, which is consistent with the previous ages from diatom biostratigraphy and K-Ar method. However, while the K-Ar age was obtained from an Ol-Cpx-Opx basalt in tuff breccia, the dated zircons were probably derived from felsic tuff including many pumice grains. Therefore, the bimodal volcanism could have occurred at c. 7 Ma in the present forearc region, indicating that the volcanic front at this age migrated south significantly, correlated to the one which also occurred coevally in the seaward side of the present volcanic front in the NE Japan arc. Further, it has been found that the eastern boundary of the Shiranuka Hills is characterized by an ESE verging, large-scale monocline based on the present field study and previous geological maps. Since the Atsunai Formation was involved in the formation of the large-scale monocline, the structure started to form after the sedimentation at c. 7 Ma. The fact implies that the ongoing collision of the Kuril forearc sliver with the NE Japan arc has propagated to this area since the latest Late Miocene.

The southeastern edge of the South China Block (SCB) experienced the subduction of the Paleo-Pacific Block, resulting in varying tectonic settings across different regions of the SCB during the subduction. To explore the tectonic settings during sediment deposition in the southeastern edge of the SCB in the early Late Cretaceous, we analyzed the Jiao Yangjing-Yetang section of the Xingning Basin, Guangdong province. This section contains a sedimentary sequence of the Upper Cretaceous Yetang Formation, facilitating lithofacies analysis, U–Pb zircon chronology, trace elements analysis, crustal thickness estimation, Hf isotopes analysis, and age distribution comparison of detrital zircons. Our findings indicate that the Late Cretaceous Yetang Formation was deposited during the early Late Cretaceous Cenomanian stage (99.6 ± 1.9 Ma), and identifies five groups of detrital zircon U–Pb ages: 2320–1810, 1100–670, 500–400, 230–160, and 130–96 Ma. The early Late Cretaceous ℇ_Hf (t) values ranged from −7.4 to 0.6, suggesting that the debris originated from the southwestern Fujian and northern Guangdong regions. Trace elements analysis of detrital zircons indicates increasing crustal thickness during the early Late Cretaceous. The Xingning Basin was in a compressive environment during the late Early Cretaceous, and the compressive condition continued until the early Late Cretaceous period (99.6 ± 1.9 Ma).

Sri Lanka occupies a pivotal geological position within East Gondwana, featuring four prominent high-grade Precambrian lithotectonic units: the Highland Complex (HC), the Wanni Complex, the Vijayan Complex (VC) and the Kadugannawa Complex. Despite reasonable geochemical and geochronological investigations into rocks in the VC, a comprehensive understanding of their petrological characteristics, particularly their pressure–temperature–time evolution, remains limited, impeding accurate terrane correlations. In this study, a detailed analysis of mineral assemblages, reaction textures, and U–Pb zircon ages in samples collected from the eastern margin of the HC to the eastern coast, in the VC were conducted. Results indicate that the VC preserved textural evidence for prograde dehydration reactions indicating upper amphibolite to granulite facies transition. The peak metamorphism is followed by a stage of near isobaric cooling stage. Conventional geothermobarometric analysis coupled with phase equilibria modeling of VC gneisses reveal peak metamorphic P–T conditions of 790°C–830°C and 7–8 kbar. Retrograde metamorphism has occurred until the temperature decreased to 740°C. The U–Pb zircon dates at ca. 1010–960 Ma suggest protolith ages of the VC coinciding with Rodinia amalgamation and dispersal, aligning metamorphic thermal peaks at ca. 575–545 Ma with the Kuunga orogeny during Gondwana amalgamation. Our data signifies connections between the VC and both the Lurio foreland in Mozambique and the Innhovde Suite and Yamato-Belgica Complex in East Antarctica, however, the differences in metamorphic conditions and geochronological signatures necessitate further investigation into its tectonic history and paleogeographic evolution.

The catastrophic Hose and Ogawatuetui landslides occurred coincidentally in 1892 following heavy rainfall in the upper reaches of the Kaifu River, Tokushima, Japan. The area is underlain by a steeply dipping coherent formation with many faults, and lies in the Shimanto Belt where deep-seated catastrophic landslides are known to occur following heavy rainfall and large subduction zone earthquakes. The main geological and geomorphological causes of the landslides were found to include local relief ≥ 350 m (1200-m-diameter analysis windows); knickpoints and slope breaks at 90–150 m above sea level, competent massive and thick-bedded sandstone with open fractures on the upper slope, and alternating thin beds of incompetent fine sandstone and shale beds of high deformability and low permeability on the lower slope.